Abstract:
A process for introducing a solution into an evaporative cooling apparatus ( 14 ), said process comprising a) positioning a selectively permeable membrane ( 16 ) between a first solution ( 18 ) and a second solution ( 20 ) having a higher solute concentration than the first solution ( 18 ), such that the solvent from the first solution ( 18 ) flows across the selectively permeable membrane ( 16 ) to dilute the second solution ( 20 ), b) introducing the second solution into an evaporative cooling apparatus ( 14 ) in which solvent is removed from the second solution ( 20 ) by evaporation, and c) recycling the second solution ( 20 ) from step b) to step a) to draw solvent from the first solution ( 18 ).

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method and apparatus for introducing a solution into a cooling apparatus. In particular, although not exclusively, the present invention relates to a method and apparatus for removing heat from a heat source. 
     2. The Prior Art 
     Heat exchangers are often used to remove excess heat from industrial processes. Typical heat exchangers include shell and tube-type heat exchangers, which comprise a length of tubing partially enclosed within a housing or shell. An industrial process stream containing excess heat is introduced into the tubing, whilst a coolant, such as water, is passed through the shell via a separate inlet and outlet. The water removes excess heat from the process stream. Thus, the water exiting the shell is at a higher temperature than the coolant entering the shell. The heated water stream is cooled in a cooling tower before it is recirculated back through the shell. In this way, heat removal can be carried out in a continuous manner. 
     Most cooling towers contain a porous filler material, known as decking (packing). Water is introduced into the top of the cooling tower and drips down through the decking, whilst air is blown through the decking, causing some of the water to evaporate. The loss of heat by evaporation (evaporative cooling) lowers the remaining water temperature. The cooled water is recirculated to the heat exchanger. 
     As evaporation occurs, contaminants, such as dissolved solids, build up in the recirculating water. Such contaminants can cause fouling, for example, as a result of biological growth, scale formation, corrosion and/or sludge deposition. The contaminant level may be reduced by removing a portion of the recirculating water from the system. The removal of water in this manner is known as blowdown. 
     To replace the total water loss from the system, make-up water is introduced. The make-up water is treated with, for example, scale inhibitors, corrosion inhibitors, biocides and dispersants. These additives tend to be expensive and have to be added continuously to the make-up water, adding to the cost of the overall process. 
     The water quality of the cooling system has a significant effect on the thermal efficiency and life of the cooling tower and heat exchangers. 
     In an air-cooler, warm air from the surroundings is blown through wet decking or packing material. Heat from the air is transferred to the wet decking material, causing the water contained in the decking to evaporate. As a result, air emerging from the cooler is at a lower temperature than air introduced into the cooler. As the water evaporates, contaminants in the water may deposit on the decking material. Such deposits have a detrimental effect on the thermal efficiency and life of the air-cooler. 
     SUMMARY OF THE INVENTION 
     According to the present invention, there is provided a process for introducing a solution into an evaporative cooling apparatus, said process comprising 
     a) positioning a selectively permeable membrane between a first solution and a second solution having a higher solute concentration than the first solution, such that the solvent from the first solution flows across the selectively permeable membrane to dilute the second solution, 
     b) introducing the second solution into an evaporative cooling apparatus in which solvent is removed from the second solution by evaporation, and 
     c) recycling the second solution from step b) to step a) to draw solvent from the first solution. 
     Preferably, the evaporative cooling apparatus is a cooling tower or an air-cooler. Suitable air-coolers include air-coolers for domestic and industrial use. 
     The selective nature of the membrane prevents undesirable solute(s) and other containments in the first solution from passing into the second solution. 
     The first solution may be an impure aqueous stream, such as seawater, brackish water, river water and waste streams from, for example, an industrial or agricultural process. When such solutions are used, water is selectively allowed to pass across the membrane to dilute the second solution. 
