Bed-to-headspace heat and mass transfer is of crucial importance in rotating drums while in bioreactors with forced aeration it is typically ignored since the air that leaves the bed is assumed to leave at the temperature of the bed and to not interact any further with the bed before leaving the bioreactor. Note that the rate of transfer depends on the driving force, and the driving force may vary in a complex way with position due to the complex flow patterns that occur with the drum head-space (see Sect. 8.4.2). Further, these flow patterns and the resulting value of the transfer coefficient may depend on how the drum is operated, especially the rotational rate and the aeration rate. Therefore it is not an easy matter to determine the bed-to-headspace transfer coefficients.
Assuming that the headspace of their rotating drum bioreactor was well mixed, Stuart and Mitchell (2003) used the work of Tscheng and Watkinson (1979) to estimate the bed to headspace heat transfer coefficient (hbg, J s-1 m-2 °C-1):
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