Hydrocarbon Engineering July 2021 issue

Page 27

Jonathan Wylde and Grahame Taylor, Clariant Oil Services, USA, examine the reaction of alkyl mercaptans with hydrogen sulfide scavengers.

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ulfur-containing species found within oil and gas production fluids are far more varied than simply hydrogen sulfide (H2S). Other species include alkyl mercaptans (alkane thiols), dialkyl sulfides (such as dimethyl sulfide), dialkyl disulfides, carbon disulfide, carbon oxysulfide, and many others. While the majority of the industry focuses on the removal of H2S largely for corrosion mitigation and health and safety purposes, the removal of alkyl mercaptans is also of great concern and interest. Mercaptans are the sulfur analogues of alcohols where the oxygen atom is replaced with sulfur. They are much more volatile than their oxygen counterparts; while methanol and ethanol are liquids at room temperature and pressure, methyl mercaptan is a gas and ethyl mercaptan is a very low boiling point liquid. This is

attributed to the much weaker hydrogen bonding between sulfur and hydrogen when compared with oxygen and hydrogen atomic species. Mercaptans have very powerful, highly offensive odours and are often removed simply for this reason alone. They are very weakly acidic: methyl mercaptan has a pKa of 10.4 and ethyl mercaptan a pKa of 10.6, whereas H2S has a pKa of 7. The concentration of mercaptans found in oil and gas produced fluids varies but it is most typical for the total mercaptan concentration to be in the range of 0 to 150 ppm. Most cases will be at the lower end of this range (<20 ppm). Further individual mercaptans can vary and typical ranges for methyl mercaptan are 0.5 to 100 ppm, ethyl mercaptan 0.5 to 120 ppm, and butyl mercaptan 0.1 to 8 ppm. To date there has been very little in the way of systematic studies into the removal of

HYDROCARBON 25

ENGINEERING

July 2021


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