The separation potential of five different commercial tubular inorganic membranes consisting of A-, T- and Y-type zeolites from Mitsui and microporous silica from ECN and Pervatech has been examined in more than 30 dehydration applications. These separations include alcohols, glycols, carboxylic acids, esters, ethers, ketones, amines, nitrites, and halogenated hydrocarbons. The membranes combine excellent permeate flux with very good selectivity up to values of several thousand. Water permeances valuing between 10 and 20 kg /m(2) h bar partial pressure difference represent a new dimension in solvent dehydration. The pervaporation separation index (PSI) of standard polymeric membranes is around 100 while for inorganic materials index values of up to 50,000 are achieved. The performance ranking of the membranes in term of selectivity decreases in the sequence zeolite A > zeolite T > Pervatech silica > ECN silica. Regarding the flux the order is reversed with ECN silica > Pervatech silica > zeolite A > zeolite T. The reason for this behavior is founded in the different membrane structure and adsorption characteristics. The separation layer of the zeolite membranes is at least 10 mu m thick whereas the thickness of the silica membranes is below 200 nm. On the other hand, zeolite crystals have a strictly defined pore size allowing a sharper separation in comparison to the amorphous silica with a pore size distribution. The water adsorption on zeolites follows a Langmuir isotherm while for silica it is in the Henry region. Thus, concentration polarization effects are more severe for the latter material and viscosity of the systems has a major impact on the transport resistances in the boundary layer. These conclusions were confirmed by a comparison of single component to mixture permeation data. A simple transport model based on normalized permeate flux is used to calculate the influence of operation parameters, such as concentration, temperature and permeate pressure, on the separation characteristic from a single measurement. Water can be separated with high efficiency from complex solvent and reaction mixtures including methanol by A-type zeolite membranes. According to process needs the cut-off can be shifted to the separation of both water and methanol by use of a Y-type zeolite or amorphous silica membrane. The chemical stability of all tested membranes in aprotic solvents is excellent while their acid and base resistance is limited. For the A-type zeolite an acidic environment has to be avoided by all means, whereas the T-and Y-type zeolite can withstand a lower pH to some extent. Standard silica is stable down to a pH value of 3. Durability can be further improved by chemical modifications of the sol. In basic environment, the gamma-alumina support is affected at a pH greater than It. Membranes show no signs of thermal degradation up to 150 degrees C. In pervaporation, micro-cracks have been detected at high permeation rates above 30 kg/m(2) h. A temperature difference is necessary to transfer the heat for the evaporation of the permeating components from feed to permeate side. Hence, thermal stress is a likely cause for the observed destruction of the membrane. Tubular membranes can withstand normal operation pressures of up to 30 bar without any problem. Contact of the membrane with solid particles should be avoided to guarantee mechanical stability. This can be achieved by introducing a feed filter in the range of 10-100 mu m into the feed stream. (c) 2005 Elsevier B.V. All rights reserved