Dithiazine vapour saturation in natural gas mixtures

Abstract

Dithiazine is a reaction product resulting from the use of triazine as a scavenging agent to remove H2S from oil and gas streams at well-head or at gas treating facilities using contactors. In some cases, MEA-dithiazine product has been found in the sales gas in downstream transmission and distribution pipeline systems. MEA-dithiazine can subsequently precipitate when the solubility conditions become unfavourable, e.g. within regulating valves on a fuel gas skid at a gas turbine driven compressor station or at distribution regulating stations. The present work utilizes experimentally-obtained solubility data in methane and lean natural gas mixtures, along with fundamentals of chemical equilibrium based on equality of fugacity, and a closed-form analytical expression of Peng-Robinson EOS. The data shows that natural gas composition does not appear to have a significant effect on the solubility of dithiazine up to ~4 MPa; however, as pressure increases above this range, dithiazine tends to be more soluble in richer than in leaner gas mixtures. More importantly, the solubility isotherms of dithiazine in natural gas mixtures exhibit minima at 2–3 MPa. The implication of this is significant in terms of the amount of dithiazine that could potentially precipitate out of the gas stream if the gas undergoes throttling at pressure regulating valves. As an example, at a gas flow rate of 20 × 106 m3/day, the rate of dithiazine precipitation may be as high as 3.3 kg/day.