     The second solution may comprise seawater, brackish water and industrial process streams. Suitable industrial process streams may be derived from, for example, the salty residues of desalination plants, such as thermal desalination and/or reverse osmosis plants, and aqueous effluents, such as those typically employed as make-up water for conventional cooling towers. The seawater, brackish water and industrial process streams employed may be concentrated prior to use. Alternatively or additionally, the seawater, brackish water and/or industrial process streams employed may be concentrated during the course of the process of the present invention, such that the solution in contact with the membrane in step a) has a higher solute (total dissolved salts) concentration than the first solution. For example, the evaporative cooling apparatus may be employed to remove solvent from the second solution by evaporation to produce a concentrated second solution that can be used to draw solvent from the first solution in step a). By recycling the second solution between steps a) and b) in a closed loop, solutes dissolved in the second solution may be “immobilised”. Thus, it may be possible to recycle industrial process streams containing, for example, undesirable impurities, such as toxic and radioactive materials in such a closed loop which isolates the impurities from the surrounding environment. 
     When seawater, brackish water and industrial process streams are used as the second solution, it is desirable to add additives, such as scale inhibitors, corrosion inhibitors, biocides and dispersants to reduce or avoid fouling and corrosion in the process. These additives may be “immobilised” in the system when the second solution is recycled between steps a) and b) in a closed loop. Thus, it may not be necessary to continuously add such additives to the second solution. 
     The second solution may have a known composition. For example, in one embodiment, the second solution is formed by introducing a known quantity of a solute into a known quantity of solvent. Thus, the second solution may consist essentially of a selected solute dissolved in a selected solvent. This second solution may be formed prior to step a). By forming the second solution in this manner, a substantially clean solution may be produced. Thus, the second solution may have a reduced concentration of suspended particles, biological matter and/or other components that may cause fouling of the cooling system. More preferably, the second solution is substantially free of such components. In one embodiment, additives such as scale inhibitors, corrosion inhibitors, biocides and/or dispersants are included in the second solution. The second solution may be recirculated in a closed-loop, for example, such that it is continuously reused in steps a) and b). In such an embodiment, the components of the second solution are effectively “immobilised” within the loop. Thus, once the second solution is formed, further addition of solute and/or additives such as scale inhibitors, corrosion inhibitors, biocides and/or dispersants may not be necessary. 
     The solvent in the second solution is preferably water. 
     The solute (osmotic agent) in the second solution is preferably a water-soluble solute, such as a water-soluble salt. Suitable salts include salts of ammonium and metals, such as alkali metals (e.g. Li, Na, K) and alkali earth metals (e.g. Mg and Ca). The salts may be fluorides, chlorides, bromides, iodides, sulphates, sulphites, sulphides, carbonates, hydrogencarbonates, nitrates, nitrites, nitrides, phosphates, aluminates, borates, bromates, carbides, chlorides, perchlorates, hypochlorates, chromates, fluorosilicates, fluorosilicates, fluorosulphates, silicates, cyanides and cyanates. One or more salts may be employed. In a preferred embodiment, the solute of the second solution is a sodium and/or potassium salt. Thus, the second solution may be formed by dissolving a known amount of a sodium and/or potassium salt in water. In one embodiment, the second solution is formed by dissolving a sodium chloride in water. In a further preferred embodiment the second solution may be a solution of ammonia and carbon dioxide, with resultant aqueous species: ammonium carbonate, ammonium bicarbonate and ammonium carbamates (see WO 02/0608025). The second solution initially used in step a) may have a solute or total dissolved salts (TDS) concentration that is higher than the solute or TDS concentration of the first solution. 
     In step a) of the present invention, the first solution is placed on one side of a semi-permeable membrane. A second solution having a higher solute concentration (and, therefore, a lower solvent concentration) is placed on the opposite side of the membrane. As a result, solvent passes across the membrane from the side of low solute concentration (high solvent concentration) to the side of high solute concentration (low solvent concentration). The flow occurs along a concentration gradient. Thus, high pressures are not required to induce solvent flow. However, a pressure differential across the membrane may be applied, for example, to increase the flux of water. 
     After solvent (e.g. water) from the first solution has passed into the second solution, the second solution may be at an elevated pressure (osmotic pressure when water is used as a solvent), even when a pressure is not applied to induce solvent flow from the first solution to the second solution. This is because the flow of solvent from the first solution into the second solution occurs along a concentration gradient. This pressure may be used to aid the transfer of the second solution to subsequent processing steps of the present invention. This pressure may be sufficient to transfer the second solution to subsequent processing steps, for example, without the aid of pumps. In one embodiment, excess pressure is converted into mechanical work. Thus, the pressure (e.g. osmotic pressure) generated in the second solution may be used to reduce the power consumption and/or increase the heat transfer efficiency of the overall process. 
