Advantages of Airlift Bioreactors

For the growth of microorganisms, ALRs are considered to be superior to traditional stirred-tank fermenters despite the fact that the conventional fermenters provide the major requirements for culturing microorganisms: gasmedium interface for the supply of oxygen and the removal of waste gases; means of agitation to ensure proper nutrient distribution and to minimize damage resulting from addition of concentrated acid or base (for pH control); means of heat transfer (for temperature control); and a contamination-free environment. Therefore, the reason for the more successful growth reported in ALRs (3,4) appears to lie in the difference in the fluid dynamics between ALRs and the more conventional fermenters. In conventional stirred tanks or bubble columns, the energy required for the movement of the fluids is introduced focally, at a single point in the reactor, via a stirrer or a sparger, respectively Consequently, energy dissipation is very high in the immediate surroundings of the stirrer and decreases away from it toward the walls. Similarly, shear will be greatest near the stirrer (5), since the momentum is transferred directly to the fluid in that region (6), which, in turn, transfers this energy to the slower-moving, more distant elements of the fluid. This results in a wide variation of shear

Figure 2. Different types of gas separators.

forces; for example, the maximum shear gradient in a stirred tank with a flat-blade turbine has been reported to be approximately 14 times the mean shear gradient (7).

Cells in culture may thus be exposed to contrasting environments in a mechanically stirred vessel, either to minimal shear forces that may generate potentially undesirable gradients in temperature and in substrate, metabolite, and electrolyte concentrations or, alternatively, to highly turbulent zones, with no problems of heat or mass transfer, but with very high shear gradients that may endanger cell integrity or exert some influence on cell morphology and metabolism (6). Changes in the morphology of microorganisms associated with high shear forces in the medium have frequently been observed (8-10). The nature of the relationship between such morphological changes and the rates of growth and metabolite production is still not properly understood, although it may be of great importance in the design and scale-up of bioreactors.

In ALRs, as in bubble columns, the gas is injected at a single point, but in ALRs the direct contribution of gas injection to the dynamics of the system is small; circulation of liquid and gas is facilitated by the difference in gas holdup between the riser and the downcomer, which creates a pressure difference at the bottom of the equipment:

where DPb is the pressure difference, qL is the density of the liquid (the density of the gas is considered to be negligible), g is the gravitational constant, and yr and ud are the fractional gas holdup of the riser and downcomer, respectively. The pressure difference forces the fluid from the bottom of the downcomer toward the riser, generating fluid circulation in the ALR. Since ur and ud are both average values integrated along the height of the reactor, it follows that there are no focal points of energy dissipation and that shear distribution is homogeneous throughout the ALR. There is thus a relatively constant environment, with minimization of sharp changes in the mechanical forces acting on suspended particles. Because good mixing is required, shear forces cannot be avoided completely. One of the most critical points is the bottom, where there is a sharp 180° turn.

Shear-sensitive mammalian and plant cells in culture should benefit from such an environment. Currently, the research and development of new bioreactors for mammalian cells is indeed focusing on the issue of shear-related damage to suspended cells (8,11-24).

Mammalian and plant cells in culture are more susceptible than microorganisms to the reactor conditions. Mammalian cells, which lack the rigid cell wall of microorganisms, have a larger size (one order of magnitude) than microorganisms and are very sensitive to mechanical stress. Plant cells have a rigid cellulose wall, but they are also much larger than microorganisms (usually by about an order of magnitude) and are therefore also sensitive to reactor conditions. Kolmogoroff's model of isotropic turbulence (25) indicates that serious damage may occur at relatively large values of the length scale. The last length is a parameter of the model and indicates the size of the eddy where energy starts to be dissipated by viscous resistance. Indeed, it has been observed that plant cells, in spite of their rigid wall, are shear-sensitive, and difficulties have been found in stirred-tank cultures. This is especially true when large-scale systems are considered. Although high agitation rates may be detrimental to cell growth, low agitation rates lead to an increase in the number and size of cell aggregates—also an undesirable phenomenon. The aggregates are formed as a result of daughter cells failing to separate after division and as a consequence of the stickiness of the polysaccharides excreted by the cells, especially at the end of the growth phase. An optimal shear rate between these two extremes must be found for each culture.

It has recently been shown experimentally that velocity fluctuations related to turbulent shear are relatively homogeneously distributed in an ALR (26,27). The measurements of fluctuating velocity made by Tan et al. (26) show that the liquid turbulence in ALRs is homogeneously distributed in both the riser and the downcomer. It thus seems reasonable to assume that the homogeneity of the stress forces is the main advantage offered by ALRs and that this homogeneity is responsible for the success of shear-sensitive cultures in the ALR type of fermenter (3,28-31).

Another advantage of the ALR is the mechanical simplicity of the device. The absence of a shaft and of the associated sealing, which is always a weak element from the point of view of sterility, confers on the ALR an obvious advantage over agitated tanks. This consideration is es pecially important in processes involving slow-growing cultures, such as animal and plant cells, for which the risk of contamination is large.

All the points mentioned above are particularly relevant for sophisticated processes in which the product is usually of high value. But ALRs may be used also for processes involving low-value products, in which case efficiency of energy use may well become the key point for design, as in the use of ALRs for wastewater treatment (32). The superiority of ALRs over mechanically agitated contactors in terms of mass transfer rates for a given energy input has been demonstrated by Legrys (33). Comparison of the efficiency of oxygen transfer, that is, the mass of oxygen absorbed per unit energy invested and unit time, showed that the efficiency of the ALR is among the highest in agitated systems (32). The ALRs are particularly suited to processes with changing oxygen requirements because aeration efficiency and performance are relatively insensitive to changes in operating conditions. Performance decreases markedly in mechanically stirred systems as the energy input (or oxygen transfer rate) increases, but it is quite constant in ALRs (34) (Fig. 3).

The efficiency of ALRs decreases relatively slowly as the energy input per unit volume of reactor is increased, as is shown in Figure 4 (32). In contrast, in the operation of stirred tanks, the mass transfer rate can be easily increased by increasing the power input, but this improvement is achieved at the cost of a considerable decrease in the efficiency of oxygen transfer. This decrease may constitute a crucial disadvantage in a process like wastewater treatment, where the energy input is an important element in the cost of the final product and flexibility of operating conditions is required because of the constant change of feed composition and flow rate.

Energy economy in the ALR may be improved by placing a second sparger in the upper part of the downcomer (32,35,36). If the liquid velocity is greater than the free rising velocity of the bubbles generated, the gas is carried down, resulting in a longer contact time between the bubble and the liquid. This diminishes the energy requirements, since part of the gas is injected against a lower hydrostatic pressure.

The advantages described above counterbalance the obvious disadvantage of ALRs, which is the requirement for a minimum liquid volume for proper operation. Indeed, the changes in liquid volume in these reactors are limited to the region of the gas separator, since the liquid height must always be sufficient to allow liquid recirculation in the reactor and must therefore be above the separation between the riser and the downcomer.

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