Mixers-settlers vessels, agitated

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. 

In relatively small-scale operations, where frequent shutdowns may be desirable, as at night or during week ends, the mixer-settler plant offers the advantage that concentration gradients are maintained within the plant indefinitely even though operations have stopped. On start up, steady-state conditions are established immediately, product specifications are met with the first effluents, and recycling of the first products withdrawn is unnecessary. The agitated vessel is also eminently suitable for purely batch processes, which are likely to be on a small scale. 

The most widely used extractor is the mixer-settler that is a cylindrical vessel with one or several agitators. The vessel is usually equipped with four equally spaced baffles to prevent the vortex... 

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)]. 

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... 

The mixer and settler can be combined in one vessel or separated. In the combined case, the agitator is turned off after extraction so that the two phases can separate. For continuous operation, the mixer and settler are separated. A settler is nothing but a large tank. 

The extraction equipment consists of a mixer, where the solvent and the mixture of solutes are mixed in an agitated vessel, after which the two layers are made to separate in a settler tank. The equipment may be operated in batch or continuous mode. For batchwise extraction the mixer and the settler may be the same unit, whereas for continuous operation the mixer and the settler must be separate pieces of equipment. If several contact stages are required, a series... 

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