     In one embodiment, the diluted second solution from step a) may be contacted with one side of a further selectively permeable membrane, whilst a third solution having a higher solute concentration than the diluted second solution is contacted with the other side of the membrane. As the second solution has a higher solvent concentration than the third solution, solvent from the second solution flows across the membrane to dilute the third solution. Like the second solution, the third solution may consist essentially of a selected solute dissolved in a selected solvent. Thus, by repeating steps (a) one or more times, the composition of the solution introduced into the cooling tower may be better controlled. 
     The third and/or subsequent solution may be formed of any of the solutions described above in relation to the second solution. Thus, the solvent in the third and/or subsequent solution is preferably water. 
     The solute (osmotic agent) in the third and/or subsequent solution is preferably a water-soluble solute, such as a water-soluble salt. Suitable salts include salts of ammonium and metals, such as alkali metals (e.g. Li, Na, K) and alkali earth metals (e.g. Mg and Ca). The salts may be fluorides, chlorides, bromides, iodides, sulphates, sulphites, sulphides, carbonates, hydrogencarbonates, nitrates, nitrites, nitrides, phosphates, aluminates, borates, bromates, carbides, chlorides, perchlorates, hypochlorates, chromates, fluorosilicates, fluorosilicates, fluorosulphates, silicates, cyanides and cyanates. One or more salts may be employed. In a preferred embodiment, the solute of the third and/or subsequent solution is a sodium and/or potassium salt. Thus, the third and/or subsequent solution may be formed by dissolving a known amount of a sodium and/or potassium salt in water. In one embodiment, the third and/or subsequent solution is formed by dissolving a sodium chloride in water. In another embodiment the third/and or subsequent solution may be a solution of ammonia and carbon dioxide, with resultant aqueous species: ammonium carbonate, ammonium bicarbonate and ammonium carbamates (see WO 02/060825). 
     The third and/or subsequent solution may be contain the same solute(s) and solvent(s) as the second solution. It may also be possible to use different solutions as the second, third and/or subsequent solutions. 
     In one embodiment, additives such as scale inhibitors, corrosion inhibitors, biocides and/or dispersants are included in the third and/or subsequent solution. As will be described in further detail below, the third and/or subsequent solution may be recirculated in a closed-loop, for example, such that it is continuously reused in steps a) and b). In such an embodiment, the components of the third and/or subsequent solution are effectively “immobilised” within the loop. Thus, once the third and/or subsequent solution is formed, further addition of solute and/or additives such as scale inhibitors, corrosion inhibitors, biocides and/or dispersants may not be necessary. 
     In step b), the second solution is introduced into an evaporative cooling apparatus. The cooling apparatus preferably comprises supporting material from which solvent (e.g. water) can evaporate. The supporting material is preferably porous and may advantageously have a large surface area. The supporting material may be made from plastic, metal, ceramic and natural materials, such as wood. 
     In use, the second solution is contacted with the supporting material. A gas, such as air, may then be passed through the wet supporting material causing the solvent of the second solution to evaporate. Depending on the relative temperatures of the solution and the gas, the temperature of either the solution or the gas is reduced as a result of the evaporative cooling. The cooled solution or gas may be used as a coolant, for example, to remove heat from a heat source or to cool the surrounding atmosphere. 
     Solution emerging from step b) is recycled in step a). The second solution from step b) may be directly recycled to step a) to draw the solvent from the first solution. Alternatively, the second solution may be recycled to step a) after one or more intermediate steps. For example, the second solution from step b) may be used to remove heat from a heat source prior to being recycled to step a). In one embodiment, the second solution is used as a coolant in a heat exchanger prior to being recycled to step a). It may be possible to recirculate the second solution, for example, in a closed loop. Optionally, additional components, such as solvents, solutes and additives selected from, for example, scale inhibitors, corrosion inhibitors, biocides and/or dispersants may be added to the closed loop. 
     Examples of suitable evaporative cooling apparatuses include cooling towers and air-coolers, such as air-coolers for domestic and industrial use. 
