Spray contactors

Humidification processes also occur in spray contactors often used to scmb minor components from a gas stream. Here the gas passes through successive sprays of hquid. The hquid is often water but may be specially compounded to enhance absorption of the component to be removed. 

Spray Towers as Direct Contact Condensers Similarly spray contactors can be highly effective for direct contact condensers, which are also liquid-limited. The high transfer rate in the initial formation of sprays is the key. Kunesh [Ind. Engr. Chem. Res., 32, 2387-2389 (1993)f reported a 97 percent approach to equilibrium in a hydrocarbon system in the 6-in space below the discharge of a row of hollow cone spray nozzles. 

In an actual plant good contact must be made between the large volumes of flue gas and the relatively small melt stream. However, power plant integration requirements make it important to impose as small a pressure drop as possible on the gas stream. Because of this, a spray contactor has been selected as the molten salt scrubber concept. The spray contactor uses spray nozzles to break up the melt into small droplets for good gas-liquid contact and a very efficient mist eliminator to prevent the gas stream from carrying melt mist out of the scrubber. 

Tray Columns Packed Columns (Random) Spray Contactors... 

The secood step is sdectiou of the type of contactor lo be used. This may be accomplished ou the hasis of system requirements and experience factors such as those listed in Table 6.1-3 or. again, na economic comparison may be required to evaluate several alternatives. This step includes at least a preliminary selection of the specific tray, packing, or spray contactor configuration to be employed. 

To determine the required size of an absorption or stripping nrtl, it is necessary to know not only the equilibrium soluhility of the solute in the solvent and the material balance atound the column bas also the rate at which solute is transferred from one phase to the other within the tower. This rale directly affects the volume of packing needed in a packed tower, the degree of dispersion requited in a spray contactor, and (somewhat less directly) the number of trays required in a nay tower. The last effect occurs as a result of the influence of mass transfer rms on tray efficiency which is discussed in a later section. Because of its direct effect ou packed tower design and the importance of this type of contactor in absoiption. this discussion of mass transfer is aimed primarily at the packed tower case. A more detailed review of mass transfer theoty is given in Chapter 2. 

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. 

It is not unusuel for exhaust gas scrubber Systems to incorporate more than cue type of spray contactor. In fact, combinations with trays and packed sections are used extensively. Figure 6.4 6 shows diagrams for four typical scrubber systems used for removing HF and SiF, from phosphoric acid plants. The following systems are illustrated. 

Attempts have been made to develop design equations for preformed spray contactors on the basis of a more foudemental approach by considering muss transfer to individual drops. However, this requires a characterization of the drop size distribution, time of flight, and other factors. The approach is useful when test data are not available bat cannot be expected to provide accurate results for commercial equipment because the effects of drop agglomeration, entrainment, wall contact, gas recirculation, drop velocity changes, and other factors are very difficult to model analytically. 

In this chapter, the theories for gas absorption plus reaction are presented first for relatively slow reactions and then for fast reactions and for instantaneous reactions. Performance data and scaleup criteria for several types of gas-liquid reactors are then reviewed. For a given reaction, the intrinsic kinetics of the liquid-phase reaction are the same for all types of reactors, including stirred tanks, packed columns, bubble columns, and spray contactors, but the mass transfer coeffieients differ greatly, and the... 

When carrying out a gas-liquid reaction, the gas may be dispersed in the liquid, as in bubble-column reactors or stirred tanks, or the gas phase may be continuous, as in spray contactors or trickle-bed reactors. The fundamental kinetics are independent of the reactor type, but the reaction rate per unit volume and the selectivity may differ because of differences in surface area, mass transfer coefficient, and extent of mixing. In the following sections, gas holdup and mass transfer correlations and other performance data for gas liquid reactors are reviewed and some problems of scaleup are discussed. 

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