Thermodynamic and Physical Properties of MonomethylamineLithium Thiocyanate System ROBERT A. MACRISS, DHARAMVIR PUNWANI, WILLIAM F. RUSH, and WENDELL J. BIERMANN' Institute of Gas Technology, Chicago,
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Vapor pressure data obtained in the concentration range of 0 to about 56 weight O h lithium thiocyanate in monomethylamine and for pressures of 1 to 200 psia show extremely high negative deviations from Raoult's law. The data were used to compute heats of vaporization from these solutions. Viscosity data were obtained over a temperature range of 75' to 2OOOF and concentrations to about 56 weight 'Yo salt. The data fit Andrade's equation very well and produce straight lines which are amenable to extrapolation to higher temperatures. The densities of the solution were also measured.
M o s t of the investigations of thiocyanate salts in the literature involve aqueous solutions. Although dilute aqueous solutions have contributed greatly to the development of physical chemistry, the behavior of very concentrated nonaqueous electrolytic solutions has not been completely understood. Progress in formulating theoretical descriptions of these concentrated solutions requires the availability of experimentally determined data, which a t present are in meager supply in the literature. Such measurements are reported here for solutions of lithium thiocyanate in liquid monomethylamine. Experimental data on vapor pressures, solubility, density, and viscosity are given between 75" and 285" F, from 100 to 55 weight 7%methylamine, and pressures from 1 to 200 psia. This particular system was chosen for study because of its possible application to absorption cooling in a gas-fueled air-cooled system. Most of the literature studies involving methylamine have been carried out at or near its normal boiling point, to avoid experimentation a t high pressures. Because the present work was carried out in a range of temperatures extending much above room temperature, special equipment was constructed to facilitate measurements a t high pressures. DISCUSSION Vapor Pressures. Vapor pressure was measured by using a 25-mm-bore, 90-cu-cm-capacity glass cell attached to a glass-to-metal pressure connector through an O-ring assembly. The cell was pressure-tested to 34 atm. In operation, a known amount of dry lithium thiocyanate salt is charged to the cell, and, after evacuation and cooling, methylamine is distilled into it until 75 to 90% of the volume of the
cell is occupied by the resulting liquid mixture. The contents of the cell (Figure 1) are thoroughly mixed by returning refrigerant vapor (methylamine) from the top of the cell to the bottom of the liquid layer and bubbling the vapor through the liquid. Circulation is accomplished by a magnetically operated plunger pump which is in a closed loop with the equilibrium cell. The equilibrium cell is permanently fixed in a constant-temperature bath. The pres-
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TO PRESSURE MEASURING DEVICE
EQUILIBRIUM CELL
Present address, Research Division, Carrier Corp., Syracuse, Figure 1 . Vapor pressure cell assembly
N . Y. 466
Journal of Chemical and Engineering Data, Vol. 15, No. 4, 1970
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Table I. Vapor Pressures of Monomethylamine-Lithium Thiocyanate Solutions
21.1 Wt % Salt" Temp., Pressure, "F psia 80.4 112.5 120.0 130.6 100.6 138.4 148.4 158.5 169.2
50.2 81.1 94.5 107.7 68.2 121.9 139.3 159.4 191.4
39.9 Wt % Salt' Temp.,
O F
13
14
15
I6
17
18
19
RECIPROCAL ABSOLUTE T E M P , O R - ~ Xlo4
Figure 2. Vapor pressure data of monomethylamine-lithium thiocyanate solutions
83.2 102.3 102.3 138.6 138.8 160.3 177.8 198.2 223.1 223.0 248.0 248.1
29.5 Wt 70~Salt" Temp., Pressure, "F psia 81.4 81.6 108.7 108.9 128.5 128.7 150.4 150.4 169.0 187.8
34.4 34.3 55.3 55.6 77.6 77.8 107.7 108.2 135.4 168.0
46.5 Wt %IC Salt'
Pressure, psia
Temp., OF
18.50 20.30 23.10 26.17 26.37 33.3 44.7 63.9 100.8 102.1 135.2 138.0
83.3 83.3 129.8 147.2 176.3 199.5 229.2 251.9
Pressure, psia 4.72 4.77 6.29 7.81 18.87 29.84 48.9 70.3
34.6 Wt % Salt* Temp., Pressure, O F psia 93.5 93.4 93.5 129.2 128.3 150.6 173.6 197.0
21.43 21.63 21.53 41.4 42.3 61.5 87.5 122.0
51.2 Wt %IC Saltd Temp.,
O F 78.4 102.4 128.2 156.9 178.7 198.4 222.3 244.9 265.8 285.4
Pressure, psia 0.93 1.13 1.57 3.74 6.19 9.34 14.05 23.05 34.05 48.35
56.1 Wt '% Salt'
sure of the mixture a t low- and high-pressure ranges is read with a manometer or a dead-weight tester, depending on the pressure range. The 1000-psi dead-weight tester was supplied with a calibration accurate to 1 0 . 1 psia. Measurements, however, were only approximately accurate to k 0 . 5 psia because of the insensitivity of the pressure transducer in the system. For low pressures, a 50-inch mercury manometer was used and was read to + O . l mm of Hg. Temperatures were measured with copper-constantan thermocouples accurate to within 10.1" F in the temperature range of 70" to 300" F. The thermocouples were calibrated by comparison to a platinum resistance thermometer from the National Bureau of Standards. The methylamine used had the following impurities: Trimethylamine Dimethylamine Ammonia
0.11 mole 7: 0.6 mole 7: 0.37 mole %;c
The lithium thiocyanate was purchased from a supplier who prepared it by using the method outlined by Lee ( 4 ) . The wet lithium thiocyanate crystals were further dried at I G T a t 130"C in a rotary kiln in a nitrogen atmosphere until the amount of water in the salt was
