Liquid / Liquid extraction is a process for separating the components of a mixture and is closely related to distillation. It may be used as a complementary or alternative technique when distillation is difficult such as in the separation of thermally sensitive materials or when materials have similar boiling points. The use of liquid/liquid extraction process for materials separation continues to gain importance both technically and economically. Low power requirements and the possibility provided for selective separation of components means that the fields of application for liquid/liquid extraction process are continually being extended. We have gained considerable experience in the construction of glass extraction units. On the basis of this know-how a new range of production units has been developed. Mixer-settlers are part of this range.
Mixer settlers are generally multi-stage and the two phases flow through the entire unit concurrently. It is also possible to use a cross flow arrangement, treating each stage with fresh solvent. The continuous and dispersed phases are fed into a mixing zone where a turbine stirrer with variable speed drive unit mixes the two phases, dispensing one phase in the form of small droplets in the other. Mass transfer takes place during dispersion or immediately after it. The residence time of the two phases in the mixing zone is of great importance, as is the droplet size of the dispersed phase which is inversely related to the stirrer speed. There is , however, a limit to the stirrer speed since, if the droplets are too small, an emulsion may form which complicates or even prevents coalescence of the droplets in the settler zone. Correct sizing of the mixing compartment and suitable selection of the stirrer speed is, therefore, of great importance in creating optimum conditions for mass transfer. Separation of the phase takes place in two stages. Firstly, the turbulent flow in the mixing zone must be brought under control and converted into axial flow. Then the mixture passes into the separation zone where the two phases separate, due to their specific gravity difference. The available separation time is determined, into a large extent, but it is also dependent on the diameter of the mixer-settler and the position of the interface. The position of the interface is a function of the ratio of the volumetric feed rates of the two feed phases.
The mixing zone of the mixer-settler consists of a special glass adaptor on which the variable speed drive is mounted. This drives the ptfe turbine stirrer inside the adaptor. The head of the stirrer is in the form of a double turbine which not only sucks in the two phases in combination with a specially designed feed pipe, but also achieves thorough mixing in the zone above the stirrer head. Between the mixing and settling zones, a perforated plate and wire is fitted to remove turbulence from the flow. In mixer settlers upto DN 300, this is made of glass and ptfe and in the DN 450 version, it is ptfe. Depending upon the size, the separation zone is made of one or more pipe sections and a second glass adaptor closes off the other end. This is fitted either with a fixed overflow for the heavy phases (DN 100) or a sealed-in overflow valve (DN 150 and above). In addition, there are necks for the light phase and drain. If special separation aids (porus media, mesh etc...) are required in the mixing zone, they can be fitted in a pipe section approximately 500 mm long with a second perforated plate at one end. If the sealed-in overflow valve is not large enough for the required throughput of heavy phase, it can be replaced by a capacitive measuring probe which detects the level of interface and controls a run-off valve. A further alternative is an overflow fitted outside the mixer-settler. This is used when the separation interface lies outside the range of adjustment of the seal-in valve, from approximately 0.33 to 0.66 of the liquid height.
Although mixer-settlers have been in use in the chemical industry for a number of years, the complexity of liquid/liquid extraction processes and the lack of reliable physical property data on novel systems makes the theoretical design of unit difficult. For this reason, extensive pilot plant test are carried out to establish the maximum load rating and separation effect of mixer-settlers. These tests are carried out by our own engineers, using customer materials. Alternatively, units are available for customers to borrow in order to carry out their own experimental work. The results of pilot plant tests establish optimum values of mixer-settler design parameters at which satisfactory extraction is achieved. These results are then used by our own engineers to design the production scale unit.
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