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Figure 9. Results of the mathematical model for a tapered fluidized-bed producing ethanol from glucose with Zymomonas mobilis cells immobilized in carrageenan beads. Internal concentration profiles with the dimensionless fermenter height: experimental values (points) and values predicted by the model (solid lines). Dotted lines represent the calculated CO2 flow rate produced by fermentation. Source: From Ref. 10.

operation, hydrodynamic behavior and evolution of the immobilized biocatalyst), and the time required in any industrial process to implement any change in technology. Some specific examples from different biotechnological processes illustrate the feasibility of the fluidized bed at a pilot plant and on the industrial scale. The area of waste-

water treatment provides examples of high-volume operation and has made use of three-phase fluidized-bed bio-reactors, as well as other related designs (such as the up-flow anaerobic sludge blanket reactor), to a significant extent. The wastewater from the baker's yeast industry, particularly from the Gist-brocades company in Delft,

Netherlands, has been treated with two anaerobic fluidized-bed reactors in series, each one 21m high and with 390 m3 total volume, operating with bacterial cells immobilized by attachment onto 0.25- to 0.5-mm-diameter sand particles, a support that is typical for other fluidized-bed reactors applied to anaerobic and aerobic wastewater treatment (34). The volume of the reactor employed for the disengagement of the CH4 gas obtained was important, and the fluidized-bed volume was 215 m3. The carbon-oxygen demand (COD) conversion of these reactors is 22 kg/(m3 day), which is 65% of the COD in the load. A first set of these reactors started operation in 1984, and a second set in 1986. A picture of these industrial units is given in Figure 10. More recently, data from two full-scale an aerobic three-phase fluidized-bed plants have also been reported (33). A 700-m3-total-volume reactor (500-m3 working volume) has been installed for wastewater treatment at a sugar beet factory of United Northern Sugar Factories Co. in Clauen, Germany, using cells immobilized onto pumice and having COD conversion of 20 kg/(m3 day). A 734-m3-total-volume multistage reactor (650- to 700-m3 working volume) has been installed for wastewater treatment at a yeast factory of Hamburg Co., in Hamburg, Germany, using free cells and a new principle that allows control, in each stage, of the ratio of the recirculated gas amount with respect to the rising gas flow. This enables optimization of the hydrodynamic conditions in each stage, according to the reactor loading. Nitrification of the effluent from a municipal wastewater treatment plant in order to eliminate ammonium salts has also been carried out in large-scale fluidized-bed reactors (35) with a cross-sectional area of 2.2 m2 and a height of 5 m, using 5 tons of sand particles 0.4-0.6 mm in diameter as support particles for cell biofilm development. The reported results show the feasibility of the system, from both the technical and economic points of view.

Ethanol fermentation has been studied intensively in fluidized-bed reactors and brought up to the pilot-plant level in several cases. A pilot plant with a volume of 1 m3, using Saccharomyces cerevisiae cells immobilized in car-rageenan beads has been successfully operated for half a year (36). As an alternative to yeast, the bacteria Zymo-monas mobilis can also be used. Very promising results have been reported on the operation of a series of two pilot reactors of 55 L each, under nonsterile conditions (37). The support for the immobilization in this case was a macro-porous glass particle, Siran, that allowed cell attachment to the internal surface of the pores. After an initial period of sterile colonization of the beads in a conventional stirred tank, the particles, with a high Z. mobilis cell concentration, can be transferred to the fluidized bed, which then can be operated at high dilution rates under nonsterile conditions because all the possible contaminants in the feed (a solution of hydrolyzed B-starch) are washed out from the reactor. Complete conversion of 120 g/L sugar concentration with a 4.25-h residence time can be consistently obtained, with ethanol productivities of 18 g/(L h), calculated with the total reactor volume as a basis. A complete technical and economic evaluation of the use of fluidized-bed reactors for the production of ethanol with Z. mobilis has shown promising features (38).

Other examples of the use of fluidized-bed bioreactors on the pilot scale in fermentation processes include the production of alcohol-free beer with immobilized yeast in a 50-L pilot-plant fluidized-bed reactor (39) and the production of penicillin using Penicillium chrysogenum immobilized in urethane particles in a 160-L pilot-plant reactor (40). Penicillium chrysogenum cells have also been used in different studies with fluidized-bed reactors with celite as the support material (41). Also, the area of animal cell culture provides a number of examples of the application of fluidized-bed technology, taking into account that smaller production volumes are usually required in this case. The use of immobilized cells is particularly required for those cells that are anchorage dependent, and a variety of porous carriers and beads have been developed for this purpose. The production of a human anti-HIV-1 antibody using recombinant Chinese hamster ovary cells grown onto a macroporous polyethylene carrier has been carried out in an 84-L-volume pilot-scale fluidized-bed novel reactor equipped with a low shear stress internal impeller for the recirculation of the liquid (42).

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