**1. Introduction**

16 Mass Transfer in Chemical Engineering Processes

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The industrial importance of penetrable and/or impenetrable polymer systems is evident when one faces with a huge number of publications considering various aspects of diffusion phenomenon. Strong worldwide interest to realize more details about the fundamental of the process, generalize the governed laws to new findings, and find fast and reliable techniques of measurement, makes motivation to follow in this field of science.

Polymers are penetrable, whilst ceramics, metals, and glasses are generally impenetrable. Diffusion of small molecules through the polymers has significant importance in different scientific and engineering fields such as medicine, textile industry, membrane separations, packaging in food industry, extraction of solvents and of contaminants, and etc. Mass transfer through the polymeric membranes including dense and porous membranes depends on the factors included solubility and diffusivity of the penetrant into the polymer, morphology, fillers, and plasticization. For instance, polymers with high crystallinity usually are less penetrable because the crystallites ordered has fewer holes through which gases may pass (Hedenqvist and Gedde, 1996, Sperling, 2006). Such a story can be applied for impenetrable fillers. In the case of nanocomposites, the penetrants cannot diffuse through the structure directly; they are restricted to take a detour (Neway, 2001, Sridhar, 2006).

In the present chapter the author has goals of updating the theory and methodology of diffusion process on recent advances in the field and of providing a framework from which the aspects of this process can be more clarified. It is the intent that this chapter be useful to scientific and industrial activities.
