Mixers-settlers coalescence

Density. The difference in density between the two hquid phases in eqiiilibrium affects the countercurrent flow rates that can be achieved in extrac tion equipment as well as the coalescence rates. The density difference decreases to zero at a plait point, but in some systems it can become zero at an intermediate solute concentration (isopycnic, or twin-density tie line) and can invert the phases at higher concentrations. Differential types of extractors cannot cross such a solute concentration, but mixer-settlers can. 

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. 

Solvent extraction carried out in conventional contactors like mixer-settlers and columns has certain limitations, including (a) controlling optimum dispersion and coalescence, (b) purifying both phases to ensure that stable emulsions are avoided (c) temperature control within a narrow band (d) high entrained solvent losses and related environmental and process economic effects and (e) large equipment dimensions and energy requirements when the density differential or selectivity is low. 

Mixer-settlers have been the more common type of equipment and, with the development of hydrometallurgy over the past 20 years, designs have improved considerably. To select the appropriate equipment, a clear understanding of the chemical and physical aspects of the process is required. Also the economics must be considered relative to the type of equipment to suit particular conditions of given throughput, solution and solvent type, kinetics and equilibrium, dispersion and coalescence, solvent losses, number of stages, available areas, and corrosion. 

The types of equipment used, which range from stirred tanks and mixer-settlers to centrifugal contactors and various types of columns, affect both capital and operating costs [9]. In the decision to build a plant, the choice of the most suitable contactor for the specific situation is most important. In some systems, because of the chemistry and mass transfer rates involved, several alternative designs of contacting equipment are available. In the selection of a contactor, one must consider the capacity and stage requirements solvent type and residence time phase flow ratio physical properties direction of mass transfer phase dispersion and coalescence holdup kinetics equilibrium presence of solids overall performance and maintenance as a function of contactor complexity. This may appear very complicated, but with some experience, the choice is relatively simple. 

In one mixer-settler design, the mixed phases flow down a shallow trough placed over the settler, which gives them an opportunity to coalesce and separate before entering the settler. In this way, the capacity of the settler is markedly increased, with a concomitant reduction in the inventory of solvent required for a given duty. 

Scrubbing. In a technical scale, some entrainment of the organic phase after extraction can not be avoided. In scrubbing, the main portion will be removed. The scrubbing raffinate contains low concentrations of sodium sulfate. A typical aqueous organic ratio is 1 20. A combination of coalescer and settler can be used for entrainment removal besides mixer settlers. 

The dispersion in the mixer sections is swept under the dividing partitions into the adjacent transfer sections. If the dispersed phase is the less dense of the two its droplets rise up through the transfer sections against the down-coming flow of continuous phase after which they pass to the next mixer section where the process is repeated. Unlike a mixer-settler the ROF contactor does not depend for its operation on the coalescence of the dispersed phase in separating compartments and in its application to the nitration stage of the TNT process nitrobody separates out only at the product end where a smaller unstirred compartment operates as a true separator. 

Stagewise contact with controlled coalescence redispersion cycles Tray column Pulsed sieve column. Pulsed Mixer-Settler-cascade, Extraction tower with controlled cycle Scheibel column, ARD-Extractor, Leisibach column, Mixer-Settler cascade ... 

Low shear process will produce larger liquid droplets (>30 microns), and heavy/light liquids can be separated using gravity settling method. Examples of low shear process are solvent extraction, mixer settlers, steam stripping, washing process, and counter-current tower. Coalescer pad can be used to speed up the liquid/liquid separation and reduce the separator size. 

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