(723i) Supported Liquid Membranes with Strip Dispersion for the Recovery of Cephalexin

Cephalexin is an important and widely used semisynthetic cephalosporin. Cephalosprorins along with penicillins are Beta-lactam antibiotics, which account for the majority of the antibiotic world market. Cephalexin is traditionally produced by a ten-step chemical synthesis. An enzymatic synthesis for Cephalexin has been developed, and offers several advantages over the classical route. The enzymatic synthesis reduces energy and solvent waste, but has been used in industrial production on a limited basis due to its difficult separation. Liquid membranes, in particular supported liquid membranes (SLMs), are a promising solution to the separation. SLMs are not used commercially, as they are still plagued with problem of long term instability. SLM with strip dispersion has been a recent development to solve the issue of stability.

SLMs with strip dispersion are formed when an aqueous strip solution is dispersed in an organic membrane solution by a mixer, and passed onto one side of a membrane support. When a microporous hydrophobic support is used, the organic phase of the dispersion becomes imbedded in the pores of the support, forming a stable SLM. Stability is maintained by having a constant supply of organic membrane solution to the pores. An aqueous feed solution is passed on the other side of the membrane support, and the target species is extracted into the organic solution by a selective carrier. The target species is then stripped by the aqueous strip solution. For final recovery of the target species, the mixer is turned off or a settler is used, and the dispersion quickly separates.

In this study, Cephalexin has been separated and concentrated from an aqueous solution using SLMs with strip dispersion. The following key parameters were investigated: feed and strip dispersion flowrate, strip dispersion mixing rate, carrier concentration, counter ion concentration, pH, and the volume of aqueous strip solution. Experiments used a Liqui-Cel® hollow fiber module as a microporous support. The organic membrane solution of the SLM consisted of Aliquat 336, Isopar L (isoparaffinic hydrocarbon solvent), and 1-decanol. The aqueous strip solution was composed of potassium chloride and citrate buffer. An enrichment factor of 3.2 was observed in the aqueous strip solution while achieving over 99% extraction and 96.2% total recovery. In this case, the aqueous feed solution of 5500 ppm (15 mM) was lowered to 30 ppm when using an organic membrane solution containing 2.5% Aliquat 336. The resulting overall mass transfer coefficient was 1.6 x 10-5 cm/sec. The mass flux of Cephalexin for this system was found to be independent of aqueous feed and strip dispersion flowrates, suggesting a major mass transfer resistance due to chemical reaction kinetics, which is supported by calculated individual mass transfer resistances. The pH of the aqueous strip phase was found to play a more significant role when trying to achieve higher enrichment ratios. It was observed that the highest stripping efficiency occurs when the pH of the aqueous strip phase is between the values of 5 and 6.