     Air-coolers typically comprise a housing containing a porous filler material (e.g. decking or packing). The second solution is introduced into the air-cooler and wets the filler material. When warm air from the surroundings is blown through the filler material, some of the solvent (e.g. water) of the second solution evaporates. The loss of heat by evaporation (evaporative cooling) lowers the temperature of the air. Thus, the temperature of the air emerging from the air-cooler is lower than that of the air introduced into the air-cooler. The air emerging from the air cooler may be used as a coolant for, for example, a heat exchanger. Alternatively, the emerging air may be used to cool an enclosed space, such as a room. 
     Cooling towers typically contain a porous filler material, known as decking (packing). The second solution is introduced into the top of the cooling tower and drips down through the decking, whilst a coolant, such as air, is blown through the decking, causing some of the solvent of the second solution to evaporate. The loss of heat by evaporation (evaporative cooling) lowers the temperature of the remaining second solution. 
     As evaporation occurs, the concentration of the second solution increases. If contaminants are present in the second solution, they may be at least partially removed by removing a portion of the second solution entering the cooling tower (e.g. as a bleed). This removal is known as blowdown. 
     Any suitable cooling tower may be employed in the process of the present invention. Examples of suitable cooling towers include natural draft and mechanical draft cooling towers. 
     After step a), the second solution may be used to remove excess heat from a heat source (step d). Thus, according to a preferred embodiment of the present invention, the present invention provides a method for removing heat from a heat source. Step d) may be carried out before and/or after the second solution is introduced into the evaporative cooling apparatus in step b) provided that the second solution used in step (d) is at a lower temperature than the heat source. In one embodiment, step d) is carried out before and/or after the second, solution is cooled in a cooling tower in step b). 
     In one embodiment, the second solution is used as a coolant in a heat exchanger to remove heat from an industrial process stream, such as steam from a power plant. For example, the heat exchanger may be a shell-and-tube-type heat exchanger, which comprises a length of tubing partially enclosed within a housing or shell. The industrial process stream is introduced into the tubing, whilst the second solution is passed through the shell via a separate inlet and outlet. The second solution removes excess heat from the process stream. Thus, the second solution exiting the shell is at a higher temperature than the second solution entering the shell. 
     Once the second solution has been heated in the heat removal step (d), it may be reused in step a). However, if the second solution is reused in step a), the overall concentration of solute in the second solution in contact with the selectively permeable membrane should be higher than the concentration of solute in the first solution, so that solvent from the first solution will pass across the selectively permeable membrane into the second solution. In a preferred embodiment, therefore, the removal of solvent from the second solution is controlled to ensure that the second solution in contact with the selectively permeable membrane has a desired concentration. In one embodiment, the second solution may be cooled prior to reuse in step a) (e.g. in a cooling tower). 
     In step c), the solution used in steps a), b) and, optionally, d) may be recirculated in a closed loop. Optionally, additional components, such as solvents, solutes and additives selected from, for example, scale inhibitors, corrosion inhibitors, biocides and/or dispersants may be added to the closed loop. 
     Any suitable selectively membrane may be used in the process of the present invention. An array of membranes may be employed. Suitable membranes include cellulose acetate (CA) and cellulose triacetate (CTA) (such as those described in McCutcheon et al., Desalination 174 (2005) 1-11) and polyamide (PA) membranes. The membrane may be planar or take the form of a tube or hollow fibre. Thin membranes may be employed, particularly, when a high pressure is not applied to induce solvent flow from the first solution to the second solution. If desired, the membrane may be supported on a supporting structure, such as a mesh support. 
     In one embodiment, one or more tubular membranes may be disposed within a housing or shell. The first solution may be introduced into the housing, whilst the second solution may be introduced into the tubes. As the solvent concentration of the first solution is higher than that of the second, solvent will diffuse across the membrane from the first solution into the second solution. Thus, the second solution will become increasingly diluted and the first solution, increasingly concentrated. The diluted second solution may be recovered from the interior of the tubes, whilst the concentrated first solution may be removed from the housing. 
     When a planar membrane is employed, the sheet may be rolled such that it defines a spiral in cross-section. 
     The pore size of the membrane may be selected depending on the size of the solvent molecules that require separation. It may be possible to use a membrane having a pore size that allows two or more different types of solvent molecules to pass through the membrane. Preferably, the pore size of the membrane is selective to the passage of water. The pore size of the membrane is preferably selected to prevent the flow of solute and other contaminants from the first solution to the second solution. Typical pore sizes range from 1 to 100 Angstroms, preferably 5 to 50 Angstroms, for example 10 to 40 Angstroms. 
