Apparatus for desubliming a target compound from a gas mixture

Abstract

The present invention provides an apparatus for desubliming a target compound from a first gas mixture containing the target compound, comprising a desublimation means comprising a surface onto which desublimation of the target compound can occur; a cooling means that cools the desublimation means; an inlet through which the first gas mixture can enter the apparatus such that it comes into contact with the surface of the desublimation means in a contact region; a target compound recovery region; an outlet through which the target compound can leave the target compound recovery region; a first path for the desublimation means that extends from the cooling means to the contact region before returning to the cooling means without passing through the target compound recovery region; and a second path extending from the first path to the target compound recovery region and then back to the first path, wherein the desublimation means is movable along said paths.

Description

[0001] APPARATUS FOR DESUBLIMING A TARGET COMPOUND FROM A GAS MIXTURE

[0002] The present invention concerns an apparatus for desubliming a target compound from a gas mixture, as well as a method for doing the same.

[0003] Separation of a target compound such as carbon dioxide from gas mixtures can use absorption, dissolution in solvents, permeation through membranes or physical separation by condensation or desublimation. Factors that determine the most appropriate method include the concentration of the target compound in the gas mixture, the temperature and pressure of the gas mixture, and the presence of other chemical species and contaminating particulates or droplets.

[0004] Industrial processes for the treatment of natural gas often include the separation of carbon dioxide from the raw gas. Similarly, the production of hydrogen requires the separation of carbon dioxide from a process gas mixture. These separations add complexity and increase the energy consumption of these processes.

[0005] With the growing concerns regarding pollution and global warming, the removal of carbon dioxide (a known greenhouse gas) from waste gases is becoming more important. However, including an additional step to remove carbon dioxide in any industrial process can unacceptably increase the cost, complexity and energy consumption of the overall process.

[0006] Established processes for the removal of carbon dioxide from gas streams include absorption in aqueous solutions of amines or inorganic salts such as potassium carbonate or dissolution of the gas into various organic solvents. The absorption processes require tall columns to achieve desired capture effectiveness of over 80%, have high energy consumption for the stripper column to recover the carbon dioxide and may be degraded by trace contaminants in the exhaust stream such as nitrogen or sulphur oxides. The dissolution processes generally operate at elevated pressure, necessitating the compression of the gas stream incurring significant energy costs.

[0007] Permeation through membranes can also be used to separate carbon dioxide from natural gas. However the process requires large pressure differences to drive the separation and currently available membranes have limited selectivity so that multiple stages are generally necessary to achieve adequate separation and purity of the separated carbon dioxide. This means that the process has a high energy cost for compression for most applications. At pressures below its triple point at about 5.2 bara, when carbon dioxide is cooled, it desublimes as frost rather than condensing as a liquid. Various cryogenic processes that exploit this behaviour have been suggested to separate carbon dioxide from a flue gas. However, these processes often require increased complexity and higher energy consumption.

[0008] The CRYOPUR process treats biogas containing a high level of carbon dioxide by cooling the gas in a low temperature heat exchanger. The carbon dioxide is deposited as a frost on the heat exchanger surface and is allowed to build up to a certain level. The biogas is then diverted through a second similar heat exchanger to continue deposition while the first heat exchanger is isolated from the gas stream and warmed to recover the carbon dioxide as a liquid. The cycle of freezing and thawing of alternating heat exchangers is used to maintain continuous separation of carbon dioxide from the biogas. The energy consumption of refrigeration for the process is increased significantly by the thermal resistance of carbon dioxide frost layer on the heat exchanger surface while the cost of the plant is increased by the need for at least two low temperature heat exchangers which must be constructed to withstand repeated thermal cycling at low temperatures.

[0009] WO 2018 / 033737 discloses a process for capture of a target compound such as carbon dioxide from a gas mixture. This process describes an invention in which a circulating bed of material which is first cooled to a low temperature contacts the gas mixture containing a target compound such as carbon dioxide so that the target compound is deposited as a frost on the surface of the bed. The bed then passes out of the contact region with the gas stream to be warmed to the point where the target compound sublimes or melts off the surface and is recovered. The circulating bed returns to be cooled and contact the gas mixture again. However, this process incurs significant energy costs to warm and cool the inert bed material between the conditions for desublimation and recovery of the target compound.

[0010] This process also uses the compression and expansion of the gas mixture as a means of providing the conditions for the phase changes of the target compound that are necessary to achieve the desired separation. This adds complexity and limits the flexibility of the process to handle a range of gas mixture compositions. An enhanced process that separates a target compound from a gas mixture with reduced complexity and energy consumption, as well as having a greater flexibility to handle the range of gas mixture compositions, is therefore required.

[0011] According to a first aspect of the present invention, there is provided an apparatus for desubliming a target compound from a first gas mixture containing the target compound, comprising: a desublimation means comprising a surface onto which desublimation of the target compound can occur; a cooling means that cools the desublimation means; an inlet through which the first gas mixture can enter the apparatus such that it comes into contact with the surface of the desublimation means in a contact region; a target compound recovery region; an outlet through which the target compound can leave the target compound recovery region; a first path for the desublimation means that extends from the cooling means to the contact region before returning to the cooling means without passing through the target compound recovery region; and a second path extending from the first path to the target compound recovery region and then back to the first path, wherein the desublimation means is movable along said paths.

[0012] The path of the desublimation means is a continuous path moving through the cooling means, the contact region, and / or the target compound recovery region. By "continuous" it is meant that the desublimation means can circulate around the path, either along the first or second paths outlined above, in a continuous manner, without leaving the apparatus.

[0013] The second path may extend from the first path and rejoin the first path at any point along the first path.

[0014] The second path may extend from the first path after or within the contact region, with reference to the direction of movement of the desublimation means. The second path may rejoin the first path before or within the cooling means with reference to the direction of movement of the desublimation means. Thus, the second path may extend through the cooling means, the contact region and the target compound recovery region. Alternatively, the second path may may rejoin the first path before or within the contact region with reference to the movement of the desublimation means. Thus, the second path may extend through the contact region and the target compound recovery region, without extending through the cooling means.

