Pressure Gradients in Packed Beds

This phenomenon, introduced in Chap. 7.2.4, is of particular importance in packed-beds due to the combination of static operation with forced aeration. The static operation means that the hyphae that grow into the inter-particle spaces are not disrupted or squashed onto the particle surface, and therefore these hyphae represent an extra impediment to air flow, increasing the pressure drop. The maximum pressure drop expected during the fermentation is an important consideration because it will affect the pressure that the blower or compressor must be capable of supplying.

Excessive pressure drop tends not to be a problem in intermittently agitated packed-beds because the agitation prevents the hyphae from binding the substrate bed into one large mass and it also squashes the hyphae onto the particle surface. In fact, infrequent agitation events might be used in packed-beds with the major purpose of decreasing the pressure drop. Figure 7.10 illustrates this point.

Despite its potential importance at large scale, pressure drop has received most attention in small-scale packed-bed bioreactors, and in these experiments the interest was in using the pressure drop to quantify the growth.

Auria et al. (1993, 1995) used a column of 6.5-cm height and 2-cm internal diameter, and a superficial velocity (calculated as volumetric flowrate divided by the total cross-sectional area of the column) of 0.435 cm s-1. The substrate was an artificial substrate based on an amberlite resin impregnated with nutrients. The maximum pressure drop observed during the fermentation ranged from 0.21 to 0.69 cm-H2O cm-bed-1, for various different initial nutrient concentrations. With bagasse as the substrate and a superficial velocity of 0.379 cm s-1 the maximum pressure drop obtained in the same column was 2.75 cm-H2O cm-bed-1. In this case the pressure drop was already 0.45 cm-H2O cm-bed-1 at the beginning of the fermentation. In a larger column of 15-cm height and 4-cm diameter, they obtained a maximum pressure drop of 0.12 cm-H2O cm-bed-1 with a wheat bran substrate and a superficial velocity of 0.675 cm s-1. With a much higher superficial velocity of 11.2 cm s-1 Gumbira-Sa'id et al. (1993) obtained a pressure drop of 1.38 cm-H2O cm-bed-1 with a substrate based on cooked sago-beads.

Fig. 7.10. Typical temporal profiles for the pressure gradient in the bed, based on the results of Gumbira-Sa'id et al. (1993) for the growth of Rhizopus oligosporus on a substrate based on sago beads. (—) Evolution of the pressure drop in a fermentation in which the bed was not disturbed by the removal of samples. Note that the decrease in the pressure drop after 40 h is due to the substrate bed shrinking and pulling away from the bioreactor wall; (- - -) Evolution of the pressure drop in a fermentation in which the bed was periodically disturbed by the removal of samples. Adapted from Gumbira-Sa'id et al. (1993) with kind permission of Elsevier

Fig. 7.10. Typical temporal profiles for the pressure gradient in the bed, based on the results of Gumbira-Sa'id et al. (1993) for the growth of Rhizopus oligosporus on a substrate based on sago beads. (—) Evolution of the pressure drop in a fermentation in which the bed was not disturbed by the removal of samples. Note that the decrease in the pressure drop after 40 h is due to the substrate bed shrinking and pulling away from the bioreactor wall; (- - -) Evolution of the pressure drop in a fermentation in which the bed was periodically disturbed by the removal of samples. Adapted from Gumbira-Sa'id et al. (1993) with kind permission of Elsevier

As yet there is insufficient information to predict the pressure drops that can be expected during a large-scale fermentation, although the experimental values reported here give some idea of the orders of magnitude that might be expected. The initial pressure drop will depend on the substrate and how it packs together, which in turn will depend on how the substrate is prepared. The maximum pressure drop achieved during the fermentation will depend on how the microorganism grows within the bed, although it is also a function of the superficial velocity.

Note that the pressure drop across the bed can decrease later in the fermentation. This can happen due to the bed pulling away from the walls, leaving a gap through which the air can pass (Gumbira-Sa'id et al. 1993; Weber et al. 2002). As described in the next subsection, this is undesirable because it will lead to heat and mass transfer limitations within the bed.

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