Synopsis of the Model

The two types of operation, that is, static operation and the mixing event, are modeled separately. During static operation, the system is treated as a packed-bed bioreactor (Fig. 25.1). It is assumed that the bioreactor is sufficiently wide such that heat transfer to the side walls makes a negligible contribution, and therefore only heat and mass transfer in the axial direction are described. Both temperature and water balances are done; in both cases separate balances are written for the air and solid phases. Note that the terms of the balance equations are per cubic meter of bioreactor, and biomass and water contents are expressed on the basis of the total dry solids per cubic meter (S), which is equal to the sum of the dry biomass and dry substrate. The definitions of symbols used are given in Table 25.1.

The model equations are summarized in Fig. 25.2. The balance on the overall mass of dry solids (i.e., the sum of dry biomass and dry residual substrate, Fig 23.2, lower right) is necessary since not all the consumed substrate is converted into biomass; a proportion is lost in the form of CO2. This is Eq. (16.11), although without the maintenance term.

(a) Static operation when the water activity of the outlet gas falls below a set point, mixing is triggered z=Z

air flow when the water activity of the outlet gas falls below a set point, mixing is triggered negligible heat removal through the side walls of the bioreactor after 15 min returns to static operation

air flow

Well-mixed operation air flow air flow

gas

solid

phase (g)

phase (s)

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