Patent Application
20100155660
This invention relates to mixtures of ammonia and ionic liquids for use as absorption cooling fluids and ammonia storage.
The absorption refrigeration cycle is more than a 100 year old technique. Although the vapor compression cycle took over most of air-conditioning and refrigerating applications, the well-known refrigerant-absorber systems (H2O/LiBr and NH3/H2O) are still being used for certain applications, particularly in the field of industrial applications or large-scale water chiller systems. Recently, more attention has been directed toward recovery of waste heat using the NH3/H2O system (Erickson, D. C. et al, “Heat-Activated Dual-function Absorption Cycle”, ASHRAE Trans. 2004, 110). Inherent drawbacks to using LiBr and NH3 as refrigerants include the corrosiveness of LiBr and the toxicity and flammability of NH3. In the late 1950s, some pioneering works proposed new refrigerant-absorbent pairs for the absorption cycle, using fluoroalkane refrigerants with organic absorbents (Eiseman, B. J., “A Comparison of Fluoroalkane Absorption Refrigerants”, ASHRAE J. 1959, 1, 45; Mastrangelo, S. V. R., “Solubility of Some Chlorofluorohydrocarbons in Tetraethylene Glycol Ether”, ASHRAE J. 1959, 1, 64). Such studies continue actively even at the present time, especially among academic institutions. One drawback to using fluorinated hydrocarbons as refrigerants is the potentially adverse environmental impacts that may result from their use. Needed are new refrigerant-absorber systems.
Room-temperature ionic liquids (RTILs) are a new class of solvents and molten salts with a melting point of less than about 100° C. Because of the negligible vapor pressure, they are often called (environmentally-friendly) “green solvents”, compared with ordinary volatile organic compounds (VOCs). For the past several years, worldwide research on thermodynamic and transport properties of pure RTILs and their mixtures with various chemicals have been conducted. As a new type of solvent with immeasurable vapor pressure, room-temperature ionic liquids are being considered as absorbers with various refrigerants. For instance, Shiflett et al, US 2006/0197053 A1 disclose the use of ionic liquids as absorbents with fluorinated hydrocarbons as the refrigerant in absorption cycles. Although several other refrigerants are mentioned, including the possibility of ammonia, no example or data enabling the possibility were disclosed. Knowledge of solvent phase behaviors is highly important to determine the attractiveness of using ionic liquids in these applications as well as in new applications such as absorption cooling or heating.
Another need is a medium to store and transport volatile materials. Ammonia, for instance, is typically stored in high-pressure cylinders; or in water, as ammonium hydroxide. However, in applications where water, a medium with a significant vapor pressure at room temperature, can not be tolerated, ammonium hydroxide is not a suitable medium for storing ammonia. Conventional adsorbents, such as surface-modified active carbons and ion-exchanged zeolites, have been used for storage of ammonia. However, the ammonia storage capacities are not very high, for instance, for Cu form of Y-zeolite the storage capacity is about 5 millimol of ammonia per gram (Ind. Eng. Chem. Res. 2004, 43, 7484-7491).
Alkaline earth halides and their hydrated forms MgClOH, CaCl2, CaBr2, and SrBr2 have been found to have higher capacities on the order of 25 to 40 millimol per gram (i.e. MgClOH is 26 millmol per gram). One issue with the alkaline earth halides is the adsorption requires heat to completely remove the ammonia from the surface in order to regenerate the solid. For instance, MgCl2—CaCl2 at 298 K adsorbs about 46 millimol of ammonia per gram of solid at 80 kPa; and further increase in pressure results in no further increase in ammonia adsorbed. Release of the pressure and evacuation of the adsorbent, followed by a second adsorption measurement shows far less ammonia can be adsorbed. For example, a second adsorption measurement resulted in 14 millimol of ammonia per gram of solid at the same temperature (298 K) and pressure (80 kPa). This indicates that the absorption process is irreversible even after 1 hour of evacuation to remove all the ammonia from the first adsorption experiment. Needed are mediums that can reversibly store significant quantities of ammonia and also have very low or no vapor pressure themselves.
One aspect of the invention is a composition comprising ammonia and at least one ionic liquid wherein the composition comprises about 1 to about 99 mole % of ammonia over a temperature range from about −40 to about 130° C. at a pressure from about 1 to about 110 bar.
