Crosscurrent flow

For crosscurrent flow, shown in Fig. 26.96, there will be a definite reaction plane in the solids whose angle depends solely on the stoichiometry and the relative feed rate of reactants. In practice, heat transfer characteristics may somewhat modify the angle of this plane. 

Repeat Example 1.1 for a solvent for which E = 0.90. Display your results with a plot like Fig. 1.15. Does countercurrent flow still have a marked advantage over crosscurrent flow It is desirable to choose the solvent and solvent rate so that E > 1 Explain. 

Figure 1-64 shows a cascade of three stages operated at steady-state in a crosscurrent flow process where heat and mass transfer occur. 

The fuel-bed system may also be characterised with respect to what is here called fuel-bed configuration. Fuel-bed configuration may be defined as the relative movement between air flow (interstitial gas phase) and solid phase flow [19,38], Three basic configurations arise as a consequence of the possible 90°-combinations and are commonly referred to as cocurrent, countercurrent and crosscurrent., see Figure 27. 

Figure 27 The three basic updraft configurations between air-fuel flow, A. countercurrent, B- crosscurrent and C-countercurrent.

Cocurrent means that the air flow and the solid phase have the same direction, see Figure 27A. Countercurrent correspond to opposite flows, see Figure 27C. In a crosscurrent system the solid phase feed and the air flow cross each other, see Figure 27B. 

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.

Cocurrent and Crosscurrent Plug Flow of Gas and Solids. In cocurrent flow, shown in Fig. 26.9a, all reaction occurs at the feed end, and this represents a poor method of contacting with regard to efficiency of heat utilization and preheating of entering materials. 

In this process the feed and subsequently the raffinate are treated in successive stages with fresh solvent. The sketch is with Example 14.3. With a fixed overall amount of solvent the most efficient process is with equal solvent flow to each stage. The solution of Example 14.3 shows that crosscurrent two stage operation is superior to one stage with the same total amount of solvent. 

Multistage Crosscurrent Extraction In multistage crosscurrent extraction, the raffinate is successively contacted with fresh solvent which can be done continuously or in batch (Figure 10.4). With the given flow rates of the feed and the solvent streams, the... 

Fig. 10.4 Flow diagram for the crosscurrent multistage mixer-settler.

An attractive property of monolithic reactors is their flexibility of application in multiphase reactions. These can be classified according to operation in (semi)batch or continuous mode and as plug-flow or stirred-tank reactor or, according to the contacting mode, as co-, counter-, and crosscurrent. In view of the relatively high flow rates and fast responses in the monolith, transient operations also are among the possibilities. 

Crosscurrent extraction (Fig. 5a) consists of repeated contacts of the feed solution with fresh solvent, resulting in a series of extract streams of gradually diminishing concentration. It is a simple arrangement and can be readily applied batchwise (as a laboratory operation) or with continuous flow of feed and solvent. It is more effective than a single-stage fed with the same total flows of solvent and feed. 

An example of this type of configuration is the Arbiter Process (2) for ammoniacal leaching of copper concentrates. The lixiviant consists of aqueous ammonia flowing counter-currently and oxygen sparged crosscurrently. 

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... 

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