A system and method for the drying of liquid or solid substances

Abstract

A system and method for the drying of liquid or solid substances. The system comprises a dryer for the removal of water from the to be dried substance by evaporation, a moisture collection system for absorbing the product moisture into a liquid absorbent and a liquid absorbent dryer for separating and discharging the absorbed product moisture from the liquid absorbent. The drying system is near adiabatic, because the temperature of the to be dried substance and removed water are the same before and after the drying process, as is the physical state of the removed water before and after the drying process. The system further avoids the discharge of moist air from the enclosure of the dryer.

Description

[0001] Title: A system and method for the drying of liquid or solid substances

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to a system and method for the drying of liquid or solid substances, for example food products, chemicals, agricultural products, paper and wood.

[0004] BACKGROUND TO THE INVENTION

[0005] Drying processes usually involve the removal of water from liquid or solid substances by evaporation of the water, for example to adapt the product’s consistency or to extent the shelf life. Drying processes can for example be carried out inside a belt dryer, a fluid bed dryer, on a cylindrical drum, inside a screw conveyor or inside a spray dryer. After removal, the evaporated product moisture is usually dissolved in an excess amount of air, sufficiently high to achieve a for the drying process sufficiently low relatively humidity. The air, dissolved product moisture and possibly other volatile and odorous components are usually discharged from the enclosure of the dryer directly to the atmosphere or, if necessary indirectly via an air treatment system. The energy consumption of a dryer consists of various contributions, such as for the sensible heating of the incoming substance to the drying temperature, the latent heat for evaporation of the product moisture, heating of the fresh air supply and insulation losses. This energy can be supplied by convective heat transfer from the relatively warm drying air to the to be dried substance, by radiation from infrared heaters or burners, by microwave heating and I or by conduction from a heated drum or heated closed conveying belt carrying the to be dried product. The total amount of energy for drying processes ranges typically from 1.2 to 2.5 times the latent heat for evaporation of the product moisture (typ. 2258 kJ / kg). Drying processes hence require typically 2700 to 5600 kJ energy per kg of evaporated water and are therefore known as energy intensive processes, on the one hand due to the high latent heat to evaporate the product moisture and on the other hand due to the usually considerable losses associated with the discharge of relatively warm, moist air.

[0006] Various attempts have been made to reduce the energy demand of drying processes. Quijano et al [lit 1] describe for example a spray drying process whereby the moist air discharged from the drying process is demoisturised and reused for the drying process, i.e. in a closed loop, avoiding energy losses associated to the discharge of warm air. The air demoisturisation takes place inside a column where the air is brought into direct contact with a hygroscopic liquid absorbent, which absorbs the product moisture from the air. The dilute liquid absorbent is subsequently heated up to 400°C inside a regenerator, to a sufficiently high temperature to boil out the dissolved product moisture and to generate steam that can be recovered elsewhere. A disadvantage of this method is that no suitable combination of a high temperature resistant liquid absorbent and corrosion resistant construction materials for the regenerator were found. Ko et al [lit 2] describe a batchwise drying process, involving the re-use of drying air in a closed loop and a liquid absorbent which absorbs the product moisture from the drying air for a certain period, followed by a regeneration step of the liquid absorbent in a separate loop, where the liquid absorbent is heated to a sufficiently high temperature to discharge the absorbed moisture again by evaporation.

[0007] Both described methods require a heating step of the dilute liquid absorbent to evaporate the product moisture from this liquid absorbent. The theoretical minimal energy requirement of these optimized drying processes is therefore at least equal to the of latent heat for evaporation of the product moisture plus the enthalpy of dissolution, i.e. the binding energy of the water and the absorbent salt, together at least typically 2500 kJ per kg removed product moisture. This is still considerably high in view of the fact that the specific energy of respectively water and a dry liquid or solid substance does not change significantly when the water is mixed with (or dissolved in) this liquid or solid substance. Analogously, a drying process, which essentially means separating the water from the substance, should therefore not have to cost any considerable amount of energy either, on the condition that the temperature of the to be dried substance and removed water are the same before and after the drying process, that the physical state of the removed water is the same as that in the to be dried product, i.e. liquid and that no warm, moist air is discharged from the drying process enclosure to the environment.

[0008] Other examples of drying systems are known from CN111947443, CN212362660U and FR939336.

[0009] SUMMARY OF THE INVENTION

[0010] It is an object to provide a drying system for the drying of liquid or solid substances, and / or a method for drying of liquid or solid substances, that obviates, or at least diminishes the disadvantages mentioned above. More in general, it is an object to provide an energy efficient drying system and / or method for the drying of liquid or solid substances, without involving the discharge of moist, odorous air to the environment. Herein drying can also be defined as demoisturising, and vice versa.

[0011] The invention is defined by the claims. According to a first aspect there is provided a product drying system, comprising:

[0012] - a product dryer including a product inlet for receiving wet product, wherein the product dryer is configured to remove moisture from received wet product by evaporation, wherein the product dryer includes a product outlet for discharging dried product and a first moisture outlet for discharging evaporated product moisture;

[0013] - a moisture collection system including a moisture inlet for receiving product moisture from the product dryer, and liquid absorbent inlet for receiving concentrated liquid absorbent, wherein the moisture collection system has a conditioner that is configured to absorb the product moisture into the Equid absorbent, in particular to generate dilute liquid absorbent; and

[0014] -a Equid absorbent dryer configured to receive dilute liquid absorbent from the moisture collection system, and to separate the absorbed moisture from the liquid absorbent, wherein the Equid absorbent dryer includes a second moisture outlet for discharging separated moisture to an environment, wherein the Equid absorbent dryer is preferably configured to discharge the product moisture in a Equid state, for example to a drain; wherein the product drying system is configured such, that:

[0015] -the product dryer, the moisture collection system and the liquid absorbent dryer are mutually arranged to provide near adiabatic drying of the product; and / or

[0016] - the product drying system has a fluid-tight system boundary that encloses each of the product dryer, the moisture collection system and the liquid absorbent dryer, wherein the fluid-tight system boundary is only crossed by the product inlet of the product dryer, the product outlet of the product dryer, the moisture outlet of the liquid absorbent dryer, and an optional outlet of a vacuum pump of the liquid absorbent dryer.