Another aspect of the invention is an absorption cycle comprising a composition of the invention useful for heating or cooling.
Another aspect of the invention is a process for storing ammonia comprising absorbing ammonia in an ionic liquid to provide a composition comprising about 1 to about 99 mole % of ammonia over a temperature range from about −40 to about 130° C. at a pressure from about 1 to about 110 bar.
All patents and patent applications cited herein are hereby incorporated by reference. Herein all trademarks are designated with capital letters.
In this disclosure, a number of terms are used for which the following definitions are provided.
An “alkane” is a saturated hydrocarbon having the general formula CnH2n+2, and may be a straight-chain, branched or cyclic.
An “alkene” is an unsaturated hydrocarbon that contains one or more carbon-carbon double bonds, and may be a straight-chain, branched or cyclic. An alkene requires a minimum of two carbons. A cyclic compound requires a minimum of three carbons.
An “aromatic” is benzene and compounds that resemble benzene in chemical behavior.
A “fluorinated ionic liquid” is an ionic liquid having at least one fluorine on either the cation or the anion. A “fluorinated cation” or “fluorinated anion” is a cation or anion, respectively, comprising at least one fluorine.
A “halogen” is bromine, iodine, chlorine or fluorine.
A “heteroaryl” group is an alkyl group having a heteroatom.
A “heteroatom” is an atom other than carbon or hydrogen in the structure of an alkanyl, alkenyl, cyclic or aromatic compound.
An “ionic liquid” is an organic salt that is fluid at about 100° C. or below, as more particularly described in Science (2003) 302:792-793.
“Optionally substituted with at least one member selected from the group consisting of”, when referring to an alkane, alkene, alkoxy, fluoroalkoxy, perfluoroalkoxy, fluoroalkyl, perfluoroalkyl, aryl or heteroaryl, means that one or more hydrogens on the carbon chain may be independently substituted with one or more of one or more members of the group. For example, substituted C2H5 may, without limitation, be CF2CF3, CH2CH2OH or CF2CF2I.
Ionic liquids can be synthesized, or obtained commercially from several companies such as Merck KGaA (Darmstadt, Germany) or BASF (Mount Olive, N.J.). The synthesis of several ionic liquids useful in the compositions of the invention is disclosed in Shiflett et al, US 2006/0197053 A1.
In one embodiment of the invention, the ionic liquid has a cation, herein defined as Group A Cations, selected from the group consisting of:
wherein R, R1, R7, R8, R9, and R10 are independently selected from the group consisting of:
R2, R3, R4, R5, and R6 are independently selected from R and a halogen;
R11, R12, R13, and R14 are independently selected from R with the proviso that R11, R12, R13, and R14 are not hydrogen; and
wherein, optionally, at least two of R, R1, R2, R3, R4, R5, R6, R7, R8, R9, R10, R11, R12, R13, and R14 can together form a cyclic or bicyclic alkanyl or alkenyl group; and
an anion, herein defined as Group A Anions, selected from the group consisting of [CH3CO2]−, [HSO4]−, [CH3OSO3]−, [C2H5OSO3]−, [AlCl4]−, [CO3]2−, [HCO3]−, [NO2]−, [NO3]−, [SO4]2−, [PO4]3−, [HPO4]2−, [H2PO4]−, [HSO3]−, [CuCl2]−, Cl−, Br−, I−, SCN−, and a fluorinated anion.
In another embodiment, ionic liquids useful for the invention comprise fluorinated cations wherein at least one member selected from R, R1, R2, R3, R4, R5, R6, R7, R8, R9, R10, R11, R12, R13 and R14 comprises one or more fluorines. Included in these materials are fluorinated cations wherein one or more R2, R3, R4, R5, and R6, may be fluorine; and wherein one or more R, R1, R2, R3, R4, R5, R6, R7, R8, R9, R10, R11, R12, R13 and R14 may be an alkyl, alkenyl or an aromatic group containing one or more fluorinated carbon atoms; including perfluorinated alkyl, alkenyl and aromatic groups.
