U.S. patent application number 16/195696 was filed with the patent office on 2019-04-18 for eutectic mixtures of ionic liquids in absorption chillers.
The applicant listed for this patent is Yazaki Corporation. Invention is credited to Stefan Maat, Amirhossein Mehrkesh, George G. Tamas.
Application Number | 20190113257 16/195696 |
Document ID | / |
Family ID | 60411664 |
Filed Date | 2019-04-18 |
United States Patent
Application |
20190113257 |
Kind Code |
A1 |
Mehrkesh; Amirhossein ; et
al. |
April 18, 2019 |
EUTECTIC MIXTURES OF IONIC LIQUIDS IN ABSORPTION CHILLERS
Abstract
This invention relates to using a eutectic mixture of two ionic
liquids, as an absorbent material in an absorption chiller. The
invention provides an absorption chiller comprising a mixture of a
refrigerant and an absorbent, and the absorbent is a eutectic
mixture of two ionic liquids.
Inventors: |
Mehrkesh; Amirhossein;
(Camarillo, CA) ; Maat; Stefan; (Camarillo,
CA) ; Tamas; George G.; (Camarillo, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Yazaki Corporation |
Tokyo |
|
JP |
|
|
Family ID: |
60411664 |
Appl. No.: |
16/195696 |
Filed: |
November 19, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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15607079 |
May 26, 2017 |
10168080 |
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16195696 |
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62341736 |
May 26, 2016 |
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62350968 |
Jun 16, 2016 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
Y02A 30/27 20180101;
C09K 5/00 20130101; C07D 213/18 20130101; F25B 15/006 20130101;
C09K 5/04 20130101; Y02P 20/10 20151101; F25B 15/06 20130101; Y02B
30/62 20130101; C07D 233/54 20130101; F25B 15/002 20130101; C09K
5/02 20130101; F25B 15/025 20130101; C09K 5/047 20130101 |
International
Class: |
F25B 15/02 20060101
F25B015/02; C09K 5/02 20060101 C09K005/02; C09K 5/04 20060101
C09K005/04; C07D 213/18 20060101 C07D213/18; F25B 15/00 20060101
F25B015/00; F25B 15/06 20060101 F25B015/06; C07D 233/54 20060101
C07D233/54; C09K 5/00 20060101 C09K005/00 |
Claims
1. An absorption chiller comprising a mixture of a refrigerant and
an absorbent, and the absorbent is a eutectic mixture of two ionic
liquids, wherein the two ionic liquids have a concentration ratio
at the eutectic point of the eutectic mixture.
2. The absorption chiller of claim 1, wherein the refrigerant is
water or ethanol.
3. The absorption chiller of claim 1, wherein the absorbent has a
melting point of 290 K or less.
4. The absorption chiller of claim 1, wherein the mixture of the
refrigerant and the absorbent has a viscosity lower than 15
centistokes.
5. The absorption chiller of claim 1, wherein the two ionic liquids
have enthalpies of fusion within 20 kJ/mol.
6. The absorption chiller of claim 1, wherein the two ionic liquids
have melting points within 50K.
7. The absorption chiller of claim 1, wherein the two ionic liquids
have the same cation.
8. The absorption chiller of claim 7, wherein the cation is
imidazolium or guanidinium.
9. The absorption chiller of claim 8, wherein the two ionic liquids
are hexamethylguanidinium acetate and hexamethylguanidinium
dicyanamide.
10. The absorption chiller of claim 1, wherein the two ionic
liquids have the same anion.
11. The absorption chiller of claim 10, wherein the anion is
bromide, chloride, acetate, or dimethylphosphate.
Description
[0001] This application is a continuation of U.S. application Ser.
No. 15/607,079, filed May 26, 2017; which claims the benefit of
U.S. Provisional Application Nos. 62/341,736, filed May 26, 2016,
and 62/350,968, filed Jun. 16, 2016. The contents of the
above-identified applications are incorporated herein by reference
in their entireties.
TECHNICAL FIELD
[0002] This invention relates to an absorption chiller comprising a
eutectic mixture of two ionic liquids as the absorbent
material.
BACKGROUND
[0003] Absorption chillers are designed to generate cooling
(chilling) effect by means of generating chilled water which can be
used to extract heat from an air flow (e.g. in an air conditioning
system). Absorption chillers create a chilling effect by going
through a complete absorption-refrigeration cycle. The simultaneous
heat and mass transfer of the refrigerant to and from its mixture
with the absorbent is the main mechanism of producing the chilling
effect in an absorption chiller. The absorbent in the system should
have a great tendency towards the refrigerant by dissolving it
readily under the operating conditions of the system. The
absorption process will make it possible for the system to work at
sub-atmospheric pressures (between 0.01-0.1 atm for a water-based
absorption chiller) leading to the evaporation of the refrigerant
at much lower temperatures than its normal boiling point.
