Dehydration zeolite membranes

An important driver for zeolite membrane apphcations has been the commercialization of the NaA membranes for dehydration. However, for these membranes, the quality required is not as high as compared to gas-phase molecular sieving... 

The inorganic silica membranes, also commercial, have solved the problem of thermal and chemical stability however, these membranes are only used for dehydration purposes, leaving the problem of separation of organic mixtures unsolved. As we have seen previously, due to the versatility and special feamres of zeolites, new applications in pervaporation that are not possible with other membranes could be developed with zeolite membranes. GaUego-Lizon et al. [110] compared different types of commercial available membranes zeolite NaA from SMART Chemical Company Ltd., sUica (PERVAP SMS) and polymeric (PERVAP 2202 and PERVAP 2510) both from Sulzer Chemtech GmbH, for the pervaporation of water/f-butanol mixtures. The highest water flux was obtained with the silica membrane (3.5 kg/m h) while the zeolite membrane exhibited the highest selectivity (16,000). 

Pervaporation Performance in Organic Dehydration of Solvents with Zeolite Membranes... 

In spite of all these hurdles, there are already industrial-scale applications of zeolite membranes for solvent dehydration [106] by pervaporation plants using tubular zeolite A membranes with 0.0275 m of permeation area each (see Section 10.2.3). Li et al. [280] have prepared large area (0.0260 m ) ZSM-5 membranes on tubular a-alumina supports. This work is also interesting from the industrial point of view because the authors used inexpensive n-butylamine as template. Indeed, the cost required for industrial modules, on a general basis, is still far from clear. However, it must be noted that most of the costs can be ascribed to the module, and only 10%-20% to the membrane itself [3]. This underlines again the importance of preparation of zeolite membranes on cheaper, alternative supports that can also pack more area per unit volume. 

Gallego-Lizon T, Edwards E, Lobiundo G, and dos Santos LF. Dehydration of water/t-butanol mixtures by pervaporation Comparative study of commercially available polymeric, microporous silica and zeolite membranes. J Membr Sci 2002 197 309-319. 

According to a recent conference given by Prof. Kita [162], the classical synthesis method currently used by Mitsui allows to produce about 250 zeolite membranes per day. Both the LTA and T types (Na K) membranes are now commercial and more than 80 pervaporation and vapor permeation plants are operating in Japan for the dehydration of organic liquids [163]. A typical pervaporation system, similar to the one described in [8], is shown in Fig. 11. One of the most recent applications concerns the production of fuel ethanol from cellulosic biomass by a vapour permeation/ pervaporation combined process. The required heat is only 1 200 kcal per liter of product, i.e. half of that of the classical process. Mitsui has recently installed a bio-ethanol pilot plant based on tubular LTA membranes in Brazil (3 000 liters/day) and a plant with 30 000 liters/day has been erected in India. The operating temperature is 130 °C, the feed is 93 % ethanol, the permeate is water and the membrane selectivity is 10 000. 

S. lkeda, S. Nakane, M. Matsukata, Recent advances of zeolite membrane Technologies for dehydration, in Proc. 3rd International Zeolite Membrane Meeting, R. Noble (Ed), Breckenridge, CO USA, July 25-28, 2004. 

Besides producing mixed-hydrocarbons (ultra-clean diesel), F-T process can also selectively produce mixed-alcohols (oxygenated fuel). The addition of mixed-alcohols into gasoline can effectively reduce HC and CO emissions. However, before directly used as fuels or blended with conventional fuels, the water content in the as-produced F-T mixed-alcohols must be reduced below 0.5wt.%. This dehydration step is essential but difficult since most of the contained alcohols form azeotropies with water. In our group, we studied the dehydration performance of microwave synthesized NaA zeolite membrane toward F-T produced mixed-alcohols [24, 25]. The membrane also showed excellent pervaporation performance toward dehydration of simulated F-T produced mixed-alcohols. The permeate consisted of only water and little methanol (< 10%) in aU the range of feed composition. This result confirmed that NaA zeolite membrane based pervaporation (or vapor permeation) process could be an effective technology for dehydration of F-T produced mixed-alcohols. 

