[0001] The present invention relates to freeze concentration systems and more particularly, to indirect freeze concentration systems working in accordance with reversible heat pump cycle.

DESCRIPTION OF THE BACKGROUND ART

[0002] Freezing technology is generally well known for generating substantially pure water from its parent mother liquor/aqueous solution. Freezing technology involves freeze concentration of aqueous solution to obtain crystallized water in the form of ice and thereafter, separating the ice from the mother liquor/aqueous solution. A prominent benefit that could be derived out of the freezing technology is to reduce the volume that is to be handled during storage, transportation and sale. Applications of freezing technology could be easily found in beverage industries and specifically, in desalination systems to generate substantially pure water in the form of ice upon freezing of seawater.

[0003] Common freezing methods include direct, vacuum, and indirect freezing. In direct freezing method, refrigerant comes directly in contact with the aqueous solution, for example seawater, thus eliminating the cost of a heat and mass transfer surface. Evaporation of refrigerant cools the seawater below its freezing point to form ice crystals of substantially pure water. Thereafter, the refrigerant is condensed for reuse. This method has the advantage of lower energy consumption of the order of 8 to 9 kWhJm ³ .

[0004] In vacuum freezing method, the surface of seawater is maintained at a temperature lower than triple point. This causes evaporation of water and selective freezing of water from the seawater. The generated vapor is compressed and condensed with the help of ice frozen from seawater which is in the ice chamber. The combined liquid due to condensation of vapor and melted ice is used as potable water. The main energy required in the process is for compressing the vapor, however, in order to do so design of the compressor becomes very complex owing to the large specific volume of water vapour, which is about 20,600 m ³ per ton of water.

[0005] However, both direct as well as vacuum freezing methods suffer from a major drawback that is separation of ice from mother liquor. The formed ice crystals, which are generally flat in shape and smaller in size, are generated as a suspension in the mother liquor/brine and therefore are highly prone to be contaminated by wetting with mother liquor/brine. Generally, it becomes really difficult to remove the traces of the mother liquor/brine. A common method to remove the traces of the mother liquor/brine is to wash the same with water in a decanter/wash column thereby causing significant melting loss which reduces the yield.

[0006] In order to address the above drawbacks, an indirect freeze concentration scheme is generally used in which the refrigerant and the aqueous solution are separated by heat transfer surfaces. The refrigerant and aqueous solution on opposite sides of heat transfer surface exchanges heat which result in formation of ice layer on heat transfer surfaces. However, such indirect freezing techniques also suffer from some major drawbacks that need to be addressed by industries using such freeze concentration systems. [0007] One of the major problems related to the heat exchangers used in such freeze concentration system that are open and are exposed to atmosphere. Due to this open structure of the heat exchanger, a significant amount of inclusion/solute of the feed stream is included in the layer if ice is formed on the heat transfer surface. In open structure heat exchanger as the flow of solution is due to gravity, this limits the feed velocity and the mixing/turbulence.. This thus has lower heat and mass transfer coefficients. Further, lower velocity results in increase of inclusion of solute in the frozen layer.

[0008] Another drawback is that power consumed in such freeze concentration systems is high. Generally, power driven mechanical scrapers are used to remove the ice layer formed on the heat transfer surfaces. Such scrapers add extra load on the cooling unit that increases overall power consumption of the freeze concentration system. Some freeze concentration systems use double heat exchangers operably connected with each other and operating on the principle of heat pump. In this system, ice is formed in one heat exchanger while a pre-formed frozen ice, formed during a previous heat pump cycle, is melted simultaneously in another heat exchanger. However, such systems use two compressors arrangement that are connected in parallel with each other and fluidly connected with the evaporator side that is first heat exchanger.

[0009] The first compressor compresses a low pressure refrigerant coming out from the first heat exchanger so that its heat could be rejected at the second heat exchanger for melting the pre-formed ice therein. Thus, the heat is rejected at nearly 0°C. Whereas, the second compressor compresses the refrigerant to ambient temperature to reject extra heat acquired by the low pressure refrigerant, in the process of pre-cooling the incoming feed stream within the first tube-tube heat exchanger, compressor power and the heat gain from ambient. Thus, this heat is rejected slightly above ambient heat sink. The second compressor thus operates at higher pressure ratio than the first compressor, which increases its power consumption and increases extra heat load. Further, higher pressure ratio results in lower volumetric efficiency.

