Particularities of Membrane Gas Separation Under Unsteady State Conditions

Abstract

Membranes become the key component of modern separation technologies and allow exploring new opportunities and creating new molecular selective processes for purification, concentration and separation of liquids and gases (Baker, 2002, 2004). Particularly the development of new highly effective processes of gas separation with application of existing materials and membranes takes specific place. In present time special attention devotes to purification of gas and liquid waste streams from ecologically harmful and toxic substances such as greenhouse gases, VOCs and others. From the fundamental point of view the development on new highly effective processes of gas separation demands the investigation of mass transfer in the unsteady (kinetic) area of gas diffusion through a membrane. This approach allows in some cases to obtain much higher selectivity of separation (using the same membrane materials) compared to traditional process where steady state conditions are applied. First studies of membrane separation processes under unsteady state conditions have demonstrated both opportunities and problems of such approach (Beckman, 1993; Hwang & Kammermeyer, 1975; Paul, 1971). It was shown that effective separation in unsteady membrane processes is possible if residence times of mixture components significantly differ from each other that is the rare situation in traditional polymeric materials but well known for liquid membranes with chemical absorbents (Shalygin et al., 2006). Nevertheless similar behavior is possible in polymeric membranes as well when functional groups which lead to partial or complete immobilization of diffusing molecules are introduced in polymer matrix. Moreover the functioning of live organisms is related with controllable mass transfer through cell membranes which “operate” in particular rhythms. For example scientific validation of unsteady gas transfer processes through membranes introduces particular interest for understanding of live organisms’ breathing mechanisms. It can be noticed that development of highly effective unsteady membrane separation processes is far from systematic understanding and practical evaluation. Therefore the evolution of investigations in this area will allow to accumulate new knowledge about unsteady gas separation processes which can be prototypes of new pulse membrane separation technologies. Theoretical description of unsteady mass transfer of gases in membranes is presented in this work. Examples of binary gas mixture separation are considered for three cases of gas