Single-stage extraction

In a single stage extraction process the solution to be extracted, i. e., the feed F consisting of carrier substance T and the key component S (transfer component), is intensively mixed in the mixing section of the extraction device with the total amount of solvent After the distribution equilibrium is reached, the extract and raffinate phases leave the separation and settler zone of the extractor. The extract phase mainly consists 

Sufficient loading capacity of key component per weight solvent (favorable distribution equilibrium, with chemically active solvents favorable distribution and reaction equilibrium). 

As low a miscibility as possible of the extraction solvent with the carrier. 

Easy and high separation of solvent from the extract phase (large difference of boiling points of solvent and key component, no formation of an azeotropic mixture). 

Large density difference of heavy and light phase to simplify phase separation and to avoid emulsion forming. 

Fig 10.3 Flow diagram for the single-stage mixer settler. 

If inlet conditions (F, S, xF, and ys) are known, we have four unknown variables (R, E, x, and y). However, since we have only three equations, we need additional information to be able to solve for the unknown variables, which are the equilibrium data of the ternary system solute, solvent, and diluent, which are usually described graphically in triangular coordinates (Treybal, 1980). 

In bioseparations, the solute concentration in the feed is usually low, therefore, the changes of the extract and the raffinate streams are negligible. We can assume that F = R and S = E. In that case, we have only two unknown variables, x and y, so we can solve it to obtain, 

Another parameter that is important in extraction is the recoverability of solute. Even though Y is high, if the recoverability of solute is low, the extractor cannot be regarded as an efficient unit. The recoverability yean be defined as the fraction of the solute recovered by an extractor as 

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. 

Calculate flows and compositions for single-stage extraction. 

Analyze countercurrent extraction cascades without reflux. 

The quantities of extract and raffinate can be computed from the lever-arm rule, or by the materia] balance for C  

This nomenclature is shown with Example 14.3. On the triangular diagram, the proportions of feed and solvent locate the mix point 

The extract E and raffinate R are located on opposite ends of the tieline that goes through M. 

In this process the feed and subsequently the raffinate are treated in successive stages with fresh solvent. The sketch is with Example 14.3. With a fixed overall amount of solvent the most efficient process is with equal solvent flow to each stage. The solution of Example 14.3 shows that crosscurrent two stage operation is superior to one stage with the same total amount of solvent. 

The distribution of a solute between two mutually immiscible solvents can be represented by the simple equation, 

When K is not truly constant, some kind of mean value may be applicable, for instance, a geometric mean, or the performance of the extraction battery may be calculated stage by stage with a different value of K for each. The material balance around the first stage where the raffinate leaves and the feed enters and an intermediate stage k (as in Fig. 14.8, for instance) is 

X = mass of solute/mass of diluent, y = mass of solute/mass of solvent. 

The state of the mixture Mj results from a mass balance of the transfer component  

the amount of solvent Eq determines the position of the mixing point in the concentration space. The point Mj represents the overall concentration of the two phases. It has to lie within the two-phase region (miscibility gap). Points R and define the concentrations of the raffinate and the extract, respectively. Their amounts are determined via mass balances or, graphically, via lever rule. 

The operation parameter of solvent extraetion is the amount of the solvent. The larger the the lower the residnal eoneentration in the raffinate. However, the eoneentration of transfer eomponent in the extract becomes very low, too. 

The process of leaching is shown in Fig. 6.2-2. The thermodynamic description is fully equivalent to solvent extraction. The raffinate phase lies on the residuum line 

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Acetonitrile serves to greatly enlarge the spread of relative volatilities so that reasonably sized distillation equipment can be used to separate butadiene from the other components in the C4 fraction. The polar ACN acts as a very heavy component and is separated from the product without much difficulty.The feed stream is carefully hydrogenated to reduce the acetylene level rerun, and then fed to the single stage extractive distillation unit. Feed enters near the middle of the extractive distillation tower, while (lean) aqueous ACN is added near but not at the top. Butenes and butanes go overhead as distillate, with some being refluxed to the tower and the rest water washed for removal of entrained ACN. 

