Equilibrium extractor

What are the flow rates and compositions ofthe products, and how many equilibrium extractor stages are required to meet the solute reduction specification if the feed rate is 90 kmol/h and the solvent rate is 50 kmol/h The phase equilibrium diagram in Figure 11.4 may be used for this system. 

Several pairs of reactants are possible. The aldol reaction between two molecules of the same aldehyde is generally quite successful, since the equilibrium lies far to the right. For the analogous reaction of ketones, the equilibrium lies to the left, and the reaction conditions have to be adjusted properly in order to achieve satisfactory yields (e.g. by using a Soxhlet extractor). 

Reaction between Two Molecules of the Same Ketone. In this case, the equilibrium lies well to the left and the reaction is feasible only if the equilibrium can be shifted. This can often be done by allowing the reaction to proceed in a Soxhiet extractor (e.g., see OS I, 199). Two molecules of the same ketone can also be condensed without a Soxhiet extractor, by treatment with basic Al203. Unsymmetrical ketones condense on the side that has more hydrogens. (An exception is butanone, which reacts at the CH2 group with acid catalysts, though with basic catalysts, it too reacts at the CH3 group.)... 

Kl is the mass transfer coefficient for the L phase (m/s), a is the interfacial area per unit volume (m /m ), referred to the total liquid volume of the extractor, V is the total holdup of the tank, and is equal to (Vl+Vq). X is the equilibrium concentration, corresponding to concentration Y, given by... 

An alternative approach to the solution of the system dynamic equations, is by the natural cause and effect mass transfer process as formulated, within the individual phase balance equations. This follows the general approach, favoured by Franks (1967), since the extractor is now no longer constrained to operate at equilibrium conditions, but achieves this eventual state as a natural consequence of the relative effects of solute accumulation, solute flow in, solute flow out and mass transfer dynamics. 

A wide variety of extraction column forms are used in solvent extraction applications and many of these, such as rotary-disc contactors (RDC), Oldshue-Rushton columns, and sieve-plate column extractors, have rather distinct compartments and a geometry, which lends itself to an analysis of column performance in terms of a stagewise model. As the compositions of the phases do not come to equilibrium at any stage, however, the behaviour of the column is therefore basically differential in nature. 

The modelling approach to multistage countercurrent equilibrium extraction cascades, based on a mass transfer rate term as shown in Sec. 1.4, can therefore usefully be applied to such types of extractor column. The magnitude of the... 

The modelling approach to multistage countercurrent equilibrium extraction cascades, based on a mass transfer rate term as shown in Section 1.4, can therefore usefully be applied to such types of extractor column. The magnitude of the mass transfer capacity coefficient term, now used in the model equations, must however be a realistic value corresponding to the hydrodynamic conditions, actually existing within the column and, of course, will be substantially less than that leading to an equilibrium condition. 

EQEX - Simple Equilibrium Stage Extractor System... 

Weight fraction of volatile component at interface Equilibrium weight fraction of volatile component Weight fraction of volatile component at inlet to extraction section Weight fraction of volatile component at exit of extractor... 

In general, the reactions in the addition phase of both the base- and acid-catalyzed mechanisms are reversible. The equilibrium constant for addition is usually unfavorable for acyclic ketones. The equilibrium constant for the dehydration phase is usually favorable, because of the conjugated a,/ -unsaturated carbonyl system that is formed. When the reaction conditions are sufficiently vigorous to cause dehydration, the overall reaction will go to completion, even if the equilibrium constant for the addition step is unfavorable. Entry 3 in Scheme 2.1 illustrates a clever way of overcoming the unfavorable equilibrium of the addition step. The basic catalyst is contained in a separate compartment of a Soxhlet extractor. Acetone is repeatedly passed over the basic catalyst by distillation and then returns to the reaction flask. The concentration of the addition product builds up in the reaction flask as the more volatile acetone distills preferentially. Because there is no catalyst in the reaction flask, the adduct remains stable. 

We refer to Fig. 6.7-1. Reaching once equilibrium between the supercritical fluid SCF1 and the feed in the extractor El is enough for separation. By changing pressure and temperature the produced extract EX1 and raffinate R1 concentrations can be varied following the ternary phase equilibrium. The supercritical solvent-to-feed flow rate ratio affects the amounts of products obtained from a given feed. The apparatus required to apply this method are a normal stirred reactor, where contact of the two phases takes place, followed by a separator eliminating the extract from the extraction gas, which is recycled back to the extractor. 

