A molecular-thermodynamic correlation has been developed for calculating the inhibition effect of methanol on the formation of hydrates in moist gas mixtures. Six phases are potentially present in these mixtuies: gas, aqueous liquid, hydrocarbon liquid, ice, and hydrate structures I and II. For a given temperature and system composition, the molecular-thermodynamic method described here allows computation of the hydrate-point pressure as well as relative amounts and compositions of all coexisting phases. Good agreement is obtained when calculated results are compared with diverse experimental data reported in the literature. The correlation given here may be useful for computeraided design for gas processing and transportation.
F. E.
SCOPENumerous articles are concerned with gas/ hydrate phase equilibria (e.g., Berecz and Balla-Achs, 1983). Where no inhibitor is present, correlations for hydratesystem properties are restricted to selected parts of the general, multiphase-multicomponent diagram. A number of these studies are directed at the threephase system hydrate/vapor/aqueous-liquid (or ice) (Parrish and Prausnitz, 1972;Ng and Robinson, 1976). Others have directed attention to the two-phase system hydrate/gas (Sloan et al., 1976; Ng and Robinson, 1980; Aoyagi et al., 1980;Song and Kobayashi, 1982). Limited studies of the three-phase system hydrate/ aqueous-liquid/hydrocarbon-liquid have been made (Ng and Robinson, 1976, 1977). While several proprietary correlations exist (Gas Processors Assn.), no thermodynamically consistent overall multicomponentmultiphase correlation for this hydrate inhibition problem has appeared in the literature. Previous correlations for the effects of inhibitors on hydrate formation (Menten et al., 1981) have assumed that only condensed aqueous phases coexist with the gas and hydrate phases. This work covers all of the above-mentioned equilibria in a unified framework suitable for further development for computer-aided process design.
CONCLUSIONS AND SIGNIFICANCEThis work presents a method for predicting the amount of inhibitor that must be added to prevent hydrate formation in gas processing. The method, based on molecular-thermodynamic models and a computational framework, calculates hydrate-point pressures and temperatures, and coexisting-phase fractions and compositions for gas/water/methanol mixtures in the region from 220 to 320 K and pressures to 500 bar (50 MPa). Consideration is given to the possible existence of all of the six potential phases: gas, aqueous liquid, hydrocarbon liquid, ice, and hydrate structures I and II; these phases may occur in various combinations. The calculation procedure includes nitrogen, methane, ethane, propane, n-butane, isobutane, n-pentane, carbon dioxide, and hydrogen sulfide along with methanol and water.Fugacities of all components in the gas phase are calculated using a modification of the Redlich-Kwong (1949) equation of state similar t o that of de Santis et al. (1974). Fugacities for all condensible components are calculated using activ...