Preferred fluorinated anions for the compositions of the invention, defined here as Group B Anions, are selected from the group consisting of: [BF]−, [BF3CF3]−, [BF3C2F5]−, [PF6]−, [PF3(C2F5)3]−, [SbF6]−, [CF3SO3]−, [HCF2CF2SO3]−, [CF3HFCCF2SO3]−, [HCClFCF2SO3]−, [(CF3SO2)2N]−, [(CF3CF2SO2)2N]−, [(CF3SO2)3C]−, [CF3CO2]−, [CF3OCFHCF2SO3]−, [CF3CF2OCFHCF2SO3]−, [CF3CFHOCF2CF2SO3]−, [CF2HCF2OCF2CF2SO3]−, [CF2ICF2OCF2CF2SO3]−, [CF3CF2OCF2CF2SO3]−, [(CF2HCF2SO2)2N]−, [(CF3CFHCF2SO2)2N]−; and F−.
In another embodiment, ionic liquids useful in the invention comprise a Group A Cation as defined above; and a Group B Anion as defined above.
In another embodiment, ionic liquids useful in the invention comprise a Group A Cation as defined above, wherein at least one member selected from R, R1, R2, R3, R4, R5, R6, R7, R8, R9, R10, R11, R12, R13 and R14, comprises one or more fluorines; and an anion selected from Group A Anions, as defined above. In a preferred embodiment, the ionic liquids useful in the invention consists essentially of Group A Cation as defined above, wherein at least one member selected from R, R1, R2, R3, R4, R5, R6, R7, R8, R9, R10, R11, R12, R13 and R14 comprises one or more fluorines; and an anion selected from Group A Anions, as defined above.
In another embodiment, ionic liquids useful in the invention comprise Group A Cation as defined above, wherein at least one member selected from R, R1, R2, R3, R4, R5, R6, R7, R8, R9, R10, R11, R12, R13 and R14 comprises one or more fluorines; and an anion comprises a Group B Anion, as defined above.
In another embodiment, preferred ionic liquids useful for the invention comprise an imidazolium as the cation, and an anion selected from the group consisting of Group B Anions, as defined above, and [CH3OSO3]−.
In a preferred embodiment, the ionic liquids useful for the invention consist essentially of an imidazolium as the cation, and an anion selected from the group consisting of Group B Anions, as defined above, and [CH3OSO3]−.
In another embodiment, preferred ionic liquids useful for the invention comprise 1-butyl-3-methylimidazolium as the cation, and an anion selected from the group consisting of Group B Anions, as defined above, and [CH3OSO3]−.
In another embodiment, preferred ionic liquids useful for the invention comprise 1-ethyl-3-methylimidazolium as the cation, and an anion selected from the group consisting of Group B Anions, as defined above, and [CH3OSO3]−.
In another embodiment, preferred ionic liquids useful for the invention comprise 1-ethyl-3-methylimidazolium as the cation, and [(CF3CF2SO2)2N]−, [PF6]−, or [HCF2CF2SO3]− as the anion.
In another embodiment, preferred ionic liquids useful for the invention comprise 1,3-dimethylimidazolium as the cation, and an anion selected from the group consisting of Group B Anions, as defined above, and [CH3OSO3]−.
In another embodiment, preferred ionic liquids useful in the invention comprise a Group A Cation as defined above; and the anion is [CH3CO2]−. More preferred ionic liquids within this group are those wherein the cation is an ammonium cation. In a preferred embodiment, ionic liquids useful in the invention consist essentially of an ammonium cation; and the anion is [CH3CO2]−. An especially preferred ionic liquid is wherein the cation is N,N-dimethylammonium ethanol.
Mixtures of ionic liquids may also be useful for mixing with ammonia for use in absorption cooling cycles, for storage of ammonia.
A useful method for characterization of the ionic liquids useful in the invention is the determination of viscosity using a capillary viscometer (Cannon-Manning semi-micro viscometer) over a temperature range (283.15 to 373.15 K) as disclosed in “Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids and the Calculation of Dynamic Viscosity”, ASTM method D445-88. Preferably the ionic liquid useful in the invention has a viscosity, as measured by ASTM method D445-88 method, at 25° C., of less than 100 centipoise (cp). The lower the viscosity of the ionic liquid, the lower the pumping power required to move a composition through an absorption cycle. Lower pumping power increases the overall efficiency of an absorption cycle. The calculated coefficient of performance (COP), as described in the examples, does not factor-in pumping power requirements. Table A lists the viscosity of several ionic fluids useful in the invention.
