Thermodiffusive separation device

Abstract

Disclosed is a thermodiffusion device for a liquid comprising at least one diffusing species, the device for separating the liquid into a fraction that is more concentrated with the diffusing species, and a fraction that is less concentrated with the diffusing species, the device comprising: two opposing walls, each comprised of a thermally conductive material, one of the walls being at a higher temperature than the other of the walls in use; a plurality of sidewalls located between the two walls and arranged to form a plurality of channels through which the liquid flows in use, the plurality of sidewalls each comprised of a material that has a thermal conductivity that is lower than the thermal conductivity of each of the two opposing walls; an inlet configured to allow the liquid to flow into the plurality of channels in use; and at least two outlets configured to collect the more concentrated fraction and the less concentrated fraction.

Description

[0001] THERMODIFFUSIVE SEPARATION DEVICE

[0002] TECHNICAL FIELD

[0003] This disclosure relates to devices and methods for the thermodiffusive separation of diffusing species in a liquid. This disclosure also relates to the use of the devices and methods for thermodiffusive desalination.

[0004] BACKGROUND ART

[0005] Desalination is a $20 billion-dollar (USD) industry that mitigates water scarcity in many regions in the world (including Australia, the Middle East, and South and North America). Currently, 1.1 billion people worldwide lack access to water. One-third find water scarce for at least one month per year. The problem of water scarcity is becoming more extreme due to population growth and climate change.

[0006] Currently, desalination is used to produce more than 100 million cubic metres of freshwater per day. However, this amounts to a few percentage points of human needs. Desalination using conventional methods is expensive in terms of operational costs (high energy consumption and maintenance costs) and capital costs (desalination plants require expensive pumps, membranes and advanced equipment, as well as dedicated electric power stations).

[0007] Most concepts of desalination fall into two categories: phase-change-based thermal desalination and functional-material-based desalination. Reverse osmosis (RO) is the current state of the art in the desalination industry with regard to functional-material-based desalination and produces more than 74% of the desalinated water worldwide. However, RO relies on membranes that degrade over time due to fouling and scaling. RO is also a process that consumes vast amounts of electric power, which makes this method inaccessible for most of the population suffering from water scarcity. Materials degradation in RO is unavoidable and also adds to the costs. Membrane-less desalination methods have been proposed, but these are chemical based methods that are not scalable. Phase- change-based thermal desalination processes, such as multi-stage flash distillation and freeze distillation, are generally associated with higher energy consumption due to the phase-change requirements.

[0008] It is to be understood that, if any prior art is referred to herein, such reference does not constitute an admission that the prior art forms a part of the common general knowledge in the art, in Australia or any other country.

[0009] SUMMARY

[0010] Disclosed herein in a first aspect is a thermodiffusion device for a liquid comprising at least one diffusing species. In the context of this specification, the term “diffusing species” should be understood as being any species that is capable of diffusing in the liquid that comprises the diffusing species when the liquid is subjected to thermodiffusion as set forth herein. In this regard, it should be understood that the diffusing species may comprise an ionic species. Alternatively or additionally, the diffusing species may comprise a non-ionic species. The device can be used for separating the liquid into a fraction that is more concentrated with the diffusing species, and a fraction that is less concentrated with the diffusing species. The thermodiffusion device may be suitable for use in a variety of applications. For example, the thermodiffusion device may be suitable for reducing the concentration of diffusing species in a liquid. Alternatively, the thermodiffusion device may be suitable for concentrating a diffusing species in a liquid. For example, the device may be used to increase the concentration of a diffusing species in aqueous solutions or complex mixtures such as biofluids, including protein aqueous solutions, animal blood and animal plasma. As another example, the device may be used to increase the concentration of a diffusing species in non-aqueous solutions, such as polymeric solutions. Accordingly, in the context of this specification, the term “solutions” should be understood as comprising both aqueous and non-aqueous solutions.

[0011] The thermodiffusion device can comprise two opposing walls. Each opposing wall can be comprised of a thermally conductive material. It should be understood that this does not preclude either one of the two opposing walls also being comprised of an insulating material or being comprised of two types of materials. For example, in some embodiments, each of the opposing walls can be partly comprised of a thermally conductive material and partly comprised of a thermally insulating material. In use, one of the walls can be at a higher temperature than the other of the walls. In this way, in use, a temperature gradient between the two opposing walls can be established. As will be appreciated, such a temperature gradient is the driving force for thermodiffusion and is therefore necessary for thermodiffusion to occur.

[0012] The thermodiffusion device can also comprise a plurality of sidewalls located between the two walls. The plurality of sidewalls can be arranged to form a plurality of channels through which the liquid flows in use. The plurality of sidewalls can each be comprised of a material having a thermal conductivity that is lower than the thermally conductive material of each of the two opposing walls. It has been advantageously found that, by providing sidewalls with a lower thermal conductivity than the thermally conductive material of the two opposing walls, the heat flux through the liquid can be increased. This can increase the thermal energy efficiency of the thermodiffusion device. For example, in some embodiments, the material of the sidewalls may be significantly less thermally conductive than the thermally conductive material of the opposing walls.

[0013] The thermodiffusion device can further comprise an inlet configured to allow the liquid to flow into the plurality of channels in use. The thermodiffusion device can also comprise at least two outlets. For example, at least one of the outlets may be configured to collect the more concentrated fraction and at least one other of the outlets may be configured to collect the less concentrated fraction.

[0014] As will be known and appreciated to those skilled in the art, thermodiffusion is the phenomenon by which species migrate under the presence of a temperature gradient. Although the use of thermodiffusion for desalination (commonly known as thermodiffusive desalination or TDD) is known, until now, TDD has never been used in practical desalination applications. This is because thermodiffusion is a weak phenomenon, which, historically, has required significant amounts of time to produce very small reductions in salinity of a liquid.

[0015] Unlike other thermal desalination processes, TDD is operated entirely in the liquid phase with moderate temperature differentials driving the process and without the need for an energy -intensive phase change. For example, temperatures of between about 20 °C to about 90 °C may be sufficient to drive the TDD process. It is thought that, at temperatures below about 20 °C, thermodiffusion may be very slow. At temperatures above about 90 °C, bubbles may form in the liquid at one atmosphere, reducing the efficiency of the process.

[0016] In contrast, methods of the prior art based on evaporation-condensation are energy intensive, corrode materials of construction, and produce solid salts that are tedious to extract from the system. Of further advantage is that TDD does not require membranes or other functional materials. Material degradation and fouling are intrinsic issues of desalination processes such as RO because the selectivity of the membrane is dependent on its microporous structure. These issues can increase capital and operating costs. In contradistinction, TDD may provide a robust and low-maintenance technology because it is not based on functional materials such as membranes.

[0017] However, TDD has not gained attention in high-throughput applications because the concentration variation that could be induced by thermodiffusion in the prior art has been very small, with such a concentration gradient being susceptible to disturbances, e.g., due to convection or vibrations. A proof-of-concept study has recently been undertaken on the use of single channel TDD devices. The results of the proof-of-concept study are published as “Shuqi Xu, Alice J. Hutchinson, Mahdi ar Taheri, Ben Corry & Juan F. Torres, Thermodiffusive Desalination, Nature Communications, (2024)15:2996” (hereinafter referred to as “the single pass Nature Communications study”), the entire contents of which publication are incorporated herein by way of cross-reference. Whilst the publication provides a purely theoretical indication that a multichannel device may provide a pathway for scaling up single channel TDD devices, the publication provides no guidance on how such a device should be configured. Notably, in the past, such multiscale diffusive devices have only been applied to gaseous feeds. As will be appreciated by those skilled in the art, the design considerations for gaseous feeds are typically different from liquid feeds given the differences in density, thermal conductivity, diffusivity of species, etc. between gases and liquids. The present inventors have advantageously found that, by providing multiple channels in which the sidewalls of the channels are comprised of a sufficiently low thermal conductivity material, multichannel devices can be effectively used for the thermodiffusive separation of liquids. This is because, when the thermal conductivity of the sidewalls is sufficiently low, at least a portion of the thermal energy from the heated wall may be caused to flow through the liquid. In addition, the present inventors have found that, by employing a plurality of channels, the issues associated with the use of single channels (i.e., low throughput) may be reduced and / or eliminated. In this regard, the use of a plurality of channels in a TDD device can enable an easier pathway for scaling up the device.

[0018] It has also been found that the multichannel device of the present application can be used as a thermodiffusive salination (TDS) device to provide a concentrated fraction of liquid. Again, the present inventors have advantageously found that, by providing multiple channels in which the sidewalls of the channels are comprised of a sufficiently low thermal conductivity material, multichannel devices can be effectively used for the thermodiffusive concentration of a diffusing species in liquids.

[0019] In some embodiments of the first aspect, the two opposing walls may be arranged substantially parallel to one another. In such embodiments, the height of the channels may remain approximately consistent throughout a length and width of the device.

[0020] In some embodiments of the first aspect, the plurality of channels may be arranged in a Burger cascade. It has been advantageously found that the use of a Burger cascade can enhance the separation of the at least one diffusing species to the concentrated fraction. Burger cascades have been used for gaseous thermodiffusion applications in the past. The use of a Burger cascade for liquid processes, such as desalination, has been hypothesised with the use of purely theoretical computational calculations. However, to the inventors’ knowledge, the practical application of a Burger cascade in a composite structure configuration (with a plurality of wall materials) to desalination (or other liquid) processes has not yet been reported on.

[0021] In particular, it has been found that Burger cascade devices designed for gaseous applications can lack utility when directly applied to liquid systems. As will be appreciated by those skilled in the art, liquids and gases can have very different physical properties. In particular, it has been found that, in Burger cascade devices designed for gaseous applications that are applied to a liquid, the thermal energy tends to be transferred from the higher temperature wall to the lower temperature wall via the sidewalls. As a consequence, the thermal energy tends not to be transferred to the liquid resulting in low heat flux through the liquid. As a consequence, the temperature differential established within the liquid is small, meaning the driving force for thermodiffusion within the liquid is small. It has been surprisingly and advantageously found that, by providing the plurality sidewalls with a material with a lower thermal conductivity than the thermally conductive material of the two opposing walls, the transfer of energy through the liquid rather than through the sidewalls can be promoted. As a result, a larger temperature gradient through the liquid can be established, providing a larger driving force for thermodiffusion. This can also provide a reduction in heat loss, i.e., the amount of parasitic heat that is transferred through the device rather than through the liquid and which does not contribute to thermodiffusive separation may be reduced.

[0022] In some embodiments of the first aspect, the plurality of sidewalls may be comprised of a material with a thermal conductivity of less than about 1 W m-1K-1. The inventors note that this thermal conductivity is considerably lower than the thermal conductivities employed for the sidewall material in prior art Burger cascades which have been used exclusively in gaseous applications. To the inventors’ knowledge a thermal conductivity of less than about 1 W m-1K-1is also lower than the thermal conductivities employed in single or other multiple channel liquid thermodiffusion devices of the prior art. For example, liquid thermodiffusion devices of the prior art tend to use materials such as stainless steel for the sidewalls, which has a thermal conductivity of about 15 W m-1K-1.

[0023] In some embodiments of the first aspect, the plurality of sidewalls may each be comprised of a non-metallic material. For example, the plurality of sidewalls may each be comprised of: a polymer or a ceramic or a porous material with closed porosity. It is noted that porous materials with a closed porosity comprise pores with air or a vacuum therewithin. Notably, air has a very low conductivity. Therefore, the thermal insulation of a porous material with air placed within the voids may be improved (i.e., compared to an open porosity in which the voids become filled with the liquid in use). Furthermore, it can be beneficial to provide a porous material with closed porosity to reduce the risk of cross-contamination via diffusion between the channels.

[0024] In some embodiments of the first aspect, the two opposing walls may be comprised of at least one material with a thermal conductivity greater than about 50 W m-1K-1. For instance, in some embodiments, the two opposing walls may be comprised of a material with a thermal conductivity greater than about 200 W m-1K-1. For example, the two opposing walls may be comprised of: aluminium and / or copper and / or other high thermal conductivity materials such as graphene (-4000 W m-1K-1). As another example, the high thermal conductivity material can be composed of a plurality of high thermal conductivity materials as in a composite structure or multi-layer material. In this regard, it will be appreciated that the thermal conductivity of the two opposing walls may be significantly (i.e., 200 times or more) higher than the thermal conductivity of the sidewalls. In some embodiments of the first aspect, a portion of the opposing walls may be comprised of a material with a low thermal conductivity. That is, in such embodiments, the opposing walls may be comprised of multiple materials with different thermal conductivities. As above, a portion of the opposing walls may be comprised a thermally conductive material, so as to allow the temperature gradient through the liquid to be established in use. Another portion of the opposing walls may be comprised of a material which has a low thermal conductivity. It has been surprisingly and advantageously found that, when a portion of the opposing walls are comprised of the material with a low thermal conductivity, the energy requirements of the device can be reduced, without a significant decrease in the separation efficiency of the device. This can reduce the operating costs of the device.

[0025] In some of these embodiments, the low thermal conductivity material may comprise a material that is classified as an insulator. In this regard, the material may have a thermal conductivity of less than about 1 W m-1K-1. Accordingly, in some of these embodiments, at least a portion of the opposing walls may be comprised of a thermally insulating material. In some of these embodiments, up to 50% of the opposing walls may be comprised of a thermally insulating material.

[0026] In some embodiments of the first aspect, one or both of the opposing walls may comprise one or more grooves. In particular, each of the one or more grooves may define a surface of each of the one or more channels. In some of these embodiments, a width of each groove may be less than a width of the respective channel. It is thought that the grooves may comprise any shape including, but not limited to, triangular grooves of different pitches, rectangular, trapezoidal, or combinations thereof. The inventors have found that the presence of these grooves can advantageously improve the temperature homogeneity within the area of the channel in proximity to the opposing walls in-use, thereby creating a non-linear temperature profile in the liquid in the channel, which can increase the efficiency of the device. In particular, it is thought that the grooves can be particularly advantageous when a width to height aspect ratio of the channels is large, as explained in more detail below.

[0027] In some embodiments of the first aspect, each of the sidewalls may comprise a first portion located adjacent to one of the two opposing walls. The first portion may be comprised of a material of higher thermal conductivity than a remaining portion of the sidewall. In some of these embodiments, each of the sidewalls may further comprise a second portion located adjacent to the other one of the two opposing walls. The second portion may be comprised of a material of higher thermal conductivity than the remaining portion of the sidewall. It is thought that the provision of the first and / or the second portion in each of the sidewalls can result in a non-linear temperature profile within the channels of the device. Surprisingly, it has been observed that this may allow for the extraction of the lowest and highest concentrated fractions of the liquid and may increase the efficiency of the device.

[0028] In some embodiments of the first aspect, the material of higher thermal conductivity of the first portion and the second portion (when present) may comprise the same material as the two opposing walls. For example, the material of higher thermal conductivity of the first portion and the second portion (when present) may be a continuation of the material of the two opposing walls. In other words, ends of one or both of the opposing walls may continue on into the sidewalls to thereby define the first and second portions.

[0029] In some embodiments of the first aspect, the first portion and the second portion (when present) may each comprise from about 30% to about 90% of a total height of the channel.

