The bioreactor can be treated as a whole by drawing a system boundary around the outside of the bioreactor and only considering the exchanges of mass and energy between the bioreactor and its surroundings (Fig. 4.1). The air stream carries mass (N2, CO2, O2, and water vapor) and energy into and out of the bioreactor, with the amount of energy carried depending on its humidity and temperature. The tendency is for the air to leave hotter and carrying more water than when it entered, and both the higher temperature and higher humidity contribute to the overall heat removal from the bioreactor. The composition of the air may be different at the air inlet and outlet. The outlet air is likely to have more CO2, less O2, and more water than the inlet air. Therefore, in terms of mass transfer, the effect of the airflow is not only to provide O2 and remove CO2, but also to dry the bioreactor. Note that it is almost impossible to prevent this drying effect if the air heats up as it passes through the bioreactor.
Energy can be exchanged by convective heat transfer between the bioreactor wall and surroundings, which could be air or could be cooling water in a water jacket. The convection of heat away from the outside of the bioreactor wall will occur by free convection if the outside wall of the bioreactor is in contact with the surrounding air and there is no forced flow of this air past the bioreactor. If the bioreactor is jacketed and water is pumped through the jacket, or if air is blown past the bioreactor surface, then the heat will be removed by forced convection. The significance of the contribution of this heat transfer to overall heat removal depends on the scale of the bioreactor. In the laboratory, small bioreactors have large surface-to-volume ratios, and this heat removal can make a large contribution. At large scale, the surface area-to-volume ratio will be smaller; therefore the contribution of this mechanism to overall heat removal may be very small or even negligible.
The change in energy of the bioreactor itself will manifest itself as a change in temperature ("sensible energy") or a change in phase of water between the liquid and vapor states within the bed ("latent energy").
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