Bubble Columns and Air-Lift Reactors

A second important category is non-stirred bioreactors, of which bubble columns and, to a lesser extent, the air-lift reactor are the most important members. The bubble column is driven by gas, and the demand for sufficient gas energy can be higher than in a stirred tank. Because of absence of rotating parts and simpler construction, the vessel sizes can be bigger more than 1000 m has been reported. 

This type of reactor operates well with low-viscosity mixtures, such as yeast and nonfilamentous bacteria. Citric acid and bakers yeast fermentations are well known in this respect. Also, beer fermentation can be considered as a special case, because the gas is process-generated CO2 and not air. At low viscosities, mixing can be much better than a stirred tank, while the mass transfer is similar or even better [4]. However, at higher viscosities, the performance of bubble columns will sharply fall, and so this type will lose functionality against the stirred tank. 

Also in these reactors, the cellular environment varies continuously, which needs to be assessed for successful development and scale-up. 

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Table 1 shows a number of aerobic fermentation systems which are schematically classified into (i) internal mechanical agitation reactors, (ii) external circulation reactors, and (Hi) bubble column and air-lift loop reactors. This classification is based on both agitation and aeration as it relates to oxygen supply. In this table, reactor 1 is often used at the industrial level and reactors (a)2, (b)2, (c)2, and (c)3, can be fitted with draught tubes to improve both mixing and oxygen supply efficiencies. 

Class 1 equipment are also called column-type equipment. Under this category, there are the various multiphase contactors. Gas-liquid contactors include bubble columns, packed bubble columns, internal-loop and external-loop air-lift reactors, sectionalized bubble columns, plate columns, and others. Solid-fluid (liquid or gas) contactors include static mixers, fixed beds, expanded beds, fluidized beds, transport reactors or contactors, and so forth. For instance, fixed-bed geometry is used in unit operations such as ion exchange, adsorptive and chromatographic separations, and drying and in catalytic reactors. Liquid-liquid contactors include spray columns, packed extraction... 

Bubble column, packed bubble column, sectionalized bubble column, plate column, external- and internal-loop air-lift reactors, static mixer, venturi scrubbers... 

In addition to low pressure drop, external-loop air-lift reactors have several advantages over bubble-column reactors. The former offer much flexibility in design in terms of height-to-diameter ratio, area ratio of downcomer to riser, sparger locations, and so on. Conditions can be manipulated... 

This case study on oxidation of sodium sulfide illustrates the design of a variety of gas-liquid reactors and compares their performances. Bubble column reactors are particularly attractive, as they offer advantages such as simplicity of construction and operation, but they suffer from such drawbacks as high pressure drop and backmixing in the liquid phase. To reduce the pressure drop, two modifications have been considered an external-loop air-lift reactor and a horizontal sparger reactor. Both result in substantial energy savings (because of low AP) under similar conditions of capacity and conversions in the gas and liquid phases. 

In the simplest type of bubble column, gas is dispersed at the bottom and bubbles are present throughout the reactor, as shown in Figure 7.10a. In a loop or air-lift reactor (Fig. 7.10b), gas is introduced beneath a central draft tube, and rising bubbles carry liquid upward. After most of the bubbles disengage, liquid flows downward in the annulus. The direction of flows could be reversed by feeding gas to the annulus. The columns may have internal coils for heat transfer and baffles to decrease axial mixing. 

Figure 1. Schematic representation of a bubble column (a), internal loop (b) and external loop (c) air lift reactors.

External and internal loop air-lifts and bubble column reactors containing a range of coalescing and non-Newtonian fluids, have been studied (52,53). It was shown that there are distinct differences in the characteristics of external and internal loop reactors (54). Overall, in this type of equipment... 

Gas-liquid systems of particular interest to the chemical engineer are encountered in bubble columns, spray columns, air lift, falling film, and stirred tank reactors. Usually the form of these reactors corresponds to that of vessels or columns. From the perspective of the chemical engineer, who is concerned with the conversion and selectivity of chemical transformations, it is of utmost importance that an intensive contact between a gas and a liquid be achieved and therefore very often one phase is continuous whereas the other is disperse. Therefore, the interfacial area and the size of the disperse phase elements constitute very important aspects of CFD modeling of these types of systems. 

A stainless steel single-stage concurrent bubble column air lift loop reactor, 15 cm in dieuneter, with a bubbling layer, 275 cm high, and a stainless steel porous plate were used for the cultivation of Hansenula polymorpha (1) and Escherichia coli (2) (Fig. 1)... 

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