[0017] It has been found that in this way an energy efficient drying system for the drying of product can be achieved, in particular without involving the discharge of moist, odorous air to the environment. In particular, the system boundary can be a hermetically (e.g. gas-tight) enclosure or housing (e.g. defined by respective closed outer walls) of the system, as will be appreciated by the skilled person.

[0018] The fluid-tight system boundary in particular does not include any gas inlet and does not include any gas outlet. One or more gas lines (if any), e.g. one or more gas ducts for transport of heated gas or heated air (if any) and / or one or more vapour ducts for transport of evaporated moisture (vapour), are only present within the enclosure of the system boundary. In a preferred embodiment, the moisture collection system includes a dilute liquid absorbent outlet for discharging the dilute absorbent.

[0019] It follows that during operation, the system preferably does not discharge any gas or gas mixture.

[0020] According to an embodiment, there can be provided a near adiabatic drying system for the demoisturisation of liquid or solid substances, comprising:

[0021] - a product dryer, to remove moisture from the to be dried substance;

[0022] - a moisture collection system, to collect the product moisture and transfer the collected product moisture to a liquid absorbent;

[0023] - a liquid absorbent drying system, to separate the collected product moisture from the liquid absorbent and to discharge this liquid moisture.

[0024] According to an embodiment, the system comprises a dryer for the removal of water from the to be dried substance by evaporation, a moisture collection system for absorbing the product moisture into a liquid absorbent and a liquid absorbent dryer for separating and discharging the absorbed product moisture from the liquid absorbent.

[0025] It is preferred that during operation, the product drying system receives wet product via the product inlet at a certain product temperature, wherein the product drying system discharges removed moisture (in a liquid state) via the second moisture outlet such that the discharged moisture has a temperature that is substantially the same as the product temperature.

[0026] The system further preferably avoids the discharge of moist air from the enclosure of the dryer.

[0027] For example, the product dryer can include a belt dryer, a fluid bed dryer, a cylindrical drum dryer, a screw conveyor dryer or a spray dryer.

[0028] According to an embodiment, the overall system has an enclosure (i.e. said system boundary) and is foreseen with a product inlet for supply of the to be dried substance and a product outlet for the discharge of the dried substance. During system operation, the moisture dissolved in the to be dried substance is, at least for a part, evaporated inside this enclosure (i.e. the system boundary), after which the formed vapour can be dissolved and for example diluted in a certain amount of air present inside the enclosure to a moisture containing gas mixture, to accomplish a relative humidity in this moist gas low enough for the drying process. The moist gas can be discharged from the dryer (for example from an enclosure of the dryer) and a similar amount of fresh, dry air can be supplied to the dryer enclosure (the dry air in particularly being generated inside the system boundary).

[0029] It should be noted that air is not required for the drying of the product (by the dryer). In particular, for drying processes at relatively high temperatures, usually a temperature > 100°C, it is possible that the evaporated product moisture is not diluted in air. The moist gas consists in that case of product moisture alone and is discharged as such from the dryer to the downstream a moisture collection system. These processes also not require any supply of fresh air to the dryer (e.g. to an enclosure of the dryer).

[0030] In particular, the invention provides a product drying system and method comprising adiabatic chemical absorption of product moisture by a liquid absorbent and a near adiabatic removal of the moisture from this liquid absorbent. More in particular, the invention provides a near adiabatic system and method for the drying of liquid or solid substances. In particular, an aspect of the invention provides a closed character of the drying process, avoiding the discharge of moist, odorous air to the environment.

[0031] For example, the moisture collection system can comprise a conditioner, configured for absorbing the product moisture vapour into a liquid absorbent. The liquid absorbent can be e.g. a concentrated salt solution of for example lithium chloride, calcium chloride, and / or lithium bromide in water, a will be clear to the skilled person. The moist gas outlet of the product dryer can be connected to a moist gas inlet of the conditioner. During operation, the moist gas can be brought into contact with the liquid absorbent inside an enclosure of the conditioner, whereby the moisture from the moist gas is absorbed by the liquid absorbent.

[0032] According to an embodiment, the conditioner can include a dry air outlet, for the discharge of any remaining air after having transferred at least a part of the dissolved moisture to the liquid absorbent. This dry air is then preferably returned to the product dryer, e.g. via a dry air inlet on an enclosure (e.g. housing) of the dryer.

[0033] The conditioner can e.g. include a packed column, to enlarge the moisture exchanging interfacial surface between the moist gas and the receiving liquid absorbent. The conditioner is in particular foreseen with a liquid absorbent outlet for the discharge of the dilute liquid absorbent, containing the dissolved product moisture, to a liquid absorbent dryer. The conditioner can also be foreseen with a liquid absorbent inlet, for the return of concentrated liquid absorbent from the liquid absorbent dryer, after removal of at least a part of the dissolved product moisture from the liquid absorbent. The moisture collection system can further comprise means for transferring energy from the dilute liquid absorbent to the dry air (if any).

[0034] The liquid absorbent drying system is preferably configured for evaporating the product moisture, absorbed into the liquid absorbent, from the liquid absorbent to become evaporated moisture. The liquid absorbent drying system can include one or more liquid absorbent sprayers and a packed column, in order to enlarge the moisture releasing surface area of the dilute liquid absorbent from which the dissolved product moisture can evaporate.

[0035] The liquid absorbent drying system preferably comprises a condenser configured for condensing the evaporated moisture. The condenser can include a surface area to condense the product moisture on, the surface area e.g. being provided by a surface of at least one heat transfer tube and / or one heat transfer plate.

[0036] The condenser can also include a condensate pan for collecting condensed product moisture discharged from the condensing surface . This condensate pan can be connected to a condensate outlet of the liquid absorbent drying system.

[0037] The liquid absorbent drying system preferably comprises a heat transfer medium flow circuit arranged such that heat removed from the evaporated moisture during condensation thereof in the condenser is absorbed by the heat transfer medium, and such that said absorbed heat is used in evaporation of the absorbed moisture from the liquid absorbent.

