Air-lift loop reactors

In contrast to the bnbble colnnm, the air-lift loop reactor has a hydrodynamic flow pattern which can be well described and which is controlled by the gas flow. 

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. 

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)... 

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... 

Loop reactor modified air lift, pump transport the air and fluid through the vessel. 

The air-lift consists of two pipes, intercoimected at top and bottom. In one of the pipes (the riser) air is sparged at the bottom. The air rises and escapes at the top. Therefore, nnder most circnmstances there is no air present in the other pipe (the downcomer). The density difference between riser and downcomer canses an intensive liquid circulation. Two designs can be used, i.e., the internal (Figure 11.8A) and the external loop reactor (Figure 11.8B). 

A novel bioreactor, especially designed to work with two liquid phases, is the liquid-impelled loop reactor (Figme 11.10), in which the advantages of air lifts and... 

An example of integration of part of the downstream processing in the actual bioreactor is two-liquid-phase biocatalysis (see above), in which the organic-solvent phase is used as extractant for the product. The liquid-impelled loop reactor (Fig. 7.3) is specifically designed for this purpose. It is based on the well-known air-lift principle, but instead of air, a water-immiscible, heavier or lighter organic solvent is injected. 

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... 

FIGURE 11.23 External-loop air-lift reactor (a) riser sparging, (b) downcomer sparging. 

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... 

When the total reactor volume is small (< 5 to 10 m ), an air-lift reactor can be used in the form of an internal loop as shown in Eigure 11.24. 

As indicated in Section 11.4.2.1.4, the energy requirement of a bubble column can be reduced with an external-loop air-lift reactor (Figure 11.23a). As an illustration, consider case 2 in Table CS7.2. This case was shown to be optimum on the basis of bottom pressure or the pressure at which the gas phase enters the column. Furthermore, there is no complication of desorption. 

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. 

Flg.1. Measurements during the cultivation of recombinant E. coli K-12 MF cells in a 60-1 working volume air lift tower loop reactor without gene expression using SE9 antifoam agent (AFA) [50,51]. a Variation in the mean bubble velocity. Bubble velocity measured in situ by an ultrasound Doppler technique. AFA added to the medium, b Variation in the specific interfacial area (m ) measured in situ by an ultrasound technique... 

Choice of reactor configuration aerated STR bubble reactors jet loop or air lift fluidized... 

Air lift tower loop reactors are often used to carry out cultivation of microbial cells (2,3,4). In these reactors the medium is recirculated from the top of the tower to its bottom. 

Four different tower loop reactor types were used for the investigations Two concurrent air lift tower loop systems (a single-stage and a ten-stage reactor) and two countercurrent tower loop systems (a single-stage and a three-stage reactor). 

Fig,ISchematic view of the single-stage concurrent (air lift) tower loop reactor (Reactor A )... 

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