Liquid Extraction Towers

Equipment Control 43 Heat Transfer Equipment 44 Distillation Equipment 47 Liquid-Liquid Extraction Towers 50 Chemical Reactors 53... 

A liquid-liquid extraction tower broke in half. The detonation of the resulting cloud of hydrocarbons killed most of the victims. Figure 39.2 shows the process the extraction of hydrogen sulfide by MEA (mono-ethanol-amine). A liquid propane stream containing... 

Available in metal and plastic, used in large and small towers for distillation, absorption, scrubbing, liquid extraction. High efficiency, low HETP, low pressure drop. Limited data available. 

Extraction (sometimes called leaching) encompasses liquid-liquid as well as liquid-solid systems. Liquid-liquid extraction involves the transfer of solutes from one liquid phase into another liquid solvent it is normally conducted in mixer settlers, plate and agitated-tower contacting equipment, or packed or spray towers. Liquid-solid extraction, in which a liquid solvent is passed over a solid phase to remove some solute, is carried out in fixed-bed, moving-bed, or agitated-solid columns. 

The problems relating to mass transfer may be elucidated out by two clear-cut yet different methods one using the concept of equilibrium stages, and the other built on diffusional rate processes. The selection of a method depends on the type of device in which the operation is performed. Distillation (and sometimes also liquid extraction) are carried out in equipment such as mixer settler trains, diffusion batteries, or plate towers which contain a series of discrete processing units, and problems in these spheres are usually solved by equilibrium-stage calculation. Gas absorption and other operations which are performed in packed towers and similar devices are usually dealt with utilizing the concept of a diffusional process. All mass transfer calculations, however, involve a knowledge of the equilibrium relationships between phases. 

Ruby, C. L. and Elgin, J. C. Mass transfer—Transport properties. Chem. Eng. Prog. Symp. Series No. 16, 51 (1955) 17. Mass transfer between liquid drops and a continuous liquid phase in a countercurrent fluidized system. Liquid-liquid extraction in a spray tower. 

The GS enriching process is a counter-current gas-liquid extraction done at a pressure of 2000 kPa in a sieve tray tower with the upper half operating at 30 C and the lower at 130 C. ( 5) In the top half of the tower, feedwater extracts deuterium from the upflowing cold H2S, reaching a maximum at the centre of the tower. The recycled lean H2S entering the lower hot half of the tower strips deuterium from the water, which then leaves the system depleted in deuterium. A cascade of several stages is used to reach the desired feed concentration for the final water distillation or finishing unit. Transfer between cascades can be either by gas or liquid from the centre of the tower. 

As the potentialities of liquid extraction as a separation method were developed, the need for efficient, continuously operated, multistage equipment became apparent. It was natural therefore to turn to devices which had been so successful in other similar fluid-contacting operations, such as the bubble-tray tower and the packed tower of distillation. These devices have proved to be disappointing in liquid-extraction service, however for example, bubble-tray towers provide tray efficiencies in liquid-extraction operations of less than 5% (S7), and conventional packed towers show heights of transfer units of 10 to 20 ft. or more (T3). 

The reason for the disparity in performance of such devices in the two services has been clearly outlined by Hachmuth (HI). Bubble-tray towers for distillation, for example, use as the source of energy for dispersion of the gas and for developing the desirable turbulent flow conditions both the expansion of the vapor as it experiences a pressure drop in flowing through the tray, and the liquid head available between trays. In liquid extraction only the liquid head is available. When it is considered that the difference in densities of the contacted phases in distillation may be of the order of 50 to 60 lb./cu. ft., whereas in extraction it is more likely to be of the order of 5 or less, it is easy to understand that in the latter case there is simply insufficient energy available from this source to provide for adequate dispersion and interphase movement. Interfacial area between phases remains small, turbulences developed are of a low order, and mass transfer rates are disappointingly small. 

Figure 3.16. Extraction tower control, (a) Operation with heavy solvent, interface in the upper section, top liquid level on LC. (b) Same as part (a) but with overflow weir for the light phase, (c) Same as part (a) but with completely full tower and light phase out at the top. (d) Operation with interface on ILC in the lower section, removal of the light phase from the upper section by any of the methods of (a), (b), or (c).

J.S. Eckert, Extraction, liquid-liquid, packed tower design, Encycl. Chem. Process. Des. Zl, 149-166 (1984). 

In liquid-liquid extraction one or more components are removed from a liquid mixture by intimate contact with a second liquid that is itself nearly insoluble in the first liquid and dissolves the impurities and not the substance that is to be purified. In other cases, the second liquid may dissolve i.e., extract from the first liquid, the component that is to be purified, and leave associated impurities in die first liquid. Liquid-liquid extraction may be earned out by simply mixing die two liquids widi agitation and dien allowing diem to separate by standing. It is often economical to use counter-current extraction, in which the two immiscible liquids are caused to flow past or dirough one another in opposite directions. Thus fine droplets of heavier liquid can be caused to pass downward through the higher liquid in a vertical tube or tower,... 

The purest caustic solution is obtained by extraction with ammonia. The initial 50 per cent caustic solution is led to the top of an extraction tower and flows countercurrently to a mixture of 75 per cent ammonia and 25 per cent water which rises from below. The two solutions are immiscible. The ammonia solution extracts almost completely the chloride and sodium chlorate from the caustic liquor so as to leave no more than 0.08 per cent NaCl (related to 100 per cent NaOH) and practically no chlorate. Ammonia regeneration equipment complements the extraction unit. If anhydrous liquid ammonia is used instead of an ammonia solution a certain degree of concentration of the caustic solution can be achieved at the same time as ammonia also extracts water from the caustic solution. 

FIGURE 14 Sieve-tray extraction tower arranged for light liquid dispersed. Reprinted from Treybal5 with permission of The McGraw-Hill Companies. 

Description Hydrocarbon feed is pumped to the liquid-liquid extraction column (1) where the aromatics are dissolved selectively in the sulfolane water-based solvent and separated from the insoluble non-aromatics (paraffins, olefins and naphthenes). The non-aromatic raffinate phase exits at the top of the column and is sent to the wash tower (2). The wash tower recovers dissolved and entrained sulfolane by water extraction and the raffinate is sent to storage. Water containing sulfolane is sent to the water stripper. 

J.S. Eckert, Extraction, liquid-liquid, packed tower design, Encycl. 

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