[0010] Thus, there is a need to have a freeze concentration system that addresses at least some of the above mentioned drawbacks. SUMMARY OF THE INVENTION

[0011] Disclosed herein is an indirect freeze concentration system operating in accordance with a reversible-heat pump cycle including a first tube-tube heat exchanger receiving a selectively supplied feed stream and a selectively supplied low pressure low temperature refrigerant, heat exchange between the feed stream and the low pressure low temperature refrigerant enables a portion of the solvent from the feed stream to freeze therein while vaporizing the low pressure low temperature refrigerant, a first compressor disposed to receive and compress the low pressure vapor refrigerant to an intermediate pressure, heat of the intermediate pressure refrigerant being rejected at a temperature slightly higher than that of melting temperature of pre- formed frozen solvent, a second tube-tube heat exchanger having the pre-formed frozen solvent and disposed to receive the intermediate pressure refrigerant, heat exchange between the preformed frozen solvent and the intermediate pressure refrigerant melts the frozen solvent and while partially condensing the intermediate pressure refrigerant, and a second compressor is disposed to receive and compress the vapours of the partially condensed intermediate pressure refrigerant to a high pressure refrigerant, heat of the high pressure refrigerant is rejected to ambient temperature heat sink to obtain a condensed refrigerant, the condensed refrigerant is re-circulated to the first tube-tube heat exchanger.

[0012] In some embodiments, a feed stream regulating device disposed in between the first tube-tube heat exchanger and the second tube-tube heat exchanger to selectively supply the incoming feed stream.

[0013] In some embodiments, on reversing the heat pump cycle, the regulating device enables the incoming feed stream to enter the second tube-tube heat exchanger for freezing of a solvent of the feed stream therein, and wherein the melted form of the pre-formed solvent is received from the first tube-tube heat exchanger. [0014] In some embodiments, a liquid separator connected with the second tube-tube heat exchanger and the second compressor, the liquid separator receiving the partially condensed intermediate pressure refrigerant therein to separate and retain liquid portion therefrom, vapor portion of the partially condensed intermediate pressure refrigerant being received by the second compressor.

[0015] In another embodiment of the present invention, each of the first and the second tube-tube heat exchangers include plurality of tubular conduits arranged adjacent to each other, one of the tubular conduits has adjacent thereto a flow of the feed stream or the frozen solvent whereas, an immediately positioned another tubular conduit carries the low pressure low temperature refrigerant or the intermediate pressure refrigerant therethrough.

[0016] In another embodiment of the present invention, the feed stream and the refrigerant carrying conduits are in physical contact to each other with a thermal boding material applied therebetween, both the physical contact and the thermal boding material act as a heat transfer surface for allowing heat exchange between the feed stream and the refrigerant. [0017] In some embodiments, the first tube-tube heat exchanger further includes a sufficiently cold and concentrated feed stream flowing out from the feed stream carrying conduit, and wherein the second tube-tube heat exchanger further includes a melted form of the preformed frozen solvent flowing out from the corresponding feed stream carrying conduit, both the cold and concentrated feed stream and the molten solvent being introduced into a third tube-tube heat exchanger.

[0018] In some embodiments, an external condenser operates near at ambient heat sink temperature and connected to the second tube-tube heat exchanger and the second compressor, the external condenser allowing heat exchange between the cold concentrated feed stream and the high pressure vaporized refrigerant so as to completely condense the high pressure vaporized refrigerant.

[0019] Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.

[0020] It is to be understood that both the foregoing general description and the following detailed description of the present embodiments of the invention and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention and are incorporated into and constitute a part of this specification. The drawings illustrate various embodiments of the invention and together with the description serve to explain the principles and operation of the invention.

A BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The above-mentioned and other features and advantages of the various embodiments of the invention, and the manner of attaining them, will become more apparent will be better understood by reference to the accompanying drawings, wherein:

[0022] FIG. 1 is a schematic representation of the freeze concentration system according to an embodiment of the present invention; and

[0023] FIG. 2 shows a partial cross-section view of an arrangement of the tubular conduits within a heat exchanger of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS [0024] FIG. 1 illustrates a schematic representation of an indirect freeze concentration system 100 based on a reversible-heat pump cycle according to an embodiment of the present invention. The freeze concentration system 100 primarily includes a first tube-tube heat exchanger 102 and a second tube-tube heat exchanger 104 operably connected with each other. The indirect freeze concentration system 100 has a feed stream 106 supplied therein from a reservoir (not shown) and operates in heat exchange relationship with a refrigerant within the first tube-tube heat exchanger 102 and the second tube-tube heat exchanger 104. The refrigerant circulates within the freeze concentration system 100 in between the first tube-tube heat exchanger 102 and the second tube-tube heat exchanger 104. Further, when the freeze concentration system 100 operates under a normal mode of the heat pump cycle, evaporation and condensation functions are selectively performed at the first tube-tube heat exchanger 102 and the second tube-tube heat exchanger 104, respectively. Furthermore, it is also well known that when the freeze concentration system 100 operates under a reversible mode of the heat pump cycle, evaporation and condensation takes place at the second tube-tube heat exchanger 104 and the first tube-tube heat exchanger 102, respectively. Here, both the first and the second tube-tube heat exchangers are generally latent heat exchangers because of the fact that they handle latent heat duty within the freeze concentration system 100. Additionally, the feed stream 106 may be pumped within the freeze concentration system 100 by a pump operated under lesser power.

[0025] Examples of the feed stream 106 includes contaminated water, sea water, brackish water, industrial waste water, chemical process streams containing salts or other chemicals in suspension or in solution, solutions containing food matter or suspensions in orange juice, coffee, sugarcane juice, milk and their like. In various embodiments of the present invention discussed in the foregoing description, the feed stream 106 is an aqueous medium. However, it would be appreciated by a skilled person that the feed stream 106 should not be construed to be limited to aqueous medium only. Rather, non-aqueous mediums are also considered within the scope of the invention.

[0026] A feed stream regulating device 110, which is generally a three-way valve, is disposed within the freeze concentration system 100 and fluidly connected 112 in between the first tube-tube heat exchanger 102 and the second tube-tube heat exchanger 104. The feed stream regulating device 110 switches the flow of feed stream 106 in between the first tube-tube heat exchanger 102 and the second tube-tube heat exchanger 104 depending on the mode in which the reversible heat pump cycle is operating. However, for the sake of bringing in clarity while describing the various embodiments of the invention in the foregoing description, it is assumed that the heat pump cycle is operating in the normal mode. In the normal mode, the feed stream 106 is allowed to enter the first tube-tube heat exchanger 102 for evaporation of the refrigerant and simultaneously freezing of the incoming feed stream 106.

[0027] Constructional features of the tube-tube heat exchangers, namely the first tube- tube heat exchanger 102 and the second tube-tube heat exchanger 104 are shown in FIG. 2 will now be described herein in detail. Each of the first and the second tube-tube heat exchangers 102 & 104 has a plurality of longitudinal tubular conduits 114 that may be used for carrying various kinds of fluids therethrough. Further, each of the plurality of tubular conduits 114 is arranged adjacent to each other in such a manner that outer surfaces 116 of any two of the adjacently placed tubular conduits are in physical contact with each other. Preferably, a thermal bonding material 118 is also applied in between the two adjacently disposed tubular conduits. Both the physical contact as well as the thermal bonding material 118 acts as a heat transfer surface 120 to allow heat exchange between any two fluids passing through the two adjacently disposed tubular conduits 122 and 124, respectively. Preferably, each of the first tube-tube heat exchanger 102 and the second tube-tube heat exchanger 104 has therein at least two longitudinal tubular conduits defining one heat transfer surface 120. Usage of the tube-tube heat exchanger allows efficient freezing of the solvent of the feed stream 106 and reduce the chances of solute inclusion during formation of ice. . This owes to the design of the tube-tube heat exchangers that allows effective freezing of the solvent. The plurality of the tubular conduits 114 has an elongated structure of sufficient length to provide adequate surface area for freezing of the solvent during heat exchange between the incoming feed stream 106 and a low pressure refrigerant 126.