[0030] It will be appreciated that the dimensions of the channel can affect the efficiency of the device. Therefore, the channel width and the channel height can be selected based on, for example, a required concentration of one or both of the outlet streams. In some embodiments of the first aspect, a width of each channel may be from about 0.25 mm to about 50 mm. For example, the width of each channel may be about 5 mm. As another example, the width of each channel may be about 23.4 mm. In some embodiments of the first aspect, a height of each channel may be from about 0.1 mm to about 5 mm. For example, the height of each channel may be about 2.1 mm. It has also been found that the width to height aspect ratio of the channel can affect the efficiency of the device. Accordingly, the channel width and the channel height can be selected so as to provide a desired width to height aspect ratio (e.g., that maximises device efficiency). For instance, in some embodiments of the first aspect, the channel width and the channel height may be selected so as to achieve a liquid region with near-homogeneous temperature in proximity to the opposing walls such that a non-linear temperature profile of the liquid is established within the channel. It is thought that, when there is a nonlinear temperature profile within the channel, the energy efficiency of the device is enhanced. It is thought that this is because the flow rate per heat flux is sufficiently high. In this regard, in some embodiments, the width to height aspect ratio of the channel is between about 0.25 to 5, such as between about 0.25 to 2, such as between about 0.25 to 1.

[0031] In embodiments of the first aspect in which the plurality of channels are arranged in a Burger cascade, the device comprises further dimensions such as the length of each column, the number of rows, the number of columns, and the diagonal length between adjacent columns. A ‘column’ in this context will be understood by those skilled in the art to refer the channels arranged in a direction that is normal to the flow direction. A ‘row’ in this context will be understood by those skilled in the art to refer to the channels arranged in a direction that is parallel to the flow direction.

[0032] In some of these embodiments of the device, a length each column in the Burger cascade may be from about 10 mm to about 300 mm. For example, the length of each column may be about 52 mm. As another example, the length of each column may be about 203.4 mm. In some of these embodiments of the device, a diagonal length between adjacent columns of the Burger cascade may be from about 0 mm (i.e., the Burger cascade does not comprise a diagonal length) to about 40 mm. The diagonal length defines the length travelled by the liquid in use as it passes from one column to an adjacent column within the cascade. For example, the diagonal length may be about 8.8 mm. As another example, the diagonal length may be about 17 mm. In some of these embodiments of the device, the Burger cascade may comprise at least 4 columns. In some of these embodiments of the device, the Burger cascade may comprise at least 4 rows. However, in other embodiments, the number of rows and columns may be as much as several hundred or thousand. For example, in larger scale Burger cascade devices, the number of rows and / or columns may be increased. In some of these embodiments, the Burger cascade may be comprised of one or more modules. The one or more modules may be connected in series. In addition, the one or more modules may be stacked vertically, so as to reduce a footprint of the device.

[0033] Disclosed herein in a second aspect is a method for thermodiffusive separation in a liquid comprising at one diffusing species that is ionic or non-ionic. The method can be used for separating the liquid into a fraction that is more concentrated with the diffusing species, and a fraction that is less concentrated with the diffusing species.

[0034] The method can comprise providing a Burger cascade device. The method can also comprise heating a first wall of the Burger cascade device and cooling a second wall of the Burger cascade device to thereby form a temperature gradient between the first wall and the second wall. It will be understood that, in a Burger cascade device, the first and second walls form opposing walls. The method can further comprise allowing the liquid to flow through the Burger cascade device such that, as the liquid flows from an inlet to an outlet thereof, the at least one diffusing species therein diffuses to form the more concentrated fraction and the less concentrated fraction.

[0035] In some embodiments of the second aspect, the method may further comprise cooling the second wall to a temperature above an inversion temperature of the one or more diffusing species. As will be known and appreciated to those skilled in the art, the inversion temperature is the temperature at which thermodiffusive transport changes from thermophobic to thermophilic; thus, thermodiffusive transport does not occur. This can cause the one or more diffusing species to diffuse toward the second wall. For example, when the liquid comprises seawater, the second wall may be cooled to a temperature of above about 10 °C (i.e., the inversion temperature of natural seawater).

[0036] In some embodiments of the second aspect, the method may further comprise heating the first wall to a temperature below a boiling temperature of the liquid. For example, in embodiments where the device is operated at a pressure of about one atmosphere, the first wall may be heated to a temperature below about 100 °C (i.e., the boiling point of water at about one atmosphere). However, in other embodiments, the device may be operated at an elevated pressure. For example, the device may be operated at a pressure of about 10-20 bar, such as at about 16 bar. It is though that, in some embodiments, the device may be operated at pressures as high as 50 bar or even 100 bar or more. As a result, the first wall may be heated to higher temperatures because the boiling point of the liquid increases when the operating device of the pressure is increased.

[0037] In some embodiments of the second aspect, the method may further comprise orienting the Burger cascade device such that the first wall is located above the second wall. That is, the first wall may define an upper wall of the Burger cascade device and the second wall may define a lower wall of the Burger cascade device. In particular, in embodiments where the second wall is cooled to a temperature above the inversion temperature of the one or more diffusing species, orienting the Burger cascade device such that the cooler wall defines the lower wall of the device can minimise the extent to which natural convection occurs within the device. This is because, typically, the more concentrated fraction will tend to have a higher density than the less concentrated fraction. By orienting the device such that the more concentrated (denser) fraction is within the lower portion of the channel, convection due to density differences within the channel (i.e., which may otherwise oppose the separation) may be minimised and / or eliminated altogether.

[0038] In some embodiments of the second aspect, the liquid may comprise natural seawater or natural seawater brine. In these embodiments, the thermodiffusive separation may therefore comprise thermodiffusive desalination (TDD). Advantageously, TDD requires only a fraction of the electrical power consumption compared to other desalination processes, such as RO. In addition, waste heat from industrial processes can be used as the energy source to heat the first wall, further increasing the economic viability of TDD. The use of a singlephase process (as well as reducing the energy consumption) can allow simplification of the desalination process. An all-liquid thermal desalination process is more amenable for industrialisation as it removes the requirement for more complex systems, such as boilers, condensers, etc.

[0039] In some embodiments of the second aspect, the diffusing species may comprise an aqueous solution of lithium chloride, potassium sulphate and / or sodium hydroxide. In some other embodiments of the second aspect, the diffusing species may comprise a non-aqueous solution of lithium chloride, potassium sulphate and / or sodium hydroxide and / or any type of biofluids such as dissolved proteins and / or a polymeric solution that can be organic or inorganic.

[0040] In some embodiments of the second aspect, the Burger cascade device of the method may comprise the device of the first aspect.

[0041] Also disclosed herein are uses of a thermodiffusion device of the first aspect. For example, the thermodiffusion device of the first aspect may be used for the desalination of water. The uses of the thermodiffusion device may employ the method of the second aspect. As another example, the thermodiffusion device of the first aspect may be used for thermodiffusive salination.

[0042] BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the thermodiffusion device and method will now be described, by way of example only, with reference to the accompanying drawings in which:

[0043] Figure 1 is a CAD model of an embodiment of a multiple channel thermodiffusive desalination device in which the channels are arranged in a Burger cascade arrangement;

[0044] Figure 2 is a schematic side view of one of the channels from the device of Figure 1;

[0045] Figures 3A and 3B are schematics of the channel arrangement of the device of Figure 1 with arrows indicating the direction of liquid movement between the channels;

[0046] Figures 4A to 4C are schematics of different embodiments of multiple channel thermodiffusive desalination devices comprising Burger cascades in which the sidewalls of the channels are comprised of different materials;

[0047] Figures 5A to 5D illustrate the effect of nonlinear temperature profile on the performance of a full-scale device which employs a Burger cascade;

[0048] Figure 6A is an experimental set-up in which a laboratory-scale device employing a Burger cascade was used for the desalination of water; and Figure 6B is a photo of the machined parts of the laboratory-scale Burger cascade;

[0049] Figures 7A to 7F are results showing the effectiveness of the Burger cascade device of Figure 6B in separating a less concentrated fraction from a liquid containing more than one diffusing species;

[0050] Figure 8a to 8c are results from modelling studies on the effect of changing the number of rows and / or columns in a Burger cascade on the concentration profile therewithin;

[0051] Figures 9A to 9D are schematics of different embodiments of multiple channel thermodiffusive desalination devices comprising Burger cascades in which the sidewalls of the channels are comprised of different materials and have different configurations;

[0052] Figures 10A to 10C are images of sidewalls of Burger cascade devices in which the channels are arranged in a serpentine arrangement;

[0053] Figures 11A and 11B are results from modelling studies on the effect of changing the width to height aspect ratio of the channel on the temperature profile within the cross-section of the channel (normal to the flow) when different fractions of the sidewalls are comprised of a material of high thermal conductivity;

[0054] Figures 12A and 12B are results from modelling studies on the effect of changing channel shape on the temperature profile across the channel crosssection normal to the flow;

[0055] Figures 13A to 13C are results from modelling studies on the effect of partially thermally insulating the opposing walls of some channels;

[0056] Figures 14A to 14C are results from modelling studies on the effect of partially thermally insulating the opposing walls of some channels;

[0057] Figures 15A and 15B are schematics showing flow behaviour at the end of each channel within a multichannel Burger cascade device;

[0058] Figures 16A and 16B show a CAD model of an embodiment of a prototype of a liquid Burger cascade device; and Figure 16C shows a concentration profile in leftmost and rightmost channels of the Burger cascade device;

[0059] Figures 17A to 17D are results from modelling studies on liquid Burger cascade performance under atmospheric pressure;

[0060] Figures 18A to 18D are results from modelling studies on liquid Burger cascade performance at an elevated pressure of 15.5 bar; Figures 19A and 19B are results from modelling studies on a liquid Burger cascade when the diffusing species is lithium chloride;

[0061] Figure 20A and 20B are results from modelling studies on a liquid Burger cascade when the diffusing species is potassium sulphate;

[0062] Figure 21A and 21B are results from modelling studies on a liquid Burger cascade when the diffusing species is sodium hydroxide;

[0063] Figures 22A, 22B and 22C are results from modelling studies on a liquid Burger cascade when the diffusing species is lithium chloride showing how the concentration yield changes for a given feed concentration Co when the number of rows, columns or channels is changed;

[0064] Figures 23A, 23B and 23C are results from modelling studies on a liquid Burger cascade when the diffusing species is potassium sulphate showing how the concentration yield changes for a given feed concentration Co when the number of rows, columns or channels is changed;

[0065] Figures 24A, 24B and 24C are results from modelling studies on a liquid Burger cascade when the diffusing species is sodium hydroxide showing how the concentration yield changes for a given feed concentration Co when the number of rows, columns or channels is changed;

[0066] Figure 25 is a concentration profile in a liquid Burger cascade which includes an insulated area when the diffusing species is lithium chloride obtained from modelling studies;

[0067] Figure 26 is a concentration profile in a liquid Burger cascade which includes an insulated area when the diffusing species is potassium sulphate obtained from modelling studies; and

[0068] Figure 27 is a concentration profile in a liquid Burger cascade which includes an insulated area when the diffusing species is sodium hydroxide obtained from modelling studies. DETAILED DESCRIPTION

[0069] In the following detailed description, reference is made to accompanying drawings which form a part of the detailed description. The illustrative embodiments described in the detailed description, depicted in the drawings and defined in the claims, are not intended to be limiting. Other embodiments may be utilised and other changes may be made without departing from the spirit or scope of the subject matter presented. It will be readily understood that the aspects of the present disclosure, as generally described herein and illustrated in the drawings can be arranged, substituted, combined, separated and designed in a wide variety of different configurations, all of which are contemplated in this disclosure.

[0070] Disclosed herein is a thermodiffusion device and method for a liquid comprising at least one diffusing species, either ionic or non-ionic. The device and method separate the liquid into a fraction that is more concentrated with the diffusing species, and a fraction that is less concentrated with the diffusing species. As set forth above, the “diffusing species” may be any species that is capable of diffusing in the liquid that comprises the diffusing species when the liquid is subjected to thermodiffusion as set forth herein.

[0071] The device as disclosed herein comprises: two opposing walls, each comprised of a thermally conductive material, one of the walls being at a higher temperature than that of the opposing wall in use; a plurality of sidewalls located between the two walls and arranged to form a plurality of channels through which the liquid flows in use, the plurality of sidewalls each comprised of a material that is less thermally conductive than the two opposing walls; an inlet configured to allow the liquid to flow into the plurality of channels in use; and at least two outlets configured to collect the more concentrated fraction and the less concentrated fraction.

[0072] The thermodiffusion device and method as disclosed herein can be advantageously used for thermodiffusive desalination (TDD). As described above, TDD has not gained attention in high-throughput applications because the concentration variations that can be induced by thermodiffusion are typically very small and require significant amounts of time to establish. In addition, such a concentration gradient is susceptible to disturbances by, e.g., convection. This can result in weak separation and the need for a long resilience time. For example, in the original thermodiffusion experiments performed by Soret, it took 56 days to produce a 1.2% relative drop in the salinity. In contradistinction, by employing the thermodiffusion device and method in accordance with embodiments of the present disclosure, a 90% relative drop in salinity may be obtained within hours. In such applications, it will be appreciated that the one or more diffusing species comprise one or more ionic species.

[0073] The thermodiffusion device and method as disclosed herein can also be advantageously used for thermodiffusive salination (TDS). In TDS applications, the fraction that is of interest is the more concentrated fraction. In the below description, reference is primarily made to TDD applications. However, it should be understood that the same devices and methods can alternatively be employed for TDS applications.

[0074] Turning first to Figures 1 and 2, Figure 1 shows a CAD image of an embodiment of a TDD device 10 and Figure 2 shows a longitudinal cross-section of a single channel 16 of the device 10 of Figure 1. The device 10 of the illustrated embodiment of Figure 1 comprises two opposing walls 12, 14. In the illustrated embodiment of Figure 1, the two opposing walls 12, 14 are substantially parallel. Arranged between the two walls 12, 14 are a series of sidewalls 30. In the illustrated embodiment of Figure 1, there are sidewalls 30a attached to the upper side of wall 14 and sidewalls 30b attached to the underside of wall 12. The sidewalls 30a, 30b are in the form of a laminae template having a series of channels 16 formed therethrough and configured such that, when the walls 12, 14 are placed together, such that the sidewalls 30a, 30b make contact, the channels in the sidewalls 30a, 30b correspond, thereby forming the series of channels 16 between the walls 12, 14. The channels 16 in each laminae of the sidewalls 30a, 30b can be laser cut, machined, moulded, etched, 3D-printed, etc.

[0075] In this regard, the sidewalls 30a, 30b are arranged so as to define the series of channels 16 whereby the channels 16 become defined and located between the two walls 12, 14. In the illustrated embodiment of Figure 1, the sidewalls 30a, 30b are arranged such that the channels 16 form a Burger cascade. The Burger cascade is further illustrated in Figures 3 and 4. However, it will be appreciated that other arrangements of channels 16 are possible. For example, the sidewalls may instead be arranged to form multiple parallel channels.

[0076] The channels 16 are configured to allow a liquid (e.g., the liquid to be desalinated or a liquid to be concentrated, e.g., for salination) to flow through the device 10 in use. In particular, in use the liquid 18 enters the device 10 through an inlet 36 located at a first end 20 of the device 10. In the illustrated embodiment of Figure 1, there is one inlet 36. The inlet 36 is configured to divide the liquid 18 into substantially equal portions and to pass each portion to a respective channel 16 of the device 10. In this regard, the inlet 36 typically comprises a manifold (not shown) which divides the liquid into substantially equal portions. The manifold is fluidly connected to the channels 16 and causes each equal portion of liquid to flow to a respective channel 16 within the device 10.

