Hydrophilic zeolite membranes

In general, most of the high-separation factors reported for zeolite membranes are associated with pervaporation processes (see Section 10.5) or with vapor-separation applications where the permeated component is preferentially adsorbed. This has given rise to a variety of works in which the membranes have been used for equilibrium displacement by selective product permeation. The largest group probably corresponds to esterification processes, where hydrophilic zeolite membranes are employed to remove the product, water, replacing the extensively studied polymer membranes [187-192]. 

This reaction is exothermic, but for kinetic reasons it is most favorably performed at temperatures up to 700 K. The removal of water by, e.g., hydrophilic zeolite membranes would shift the reaction toward completion and reduce catalyst deactivation. 

A- HYDROPHILIC ZEOLITE MEMBRANE B- HYDROPHOBIC ZEOLITE MEMBRANE... 

MTBE synthesis from /-butanol and methanol in a membrane reactor has been reported by Salomon et al. [2.453]. Hydrophilic zeolite membranes (mordenite or NaA) were employed to selectively remove water from the reaction atmosphere during the gas-phase synthesis of MTBE. This reaction was carried out over a bed of Amberlyst 15 catalyst packed in the inside of a zeolite tubular membrane. Prior to reaction, the zeolite membranes were characterized by measuring their performance in the separation of the equilibrium mixture containing water, methanol, /-butanol, MTBE, and isobutene. The results obtained with zeolite membrane reactors were compared with those of a fixed-bed reactor (FBR) under the same operating conditions. MTBE yields obtained with the PBMR at 334 K reached 67.6 %, under conditions, where the equilibrium value without product removal (FBR) would be 60.9%. 

An alternative way to utilize renewable biomass is the so-called biomass-to-liquids (BTL) process. Parallel with GTL and CTL, BTL is essentially a variant of the F-T process that uses biomass as a feedstock. Hydrophilic zeolite membrane can also play an important role in the downstream dehydration process. 

THF is an important indnstrial solvent and forms an azeotropic mixture at 5.3 wt.% with water (see Table 11.2). In order to separate water/THF, Li et al. [206] tested the pervaporation performance of different hydrophilic zeolite membranes, zeolite A, zeolite Y, MOR, and ZSM-5. The preliminary test showed that the separation factor increased as the Si/Al ratio of the zeolite decreased, except for the case of zeolite A. This observation is probably due to the lower quality of this membrane with respect to the others. In fact, the permeation of TIPB showed the highest flux, 3.1 g/(m h), indicating the presence of nonselective defects. Therefore, the best results were obtained with zeolite Y, rendering a separation factor of 300 with a water flux of 2.24 kg/(m h) at 60°C. The water flux increased with water concentration in the feed, up to a value of 15 wt.%, indicating that the zeolite was saturated, as was the same for the case of water/etha-nol mixtures in zeolite A, previously described. At the same time, the separation factor decreases as water concentration decreased. The stability of the membrane was also studied, showing a stable performance after 35 h of operation. 

Outstanding performance of hydrophilic zeolite membranes for dehydration of ethanol has been reported (Caro, Noack, Kolsch, Schafer, 2000 Caro, Noack, Kolsch, 2005). The reported permeate fluxes are about 7 kg/m /h with separation factors of about 10,000 for feed concentrations of 90% (w/w) ethanol solution. The application of 16 pervaporation modules in a multipurpose plant for dehydration of alcohols was reported by Morigami, Kondo, Abe, Kita, and Okamoto (2001). Alcohol purification from 90% to 99.8% (w/w) was achieved in this stody. 

Using a hydrophilic zeolite membrane such as LTA and zeolite T to remove the water from the reaction system by sweep gas or vacuum (pervaporation), the esterification reaction would drive toward the product side with the result of enhanced yields. Beneficial aspects of PVMRs also include low energy consumption and the possibility of carrying out esterification at a selected temperature. 

