Homogeneously catalyzed gas-liquid reactions

These flows were guided into a wound delay tube for reaction with ethylene glycol/ water (60/40 wt%) and hydrogen [324,326]. Efficient mass transfer in a kinetically controlled manner was achieved by the large specific gas-liquid interfaces of the foams of up to 50000m2/m3. 

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Romanainen, J.J and Salmi T (1992) The Effect of Reaction Kinetics, Mass Transfer and Flow Pattern on Noncatalytic and Homogeneously Catalyzed Gas-Liquid Reactions in Bubble Columns, Chem Eng Sci, 47 2493. 

To simulate the effects of reaction kinetics, mass transfer, and flow pattern on homogeneously catalyzed gas-liquid reactions, a bubble column model is described [29, 30], Numerical solutions for the description of mass transfer accompanied by single or parallel reversible chemical reactions are known [31]. Engineering aspects of dispersion, mass transfer, and chemical reaction in multiphase contactors [32], and detailed analyses of the reaction kinetics of some new homogeneously catalyzed reactions have been recently presented, for instance, for polybutadiene functionalization by hydroformylation in the liquid phase [33], car-bonylation of 1,4-butanediol diacetate [34] and hydrogenation of cw-1,4-polybutadiene and acrylonitrile-butadiene copolymers, respectively [10], which can be used to develop design equations for different reactors. 

A general application area of microreactors is screening. Screening applications have been described, for example, for catalyst search in the area of heterogeneously catalyzed gas-phase reactions [23] and homogeneously catalyzed gas-liquid reactions and for parameter screenings in process development. 

Homogeneous and homogeneously catalyzed gas-liquid reactions take place in the liquid phase in which gaseous reactants dissolve and react with other reactants that are primarily present in the liquid phase. Typical constructions to be used as gas-liquid reactors are column and tank reactors. Liquid-liquid reactors principally resemble gas-liquid reactors, but the gas phase is replaced by another liquid phase. The reactions can principally take place in either of, or even both, the liquid phases. 

For a review of noncatalytic or homogeneously catalyzed gas-liquid reactions, see Table 7.1 [2-4]. 

For noncatalytic and homogeneously catalyzed gas-liquid reaction systems, BRs are frequently used. Provided that the reactor operates at the kinetic regime (mass transfer resistances and reactions in the films are negligible see Chapter 7), the component mass balance is given by... 

In all the above mentioned cases conversion can only take place when the components are transferred to the catalytic phase or at least to the interface in which the reaction proceeds. Transport from one phase to the other(s) requires a driving force, i.e., the existence of concentration gradients. Figure 2 shows schematically the principal steps of a homogeneously catalyzed gas-liquid-liquid reaction (eq.(7)), where the reaction product P, is formed by the reaction between a gaseous reactant Ai and reactant A2 in the liquid phase 1 in presence of a second liquid phase which contains the catalyst. Both liquid phases are immiscible and Ai is only soluble in liquid phase 1. 

Figure 2. Principal steps of mass transfer and chemical reaction during the homogeneously catalyzed gas/liquid/liquid reaction (eq. (13)).

In homogeneously catalyzed gas/liquid-phase reactions the overall reaction rate is determined by the actual chemical reaction rate and by mass transfer processes [lb]. Depending on the magnitude of the rates of the catalytic reaction and of the transfer rate of the gaseous reactants, severe concentration gradients may exist near the gas-liquid interface. These phenomena are shown in Figure 1 for the reaction... 

In future, a complete quantitative analysis on the basis of chemical reaction engineering principles of homogeneously catalyzed gas-liquid-liquid reactions is needed to improve known aqueous biphasic reactions as well as to find new, highly active and selective homogeneous catalysts for organic synthesis. 

Reacting gases may be in excess if they react with solids and do not condense in liquid phases, but supercritical media are clearly not the subject of solvent-free chemistry and deserve their own treatment. For practical reasons, this book does not deal with homogeneous or contact-catalyzed gas-phase reactions. Furthermore, very common polymerizations (except for solid-state polymerizations), protonations, solvations, complexations, racemizations, and other stereo-isomerizations are not covered, to concentrate on more complex chemical con-... 