[0077] The liquid 18 then flows through the channels 16 from the first end 20 to a second end 22 of the device 10. The liquid 18 exits the device 10 at the second end 22 via at least two outlets (i.e., at least one outlet by which the more concentrated fraction exits the device 10 and at least one outlet by which the less concentrated fraction exits the device 10). It should be appreciated that two outlets for each channel could be provided, however, in practice, the respective liquid streams from the channels can be combined in various ways to then pass out of the device 10 via a lesser number of outlets. In some embodiments, the outlets may be configured such that at least 10% of the seawater 18 is recovered as the less concentrated fraction. Typically, the device 10 comprises one outlet by which the more concentrated fraction exits the device 10 and one outlet by which the less concentrated fraction exits the device 10. Those skilled in the relevant art will understand that each outlet typically groups many internal channels within the Burger cascade. Accordingly, the recovery rate of the device 10 can be set to a desired value depending on how the internal channels are grouped. For the application of desalination, it is noted that, by grouping the channels such that the recovery rate is increased (i.e., a greater proportion of the incoming seawater 18 reports to the less concentrated fraction), the salinity reduction will be decreased. As such, the concentration of the ionic species in the less concentrated fraction will be higher. Thus, for desalination applications, there is a trade-off between the recovery rate and the concentration of the less concentrated fraction, with higher recovery rates resulting in higher concentrations of the one or more ionic species within the less concentrated fraction. For example, if a Burger cascade comprises ten rows, then the outlet for the less concentrated fraction can comprise the two rows with the lowest concentration. The outlet for the more concentrated fraction will comprise the remaining eight rows. In this scenario, the recovery rate is 20%. Alternatively, the outlet for the less concentrated fraction could comprise only one row with the lowest concentration, with the remaining nine rows comprising the more concentrated fraction. In this variation, the recovery rate is only 10%, but the concentration of the less concentrated fraction will be lower. The inventors note that the device 10 can also be used in salination applications. In such applications, decreasing the recovery rate (i.e., increasing the proportion of incoming liquid that reports to the more concentrated fraction) will tend to increase the concentration of the more concentrated fraction.

[0078] In the device 10, the wall 12 is heated and the wall 14 is cooled. For example, the wall 12 can be heated by passing hot water (or any other heating medium - e.g., hot off-gases, process liquids, etc.) therethrough. Any suitable heat source may be employed to provide thermal energy to the hot water (or other heating medium). For example, solar, waste heat, heat from the surrounding environment, etc. Similarly, the wall 14 can be cooled by passing cold or cool water (or any other cooling medium) therethrough. In this regard, in the illustrated embodiment, the walls 12, 14 can each take the form of a chamber (as shown) through which (respectively) hot water and cold water are able to be circulated in use. Thus, reference to a ‘wall’ should be understood as including a wall of such a chamber. The heated wall 12 comprises a cavity 13, which allows hot water (or any other heating medium) to be passed through therethrough. Similarly, the cooled wall 14 comprises a cavity (not shown in Figure 1 due to sidewall 30a) which allows cold or cool water (or any other cooling medium) to be passed therethrough.

[0079] The walls 12, 14 are each encased by thermal insulation 32, 34 which acts to minimise heat losses from the device 10 to the surrounding environment, or heat encroachment and temperature preservation in the case of cooled wall 14. One or more seals 39 may be placed between the thermal insulation 32, 34 and the walls 12, 14, as well as between the heated wall 12 and the cooled wall 14 to reduce the risk of fluids (e.g., the hot water, cold water or liquid to be treated in the device) leaking from the device 10.

[0080] To help maintain the appropriate temperature of the walls 12, 14 in use, one or more thermocouples may be installed in the device 10. In the illustrated embodiment of Figure 1, the device 10 comprises ten thermocouples 36 mounted within the heated wall 12 and evenly distributed around the cavity 13. Similarly, the device 10 comprises ten thermocouples 38 mounted within the cooled wall 14 and evenly distributed around the cavity of the cooled wall 14. In this regard, the thermocouples 36, 38 are evenly spaced within the respective wall 12, 14. However, those skilled in the art will appreciate that other thermocouple arrangements may alternatively or additionally be employed. As will be explained in further detail below, temperature readings from the thermocouples 36, 38 can be used to control the flow of the hot water and the cold water through the chambers of the walls 12, 14 respectively. Typically, as in the illustrated embodiment of Figure 1, the device 10 is oriented in use such that the heated wall 12 is located toward an upper portion of the device and the cooled wall 14 is located toward a lower portion of the device. This is because, under the temperatures employed for desalination, the ions in the liquid 18 tend to be thermophobic. As such, the ions in the liquid 18 diffuse toward the cooled (lower) wall 14 of the device 10. As a result, the liquid that is located toward the cooled wall 14 becomes more saline and, as a consequence, denser than the liquid located toward the heated wall 12. It has been found that, by configuring the device 10 such that the more saline / denser liquid tends to be located toward the lower wall 14, convection of the liquid within the channel due to density differences can be minimised and / or eliminated altogether. This can reduce mixing within the channel, thereby enhancing separation and improving the desalination (or other ionic separation process).

[0081] However, it will be appreciated that, in other embodiments (such as when the device is operated such that the ions are thermophilic), the device can instead be oriented such that the heated wall is located toward the lower portion of the device and the cooled wall is located toward the upper portion of the device. Again, by configuring the device such that the ions diffuse toward the lower portion of the device, convection of the liquid within the channel due to density differences can be minimised and / or eliminated altogether.

[0082] As can be more clearly seen from Figure 2, located at the second end 22 of each channel 16 is a splitter 24. The splitter 24 divides the liquid 18 into a top stream 26 and a bottom stream 28. The splitter 24 is configured to split the liquid 18 such that mixing of the liquid 18 is minimised. It will be appreciated that the height of the splitter 24 within the channel 16 can be adjusted. For example, based on the desired or required relative proportions of the top stream 26 and bottom stream 28. It is noted that there may be a trade-off between the relative salinity of the top stream 26 and the recovery of the liquid 18 when selecting the splitter height 24. For instance, when the splitter 24 is located closer to the wall 12, the recovery of the liquid 18 to the top stream 26 is reduced. However, in desalination applications, this can also result in a lower salinity of the top stream 26. Typically, however, the splitter is located at a height of about half the total channel height.

[0083] Referring now to Figures 3A and 3B, those skilled in the art will appreciate that, because the channels are arranged in a Burger cascade, the device 10 will comprise a number of channels 16 arranged in columns 60 (being arranged normal to the flow direction) and rows 58 (being arranged in the flow direction). In this regard, at the end of each channel 16, one of the top stream 26 or the bottom stream 28 is also diverted to the left, with the other one of the top stream 26 or the bottom stream 28 diverted to the right, i.e., so as to form the Burger cascade. This configuration can be more clearly seen in Figures 3 A and 3B, in which arrows are used to indicate the different flow directions between channels. In the configuration of Figure 3 A, the top (hotter) stream 26 is diverted toward the left of the cascade, whilst the bottom (cooler) stream 38 is diverted toward the right of the cascade. In Figures 3A and 3B, the top (hotter) stream comprises the less concentrated fraction, whilst the bottom (cooler) stream comprises the more concentrated fraction when the diffusing species is thermophobic (i.e., such that the diffusing species diffuses toward the cooled wall). On the other hand, the top (hotter) stream comprises the more concentrated fraction, whilst the bottom (cooler) stream comprises the less concentrated fraction when then diffusing species is thermophilic (i.e., such that the diffusing species diffuses towards the heated wall).

[0084] Figures 15A and 15B are schematics further illustrating the flow behaviour at the end of a channel 16. Figure 15A is a schematic of a front view from the direction of liquid flow. The top image shows a desired flow regime whereby the flow in the top-left of the channel 16 is caused to flow horizontally to the top-right, and whereby the flow in the bottom-right of the channel 16 is caused to flow horizontally to the bottom-left. This flow regime occurs at the end of the channel 16, as the liquid 18 is split into the top stream 26 and the bottom stream 28 by the splitter 24. As above, one of the streams is also diverted to the left, whilst the other one of the streams is diverted to the right, i.e., so as to form the Burger cascade. Ideally, vertical mixing of the liquid 40 located toward the (heated) upper wall 12 with the liquid 42 located toward the (cooled) lower wall 14 is minimised as the liquid 18 is split by the splitter 24. This is because mixing of the liquids 40, 42 reduces the efficiency of the separation of the more concentrated fraction and the less concentrated fraction. The bottom image of Figure 15A is a schematic illustrating an undesired flow at the end of the channel 16, whereby some of the flow in the top-left flows downwards while some of the flow in the bottom-right flows upwards. That is, in Figure 15A some of the liquid 40, 42 is allowed to mix as the liquid 18 is being split by the splitter 24. Such mixing is typically undesirable.

[0085] Figure 15B is a schematic of a top view of the channel 16 at the bifurcation point, i.e., at the point where the splitter 24 splits the liquid 18 into the top stream 26 and the bottom stream 28. In the embodiment of Figure 15B, the top stream 26 is diverted to the left and the bottom stream 28 is diverted to the right, i.e., so as to form the Burger cascade. The left-hand side schematic shows an abrupt (less desirable) configuration of a splitter in which the sidewalls 30a, 30b are normal to the flow of the liquid 18. This can cause vertical mixing of the liquid within the channel 16 which, as above, can reduce the efficiency of the separation of the more concentrated fraction and the less concentrated fraction. The right-hand side schematic shows a smoother configuration of a splitter in which the sidewalls 30a, 30b are oblique so as to smoothly direct the flow of the top stream 26 and the bottom stream 28 in a diagonal manner. This configuration can minimise vertical mixing of the fluid as it is split into the top stream 26 and the bottom stream 28.

[0086] The operation of the device 10 will now be explained in more detail with specific reference to its application in TDD. However, those skilled in the art will appreciate that the device 10 is not limited to TDD. For example, the device 10 may instead be used for increasing the salinity of a brine. Alternatively, the device 10 may be used to purify liquids comprising ions. Referring again to Figures 1 and 2, in use, a seawater feed 18 (or other liquid comprising at least one diffusing species) is introduced into the device 10 via the inlet 36. For example, the seawater 18 may be pumped into and through the device 10. Typically, the seawater 18 is pumped into and through the device 10 at a steady rate. This is because the volumetric flowrate of the seawater 18 can influence the efficiency of the desalination. In particular, increasing the volumetric flowrate can decrease the concentration difference between the less concentrated fraction and the seawater 18. In addition, typically, the flow through each of the channels has a Reynolds number that does not exceed the critical threshold of transition from laminar to turbulence. That is, the flow through each of the channels is laminar. By employing a laminar flow, mixing of the seawater 18 within each of the channels 16 due to turbulence may be minimised.

[0087] At the inlet 36 of the device 10, the seawater 18 is homogenous. The channels 16 are typically configured such that the flow of the seawater 18 therethrough becomes fully developed near the inlet. As will be described in further detail below, as the seawater 18 flows through the channels, the ions diffuse toward the cold lower regions, forming a more concentrated fraction toward the cooled wall 14 and a less concentrated fraction toward the heated wall 12.

[0088] As above, the channels 16 within the device 10 are arranged as a Burger cascade, comprising a number of columns and rows. The inlet 36 is configured such that seawater 18 flowing into the device 10 is caused to be approximately evenly distributed between the columns of the Burger cascade. For example, the inlet 36 can comprise a manifold configured to distribute the seawater 18 approximately evenly between the columns. In some embodiments, a degassing device (not shown) can be placed before the inlet 36. The degassing device can ensure the liquid fed to the device 10 does not comprise any gas in the form of bubbles, which can decrease the efficiency of the device.

[0089] Hot water is circulated through the cavity 13 of the heated wall 12. For example, the hot water can be circulated through the cavity 13 by pumping. The hot water is at an elevated temperature. The elevated temperature is typically below the boiling temperature of the seawater 18. For example, when the device 10 is operated at an atmospheric pressure, the elevated temperature is typically below a temperature of about 100 °C and, more typically, below about 90 °C. This is because boiling of the liquid 18 within the channels 16 reduces the efficiency of the device 10, as well as decreasing the energy efficiency of the desalination process. It has been observed that, at temperatures above 90 °C, bubbles may start to form. For example, if the seawater 18 is not properly degassed. It is thought that higher elevated temperatures (and higher temperature differentials between the heated wall and the cooled wall) may be used if the operating pressure of the device 10 is increased (i.e., such that the boiling point of the seawater 18 is increased).

[0090] Without being bound by theory, it is thought that the device 10 may be operated such that the absolute pressure of the seawater is increased to above atmospheric pressures. For example, in some embodiments, the device 10 may be operated at elevated pressures of 16 bar or more. It will be appreciated that increasing the pressure of the seawater increases the boiling temperature of seawater. As a result, the heated wall can be operated at a higher temperature without the seawater being caused to boil, thereby allowing a larger temperature difference between the first wall and the second wall. For example, at absolute pressures greater than 100 bar, the boiling temperature of seawater is 300 °C, allowing a greater temperature difference to be applied, while maintaining the seawater in the liquid phase. It is thought that the pressure difference between the inlet and outlet of the Burgers cascade may need to be relatively small, such that a strong flow does not develop.

[0091] As the hot water is circulated through the cavity 13 of the heated wall 12, heat is transferred via the heated wall 12 from the hot water to the seawater 18 flowing through the channels 16. This causes the hot water to be cooled and seawater 40 in proximity to the heated wall 12 to become heated. In this regard, the hot water provides a heat source for the seawater 18 flowing through the channels 16. The hot water circulating through the cavity 13 of the heated wall 12 may be continuously recycled. For example, by collecting the (now cooler) water exiting the heated wall 12, reheating the water to the elevated temperature and passing the heated water through the cavity 13 of the heated wall 12. The flowrate of the hot water can be controlled using a feedback loop which uses the temperature(s) measured by the thermocouples 36 as an input. For example, when a measured temperature of the wall is greater than a setpoint temperature, the flowrate of the hot water can be decreased. On the other hand, when the measured temperature of the wall is less than the setpoint temperature, the flowrate of the hot water can be increased.

[0092] At the same time, cooled water is circulated through the cavity of the cooled wall 14. For example, the cooled water can be circulated through the cavity by pumping. The cooled water is at a temperature which is cool relative to the hot water. That is, it will be appreciated that the cooled water need not necessarily be ‘cold’. It will be appreciated that the seawater will comprise a number of diffusing species such as sodium, magnesium, potassium, chloride, etc. In the case of seawater, these diffusing species are typically in the form of ions. Typically, the cooled water is above the inversion temperature of the seawater, such that the ions exhibit thermophobic behaviour. In this regard, the cooled water is typically at a temperature of about 12 °C or above. The inventors note, however, that at temperatures between 12 °C and 20 °C, thermodiffusion in seawater is typically very weak. As such, the cooled water is typically at a temperature greater than 20 °C.

[0093] As the cooled water is circulated through the cavity of the cooled wall 14, heat is transferred from the seawater flowing through the channels 16 (which is heated due to heat transferred from the hot water) to the cooled water via the cooled wall 14. This causes the seawater 42 in proximity to the cooled wall 14 to become cooled. At the same time, the cooled cavity water becomes heated. In this regard, the cooled cavity water provides a heat sink for the seawater flowing through the channels, with the thermal energy from the heated wall 12 being conducted to the cooled wall 14 via the seawater 18.

[0094] The cooled water circulating through the cavity of the cooled wall 14 may be continuously recycled. For example, by collecting the (now heated) cooled water exiting the cooled wall 14, re-cooling the water and passing the cooled water through the cavity of the cooled wall 14. The flowrate of the cooled water can be controlled using a feedback loop which uses the temperature(s) measured by the thermocouples 38 as an input. For example, when a measured temperature of the wall is greater than a setpoint temperature, the flowrate of the cooled water can be increased. On the other hand, when the measured temperature of the wall is less than the setpoint temperature, the flowrate of the cooled water can be decreased.

