CA1272587A - Absorbent composition for refrigeration and heating systems - Google Patents
Absorbent composition for refrigeration and heating systemsInfo
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- CA1272587A CA1272587A CA000496119A CA496119A CA1272587A CA 1272587 A CA1272587 A CA 1272587A CA 000496119 A CA000496119 A CA 000496119A CA 496119 A CA496119 A CA 496119A CA 1272587 A CA1272587 A CA 1272587A
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- refrigeration
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- absorbent
- refrigerant
- mpl
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Abstract
ABSORBENT COMPOSITION FOR REFRIGERATION AND HEATING
ABSTRACT OF THE DISCLOSURE
The invention provides a working fluid for an absorption refrigeration, cooling or heating system comprising a mixture of dimethyl formamide and N-methyl-2-pyrrolidone as the absorbent therein.
ABSTRACT OF THE DISCLOSURE
The invention provides a working fluid for an absorption refrigeration, cooling or heating system comprising a mixture of dimethyl formamide and N-methyl-2-pyrrolidone as the absorbent therein.
Description
~ ~'7~ 5 ~
.
The present invention relates to a working fluid for use in absorption refrigeration, cooling or heating systems.
More particularly the present invention relates to a novel absorbent composition having improved performance properties.
Absorption refrigeration systems as well as absorption heat pumps and absorption cooling systems including a liquid absorbent which remains in liquid form throughout the operation cycle and a refrigerant having both a liquid phase and a vapor phase, wherein the refrigerant is dissolved in the absorbent and, in well known thermodynamic steps, is successively boiled off in a generator, condensed9 evaporated and reabsorbed into a weak solution of the absorbent, are well known in the art.
With advancing absorption technology and, e.g., the ever increasing demand for air-conditioning and more sophisticated refrigeration machines, there is a continuing need and search for improved working fluids in general, and for improved absorbent materials in particular.
As is known, conventional refrigeration machines have primarily been operated with either ammonia water or water-lithium bromide solutions.
Ammonia-water refrigeration systems, however, suffer from the following known disadvantages:
a) Water is a relatively volatile absorbent, therefore, to obtain pure refrigerant vapors for proper operation of the machine, it is necessary to separate vapors by rectification;
b) Ammonia is a toxic gas. Its use in household applications is consequently prohibited in some countries and limited to very small machines in others;
c) Ammonia-water is uns~ble at temperatures above ~140C.
~ ~'7~ 5 ~7 d) Because of safety limitations, ammonia cannot be used for refrigeration in motorized vehicles~
e) For refrigeration in direct-contact cooling applications e.g., freezing desalination, ammonia cannot be used with water or aqueous solutions (e.g. sea water).
Similarly, water-lithium bromide systems have the following known disadvantages:
a) Because water is the refrigerant, it is not possible to obtain sub-zero (C) tempera~uresO
b) Because of crystallization of LiBr from aqueous solutions, operation must be several degrees above 0C.
c) ~ater-LiBr solutions are very viscous, therefore operation of the absorbent in the system is problematic, because of the required energy of pumping.
d) For direct-contact applications, the same problems exist as mentioned before with regard to ammonia water systems.
To overcome the disadvantages of ammonia-water and water-LiBr solutions, several working fluids have been proposed, such as:
1) Methanol-LiBr which permits obtaining sub-zero temperatures, but which is unstable above 120C. Furthermore9 the same viscosity problem exists as mentioned above and the refrigerant is both toxic and flammab]e.
.
The present invention relates to a working fluid for use in absorption refrigeration, cooling or heating systems.
More particularly the present invention relates to a novel absorbent composition having improved performance properties.
Absorption refrigeration systems as well as absorption heat pumps and absorption cooling systems including a liquid absorbent which remains in liquid form throughout the operation cycle and a refrigerant having both a liquid phase and a vapor phase, wherein the refrigerant is dissolved in the absorbent and, in well known thermodynamic steps, is successively boiled off in a generator, condensed9 evaporated and reabsorbed into a weak solution of the absorbent, are well known in the art.
With advancing absorption technology and, e.g., the ever increasing demand for air-conditioning and more sophisticated refrigeration machines, there is a continuing need and search for improved working fluids in general, and for improved absorbent materials in particular.
As is known, conventional refrigeration machines have primarily been operated with either ammonia water or water-lithium bromide solutions.
Ammonia-water refrigeration systems, however, suffer from the following known disadvantages:
a) Water is a relatively volatile absorbent, therefore, to obtain pure refrigerant vapors for proper operation of the machine, it is necessary to separate vapors by rectification;
b) Ammonia is a toxic gas. Its use in household applications is consequently prohibited in some countries and limited to very small machines in others;
c) Ammonia-water is uns~ble at temperatures above ~140C.
