Selectivity Knudsen

Since water is the byproduct, and also has an undesired inhibitory effect on catalyst activity, it must be separated efficiently from the reaction mixture. To achieve this, both conventional reactive distillation and reactive membrane separation are considered as process alternatives. In the latter process, a Knudsen-membrane is applied. Consequently, the mass transfer matrix [/c] has a diagonal structure and the diagonal elements are the Knudsen-selectivities - that is, the square-roots of the ratios of the molecular weights Mr. 

In summary, the main goal of the present work is the development of a hydrothermally stable microporous silica membrane with prescribed transport properties. Preferably, these steam stable membranes should have very high permselectivities. Because the permselectivity of a molecular sieving silica membrane will drop to the Knudsen value of the y-alumina supporting membrane when the silica membrane deteriorates under steam reforming conditions, a selectivity of the silica layer higher than the Knudsen selectivity is sufficient. In this way the measurement of the permselectivity is a powerful tool to assess the hydrothermal stability of a supported microporous membrane. 

First results are, however, rather positive. It was possible to coat silica layers with a selectivity, which was somewhat above the Knudsen selectivity. This shows the presence of at least some degree of microporosity with, fortunately, still a relatively high N2 permeance compared to that of the centrifugal cast tubes described in chapter 4. Coating with just 1 silica layer from an undiluted silica coating solution showed to give best results. Coating with a second layer, did not improve selectivity, it did only reduce the N2 permeance of the membrane with... 

Membrane treatment Gas Permeability x 10 (cm (STP)-cm/s-cm2-cm Hg) Measured selectivity Knudsen selectivity... 

Microporous inorganic membranes have pores that can be tuned to the molecular size. This enables zeolite membranes to carry out separations (i.e., the separation of isomer compounds) that are not possible with membranes in which only Knudsen selectivity is possible. Moreover, zeolite microporous membranes can compete with traditional energy costly separation methods, such us distillation of mixmres of close boiling point components, separation of mixtures of low concentration, and azeotropic distillation. 

The application of a membrane reactor with a hydrogen permselective membrane to intensify H2S decomposition to hydrogen and sulphur at a lower temperature was first considered in the eighties [50, 73, 74]. However, the expected positive effects on H2S conversion observed with the early Vycor-based membranes were insignificant. Also, a zirconia-silica membrane developed more recently and applied to decomposition of H2S by Ohashi et al. [75] had only Knudsen selectivity and produced similar results. 

In Knudsen diffusion (Fig. 11.9a), the pore size forces the penetrant molecules to collide more frequently with the pore wall than with other incisive species [26]. Except for some special applications as membrane reactors, Knudsen-selective membranes are not commercially attractive because of their low selectivity [27]. In surface diffusion mechanism (Fig. 11.9 b), the pervasive molecules adsorb on the surface of the pores so move from one... 

Pores Even porous membranes can give very high selectivity. Molecular sieve membranes exist that give excellent separation factors for gases. Their commercial scale preparation is a formidable obstacle. At the other extreme, UF,3 separations use Knudsen flow barriers, with aveiy low separation factor. Microfiltration (MF) and iiltrafiltra-tion (UF) membranes are clearly porous, their pores ranging in size from 3 nm to 3 [Lm. Nanofiltration (NF) meiTibranes have smaller pores. 

Selective gas permeation has been known for generations, and the early use of p adium silver-alloy membranes achieved sporadic industrial use. Gas separation on a massive scale was used to separate from using porous (Knudsen flow) membranes. An upgrade of the membranes at Oak Ridge cost 1.5 billion. Polymeric membranes became economically viable about 1980, introducing the modern era of gas-separation membranes. Hg recoveiy was the first major apphcation, followed quickly by acid gas separation (CO9/CH4) and the production of No from air. 

Knudsen I M B, Hockenhull J, Jensen D F, Gerhardson B, Hokeberg R, Tahvonen R, Teperi E, Sundheim L and Henriksen B (1997), Selection of biological control agents for controlling soil and seed-borne diseases in the field , Europ. J. Plant Pathol., 103, 775-784. 

The optimum data are selected in step l by using a precalculated standard deviation, which is directly related to the constancy of temperature. These selected sets of data are used in step 2 for calculation of equilibrium vapor pressures by means of the Knudsen equation. A regression analysis of these vapor pressure data is then carried out in step 3. 

Summarizing it can be stated that the separation by gas phase transport (Knudsen diffusion) has a limited selectivity, depending on the molecular masses of the gases. The theoretical separation factor is decreased by effects like concentration-polarization and backdiffusion. However, fluxes through the membrane are high and this separation mechanism can be applied in harsh chemical and thermal environments with currently available membranes (Uhlhorn 1990, Bhave, Gillot and Liu 1989). 

We should add a note of caution here, however, for in the Knudsen flow region De is proportional to the pore radius. When the pores are sufficiently small for Knudsen diffusion to occur then the selectivity will also be influenced by pore size. Maximum selectivity would be obtained for small particles which contain large diameter pores. 

Knudsen, H.N., Nielsen, P.A., Clausen, P.A., Wilkins, C.K. and Wolkoff, P. (2003) Sensory evaluation of emissions from selected building products exposed to ozone. Indoor Air, 13 (3), 223-31. 

It follows from Equation (2.107) that the permeability of a gas (/) through a Knudsen diffusion membrane is proportional to 1 The selectivity of this... 

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