[0095] By continuously circulating hot water through the cavity of heated wall 12 and cooled water through the cavity of the cooled wall 14, a temperature differential within the seawater flowing through the channel 16 is established (i.e., because the temperature of the seawater 40 located toward the heated wall 12 becomes higher relative to the seawater 42 located toward the cooled wall 14, with thermal energy being transferred from the heated seawater 40 toward the cooled seawater 42). This temperature gradient provides the driving force for the diffusion of the ionic species present in the seawater 18. In particular, when the device 10 is operated above the inversion temperature of the seawater 18, the ionic species present in the seawater 18 are caused to diffuse from the heated seawater 40 toward the cooled seawater 42. As a result, the cooled seawater 42 becomes more concentrated in the ionic species (i.e., it has a higher salinity) and the heated seawater 40 becomes less concentrated in the ionic species (i.e., it has a lower salinity). In some embodiments, the hot water and the cooled water flow in a counter-current direction. It will be appreciated that counter-current flow can provide a relatively constant temperature difference across the length of the device 10. However, it will be appreciated that co-current flow arrangements can also be employed as an alternative. For example, if the mean temperature through the device does not fluctuate. As the seawater 18 continues to flow through the channel 16, thermodiffusion continues to occur, due to the temperature differential. At the end of the channel 16, the seawater 18 is separated into a more saline bottom stream 28 and a less saline top stream 26 by the splitter 24. The bottom stream 28 and the top stream 26 may each be passed to respective outlets of the device 10. Alternatively, the bottom stream 28 and the top stream 26 may each be passed to a further row of the Burger cascade.

[0096] The seawater 18 continues to flow through the columns of the Burger cascade. As can be seen from Figure 1, for the entire Burger cascade, the top of the channels are heated (i.e., via the heated wall 12) and the bottom of the channels are cooled (i.e., via the cooled wall 14). As a consequence, as the seawater 18 continues to flow through the columns, the temperature differential between the seawater 40 and the seawater 42 is maintained and diffusion of the ionic species toward the cooled seawater 42 continues.

[0097] At the second end 22 of the device 10, a heated seawater stream (now less concentrated in the ionic species compared to the seawater 18) is collected from an upper outlet of the device 10 (not shown). At the same time, a cooled seawater stream (now more concentrated in the ionic species compared to the seawater 18) is collected from a lower outlet of the device 10 (not shown). If appropriate, some of the heat of the heated seawater stream may be captured by heat exchange for use in at least partially heating the water for the chamber of the heated wall 12.

[0098] It will be appreciated that increasing a retention time of the seawater 18 within the device 10 can increase the efficiency of the device 10, because increasing the retention time increases the time over which diffusion can occur. That is, increasing the seawater retention time increases the separation of the ionic species toward the cooled seawater 42, thereby decreasing the concentration of the ionic species in the heated seawater 40. As a result, the concentration drop between the incoming seawater 18 and the cooled seawater 42 is increased. The inventors have advantageously found that, by using a Burger cascade, a higher concentration drop may be achieved compared to the use of a single channel device. For instance, it is thought that the Burger cascade can provide a concentration drop along the length of the device that is four to five times greater than the concentration drops provided from a single channel of the same length. In particular, the use of a Burger cascade can improve the recovery rate and increase the drop in the concentration of the ionic species in the cooled seawater 42, compared with the use of multiple single-pass channels. For example, as discussed in the single pass Nature Communications study, the use of a single pass column can provide a 50% recovery rate, but only a -600 parts per million (ppm) concentration drop (when the initial seawater has a salinity of 60,000 ppm). To increase the concentration drop, multiple passes must be used, which decreases the recovery. For example, after three passes, the recovery rate drops to about 15% and the concentration drop increases to 2,000 ppm. On the other hand, it is thought that a device according to the present disclosure which comprises a Burger cascade can provide a concentration drop of about 2,000 ppm with a recovery rate of about 50%. Furthermore, if the recovery rate is decreased to about 20%, then it is thought that concentration drops of about 4,000 ppm can be achieved. It is thought that this increase in efficiency is due to the configuration of the channels within the Burger cascade.

[0099] Those skilled in the art will appreciate that the retention time of the seawater 18, for a given volumetric flowrate, is affected by the cross-sectional area of the channels and the length of the flow pathway. Without being bound by theory, it is thought that an optimal channel width may be between about 1 mm to about 50 mm, such as between about 5 mm to 25 mm. For example, an optimal channel width may be 23.4 mm. It is also thought that an optimal channel height may be between about 0.25 mm to about 5 mm, such as about 2.1 mm. It has also been found that the width to height aspect ratio of the channel can affect the efficiency of the device. This is because the width to height aspect ratio can influence the temperature distribution through the liquid within the channel. It is thought that it can be optimal to select the channel width and the channel height so as to achieve a liquid region with near-homogeneous temperature in proximity to the opposing walls 12, 14, such that a non-linear temperature profile of the liquid within the channel is established. It is thought that, when such a region is present, the energy efficiency of the device 10 is enhanced. It is thought that this is because the flow rate per heat flux is sufficiently high. The inventors note that, when the width to height aspect ratio is too high, there is a tendency for a more linear temperature profile to be formed. However, having a width to height aspect ratio that is too low can result in manufacturing difficulties. In this regard, in some embodiments, the width to height aspect ratio of the channel is between about 0.25 to about 5, such as between about 0.25 to about 2, such as between about 0.25 to about 1. In some embodiments, the width to height aspect ratio may be less than 1. In other embodiments, the width to height aspect ratio may be 1.

[0100] The length of the flow pathway is influenced by both the length of each individual channel, as well as the number of rows within the Burger cascade. Without being bound by theory, it is thought that an optimal channel length may be between about 10 mm to about 300 mm, such as about 52 mm. It is thought that, for seawater comprising 35,000 ppm of ionic species, a Burger cascade comprising at least 4 columns can provide good recovery and decrease the salinity in the less concentrated fraction to a useable level. For example, the less concentrated fraction may be suitable for use as a feedstock to further desalination processes. It will be appreciated that these dimensions can of course change when the device is scaled to industrial usage.

[0101] It is noted that the optimal flowrate through device 10 is dependent on the dimensions of the one or more channels 16. For example, when a higher flowrate is required, the length of each channel and / or the number of columns may be increased (i.e., so as to provide the same resilience time that enables efficient thermodiffusive separation). As another example, when a higher flowrate is required, the width of each of the one or more channels may be increased. As above, it can be preferable to maintain the seawater in a laminar flow so as to minimise mixing within the channel due to turbulence. In this regard, the flowrate of the seawater may be selected such that the Reynolds number is below the transition point. It will be appreciated that the transition point will depend on the dimensions of the channel.

[0102] It will be appreciated that, for given flowrate and channel dimensions, increasing the number of columns will increase the concentration of the ionic species in the more concentrated fraction (i.e., the cooled seawater 42) and, likewise, decrease the concentration of the ionic species in the less concentrated fraction (i.e., the heated seawater 40). This is because increasing the number of columns increases the retention time. In this regard, it will be appreciated that, depending on the requirements for the less concentrated fraction, the dimensions of the Burger cascade may be adjusted accordingly. For instance, if it is desired to provide a less concentrated fraction that is suitable for drinking, the Burger cascade can include a larger number of columns.

[0103] The diagonal length 59 (see Figure 3B) between columns within the Burger cascade may be adjusted, for example, to ensure laminar flow between columns. In some embodiments, the diagonal length between columns of the Burger cascade is from about 0 mm to about 40 mm, such as about 8.8 mm or, in some embodiments, 35.7 mm. For example, the diagonal length between columns may be from about 7 mm to about 38 mm. That is, in some embodiments, there is no diagonal length, with the seawater flowing directly from one column into an adjacent column. However, without being bound by theory, it is thought that, by not having the diagonal geometry, this may introduce disturbances into the flow, which may reduce the efficiency of the separation of the less concentrated fraction and the more concentrated fraction.

[0104] At the same time, those skilled in the art will appreciate that the efficiency of the device 10 is also influenced by the number of rows. In particular, it has been found that, for a given number of columns, there is a maximum concentration drop that can be achieved by increasing the number of rows. As the number of columns increases, so does the maximum concentration drop that can be achieved. However, at the same time, the number of rows required to achieve the maximum concentration drop also increases. In some embodiments, the Burger cascade comprises at least 4 rows and at least 4 columns. It has been found that, by employing a Burger cascade with at least 4 rows and at least 4 columns, salinity drops of about 2000 ppm may be obtained. The inventors noted that such a salinity drop is tangible in desalination applications and such a device may be useful for some applications. For instance, those in which only small salinity drops are required. However, in some embodiments, the Burger cascade may comprise hundreds, if not thousands, of rows and / or columns. For example, in applications where larger salinity drops are required and / or larger throughputs are required.

[0105] In some embodiments, and as illustrated in Figures 16A and 16B, it is thought that the Burger cascade can comprise a modular device which is comprised of a plurality of modules I la, 1 lb,...,1 In. In this regard, Figure 16A shows a device 10a which is comprised of nine modules I la, 11b,..., Hi and Figure 16B is a schematic of the channel structure of each module. Each module is about 1.5 m in length. Thus, the total length of the Burger cascade is 13.5 m. However, because the modules I la, 11b,..., Hi are stacked vertically, the footprint of the device 10a is reduced. That is, the footprint of the device 10a is equivalent to only one module. Such a configuration is particularly advantageous when the Burger cascade comprises hundreds or thousands of channels.

[0106] Each of the modules I la, 11b,..., Hi is comprised of a device 10 with two walls 12, 14, each comprising sidewalls 30 which form channels 16 when the two walls 12, 14 are joined. As above, in-use, one of the walls is heated and the other one of the walls is cooled. The heated walls of each module I la, 11b,..., Hi can be heated using the same heat source, e.g., hot water. Alternatively, the heated walls can be heated using one or more different heat sources, for example, when it is required or desirable to heat the heated walls of different modules I la, 1 lb,. . .,1 li to different temperatures. Similarly, the cooled walls of each module I la, 11b,..., Hi can be cooled using the same cooling source, e.g., cooled or chilled water. Alternatively, the cooled walls can be cooled using one or more different cooling sources, for example, when it is required or desirable to cool the cooled walls of different modules I la, 1 lb,. . .,1 li to different temperatures.

[0107] The device 10a comprises an inlet 36 at a bottom corner of the first module 1 la at a first end 13a thereof. As above, liquid 18 entering the inlet 36 is evenly distributed through each of the channels, e.g., by a manifold. Thus, in the illustrated embodiment, the liquid 18 is split into nine portions, as there are nine channels 16 (see Figure 16B). The liquid 18 flows through the first module I la to the second end 15a thereof. At the second end 15a of the first module I la, the liquid 18 is directed into the second end 15b of the second module 1 lb. The liquid 18 then flows through the second module 11b to the first end 13b thereof. The liquid 18 is then directed into the first end of the third module and so on. Once the liquid 18 reaches the second end 15i of the ninth module Hi, the concentrated fraction is collected from the first outlet 35 and the less concentrated fraction is collected from the second outlet 37.

[0108] Figure 16B is a schematic of the channel configuration of a module I la, 11b,..., Hi. Each module I la, 11b,..., Hi comprises the same channel configuration. In the illustrated embodiment, each module I la, 11b,..., Hi comprises three pairs of two rows of channels (i.e., each module comprises six rows of channels in total). Accordingly, the device 10a comprises a total of 27 pairs, each pair comprising two rows of channels, i.e., because it comprises nine vertically stacked modules. The device 10a comprises nine rows in each module I la, 1 lb,. . .,1 li. Each channel has a length of 203.4 mm, a width of 23.4 mm and a height of 2 mm. The width of the module 11 is 325.5 mm and the length of the module is 1430 mm. The module comprises sidewalls that are 5 mm thick. The total height of each module is 200 mm. Figure 16C shows the expected concentration profile across the pairs of channels (i.e., with 27 pairs of channels in the device 10a) and indicates that 27 pairs of channels is sufficient to provide good separation of the liquid into a more concentrated fraction and a less concentration fraction. It will be appreciated that the number of rows and columns of channels in each module, as well as the number of modules, can be varied. For example, to achieve one or more of a required residence time, concentration in one of the product streams, etc. In addition, the dimensions of the channels can be varied.

[0109] Without being bound by theory, it is thought that the less concentrated fraction produced by the Burger cascade devices of the present disclosure may be suitable for a number of end-uses. For example, the less concentrated fraction may be suitable for use as drinking water. Alternatively or additionally, the less concentrated fraction may be suitable for use as a feed into further desalination processes. In this regard, in some scenarios, the inventors note that it may be beneficial to design the device 10 such that the less concentrated fraction is suitable for use as feed into further desalination processes. This can reduce the energy consumption of these further desalination processes. For example, the less concentrated fraction may be used as feed to an RO process. It is known that reducing the feedwater concentration to an RO process can improve the efficiency of the process and reduce the frequency of maintenance. As such, a less concentrated fraction produced from the device 10 may be advantageously used as feed to an RO process. Also, when the less concentrated fraction is to be used as a feed for further desalination processes rather than as drinking water, the device 10 may be able to be smaller. In this regard, the dimensions of the Burger cascade may be adjusted to suit the operational requirements. In addition, the discharged brine from the RO process may be treated in a thermodiffusive device of the present application to form a more concentrated fraction and a less concentrated fraction. In particular, the discharged brine may undergo salination. This can result in lower volume brines and can improve the overall recovery rate of the system. In this regard, it is thought that the thermodiffusive device may be employed in hybrid RO-thermodiffusion and / or hybrid electrodialysis- thermodiffusion type systems.

[0110] Material Selection

[0111] The present inventors have discovered that, for the Burger cascade device to be able to be effectively used for liquid applications (e.g., desalination), the selection of the materials used for the walls 12, 14 and the sidewalls 30 is important. In particular, the two walls 12, 14 should be primarily comprised of a thermally conductive material, i.e., so as to allow heat transfer to occur between the respective walls 12, 14 and the seawater 18, as the seawater 18 flows through the channels 16. The thermally conductive material that the two walls 12, 14 is primarily comprised of should be of a higher conductivity than the material used to form the sidewalls 30, such that heat flux through the sidewalls 30 is minimised. Again, this is to promote the flow of thermal energy through the liquid. In particular, this composite structure can reduce heat transfer through the sidewalls 30 while promoting heat transfer through the seawater 18 passing through the channels.

[0112] Burger cascade devices of the prior art typically use stainless steel or brass for the sidewalls. However, it is noted that stainless steel is not a good choice for liquid applications because it is neither a good insulation material (as required for the sidewalls) or a good conductor (required for the heated and cooled walls). The present inventors have found that, for the Burger cascade device to be able to be effectively used for liquid applications (e.g., desalination), the sidewalls 30 should be comprised of a material with a sufficiently low thermal conductivity, such that the thermal resistance of the sidewalls is high (i.e., it is an insulator). This can minimise the heat flux through the sidewalls.

[0113] It has been found that if the thermal conductivity of the material used to form the sidewalls 30 is too high, then thermal energy from the heated wall 12 to the cooled wall 14 is preferentially conducted through the sidewalls 30, rather than being conducted through the seawater 18. As a consequence, the heat flux through the seawater 18 is small. This results in a low (even negligible) temperature differential between the seawater 40 and the seawater 42 resulting in poor separation of the ionic species. On the other hand, the present inventors have surprisingly and advantageously found that, when the sidewalls 30 are comprised of a sufficiently low conductivity material, thermal energy from the heated wall 12 can be conducted through the seawater 18.

