Agitated vessels liquid extraction

An equihbrium, or theoretical, stage in liquid-liquid extraction as defined earlier is routinely utilized in laboratory procedures. A feed solution is contacted with an immiscible solvent to remove one or more of the solutes from the feed. This can be carried out in a separating funnel, or, preferably, in an agitated vessel that can produce droplets of about 1 mm in diameter. After agitation has stopped and the phases separate, the two clear liquid layers are isolated by decantation. 

Scale-Up of Mixers For the details associated with the design and scale-up of agitated vessels, the reader is referred to Section 18 which covers this topic in great detail. The intention here is to provide only some of the generalprinciples involved which have particular apphcation to liquid-hquid extraction. 

Mixing in Agitated Vessels Agitated vessels may frequently be used for either batch or continuous service and for the latter may be sized to provide any holding time desired. They are useful for liquids of any viscosity up to 750 Pa s (750,000 cP), although in contacting two liquids for reaction or extraction purposes viscosities in excess of 0.1 Pa s (100 cP) are only rarely encountered. 

The archetypal, stagewise extraction device is the mixer-settler. This consists essentially of a well-mixed agitated vessel, in which the two liquid phases are mixed and brought into intimate contact to form a two phase dispersion, which then flows into the settler for the mechanical separation of the two liquid phases by continuous decantation. The settler, in its most basic form, consists of a large empty tank, provided with weirs to allow the separated phases to discharge. The dispersion entering the settler from the mixer forms an emulsion band, from which the dispersed phase droplets coalesce into the two separate liquid phases. The mixer must adequately disperse the two phases, and the hydrodynamic conditions within the mixer are usually such that a close approach to equilibrium is obtained within the mixer. The settler therefore contributes little mass transfer function to the overall extraction device. 

Continuous-flow agitated liquid-liquid systems are commonly employed for extraction applications. Oldshue and Rushton (05) and Karr and Scheibel (K3) studied the performance of compartmented columns agitated by many impellers on a common shaft. Flynn and Treybal (F3) studied continuous extraction in a system consisting of one agitated vessel. 

Overcashier and co-workers (08) also studied agitated-vessel performance as related to liquid-liquid extraction. They used a 14.75-in. 

Liquid extraction is an operation used to separate the components of a liquid mixture of two or more species. In the simplest case, the mixture contains two components a solute (A) and a liquid solvent (B). The mixture is contacted in an agitated vessel with a second liquid solvent (C) that has two key properties A dissolves in it, and B is immiscible or nearly immiscible with it. (For example. B may be water, C a hydrocarbon oil, and A a species that dissolves in both water and oil.) Some of the A transfers from B to C, and then the B-rich phase (the raffinate) and the C-rich phase (the extract) separate from each other in a settling tank. If the raffinate is then contacted with fresh C in another... 

Schindler H.D, Treybal R.E., Continuous-Phase Mass-Tranter Coifflcients for Liquid Extraction in Agitated Vessels, AIChE J. 14 (1968) 5, p. 790-798... 

Special features/comments temperature can be set independently for each rv can be converted to parallel evaporator, see also Section 8.4 and Table 7 temperature/time programs possible top fdtration unit available fully enclosed reagent/solvent addition under agitation possible parallel evaporation on the instrument liquid-liquid extraction tool parallel filtration (vessel to vessel) push-button or PC control this is an upgradable light version of the fully automated Chemspeed ASW 2000 (see Table 5)... 

Liquid-Liquid Mixer Design Many different types of impellers are used for liquid-liquid extraction, including flat-blade and pitched-blade turbines, marine-type propellers, and special pump-mix impellers. With pump-mix designs, the impeller serves not only to mix the fluids, but also to move the fluids through the extraction stages of a mixer-settler cascade. The agitated vessel should be baffled if the vessel is operated with a gas-liquid surface, to avoid forming a vortex. As noted earlier in reference to Eq. (15-172), baffles are not needed if the vessel is operated with the liquid full [Weinstein and Treybal, AIChEJ., 19(2), pp. 304-312 (1973)]. 

The presence of two liquid phases mey be (he result of an extraction, washing, or reaction operation. The two phases may have been contacted In pipas. static mixers, agitated vessels, or at ihe impeller of a pamp. The type of contacting, the level of shanr forces applied, the concentration of the phases, and the time of contacting all influence the difficulty of the separation prceess. 

Extraction equipment may be operated batchwise or continuously. A quantity of feed liquid may be mixed with a quantity of solvent in an agitated vessel, after which the layers are settled and separated. The extract is the layer of solvent plus extracted solute, and the raffinate is the layer from which solute has been removed. The extract may be lighter or heavier than the raffinate, and so the extract may be shown coming from the top of the equipment in some cases and from the bottom in others. The operation may of course be repeated if more than one contact is required, but when the quantities involved are large and several contacts are needed, continuous flow becomes economical. Most extraction equipment is continuous with either successive stage contacts or differential contacts. Representative types are mixer-settlers, vertical towers of various kinds... 

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