Decarbonisation Technology - February 2022 Issue

quench overhead is recycled back to the AGRU through a compressor, where H 2 S and CO 2 are separated, and as a result there is no SO 2 emission from the SRU/TGU. If not properly destroyed, hydrocarbons in the acid gas feed often cause carbon laydown on catalyst, along with generation of undesired high concentrations of COS and CS 2 . In addition, ammonia in the acid gas feed often causes deposition of complex ammonia/sulphur salts in cooler parts of the plant. These undesired phenomena could either cause unscheduled plant shutdown, reduced sulphur recovery, or shortened catalyst life. Oxygen enrichment raises the reaction furnace temperature, which ensures complete destruction of heavy hydrocarbons and ammonia, reduces formation of COS and CS 2 , and shortens gas residence time requirements for destruction of contaminants. In the case of lean acid gas feed contaminated with high levels of heavy hydrocarbons, oxygen enrichment offers an inexpensive and simple solution. Quantitatively, based on practical experience, the oxygen burner has proven to be very effective at destroying ammonia and hydrocarbons in Claus plants. Oxy-fuel burners are also widely used in the metals and minerals industries and the chemical and refining industries to burn a range of fuels, including gases, liquids, and pulverised solids. One of their most attractive features is the ability to burn heavy residual hydrocarbons cleanly.

the SRU. Therefore, even using 100% oxygen enrichment, neither staged combustion or recycling are required to maintain the temperature of refractory, as single combustion is adequate. COS requires hydrolysis to convert to H 2 S, to improve emissions and overall sulphur recovery. Traditionally, if titanium dioxide-based catalyst is used in the first Claus reactor, about 98% of the COS will be hydrolysed, the remainder being hydrolysed in the tail gas hydrogenation reactor. Sometimes, owing to different feed compositions from the upstream units, it may be necessary to have a separate COS hydrolysis reactor. In summary, the following units were considered for this project: • Acid gas removal scheme with Selexol physical solvent or similar • SWS and condensate stripper is a unique design, where CO 2 was used as the stripping gas • SRU is designed based on 100% oxygen enrichment, single combustion • TGU contains an additional reactor for COS hydrolysis and to process the feed stream directly to maintain a high temperature in the SRU • TGU is designed to recycle the quench overhead to the acid gas scrubber, whereby the tail gas amine portion is eliminated and zero sulphur emissions are achieved. Detailed description Figure 2 represents sulphur recovery with 100% oxygen enrichment, plus the hydrogenation and hydrolysis reactors and the quench system. The

AGRU-Selexol

Quench system

H or syngas

Hydrolysis

Hydro- genation

Claus

Caustic

Desuperheater

Figure 2 Sulphur recovery with 100% oxygen enrichment, plus the hydrogenation and hydrolysis reactors and the quench system