Scale-up of continuous fermentation with cell-recycle for lactic acid production and process economics

Abstract

Lactic acid is a natural organic hydroxy acid with many uses in food, chemical and pharmaceutical fields. It can form a linear thermoplastic polyester, polylactic acid, which can be used in biodegradable films and strong fibers. Because of the environmental needs the production of lactic acid could play an increasing role in industry in the near future. In order to increase the bioreactor performance for the production of lactic acid, continuous lactic acid fermentation system coupled with membrane cell separation technique (MCRB) has been studied. By greatly increased density and reaction time of the cells in the reactor, volumetric productivity could be increased over ten times than the conventional batch and continuous fermentation. However, the concentration of lactic acid produced, major factor for the economical feasibility, could not be increased higher than 95g/L by this system because at that high lactate concentration cell growth is inhibited almost completely. When scale-up of two stages MCRB was performed, 93g/L lactate could be produced with a productivity of 55.8g/$(L\cdot h)$. Based on the annual lactic acid production capacity of 10,000 MT per year, production cost for the developed 2-stage MCRB was estimated in comparison with 1-stage MCRB and batch. The raw material cost in all types of reactor is almost the same because the lactic acid yields on glucose consumption were similar. The cost for consumables(membrane cost) was zero in batch fermentation while significant contributions observed in MCRBs. The capital investments of 1-stage and 2-stage MCRB increased almost identically while that of batch fermentation increased more rapidly than the MCRBs. Because of this high capital investments the unit production cost of batch fermentation was the highest among the reactor types. The economical advantage of 2-stage MCRB grew rapidly as the production capacity increased. If the membrane cost could be substantially reduced due to the advances of...