~ ~'7~ 5 ~7 d) Because of safety limitations, ammonia cannot be used for refrigeration in motorized vehicles~
e) For refrigeration in direct-contact cooling applications e.g., freezing desalination, ammonia cannot be used with water or aqueous solutions (e.g. sea water).
Similarly, water-lithium bromide systems have the following known disadvantages:
a) Because water is the refrigerant, it is not possible to obtain sub-zero (C) tempera~uresO
b) Because of crystallization of LiBr from aqueous solutions, operation must be several degrees above 0C.
c) ~ater-LiBr solutions are very viscous, therefore operation of the absorbent in the system is problematic, because of the required energy of pumping.
d) For direct-contact applications, the same problems exist as mentioned before with regard to ammonia water systems.
To overcome the disadvantages of ammonia-water and water-LiBr solutions, several working fluids have been proposed, such as:
1) Methanol-LiBr which permits obtaining sub-zero temperatures, but which is unstable above 120C. Furthermore9 the same viscosity problem exists as mentioned above and the refrigerant is both toxic and flammab]e.
2) Ammonia-NaS~N solutions, while not requiring rectification, pose the same problems of high viscosity and high costs.
3) Normal hydrocarbons such as butane as refrigerant, and a higher hydrocarbon as absorbent. However, due to safety hazards and low efficiency, this proposal has equally proven to be impractical.
~ ~ 7~ ~ ~7 More recently, the use has been recommended in the literature of commercial halocarbon refriyerants of the Freon family in combination with various absorbents.
Thus, e.g., the following working-fluid pairs with the hereinafter mentioned advantages and disadvantages have been suggested:
1) R22 (difluorochloromethane) and a dimethyl ether of tetraethylene glycol (El~ relatively good solubility, no need for rectification and stable at high temperatures, however, suffers from high cost of E181 and high viscosity impaired absorption and increased pressure drops);
2) R22 and di-butyl phthalate (DBP) - no rectification necessary, however has the disadvantage of relatively poor efficiency, very high viscosity and high cost of DBP;
3) R22 and dimethyl formamide (DMF) - good solubility (efficiency), low viscosity, low cost of DNF, but relatively low volatility of DMF - makes rectification or at least dephlegmation of vapors necessary.
~) R22 and mixture of DBP - DMF, the authors expected to over-come the disadvantages oF R22-DBP and R22-DMF, and to benefit from the advantages of both. However, what has so far been reported in the literature is merely the thermodynamics of this mixture as a working fluid.
While selecting new working fluids for absorption refrigeration operated by low thermal-potential energy sources, the properties of the pure refrigerant and pure absorbent are important, the properties of their solutions are even more decisive, determining as they do the ability of the systems to provide desirable performance characteristics of the refrigeration machines~ Several properties of the working fluids have to be considered, such as their chemical stability, toxicity9 corrosivity, and tendency to exhibit negative deviations from Raoult's law.
Although with respect to R22 as the refrigerant of choice most mentioned in the recent literature, the dimethyl ether of tetraethyleneglycol (DMETEG) has so for been considered to be the best absorbent in terms of the overall properties of its solutions as a working fluid in an absorption refrigeration machine, it was nevertheless found desirable to use a cheaper and more extensively used solvent than DMETEG.
Thus, e.g. one finds various suggestions for novel absorbents and working fluids in U.S. Patents 4,005,584; 4,042,524; 4,072,027;
~ ~ 7~ ~ ~7 More recently, the use has been recommended in the literature of commercial halocarbon refriyerants of the Freon family in combination with various absorbents.
Thus, e.g., the following working-fluid pairs with the hereinafter mentioned advantages and disadvantages have been suggested:
1) R22 (difluorochloromethane) and a dimethyl ether of tetraethylene glycol (El~ relatively good solubility, no need for rectification and stable at high temperatures, however, suffers from high cost of E181 and high viscosity impaired absorption and increased pressure drops);
2) R22 and di-butyl phthalate (DBP) - no rectification necessary, however has the disadvantage of relatively poor efficiency, very high viscosity and high cost of DBP;
3) R22 and dimethyl formamide (DMF) - good solubility (efficiency), low viscosity, low cost of DNF, but relatively low volatility of DMF - makes rectification or at least dephlegmation of vapors necessary.
~) R22 and mixture of DBP - DMF, the authors expected to over-come the disadvantages oF R22-DBP and R22-DMF, and to benefit from the advantages of both. However, what has so far been reported in the literature is merely the thermodynamics of this mixture as a working fluid.
