Gas 2023 Issue

COS and mercaptans removal from gases

Challenges with removing trace sulphur species can be better understood and resolved with a new kinetic model validated by plant data in its ability to predict COS removal

Prashanth Chandran, Harnoor Kaur, Jeffrey Weinfeld and Ralph Weiland Optimized Gas Treating, Inc.

G as and liquid hydrocarbon streams from refineries and gas plants must be well cleaned of sulphur com- pounds such as H 2 S, COS, and mercaptans (RSH), dictated mainly by environmental concerns. LPG can be sub- tle because although a copper strip test may indicate accept- able sulphur content today, COS slowly reverts to H 2 S and CO 2 in the presence of water, so the same test administered tomorrow may fail. This contribution offers a new model for COS absorption into alkaline solvents based on mass transfer rates enhanced by reaction kinetics – the first time commercial software has had the ability to simulate this aspect of COS absorption accurately. As part of the reported work, a compilation of plant performance test data on mercaptans removal is also presented. It shows plenty of room for improvement in reli- ability and accuracy of simulation tools. Amines are excellent solvents for H 2 S, but, by and large, they are horrible for removing other less acidic, trace sulphur species such as COS and mercaptans. Until now, no simula- tor has been able to model COS and mercaptans adequately. With mercaptans, the basic problem seems to be insuffi - cient, inaccurate phase-equilibrium data. Almost all the pub- lic domain mercaptans solubility data are academic in origin, which may explain why there is so little of it. Very few academic institutions welcome researchers who handle mercaptans – academia is generally ill equipped to handle them safely and therefore avoids the risk associated with their malodorous and toxic nature. Good-quality data in the range of commercial interest are hard to come by. For COS, one of the main issues has been that simulators have ignored its reactive nature in aqueous amine solu- tions, treating its chemistry in an over-simplified way as a purely physically dissolved, non-reacting solute. The COS absorption rate is thus wrongly computed because the calculations fail to account for significant absorption rate enhancement that results from the chemical reactions of COS with non-tertiary amines. Reactions of COS and mercaptans Reaction kinetics of H 2 S and CO 2 in aqueous amines are too well known to benefit from further discussion here. RSH merely dissociates in aqueous media. But to describe the decomposition of COS in water just by the reaction COS + H 2 O → CO 2 + H 2S is deceptively oversimplified. The reaction

mechanisms and kinetics of COS in amines are much more complex than that and can benefit from a brief explanation: RSH ⇌ H + + RS – (1) COS + H 2 O ⇌ H + + HCO 2 S – (2) HCO 2 S – + H 2 O � H + + HCO₃ – + HS – (3) Reaction (1) is a simple dissociation involving a single hydrogen ion and, as such, is known to be essentially instan- taneous. Thus, it is always at equilibrium. The limitation with RSH is that it is an extremely weak acid, so even a low level of acidification of the solvent will drive Reaction (1) back towards the formation of molecular RSH, and mercaptans have very low physical solubility in water. Significant acidifi - cation can be had even with a modest amount of dissolved CO 2 or H 2 S. In regenerative caustic solutions, the CO 2 and H 2 S spend the caustic from its intended purpose of RSH removal. The significance of these effects is discussed in the next section by looking at the vapour-phase profile of mer - captans in a typical absorber. COS reacts in aqueous solutions first to form thiocarbon - ate Reaction (2), which further hydrolyses to bicarbonate and bisulphide Reaction (3). The combined form of Reactions (2) and (3) along with other speciation reactions of CO 2 and H 2 S is equivalent to the overall simplified hydrolysis of COS to CO 2 and H 2 S already mentioned. Reactions (2) and (3) are very slow unless a base is present in the solution to catalyse them. In the presence of amines, it is postulated that COS reacts by a base-catalysed mechanism according to: COS + Am + H 2 O ⇌ AmH + + HCO 2 S – (4) HCO 2 S – + Am + H 2 O � AmH + + HCO₃ - + HS – (5) In addition to these reactions, COS forms thiocarba- mate with primary and secondary amines via a zwitterion mechanism: COS + AmH + ⇌ AmH + COS – (6) AmH + COS – + B � AmCOS – (thiocarbamate) + BH + (7) Reaction (6) represents zwitterion formation (AmH + in Reaction (6) stands for the primary or secondary amine with at least one mobile hydrogen). Reaction (7) describes the zwitterion’s deprotonation to thiocarbamate, AmCOS – .

Gas 2023