Amorphous Polymeric Dithiazine apDTZ Solid Fouling: Critical Review, Analysis and Solution of an Ongoing Challenge in Triazine-Based Hydrogen Sulphide Mitigation

Abstract

Despite attempts to inhibit or avoid the formation of fouling deposits (polymeric amorphous dithiazine or apDTZ for short) from the use of MEA triazine, this remains a major operational problem and limits the use of this most popular and ubiquitous hydrogen sulphide (H2S) scavenger. This paper (a) reviews and summarizes previous work, (b) provides fresh insights into the reaction product and mechanism of formation, (c) proposes an effective method of removal, and (d) proposes some mechanisms of apDTZ digestion. The mechanism of apDTZ formation is discussed and reasoning is provided from a variety of perspectives as to the mechanism of MEA-triazine reaction with H2S. These include basicity and nucleophilic substitution considerations, steric properties and theoretical calculations for electron density. Novel procedures to chemically react with and destroy this solid fouling are presented with an in-depth study and experimental verification of the underlying chemistry of this digestion process. A review of agents to chemically destroy apDTZ is undertaken and a very effective solution has been found in peroxyacetic acid, which is much more powerful and effective than previously suggested peroxides. The structure of amorphous polymeric dithiazine is emphasized and the reason why this fouling cannot be 1,3,5-trithiane is stressed. This work therefore overcomes a current industry misconception by providing insight on two major paradoxes in the reaction pathway; namely i) why the thiadiazine reaction product from tris hydroxyethyl triazine (MEA triazine) is never observed and ii) why does the dithiazine in all cases never progress to the trithiane (3rd sulphur molecule substitution)? The latter issue is probably the biggest misconception in the industry and literature regarding triazine and H2S reactions. Many reasons for this are put forward and the common misconception of "overspent" triazine is refuted. A very effective chemical reaction that results in soluble by-products, counteracting the problems produced by this intractable polymer is found and their composition is proposed and experimentally verified.