MEMBRANE CONCENTRATOR FOR ABSORPTION CHILLERS

BACKGROUND OF THE INVENTION

[0001] The invention relates generally to the field of absorption chiller systems. More specifically, the invention relates to an absorption chiller system that obviates the high temperature boiler and condenser needed for absorbent solution reconcentration .

[0002] The basic absorption cycle employs a refrigerant and an absorbent. Typically, water is used as the refrigerant and lithium bromide (LiBr) is used as the absorbent. These fluids are separated and recombined during the absorption cycle.

[0003] In the absorption cycle, refrigerant vapor is absorbed into the absorbent releasing a large amount of heat. The concentration of the diluted (weak) absorbent solution may be 55 percent weight or higher. The weak absorbent solution is pumped to a high temperature boiler. The added heat causes the refrigerant in the weak absorbent to desorb from the absorbent and vaporize. The vapors flow to a condenser, where heat is rejected, and condenses to a liquid. The liquid is metered to the lower pressure in the evaporator where it evaporates by absorbing heat and provides useful cooling. The remaining liquid absorbent in the boiler is recombined with the refrigerant vapors returning from the evaporator so the cycle can be repeated.

[0004] Typical absorption chiller designs require a high temperature boiler and condenser operated under deep vacuum in order to boil the weak absorbent solution at a low temperature

such as 80 ⁰ C. A corrosion inhibitor is usually employed to ameliorate the corrosion issue caused by the absorbent solution, which can be very corrosive to metals especially in the high temperature boiler. This has resulted in heavy maintenance requirements for absorption chillers.

SUMMARY OF THE INVENTION

[0005] The inventor has discovered that it would be desirable to have an absorption refrigeration system that does not require a high temperature boiler and condenser to concentrate the weak absorbent solution in an absorption cycle. The invention uses membrane distillation to concentrate the weak absorbent solution back to a normal concentration level using a membrane contactor.

[0006] Absorption refrigeration systems according to this aspect of the invention include an evaporator for cooling a fluid and generating a refrigerant vapor, an absorber carrying an absorbent solution for absorbing refrigerant vapor from the evaporator to produce a refrigerant-absorbent solution (weak absorbent solution) , and a membrane distiller for removing refrigerant from the weak absorbent solution to provide concentrated absorbent solution for the absorber and refrigerant for the evaporator .

[0007] Another aspect of the absorption refrigeration system is where the membrane distiller comprises a membrane contactor.

[0008] Another aspect of the absorption refrigeration system is where the membrane contactor comprises microporous membranes having hydrophobic inner and outer surfaces.

[0009] Another aspect of the absorption refrigeration system is where the microporous membrane material is selected from the group consisting of polypropylene, polyvinylidene difluoride (PVDF) , polytetrafluoroethylene (PTFE) or other thermoplastic polymers .

[0010] Another aspect of the absorption refrigeration system is where the microporous membranes in the membrane distiller are arranged to have the weak absorbent solution flow on one side and the refrigerant flow on the other side of the microporous membranes .

[0011] Another aspect of the absorption refrigeration system further comprises a heater configured to heat the weak absorbent solution to a predefined temperature before entering the membrane distiller and a cooler configured to cool the refrigerant to a predefined temperature before entering the membrane distiller.

[0012] Another aspect of the absorption refrigeration system is where the weak absorbent solution predefined temperature determines a weak absorbent solution vapor pressure and the refrigerant predefined temperature determines a refrigerant vapor pressure.

[0013] Another aspect of the absorption refrigeration system is where the weak absorbent solution vapor pressure is greater than the refrigerant vapor pressure, and refrigerant in the weak absorbent solution vaporizes on one side of the membrane contactor where the weak absorbent solution is circulated and

the vaporized refrigerant is transported through the membrane by vapor pressure difference and condenses on the other side of the membrane contactor where the refrigerant is circulated.

[0014] Another aspect of the absorption refrigeration system is where if the weak absorbent solution vapor pressure is less than or equal to the refrigerant vapor pressure, the weak absorbent solution heater output temperature is increased.

