Operating crosscurrent operation

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.

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 Adsorption The amount of adsorbent required for the separation of a given amount of solute can be decreased by employing multistage crosscurrent contact, which is usually operated in batch mode, although continuous operation is also possible. The required adsorbent is further decreased by increasing the number of stages. However, it is seldom economical to... 

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. 

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... 

Countercurrent operation of shaft and rotary kiln furnaces leads to extensive heat transfer between feedstock and outgoing products. The sensible heat of the products remains within the system which enhances its thermal yield. Cocurrent operation shows none of these advantages yet it is a simple and efficient method to avoid tar problems. Indeed the tar is extensively cracked while moving through the hearth zone. Also the problem of wastewater is somewhat simplified. Crosscurrent operation is only seldom used. 

The operational modes of the three-phase fluidized beds can be classified by the directions of the gas and liquid flows cocurrenf upward, cocurrent downward, countercurrent, or crosscurrent as shown in Fig. 

Direct Roto-Louvre dryer. It is, perhaps, the most important of the special types, as the solids progress in a crosscurrent motion to the gas, and it is suitable for low- and medium-temperature operations. 

Dryers are classified briefly into three types parallel current, countercurrent, and crosscurrent, by the direction of hot airstream and materials movement. The parallel-current type is introduced when fast drying rate is necessary in the earlier drying period. The countercurrent type is available for the case where drying operation is conducted to maintain a slow rate in the earlier period and sequentially increasing the drying rate at a later period. Batch dryers in a broader sense can be considered essentially as one of the crosscurrent type. In this section, the outline of the dryerdesigning procedure is described for the most popular hot air heating. 

Example 1 In an API manufacture, a stirred tank of 5 m capacity is used as a reactor. It is charged with a solvent and cooled to 5 °C from the ambient temperature. The reactants are added and the reaction is carried about for 8 h under constant stirring. At the end of the reaction, the reaction mixture is washed with the same volume of water at 10 °C and then the wash water is decanted. The washing is repeated 6 times. Then, the vessel is cleaned for the next operation to meet regulatory requirements. The total batch time is about 40 h. Thus the vessel is occupied for 40 h. These washings amount to batch crosscurrent multistage extraction. There is a loss of the API as well as of the solvent (in which the API is held) in each of these washing. The yield is only about 60%. 

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

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