Blower Compressor Selection and Flow Rate Control

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The air blower is likely to be responsible for a large proportion of the energy consumed in bioreactor operation and therefore selection of an appropriate blower is very important. In general, SSF bioreactors need high air flow-rates at low pressures. If the pressure drop of the whole system, including piping, accessories, and bioreactor, is equal to or less than 35 cm of water, a fan is the best device. The power consumed by a fan (P, kW) can be calculated as follows:

where F is the air flow-rate (m3 h-1) and p is the operating pressure of the fan (cm-H2O). Manufacturers usually supply operational curves and installation details.

For higher pressure-drops, a centrifugal compressor would be the best choice, however, the energy consumption would be prohibitive in many cases. If possible, it is advisable to design and operate the bioreactor and air preparation system in such a manner as to minimize the pressure drop, such that it is possible to use a fan, rather than to work with a compressor.

Usually fans work at fixed velocity whereas the aeration requirements change over time. Therefore a flow control valve (FCV), placed in the air line between the blower and the bioreactor, is required. For air at low pressure a butterfly valve is the best choice. Centrifugal compressors can be operated in a similar manner or, alternatively, their velocity can be controlled with inverter drives.

Both fans and centrifugal compressors work under a "characteristic curve" that gives the flow rate provided by the equipment as a function of the pressure in the air line. Note that the pressure in the air line depends on the pressure drop suffered by the air as it flows from the blower, through the system, to the air outlet (that is, the pressure at the air inlet must be at least equal to the sum of the pressure at the air outlet and the pressure drop within the system). Both these types of blowers will provide larger flow rates at lower pressure, with the flow rate reducing as the pressure in the air line increases. The characteristic curve depends on the type and the design of the blower and should be provided by the manufacturer.

The required air flow rate for the bioreactor will depend on heat removal needs, as was clearly demonstrated in the various modeling case studies presented in Chaps. 22 to 25. The blower must be capable of producing the flow rate required at the time of maximum heat production within the substrate bed. Obviously, models of the type presented in these chapters are useful tools in deciding on the requirements of the blower.

Note that the pressure drop that the system must be capable of overcoming is that which is present in the system at the time of maximum heat production. Potentially, the pressure drop as air flows through the bed can make a significant contribution to the overall pressure drop in the aeration system in those bioreactors in which the bed is forcefully aerated. The pressure drop across the bed depends on the substrate, the microorganism, and their interaction during the process and, due to our relatively poor understanding of these phenomena, it is not possible to use a set of theoretical calculations to predict the magnitude of the pressure drop that can be expected. Therefore, some experimental assays at laboratory scale will be necessary in order to estimate the pressure drops for a particular system. In systems in which the bed is agitated, a maximum allowable pressure drop can be set as a parameter for triggering mixing events, and this should prevent the pressure drop across the bed from reaching high values.

The best strategy is to select the equipment that provides the largest pressure range for the maximum required flow-rate, this flow-rate being deduced from the energy balance model. It is then necessary to check whether the equipment will operate economically in terms of energy consumption at the required combination of pressure and flow rate. If energy consumption is too high then possibly an inferior blower will need to be selected. This may not be capable of meeting the aeration needs during the periods of peak heat generation, so the performance of the process may be deleteriously affected. As stated above, many of the products obtained by SSF have low profit margins; in this case the energy consumption of the aeration system is a crucial factor in determining process profitability.

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