[0004] In absorption chillers, the need for an electricity
consuming part (i.e. a compressor) to pressurize the refrigerant is
addressed through the use of an appropriate absorbent. Latent heat
is consumed for the evaporation of the refrigerant, which provides
a means of chilling. The low pressure in the evaporator provides
the benefit of easy evaporation of the refrigerant (i.e. liquids
evaporate easier at lower pressures), thereby making the system
capable of producing a chilling effect at low temperatures.
However, the very low pressure of the evaporator makes the
condensation process of the vapor phase (in order for the cycle to
be continued) more challenging. This is where an efficient
absorbent is needed to thoroughly absorb the refrigerant vapor
(which previously has been cooled by releasing latent heat to a
cooling water stream) and to change it back into the liquid
phase.
[0005] Like any other chemical/physical system, absorption chillers
have their own drawbacks and limitations. Certain factors such as
the crystallization of the absorbent in the system, or the heat
loss from different compartments of the system, can make the system
deviate from the ideal performance predicted by thermodynamic-based
models. The benefits and drawbacks of conventional absorption
chillers are described as follows.
Benefits of an Absorption Chiller:
[0006] Low electricity cost--The only electricity consuming part in
the system is a relatively small pump, which is used to circulate
the absorbent-refrigerant mixture within the system. This fact
makes absorption chillers an ideal choice for countries which do
not have well developed infrastructures for the generation of
electricity. [0007] It is a closed system in which almost no
refrigerant (commonly water) is wasted. [0008] Ability to work in
both dry and humid climates.
Drawbacks:
[0009] Water-lithium bromide (LiBr) salt is a commonly used
refrigerant-absorbent (working) pair in absorption chillers. LiBr
is a very efficient absorbent for water refrigerant due to its high
hygroscopicity. LiBr, which as a pure salt has a melting
temperature of 552.degree. C., can absorb water to a high enough
degree such that it becomes completely dissolved in the water it
has absorbed.
[1]
[0010] However, absorption chillers working with LiBr absorbent can
only operate within a relatively narrow range of the concentration
of LiBr in water. The process is impaired if the solution of LiBr
in water is either too concentrated or too dilute. On the one hand,
a very low amount of water is insufficient to keep LiBr in the
liquid phase due to the high melting point of LiBr (552.degree.
C.), causing the absorbent to crystallize out of the liquid working
pair [2]. On the other hand, a very high amount of water (too
dilute of a solution) will completely cover and solvate the
Li.sup.+ cations and Br anions, disturbing the capability of the
system to work continuously and efficiently. A narrow (.about.5%)
change in LiBr concentration in the water (from .about.57% LiBr/43%
water in the diluted stream to .about.62% LiBr/38% water in the
concentrated stream) is typically required to produce an acceptable
amount of cooling load while preventing the solution from being too
concentrated or too dilute.
[0011] Another drawback of LiBr salt as an absorbent is its
corrosiveness, necessitating the use of costly corrosion inhibitors
and copper piping. Due to the corrosive nature of LiBr and the
involved control procedures needed to avoid its crystallization
within the system, there is a need for absorption chillers having
less problematic absorbent materials compared with LiBr.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 illustrates an absorption chiller.
[0013] FIG. 2 shows the crystallization behavior of a mixture of
two ionic liquids, Bmim Br and EPy Br.
[0014] FIG. 3 shows the crystallization temperature vs. mass
percent of the pure Bmim Br, pure EPy Br, and the eutectic mixture
of the two ionic liquids in the ethanol refrigerant.
[0015] FIG. 4 shows the reaction scheme for synthesis of
N-[chloro(dimethylamino)methylene]-N,N-dimethylchloride (4MeUCl)
from 1,1,3,3-tetramethylurea (4MeUrea) and oxalyl chloride
(OxalylCl).
[0016] FIG. 5 shows the reaction scheme for synthesis of
N,N,N',N',N'',N''-hexamethylguanidinium chloride (6MeGuaCl) from
4MeUCl in presence of 1:2 excess N,N-dimethyltrimethylsilylamine
(TMSN2Me), using extra-dry THF as solvent.
[0017] FIG. 6 shows the reaction scheme for synthesis of
N,N,N',N',N'',N''-hexamethylguanidinium acetate (6MeGuaOAc) from
6MeGuaCl via a metathesis reaction in presence of equimolar amount
of silver acetate.
DETAILED DESCRIPTION OF THE INVENTION
[0018] The inventors have discovered that substituting a
conventional absorbent with a eutectic mixture of two ionic liquids
in an absorption chiller avoids the crystallization of the
absorbent in the system and therefore improves its efficiency.