[0015] The second path may extend from the first path after or within the cooling means. If the second path rejoins the first path before the cooling means, then the second path will extend through the cooling means and the target compound recovery region, without extending through the contact region.

[0016] The second path may not extend through the cooling means and the contact region.

[0017] The first gas mixture comprises one or more other components in addition to the target compound. The apparatus of the present invention provides a continuous means of separating a target compound from a first gas mixture comprising one or more other components. Continuous operation is more cost effective than the use of alternating desublimation and recovery of the target compound, reducing the equipment scope and the energy necessary to cycle the temperatures of the process units compared with arrangements of the prior art.

[0018] The apparatus of the present invention exploits the intensity of heat and mass transfer in closely packed elements of the desublimation means so that it is much smaller than that required in the prior art where columns up to 20 m high are necessary to achieve the desired separation. This enables the apparatus to be simpler, cheaper and much more compact than the prior art.

[0019] The division of the desublimation means between the first path returning to the cooling means from the contact region and the second path circulating through the target compound recovery region enables process performance to be improved. Firstly, the reduction in the mass of the desublimation means being warmed to recover the target compound and then re-cooled reduces the duty of the refrigeration process and exergy losses in heat transfer with the desublimation means.

[0020] This also means that a lower volume of desublimation means is needed to pass through the target compound recovery region to recover the same amount of target compound compared to arrangements in the prior art. This improves the efficiency of separating the regions using sealing means such as pressure locks or valves. Secondly, the process means that at least a portion of the desublimation means entering the cooling means already has some target compound thereon. The heat transfer in the cooling means is surprisingly enhanced by the presence of the frosted target compound on the surface of the desublimation means. In the prior art, the heat transfer between the desublimation means and the cooling means is dominated by the thermal conductivity of the first gas mixture in the interstices of the desublimation means. In the present invention, the presence of the solid target compound enables heat transfer by local sublimation from warmer surfaces within the desublimation means and subsequent desublimation of the target compound onto colder surfaces of the cooling means. This effect improves heat transfer compared with the prior art by a factor of between 2 and 50, depending on local conditions, thereby enhancing performance and reducing the size and cost of the apparatus.

[0021] The desublimation means may comprise a granular material, such as a plurality of balls, optionally wherein the balls are ceramic or metallic. This provides a larger surface area onto which the target compound can desublime. This also allows the desublimation means from the first path to mix efficiently with the desublimation means from the second path, so that large patches of different temperatures are not created at the point at which the paths meet.

[0022] At least the first path may extend from the contact region at a point at which the target compound has desublimed onto the desublimation means. Both the first and second paths may be positioned such that they comprise desublimation means onto which the target compound has desublimed in the most recent circulation through the contact region. The desublimation means in the first and second paths may be at different temperatures or at the same temperature.

[0023] The first and second paths may split after the contact region, such that there is only one outlet for the desublimation means from the contact region that subsequently splits into the first and second paths. Alternatively, the contact region may have two or more outlets for the desublimation means, with the first and second paths extending from different outlets. The outlet for the first path may be in the half of the contact region closest to the inlet through which the first gas mixture enters the contact region, preferably adjacent to said inlet. The outlet for the second path may be at any point within the contact region.

[0024] The desublimation means may move continuously at a single speed or, preferably, the speed may vary. The overall flow rate of the desublimation means and / or the flow rate of the desublimation means within certain regions can be adjusted and can therefore be determined by desired local conditions. The desublimation means may move in stages, such that portions of the desublimation means have dwell times within the certain regions where the desublimation means is moving more slowly or not moving. For example, the desublimation means may move more slowly through the contact region. This may increase mass transfer. Additionally or alternatively, the desublimation means may move in stages such that portions of the desublimation means move more rapidly through certain regions. For example, the desublimation means may move more rapidly through the cooling means. This may enhance heat transfer.

[0025] The overall flow rate of the desublimation means and / or the flow rate of the desublimation means within certain regions can be adjusted to match demands on the process. For example, the flow rate of the desublimation means may be reduced when the concentration of the target compound in the first gas mixture is low and / or the flow rate of the first gas mixture is low. Alternatively, the flow rate of the desublimation means may be increased when the flow rate of the first gas mixture is high.

[0026] As a surface onto which the target compound desublimation may occur circulates around this continuous path, it may move relative to other parts of the desublimation means or may be held in a certain position relative to other parts of the desublimation means as the entire means circulates.

[0027] In contrast to many arrangements of the prior art, the apparatus of the present invention does not have to be run at atmospheric pressure or high pressures and instead can make use of modestly elevated pressures, typically in the range of 2 to 10 bara throughout. Any pressure may be used, depending on the performance required and provided that the partial pressure of the target compound in the first gas stream is less than its triple point pressure. This further reduces the cost, complexity and size of the apparatus. Additionally, this avoids the risk involved in running the apparatus at higher pressures.

[0028] The inlet may be arranged to provide a flow of gas around the path of the desublimation means in the opposite direction to the movement of the desublimation means. The inlet may be located between the cooling means and the target compound recovery region on the path of the desublimation means, but before the first and second paths diverge. The inlet may be within the contact region. The desublimation means may not be fluidised, so that the temperature gradients produced by the flow of the gas in the opposite direction to the movement of the desublimation means are not lost. The contact region may comprise a temperature gradient, which may be created by the flow of the gas from the inlet in the opposite direction to the movement of the desublimation means. The properties of the temperature gradient, such as its position and the temperatures involved, may be varied to control the process performance.

[0029] The desublimation means can be divided between the first and the second path by a splitter. The division between the first and second paths may be random, such that portions of the desublimation means are randomly selected for movement along one of the two alternative paths.

