Separation zeohte membranes

The separation factors are relatively low and consequently the MR is not able to approach full conversion. With a molecular sieve silica (MSS) or a supported palladium film membrane, an (almost) absolute separation can be obtained (Table 10.1). The MSS membranes however, suffer from a flux/selectivity trade-off meaning that a high separation factor is combined with a relative low flux. Pd membranes do not suffer from this trade-off and can combine an absolute separation factor with very high fluxes. A favorable aspect for zeoHte membranes is their thermal and chemical stability. Pd membranes can become unstable due to impurities like CO, H2S, and carbonaceous deposits, and for the MSS membrane, hydrothermal stability is a major concern [62]. But the performance of the currently used zeolite membranes is insufficient to compete with other inorganic membranes, as was also concluded by Caro et al. [63] for the use of zeolite membranes for hydrogen purification. 

Separation through zeohte membranes proceeds through three different mechanisms (i) molecular sieving, (ii) diffusion-controlled permeation, (iii) adsorption-controUed permeation [7, 8]. Figure 10.5 gives examples of common zeohte separations that fall within each category. The simplest of the three is molecular... 

In other respects, we can consider zeohte membranes as pertaining to the ceramic material category. Indeed zeolites are classified for the most part as microporous, crystalline silico-aluminate stmctures with different alumininum/silicon ratios. Thus, the chemical compositions are close to those of ceramic oxide membranes, in particular of microporous silica and alumina membranes. On the other hand, zeohtes are crystalline materials and they have a structural porosity very different from microporous amorphous silica [124]. Zeohte membranes are well adapted to the separation of gases, in particular H2 from hydrocarbons, but these membranes are not very selective for the separation of mixtures of noncondensable gases. 

Okamoto et al. [141] studied several water/organic systems that are listed in Table 10.6, and the performance of the zeolite A membrane was excellent for aU the separations. These results could be also compared with the ones obtained using microporous sflica membranes [153]. Sflica membranes, for a water/dioxane (10/90 wt%) mixture at 60°C, showed a separation factor of 125 and a water flux of 2.2 kg/m h. For dymethilformamide, (DMF), the results obtained for a mixture of water/DMF (13.2/86.8 wt%) were 30 and 0.225 kg/m h for the separation factor and water flux, respectively. In both separations, zeohte A outperforms the microporous silica membrane. 

Meindersma GW and de Haan AB. Economical feasibihty of zeohte membranes for industrial scale separations of aromatic hydrocarbons. Desalination 2002 149 29-34. 

Tomita T, Nakayama K, and Sakai H. Gas separation characteristics of DDR type zeohte membrane. Micropor Mesopor Mater 2004 68 71-75. 

Bernal MP, Coronas J, Menendez M, and Santamarfa J. Separation of CO2/N2 mixtures using MEI-type zeohte membranes. AIChE J 2004 50(1) 127-135. 

Hasegawa Y, Watanabe K, Kusakabe K, and Morooka S. The separation of CO2 using Y-t)fpe zeohte membranes ion-exchanged with alkali metal cations. Sep PurifTechnol 2001 22-3(l-3) 319-325. 

Kalipcilar H, Gade SK, Ealconer JL, and Noble RD. Synthesis and separation properties of B-ZSM-5 zeohte membranes on monohth supports. J Membr Sci 2002 210(1) 113-127. 

Piera E, Brenninkmeijer CAM, Santamarfa J, and Coronas J. Separation of traces of CO from air using MFI-type zeohte membranes. J Membr Sci 2002 201(l-2) 229-232. 

Aoki K, Kusakabe K, and Morooka S. Separation of gases with an A-type zeohte membrane. Ind Eng Chem Res 20(X) 39(7) 2245-2251. 

Piera E, Coronas M, Menendez M, and Santamaria J. Gas separation selectivity with imperfect zeohte membranes. Chem Common 1999 14 1309-1310. 

Liu Q, Noble RD, Falconer JL, and Funke HH. Organics/water separation by pervaporation with a zeohte membrane. J Membr Sci 1996 117 163-174. 

Jia, W. and Murad, S. (2004). Molecular dynamics simulations of gas separations using faujasite-type zeohte membranes. J. Chem. Phys., 120, 4877—85. 

Table 153 Organic mixtures separation results by zeohte membranes ...

It is therefore likely that eoming years will see the appheation of zeolite membranes in the purifieation of solvents by per vaporisation, the ultrafiltration of maeromoleeules (sueh as dendrimer catalysts) from solutions and the separation of similar hydrocarbon isomers on the basis of molecular shape. In addition, the development of zeohte membranes for use in membrane reactors, in which a catalytic fimctionahty is included into the zeolite, offers potential advantages. In principle, it should be possible to enhance yields in equilibrium... 

Although Knudsen diffusion, shape selectivity, and molecular sieving play an important role in the separation of mixtures, the mechanisms that control the majority of the multicomponent separations in zeoHte membranes are surface diffusion and, sometimes, capillary condensation. In addition, molecular simulations and modeling of the M-S diffusion in zeolites [102,103] show that the slower-moving molecules are also sped up in some mixtures [104,105] in the presence of fast-diffusing molecnles, and other times, slower molecules inhibit diffusion of faster molecules because molecules have difficulty passing one another in zeohte pores [106]. 

Kusakabe K, Kuroda T, Morooka S. Separation of carbon dioxide from nitrogen using ion-exchanged faujasite-type zeohte membranes formed on porous support tubes. J Membr Sci 1998 148(1) 13-23. 

Dong J, Lin YS, Liu W. Multicomponent hydrogen/hydro-carbon separation by MFI-Type zeohte membranes. AIChE J 2000 46(10) 1957-1966. 

Kalipcilar, H., Bowen, T.C., Noble, R.D. and Falconer, J.L. 2002. Synthesis and separation performance of SSZ-13 zeohte membranes on tubular supports. Chem Mer 14 ... 

Li SG, Tuan VA, Falconer JL. (2001b). Effects of zeohte membrane sfructure on the separation of 1,3-propanediol from glycerol and glucose by pervaporation. Chem Mater, 13, 1865-1873. 

Liangxiong L, Junhang D., Tina M. N. Transport of water and alkali metal ions through MFI zeohte membranes during reverse osmosis. Separation and Purification Technology 53(1), 42-48 (2007). 

Jeong, Sotowa, and Kusakabe (2004) simulated the catalytic dehydrogenation of cyclohexane in an FAU-type zeolite membrane reactor. The cyclohexane conversion enhanced in the zeohte membrane reactor, which was more dependent on the permeance than the separation factor. Table 21.3 presents a summary of some of the membrane reactors used for cyclohexane dehydrogenation. 

A comprehensive review of the hterature on this subject until 2002 is provided in Lin et al. (2002). A more recent review on the apphcation of silica and zeohte membranes for H2 separation is provided in Verweij et al. (2006). 

Structures at all relevant length scales, as described in Sections 34.2.1-34.2.4, can be classified further into organized and random packing stmetures. For dense materials, structural organization is expressed in the presence (or absence) of a periodic crystal lattice. For microporous materials there is a clear distinction between crystalline zeohtes and amorphous sihea with a very short range order. Zeohte membranes may consist of a three-dimensional mosaic of crystaUites that may be either randomly orientated with respect to each other or possess a certain preferred orientation or texture (Lai et al., 2003). The polycrystaUine nature of and presence of texture in zeohte membranes can have important consequences for flux and separation behavior. 

---