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Ind. Eng. Chem. Res. 2000, 39, 3364-3369
Liquid-Liquid Extraction of trans-Aconitic Acid from Aqueous Solutions with Tributyl Phosphate and a Mixed Solvent at 303.15 K† Norma G. Barnes, Mo´ nica B. Gramajo de Doz, and Horacio N. So´ limo* Departamento de Fı´sica, Facultad de Ciencias Exactas y Tecnologı´a, Universidad Nacional de Tucuma´ n, Avenida Independencia 1800, 4000 San Miguel de Tucuma´ n, Argentina
The phase diagram of the water + trans-aconitic acid + tributyl phosphate ternary system was obtained at 303.15 K. Experiments were also conducted on the equilibrium distribution of transaconitic acid between its aqueous solutions and a mixed solvent: tributyl phosphate (TBP) + hexane (H) (with a volume ratio of 60/40 TBP/H). Analysis of the results shows that the diluent (H) decreases the selectivity and the distribution coefficient when compared with pure TBP. However, this is not relevant because these properties are still appropriate for extraction purposes. Additionally, a multistage cross-flow extraction process was performed in order to verify the accuracy of the basic equilibrium data and to obtain the number of stages required to extract trans-aconitic acid from its aqueous solutions. This number was also graphically determined by using a distribution diagram in Bancroft’s coordinates and analytically calculated assuming virtual immiscibility between the feed and the extraction solvents. The extraction process was also simulated following the procedure contained in the ChemCAD Process Flowsheet Simulator 4.0 and compared with the experimental results. 1. Introduction The recovery of some chemical products contained either in effluents or in industrial waste residuals by conventional methods implies the use of energy-expensive processes mainly because of the usually low concentrations of these chemicals. In this situation, the technology for liquid-liquid extraction becomes important because of its lower energy consumption and for environmental protection reasons. trans-Aconitic acid (1,2,3-propenetricarboxylic acid, C6H6O6) is a tribasic unsaturated organic acid that has several important industrial uses, especially in the fields of plastics and synthetic rubber. It was identified for the first time in sugar cane in 1877 by Behr,1 and Yoder,2 McCalip and Seibert,3 and Balch et al.4 also reported the aconitic acid in sugar cane juice and pointed out that it is the predominant organic acid, excluding amino acids, in sugar cane. Regna and Bruins5 have studied the extraction of trans-aconitic acid from sugar cane molasses using methyl isobutyl ketone and ethyl methyl ketone as solvents. Their phase equilibria diagrams were reported, but they only analyze each liquid-liquid equilibrium region without showing their complete phase diagrams because all liquid-solid equilibrium regions were omitted. On the other hand, Azzam and Radwan6 have developed a lab process for the direct extraction of aconitic acid from acidified molasses using ethyl acetate as the solvent, but they do not determine its phase equilibria either. Recently, Barnes et al.7 studied ternary aqueous systems using trans-aconitic acid as the solute and weak † The material of this paper is part of a thesis by Norma G. Barnes, written in partial fulfillment of the requirements for the Doctoral degree in Chemical Engineering at the Universidad Nacional de Tucuma´nsArgentina. * To whom all correspondence should be addressed. Tel.: (54-381) 4364093 ext. 770. Fax: (54-381) 4363004. E-mail:
[email protected].
