Staged operations crosscurrent

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. 

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

Staged operations (a) single stage (b) co-current (c) countercurrent (d) crosscurrent. 

This example shows the distinct advantage that tiie countercurrent process holds, not only over the single-stage operation, which was expected, but also over its crosscurrent counterpart Consumption by the latter is higher by a factor of 2.2/0.87 = 2.5. The principal advantage of the crosscurrent cascade is that it is more easily adaptable to batch processing, whereas the counter-current cascade is by necessity continuous. 

The color inteosity was measured on an arbitrary scale, proportional to the concentration of the colored substance. It is desired to reduce the color to 10% of its origmal value, 9.6. Determine the quantity of fresh carbon required per 1000 kg of solution for a single-stage operation, for a two-stage crosscurrent process using the nninimum total amount of carbon, and for a two-stage countercurrent operation. 

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... 

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. 

FIGURE 15.14 Graphical solution to determine minimum sorbent required in two-stage crosscurrent operation. 

The single equilibrium stage is the key unit on which the more complex configurations such as the crosscurrent and countercurrent cascades are based. We use it here to introduce the reader to some basic notions of equilibrium stage processes and to make a first presentation of a key tool, the operating diagram. 

Operating diagram for a two-stage crosscurrent adsorption process using fresh adsorbent. 

The optimization problem considered indicates that the best way to operate a crosscurrent cascade is by equal subdivision of solvent or adsorbent. The reader should be aware, however, that the complete optimization of a plant must also consider the cost of the stages, the cost of solvent recovery, and the value of the extracted solute itself. Thus, in addition to optimmn solvent use, we also need to determine the optimum number of stages, and this... 

Figure 11.19 Solution to Eq. (11.13). Minimum total adsorbent, two-stage crosscurrent operation.

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