Mechanism mass transport phenomena

Surface diffusion is yet another mechanism that is often invoked to explain mass transport in porous catalysts. An adsorbed species may be transported either by desorption into the gas phase or by migration to an adjacent site on the surface. It is this latter phenomenon that is referred to as surface diffusion. This phenomenon is poorly understood and the rate of mass... 

Many macromolecular compounds exhibit very complex mass transport mechanisms. It was suggested that this phenomenon originates from the contribution... 

Gas-to-liquid mass transfer is a transport phenomenon that involves the transfer of a component (or multiple components) between gas and liquid phases. Gas-liquid contactors, such as gas-liquid absorption/ stripping columns, gas-liquid-solid fluidized beds, airlift reactors, gas bubble reactors, and trickle-bed reactors (TBRs) are frequently encountered in chemical industry. Gas-to-liquid mass transfer is also applied in environmental control systems, e.g., aeration in wastewater treatment where oxygen is transferred from air to water, trickle-bed filters, and scrubbers for the removal of volatile organic compounds. In addition, gas-to-liquid mass transfer is an important factor in gas-liquid emulsion polymerization, and the rate of polymerization could, thus, be enhanced significantly by mechanical agitation. 

Diffusion is one of the basic mass transport mechanisms, which is involved in the control of drag release from numerous drag delivery systems (14-16). Pick was the first to treat this phenomenon in a quantitative way (21), and the textbook of Crank (22) provides various solutions of Pick s second law for different device geometries and initial and boundary conditions. A very interesting introduction into this type of mass transport is given by Cussler (23). 

The chemical approach to increasing transport rates involves the manipulation of the PIM composition. It has been observed that some PIM compositions provide much higher transport rates than their SLM counterparts, but the reasons for this phenomenon remain unclear. In order to understand the chemical processes occurring in PIMs, a number of researchers have investigated the structure of PIMs with a view to obtaining information regarding the way the carrier and other membrane components interact and the mechanisms for mass transport within the membrane. It is anticipated that once there is a better understanding of the structure of PIMs, it will be possible to better formulate the composition to provide optimum transport rates. 

At lower stresses and high temperatures, diffusion-controlled mechanisms will be active. Atomic-level mass transport by vacancies and interstitials can cause deformation at the macroscopic level. Vacancies can assist the motion of dislocations by cross-slip processes. As vacancy mobility and/or the density increases, the phenomenon of grain boundary migration will be observed. 

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