Single-Phase Reactions in Fixed Beds

The catalytic vapor-phase oxidation of propylene is generally carried out in a fixed-bed multitube reactor at near atmospheric pressures and elevated temperatures (ca 350°C) molten salt is used for temperature control. Air is commonly used as the oxygen source and steam is added to suppress the formation of flammable gas mixtures. Operation can be single pass or a recycle stream may be employed. Recent interest has focused on improving process efficiency and minimizing process wastes by defining process improvements that use recycle of process gas streams and/or use of new reaction diluents (20-24). 

Computational fluid dynamics (CFD) is rapidly becoming a standard tool for the analysis of chemically reacting flows. For single-phase reactors, such as stirred tanks and empty tubes, it is already well-established. For multiphase reactors such as fixed beds, bubble columns, trickle beds and fluidized beds, its use is relatively new, and methods are still under development. The aim of this chapter is to present the application of CFD to the simulation of three-dimensional interstitial flow in packed tubes, with and without catalytic reaction. Although the use of... 

Chromatographic fixed-bed reactors consists of a single chromatographic column containing a solid phase on which adsorption and reaction take place. Normally a pulse of reactant is injected into the reactor and, while traveling through the reactor, simultaneous conversion and separation take place (Fig. 3). Since an extensive overview of the models and applications of this type of reactor was presented by Sardin et al. [ 132], only a few recent results will be discussed here. Most of the practical applications have been based on gas-liquid systems, which are not applicable for the enzyme reactions, but a few reactions were also reported in the liquid phase. One of these studies, performed by Mazzotti and co-workers [ 141 ], analyzed the esterification of acetic acid into ethyl acetate according to the reaction ... 

So far, no reference has been made to the presence of more than one phase in the reactor. Many important chemicals are manufactured by processes in which gases react on the surface of solid catalysts. Examples include ammonia synthesis, the oxidation of sulphur dioxide to sulphur trioxide, the oxidation of naphthalene to phthalic anhydride and the manufacture of methanol from carbon monoxide and hydrogen. These reactions, and many others, are carried out in tubular reactors containing a fixed bed of catalyst which may be either a single deep bed or a number of parallel tubes packed with catalyst pellets. The latter arrangement is used, for exjimple, in the oxidation of ethene to oxiran (ethylene oxide)... 

By choosing a suitable solvent and suitable conditions (concentrations, temperatures, and pressures), the solvent, the substrate, the product, and the hydrogen can form a single phase. When such a single phase is fed to a fixed-bed reactor, extremely high reaction rates have been achieved for very large molecules [28 - 30]. These reaction rates, expressed on a molar basis, are the same as those achieved in gas-phase reactors for small molecules [30]. 

Reaction engineering may be experiencing the same thing. If we continue to work on fixed-bed single-phase reactors with multiple steady states in... 

The reactor models developed for fixed-bed reactors have been exploited for use in other situations, for example, CVD reactors (see Vignette 6.4.2) for microelectronics fabrication. These models are applicable to reaction systems involving single fluid phases and nonmoving solids. There are numerous reaction systems that involve more than one fluid phase. Figure 10.4.3 illustrates various types of reactors... 

Fixed-bed catalytic reactors are widely applied to reaction systems in which the reactants are present in a single vapor phase. The scale-up and performance of commercial reactors can be predicted from experiments in small-scale reactors. On the other hand, the mixed-phase trickle bed reactor is considerably more complex to analyze and scale up. The performance of trickle bed reactors is influenced by many factors associated with mixed-phase (gas-liquid-solid) processing. Some of... 

Catalytic distillation (CD) is an unit operation combining reaction and separation in a single reactor/distillation column. CD belongs to the general class of two-phase flow fixed-bed catalytic reactor. An upward flow of vapor and downward flow of liquid conqjrise the two flowing phases in the CD reactor. Solid catalyst packed in a distillation column not only accelerates a chemical reaction but also supplies a packing surface for vapor-liquid mass transfer to separate the reactants... 

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