Vaporization Equilibrium Ratios for C02 and H2S in Paraffinic, Naphthenic, and Aromatic Solvents C. J. Mundis Oklahoma State University, Stillwater, Oklahoma 74074
L. Yarborough Amoco Production Company, Tulsa, Oklahoma 74 102
R. L. Robinson, Jr.’ Oklahoma State University, Stillwater, Oklahoma 74074
Vaporization equilibrium ratios (K values) were determined by the chromatographic technique for C 0 2 and H2S at essentially infinite dilution in the methane n-heptane, methane methylcyclohexane, and methane toluene systems at 20, 0, -20, and -40 O F and in the methane 4- n-octane system at 0 and -20 O C . Measurements covered the pressure range of 100 to 1500 psia. For the methane n-octane system, the chromatographic data were confirmed by measurements made by classical techniques in a variable-volume windowed equilibrium cell. For COz, the Kvalues in the n-heptane and toluene solvents were nearly identical, while methylcyclohexane enhanced the C 0 2 Kvalue by 2 5 % . For H2S, the Kvalues in n-heptane and methylcyclohexane were quite similar, while toluene reduced the Kvalues by 40 to 60 YO.These results illustrate the effects that various types of chemical components present in solvent oils could exert on the behavior of C 0 2 and H2S. The data of this study were correlated by a modified version of the Redlich-Kwong equation of state. Average errors in the predicted K values of C 0 2 and H2S were approximately 5 % .
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Introduction Information is scarce on the vapor-liquid equilibrium behavior of carbon dioxide and hydrogen sulfide in heavy solvents a t subambient temperatures and moderate pressures. Such information is needed because the behavior of COi and HJS has proven more difficult to predict than that of the hydrocarbons with which they naturally occur. In particular, data are scarce on the effect of the chemical nature of solvent oils on the COj and HlS K values. In the present study, chromatographic measurements were made on K values of CO2 and HjS a t essentially infinite dilution (at pressures to 1500 psia) in the following systems: methane n-heptane (20, 0, -20, -40 O F ) ; methane n octane (0, -20 “C); methane methylcyclohexane (MCH) (20, 0, -20, -40 OF); methane + toluene (20, 0, -20, -40 OF). The above measurements were designed to provide information on the effects that paraffinic, naphthenic, and aromatic components in absorber oils exert on CO, and HrS vaporization behavior, but the results are applicable to other systems where COj and HzS occur with heavy solvents.
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Experimental Section Gas-Liquid Partition Chromatography. The chromatographic studies were performed on a recently constructed apparatus quite similar to one employed earlier by Yudovich et al. (1971) in the OSU laboratory. Only modifications to the apparatus and procedure of Yudovich will be discussed here. The new apparatus contained the following modifications. (1) The sample valve was of the design described by Yarborough and Vogel(1967). A sample of the solute to be injected was trapped beneath the valve stem in a cavity in the valve body. In this work, the volume of the cavity was approximately 40 wL. The valves were supplied by Jacoby (1970), who has 254
Ind. Eng. Chem., Process Des. Dev., Vol. 16, No. 2 , 1977
recently described their use. (2) The chromatograph column and presaturator were contained in a liquid ethanol bath where temperatures were controlled to f O . l “C, measured on a nine-junction copper-constantan thermopile. Bath temperature was controlled by a commercial constant-temperature bath (Lauda Model KB-20). ( 3 )Sample peaks were detected on a thermistor detector (Gow Mac Model 40-002, with Model 13-504 thermistors) and recorded on a strip chart recorder. The chromatograph columns were 10 to 15 in. lengths of %-in. 0.d. stainless steel tubing, packed with 30/60 mesh acid-washed firebrick (Tek Lab, Cat. No. CP-23060) and coated with 30 wt % of the desired solvent. Presaturator columns were identical with the above, except that they were 48 in. in length. Fresh columns were used for each isotherm. Loss of solvent from the column was never more than 1%during an experimental run. Experimental runs consisted of injecting sequentially samples of the gases studied (He, Ar, COz, H2S) into a stream of methane which flowed a t controlled pressure and rate through the column. The time required for each solute to pass through the column was recorded, along with the temperature, pressure, and methane flow rate (measured by bubble meter). The K values were calculated by the following relationships, which have been described elsewhere (e.g., Asano et al., 1971; Stalkup and Kobayashi, 1963; Yudovich et al., 1971).
K, =
W L
(1- X d P ( V , - V,)
(1)
where K , = vapor-liquid equilibrium ratio ( y / x ) ,W I ,= moles of pure solvent liquid on the chromatograph column packing, X I = solubility (mole fraction) of the carrier gas (methane) in the solvent liquid at column conditions, p = molar density of the carrier gas at column conditions, V , = “retention volume” of component i in the column, and V , = “retention volume”
of an hypothetical, insoluble substance in the column. Retention volumes were calculated from experimental measurements by the relationship
shown in Figures 1 through 3 The static cell data dre g i v n i I I I Table 11. Direct comparisons ofthe data of this work for (3) ami t i h are possible with the data of Asano et al. (1971 1 f o r I ~ 5' P t t where n-octane is the solvent. Figure 4 present5 siich
