Biological bubble column

The main part of the report describes the results of systematic investigations into the hydrodynamic stress on particles in stirred tanks, reactors with dominating boundary-layer flow, shake flasks, viscosimeters, bubble columns and gas-operated loop reactors. These results for model and biological particle systems permit fundamental conclusions on particle stress and the dimensions and selection of suitable bioreactors according to the criterion of particle stress. 

Special reactors are required to conduct biochemical reactions for the transformation and production of chemical and biological substances involving the use of biocatalysts (enzymes, immobilised enzymes, microorganisms, plant and animal cells). These bioreactors have to be designed so that the enzymes or living organisms can be used under defined, optimal conditions. The bioreactors which are mainly used on laboratory scale and industrially are roller bottles, shake flasks, stirred tanks and bubble columns (see Table 1). 

The term three-phase fluidization, in this chapter, is taken as a system consisting of a gas, liquid, and solid phase, wherein the solid phase is in a non-stationary state, and includes three-phase slurry bubble columns, three-phase fluidized beds, and three-phase flotation columns, but excludes three-phase fixed bed systems. The individual phases in three-phase fluidization systems can be reactants, products, catalysts, or inert. For example, in the hydrotreating of light gas oils, the solid phase is catalyst, and the liquid and gas phases are either reactants or products in the bleaching of paper pulp, the solid phase is both reactant and product, and the gas phase is a reactant while the liquid phase is inert in anaerobic fermentation, the gas phase results from the biological activity, the liquid phase is product, and the solid is either a biological carrier or the microorganism itself. 

The 1980 s and the early 1990 s have seen the blossoming development of the biotechnology field. Three-phase fluidized bed bioreactors have become an essential element in the commercialization of processes to yield products and treat wastewater via biological mechanisms. Fluidized bed bioreactors have been applied in the areas of wastewater treatment, discussed previously, fermentation, and cell culture. The large scale application of three-phase fluidized bed or slurry bubble column fermen-tors are represented by ethanol production in a 10,000 liter fermentor (Samejima et al., 1984), penicillin production in a 200 liter fermentor (Endo et al., 1986), and the production of monoclonal antibodies in a 1,000 liter slurry bubble column bioreactor (Birch et al., 1985). Fan (1989) provides a complete review of biological applications of three-phase fluidized beds up to 1989. Part II of this chapter covers the recent developments in three-phase fluidized bed bioreactor technology. 

Hano, T., Matsumoto, M., Kuribayashi, K., and Hatate, Y., Biological Nitrogen Removal in a Bubble Column with a Draught Tube, Chem. Eng. Sci., 41 3131 (1992)... 

In this pi-space, measurements were evaluated which were performed in a bench-scale flotation cell (Fig. 3 a) of D = 0.6 m. The flotation cell input consisted of biologically purified waste water, containing = 3 g TS/1 activated sludge (TS - total solids), which was processed in the 30 m high bubble columns, the so-called Tower Biology of BAYER AG/Leverkusen, Germany. 

Handa-Corrigan A, Emery EN Spier RE (1987) On the evaluation of gas-liquid interfacial effects on the hybridoma viability in bubble column reactors. Developments in Biological Standardization 66 241-253. 

Direct bubbling with a mixture of air and C02 is used in many closed photobioreactors. The mass transfer capacity (as measured by the volumetric oxygen transfer coefficient, kLa) increases with decrease in the bubble diameters (achieved by using spargers with very small diameter pores) and with increase in the aeration rate. In vertical columns (Fig. 4), the gas transfer from the gas bubbles to the medium is high since the gas bubbles remain submerged in the broth until they exit the reactor. Thus bubble column bioreactors have been used for many biological processes. 

Sintered glass or ceramic plates and perforated metal plates are classical gas spargers (dispersers) for bubble columns. Static mixers and nozzles became available with the emergence of biological waste water purification. They all realize gas... 

Table 1. Values of for biological systems and nutrient salt solution in a bubble column at WsG = 4 cm s". The reference is the nutrient salt solution with Desmophen which has the lowest kLa value [40]...

The exact reactor choice is highly dependent on the type of reaction required by the process. Since biological applications are not very fast, the choice is going to be more dependent on the microorganism s environmental requirements. For biological applications, fixed bed reactors have traditionally been used for shear-sensitive microbes and cells (especially mammalian). In contrast, shear-resistive strains would be expected to experience better results in a bubble column or airlift reactor since these devices operate at much higher gas and liquid flow rates. 

Owing to slight density differences between fermentation liquid and biomass the particles have the tendency to settle. Thus a biomass concentration profile along the tower may result. The pertinent model to account for biomass concentration profiles is the sedimentation-dispersion model (74,75). This model involves two parameters, neimely, the solid dispersion coefficient 3 and the mean settling velocity Us of the biomass particles in the swarm. Both parameters were determined by Kato et al. (75) in bubble columns for glass beads of 75 and 163 urn diameter. The authors presented their results by empirical correlations for both Eg and ug. Until other data for smaller density differences are available the application of the correlations of Kato and coworlcers is recommended for biological systems also. It should be pointed out that the solid phase dispersion coefficient Eg almost completely agrees with El i.e. the liquid phase dispersion coefficient. ... 

The mechanisms by which freely rising bubbles interact with each other in relatively low-viscosity liquids and, specifically, how they approach, contact, and coalesce or break up are important aspects of multi-phase flow. Coalescence and breakup can control the interfacial area and mass transfer rate in bubble columns and gas-sparged chemical and biological reactors. Bubble interaction is fundamental in two-phase flow instability that plagues boilers and oil and gas wells. But bubble interaction remains a relatively mysterious area. 

A cursory inspection of Table 1 shows that in most instances polymer solutions (carboxymethyl cellulose, polyacrylamide, xanthan) have been used to mimic the non-Newtonian features of biological systems, encompassing wide ranges of shear thinning conditions (though viscoelastic effect have been studied only scantily) in bubble columns up to as large as 760mm in diameter. 

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