When performing single-stage extraction on the laboratory scale, the chemist employs a separating funnel as mixing and precipitating vessel [23, 31 ].
On an industrial scale, the following requirements for extraction equipment are called for:
- generation of small droplets
- high turbulence in the mixing zone
- homogeneous droplet distribution
- generation of high mass transfer coefficient
- prevention of axial back-mixing
- fast phase separation after solute transfer.
In technical applications, mixing of the two phases is achieved by
- blenders, intensive mixers or high-speed mixers
- static blender of a centrifugal or jet pump
- mixing centrifuges
- or, until the distribution equilibrium is reached, with sound waves and electric discharge, similar to solid-liquid extraction .
Gravitational forces are used for separating the two phases. Horizontal chambers with mixers are selected for dispersion purposes. In the subsequent settler, the drops coalesce forming a separate layer, resulting in the name mixer-settler. Good extraction efficiency is obtained, apart from high interfacial area, if a certain minimum residence time in the mixer is achieved. In the settler, the dispersed phase must coalesce and form a homogeneous phase layer. Most settlers consist of horizontal vessels, as experience has shown that phase separation efficiency is proportional to the interface area. Improvements in separation have been accomplished by installing settling aids that must be wetted by the dispersed phase. The throughput is therefore dominated by the settling process. The reinforcement ratio of a mixer-settler is in the range of 0.8 to 1. For detailed studies see [31, 33].
For high-stage efficiency and rapid phase separation, centrifugal forces constitute a suitable tool. Mixing and separation can be performed with a centrifugal mixer and separator. Self-cleaning or nozzle separators are employed to discharge any solids or sludges present. The former are equipped with a hydraulic bowl-opening mechanism for intermittent solids. Nozzle separators feature continuous sludge discharge.
For this technique, several units are arranged together in such a way that the effluents of the two phases flow countercurrently. All mixing and separation equipment of single-stage extraction can be employed in the corresponding countercurrent mode. Extraction batteries have large space and material requirements. They have, nevertheless, widespread application in the flavour industry, as they can be employed universally. They can also be used with larger extract requirements in multi-stage liquidliquid extraction and, depending on arrangement, both continuously and discontinu-ously.
The earliest extraction units were mixer-settlers that involved separate mixing and settling vessels. The disadvantage of these units was their high space requirement, and soon different configurations were developed. In this context, box or tower mixer-settler extractors found a broad range of application . The most common unit is the mixer-settler battery consisting of a mixer chamber and an integrated downstream settler chamber. The settler is separated from the mixer chamber by a slotted baffle. The separated phases are removed at the end of the settler chamber according to their densities and then pumped into the next unit. The advantages of mixer-settler units are the simple addition of further stages and the broad loading capacity with the possibility of extreme phase ratios.
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