[0038] According to an embodiment, temperatures, volumes and / or pressures (of the fluid or fluids) in the liquid absorbent drying system can be chosen such that the absorbed moisture evaporates from the liquid absorbent in the liquid absorbent drying system, and / or such that the evaporated moisture condenses on a condensing surface of a respective condenser. The temperatures, volumes and / or pressures (of heat transfer fluid) in a heat transfer medium flow circuit (if any) can be chosen such that the heat transfer medium condenses in a heat exchanger indirectly against the dilute liquid absorbent, and / or such that the heat transfer medium evaporates in the condenser. Said heat transfer medium flow circuit preferably comprises a compressor (e.g. downstream of the at least condenser, e.g. the one cooling tube and / or cooling plate), arranged to increase a pressure of the heat transfer medium to the pressure inside said heat exchanger.

[0039] Optionally, a first heat exchanger is installed in a dilute liquid absorbent outlet of the moisture collection system (i.e. the dilute liquid absorbent outlet of the moisture collection system can include a first heat exchanger). Optionally, a second heat exchanger is installed in a dried air outlet (if any) of the moisture collection system (i.e. the dried air outlet can include a second heat exchanger).

[0040] Optionally, the heat exchanger of the dilute liquid absorbent outlet of the moisture collection system is connected to the heat exchanger of the dried air outlet of the moisture collection system, to exchange energy (i.e. heat) between dilute liquid absorbent (flowing along or through the first heat exchanger during operation) and dried air (flowing through or along the second heat exchanger during operation).

[0041] Optionally, the (second) heat exchanger installed in the dried air outlet of the moisture collection system is installed inside an enclosure of the product dryer.

[0042] Further, an aspect of the invention provides a method that is characterized by the features of claim 16. In this way, above-mentioned advantages can be achieved.

[0043] In particular, according to an embodiment there is provided a method for the near adiabatic drying of liquid or solid substances, such as food products, chemicals, agricultural products, paper and wood. The method comprises the removal of product moisture from liquid of solid substances by evaporation of this moisture. The method further comprises absorbing the removed moisture from the product into a liquid absorbent, i.e. preferably a concentrated salt solution of for example lithium chloride, calcium chloride, and / or lithium bromide in water. The method comprises the separation the absorbed product moisture from the liquid absorbent by evaporation of the product moisture (by a liquid absorbent dryer).

[0044] The method in particular comprises condensing the evaporated product moisture (in particular by a liquid absorbent dryer), wherein heat released during condensation of the evaporated moisture is preferably absorbed by a heat transfer medium, and wherein said absorbed heat is preferably used in evaporation of the absorbed product moisture from the liquid absorbent. Thus, the energy required for demoisturising the liquid absorbent, i.e. removal of the absorbed product moisture by evaporation, can be minimized by reclaiming the energy released by the condensation of the product moisture vapour.

[0045] Optionally, the method further comprises the exchange of heat from the dilute liquid absorbent to dried air (if any). Optionally, this heat exchange takes place inside an enclosure of the product dryer.

[0046] It will be appreciated that any of the aspects, features and options described in view of the (near adiabatic) product drying system apply equally to the method for (near adiabatic) product drying , and vice versa. It will also be clear that any one or more of the above aspects, features and options can be combined. In the drawings, similar or corresponding features are denoted by similar or corresponding reference signs.

[0047] BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Non-limiting embodiments of the present invention will now be described in detail with reference to the accompanying drawings in which:

[0049] Figure 1 shows a schematic representation of a typical drying system for liquid or solid substances according to the present state of the art, the system boundaries and the streams crossing these system boundaries;

[0050] Figure 2 shows a schematic representation of the external boundaries and the functional elements within of an example of a near adiabatic drying system for the drying of liquid or solid substances and the streams crossing the respective element boundaries;

[0051] Figure 3 shows an example of a schematic representation of a working principle of a further example of a near adiabatic drying system in combination with a belt type product dryer;

[0052] Figure.4A shows a dryer section of the example of Figure 3 in more detail; Figure 4B shows a moisture collection system of the example of Figure 3 in more detail; and

[0053] Figure 4C shows an absorbent drying system of the example of Figure 3 in more detail.

[0054] DETAILED DESCRIPTION

[0055] Figure 1 shows an illustration of a drying system according to the present state of the art, which has some disadvantages which the present invention seeks to alleviate. The system encompasses the elements within the external boundary 1, such as an enclosure 2 with an infeed opening 3 for the supply of the to be dried wet product 4 and an outfeed opening 5 for the discharge of the dried product 6. The enclosure 2 can also be foreseen with an air infeed opening 6A for the supply of air 7 and an outfeed opening 8 for the discharge of a product moisture containing gas 9. This moist gas 9 consists of a mixture of the evaporated product moisture 10 and the supplied air 7. It is also possible that no fresh air is supplied to the enclosure 2. In that case, the moist gas 9 only consists of the product moisture 10. The product drying takes place by the evaporation of product moisture 10 from an interfacial product surface 11. The latent heat for the evaporation of the product moisture can be supplied to the product surface 11 by convection, i.e. heat transferred from the airstream 7. In that case, air stream 7 is initially brought to a sufficiently high temperature inside a heat exchanger 12 by means of an energy source 13, such as for example electricity, steam, hot water or a hot oil. The heat exchanger 12 can be located outside or inside the enclosure 2. It is also possible that the latent heat for the evaporation of the product moisture is supplied by conduction, by direct contact of the product with a relatively hot surface underneath (i.e. a heated surface, a heated support surface). In that case, heat exchanger 12 can be part of the transport device that carries the product through the dryer, such as a cylindrical internally heated drum or a transport belt with heat supply underneath. Another option for the heat supply is radiation, from gas fired or electrically heated radiative surfaces which are placed inside the enclosure 2.

[0056] The driving force for moisture transfer from the product surface 11 to the receiving gas 14 is the positive difference in partial pressure of the moisture at the product surface 11 and inside receiving gas 14. This positive difference is usually maintained by minimizing the partial vapour pressure of the receiving gas 14 to values sufficiently below the partial vapour pressure of the evaporating product surface 11. This can be accomplished by or by a combination of two measures: by reducing the moisture concentration in the receiving gas by increasing the mass ratio of fresh air supply 15 to recirculated air 16, for example by adjusting the flow rates of the exhaust fan 17 and recirculation fan 18, or by increasing the temperature of the supplied air 7 to the drying process. The energy required for drying processes usually consists of the following two main components: the latent heat for evaporation of the product moisture and the sensible heat associated with the heating of the fresh air supply 15 from the inlet temperature of stream 15 to the outlet temperature of stream 19. The thermal energy consumption of drying processes usually amounts up to 1.2 to 2.5 times the amount latent heat for evaporation of the product moisture alone (2258 kJ / kg), i.e. to 2700 to 5600 kJ per kg of evaporated water. This thermal energy demand is supplied by the heating circuit 13 via the heater 12. Also, drying processes typically involve the discharge of moist, odorous air 19 to the environment 20 which can cause nuisance to the neighbourhood.

