Facilitated diffusion possible separations

It was proposed that the temperature dependence of polymer 5 arises from the temperature dependence of the kA step. Specifically, it was suggested that the polymer segments to which the radicals are attached are conformationally stressed. There are two possible modes for the newly formed radicals to relax and become separated They can rotate or recoil away from each other (Scheme 9). These secondary motions of the polymer arise from the relaxation of unfavorable bond conformations that are formed during the polymer casting process. The increased thermal energy facilitates the rotation and recoil relaxation processes, which effectively increases the rate constant for diffusion of the radicals out of the cage, kA. This leads to decreased radical-radical recombination and consequently an increase in photodegradation efficiency. 

A well-defined amount of co-crystallization is possible across the interface of two adjacent crystals by annealing two stacked, completely wetted, solution-cast films of UHMW-PE [32]. It was found that doubling of the lamellae across the interface enhances the peel energy to such a level that the films could not be separated anymore. By contrast, pre-annealing one side of the film prohibited co-crystallization across the interface and these films could still be separated easily. It was therefore concluded that a limited amount of chain diffusion across the interface occurs during doubling of the lamellae, as facilitated by the well-defined structure of the interphase due to the adjacent reentry that occurs upon crystallization from solution. 

In facilitated transport, unlike solvent extraction and other equihbrium stage wise processes, the overall mass transfer rate is not governed by the usual equihbrium considerations alone. Instead, the solute transport process is controlled by a combination of the diffusion rate and the complexation reaction rate and in case of coupled transport the solute can be transported against its concentration gradient thus opening up the possibilities of separation from even very dilute solute solutions. 

The phenomenon of surface diffusion, demonstrated by Barter and Strachan (1955) may be used to advantage for the separation of some hydrogen-hydrocarbon mixtures (Sircar, 1993b). Thus, thin microporous carbon membranes (<5 pm), supported by macroporous sheets of graphite assembled into a plate and frame membrane module, were able to separate H2 at 63% recovery from a refinery waste gas. The feed was passed at high pressure over the membrane unit which preferentially adsorbed the hydrocarbons and facilitated surface diffusion of these species through the membrane to the low pressure side of the module. Adsorption, accompanied by surface diffusion through a thin microporous membrane, thus offers possible future applications. 

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