[0114] In this regard, in some embodiments, the thermal conductivity of the material used to form the sidewalls 30 is less than about 20 W m'1K'1, for example, less than about 10 W m'1K'1. In some embodiments, the thermal conductivity of the material used to form the sidewalls is less than about 1 W m'1K'1. Typically, the sidewalls 30 are formed from a non-metallic material, such as a polymer. For example, in some embodiments, the sidewalls can be comprised of polyoxymethylene (POM). However, without being bound by theory, it is thought that any type of plastic may be used. For example, acrylic may be used. Materials such as acrylic can show good bonding strength with the selected metal for the heated and cooled walls.

[0115] It has been found that when the sidewalls are comprised of POM, which has a thermal resistance of 0.36 K W'1, about 46% of the heat flux goes through the liquid. The inventors have found that this heat flux is sufficient to effect good separation of the diffusing species within the seawater. Alternatively, the sidewalls 30 may be formed from another material with low thermal conductivity, such as a ceramic or a porous material with closed pores. It is noted that, by employing a sidewall material with low thermal conductivity, the quantity of thermal energy which passes through the liquid can be beneficially maximised.

[0116] In this regard, the present inventors have found that Burger cascade devices of the prior art tailored for gaseous applications are not effective for liquid applications, because the gaseous Burger cascade devices of the prior art comprise sidewalls of much higher thermal conductivity (i.e., the sidewalls are made of thermally conductive metals). It is noted that gases have an extremely low thermal conductivity compared to liquids. However, it is thought that, even if there is a relatively large heat loss for gases (through the device), the separation will be effective because the gas component does not contribute to thermal energy consumption. In contrast, for a liquid Burgers cascade, the heat transfer occurs through both the liquid mixture and solid structure (i.e., the sidewalls). The heat transfer through the liquid mixture is relatively large compared to gases. As such, minimising heat transfer through the solid sidewall structure can reduce the thermal energy consumption. That is, the present inventors have surprisingly found that, for liquid applications, the Burger cascade needs to be tailored in a way that heat is not transferred unnecessarily between the heated wall and the cooled wall via the sidewalls.

[0117] In this regard, at the same time, it has been found that the thermal conductivity of the material used to form at least part of the surface area of the walls 12, 14 should have a higher conductivity than the sidewalls 30. In particular, at least part of the surface area of the walls 12, 14 should have a low resistance to allow thermal energy to be efficiently transferred from the hot water to the seawater (i.e., through the heated wall 12) and from the seawater to the cooled water (i.e., through the cooled wall 14). In this regard, at least part of the surface area of the walls 12, 14 is typically comprised of a metallic material such as aluminium and / or copper. However, those skilled in the art will appreciate how to select alternative materials with the desired properties. In some embodiments, the thermal conductivity of the high thermal conductivity material is greater than about 100 W m-1K-1, such as greater than about 150 W m-1K-1. In one embodiment, the thermal conductivity of the material is greater than about 200 W m-1K-1. In some embodiments, the high thermal conductivity material may be composed of a plurality of high thermal conductivity materials, as in a composite structure or multi-layer material.

[0118] The inventors have surprisingly and advantageously found that it is possible to thermally insulate at least some of the surface area of the walls 12, 14 without significantly affecting the efficiency of the separation. Thermally insulating at least some of the surface area of the walls 12, 14 can increase the energy efficiency of the device 10 in use. More specifically, by thermally insulating the surface area of the walls 12, 14 located in a region in which the concentration gradient is minimal even when the walls 12, 14 are comprised of a thermally conductive material, the energy efficiency of the device 10 can be enhanced, without significantly affecting the overall separation efficiency. It is noted that, for channels bounded by walls comprising a thermally insulating material, little to no thermodiffusion occurs therewithin because no temperature gradient is established across the channel.

[0119] The thermally insulating material can comprise the same material as the sidewalls. In this regard, in some embodiments, the thermal conductivity of the thermally insulating material is less than about 20 W m'1K'1, for example, less than about 10 W m'1K'1. In some embodiments, thermal conductivity of the material is less than about 1 W m'1K'1. Typically, the thermally insulating material comprises a non-metallic material, such as a polymer.

[0120] In this regard, the thermal insulation may be in the form of an additional layer of material, the additional layer of material being a thermal insulator. Alternatively, the walls can themselves be comprised of multiple materials, i.e., the thermally conductive material and the thermally insulating material.

[0121] Sidewall Composition

[0122] Without being bound by theory, it is thought that, by providing sidewalls in which at least one portion of the sidewall in proximity to the heated and / or cooled walls is comprised of a higher thermal conductivity material, the efficiency of the device 10 may be improved. In this regard, Figure 4 provides perspective views of (an underside of) the heated wall (upper images) and cross-sectional detail views of single channels (lower images) of three Burger cascade configurations 10, 10b and 10c. The cross-sections show a top view of the heated wall 12, with the plurality of sidewalls 30 which define the channels 16 attached thereto. A cross- section of three different configurations of side walls 30, 30b, 30c, corresponding to each of the Burger cascade configurations 10, 10b and 10c is provided below.

[0123] Referring first to Figure 4 A, the device 10 previously described with reference to Figures 1 to 3 is shown. As explained above, in the device 10, the sidewalls 30 are comprised of a low thermal conductivity material 50 and the cooled wall 14 is comprised of a high thermal conductivity material 52. Without being bound by theory, the inventors noted that, under these conditions, the flow of seawater (or other liquid) through the channels 16 is thought to assume a Poiseuille plane flow (as seen in Figure 5a.1) with a linear temperature profile. Under a linear temperature profile, the vertical concentration distribution of the ionic species within the liquid is thought to be quasi-linear (as shown in Figure 5a.2), when the temperature dependence of the thermophysical properties is ignored. This can result in the coupling of the lowest concentration fluid with a stagnant flow (i.e., zero velocity) at each of the walls. That is, the lowest concentration fluid is associated with a stagnant (zero velocity) flow. As a result, this configuration may not allow for the extraction of the liquid with the lowest concentration of the ionic species, which can reduce the effectiveness of TDD.

[0124] Without being bound by theory, it is thought by changing the linear temperature profile and providing a high-temperature region within the seawater closest to the heated wall which has a temperature similar to the heated wall, the less concentrated fraction may have a non-zero velocity, enabling its extraction. Similarly, in applications where the device 10 is being used for salination, it is thought that changing the linear temperature profile and having a low-temperature region within the seawater closest to the cooled wall and which has a temperature similar to the cooled wall can enable extraction of the more concentrated fraction.

[0125] In this regard, Figure 4B shows a device 10b in which the sidewalls 30b each comprise a first portion 54b located adjacent the heated wall 12b. The first portion 54b is comprised of a material of higher thermal conductivity than a remaining portion 56b of the sidewall 30b. In the illustrated embodiment of Figure 4B, the first portion 54b is comprised of the same material as the heated wall 12b and is formed by machining cut(s) or grooves or the like into the material of the heated wall during the manufacturing process. In this regard, the higher conductivity material of the first portion 54b is a continuation of the material of the heated wall 12b. However, it will be appreciated that the first portion 54b may be otherwise connected or attached to the heated wall during the manufacturing process.

[0126] Although not shown, the sidewall can alternatively or additionally comprise a second portion which is located adjacent the cooled wall. The second portion can be comprised of a material of higher thermal conductivity than the remaining portion of the sidewall. The second portion can be comprised of the same material as the cooled wall and can be formed by machining cut(s) or grooves or the like into the material of the cooled wall during the manufacturing process. In this regard, the higher conductivity material of the second portion can be a continuation of the material of the cooled wall. However, as with first portion, the second portion may be otherwise connected or attached to the cooled wall during the manufacturing process.

[0127] Turning now to Figure 4C, a device 10c is shown in which the sidewalls 30c each comprise a first portion 54c located adjacent the heated wall 12c. The device 10c differs from that of the device 10b only in the height of the first portion 54c relative to the remaining portion 56c of the sidewall 30c. In this regard, in the illustrated embodiment of Figure 4, each of the sidewalls has a height of 1 mm. In Figure 4B, the first portion 54b has a height of about 0.3 mm, i.e., about 30% of the total sidewall height, whereas in Figure 4C, the first portion 54c has a height of about 0.5 mm, i.e., about 50% of the total sidewall height. However, it will be appreciated that other dimensions for the first portion can be employed. For example, it is thought that the first and / or second portions can (together) comprise up to 90% of the total sidewall height. It is also noted that, depending on the sidewall height, the height of the first portion can be adjusted. The effect of providing a first portion and / or second portion on the efficiency of the desalination process is illustrated by way of the Examples below. It is noted that, by providing sidewalls with first and / or second portions, the energy requirement for the device 10 may increase. This is because, the first and / or second portions of the sidewalls have a higher thermal conductivity. As a result, the thermal resistance of the first and / or second portions is small (compared to the remaining portion of the sidewalls which is comprised of a material with a low thermal conductivity). As such, when the same temperature difference is applied to the device, the temperature difference is effectively applied to a smaller height, being the height of the remaining portion (i.e., the portion of the sidewall that is comprised of the lower thermal conductivity material). This results in larger heat flux through the liquid, for the same temperature difference. It has been surprisingly found that the volumetric flowrate through the device can be increased, whilst still maintaining good separation efficiency. In particular, the throughput can be about 8 times higher while the thermal energy consumption is about 5 times higher, compared to the case when the sidewall is entirely made of the low thermal conductivity material.

[0128] It is noted that, although in each of the channels 16, 16b and 16c the sidewalls 30, 30b and 30c are approximately perpendicular to the plate 12 in the illustrated embodiment of Figure 4, it will be appreciated that other configurations are possible. For example, the sidewalls 30, 30b and 30c may be at a different angle to the cooled wall 14 or the join therebetween may be curved.

[0129] As an alternative to providing a first and / or second portion of higher thermal conductivity material on the sidewall, it is thought that a moving wall may be employed to carry the low-concentration liquid attached to the heated wall (i.e., due to the no-slip condition in fluid mechanics). Alternatively or additionally, a moving wall, e.g., in the form of a conveyor, may be employed to carry the high- concentration liquid attached to the cooled wall.

[0130] Sidewall and Opposing Wall Configurations

[0131] As above, it is thought that, by providing sidewalls by which a non-linear temperature profile is established within the channel 16, the efficiency of the device 10 may be improved. Figure 9 provides cross-sectional detail views of single channels of four different Burger cascade channel configurations 10, 10b, lOd, lOe.

[0132] The configuration of the sidewalls 30 and opposing walls 12, 14 of the device 10 of Figure 9A has already been described in detail above, e.g., with reference to Figure 1. As explained above, in the device 10, the sidewalls 30 are comprised of a low thermal conductivity material 50 and the cooled wall 14 and the heated wall 12 are each comprised of a high thermal conductivity material 52. However, the temperature profile of the device 10 is linear.

[0133] To increase the efficiency of the device, the device can instead comprise first and / or second sidewall portions that are comprised of a higher thermal conductivity material. For example, in the device 10b of Figure 9B (which has been described above in detail with reference to Figure 4B and 4C), the sidewalls 30b comprise a first portion 54b and, optionally, a second portion 55b each comprised of a material of higher thermal conductivity than a remaining portion 56b of the sidewall 30b. In the illustrated embodiment of Figure 9B, the first portion 54b and the second portion 55b are each comprised of the same material as the heated wall 12b and the cooled wall 14b (respectively) and are each formed by machining, etc. cut(s) or grooves or the like into the material of the heated wall during the manufacturing process. In this regard, the higher conductivity material of the first portion 54b is a continuation of the material of the heated wall 12b. Similarly, the higher conductivity material of the second portion 56b is a continuation of the material of the cooled wall 14b. However, it will be appreciated that the first portion 54b and / or the second portion 55b may be otherwise connected or attached to the heated wall during the manufacturing process. It is noted that the device 10c of Figure 4C is similar to the device 10b of Figure 4B (and Figure 9B), differing only by the height of the first and / or second portions. As an alternative to the variation shown in Figure 4B, it is thought that a nonlinear temperature profile may instead be established by machining, cutting, 3D printing, etc. a groove into the heated wall and / or the cooled wall (i.e., by machining, cutting, 3D printing, etc. a groove into one or more of the opposing walls). Turning now to Figure 9C, a further variation of the device lOd is shown. The sidewalls 30d of the device lOd are comprised of a lower thermal conductivity material and the walls 12d, 14d are comprised of a higher thermal conductivity material. Cut into each of the walls 12d, 14d are grooves 13d, 15d respectively. The grooves can be machined (or otherwise cut) into the respective wall during manufacturing. It is envisaged that the grooves run longitudinally through each of the channels 16d and define a lower (or upper) boundary of the respective channel 16d. The width of each groove 13d, 15d is less than a width of the respective channel 16d. As a result, the volume of the liquid that is within the groove 13d, 15d is subjected to the higher (or lower) temperature of the heated (or cooled) wall. This creates a non-linear temperature gradient within the channel 16.

[0134] Shown in Figure 9D is a device lOe which is a hybrid of the devices 10b and lOd. The device lOe comprises sidewalls 30e which are comprised of a lower thermal conductivity material. The sidewalls 30e comprise a first portion 54e and, optionally, a second portion 55e, each comprise of a higher thermal conductivity material. In the illustrated embodiment of Figure 9D, the first portion 54e and the second portion 55e are each comprised of the same material as the heated wall 12e and the cooled wall 14e and are each formed by machining etc. cut(s) or grooves or the like into the material of the heated wall during the manufacturing process. In this regard, the higher conductivity material of the first portion 54e is a continuation of the material of the heated wall 12e. Similarly, the higher conductivity material of the second portion 56e is a continuation of the material of the cooled wall 14e. However, it will be appreciated that the first portion 54e and / or the second portion 55e may be otherwise connected or attached to the heated wall during the manufacturing process. In addition, cut into each of the walls 12e, 14e are grooves 13e, 15e respectively. The grooves can be machined (or otherwise cut) into the respective wall during manufacturing. It is envisaged that the grooves will run longitudinally through each of the channels 16e and define a lower (or upper) boundary of the respective channel 16e. The width of each groove 13e, 15e is less than a width of the respective channel 16e.

[0135] The grooves illustrated in Figures 9C and 9D are each rectangular in shape. However, the inventors note that the grooves can comprise other geometries. For example, the grooves can be triangular, square, trapezoidal, circular or any combination of these or other shapes. It is noted that the use of grooves can be particularly advantageous when the width to height aspect ratio of the channels is large such that a linear profile within the channels tends to be formed. The presence of the grooves can help promote the non-linear temperature profile within the channels in these scenarios. However, it is also noted that grooves may promote mixing within the channels, which can disturb the temperature profile within the channels. Therefore, the grooves should be configured to promote the non-linear temperature profile whilst minimising mixing.

[0136] Without being bound by theory, it is thought that combinations of the configurations illustrated in Figures 9A to 9D may be employed. For example, the portion of the sidewall nearest the heated wall may comprise the sidewall configuration of 9D, whilst the portion of the sidewall nearest the cooled wall may comprise the sidewall configuration of 9A. It is thought that the configuration of the sidewall(s) can be selected based on many factors, including (but not limited to): the application of the device, a required separation efficiency, a required thermal efficiency, ease of manufacturing, etc.

[0137] In each of the configurations, it is noted that the high thermal conductivity material may be composed of a plurality of high thermal conductivity materials, as in a composite structure or multi layer materials.

[0138] Channel Configurations In the Burger cascade device 10 of Figure 1, it will be appreciated that the rows of the cascade are arranged along a length of the device 10. It is noted that, in some applications, this can result in a very long and narrow device (e.g., where the device requires a large number of rows). Without being bound by theory, it is thought that, by arranging the rows in a serpentine configuration, the overall length of the device 10 may be reduced. It is envisaged that, in the serpentine arrangement, the rows become stacked across the width of the device, forming a wider but shorter device.