While selecting new working fluids for absorption refrigeration operated by low thermal-potential energy sources, the properties of the pure refrigerant and pure absorbent are important, the properties of their solutions are even more decisive, determining as they do the ability of the systems to provide desirable performance characteristics of the refrigeration machines~ Several properties of the working fluids have to be considered, such as their chemical stability, toxicity9 corrosivity, and tendency to exhibit negative deviations from Raoult's law.
Although with respect to R22 as the refrigerant of choice most mentioned in the recent literature, the dimethyl ether of tetraethyleneglycol (DMETEG) has so for been considered to be the best absorbent in terms of the overall properties of its solutions as a working fluid in an absorption refrigeration machine, it was nevertheless found desirable to use a cheaper and more extensively used solvent than DMETEG.
Thus, e.g. one finds various suggestions for novel absorbents and working fluids in U.S. Patents 4,005,584; 4,042,524; 4,072,027;
4,172,043 and 4,251,382 issued to Allied Chemical Corporation in recent years.
EP-A-0062516 discloses a composition for absorption refrigeration comprising a fluorinated hydrorcarbon (refrigerant), an amide or glycol ether (absorbent) and 0.05 to 0.5 weight ~ of a phosphite (stabiliser).
EP-A-0030127 discloses an absorption refrigerant composition cornprising 1,1,1,2-tetrafluoroethane and an organic solvent therefor, e.g. at least one of tetraethylene glycol dimethyl ether, dimethyl formamide and methyl ethyl ketone.
EP-A-0124079 discloses N-methyl pyrrolidone as a solvent and a refrigerant, such as 1~1.1 trifluoroethanol.
After years of research and development, only partially described hereinafter, it has been surprisingly discovered that an unexpected synergistic effect is achieved when dimethyl formamide (DMF) is admixed with N-methyl 2-pyrrolidone (MPL) to form a binary absorbent and accordingly the present invention provides a working fluid for an absorption refrigeration9 cooling or heating system comprising mixtures of dimethyl formamide and N-methyl-2-pyrrolidone as the absorbent therein.
Within the framework of the search for a safe, non-toxic refrigerant with high chemical stability for absorption units powered by low-grade heat sources, volatile, hydrogen-containing halogenated aliphatic hydrocarbons were found to be preferable, especially difluoromonochlormethane.
.i : ,., : .
- ' ?, .~ = ' .
. . , ~ 7 Initially, the search for refrigerant-absorbent pairs was extended to mostly polar absorbents, some of them selected from the group of esters of phosphoric and phosphonic acids. It should be emphasized that this group of absorbents has not been considered for refrigeration applications prior to the present investigation. In selecting the latter, it was expected that their ability to form hydrogen bonds through the oxygen as well as the phosphor atoms could contribute towards a negative deviation from Raoult's law.
Furthermore, these materials generally have high boiling points, in ~e range 200-400C:
Dimethyl methylphosphonate (DMNP) 181C
Trimethyl phosphate (TMP) 197C
Triethyl phosphate (TEP) 215C
Tributyl phosphate (TBP) 289C
Tricresyl phosphate (TCP) 410C
All these materials are chemically stable. Their stability in prolonged contact with the refrigerant at temperatures up to 120C has been tested by comparing their infrared and nuclear magnetic resonance spectra before and after contact. No traces of decomposition could be detected and the spectra in all cases were identical.
All of these compounds are used extensively in industry and are less expensive than DMETEG, The only toxic substance in this group is o-tricresyl phosphate, while the m- and p- derivatives as well as the trialkyl phosphates are practically nontoxic. In addition, these solvents are noncorrosive with respect to metals, although they may plasticize some polymeric materials at high temperatures.
~ 5 ~
Evaluation of potential combinations of refrigerant-absorbents, including absorbents from the esters of phosphoric and phosphonic acids was carried out on the basis of the considerations stated above. Since there are no data available in the literature on the vapor-liquid equilibria of these combinations (and hence of the ~thalpy-concentration diagrams, which are necessary for evaluation of the performance characteristics of absorption-refrigeration installations operating with these combinations), experimental determination of these equilibria was required. With difluorochloromethane (R22) as refrigerant, the vapor-liquid equilibrium relationships were measured over a broad range of pressures, temperatures and concentrations of its solutions with N-methyl-~-caprolactam (MCL), N-methyl-2-pyrrolidone (MPL), triethyl phosphate (TEP), tri-n-butyl phosphate (TBP), trimethyl phosphate (TMP), triallyl phosphate (TAP); 1,3-dichlorobenzene (DCB), hexamethylphosphoric triamide (HMPA) and dimethyl methylphosphonate (DMMP) Furthermore, the data for the pair R22-dimethyl ether of tetraethylene glycol (DMETEG), was redetermined. Among these, the system R22-DCB exhibited a positive deviation from Raoult's law and hence would not be applicable for absorption refrigeration. The absorbents TAP and HMPA wer~
unstable, so the pairs R22-TAP and R22-HMPA. too~would not be suitable for refrigeration uses.
