DMD-3 process

The process is used for treating straight-run gasoline fraction (62─180^о C fraction) or catalytic cracking gasoline with high mercaptan sulfur content for hydrogen sulfide and mercaptans.

The process consists in preliminary removal of hydrogen sulfide and low molecular weight (С₁-С₄) mercaptans, using a sodium hydroxide solution, with subsequent oxidation of high molecular weight mercaptans to disulfides with air. The process provides reduction of total sulfur content by 20–30 % and mercaptan sulfur content to 5 ррm.

The gasoline fraction is supplied to the bottom of pre-alkalization apparatus V-1, where hydrogen sulfide and naphthenic acids are removed from the gasoline using a 1–3 % sodium hydroxide solution. From the top of V-1 the gasoline is sent to the bottom of extractor V-2, where light mercaptans, constituting approximately 50 % of all mercaptans contained in the gasoline, are removed, using a catalyst complex (10–20 % sodium hydroxide solution with 0.005 % of IVKAZ catalyst). From the extractor the gasoline is sent to feed tank V-3.

The CC, containing mercaptides, is sent from the bottom of V-2 to the bottom of regenerator R-1, wherein an estimated quantity of air at a pressure of 0.4–0.5 MPa is also supplied. The alkali regeneration with disulfide formation takes place in the regenerator. From the top of R-1 the CC and air pass into air separator V-4. The spent air is discharged from the top of the separator to the nearest incinerator. From the bottom of V-4 the catalyst complex with the disulfides flows to disulfide separator V-5, where the disulfides are separated from the CC. The upper layer – the disulfides – is sent from separator V-5 to a storage tank and the lower layer – the regenerated CC – is pumped with pump Р-1 to extractor V-2. The CC spent on the extraction stage is discharged periodically from the circulation system to pre-alkalization apparatus V-1 for further use. After the alkali is completely spent, the CC is discharged periodically from V-1 and sent to a unit for sulfur-alkaline waste water processing.

The gasoline, pre-treated for mercaptans, is supplied from feed tank V-3 with pump Р-2 to demercaptanization reactor R-2. Before the reactor inlet the gasoline is mixed with an estimated quantity of air and catalyst complex.

Oxidation of the mercaptans to disulfides proceeds in the reactor. Temperature in the reactor is kept at the level not exceeding 60^о С and pressure is equal to 1.0–2.0 MPa depending on the air flow rate. From the top of the reactor the demercaptanized gasoline together with the CC passes to settler V-6. From the bottom of the settler the CC is supplied to the reactor with pump Р-3 and the gasoline is sent to sand filter F-1 for removal of alkali carried-over in the form of mist.

DMD-3 process flow diagram