Counter current

U. von Stockar and Xiao-Piug Lu,M Simple andMccurate Short-cut Procedure toMccountforMxial Dispersion in Counter-current S eparation Columns, Ind. Eng. 

Until separation techniques such as chromatography (28,29) and counter-current extraction had advanced sufficientiy to be of widespread use, the principal alkaloids were isolated from plant extracts and the minor constituents were either discarded or remained uninvestigated. With the advent of, first, column, then preparative thin layer, and now high pressure Hquid chromatography, even very low concentrations of materials of physiological significance can be obtained in commercial quantities. The alkaloid leurocristine (vincristine, 22, R = CHO), one of the more than 90 alkaloids found in Catharanthus roseus G. Don, from which it is isolated and then used in chemotherapy, occurs in concentrations of about 2 mg/100 kg of plant material. 

Zirconium and hafnium have very similar chemical properties, exhibit the same valences, and have similar ionic radii, ie, 0.074 mm for, 0.075 mm for (see Hafniumand hafnium compounds). Because of these similarities, their separation was difficult (37—40). Today, the separation of zirconium and hafnium by multistage counter-current Hquid—Hquid extraction is routine (41) (see Extraction, liquid—liquid). 

A beryUium concentrate is produced from the leach solution by the counter-current solvent extraction process (10). Kerosene [8008-20-6] containing di(2-ethylhexyl)phosphate [298-07-7] is the water-immiscible beryUium extractant. The slow extraction of beryUium at room temperature is accelerated by warming. The raffinate from the solvent extraction contains most of the aluminum and aU of the magnesium contained in the leach solution. 

Removal of Free Fatf Fields. Alkali treatment of the oil is accompHshed by the use of caustic soda solutions to neutralize the excess free fatty acids. Because castor oil readily forms emulsions with water and/or alkaline solutions, special techniques have been developed to neutralize the acids. A continuous counter-current process was developed using a stationary contact reactor (15). Treatment in the presence of a solvent is also utilized (16). 

A. Absorption, counter-current, liquid-phase coefficient Hi, Sherwood and Holloway correlation for random packings... 

B. Absorption counter-current, gas-phase coefficient Hq, for random packing... 

C. Absorption, counter-current, gas-liquid individual coefficients and iuterfacial area, Shulmau data for random pacldugs... 

E. Distillation and absorption, counter-current, random packings, modification of Onda correlation, Bravo and Fair correlation... 

Counter-current flow. Structured packings. Gauze-type with triangular flow channels, Bravo, Rocha, and Fair correlation... 

The hydrolysis of fats is improved by running in a counter-current extraction column (Donders et al., Proc. 4th Europ. Symp. Chem. React. Eng., Pergamon, 1968, pp. 159-168). 

The best known use of the hairpin is its operation in true counter-current flow which yields the most efficient design for processes that have a close temperature approach or temperature cross. However, maintaining countercurrent flow in a tubular heat exchanger usually implies one tube pass for each shell pass. As recently as 30 years ago, the lack of inexpensive, multiple-tube pass capability often diluted me advantages gained from countercurrent flow. 

Fields of Application One of the major advantages of the gravity-bed technique is that it lends itself well to true intimate counter-current contacting of solids and gases. This provides for efficient heat transfer and mass transfer. Gravity-bed contacting also permits the use of the sohd as a heat-transfer medium, as in pebble heaters. 

FIG. 19-46 Magnetic-field distribution charts, a) Concurrent and counter-current wet drum magnetic separator, 1.2-m diameter, (h) Counterrotation wet drum magnetic separator, 1.2-m diameter. (Coutiesy of Sala International, Inc.)... 

Solids may be processed continuously or semicontinuously by pumping slurries or by using lock hoppers. An example is the separation of insoluble polymers by floatation with a variable-density SCF. For liquid feeds, multistage separation may be achieved by continuous counter-current extraction, much like conventional liquid-hquid extraction. The final produces may be recovered from the extract phase by a depressurization, a temperature change, or by conventional distillation. 

FIG. 23-25 Typ es of industrial gas/Hqiiid reactors, (a) Tray tower, (h) Packed, counter current, (c) Packed, parallel current, (d) Falling liquid film, (e) Spray tower, if) Bubble tower, (g) Venturi mixer, h) Static in line mixer, ( ) Tubular flow, (j) Stirred tank, (A,) Centrifugal pump, (/) Two-phase flow in horizontal tubes. 

Figure 16.1 Simple dry cell battery. Electrons are conducted along the external circuit (4), which physically connects the active (2) and noble (1) materials. An equivalent ionic counter-current is conducted through the electrolyte (3), thereby completing the circuit.

Trichloroethane and 1,1,1,2-tetrachloroethane can be removed by counter-current extraction with EtOH/water. 

Biphenylyl diphenyl phosphate [132-29-6] M 302.4, n l.5925. Vacuum distd, then percolated through an alumina column. Passed through a packed column maintained at 150° to remove residual traces of volatile materials by a counter-current stream of nitrogen at reduced pressure. [Dobry and Keller J Phys Chem 61 1448 1957.]... 

Sodium chlorite [7758-19-2] M 90.4, m 180°(dec). Crystd from hot water and stored in a cool place. Has also been crystd from MeOH by counter-current extraction with liquid ammonia [Curti and Locchi Anal Chem 29 534 1957]. Major impurity is chloride ion can be recrystallised from O.OOIM NaOH. 

Tri-p-tolyl phosphate [20756-92-7, 1330-78-5 (isomeric tritolyl phosphate mixture)] M 368.4, b 232-234 , d 1.16484, n 1.56703. Dried with CaCl2, then distd under vacuum and percolated through a column of alumina. Passage through a packed column at 150°, with a counter-current stream of nitrogen, under reduced pressure, removed residual traces of volatile impurities. 

Geranylgeranyl pyrophosphate [6699-20-3 (NH4 salt)] M 450.5, pKEst(i)-<2, pKesk2) <2, pKEst(3) 3.95, pKes,(4) 6.26. Purified by counter-current distribution between two phases of a... 

In the steady-state operation of the OXITOX reactor, pelletized solid of catalytieally aetivated sodium earbonate slides down a Silo type reaetor. Counter-current to the solid flow, the polluted air rises through the sliding bed of solids. At reaction temperature the following reaetion oecurs ... 

Taking the upward direction as positive, when both Fy < 0 and Fs<0, represents a co-current downflow operation, if Fs < 0, but Fy > 0 means a counter-current operation. The rest of the equation defining the system can be seen in Berty (1997). Integration was done by the Romberg method as is used in Mathcad PLUS 6 (1996) software. 

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