Changing of the mode of bioreactor operation from batch to continuous saves the time required for loading, discharging, and cleaning of the bioreactor, since in continuous operation loading and discharging proceed simultaneously with the fermentation whereas in batch processes the bioreactor is not producing product while these "turnaround" operations are taking place. Assuming the same fermentation time for both batch and continuous operations, the saved time means that the volumetric productivity of the continuous plug-flow bioreactor is higher.
The extent of the increase in productivity depends on ratio of the turnaround time to the fermentation time. For example, assuming that the turnaround and fermentation times are 10 and 40 h, respectively, the volumetric productivity will increase by 25% upon changing from batch to continuous mode. The productivity increase is smaller for higher ratios of fermentation time to turnaround time. For example, with the same turnaround time of 10 h but a fermentation time of 70 h, the increase in productivity gained by changing from batch to continuous operation is only 14%.
As mentioned previously, replacing a single large batch bioreactor with several small ones operating in a staggered manner will reduce the difference in productivities. However, once again, it must be highlighted that this implies higher investment costs.
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