[0019] FIG. 1 illustrates the schematic of an absorption chiller.
An absorption chiller is a machine that utilizes a heat source
(e.g., direct flame, hot water, steam, solar energy, waste heat
etc.) to drive a cooling process. A mixture of a refrigerant and an
absorbent is present in the absorber compartment and the generator
compartment of the system.
[0020] The present invention relates to a mixture of a refrigerant
and a eutectic mixture of two ionic liquids and the use of the
mixture in an absorption chiller. The present invention provides an
absorption chiller comprising an absorber compartment and a
generator compartment, wherein both compartments comprise a mixture
of a refrigerant and a eutectic mixture of two ionic liquids as an
absorbent.
[0021] In the absorption chiller of the present invention, a
working pair comprises an absorbent, which is paired (dissolved)
with a liquid refrigerant. A refrigerant is a liquid compound used
to undergo evaporation in the evaporator compartment of an
absorption chiller to produce a chilling effect. A refrigerant in
general has appropriate properties for use in such a system, such
as low melting point, low-to-medium boiling point, low toxicity,
low flammability, low corrosivity, low viscosity, high thermal
conductivity, high wettability, and high heat of evaporation. An
absorbent has the role of absorbing the refrigerant vapor in the
absorber compartment and transferring the refrigerant from a vapor
phase to a liquid phase. The generator compartment has the sole
role of transferring a portion of the refrigerant from the liquid
phase (in solution with the absorbent) to the vapor phase (partial
evaporation), thereby performing a vapor-liquid separation
procedure. A pure refrigerant is needed for chilling purposes in
the evaporator compartment, and therefore, needs to be evaporated
from the liquid solution containing the absorbent. The absorbent
material generally has a negative role in the generator
compartment, since it decreases the vapor pressure of the
refrigerant, hindering its evaporation. However, the existence of
absorbent in the generator compartment cannot be avoided due to the
fact that it is dissolved in the refrigerant stream (working pair
solution) incoming from the absorber compartment. An
absorption-refrigeration cycle can be accurately modeled using
fundamental thermodynamics.
[0022] Water is a preferred refrigerant because it is cheap and
readily available. Water is non-toxic, non-flammable, and
non-explosive, and has a relatively high liquid range. Water also
has an exceptionally high enthalpy of vaporization and specific
heat capacity. Due to this combination of properties, water is a
good heat transfer medium for heat exchange purposes.
[0023] However, despite the general suitability of water as a
refrigerant in commercial absorption chillers, it is still
desirable that the operating pressure and temperature of these
systems be reduced, preferably near or at atmospheric conditions.
In this case an organic compound possessing aforementioned
properties may be used instead. Ethanol is another example of a
refrigerant which can be used in the present invention, having
higher volatility than water, which may allow system operation
closer to atmospheric pressure and temperature.
[0024] A eutectic mixture of chemical compounds is generally
defined as a mixture of two chemical substances which do not
interact to form a third chemical component but, at a certain
ratio, inhibit the crystallization process of one another resulting
in a system having a lower melting point than either of the
individual components. A eutectic mixture of two components may be
achieved at a certain concentration ratio of these components. The
eutectic mixtures of compounds are generally desired for their
lower crystallization temperatures, thereby expanding the liquid
range. In eutectic mixtures, the two components will crystallize at
the same time (i.e., as if it were a single substance), without
undergoing a phase separation in which one component partially
crystallizes and partially remains in solution with the other
component. The phase diagram of a eutectic mixture of compounds
will not show regions having one of the constituents present as a
solid phase while the other constituent is still dissolved in a
liquid phase, as in the case of non-eutectic mixtures of two or
more compounds.
[0025] An ionic liquid (IL) is a multi-atomic salt with organic or
inorganic cations and anions, usually defined as having a melting
temperature of 100.degree. C. or lower. In a eutectic mixture of
two ionic liquids (IL.sub.1 and IL.sub.2), ions from the two
compounds are randomly distributed, and the mixture exhibits the
solid-liquid phase change behavior of a single substance (i.e. the
two components in the eutectic mixture melt or solidify together)
without a phase separation. One benefit of a eutectic mixture is
that its melting point is lower than the melting point of its
constituents. An absorbent with a low melting point is desired to
lower the risk of crystallization in the absorption chiller
systems.
[0026] Many ionic liquids (ILs) are not strongly hydrophilic due to
the organic nature of their cations, the larger size of both their
cations and anions compared to water molecules, and the limited
amount of mass-based solubility of water in ionic liquids due to
their relatively large molecular weight. This renders most ILs
unsuitable to use as absorbents with water as the refrigerant in an
absorption chiller, and the identification of suitable ILs for this
purpose is not a simple task.