[0030] Alternatively, the apparatus may further comprise a means for selecting a portion of the desublimation means for movement along the first path and a portion of the desublimation means for movement along the second path. The selection of each portion may be based on the amount of desublimed target compound on the desublimation means on each portion. For example, in the case of a granular bed material, the granules with more desublimed target compound may be selected for the second path while the granules with less desublimed target compound may be selected for the first path.

[0031] The portions of the desublimation means may be selected by weight, as the heavier the portion, the more desublimed target compound is present. This can be done by any suitable method known in the art, for example by using an inertial separator such as a cyclone. Additionally or alternatively, the portions may be selected based on the size of the particles of the desublimation means, as the larger the particles, the more desublimed target compound is present. This can be done by any suitable method known in the art, for example by using a sieve or screen. Density can also be used to select the portions of the desublimation means, depending on the relative density of the desublimation means and the desublimed compound.

[0032] The first path may be designed to carry more desublimation means than the second path. The first path may carry a first portion of the desublimation means and the second path may carry a second portion of the desublimation means. The first portion may comprise 75% or more of the total desublimation means, preferably between 75% and 95%. The second portion may comprise 25% or less of the total desublimation means, preferably between 25% and 5%.

[0033] The means for selecting the first and second portions of the desublimation means may be automatic. Thus, the quantity of the total desublimation means in the first or second portion may be adjusted automatically for more efficient sublimation of the target compound on the desublimation means. This simplifies control of the process during start-up and operation, particularly under varying demand.

[0034] Additionally or alternatively, the means for selecting the first and second portions of the desublimation means may select the portion of the desublimation means with the top 25%, or between 5 and 25% of desublimed target compound for movement along the second path.

[0035] The apparatus may further comprise an outlet through which a second gas mixture can leave the path of the desublimation means at a location such that at least part of the second gas mixture is removed from the path before it reaches the cooling means, with reference to the flow of gas around the path. The outlet may extend from the contact region. The outlet may extend from the path of the desublimation means, between the contact region and the cooling means. The second gas mixture may be recovered using porous or perforated injectors.

[0036] The part of the path of the desublimation means from which the outlet extends may have different conditions of temperature and / or pressure compared to the target compound recovery region. Said part may be isolated from the target compound recovery region.

[0037] The target compound recovery region may be isolated from the rest of the apparatus, in that it may have different conditions of temperature and / or pressure. The target compound recovery region may be physically isolated by a sealing means such as a valve or the like. Each of the regions may be physically isolated by a sealing means. This means that different regions on the path can have different pressures and allows the pressure of each region to be controlled, thereby allowing the conditions in each region to be tailored to the desired physical state of one or more of the components of the first gas mixture. The sealing means additionally may act to isolate regions of the apparatus to minimise leakage into the second gas mixture of the target compound that has been separated.

[0038] Some or all of the regions may comprise the same or similar pressures. For example, the target compound recovery region may have the same pressure as one or more other regions of the apparatus. The target compound recovery region may not be isolated in any way from one or more other regions. As used herein, the second gas mixture is the first gas mixture once some or all of the target compound has been removed using the invention disclosed herein. The second gas mixture may comprise one or more components, but comprises a lower amount of the target compound than the first gas mixture. The second gas mixture may be the gas that is released from the apparatus. The second gas mixture may be a waste product or may be a desired cleaned product.

[0039] The apparatus may comprise a secondary cooling means. The secondary cooling means may cool the desublimation means at a different point to the cooling means. The secondary cooling means may cool the desublimation means in the contact region.

[0040] The secondary cooling means may be on the first and / or the second path. The secondary cooling means may be on the second path, and may be located after the target compound recovery region with reference to the movement of desublimation means, optionally before the cooling means. This cools the desublimation means in either or both paths, preferably to around the temperature of the desublimation means as it leaves the contact region. This can ensure that the desublimation means in both the first and the second paths are around the same temperature.

[0041] The second gas mixture may be used to lower the temperature of the desublimation means. This can be done in the cooling means, or in a secondary cooling means in the apparatus. The first gas mixture may be compressed. If the first gas mixture is compressed, the second gas mixture may be expanded or its pressure reduced before it is used to lower the temperature of the desublimation means. The second gas mixture may be expanded through a turbine to recover the energy.

[0042] The cooling means and / or the secondary cooling means preferably comprises an indirect heat exchanger, particularly when the target compound is carbon dioxide. Additionally or alternatively, the secondary cooling means may comprise directly contacting the desublimation means with a cooled gas. The cooled gas may be the second gas mixture. The cooling means and / or the secondary cooling means may lower the temperature of the desublimation means using a separate refrigeration cycle, which does not make use of the second gas mixture. Instead, the separate refrigeration cycle may use an isolated refrigerant stream for heating and cooling using indirect heat exchange.

[0043] The isolated refrigerant stream may interact directly with the cooling means, or there may be an intermediate fluid exchanging heat between the isolated refrigerant stream and the cooling means. The refrigerant stream may be a refrigerant gas stream and may comprise nitrogen, dry air, a noble gas such as argon or a hydrocarbon gas or a mixture of such gases. Alternatively, the refrigerant gas stream may comprise a volatile liquid such as a hydrocarbon, fluorinated hydrocarbon or noble gas and / or its vapour. The refrigerant gas of the refrigerant gas stream may be compressed, optionally condensed, and expanded in a separate system to provide cooling and / or heating, as necessary. Thus, different portions of the refrigeration cycle may be at different pressures. Heat exchange may occur between different portions of the refrigerant gas stream. The refrigerant gas stream may cool the desublimation means in the cooling means and / or the secondary cooling means, and / or may reject heat in the target compound recovery region to raise the temperature of the desublimation means.

[0044] The refrigerant stream from the cooling means may cool the refrigerant stream in another portion of the separate refrigeration cycle, such as the stream that has raised the temperature in the target compound recovery region to aid recovery of the target compound. Between the two, the refrigerant may have been expanded to cool the refrigerant stream and optionally to provide energy, for example to drive a generator or a compressor. Another heat exchange may occur in the process, in which the refrigerant stream is cooled before it enters the heat exchanger with the target compound recovery region by another portion of the refrigerant stream.

