Study On Composite Membranes For FCC Gasoline Desulphurization

Gasoline desulphurization by pervaporation (PV) membrane process is a newly-emerged desulphurization technology. And this process has the following advantages: low investment and operating cost, depth desulphurization, modular design, easily magnification and construction etc. The technology has attracted the attention of petrochemical field. In this study, a polyethylene glycol (PEG)/ poly (vinylidene fluoride) (PVDF) composite membrane that can be applied on a commercial (or scale up) plant for fluid catalytic cracking (FCC) gasoline desulphurization was prepared. The process simulation and scale-up experiments were conducted too.The composite membrane has a clear-cut boundary surface between the dense active layer and the porous support layer by SEM photos, and the thickness of active layer is about 16μm. The study showed that the pre-wetting method could effectively confine the intrusion of PEG solution to porous PVDF support layer in coating process. Effects of operation conditions on the PV performance revealed that flux decreased with increasing permeate pressure while increased with the increasing operating temperature. Sulfur enrichment factor increased firstly and decreased then when temperature rose. The peak value occurred at about 373K. Under the feed flow rate of 100ml/min, the membrane showed better and stable performance. Flux increased while sulfur enrichment factor decreased as the feed sulfur content increased. PV experiments indicated that the membrane, with the crosslinking agent amount of 18% and solids content in active layer solution of 16%, had a stable performance for FCC desulphurization. The sulfur enrichment factor came to 3.6, and the total permeation flux was 2.7 kg /(m2·h).Process simulation model was developed for gasoline desulphurization by PV on a pilot scale. Based on the simulation results as well as discussions on considerable base data, scale-up plant was designed and established. The optimal operating conditions for scale-up plant were 373-383K of feed temperature, 5-10kPa of permeate pressure in vacuum shell and 18-21L/h of feed flow rate. The scale-up experiments under above conditions showed that the sulfur content of FCC gasoline was reduced from 710μg/g to 70μg/g after about 20h of continuous running, when the retentate rate was above 70%. Meanwhile, it was found that the desulphurization capacity for membrane sheets along the feed flow reduced, which correspond to the simulation results. Octane number has attracted more attention for gasoline desulphurization technology. Whether or not there is octane number loss for desulphurization process with PEG membranes is a necessary investigation from economic considerations. According to GC hydrocarbon group analysis, the octane numbers of retentate stream were a little higher than those of feed. However, the octane numbers for feed, retentate stream and permeate products changed little as a whole.