Acid salt corrosion in a hydrotreatment plant of a petroleum refinery

The operation of a diesel hydrotreatment plant was stopped after only 18 months of service due to the obstruction of a set of heat exchangers, which are responsible for cooling of the stream that flows out from reactors. In particular, the obstruction occurred mainly in one of them that operated with a low temperature. Chemical analysis of fouling of samples obtained from obstructed tubes identified the presence of NH4Cl and the absence of NH4HS, whose presence is expected in these units. The stream that flows out of the reactors has mainly H2S and NH3; however, the chloride contents lower than the sulfide contents are sufficient to precipitate NH4Cl due to the differences between the stability of both salts. The steel used in the equipment of this unit is austenitic stainless steel. Thus the presence of a hygroscopic salt must be eliminated since its hydrolysis generates HCl, and severe restriction to the HCl content in the plant feedstocks must be observed considering that only a very small amount of this compound is expected from the organic chloride.