Multistage contactors

In an ideal stage, the gas and liquid are brought imo intimate contact, attain equilibrium, and then ate separated. In most tray columns, where the liquid flows horizontally across each tray while the gas flows 

A similar efficiency can be defined for the liquid phase ml however, the vapor-phase efficiency is used more widely in column design calculations. 

Any consistent set of units may be used to define the concentrations of solute in the liquid and gas phases however, the same units must be used for both the equilibrium and operating lines. For purposes of the illustrated example, the concentration units commonly used by the U.S. natural gas industiy are enmloyed. The feed gas is assumed to be saturated with water at 90°F (32.2 C) and 500 psia (3.447 X l(r k/Pa ) (i.e., it contains 80 lb , HjO/lO scf of gas). It is desired to produce a gas containing no more than 10 lb H2O/10 scf [dew point 28 F (-2.2 Q] using triethylene glycol absorbent which has been regenerated to a concentration of 98.5% (0.0132 lb H20/lb glycol) in a distillation column. To fix the 

FIGURE 6.1-9 Temperature and composition profiles for absorber empitqring a reactive solvent to purify a gas containing low concentrations of HjS and CXDj. (From Kohl and Riesenfeld, copyright 1985 by Golf Publishing Co., Houston, TX. All rights reserved us with permission.) 

Experience with absorbers of this type has indicated that actual trays have a Muiphree vapor-phase efficiency on the order of 40%. This can be taken into account by drawing a pseudoequilibrium curve 40% of the distance (vertically) from the operating line to the equilibrium curve and stepping off the trays as before. Results indicate that six actual trays will be required. Heat effects, of course, must be consisted in establishing the final position of the equilibrium curve but, in this case, the effects are negligible because the large volume of gas carries away the heat of reaction with only a very small temperature increase. 

The graphical technique for earimatmg die number of trays illustrated in Fig, 6.2-1 can be used even if die operating nad equilibrium lines are curved. For the special cese where both are straight, an analytical soluijou has been derived by Kremser1 and improved by Souders and Brown.1 Their equation is based on 

FIGURE 6.2-1 Graphical deletraiiulion of number of trays for gas dehydration column employing tri-ethylene glycol. 

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Fig. 5. Arrangement of multistage contactors where F = feed flow (A-rich), R = raffinate flow, 5 = solvent flow (B-rich), and E = extract flow, (a)...

Glycerol, Glycol, and Other Polyhydnc Alcohols. These alcohols (Class 3, regenerative) are widely used to dry gases (20,21). However, they can only produce dew points in the range of —15 to 0°C (see Antifreezes and deicing fluids). They have somewhat lower capacity than sulfuric acid (Fig. 2) but are effective when either injected or employed in a multistage contactor to achieve dew point depression. 

This section discusses the design and operation of a single stage (Figure 10.2), followed by a multistage contactor (Figure 10.10). Other design and operational considerations are also reviewed. 

This section concerns other design and operational matters for both single- and multistage contactors. 

Paca et al. (1976) showed that for a 0.51 m i.d. multistage contactor with three impellers and for a non-Newtonian liquid with an apparent viscosity range of 0.001 -0.05 Pa S, the mixing time decreases with an increase in gas velocity. 

The studies of Juvekar (1976) and Ramanarayanan and Sharma (1982) with a 0.2 m i.d. multistage contactor with d,/dx = 0.5 showed that both aL and kLaL are independent of superficial gas velocity above the critical speed of agitation, and they vary linearly with impeller speed. Both aL and kLaL were independent of the number of stages under otherwise identical conditions of dfdj, N, ug, and gas-liquid system. Thus, the results for a single stage are applicable to multistage systems. This conclusion needs to be verified for larger-diameter columns. 

In large-size mechanically-agitated multistage contactors, the speed of the agitator is kept at a relatively low level and horizontal baffles are provided. Here, RTD in the gas and liquid phases may show different behavior compared to a small-scale unit. 

The domain of the search for an improved separation process was defined by certain criteria (a) isotopic fractionation should be achieved by means of a two-phase, chemical exchange reaction which was amenable to countercurrent operation in a multistage contactor at ambient temperature and pressure (b) the single-stage isotopic fractionation factor for the reaction should be appreciably larger than that for the distillation of Me20 BF3 (c) the molecular species in each process stream should be thermally refluxable—i,e, convertible from one species to the other by the addition or removal of heat alone (d) process materials should be more stable with respect to irreversible decomposition than those used in the (CH3)20 process and (e) the chemical form of the product should permit a ready, quantitative conversion of the separated isotopes to the elemental state. 

With ferrous ion or cathodic reduction, conversion of plutonium from Pu to Pu is so rapid that back extraction of plutonium to the aqueous phase and reduction there to Pu can be carried out simultaneously in a single multistage contactor. With tetravalent uranium, reduction of plutonium is slower, so that additional contactor volume is desirable to complete back extraction. With hydroxylamine, reduction of plutonium is so much slower that it is preferable first to return both uranium and plutonium to the aqueous phase by stripping with dilute nitric acid and then to reduce the plutonium in equipment providing sufficient residence time for reduction to proceed to completion. Finally, the uranium is reextracted by TBP. 

Depending on the relative rates of mass transfer and the chemical reaction (as well as many process-specific conditions), the appropriate equipment for gas-liquid systems may conform to many different geometries. The conventional types include bubble columns, spray columns, multistage contactors (sieve or bubble plate, mechanically agitated etc.) and packed towers whose design aspects will be discusses in the two published volumes of this ASl. 

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