Pervaporation of alcohol-water and dimethylformamide-water mixmres using hydrophilic zeolite NaA membranes mechanisms and experimental results. /. Membr. Sci., 179 (1-2),... 

D. Shah, K. Kissick, A. Ghorpade, R. Hannah and D. Bhattacharyya, Pervaporation of Alcohol-Water and Dimethylformamide-Water Mixtures using Hydrophilic Zeolite NaA Membranes Mechanisms and Results, J. Membr. Sci. 179, 185 (2000). 

Some efforts have already been made to develop ceramic pervaporation membranes, especially silica and zeolite membranes, which are both hydrophilic membranes. Silica pervaporation membranes have been developed by ECN, The Netherlands. The membranes were tested in a pilot installation of 1 m2 membrane surface at Akzo Nobel and other companies in the Netherlands [34, 35]. 

Ad Figure 2.6. A fimdamentally different system is to load a polymer film (e g. a siloxane) with zeolite crystals. Especially the Twente group studied such composite membranes. The zeolite crystals then just contribute to the permeation by acting as selective reservoirs of components. Hydrophobic (silicalite-1) as well as hydrophilic (zeolite A) zeolites have been studied in such a configuration. 

The transport mechanisms through zeolite membranes depend on different variables such as operation conditions (especially temperature and pressure), membrane pore size distribution, characteristics of the pore surface of the zeohtic-channel network (hydrophilicity/hydrophobicity ratio), as well as the characteristics of the crystal boundaries and the characteristics of the permeating molecules (kinetic diameter, molecular weight, vapor pressure, heat of adsorption), and their interactions in the mixture. 

Generally speaking, two terms, hydrophobic and hydrophilic, are employed in pervaporation in zeolite membranes, to refer to the affinity of organophUic and water molecules, respectively, toward the zeolite. In this way, a hydrophUic zeohte adsorbs and preferentially permeates water. Gyaya et al. [118] defined a hydrophobicity index (HI), which is the ratio between the amount of organic and the amount of water that a sohd adsorbs ... 

MasudaT, Otani S, Tsuji T, KitamuraM, andMukai SR. Preparation of hydrophilic and acid-proof silicalite-1 zeolite membrane and its application to selective separation of water from water solutions of concentrated acetic acid by pervaporation. Sep Purif Technol 2003 32(1-3) 181-189. 

Zeolite membranes have been demonstrated for many applications. Applications such as separation membranes, membrane reactors, adsorption, and catalysis have been covered in several reviews. In this entry, we focus on new applications including sensors, low-dielectric constant (low-k) films, corrosion resistant coatings, hydrophilic coatings, heat pumps, and thermoelectrics. 

A membrane reactor using a H-ZSM5 membrane was used by Bernal et al. [3.42] to carry out the esterification reaction of acetic acid with ethanol. An equimolar etha-nol/acetic acid liquid mixture was fed in the membrane interior, while He gas was used as an inert sweep on the shell-side. In this particular application the membrane, itself, provides the catalysis for the reaction. NaA and T-type zeolite membranes have been utilized for esterification reactions in a PVMR and in a vapor permeation membrane reactor (VPMR) by Tanaka et al [3.43, 3.44]. Both membranes are hydrophilic and show good separation characteristics towards a number of alcohols. The NaA membrane was used to study the oleic/acid esterification in a vapor permeation membrane reactor (VPMR) at 383... 

K. Soontarapa, Hydrophilic membranes for pervaporation An analytical review, Desalination, 1997, 110, 251-286 T.M. Aminabhavi, R.S. Khinnavar, S.S. Harogop-pada and U.S. Arithal, Pervaporation separation of organic-aqueous binary mixtures, J. Macromol. Sci., Macromol. Chem. Phys. 1994, C34(2), 139-204 H. Kita, T. Inoue, H. Asamura, K. Tanaka and K. Okamoto, NaY zeolite membrane for the pervaporation separation of methanol-methyl tert-butyl ether mixture, J. Chem. Soc., Chem. Commun., 1997, 45-46. 

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