Such an assembly of mixing and separating equipment is represented in Figure 14.3(a), and more schematically in Figure 14.3(b). In the laboratory, the performance of a continuous countercurrent extractor can be simulated with a series of batch operations in separatory funnels, as in Figure 14.3(c). As the number of operations increases horizontally, the terminal concentrations E1 and R3 approach asymptotically those obtained in continuous equipment. Various kinds of more sophisticated continuous equipment also are widely used in laboratories some are described by Lo et at. (1983, pp. 497-506). Laboratory work is of particular importance for complex mixtures whose equilibrium relations are not known and for which stage requirements cannot be calculated. 

Inlerfacial Mass-Transfer Coefficients. Whereas equilibrium relationships arc importanl in determining the ultimate degree of extraction attainable, in practice the rate ol extraction is of equal importance. Equilibrium is approached asymptotically with increasing contacl lime in a hatch e.xlrac-lion. In conlinuous extractors the approach to equilibrium is determined primarily by the residence Lime, defined as the volume of the phase contact region divided by the volume flow rjte of the phases. 

With a given system of constant K, a decrease of E/R increases y, but decreases y. Therefore, an optimum operation condition has to be determined based on the various factors affecting the economy of the separation processes, such as the value of products, equipment costs, and operating costs. It is interesting to note that Y depends on the ratio E/R, but not on the values of E and R. Can we increase E and R indefinitely to maintain the same y as long as E/R is constant for a continuous extractor The answer is "no." We should remember that Eq. (10.13) is based on the assumption that the extractor is in equilibrium. Therefore, the increase of E and R will shorten the residence time as a result, the extractor cannot be operated in equilibrium and y will decrease. 

Point T, however, will separate into two liquid phases. If the extractor consists of one true equilibrium stage, this mixture (point T) separates into two phases represented by points R and E. 

This phenomenon has been applied to many an operating plant extractor, even after the plant has been designed for other criteria. Indeed, some plants have added parallel extractors to achieve this lower solute concentration operating practice. One major reason so many operators pursue this lower solute concentration is that as the solute concentration increases above 25 or 30%, the equilibrium of both the solvent in the feed-raffinate and the feed-raffinate in the solvent increases. This increased switching of the feed and solvent means the liquid phases become less immiscible and therefore a very poor extractor separation is made. Remember that as the plait point P is approached, only one liquid... 

The subscript 1 and 2 denote the inlet and outlet of the liquid-phase streams. Take note that these subscripts denote the equilibrium curve points (points 1 and 5 in Fig. 7.2). Throughout this book the extractor is assumed to be countercurrent flowing raffinate and solvent streams. Y is therefore the weight percentage of solute in the inlet solvent stream that is in equilibrium with the Xi stream. Xi denotes the weight percentage of the solute in the raffinate stream leaving the extractor. The... 

Equations (7.11) and (7.12) may be used to determine the pounds of solute at any point on the respective equilibrium curve, including the entrance and exit points of the extractor. KD may also be calculated from ... 

Equation (7.13) is one of the more useful equations in field operation data gathering. For any actual extraction process over time, numerous Kd values can be calculated. These are excellent and reliable data to use for plant expansions or new extractor units, or for rating any similar existing extractor. Another way in which KD equilibrium data can be obtained is through actual operating data. 

Step 2. Select solvent inlet purity. The solvent (benzene) enters the extractor at a point that must be reasonably true and obtainable for the equilibrium curve. Please note that we cannot cross the equilibrium curve and that the McCabe-Thiele steps will be to the right of the curve. The left side would be for points of miscible liquids, not two-phase. Since the water is to contain 1.0% solute at exit, a good estimate for the solvent entering is 0.5 wt % acid. Reviewing the Fig. 7.2 equilibrium curve, in order to meet this treated water 1.0% acid specification, the concentration of acid in the inlet counterflow solvent must be below 1.0%. We may assume the solvent has been nearly stripped free of acetic acid. Thus, 0.5% acid in the inlet solvent (benzene) is a reasonable assumption. Plotting the McCabe-Thiele equilibrium steps will show that this is a good and reasonable estimate to achieve the treated water specification of 1.0% acid remainder. Then,... 

Now notice how small an equilibrium stage step was made for the third step in Fig. 7.6. It is very obvious that the next stage step will be even smaller, yet will require as much of the extractor column physical size as the first equilibrium stage step required. Thus, for a critical specification of 1.0% or less as stated here, a fourth stage is required. This fourth stage ensures that the specification will be met. 

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