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 membrane technology, the economical advantage of 2-stage MCRB will become higher than other configurations.
젖산은 식품, 화학, 의약분야등 여러 분야에서 범용적으로 사용되는 유기산이다. 또한 생분해성 고분자인 polylactic acid의 원료로 사용된다. 자연친화적인 물질의 생산에 대한 관심이 높아지고 있는 현재 생분해성 원료인 젖산의 생산은 중요한 이슈가 될 것이다.
젖산 생산량을 늘리기 위해 membrane을 이용해 cell을 분리하는 기법이 연구되었다. 반응기 내의 cell 농도가 증가함에 따라 일반적인 batch나 연속공정보다는 생산량이 10배 이상 증가하였다. 발효시 생산되는 젖산은 일반적으로 cell 의 성장을 방해하므로 연속공정의 경우 젖산의 농도가 95g/L 이상은 증가하지 않는다. 2단 MCRB(Membrane Cell Recycle Bioreactor)의 스케일업을 수행한 결과 93g/L 의 젖산이 얻어졌고 생산량은 55.8 g/(L·h) 였다.
연간 생산량 10,000 MT 기준으로 2단 MCRB의 경제성을 batch와 1단 MCRB와 비교해 보았다. glucose의 수율이 각 타입별로 유사하므로 필요한 재료비는 거의 비슷했다. batch의 경우는 membrane을 사용하지 않으므로 consumable 비용이 필요하지 않으나 1단과 2단 MCRB는 consumable 비용이 필요했다. batch는 생산량이 증가함에 따라 capital cost가 MCRB보다 급격히 증가하므로 생산량이 많을 경우 적합하지 않았다. 그러나 2단 MCRB의 경우 membrane 가격이 문제가 되는데 만약 membrane 가격이 적절하다면 2단 MCRB은 batch보다 더 경제적일 것이다.
