Threephase Airlift Reactors

The special qualities of the ALR stem, as stated before, from its fluid dynamic characteristics. One such characteristic is the directionality of the liquid flow. Independently of superimposed fluctuations, a clear net flow is present in the reactor, with exception of the gas separator in internal-loop designs. Therefore, it is to be expected that the fluidization capacity of the ALR will be markedly superior to that of a bubble column. Several studies have been conducted on the suspension of solids in ALRs, particularly on the use of this type of device for catalytic processes in the chemical industry, where the solid support is usually heavy (61,159-163). In this regard, a very important point is the minimum gas superficial velocity that leads to complete solid fluidization (61,87,159,162,164166). Hysteresis has been observed in some cases; once total fluidization has been attained, the superficial velocity can be reduced to values lower than that required to reach this state. This is due to the high pressure drop related to passage of liquid through a bed of solids, before fluidiza-tion, as compared to the drag forces required to maintain the solid in suspension after all the solids are suspended. Contradictory data on the effect of the suspended solids on the reactor performance have been reported. Fan et al. (167) claimed that the overall gas holdup increased due to the presence of the solids, whereas Koide et al. (168,169) showed the opposite effect on the gas holdup and reported a small decrease in kLa as well. It is possible that these discrepancies are due to the use of different solids. One of the properties of solids that is often overlooked is wetta-bility. Small bubbles may adhere on wettable solids, leading to a change in the apparent density of the particle and thus changing their solid circulation velocity.

In the case of suspended solids that take an active part in the process, the mass transfer rate from the liquid to the solid may become the limiting step. The dependence of particle size on the mass transfer to the suspended solids has been studied by several authors (19,170,171).

All the comments made above relate to heavy solids. This situation is not very frequent in biological processes, with the exception of biolixiviation (172) or the special case of microbial desulfuration of coal (173). In most of the biological processes that may take place in an ALR, the solids are either cells, clumps of cells, or supports that are not much more dense than the medium. Therefore, neither flu-idization nor the distribution of solids in the reactor constitutes a problem. Assa and Bar (174) found very small variations in the axial distribution of animal and plant cells suspended in an internal-loop ALR. This is due to the small free-falling velocity of the solids, which is the reason for the difference in loading between the riser and the downcomer when heavy particles are used (160-162). Because of the small difference between solid and medium density, the movement of the particles usually present in biological processes is not as dependent on gravity forces as on liquid and bubble movement. In this case, the transport of liquid in the wake of the bubbles may be considered to be the prevailing transport mechanism (175). Snape and Thomas (176) proposed a Monte Carlo algorithm for modeling particle movement by this mechanism in bubbly flow to predict distribution of circulation times in the reactor. Koide et al. (169) conducted a broad study of gas holdup and mass transfer rates in an internal-loop ALR containing Ca-alginate beads, which are often used for cell immobilization. The authors found that solid loading had a negative effect on both parameters, but the particle diameter had no influence in the range studied (1.8 mm < dp < 3.98 mm). Chang et al. (31) studied the influence of suspended pellets (cylindrical pellets of immobilized penicillin acylase) on the mixing time in an internal-loop ALR. They found mixing time increased when the solid volumetric concentration was raised up to 15%. For higher concentrations the trend inverted, and the mixing time decreased. No such effects were observed with heavier solids. Increases in gas superficial velocity and in top clearance both lead to decreases in mixing time in all cases.

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