Equilibrium data must be obtained for material balance showing raffinate and extracted phases. A simple separation funnel for single-stage extraction using amyl acetate as organic solvent is shown in Figure 7.13. 

C = CM/CAo, where product is shown in the lower phase and Sc = CAu/CAo, where product partitions to the upper phase. When single-stage extraction does not give sufficient recovery, repeated extraction can be carried out in a chain or cascade of contacting and separation units. 

While offering a more inherently realistic method of solution, however, the technique may cause some additional problems in the numerical solution, since high values of Kl can lead to increased stiffness in the differential equations. Thus in using this technique, a compromise between the approach to equilibrium and the speed of numerical solution may have to be adopted. Continuous single-stage extraction is treated in the simulation example EQEX. Reaction with integrated extraction is demonstrated in simulation example REXT. 

The single-stage extraction is modelled by balances for P in both phases X and Y. 

Continuous single-stage extraction is treated in the simulation example EQEX. Chemical reaction with integrated single stage extraction is demonstrated in the simulation example REXT. 

Table 8.1 Effect of Phase Ratio in Single Stage Extraction... 

A single-stage extraction using the same total volume of solvent achieves only 92% extraction, and the extract concentration is only 0.23, vs. nearly 0.25 for the cross-flow extraction. The use of four cross-flow extraction stages is clearly preferable to a single extraction. Equally, of course, the use of more than four extraction stages, each with a proportionately smaller volume, would improve the performance. In the limit, one would seek a differential contacting process similar to the Soxhlet extractor employed for extraction from solid phases, but such a contactor has not found use in solvent extraction. 

The volume of solvent needed for a particular duty depends on the system adopted for the extraction in general, a single-stage extraction requires more solvent phase than a cross-flow cascade, which in turn needs more than a countercurrent cascade. 

In the case of the oil oontamined sand, a single stage extraction yielded a total recovery of hydrocarbons of 99% and a dean sand (hardly containing 0.1% of hydrocarbons), thus safe to be returned to the environment. 

Calculation of Stage Requirements 463 Single Stage Extraction 463 Crosscurrent Extraction 464 Immiscible Solvents 464... 

Consider a single stage extraction process, Eig. 2A, where a feed liquid (i.e., water) containing a solute (i.e., acetone) contacts an extraction solvent (i.e., chloroform). In this... 

If the performance of a single-stage extraction is not adequate, repeated cross-current extractions can be carried out to increase solute recovery or removal. For this configuration, the reduction factor is given by... 

Fig, 2,8 Depiction of a single-stage extraction in a triangle diagram 2.1.1.3.2 Multi-Stage Liquid-Liquid Extraction... 

When performing single-stage extraction on the laboratory scale, the chemist employs a separating funnel as mixing and precipitating vessel [23, 31 ]. 

Other important features in assessing industrial extractions are whether one or more extraction stages are to be used. This will depend in part on the partition coefficient of the system of interest, and on the flow pattern of the solvent and extracted phase (the raffinate) to be used, if more than one extraction stage is employed. Single-stage extraction requires a mixer to bring about intimate contact between the two phases, followed by a settler which allows phase separation and a means for independent removal of the two phases (Fig. 10.7). 

By substitution of known values into this expression, the concentration is found to be 9 X 10 M. Thus, one extraction stage as described decreases the benzoic acid concentration in the effluent to just under one-half the original value. Similarly, by substituting this concentration value and Kp into Eq. 10.36, the concentration of benzoic acid obtained in the benzene extract is found to be 0.04 M, a ratio corresponding to the benzoic acid partition factor between these two liquids. The same equation may also be used to calculate the single-stage extraction parameters for any other solute of interest, provided that Kp for the system is known, or is determined experimentally. 

Volume of benzene (L) Single-stage extraction Three-stage crosscurrent Countercurrent ... 

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