Trickle membrane reactor

In membrane reactors plugging is an ever-present problem because any membrane is also a good filter. In bubble, drop, emulsion, and trickle bed reactors surface-active agents can cause formidable problems with foaming. Traces of soap in liquid feeds are difficult to avoid, and their result is similar to too much detergent in a washing machine. 

The concept of process intensification aims to achieve enhancement in transport rates by orders of magnitude to develop multifunctional modules with a view to provide manufacturing flexibility in process plants. In recent years, advancement in the field of reactor technology has seen the development of catalytic plate reactors, oscillatory baffled reactors, microreactors, membrane reactors, and trickle-bed reactors. One such reactor that is truly multifunctional in characteristics is the spinning disk reactor (SDR). This reactor has the potential to provide reactions, separations, and good heat transfer characteristics. 

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... 

Another use of nonpermselective membranes is in multiphase organic reactions involving trickle-bed type reactors (Harold et al., 1989, 1994 Cini and Harold, 1991). The reactor consists essentially of a hollow macroporous membrane tube coated on the inside with a hollow mesoporous catalyst layer, as shown in Figure 24.Id. Liquid and gas are allowed to flow on opposite sides of the membrane. Because the gas comes into direct contact with the liquid-fllled catalyst, it resembles a trickle-bed reactor. However, because there is no separate liquid film to hamper the supply of gas to the catalyst sites, it performs better than the traditional trickle-bed reactor. 

The tubular multiphase hollow membrane wall reactor briefly described before and sketched in Figure 24.1 h is a multiphase reactor design very similar to the trickle-bed reactor. In a regular trickle-bed reactor, the liquid flows over a partially wetted pellet as a thin film and supplies the liquid-phase reactant to the catalyst pores. This action, however, has the effect of hindering pore access to... 

Reactions requiring both a gaseous and a liquid reactant are usually performed in trickle-bed reactors in which the gas and liquid are pumped counter- or co-currently through a bed of catalyst particles [53, 54). Many of these systems encounter mass-transfer limitations as a result of intraparticle mass-transfer resistance, liquid-film resistance, liquid maldistribution and channelling. To overcome these problems, membrane reactors have been used for chemical reactions as well as biological conversions. 

In conventional industrial multi-phase reactors, the heterogeneous catalyst can be organized as a packed (or fixed) bed of catalyst particles (e.g., in trickle-bed reactors or in submerged up-flow reactors), as catalyst particles suspended or fluidized in one of the two phases (in the hquid phase of a three-phase reactor, as for example in a slurry-stirred reactor and a slurry-bubbling reactor) or finally as a structured catalyst (e.g., monolith and membrane reactors). Structured catalysts are regular solid structures which reduce randomness through a well-defined structure and shape at a reactor level. The selection of the most appropriate traditional multi-phase... 

As can be observed, the main difference between conventional three-phase reactors and catalytic membrane reactors hes in the relative positions of the mass transfer resistances with respect to the catalytic phase. In a conventional porous catalyst the catalytic sites in the pores have only one way or path of access. The gaseous reactant will encounter the first two mass transfer resistances at the gas-liquid interface, where the solvation equilibrium of the species from one phase to the other wiU take place. The dissolved species will diffuse towards the surface of the catalytic pellet for quite a long path in the hquid phase and will meet an additional mass transfer resistance at the hquid-sohd catalyst interface. It then needs to diffuse and react in the porous structure of the catalyst as well as the other reactant already present in the liquid phase. In the case of a traditional three-phase reactor (Fig. 4.3a), the concentration of at least one of the reacting species is hmited by its solubility and diffusion in the other fluid phase with a long diffusion path and in some cases unknown interfadal area (e.g., bubbles with variable size depending on the type of the gas feeding and distribution device in slurry reactors, not uniform phase contact and distribution in trickle-bed reactors). 

Qrgill, J. J., Atiyeh, H. K., Devarapalli, M., Phillips, J. R., Lewis, R. S., Huhnke, R. L. (2013). A comparison of mass transfer coefficients between trickle-bed, hollow fiber membrane and stirred tank reactors. Bioresource Technology, 133, 340—346. 

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