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Show • Adsorption Processes - Activated Carbon & Aluminas, Molecular Sieves. • Chemical Reaction - in Liquid (Redox) or Vapor phases, both Regenerable and non- Regenerable (Batch). • Chemical Reaction - of Sulfur Species to H~, e.g. Hydrogenation and! or Hydrolysis. • Sulfur Plant Tail Gas Treating (TGT) Units - Combinations of many of the above. • Selective Permeation - Membranes 2.0 Promising Areas for RFG A number of the process areas show promise both technically and economically for RFG. The following conclusions were drawn from the technological survey : 2.1 Caustic Based Processes Many of the sulfur species, notably H~ and CO2 , react irreversibly with caustic (NaOH) and form salts which can create large volumes of spent caustic. Mercaptans (RSH's) react with NaOH to form sodium mercaptides via a reversible reaction. The effects of other sulfur compounds such as COS are undefined at this time. If the contained sulfur in an RFG stream can be condensed down to mercaptans only, a regenerative caustic unit is then a viable and economic process. Two competitive processes are Merox (UOP) and niolex (Merichem). These processes catalytically oxidize mercaptides to insoluble disulfides and thereby -regeneratethe NaOH. Spent caustic is now classified as a hazardous waste, and its disposal is becoming increasingly difficult and more expensive. Some chemical companies are working on processes to recover the contained salts and thereby regenerate the caustic for recycle. This will most likely prove to be the most viable, long term solution for caustic processes. 2.2 Amine Based Processes The standard amine unit being employed uses DEA (diethanolamine), and when properly designed and operated, it achieves essentially quantitative removal of H~. However, DEA removes RSH's and COS with only about a 50-60 % efficiency. MEA (mono) shows some promise in increasing the removal of COS (80-90 %) at the cost of increased amine degradation and consumption. DGA (diglycolamine) and enhanced MDEA (methyldiethanolamine) show the potential for some increase in COS and RSH removal, but require more sophisticated amine cleanup systems. MDEA and DGA offer larger acid gas absorptive capacity per unit of flow due to much higher solution strengths - about 50 % vs 15-20 % for MEA and about 25 % for DEA. 2.3 Other Absorption Processes Amines with added physical solvents, such as a Sulfinol solution or an enhanced MDEA (Ucarsol and others) can increase the removal efficiencies of the major sulfur compounds. Sulfinol has shown removal efficiencies of ~90 % on H2S, COS and RSH in RFG service at one California refinery. The absorptive processes do not seem to be effective on the organic sulfides, such as dimethyl disulfide (DMDS). The Selexol physical solvent (a dimethyl ether) process can remove 99 %+ of the H~ and RSH, and about 85 % of the COS; according to its licensor. High investment and operating costs due to compression and refrigeration can be panially offset by the potential for the recovery of heavier, more valuable hydrocarbons. One disadvantage of most physical solvent processes is the greater carryover of hydrocarbon into the solvent due to higher solubilities relative to amine-only solutions. Another is the need for higher pressures to maintain -8- |
