Process design studies of molasses fermentation have shown that the investment cost was considerably reduced when continuous rather than batch fermentation was employed, and that the productivity of ethanol could be increased by more than 200%. Continuous operations can be classified into continuous fermentation with or without feedback control. In continuous fermentation without feedback control, called a chemostat, the feed medium containing all the nutrients is continuously fed at a con­stant rate (dilution rate D) and the cultured broth is simultaneously removed from the fermentor at the same rate. The chemostat is quite useful in the optimization of media formulation and to investigate the physiological state of the microorganism [71]. Continuous fermenta­tions with feedback control are turbidostat, phauxostat, and nutristat. A turbidostat with feedback control is a continuous process to maintain the cell concentration at a constant level by controlling the medium feeding rate. A phauxostat is an extended nutristat, which maintains the pH value of the medium in the fermentor at a preset value. A nutristat with feedback control is a cultivation technique to maintain nutrient con­centration at a constant level [71].

When lignocellulosic hydrolyzates are added at a low feed rate in con­tinuous fermentation, low concentration of bioconvertible inhibitors in the fermentor is assured. In spite of a number of potential advantages in terms of productivity, this method has not developed much yet in fermentation of the acid hydrolyzates. One should consider the following points in continuous cultivation of acid hydrolyzates of lig — nocelluloses:

■ Cell growth is necessary at a rate equal to the dilution rate in order to avoid washout of the cells in continuous cultivation.

■ Growth rate is low in fermentation of hydrolyzates because of the presence of inhibitors.

■ The cells should keep their viability and vitality for a long time.

The major drawback of the continuous fermentation is that, in contrast to the situation in fed-batch fermentation, cell growth is necessary at a rate equal to the dilution rate, in order to avoid washout of the cells in continuous cultivation [21]. The productivity is a function of the dilu­tion rate, and since the growth rate is decreased by the inhibitors, the productivity in continuous fermentation of lignocellulosic hydrolyzates is low. Furthermore, at a very low dilution rate, the conversion rate of the inhibitors can be expected to decrease due to the decreased specific growth rate of the biomass. Thus, washout may occur even at very low dilution rates [18]. On the other hand, one of the major advantages of continuous cultivation is the possibility to run the process for a long time (e. g., several months), whereas the microorganisms usually lose their activity after facing the inhibitory conditions of the hydrolyzate. By employing cell-retention systems, the cell-mass concentration in the fermentor, the maximum dilution rate, and thus the maximum ethanol productivity increase. Different cell-retention systems have been inves­tigated by cell immobilization and encapsulation, and cell recirculation by filtration, settling, and centrifugation. A relatively old study [72] shows that the investment cost for a continuous process with cell recir­culation has been found to be less than that for continuous fermentation without cell recirculation.

Biostil® is the trade name of a continuous industrial process for ethanol production with partial recirculation of both yeast and waste­water. The fermentor works continuously; the cells are separated by using a centrifuge, and a part of the separated cells is returned to the fermentor. Most of the ethanol-depleted beer including residual sugars is then recycled to the fermentor. In this process, besides providing enough cell concentration in the fermentor, less water is consumed and a more concentrated stillage is produced. Therefore, the process has a lower wastewater problem. However, the process needs a special type of centrifuge (which is expensive) in order to avoid deactivation of the cells [47, 73].

Application of an encapsulated cell system in continuous cultivation has several advantages, compared to either a free-cell or traditionally entrapped cell system, e. g., in alginate matrix. Encapsulation provides higher cell concentrations than free-cell systems in the medium, which leads to higher productivity per volume of the bioreactor in continuous cultivation. Furthermore, the biomass can easily be separated from the medium without centrifugation or filtration. The advantages of encap­sulation, compared to cell entrapment, are less resistance to diffusion through beads/capsules, some degree of freedom in movement of the encapsulated cells, no cell leakage from the capsules, and higher cell con­centration [74].