Homogeneously catalyzed gas-liquid

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. 

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)).

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. 

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. 

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... 

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]. 

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-... 

The two-film model representation can serve as a basis for more complicated models used to describe heterogeneously catalyzed RSPs or systems containing suspended solids. In these processes a third solid phase is present, and thus the two-film model is combined with the description of this third phase. This can be done using different levels of model complexity, from quasi-homogeneous description up to the four-film presentations that provide a very detailed description of both vapor/gas/liquid-liquid and solid/liquid interfaces (see, e.g., Refs. 62, 68 and 91). A comparative study of the modeling complexity is given in Ref. 64 for fuel ether synthesis of MTBE and TAME by CD. 

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). 

Product separation and catalyst recovery at the end of the homogeneous catalyzed reactions, as explained, are in most cases carried out by crystallization, filtration, distillation liquid-liquid extraction, or gas-liquid absorption. These unit operations can be performed in batch or continuous mode. The salient features of these operations are described in the following. 

In other words, of the initial charge of isopropyl chloride, 34% of the carbon is converted to propane under these conditions. A comparable sample of 1-chloropropane under identical reaction conditions, of excess alkyl halide Lewis acid, was converted after five minutes to the extent of 18% to propane. After 15 minutes the relative amount of "propane" had decreased as a result of further acid catalyzed polycondensation reactions. Similarly, isopropyl chloride reacts in HBr-AlBr3 at room temperature to give a gas product which is entirely propane after 2 min. In this system we again begin to see a buildup of heavier hydrocarbon species with time. The fact that the reaction proceeds so rapidly here can probably be attributed to a homogeneous hydrocarbon/acid liquid phase. This last result and additional experiments are summarized in Table IV. 

In a mure specific definition, the carbon atom introduced originates from synthesis gas (CO/Hj) in a homogeneously-catalyzed liquid phase reaction. Substrates accessible for homologation are, in particular, alcohols but can also be ethers, esters.acids, aldehydes, and ketones [1"6]. 

Fig. 8.21. Concentration profile of a reactant in the vicinity of a gas-liquid interface with a homogeneously catalyzed reaction in the liquid phase.

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