Zeolite membrane forces

For a packed-bed membrane reactor (PBMR) the membrane is permselective and removes the product as it is formed, forcing the reaction to the right. In this case, the membrane is not active and a conventional catalyst is used. Tavolaro et al. [45] demonstrated this concept in their work on CO2 hydrogenation to methanol using a LTA zeolite membrane. The tubular membrane was packed with bimetallic Cu/ZnO where CO2 and H2 react to form EtOH and H2O. These condensable products were removed by LTA membrane which increased the reaction yield when compared to a conventional packed bed reactor operating under the same conditions [45]. 

Colloidal zeolites have been used as building blocks to fabricate hierarchical porous materials. Infiltrating ethanol sol of zeolite nanoparticles into an ordered array of polystyrene spheres resulted in macroporous zeolites, which involves a self-assembly process. After ethanol evaporation, zeolite nanoparticles were aggregated by capillary forces. High concentration of external silanol groups favored the formation of hydrogen bonds between particles and eventually Si-O-Si bonds after calcination. The method has been further developed to produce transparent and self-standing zeolite membranes with controlled mesoporosity. Concurrently, the preformed zeolite-coated polystyrene spheres have been... 

Conversely, the correct approach to formulate the diffusion of a single component in a zeolite membrane is to use the MaxweU-Stefan (M-S) framework for diffusion in a nonideal binary fluid mixture made up of species 1 and 2 where 1 and 2 stands for the gas and the zeohtic material, respectively. In the M-S theory it is recognized that to effect relative motions between the species 1 and 2 in a fluid mixture, a force must be exerted on each species. This driving force is the chemical potential gradient, determined at constant temperature and pressure conditions [68]. The M-S diffiisivity depends on coverage and fugacity, and, therefore, is referred to as the corrected diffiisivity because the coefficient is corrected by a thermodynamic correction factor, which can be determined from the sorption isotherm. 

A series of original synthesis strategies has been also reported recently such as flow-through reactors for the homogeneous synthesis of zeolite membranes [77], centrifugal force field [114] or electrophoresis [115] for the preparation of A-type membranes, and pulse laser deposition (PLD) for the secondary growth of oriented MCM-22 membranes [116]. 

In many processes, including those in nature, transport proceeds via diffusion rather than convection. Substances diffuse spontaneously from a high to a low chemical potential. Processes which make use of a concentration difference as the driving force are gas separation, vapour permeation, pervaporation, dialysis, diffusion dialysis, carrier mediated processes and membrane contactors (In pervaporation, gas separation and vapour permeation it is preferred to express the driving force as a partial pressure difference or an activity difference rather than concentration difference). On the basis of differences in structure and functionality it is possible to distinguish between processes that use a synthetic solid (polymeric or sometimes ceramic or zeolitic) membrane (gas separation, dialysis and pervaporation) and those that use a liquid (with or without a carder) as the membrane. 

The simplest model for permeation through a zeolite membrane assumes a linear equilibrium isotherm and a constant diffusivity. The driving force is provided by the difference in partial pressure across the membrane so ... 

Intra-crystaUine permeation through a zeolite membrane can be described using different approaches (Krishna, 2006). For example, in the Fickian approach, the concentration gradient is the driving force through the zeolite membrane whereas in the Maxwell-Stefan (MS) approach the gradient of the thermodynamic potential is the driving force. The MS approach... 

Tiscareno-Lechuga F, Tellez C, Menendez M, and Santamaria J. A novel device for preparing zeolite—a membranes under a centrifugal force field. J Membr Sci 2003 212(1-2) 135-146. 

The available range of membrane materials includes polymeric, carbon, silica, zeolite and other ceramics, as well as composites. Each type of membrane can have a different porous structure, as illustrated in Figure 5.2. Membranes can be thought of as having a fixed (immovable) network of pores in which the gas molecule travels, with the exception of most polymeric membranes [28,44]. Polymeric membranes are composed of an amorphous mix of polymer chains whose interactions involve mostly van der Waals forces. However, some polymers reveal a behaviour that is consistent with the idea of existence of opened pores within their matrix. This is especially true for high free volume, high... 

Figure 12.25 Membrane reactors for FT synthesis from the literature (a) distributed feeding of reactants A and B, (b) in situ water removal by selective membrane (F, feed S, sweep), (cl) plug-through contactor membrane (PCM) with wide transport pores, (c2) forced-through flow membrane contactor, product and heat removal by circulated liquid product, (d) zeolite encapsulated FT catalyst, P, modified product [123].

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