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water from warm or cool reservoir cool or warm saturated air to bioreactor

cool or warm saturated air to bioreactor system controlled such that either • pump 1 on, valve 1 open, pump 2 off, valve 2 closed or pump 1 off, valve 1 closed, pump 2 on, valve 2 open excess water back to the corresponding reservoir

^ ' cool reservoir warm reservoir hi" -w

Fig. 29.4. Configuration used for an air preparation system for a 200-L bed capacity pilot-scale SSF bioreactor

Filter. The filter selected is a HEPA (High Efficiency Particulate Air filter), with a minimum efficiency of 99.97% for particles larger than 0.3 |im. The dimensions of the filter cartridge are 305 mm by 305 mm, with an overall thickness of 78 mm. The pressure drop caused by this filter is equal to a 25 mm water column. In order to protect the filter, a dust pre-filter is included in the cartridge.

Reservoirs. The reservoirs of warm and cool water are large (1 m3 each) in order to make the regulation of their temperature easier, that is, the reservoirs have a large thermal inertia. Note that the intention is to maintain the water temperature of each reservoir constant at the desired set point during the whole fermentation; manipulation of the reservoir temperatures is not part of the control strategy. For the saturation, at 40°C, of 283.5 kg-air h-1, the evaporation rate will be 7.7 kg-H2O h-1 (an inlet relative humidity of 80% was assumed, based on local weather information; obviously it can vary significantly with location). If the make-up water is provided at this rate but at 10°C, then heat must be added at the rate of 270 W in order to maintain the temperature of the water in the warm reservoir at 42°C. By placing a resistance heater of 1000 W in the warm water tank and one of 700 W in the cool water tank, the water temperature can be controlled easily (this can be assured even without making specific calculations about heat losses since the reservoirs are made of polypropylene, which has insulating properties, and also the tanks are covered to prevent evaporation to the surrounding air). This extra capacity also allows a faster warm-up of the reservoir at the beginning of a fermentation.

Humidification column. A computer program was used to determine the minimum design necessities for the humidification column. For a column diameter of 40 cm, a water flow rate through the column of 1.5 m3 h-1, a packing consisting of 25 mm Raschig rings, an inlet water temperature of 42°C and an inlet air temperature of 20°C, Fig. 29.5 shows the resulting predictions for the air and water temperature and the air humidity as functions of height within the column.

A 35 cm high column is sufficient to saturate the air at 40°C. For this height the pressure drop is equivalent to 2 mm of water. The air temperature can be manipulated by changing the water temperature or flow rate, although any such change will be done between different fermentation runs and not during a given fermentation (temperature change during a run is impractical due to the large thermal inertia of the reservoir). With this particular set of parameters, the air velocity through the column is 0.45 m s-1, far below the air velocity that would cause flooding (1.5 m s-1). Our column was designed with a packing height of 70 cm in order to guarantee saturation of the air even if we use different operating conditions.

Fig. 29.5. Predicted performance of the humidification column operating under the conditions given in the text. (a) Temperatures of the (—) air and (---) water as a function of height within the column; (b) Humidity of the air as a function of height in the column.

(---) saturation humidity, which changes due to the change in air temperature as the air passes through the column; (—) actual humidity of the air. This graph is used to decide on the height of the bed in the humidification column

Fig. 29.5. Predicted performance of the humidification column operating under the conditions given in the text. (a) Temperatures of the (—) air and (---) water as a function of height within the column; (b) Humidity of the air as a function of height in the column.

(---) saturation humidity, which changes due to the change in air temperature as the air passes through the column; (—) actual humidity of the air. This graph is used to decide on the height of the bed in the humidification column

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