Thin layer extraction
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Thin layer extraction is a time-periodic reactive liquid extraction process that provides excellent mass transfer, while maintaining phase separation.[1] It is performed via a periodic batch production process utilizing full control of the time of each chemical reaction.
Concept
With a typical liquid mass diffusivity in the order of 10-9 m2/s,[2] the characteristic time for diffusion through a 20 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 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.
Equipment
Thin layer extraction is used in specialized equipment operated as robots consisting of:
- The cartridge. Consisting of a permeable, open, macro-porous matrix, 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.
- 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).
- A programmable control system.
A thin layer extraction cell consists of a section of the matrix that takes turn 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.
