Liquid-liquid extraction (LLE) is a crucial step in the manufacturing of a wide range of products. Such processes are used for the extraction of one compound and as well as for separation between two or more compounds (fractional extraction).
The LLE process is simple in concept and usually requires the contacting of a feed containing the solute to be extracted with a solvent. This solvent/feed mixture is usually immiscible, but may be partially miscible.
The extraction and the stripping involve liquid-liquid contacting in which the droplets of one phase are initially dispersed in a second phase to facilitate mass transfer across the liquid-liquid boundary. Basically, there are two types of LLE units, those in which each individual stage is a separate unit termed "mixer settlers" and those in which several stages are integrated into one column. Multi-stage columns can be simple spray or packed columns, or can have stages equipped with various types of mixing devices separated by coalescence sections. The stage efficiency and the throughput of such devices are, obviously, directly related to the mass transfer and the coalescence rates.
To form small drops and ensure good contact between the phases, in slow mass-transfer systems, high intensity mixing is required. However, the shear stress induced by such a mixing can, in many cases, damage high molecular weight molecules. In addition, the intense mixing forms fine dispersions which reduce the coalescence rate or, in the presence of surface active impurities, may even cause a "stable emulsion", one of the operating hazards of solvent extraction equipment.
From the point of view of LLE processes, the stability of the dispersion is its most important property, since the phases must separate at each extraction stage. For all practical purposes, the breakup time or the coalescence rate will determine the workable throughput of the extraction equipment. In countercurrent column-type contactors, steady operation is possible only when the rate of droplets arrival does not exceed the coalescence rate at the main interface; otherwise the dispersed band will extend over the entire column, leading to flooding. In mixer-settler contactors, the dimensions of the settler are designed according to the coalescence rate, and increasing the throughput above the coalescence rate will result in flooding of the settler. It is clear, therefore, that systems with emulsification tendencies cannot be operated by conventional extractors. Commonly such systems are handled in centrifugal extractors or by filtration. In some cases, adding compounds that break the emulsion can minimize the problem. This, however, makes complex the final purification of the product.
Emulsion formation is a common problem in the pharmaceutical industry, where the desired products are frequently extracted from the fermentation broth by organic solvents and in extraction processes where mechanical agitation is used to increase the mass transfer rates.
U.S. Pat. No. 4,954,260 of Z. Ludmer, R. Shinnar, and V. Yakhot ("the U.S. Pat. No. '260 patent") describes a process that overcomes some of these difficulties. In that case, special solvents are used that at one temperature form a homogeneous, one-phase mixture and, at a higher or lower temperature, form two phases, one solvent-rich and the other water-rich. See, also, Ullmann, Ludmer and Shinnar, "Novel Separation Process Using Solvents with a Critical Point of Miscibility," Proc. A.I.Ch.E. Conference, Miami, Fla. (1993).
The process is composed of two stages, the heating Stage and the cooling stage where the mixture is cooled across the coexistence curve. Only very mild agitation is required. Compared to the mixing stage of the conventional isothermal extraction process, the heating stage provides a greatly improved contacting area, since in such stage there is but a single phase. Even more importantly, in the process of the U.S. Pat. No. '260 patent, the cooling stage has great advantage over the settling stage of the conventional extraction process: namely, when the cooling is fast enough, rapid phase-separation occurs even in the presence of impurities and cell debris. Accordingly, the need to use centrifuges is eliminated.
Unfortunately, as a practical matter the process has the disadvantage of requiring continual and rapid heating and cooling.