[0139] Figures 10A to 10C provide images of a sidewall configuration 300 for location between the heated wall and the cooled wall of a Burger cascade device. Figure 10B shows the sidewall configuration 300 located on one such plate 301, and another such plate would then overlay the sidewall configuration 300 to form the device 10. In the sidewall configuration 300, the rows 302 of the cascade are further arranged in a serpentine configuration. The sidewall configuration 300 can, for example, be 3D printed, machined, moulded, etc. from a polymer, elastomer, etc.

[0140] In the illustrated embodiment of Figures 10A to 10C, the serpentine of the sidewall configuration 300 comprises a first set of rows 304. The first set of rows 304 is arranged across a length of the device 10. A second set of rows 306 is likewise arranged across a length of the device 10 but is arranged adjacent the first set of rows 306. Between the sets of rows 304, 306 is a curved row 308 which joins the first set of rows 304 and the second set of rows 306 of the cascade.

[0141] Further sets of rows, joined by curved rows, can be added to the sidewall configuration 300 as needed, such that in use, the liquid can flow from the inlet to the outlet(s) of the device 10. It will be appreciated that, for a given number of rows, as the number of sets of rows arranged across the width of the device increases, the sidewall configuration 300 (and thus the device 10) becomes wider but shorter in length. As such, the configuration of the serpentine (i.e., the number of rows across the length of the device and the number of sets of rows) can be selected based on a desired total length and / or width of the device.

[0142] In use, liquid enters the device 10 via its inlet (not shown) and is distributed into the channels (see commencement of arrow 310). The liquid passes along the first set of rows 304 in the direction indicated by the arrow 310. The liquid then passes from the first set of rows 304 to the second set of rows 306 via the curved row 308 (i.e., in the direction of the arrow 312), and then passes through the second set of rows 306 in the direction 314. In this regard, it will be appreciated that the liquid flows in opposite directions through adjacent sets of rows. The liquid continues passing through the serpentine configuration in this manner, until it reaches the outlet of the device (not shown).

[0143] Of course, it will also be appreciated that other configurations of channels are possible. For example, as discussed above with reference to Figure 16, the Burger cascade device can instead be in the form of a series of stacked modules.

[0144] Examples

[0145] The TDD and the method for desalination will now be explained in further detail through the following non-limiting Examples.

[0146] Example 1 - Laboratory scale Burger cascade TDD device

[0147] In this Example, a laboratory scale Burger cascade TDD device was constructed and tested. The device had a similar configuration to the device 10 described above with reference to Figures 1 to 3.

[0148] The lab-scale Burger cascade TDD device consisted of four columns and four pairs of rows, each pair of rows comprising four columns, and its CAD model, is shown in Figure 1 and Figure 6B. The Burger cascade TDD device was a 4x4 device with a single inlet flow that bifurcated equally to the four pairs of rows, and five outlet flows. Each channel was 5 mm in width, 2 mm in height and 52.2 mm in length. The diagonal length between the rows of channels was 8.8 mm.

[0149] It was noted that the five outlets were employed for the purpose of understanding how the device works. In particular, the inventors noted that employing five outlets allowed better observation of how the seawater behaved and separated within the device. For instance, it allowed the salinity drop along the rows to be observed. However, the use of five outlets would be unlikely in a practical setting. It was noted that, for an industrial device, the device would comprise just two outlets: an outlet for the more concentrated fraction and an outlet for the less concentrated fraction. Each outlet would be formed from one or more of the internal channels of the Burger cascade.

[0150] The device comprised two aluminium blocks, which served as water chambers for heating and cooling circulating water baths (i.e., corresponding to the heated wall 12 and the cooled wall 14 of Figure 1). Two outer insulation layers (i.e., corresponding to the layers 32, 34 of Figure 1) were provided. Each insulation layer was comprised of high-density polypropylene (POM), and each insulation layer acted as thermal insulation and also as a lid for thermostatically controlled water located in its respective chamber. The sidewalls were comprised of a material with low thermal conductivity.

[0151] It was noted that the example Burger cascade had a different manufacturing process and material selection from the gas Burger cascade devices reported in the literature, the latter which were entirely made of stainless steel. In this regard, it was noted that the composite material structure of the example Burger cascade reduced heat transfer through the solid parts of the device (other than at the walls 12, 14) while promoting heat transfer through the seawater solution that passed through the channels 16 in use.

[0152] One-dimensional conduction models were used to assess heat transfer through the device. The water flow rate and vertical aspect ratio were small, so the conduction problem was simplified to a thermal resistance network in which the thermal resistance was dependent on the resistance of the fluid and the resistance of the sidewalls. The calculation showed that if stainless steel was used, only 1.2% of the heat flux would pass through water. It was noted that this would not be acceptable from a practical perspective. For liquid applications, since the thermal conductivity of stainless steel is about 25 times that of water, conductive sidewalls would result in unacceptable heat loss. Advantageously, the calculation also showed that, when the sidewall material was instead a polymer such as POM, 46% of the heat flux would pass through the water. It was noted that this would be acceptable from an energy efficiency perspective. In this regard, it was found that the issue of improving heat flux through the liquid can be addressed by using a low-conductivity material, such as POM, for the sidewalls. It was further noted that the low conductivity material should have good mechanical strength and manufacturability. It was additionally noted that glue additives that could be present (e.g., on the gaskets and / or walls) increase the thermal barrier between the water and the metallic upper and lower channel walls were removed during the manufacturing process.

[0153] The entire Burger cascade structure consisted of six layers and three materials. The middle two layers were the sidewall layers (i.e., the sidewalls 30a, 30b of Figure 1) which were comprised of POM. Each sidewall layer had a height of 1 mm (i.e., such that each channel had a total height of 2 mm). The sidewall layers were joined together to form the channel structure therebetween. The sidewall layers were located between two aluminium blocks which formed the heated wall and the cooled wall (i.e., walls 12, 14 respectively of the device 10 of Figure 1). The ‘outside’ of each aluminium block has a respective chamber 13 formed (e.g., machined, etc.) therein (i.e., each chamber locates adjacent to its respective wall 12, 14). The aluminium used was alloy 6061 which had a thermal conductivity of about 200 W m-1K-1. The upper and lower halves of the device were manufactured separately and then compressed together with a single gasket and bolts along the perimeter to avoid leakage. For example, the POM sheet was glued onto the aluminium block with epoxy glue before being machined with a computer numerical control (CNC) manufacturing method that formed the channel pattern.

[0154] Feed Preparation

[0155] Sodium chloride feed samples comprising water and sodium chloride (NaCl) were prepared gravimetrically using NaCl (Ajax Finechem, Thermo Fisher Scientific, minimum assay 99.7%) and deionised water (Pacific TII 12, Thermo Scientific). The balance had an accuracy of 1 mg and a precision of 0.1 mg (ATX224, Shimadzu). Natural seawater was harvested from Jervis Bay, New South Wales, Australia, and natural seawater brine was obtained by evaporating approximately half the volume of natural seawater.

[0156] Burger cascade Experimental Set-up

[0157] The experimental set-up employed is shown in Figure 6A. A TDD device 100 (which was prepared according to the above methodology) was connected to a hot water bath (101) and a cold-water bath (103). The hot water bath 101 supplied hot water which flowed 102 through the top block of the device 100. The cold-water bath 103 supplied cold water which flowed 104, 105 through the bottom block of the device 100, and in an opposite direction to the hot water 102, 107. The liquid feed (i.e., comprising the NaCl water, the natural seawater or the natural seawater brine) was stored in a bottle 106, which was connected to the device 100 via a degassing unit 108 and a peristaltic pump 110. Outlets of the device 100 were connected to collection bottles 112. It was noted that the collected liquid corresponded to less concentrated fractions of the liquid feed. The thermocouples were connected to an external temperature acquisition unit 116 which allowed the temperature being measured by the thermocouples to be displayed and recorded. The concentration of the brine feed and the collected liquid were measured using ex situ phase-shifting interferometry 118.

[0158] It was observed that the success of the Burger cascade experiments was conditional on the prevention of large bubbles in the channels and the production of a similar volumetric flow velocity in all streams. First, to avoid pushing the bubbles into the device during the initial filling process, a three-way valve (not shown) before the inlet of the device first discharged the liquid feed to an external collector (not shown) until all bubbles were removed. Then, the valve was configured such that the liquid was allowed to flow from the bottle 106 to the device 100 via a degassing unit 114, which removed any bubbles that were still present. It was noted that the device could also be tilted while filling with the initial liquid feed so that no air was trapped inside the Burger cascade structure.

[0159] It was noted that, in the device used, the inlet was in the bottom aluminium block while all the outlets were in the top aluminium block to help air escape during the initial filling process. However, it was thought that the inlet / outlets could be otherwise arranged. For example, the inlet may be in the top aluminium block and / or the outlets may be in the bottom aluminium block. Once filled, the device 100 was returned to a horizontal position. The hot water system 101 was connected to the heated wall and the cold-water system 103 was connected to the cooled wall. The hot / cold water were each allowed to circulate through the respective wall, with the hot water flowing in a first direction 102, 107 and the cold water flowing counter-currently in a second direction 104, 105. It was observed that a temperature difference AT was established between the heated wall and the cooled wall. Liquid feed continued to be allowed to flow through the device 100, with the outflowing liquid collected in collection bottles 112. There were five collection bottles 112, each collection bottle corresponding to a respective outlet of the device 100.

[0160] About 30 min after the temperature difference was established, the flow of the liquid feed was stopped and the outlet tubes 112 were clamped to ensure that there was no backflow. Any liquid in the collection bottles 112 at this point was discarded. This is because it was noted that the device 100 had, up until this point, been in thermal regulation and was not yet operational. As a result, none of the ionic species separation had occurred. The clamps were then removed, and the liquid feed flow to the device was resumed. The experiment was run until there was around 50 mL of fluid in each collection bottle 112. During the experimental run, the thermocouple readings were recorded by the temperature acquisition unit 116. After each experiment, the saline water was first pumped out of the system as much as possible. Then deionised water was used to flush the entire system for 15 min at a flow rate of about 20 mL / min. Then compressed dry air flows were used to dry the system for about 30 min. To confirm that this was sufficient to remove all the salt and leave the channels completely dry, the channels were disassembled and examined.

[0161] Thermodiffusion Experiments

[0162] Following the above methodology, eight experiments were performed using four different types of liquid feed at two different flow rates. The liquid feeds tested were: NaCl / water solution at initial concentrations of 35 000 ppm and 70 000 ppm, natural seawater, and natural seawater brine. It was noted that the effective length for thermodiffusive separation was 52.2 mm and the cross-sectional area for each row was 40 mm2.

[0163] Figures 7A to 7D show the concentration difference between the liquid at each of the outlets (the five outlets being labelled j = 0.5, 1.5, 2.5, 3.5, 4.5) and the initial feed liquid for each of the liquid feeds tested at two different flowrates. Figures 7E and 7F show the concentration of specific ions at the outlets for the natural seawater and natural seawater brine experiments respectively. Two different flowrates were used. The concentration of the ionic species was determined using inductively coupled plasma (ICP) spectroscopy.

[0164] In each case, linear fits were applied indicating a monotonic decrease in the concentration of the ionic species as the number of outlets increased. This indicated that the device 100 was effective as a thermodiffusive desalination device, with the concentration of the ionic species at each of the outlets decreasing. It was also observed that the thermodiffusion of each ionic species was coupled, with each species tending to diffuse at similar rates.

[0165] It was noted that the device appeared to provide a more effective separation when the concentration of the seawater was increased from 35,000 ppm (seawater) to 200,000 ppm (brine close to saturation). In particular, the concentration difference increased almost linearly with the mean concentration of the liquid. It was noted that this was a key characteristic of thermodiffusion. Based on this, it was noted that thermodiffusion could be used for brine treatment, as well as desalination. For example, thermodiffusion could be used to increase the concentration of the diffusing species in one of the fractions of the liquid. In addition, it was noted that when the liquid was a multicomponent ionic solution of seawater, the separation was more effective than when the liquid comprised only a single salt (e.g., NaCl).

[0166] It was observed that the concentration of ionic species in the less concentrated fraction decreased linearly as the outlet number was increased. Therefore, a less concentrated fraction that had an even lower concentration of ionic species could be obtained by increasing the number of columns in the Burger cascade. It was noted that, to the inventors’ knowledge, and until now, there was no experimental evidence showing that a Burger cascade could be used in applications of liquid separation based on thermodiffusion.

[0167] Referring to Figures 7A and 7B, a concentration change of approximately 1000 ppm was achieved from normal salinity seawater (35 000 ppm) and 2000 ppm for brine (70 000 ppm). It was observed that these concentration drops were obtained within hours. It was noted that the recovery rate was 3.2 times higher than in single channel thermodiffusive unit recirculation experiments previously undertaken in which the low concentration stream was recirculated four times. It was noted that this was a significant result, given that the Burger cascade tested was small.

[0168] Example 2 - Full-scale Burger Cascade Device

[0169] An in-house MATLAB code was developed (which has been made open source on [GitHub]) which could calculate the performance of a full-scale liquid Burger cascade based on user requirements, including total footprint area, recovery rate, feedwater and yield concentration, concentration separation ratio, linear temperature percentage, etc. The code was used to simulate Burger cascade devices with fully developed flow profiles. It was noted that the analysis did not account for heat losses through the sidewalls and that the total width corresponded to the individual channel width multiplied by the total number of columns. It was observed that the performance metrics for the liquid Burger cascade could be determined by its thermal efficiency, depending on the availability and the quality of its heat source.

[0170] The effect of the number of rows and columns on the efficiency of the liquid Burger cascade was tested by changing the number of rows and / or columns in the computation model. Figure 8A is a contour plot of the concentration profile inside a Burger cascade with 150 columns and 6000 rows. It was observed that the concentration of the ionic species within the liquid started to change from the outside columns of the device and gradually propagated toward the centre of the device as the number of rows increased. The concentration of the ionic species continued to change until an equilibrium concentration was reached (marked as Meq on Figure 8A). It was noted that the number of rows corresponding to the equilibrium concentration was optimal, because further increasing the number of rows did not significantly affect the concentration profile of the ionic species within the device.

[0171] Figure 8B shows the number of rows required to reach the equilibrium concentration for different numbers of columns and the corresponding equilibrium concentration drop (i.e., the difference in the concentration of the ionic species between the liquid feed and the less concentrated fraction produced from the Burger cascade). It was noted that the number of rows required to reach equilibrium increased with the number of columns. The equilibrium concentration drop also increased with the number of columns. This suggested that increasing the number of columns can decrease the concentration of the ionic species present in the less concentrated fraction (i.e., because the equilibrium concentration drop increases). However, a larger number of rows is required to reach the equilibrium concentration drop. It was noted that the equilibrium concentration drop increased as the number of columns was increased until the Burger cascade included about 100 columns. When the number of columns was further increased (i.e., to above 100 columns) it was observed that the equilibrium concentration drop did not significantly change and appeared to plateau. This suggested that increasing the number of columns above 100 would not significantly improve the performance of the device. The inset of Figure 8B shows the ratio of the equilibrium concentration drop and the equilibrium row number as a function of the number of columns.

[0172] Figure 8C shows the concentration drop as a function of the row in the Burger cascade. It was observed that the maximum concentration drop achievable in the Burger cascade corresponded to the equilibrium concentration drop for the number of columns in the Burger cascade. That is, once the equilibrium concentration drop was achieved, increasing the number of rows did not have a significantly improve the concentration drop. It was noted that, with fewer columns, a lower concentration drop (and thus a lower equilibrium concentration) was attainable. However, the equilibrium concentration drop was able to be achieved in fewer rows, resulting in a smaller Burger cascade.