[0015] Another aspect of the absorption refrigeration system is where if the weak absorbent solution vapor pressure is less than or equal to the refrigerant vapor pressure, the refrigerant cooler output temperature is decreased.

[0016] Another aspect of the invention is a method for absorption refrigeration. Methods according to this aspect of the inventions start with circulating an absorbent solution through an evaporator/absorber, generating a refrigerant vapor, absorbing the refrigerant vapor producing a refrigerant-absorbent solution (weak absorbent solution) , circulating the weak absorbent solution and a refrigerant through a membrane distiller, removing the refrigerant from the weak absorbent solution in the membrane distiller, and providing a concentrated absorption solution for the evaporator/absorber .

[0017] The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims .

BRIEF DESCRIPTION OF THE DRAWINGS [0018] FIG. 1 is an exemplary absorption refrigeration machine.

[0019] FIG. 2 is an exemplary absorption refrigeration system using membrane distillation to concentrate a weak absorbent solution.

[0020] FIG. 3 is an exemplary microporous membrane distiller.

[0021] FIG. 4 is a photomicrograph of a single microporous membrane cross section in a partial tube-and-she11 arrangement with other fibers.

[0022] FIG. 5 is a photomicrograph of the microporous membrane wall shown in FIG. 4.

DETAILED DESCRIPTION

[0023] Embodiments of the invention will be described with reference to the accompanying drawing figures wherein like numbers represent like elements throughout. Before embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of the examples set forth in the following description or illustrated in the figures. The invention is capable of other embodiments and of being practiced or carried out in a variety of applications and in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of "including," "comprising," or "having," and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. The terms "mounted, " "connected, " and "coupled, " are used broadly and encompass both direct and indirect mounting, connecting, and coupling. Further, "connected, " and "coupled" are not restricted to physical or mechanical connections or couplings .

[0024] By way of background, absorption refrigeration is a process that is different from compression refrigeration. The absorption process uses heat as a driving force instead of electrical or shaft power.

[0025] FIG. 1 shows a simplified absorption chiller machine 101. The machine 101 includes an evaporator 103 and an absorption section 105.

[0026] The refrigerant 107 in this example is water which is metered into the evaporator section 103. A refrigerant circulating pump 109 circulates the water through spray heads 111 to be sprayed over a chilled water tube bundle 113. This wets the tube bundle 113 through which circulating water from a cooling water system passes. The heat from the system water 113 evaporates the refrigerant 107 to create water vapor schematically illustrated at 115. Water is constantly being evaporated and must be made up.

[0027] In the absorption section 105, the absorbent (LiBr) solution 117 has a lower vapor pressure than that of the evaporated water from the evaporator section 103, and readily absorbs the water vapor 115 into the solution 117. The LiBr solution 117 is recirculated via a LiBr circulating pump 119 through spray heads 121 to give the solution more surface area to attract the water vapor 115. As the solution 117 absorbs water, it becomes diluted. If the water is not removed, the solution 117 will become so diluted that it will no longer have any attraction potential and the absorption process will stop. Another pump 123 constantly removes some of the solution 117 and pumps it to a concentrator 125. The solution that is pumped to the concentrator 125 is referred to as the weak solution because it contains water absorbed from the evaporator 105.

[0028] The concentrator (generator) 125 includes a boiler 127 and a condenser 129. The generator 127 requires a heat source which may be either steam or hot water 131. The condenser 129 requires a stream of cool water usually from a cooling tower system 133. The weak solution is pumped into the concentrator 125 where it is boiled. The boiling action changes the water to

a vapor which leaves the absorbent solution and is attracted to the condenser coils 129. The water vapor is condensed to a liquid where it gathers and is metered back to the evaporator section 103 through an orifice 135. The absorbent solution becomes concentrated 137 and is drained back through line 139 to the absorption section 105 for circulation by the absorbent pump 119.

[0029] The absorption process 101 is simple considering that the only moving parts are the pump motors and pump impellers. The absorption chiller may include more than one stage which results in an absorption machine that is more efficient than a single-stage design.