[0027] For example, 1-butyl-3-methylimidazolium
bis[(trifluoromethyl)sulfonyl]imide, ([Bmim].sup.+[Tf2N].sup.-), is
a well-known ionic liquid with a molecular weight of 419.4 g/mol.
The high molar mass of this ionic liquid means that an equimolar
solution with water (1:1), in which 1 mole of ionic liquid is
dissolved in 1 mole of water, is equivalent to a solution with only
4.11% by mass of water. In this case, the "working pair" of Bmim
Tf2N (absorbent) and water (refrigerant) contains an insufficient
amount of refrigerant (water) to be of practical use in absorption
chillers.
[0028] Instead of using a single IL, the inventors select suitable
ionic liquids to form a eutectic mixture to be used as absorbent in
an absorption chiller based on the following criteria. At least one
of the ionic liquids should be a good water absorbent. The final
mixture of the eutectic mixtures of ionic liquids (EMIL) and water
(i.e., the "working pair" of absorbent and refrigerant) should have
sufficiently low viscosity such that the circulation of the
absorbent-refrigerant mixture within the system does not create an
unreasonable strain on the system components. The crystallization
temperature of the EMIL in the refrigerant should be lower than the
temperature of the working temperature range of the absorption
chiller to avoid the crystallization of EMIL in the system. At
least one of the ionic liquids for use in a eutectic mixture in
absorption chillers should have low viscosity and high solvency
power towards the refrigerant of interest.
[0029] In absorption chillers, the mass basis concentration of the
refrigerant (e.g. water) in the absorbent-refrigerant mixture needs
to be reasonably high. Therefore, suitable ionic liquids for the
present invention in general have low molecular weights, preferably
lower than 350 g/mol, and more preferably lower than 250 g/mol.
[0030] Two ionic liquids that are suitable to form a eutectic
mixture will generally have similar values for enthalpy of fusion
(melting). In one embodiment, the two ionic liquids in a eutectic
mixture have enthalpies of fusion within 20 kJ/mol, or 15 kJ/mol,
or 10 kJ/mol of each other.
[0031] Moreover, two ionic liquids (IL1 and IL2) that have
identical or similar cations (e.g., cations of IL1 and IL2 are
imidazolium, cations of IL1 and Il2 are guanidinium, etc.) or
similar anions (e.g., anions of IL1 and IL2 are bromide, anions of
IL1 and IL2 are chloride, anions of IL1 and IL2 are acetate, anions
of IL1 and IL2 are dimethylphosphate, etc.), are more inclined to
form a eutectic mixture. In addition, cations and anions of ionic
liquids (IL1 and IL2) having dissimilar sizes between their
constituent ions are typically more likely to be paired with the
counterions from another ionic liquid, forming a eutectic mixture
at a certain concentration ratio (e.g., cation of IL1 is dissimilar
in size compared to anion of IL1 and/or cation of IL2 is dissimilar
in size compared to anion of IL2; or cation of IL1 is dissimilar in
size compared to cation of IL2 and/or anion of IL1 is dissimilar in
size compared to anion of IL2).
[0032] In one embodiment, one or both ionic liquids in a eutectic
mixture have dissimilar sizes of anions and cations. For
multiatomic or monatomic cations and anions, Van der Waals radius
is used as an approximate yet well-accepted measure of the size.
For the formation of eutectic mixtures of ionic liquids, it is
preferable to have dissimilar sizes of cations and anions in each
ionic liquid. The inventors have discovered that a ratio of the
size of anion to the size of cation or vice versa is preferably at
least 1.5.
[0033] For example, in the case of the eutectic mixture of
hexamethylguanidinium acetate (IL1) and hexamethylguanidinium
dicyanamide (DCA), the Van der Waals radius of the cation, which is
the same for both ionic liquids, is .about.0.82 nm, and the size of
acetate and dicyanamide anions were calculated to be 0.507 nm and
0.52 nm, respectively. For hexamethylguanidinium acetate (IL1) the
ratio of the size of cation to the size of anion is 1.615 and for
hexamethylguanidinium dicyanamide (DCA) this ratio is 1.57.
[0034] The eutectic mixture of ionic liquids suitable for use as an
absorbent in a water-based absorption chiller (i.e. with water as
the refrigerant) has a high hygroscopic effect comparable to that
of LiBr. The mixture of the eutectic mixture of ionic liquids with
water in the absorbent-refrigerant working pair has a reasonably
low viscosity, and the eutectic mixture exhibits a lower risk of
crystallization due to its lower melting point.
