The air preparation system presented in Fig. 29.3 represents a compromise between technical specifications and costs in the sense that, while it does not allow as flexible a control of the conditions of the air supplied to the bioreactor as the system shown in Fig. 29.2, it will be much cheaper to build and operate than that system and it will allow better control than the system presented in Fig. 29.1. On the basis of these considerations, the system shown in Fig. 29.4 was recently constructed for a pilot-scale SSF bioreactor with a 200-L substrate bed. Although the bioreactor has not yet gone into operation, it is worthwhile to describe briefly the calculations that were used to design the system.

Maximum air flow rate requirement. The maximum air flow rate that would be needed was calculated on the basis of heat removal considerations. Assuming logistic growth kinetics, the maximum heat generation rate (Rq, kJ h-1) is given by (Mitchell et al. 1999):

The substrate packing density (pb) was estimated as 350 kg-dry-substrate m-3, the maximum biomass content (Xmax) as 0.125 kg-dry-biomass kg-dry-substrate-1, the metabolic heat yield coefficient (Yq) as 107 J kg-dry-biomass-1, the maximum value of the specific growth rate constant (pmax) as 0.324 h-1, and the bed volume (Vb) as 0.2 m3. The calculation gave a value of Rq of 7.1 MJ h-1.

A conservative estimate of the capacity of the air to remove heat from the bed was made as Qrem = 5 kJ kg-air-1 °C-1. This represents the sum of the heat capacity of humid air (~1.0 kJ kg-dry-air-1 °C-1) and the contribution of evaporative cooling of "2.(dHsat/d7)" where A. is the enthalpy of evaporation of water (2500 kJ kg-water-1) and dHsaJdT (kg-vapor kg-dry-air-1 °C-1) is the change in the water-carrying capacity of air with a change in temperature (see Sects. 18.5.2.2 and 19.4.1). Use of Eq. (19.20) shows that dHJdT varies from 0.0016 kg-vapor kg-dry-air-1 °C-1at 30°C to 0.0048 kg-vapor kg-dry-air-1 °C-1 at 50°C. Using the value of dHsJdT at 30°C therefore leads to a more conservative estimate and with this value "A.(dHsat/dT)" is calculated as 4 kJ kg-air-1 °C-1.

The mass flow rate of air required (Wair, kg h-1) was then calculated as:

Crem

Qrem &T

where AT is the maximum allowable rise in temperature of the air as it flows through the bed. This was taken as 5°C. Substituting the values of Rq and Qrem, Wair was calculated as 283.5 kg h-1 (235 m3 h-1 at 15°C and 1 atm). Since a conservative value was used for dHsatldT, this is probably an overestimate, but will therefore allow a reasonable margin for error.

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