Spray-columns countercurrent

The earliest large-scale continuous industrial extraction equipment consisted of mixer—settlers and open-spray columns. The vertical stacking of a series of mixer—settlers was a feature of a patented column in 1935 (96) in which countercurrent flow occurred because of density difference between the phases, avoiding the necessity for interstage pumping. This was a precursor of the agitated column contactors which have been developed and commercialized since the late 1940s. There are several texts (1,2,6,97—98) and reviews (99—100) available that describe the various types of extractors. 

Figure 1. Slurry reactors classified by the contacting pattern and mechanical devices (a) slurry (bubble) column (b) countercurrent column (c) co-current upflow (d) co-current downflow (e) stirred vessel (C) draft tube reactor (g) tray column (h) rotating disc or multi-agitated column reactor (i) three-phase spray column — liquid flow —> gas flow.

Spray contactors ate particularly important for the absorption of imparities from large volumes of Hue gas where tow pressure drop is of key importance. They are used where materials in the liquid phase (e.g., particles of limestone) or in the gas phase (e.g., droplets of tar) may cause plugging of packing or trays. Other important applications of spray contactors (which are outside the scope of this discussion) include particulate removal and hot gas quenching. When used for absorption, spray devices are not applicable to difficult separations and geueratty are limited to about Four transfer units even with countercurrent spray column designs. The tow efficiency of spray columns is believed to be due to entrainment of droplets in the gas and beckmixing of the gas induced by the sprays. 

Some degree of superheating is required before nucleation sets in in the drop, depending on drop size as well as the physical and chemical purity of the fluids. Similar phenomena have also been observed in countercurrent spray column studies (S9), where the temperature of the continuous phase can be lowered to the desired value only after some evaporation starts. As is to be expected, the time required for complete evaporation of the drop is inversely proportional to the temperature driving-force. A similar relationship exists for the length of the evaporation path of single drops. This, however, may not be directly extended to populations of drops, where the onset of nucleation is not simultaneous but rather depends on the dispersed phase flow rate, holdup, and degree of turbulenee of the system. 

At low holdups, longitudinal dispersion due to continuous-phase velocity profiles controls the amount of mixing in the countercurrent spray column whereas at higher holdups the velocity profile flattens, and the shed-wake mechanism controls. Above holdups of 0.24, the temperature jump ratio is linearly proportional to the dispersed-to-continuous-phase flow ratio, and all mixing is caused by shed wakes into the bulk water and coalescence of drops. As column size decreases, it approaches the characteristics of a perfect mixer, and the jump ratio approaches unity (as compared with the value of zero for true countercurrent flow). It is interesting to note that changing the inlet temperature of dispersed phase by about 55°F hardly affected the jump ratio, probably due to the balancing effects of reduced viscosities and a decrease of drop diameter. 

Multi-stage extraction is used to achieve a higher efficiency of separation, in which the product is almost completely removed from the raffinate. The solvent is split up into several portions and fed to a series of mixers and settlers. The disadvantage of this method is the need to use large volumes of solvent. A more complicated system, called countercurrent, multi-stage extraction, uses a series of mixers and settlers arranged as before, but the feed liquid and pure solvent are passed through the system in opposite directions, that is counter-currently. Continuous countercurrent operation may be carried out by means of spray columns, packed columns (similar to those used in distillation), plate columns, or, sometimes, bubble cap columns. 

Several constructions are available for gas-liquid reactors because of the large number of different application areas. Some of the main reactor types are illustrated in Figure 7.2 [5]. Spray columns, wetted wall columns, packed columns, and plate columns are mainly used for absorption processes. The gas concentrations are low in the case of absorption processes, and to enhance the absorption process, a large interfacial contact area between the gas and the liquid is important. This area is obtained in the previously mentioned reactor types. These column reactors usually operate in a countercurrent mode. The countercurrent operation is the optimal operating mode, because at the gas outlet where the gaseous component concentration is the lowest, the gas comes into contact with a fresh absorption solution. The low concentration of the gaseous component can then partly be compensated by the high concentration of the liquid component. 

Types of equipment classified as spray contactors include countercurrent spray columns, venturi scrubbers, ejectors, cyclone scrubbers, and spray dryers. The use of spray dryers as absorbers is of particular interest in the removal of sulfur dioxide from hot flue gas (see Chapter 7). 

The use of physical absorption to remove V(X)s from exhaust air streams has found limited application. In Germany, where the limits for solvents in air were reduced to 20 mg/m as of Jan. 1, 1989, one plant has been built to absorb 10-200 g/m dichloromethane from a 30 m /h air stream at ambient temperature using tetraethylene glycol dimethyl ether as the absorbent. Absorption is accomplished in a countercurrent spray column. Desorption occurs in an electrically heated column at 100°-130°C and 80-100 millibar pressure. The operation requires 11 kW power and 5 m /h cooling water (Anon., 1989). 

Both spray columns and packed columns are seriously compromised by flooding. In flooding, the feed and solvent streams do not flow evenly and countercurrently past each other, but both simply gush out one end of the column. Flooding is a more serious risk in extraction than in absorption because of the smaller density difference between the two fluids. This density difference is typically less than 0.1 g/cm, about 10 times less than that common in gas absorption. As a result, countercurrent flows that are routine in gas absorption will be difficult to realize in liquid-liquid extraction. 

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