Fluid-solid contacting

If larger particles are used in a moving-bed reactor, there is some sacrifice over temperature control and fluid-solid exchange. However, the pressure drop is much less than in bubbling fluidized beds, and erosion by particles is largely avoided. Furthermore, the fluid-solid contacting is close to ideal, and so performance is enhanced. 

Various methods of contacting fluids with particulate solids are shown in Figure 5.13. These contacting methods include countercurrent, crosscurrent, and cocurrent plug flow as well as mixed solids flow-intermediate gas flow, and semi-batch operations. Consideration of the residence time distribution for each type of fluid-solid contact is necessary to understand its effect on the conversion. As a result of given residence time distribution, E t), the average conversion of B, Xb, is given by... 

Adsorber Vessel The most frequently used method of fluid-solid contact for adsorption operations is in cylindrical, vertical vessels, with... 

Reactor Type Sampling and Analysis Isothermality Fluid-Solid Contact Decaying Catalyst Ease c Construction... 

The term adsorption operations, as used in this chapter, is intended to refer to all methods of separation that can be carried out with arrangements of apparatus characteristic of adsorption, or appropriate to it. Any separation operation based upon fluid-solid contact is thus somewhat related to adsorption, and the calculation methods developed for adsorption may prove applicable to it. 

Heterogeneous catalysis relies on fluid-solid contact, and catalysts are prepared with as high as possible surface area in order to maximise such contact. This structure is thermodynamically unstable and, given sufficient... 

In transient two-phase flows, one phase replaces the other and the dynamics of the wettingdewetting of the surface, which is influenced by the fluid-fluid interfacial tension, solid-fluid interfacial tensions, and the solid-surface forces, must be closely examined. The research on the dynamics of the contact line (fluid-fluid-solid contact line) has been advanced in the last decade. 

Due to the small characteristic dimension, the flow in microchemical systems is laminar. As a result, mixing relies only on molecular diffusion instead of the more efficient turbulence that large-scale systems typically exhibit. At the same time, the diffusion time scale is much shorter due to the small size of a microscale device. However, structural elements that play the role of static micromixers may be necessary to spread fast flows, enhance fluid-solid contact, increase mixing of incoming gases, etc. One such example is the post-micromixer discussed in Ref. [5]. 

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. 

The conventional fluidized beds also possess some serious deficiencies, however. The bubbles that are responsible for many benefits of a fluidized bed represent the fluid bypassing and reduction of fluid-solids contacting. The rapid mixing of solids in the bed leads to nonuniform solids residence time distribution in the bed. The rigorous solids mixing in the bed also leads to attrition of bed material and increases the bed material... 

In practical operations, maximum capacity of adsorbent cannot be fully utilized because of mass transfer effiects involved in actual fluid-solid contacting processes. In order to estimate practical or dynamic adsorption capacity, however, it is essential, first of all. to have information on adsorption equilibrium. Then kinetic analyses are conducted based on rate processes depending on types of contacting processes. The most typical of the rate steps in solid adsorbents is the intraparticle diffusion which is treated in the next chapter. 

Since the first introduction of these techniques for fluid-solid contact in 1948, with the principal application in the catalysis of gaseous chemical reactions, a very large technology for their design has developed. This is still not in a condition which permits summary for design purposes in the space available here, however, and more extended works should be consulted for such details [16, 54, 61, 103). The discussion here is limited to the computation of stage requirements and does not include the mechanical design. 

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