[0030] FIG. 2 shows an absorption chiller machine 201 that uses a membrane distiller 203 instead of a concentrator 125. The absorption chiller 201 includes an evaporator section 205, an absorber section 207, a weak absorbent solution heater 209, a refrigerant heat exchanger (cooler) 211, a refrigerant circulating pump 221, an absorbent solution circulating pump 212, and an absorbent solution recuperator 213. A recuperator is a special purpose counter-flow heat exchanger that is used to recover waste heat and serves to recuperate or recycle this heat. The absorbent solution used in the exemplary embodiment is LiBr, but other absorbents may be used. The refrigerant used in the exemplary embodiment is water, but other refrigerants may be used.

[0031] In the evaporator 205, a chilled water 215 flows through tubes 217 within the evaporator 205, and the refrigerant 219 circulated by the refrigerant pump 221 is sprayed on the

outsides of the tube bundle 217 from a spray tree 223 so that heat is removed from the chilled water 215 flowing through the tube bundle 217 by evaporating the refrigerant 219.

[0032] In the absorber 207, an absorbent solution 225 having a vapor pressure lower than that of water vapor 227 functions to absorb refrigerant vapor 227 generated from the evaporator 205 at a fairly low temperature. In the absorber 207, the refrigerant vapor 227 evaporated in the evaporator 205 is absorbed by the absorbent solution sprayed for example from a spray tree 229 on the outsides of a cooling pipe 231 of the absorber 207, and the absorption heat generated at that time is cooled by a cooling water 233 flowing through the cooling pipe 231.

[0033] The absorbent solution 225, the concentration of which has been lowered by absorbing refrigerant, is decreased in its absorption capacity (weak solution) .

[0034] The weak absorbent solution 225 is input to the absorbent recuperator 213. The recuperator 213 preheats a mixed absorbent solution 247 formed from concentrated absorbent solution 245 output by the membrane distiller 203 and cooled, weak absorbent solution 226 output by the recuperator 213. The recuperator 213 cools the weak absorbent solution 225 while warming the mixed absorbent solution 247. The mixed absorbent solution 235 is heated in the weak absorbent solution heater 209. The heater 209 is heated to a predefined temperature, for example 85 ⁰ C, using a hot water or steam source 237. The heated weak solution 239 is input to the membrane distiller 203 to concentrate the absorbent solution 239.

[0035] FIG. 3 shows a cut-away view of the membrane distiller 203. The membrane distiller 203 is a membrane contactor having a construction analogous to that of a tube-and-shell exchanger where membrane tubes constructed of hydrophobic microporous membranes are arranged. Since the wall surfaces of the microporous membrane are hydrophobic, the membrane will not allow liquid water to pass through the pores to the opposite sides of the membrane. The microporous membranes, or at least the surface material of the membrane, may be selected from the group of polypropylene, polyvinylidene difluoride (PVDF) , polytetrafluoroethylene (PTFE) or other thermoplastic fluoropolymers .

[0036] Membrane contactors are devices that allow a gaseous phase and a liquid phase to come into contact with one another for the purpose of heat and mass transfer between the phases, without dispersing one phase into the other. The invention uses membrane distillation to concentrate the heated, weak absorbent solution 239. The membrane distillation process employs low temperature heat to vaporization water from the weak absorbent solution on one side of the membrane contactor which may be the shell side, and condenses the vapor on the other, or tube side, of the membrane contactor where refrigerant 241 is circulated.

[0037] FIG. 4 shows a cross section of one tube in the distiller 203. The hot, weak absorbent solution 239 is shown flowing across the "tubes" in the "shell" side. The cold refrigerant 241 is shown flowing within each tube. FIG. 5 shows an enlarged view the tube wall shown in FIG. 4 where the transition from the hot, weak absorbent solution 239, to refrigerant vaporization and refrigerant condensation 243 occurs.

[0038] The microporous membrane has a pore size in the range of from about 0.1 to 0.6 micrometer and a porosity of greater than 50 percent. The membrane acts as a selective barrier between the two phases of the heated weak absorbent solution 239 and the liquid refrigerant. The membrane's surface energy is sufficiently less than the lesser of the weak absorbent solution's surface tension or the refrigerant's surface tension. A typical value is 10 dyne/cm or greater. The driving force of the refrigerant vapor transport through the membrane is by a differential vapor pressure across the membrane.

