Drying process in the formation of sol-gel-derived TiO2 ceramic membrane

Accurate drying data for thin titania gel layers dried at 40°C and 20% relative humidity (RH) are given. The drying rate versus free moisture content diagram should show three regions as predicted by the classical drying theory. They are the constant rate period, the first falling rate period and the second falling rate period. The second falling rate period was not observed in the present case, because at 40°C and 20% RH the equilibrium moisture content will be enough to provide a continuous fluid network in the gel. The total drying time in the falling rate period increases with layer thickness. The drying mechanism in the first falling rate period was identified as capillary flow

Catalysis with inorganic membranes

Catalytic inorganic membranes are among the most challenging and intriguing porous materials. Consisting of a thin film of mesoporous or microporous inorganic material deposited on a macroporous material, catalytic membranes are multifunctional materials that must be engineered for both chemical and physical properties. New approaches to carrying out chemical reactions are possible by tailoring the membrane catalytic activity and selectivity, permselectivity, and other thin film properties. Readers are referred to several recent reviews of inorganic membranes, in particular, Zaspalis and Burggraaf, Armor, Gellings and Bouwmeister, Hsieh, Stoukides, and Tsotsis et al. Inorganic membranes are most conveniently classified according to pore size (see introductory article). Of particular importance is the ratio of the pore size to the molecular mean free path (MFP). Decreasing pore dimensions lead to increased selectivity with corresponding loss of permeability. Macroporous membranes have a pore size much larger than the MFP, leading to molecular (bulk) diffusion or viscous flow. Knudsen diffusion dominates in the mesoporous regime, where the pore size is comparable to the MFP. In addition, surface diffusion of the molecules along the pore walls may contribute, leading to an enhanced flux of the adsorbed species along the walls. The microporous regime is encountered when the pore size is comparable to the molecules. This regime makes possible much higher permselectivities, which depend on both molecular size and specific interactions with the solid. Finally, in dense membranes, molecular transport occurs through a solution-diffusion mechanism, which also involves specific interactions between the solute and membrane

Intermediate product yield enhancement with a catalytic inorganic membrane: I. Analytical model for the case of isothermal and differential operation

A simple model is developed to examine the performance of a supported catalytic membrane within which occurs the consecutive-parallel reaction system given by A + B → R, with rate = k1pαA1ApαBB, and A + R → P, with rate = k2pαA2ApαRR. Closed-form solutions reveal that segregation of reactants A and B to opposite sides of the membrane is an effective strategy for increasing the desired product (R) point yield. However, increases in the component R yield come at the expense of the point catalyst utilization, due, in part, to depletion of reacting components B and R. The membrane performance is sensitive to the relative reaction orders with respect to component A for the special case in which the rates are zeroth-order with respect to B and R (αB = αR = 0). The segregation strategy is shown to be most beneficial if three requirements are met: (i) αA1 < αA2, (ii) k1, k2 sufficiently large and (iii) active layer sufficiently thin compared to support. Under favorable conditions [requirements (i)-(iii) met], component R is selectively produced near the active layer surface, and diffuses out of the membrane before further reaction to undesired product (P). The simulations indicate that the fractional increases in the R yield attained, as the degree of segregation is increased, exceed the fractional decreases in catalyst utilization. A secondary benefit of the membrane design is the confinement of reaction products in the bulk stream on the active layer side, thus reducing the downstream separation needs

Mixture Homogeneity and its Influence on the Sintering and Magnetic Performance of MnZn-Ferrites

The homogeneity of the green mixture, during the manufacturing of MnZn-Ferrites, is a process parameter which can be measured in a quantitative way. This parameter, depends on the morphological properties of the raw materials and determines the final power losses of the sintered products. By influencing this parameter with appropriate techniques good magnetic performances can also be achieved without the usage of extra sintering dopants