Reactors for Fluid-Solid Systems

Heterogeneous systems can be subdivided into three classes based on the number of phases involved  

The fluid-solid reactions are presented in this chapter while fluid-fluid (gas-liquid, liquid-liquid) and three phase reactions are presented in the next two chapters. Fluid-solid systems cover a major class of chemical reactions and encompass both liquid-solid and gas-solid systems. In either case, the fluid phase is a single homogeneous fluid. The solid phase acts as a catalyst and its arrangement in the main reactions zone is an important and complex task. 

Conventionally, fluid-solid reactions are carried out in various types of reactors, such as packed beds, fluidized/slurry, and monolith reactors as summarized in Table 6.1 [1]. Packed bed reactors are relatively simple, easy to operate and suitable for reactions that require relatively large amounts of catalyst, as they provide a high volumetric catalyst fraction of about 60%. The characteristic feature of packed bed reactors is the pressure drop of the fluid flowing through the catalytic bed. To avoid excessive pressure drop the use of catalyst pellets of 2-6 mm is necessary. But, large porous particles lower the transformation rate through diffusion limitations in the porous network and may decrease product selectivity and yield as discussed in Chapter 2. 

An important issue for packed bed reactors is temperature control. Insufficient heat removal may lead to local overheating of the catalyst pellets with the consequence of rapid deactivation. Therefore, multitubular reactors with up to 35 000 parallel tubes are used in chemical industry for the temperature control of highly exothermic reactions. 

Fluidized bed reactors allow improved heat management for fast exothermic reactions thus increasing the performance of gas-solid reactors, but within narrow operating windows. Fluidized bed reactors impose special demands on the mechanical stability of the catalyst and are difficult to scale-up. 

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I 6 MIcrostructured Reactors for Fluid-Solid Systems order process, the conversion is given by ... 

A reactor model based on solid particles in BMF may be used for situations in which there is deliberate mixing of the reacting system. An example is that of a fluid-solid system in a well-stirred tank (i.e., a CSTR)-usually referred to as a slurry reactor, since the fluid is normally a liquid (but may also include a gas phase) the system may be semibatch with respect to the solid phase, or may be continuous with respect to all phases (as considered here). Another example involves mixing of solid particles by virtue of the flow of fluid through them an important case is that of a fluidized bed, in which upward flow of fluid through the particles brings about a particular type of behavior. The treatment here is a crude approximation to this case the actual flow pattern and resulting performance in a fluidized bed are more complicated, and are dealt with further in Chapter 23. 

The analysis of this type of reactor requires a uniform composition of fluid phase throughout the volume. While this is easily achieved by standard agitation devices for liquid-solid systems, i.e. impellers, it requites special design to be achieved for gas-solid systems. This type of reactor is basically used for laboratory experimentation. 

In fluid-solid systems the interparticle gradients - between the external surface of the particle and the adjacent bulk fluid phase - may be more serious, because the effective thermal conductivity of the fluid may be much lower than that of the particle. For the interparticle situation the heat transfer resistances, in general, are more serious than the interparticle mass transfer effects they may become important if reaction rates and reaction heats are high and flow rates are low. Hie usual experimental test for interparticle effects is to check the influence of the flow rate on the conversion while maintaining constant the space velocity or residence time in the reactor. This should be done over a wide range of flow rates and the conversion should be measured very accurately. 

We will now stop and consider reactor design for a fluid-solid system with decaying catalyst. To analyze these reactors we only add one step to our algoritlhm, that is, determine the catalyst decay law. The sequence is shown below. 

Equation (8.4) is suitable for homogeneous system, equation (8.5) is useful for fluid/solid catalyst reactors, while equation (8.6) is applicable for gas/liquid or liquid/liquid reactors. 

The kinetics of the triphase reaction is complicated and not yet completely described by that of the classical fluid-solid system. These experimental results revealed that it is preferable to use an SR rather than an FBR for a triphase reaction but the reactor volume of SR was much larger than that of FBR. The goal of designing an SR could be to reduce the reactor volume and stop the catalyst from flowing out of the reactor. If no premix was set before an FR, the length of reactor was too short to reduce llte performance of the FR. 

Catalysts for fluidized-bed reactors have to be spherical as well. The appropriate particle size fraction for gas-solid systems can be estimated after Geldart [1] from the density difference between soKd and gas. Most widely used catalysts for fluidized beds and risers are Geldart-type B powders with particle diameters ranging from 40 to 500 pm or solid densities between 1.4 X 10 and 4 x 10 kg/m, respectively. When fluidization is provided by a Kquid as in ebullated-bed reactors, the particle sizes may be substantially larger because of the higher buoyancy in these systems. However, all types of fluidized-bed catalysts must exhibit high mechanical stability because they are exposed to abrasion on reactor walls and internals, collisions between particles and shear forces exerted by the surrounding fluid. 

The hydrodynamic characteristics of three-phase reactors, such as pressure drop and residence time distribution, can be determined from those for fluid-solid and fluid-fluid reactors. The difference between the gas-liquid and gas-liquid-solid systems is that due to the reaction at the surface of the catalyst, there is always a concentration gradient in the liquid phase in the latter case. Unlike in gas-liquid reactions, it is always important to saturate the liquid film with the gaseous... 

Table 11.4 lists reactors used for systems with two fluid phases. The gas-liquid case is typical, but most of these reactors can be used for liquid-liquid systems as well. Stirred tanks and packed columns are also used for three-phase systems where the third phase is a catal5hic solid. The equipment listed in Table 11.4 is also used for separation processes, but our interest is on reactions and on steady-state, continuous flow. 

On the basis of different assumptions about the nature of the fluid and solid flow within each phase and between phases as well as about the extent of mixing within each phase, it is possible to develop many different mathematical models of the two phase type. Pyle (119), Rowe (120), and Grace (121) have critically reviewed models of these types. Treatment of these models is clearly beyond the scope of this text. In many cases insufficient data exist to provide critical tests of model validity. This situation is especially true of large scale reactors that are the systems of greatest interest from industry s point of view. The student should understand, however, that there is an ongoing effort to develop mathematical models of fluidized bed reactors that will be useful for design purposes. Our current... 

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