[0045] The separate refrigeration cycle may comprise an aftercooler, which rejects waste heat to the ambient, thereby further cooling the refrigerant stream. This may be located after a compressor in the separate refrigeration cycle. Alternatively, the separate refrigeration cycle may reject heat to a secondary refrigeration cycle which in turn directly or indirectly rejects heat to the ambient.

[0046] The refrigerant gas of the refrigerant gas stream may be delivered at a pressure above atmospheric pressure to a first compressor. From the first compressor, the high pressure warm refrigerant gas may enter an aftercooler, to reject waste heat to the ambient, followed by a first recuperator, where the high pressure gas is cooled by an intermediate pressure portion of the refrigerant gas. The cooled high pressure refrigerant gas may then flow from the first recuperator through a heat exchanger, which rejects heat to the target compound recovery region to recover the target compound, for example to a melting or desubliming process to recover the target compound from the frosted surface of the desublimation means. The cooled high pressure refrigerant gas may then flow into a second recuperator where it is cooled by low pressure refrigerant gas leaving a cooling means heat exchanger. The high pressure refrigerant gas is then expanded in a turbine, which can extract energy from the gas stream to drive a generator or loading compressor, thereby cooling the gas stream. The cold low pressure gas stream then indirectly cools the desublimation means in the cooling means. The gas gives up further heat in the second recuperator to the incoming higher pressure gas stream before being compressed in a second compressor. The intermediate pressure gas flows through the first recuperator to cool the high pressure gas, before flowing into the first stage compressor to complete its circulation.

[0047] Alternative refrigeration cycles may use a refrigerant mixture of hydrocarbon gases. This mixture may include at least one of methane, ethane, or ethylene. The mixture may further comprise constituents such as fluorinated hydrocarbon gases or inert gases such as nitrogen.

[0048] The refrigerant mixture can be condensed and flashed during circulation. For example, from the first compressor, the high pressure warm refrigerant gas may enter an aftercooler where it is cooled to around ambient conditions. The gas may then flow to a first recuperator where it is cooled further by the returning refrigerant gas. The cooled refrigerant gas may then flow from the first recuperator through a heat exchanger where heat is rejected to recover the target compound, for example by melting or subliming the target compound off the desublimation means, and wherein the refrigerant gas is partially condensed. The resulting refrigerant gas and liquid may then flow into a second recuperator wherein it is further cooled by low pressure refrigerant gas leaving a cooling means heat exchanger and wherein it is further condensed. The refrigerant gas and liquid is then flashed down to a lower pressure through a Joule-Thompson valve resulting in a colder mixture of the refrigerant gas and liquid.

[0049] The cold mixture of refrigerant gas and liquid may then indirectly cool the desublimation means in the cooling means, evaporating the liquid refrigerant and warming the refrigerant gas. The gas may then give up further heat in the second recuperator to the incoming higher pressure refrigerant gas and liquid before, optionally, being compressed in a second compressor. The intermediate pressure refrigerant gas then flows through the first recuperator to cool the incoming high pressure gas, before flowing into the first stage compressor to complete its circulation.

[0050] The separation of the refrigeration cycle means that the heat exchangers and machinery are not exposed to dirty and potentially corrosive components in the second gas mixture and allows the conditions in the refrigeration cycle to be more closely controlled, thereby giving higher efficiency (i.e. a lower energy consumption). The segregation allows a wider range of process operating conditions as the refrigeration cycle gas flows are not limited to the flow of the second gas mixture. This means that a wider range of heating and cooling duties, for example for different carbon dioxide contents of the first gas mixture, can be managed successfully. Segregation of the refrigeration process also allows different refrigeration cycles to be employed, for example using hydrocarbon refrigerants in a vapour compression cycle, thereby enabling higher performance cycles to be used. Segregation of the refrigeration cycle also allows different conditions to be adopted for the refrigeration process itself. For example, higher pressures would allow smaller and hence cheaper heat exchangers to be used, vapour compression cycles would minimise the capacity and duty of compressors. Segregation will also allow refrigeration components to be drawn from standard product ranges rather than being specially designed for each application. Thus, segregation of the refrigeration cycle enables both energy consumption and equipment sizes and costs to be reduced.

[0051] The secondary cooling means may utilise the second gas mixture, while the cooling means utilises an isolated refrigerant stream.

[0052] The second gas mixture may be used to cool the first gas mixture before it enters the contact region. The second gas mixture may be passed through an expander before it is released from the apparatus and / or used to cool the first gas mixture.

[0053] A third path may provide additional cooled desublimation means after the recovery region or secondary cooling means. The third path may provide the additional cooled desublimation means to a point on the path of the desublimation means after the desublimation means has circulated through the recovery region and / or secondary cooling means. Additionally or alternatively, the secondary cooling means may provide the additional cooled desublimation means.

[0054] This helps to stage the flow of the desublimation means through the contact region and therefore to maintain the flow of the desublimation means through the contact region at or below a desired level. Additionally, this may tailor the temperature profile of the desublimation means as it passes along the first and / or second paths to minimise energy consumption.

[0055] Preferably, the conditions of the target compound recovery region can be such that the target compound leaves the target compound recovery region as a liquid or a gas. Thus, mechanical removal may not be required. The target compound recover region may be isolated from the other regions in the apparatus. This can be achieved in use by the partial pressure gradient of the gases involved, but it may also be achieved using physical sealing means, such as valves and other sealing devices. The efficiency of these physical isolation means is improved due to the lower volume of desublimation means passing through the target compound recovery region.

[0056] The target compound may be recovered in the target compound recovery region using porous or perforated injectors. The porous injectors may comprise porous metal plates which may comprise sintered metal particles or sintered metal mesh. The perforated injectors may comprise metal plates which may comprise fine holes or slots.