     The flow of solvent across a selectively membrane is generally influenced by thermal conditions. Thus, the solutions on either side of the membrane may be heated or cooled, if desired. The solutions may be heated to higher temperatures of 40 to 90° C., for example, 60 to 80° C. Alternatively, the solutions may be cooled to −20 to 40° C., for example, 5 to 20° C. The solution on one side of the membrane may be heated, while the other side cooled. The heating or cooling may be carried out on each solution independently. Chemical reactions may also be carried out on either side of the membrane, if desired. 
     To improve the efficacy of the osmosis step, the first and/or second solution may be treated to reduce fouling and scaling of the membrane. Accordingly, anti-scaling and/or anti-fouling agents may be added to one or both solutions. Although not required, pressure may be applied to the first solution side of the membrane to increase the rate of flux of water across the membrane. For example, pressures of 1×10 5  Pa to 5×10 5  Pa [1 to 5 bar] may be applied, preferably pressures of 2×10 5  Pa to 4×10 5  Pa [2 to 4 bar]. Additionally or alternatively, the pressure on the second solution side of the membrane may be reduced. For example the pressure may be less than 1×10 5  Pa [1 bar], preferably less than 0.5×10 5  Pa [0.5 bar]. 
     The viscosities of the first solution and/or the second solution may also be modified to improve the rate of flux across the membrane. For example, viscosity modifying agents may be employed. 
     The process of the present invention may further comprise a, pre-treatment step of removing contaminants, such as suspended particles and biological matter, from the first solution. Additionally or alternatively, a threshold inhibitor to control scaling may be added to the first solution. Pre-treatment steps to alter the pH of the first solution may also be employed. When seawater is used as a feed, it is preferable to use a deep sea intake, as deep seawater typically contains fewer contaminants. 
     According to a further embodiment of the present invention, there is provided an apparatus for introducing a solution into an evaporative cooling apparatus, said apparatus comprising 
     a housing comprising a selectively permeable membrane for separating a first solution from a second solution having a higher solute concentration than the first solution, said membrane being configured to selectively allow solvent to pass from the first solution-side of the membrane to the second solution-side of the membrane, 
     an evaporative cooling apparatus, and 
     means for removing second solution from the housing, and 
     means for introducing the second solution into the evaporative cooling apparatus. 
     The apparatus of the present invention may further comprise a heat exchanger. 
     These and other aspects of the present invention will now be described with reference to the accompanying drawings, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an apparatus according to a first embodiment of the present invention, 
         FIG. 2  is a schematic diagram of an apparatus according to a second embodiment of the present invention, 
         FIG. 3  is a schematic diagram of an apparatus according to a third embodiment of the present invention, and 
         FIG. 4  is a schematic diagram of an apparatus according to a fourth embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 1 , there is provided an apparatus  10  for producing a cool stream of air. 
     The apparatus  10  comprises a housing  12  and an air cooler  14 . The housing  12  comprises a selectively permeable membrane  16  for separating seawater  18  from a solution  20  formed by dissolving a known amount of sodium chloride in water. 
     In use, seawater  18  is circulated through the housing  12  on one side of the membrane  16 , whilst sodium chloride solution  20  is circulated through the housing  12  on the opposite side of the membrane  16 . The sodium chloride solution  20  in contact with the membrane  16  has a higher total dissolved salt (solute) concentration than the seawater  18 . Thus, water flows from the seawater-side of the membrane  16  to the solution-side of the membrane  16  by osmosis. 
     The flow of water across the membrane  16  dilutes the sodium chloride solution  20 . The diluted solution  20  is removed from the housing  12  and is introduced into the air cooler  14 . The air cooler  14  contains a porous filler material (not shown). The solution  20  is introduced into the top of the air cooler  14  and wets the porous material. 
     When warm air  22  from the surroundings is blown through the wet porous material, heat from the air  22  is transferred to the wet porous material, causing water in the solution  20  to evaporate. As a result, the air  24  emerging from the cooler  14  is at a lower temperature than the air  22  introduced into the cooler  14 . The emerging air  24  may be used to cool an enclosed space, such as a room. 