Lewis bases (1-pentanol (P), methyl isobutyl ketone (MIK), and isobutyl acetate (iBuAc)) as the solvents and showed that the acid is always preferentially distributed in the aqueous phase with distribution coefficients smaller than unity practically in the whole solute concentration range (0.78 e mP e 1.64, 0.54 e mMIK e 0.63, 0.13 e miBuAc e 0.17, respectively). Although the selectivities of these solvents are relatively high, particularly for methyl isobutyl ketone (2.9 e βP e 12.5, 4.0 e βMIK e 14.5, 4.5 e βiBuAc e 9.3, respectively; an error in the caption corresponding to Figure 4 of this reference was detected, in which the symbols ∆ and ( were erroneously interchanged), it is desirable to make studies with considerably stronger Lewis bases such as tributyl phosphate (TBP). Because aconitic acid is a byproduct of the sugar cane, which is produced in tropical and subtropical regions where the average temperature is around 30 °C, this temperature was chosen to carry out the experimental determinations. The goals of this paper are (i) to obtain the phase diagram of the water (W) + trans-aconitic acid (AA) + tributyl phosphate (TBP) ternary system at 303.15 K, by analyzing the extractive properties of TBP, (ii) to analyze the influence of the addition of a diluent (hexane, H) to TBP in order to reduce its viscosity and density, (iii) to perform an extraction process (lab scale) to extract the trans-aconitic acid from aqueous solutions with a mixed solvent (TBP/H), obtaining the number of stages necessary to achieve predetermined conditions for both the feed and solvent using a multistage crossflow process, (iv) to verify the applicability of an idealized extraction model which assumes that the feed solvent is virtually insoluble with the extraction solvent, to calculate the required number of stages needed to obtain prefixed conditions, and to compare these results with the experimental ones, and (v) to simulate the extraction process using the ChemCAD Process Flowsheet Simulator 4.0 and to compare the results with the experimental ones.
10.1021/ie000103q CCC: $19.00 © 2000 American Chemical Society Published on Web 08/09/2000
Ind. Eng. Chem. Res., Vol. 39, No. 9, 2000 3365 Table 1. Liquid-Liquid and Liquid-Solid Equilibrium Compositions, Distribution Coefficients (m), and Selectivities (β) for the Water (1) + trans-Aconitic Acid (2) + Tributyl Phosphate (3) Ternary System at 303.15 ( 0.05 K and Viscosities (η) of Both Phases (a) Liquid-Liquid Equilibrium aqueous phase (R)a
organic phase (E)a
η/ η/ 100w1R 100w2R (mPa s) 100w1E 100w2E (mPa s) 99.5 98.2 96.0 92.6 88.5 86.5 79.0 76.1b
0.0 0.4 2.8 7.1 9.9 12.1 18.9 24.8b
0.86 0.78 0.85 0.92 0.99 1.00 1.25 1.34
5.3 5.2 5.8 4.8 4.5 4.8 5.3 5.0b
0.0 5.4 11.1 12.4 14.3 16.4 17.6 18.8b
3.6 7.4 15.5 19.2 21.7 24.2 27.3 31.4
m
β
13.5 4.0 1.7 1.4 1.4 0.9 0.8
254.9 66.2 32.8 27.5 25.2 13.4 12.2
(b) Liquid-Solid Equilibrium aqueous phase
organic phase
100w1R
100w2R
100w1E
100w2E
75.1c
24.9c
0.9 3.0 0.0d
17.0 18.2 14.5d
a Percent mass fractions of component i in the corresponding phase. b Invariant points in the 2L + S region. c Aqueous solubility of trans-aconitic acid at 303.15 K. d Organic solubility of transaconitic acid at 303.15 K.