Among the absorbents investigated were representatives of groups of different chemical nature, N-methyl-2-pyrrolidone (MPL)- boiling point 203C; methyl e-caprolactam (MCL) -boiling point 235.8C, dimethyl methylphosphonate (DMMP) -~ ~ 7~ S ~'~
b.p. 190C, dimethyl ether of tetraethyleneglycol (DMETEG),b.p. 275.3C and dimethyl formamide (DMF), b p 152C.
Combinations of R22 with these absorbents eliminates the safety problems encountered with ammonia-water systems and the crystallization and corrosion problems of water-lithium bromide systems. With the first three absorbents mentioned, no substantial rectification is envisaged. A
further advantage of R22 is its low solubility in water, which permits its use in direct-contact heat exchange for cooling and heating purposes, resulting in a reduction of the cost of the absorption unit.
Selection of candidate refrigerant-absorbent pairs for absorption refrigeration and heat pumping is based on cycle analysis, from which the following performance characte-ristics can be derived: a) coefficient of performance (COPp), for refrigeration, defined as the ratio of the heat extracted from the evaporator (QE) to the energy input in the generator (QG) plus the energy consumed by the solution pump (Wp):
QE
coP
R QG ~ WP ( 1 ) b) circulation ratio (f), defined as the ratio of mass flow rate of the concentrated solution (ms) to the mass flow rate of the pure refrigerant (mr), ms f mr (2) The ai~ of the cycle analysis of the absorption system was evaluation of the highest COP and the lowest f ob~ainable under different generator temperatures and constant tempe-ratures of heat rejection (condensing and absorbing) for various working fluids. For heat pumpsjthe highest COP and the lowest f were considered for different heat distribution temperatures and fixed evaporator temperatures.
~ ~ 7~ 7 For refrigeration and heat pumps, the performance characteristics of the model absorption cycle were evaluated over the following operating conditions: generator temperatures, 60 <TG < 140C; evaporator temperatures, -20 < TE ~ 5C and condenser temperatures, 25 < TG ' 50C. For the combinations R22 with five of the candidate absorbents investigated, namely MPL, DMMP, MCL, DMF and DMETEG, the coefficients of performance and the circulation ratios are presented for refrigeration applica-tion, i.e., TE = 0C, Tc = 30C. The COPR and f curves in Fig. 1 are similar in form for all refrigerant~absorbent combinationsO
All COPR curves have a m;nimum generating temperature below which operation of the cycle is not possible. The best performance characteristics in terms of COPR and f are shown by the combination R22-DMF, followed by R22-MPL, R22-DMMP, R22-DMETEG, and finally by R22-MCL. In the higher range of generator temperatures the f curves for R22-DMMP practically merge with those for R22-DMETEG, and the COPHp curves for R22-DMETEG
coincide with the R22-MCL curves.
A very important consideration in the design of absorption units is the cost of the heat source and of the circulation pump. If the heat is inexpensive, the system should be designed so as to minimize the circulation ratio at the possible expense of a lower COP, to minimize the cost of the pump.
Since the energy requirements for the desorption process in the generator depend on the absolute value and on the temperature dependence of the excess enthalpy of the solutions, better performance characteristics in terms of COPR can be expected with the refrigerant-absorbent combinations for 7~3~ ~
~hich both the absolute value of the excess enthalpy and its temperature dependence are small, as is the case with the combination R22 DMF and R22-MPL compared to the other working pairs studied, which exhibited strong temperature dependence.
The required properties of working pairs have ~o b~
derived from the requirements of the specific application and, secondly, from the chosen cycle, the latter being also a function of the chosen working pair.
The cycle analysis has shown that the best performance in terms of COP and f can be achieved with R22-DMF, with DMF exhibiting a chemical affinity towards R22-higher than any other of the investigated absorbents. However, dimethyl formamide has a relatively low boiling point~ i,e., 152C.
To avoid the necessity of using a rectifier, the boiling point of the absorber should be at least 200C higher than that of the reFrigerant and thus even R22-DMF is not a totally satisfactory working fluid.
It was in this context that mixtures of absorbents were tested and that, among the various absorbent mixtures tested, a mixture of MPL and DMF was prepared. It was expected that the performance characteristics, COP and f,of the absorption refrigeration cycle with R22 and the binary absorbent (mixture of DMF and MPL) would be in between the performance characteristics of R22 with each of the absorbents DMF and MPL, as the physical, thermal and transport properties of mixtures are ~ually in between the properties of the single component COPMPL ~ CPmi x < COPDMF
fMPL fmix fDMF -~.~'7~58~
The vapor liquid equilibrium data for the pair R22 and mixtures of MPL and DMF at five different weight fractions, i.e. MPL/DMF 70/30, 60/40, 50/50, 40/60 and 30/70 were experimentally determined. The data was processed to obtain the enthalpy-concentration diagrams, necessary for evaluation of these new working fluids.
