NaA zeolite membrane

Figure 10.2 NaA zeolite membrane (a) cross-section and (b) top view [11]. (Reprinted with permission of Elsevier.)...

M. Kondo, M. Komori, H. KitaandK.-I. Okamoto, Tubular-type Pervaporation Module with NaA Zeolite Membrane, J. Membr. Sci. 133, 133 (1997). 

Even if the problems of poor crystal intergrowth due to local exhaustion of reactants in the autoclave and synthesis of zeolite material in the bulk of the solution were solved, an important problem remains, related to the fact that several batch synthesis cycles (with their associated heating and cooling processes) are often required to achieve a zeolite membrane of good quality. Thus, a synthesis procedure in which reactants are continuously supplied to the synthesis vessel while this is maintained at a constant temperature would clearly be desirable not only for performance but also for the feasibility of the scale-up. This type of approaches has already been tested for inner MFI and NaA zeolite membranes [33-35], and the results obtained indicate that the formation of concomitant phases and the amount of crystals forming in the liquid phase are greatly reduced. Similarly, the continuous seeding of tubular supports by cross-flow filtration of aqueous suspensions [36-37] has been carried out for zeolite NaA membrane preparation. 

Pina MP, Arruebo A, Felipe A, Fleta F, Bernal MP, Coronas J, Menendez M, and Santamaria J. A semi-continuous method for the synthesis of NaA zeolite membranes on tubular supports. J Membr Sci 2004 244 141-150. 

In addition to the preparation of packed beds and monoliths, wall coating is an alternative method forthe introductionofcatalysts into continuous flow systems, due to the short diffusion distances obtained within micro reaction channels. An early example of this was demonstrated by Yeung and co-workers [59]. who employed a stainless-steel micro reactor [channel dimensions = 300 pm (width) x 600 pm (depth) x 2.5 cm (length)] coated with an NaA zeolite membrane, followed by a layer of... 

A common feature of all catalysis for F-T synthesis, whether they are cobalt or iron based, is that the catalytic activity is reduced due to the oxidation of active species. Under the typical reaction conditions, this oxidation may be caused by water, which is one of the primary products in the F-T process. On the other hand, at low partial pressure water can also help to increase the product quality by increasing the chain growth probability. Thus, in situ removing some of the water from the product and keeping the water pressure at an optimal value may improve the catalysis activity and promote the reaction rate. Zhu and coworkers [22] have evaluated the potential separation using NaA zeolite membrane to in situ removal of water Irom simulated F-T product stream. High selectivity for water removal from CO, H2 and CH4 were obtained. This result opened an opportunity for in situ water removal from F-T synthesis under the reaction conditions. 

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. 

Generally, zeolite membranes are synthesized on tubular supports, and used as tubular-type modules. Packing density (i.e. membrane separation area/module volume ratio) of the tubular module was low as it was compared with that of the polymeric membranes. Xu et al. [32] synthesized of NaA zeolite membrane on a ceramic hollow fiber with an outer diameter of 400 p,m, a thickness of 100 p,m and an average pore radius of 0.1 (rm. The quality of the as-synthesized NaA zeolite membrane held a He/N2 separation factor of 3.66 and He permeance of 10.1 X 10 mol/(m. s.Pa). 

Aiming at practical applications, we used microwave heating (MH) technique to prepare NaA zeolite membrane [13], The permeance of the NaA zeolite membrane synthesized by MH was four times higher than that of the NaA zeolite membrane synthesized by conventional heating, while their permselectivities were comparable. 

Cation-exchanged zeolites show a strong tendency to adsorb water compared to organic molecules, as described in Chapter 7. As a consequence, water can be preferentially removed from solvents. By the use of porous tubes coated in zeolite membranes, water can be removed from the solvent on one side of the membrane and evaporated from the other side. The Japanese Mitsui company, for example, have commercialised the first large-scale pervaporation plant that produces over 500 Ih of solvents with less than 0.2 wt% water from simple alcohols containing 10 wt% water. The plant makes use of over 100 individual sections of NaA zeolite membrane operating at 120 °C. 

Figure 11.3a and b shows, respectively, the XRD spectra of a supported NaA zeolite membrane and of the resulting powder after filtering and drying the liquid phase. SEM observations allow the evaluation of membrane thicknesses, shape, orientation and size of the crystals, homogeneity and uniformity of the zeolite layer, and a morphological analysis on the existence of intercrystalline defects. SEM-energy-dispersive x-ray analysis (EDX) can be used to measure qualitatively and quantitatively the atomic composition of... 

Among the publications related to this subject, Jafar et al. [268] used a tubular NaA zeolite membrane prepared on a carbon-zirconia support in the vapor space of an esterification reactor to remove the product, water, formed in the reaction of lactic acid with ethanol catalyzed by p-toluenesulfonic acid. This protected the sensitive zeolite A membrane from acid attack and still allowed high permeation fluxes responsible for the enhanced yields of ethyl lactate. The same group [269] applied a... 

Aguado S, Gascon J, Jansen JC, Kapteijn F. Continuous synthesis of NaA zeolite membranes. Aguado Micropor Mesopor Mater 20W-, 120(1-2) 170-176. 

Sato K, Sugimoto K, Nakane T. Preparation of higher flux NaA zeolite membrane on asymmetric porous support and permeation behavior at higher temperatures up to 145°C in vapor permeation. J Membr Sci 2008 307 181-195. 

Sato K, Nakane T. A high reproducible fabrication method for industrial production of high flux NaA zeolite membrane. 

A. Malekpour, M.R. MUlani, M. Kheirkhah, Synthesis and characterization of a NaA zeolite membrane and its applications for desalination of radioactive solutions. Desalination 225, 2008, 199-208. 

Kita et al. (2003) reported on a tubular-type PV and vapor permeation module with zeolite membranes for fuel EtOH production. They used two types of zeolite membranes (i) NaA-type zeolite membrane, which was grown on the surface of a porous cylindrical mullite support and (ii) T-type zeolite membrane, which was also grown hydrothermally on the mullite support. Both membranes were studied for the flux and the separation factor of PV and vapor permeation for water-alcohol mixtures at 50°C and 75°C. The membranes were selective for permeating water preferentially with the high permeation flux. The separation factor of the T-type zeolite membrane was slightly smaller than the NaA zeolite membrane. They also claimed that this can provide more energy-efficient concentration of the EtOH to fuel grade EtOH. 

Figure 3.4 Schematic diagram for synthesis of NaA zeolite membrane together with microwave heating and conventional heating method. Reproduced from [26]. With permission from Elsevier.

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