Study On Pervaporation Membrane And Mass Transfer And Hybrid Process For Separating Hydrocarbon (Ether)-Methanol Mixtures

In this paper, the pervaporation separation of hydrocarbon(ether)-methanol mixtures using various kinds of membranes from cellulose acetate, cellulose acetate/poly(vinyl butyral) blend and methyl mathacrylate-acrylic acid-butyl acrylate terpolymer, and the effect of concentration polarization on pervaporation transfer process were studied. Finally, a hybrid process of pervaporation with distillation was designed and optimized, and the costs of tradition technique and hybrid process were analyzed. 1. Separation performance of cellulose acetate membraneUnder different operating conditions, a trade-off trend was found for cellulose acetate (CA) membranes. The separation factor decreased with increasing operating temperature and methanol concentration in feed as the flux increased, or vice versa. The permeation flux increased and the methanol selectivity decreased with increasing molecular weight of polymer since the package holes of the polymer with different molecular weights were different.The transmitted light intensity of casting solution had been measured at different environment parameter of membrane preparation by lab-made light transmission device. It was found that the relative humidity influenced transmitted light intensity of casting solution. The intrinsic viscosity of CA dilute solution in dioxane-water mixed solvent decreased firstly, and then increased with the increase of water concentration in solvent. It was shown that there is association effect in dilute solution as the good solvent becomes the poor solvent owing to the introduction of water. So, the morpholotical structure of membrane and thus its pervaporation characteristics would change with relative humidity With increasing relative humidity, the flux decreases, but the separation factor increases.Viscosity behavior of CA in four solvents (tetrahydrofuran, dioxane, acetone and butanone) was investigated. It was found that the viscosity slope coefficients (b) and permeation flux for CA membranes with different solvents were different, and the permeation fluxes were inversely proportional to the b values. The opposite relationship between permeation flux and b could be due to the solvent solvation power, which alters b, and also the degree of coil overlap and the bulk density ofpolymer membranes. So, the separation performance of CA membrane can be improved by adjusting the composition of casting solvent. As the methanol content in casting solvent was 15vol%, the methanol flux and separation factor of CA membrane were 138g.m"2.h'' and 161, respectively.The selectivity and flux may be improved by immersing the CA membrane in alcohol-containing solvents. For the CA membrane immersed in non-alcohol solvents, the flux increased, but the selectivity decreased. The results of DSC showed that there was crystallization phenomenon for the CA membrane immersed in alcohol solvents.There were high separation factor and methanol flux as CA3 membrane was used to separate methanol/Cs mixture, and it could be applied in industry. Two CA3 membranes with different areas of 0.00159m2 and 0.0748m2 are used for pervaporation separation of methanol/C5 mixture. They are placed in two modules with different structures, and then the current statuses of feed are disparate, the velocity distributions of the feed in both modules are simulated by the ANSYS software. At the same current velocity, the holding time of the feed in the larger module is longer than that in smaller module, and there are more channel and dead angle in the smaller module. It caused that the separation factor is higher, and the flux is lower in the larger module. 2. Separation performance of cellulose acetate-poly(vinyl butyral) blend membrameThe membranes were prepared from different types of CA and PVB by solution blending. The compatibility of the polyblends was studied by FTIR, viscometry and measurement of density of blend membrane. It was found that the compatibility region of CA1-PVB was W|>VB<30wt%, and these of CA2-PVB, CA3- PVB and CA4-PVB were WPVB=30~70wt%, 20~70wt% and 20~60wt% respectively. From FTIR, it was discovered that there was hydrogen bond between CA and PVB in compatibility region, then, the OH group content in CA could influence the compatibility of CA-PVB. Additionally, the compatibility of CA-PVB blend was influenced by their molecular weights.The effect of compatibility of CA-PVB on interfacial tension and hydrophilicity of blend membrane was investigated by measuring water contact angle. The inter-molecular interaction between CA and PVB enhanced, and interfacial tensionand hydrophilicity of blend membranes increased with the increase of compatibility. For the incompatible blend membranes, the interfacial force weakened, and the hydrophilicity decreased.The CA-PVB blend membranes were used to