[0039] The hot, weak absorbent solution 239 may have a 55 percent weight concentration and a corresponding vapor pressure

^V P _de rni _{er veakso} iun _o n ^of < ^for example, 33.33 kPa at 82 ⁰ C. The vapor pressure of the weak absorbent solution 239 must be higher than the refrigerant 241 vapor pressure vp _{dιsllllerrefngeranl} which may be, for example, 3.33 kPa at 32 ⁰ C after passing through the refrigerant cooler 211.

[0040] The differential vapor pressure drives the vapor transport through the membrane pores . The weak absorbent solution side 241 of the membrane is at a temperature high enough to generate a vapor pressure that is higher than that of the refrigerant on the refrigerant side of the membrane.

[0041] The refrigerant temperature t _λ ° _{sllllerrefngeranl} in the membrane distiller 203 determines the refrigerant vapor pressure

^V P _ώsh ii _{er re} f _r i _gerant ■ ^o concentrate the weak absorbent solution , the vapor

pressure of the weak LiBr solution must be greater than the vapor pressure of the refrigerant in the membrane distiller 203.

[ 0042 ] ^V P distiller weak solution ^{> V} P distiller refrigerant ( 1 )

[0043 ] The percent concentration weight of the weak absorbent solution is known, and using the percent concentration weight and weak solution temperature t _ώ ° _{stlllerweaksolulιoπ} in the membrane distiller 203 , the weak solution vapor pressure vp _{ώstlllerweaksolutιm} may be found. The conversions from temperature and concentration, to vapor pressure may be found either using an equation or a memory look-up table.

[0044] The weak solution heater 209 and the refrigerant cooler 211 are configured to output the weak solution 237 and cool refrigerant 241 at predefined temperatures t _d ° _{ιstlller weaksolutlon} , t _d ° _{ιsllllerrefrιgerant} that correspond to predefined vapor pressures vp _{dιslιllerweaksolullon} ,

^V P _d i _st i _llerre f _r i _gerant ^{for a} given capacity absorption refrigeration system, ensuring that the membrane distiller 203 functions to that capacity. If a system perturbation occurs and the vapor pressure relationship (1) is not met, the weak solution heater 209, for example, may be thermostatically controlled such that the weak absorbent solution temperature t _d ° _{lsllllerweaksoluUon} will be increased, in turn increasing the weak absorbent solution vapor pressure VPJai _{ler weaksolut} i _on ■ Conversely, if the relationship (1) is not met, the refrigerant cooler 211, for example, may be thermostatically controlled such that the refrigerant temperature t _{dιsllllerrefngeranl} will be decreased, thereby decreasing the refrigerant vapor pressure

^V P _d eni _{er re} fri _gerant - ^{In thi} s manner, the relationship (1) will be maintained throughout any system perturbation. Control arrangements controlling both the weak solution heater 209 and the refrigerant cooler 211 may be provided.

[0045] The weak absorbent solution after passing through the membrane distiller 203 becomes concentrated 245, recovering its absorption capacity. The recovered absorbent solution 245 is mixed with the weak, heated absorbent solution 226 and is circulated by the absorbent solution circulating pump 212 through the absorbent recuperator 213 and sprayed in the absorber 207 completing the absorbent solution cycle.

[0046] The refrigerant pump 221 circulates the refrigerant 243 through the cooler 211 which is cooled by cooling tower water 233. A portion of the cooled refrigerant 241 is bypassed to the membrane distiller 203 while the remaining portion of the refrigerant is returned to the evaporator 205.

[0047] An absorption chiller machine using the membrane distiller of the invention 203 may have a coefficient of performance (COP) similar to or greater than absorption chillers using the current concentration technology, a boiler 127 and a condenser 129. The coefficient of performance of an absorption chiller system using membrane distillers can be defined by the following equation,

[0049] where Q is the useful heat removed from the incoming chilled stream by the evaporator and Q _1n is the thermal energy- input to the generator. The COP may be greater than 1.0 with multi-effect generation, compared to a COP of 0.7 for a single effect generation.

[0050] One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.