Countercurrent separations

Countercurrent Separation and Elutriation. The process known as elutriation in cell separation is a refined method for separation of cells having close mass densities. Cells can be separated by making use of differences in the critical velocity of cells. If the mass densities of two cells are identical, but the sizes are different, then the larger particle has a higher critical velocity than the smaller one. 

Cell separation techniques that use an inward flow component are referred to as countercurrent separation techniques. The concept of countercurrent separation is compHcated by biological variations of all parameters in equation 4. 

H. T. C. Pratt, Countercurrent Separation Processes, American Elsevier Publishing Company, Inc., New York, 1967. 

Practical separation techniques for hquid particles in gases are discussed. Since gas-borne particulates include both hquid and sohd particles, many devices used for dry-dust collection (discussed in Sec. 17 under Gas-Sohds Separation ) can be adapted to liquid-particle separation. Also, the basic subject of particle mechanics is covered in Sec. 6. Separation of liquid particulates is frequently desirable in chemical processes such as in countercurrent-stage contacting because hquid entrainment with the gas partially reduces true countercurrency. Separation before entering another process step may be needed to prevent corrosion, to prevent yield loss, or to prevent equipment damage or malfunc tion. Separation before the atmospheric release of gases may be necessaiy to prevent environmental problems and for regula-toiy compliance. 

Pratt, H. R. C. Countercurrent Separation Processes (Elsevier, Amsterdam, 1967). 

Bieser, H.J. and De Rosset, A.J. (1977) Continuous countercurrent separation of saccharides with inorganic adsorbents. Staerke, 29 (11), 392-397. 

Several tests using countercurrent separation in mixer-settler or centrifugal extractors with simulated and genuine high-level waste showed that the recovery of An(III, IV, VI) is quantitative. The back extraction of An(III) was complete. These early flow sheets were not designed to strip U(VI) and Pu(IV). The distribution ratios of these ions at low acidities are lower than those measured with CMPO and one can guess that the stripping... 

The many laboratories involved in this work allowed a large experimental program to be set up to test the behavior of these soft donor extractants in countercurrent separations. The trivalent actinide-lanthanide separation process was named SANEX and four chemical systems have been explored (Fig. 12.20) ... 

Figure 13.27. Separation of the azeotropic mixture of acetonitrile and water which contains approximately 69 mol % or 79.3 wt % of acetonitrile. (Pratt, Countercurrent Separation Processes, Elsevier, New York, 1967, pp. 194, 497). (a) A dual pressure process with the first column at 100 Torr and the second at 760 Torr. (b) Process employing trichlorethylene as entrainer which carries over the water in a ternary azeotrope that in turn separates into two phases upon condensation.

FIGURE 13 Schematic diagram showing the sequence of column interchange in a periodic countercurrent separation process. 

The same principles apply to distillation processes as illustrated in in Fig. 5.2, and to other countercurrent separation processes [18, 20, 30, 40]. 

Two fundamental principles applicable to the design of continuous countercurrent separation processes are known. They are identifiable with (i) flow reversal of the separated mixture at the edge of the contact sys-... 

Fig. 4 Countercurrent separation of various samples by the coil planet centrifuge, (a) Separation of basic dyes with an organic/aqueous two-phase solvent system. M.G. methyl green M.B. methylene blue N.R. neutral red F.B. basic fuchsin. (From Ref. [3].) Solvent system consisted of isoamyl alcohol/ethanol/acetic acid/water (4 2 1 5, v/v).

The aqueous strip solution will form directly the feed solution of the Am, Cm/RE countercurrent separation step, to be performed according to the operating conditions of the TALSPEAK process (15). The presence of a complexing agent like DTPA will prevent the extraction of Am and Cm whereas the RE will be more easily extracted by di(2-ethylhexyl)phosphoric acid (HDEHP) in n-dodecane (pH = 3). 

When the distribution ratio is not highly favorable, it is still possible to obtain a quantitative and selective separation through the use of a countercurrent liquid extraction approach. Although such approaches are no longer practical, having largely been supplanted by instrumental techniques such as preparative HPLC and continuous solvent extraction, countercurrent separations are conceptually useful. These approaches can be applied to preliminary separation of complex mixtures or in the isolation of compounds that do not perform well in LC because of undesirable interaction with the stationary phase (irreversible adsorption, denaturation, etc.). For these reasons, most applications of countercurrent separations involve the isolation of natural or biochemical products from plant or animal extracts. As will be described below, countercurrent extractions form the theoretical basis for LLE cartridges. 

HTU. Abbreviation for height of a transfer unit the height of a distillation column or fractionating tower in which unit separation is achieved by transfer from liquid to vapor or vice versa, of the materials being separated. Unit separation is defined by the differential equation that takes into account the varying concentrations along the column. HTU is also applied to extraction and other countercurrent separation processes. 

One of the important methods of critical fiuid fractionation involves the countercurrent separation of phospholipids from a vegetable oil. A system to achieve this end is presented in Figure 7. Here, high pressure CO2 is fed into a pressure vessel packed with segmented gauze mesh packing (the refining... 

FIGURE 14.3-1 Schematic of equipment Tor conn-cercurrent and simulated countercurrent separation. 

Countercurrent separation of the alkaloids of Palicourea alpina (Sw.) DC. (Rubiaceae) furnished palinine C27H32N2O10 mp 166.5-168° [a]D-252.3° (MeOH). The UV spectrum, Amax(log e) 236.5 (4.62), 290.5 (4.20), and 394 (3.61) nm, with a bathochromic shift in acid strongly supported the presence of / -carboline chromophore in the glycoalkaloid. Palinine on acetylation with pyridine and acetic anhydride gave the pentaacetate 77 which on purification with silica gel afforded the tetraacetate 76. The NMR spectrum of the latter showed close resemblance to deoxycordifoline... 

The countercurrent separation process may be defined as a process in which two phases flow in countercurrent contact to produce a concentration gradient between inlet and outlet. The dominant feature is the concentration gradient the purpose of the process is to achieve a change in concentration of one or more components of the feed. 

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