separate methanol/MTBE mixture with 5.5wt% methanol. The results showed that the permeation flux increased and the selectivity decreased with increasing PVB content in blend membranes. At the region of compatibility, the negative deviation of permeation flux for CA membrane and PVB membrane was displayed, and at the region of incompatibility, the positive are demonstrated. The pervaporation performances of blend membranes were improved. Their compositions were CA1:PVB=9:1, CA2:PVB=7:2, CA3:PVB=8:2 and CA4:PVB=7:3, respectively. And the permeation flux of blend membrane was more than that of CA membrane, the selectivity of blend membrane was better than that of PVB membrane.3. Separation performance of methyl methacrylate-acrylic-butyl acrylate terpolymer membrameSeveral MMA(methyl methacrylate)-AA(acrylic acid)-BA(butyl acrylate) terpolymers with different compositions were synthesized. The molecular weights of these copolymers were measured by gel chromatography. The molecular weights of all the terpolymers were greater than 105.Swelling experiments of these terpolymers in pure methanol, MTBE, C5 and methanol/MTBE/Cs mixtures were investigated, respectively. The results showed that there was a strong swelling ability of copolymer with higher content of AA in methanol, and there was an interaction between MTBE and copolymer with higher content of BA.The pervaporation results showed that the separation factor increased, and the methanol flux decreased with the increase of the AA content in membranes. The total flux and methanol flux all increased with the increase of BA content in membranes. Furthermore, the fluxes were influenced by the molecular weights of membranes.The copolymer membranes with 3.1:7:6 and 3.1:7:3 MMA-AA-BA weight ratios were used to separate methanol/MTBE mixture at 30°C. The separation factor were 481.57 and 943.32, and the methanol fluxes were 202.98g.m'2.h"' and 107.23g.m"2.h'', respectively.4. Concentration polarization phenomenonThe dilution diffusion coefficients of methanol in MTBE and C5 were obtained from Wilke-Chang equation. The value of diffusion coefficients of methanol in C5 was more than that of methanol in MTBE. From Vignes equation and experimental results, it was found that there was concentration polarization in liquid boundary-layer as the methanol concentrations in the C5 and MTBE were less than 0.5wt% and 1 .Owt%, respectively. The concentration polarization could be weakened by reducing the thickness of liquid boundary-layer.The pervaporation separation properties for the methanol/MTBE/Cs ternary mixtures were studied. The results showed that the methanol flux decreased with the increase of MTBE concentration in the ternary mixtures. Based on Hansen's solubility parameter theory, the interaction between methanol and MTBE was higher than that between methanol and C5. From Stefan-Maxwell equation and the values of pervaporation experiment, the methanol concentration profiles in liquid boundary-layer were calculated, resulting in a more significant decrease of methanol concentration in boundary-layer with increasing C5 content in mixture. And, for the mixtures with high C5 content, the control step for pervaporation process was methanol diffusion in liquid boundary-layer. The dilute diffusion coefficient with composition of methanol/MTBE/C.s ternary system was calculated from Leffler-Cullinan equation, and the value of diffusion coefficient decreased, with the decrease of C> in ternary system.5. Hybrid process of distillation and pervaporationA hybrid process of distillation and pervaporation was designed and simulated by HYSIM software for separating methanol from MTBE and C4. The pervaporation unit in the hybrid process was used to remove methanol and to make the distillation gotten over azeotropic point. In the case, the qualified product of C4 and MTBE with methanol mole fraction less than 10"4 could be obtained by the hybrid process. The pervaporation was side-connected with distillation in rectifying section, and the retentate was pumped back to the upsection of side-connecting point. In order to improve the pervaporation behavior and reduce the membrane area, it was required that the methanol concentration in retentate should be high as 0.025.The total annual costs of traditional process and hybrid process for productionof MTBE were estimated. The results showed that the total annual cost of de-butanizer was similar with that of water scrubbing and methanol recovery column in traditional process, and the total annual cost of de-butanizer was 98% of that of the hybrid process. The energy consumption was the main part of annual operation cost. The annual operation cost of hybrid process was depended on the reflux of de-butanizer. As reflux ratio was less than 2.0, the operation cost of hybrid process was less than that of traditional process. And, the cost of hybrid process decreased with the decrease of reflux ratio.