Membrane cell retention systems for continuous production of l-carnitine using Proteus sp

Membrane reactors with a Proteus sp. strain as the biocatalyst were used to attain continuous l-carnitine production from crotonobetaine. Hollow fiber cartridges with fibers of polysulfone provided adequate filtration rates and were reusable after long-term operation (consecutive runs of 140 h followed). Running under optimum medium composition, these systems enabled high cell densities (35–45 g dry weight/l) to be reached after 35–40 h at dilution rates ranging from 0.1 to 1.5 h−1, while cross-flow filtration units with flat-sheet membranes made of polyvinylidene difluoride (PVDF) showed lower cell concentrations (up to 25 g dry weight/l) and levels of l-carnitine production. The composition of the hollow fiber membranes probably improved filtration flow rates and definitely speed up the cleaning-up procedure because cake layers did not adhere tightly to the membranes. Furthermore, resting cell-cycles were maintained in continuous operation for 2–3 days with crotonobetaine and phosphate buffer as the only medium components, followed by cycles of cell re-activation, meaning that the system was stable for long periods of time (140–200 h). During resting cell-cycles, high cell decay rates were observed due to nutrient shortages. The hollow fiber system was able to produce high levels of l-carnitine during the growing/resting cycles with 35–50% conversion and productivities as high as 40.5 g/(l h). The optimum dilution rate was 1.0 h−1 as mass-transfer problems appeared at higher dilution rates because of high cell densities. The process described is presented as a way of recycling d-carnitine from the chemical industry, since, when dehydrated, this compound renders crotonobetaine.