Thermal membrane crystallization

In thermal membrane crystallization, the membrane is in contact with the warm crystaUizing solution at the retentate side and with a cold condensing solution at the distillate side, so that the driving force is generated by a temperature difference. Membrane crystallization is particularly attractive for the preparation of heat-sensitive catalysts such as enzymes. 

PLA membranes with different crystalline structures were prepared by regulating the heating and cooling conditions during membrane preparation [1], Thermally induced crystallization of PLA membranes was conducted by casting under vacuum for 48 hours at 70°C, 80°C, 90°C, and 150°C. The thermally treated PLA membranes were then cooled to room temperature under atmospheric pressure. The properties of the PLA membranes prepared in the study of Sawada et al. are summarized in Table 3.1 [1]. The membrane density increased from 1.257 to 1.273 g cm" as the treatment temperature increased from 70°C to 150°C. 

A crystalline or semicrystalline state in polymers can be induced by thermal changes from a melt or from a glass, by strain, by organic vapors, or by Hquid solvents (40). Polymer crystallization can also be induced by compressed (or supercritical) gases, such as CO2 (41). The plasticization of a polymer by CO2 can increase the polymer segmental motions so that crystallization is kinetically possible. Because the amount of gas (or fluid) sorbed into the polymer is a dkect function of the pressure, the rate and extent of crystallization may be controUed by controlling the supercritical fluid pressure. As a result of this abiHty to induce crystallization, a history effect may be introduced into polymers. This can be an important consideration for polymer processing and gas permeation membranes. 

Conditions sometimes exist that may make separations by distillation difficult or impractical or may require special techniques. Natural products such as petroleum or products derived from vegetable or animal matter are mixtures of very many chemically unidentified substances. Thermal instability sometimes is a problem. In other cases, vapor-liquid phase equilibria are unfavorable. It is true that distillations have been practiced successfully in some natural product industries, notably petroleum, long before a scientific basis was established, but the designs based on empirical rules are being improved by modern calculation techniques. Even unfavorable vapor-liquid equilibria sometimes can be ameliorated by changes of operating conditions or by chemical additives. Still, it must be recognized that there may be superior separation techniques in some cases, for instance, crystallization, liquid-liquid extraction, supercritical extraction, foam fractionation, dialysis, reverse osmosis, membrane separation, and others. The special distillations exemplified in this section are petroleum, azeotropic, extractive, and molecular distillations. 

Small-angle X-ray diffraction was used to identify the time-averaged location of amiodarone in a synthetic lipid bilayer. The drug was located about 6 A from the center of the lipid bilayer (Figure 4.13) [125, 126]. A dielectric constant of k = 2, which is similar to that of the bilayer hydrocarbon region, was used to calculate the minimum energy conformation of amiodarone bound to the membrane. The studies were performed below the thermal phase transition and at relatively low hydration of lipid. The calculated conformation differed from that of the crystal structure of amiodarone. Even though the specific steric effects of the lipid acyl chains on the confor-... 

A sharp change in porosity of the titania membranes occurs at 350 C. This may be attributed to the crystallization of the constituent titania particles [Xu and Anderson, 1994]. Thus crystallization is likely to be a key factor affecting thermal stability of sol-gel derived titania membranes. 

Silica membranes have also been studied by several investigators for use in gas separation and membrane reactors. They arc thermally very sublc up to about 500°C. Sintering and densification temperatures of silica membranes depend on the water/alkoxide ratio in the sol-gel process for making the membranes (Langlct et al., 1992]. Crystallization of amorphous silica particles in the membranes takes place at temperatures around 1,000°C [Larbot et al., 1989]. However, pore growth can gradually... 

Separation processes are not only of great importance in refineries, but also in the chemical, petrochemical, gas processing, and pharmaceutical industries. Although the reactor can be regarded as the heart of a chemical plant, in most cases, 60-80% of the total cost is taken up by the separation step. This step involves one or more thermal separation processes such as distillation, extraction, absorption, crystallization, adsorption, membrane processes, c/c., which are used to obtain the products at the required purity. 

Pure Pd membranes become brittle upon thermal cycling in a H2 atmosphere due to a phase transition between the different Pd hydride phases (a and P) with distinctly different crystal lattice parameters (see Fig. 11.3). Allojting with various other metals (for example Ag, Ru, Rh) tends to lower the phase transition temperature and under some circumstances improves the hydrogen permeability. The phase transition problem is not technically insurmountable, if good care is exercised during start-up (when the membrane is first exposed to hydrogen)... 

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