Crosscurrent processes

The flowsketch of the crosscurrent process is shown. Feed to the first stage and water to both stages are at 100 kg/hr. The extract and raffinate compositions are on the tielines passing through mix points and Mi- Point M is for one stage with the same total amount of solvent. Two stage results are ... 

Despite the success of the counter-current process in comparison with single batch or crosscurrent processes, a DFl value of 10 is modest in view of the hundredfold difference between and. Better decontamination can be achieved if a washing or scrub section is incorporated in the process. This involves contacting the loaded organic product stream with an aqueous phase having a low or zero Ln concentration. The raffinate from this section is then combined with the aqueous feed to the extraction section, as illustrated in Figure 31. The mass fraction, of... 

Conversion of crosscurrent process into countercurrent process... 

Alternative The washing can be done in a Podbielniak centrifugal extractor, which is continuous countercurrent extraction. Alternately, the same can be done in a cascade of annular centrifugal extractors configured in the countercurrent mode. It is known that the amount of cold water required and the loss of the API would be far less compared with values for the crosscurrent process [18]. 

Washing and adsorption The belt filters are used for washing and dewatering of fine solids in the manufacture of catalysts, zeolites, alumina and other crystalline substances, and so on. The belt filter requires the associated equipment — several vacuum pumps and separation drums. Through-circulation tray driers and through-circulation rotary driers are used to dry solids. All these are continuous crosscurrent processes. These could be replaced with a moving-bed, Hildebrand (screw) conveyer or Kennedy extractor [18], each of which is a countercurrent process. Their potential for PI has not been explored. 

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. 

The crosscurrent scheme is not generally economically attractive for large commercial processes because solvent usage is high and solute concentration in the combined extract is low. 

The objective of Lamb et al s experimental work [4] was to obtain measurement data on burning rates (conversion rates) of a simulated crosscurrent moving bed. The experimental data would be applied to the combustion process in an inclined grate furnace. 

The unsteady dynamics of a fixed (over-fired) batch bed process has been proven to generate a conversion process structure similar to that in a crosscurrent moving bed [80], see Figure 57. 

Figure 57 An idealised stationary bed process structure of a crosscurrent configuration on a travelling grate. [12]...

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 5 Multistage extraction processes (a) crosscurrent extraction, (b) countercurrent extraction, and (c) countercurrent fractional extraction. 

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. 

I u.o rossiDie arrangements ot mixer settlers ana process streams Tor muitisx processes, (a) Crosscurrent arrangement, (b) Countercurrent arrangement. 

Two liquid phases may be contacted in different ways to achieve separation. In the laboratory, the simplest example of LLE involves the use of a separatory funnel where a feed mixture, perhaps an aqueous phase from a reactor, is contacted with several washes of solvent (say, diethyl ether) to extract a solute. If the same aqueous phase is contacted repeatedly with fresh amounts of extracting solvent, such a process is said to be crosscurrent. 

FIGURE 21.1-10 Schemetic representation of a multistage dialysis process For couiinuous fractionetion of solutes. The process combines a nouseiective solvent stripper (concentrator) with two crosscurrent dialysis cascades (Dt-Du and Di-Da ). (Reprinied by permission of the American Institute of Chemical Engineera.)... 

This is an extension of single-stage extraction wherein the raffinate is successively contacted with fresh solvent, and may be done continuously or in batches. Figure 7.9 is a schematic diagram for a three-stage crosscurrent extraction process. A single final raffinate results, and the extracts can be combined to provide a composited extract, as shown. As many stages as necessary can be used. 

The composited extract flows at the rate of 52.58 kg/h, and its composition is 5.4 wt% water, 45.1 wt% chloroform, and 49.5 wt% acetone. Notice that the acetone content of the final raffinate (18.5 wt%) is slightly lower than the acetone content of the raffinate obtained in Example 7.2 in a single-stage extraction with 50 kg/h of chloroform. Remarkably, this is true even though the total amount of solvent used in the multistage crosscurrent extraction process is only 3 x 8 = 24 kg/h. 

Consider the extraction process of Problem 7.3. Calculate the total amount of solvent required if the extraction is done in a crosscurrent cascade consisting of 5 ideal stages. Use equation (3-68) from Problem 3.21. 

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