Insights the Model Gives into Operation of Intermittently Mixed Bioreactors

25.3.1 Predictions about Operation at Laboratory Scale

The base case simulation with the model uses the values given in Tables 25.1 and 25.2. This first simulation, although not intended specifically to model the bioreactor of Ghildyal et al. (1994), is for a similar situation. They used a bioreactor of 34.5 cm height and superficial velocities from 0.0047 m s-1 to 0.0236 m s-1. In the base case simulation, the bioreactor is 30 cm high, and the air flux used, 0.02 kg-dry-air m-2 s-1, corresponds to a superficial velocity of 0.0175 m s-1 (i.e., 0.02 kg-dry-air m-2 s-1/(1.14 kg-dry-air m-3)).

Figure 25.3(a) illustrates the advantages of intermittent mixing over true packed-bed operation. In the absence of any mixing event, the average biomass curve suddenly decelerates at 20 h. This deceleration is due to the bed drying out to water activities that greatly restrict the growth rate. In the intermittent-mixing mode, water is added during the mixing events at 20.35 and 30.8 h, these mixing events being indicated by the sudden drops in bed temperature in Fig. 25.3(c). During a mixing event, the water activity of the solids in the bed is brought back to values that favor fast growth, and the average biomass profile does not deviate too far from the dashed curve in Fig. 25.3(a), which was obtained by plotting the logistic equation with n always equal to the maximum possible value, 0.236 h-1.

Table 25.1. Values used for the base case simulation of those parameters and variables that can be changed in the accompanying model of an intermittently-mixed forcefully-aerated bioreactor

Symbol Significance

Base case value and unitsa

Design and operating variables and initial values of key system variables

Table 25.1. Values used for the base case simulation of those parameters and variables that can be changed in the accompanying model of an intermittently-mixed forcefully-aerated bioreactor

Design and operating variables and initial values of key system variables

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