Sorption components

Thus, while individual indices such as y or yv have been successful in modeling the sorption of non-polar hydrophobic compounds, they are not by themselves good indicators of any non-hydrophobic sorption component, since, for a given class of compounds, all MCIs show a general increase with molecular size. 

The experimental and theoretical data presented in the literature show that selective permeability of mbbery polymers for hydrocarbons follows the general mle of being dominated by the sorption component. Selective permeability of glassy polymers for hydrocarbons follows the general mle of being dominated by the diffusion component. 

The permselectivity of hydrocarbon vapors, p, is dominated by the sorption component, and sorption of hydrocarbon vapors by rubbery polymers is determined by the condensability of their vapors. It can be seen from Table 9.3, that in organosilicon polymers the propane/methane sorption selectivity, is 10.5-16.2, whereas diffusion selectivity, is only 0.16-0.41. Refs. [39 3] report values of permselectivity of hydrocarbon mixtures with nitrogen for organosilicon membranes produced by GKSS (see Figure 9.10). It can be seen that separation selectivity increases with rising boiling temperature of the hydrocarbon, which points to domination of the sorption component of selectivity. 

In selective separation of hydrocarbons from their mixtures with air or from their aqueous solutions, it makes sense to use membranes based on rubbery polymers, whose permeability increases with the decrease in glass transition point. Permselectivity of rubbery polymers is dominated by the sorption component, which increases with condensability of the hydrocarbon penetrant. Higher activity of the component being separated in the feed mixture results in plasticization of the membrane and can make it swell. This can produce a non-monotonic dependance of selective properties of the membrane on activity of the component being separated. As a rule, permselectivity for mixtures of penetrants is significantly lower than their ideal values. Negative values of sorption heat of easily condensable hydrocarbons can result in existence of non-monotonic temperature dependencies of mass transfer coefficients. 

The polymer materials mainly used for the membranes are glassy polymers, the first and foremost polyimides. The use of glassy polymers having a rigid ensemble of macromolecules results in high separation effectiveness. Separation effectiveness in pervaporation processes is characterized by the separation factor, /3p, which is determined by the diffusion component, /3d, and the sorption component, /3s [8,55]. Let us consider the effect of chemical composition of polymer membranes on their transport properties with respect to aromatic, alicyclic, aliphatic hydrocarbons and analyze ways to improve these properties. 

Increasing the Sorption Component of the Separation Factor by Introducing a Homogeneously Distributed Electron Acceptor into the Polyimide Matrix... 

To increase the sorption component of the separation factor, homogeneously distributed tetracyanoethylene, a strong electron acceptor having high affinity for electron donors, was added to the polyimide matrix [77]. It can be seen from data presented in Table 9.12 that this is accompanied by an increase in the sorption component /3s (benzene/cyclohexane) by a factor of 1.5 probably as a result of selective sorption of aromatic compounds by tetracyanoethylene with a simultaneous increase in the diffusion component /3d. The prepared membranes showed good pervaporation properties with respect to benzene/cyclohexane, toluene/isooctane mixtures. For example, for a two-component 50/50 wt% benzene/cyclohexane mixture at 343 K, the flux was 2 = 0.44 kg p,m/m h, and /3p (benzene/cyclohexane) = 48 and for a two-component toluene/isooctane mixture, 45/55 wt%, at 343 K the flux was 2 = 1-1 kg p-m/m h, and /3p (toluene/wo-octane) = 330. 

Ethynyi Groups According to Ref. [77], acetylene fragments introduced into polyimides, e.g., 2,2 -diethynylbenzidine, DEB, have TT-electron affinity for aromatic compounds. It can be seen from the data presented in Table 9.12 that the increase in the content of DEB component results in an increase in the separation factor of the benzene/cyclohexane mixture. This is probably caused not only by the growth of the diffusion component /8d (resulting from thermal cross-linking through the unsaturated bonds) but also by enhancement of the sorption component /Ss-... 

Carboxylic Groups Pervaporation separation of toluene/i-octane mixmres using copolyimide membranes containing 3,5-diaminobenzoic acid (DABA) was investigated in Ref. [128]. It was established that introduction of diaminobenzoic acid into the 6FDA-TrMPD polyimide improves membrane selectivity. The sorption component of the separation factor /3s is hnearly correlated with the membrane solubility parameter and with DABA content in the copolymer (/3s = 3.2, 3.3,4.3, 5.2 for DABA contents 0%, 10%, 33%, 60%, respectively). 

Phosphates of zirconium were selected as additional active sorption components to be introduced into the porous matrices of the sorption electrodes. This choice was justified by the following reasons ... 

Consequently, the polarization current i(f) consists of nucleative and sorption components which depend on each other and, therefore, cannot be clearly separated. However, it can be assumed that after a critical time, tc, qep(t) is approaching a steady state final value ep( -> °°) which is of the order of magnitude of the metastable equilibrium charge amount ep at tiE/. This means, d ep(0/df tends to zero for t tc. On the other hand, i nuc(0 tends to zero for f -> 0 and t oo in conventional 2D... 

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