It is very well-known that the viscosity of the raffinate and extract phases should be as low as possible in order to favor the mass transfer in the extraction processes. Taking into account that the experimental results show that the viscosity of the extract phase for the ternary system W + AA + TBP reaches values higher than 30 mPa s for some acid concentrations as can be seen in Table 1, several mixtures of TBP/H were prepared by obtaining quaternary tie lines W + AA + TBP + H at 303.15 K for each one of their TBP/H ratios, where hexane was chosen because it is nonpolar, has a low price, and is virtually insoluble in water. When the experimental viscosity and density of both equilibrium phases, as well as the distribution coefficients and selectivities in each case, were determined, the TBP/H ratio with a TBP proportion as high as possible was determined. Four different aqueous trans-aconitic acid solutions with compositions inside its solubility region were used in order to determine the solvent ratio for the best TBP/H ratio among those studied in a single-stage assay. When the solvent ratio was changed and the percentage of extracted acid was analyzed, the best solvent ratio was obtained. A multistage cross-flow extraction was carried out to determine the number of experimental stages needed to reach predetermined final conditions. The number of theoretical stages (NTS) was evaluated both analytically (eq 1) and graphically, using a distribution diagram in Bancroft’s coordinates (see Figure 1), and compared with experimental results. 2. Experimental Section 2.1. Materials. TBP (Fluka, puriss. 99% GC), H (Dorwil, ACS), bidistilled water, and trans-aconitic acid (Fluka, puriss.) were used. The purity of the liquids was verified by comparing their experimental densities and refractives indexes with those of the literature. An excellent agreement was always obtained and, conse-
quently, they were used as supplied. On the other hand, the purity of aconitic acid was checked with standard sodium hydroxide (0.1 N) using phenolphthalein as the indicator, which showed it to be better than 99%. It was therefore also used without further purification. 2.2. Apparatus and Procedures. 2.2.1. LiquidLiquid and Liquid-Solid Equilibria for the Ternary System W + AA + TBP. Isothermal solubility curves and tie lines were obtained simultaneously in the liquid-liquid region. Mixtures with compositions inside the heterogeneous area were vigorously shaken for at least 1 h at a constant temperature of 303.15 ( 0.05 K in an equilibrium cell equipped with a magnetic stirrer and a jacket for circulating the isothermal water at the desired temperature, using a sample size of approximately 30 mL. After decantation, samples of both phases were drawn with hypodermic syringes and the compositions of trans-aconitic acid and water present in each conjugated phase were determined by titration with 0.1 N NaOH (using phenolphthalein as the indicator, with an accuracy of (0.001 in mass fraction) and Karl Fischer titration (Mettler DL 18 Karl Fischer titrator using Hydra Point Titrant 5 and Hydra Point Solvent G both Baker A. R., with an accuracy of (0.0001 in mass fraction), respectively. Consequently, these compositions were expressed in mass fractions with an accuracy of (0.001, taking into account the propagation errors of both titrations. The solid-liquid solubility curves and invariant points were obtained from saturated solutions in the L + S and 2L + S regions, respectively. Their liquid phases were analyzed with the same analytical procedure as that used for the liquid-liquid equilibria. 2.2.2. Determination of the Best TBP/H Ratio. Mixtures with 20/80, 40/60, 60/40, and 80/20 volume ratios of TBP/H were prepared and their densities and viscosities measured at 303.15 ( 0.05 K. Several quaternary tie lines in the L + L region were obtained for each TBP/H ratio, determining the viscosity, density, and trans-aconitic acid and water concentrations for both conjugated phases. Densities were measured using a KEM DA-300 densitometer with an accuracy of (0.1 kg m-3 using samples of approximately 1 mL, while viscosities were measured with a Cannon-Fenske viscometer (calibrated with bidistilled water and benzene) using samples of approximately 10 mL, with an accuracy of (0.01 mPa s. The TBP/H ratio was obtained by taking into account the following parameters: (a) difference of the densities of both conjugated phases, (b) viscosities of each phase, (c) distribution coefficients, and (d) selectivities. The ratio 60/40 for the mixture TBP/H is probably not the optimum ratio but rather is the best among those studied. 2.2.3. Solvent Ratio Determination. To determine the solvent ratio, Sr ) S′/F′, (where S′ is the mass of the extraction solvent and F′ is that of the feed solvent), a 50 mL separating funnel provided with a jacket for circulating water from a bath at 303.15 ( 0.05 K was used. After thermal equilibrium was reached, the separating funnel was shaken manually every 10 min for a total of approximately 30 min. Four feed aqueous solutions containing 3, 6, 12, and 20% in mass of acid were tested in a single contact extraction using solvent ratios of 0.3, 0.4, 0.6, and 0.7. The acid percentage was determined in each conjugated phase in order to calculate the extracted acid. The solvent ratio adopted for each pair, feed concentration-solvent ratio studied, was the one that provided a percentage of extracted acid higher than 25% in the first stage.