[0057] During target recovery, the target compound may pass upwards through the interstices of the desublimation means to be extracted via the injector bodies. The dimensions of the slots or porosity of the metal may be selected to prevent the desublimation means leaking into the injector bodies. The injectors may be of the same design as those used to introduce the first gas mixture to and remove the second gas mixture from the contact region.

[0058] Additionally or alternatively, the target compound may be removed from the desublimation means mechanically. The target compound may then be transferred to a storage region.

[0059] The temperature of the target compound recovery region may be increased relative to the contact region. This temperature increase may result from indirect heating. There may be no direct heating means that increases the temperature of the target compound recovery region. Indirect heating can be achieved by cooling the initial inflow of the first gas mixture and / or another stream of gas at a higher temperature. The temperature increase may be achieved using the separate refrigeration cycle.

[0060] The first gas mixture may be dried before it is fed into the apparatus. The first gas mixture may be dried by any suitable means. For example, the first gas mixture may be dried using desiccant liquids, such as tri-ethylene glycol and propylene glycol, or by using solid desiccants, for example in alternating beds or as desiccant wheels.

[0061] Alternatively, the apparatus may further comprise a water removal region, in which water is removed from the first gas mixture. This means that the first gas mixture does not need to be dried before it is fed into the apparatus, thereby reducing the cost and complexity of the process. The water may be removed as water vapour. The first gas mixture may pass through this region before it enters the contact region. This region may comprise a drying means, which may comprise a surface onto which the water condenses or freezes, or a desiccant. The region may therefore involve conditions of temperature and pressure under which water is a liquid or a gas. This can improve the recovery of water as the temperatures and flows may be different to those involved in the recovery of the target compound. The path of the desublimation means may extend through this region. The water may subsequently be recovered in a water recovery region, which may be downstream of the water removal region relative to the movement of the desublimation means or the drying means. The water may be recovered as a liquid or a gas.

[0062] The target compound is a sublimable compound. The target compound may be carbon dioxide.

[0063] The first gas mixture may be cooled before it enters the contact region. Thus, the apparatus may comprise a cooling means for cooling the first gas mixture before it enters the contact region. This may be done using the second gas mixture. For example, by using the second gas mixture within a heat wheel. Additionally or alternatively, the first gas mixture may be cooled using a separate refrigeration circuit. The first gas mixture may also be compressed before it enters the contact region, such that it is at a pressure of above atmospheric pressure. The first gas mixture may be compressed and cooled at the same time. Alternatively, the means for cooling the first gas mixture may be before or after the compressor. Preferably, the means for cooling the first gas mixture is after the compressor, such that it cools the compressed gas. The means for cooling the first gas mixture may comprise two separate components such that the first gas mixture is cooled in two separate stages before it enters the apparatus.

[0064] The temperature of the first gas mixture may be cooled to close to, but greater than, the desublimation temperature of the target compound. This means that the desublimation means extracted throughout the contact region will contain at least some of the target compound. This also means that less cooling is required by the desublimation means in order to desublime the target compound, thereby reducing the energy consumption required to remove the target compound from the first gas mixture.

[0065] The heating of the target compound recovery region such that the target compound is recovered as a liquid or a gas may also be used to cool another part of the apparatus or system, such as the first gas mixture or refrigerant gas. According to a second aspect of the present invention, there is provided a method of removing a target compound from a first gas mixture containing the target compound, including the steps of: providing a desublimation means in a continuous path that passes through a cooling means, a contact region in which it comes into contact with the first gas mixture and / or a target compound recovery region; cooling a portion of the desublimation means within the cooling means to a temperature below the desublimation temperature of the target compound; moving the cooled desublimation means to a contact region; providing the first gas mixture to the contact region such that it contacts a surface of the cooled portion of the desublimation means, thereby causing at least a portion of the target compound in the first gas mixture to desublime onto the surface of the desublimation means; moving a first portion of the desublimation means along a first path such that a surface of the desublimation means comprising the desublimed target compound returns to the cooling means without passing through the target compound recovery region; moving a second portion of the desublimation means along a second path such that a surface of the desublimation means comprising the desublimed target compound circulates into the target compound recovery region from the first path; removing the target compound from the surface of the desublimation means in the target compound recovery region; returning the second portion of the desublimation means to the first path; and re-cooling the portion of the desublimation means to a temperature below the desublimation temperature of the target compound.

[0066] The method may be carried out using the apparatus described above. As such, the features outlined above in relation to the apparatus may apply equally to the method.

[0067] The target compound may be carbon dioxide.

[0068] The inlet provides a flow of the first gas mixture into the continuous path of the desublimation means. The movement of the desublimation means may be in the opposite direction to the flow of the gas around the path of the desublimation means, as discussed above in relation to the apparatus. The cooled portion of the desublimation means may be in the contact region for an amount of time sufficient for the target compound to desublime on the desublimation means. The method may comprise varying the speed at which the desublimation means moves in order to vary the amount of time that the cooled portion of the desublimation means is present in the contact region.

[0069] The desublimation means may move in stages, such that portions of the desublimation means have dwell times within certain regions where the desublimation means is not moving or is moving more slowly or such that portions of the desublimation means move more rapidly within certain regions. Advantageously, this may optimise heat and mass exchange within the certain regions. Alternatively, the desublimation means may move continuously at a single speed.

[0070] The method may include the step of randomly selecting the first and second portions of the desublimation means. This can be done by a splitter, or by any other conventional means for splitting the portions.

[0071] Alternatively, the method may include the step of selecting the first and second portions of the desublimation means. This selection may be based on the amount of desublimed target compound on each portion. The portions of the desublimation means may be selected by weight, density or by size of particles of the desublimation means.

[0072] The method may further comprise the step of removing a second gas mixture. At least part of the second gas mixture may be removed from the path before it reaches the cooling means. At least part of the second gas mixture may be removed from within the contact region, or from between the contact region and the cooling means with reference to the flow of the gas. The second gas mixture may be removed at a point in the contact region at which the target compound has desublimed onto the desublimation means.

