Theoretical models allow the simulation and data generation from “standard- ized” batch tests in some other widely used mechanisms that have been extensively researched. For instance, the mass transfer occurring in the con- tinuous solid–liquid and liquil–liquid contacting inside mixed vessels can be reliably designed from the kinetic batch reaction curves obtained in bench- scale tests in well-defined mixing conditions.
Batch Aerobic Fermentation — A particular case of increasing industrial importance is the batch aerobic fermentation involving the mixing of a liquid solution with dispersed microorganism particles, chemical additives, and air
bubbles. In such a process, from the chemical engineering point of view, oxygen from the air bubbles is continuously dissolved and consumed by the microorganisms (the “bugs” in operators’ jargon), CO2 is generated and evaporated, carbohydrates are reacted and consumed, a soluble valuable (desired) component is produced, and a lot of other side reactions may be occurring, all simultaneously with the release of heat. A significant cooling
capacity is critical. The flow rate of the air, the pH, and the temperature of
the mixture are generally maintained and controlled by external means and the excess gases are vented. Mixing is very important in maintaining the aqueous solution more or less uniform; it can be internal or external and is generally combined with the cooling system (jacket or heat-exchangers).
The batch period is a matter of days. Considering a large tonnage plant (say, 50,000 metric tons/year [MTY]) with a batch turnover of, say, 4 days, and a product concentration of the order of 10% in the fermentation broth, the net internal volume inside all the fermentors is considerable, about 6800 m3 or 34 fermentors of 200 m3 each. Therefore, any improvement in the average batch period or in the final broth concentration can have a serious economic effect. The hydrostatic pressure at the bottom of such a large fermentor (say, 5 m diameter and up to 10 m in height or even more) is an important operating parameter. It affects not only the supply pressure and, therefore, the cost of the compressed air, but also the solubility of the different gases in the solution and possibly also the biological functioning of the microorganisms. Different models of large fermentors are used in industry, each with its apparent advantages and disadvantages. (Only apparent since many of the important features relevant to their operation have not been released for publication by the corporations operating them.) In any case, the internal inspection and periodical cleaning is essential.
Figure 6.1 illustrates the principle of a draft tube circulation with a cool- ing jacket, which is using the inlet air in the draft tube to promote the circulation of the media. The air/liquid contact inside the draft tube is short, but at a higher turbulence regime. Such type of fermentors have limited size and limited cooling capability and, therefore, they were used mostly for smaller production capacities, since their upscaling is estimated to reach a limit around 60 to 100 m3.
Figure 6.2 shows the main features of a fermentor in which the com- pressed air is sparged from the bottom and an external pumping circuit takes the media around through the heat exchangers. These features have more design options and scale up possibilities, but the passage of the microorgan- isms through a (positive flow) pump and through the heat exchangers has been hotly debated.
The composition of the solution in the batch fermentor is changing all the time and is monitored by the operator to detect any unexpected trend. As the final trend in composition is asymptotic, the main operating issue is
how and when to stop the “reaction” (“dropping” the fermentor), since in many
cases, the later period of operation produces little valuable component but a lot of impurities, which can complicate the recovery.
Figure 6.1 Fermentor with a cooling jacket and draft tube circulation.
Figure 6.2 Fermentor with sparging air and external cooling cycle.
gases out froth air in CW out CW in cooling jacket draft tube gas separation air in CW in gases out gases separation cooler
In most new implementations, a batch fermentor is a prudent starting choice, but it is generally expected that when the industrial process will be well in hand, a number of such fermentors (four to eight) would be connected and operated in series in a continuous fashion. This possibility should be a basic condition for the study and that option should be provided in the plant design. Note that an industrial setup should also include a smaller special “inoculum” fermentor and one or more nonaerated “drop-tanks” into which the content of a finished fermentor is transferred to stop the fermentation, in addition to means for pasteurization (“steaming”) of all incoming streams and all equipment and piping, an acceptable waste disposal treatment for the “bugs,” and, in many cases, the supply of chilled water.
Once a particular process is defined and a model of fermentor is chosen, the study and design of quite large industrial units can be done from a straightforward quantitative model based on the data generated in a pilot fermentor of 10 to 100 L. Such a pilot is often made in the form of very high vertical glass pipes of 7 to 10 cm diameter, with induced circulation to duplicate the changing hydrostatic pressure effects.
Separation of Solids — The rate of separation (or concentration) of solids from a slurry in a continuous solid–liquid thickener depends on the “filtering” velocity of a liquid flow through a dilute solid bed. It has been modeled by Kinch18 long ago and can still be calculated from a standardized slurry settling curve in a 1 L graduated glass cylinder. This method is used routinely for the study of the effects of flocculating agents or other pretreat- ments on the settling rate of the slurry and on the capacity of the thickener. (See Chapter 5, Figure 5.2.)
Vacuum Filters — Large industrial continuous vacuum filters can be designed from standardized, bench-scale, batch-filtering tests.
Rate of Continuous Separation — The specific rate for the continuous separation in industrial liquid–liquid settlers can be predicted from stan- dardized batch tests following the Barnea-Mizrahi model.19–23