Continuous countercurrent extraction

FSL solutions have been developed for continuons countercurrent extractions in which the effects of flow maldistribution and anial dispetaion are negligibly small.1 4 These solutions involve the following infinite series  

Values of (I - i)) obtained from Eqs. (10,13-1), (10.13-2), and (10,13-3) are plotted versus t, Tot spherical particles with a as a parameter in Fig. 10.13-1. It can be seen that when a 1.0 the maximum valne of i) is a. The necessary extractor voluma is 1 - e). Typical length to diameter tatios Tor continuous countercurrent extractors (i.e., tower extractors and slopa extractors) range betwaan 2 1 and 4 1.  

Valons of X, and f the solid and liquid conesnuations ai positions along the length of the extractor, ate provided by the following equations  

13 1 Continuous Extraction Versus Multistage Batch Extraction 

Different C, must be used in (10.13-2), but the g, remain the same as in (10.13-1). Defining equations for C and q and Q and q, and values for G and P are presented in Tables 10.13-1 and 10.13-2 for various slupes of solids. These defining equations are valid only when Bi is infinite, which is a good approximation for most continuous leaching systems. Alternative values for these parameters, which are valid when Bi is finite, are available in some cases. Extraction yields, q = ET y,/R)C are given by the following equations  

R as a parameter for spherical particles in a continuous countercurrent leaching qrstem. 

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Extraction from Aqueous Solutions Critical Fluid Technologies, Inc. has developed a continuous countercurrent extraction process based on a 0.5-oy 10-m column to extract residual organic solvents such as trichloroethylene, methylene chloride, benzene, and chloroform from industrial wastewater streams. Typical solvents include supercritical CO9 and near-critical propane. The economics of these processes are largely driven by the hydrophihcity of the product, which has a large influence on the distribution coefficient. For example, at 16°C, the partition coefficient between liquid CO9 and water is 0.4 for methanol, 1.8 for /i-butanol, and 31 for /i-heptanol. 

As Sherwood and Pigford(3) point out, the use of spray towers, packed towers or mechanical columns enables continuous countercurrent extraction to be obtained in a similar manner to that in gas absorption or distillation. Applying the two-film theory of mass transfer, explained in detail in Volume 1, Chapter 10, the concentration gradients for transfer to a desired solute from a raffinate to an extract phase are as shown in Figure 13.19, which is similar to Figure 12.1 for gas absorption. 

An important feature in the construction of continuous countercurrent extraction towers is the design of the nozzles through which the solvent and oil feed enter. Nozzles should be designed to distribute the stream evenly over the cross-sectional area of the tower in order to minimize channeling and to make maximum use of the packing near the inlet. Consideration should also be given to the use of intercoolers to enforce a temperature gradient. 

Figure 14.3. Representation of countercurrent extraction batteries, (a) A battery of mixers and settlers (or separators), (b) Schematic of a three-stage countercurrent battery, (c) Simulation of the performance of a three-stage continuous countercurrent extraction battery with a series of batch extractions in separatory funnels which are designated by circles on the sketch. The numbers in the circles are those of the stages. Constant amounts of feed F and solvent S are mixed at the indicated points. As the number of operations is increased horizontally, the terminal compositions Et and R3 approach asymptotically the values obtained in continuous countercurrent extraction (Treybal, 1963, p. 360).

Downstream processing After removing the mold mycelium, the penicillin is separated from the broth by means of a two-stage continuous countercurrent extraction with amyl or butyl acetate. 

Flooding Hydrodynamic instability occurring in continuous countercurrent extraction due to excessive flow rates supplied to the process. 

The important question for the designer of a continuous countercurrent extraction process is how many equilibrium stages are needed, given the flow rates and inlet and outlet concentrations A simple graphical procedure is possible for a single-solute extraction when the two... 

Supercritical fluid extraction has been focused for the deterpenation of citrus oil as a lower temperature process [1-6]. Coppella and Barton [4], Stahl and Gerard [5], and Temelli et al. [6] studied the extraction process for the removal of terpenes in citrus oil. However, the simple extraction process does not give sufficient selectivity and yield A continuous countercurrent extraction process is one of the method to achieve higher selectivity between terpenes and oxygenated compounds. Perre et al. [7] and Sato et al. [8] succesfully developed the continuous extraction process. 

Salex A process for purifying sodium chloride. Continuous countercurrent extraction with brine removes the impurities without dissolving the salt. There are three variants SALEX-B, SALEX-C, AND SALEX-M. Developed by Krebs Swiss and first operated in 1982. 

When processing apples, the recovery of juice solids can also be achieved by continuous countercurrent extraction with water. These extraction systems were derived from the extraction of sugar beets in the sugar industry and they work either as hot or as cold extraction. 

H, G. Schwanzberg, Continuous Countercurrent Extraction in the Food Indusity. Chem. Eng,... 

Example 20.1. By extraction with kerosene, 2 tons of waxed paper per day is to be dewaxed in a continuous countercurrent extraction system that contains a number of ideal stages. The waxed paper contains, by weight, 25 percent paraffin wax and 75 percent paper pulp. The extracted pulp is put through a dryer to evaporate the kerosene. The pulp, which retains the unextracted wax after evaporation, must not contain over 0.2 lb of wax per 100 lb of wax-free pulp. The kerosene used for the extraction contains 0.05 lb of wax per 100 lb of wax-free kerosene. Experiments show that the pulp retains 2.0 lb of kerosene per pound of kerosene- and wax-free pulp as it is transferred from cell to cell. The extract from the battery is to contain 5 lb of wax per 100 lb of wax-free kerosene. How many stages are required ... 

Studies were made at room temperature (27°C) of the continuous countercurrent extraction of catalytic reformate with sulfolane and with 1,3-dicyanobutane. In this study a 22-stage, 1 inch diameter York-Scheibel extraction column was used. The hydrocarbon mixture was introduced into the bottom of the column, and the polar solvent was introduced at the top of the column. The effluent streams emerged just beyond the feed points. The streams were metered into the column under a hydraulic head plus 2 psi of air from reservoirs through needle valves and calibrated rotameters. The stream rates were well below flooding... 

Krukonis, V.J. Design of a Continuous Countercurrent Extraction Process for Producing Concentrated EPA Ethvl Esters 1987, Appendix submitted to National Marine Fisheries Service, Seattle, WA. 

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]. 

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