[0035] The eutectic mixture of the present invention typically has
a melting point (T.sub.m) 10 to 30.degree. C. lower than the
T.sub.m of each of the two individual ionic liquids in their pure
form. The eutectic mixture of the present invention typically has a
melting point (T.sub.m) lower than 300 K (27.degree. C.),
preferably lower than 290 K (17.degree. C.), and more preferably
lower than 273 K (0.degree. C.). For example, the melting point of
a eutectic mixture of the present invention is between 250 K and
290 K for a water-based absorption chiller.
[0036] The eutectic mixture of the present invention when mixed
with a proper amount of refrigerant such as water typically has a
kinematic viscosity less than 15 centistokes, preferably less than
10 centistokes.
TABLE-US-00001 TABLE 1 Examples of eutectic mixtures of ionic
liquids. IL.sub.1 IL.sub.2 1-ethyl-3-methylimidazolium acetate
1-ethylpyridinium acetate 1-butyl-3-methylimidazolium acetate
1-butylpyridinium acetate 1-ethyl-3-methylimidazolium
1-ethylpyridinium dimethylphosphate dimethylphosphate
1-butyl-3-methylimidazolium 1-butylpyridinium dimethylphosphate
dimethylphosphate 1-ethyl-3-methylimidazolium bromide
1-ethylpyridinium bromide 1-butyl-3-methylimidazolium bromide
1-butylpyridinium bromide 1-butyl-3-methylimidazolium bromide
1-ethylpyridinium bromide Tetramethylammonium chloride
1-ethyl-3-methylimidazolium chloride Tetraethylammonium chloride
1-ethyl-3-methylimidazolium chloride 1-ethyl-3-methylimidazolium
acetate 1-ethyl-3-methylimidazolium dimethylphosphate
1-butyl-3-methylimidazolium bromide 1-butyl-3-methylimidazolium
chloride hexamethylguanidinium acetate hexamethylguanidinium
dicyanamide
[0037] Table 1 provides examples of eutectic mixtures of ionic
liquids (IL.sub.1 and IL.sub.2) suitable for use in water-based
and/or ethanol-based absorption chillers.
[0038] The eutectic point of two ionic liquids is mainly a function
of the properties of the individual ionic liquids. In normal
situations in which the refrigerant does not react or otherwise
influence the eutectic mixture of ionic liquids, the type of
refrigerant chosen would not affect the eutectic temperature and
eutectic composition.
[0039] The inventors have discovered that hygroscopic ionic liquids
with low molecular weight can resolve the issue of limited
(mass-based) water solubility of ionic liquids. For example,
cations with a high degree of water tendency (e.g.
guanidinium-based cations) can first be functionalized by the
addition of appropriate functional groups having the desired
properties. They can then be combined with a desired anion such as
acetate, which exhibits ultra-high hygroscopic effects and low
melting point, to create a new absorbent with desired
properties.
[0040] Effective eutectic mixtures of ionic liquids can be designed
based on quantum chemistry calculations and thermodynamic-based
models, and then synthesized so that the final absorbent has a
desirable melting point lower than those of the separate individual
ionic liquids. The eutectic mixture can inherit its desired
thermodynamic properties (e.g. high tendency towards water) from
one of its components; for example, guanidinium-based acetate ionic
liquids, and its desired physical properties (e.g. low viscosity)
from the other component, for example, guanidinium-based
dicyanamide ionic liquids.
[0041] The following examples further illustrate the present
invention. These examples are intended merely to be illustrative of
the present invention and are not to be construed as being
limiting.
EXAMPLES
[0042] The thermo-dynamic COP (Coefficient of Performance) of an
absorption chiller is defined as the amount of cooling load
generated in the evaporator, Q.sub.E, (in kilowatt [kW]) divided by
the amount of thermal/heat energy, Q.sub.G, (in kilowatt [kW]) used
to heat up the dilute solution in the generator in order to release
refrigerant vapor. A high COP is desirable meaning that for a given
expense being paid (thermal energy being used), more work (cooling
load) is being delivered. The COP however does not take into
account the quality or cost of the thermal/heat energy Q.sub.G
used.
[0043] The thermodynamic ECOP (Exergetic COP) takes the quality of
heat being used into consideration.
ECOP = COP ( T 0 T E - 1 ) ( 1 - T 0 T h ) ##EQU00001##
Where T.sub.0=298 K is the room temperature, T.sub.E the
temperature in the evaporator and T.sub.h=T.sub.G+ (5 to 10 K) is
the heat source temperature, with T.sub.G being the temperature in
the generator compartment. [2] Use of waste heat or a thermal
stream with a lower temperature (i.e., a lower T.sub.G) will
eventually increase the ECOP pointing towards a more economical
process.