[0057] Various attempts have been described in the literature to reduce the energy requirements of drying processes, for example by respectively transferring the moisture from the exhausted gas 19 to a liquid absorbent and reusing the demoisturised gas as fresh make-up, hence in a closed loop towards inlet 15. This measure could reduce the loss of energy associated with the net heating of the fresh air and emission of odours from the drying process to the environment, but what remains is a considerable amount of energy for the evaporation of the product moisture.

[0058] The specific energies of respectively water and a liquid or solid substance do not change significantly when the water is mixed with (or dissolved in) the liquid or solid substance. Analogously, a drying process, which essentially means (partially) separating water from a liquid or solid substance does not have to cost a considerable amount of energy either, if the following conditions are met: the temperature of the to be dried substance and removed water are the same before and after the drying process, the physical state of the removed water after the drying process is the same as before , i.e. liquid and no warm, moist air is discharged from the drying process enclosure to the environment.

[0059] Figure 2 shows an overview of main components and their mutual connections of a drying system according to a non-limiting example of the present invention which meets these demands.

[0060] In particular, Figure 2 shows a product drying system, comprising a product dryer 22 including a product inlet for receiving wet product. The product dryer 22 is configured to remove moisture from received wet product by evaporation, wherein the product dryer 22 includes a product outlet for discharging dried product and a first moisture outlet for discharging evaporated product moisture. The product dryer 22 can e.g. be installed inside or be provided with a respective dryer enclosure 2 (e.g. a housing or the-like).

[0061] The system includes a moisture collection system 23 including a moisture inlet for receiving product moisture from the product dryer 22, and liquid absorbent inlet for receiving concentrated liquid absorbent. The moisture collection system has a conditioner 28 that is configured to absorb the product moisture into the liquid absorbent, in particular to generate dilute liquid absorbent.

[0062] Further, the system has a liquid absorbent dryer 24 configured to receive dilute liquid absorbent from the moisture collection system 23, and to separate the absorbed moisture from the liquid absorbent. The liquid absorbent dryer 24 includes a second moisture outlet 58 for discharging separated moisture to an environment. The liquid absorbent dryer 24 is preferably configured to discharge the product moisture in a liquid state 25, for example to a drain.

[0063] As will be explained in the following, the product drying system 22,

[0064] 23, 24 is configured such, that: the product dryer 22, the moisture collection system 23 and the liquid absorbent dryer 24 are mutually arranged to provide near adiabatic drying of the product; and / or the product drying system has a fluid-tight system boundary 21 that encloses each of the product dryer 22, the moisture collection system 23 and the liquid absorbent dryer 24, wherein the fluid-tight system boundary 21 is only crossed by the product inlet of the product dryer 22, the product outlet of the product dryer 22, and the moisture outlet of the liquid absorbent dryer

[0065] 24.

[0066] In particular, the near adiabatic drying system comprises within its encapsulating system boundaries 21 said product dryer 22, to remove moisture from the to be dried substance, said moisture collection system 23, to collect the product moisture and transfer the collected product moisture to a liquid absorbent and said absorbent drying system 24, to remove the product moisture from the liquid absorbent. A characteristic aspect of the present near adiabatic drying system is that the system boundary 21 is only crossed by an inwardly directed mass flow of the to be dried product 4, an outwardly directed mass flow of dried product 6 and an outwardly directed mass flow 25 of product moisture in liquid state.

[0067] Preferably the product dryer 22 includes a belt dryer, a fluid bed dryer, a cylindrical drum dryer, a screw conveyor dryer or a spray dryer. The product dryer 22 can have a respective enclosure 2 being provided with the product inlet 3 for supply for the to be dried substance 4 and the product outlet 5 for the discharge of the dried substance 6. The enclosure 2 of the dryer 22 can have a moist gas outlet connection 8 for the discharge of moist gas 9, which can consist of product moisture 10 in pure form, or of product moisture dissolved in air. The dryer enclosure 2 can have an optional inlet connection 6 A for the supply of dry air 7 (to an interior of the dryer). The embodiment according to Figure 2 contains several items with similar names and functions as in the embodiment of Figure 1. These similar items are indicated by the same reference numerals. The explanation of the working principle of the embodiment according to Figure 2 is limited to the differences with Figure 1 for conciseness.

[0068] It is preferred that during operation of the dryer 22, a driving force for moisture transfer from the product surface 11 (of product inside the dryer 22) to the receiving gas 14 (for receiving the moisture) is a positive difference in partial pressure of the moisture at the product surface 11 and partial pressure inside the receiving gas 14. This positive difference is preferably maintained by the downstream moisture collection system 23, which is connected to the moisture (i.e. moist gas) discharge of the dryer 22 at a connection point 26 (the connection 26 being located internally, within the external boundary 21 of the system).

[0069] It is preferred that the moisture collection system 23 includes a dry air outlet, connected e.g. via a transfer point 27 (internally, within the external boundary 21 of the system ) to the dryer 22 to return relatively dry air 7 to the product dryer enclosure 2 (after having collected at least a part of the product moisture 10).

[0070] It will be appreciated that the energy required for the drying process essentially consists of the latent heat for evaporation of the product moisture, i.e. 2258 kJ energy per kg of evaporated water plus possibly some energy for heating up the incoming product 4 to the drying temperature. Another preferred characteristic aspect is that said energy is provided by the moisture collection system 23 and not through an external energy source (i.e. an energy source located outside the boundary 21). Preferably, the drying process itself is adiabatic and hence according to the invention not equipped with any external thermal energy supply, sourcing from out of the external boundary 21.

[0071] The moisture collection system 23 preferably comprises a so-called conditioner 28, configured for absorbing the product moisture 10 into a receiving interfacial surface 29 of liquid absorbent, a dilute liquid absorbent outlet 30 and a concentrated liquid absorbent inlet 31. Thereto, a product moisture vapour inlet 26 of the moisture collection system can be connected to moist gas outlet 9 of the dryer.