The same as F-T produced mixed-alcohols, low purity bio-ethanol extracted from fermentation broth must be refined into high purity fuel grade ethanol. The pervapo-ration dehydration pilot plant based on NaA zeolite membrane was set up by Mitsui Engineering Shipbuilding Co., Ltd. (MES) in 1999. Recently, a pilot-scale NaA zeolite membrane based vapor permeation dewatering installation has been setup in our group with a handling capacity of 250 L/D. This installation can continuously produce 225 L 99.7wt.% ethanol per day. Meanwhile the permeate is nearly pure water. 

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. 

Zeolite membranes are not the only kind of membranes that have been used in pervaporation, and organic and other types of inorganic membranes, different from zeolites, have been employed. Polymeric membranes of polyvinylalco-hol (PVA) have been widely employed for dehydration and separation of organic mixtures however, their main limitations are related to their low thermal and chemical stability. When the water content in the feed mixture is high, polymeric membranes suffer from swelling moreover, in the separation of organic mixtures, they usually present a low selectivity. 

The different applications of these zeolites in pervaporation include alcohol dehydration water removal in acid solutions organic dehydration separations such as water/ tetrahydrofuran (THF), water/dioxane, or water/dimethyl-formamide (DMF) removal of organics from water and organic/organic separations such as methanol/methyl-tert-butyl ether (MTBE) or p-xylene/o-xylene. In the following subsections, the mechanism of pervaporation in zeolite membranes will be briefly described and we will provide the details about the different applications. 

Hasegawa Y, Abe C, Nishioka M, Sato K, Nagase T, Hanaoka T. Formation of high flux CHA-type zeolite membranes and their application to the dehydration of alcohol solutions. J Membr Sci 2010 364(1-2) 318-324. 

Wee et aL (2(X)8) wrote a review on Monhrane Separation Process-Pervaporalion Through Zeolite Monbrane. The focus of this review was on zeolite membrane covering (i) Synthesis of zeolite monbranes (ii) Membrane characterization (iii) PV studies and (iv) Applications in alcohol dehydration, organic-organic separations. 

Membranes that have high selectivity and high flux are not commonly available. Polymeric membranes have been limited to dehydration of solvents due to insufficiency of their thermal, mechanical, and chemical stabilities. The development of zeolite membranes has made it possible to overcome this limitation. 

Type A zeolite membranes are suited for organic dehydration because they are highly hydrophilic (Kondo et al. 1997 Okamoto et al. 1996, 2001 Morigami... 

Zeolite membranes show high thermal stability and chemical resistance compared with those of polymeric membranes. They are able to separate mixtures continuously on the basis of differences in the molecular size and shape [18], and/or on the basis of different adsorption properties [19], since their separation ability depends on the interplay of the mixture adsorption equilibrium and the mixture. Different types of zeolites have been studied (e.g. MFI, LTA, MOR, FAU) for the membrane separation. They are used still at laboratory level, also as catalytic membranes in membrane reactors (e.g. CO clean-up, water gas shift, methane reforming, etc.) [20,21]. The first commercial application is that of LTA zeolite membranes for solvent dehydration by pervaporation [22], Some other pervaporation plants have been installed since 2001, but no industrial applications use zeolite membranes in the GS field [23]. The reason for this limited application in industry might be due to economical feasibility (development of higher flux membranes should reduce both costs of membranes and modules) and poor reproducibility. 

For zeolite membranes, the separation of water and alcohol molecules can be explained by strong interactions between the water molecules and ionic sites in the zeolite crystal lattice and the partial sieving achieved by the zeolite channels (Shah, Kissick, Ghorpade, Hannah, Bhattacharyya, 2000). Macroscopic transport equations describing the mass transfer through such composite membranes are often Maxwell— Stefan based (Krishna van Den Broeke, 1995). Wee, Tye, and Bhatia (2008) listed several zeolite materials for the dehydration of alcohols, such as silicalite or mordenite. Most of these materials were supported by an a-Al203 porous support membrane. 

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. 

Another type of inorganic membranes used to the PV separation is a zeolite membrane. Na-type zeolite membranes have been applied for dehydration of aqueous alcohol. Kita et al. [9] reported that a permeation flux of 3kgm h and separation factor (a) over 10 000 isopropyl alcohol aqueous solution (90wt% isopropyl alcohol), which corresponds to much larger flux and selectivity compared with polymeric membranes (normally a 1000 flux <0.1 kgm h h) On the other hand, a silicalite membrane, which is hydrophobic, preferentially permeates alcohol over water, showing a selectivity of 60 and flux of 0.8kgm h - at 5wt% of ethanol at60°C [8). 

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