Note: Descriptions are shown in the official language in which they were submitted.
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.
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.
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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.
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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.
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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.
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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.
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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) -
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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.
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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
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~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 -
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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
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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
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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.
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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~