[0072] The moisture collection system 23 can also be foreseen with a dry air outlet 27, to return relatively the dry air 7 back to the product dryer enclosure 2, after having transferred at least a part of the dissolved product moisture 10 to the receiving liquid absorbent 29. Preferably, a driving force for this moisture transfer from the moist gas 32 to the receiving interfacial surface area 29 of the liquid absorbent is a positive difference of the partial pressure of the moisture into the moist gas 32 and the partial pressure of the moisture in the receiving liquid absorbent 29.

[0073] According to an embodiment, the liquid absorbent can comprise one or more salt solutions, for example a lithium chloride solution, calcium chloride solution, and / or a lithium bromide solution. It will be appreciated that once the chemical composition of the liquid absorbent has been chosen, the latter partial pressure can only be decreased to sufficiently low values by either increasing the mass fraction of dissolved salt or by reducing the temperature of the receiving liquid absorbent.

[0074] In particular, the absorption of the product moisture 10 into the liquid absorbent 29 is an exothermic process (with heat release from two sources: latent heat from the phase change of the product moisture from vapour to liquid and dissolution energy, i.e. the chemical binding energy of the product moisture to the salt ions). Also, preferably, the moisture transfer process within the moisture collection system 23 is adiabatic, i.e. without external energy supply or energy discharge from out of the external boundary 21. A preferred aspect of the near adiabatic drying system is that the exothermic heat released inside / by the moisture collection system 23 is reused by the dryer 22, i.e. as heat source for the drying process 22, for example by transferring energy from the outgoing dilute liquid absorbent 30 to the outgoing dry air 7, or as direct contact conduction heat source below surface 11 in case of a belt drying system of drum dryer.

[0075] Preferably, during operation, the moisture concentration in the liquid absorbent in the moisture collection system 23 is maintained at a constant, sufficiently low level by a downstream liquid absorbent drying system 24. This absorbent drying system 24 is preferably connected via a connection point 30 (located internally, within external system boundary 21) to the dilute liquid absorbent outlet of the moisture collection system, for receiving dilute liquid absorbent. Also, absorbent drying system 24 is preferably connected to a concentrated liquid absorbent inlet 31 (located within the external boundary 21) of / to the moisture collection system 23, for feeding concentrated liquid absorbent to the moisture collection system 23.

[0076] The liquid absorbent drying system 24 is preferably configured for evaporating the product moisture 10 (absorbed into the liquid absorbent) from the liquid absorbent to become evaporated moisture, which evaporation process preferably takes place inside an enclosure 32 (e.g. housing) of the liquid absorbent drying system 24.

[0077] The liquid absorbent drying system 24 can include one or more liquid absorbent sprayers and a packed column, in order to enlarge an interfacial surface area 33 of the dilute liquid absorbent from which the dissolved product moisture 10 can evaporate.

[0078] The liquid absorbent drying system preferably comprises a condenser 34 configured for condensing the evaporated moisture and the collection and discharge of the formed condensate 25 to an external drain 35. As will be appreciated, during operation of the system, a driving force for the evaporation of moisture from the dilute liquid absorbent is the positive difference of the partial pressure of the moisture at the evaporating surface 33 of the liquid absorbent and the partial pressure of the moisture at condenser surface 34. The first partial pressure depends on a combination of water activity of the liquid absorbent and its temperature at the evaporating surface 33. The latter partial pressure depends on the temperature of the condenser surface 34. The positive difference of these respective surfaces is preferably maintained by transferring latent heat released at the condenser surface 34 to latent heat required for evaporation at the evaporating surface 33. The only remaining net heat demand, for evaporation minus condensation, is the chemical binding energy of the dissolved product moisture in the dilute liquid absorbent, which can potentially be retrieved from the exothermic energy released at the moisture absorption process inside the conditioner 28. This means that potentially no net energy demand is required for the combined processes within the boundary 21 of the overall system, if the temperature of the to be dried substance 4, dried substance 6 and removed water 25 are the same before and after the drying process and if that the physical state of the removed water 25 is the same as that in the to be dried product, i.e. liquid. In addition, the process is substantially closed, without any discharge of moist air or odours to the atmosphere.

[0079] Figures 3, 4A, 4B, 4C show an example of a further embodiment, e.g. including a belt dryer, according to the present invention. The embodiment according to Figure 3 contains several items with similar names and functions as in the embodiment of Figure 1 and Figure 2. These similar items are indicated by the same reference numerals. The explanation of the working principle of the embodiment according to Figure 3 is limited to the differences with Figure 2 for conciseness.

[0080] The product dryer 22 can include a product conveyor 37 configured to convey the (liquid or solid) product along a product drying path between the product inlet and the product outlet. The product conveyor 37 is preferably configured to support the product along the product drying path.

[0081] In the embodiment of Figure 3, the to be dried product 4 is supplied to and carried through the dryer enclosure 2 on a transport belt 37 (e.g. an open conveying belt 37 or an open mesh conveying belt 37), which also transports the dried product 6 from (out of) the dryer enclosure 2. An air stream 7 from the moisture collection system 23 (see Fig. 4B) can be supplied to the dryer 22 (see Fig. 4A) and dryer enclosure 2 via one or more inlets 6A at a relatively high temperature, of for example 50 to 90°C, and a relatively low absolute humidity ratio, for example a ratio of 10 to 20 g / kg.

[0082] In the present example, wherein the product conveyor is a belt conveyor 37 (e.g. belt dryer), air can be transported upwardly through a porous product layer, thereby adiabatically taking up moisture from the product and convectively transferring energy to the product. The dryer 22 is configured such that a resulting moist gas stream 9 can leave the dryer enclosure 2, e.g. by means of a discharge fan 17, through one or more outlet openings 8, for example at a reduced temperature, e.g. a temperature of in the range of 35 to 50°C and for example at an elevated absolute moisture ratio of for example 30 to 50 g / kg. The resulting moist gas stream 9 can be transferred as such to the downstream moisture collection system 23. The combination of processes within the boundaries 22 of the dryer are therefore preferably also adiabatic, i.e. without external heat energy supply from any external source.

