Gas 2023 Issue

with each other. Similarly, only one data point by Yabroff 2 is available for the pK a of BuSH. The pK a of EtSH was studied thoroughly by Tsonopoulos et al . 3, covering a wide tempera- ture range of 298 to 423 K. To make an accurate determina- tion of pK a , it is important to have consistent experimental data for all mercaptans covering a broad temperature range, which is currently lacking in the literature studies. One of the main results of this study is emphasis on the very poor quality of mercaptans solubility data and the tre- mendous scatter in what little data exist. Since the data avail- able in the literature are not measured at isobaric conditions, we have presented these data sets in our study in terms of an Apparent Henry’s Law constant for better representation: Apparent Henry’s Constant, H (in Pa) = y RSH P x RSH where y RSH = vapour phase mole fraction of mercaptan, P= total system pressure, x RSH = liquid phase mole fraction of mercaptan. Apparent Henry’s Constant data for the four mercaptans in aqueous MDEA and DEA solutions are very scattered and do not follow a generic trend as a function of temperature. Another drawback of these experimental data sets is the limited amount of data available at the acid gas loadings of interest. For example, no experimental data are available for MeSH or EtSH in aqueous DEA in the loading range of 0.05 to 0.4. This range is critical for accurate determination of speciation and VLE behaviour, and lack thereof could lead to erroneous estimation of thermodynamic properties. Simulation accuracy relies directly on the quality of the data underlying the models. The thermodynamic and mass transfer models themselves are quite sound, but the cor- relational data are weak. The apparent randomness in the plots is a manifestation of data inadequacies highlighted by the fact that each data set is for an absorber in a unique situation involving simultaneous absorption of CO 2 and H 2 S under non-isothermal conditions. There are too many factors at play to ascribe randomness to any one or two of them. Thus, we would encourage the collection of more extensive fundamental mercaptans data, such as accurate pK a and VLE data as a function of temperature. Conclusion The fundamentals behind the absorption of COS and mer- captans in aqueous amines were discussed. With this work, a new and accurate kinetic model for predicting the removal of COS has been introduced. Users of Legacy simulators have complained for years that predicted COS removal has been far removed from observations. That deviancy has now been rectified; ProTreat’s Kinetic Model predictions conform well. Because of the slow kinetics, COS is expected to be severely mass transfer rate limited, even more so than CO 2 . The model predictions were validated with a range of plant data. For MDEA systems, the level of COS pick-up will be influenced by the degree of solvent degradation and/or feed contamination with primary or secondary amines, which can vary considerably. For mercaptans, it was demonstrated that the lack of good-quality public data on solubility is a big roadblock to

10.95

MeSH EtSH PrSH PrSH BuSH

10.9

10.85

10.8

10.75

10.7

10.65

10.6

10.55

Carbon number of n-mercaptan

Figure 6 pK a of mercaptans as a function of carbon number

developing a fundamental model that can predict mercap- tans removal with a high level of accuracy and reliability. The authors appeal to the community to make high-quality experimental measurements on the pK a and VLE of mercap- tans across a range of temperatures and acid gas loading. ProTreat is a mark of Optimized Gas Treating, Inc. References 1 Gokel, G. W., Dean, J. A., Dean’s Handbook of Organic Chemistry , 2nd ed, McGraw-Hill, New York, 2004. 2 Yabroff, D. L., Extraction of mercaptans with alkaline solutions, Indus- trial & Engineering Chemistry, 32.2, 1940, 257-262. 3 Tsonopoulos, C., Coulson, D. M., Inman L. B., Ionization constants of water pollutants, Journal of Chemical and Engineering Data , 21.2, 1976, 190-193. 4 Jou, F.-Y., Mather, A. E., Ng, H.-J., Effect of CO 2 and H 2 S on the solubil- ity of methanethiol in an aqueous methyldiethanolamine solution, Fluid Phase Equilibria, 158, 1999, 933-938. 5 Coquelet, C., Boonaert, E., Valtz A., Huang S., Effect of methane, CO 2 , and H 2 S on the solubility of methyl and ethyl mercaptans in a 25 wt% methyldiethanolamine aqueous solution at 333 and 365 K, 66.11, 2021, 4000-4017. 6 Jou, F.-Y., Mather, A. E., Schmidt, K. A. G., Ng, H.-J., Vapour-liquid equi- libria in the system ethanethiol + methyldiethanolamine + water in the presence of acid gases, Journal of Chemical and Engineering Data 44.4, 1999, 833-835. 7 Coquelet, C., Awan, J. A., Boonaert E., Valtz, A., Théveneau P., Richon D., Vapour-liquid equilibrium studies of organic sulphur species in MDEA, DEA aqueous solutions, GPA RR-207, 2011. 8 Jou, F.-Y., Mather, A. E., Ng, H.-J., Solubility of methanethiol and eth- anethiol in a diethanolamine solution in the presence of acid gases, Jour- nal of Chemical & Engineering Data 45.6, 2000, 1096-1099. Prashanth Chandran is Software Development Team Lead at Opti- mized Gas Treating, Inc. He holds a BTech in chemical engineering from Anna University, India and an MS in chemical engineering from Okla- homa State University. Harnoor Kaur joined Optimized Gas Treating in 2020. She holds a PhD in chemical engineering from Texas Tech University and a BE degree in chemical engineering from Panjab University, Chandigarh, India. Jeffrey Weinfeld is a Development Engineer at OGT. He holds a BSc in chemical engineering from the University of Rochester and a PhD in chemical engineering from the University of Texas at Austin.

Gas 2023