The calculated performance characteristics of the mixtures of R22~DMF+MPL were compared with those of R22+DMF and R22+MPL and are shown in the following Table 1.
Table 1 Performance characteristics for R22-MPL, R22-DMF and their mixtures w%MPL 100 70 60 50 40 30 0 __~__ ____ ____ ____ ____ ____ ____ ____ w%DMF 0 30 40 50 60 70 100 =====================================================================
COP .4928 .5230 .5200 .5208 .5226 .5283 .5353 f 5.396 4.191 4.128 4.074 4.1 4.03 4.582 mS kg/hr 375.1 291.3 287.0 283.2 285.0 280.1 318.5 Vs GPM1.443 1.130 1.113 1.101 1.110 1.091 1.245 A In 1 . 466 .989 1.002 1.007 .992 .975 1.389 9 .497 .479 .485 .486 .489 .498 .547 a 590 .603 .610 .612 .614 .623 .646 Tb C 203 177.5 173.5 166 163 160 153 ======================================================================
'..~
, 2 7~ 3~7 The comparison was carried out under the following conditions:
Generator temperature ~90C
Absorber temperature '28C
EvaporaDr coil temperature -3C
In table l, A is the total heat transfer surface (m2) of the inner heat exchangers in the evaporator~mS is the pump mass flow rate in kg/hr; VSGPM is the volumetric -flow rate in gallons per minute;Tb is the boiling temperature of the mixture; g a are the weight fractions of R22 in the solution at the generator and absorber exit respectively.
The evaluation of the performance characteristics of the pure fluids and mixtures are based on cycle analysis.
From the above table and from figure 2 in which COP and f as a function of generator temperature are plotted for the various mixtures of R22 - MPL - DMF as set forth in table 1, it can be seen that while the COP of R22 with the binary mixture of absorbents is in between those of R22 - DMF and R22 - MPL the circulation ratio f of the various synergistic mixtures oF R22 plus DMF and MPL is appreciably lower than that of DMF which itselF is lower than than of MPL.
Furthermore, the total heat transfer area of inner heat exchangers for l kg/hr oF refrigeration in the evaporator is also surprisingly lower than that of MPL or DMF alone. The mass flow rate of the solutions is lower than that of R22-DMF. This allows use of a smaller solution pump.
In addition, as shown in table 1 the new mixtures of R22+DMF+MPL
haue boiling points in the range of 160 to 177.5C which is 7-24.5C
higher than that of R22-~DMF alone so that depending on the ratio of the components and the characteristics of the other equipment used, a rectifier will be unnecessary in most instances. Furthermore, the performance characteristics and the total area of heat transfer of the refrigeration unit is closer to that of R22+DMF while the mass flow rate is lower than that for R22+DMF.
, .
., 2~5~3~7 The above-mentioned indicators are significant for the evaluation of the economic feasibility of the refrigeration unit. The total area of heat transfer and the size of the solution pump are indicative of the investment costs for the refrigeration system and affect the operating costs through capital charges. The general requirement is a reduction of the equipment costs and of power consumption for pumping.
Thus in preferred embodiments of the present invention there is provided a working fluid for an absorption refrigeratin cooling or heating system comprising a mixture of about 30 to 70 DMF w/w% and about 70 to 30 w/w% MPL as the absorbent therein, and difluoromonochloromethane (R22) as refrigerant.
In an especially preferred embodiment of the present invention, there is provided a working fluid for an absorption refrigeration cooling or heating system comprising a mixture of about 40 to 60 w/w%
DMF and about 60 to 40 w/w% MPL as the absorbent therein, and difluoromonochloromethane (R22) as refrigerant.
As can be seen from the above table the most preferred embQdiment of the present invention is a working fluid comprising a mixture of substantially equal amounts of dimethyl foramide and N-methyl-2-pyrrolidone as the absorbent therein, and difluoromonochloromethane as refrigerant.
It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrative examples and that the present invention may be embodied in other specific forms without departing from the essential attributes thereof, and it is therefore desired that the present embodiments and examples be considered in all respects as illustrative and not restrictive, reference being made to the appended claims, rather than to the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein~
EP-A-0062516 discloses a composition for absorption refrigeration comprising a fluorinated hydrorcarbon (refrigerant), an amide or glycol ether (absorbent) and 0.05 to 0.5 weight ~ of a phosphite (stabiliser).