[0028] As shown in FIG.l, once the feed stream 107 is received by the feed stream regulating device 110, the regulating device switches the flow of the feed stream 107 to the first tube-tube heat exchanger 102 (shown by dark and bold lines). The incoming feed stream 107 enters one of the tubular conduits 122 whereas, an immediately positioned another tubular conduit 124 has a flow of a low pressure low temperature refrigerant 126 flowing therein. The refrigerant circulates within the system 100 in accordance with the heat pump cycle operating in normal mode. Prior to entering the first tube-tube heat exchanger 102, the low pressure low temperature refrigerant 126 was being expanded through low pressure expansion valve 128. The low pressure expansion valve 128 is suitably connected 130 with the tubular conduit 124 for carrying the low pressure low temperature refrigerant 126 within the first tube-tube heat exchanger 102.

[0029] In the first tube-tube heat exchanger 102, heat exchange between the incoming feed stream 106 and the low pressure low temperature refrigerant 126 takes place via the heat transfer surface 120 that has the bonding material 118 applied therebetween. The heat exchange allows the feed stream 107 to cool by rejecting its heat to the low pressure low temperature refrigerant present in the adjacent tubular conduit 124. As this process continues, the incoming feed stream 107 reaches to a state where a solvent, which is generally the water component of the feed stream 107, freezes on an inner surface 132 of the tubular conduit 122 receiving the incoming feed stream (See FIG. 2). Simultaneously, the low pressure low temperature refrigerant 126 vaporises and becomes hot and reaches to a state where it is transformed into a low pressure vaporised refrigerant 134. A relatively cold concentrated feed stream 136 is received from the outlet of the first tube-tube heat exchanger 102. This freezing technique across the heat transfer surface 120 will be clear to a skilled person by considering the example described below where water (solvent) freezes from the seawater.

[0030] In freezing technology, seawater is cooled below its freezing temperature, which is a function of its salinity. As is very well known, the seawater broadly includes a solvent component, which is water, and a solute component, which is salt. As the salt concentration in normal seawater is 3.5% wt NaCl is much less than its eutectic composition of 23.3% wt NaCl, ice crystals of pure water are formed when it starts freezing by rejecting its heat to the low pressure low temperature refrigerant 126 present in the adjacent tubular conduit. Further, the latent heat of freezing and the cooling heat load of the seawater convert the low pressure low temperature two-phase refrigerant 126 to the low pressure low temperature refrigerant vapour 134. The ice crystals are formed as a layer of frozen ice on the inner surface 132 of tubular conduit 122 of the first tube-tube heat exchanger 102. Once the water starts freezing, concentrated brine leaves from the first tube-tube heat exchanger 102. Typically, this ice formation is limited to 50% of the feed stream and therefore the brine leaving out from the first tube-tube heat exchanger 102 is twice in concentration of inlet feed.

[0031] The elongated tubular structure of the first tube-tube heat exchanger 102 allows the incoming feed stream 106 to flow at a higher velocity because of possible feed flow at higher pressure and its close passage (shown in FIG. 2). Such high velocity cannot be achieved with prior art heat exchangers because of open structures. In the embodiments of the present invention, at higher flow rate of the incoming feed stream 107, the turbulence in the feed stream 107 increases and due to which the heat and mass transfer coefficient also increases. This reduces heat and mass transfer area required for first tube-tube heat exchanger 102. Further, this also reduces the chances of solute inclusion in the ice layer formed. Inclusions are the amount of solute present in the frozen solvent. As a result of high velocity and resultant turbulence average distribution coefficient and the inclusion reduces. Again, nearly pure frozen solvent is separated out from the incoming feed supply. This frozen solvent is adheres to the inner surface 132 of the tubular conduit 122 and as the freezing process continues, thickness of the frozen solvent goes on increasing and the process continues till temperature and pressure of refrigerant decreases to the lower set values. Thus, a skilled person in the art would recognize that frozen solvent obtained out from such tube-tube heat exchangers 102 & 104 have low inclusions as compared to the heat exchanger in the prior art.