[0173] Example 3 - Non-linear Temperature Profile

[0174] The same MATLAB model was used to simulate Burger cascade devices in which the sidewalls have a first portion comprised of a material of higher thermal conductivity than a remaining portion and, optionally, a second portion comprised of a material of higher thermal conductivity than a remaining portion (e.g., as in the embodiments of the device 10b, 10c described above with reference to Figure 4). Figure 5 shows simulation results. It was noted that these simulation results were based on experimentally validated models. It was assumed that, in each channel, there was a fully developed parabolic velocity profile (as seen in Figure 5a.1). In the model, the channel top and bottom walls were made of aluminium allow (the top wall being heated and the bottom wall being cooled) and the side walls were made of POM. It was noted that, under a linear temperature profile, the concentration profile was quasi-linear (as seen in Figure 5a.2 which shows the normalised concentration at different channel heights for a linear temperature profile). As a result, it was noted that the liquid with the lowest concentration coincided with the liquid with zero velocity (i.e., due to the no-slip boundary condition).

[0175] It was noted that, in order to couple the low concentration liquid with a non-zero velocity, the temperature profile could be changed. This could be achieved by, for example, changing part of the sidewall material (i.e., to be a material of higher thermal conductivity, such as aluminium alloy). Figure 5a.3 shows the normalised concentration (i.e., concentration compared to the initial value) at different channel heights for a non-linear temperature profile where a portion of the sidewall adjacent the heated wall is comprised of a higher thermal conductivity material than a remaining portion of the sidewall (such as the material from which the heated wall is comprised). For example, a normalised concentration of 2.0 represents a concentration that is twice the initial concentration. Figure 5c.1 shows the temperature profile as a function of the separation ratio for different values of the linear temperature ratio. The separation ratio describes an incomplete separation to a complete separation. It is noted that the separation ratio can be modified by increasing the volumetric flowrate. The linear temperature ratio is the ratio between the vertical distance in which the temperature varies linearly and the total height of the channel.

[0176] The configurations of Figures 5a.3 and 5c.1 are deemed “asymmetric” as only a first portion of the sidewall material is substituted for the material of higher thermal conductivity. In this regard, Figures 5a.4 and 5c.2 correspond to a “symmetric” configuration in which a second portion of the sidewall adjacent to the cooled wall is comprised of a higher thermal conductivity material than the remaining portion of the sidewall (such as the material from which the cooled wall is comprised).

[0177] Figures 5b.1 and 5b.2 show the effect of the non-linear temperature profile (i.e., of

[0178] Figures 5a.3 and 5a.4) on the separation as a function of the linear temperature ratio (defined by the percentage of the channel that is under a linear temperature profile) under symmetric and asymmetric conditions respectively. That is, as the height of the first and / or second portions increases, the linear temperature ratio decreases. For example, the linear temperature ratio for Figures 5a.3 and 5a.4 is 60%, whilst the linear temperature ratio for Figure 5a.2 is 100%. It was noted that, for the symmetric case, the normalised concentration drop decreased as the linear temperature ratio increased. For the asymmetric case, the normalised concentration drop increased as the linear temperature ratio was increased to about 70% and then decreased. This indicated that, for the asymmetric case, a linear temperature ratio of about 70% could increase the overall separation of the device.

[0179] It was further noted that, the presence of first and / or second portions in the sidewall affected the utilisation of the thermal energy. Figure 5d shows the normalised volumetric flow rate (left axis) and the normalised flow rate per heat flux (right axis) as a function of the linear temperature percentage. It was observed that the symmetric case always showed a larger normalised volumetric flow rate compared to the asymmetric case and a higher normalised flow rate per heat flux.

[0180] It was noted that increasing the size of the region of the sidewall that was comprised of the higher thermal conductivity material allowed a wider range of low-concentration liquids which have a finite velocity. In particular, when the device was employed for desalination, increasing the size of the portion of the sidewall adjacent to the heated wall that was comprised of the higher thermal conductivity material allowed for the low-concentration liquid to be collected. In this way, the low-concentration liquid could be more effectively extracted from the system compared to case without the portion of conducting sidewall. In contrast, if the sidewalls were only comprised of the low thermal conductivity material, then the lowest concentration liquid would be attached to the heated wall, due to the no-slip condition. This inhibited the method from extracting the lowest concentration seawater from the system in a desalination application. It was noted that, for applications where brine was sought to be concentrated, a region of the sidewall closest to the cooled wall could be comprised of the higher thermal conductivity material. This would increase the amount of liquid in the channel that had the highest or lowest concentration with a finite velocity.

[0181] The inventors noted that a combination of linear and non-linear temperature profiles (i.e., by providing first and / or second portions within the sidewall) along the channel heights could enable volumetric flowrates around ten times higher to be used compared to when the sidewalls were comprised of only the material of lower thermal conductivity, whilst maintaining the same or better separations. It was noted that the thermal energy consumption of devices with non-linear temperature profiles (e.g., as in Figures 5a.3 and 5a.4) may be up to five times higher than those with linear temperature profiles. However, the thermal efficiency of such a configuration was still sufficiently high, given that the throughput of such a device is also higher.

[0182] Example 4 - Non-Linear Temperature Profile

[0183] In this example, the same MATLAB model was used to simulate Burger cascade devices with grooves in the opposing walls. In particular, a Burger cascade with triangular grooves with varying pitches was simulated. The temperature of the heated wall was 35 °C and the temperature of the lower wall was 25 °C. The channel width was 5 mm and the channel height was 2 mm. For each simulation, a triangular groove with a width of 1 mm in each of the opposing walls was used. The temperature profile in the channels is shown in Figure 12. In Figure 12, the opposing walls 12, 14 have grooves 15 cut thereinto. The contour lines denote the temperature profile within the channel. In the first simulation (Figure 12A), the triangular grooves each had a 30° pitch and in the second simulation (Figure 12B), the triangular grooves each had a 60° pitch.

[0184] It was observed that, when the grooves were present in the opposing walls, there was a non-linear temperature profile in the channels. That is, there was an area of liquid (denoted 201, 202 in Figures 12A and 12B) near the opposing walls which had an approximately homogeneous temperature. However, the non-linear temperature profile was more significant when the pitch of the triangular groove was 60° compared to when the pitch of the triangular groove was 30°. In particular, there was a larger region near the opposing walls in which the liquid had a homogeneous temperature when the pitch of the triangular groove was larger. This indicated that the presence of grooves could allow for homogeneous temperature regions 201, 202 to be developed in the liquid near the opposing walls which can increase the separation efficiency. The inventors noted that the presence of such grooves can be particularly beneficial when the channels had a larger width to height aspect ratio, because a larger width to height aspect ratio tended to decrease the volume of the homogeneous temperature region.

[0185] Example 5 - Aspect Ratio of Channel

[0186] In this example, the same MATLAB model was used to simulate Burger cascade devices in which the channels had width to height aspect ratios that were varied between 0.25 to 5. Figures 11A and 11B show how changing the aspect ratio affects the average temperature in the regions of homogeneous temperature 201, 202 closest to the opposing walls when 50% of the sidewall height is comprised of a high thermal conductivity material and when 25% of the sidewall height is comprised of a high thermal conductivity material respectively. In this set of experiments, the heated wall was set to a temperature of 60°C and the cooled wall was set to a temperature of 20°. The y-axis shows the average temperature in a u- shaped region of the channel, being the region bounded by the high thermal conductivity material.

[0187] It was observed that, as the width to height aspect ratio of the channel increased, the average temperature in the u-shaped region of the channel decreased, as did the volume of liquid in the region of homogeneous temperature 201, 202 closest to the opposing walls. This indicated that the temperature profile became more linear as the width to height aspect ratio of the channel increased. The inventors noted that, as explained above, it can be advantageous to have a non-linear temperature profile within the channel, because it can increase the efficiency of the device. It was noted, however, that there may be manufacturing limitations associated with channels with very low width to height aspect ratios. Thus, there is likely to be a trade off between selecting an aspect ratio that provides a good separation efficiency (i.e., by allowing a non-linear temperature profile within the channel to develop) and that is able to be manufactured.

[0188] Example 6 - Thermal Insulation of Opposing Walls

[0189] In this example, the same MATLAB model was used to simulate Burger cascade devices in which sections of the opposing walls were comprised of a thermally insulating material. For each of these simulations, a Burger cascade of 40 rows and 10 columns was used. The temperature difference between the opposing walls was 40 °C, with the heated wall being at a temperature of 60 °C. It was assumed that the concentration profile was partially developed, with a concentration separation ratio of 78% used for the simulation, and that the sidewalls were symmetric with 40% of the sidewall being comprised of a material of higher thermal conductivity. An initial seawater feed concentration of 35,000 ppm was used.

[0190] Figure 13 A shows the concentration profile in a Burger cascade with no thermal insulation. Figure 13B shows the concentration profile in a Burger cascade in which different regions are insulated, with the contour plot corresponding to the concentration profile corresponding to the case when 20% of the channels are insulated. Figure 13C shows the concentration in the last row of the Burger cascade when different amounts of thermal insulation are used.

[0191] It was noted that insulating the channels could be achieved by providing the opposing walls that define the channels with a thermally insulating material. The thermally insulating material could comprise the wall material, or be applied, attached, connected, etc., to the surfaces of the opposing walls, i.e., as an additional layer. Alternatively, the opposing walls could be comprised of different materials, such as a material with high thermal conductivity (where it is desired not to insulate the channels) and a thermally insulating material (where insulation of the channels is desired).

[0192] Figure 13 A shows that, when no thermally insulating material is used on the opposing walls, there is a significant area in which no species separation occurs, located in the lower middle part of the Burger cascade. That is, there is a significant area in which the concentration of the ions remains approximately constant. This occurs because of mixing between the low and high concentration streams at the inlet of the individual channels. The inventors noted that it was possible to thermally insulate this area of the Burger cascade with minimal impact on the development of the concentration profile and yield performance.

[0193] Figure 13B shows the concentration profile that is obtained from a simulation in which the Burger cascade is partially thermally insulated. The solid, dash-dotted, and dotted lines represent insulating the original channels in the range 34,000 - 36,000 ppm (corresponding to 20% of the channels being insulated), insulating the original channels in the range 32,000 - 38,000 ppm (corresponding to 34% of the channels being insulated), and insulating the original range 30,000 - 40,000 ppm (corresponding to 45% of the channels being insulated). The concentration ranges correspond to the concentrations found in the channels of the non-insulated Burger cascade of Figure 13 A. The fraction of the Burger cascade that is insulated was calculated by considering the ratio between the number of insulated channels and the total number of channels. The contour plot of Figure 13B is the concentration profile corresponding to the case where the 20% of the channels were insulated. It was observed that the concentration profile obtained when 20% of the channels were thermally insulated was not significantly different from the concentration profile obtained when no channels were thermally insulated.

[0194] Figure 13C shows the concentration in the last row when different percentages of the channels were thermally insulated. It was observed that, when the Burger cascade comprised 34% and 45% insulation, the un-insulated area within the Burger cascade was essentially divided into two parts, left and right. It was noted that these two parts effectively worked as individual Burger cascades. In the insulated region which divided the two parts, it was noted that the diffusing species was transported via diffusion (not thermodiffusion) since there was no temperature difference in this region. It was thought that this accounted for the slight increase in concentration observed between columns 4 to 7.

[0195] The inventors noted that thermally insulating at least some of the channels did not significantly affect the performance of the Burger cascade. However, the presence of the thermal insulation could reduce the energy consumption of the Burger cascade. In addition, it was noted that, within the thermally insulated areas, the sidewalls did not need to have complex configurations (such as grooves), because there was no thermodiffusion occurring in these sections. It was noted that this could simplify the manufacturing process and reduce the cost of manufacturing the Burger cascade. However, it was also noted that thermally insulating certain areas of the Burger cascade could add to design and manufacturing complexity and, additionally, may not always be necessary.

[0196] Example 7 - Applications of Thermal Insulation of Opposing Walls

[0197] In this example, the same MATLAB model was used to simulate Burger cascade devices in which sections of the opposing walls were comprised of a thermally insulating material. For each of these simulations, a Burger cascade of 40 rows and 10 columns was used. The temperature difference between the opposing walls was 40 °C, with a mean temperature of 60 °C. It was assumed that the concentration profile was partially developed, with a concentration separation ratio of 78% used for the simulation, and that the sidewalls were symmetric with 40% of the sidewall being comprised of a material of higher thermal conductivity. An initial seawater feed concentration of 35,000 ppm was used. In this set of simulations, the area of the Burger cascade that was thermally insulated was changed, depending on the application of the Burger cascade.

[0198] Figure 14A shows the concentration profile for a partially thermally insulated Burger cascade which is being used for a thermodiffusive desalination application. It was observed that, for TDD applications, up to 49.1% of the area of the Burger cascade could be thermally insulated without significantly impacting the yield concentration and volumetric flow rate, assuming an optimal recovery of 50%. In particular, the thermal insulation was provided in the bottom-middle triangular area (a-i) of the Burger cascade because this was observed to comprise the area with the lowest concentration difference to the liquid feed. The left hand side area (a-ii) was also insulated, because this was the area of highest concentration, which was not of interest in TDD applications. This was because, in TDD applications, it was the lower concentration fraction that was of interest. Additionally, a small area in the upper right area (a-iii) of the Burger cascade was insulated because no further separation was required in that area. It was noted, however, that the exact boundary of the insulation could be adjusted through trial and error, e.g., based on the desired concentration of the lower concentration fraction, the recovery, etc.

[0199] Figure 14B shows the concentration profile for a partially thermally insulated Burger cascade which is being used for a thermodiffusive salination application. It was observed that, for TDS applications, up to 40.1% of the area of the Burger cascade could be insulated without significantly impacting the yield concentration and volumetric flow rate, assuming an optimal recovery of 30%. For TDS applications, it was noted that bottom middle area (b-i) could be insulated, because this was observed to comprise the area with the lowest concentration difference to the liquid feed. In addition, the rightmost region (b-ii) of the Burger cascade could be insulated because, in TDS applications, it was the higher concentration fraction that was of interest. Therefore, the area of the Burger cascade that typically had low concentrations could be thermally insulated. It was noted, however, that the exact boundary of the insulation could be adjusted through trial and error, e.g., based on the desired concentration of the higher concentration fraction, the recovery, etc.

[0200] It was noted that the optimal thermal insulation area and the placement of the thermal insulation in the Burger cascade was different for TDD and TDS applications because in TDD applications the lower concentration fraction was the desired product whilst in TDS applications the higher concentration fraction was of interest.

[0201] Figure 14C shows the concentration in the last row of the Burger cascade when only the middle section of the Burger cascade is insulated (as in Figure 13B) and when the Burger cascade comprises insulation for TDD and TDS applications (i.e., as explained in relation to Figures 14A and 14B respectively). It was observed that the concentration in the last row was similar for all insulation conditions. However, the end rows that are within the insulated regions of the Burger cascade (i.e., the insulated regions corresponding to a-ii, a-iii, b-ii), tended to have a flatter concentration profile. It was noted that the results indicated that part of the Burger cascade could be thermally insulated without significantly affecting the yield concentration of the lower or higher concentration fractions. It was also noted that, because the area that was thermally insulated had no heat flux, the energy consumption was reduced when thermal insulation was used. It was noted that the reduction in energy use was proportional to the area of the Burger cascade that was insulated.

[0202] Example 8 - Effect of Pressure on Separation

[0203] In this example, the same MATLAB model was used to simulate Burger cascade devices in which an operating pressure of the device was changed. For each of these simulations, a Burger cascade of 40 rows and 10 columns was used.

