Heterogeneous Reactors—Mass Transfer Equipment

Unlike homogeneous reactors in which all the reacting chemical compounds are present in one single phase, in the heterogeneous reactors, the reactants are distributed in different phases. Heterogeneous reactors are multiphase reaction vessels that are designed to handle reachons occurring between chemical compoimds present in two or more different phases. Consider a chemical reaction between compoimd A in phase PI and compound B in the phase P2. 

In multiphase reactions, the mass transfer rate plays a crucial role in determining the overall rate of conversion of reactant to product. In the multiphase reaction between compound A and B discussed above, A moves from phase PI to phase P2 and the reaction between A and B occurs in phase P2. Compound A is the limiting reactant as the overall conversion of A to final product is limited by the availability of A in phase P2, in which the reaction occurs. The larger the rate of mass transfer of A from phase PI to phase P2, the higher the concentration of A in phase P2 and the greater the rate of reaction in phase P2. Thus, the mass transfer rate has a stronger influence on the overall conversion. 

Any mass transfer equipment that is used for contacting two different phases PI and P2 can also be used as a heterogeneous reactor for carrying out reactions involving these phases. For example, consider a chemical reaction between CO2 (compound A) in gas phase (PI) and NaOH solution (compound B) in liquid phase (P2) 

Consider the chemical reaction between FeS2 in the powder form (solid phase) and O2 in the gas phase  

Compound A is transferred from phase PI to phase P2 across the phase boundary and it happens at a rate Xy which is proportional to e concentration difference driving force (Cm - Q2)/ thaf is 

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In many of these operations the engineer is concerned primarily with prediction of pressure losses. However, the heat transfer rate through the tube wall into the gas or the liquid phase is also of major concern in heat-exchange equipment. In the design of chemical reactors for heterogeneous gas-liquid systems, it is necessary to be able to predict not only pressure drops and rates of heat transfer into or out of the channel, but also the rates of mass transfer from the gas into the liquid phase. 

Example 44 Dimensioning of a tubular reactor, equipped with a mixing nozzle, designed for carrying out competitive-consecutive reactions 193 Example 45 Mass transfer limitation of the reaction rate of fast chemical reactions in the heterogeneous material system gas/liquid 197... 

Heterogeneous catalytic reactors are the most important single class of reactors utilized by the chemical industry. Whether their importance is measured by the wholesale value of the goods produced, the processing capacity, or the overall investment in the reactors and associated peripheral equipment, there is no doubt as to the prime economic role that reactors of this type play in modem industry. The focus of this chapter is the design of heterogeneous catalytic reactors. Particular emphasis is placed on the concept of catalyst effectiveness factors and the implications of heat and mass transfer processes for fixed bed reactor design. 

These processes are carried out in a variety of equipment ranging from a bubbling absorber to a packed tower or plate column. The design of the adsorber itself requires models characterizing the operation of the process equipment and this is discussed in Chapter 14. The present chapter is concerned only with the rate of reaction between a component of a gas and a component of a liquid—it considers only a point in the reactor where the partial pressure of the reactant A in the gas phase is and the concentration of A in the liquid is C, that of B, Cg. Setting up rate equations for such a heterogeneous reaction will again require consideration of mass and eventually heat transfer rates in addition to the true chemical kinetics. Therefore we first discuss models for transport from a gas to a liquid phase. 

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