[0032] As shown in FIG. 1, a first compressor 138 is suitably connected 140 with the first tube-tube heat exchanger 102 to receive the low pressure vapour refrigerant 134 therefrom via a first four way valve 142. The low pressure vapour refrigerant 134 is compressed by the first compressor 138 to an intermediate pressure refrigerant 144. The intermediate pressure refrigerant 144 is introduced in the second tube-tube heat exchanger 104 via a second four way valve 146. The second tube-tube heat exchanger 104 has the pre-formed solvent/ice formed on the inner surface 132 of one of the tubular conduits during a previous heat pump cycle working in the reversible mode.

[0033] Within the second tube-tube heat exchanger 104, the intermediate pressure refrigerant 144 exchanges heat with the pre-formed solvent via another heat transfer surface 120 formed in a similar manner as that in the first tube-tube heat exchanger 102 explained above. This allows rejection of heat from intermediate pressure refrigerant vapour 144 to the pre-formed frozen ice and results in melting of preformed solvent 148. The molten solvent 148 taken out from the second tube-tube heat exchanger 104. Further, the intermediate pressure vapour refrigerant 144 is partially condensed within the second tube-tube heat exchanger 104. The complete condensation of low pressure vapour refrigerant 144 is not possible at second tube-tube heat exchanger 104. The maximum heat to be rejected in second tube-tube heat exchanger 104 is equivalent to sensible heating and melting heat load of formed ice on inner surface 132 of conduit 124, however heat to be rejected has heat gain while freezing of feed stream in first tube-tube heat exchanger 102, compressor work at 138 and heat gain by low pressure refrigerant in first tube- tube heat exchanger 102 and second tube-tube heat exchanger 104 from the ambient. It is to be noted that freezing of the solvent of the feed stream 107 in the evaporator and the melting of the frozen solvent in the condenser occurs simultaneously. Modular nature of the first and the second tube-tube heat exchangers 102 and 104 allow easy scaling up capacities of freeze concentration system 100.

[0034] According to another embodiment of the present invention, the molten solvent

148 from the second tube-tube heat exchanger 104 and the cold concentrated feed stream 136 from the first tube-tube heat exchanger 102 is introduced into a third tube-tube heat exchanger 150 via a third four way valve 152. The third tube-tube heat exchanger 150 is disposed in fluid connection 154, with the first and the second tube-tube heat exchangers 102 & 104. Further, the third tube-tube heat exchanger 150 is designed to have three tubular conduits for carrying fluids therein and having two heat transfer surfaces 120 in between the three tubular conduits. The two heat transfer surfaces 120 are formed in a similar fashion as that of the first and the second tube- tube heat exchangers 102 & 104.

[0035] A first tubular conduit 156 is fluidly connected to receive the cold concentrated feed stream/brine coming out from the first tube-tube heat exchanger 102 whereas, a second tubular conduit 158 is fluidly connected with the molten solvent 148 coming out from the second tube-tube heat exchanger 104. A middle tubular conduit 160 receives the incoming feed stream 106 from the reservoir and transports the same to the fluid regulating device 110 which in turn allows the fed stream 107 to either of the first or the second tubular heat exchangers 102 & 104. As the first and the second tubular conduits 156 & 158 carry relatively cold fluids therein as compared to the incoming feed stream 106, which is at least disposed at room temperature, heat exchange between all of the fluids allow the incoming feed stream 106 to pre-cool to the feed stream 107. Thus, embodiments of the freeze concentration system 100 according to the present invention have advantages that it obviates external pre-cooling mechanism. As explained above, this pre-cooled feed stream 107 is supplied to the feed stream regulating device 110.

