The air stream not only must be supplied at proper conditions of temperature and humidity as addressed above, it also must overcome the pressure drop caused by the bed, piping, and other accessories. All these aims have to be achieved at low cost, since many SSF processes have low profit margins. At this point, we can examine some alternatives for air preparation, starting with the very simple alternative presented in Fig. 29.1.
The system presented in Fig. 29.1 will only be appropriate if aseptic conditions are not required, since it has only a simple dust collector, like those found in home air conditioning units. A porous plate is necessary for efficient air distribution in the humidifier. Porous stainless steel plates with high permeability are available in the market; they are very efficient but are not cheap. Sintered ceramic plates are also available; they are cheaper than metallic plates, however, they require a mechanical support in order to withstand the mass of the water in the tank. In this system the temperature of the air leaving the humidification tank will be close to the water temperature, therefore it is desirable to control the water temperature, which can be done using electrical resistance heaters and an ordinary thermostat.
This simple system has three important features. Firstly, it is not practical to obtain air temperatures below the ambient temperature since no cooling unit is included in the system. Secondly, it will not allow a fast change of the temperature of the air fed into the bioreactor due to the high thermal inertia of the water mass. Thirdly, high air flow rates will cause the evaporation of large amounts of water and therefore water must be replenished periodically in order to prevent the tank from drying out.
Since the system presented in Fig. 29.1 does not allow a rapid manipulation of the air temperature, another possible arrangement is presented in Fig. 29.2.
Aseptic cultivation will be easier to achieve with this arrangement. Firstly, the cooling of the air before filtering will eliminate part of the microorganisms with the purged water. Secondly, the micro-porous filter will not allow particles larger than 0.3 |im to pass, which is sufficient to remove fungal spores, bacterial cells, and any larger organisms.
In this system a pair of valves and two sets of electrical resistances can be used to control the air temperature. Typically the process will require saturated air at temperatures higher than the ambient during the lag phase while during the rapid growth phase it will be necessary to supply air at a temperature below the optimum temperature for growth in order to promote heat removal. The highest rates of heat removal will be obtained by supplying cold, dry air in order to promote evaporative cooling, however, this will also promote drying of the bed. It is possible to inject steam into the cold air; but it is not easy to generate steam at temperatures around say 20°C. Finally, it is important to note that it is not a simple matter to produce saturated air by direct mixing of steam and dry air, since it is not easy to design a mixing device that does not produce condensation.
A third alternative is presented in Fig. 29.3, where a humidification column replaces the steam-air mixer. A well-designed humidification column will guarantee saturated air. The heater for producing cooler dry air that appeared in Fig. 29.2 is not present since only saturated air will be provided by this system. In this system the temperature of the outlet air will be very close to the water temperature, as was the case for the system presented in Fig. 29.1. It is therefore interesting to work with two reservoirs, one with hot water and other with cold water. A set of synchronized solenoid valves can be used to change from circulation of hot water to circulation of cold water and vice-versa, proving saturated air at a higher temperature at the beginning of the fermentation and saturated air at lower temperatures for cooling the reactor during the rapid growth period. It is important to remember that even with the use of saturated air the bed will still dry out, since the air is heated as it passes through the bed and therefore its capacity to carry water increases (see Fig. 4.3). In other words, the use of saturated air will reduce the frequency with which water must be replenished but water replenishment will still be necessary. Note that aseptic operation is unlikely to be feasible due to the difficulty in operating the entire humidification system (reservoirs and column) in an aseptic manner.
Of course, combinations and variations of the alternatives presented in this section can be worked out. The suggested configurations demonstrate the advantages and disadvantages of selecting a particular combination of devices. The decision for a particular arrangement must be based on criteria of economic performance of the process, which will depend on the capital cost of the selected devices and operating costs related to energy consumption for the various unit operations such as blowing the air, heating or cooling the air, producing steam, and heating water. In fact, great care should be taken in computing energy costs, since they can compromise the feasibility of the project, particularly when working with products that have low profit margins. The following sections give some further advice about various aspects of the design of the air preparation system.
water from cold water from cold
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