External-loop air-lift reactor

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... 

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... 

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. 

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

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... 

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

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). 

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. 

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