EP-A-0030127 discloses an absorption refrigerant composition cornprising 1,1,1,2-tetrafluoroethane and an organic solvent therefor, e.g. at least one of tetraethylene glycol dimethyl ether, dimethyl formamide and methyl ethyl ketone.
EP-A-0124079 discloses N-methyl pyrrolidone as a solvent and a refrigerant, such as 1~1.1 trifluoroethanol.
After years of research and development, only partially described hereinafter, it has been surprisingly discovered that an unexpected synergistic effect is achieved when dimethyl formamide (DMF) is admixed with N-methyl 2-pyrrolidone (MPL) to form a binary absorbent and accordingly the present invention provides a working fluid for an absorption refrigeration9 cooling or heating system comprising mixtures of dimethyl formamide and N-methyl-2-pyrrolidone as the absorbent therein.
Within the framework of the search for a safe, non-toxic refrigerant with high chemical stability for absorption units powered by low-grade heat sources, volatile, hydrogen-containing halogenated aliphatic hydrocarbons were found to be preferable, especially difluoromonochlormethane.
.i : ,., : .
- ' ?, .~ = ' .
. . , ~ 7 Initially, the search for refrigerant-absorbent pairs was extended to mostly polar absorbents, some of them selected from the group of esters of phosphoric and phosphonic acids. It should be emphasized that this group of absorbents has not been considered for refrigeration applications prior to the present investigation. In selecting the latter, it was expected that their ability to form hydrogen bonds through the oxygen as well as the phosphor atoms could contribute towards a negative deviation from Raoult's law.
Furthermore, these materials generally have high boiling points, in ~e range 200-400C:
Dimethyl methylphosphonate (DMNP) 181C
Trimethyl phosphate (TMP) 197C
Triethyl phosphate (TEP) 215C
Tributyl phosphate (TBP) 289C
Tricresyl phosphate (TCP) 410C
All these materials are chemically stable. Their stability in prolonged contact with the refrigerant at temperatures up to 120C has been tested by comparing their infrared and nuclear magnetic resonance spectra before and after contact. No traces of decomposition could be detected and the spectra in all cases were identical.
All of these compounds are used extensively in industry and are less expensive than DMETEG, The only toxic substance in this group is o-tricresyl phosphate, while the m- and p- derivatives as well as the trialkyl phosphates are practically nontoxic. In addition, these solvents are noncorrosive with respect to metals, although they may plasticize some polymeric materials at high temperatures.
~ 5 ~
Evaluation of potential combinations of refrigerant-absorbents, including absorbents from the esters of phosphoric and phosphonic acids was carried out on the basis of the considerations stated above. Since there are no data available in the literature on the vapor-liquid equilibria of these combinations (and hence of the ~thalpy-concentration diagrams, which are necessary for evaluation of the performance characteristics of absorption-refrigeration installations operating with these combinations), experimental determination of these equilibria was required. With difluorochloromethane (R22) as refrigerant, the vapor-liquid equilibrium relationships were measured over a broad range of pressures, temperatures and concentrations of its solutions with N-methyl-~-caprolactam (MCL), N-methyl-2-pyrrolidone (MPL), triethyl phosphate (TEP), tri-n-butyl phosphate (TBP), trimethyl phosphate (TMP), triallyl phosphate (TAP); 1,3-dichlorobenzene (DCB), hexamethylphosphoric triamide (HMPA) and dimethyl methylphosphonate (DMMP) Furthermore, the data for the pair R22-dimethyl ether of tetraethylene glycol (DMETEG), was redetermined. Among these, the system R22-DCB exhibited a positive deviation from Raoult's law and hence would not be applicable for absorption refrigeration. The absorbents TAP and HMPA wer~
unstable, so the pairs R22-TAP and R22-HMPA. too~would not be suitable for refrigeration uses.
Among the absorbents investigated were representatives of groups of different chemical nature, N-methyl-2-pyrrolidone (MPL)- boiling point 203C; methyl e-caprolactam (MCL) -boiling point 235.8C, dimethyl methylphosphonate (DMMP) -~ ~ 7~ S ~'~
b.p. 190C, dimethyl ether of tetraethyleneglycol (DMETEG),b.p. 275.3C and dimethyl formamide (DMF), b p 152C.
Combinations of R22 with these absorbents eliminates the safety problems encountered with ammonia-water systems and the crystallization and corrosion problems of water-lithium bromide systems. With the first three absorbents mentioned, no substantial rectification is envisaged. A
further advantage of R22 is its low solubility in water, which permits its use in direct-contact heat exchange for cooling and heating purposes, resulting in a reduction of the cost of the absorption unit.
