The process is used for treating pentane-hexane fraction for mercaptans and dimethyl sulfide.
The process is based on removal of mercaptans from the feedstock using DMD-2M process and subsequent azeotrope rectification of dimethyl sulfide concentrate.
The pentane-hexane fraction at a temperature of 30–40оС and a pressure of 0.5–0.6 MPa is supplied to the bottom of extractor V-1. An alkaline solution of IVKAZ catalyst (CC) is fed to the top of the extractor. Removal of C2 – C5 mercaptans with the alkali, followed by mercaptide formation, takes place in the extractor. The CC solution, containing the mercaptides, passes from the bottom of extractor V-1 to the bottom of regenerator R-1, wherein an estimated quantity of air is injected via disperser for oxidation of the mercaptides and regeneration of the alkali. Oxidation of the mercaptides with formation of disulfides takes place in the regenerator at a temperature of 40–45о С and pressure of 0.4–0.5 MPa.
From the top of R-1 the CC with the disulfides and air are sent to separator V-3, where separation of the air from the liquid phase takes place. From the top of the separator the air is sent for burning and the CC with the disulfides are sent to disulfide separator V-4. The disulfides are discharged from the top of V-4, and the regenerated CC is supplied to extractor V-1 with pump P-1.
From the top of extractor V-1 the demercaptanized feedstock flows to separator-demister V-2 for removal of carried-over alkali droplets and then to a column of azeotrope rectification К-1 for distillation of dimethyl sulfide and a part (5–6 %) of iso-pentane. A temperature in the column is 50–90 оС, pressure is 0.3–0.5 MPa. A mixture of the dimethyl sulfide and iso-pentane (5 % of the total amount in the feedstock) is discharged from the top of column K-1.
The pentane-hexane fraction, treated for mercaptans and dimethyl sulfide, is discharged from the bottom of column К-1 via cooler С-1 to a tank farm.
The DMD-4 process provides production of iso-pentane with total sulfur content of below 30 ppm. The process was tested under commercial conditions on Novokuibyshev petrochemical plant and Sterlitamak plant of synthetic rubber.
DMD-4 process flow diagram