Marine propeller

Static mixer

Gas input Gas input Gas input

Figure 38. Modifications proposed in ALRs.

External-loop ALR

Gas output

Gas output

Helical flow promoter

Gas output

Figure 39. Comparison between a bubble column an ALR and an ALR with helical flow promotors. From Schlotelburg et al. (196).

Different types of static mixers, usually located in the riser, have also been used for enhancing the performance of ALRs (Fig. 38). Potucek (189) studied the influence of static mixers on the gas holdup and interfacial gas-liquid area in a concentric-tube ALR and found an improvement in interfacial area. Enhancement of kLa has been also obtained, especially in viscous liquids, as reported by Gas-pillo and Goto (190) and Chisti et al. (191). Zhou etal. (192) showed better growth of Cephalosporum acremonium in an ALR with static mixers.

As mentioned above, it has been shown repeatedly that the mass transfer rate in a bubble column is usually higher than that in the conventional ALR. It therefore makes sense to try to bring into the ALR some of the characteristics of bubble columns in a controlled fashion. This was done by Bando et al. (193,194), who tested a perforated draft tube in a concentric-tube ALR. The perforations in the draft tube facilitated communication between the less-well aerated liquid in the downcomer and the better aerated riser (Fig. 38). The reported improvements in mass transfer rates were undoubtedly obtained at the cost of a reduction of circulation velocity.

Another variant tested is the converging-diverging tube ALR (195), which can be seen in Figure 38. The authors report that the changes in cross-section in the riser produced an increase in the gas holdup.

One of the advantages of the directionality of flow in ALRs is the improved fluidization capacity. The strengthening of this advantage was the aim of another modification, the helical flow promoter, proposed by Gluz and Mer-chuk (170). The helical flow promoter causes the fluid to flow down in the downcomer (Fig. 39) in a helical pattern. The device comprises several fins or baffles, which have the effect of modifying the flow paths; instead of going in straight lines along the axis, the flow paths move along an helix. The baffles may be installed in a small section at almost any place along the riser or the downcomer, and the effect is perceived throughout the reactor. One of the best positions for the helical flow promoter is the top of the riser.

A helical flow is then generated in the downcomer to produce a swirl at the bottom and a corkscrew-like path in the riser. This has a strong potential for the culture of photo-synthetic microorganisms. The helical movement causes secondary flow, which leads to enhanced radial mixing, and therefore more homogeneous distribution of light and heat among liquid elements and suspended particles. With the helical flow promoter, it is thus more likely that all the elements of the fluid get the same exposure to light and heat exchange.

One of the most important characteristics of the helical flow promoter is the enhanced capacity for fluidizing solid particles. This is due to the swirls that develop at the bottom of the reactor. Thus, this modification is especially suited to processes operating with cells immobilized on a solid. The minimal gas flow rate for complete fluidization of solids in an ALR may be reduced drastically by the use of a helical flow promoter (170). In addition, the mass transfer rate to suspended solids may be enhanced up to 50%, because of the higher relative velocity between the particles and solids.

Another interesting aspect of the performance of helical flow promoters can be seen in Figure 39 (196). Mass transfer coefficients obtained in a bubble column, an ALR, and an ALR with HFP were compared, for water and two solutions of CMC: 0.2% and 0.04%. The corresponding effective viscosities for y = 50 s21 are 16 and 38 mPa s respectively. It is clearly seen that although with water there is a consistent difference in favor of the bubble column, the differences diminish as the CMC concentration increases, and for the highest concentration tested the difference is negligible. This implies that under these conditions the disadvantage of the HFP in mass transfer vanishes, whereas all the advantages mentioned before subsist.

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