Composite Hydroxyethyl Cellulose Membrane For Sulfur Removal From FCC Gasoline

Gasoline desulphurization by pervaporation (PV) membrane process is a newly-emerged desulphurization technology. The technology has the following advantages: low investment and operating cost, depth desulphurization, modular design, easily magnification and construction etc. It has attracted the increasing attention of petrochemical field, but little was reported on industrialzation application of the technology in China. The researches on desulfurization mechanism in composite membrane were also seldom reported. Therefore, it is very important significance in theory and application to deeply study the mass transfer mechanism of pervaporation process, and to prepare with independent intellectual property rights, the high permeable and high selective membranes in order to propel the newly-emerged desulphurization technology into practical application.The membrane material HEC for desulfurization was studied by using the homogeneous plate membrane. The effects of the factors such as the intrinsic viscosity of HEC, crosslinkers, content of crosslinker, feed type, feed temperature and solvent vaporing time on PV performance of HEC membranes were investigated. Through these studies, the suitable formula and operation conditions for preparing the active layer of composite membrane were found. According to FTIR spectra of different HEC membranes, it was found that the crosslink reaction took place in HEC membrane, and different acrylate compounds and HEC reacted to generate the similar characteristics group. Moreover, the density of crosslink increased with the increasing content of crosslinker. The HEC membranes prepared showed the total fluxs of 0.13, 0.6 and 0.267 kg·m-2·h-1 for model gasoline and 2 FCC gasolines, and sulfur enrichment factors were 26, 3.57 and 5.873, respectively.At 25°C, the sorption curves of the typical gasoline components heptane, cyclohexane, cyclohexene, toluene and thiophene in the different crosslinked HEC membranes indicated that, n-heptane and cyclohexane in HEC was a typical non-Fick diffusion, whereas cyclohexene, toluene and thiophene was a typical example of the Fick diffusion. The increasing degree of crosslink did not change the diffusion mode of the gasoline component in HEC, but just reduced the diffusion coefficient and solubility of gasoline components.PVDF support layer were prepared. The influnces of the concentration of polymers, the content of additive and solvent vaporing time on the structure and performance of support layer were investigated. According to the orthogonal experiments, a significant degree order, these factors affecting on PVDF membrane structure and performance, was PVDF concentration > additive content > solvent vaporing time. HEC solution was coated on the self-prepared PVDF membranes and made of composite membranes. PV performances of composite HEC membranes were tested and the suitable formula and operation conditions for preparing the PVDF membranes were also found as follows: 12.72% concentration of polymers, additive mass fraction of 1.969%, solvent vaporing time 5s. Seen from the effect of concentrations of polymer and additive agent on PV performance of composite HEC membrane, when the polymer concentration was not more than 12.7%, or the mass fraction of additive agent was less than 1.959%, hydrocarbon compounds appeared the capillary agglutination in the process of mass transfer through the PVDF membrane.The prediction abilities of UNIFAC-FV model, UNIFAC-ZM model and Entropic-FV model for the solubility behavior of n-heptane and thiophene in HEC membrane were evaluated. When sulfur mass fraction was no more than 4000μg·g-1, UNIFAC-FV model forecast best. Based on free volume theory, Vrentas-Duda diffusion model was used to calculate the diffusion coefficient of heptane, thiophene in the HEC membranes at 25, 40, 60°C. The calculated diffusion coefficient was good coincident with the experimental data, indicating that UNIFAC-FV model and Vrentas-Duda model were higher feasible methods studing the solubility and diffusion behavior of small molecules in the polymer.The partition coefficient of heptane, thiophene in the liquid phase-HEC membrane phase and the diffusion coefficient in the polymer were calculated by using UNIFAC-FV model and Vrentas-Duda model, respectively. The henry’s contants of n-heptane, thiophene in the PVDF membranes were computed by UNIFAC-FV model. According to the partition and diffusion coefficients and henry’s contants obtained, mass transfer resistances of heptane and thiophene in the active layer and support layer were both estimated. On resistance-in-series model, the effects of temperature, liquid sulfur mass fraction, the thickness of the active layer, and the thickness of the support layer on the mass transfer of n-heptane, thiophene in the composite membrane were investigated. The analysis results indicated that temperature and liquid sulfur mass fraction impacted a little bit on the mass transfer of heptane and thiophene in the composite membrane. The thickness of active layer had a greater influence on the mass transfer of thiophene but had a less impact on the mass transfer of n-heptane. When the active layer was very thin, the thickness of support layer played an important role in the mass transfer process of n-heptane, thiophene through the composite membranes. The importance was dramatically weakened by the increasing thickness of active layer. The diffusion mode of gasoline components in the support layer depended on the pore sizes of support layer.