[0083] As follows from the drawings, from the internal connection point 26 with the moisture collection system, the moist gas stream can be split into a part 37 which is supplied to the conditioner 28 and part 16 which is redirected towards the dryer 22, after e.g. being mixed with a dry air stream 38 (if any) from the exhaust fan 39 from the conditioner. By this measure, the volume flow rate of air through the dryer, through the product layer and discharged from the dryer by fan 17 can be dimensioned larger (or smaller) than the flow rate from the conditioner, from fan 39.

[0084] The conditioner 28 is preferably equipped with sprayers 46 to divide the supplied concentrated liquid absorbent 49 over a packed column 47 for enlargement of the moisture receiving interfacial surface area of the liquid absorbent, a demister 48 to avoid the carry over of liquid absorbent droplets to the dry air 38 from the conditioner and an exhaust fan 39.

[0085] The system can be configured such that the moist gas stream 37 is supplied to the conditioner 28 underneath the packed column 47, and such that the moist gas stream 37 is transported upwardly through this packed column in counterflow with the moisture receiving liquid absorbent. During operation, moisture from the moist gas stream is preferably transferred to the liquid absorbent by an exothermic adiabatic process, whereby the released latent heat and binding energy are taken up mainly by the dilute liquid absorbent 45 and for a part by the dried air 38 (if any). A for this example suitable liquid absorbent is lithium chloride dissolved in water, e.g. supplied to the conditioner 28 at a concentration of about 40 to 44%, with a supply temperature to the conditioner of for example 20 to 60°C. The dilute liquid absorbent is preferably discharged after having absorbed a part of the moisture from the air stream at a concentration of for example 36 to 40% and a temperature of for example 60 to 130°C.

[0086] According to an aspect of the invention, a heater 40 and cooler 41 can be foreseen in respectively the dilute liquid absorbent outlet 45 of the conditioner 28 and the dry air supply 7 to the dryer. This heater and cooler can be connected via a heat transfer loop 42, to reclaim heat from the absorption process inside the conditioner 28 as heat source for the drying process inside enclosure 2. The heat carrying medium inside this heat transfer loop can be a liquid, such as water, circulated by means of a liquid pump 43. The heat carrying medium can also be a refrigerant, which evaporates inside the cooler 41 (i.e. the cooler being an evaporator) and condenses inside the heater 40 (i.e. the heater being a condenser), at adequately selected temperatures and pressures, enabled by means of a compressor 43 (i.e. said pump being a compressor) to transport the refrigerant gas from a lower pressure in the cooler 41 to a higher pressure in the condenser 40 and a throttle valve 44 for flow and pressure adjustment. According to the invention, heater 40 can also be placed inside the enclosure 2 of the dryer. It is also possible that the latent heat for the evaporation product is supplied by conduction, by direct contact of the product with a relatively hot surface underneath. In that case, heat exchanger 40 can be part of the transport device 37 that carries the product through the dryer, such as a cylindrical internally heated drum or a transport belt with heat supply underneath.

[0087] During system operation, the combination of processes within (the boundaries) of the moisture collection system 23 are therefore preferably also adiabatic, i.e. without external heat energy supply from any external source sourcing out the external boundaries 21.

[0088] The regeneration of the liquid absorbent preferably takes place within (the boundaries of) the liquid absorbent drying system 24 (see Fig. 4C), to which the dilute liquid absorbent can be supplied at an internal transfer point 31 and from which concentrated liquid absorbent returns via an internal transfer point 30. Such a drying system per se, applied for air demoisturisation processes, has been described patent application PCT / NL2022 / 050195, which is deemed to be incorporated in the present application in its entirety by reference.

[0089] The dilute liquid absorbent 45 can be supplied to a holder 32 (e.g. a vessel or container), wherein the liquid absorbent can be distributed e.g. by means of sprayers 50 over a packed column 51 on the interfacial surface of which the drying process of the liquid absorbent takes place during operation. A resulting concentrated liquid absorbent 52 can be discharged from the holder 32 of the liquid absorbent drying system 24, e.g. by means of a pump 36, preferably for a part towards a transfer point 31 to the conditioner 28 and for a part recirculated back to the inlet of the holder 32 via throttle valve 53.

[0090] Water vapour from the liquid absorbent is preferably condensed on the external surface of a vapour condenser 34 which can for example be situated in the upper part of the holder 32 of the liquid absorbent drying system 24. This vapour condenser 34 can comprise a bundle of multiple internally cooled tubes and / or multiple internally plates. It is preferred that a heat transfer medium 54, usually a refrigerant, evaporates inside the tubes and / or plates of the vapour condenser 34, so that the latent heat released by the condensation of the moisture from the liquid absorbent is discharged from the condenser. The refrigerant vapour is transported from the condenser to a respective refrigerant compressor 55 and is pressurized by this compressor 55 to a sufficiently high pressure, such that a second phase change of the refrigerant, from vapour to liquid, can take place inside a heat exchanger 56. The heat exchanger 56 is preferably arranged to pre-heat the dilute liquid absorbent before entering holder 32, thereby transferring the energy from the evaporated moisture during condensation in the condenser 34 to the dilute liquid absorbent, as an energy source for the evaporation of the absorbed moisture from the liquid absorbent. Again, this drying process is highly energy efficient, and in particular does not cost any energy, other than e.g. a limited amount of electricity for e.g. a drive motor of the compressor 55 and an optional vacuum pump 60 (usually not more than 5 to 25% of the energy normally required to evaporate a unit mass of water). The formed condensate in the condenser 34 is in this example collected on an inclined condensate pan

[0091] 57 underneath the tube bundle and is discharged via a condensate outlet pipe

[0092] 58 and a condensate pump 59 to a condensate drain 35.

[0093] It is preferred that a pressure inside holder 32 is sub atmospherical, for example in the range of 20 to 200 mbar, in partiuclar depending on a partial pressure of the condensed moisture on the condenser 34. The holder can be connected to a vacuum pump 60 for achieving the subatmospherical pressure. During operation, non-condensable gases can be discharged to the atmosphere (i.e. an environment of the system) by means of the vacuum pump 60. Apart from some electrical energy for the drive motor of the compressor, the combination of processes within the boundaries 24 of the liquid absorbent drying system is adiabatic, i.e. without external heat energy supply from any external source sourcing out the external boundaries 21. Taking into account the said limited electrical energy for the drive motor of the compressor (and e.g. the vacuum pump), the processes inside the boundaries of the liquid absorbent drying process are therefore nearly adiabatic, only requiring 5 to 25% of the energy normally required per unit mass of evaporated water.

