Modeling phase equilibria for acid gas mixtures using the CPA equation of state. I. Mixtures with H₂S

Abstract

in Wiley Online Library (wileyonlinelibrary.com).The Cubic-Plus-Association (CPA) equation of state is applied to a large variety of mixtures containing H 2 S, which are of interest in the oil and gas industry. Binary H 2 S mixtures with alkanes, CO 2 , water, methanol, and glycols are first considered. The interactions of H 2 S with polar compounds (water, methanol, and glycols) are modeled assuming presence or not of cross-association interactions. Such interactions are accounted for using either a combining rule or a cross-solvation energy obtained from spectroscopic data. Using the parameters obtained from the binary systems, one ternary and three quaternary mixtures are considered. It is shown that overall excellent correlation for binary mixtures and satisfactory prediction results for multicomponent systems are obtained. There are significant differences between the various modeling approaches and the best results are obtained when cross association is explicitly accounted for, especially using the cross-association energy from independent experimental studies rather than from combining rules. Figure 3. H 2 S-water liquid-liquid equilibria.Experimental data 37 (points) and CPA calculations assuming that H 2 S is self-associating fluid and using 3B associating scheme (solid lines) and that H 2 S in non-self-associating fluid, but has two proton donors able to cross associate with water (dotted lines). Calculations using first and second approach.Calculations using different association schemes for H 2 S and the zeroth approach (i.e. no solvation is included and only one interaction parameter is used) described in ''The CPA Equation of State'' section (experimental data [43][44][45][46] ). d, proton donor; a, proton acceptor, %AAD ¼ 1, where X stands for pressure, P, or mole fraction, y, and n is the number of experimental data points, DT ¼ 1, where T is the temperature and n is the number of experimental data points. 100, where X stands for mole fractions, x or y, and n is the number of experimental data points.

Towards predictive association theories

“…water-alcohol-hydrocarbons has been illustrated [6,7,26], only very recently multicomponent mixtures with acid gases (H 2 S) have been considered (Tsivintzelis et al [27]). We evaluate here the performance of the model for multicomponent mixtures with CO 2 .…”

“…CO 2 was modeled assuming one site. Pure fluid parameters were adopted from previous studies [27,36], unless otherwise indicated in the text. All binary interaction parameters were adopted from the corresponding binary systems (using VLE and/or LLE data) and they are presented, for the three systems, in Tables 1-3. Takahashi et al [28] report the solubility of CO 2 in aqueous TEG solutions at various temperatures as a function of pressure.…”

Measurement and correlation of solubility of water in carbon dioxide-rich phase

“…For pure component parameters, we used the values reported by Tsivintzelis [20,21] and listed in Table 2. Water is considered as 4-site associating components (two electron donor and two electron acceptor) whereas carbon dioxide only contains two electron acceptor.…”

“…It is therefore assumed that carbon dioxide molecules cannot form hydrogen bond among themselves; they only form cross hydrogen bonding with water. For the values of cross hydrogen bonding parameters, we used the values provided by Tsivintzelis [20,21] and listed in Table 2. …”

“…Even with advanced equations of state approaches it was still difficult to properly describe the complex behavior of such mixtures. Recently, the cubic-plusassociation equation of state (CPA EOS) proposed by Kontogeorgis and coworkers are attracting attention among researchers with the ability to represent complex systems [19][20][21][22]. A modified version of the CPA EOS by Perakis et al [22]; called CPA-PR EOS, was reported to give very satisfactory results for water-carbon dioxide-acetic acid and water-ethanol-carbon dioxide systems by treating carbon dioxide as a solvating molecule (only cross-associating with other self-associating molecule).…”