Ind. Eng. Chem. Res. 2007, 46, 1605-1610
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Ammonia Solubilities in Room-Temperature Ionic Liquids A. Yokozeki DuPont Fluoroproducts Laboratory, Chestnut Run Plaza 711, Wilmington, Delaware 19880
Mark B. Shiflett* DuPont Central Research and DeVelopment, Experimental Station, Wilmington, Delaware 19880
The solubilities of ammonia in the room-temperature ionic liquids 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]), 1-hexyl-3-methylimidazolium chloride ([hmim][Cl]), 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([emim][Tf2N]), and 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]) have been measured for the first time. New static phase equilibrium cells have been constructed for the present experiment. Isothermally fixed temperatures are ∼283, 298, 323, 348, and 355 K, and six mixture compositions are prepared (from ∼10 mol % to 85 mol % ammonia) at the ambient temperature. The observed solubilities are high: in fact, they are among the highest ever reported in the literature for room-temperature mixtures that contain ionic liquids. Observed pressure-temperature-composition (PTx) data have been successfully correlated with our previous equation of state (EOS) model. Possible applications of the present systems to the absorption cooling/heating cycle have been discussed, in comparison to the traditional ammoniawater system. 1. Introduction Room-temperature ionic liquids (RTILs) are a new class of solvents and molten salts with a melting point of less than ∼100 °C. Because of the negligible vapor pressure, they are often called (environmentally friendly) “green solvents”, in comparison to ordinary volatile organic compounds (VOCs). For the past several years, worldwide research on the thermodynamic and transport properties of pure RTILs and their mixtures with various chemicals has been conducted.1 For the last two years, we have intensively studied the solubilities and diffusivities of various hydrofluorocarbon (HFC) and RTIL mixtures.2-8 For the first time, it has been determined that (i) some HFCs have very high solubilities in RTILs2,3 and (ii) some HFCs exhibit partial immiscibility with a lower critical solution temperature (LCST), which is a rather rare phenomenon. These findings have led us to consider several practical applications for HFC and RTIL mixtures. Among them, efficient separations of HFC azeotropes,9 extractive distillations of closeboiling-point materials, and absorption (cooling/heating) cycles10 seem quite attractive and promising. The present purpose is to find the feasibility of ammonia absorption cycles using RTILs as an absorbent. This is a replacement of the traditional ammonia-water absorption cycle, where one of the problems is the high vapor pressure of water as an absorbent; thus, it requires a costly rectifier unit. Therefore, we must know the ammonia solubilites in RTILs. To the best of our knowledge, there is no single solubility datum of ammonia in RTILs in the literature. In this report, we have investigated ammonia solubilities (pressure-temperature-composition (PTx) data) in four RTILs: 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]), 1-hexyl-3-methylimidazolium chloride ([hmim][Cl]), 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([emim][Tf2N]), and 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]). Amazingly high solubilities of ammonia in these RTILs have been observed, and the high solubility is almost comparable to the case of ammonia-water * To whom correspondence should be addressed. Tel.: 302-6952572. Fax: 302-695-4414. E-mail:
[email protected].
mixtures. The PTx data have been well-correlated with our equation of state (EOS) model.7,8,11 The possible applications of ammonia-RTIL mixtures to the absorption cycle will be briefly discussed. 2. Experimental Section 2.1. Materials. High-purity anhydrous ammonia (purity of g99.999%, semiconductor grade; CAS No. 2664-41-7) was obtained from MG Industries (Philadelphia, PA). The [bmim][PF6] (assay g96%, C8H15F6N2P, Lot and Filling Code No. 1242554 41106104, CAS No. 174501-64-5), [hmim][Cl] (assay g97%, C10H19ClN2, Lot and Filling Code No. 1086333 41705081, CAS No. 171058-17-6), [emim][Tf2N] (assay g97%, C8H11F6N3O4S2, Lot and Filling Code No. 1220095 33505239, CAS No. 174899-82-2), and [bmim][BF4] (assay g97%, C8H15F4N2B, Lot and Filling Code No. 455283/1 24203328, CAS No. 17450165-6) were obtained from Fluka (Buchs, Switzerland). The ionic liquid samples were dried and degassed by first placing the samples in borosilicate glass tubes and pulling a course vacuum on the samples with a diaphragm pump (Pfeiffer, model MVP055-3) for ∼3 h. Next, the samples were fully evacuated using a turbopump (Pfeiffer, model TSH-071) to a pressure of ∼4 × 10-7 kPa while simultaneously heating and stirring the ionic liquids at a temperature of ∼348 K for 48 h. 2.2. Apparatus and Measurements. Six static phase equilibrium cells have been constructed, as shown in Figure 1. Each cell was made using Swagelok fittings, two Swagelok ball valves (SS-426S4), stainless steel tubing, and a pressure transducer (Dwyer Instruments, model 682-5). The internal volume of each cell was calculated by measuring the mass of methanol required to fill the cell completely and knowing the density of methanol at the fill temperature. The internal volume of each cell (VT) was 15.3 ( 0.1 cm3. Initially, only the lower half (part A) of the cell, as shown in Figure 1, was needed to prepare the NH3/ ionic liquid mixtures. Ionic liquid was loaded by mass (0.5-1 g) and weighed on an analytical balance with a resolution of 0.1 mg (Mettler Toledo, model AB304-S) inside a nitrogenpurged dry box. A stainless steel syringe needle (Popper & Son, Inc., model 7937, 18 mm × 152.4 mm pipetting needle) that fit through the open ball valve (valve 1) was used to fill the
10.1021/ie061260d CCC: $37.00 © 2007 American Chemical Society Published on Web 02/08/2007
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Ind. Eng. Chem. Res., Vol. 46, No. 5, 2007
Figure 1. Schematic diagram of a sample holder.