The compositions comprising ammonia and ionic liquid can be prepared adding a weighed amount of ionic fluid to a sealable vessel, followed by applying a vacuum, with heating if so desired, to remove any residual water. The vessel can be tared and then ammonia gas added. The vessel is sealed and the mixture equilibrated with occasional agitation to provide a solution of ammonia in the ionic liquid.
The ammonia solutions can be used as a storage medium for anhydrous ammonia. Heating the ammonia-ionic liquid mixture is sufficient to drive the ammonia into the vapor phase, leaving behind the ionic liquid that has substantially no measurable vapor pressure. The ammonia-ionic liquid composition can be heated to about 200° C., or about 150° C., or preferably about 100° C., or less, to liberate the ammonia from solution.
The compositions are also useful in absorption cycles for heating or cooling. An embodiment of the invention is an absorption cycle comprising a composition comprising ammonia and at least one ionic liquid wherein the composition comprises about 1 to about 99 mole % of ammonia over a temperature range from about −40 to about 130° C. at a pressure from about 1 to about 110 bar. A schematic diagram for a simple absorption cycle is shown in
One embodiment is an absorption cycle wherein the ionic liquid comprises a Group A Cation as defined above; and a Group A Anions as defined above.
In another embodiment the absorption cycle comprises an absorber side having an exit, and a generator side having an exit, wherein the absorber side has a concentration of ionic liquid at the exit of greater than about 70% by weight of said composition; and the generator side has a concentration of ionic liquid at the exit of greater than about 80% by weight of said composition. In this embodiment a preferred ionic liquid comprises a N,N-dimethylammonium ethanol cation.
In another embodiment, in the absorption cycle, the absorber side has a concentration of ionic liquid at the exit of greater than about 80% by weight of said composition; and the generator side has a concentration of ionic liquid at the exit of greater than about 90% by weight of said composition. In this embodiment a preferred ionic liquid comprises an imidazolium cation.
The starting volumes of the ionic and liquid and ammonia will depend on the specific system components being used in the absorption cycle.
In order to understand the absorption cycle and to evaluate the cycle performance, thermodynamic property charts such as temperature-pressure-concentration (TPX) and enthalpy-temperature (HT) diagrams are required. These charts correspond to the familiar PH (pressure-enthalpy) or TS (temperature-entropy) diagram in the vapor compression cycle analysis. However, the use of these charts may not be as straightforward as vapor compression with a compressor, where the compression process is theoretically a single isentropic path, while the absorption cycle employs the so-called generator-absorber solution circuit, and several thermodynamic processes are involved.
The PH or TS diagram in the vapor compression cycle is constructed using equations of state (EOS), and the cycle performance and all thermodynamic properties can be calculated according to the discussion and equations described in Shiflett et al, US 2006/0197053 A1. The results of these calculations for several compositions of the invention are listed in Table 9 (Example 9). The well-known refrigerant-absorbent pair, NH3/H2O also has been calculated and is for comparison. In the case of NH3/H2O, the absorbent H2O has a non-negligible vapor pressure at the generator exit, and in practical applications a rectifier (distillation) unit is required in order to separate the refrigerant from absorbent water. The effect of vapor pressure and extra power requirement due to the rectifier have been ignored; thus, the calculated COP is over-estimated for the present performance comparison. As the COP values indicate, several compositions have properties similar to the convention ammonia-water absorption cycle.
Preferred compositions for absorption cycles and storage processes have about 5 mol % to about 95 mol % ammonia; about 10 mol % to about 95 mol % ammonia; and about 25 mol % to about 85 mol % ammonia.
High purity, anhydrous ammonia (purity ≧99.999%, semiconductor grade, CAS no. 2664-41-7) was obtained from MG Industries (Philadelphia Pa.). The following ionic liquids were used in the examples:
They were obtained from Fluka (Buchs, Switzerland) also distributed by Sigma-Aldrich in the United States. The N,N-dimethylethanolammonium ethanoate (also called acetate, assay ≧99%) was obtained from Bioniqs (York, England).
