Thin Layer Extraction
Thin Layer Extraction[1] is a time-periodic reactive liquid-liquid extraction process that avoids mixing of the phases to circumvent the inherent conflict between mixing that is intended to help mass transfer but also results in the aggravation of phase separation. Reactive indicates that the extraction mechanism involves a reversible reaction between the extracted chemical species (the guests) and a host chemical species constituting, or present in, the extractant.
1. The Thin Layer Extraction Concept
A small amount of a liquid organic extractant is spread as a thin layer and firmly held by capillary or other forces onto a matrix, made of a thin microporous material, whose surfaces are freely accessible from/to the outside. This permanent thin organic layer is alternately and repeatedly brought in brief contact with thin layers of the donor and the strip aqueous liquid(s). In the extraction step, selected species present in the donor solution are transported from the donor aqueous solution to the organic phase where a reaction ensues. In the stripping step the reaction reverses and the extracted species are stripped into the strip aqueous solution(s). Two alternate product batches are generated, a raffinate and a strip product. As each of the species to be separated associates differently with the host, the composition of the raffinate and strip product is differentiated.
With a typical liquid mass diffusivity in the order of 10-9 m2/s,[2] the characteristic time for diffusion through a 20μ thick liquid layer is of the order of 0.4 s. Therefore, the thinness of both the organic and the aqueous phases causes a relatively "immediate" mass transfer of guest species to/from one phase to the other, in other words, a low mass transfer resistance. The low mass transfer resistance permits the uncoupling of effects attributed to mass transfer from those attributed to the reaction rates and also allows a relatively frequent cycling that helps mitigate the limited capacity that is due to the small batches of aqueous feed processed within each cycle.
A second characteristic of Thin Layer Extraction, arises from the batch periodic mode of operation. It permits the control in time and space over small processed elements in the course of the process, a control that is not possible in any other liquid-liquid extraction method. This control is instrumental in making possible the exploitation of differences in reaction rates of the different species (see Thermodynamic versus kinetic reaction control) and the “harvesting” of separated species early on the reaction trajectories where the relative differences in concentration are largest. This forms the basis for Kinetic Reactive Thin Layer Extraction.
The extractant, including the host, must be substantially insoluble in the processed aqueous solutions such as to avoid its being washed away. On the other hand, the difference in density between the immiscible phases, that plays an important role in conventional liquid-liquid extraction, is irrelevant in thin layer extraction.
Thin Layer Extraction is recommended for the separation of high value products that are produced in moderate volumes (for example the separation of chiral molecules[3][4]).
2. The Thin Layer Extraction Equipment
Thin Layer Extraction is implemented in specialized equipment operated as robots consisting of :
a. The cartridge. The cartridge consists of a cartridge housing containing a permeable open macro-porous matrix that is made of a microporous solid substrate, compatible with the extractant phase, and accessible from/to the external world. The replaceable microporous matrix is initially wetted with the extractant, exposing a thin liquid layer on its surface without obstructing the macro-pores.
b. Means to bring alternately small batches of the donor and strip solutions to cover as thin layers the thin extractant layer contained in defined sections of the matrix for a controlled time and then collect the product solutions. One method used consists of spraying the solutions over sections of the matrix that exchange positions periodically to be exposed alternately to the donor and the strip solutions. The aqueous layers are then shaken off the matrix and collected as two distinct products. A second method consists of pneumatically pumping at a controlled velocity, in counter-current direction alternating small batches of the donor and strips solutions through a bundle of microporous capillaries. This second method permits a programmable number of stages but does not allow changes in the organic to aqueous ratio (O/W).
c. A programmable control system.
A Thin Layer Extraction cell consists of a section of the matrix that take turns at being alternately exposed to the donor and then the strip solutions. Each cell accepts two alternating aqueous feed batches and generates two corresponding alternating batches of the products.
In multistage operation, a train of cells is operated synchronously with the products from one cell directed as feeds to a next upstream or downstream cell.
The multistage Thin Layer Extraction equipment is linearly scalable, permitting results obtained on table-top laboratory devices to be directly scaled up to full scale production plants.
References
- ↑ Lavie R., “Thin Layer Extraction – A Novel Liquid-Liquid Extraction Method”, AIChE Journal, 54, Issue 4, Pages 957-964, 2008. DOI: 10.1002/aic.11445.
- ↑ Ven_Lucassen, I., “ Diffusion coefficients in liquid systems”, Thesis – Technische Universiteit Eindhoven, 1999, ISBN 90-3862551-0.
- ↑ Lavie R. and Wajc S., “Chiral Resolution by Thin Layer Extraction”, Chemical Engineering Transactions, Volume 24, pages 739-744, 2011, DOI: 10.3303/CET1124124.
- ↑ Lavie R., "Chiral Resolution of Ionic Compounds by Thin Layer Extraction", Ind. & Eng. Chemistry Research,2011, Vol. 50, Issue 22, pages 12311-12844, DOI: 10.1021/ie201596n.
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