Synthesis reactions, zeolite membrane reactors

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%. 

Kiatkittipong et al. (2002) investigated a PV membrane reactor for the synthesis of ethyl icri-butyl ether (ETBE) from a liquid phase reaction between EtOH and TEA. Supported p-zeolite and PVA membrane were used as catalyst and as membrane in the reactor, respectively. The permeation studies of water-EtOH binary system revealed that the membrane worked effectively for water removal for the mixtures containing water lower than 62 mol%. The permeation studies of quaternary mixtures (water-EtOH-TBA-ETBE) were performed at three temperature levels of 323, 333, and 343 K. It was found that the manbrane was preferentially permeable to water. 

The application of permeable composite monolith membranes for the FT synthesis has been tested [122]. An overview of concepts associated with this reactor type has been presented (Figure 12.25) [123]. Novel uses of this concept have been advanced, and some experimental results have demonstrated the ability to operate at high CO conversion with metal FT catalysts by removal of the water produced during the synthesis [ 124] and the encapsulation of an FT catalyst by a zeolite membrane layer to effect upgrading reactions in the FT reactor [125]. The potential of this technique merits further studies to evaluate the ability to scale to a commercial level. 

Synthesis of membranes with high permeability and selectivity, that is, oriented and thin zeolite membranes. Optimal MR operation requires the membrane flux to be in balance with the reaction rate. A large number of factors - such as the support, organic additives, temperature, and profile - have a significant influence on the microstructure and overall quality of the membrane. However, the precise correlation between the synthesis procedure and conditions and the properties of the resultant zeolite membranes is not clear. In contrast, the majority of membranes synthesized so far are MFI-type zeolite membranes that have pore diameters 5 A, which are still too big to separate selectively small gaseous molecules. Zeolite membranes with pores in the 3 A range should be developed for membrane reactors, to separate small gas molecules on the basis of size exclusion. In addition, a method to produce zeolite membranes without non-zeolite pores or defects has to be found. 

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