Example 1: Performance Comparison of Guanidinium-Based Ionic
Liquids and a Eutectic Mixture
[0044] In the following example certain thermo-physical properties
of hexamethylguanidinium-based ILs, a eutectic mixture of
hexamethylguanidinium-based ILs, and LiBr are shown. Example 1
demonstrates that the properties of a eutectic mixture of ILs are
advantageous compared to those of a single IL. Further, the
absorptive properties of the eutectic are advantageous compared to
that of LiBr as crystallization is avoided due to the low melting
point of the eutectic mixture of ILs. Moreover, the presented ILs
and eutectic mixture of the ILs in water are less corrosive working
pairs than LiBr in water.
[0045] A Continuum Solvation Model (CSM) based on the concept of
dielectric constant [3-7] was used to predict the solubility
values, the melting point, and the viscosity of
hexamethylguanidinium-based ILs and the eutectic mixture of
hexamethylguanidinium-based ILs. Ab initio calculations using
density functional theory (DFT) were utilized to calculate the
molecular structure/geometry along with the electric charge density
as an input to the CSM calculations. The results of the
computations along with experimental results for LiBr are shown in
Table 2.
[0046] Table 2 further lists the computed COP and ECOP along with
the mass-based concentration of absorbent (Mass % IL) in both
dilute and concentrated streams in an absorption chiller working
with different guanidinium-based ionic liquids and water as working
pairs. Preferably, a eutectic mixture should allow at least a 5%
change in the concentration of water between the generator and
absorber compartments of an absorption chiller.
TABLE-US-00002 TABLE 2 Comparison of the performance of individual
guanidinium-based ionic liquids, the eutectic mixtures, and LiBr as
absorbents with water as the refrigerant. Temperatures of
evaporator, absorber, and condenser are 5.degree. C., 35.degree.
C., and 40.degree. C., respectively. Kinematic Viscosity Mass %
Mass % Of the concen. absorbent absorbent stream (dilute
(concentrated @ 60.degree. C. COP ECOP solution*) solution) T.sub.m
(K) cst Absorbent Predicted Predicted Predicted Predicted Predicted
Predicted Hexaethylguanidinium 0.769 0.304 85.9 90.9 312.75 20.88
Acetate Hexamethylguanidinium 0.768 0.314 82.2 87.2 335.3 10.84
Acetate Hexamethylguanidinium 0.731 0.106 94.1 99.1 316.2 6.07
Dicyanamide (DCA) Hexamethylguanidinium 0.782 0.305 88.97 93.97
377.8 31.80 DMP Hexamethylguanidinium <<0.7 (N/O.sup.#)
<<0.3 (N/O) 98 98 + 5 (N/O) <380 -- Tf.sub.2n LiBr.sup.1
(comparative) 0.775.sup.exp 0.291.sup.exp 57.sup.exp 62.sup.exp
825.sup.exp 2.04.sup.exp Eutectic Mixture: 0.791 0.182 89 94 265
.+-. 15 8.64 .+-. 1.2 [IL.sub.1]: Hexamethylguanidinium Acetate (40
.+-. 10%) [IL.sub.2]: Hexamethylguanidinium Dicyanamide (100 -
[IL.sub.1] %) *"Solution" is a mixture of absorbent (e.g., an IL, a
eutectic mixture of ILs, or LiBr) and a refrigerant liquid (e.g.,
water). A "concentrated solution" has a higher concentration of
absorbent than that of a "dilute solution." .sup.expExperimental
data; .sup.#N/O = Not operable.
[0047] As can be seen from Table 2, a large and hydrophobic anion
such as bis(trifluoromethylsulfonyl)imide (Tf.sub.2N) adversely
impacts the performance of the system due to their low tendency for
water absorption. Use of hydrophobic ionic liquids in absorption
chillers will eventually decrease the water absorption power of the
absorbent, significantly. That means that a lower amount of water
will be absorbed and then released in a one
refrigeration-absorption cycle. This adversely impacts the
performance of the system and lowers the overall COP of the
absorption chillers.
[0048] Table 2 shows that hexamethylguanidinium acetate ionic
liquid can absorb higher amounts of water compared to other ionic
liquids listed (better hygroscopic properties). This ionic liquid
can be diluted up to the point of having .about.18% (wt %) of water
in the solution and still be able to operate at the low pressures,
at which the evaporator compartment is performing (i.e., 0.01 atm).