[0073] The method may comprise a further step of cooling the desublimation means using a secondary cooling means. The secondary cooling means may cool the desublimation means at a different point to the cooling means, on the first and / or the second path. The secondary cooling means may cool the desublimation means after the target compound recovery region and optionally, before the cooling means on the second path of the desublimation means. If the first gas mixture is compressed, the second gas mixture may be expanded or its pressure may be reduced once it has been removed. The second gas mixture may be used to cool the desublimation means once it has been removed, either in the cooling means or in the secondary cooling means.

[0074] Preferably, the desublimation means may be cooled using an indirect heat exchanger, particularly if the target compound is carbon dioxide due to its volatility. The desublimation means may be cooled with direct contact with a cooled gas in a secondary cooling means. The cooling means and / or the secondary cooling means may lower the temperature of the desublimation means using a separate refrigeration cycle, which does not make use of the second gas mixture. Instead, the separate refrigeration cycle may use an isolated refrigerant stream for heating and cooling using indirect heat exchange. The separate refrigeration cycle may be as outlined above in relation to the apparatus.

[0075] The method may further comprise the step of compressing and / or cooling the first gas mixture before it is provided to the desublimation means. This may be done using a gas stream comprising the second gas mixture. Said gas stream may include an additional cold gas stream. Additionally or alternatively, the first gas mixture may be cooled by an external refrigeration circuit.

[0076] The first gas mixture may be provided to the desublimation means, the desublimation means cooled, the target compound removed and optionally also the second gas mixture removed continuously. In other words, the method runs all steps simultaneously.

[0077] The pressure and temperature of the target compound recovery region may be maintained at levels at which the target compound is a solid, gas or liquid, preferably a liquid. The second gas mixture may be removed at a point at which the target compound is a solid.

[0078] The temperatures and pressures involved may be selected to create the desired residual level of the target compound in the second gas mixture. The temperature of the desublimation means at the point at which the second gas mixture is removed from the path of the desublimation means may determine the amount of the target compound present in the second gas mixture. The temperature may therefore be selected accordingly.

[0079] The desublimation means may comprise a granular material, such as a plurality of balls, which are optionally ceramic or metallic. The method may further comprise the step of removing water from the first gas mixture. This may be done before the first gas mixture is fed into the apparatus. This may be done by any suitable means. For example, the first gas mixture may be dried using desiccant liquids, including tri-ethylene glycol and propylene glycol, or by using solid desiccants, for example in alternating beds or as desiccant wheels.

[0080] Additionally or alternatively, this may be done in a water removal region, through which the first gas mixture may pass before it reaches the contact region. Water may be recovered from the apparatus at the water removal region or at a water recovery region which, if present, is downstream of the water removal region with reference to the movement of the desublimation means. The conditions of the water recovery region can be varied to change the state in which water is recovered.

[0081] According to a third aspect of the present invention, there is provided a use of the apparatus discussed above to remove a target compound from a first gas mixture via desublimation.

[0082] The first gas mixture may be an exhaust gas from an industrial process, a vehicle or boat , a process gas stream (including a raw natural gas stream or a chemical reaction or combustion process), a natural gas, or biogas. The apparatus of the present invention can be used in post combustion capture, oxy-fuel combustion and pre-combustion decarbonisation when the target compound is carbon dioxide.

[0083] According to a fourth aspect of the present invention, there is provided a vehicle, which may be a train or rail locomotive, a boat, which may be a barge or a ship, or static marine structure such as a floating oil and gas facility, a power barge or a floating carbon capture facility, or an industrial process comprising the apparatus discussed above.

[0084] The apparatus of the present invention can also be easily retrofitted into existing systems to remove a target compound such as carbon dioxide from, for example, flue gases. This is partly due to the simplicity of the apparatus and it avoids the complex interfaces and hazardous chemicals typical of most carbon capture processes. Particularly, the apparatus only requires a gas feed, such as a flue gas feed, electric power and a cooling medium such as a cooling tower or water in order to function.

[0085] Any feature discussed above in relation to one aspect of the invention applies equally to the other aspects discussed herein. The invention will now be more particularly described with reference to the following examples and figures, in which;

[0086] Figure 1 illustrates a diagram outlining the method and apparatus according to an embodiment of the invention;

[0087] Figure 2 illustrates the improvement in thermal conductivity with a frosted desublimation means compared to a dry desublimation means at atmospheric pressure;

[0088] Figure 3 illustrates an isolated refrigeration cycle for use in the method and apparatus according to an embodiment of the invention;

[0089] Figure 4 illustrates an alternative refrigeration cycle for use in the method and apparatus according to an embodiment of the invention; and

[0090] Figure 5 illustrates the implementation of injectors to introduce the gases to the bed and to subsequently to recover them.

[0091] Figure 1 illustrates the method and apparatus according to an embodiment of the invention. Recirculated paths 1, 2 of desublimation means, in the form of a granular bed material, flow into cooling means 3 where the desublimation means are mixed. The desublimation means is cooled to the temperature desired.

[0092] The cooled desublimation means then moves along a first path into the contact region 4, where the first gas mixture enters via inlet 5 and flows in the opposite direction to the movement of the desublimation means. The first gas mixture is cooled by the desublimation means and the target compound within the first gas mixture desublimes as a frost on the surface of the particles of desublimation means. The second gas mixture 6 is extracted from the contact region 4 via porous or perforated injectors 7.

[0093] The desublimation means laden with the frost of the target compound moves along path 8 from the cooling means into stream splitter 9. The stream splitter 9 divides the desublimation means into two paths. The first path 1 returns directly back to the cooling means 3 from the stream splitter 9, without passing through the target compound recovery region 12. The second path 10 passes through a sealing device 11 to the target compound recovery region 12.

