Noncatalytic homogeneous liquid phase reaction

Most of the previously used expressions to account for incomplete catalyst wetting in trickle-beds are summarized in Table I. All of these, with the exception of the last one, are based on the assumptions of a) plug flow of liquid, b) no external mass transfer limitations, c) isothermal conditions, d) first order irreversible reaction with respect to the liquid reactant, e) nonvolatile liquid reactant, f) no noncatalytic homogeneous liquid phase reaction. 

We now consider 12 case studies that include simple homogeneous liquid-phase reactions, complex homogeneous gas-phase reactions, gas-solid catalytic and noncatalytic reactions, gas-liquid simple and complex reactions, gas-liquid-solid (noncatalytic) reactions, gas-liquid-solid (catalytic) reactions, and solid-solid reactions. The scope and coverage of each case study are summarized in Table 11.27. In the first, homogeneous reactions are considered. For these relatively simple reactions, the possibility of optimum design is discussed. 

The mechanisms and rates discussed in this section apply primarily to homogeneous, noncatalytic thermal igas-phase reactions. However, the effect of homogeneous and heterogeneous catalysts and of irradiation will be mentioned in some cases. Many of the ideas discussed apply also to liquid-phase reactions, depending on the solvent. 

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. 

One of Cauldron Chemical Company s most profitable products is made via a homogeneous, irreversible, liquid-phase reaction A —> products using a tubular, isothermal reactor packed with noncatalytic, nonporous spheres of uniform diameter dp. The spheres are believed to promote radial mixing and heat transfer. The fractional conversion of A in the effluent frxjm the current reactor is 99%. 

Although they are termed homogeneous, most industrial gas-phase reactions take place in contact with solids, either the vessel wall or particles as heat carriers or catalysts. With catalysts, mass diffusional resistances are present with inert solids, the only complication is with heat transfer. A few of the reactions in Table 23-1 are gas-phase type, mostly catalytic. Usually a system of industrial interest is liquefiea to take advantage of the higher rates of liquid reactions, or to utihze liquid homogeneous cat ysts, or simply to keep equipment size down. In this section, some important noncatalytic gas reactions are described. 

Polymerization can be catalytic or noncatalytic, and can be homogeneously or heterogeneously catalyzed. Polymers that form from the liquid phase may remain dissolved in the remaining monomer or solvent, or they may precipitate. Sometimes beads are formed and remain in suspension sometimes emulsions form. In some processes solid polymers precipitate from a fluidized gas phase. Polymerization processes are also characterized by extremes in temperature, viscosity, and reaction times. For instance, many industrial polymers are mixtures with molecular weights of 104 to 107. In polymerization of styrene the viscosity increased by a factor of 106 as conversion increased from 0 to 60 percent. The adiabatic reaction temperature for complete polymerization of ethylene is 1800 K (3240°R). Initiators of the chain reactions have concentration as low as 10-8 g-moFL, so they are highly sensitive to small concentrations of poisons and impurities. 

Note that the same mass-balance equations apply whether the reaction in the liquid phase is homogeneous or catalyzed by solid particles as in a slurry reactor. The difference between catalyzed and noncatalytic systems is accounted for in the global rate. If the reactants are introduced only in the gas phase, a mass balance is needed only for that phase. This situation exists for some slurry reactors where the liquid phase is inert and its purpose is simply to suspend the.catalyst particles. 

For noncatalytic homogeneous reactions, a tubular reactor is widely used because it cai handle liquid or vapor feeds, with or without phase change in the reactor. The PFR model i usually adequate for the tubular reactor if the flow is turbulent and if it can be assumed tha when a phase change occurs in the reactor, the reaction takes place predominantly in one o the two phases. The simplest thermal modes are isothermal and adiabatic. The nonadiabatic nonisothermal mode is generally handled by a specified temperature profile or by heat transfer to or from some specified heat source or sink and a corresponding heat-transfer area and overall heat transfer coefficient. Either a fractional conversion of a limiting reactant or a reactoi volume is specified. The calculations require the solution of ordinary differential equations. 

Part III Beyond the Fundamentals presents material not commonly covered in textbooks, addressing aspects of reactors involving more than one phase. It discusses solid catalyzed fluid-phase reactions in fixed-bed and fluidized-bed reactors, gas-solid noncatalytic reactions, reactions involving at least one liquid phase (gas-liquid and liquid-liquid), and multiphase reactions. This section also describes membrane-assisted reactor engineering, combo reactors, homogeneous catalysis, and phase-transfer catalysis. The final chapter provides a perspective on future trends in reaction engineering. 

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