Three-phase membrane reactors

Related to the experimental studies performed in our laboratory, in this review packed-bed membrane reactors were discussed. It should be mentioned that there are significant investigational activities devoted to study catalytically active membranes where the catalyst is deposited in either the membrane pores or on the inner or outer surface of the tubes [11]. Another similarly interesting and promising principle is based on using the Contactor type of membrane reactors, where the reactants are fed from different sides and react within the membrane [79]. Significant efforts have been made to exploit this principle for heterogeneously catalyzed gas-liquid reactions (three-phase membrane reactors) [80, 81]. 

M. Torres, J. Sanchez, J.-A. Dalmon, B. Bemauer and J. Lieto, Modeling and simulation of a three-phase membrane reactor for nitrobenzene hydrogenation. Ind. Eng. Chem. 

A further development aimed at increasing process integration lies in the use of three-phase membrane reactors, which were developed mainly for the recovery of organic acids [208]. In these systems, the aqueous bioconversion medium. 

Zeolite membranes indicate inorganic membranes with a selective/cata-lytic layer composed of a zeolite which is crystalline aluminosilicate with the feature of a high ordered porous structure with size comparable to molecular dimension. An example of the use of zeolites as a catalyst in a multi-phase membrane reactor can be found in Shukla and Kumar (2004) who have immobilized a lipase on a zeolite-clay composite membrane by using glu-taraldehyde as a bifunctional ligand in order to carry out the hydrolysis of olive oil. An application of a zeolite-based membrane in a three-phase membrane reactor has been reported by Wu et al. (1998), where TS-1 zeoUte crystallites were embedded in a polydimethylsiloxane (PDMS) membrane in order to catalyse the oxyfunctionalization of n-hexane (from a gas phase) with hydrogen peroxide (from a liquid phase). 

Typical reactions with three-phase membrane reactors... 

The three-phase membrane reactors have been mainly investigated for applications in both hydrogenation and partial oxidation reactions. 

Cini et al. (1991b) proposed the use of a tubular Pd/AljOj mesoporous membrane for the hydrogenation of a-methylstyrene to cumene. A comparison between the tubular catalyst and a fully-wetted pellet revealed a rate increase by up to a factor of 20. From that study, several other theoretical (Torres et al, 1994) and experimental ones confirmed that a three-phase membrane reactor can improve the mass transfer rate of gas-liquid-solid systems. 

Selective oxidation of n-paraffins has been carried out on catalytic iono-mer membranes with Fe +/H202 Fenton.The three-phase membrane reactors were constituted by a proton-conducting membrane (based on Nafion), fed on one side with a gaseous n-paraffins and, on the other side, with an aqueous solution of hydrogen peroxide and Fe " ions. The paraffin is activated by... 

Bottino, A., Capannelli, G., Comite, A., Di Felice, R., 2009. Three-phase membrane reactors and aspects of membrane contactors, in Simulation of membrane reactors, (A. Basile and F. Gallucci, Eds). Nova Science Publishers, New York, pp. 311-340. 

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. 

In catalytic multi-phase membrane reactors the catalytic membrane usually plays the role of interface between the two fluid phases. As discussed in the previous subsections, especially in the case of porous catalytic membranes, a strict control of the position of the inter-phase interface in proximity to the catalytic layer is of paramount importance in order to minimize the diffusion resistances. Figure 4.6 reports a simplified outline of a three-phase experimental rig as can be found in several publications. A req cle loop for the liquid phase has been considered in order to achieve the desired conversion. 

It is evident that in many situations the reaction rate will be directly proportional to the surface area between phases whenever mass transfer hmits reaction rates. In some situations we provide a fixed area by using solid particles of a given size or by membrane reactors in which a fixed wall separates phases Ifom each other. Here we distinguish planar walls and parallel sheets of sohd membranes, tubes and tube bundles, and spherical solid or liquid membranes. These are three-, two-, and one-dimensional phase boundaries, respectively. 

Some recent models have also appeared discussing the operation of three phase catalytic membrane reactors by Torres et al. [82]. The models which represent extension of prior models by Akyurtlu et al. [79] and Cini and Harold [80] are numerically analyzed and appear to simulate well the experimental results of the nitrobenzene hydrogenation reaction in a three phase catalytic membrane reactor. 

Three-phase catalytic membrane reactor systems, in our opinion, show significant promise, for near term application to hydrogenation reactions for fine chemicals synthesis. These reactions generally require mild operating conditions which will place less stringent requirements on the available and future commercial membranes. 

Since the first edition some reviews (and lots of patents) about the application of membranes and membrane reactors have been filed and published (for example [24]). Mostly, special aspects were in the foreground of investigations (such as the interplay of micelles or microemulsions and membranes, interfacial phenomena, three phase emulsion/solid heterogenization, or the properties of metal-based membranes [25]). 

Hydrogenation reactions have also been studied with catalytic membrane reactors using porous membranes. In this case the membrane, in addition to being used as a contactor between the liquid and gaseous reactants, could, potentially, also act as a host for the catalyst, which is placed in the porous framework of the membrane. As previously noted a triple-point interface between the three different phases (gas, liquid, and the solid catalyst) is then created in the membrane. The first application was reported by Cini and Harold... 

O. Monticelli, A. Bezzi, A. Bottino, G. Capannelli, and A. Servida, in "Hydrogenation of Cinnamaldehyde the Use of Three Phase Catalytic Membrane Reactors", Proc. Fourth Workshop Optimisation of Catalytic Membrane Reactors Systems, Oslo, Norway, May, 1997, 109. 

In Chapter 2 we discussed a number of studies with three-phase catalytic membrane reactors. In these reactors the catalyst is impregnated within the membrane, which serves as a contactor between the gas phase (B) and liquid phase reactants (A), and the catalyst that resides within the membrane pores. When gas/liquid reactions occur in conventional (packed, -trickle or fluidized-bed) multiphase catalytic reactors the solid catalyst is wetted by a liquid film as a result, the gas, before reaching the catalyst particle surface or pore, has to diffuse through the liquid layer, which acts as an additional mass transfer resistance between the gas and the solid. In the case of a catalytic membrane reactor, as shown schematically in Fig. 5.16, the active membrane pores are filled simultaneously with the liquid and gas reactants, ensuring an effective contact between the three phases (gas/ liquid, and catalyst). One of the earliest studies of this type of reactor was reported by Akyurtlu et al [5.58], who developed a semi-analytical model coupling analytical results with a numerical solution for this type of reactor. Harold and coworkers (Harold and Ng... 

Several plenary lectures were given at the conferences, including a call from Tsotsis for renewed emphasis on using membrane reactors to reduce or eliminate the separation task. An update to his 1994 review paper was given at the ISCRE-15 conference by van Swaaij, who concluded that the outlook for membrane reactors was perhaps more optimistic than a few years ago. Dalmon presented a timely survey of membrane catalysis for liquid applications, and several recent publications have focused on this, including pervaporation. Dalmon also emphasized the area of fine chemicals production, where the use of membranes for three-phase contacting could see a revival of interest. An example of this was presented at the ISCRE-15 conference. 

Figure 3 Some examples of the principle of coupling a membrane technique with an HCR. (a) Hollow-fiber membrane module (b) reactor. Case 1, separation of a soluble catalyst by NF case 2, MR contactor in two-phase reaction (only phase 1 and 2) case 3, MR contactor in three-phase reaction (phases 1, 2, and 3).

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