[0094] The division of the desublimation means between the two paths can either be done randomly, or according to the size or weight of the coated particles. If division is by weight or size, the heavier or larger particles are preferentially passed into the second path 10. The division in this embodiment is not equal and more of the desublimation means moves along the first path 1 than the second path 2.

[0095] The desublimation means is warmed in the target compound recovery region 12 to the sublimation or melting temperature of the target compound at the local pressure. The gaseous or liquid target compound is extracted from the bed via porous or perforated injectors 13.

[0096] The unloaded desublimation means then leaves the target compound recovery region along path 14 and passes through a second sealing device 15 into a cooler 16 which cools the desublimation means, preferably to the temperature at the outlet of the contact region 4. The cold unloaded stream of desublimation means then flows back along second path 2 to the cooling region 3 to be mixed with the desublimation means from the first path 1 and cooled to capture further target compound.

[0097] Representative conditions predicted for the cycle with capture of around 90% of the carbon dioxide in a gas stream with a concentration of 6.8% carbon dioxide are as shown in Table 1.

[0098] Table 1: This arrangement means that the cooling means contains a portion of the desublimation means that has the target compound frosted on a surface thereof. As illustrated in Figure 1, the thermal conductivity of the desublimation means increases with increasing temperature at atmospheric pressure when the target compound is frosted on a surface thereof. Without wishing to be bound by theory, this is thought to be because the slope of the vapour pressure curve for a target compound increases as the temperature approaches the target compound's triple point. Hence, at lower temperatures the effect of a given temperature rise on vapour pressure and thus on the driving force for diffusion is smaller than at higher temperatures. This results in a lower rate of flow of the target compound from the warmer to the cooler regions of the frosted surface, reducing the heat transfer effect on thermal conductivity.

[0099] Additionally, at all temperatures measured, the conductivity is higher with the frosted target compound than with the dry sublimation means. Without wishing to be bound by theory, this is thought to be because the presence of the solid target compound enables heat transfer by local sublimation from warmer surfaces within the desublimation means and subsequent desublimation of the target compound onto colder surfaces of the cooling means. In contrast, heat transfer between the dry desublimation means and the cooling means is dominated by the thermal conductivity of the first gas mixture in the interstices of the desublimation means.

[0100] Table 2 discloses the comparison of the arrangement shown in Figure 1 with that outlined in WO2018 / 033737 when running under similar conditions.

[0101] Table 2:

[0102] As shown in Table 2, the present arrangement reduces the heat exchange surface area required by 42% and eliminates the separate liquefication compressors and refrigeration process, thereby reducing plant capital cost by at least 30%. The energy consumption is also reduced by 34%. The overall result is likely to be a reduction in the life cost of carbon capture of around one third. The prior art was assessed by independent consulting engineers (Fichtner) to offer a saving of 50% on the life cost of capture of state of the art conventional technology using the amine process. Hence, an overall reduction in the life cost of carbon capture of 60-70% is likely to be achieved using the present arrangement.

[0103] Figure 3 illustrates an isolated refrigeration cycle, for use in the arrangement of the invention. The refrigerant gas stream 31 is delivered at a pressure above atmospheric pressure to the inlet of a compressor 32. From the compressor 32, the high pressure warm refrigerant gas flows through an aftercooler 33 to first reject waste heat to the ambient and then into a first recuperator 34. Here the high pressure gas is cooled by the intermediate pressure portion of the refrigerant gas flowing into the recuperator from the cycle. The cooled high pressure refrigerant gas flows from the first recuperator through heat exchanger 35, which rejects heat to the target compound recovery region to recover the target compound from the frosted desublimation means.

[0104] The cooled high pressure refrigerant gas then flows into a second recuperator 36 where it is cooled by low pressure refrigerant gas leaving the cooling means 40. The high pressure refrigerant gas is then expanded in a turbine 38. The turbine 38 extracts energy from the gas stream to drive generator or loading compressor 39, thereby cooling the gas stream. The cold low pressure gas stream then indirectly cools the desublimation means in the cooling means 40. The gas then cools the incoming high pressure refrigerant gas in the second recuperator 36 before being compressed in a second compressor 41. The intermediate pressure gas flows through the first recuperator 34 to cool the high pressure gas before flowing into the first stage compressor 32 to complete its circulation.

[0105] Representative conditions predicted for the cycle with capture of around 90% of the carbon dioxide in a gas stream with a concentration of 6.8% carbon dioxide are as shown in Table 3. These conditions are for producing liquid carbon dioxide.

[0106] Table 3:

[0107] Figure 4 illustrates an alternative refrigeration cycle, for use in the arrangement of the invention. The refrigerant gas stream 51 is delivered to the inlet of a compressor 52. From the compressor 52, the high pressure warm refrigerant gas flows through aftercooler 53 where it is cooled to around ambient conditions. The gas then flows to a first recuperator 54 where it is cooled further by the returning refrigerant gas. The cooled refrigerant gas then flows through a heat exchanger 55 where heat is rejected to recover the target compound and wherein the refrigerant gas is partially condensed. The resulting refrigerant gas and liquid may then flow into a second recuperator 56 wherein it is further cooled by low pressure refrigerant gas leaving a cooling means heat exchanger 59 and wherein it is further condensed. The refrigerant gas and liquid is then flashed down to a lower pressure through a Joule-Thompson valve 58 resulting in a colder mixture of refrigerant gas and liquid. The cold mixture of refrigerant gas and liquid then indirectly cools the desublimation means in the cooling means 59, evaporating the liquid refrigerant and warming the refrigerant gas. The gas gives up further heat in the second recuperator 56 to the incoming higher pressure refrigerant gas and liquid before, optionally, being compressed in a second compressor 60. The intermediate pressure refrigerant gas then flows through the first recuperator 54 to cool the incoming high pressure gas, before flowing into the first stage compressor 52 to complete its circulation. Figure 5 illustrates the implementation of injectors to introduce the gases to the bed in the contact region and to subsequently to recover them. Two alternative arrangements are shown on the diagram, one using porous metal plates 72 and the alternative using fine slots cut in metal plates 75.

