Page 302 - Gas Purification 5E

286 Gas PuriJication

Van Krevelen et al. (1949) also proposed correlations to calculate the vapor pressures of
components in carbon dioxide-ammonia-water and hydrogen sulfide-carbon dioxide-ammo-
nia-water systems and, in general, results obtained by use of the correlations agree well with
experimental data. However, it is important to note that the van Krevelen et al. correlations
apply only to ammonia-rich solutions, and are further limited to restricted ranges of acid
gadammonia ratios and to relatively low temperatures and pressures.
Edwards et al. (1975, 1978% B) established a molecular-themodynamic correlation for
calculating vapor-liquid equilibria in aqueous solutions containing one or more volatile elec-
trolytes, with special attention to the ternary systems, ammonia-carbon dioxidewater and
ammonia-hydrogen sulfide-water. Their 1978 correlation was shown to give results in satis-
factory agreement with the limited data then available for temperatures from 0" to 170°C
(32" to 338" F) and ionic strengths of about 6 molal (equivalent to total concentrations of the
electrolytes between 10 and 20 molal).
Maurer (1980) compared results calculated on the basis of the van Krevelen et al. (1949),
Edwards et al. (1978A, B), and Beutier and Renon (1978) correlations with several sets of
experimental data. The average deviation of the calculated partial pressures ranged from 10
to 50% depending upon which correlation and which set of experimental data were com-
pared. Kawazuishi and F'rausnitz (1987) updated values for some dissociation equilibrium
constants and Henry's constants used in the Edwards et al. (1978A, B) correlation with the
result that it predicted vapor pressure values in good agreement with the more recent experi-
mental data of Muller (1983).
Because of the importance of sour water processes in the development of substitute natur-
al gas supplies and in refinery operations, the Gas Processors Association (GPA) and Ameri-
can Petroleum Institute (API) supported an extensive program to obtain and correlate design
data in this field. In a joint GPA and API-sponsored project, Wilson (1978) developed a pro-
gram called SWEQ (Sour Water Equilibrium). According to Newman (1991), the SWEQ
model is used in several process simulators (e.g. PROCESS, CHEMSHARE, HYSIM, and
COADE) and is valid in the temperature range of 70" to 300°F. Newman published a series
of charts based on the SWEQ program, which are convenient to use for the preliminary
design of sour water systems. Four of his charts representing typical absorption and stripping
temperatures are reproduced as Figures 4-4,444-6 and 4-7.
After development of the original SWEQ correlation, additional experimental data were
obtained by Wilson et al. (1982) and Owens et al. (1983) under GPA sponsorship. The sub-
ject is reviewed by Wilson et al. (1985) who present new data on the partial pressure of sour
water components at temperatures from 100" to 400°F; and pressures up to 1,000 psia. Sam-
ple data are given in Table 4-5. Wilson et al. (1985) point out that at temperatures above
300"F, the measured partial pressures of NH3, H2S, and C02 diverge significantly from val-
ues predicted by the earlier correlation (Wilson, 1978). They also found the Henry's law
coefficients for the inert gases to be the same in sour water as in pure water.
The GPA program culminated with the development of a new model and computer pro-
gam called GPSWAT (GPA Sour Water Equilibria), which extends the applicability of the
precursor SWEQ program from 68"- 284°F to 68"- 600°F and extends the pressure range to
2,000 psia. This program may be purchased from the Gas Processors' Association.

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