Membrane reactors condensation reactions

For a packed-bed membrane reactor (PBMR) the membrane is permselective and removes the product as it is formed, forcing the reaction to the right. In this case, the membrane is not active and a conventional catalyst is used. Tavolaro et al. [45] demonstrated this concept in their work on CO2 hydrogenation to methanol using a LTA zeolite membrane. The tubular membrane was packed with bimetallic Cu/ZnO where CO2 and H2 react to form EtOH and H2O. These condensable products were removed by LTA membrane which increased the reaction yield when compared to a conventional packed bed reactor operating under the same conditions [45]. 

Tosh et al. proposed the use of Pd/Ag-based membrane reactors for the recovery of hydrogen and its isotopes from tritiated water in a closed loop process that includes both the forward and reverse water-gas shift reactions [24]. The aim of this process was to avoid any production of tritiated wastes and any consumption of CO. In the system, the retentate stream, rich in CO2, was recycled to the reactor. The water-gas shift stops when all the water reacts and aU the hydrogen is recovered at the permeate side. Then, hydrogen is added, and the CO2 in the stream is completely converted to CO by the reverse water-gas shift reaction, thanks to the continuous removal of the produced water in a condenser. Figure 9.13 shows the proposed process. 

With pervaporation membranes the water can be removed during the condensation reaction. In this case, a tubular microporous ceramic membrane supplied by ECN [124] was used. The separating layer of this membrane consists of a less than 0.5 mm film of microporous amorphous silica on the outside of a multilayer alumina support. The average pore size of this layer is 0.3-0.4 nm. After addition of the reactants, the reactor is heated to the desired temperature, the recyde of the mixture over the outside of the membrane tubes is started and a vacuum is apphed at the permeate side. In some cases a sweep gas can also be used. The pressure inside the reactor is a function of the partial vapor pressures and the reaction mixture is non-boiling. Although it can be anticipated that concentration polarization will play an important role in these systems, computational fluid dynamics calculations have shown that the membrane surface is effectively refreshed as a result of buoyancy effects [125]. 

PV is a promising option to enhance the conversion of reversible condensation reactions in which water is formed as a by-product. Peters et al. (2005) prepared composite catalytic membranes by a dip-coating technique. Composite catalytic membranes have been prepared by applying a zeolite coating on top of ceramic hf silica membranes. In the PV-assisted esterification reaction, the catalytic manbrane was able to couple catalytic activity and water removal. A reactor evaluation proved that the outlet conversion of the catalytic PV-assisted esterification reaction exceeded the conversion of a conventional inert PV membrane reactor, with the same loading of catalyst dispersed in the bulk liquid. Further, the performance of the zeolite-coated PV membranes can be increased by optimization of the catalytic layer thickness or by an increase in catalytic activity. 

Several authors have reported modelling of multi-phase membrane reactors and, in particular, of three-phase catalytic membrane reactors. Harold and Watson (1993) have considered the situation of a porous catalytic slab partially wetted by a liquid from one side and by a gas phase on the other side, and they have pointed out the complexity of the problem in presence of an exothermic reaction, capillary condensation and vaporization. 

Most of the research pertained on membrane reactors is carried out on gas phase reactions i using ceramic, metal or zeolite membranes. However, the concept can be used as well for liquid phase reactions. A verj specific class of reactions are the condensation or polycondensation reaction in which water is one of the products. Water is easily be removed by pervaporation (see section Vl.4.3 ) and therefore pervaporation can be applied if the reaction temperature is not too high. As example we will use here an esterification reaction [89,90]. This reaction may be carried out in a batch reactor coupled with a pervaporation unit in which water is removed constantly (see figure VI - 76). 

The benefits of the use of micromembranes for the selective removal of one or more products during reaction have been demonstrated for equdibrium-limited reactions [289]. For example, the performance of hydrophilic ZSM-5 and NaA membranes over multichannel microreactors prepared from electro-discharge micromachining of commercial porous stainless steel plates was studied by Yeung et al. in the Knoevenagel condensation [290,291] and andine oxidation to azoxybenzene [292]. For such kind of reactions, the zeolite micromembrane role consists of the selective removal of water, which indeed yields higher conversions, better product purity, and a reduction in catalyst deactivation in comparison to the traditional packed bed reactor. 

The transfer of mass within a fluid mixture or across a phase boundary is a process that plays a major role in various engineering and physiological applications. Typical operations where mass transfer is the dominant step are falling film evaporation and reaction, total and partial condensation, distillation and absorption in packed columns, liquid-liquid extraction, multiphase reactors, membrane separation, etc. The various mass transfer processes are classified according to equilibrium separation processes and rate-governed separation processes. Fig. 1 lists some of the prominent mass transfer operations showing the physical or chemical principle upon which the processes are based. 

The mesoporous nickelsilicate membranes obtained are active and very selective to benzaldehyde and condensation products in the oxidation of styrene with hydrogen peroxide. The variation of the synthesis parameters and the pretreatment of the supports affects the permeation (Table 2) and the conversion of styrene (Fig. 7). In comparison with the conventional static reactor with a control of the H2O2 feed, the conversion of hydrocarbon on membranes after 12 h reaction was lower, but the efficiency of the H2O2 and selectivity to benzaldehyde and condensation products are higher. A variation of the pressure in the feed room favors the control of the rate of the oxidation. 

Reaction cum pervaporation. This technology can be used for dewatering of organics, which is the removal of organics from waste water in place of the conventional distillation operation. A membrane is the heart of the operation. A combination of reaction and pervaporation, the latter of which occurs when placed in a loop around the reactor, replaces a condenser... 

Knoevenagel condensation between carbonyl compounds and methylene malo-nic esters on a CsNaX zeolitic coating in a microreactor demonstrated an order of magnitude higher productivity as compared with a traditional packed-bed reactor while the selectivity remained the same in both reactors [107,108]. A nearly fourfold increase in reaction conversion was obtained for the microreactor when NH2 modified CsNaX zeolitic coatings were apphed [109]. The conversion was further improved when zeolitic coatings were grown onto a stainless steel membrane (0.2 pm pores) inserted in a microreactor [110]. 

Zeolite membranes have been applied for gas permeation and separation, and liquid pervaporation. A clear advantage of microscale zeolite membranes is the higher probability of obtaining a defect-free interface, since this probability increases for smaller membrane areas [41]. In zeolite MMRs, the zeolites are incorporated as a catalyst for reaction and a membrane for separation, as well as structural material of the reactors. Reactions conducted in MMRs include mainly Knoevenagel condensation [3, 42,43] and selective oxidation reactions [39]. Supra-equilibrium conversion may be obtained in the former, while the latter displays improved performance against catalyst deactivation. 

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