Multiphase reactor

Multiphase reactors are reactors in which two or more phases are necessary to carry out the reaction. The majority of multiphase reactors involve gas and liquid phases which contact a solid. In the case of the slurry and trickle bed reactors, the reaction between the gas and the liquid takes place on a solid catalyst Sluface (see Table 12-2). However, in some reactors the liquid phase is an inert medium for the gas to contact the solid catalyst. The latter situation arises when a large heat sink is required for highly exothermic reactions. In many cases the catalyst life is extended by these milder operating conditions. 

The multiphase reactors discussed in this edition of the book are the slurry reactor, fluidized bed, and the trickle bed reactor. The trickle bed reactor which has reaction and transport steps similar to the slurry reactor is discussed in the first edition of the book and on the CD-ROM along with the bubbling fluidized bed. In slurry reactors, the catalyst is suspended in the liquid and gas is bubbled through the liquid. A slurry reactor may be operated in either a semibafch or continuous mode. 

We finally arrive at the type of reactor usually encountered in industrial practice the multiphase reactor. Obviously, a reactor has multiple phases whenever its contents do not form a single-phase solution. For two phases these may be gas-liquid, liquid-liquid, gas-solid, and liquid-solid. Listed in Table 12-1 are the names of some of the common important multiple-phase reactors. 

We have encountered many of these reactor types in previous chapters. In Chapter 2 all the reactors in the petroleum refinery were seen to be multiphase, and we will close this chapter by returning to the reactors of the petroleum refinery to see if we can now understand how they operate in a bit more detail. 

Catalytic reactors are multiphase if sohd catalysts are used as discussed in Chapter 7. Reactions that form or decompose sohds were discussed in Chapter 9, and many polymerization reactors are multiphase, as discussed in Chapter 11. Biological reactions usually take place in multiphase reactors. 

In this chapter we summarize the characteristics of these reactors and discuss the features that differ Irom single-phase systems. Obvious distinguishing features of multiphase reactors are  

Essential mass transfer steps between phases always accompany reaction steps, and these frequently control the overall rate of the chemical reactions. 

An effective hquid-liquid reactor may be designed to obtain drops that continuously break up and coalesce, or it may be designed to obtain very small drops that have very efficient mass transfer and follow the continuous phase with a low rate of coalescence. The former will require a much larger reactor, but the separation of the phases after reaction is simpler. 

Diffusion in liquids is very slow. Turbulent transport or very narrow channels are necessary for good contact between the phases. The droplets must also be very small to minimize transport hmitations within the drops. An estimation of the time constant for diffusion in a 1-mm drop is (f (10-3)2 

352 I 75 Methods for Identification of Micro- and Macroscale Physical Phenomena in Chemical Reactors 

Small drops will move with the flow and the response time for acceleration of a 1-mm liquid drop in a water-like liquid is on the order of 

This response time should be compared to the turbulent eddy lifetime to estimate whether the drops will follow the turbulent flow. The timescale for the large turbulent eddies can be estimated from the turbulent kinetic energy k and the rate of dissipation e, Xc = 30-50 ms, for most chemical reactors. The Stokes number is an estimation of the effect of external flow on the particle movement, St = r /tc. If the Stokes number is above 1, the particles will have some random movement that increases the probability for coalescence. If St 1, the drops move with the turbulent eddies, and the rates of collisions and coalescence are very small. Coalescence will mainly be seen in shear layers at a high volume fraction of the dispersed phase. 

---

Early ia the development of chemical reaction engineering, reactants and products were treated as existing ia single homogeneous phases or several discrete phases. The technology has evolved iato viewing reactants and products as residing ia interdependent environments, a most important factor for multiphase reactors which are the most common types encountered. 

Multiphase Reactors. The overwhelming majority of industrial reactors are multiphase reactors. Some important reactor configurations are illustrated in Figures 3 and 4. The names presented are often employed, but are not the only ones used. The presence of more than one phase, whether or not it is flowing, confounds analyses of reactors and increases the multiplicity of reactor configurations. Gases, Hquids, and soHds each flow in characteristic fashions, either dispersed in other phases or separately. Flow patterns in these reactors are complex and phases rarely exhibit idealized plug-flow or weU-stirred flow behavior. 

Flow Regimes in Multiphase Reactors. Reactant contacting, product separations, rates of mass and heat transport, and ultimately reaction conversion and product yields are strong functions of the gas and Hquid flow patterns within the reactors. The nomenclature of commonly observed flow patterns or flow regimes reflects observed flow characteristics, ie, armular, bubbly, plug, slug, spray, stratified, and wavy. 

A more general treatment of multiphase reactors is given in Chapter 11. 

The packed-bed reactors discussed in Chapters 9 and 10 are multiphase reactors, but the solid phase is stationary, and convective flow occurs only through the fluid phase. The reaction kinetics are pseudohomogeneous, and components balances are written only for the fluid phase. 

Multiphase reactors can be batch, fed-batch, or continuous. Most of the design equations derived in this chapter are general and apply to any of the operating modes. Unsteady operation of nominally continuous processes is treated in Chapter 14. 

The several industrial applications reported in the hterature prove that the energy of supersonic flow can be successfully used as a tool to enhance the interfacial contacting and intensify mass transfer processes in multiphase reactor systems. However, more interest from academia and more generic research activities are needed in this fleld, in order to gain a deeper understanding of the interface creation under the supersonic wave conditions, to create rehable mathematical models of this phenomenon and to develop scale-up methodology for industrial devices. 

---