From the solvent productivity point of view, batch process produced solvents with such a low productivity of 0.35-0.40 g l-1 h-1 [64]. For scaling up of such a process, high volume of broth, high capital cost, and operational cost became severe problems. This leads to uneconomical production of solvents. Continuous fermen­tation reactors show several advantages over batch reactors such as one inoculum culture is sufficient for the long time, drastically reduced sterilization and inocu­lation time. Various continuous processes, such as free cells, immobilized cells, and cell recycling and bleeding, have been investigated. However, immobilized cell process has shown significant potential with as high as 15.8 g l-1 h-1 of solvent productivity (about 40 times than batch process) [69]. Also, cell immobilization allowed long survival time to cells of C. acetobutylicum in solventogenesis phase, which resulted that 20% higher yield than conventional fermentation [30].