The absorber and evaporator are interconnected and thus have the
same operating pressure. More dilute solutions of
hexamethylguanidinium acetate in water could be made. However,
water contents of more than 18 wt % generates water vapor pressure
higher than 0.01 atm in the absorber, which decreases the cooling
quality (performance) of the absorption chiller. This is because at
higher pressures of absorber/evaporator, water will evaporate in
the evaporator at higher temperatures than the preferable
<5.degree. C. 5.degree. C. is at the higher end of an acceptable
evaporator temperature in absorption chillers. A low temperature
needs to be maintained in the evaporator in order generate a
chilled water stream (e.g., with a temperature of 8.degree. C. or
lower) suitable for air-conditioning purposes (i.e.,
.about.8.degree. C. or lower). If the temperature of the chilled
water stream rises above this level, the efficiency of the
absorption chiller is substantially reduced. It is noteworthy that
the high amount of water which can be present in the mixture with
hexamethylguanidinium acetate, compared to other ILs listed in
Table 2, will help as well to decrease the viscosity of the ionic
liquid-water (working pair) mixture.
[0049] An example of a eutectic mixture of ionic liquids exhibiting
desired properties as an absorbent in an absorption chiller, as
shown in Table 2, is a mixture of 40.+-.10% (wt %)
hexamethylguanidinium acetate with 60.+-.10% (wt %)
hexamethylguanidinium dicyanamide (DCA) ionic liquids. The eutectic
mixture has a predicted melting point of 265.+-.15 K, which is
significantly lower than the melting point of either
hexamethylguanidinium acetate (335.3 K) or hexamethylguanidinium
dicyanamide (316.2 K). A lower melting point of the eutectic
mixture lowers the risk of crystallization of the absorbent. With
the lower melting point of the absorbent (the eutectic mixture),
the operating conditions of the absorption chiller can be
re-optimized to achieve higher performance indices.
[0050] Table 2 also shows that the kinematic viscosity of the
mixture of a eutectic mixture of hexamethylguanidinium acetate and
hexamethylguanidinium dicyanamide and water in the concentrated
stream (8.64.+-.1.2 cst) is lower than the viscosity of the
concentrated mixtures of several other individual ionic liquids
with water, such as hexaethylguanidinium acetate,
hexamethylguanidinium acetate, and hexamethylguanidinium DMP
(20.88, 10.84, and 31.80 cst, respectively). By inclusion of a
low-viscosity component in the eutectic mixture, in this case
hexamethylguanidinium dicyanamide, the efficiency of the absorption
chilling process is substantially improved, by lowering the pumping
energy needed.
[0051] In summary, the eutectic mixture of hexamethylguanidinium
acetate and hexamethylguanidinium dicyanamide can be used as a less
corrosive absorbent in an absorption chiller using water as a
refrigerant. This eutectic mixture exhibits a low melting point,
lower than that of either pure hexamethylguanidinium acetate or
hexamethylguanidinium dicyanamide. The low melting point is
beneficial in avoiding crystallization of the absorbent. In the
eutectic mixture, the acetate anion is beneficial from the
thermodynamics point of view since it can increase the water
absorption power and dicyanamide anion is beneficial from the
physical properties point of view since it can decrease the
viscosity.
Example 2: Comparison on the Performance of Two Ionic Liquids and
their Eutectic Mixture with a Non-Water Refrigerant (i.e.
Ethanol)
[0052] This example shows how the eutectic mixture of two
bromide-based ionic liquids, 1-butyl-3-methylimidazolium bromide
(Bmim Br) and 1-ethylpyridinium bromide (EPy Br) can be
advantageously used as an absorbent with ethanol as a
refrigerant.
[0053] Here, existence of the same small monoatomic bromide (Br)
anion in the ionic liquids helps the formation of a eutectic
mixture.
[0054] As shown in FIG. 2 by computational analysis, the eutectic
point occurs at a composition of 55 wt % Bmim Br and 45 wt % EPy Br
in the solution with a melting temperature, T.sub.m, of 285K. The
melting point of the ionic liquids mixture at the eutectic point is
significantly lower than that of each ionic liquid individually,
listed in Table 3 below.
TABLE-US-00003 TABLE 3 Properties of ionic liquids Bmim Br, Epy Br,
and their eutectic mixture. Ionic Liquids Melting Point [K] Heat of
Fusion [kJ/mol] Bmim Br 351 22.9 EPy Br 391.3 12.8 Eutectic Mixture
285 17.7 (55% Bmim Br)
[0055] After showing the feasibility of the formation of the
eutectic mixtures of bromide-based ionic liquids, the
crystallization behavior of this eutectic mixture in ethanol
refrigerant was calculated.