[0108] The first gas mixture is introduced into the ends of the injectors 71 and flows through the porous plates 72 or the slots to be distributed into the downward moving desublimation means 73. The gases pass upwards through the interstices between the desublimation means, exchanging heat and mass according to the local conditions of the process. The second gas mixture leaves the contact region through the porous metal plate of the outlet injectors 74. The desublimation means enters and leaves the contact region by flowing between the inlet 71 and outlet 74 injectors.

Claims

CLAIMS1. An apparatus for desubliming a target compound from a first gas mixture containing the target compound, comprising: a desublimation means comprising a surface onto which desublimation of the target compound can occur; a cooling means that cools the desublimation means; an inlet through which the first gas mixture can enter the apparatus such that it comes into contact with the surface of the desublimation means in a contact region; a target compound recovery region; an outlet through which the target compound can leave the target compound recovery region; a first path for the desublimation means that extends from the cooling means to the contact region before returning to the cooling means without passing through the target compound recovery region; and a second path extending from the first path to the target compound recovery region and then back to the first path, wherein the desublimation means is movable along said paths.

2. The apparatus according to Claim 1 wherein the desublimation means comprises a granular material, such as a plurality of balls, optionally wherein the balls are ceramic or metallic.

3. The apparatus according to any one of Claims 1 or 2 wherein the inlet is arranged to provide a flow of gas around the path of the desublimation means in the opposite direction to the movement of the desublimation means.

4. The apparatus according to any one of Claims 1 to 3 wherein the desublimation means is randomly selected for movement along one of the two alternative paths.

5. The apparatus according to any one of Claims 1 to 3 further comprising a means for selecting a portion of the desublimation means for movement along the first path and a portion of the desublimation means for movement along the second path, optionally wherein the selection is based on the amount of desublimed target compound on each portion.

6. The apparatus according to Claim 5, wherein the portions of the desublimation means are selected by weight, density or by size of particles of the desublimation means.

7. The apparatus according to any one of Claims 1 to 6, wherein the first path is designed to carry more desublimation means than the second path.

8. The apparatus according to any one of Claims 1 to 7 further comprising an outlet through which a second gas mixture can leave the path of the desublimation means at a location such that at least part of the second gas mixture is removed from the path before it reaches the cooling means.

9. The apparatus according to any one of Claims 1 to 8, further comprising a secondary cooling means, optionally wherein said secondary cooling means is positioned on the second path and further optionally wherein the secondary cooling means is positioned after the target compound recovery region.

10. The apparatus according to any one of Claims 1 to 9 wherein the cooling means and / or the secondary cooling means operates using indirect heat exchange, optionally from an isolated refrigerant stream.

11. The apparatus according to Claim 10, wherein the isolated refrigerant stream cools the desublimation means and warms the target compound recovery region, as well as optionally also providing additional energy for use outside the apparatus.

12. The apparatus according to any one of Claims 1 to 11 wherein the conditions of the target compound recovery region can be such that the target compound leaves the target compound recovery region as a liquid or a gas.

13. The apparatus according to any one of Claims 1 to 12 comprising a water removal region in which water is removed from the first gas mixture.

14. The apparatus according to any one of Claims 1 to 13 wherein the target compound is carbon dioxide.

15. The apparatus according to any one of Claims 1 to 14 wherein the first gas mixture is cooled and / or compressed before it enters the contact region.

16. A method of removing a target compound from a first gas mixture containing the target compound, including the steps of: providing a desublimation means in a path that passes through a cooling means, a contact region in which it comes into contact with the first gas mixture and / or a target compound recovery region; cooling a portion of the desublimation means within the cooling means to a temperature below the desublimation temperature of the target compound; moving the cooled desublimation means to a contact region; providing the first gas mixture to the contact region such that it contacts a surface of the cooled portion of the desublimation means, thereby causing at least a portion of the target compound in the first gas mixture to desublime onto the surface of the desublimation means; moving a first portion of the desublimation means along a first path such that a surface of the desublimation means comprising the desublimed target compound returns to the cooling means without passing through the target compound recovery region; moving a second portion of the desublimation means along a second path such that a surface of the desublimation means comprising the desublimed target compound circulates into the target compound recovery region from the first path; removing the target compound from the surface of the desublimation means in the target compound recovery region; returning the second portion of the desublimation means to the first path; and re-cooling the portion of the desublimation means to a temperature below the desublimation temperature of the target compound.

17. The method according to Claim 16 including the step of randomly selecting the first and second portions of the desublimation means.

18. The method according to Claim 16 or 17 including the step of selecting the first and second portions of the desublimation means based on the amount of desublimed target compound on each portion, optionally wherein portions of the desublimation means are selected by weight, density or by size of particles of the desublimation means.

119. The method according to any one of Claims 16 to 18 further comprising the step of removing a second gas mixture at a location such that at least part of the second gas mixture is removed from the path of the desublimation means before it reaches the cooling means.

20. The method according to any one of Claims 16 to 19 wherein the first gas mixture is provided to the desublimation means, the desublimation means is cooled, the target compound is removed and optionally also the second gas mixture is removed continuously.

21. The method according to any one of Claims 16 to 20 wherein the pressure and temperature of the target compound recovery region are maintained at levels at which the target compound is liquid or a gas.

22. A use of the apparatus according to any one of Claims 1 to 15 to remove a target compound from a first gas mixture via desublimation.

23. The use according to Claim 22 wherein the first gas mixture is an exhaust gas or process gas stream from an industrial process; or a vehicle including a train or rail locomotive; or a boat including a barge or a ship, or static marine structure including an oil and gas facility, power barge or floating carbon capture facility .

24. A vehicle including a train or rail locomotive, a boat including a ship or a barge, or static marine structure including an oil and gas facility, power barge or floating carbon capture facility, or an industrial process comprising the apparatus according to any one of Claims 1 to 15.

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