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Ind. Eng. Chem. Res., Vol. 39, No. 9, 2000
Table 2. Experimental Equilibrium Data for the System Water (1) + trans-Aconitic Acid (2) + Tributyl Phosphate (3) + Hexane (4) for Different TBP/H Volume Ratios, Densities (G), and Viscosities (η) of Each Conjugated Phase, Distribution Coefficients (m), and Selectivities (β) at 303.15 ( 0.05 K aqueous phase (R) 100w1R
100w2R
F/(kg m-3)
organic phase (E) η/(mPa s)
100w1E
100w2E
F/(kg m-3)
η/(mPa s)
m
β
922.6 948.6 961.2 958.5
3.05 4.84 9.27 9.42
5.0 4.7 2.5 2.2
126.8 137.7 70.6 59.1
98.9 96.7 93.2 91.3
0.5 2.8 6.0 8.2
996.7 1006.5 1065.9 1030.1
0.76 0.79 1.06 1.08
80% TBP + 20% H 3.9 2.5 3.3 13.1 3.3 15.2 3.4 18.2
96.5 93.1 83.7 77.7
1.1 5.9 15.9 21.8
998.4 1016.3 1055.6 1080.9
0.78 0.82 0.99 1.15
60% TBP + 40% H 2.8 3.8 2.5 10.4 2.8 12.6 2.7 13.3
883.6 896.6 907.0 907.1
2.34 3.29 4.61 5.07
3.4 1.8 0.8 0.6
117.2 67.0 23.9 17.3
96.6 85.7 81.8 75.3
2.4 12.3 16.9 21.9
1004.1 1042.6 1061.7 1092.2
0.81 0.92 1.02 1.23
40% TBP + 60% H 2.1 5.7 2.5 9.9 2.2 11.5 2.0 12.4
829.6 855.6 859.8 863.1
1.73 2.65 2.85 3.22
2.4 0.8 0.7 0.6
110.4 27.4 26.0 22.6
93.7 86.5 84.3 78.0
4.0 10.7 13.1 20.1
1010.2 1035.2 1046.5 1074.6
0.81 0.90 0.95 1.11
20% TBP + 80% H 0.8 2.6 0.8 6.9 1.0 7.5 1.0 7.9
741.3 746.6 749.9 749.0
0.94 1.03 1.11 1.08
0.7 0.6 0.6 0.4
82.0 64.9 50.6 31.2
Figure 1. Equilibrium diagram for the ternary system water (W) + trans-aconitc acid (AA) + tributyl phosphate (TBP) at 303.15 ( 0.05 K. S ) solid; L ) liquid; O ) global composition for tie lines. Solid lines: splines.
2.2.4. Multistage Cross-Flow Extraction Process. The multistage cross-flow extraction process has a somewhat limited application. Industrial extraction processes almost always use a countercurrent flow for efficient solvent usage. However, the cross-flow extraction is useful to verify the accuracy of the basic data equilibria and to define the solvent dilution and operation conditions needed for a practical extraction process. To determine the number of experimental stages for the selected solvent ratio, a difference of trans-aconitic acid percentage in the raffinate phase lower than 1.5% between two successive extraction stages was adopted as the final operation condition. A procedure similar to that used for the solvent ratio determination was employed but, in this case, the same quantities of fresh solvent (with the selected TBP/H ratio) were added to each previous stage raffinate, until the final condition was reached. 3. Results and Discussion Figure 1 shows the equilibrium diagram for the ternary system W + AA + TBP at 303.15 ( 0.05 K. Because it includes solid phases, it could be classified
as type 4 in the classification proposed by Treybal.8 The liquid-liquid and liquid-solid equilibrium compositions, distribution coefficients, and selectivities for this ternary system are listed in Table 1. The distribution coefficient m, defined as the ratio between the mass fraction of the acid in the extract phase (w2E) and that in the raffinate phase (w2R), and the selectivity β, defined as β ) mw1R/w1E (where w1R and w1E are the water mass fraction in the raffinate and extract phases, respectively), are two important extractive properties of a given solvent. The experimental results show that both extraction properties present high values for the system W + AA + TBP, as can be observed in Figure 1 by the slopes of the tie lines (0.8 e m e 13.5) and the low mutual solubility between water and TBP (12.2 e β e 254.9). This fact shows that TBP is a potential solvent for the extraction of transaconitic acid from its aqueous solutions. Both extractive properties present higher values than those obtained for the previously studied systems.7 However, the viscosity of the extract phase appears greater than 30 mPa s (see Table 1) for some acid concentrations, which would