     As water evaporates from the solution  20 , the solution  20  becomes more concentrated. This concentrated solution  20  is removed from the air cooler  14  via line  26  and recirculated to the solution-side of the membrane  16  in housing  12  in a closed loop. The concentration of the solution  20  in contact with the membrane  16  is higher than that of the seawater  18  on the other side of the membrane  16 . 
     The apparatus of  FIG. 2  is similar to the apparatus of  FIG. 1 . Thus, like numerals have been used to designate like parts. Unlike the apparatus of  FIG. 1 , however, the apparatus of  FIG. 2  comprises two housings  12   a  and  12   b  are used in series. 
     The first housing  12   a  comprises a selectively permeable membrane  16   a  for separating seawater  18  from a solution  20   a  formed by dissolving a known amount of sodium chloride in water. The second housing  12   b  comprises a selectively permeable membrane  16   b  for separating solution  20   a  from the first housing  12   a  from a solution  20   b  formed by dissolving a known amount of sodium chloride in water. 
     In use, seawater  18  is circulated through the housing  12   a  on one side of the membrane  16   a , whilst sodium chloride solution  20   a  is circulated through the housing  12   a  on the opposite side of the membrane  16   a . The sodium chloride solution  20   a  in contact with the membrane  16  has a higher total dissolved salt (solute) concentration than the seawater  18 . Thus, water flows from the seawater-side of the membrane  16  to the solution-side of the membrane  16  by osmosis. 
     The flow of water across the membrane  16   a  dilutes the sodium chloride solution  20   a . The diluted solution  20   a  is circulated through the housing  12   b  on one side of the membrane  16   b , whilst sodium chloride solution  20   b  is circulated through the housing  12   b  on the opposite side of the membrane  16   b . The sodium chloride solution  20   b  in contact with the membrane  16   b  has a higher total dissolved salt (solute) concentration than the solution  20   a . Thus, water flows across the membrane  16   b  by osmosis to dilute the sodium chloride solution  20   b . The diluted solution  20   b  is introduced into an air cooler  14  in the manner described with reference to  FIG. 1 . As water flows across the membrane by osmosis, the sodium chloride solution  20   a  becomes increasingly concentrated and this is recirculated to housing  12   a.    
     In  FIG. 3 , there is provided an apparatus  100  for removing heat from an industrial process stream. 
     The apparatus  100  comprises a housing  110 , a heat exchanger  112  and a cooling tower  114 . The housing  110  comprises a selectively permeable membrane  116  for separating seawater  118  from a solution  120  formed by dissolving a known amount of sodium chloride in water. 
     In use, seawater  118  is circulated through the housing  110  on one side of the membrane  116 , whilst sodium chloride solution  120  is circulated through the housing  110  on the opposite side of the membrane  116 . The sodium chloride solution  120  in contact with the membrane  116  has a higher total dissolved salt (solute) concentration than the seawater  118 . Thus, water flows from the seawater-side of the membrane  116  to the solution-side of the membrane  116  by osmosis. 
     The flow of water across the membrane  116  dilutes the sodium chloride solution  120 . This diluted solution  120  is introduced into the cooling tower  114 . The cooling tower  114  contains a porous filler material, known as decking (not shown). The solution  120  is introduced into the top of the cooling tower  114  and drips down through the decking, whilst cool air  126  is blown through the decking, causing some of the water from the solution  120  to evaporate. The loss of heat by evaporation (evaporative cooling) lowers the temperature of the remaining solution  120 . The remaining solution, however, is more concentrated than the solution entering the cooling tower  114  because of the loss of water by evaporation. 
     The cooled solution  120  is introduced into the heat exchanger  112 . In the heat exchanger  112 , the solution  120  used as a coolant to remove heat from an industrial process stream  124 . Heat from the stream  124  is transferred to the solution  120  through the walls of the heat exchanger  112 . Thus, the temperature of solution  120  is increased. 
     The solution  120  is withdrawn from the heat exchanger  124  via line  128  and reintroduced to the solution-side of the membrane  116  in a closed loop. The concentration of the solution  120  in contact with the membrane  116  is higher than that of the seawater  118  on the other side of the membrane  116 . 
     The apparatus of  FIG. 4  is similar to the apparatus of  FIG. 3 . Thus, like numerals have been used to designate like parts. Unlike the apparatus of  FIG. 3 , solution  120  from the housing  110  is introduced into the heat exchanger  112  before it is introduced into the cooling tower  114 .