ACID GAS ENRICHMENT—MAXIMIZING SELECTIVITY

Acid gas enrichment (AGE) is a group of methods used to upgrade low quality off-gas from treating units to higher quality Claus plant feed or to a smaller volume stream suitable for reinjection. The process objective is to maximize CO2 slip and minimize the H2S leak into vent gas from the system, thereby producing a gas enriched in H2S to the greatest extent possible. The most common method to enrich is by using a separate absorber for treating the low-grade acid-gas stream coming from the regenerator. Because such streams contain very little else besides acid gases and water, virtually the entire sour gas stream can be very readily absorbed if presented with enough solvent. This makes this type of AGE unit a quite severe test of one’s ability to model selectivity. Gas flow and composition vary rapidly through the contactor, and concentration profiles can become inverted, forming an H2S bubble within the contactor. Another way to enrich the acid gas stream is simply to recycle a (substantial) part of the separated acid gas back into the feed to the contactor. This lets the absorber feed on an effectively richer sour gas and thereby produce a richer acid gas product stream. Almost nothing is free, however, and the cost may be higher solvent circulation and/or increased reboiler energy consumption. This paper addresses the operation of AGE by examining in detail the effect of sour gas composition, solvent strength including amine partial acidification, operating temperatures, the type of internals (trays, type/size of random packing, type/size of structured packing), and the total quantity of internals (tray count, packed depths) on residual H2S leak and Claus feed or reinjection gas quality. Comparisons are made with new commercial plant operating data as well as with information presented at a previous Laurence Reid Gas Conditioning Conference.