present many problems for the mass transfer in the extraction process. Keeping this in mind, H was added to TBP in volume ratios of 80/20, 60/40, 40/60, and 20/80 TBP/H. This produced a marked decrease in the density and viscosity at 303.15 K when the concentration of H was increased, as expected. Because only water and trans-aconitic acid concentrations were analytically determined in each conjugated phase for the quaternary system W + AA + TBP + H, the mixed solvent concentration was obtained by taking into account that the sum of all of the percentages must be equal to 100. Surely, the solvent (TBP) and the diluent (H) are differently distributed between conjugated phases and their ratio in each phase should be different from its initial value. This fact was not a problem because we were not interested in obtaining the quaternary phase diagram but only in analyzing the influence of H on the extraction properties for each of the quaternary sectional planes. Table 2 lists the equilibrium data for the different TBP/H ratios studied. Densities and viscosities of each conjugated phase are also shown as well as the distribu-
Ind. Eng. Chem. Res., Vol. 39, No. 9, 2000 3367
Figure 2. Percentage of extracted trans-aconitic acid as a function of the solvent ratio with a TBP/H ratio of 60/40. Feed: (, 20%; 9, 12%; 2, 6%; ×, 3% in mass of trans-aconitic acid.
tion coefficients and selectivities. As can be seen, the distribution coefficients always decrease when the concentration of trans-aconitic acid increases and the quaternary systems with TBP/H ratios of 60/40 and 40/ 60 are solutropics, as are those of the ternary system (see Figure 1). From Table 2 we can conclude that mixtures with a 80/20 ratio of TBP/H present distribution coefficients and selectivities compatible with extraction processes, but the viscosity of the extract phase is high. On the other hand, the extract phase presents low-viscosity values and acceptable selectivities for mixtures with 40/ 60 and 20/80 ratios of TBP/H, but their distribution coefficients are not favorable (m < 1). Consequently, we adopted a volume ratio of 60/40 TBP/H as the best mixture among those studied because it presents a relatively low viscosity of the extract phase, appropriate distribution coefficients and selectivities for the extraction process, and a difference of densities between both conjugated phases greater than 50 kg m-3.9 The percentage of extracted trans-aconitic acid as a function of the solvent ratio in a single contact operation was analyzed in order to define the solvent ratio. Figure 2 shows that the percentage of extracted acid increases when the solvent ratio increases and that a solvent ratio of 0.4 is needed in order to extract a percentage of acid higher than 25% in the first stage for feeds having 20% in mass of trans-aconitic acid, while for feeds having 12, 6, and 3%, the appropriate value is 0.3. Additionally, Table 2 shows that the mixed solvent with a TBP/H ratio of 60/40 is virtually immiscible with water. It is possible to apply a simplified procedure to calculate NTS for this extraction process. This can be done by assuming that (i) a constant mass of fresh solvent is always used for each extraction, (ii) the F′/S′ ratio between the masses of the feed (F′) and the extraction solvents (S′) is constant, as a consequence of their virtual immiscibility, and (iii) the distribution coefficient K ) Y/X remains constant (where Y and X are the concentrations of AA in the extract and raffinate phases, respectively, in Bancroft’s coordinates). The NTS in a multistage cross-flow extraction required to achieve a predetermined raffinate concentration can be calculated as follows:10
NTS ) log(XF/XN)/log(1 + KS′/F′)
(1)
where XF and XN are the mass ratios of AA to W in the feed and of AA to TBP/H in the final raffinate product in Bancroft’s coordinates, respectively.
