Multi-phase catalytic membrane reactions

Multi-phase catalytic membrane reactors the effectiveness factor, t], is defined in a general way as actual reaction rate... 

Typical oxidation reactions investigated by using a multi-phase catalytic membrane reactor are reported in Table 4.4 (gas-liquid systems) and in Table 4.5 (liquid-liquid systems). 

Shown in Table 8.6 arc some literature data on the use of dense membrane reactors for liquid- or multi-phase catalytic reactions. Compared to gas/vapor phase application studies, these investigations are relatively few in number. Most of them involve hydrogenation reactions of various chemicals such as acetylenic or ethylenic alcohols, acetone, butynediol, cyclohexane, dehydrolinalool, phenylacetylene and quinone. As expected, the majority of the materials adopted as membrane reactors are palladium alloy membranes. High selectivities or yields are observed in many cases. A higher conversion than that in a conventional reactor is found in a few cases. 

Multi-phase catalytic reactions have attracted some attention but the area has not in our opinion been fully exploited. Previous studies have demonstrated that the yields obtained with the catalytic membrane reactors are often better than the yields obtained with more conventional reactors. Future research in this area must involve reactions with more immediate industrial applications. Examples of such reactions could be the hydrogenation reactions studied by Gryaznov and co-workers with dense metallic membranes which we discussed earlier. New materials like zeolite membrane could offer some advantages here with their enhanced regio- or chemioselectivity. 

Key words catalytic membrane, three-phase catalytic membrane reactor, multi-phase reactions. 

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. 

Some types of membrane and catalytic membranes for three-phase reactions prepared in the Genoa membrane research group. Porous flat sheet catalytic membrane (a) unsupported (b) supported on a polymeric non-woven substrate, (c) Hollow fibres (d) single-channel Pd membrane supported on alumina porous substrate (e) three-channel catalytic zeolite membrane (f) multi-channel porous alumina substrate. 

The number of studies devoted to the application of modules with higher membrane area per unit volume in multi-phase reactions remains still very limited, for both catalytic polymeric and inorganic membranes. 

---