Bulk Gas Flow Patterns and Pressure Drops

Relatively little attention has been given to gas flow patterns in SSF bioreactors. Those studies that have been done are discussed in Chaps. 6 to 11. Only general principles are given here. Basically, there are two extremes for gas flow patterns (Fig. 4.6): at one extreme the gas phase is well mixed and at the other it undergoes plug-flow. In the case of plug-flow, there is the question of axial dispersion: if a thin plug of colored gas molecules were introduced into the bioreactor, what would exit at the other end? A thin plug of the same thickness? Certainly diffusion makes the plug wider and more diffuse, and other phenomena, such as flow through torturous pathways, can increase the amount of dispersion. In this case the flow is referred to as "plug-flow with axial dispersion". In real bioreactors flow patterns can be more complicated, with the possibility of dead spaces, turbulence, and backflow.

The phenomenon of pressure drop arises due to the viscosity of air (Fig. 4.7). Air tends to stick to the surfaces in the bed, such as the particle surface, the surface of any biofilm growing at the particle surface and the surfaces of any hyphae growing into the inter-particle spaces. This retards the flow of air, due to the loss of energy by viscous friction between various layers of air. The air must still flow through the column at a steady rate, so this resistance to flow does not decrease the kinetic energy of the air, but rather decreases the pressure of the air. In other words, the pressure of the air falls as a gas flows through a column or bed.

In order to leave the outlet, the air leaving the bioreactor must be slightly above the barometric pressure (if the bioreactor is open to the air) or even at a higher pressure (if the outlet gas passes through a filter before entering the surroundings). The greater the resistance to flow, then the greater is the pressure gradient through the bed and the greater are the costs of pumping the air through the bed. Of course, going from an empty column to a column packed with a bed of particles, the area of solid surfaces increases dramatically (Fig. 4.7). It increases even further when a fungus on the surface of the solid particle begins to fill the voids with aerial hyphae. Therefore the pressure drop is greater in a packed-bed than in a hollow column and greater still when the inter-particle spaces in a bed are full of fungal hyphae.

Was this article helpful?

0 0

Post a comment