Two-Phase Catalytic Reactors

Fluid-solid contacting towers such as a packed bed or a fluidised bed are used for carrying out two-phase catalytic reactions. Packed bed reactors are used if the catalyst is in the form of solid pellets, where as fluidised bed reactors are used if the catalyst is solid particles. 

Consider a section of the bed of length dz at a distance 2 from the fluid inlet. is the concentration of A in the gas entering this section. dC g is the change in concentration of A in the gas phase across the section. The flux of A from the gas phase to the catalyst pellet is given by Equation 4.220  

Taking a steady-state balance of A in the gas phase across this section 

As the specific surface area of the pellet a = 1/Lg, the equation for the height of the tower reduces to 

Substituting Equation 4.221 for flux into Equation 4.225, we get the final equation for the calculation of tower height Z as 

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Gas-solid reactors are the best-known two-phase catalytic reactor types operated continuously. The major reactors for solid-catalyzed gas phase reactions are the fixed-bed, the fluid-bed and the entrained-flow reactor. 

In any catalytic system not only chemical reactions per se but mass and heat transfer effects should be considered. For example, mass and heat transfer effects are present inside the porous catalyst particles as well as at the surrounding fluid films. In addition, heat transfer from and to the catalytic reactor gives an essential contribution to the energy balance. The core of modelling a two-phase catalytic reactor is the catalyst particle, namely simultaneous reaction and diffusion in the pores of the particle should be accounted for. These effects are completely analogous to reaction-diffusion effects in liquid films appearing in gas-liquid systems. Thus, the formulae presented in the next section are valid for both catalytic reactions and gas-liquid processes. 

Program for design of packed bed two phase catalytic reactor % o-Hydrogen to p-Hydrogen in Ni on A1203 catalyst % rate equation (-ra)=k(Ca - Cb/K)... 

Correlations for the dynamic liquid holdup have also been developed as function of various dimensionless numbers including the liquid and gas Reynolds number, and the two-phase pressure drop [see, e.g., Ramachandran and Chaudhari, Three-Phase Catalytic Reactors, Gordon and Rreach, 1983 and Hofmann, Hydrodynamics and Hydrodynamic Models of Fixed Bed Pieactors, in Gianetto and Silveston (eds.), Multiphase Chemical Pieactors, Hemisphere 1986],... 

Engineering of organic synthesis analysis and design of reactors for the important classes of reactions, mainly homogeneous, two-phase catalytic, two-phase noncatalytic, and three-phase catalytic and noncatalytic... 

Catalytic hydrogenation is carried out in a two-phase batch reactor, with the rate proportional to the catalyst concentration in the reaction phase. 

Most of the studies referred to in the previous discussion used two-phase (gas-liquid) systems. The considerations are substantially similar when the liquid jet ejector is to be used as a three-phase catalytic reactor, particularly because the catalyst loading commonly used in a venturi loop system is relatively low. In terms of mass transfer, besides gas-liquid mass transfer, solid-liquid mass transfer step assumes great importance. As discussed in Chapters 7A and 7B, factors relating to dispersion of the gas phase, suspension of solids, and concentration profile of the catalyst phase need to be addressed in the case of a three-phase reactor. 

In a multi-phase catalytic reactor, the reacting species are dissolved in two different fluid phases (e.g., in a gas-liquid system or in liquid-liquid system) which are separated by a phase interface, and the catalyst is located in one of the fluid phases or in a dissolved form (as for example an homogeneous catalyst) or as a third heterogeneous phase (e.g., a solid phase). When a gas-liquid system is in the presence of a solid phase catalyst, the reactor is referred to as a three-phase reactor. Multi-phase reactors represent one of the most important classes of chemical reactors and they are widely used in many industrial sectors, as for example chemical, petrochemical, biotechnological, pharmaceutical and food processing industries (Barnett, 2006 Biardi and Baldi, 1999 Henkel, 2000 Nauman, 2008). Multi-phase reactors have typical industrial application in ... 

Chapter 11 treats reactors where mass and component balances are needed for at least two phases and where there is interphase mass transfer. Most examples have two fluid phases, typically gas-liquid. Reaction is usually confined to one phase, although the general formulation allows reaction in any phase. A third phase, when present, is usually solid and usually catalytic. The solid phase may be either mobile or stationary. Some example systems are shown in Table 11.1. 

This application was performed on a lab-scale reactor and later in a miniplant-scale reactor. The reaction studied was the vapor phase catalytic amidation/cyclization step in a pesticide process. As shown in Figure 2, two reactions are taking place on the catalyst bed. 

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