Selection of candidate refrigerant-absorbent pairs for absorption refrigeration and heat pumping is based on cycle analysis, from which the following performance characte-ristics can be derived: a) coefficient of performance (COPp), for refrigeration, defined as the ratio of the heat extracted from the evaporator (QE) to the energy input in the generator (QG) plus the energy consumed by the solution pump (Wp):
QE
coP
R QG ~ WP ( 1 ) b) circulation ratio (f), defined as the ratio of mass flow rate of the concentrated solution (ms) to the mass flow rate of the pure refrigerant (mr), ms f mr (2) The ai~ of the cycle analysis of the absorption system was evaluation of the highest COP and the lowest f ob~ainable under different generator temperatures and constant tempe-ratures of heat rejection (condensing and absorbing) for various working fluids. For heat pumpsjthe highest COP and the lowest f were considered for different heat distribution temperatures and fixed evaporator temperatures.
~ ~ 7~ 7 For refrigeration and heat pumps, the performance characteristics of the model absorption cycle were evaluated over the following operating conditions: generator temperatures, 60 <TG < 140C; evaporator temperatures, -20 < TE ~ 5C and condenser temperatures, 25 < TG ' 50C. For the combinations R22 with five of the candidate absorbents investigated, namely MPL, DMMP, MCL, DMF and DMETEG, the coefficients of performance and the circulation ratios are presented for refrigeration applica-tion, i.e., TE = 0C, Tc = 30C. The COPR and f curves in Fig. 1 are similar in form for all refrigerant~absorbent combinationsO
All COPR curves have a m;nimum generating temperature below which operation of the cycle is not possible. The best performance characteristics in terms of COPR and f are shown by the combination R22-DMF, followed by R22-MPL, R22-DMMP, R22-DMETEG, and finally by R22-MCL. In the higher range of generator temperatures the f curves for R22-DMMP practically merge with those for R22-DMETEG, and the COPHp curves for R22-DMETEG
coincide with the R22-MCL curves.
A very important consideration in the design of absorption units is the cost of the heat source and of the circulation pump. If the heat is inexpensive, the system should be designed so as to minimize the circulation ratio at the possible expense of a lower COP, to minimize the cost of the pump.
Since the energy requirements for the desorption process in the generator depend on the absolute value and on the temperature dependence of the excess enthalpy of the solutions, better performance characteristics in terms of COPR can be expected with the refrigerant-absorbent combinations for 7~3~ ~
~hich both the absolute value of the excess enthalpy and its temperature dependence are small, as is the case with the combination R22 DMF and R22-MPL compared to the other working pairs studied, which exhibited strong temperature dependence.
The required properties of working pairs have ~o b~
derived from the requirements of the specific application and, secondly, from the chosen cycle, the latter being also a function of the chosen working pair.
The cycle analysis has shown that the best performance in terms of COP and f can be achieved with R22-DMF, with DMF exhibiting a chemical affinity towards R22-higher than any other of the investigated absorbents. However, dimethyl formamide has a relatively low boiling point~ i,e., 152C.
To avoid the necessity of using a rectifier, the boiling point of the absorber should be at least 200C higher than that of the reFrigerant and thus even R22-DMF is not a totally satisfactory working fluid.
It was in this context that mixtures of absorbents were tested and that, among the various absorbent mixtures tested, a mixture of MPL and DMF was prepared. It was expected that the performance characteristics, COP and f,of the absorption refrigeration cycle with R22 and the binary absorbent (mixture of DMF and MPL) would be in between the performance characteristics of R22 with each of the absorbents DMF and MPL, as the physical, thermal and transport properties of mixtures are ~ually in between the properties of the single component COPMPL ~ CPmi x < COPDMF
fMPL fmix fDMF -~.~'7~58~
The vapor liquid equilibrium data for the pair R22 and mixtures of MPL and DMF at five different weight fractions, i.e. MPL/DMF 70/30, 60/40, 50/50, 40/60 and 30/70 were experimentally determined. The data was processed to obtain the enthalpy-concentration diagrams, necessary for evaluation of these new working fluids.
The calculated performance characteristics of the mixtures of R22~DMF+MPL were compared with those of R22+DMF and R22+MPL and are shown in the following Table 1.