All of the ionic liquid samples were dried and degassed, with the exception of N,N-dimethylethanolammonium ethanoate, by placing the samples in borosilicate glass tubes and applying a course vacuum with a diaphragm pump (Pfeiffer, model MVP055-3) for about 3 h. The samples were then dried at a pressure of about 4×10−7 kPa while simultaneously heating and stirring the ionic liquids at a temperature of about 348 K for 48 h.
The syntheses of non-commercially available anions,
The following method was employed to determine if mixtures of ammonia and ionic liquids were soluble. Six static phase equilibrium cells were constructed as shown in
The NH3 gas was loaded by mass (0.02 to 0.8 g) from a high pressure gas cylinder. The NH3 gas pressure was regulated to about 500 kPa with a two-stage gas regulator (Matheson Gas Products). The sample tubing between the gas regulator and cell was evacuated prior to filling with NH3 gas. The cell was placed on an analytical balance and gas was slowly added until the desired mass of NH3 was obtained. For samples that required more than 0.1 g of NH3, the cell was cooled in dry ice to condense NH3 gas inside the cell. To obtain the final mass of NH3 added to the cell, the sample valve (valve 1) was closed and the cell was disconnected from the gas cylinder, and weighed on the analytical balance. The upper half of the cell (part B) which included the pressure transducer was connected with a Swagelok fitting to the lower half (part A). The interior volume of part B was evacuated through valve 2 using the diaphragm pump. Valve 2 was closed and capped and valve 1 was opened.
The six sample cells were placed inside a tank and the temperature was controlled with an external temperature bath, either a water bath (VWR International, Model 1160S), or an oil bath (Tamson Instruments TV4000LT hot oil bath), circulating through a copper coil submerged in the tank. The temperature was initially set at about 283 K. The sample cells were vigorously shaken to assist with mixing prior to being immersed in the tank. The water or oil level in the tank was adjusted such that the entire cell was under fluid including the bottom 2 cm of the pressure transducer. The cells were rocked back and forth in the tank to enhance mixing. The pressure was recorded every hour until no change in pressure was measured. To ensure the samples were at equilibrium and properly mixed, the cells were momentarily removed from the tank and again vigorously shaken. The cells were placed back in the tank and the process was repeated until no change in pressure was measured. In all cases the cells reached equilibrium in 4 to 8 hours. The process was repeated at higher temperatures of about 298 K, 323 K and 348 K. Additional measurements at 355 K were made for [bmim][PF6] and [bmim][BF4] examples, and 373 K measurements were made for ([emim][EtOSO3], [emim][SCN], and N,N-dimethylethanolammonium ethanoate.
The Dwyer pressure transducers were calibrated against a Paroscientific Model 760-6K pressure transducer (range 0 to 41.5 MPa, serial no. 62724). This instrument is a NIST certified secondary pressure standard with a traceable accuracy of 0.008% of full scale (FS). Also, due to the fact that the pressure transducers were submerged in the water or oil bath, the pressure calibration was also corrected for temperature effects. The Fluke thermometer was calibrated using a standard platinum resistance thermometer (SPRT model 5699, Hart Scientific, range 73 to 933 K) and readout (Blackstack model 1560 with SPRT module 2560). The Blackstack instrument and SPRT are also a certified secondary temperature standard with a NIST traceable accuracy to ±0.005 K. The temperature and pressure uncertainties were ±0.1 K and ±0.13% full scale (0-7 MPa). Liquid phase NH3 mole fractions are calculated based on the prepared feed composition and the volume of the sample container, and the detailed method is described in the following subsection.
Given that a mixture of NH3+RTIL was prepared in a container (volume VT) with a mole of NH3 (M1) and a mole of RTIL (M2), the following principles were used in order to find out a mole fraction (x1) of NH3 in the liquid phase at a given system temperature and pressure (i.e., equilibrium T and P).
The present method is based on the following liquid molar volume formula for an N-component system:
This is the same form as the mixing rule for the volume parameter (b) in the common cubic EOS with the binary interaction parameter. In the case of a binary system (N=2),
(ML1 is a NH3 mole in the liquid phase).
It should be mentioned here that eqs 1 and 2 are exact when m12=0 (or mij=0); that is when the excess volume is zero.
