Scott Alvis - Optimizing Gas Treating
Sour water stripping, nitrogen control to stop ammonia productionSour water is generally classified as either phenolic or non-phenolic. Non-phenolic water, also called HDS water because it is produced by hydro-treating in hydrodesulphurization or HDS units in refineries, contains almost exclusively ammonia, hydrogen sulfide, and possibly a trace of carbon dioxide. FCCs and Cokers produce sour water containing both organic and inorganic impurities derived from heat stable salt (HSS) precursors, especially derived from HCN. Such waters cannot be fully stripped because the HSSs permanently protonate some of the ammonia, turn it into a salt (NH4+) and thereby render it nonvolatile. Ammonia is protonated to NH4+ but can be freed by neutralizing the HSSs with caustic soda.
In this presentation, attention is turned to the WWT (waste water treatment) technology originally developed by Chevron and recently acquired by Bechtel. This is generally referenced in the literature as the Chevron WWT Process. Mass transfer rate-based simulation is used in a case study to examine two of the important factors that determine the performance of a 2-Stage Chevron WWT unit. To simplify the discussion, the case study uses HDS water because the presence of HSSs complicates the situation without adding anything to the basic findings presented here.
The purpose of using a Chevron WWT unit is to produce relatively pure ammonia and hydrogen sulfide products rather than just combusting the ammonia in the Claus furnace. It is shown that the performance of the whole plant is controlled almost entirely by the operation of the hydrogen sulfide stripper.