Mobility selectivity

Y. Y. Huang, M. P. Nicoletti, and R. A. Sailor, "The Mobil Selective Toluene Disproportionation Process (MSTDP)," 1990 Petrochemical evieiv DeWitt Company, Houston, Tex., Mar. Ill—70 1990. 

It follows from Equation 8.13 that aA/B can be expressed as the product of the diffusivity selectivity, DA/DB, and the solubility selectivity, SA/SB. Diffusion (or mobility) selectivity is governed primarily by the size difference between gas molecules and always favors smaller gas molecules. Solubility selectivity is controlled by the relative condensability of the gases in the polymer and their relative affinity for the polymer. Solubility selectivity typically favors larger, more condensable molecules. From Equation 8.13, it is seen that the product of gas mobility and solubility selectivity determines the overall membrane selectivity. It is clear that for a membrane to be C02 selective, it must have high diffusivity selectivity based on the affinity for C02 but it should be flexible enough to permeate larger molecules such... 

MSDW [Mobil selective dewaxing] A catalytic dewaxing process which uses a catalyst containing a shape-selective molecular sieve and a noble metal. 

MSTDP [Mobil selective toluene disproportionation] A process for converting toluene to benzene and a xylene mixture rich in /7-xylene. The catalyst is the zeolite ZSM-5, selectively coked to constrict the pores and thus increase the yield of //-xylene produced. Developed and licensed by the Mobil Oil Corporation and first commercialized in Sicily in 1988. See also MTDP. 

C, 10-50 atm). Xylene benzene ratios of 1-10 may be obtained. Metal catalysts were later replaced by zeolites.210,211,326-328 The most recent development is the Mobil selective toluene disproportionation process,329 which takes advantage of the high para shape selectivity of a zeolite catalyst.210 The catalyst activated by a novel procedure ensures a p-xylene content of up to 95%. After the successful com-mercialization at an Enichem refinery in Italy, the process is now licensed. The catalysts and technologies applied in toluene disproportionation may be also used for transalkylation324,325,331 [Eq. (5.74)] ... 

The process shown in Figure 9.21 was first developed by Separex, using cellulose acetate membranes. The separation factor for methanol from MTBE is high (>1000) because the membrane material, cellulose acetate, is relatively glassy and hydrophilic. Thus, both the mobility selectivity term and the sorption term in Equation (9.5) significantly favor permeation of the smaller molecule, methanol, because methanol is more polar than MTBE or isobutene, the other feed components. These membranes are reported to work well for feed methanol concentrations up to 6%. Above this concentration, the membrane is plasticized, and selectivity is lost. More recently, Sulzer (GFT) has also studied this separation using their plasma-polymerized membrane [56],... 

The permselectivity is the product of ion-exchange selectivity and mobility selectivity. The mobility of different ions is determined mainly by steric effects, that is, the size of the ions and the cross-linking density of the membrane [4],... 

Applied Voltage Effect on Mobility, Selectivity, and Resolution in Capill ... 

Applied Voltage Effect on Mobility Selectivity and Resolution in Capillary Electrophoresis... 

Equation 8 demonstrates that the ideal separation ac tor can be separated into a so-called "s lu illty selectivity", [S /Sg], and a "mobility selectivity", ID /Db]. These two ratios are also reported in Table I. Evidently, the contribution of the "mobility selectivity" is the dominant factor for all of the polymers considered except cellulose acetate in which the opposite is observed. The CO2 plasticization tendency of cellulose acetate may, in fact, be related to this polymer s apparent high "solubility selectivity". Clearly, the available data do not justify more than a tentative suggestion at this point that high "solubility selectivity" such as that seen in cellulose acetate may be associated with a tendency to be plasticized with a subsequent loss in glass-like selectivity. 

Poly(phenylene oxide) is an example of a material that might benefit from such a blending approach if an appropriate miscible "solubility selective" polymer could be discovered. As shown in Figure 2 and Table I, PPO is highly permeable to C02 and has a respectable mobility selectivity equal to 6.9 for the CO2/CH4 system. 

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