cell with ionic liquid. The ball valve was closed and the cell was removed from the dry box. The cell was reconnected to the diaphragm pump to remove residual nitrogen and weighed again to obtain the initial ionic liquid mass. The NH3 gas was also loaded by mass (0.02-0.5 g) from a high-pressure gas cylinder. The NH3 gas pressure was regulated to ∼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 >0.1 g of NH3, the cell first had to be cooled in dry ice, so that the NH3 gas would condense 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 then valve 1 was opened. The six sample cells were placed inside a Plexiglas tank, and the temperature was controlled with an external temperature bath (VWR International, model 1160S) that circulated water through a copper coil inside the tank. The bath was stirred with an agitator (Arrow Engineering Co., Inc., model 1750), and the temperature was measured using a thermocouple (Fluke 52II thermometer). The temperature was initially set at ∼283 K. The sample cells were vigorously shaken to assist with mixing prior to being immersed in the tank. The water level in the tank was adjusted such that the entire cell was under water, 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 bath and the process was repeated until no change in pressure was measured. In all cases, the cells reached equilibrium in 4-8 h. The process was repeated at higher temperatures of ∼298, 323, and 348 K (355 K for the [bmim][PF6] and [bmim][BF4] cases). The Dwyer pressure transducers were calibrated against a Paroscientific model 760-6K pressure transducer (range of 0-41.5 MPa, Serial No. 62724). This instrument is an NISTcertified secondary pressure standard with a traceable accuracy of 0.008% of full scale (FS). Also, because of the fact that the pressure transducers were submerged in the water 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 of 73-933 K) and readout (Blackstack model 1560, with a SPRT module 2560). The Blackstack instrument and SPRT module are also a certified secondary temperature standard with an NIST traceable accuracy to (0.005 K. The temperature and pressure uncertainties were (0.1 K and (0.13% FS (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. 2.3. Liquid-Phase Mole Fractions of NH3. 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), our problem is to determine a mole fraction (x1) of NH3 in the liquid phase at a given system temperature and pressure (i.e., equilibrium T and P). A key assumption in the present method is based on the following liquid molar volume formula for an N-component system:
VL )
1
N
∑ (V01 + V02)(1 - mij)xixj,
2 i,j)1
mii ) 0, and mij ) mji (1)
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),
VL ) V01x1 + V02x2 - m12(V01 + V02)x1x2
(2)
where
x1 )
ML1 ML1 + M2
(3a)
and
x2 ) 1 - x1
(3b)
(ML1 is the number of moles of NH3 in the liquid phase). It should be mentioned here that eqs 1 and 2 are exact (not an assumption) if m12 ) 0 (or mij) 0); that is, when the excess volume is zero. Thus, the key “assumption” means what value for m12 should we use in eq 2 for the present study. Later, we show that the choice of the m12 value does not present any serious errors in the present purpose. A physical liquid volume VL is given by
VL ) (ML1 + M2)VL
(4)
A mass balance equation then provides the following (assuming that the gas phase is pure NH3):
M1 ) Dg(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:
AML12 + BML1 + C ) 0 and the solution is
(6)
Ind. Eng. Chem. Res., Vol. 46, No. 5, 2007 1607
-B + xB2 - 4AC ML1 ) 2A
Table 1. Coefficients of RTIL Liquid Density in eq 11a
(7)
where A, B, and C are given by
A ≡ 1 - DgV01 B ≡ Dg[VT -
M2(V01
+
V02)(1
- m12)] + M2 - M1
C ≡ DgM2(VT - M2V02) - M1M2
(8) (9) (10)
with the following notations: Dg is the gaseous molar density (in units of mol/cm3) of NH3 at the system temperature T and pressure P; V01 the saturated liquid molar volume (cm3/mol) of NH3 at the system T; V02 the saturated liquid molar volume (cm3/mol) of RTIL at the system T; and m12 a binary interaction parameter for the mixture volume. Dg and V01 are calculated with an accurate EOS, such as that in REFPROP,12 whereas V02 is obtained from the liquid density and molecular weight of RTIL. The liquid density (F2) has been fitted to experimental data with a linear T function, and the coefficients are shown in Table 1.
F2 ) a0 + a1T
(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 purposes, it is instructive to show the following relations. The liquid volume (from eq 4), as well as the liquid (molar) quality factor R, can also be calculated:
R)
ML1 + M2 M1 + M 2
(12) V h E,
Also, it is important to note that the excess molar volume, in the present theory is given by V h E ) -m12(V01 + V02)x1x2, based on eq 2. When an excess molar volume is 10% of the total molar volume at a 50/50 (mol %) mixture, m12 will be (0.2. If we use m12 ) 0, instead of m12 ) (0.2, the maximum error in x1 then is ∼0.3 mol % at the highest T and the highest x1, and typical errors are