Table 1 Performance characteristics for R22-MPL, R22-DMF and their mixtures w%MPL 100 70 60 50 40 30 0 __~__ ____ ____ ____ ____ ____ ____ ____ w%DMF 0 30 40 50 60 70 100 =====================================================================
COP .4928 .5230 .5200 .5208 .5226 .5283 .5353 f 5.396 4.191 4.128 4.074 4.1 4.03 4.582 mS kg/hr 375.1 291.3 287.0 283.2 285.0 280.1 318.5 Vs GPM1.443 1.130 1.113 1.101 1.110 1.091 1.245 A In 1 . 466 .989 1.002 1.007 .992 .975 1.389 9 .497 .479 .485 .486 .489 .498 .547 a 590 .603 .610 .612 .614 .623 .646 Tb C 203 177.5 173.5 166 163 160 153 ======================================================================
'..~
, 2 7~ 3~7 The comparison was carried out under the following conditions:
Generator temperature ~90C
Absorber temperature '28C
EvaporaDr coil temperature -3C
In table l, A is the total heat transfer surface (m2) of the inner heat exchangers in the evaporator~mS is the pump mass flow rate in kg/hr; VSGPM is the volumetric -flow rate in gallons per minute;Tb is the boiling temperature of the mixture; g a are the weight fractions of R22 in the solution at the generator and absorber exit respectively.
The evaluation of the performance characteristics of the pure fluids and mixtures are based on cycle analysis.
From the above table and from figure 2 in which COP and f as a function of generator temperature are plotted for the various mixtures of R22 - MPL - DMF as set forth in table 1, it can be seen that while the COP of R22 with the binary mixture of absorbents is in between those of R22 - DMF and R22 - MPL the circulation ratio f of the various synergistic mixtures oF R22 plus DMF and MPL is appreciably lower than that of DMF which itselF is lower than than of MPL.
Furthermore, the total heat transfer area of inner heat exchangers for l kg/hr oF refrigeration in the evaporator is also surprisingly lower than that of MPL or DMF alone. The mass flow rate of the solutions is lower than that of R22-DMF. This allows use of a smaller solution pump.
In addition, as shown in table 1 the new mixtures of R22+DMF+MPL
haue boiling points in the range of 160 to 177.5C which is 7-24.5C
higher than that of R22-~DMF alone so that depending on the ratio of the components and the characteristics of the other equipment used, a rectifier will be unnecessary in most instances. Furthermore, the performance characteristics and the total area of heat transfer of the refrigeration unit is closer to that of R22+DMF while the mass flow rate is lower than that for R22+DMF.
, .
., 2~5~3~7 The above-mentioned indicators are significant for the evaluation of the economic feasibility of the refrigeration unit. The total area of heat transfer and the size of the solution pump are indicative of the investment costs for the refrigeration system and affect the operating costs through capital charges. The general requirement is a reduction of the equipment costs and of power consumption for pumping.
Thus in preferred embodiments of the present invention there is provided a working fluid for an absorption refrigeratin cooling or heating system comprising a mixture of about 30 to 70 DMF w/w% and about 70 to 30 w/w% MPL as the absorbent therein, and difluoromonochloromethane (R22) as refrigerant.
In an especially preferred embodiment of the present invention, there is provided a working fluid for an absorption refrigeration cooling or heating system comprising a mixture of about 40 to 60 w/w%
DMF and about 60 to 40 w/w% MPL as the absorbent therein, and difluoromonochloromethane (R22) as refrigerant.
As can be seen from the above table the most preferred embQdiment of the present invention is a working fluid comprising a mixture of substantially equal amounts of dimethyl foramide and N-methyl-2-pyrrolidone as the absorbent therein, and difluoromonochloromethane as refrigerant.
It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrative examples and that the present invention may be embodied in other specific forms without departing from the essential attributes thereof, and it is therefore desired that the present embodiments and examples be considered in all respects as illustrative and not restrictive, reference being made to the appended claims, rather than to the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein~
Claims (3)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A working fluid for an absorption refrigeration system comprising a mixture of about 30-70 w/w% dimethyl formamide and about 70-30 w/w% N-methyl-2-pyrrolidone as the absorbent therein, and difluoromonochloromethane as refrigerant.
2. A working fluid for an absorption refrigeration cooling or heating system comprising a mixture of about 40-60 w/w% dimethyl formamide and about 60-40 w/w% N-methyl-2-pyrrolidone as the absorbent therein, and difluoromonochloromethane as refrigerant.
3. A working fluid for an absorption refrigeration cooling or heating system comprising a mixture of substantially equal amounts of dimethyl formamide and N-methyl-2-pyrrolidone as the absorbent therein, and difluoromonochloromethane as refrigerant.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| IL73565 | 1984-11-28 | ||
| IL8473565A IL73565A0 (en) | 1983-11-29 | 1984-11-28 | Impact fuse |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1272587A true CA1272587A (en) | 1990-08-14 |
Family
ID=11055496
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000496119A Expired - Fee Related CA1272587A (en) | 1984-11-28 | 1985-11-25 | Absorbent composition for refrigeration and heating systems |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1272587A (en) |
-
1985
- 1985-11-25 CA CA000496119A patent/CA1272587A/en not_active Expired - Fee Related
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