Fluid-solid reactors mixing

Figure 26.1 Various contacting patterns in fluid-solid reactors a-d) countercurrent, crosscurrent, and cocurrent plug flow d) intermediate gas flow, mixed solid flow (e) semibatch operations.

Reactors for solid-solid reactions are designed in the same way as that for the fluid-solid reactors (see Section 5.10) but with these reactions the mixing of the gas does not need to be considered. 

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. 

Figure 21.4 Test of the kinetic expressions of Eq. 14 using a batch of solids and changing flow of fluid in a mixed reactor so as to keep Ca constant.

For sound process design, we need values of numerous design parameters such as fractional phase holdups, pressure drop, dispersion coefficients (the extent of axial mixing) of all the compounds, heat and mass transfer coefficients across a variety of fluid-fluid and fluid-solid interfaces depending on the type of multiphase system, type of reactor, and the rate-controlling steps. To clarify the scope of the case studies selected, their salient features are next listed. 

Gas-solid (catalytic) Differential reactor Integral reactor Mixed (Carberry) reactor Mic roreactor Fluid-bed reactor Single-pellet reactor (Chapter 7)... 

A reasonable assumption for the fluidized-bed reactor is that the fluid is partially mixed, whereas the solid is fully mixed. If we assume that the fluid is in plug flow, the residence time distribution of the solids will be given by exp(-t). As for the fixed-bed reactor, we shall first consider exponential decay. Therefore, the average rate constant to be used in solving Equation 12.46 is... 

A10 kWjij continuous reactor of interconnected fluidized beds has been discussed in Ref. [55] for CLC with biomass (Figure 5.12). The prototype is composed of a fast fluidized bed as air reactor, a cyclone and a spout-fluid bed as fuel reactor. In this case, the spout-fluid type reactor is adopted as fuel reactor in order to have a strong solid mixing between the biomass and OC particles and a long residence time. The spout-fluid reactor is designed to have two difl erent compartments. In the first part, the reaction chamber is located where the OC and the biomass are combined to produce exhaust gas and solid species (metal oxide and unconverted fuel), while the second part contains the inner seal that is located at the top and it is used to allow solids movement to the air reactor. The fuel reactor is fluidized by using exhaust gas recirculation (Table 5.2). 

In fluidized beds there are favorable conditions for rapid heat and mass transfer between the solids and the fluid. Very rapid mixing of the solids generally occurs so that the coefficients for the transfer of heat to boimdary surfaces are quite high. Fluidized beds are, therefore, used both as heat exchangers and chemical reactors particularly where close control of temperature is required and where large amounts of heat must be added to, or removed from the system. There are, of course, many other relevant applications of fluidized beds in the materials processing industry. A proper knowledge of the flow patterns of both fluid and particles is, therefore, necessary. 

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