Multiphase catalytic processes

Losey, M. W., Isogai, S., Schmidt, M.A., Jensen, K. F., Microfabricated devices for multiphase catalytic process, in Proceedings of the 4th International Conference on Microreaction Technology, IMRET 4, 5-9 March 2000, pp. 416-424, AIChE Topical Conf. Proc., Atlanta, GA (2000). 

Kapteijn F, Heiszwolf JJ, Nijhuis TA, Moulijn JA. Monoliths in multiphase catalytic processes—aspects and prospects. Cattech 1999 3 24-41. 

Interestingly, in multiphase catalytic processes the primary products can be extracted during the reaction thus modulating the product selectivity (using different substrates and reaction products solubility with the catalyst containing phase, such as dienes/monoenes and arenes/cycloalkenes). Indeed, this approach can constitute a suitable method for avoiding consecutive reactions of primary products and has been used in the partial hydrogenation of dienes and arenes by transition-metal NPs dispersed in ILs. 

The determination of kinetic parameters is a key element in catalyst research and multiphase catalytic process development. In this chapter, the experimental methods used to determine kinetic parameters, the nature of the parameters, and the areas of application and advantages of the different experimental approaches are discussed. Emphasis is placed on transient techniques and methods that provide intrinsic kinetic parameters, that is, parameters that can be directly linked to the surface com-position/structure of a heterogeneous catalyst... 

However, the right combination of catalyst, catalyst solvent, and product is crucial for the success of biphasic catalysis (Driefien-Hoelscher, 1998). The catalyst solvent has to provide excellent solubility for the catalyst complex for full catalyst immobilization but must not compete with the substrate for the free coordination sites at the catalytic center. Furthermore, a reaction system providing a miscibility gap throughout the whole conversion range is required. Finally, another prerequisite of liquid-liquid biphasic catalysis is the provision of a catalyst solvent with enough solubility for the feedstock to allow sufficient reactant concentration in the reaction phase and thus sufficient reaction rate. A technical example of hquid-liquid biphasic catalysis is given in Section 6.15. The same section discusses modern aspects of solvent development and advanced solvents for the application in liquid-liquid multiphase catalytic processes. 

The evolution of chemical engineering from petroleum refining, through petrochemicals and polymers, to new applications is de.scribed so that students can see the relationships between past, present, and future technologies. Applications such as catalytic processes, environmental modeling, biological reactions, reactions involving solids, oxidation, combustion, safety, polymerization, and multiphase reactors are also described. 

We regard the essential aspects of chemical reaction engineering to include multiple reactions, energy management, and catalytic processes so we regard the first seven chapters as the core material in a course. Then the final five chapters consider topics such as environmental, polymer, sohds, biological, and combustion reactions and reactors, subjects that may be considered optional in an introductory course. We recommend that an instmctor attempt to complete the first seven chapters within perhaps 3/4 of a term to allow time to select from these topics and chapters. The final chapter on multiphase reactors is of course very important, but our intent is only to introduce some of the ideas that are important in its design. 

High pressure catalytic processes are developed and carried out in both preformed and powdered catalysts. Preformed catalyst are useful for fixed bed operation. Preformed catalyst pellets, are used as packing in multiphase trickling flow reactors. Trickling flow reactors have been described in detail in another part of this book (see Laurent). In this section we deal with slurry catalytic reactors, where the catalyst is used in powdered form. 

In industrial practice, three-phase catalytic reactors are often used, with gases like such as H2, H2O, NH3 or O2 as reactants. The process can be classified on the basis of these gases as hydrogenation, hydration, amination, oxidation, etc [3]. Among these processes, hydrogenation is by far the most important multiphase catalytic reaction. Recently, liquid- -phase methanol synthesis and the Fischer-Tropsch process were commercialized respectively... 

Gas-liquid multiphase catalytic reactions require the reacting gas to be efficiently transferred to the liquid phase. This is then followed by the diffusion of the reacting species to the catalyst. These mass transfer processes depend on bubble hydrodynamics, temperature, catalyst activity, physical properties of the liquid phase like density, viscosity, solubility of the gas in the liquid phase and interfacial tension. 