A physical liquid volume, VL, is given by:
V
L=(ML1+M2)
Then, a mass balance equation provides, when the gas phase is pure NH3:
M
1
=D
g(VT−VL)+ML1. (5)
Inserting eq 4 into eq 5 using eqs 2 and 3, and then rearranging the equation, we can obtain the following quadratic equation for ML1:
AM
L1
2
+BM
L1
+C=0, (6)
and the solution is:
where A, B, and C are given by:
A≡1−DgV10 (8)
B≡D
g
{V
T
−M
2(V10+V20)(1−m12)}+M2−M1 (9)
C≡D
g
M
2(VT−M2V20) (10)
with the following notations,
ρ2=a0+a1 (11)
Then, by setting a proper value in m12, the solution of Eq 7 gives x1, from Eq 3. Although this information about x1 is sufficient for the present purpose, it is instructive to show the following relations. The liquid volume, Eq 4 as well as liquid (molar) quality factor α can also be calculated:
Also, the excess molar volume,
Experimental solubility (TPx) data for ammonia in [bmim][PF6] are summarized in Table 1.
Experimental solubility (TPx) data for ammonia in [bmim][BF4] are summarized in Table 2.
Experimental solubility (TPx) data for ammonia in [emim][Tf2N] are summarized in Table 3.
Experimental solubility (TPx) data for ammonia in [hmim][Cl] are summarized in Table 4.
Experimental solubility (TPx) data for ammonia in [emim][CH3COO] are summarized in Table 5.
Experimental solubility (TPx) data for ammonia in [emim][EtOSO3] are summarized in Table 6.
Experimental solubility (TPx) data for ammonia in [emim][SCN] are summarized in Table 7.
Experimental solubility (TPx) data for ammonia in N,N-dimethylethanolammonium ethanoate [(CH3)2NHCH2CH2OH][CH3COO] are summarized in Table 8.
Absorption cycle calculations were developed for compositions of invention using the computer code developed by Yokozeki in “Theoretical performances of various refrigerant-absorbent pairs in a vapor-absorption refrigeration cycle by the use of equations of state” (2005, Applied Energy, 80, 383-399). The detailed assumptions made in the cycle calculation are described in that reference, and in US 2006/0197053 A1, Shiflett et al, specifically paragraphs 0063 through 0094. Proper binary interaction parameters for the equation of state have been determined using the present PTx data. Results of the present invention for the cycle performance are compared in Table 9, together with the well-known ammonia-water system. The energy efficient performance, also called coefficient of performance (COP), is explained in detail in the above references. The ammonia-RTIL COPs are somewhat lower than that of the ammonia-water system. However, in this calculation, the extra energy cost required for a rectifier unit required to condense water, which has a significant vapor pressure, was not considered in the ammonia-water case. Because the ionic liquids have no measurable vapor pressure, a rectifier is not required in the cycle. In actual applications, ammonia+ionic liquid pairs may compete with the cycle performance of the traditional absorption cycle using ammonia and water. An additional benefit is the reduced cost of cycle equipment because no rectifier for the absorbent is required.
This example illustrates that ionic liquids can absorb large amounts of ammonia reversibly as a function of pressure.
For example, at pressures of about 1 MPa over 90 mole percent ammonia can be stored in the ionic liquid which is about 25 millimol of ammonia per gram of ionic liquid. This compares well with the best solid adsorbents and most importantly the absorption/desorption process is completely reversible with no loss of capacity in the ionic liquid to store additional ammonia. Also, other ionic liquids with a lower molecular weight such as [emim][acetate] can reach even greater concentrations closer to 50 millimol of ammonia per gram of ionic liquid. Also, if the temperature is lowered to 283 K, pressures closer to 0.5 MPa (or 5 atm) can achieve the same ammonia storage capacities of 25 to 50 millimol ammonia per gram of ionic liquid. Finally, this example is merely illustrative. Other combinations of temperature and pressure (i.e. temperatures lower than 283 K) maybe possible to reach 25 to 50 millimol ammonia per gram of ionic liquid at even lower pressures such as 80 kPa.
This application is a continuation of, and claims the benefit of the filing date of, U.S. application Ser. No. 11/615,394, filed Dec. 22, 2006, which is by this reference incorporated in its entirety as a part hereof for all purposes.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 11615394 | Dec 2006 | US |
| Child | 12688151 | US |
