16 September 2009

Contactor Separator

Browsing the chemistry journals I came across a paper titled, "Continuous Chiral Separation of Amino Acid Derivatives by Enantioselective Liquid-Liquid Extraction in Centrifugal Contactor Separators" (DOI: 10.1021/op800074w. The paper discussed the technique of enantiomerically enriching a substance based upon the greater solubility of one diastereoisomer over the other.

A closer reading of the title reveals the word continuous and it is the apparatus that the authors use to do this continuous process that is of interest.

The apparatus used by the authors was a contactor separator.
They reference US patent No 5,762,800 from 1998.

As its name suggests, the apparatus does two tasks: contacting and separating. The contacting means mixing. Take a cylinder and rotate it inside a close fitting tube; if you introduce two liquids between the gap they will mix.

The paper above is mixing two immiscible solutions(water and 1,2-dichloroethane) in order to partition the diastereoisomers between the two (remembering that there is a solubility difference and hence chiral discrimination between the two of them).

The next stage is the separation of the two immiscible liquids. In batch mode, in the lab, a chemist would do this in a separating funnel. (Or, on the plant, the reaction vessel). The solutions are put in the separating funnel, shaken (the contacting above) and left to settle.

In the continuous flow apparatus, the apparatus has the inner of the two cylinders rotating. This produces a centrifugal force (pedants not welcome on this point), which separates the two immiscible liquids. The apparatus is configured so that one liquid can be transferred to one vessel, the other liquid to another vessel.

This kit has many useful applications. The authors mention that the apparatus has been used for preparing biodiesel: this is a vast improvement on the hobbyist's batch technique. It is a lot safer, one can imagine building a piece of apparatus that only produces one or two litres of biodiesel per day which could be plumbed into your garage. Connect the oil and dilute solution of sodium hydroxide in methanol to the apparatus, flick a switch and collect a couple of litres of biodiesel the next day.

There are other examples in the patent literature where this apparatus has been used; some of which do not appear to be non-obvious and here are some links to suppliers, here and here with more explanation.

PhD Thesis of student who did the work, Enantioselective liquid-liquid extraction in centrifugal contactor separators (2008) Schuur, Boelo (apart from the hidden chapter, very useful).

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