[0204] Referring to Figure 17, the Burger cascade was operated at an atmospheric pressure. The heated wall was set to a temperature of 90 °C and the cooled wall was set to a temperature of 10 °C. The sidewalls were symmetric with 40% of the sidewall being comprised of a material of higher thermal conductivity. The concentration separation ratio was optimised to improve energy efficiency. The initial feed water concentration was 70,000 ppm.

[0205] Figure 17A shows the concentration profile for the Burger cascade with 40 rows and 10 columns. Figure 17B shows how the concentration of the most concentrated stream and the least concentrated stream varied as the liquid flowed through the Burger cascade. It was observed that, as the liquid flowed through the Burger cascade, a concentration variation developed across the columns, and the liquid could be separated into a more concentrated fraction and a less concentrated fraction.

[0206] Figure 17C shows the energy consumption per unit volume of yield for different feedwater and yield concentrations. The temperature conditions and temperature profile were held constant, with the number of channels changed to satisfy the desired concentration change within the Burger cascade, while optimising energy efficiency. Figure 17D shows the corresponding yield flow rate per channel area for the different feedwater and yield concentrations tested in Figure 17C.

[0207] Referring now to Figures 18A to 18D, a similar set of simulations was performed, but using an absolute pressure of 15.5 bar. It was noted that, because the absolute pressure was higher, higher temperatures could also be used for the heated wall, because the feed water would boil at a higher temperature. Accordingly, the heated wall was set to a temperature of 180 °C and the cooled wall was set to a temperature of 20 °C. Again, the sidewalls were symmetric with 40% of the sidewall being comprised of a material of higher thermal conductivity.

[0208] It was observed that high-pressure operation allowed the Burger cascade to operate under a higher mean temperature and with a larger temperature difference, because the feed water boiling point was higher. This improved separation was due to an increase of the Soret coefficient and due to enhanced diffusion dynamics, i.e., due to an increasing Fickian diffusion coefficient resulting in faster separation. As a result, better separation was observed when the Burger cascade was operated at an absolute pressure of 15.5 bar.

[0209] It was noted, however, that operating at higher pressures and higher temperatures can lead to higher capital cost, because there are higher demands for the mechanical strength of the Burger cascade. In addition, corrosion and scaling can be more severe at elevated temperatures and pressures. Therefore, the use of elevated temperatures and pressures may not be suitable when the liquid comprises a corrosive substance, e.g., sodium hydroxide. Higher operating temperatures and pressures can also lead to higher maintenance and operating costs, e.g., due to the increases in corrosion and scaling. Accordingly, it was noted that there should be a balance between the operating pressure and temperature, which can allow for better separation, and considerations of capital and operating costs, as well as safety concerns.

[0210] Furthermore, the inventors noted that how the Soret coefficient and the Fickian diffusion coefficient change with temperature and pressure was species and solution specific. Accordingly, for each solution, the dependencies of each coefficient on temperature and pressure should be understood.

[0211] Example 9 - Effect of Diffusing Species on Separation

[0212] In this example, the same MATLAB model was used to simulate Burger cascade devices in which the feed liquid was an aqueous solution comprising different diffusing species. For each of these simulations, a Burger cascade of 40 rows and 10 columns was used. The heated wall was set to a temperature of 90 °C and the cooled wall was set to a temperature of 10 °C. The sidewalls were symmetric with 40% of the sidewall being comprised of a material of higher thermal conductivity. The concentration separation ratio and the number of channels was optimised to improve energy efficiency.

[0213] Figures 19, 20 and 21 show the energy consumption per unit volume of yield and the corresponding yield flow rate per unit channel area with varying feedwater and yield concentration for solutions comprising lithium chloride, potassium sulphate, and sodium hydroxide, respectively.

[0214] It was observed that the energy consumption per unit volume of yield and the corresponding yield flow rate per unit channel area were different for the different solutions. This was because of the different fluid properties. In particular, for species with higher Soret coefficients, larger thermodiffusive concentrations were observed in each channel. As a result, a fewer number of channels was required to achieve a predetermined concentration change. This resulted in a smaller energy consumption (i.e., because the Burger cascade was smaller or because the fluid flow rate is higher in larger channels when the Burger cascade total area is constant but the length and width of each channel can be larger due to the fewer number of channels). It was also note that, for species with larger Fickian diffusion coefficients, faster thermodiffusive separation will occur. As a result, a higher volumetric flow of the solution through the Burger cascade can be employed, increasing the flowrate per unit channel area. The assumed Soret coefficient and Fickian diffusion coefficient for each diffusing species used in the simulations are provided in Table 1 below.

[0215] Table 1. Diffusion coefficients for diffusing species.

[0216] Figures 22, 23, and 24 show the number of rows, number of columns, and the number of channels required in the Burger cascade to achieve different concentration yields for different initial feed concentrations. It was noted that, in general, when the relative concentration change is larger, more channels are required.

[0217] Figures 25, 26 and 27 show concentrate profiles in a Burger cascade for an initial feed concentration of 150 parts per thousand (ppt) and a concentrated fraction concentration of 200 ppt. The concentration contour plots are top view of the Burgers cascade and in each case, the top opposing wall is heated and the bottom opposing wall is cooled. All Soret and (Fickian) mass diffusion coefficients were evaluated at 50 °C assuming that they followed the same temperature dependence and as the Soret and Fickian diffusion coefficients of sodium chloride. The shaded area represents the area of the Burger cascade that was partially thermally insulated (i.e., as could be achieved by thermally insulating the opposing walls in that section of the Burger cascade) to reduce energy consumption. As above, partially thermally insulating the Burger cascade does not significantly affect the concentration profile within the Burger cascade, but can reduce significantly its energy consumption. It was observed that, for the same concentration change, when the diffusing species had a higher Soret coefficient, a fewer number of rows and columns (i.e., a fewer number of channels) was required to achieve the desired concentration.

[0218] It was noted that lithium chloride was thermophilic (i.e., it had a tendency to move toward the heated plate). This was in contrast to many other species including sodium chloride, ions in seawater, potassium sulphate and sodium hydroxide. As a result, the concentration profile for the lithium chloride solution was opposite to the concentration profiles of the potassium sulphate and sodium hydroxide solutions (i.e., because the more concentrated fraction was the top stream coming out of each channel within the Burgers cascade, and those top, high-concentration streams were directed towards the right hand side by the diagonal length 59, see Figure 3B).

[0219] The inventors observed that the Burger cascade was a separation technology that could be used beyond sodium chloride aqueous solutions or seawater. For example, similar or better performance (energy efficiency and yield) can be achieved for other solutions such as lithium chloride solutions, potassium sulphate solutions and sodium hydroxide solutions. Thus, the Burger cascade of the present disclosure has a wide range of applications.

[0220] Without being bound by theory, it is also thought that the Burger cascade of the present disclosure is suitable for use with non-aqueous liquid solutions. Examples of non-aqueous solutions are thought to include biofluids such as dissolved proteins and / or a polymeric solution that can be organic or inorganic. For example, it is thought that the Burger cascade can be suitable for use with protein aqueous solutions such as animal blood and blood plasma.

[0221] Example 10 - Design Considerations It was noted that, for a full-scale liquid Burger cascade, there were many parameters that could affect the performance of the thermodiffusive desalination. The main performance metrics of the volumetric flow rate and the ratio of the flow rate to the heat flux were considered. Again, computational simulations were performed using the open-source code to determine the impact of varying the dimensions of the Burger cascade on its performance.

[0222] It was noted that, when the separation in each channel was complete (i.e., when the equilibrium separation was reached), the volumetric flowrate was proportional to the cross-sectional area divided by the channel height times the number of rows. The ratio of the flow rate to the heat flux was inversely proportional to the number of rows. It was noted that these relationships simplified the design process for a liquid Burger cascade and could allow for a great degree of freedom in the individual channel dimensions. The overall dimensions of the liquid Burger cascade may be adjusted to suit the manufacturing process or the operating conditions.

[0223] It was further noted that, when designing a Burger cascade, the channel height should not be too large. If the channel height is too large, then the time required for the flow to be fully developed within the channel will be large. A very large channel may also increase the Reynolds number beyond the critical threshold for transition from laminar to turbulent flow. However, the channel should not be too narrow or impurities such as air bubbles may block the flow of the liquid therethrough. It was also noted that the channel length can be large so as to increase the volumetric flow through the device. However, this would also increase the heat transfer rate. The inventors note that, in principle, increasing the number of channels (i.e., rows and / or columns) will allow better separation.

[0224] Variations and modifications may be made to the parts previously described without departing from the spirit or ambit of the disclosure. For example, multichannel configurations other than a Burger cascade may be employed, such as microfluidic devices. As another example, the materials from which the heated wall, the cooled wall and / or the sidewalls are comprised can be selected based on the heat flux requirements. As a further example, it is noted that the device of the present disclosure may be used for salination. That is, the more concentrated fraction may be collected and used. As a yet further example, the device of the present disclosure may be used for separating a liquid into a more concentrated fraction of a diffusing species and a less concentrated fraction of a diffusing species, wherein the diffusing species is a non-ionic species.

[0225] In the claims which follow and in the preceding description of the device and method, except where the context requires otherwise due to express language or necessary implication, the word “comprise” or variations such as “comprises” or “comprising” is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the device and method.

Claims

CLAIMS1. A thermodiffusion device for a liquid comprising at least one diffusing species, the device for separating the liquid into a fraction that is more concentrated with the diffusing species, and a fraction that is less concentrated with the diffusing species, the device comprising: two opposing walls, each comprised of a thermally conductive material, one of the walls being at a higher temperature than the other of the walls in use; a plurality of sidewalls located between the two walls and arranged to form a plurality of channels through which the liquid flows in use, the plurality of sidewalls each comprised of a material having a thermal conductivity that is lower than the thermal conductivity of each of the two opposing walls; an inlet configured to allow the liquid to flow into the plurality of channels in use; and at least two outlets configured to collect the more concentrated fraction and the less concentrated fraction.

2. A device as claimed in claim 1, wherein the two opposing walls are arranged substantially parallel to one another.

3. A device as claimed in claim 1 or 2, wherein the plurality of channels are arranged in a Burgers cascade.

4. A device claimed in any one of the preceding claims, wherein the plurality of sidewalls are each comprised of a material with a thermal conductivity of less than about 1 W m'1K'1.

5. A device claimed in claim 4, wherein the plurality of sidewalls are each comprised of a non-metallic material.

6. A device as claimed in claim 5, wherein the plurality of sidewalls are comprised of: a polymer or a porous material with closed porosity or a ceramic.

7. A device claimed in any one of the preceding claims, wherein the two opposing walls are comprised of at least one material with a thermal conductivity greater than about 50 W m'1K'1.

8. A device as claimed in claim 7, wherein the two opposing walls are comprised of: aluminium and / or copper and / or graphene.

9. A device as claimed in any one of the preceding claims, wherein a portion of the opposing walls are comprised of a material with a low thermal conductivity.

10. A device as claimed in claim 9, wherein the material with the low thermal conductivity has a thermal conductivity of less than about 1 W m'1K'1.

11. A device as claimed in any one of the preceding claims, wherein one or both of the opposing walls comprise(s) one or more grooves.

12. A device as claimed in claim 11, wherein a width of the groove is less than a width of the channel.

13. A device as claimed in any one of the preceding claims, wherein each of the sidewalls comprises a first portion located adjacent to one of the two opposing walls, the first portion being comprised of a material of higher thermal conductivity than a remaining portion of the sidewall.

14. A device as claimed in claim 13, wherein each of the sidewalls comprises a second portion located adjacent to the other one of the two opposing walls, the second portion being comprised of a material of higher thermal conductivity than the remaining portion of the sidewall.

15. A device as claimed in claim 13 or 14, wherein the material of higher thermal conductivity of the first portion and the second portion (when present) comprises the same material as the two opposing walls.

16. A device as claimed in claim 15, wherein the material of higher thermal conductivity of the first portion and the second portion (when present) is a continuation of the material of the two opposing walls.

17. A device as claimed in any one of claims 13 to 16, wherein the first portion and the second portion (when present) each comprise from about 30% to about 90% of a total height of the channel.

18. A device as claimed in any one of the preceding claims, wherein a width of each channel is from about 0.25 mm to about 50 mm.

19. A device as claimed in claim 18, wherein the width of each channel is about 5 mm or about 23.4 mm.

20. A device as claimed in any one of the preceding claims, wherein a height of each channel is from about 0.1 mm to about 5 mm.

21. A device as claimed in claim 20, wherein the height of each channel is about 2.1 mm.

22. A device as claimed in claim 3 or in any one of claims 4 to 21 when dependent on claim 3, wherein a length of each column in the Burger cascade is from about 10 mm to about 300 mm.

23. A device as claimed in claim 22, wherein the length of each column is about 52 mm or about 203.4 mm.

24. A device as claimed in claim 3 or in any one of claims 4 to 23 when dependent on claim 3, wherein a diagonal length between columns of the Burger cascade is from about 0 mm to about 40 mm.

25. A device as claimed in claim 24, wherein the diagonal length is about 8.8 mm or about 17 mm.

26. A device as claimed in claim 3 or in any one of claims 4 to 25 when dependent on claim 3, wherein the Burger cascade comprises at least 4 columns.

27. A device as claimed in claim 3 or in any one of claims 4 to 26 when dependent on claim 3, wherein the Burger cascade comprises at least 4 rows.

28. A device as claimed in claim 3 or in any one of claims 4 to 27 when dependent on claim 3, wherein the Burger cascade comprises one or more modules connected in series.

29. A method for thermodiffusive separation in a liquid comprising at least one diffusing species, the method for separating the liquid into a fraction that is more concentrated with the diffusing species, and a fraction that is less concentrated with the diffusing species, the method comprising: providing a Burger cascade device; heating a first wall of the Burger cascade device and cooling a second wall of the Burger cascade device to thereby form a temperature gradient between the first wall and the second wall; andallowing the liquid to flow through the Burger cascade device such that, as the liquid flows from an inlet to an outlet thereof, the at least one diffusing species therein diffuses to form the more concentrated fraction and the less concentrated fraction.

30. A method as claimed in claim 29, comprising cooling the second wall to a temperature above an inversion temperature of the one or more diffusing species such that the one or more diffusing species are caused to diffuse toward the second wall.

31. A method as claimed in claim 30, comprising cooling the second wall to a temperature of above about 10 °C.

32. A method as claimed in any one of claims 29 to 30, comprising heating the first wall to a temperature below a boiling temperature of the liquid.

33. A method as claimed in any one of claims 29 to 32, comprising operating the Burger cascade at atmospheric pressure .

34. A method as claimed in any one of claims 29 to 33, comprising operating the Burger cascade at an elevated pressure of up to about 50 bar.

35. A method as claimed in claim 34, comprising heating the first wall to a temperature below about 90 °C.

36. A method as claimed in any one of claims 30 to 35, comprising orienting the Burger cascade device such that the first wall is located above the second wall.

37. A method as claimed in any one of claims 29 to 36, wherein the liquid comprises natural seawater or natural seawater brine and the thermodiffusive separation comprises thermodiffusive desalination.

38. A method as claimed in any one of claims 29 to 37, wherein the diffusing species comprises lithium chloride, potassium sulphate and / or sodium hydroxide.

39. A method as claimed in any one of claims 29 to 38, wherein the Burger cascade device comprises the device as claimed in claim 3 or in any one of claims 4 to 28 when dependent on claim 3.

40. Use of a thermodiffusion device as claimed in any one of claims 1 to 28 for desalination of water.

41. Use of a thermodiffusion device as claimed in any one of claims 1 to 28 for thermodiffusive salination.

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