[0094] The combination of the processes within the boundaries 22, 23 and 24 is therefore highly efficient, requiring only 5 to 25% of the energy normally required per unit mass of evaporated water, which is substantially less than the factor 1.2 to 2.5 usually required for drying processes.

[0095] Herein, the invention is described with reference to specific examples of embodiments of the invention. It will, however, be evident that various modifications and changes may be made therein, without departing from the essence of the invention. For the purpose of clarity and a concise description features are described herein as part of the same or separate embodiments, however, alternative embodiments having combinations of all or some of the features described in these separate embodiments are also envisaged.

[0096] However, other modifications, variations, and alternatives are also possible. The specifications, drawings and examples are, accordingly, to be regarded in an illustrative sense rather than in a restrictive sense.

[0097] For the purpose of clarity and a concise description features are described herein as part of the same or separate embodiments, however, it will be appreciated that the scope of the invention may include embodiments having combinations of all or some of the features described.

[0098] In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word ‘comprising’ does not exclude the presence of other features or steps than those listed in a claim. Furthermore, the words ‘a’ and ‘an’ shall not be construed as limited to ‘only one’, but instead are used to mean ‘at least one’, and do not exclude a plurality. The mere fact that certain measures are recited in mutually different claims does not indicate that a combination of these measures cannot be used to an advantage.

[0099] Cited Literature

[0100] [1] D.P. Quijano et al. Heat recovery in milk powder drying by using a liquid sorption process. Proceedings of the 12thIEA Heat Pump Conference 2017.

[0101] [2] US Patent 4,189,848 Suk M. Ko, filed aug 4th1977

Claims

Claims1. A product drying system, comprising: a product dryer (22) including a product inlet for receiving wet product, wherein the product dryer (22) is configured to remove moisture from received wet product by evaporation, wherein the product dryer (22) includes a product outlet for discharging dried product and a first moisture outlet for discharging evaporated product moisture; a moisture collection system (23) including a moisture inlet for receiving product moisture from the product dryer (22), and liquid absorbent inlet for receiving concentrated liquid absorbent, wherein the moisture collection system has a conditioner (28) that is configured to absorb the product moisture into the liquid absorbent, in particular to generate dilute liquid absorbent; and a liquid absorbent dryer (24) configured to receive dilute liquid absorbent from the moisture collection system (23), and to separate the absorbed moisture from the liquid absorbent, wherein the liquid absorbent dryer (24) includes a second moisture outlet (58) for discharging separated moisture to an environment, wherein the liquid absorbent dryer (24) is preferably configured to discharge the product moisture in a liquid state (25), for example to a drain (35); wherein the product drying system (22, 23, 24) is configured such, that:-the product dryer (22), the moisture collection system (23) and the liquid absorbent dryer (24) are mutually arranged to provide near adiabatic drying of the product; and / or- the product drying system has a fluid-tight system boundary (21) that encloses each of the product dryer (22), the moisture collection system (23) and the liquid absorbent dryer (24), wherein the fluid-tight system boundary (21) is only crossed by the product inlet of the product dryer (22), the product outlet of the product dryer (22), the moisture outlet of the liquid absorbent dryer (24), and an optional outlet of a vacuum pump (60) of the liquid absorbent dryer (24).

2. The product drying system according to claim 1, wherein the product dryer (22) includes a product conveyor (37) configured to convey a liquid product or a solid product along a product drying path between the product inlet and the product outlet, wherein the product conveyor (37) is preferably configured to support the product along the product drying path.

3. The product drying system according to any of the preceding claims, wherein the product dryer (22) includes belt dryer, a fluid bed dryer, a cylindrical drum dryer, a screw conveyor dryer or a spray dryer.

4. The product drying system according to any of the preceding claims, including a first heater (40) for generating heat for the product dryer (22) for drying wet product.

5. The product drying system according to claim 4, including a first heat transfer medium flow circuit (42) downstream of the conditioner (28), arranged for transferring heat from liquid absorbent to the first heater (40), the first heat transfer medium flow circuit (42) preferably including a compressor (43) for compressing a heat transfer medium of the heat transfer medium flow circuit (42).

6. The product drying system according to claim 4 or 5, wherein first heater (40) is configured to heat a gas or gas mixture (7), for example air, for the drying of the product by the product dryer (22), wherein the conditioner of the moisture collection system preferably includes an outlet for feeding dry gas or dry gas mixture to the first heater (40).

7. The product drying system according to any of the preceding claims, wherein the conditioner (28) is configured to divide supplied concentrated liquid absorbent (49) over a packed column (47) for enlargement of a moisture receiving interfacial surface area of the liquidabsorbent, wherein the conditioner (28) is configured to transport supplied product moisture upwardly through the packed column (47), in counterflow with the liquid absorbent (49).

8. The product drying system according to any of the preceding claims, wherein the liquid absorbent dryer (24) includes a condenser (34) arranged for condensing liquid from the absorbent received from the moisture collection system (23), wherein the condenser (34) is preferably part of a second heat transfer heat transfer medium flow circuit (54, 55, 56), the second heat transfer heat transfer medium flow circuit (54, 55, 56) preferably being configured to transfer heat to dilute liquid absorbent upstream of the condenser (34).

9. The product drying system according to claim 8, wherein the liquid absorbent dryer (24) includes a condensed liquid collector, for example a condensate pan (57), arranged below the condenser (34) for collecting condensed liquid, and for feeding the liquid to the second moisture outlet (58).

10. The product drying system according to any of the preceding claims, wherein the liquid absorbent dryer (24) includes a holder (32), the holder (32) comprising a packed column (51) for receiving liquid absorbent to be dried, wherein the liquid absorbent dryer (24) is preferably configured to maintain a pressure inside the holder (32) at a sub atmospherical pressure, for example a pressure in a range of 20 to 200 mbar.

11. The system according to any of the preceding claims, wherein the system boundary is a hermetically, e.g. gas-tight, enclosure or housing, for example defined by respective closed outer walls of the system.

12. The system according to any of the preceding claims, wherein the system is configured such that the system boundary (21) is only crossed by an inwardly directed mass flow of the to be dried product (4), an outwardly directed mass flow of dried product (6) and an outwardly directed mass flow13. The system according to any of the preceding claims, wherein the system is configured such that exothermic heat released inside or by the moisture collection system (23) is reused by the product dryer (22).

