General Requirements and Features of Bioreactors
Bioreactors are required to perform a great number of functions to fulfill many requirements.
The requirements are met partly or to a large extent in having incorporated configurationally changes in the classical system of STR or PFRS.
As a result several new reactor types have come into existence which are either being tried in prototypes or used in large commercial operations.
1. Stirred Tank Reactors:
Principally, these are upright cylindrical vessels in which air is sparged through the agitated liquid medium. Various types of STRs are in use. Examples include single and multistage STRs and Waldhof reactors. Generally in an STR specific power consumption is high. Comparatively, Waldhof reactors consume less specific power since the center of gravity of the liquid is lower. Gas hold-up is also high since a considerable volume of air is drawn and is finely dispersed through the turbulent agitator zone.
2. Surface Reactors:
In physical configuration these are also cylindrical vessels rotating on the horizontal axis in which air is blown inside the vessel. Examples include the horizontal rotary reactor (HRR), disc reactors and film reactors. In extreme gas-liquid interactions this type of reactor has demonstrated its excellent suitability. It has been suggested that large inequalities in the distribution of DO in STRs, particularly in high viscous non-Newtonian broths, HRRs offer good solution. This reactor is significantly important on account of a number of outstanding performance.
3. Cyclone Reactors:
In this system the cells are circulated around a closed loop to effect oxygen transfer, mixing, and homogeneous cultivation instead of agitation and stirring in a tank. Special advantages are: a high gas exchange, no use of antifoam, no wall growth, and operation suitable for cultivation of aerobic and anaerobic nonpathogenic microorganisms. This reactor can be used for batch and continuous growth of synchronous and asynchronous cultures.
4. Fixed-Film Reactors:
These reactors have a performance behavior similar to that of trickling filters extensively used in waste water treatments. The ideal film thickness should correspond to the penetration depth of the limiting nutrient. Further increase in film thickness leads to no improvement in conversion efficiency.
5. Deep-Shaft Reactors:
The principle of operation is similar to that of the pressure cycle reactor except that all or most of the air is introduced into the down flow tube and no baffles are permitted to rise because of blockage risks. Depth is also considerably greater, normally in the range of 50-150 m.
Air introduced in this way is at a lower pressure than if it were introduced at the bottom; this enables savings both in capital and operating costs. Liquid circulation is established by injection of compressed air through a sparger placed at relatively shallow depth in the up flow side.
Air is then gradually switched to the down flow side. The gas void age in the top part of the up flow tube continues to balance hydraulic friction losses and net void age resistance in the lower part of the reactor. Bubble contact time, high pressure, and turbulence provide oxygen transfer as high as 3 kg of 02/h-m3. Energy requirement for this high degree of oxygen transfer is about 1 kWh.
6. Immobilized Cell Reactors:
In these reactors, enzymes/cells are either attached by adsorption, chemical bonding (cross- linked or covalently bound), or entrapment on a suitable carrier, or are encapsulated and placed/ packed in different types of vessel configurations to serve as a flow reactor.
Reactors with physical adsorption of enzymes or cells encounter practical difficulties because the adsorbed enzyme or cell is weakly bound and is lost rather easily during operation. Gel-entrapped enzyme reactor systems are associated with severe problems of diffusion resistances more exercised by the substrate than the product. Covalently or cross-linked immobilized reactor systems require mild processing conditions.
Large changes in pH and temperature are not permitted. A microencapsulated reactor system is subjected to the requirement of substrate diffusivity across the semi-permeable membrane which contains the enzyme or cells. Despite these disadvantages, the greatest advantage offered by this system is high-productivity.
7. Membrane Reactors:
Membrane reactors are one of the first novel immobilized enzyme reactors. More complex the reactor design is, the more changes of the coefficients and constants will be expected with scale-up or scale-down practices. This underlines the complicated similarity relationships of the standard STR configuration.
Although physical functions of bioreactors are determined by the geometry and mechanical inputs, the microbial activities are manifested by the established physiological and morphological picture. Total correlations between physical and mechanical functions against microbial functions are not completely available.
The release of substances by the cells into the liquid bulk is generally associated with feedback mechanism controlling the changes in the input variable and thus resulting in an altered regulatory expression in the microbial mass. The release of surface active proteinous materials affects the rheological properties of the liquid medium and thus sets up a chain of changes in the environmental conditions such as air distribution, gas hold-up, diffusion mass transfer, bulk heat transfer, and so forth. These problems change the interrelationships of constants and coefficients of reactor functions. Problems associated with aggregated mycelia in many reactor systems are well known.
The oxygen transfer pathway in a typical bioconversion process is shown in Fig. 5.1. Transport of oxygen from air bubbles to the liquid and then to the cell through the liquid pathway poses some serious problems in some particularly immiscible systems.
It is associated with the sparingly soluble oxygen and the rate-controlling step for oxygen is that from the bubble interface to the bulk liquid. The problem is aggravated by the rheological properties of the liquid. Power requirement in such a system is necessarily high. High and low power input in such systems creates different mixing zones. Creation of mixing zones in turn may result in non-homogeneity in the reaction rates.