[0056] FIG. 3 shows the crystallization temperature (K) vs. the
mass fraction of absorbent (the eutectic mixture of Bmim Br and EPy
Br) in an ethanol refrigerant. As seen in FIG. 3, the
crystallization temperature of the eutectic mixture of these two
bromide based ionic liquids is significantly lower than the
crystallization point of each individual ionic liquid (in their
pure form) when dissolved in ethanol. At the typical concentrations
of absorbent in the absorption chillers (about >60 wt % or
>0.6 mass fraction), the crystallization point of the eutectic
mixture is between 10 and 40.degree. C. lower than that of the pure
ionic liquids in ethanol refrigerant. FIG. 3 shows that the
crystallization curve of the solution of the absorbent (the
eutectic mixture) in the refrigerant is outside the working
temperature region of an absorption chiller. This is desirable
because no crystallization will occur within the system.
Example 3: Synthesis of Hexamethylguanidinium Acetate
[0057] The synthesis of N,N,N',N',N'',N''-hexamethylguanidinium
acetate (6MeGuaOAc) was accomplished through a three-step
protocol.
I. In the first step, 1,1,3,3-tetramethylurea (4MeUrea) was
converted to
N-[chloro(dimethylamino)methylene]-N,N-dimethylchloride (4MeUCl).
This reaction was performed under completely moisture-free
conditions by necessity. UHP argon was used to provide an inert
atmosphere. As shown in FIG. 4, 1,1,3,3-tetramethylurea, in
presence of 1:5 excess amount of oxalyl chloride added dropwise at
0.degree. C., generated 4MeUCl.
[0058] 6.44 mL of 1,1,3,3-tetramethylurea (99%, d=0.969 g/mL) and
40 mL toluene were added to a three-neck round bottom (3rb) flask
and left for 15 minutes under stirring to reach the ice bath
temperature.
[0059] 23.18 mL of oxalyl chloride were slowly added to the
three-neck flask with an automated syringe at a rate of 0.01
mL/min. After the addition was finished, the ice-bath was removed
and the mixture was left to stir for 24 hours at room
temperature.
[0060] A slightly yellow solid product was formed. The flask was
then taken off the condenser and the solvent (i.e. toluene) and the
excess amount of oxalyl chloride were removed in vacuo with the
rotary evaporator set at 55.degree. C. and 25 mbar. The
intermediate was left under high vacuum to fully dry for 48
hours.
II. In the second step, 4MeUCl was converted to
N,N,N',N',N'',N''-hexamethylguanidinium chloride (6MeGuaCl) in
presence of 1:2 excess N,N-dimethyltrimethylsilylamine (TMSN2Me),
using extra-dry THF as solvent, as shown in FIG. 5.
[0061] 4.81 g of 4MeUCl (99%) and 80 mL tetrahydrofuran (THF) were
added to a three-neck round bottom (3rb) flask and left for 15
minutes under stirring to reach the ice bath temperature.
[0062] 9.30 mL (97%, d=0.723 g/cm3) of
N,N-dimethyltrimethylsilylamine (TMSN2Me) were slowly added to the
three-neck flask with an automated syringe at a rate of 0.2 mL/min.
After the addition was finished, the ice-bath was removed and the
mixture was left to stir for 1 hour at room temperature and another
2 hours at 35.degree. C.
[0063] The crude product, a light yellow liquid, was then processed
in a rotary evaporator at 55.degree. C. and 25 mbar for 1 hour and
then kept under high vacuum for another 24 hours to fully remove
any trace amount of solvent. The by-product TMS-Cl, with a boiling
point of 57.degree. C., was removed during this procedure, a
chromatographic purification not being further required.
III. In the third step, 6MeGuaOAc was synthesized from 6MeGuaCl via
a metathesis reaction in presence of equimolar amount of silver
acetate, as shown in FIG. 6.
[0064] 9.24 g of 6MeGuaCl (99%) and 8.58 g of AgOAc (99%,
photosensitive) were charged to a round bottom (rb) flask. To the
rb flask, 150 mL of acetonitrile (ACS grade) were added and then
the setup was connected to a Schlenk line and wrapped in aluminum
foil. The mixture was left to stir for 24 hours at 45.degree. C.
After stirring, the hotplate was turned off and left 15 minutes for
phase separation. AgCl separated out as a gray precipitate on the
bottom of the rb flask. The slurry was gravitationally filtered
through two filter papers. The solvent was removed in vacuo with
the rotary evaporator set at 55.degree. C. and 25 mbar. 100 mL of
acetone were added to the rb and the flask was stored at low
temperature to further allow precipitation of AgCl by-product and
then vacuum filtrated. This cycle was repeated multiple times until
no AgCl was detected. The final product comprising 6MeGuaOAc was
left under high vacuum to fully dry for 48 hours.
[0065] It is to be understood that the foregoing describes
preferred embodiments of the present invention and that
modifications may be made therein without departing from the scope
of the present invention as set forth in the claims.
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