Reactors can be considered ideal if the conditions of flow and mixing are perfect, that is, not hampered by phenomena such as dispersion, short-circuiting, or dead spaces. Well-defined convectional transport is used in the BR, the CSTR, and the PFR. These reactors can be used for both homogeneous and heterogeneous reactions. The PFR is usually used for solid-catalyzed, gas-phase reactions, while in BRs and CSTRs generally at least one liquid phase is present. 

Recall that there are a number of reactions where homogeneous catalysis involves two phases, liquid and gas, for example, hydrogenation, oxidation, carbonylation, and hydroformylation. The role of diffusion becomes important in such cases. In Chapter 6, we considered the role of diffusion in solid catalyzed fluid-phase reactions and gas-liquid reactions. The treatment of gas-liquid reactions makes use of an enhancement factor to express the enhancement in the rate of absorption due to reaction. A catalyst may or may not be present. If there is no catalyst, we have a simple noncatalytic gas-liquid heterogeneous reaction in which the reaction rate is expressed by simple power law kinetics. On the other hand, when a dissolved catalyst is present, as in the case of homogeneous catalysis, the rate equations acquire a hyperbolic form (similar to LHHW models discussed in Chapters 5 and 6). Therefore, the mathematical analysis of such reactions becomes more complex. 

Hyperbolic equations were used in Chapter 6 to represent reactions catalyzed by solid surfaces. They are referred to as LHHW models and they can be empirically extended to homogeneous catalysis in liquid phase reactions. The actual rate equation to be used for a given reaction will depend on the regime of that reaction. Methods of discerning the controlling regimes for catalytic gas-liquid reactions described in the gas-liquid chapter were based on simple power law kinetics. Extension of these methods to gas-Uquid reactions catalyzed by homogeneous catalysts involves no new principles, but the mathematics becomes more... 

Reactors for Fluid-Fluid Reactions A fluid-fluid system either consists of two immiscible, or at least incompletely miscible liquids, or of a gas and a liquid. The reaction takes place in the liquid phase or in one of the two liquid phases only. For homogeneously catalyzed two-phase reactions (Section 4.8.2), the liquid catalyst is dissolved in one liquid phase, usually a solvent. In general, fluid-fluid reactions involve the mass transfer of at least one reactant into the liquid phase in which the reaction takes place. Thus, the transfer rate and interfacial area are crucial for a fluid-fluid reactor. 

The homogeneously catalyzed oxidation of butyraldehyde to butyric acid is a well-characterized gas/Hquid reaction for which kinetic data are available. It thus serves as a model reaction to evaluate mass transfer and reactor performance in general for new gas/liquid micro reactors to be tested. This reaction was particularly used to validate a reactor model for a micro reactor [9, 10]. 

Several reactor types have been described [5, 7, 11, 12, 24-26]. They depend mainly on the type of reaction system that is investigated gas-solid (GS), liquid-solid (LS), gas-liquid-solid (GLS), liquid (L) and gas-liquid (GL) systems. The first three arc intended for solid or immobilized catalysts, whereas the last two refer to homogeneously catalyzed reactions. Unless unavoidable, the presence of two reaction phases (gas and liquid) should be avoided as far as possible for the case of data interpretation and experimentation. Premixing and saturation of the liquid phase with gas can be an alternative in this case. In homogenously catalyzed reactions continuous flow systems arc rarely encountered, since the catalyst also leaves the reactor with the product flow. So, fresh catalyst has to be fed in continuously, unless it has been immobilized somehow. One must be sure that in the analysis samples taken from the reactor contents or product stream that the catalyst docs not further affect the composition. Solid catalysts arc also to be fed continuously in rapidly deactivating systems, as in fluid catalytic cracking (FCC). 

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