Ionic liquids are immiscible with many organic solvents and compounds, which lends themselves to biphasic or multiphasic catalytic reactions. Most are also immiscible with fluorous phases and some are immiscible with water. In the ideal biphasic process involving ionic liquids, a soluble polar substrate is converted to a less polar - and thus insoluble -product, which will then form a separate phase. Attaching fluorous groups to the ionic liquid cation can reverse the solubility properties. 

The following discussion shows that the industrial examples of multiphase homogeneous catalytic processes are those for which the limitations discussed above have been overcome. It is always necessary to have clearly in mind these limitations, otherwise it cannot be explained why, despite intense research in this area and the several claims of advantages in multiphase homogeneous catalysis operations, the number of industrial applications is still quite limited. 

However, we should raise some concerns regarding the sustainability of multiphase homogeneous catalytic processes. They are presented as one of the relevant directions for green chemistry. The suitability of water as a process solvent is certainly environmentally benign, and in the case of the RCH/RPs process dear advantages in terms of process simplification and reduction of energy consumption (Table 2.2) are demonstrated. However, the process has not found wider applicability and the impact on the environment, in terms of wastewater and other emissions, has to be assessed. 

A recent proposal concerns mixed organic-aqueous tunable solvents (OATS) such as dimethyl ether-water, the solubility of which for substrates can be influenced by a third component such as carbon dioxide. CO2 acts as a antisolvent and as a switch to cause a phase separation and to decant the phases from each other (preferably under pressure). This behavior makes the operation of bi- or multiphase homogeneous catalytic processes easier and more economic the preferential dissolution at modest pressure of carbon dioxide causes phase separation which results in large distribution coefEcients of target molecules in biphasic organic-aqueous systems. This extraordinary behavior lead to a sophisticated flow scheme (Figure 6) [7]. 

Although this was aimed at heterogeneous catalytic processes, we are convinced that in only a few years this statement will be also true for multiphase homc eneous catalysis. The advantages of biphasic methodologies for homogeneous catalysis have been summarized by Chaudhari et al. [2] and Driefien-Holscher et al. [3] as... 

Mass transfer with chemical reaction in multiphase systems" covers, indeed, a large area. Table 1 shows a general classification of the systems encountered. From the possible two-phase systems, solid-solid reactions, liquid-solid (reactive or catalytic) and gas-solid (reactive or catalytic) reactions are not discussed here. The first one was reviewed by Tamhankar and Doraiswamy (2) and gas-solid (reactive) systems, such as, coal gasification, calcination of limestone, reduction of ores, etc. have been treated in some detail in recent reviews (3-5). The industrially important fluid-solid catalytic processes were the topic of a previous Advanced Study Institute (6) and have been also discussed authoritatively elsewhere (5,7). Concerning solid (reactive)-liquid two-phase systems, only some interesting examples are presented in Table 2 (1). 

This review paper is concentrated on problems in scaling-up multiphase catalytic fixed bed reactors such as trickle-bed or packed bubble column reactors, in which two fluid phases (gas and liquid) pass concurrently through a bed of solid (usually porous) catalyst particles. These types of reactors are widely used in chemical and petrochemical industry as well as in biotechnology and waste water treatment. Typical processes are the hydrodesulphurization of petroleum fractions, the butinediol syntheses in the Reppe process for synthetic rubber, the anthrachinon/hydrochinon process for H202 production, biochemical processes with fixed enzymes or the oxidative treatment of waste water under pressure. 

The only rigorous way to scale-up multiphase catalytic reactors is the use of reactor models, or, more precisely, the use of mathematical models. Because these models in their most accurate versions are more complex than for other reactor types> this results in considerable expenditure of time and money usually not available in the design stages of a process. Therefore, these mathematical models should be simplified reasonably on the rational basis of knowledge about the system as e.g. presented in the preceding chapter. 

It is better to prevent waste generation than to treat or clean up waste after it has been created. The earlier messy Bechamp reductions using Fe-HCl and generating a highly acidic waste sludge are replaced by clean catalytic processes using suitably designed multiphase reactors (Section 1.3 and Chapters 7A and 8). 

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