[0036] As shown in FIG.l, the partially condensed intermediate pressure refrigerant 166 is routed to liquid separator 162 which is suitably connected by 164 through the first four way valve 142, therein liquid refrigerant of intermediate pressure refrigerant is separated and thus it retain its liquid portion. Vapour portion of the partially condensed intermediate pressure refrigerant 166 is introduced into a second compressor 170. The second compressor 170 is fluidly connected 172 with the liquid separator 162 to receive the vapour 168 of the partially condensed intermediate pressure refrigerant 166 and to compress the same to a high pressure refrigerant vapour 174.

[0037] An external condenser 176 is also fluidly connected 178 to the second compressor 170 to receive the high pressure refrigerant vapour therefrom. Further, the external condenser 176 is also connected via a suitable connection with the third tube-tube heat exchanger 150 to receive the cold concentrated feed stream 136 therefrom. The external condenser 176 rejects condensation heat of high pressure refrigerant 174 to concentrated solution 136 coming out of evaporator side. The heat exchange allows the high pressure vaporized refrigerant 174 to be completely condensed therein thereby heating cold concentrated feed stream 136. The complete condensation of high pressure refrigerant 174 can also be done through evaporative cooled condenser, or air cooled condenser. In some embodiments of the present invention, the external condenser may be an additional tube-tube heat exchanger, which has similar constructional features as described above, having at least two tubular conduits and one heat transfer surface.

[0038] It will be observed that the pressure ratio of the second compressor 170 is usually greater than the pressure ratio of the first compressor 138. This is because of the fact that the first compressor 138 compresses the low pressure refrigerant 134 to the intermediate pressure refrigerant 144. Heat of the intermediate pressure refrigerant 144 is rejected in the second tube- tube heat exchanger 104 at slightly above melting point of frozen solvent, i.e. 0°C, i.e., at the melting point of the solvent/ice. Whereas, the second compressor 170 compresses the vaporized intermediate pressure refrigerant 144 to the high pressure vaporized refrigerant 174 and the heat thereof is rejected at nearly the ambient temperature within the external condenser 176.

[0039] However, it will be appreciated by a skilled person in the art that the pressure ratio/power rating of the second compressor 170 in the above embodiments is lower than that of the prior art. This is because the volumetric efficiency and isentropic efficiency increases with reduction in pressure ratio. The present invention uses two stage compressor in place of a single stage compressor described in prior art to reject the excess heat which could not be rejected through melting of the frozen solvent.

[0040] The completely condensed refrigerant 179 from the external condenser 176 is expanded by a high pressure expansion valve 180 and introduced in the liquid separator 162 through a suitable connection so that flash vapour generated is relieved at an intermediate pressure instead of the lower pressure. Compressor work to compress this intermediate pressure vapour is lower than that to compress the low pressure refrigerant. This reduces he total power consumed by the two compressors in the present invention as compared to the two compressors in the prior art. [0041] In the second half of the reversible heat pump cycle, freezing of the feed stream

106 takes place within the second tube-tube heat exchanger 104 whereas the melting of the frozen solvent/ice, pre-formed during the previous cycle as described above in various embodiments, takes place within the first tube-tube heat exchanger 102. This is done by switching the feed stream regulating valve 110 in the other mode that allows the feed stream 106 to be supplied to the second tube-tube heat exchanger 104, as shown by dotted line. The other valves 142, 146 and 152 as mentioned above, are also operated so as to change the direction of the refrigerant flow and due to this the low pressure refrigerant 126 flows through the second tube-tube heat exchanger 104 while the intermediate pressure refrigerant 144 flows through the first tube-tube heat exchanger 102 via the first compressor 138. The partially condensed intermediate pressure refrigerant received by liquid separator 162, the second compressor 170 and the external condenser 176 works in similar manner as explained in the first half of the reversible-heat pump cycle. Thereafter, the low pressure refrigerant 126 is re-circulated to the second tube-tube heat exchanger 104. [0042] In the various embodiments of the present invention noted above, all the fluid connections or fluid connectivity should not be construed to be restricted only to a specific connection known in the art. However, all possible examples, for example pipes, tubes, conduits, or their like, should be considered within the scope of the present invention.

[0043] It will be apparent to those skilled in the art that various modifications and variations can be made to the present invention without departing from the spirit and scope of the invention. Thus it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.