14. A product drying system, for example a system according to any of the preceding claims, comprising: a product dryer (22) including a product inlet for receiving wet product, wherein the product dryer (22) is configured to remove moisture from received wet product by evaporation, wherein the product dryer (22) includes a product outlet for discharging dried product and a first moisture outlet for discharging evaporated product moisture; a moisture collection system (23) including a moisture inlet for receiving product moisture from the product dryer (22), and liquid absorbent inlet for receiving concentrated liquid absorbent, wherein the moisture collection system has a conditioner (28) that is configured to absorb the product moisture into the liquid absorbent, in particular to generate dilute liquid absorbent; and a liquid absorbent dryer (24) configured to receive dilute liquid absorbent from the moisture collection system (23), and to separate the absorbed moisture from the liquid absorbent, wherein the liquid absorbent dryer (24) includes a second moisture outlet (58) for discharging separated moisture to an environment, wherein the liquid absorbent dryer (24) is preferably configured to discharge the product moisture in a liquid state (25), for example to a drain (35); wherein the product drying system (22, 23, 24) has a fluid-tight system boundary (21) that encloses each of the product dryer (22), the moisture collection system (23) and the liquid absorbent dryer (24), wherein the system boundary is a hermetically, .e.g. gas-tight, enclosure or housing, for example defined by respective closed outer walls of the system.

15. A product drying system, for example a system according to any of the preceding claims, the system comprising:a product dryer (22) including a product inlet for receiving wet product, wherein the product dryer (22) is configured to remove moisture from received wet product by evaporation, wherein the product dryer (22) includes a product outlet for discharging dried product and a first moisture outlet for discharging evaporated product moisture; a moisture collection system (23) including a moisture inlet for receiving product moisture from the product dryer (22), and liquid absorbent inlet for receiving concentrated liquid absorbent, wherein the moisture collection system has a conditioner (28) that is configured to absorb the product moisture into the liquid absorbent, in particular to generate dilute liquid absorbent; and a liquid absorbent dryer (24) configured to receive dilute liquid absorbent from the moisture collection system (23), and to separate the absorbed moisture from the liquid absorbent, wherein the liquid absorbent dryer (24) includes a second moisture outlet (58) for discharging separated moisture to an environment, wherein the liquid absorbent dryer (24) is preferably configured to discharge the product moisture in a liquid state (25), for example to a drain (35); wherein the product drying system includes a first heater (40) for generating heat for the product dryer (22) for drying wet product, wherein the system includes a first heat transfer medium flow circuit (42) downstream of the conditioner (28), arranged for transferring heat from liquid absorbent to the first heater (40), the first heat transfer medium flow circuit (42) preferably including a compressor (43) for compressing a heat transfer medium of the heat transfer medium flow circuit (42), wherein the liquid absorbent dryer (24) includes a condenser (34) arranged for condensing liquid from the absorbent received from the moisture collection system (23), wherein the condenser (34) is part of a second heat transfer heat transfer medium flow circuit (54, 55, 56), the second heat transfer heat transfer medium flow circuit (54, 55, 56)preferably being configured to transfer heat to dilute liquid absorbent upstream of the condenser (34); wherein the product drying system (22, 23, 24) is configured such, that:-the product dryer (22), the moisture collection system (23) and the liquid absorbent dryer (24) are mutually arranged to provide near adiabatic drying of the product; and / or- the product drying system has a fluid-tight system boundary (21) that encloses each of the product dryer (22), the moisture collection system (23) and the liquid absorbent dryer (24), wherein the fluid- tight system boundary (21) is only crossed by the product inlet of the product dryer (22), the product outlet of the product dryer (22), the moisture outlet of the liquid absorbent dryer (24), and an optional outlet of a vacuum pump (60) of the liquid absorbent dryer (24).

16. A method for drying a wet product, for example utilizing a system according to any of the preceding claims, the method including:- feeding wet product to a product dryer (22), wherein the product dryer (22) removes moisture from the received wet product by evaporation, and discharges dried product, wherein product dryer (22) discharged evaporated product moisture to a moisture collection system (23); absorbing the product moisture into a liquid absorbent, by a conditioner (28) of the a moisture collection system (23), thereby in particular generating dilute the liquid absorbent; and separating the absorbed moisture from the liquid absorbent by a liquid absorbent dryer (24); wherein the liquid absorbent dryer (24) discharges separated moisture to an environment in a liquid state (25), for example to a drain (35); wherein the method is carried out such that:- near adiabatic drying of the product is achieved;and / or- a fluid-tight system boundary (21) is utilized, the a fluid-tight system boundary (21) enclosing each of the product dryer (22), the moisture collection system (23) and the liquid absorbent dryer (24), wherein the fluid- tight system boundary (21) is only crossed by a product inlet of the product dryer (22), a product outlet of the product dryer (22), a moisture outlet of the liquid absorbent dryer (24) and an optional outlet of a vacuum pump (60) of the liquid absorbent dryer (24).

17. The method according to claim 16, wherein exothermic heat is released inside the moisture collection system (23), wherein the exothermic heat is reused as heat source for the drying process by the product dryer (22), for example by transferring energy from outgoing dilute liquid absorbent (45) to the outgoing dry air (7), or as a direct contact conduction heat source for and / or below a surface (11) of a product conveyor (37) of the product dryer (22).

18. The method according to claim 16 or 17, including reclaiming heat from the absorption process inside the conditioner (28) as heat source for the drying process inside an enclosure (2) of the product dryer (22).

19. The method according to claim 18, including using a heat transfer loop having a liquid heat carrying medium, for example water or a refrigerant, in the reclaiming of the heat the absorption process.

20. The method according to any of claims 16-19, wherein processes inside the liquid absorbent dryer (24) are near adiabatic, in particular only requiring 5 to 25% of energy normally required per unit mass of evaporated water and preferably less that the latent heat of water and in particular less than a usually required 1.2 to 2.5 times this amount of energy.

21. The method according to any of claims 16-20, wherein the drying process itself is not equipped with any external thermal energy supply.

22. The method according to any of claims 16-21, wherein the product is a liquid or solid substance, the product that is fed to the dryer in particularly not being a gas or gas mixture.

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