Figure 3. Multistage cross-flow extraction with immiscible solvents: +, experimental equilibrium distribution in Bancroft’s coordinates; thick solid line, splines; s, operating line for a solvent ratio Sr ) 0.4 and wF ) 0.20; - -, linear behavior corresponding to a constant K distribution coefficient. Table 3. Average Distribution Coefficients K h , Number of Stages (NTS) Obtained Experimentally, by Using Eq 1, and Graphically, and Solvent Ratios for a TBP/H Ratio of 60/40 NTS feed
K ha
exptl
eq 1
graphically
solvent ratio
20 12 6 3
1.25 1.78 2.62 3.74
6 5 3 2
6.2 4.5 3.6 2.6
5.9 4.8 3.0 1.6
0.4 0.3 0.3 0.3
a Average value obtained assuming a linear plot and considering all experimental values between the origin and the corresponding feed (i.e., dashed line in Figure 3).
An aconitic acid mass balance for the ith stage gives
Yi ) (F′/S′)(Xi - Xi-1)
(2)
Figure 3 shows the distribution curve and the operating lines corresponding to the solvent ratio 0.4 for a feed of 20% in mass of trans-aconitic acid. The distribution curve was drawn by plotting all of the experimental tie lines (including those listed in Tables 2 and 4 for a TBP/H ratio of 60/40) while the operating lines were determined using eq 2 with slopes F′/S′. As can be seen, the distribution curve presents a nonlinear behavior, in contradiction with the simplified model, which is surely due to the lack of constancy of the distribution coefficient K. However, this is not an important problem for this system, because the values calculated using eq 1 are in agreement with the experimental ones, as can be seen in Table 3. The slope of the straight line in Figure 3 was obtained by equating to zero the sum of the differences between experimental (equilibrium curve) and calculated (linear behavior) values, to minimize their deviations. Similar plots were obtained for the other feeds. The numbers of stages obtained experimentally are 6, 5, 3, and 2 for feeds of 20, 12, 6, and 3% in mass of trans-aconitic acid, respectively. On the other hand, the number of stages obtained graphically using Bancroft’s coordinates was in good agreement with the experimental results (which confirms that the equilibria in each stage were reached and that the hypothesis of virtual
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Ind. Eng. Chem. Res., Vol. 39, No. 9, 2000 Table 4. Comparison between Experimental (me) and Calculated (mc) Distribution Coefficients, Obtained Using the Experimental Equilibrium Data and the ChemCAD Process Flowsheet Simulator 4.0, Respectively, and Their Standard Deviation (∆m) experimental value
Figure 4. Comparison between experimental (() and ChemCAD calculated (9) concentrations of the component AA in the extract and raffinate phases for a feed solution of 20% trans-aconitic acid, a TBP/H ratio of 60/40, and a solvent ratio of 0.4.
immiscibility between feed and extraction solvents is correct), while the number of stages obtained analytically was approximately the same (see Table 3). We can conclude, therefore, that the simplified procedure given by eq 1 is appropriate to this extraction process. Table 3 shows the average distribution coefficients used in eq 1 for all feeds, together with the experimental and calculated NTS and the experimental solvent ratio. Table 3 also shows that the number of stages calculated both using eq 1 and graphically are in good agreement with experimental values. ChemCAD Process Flowsheet Simulator 4.0 for Windows11 was also used to make a simulation of the multistage cross-flow extraction process studied in this work. In the simulation, the compositions and solvent ratios of the feed streams had the same values as those previously stated, while the numbers of stages were equal to those experimentally obtained. A comparison of the experimental and simulated values of the raffinate and extract compositions of trans-aconitic acid can be seen in Figure 4, for a feed solution of 20% in mass of trans-aconitic acid, a TBP/H ratio of 60/40, and a solvent ratio of 0.4. Similar plots were obtained for the other feeds. To use ChemCAD Process Flowsheet Simulator 4.0,11 the equilibrium equation was obtained using the UNIQUAC model12 with parameters for liquid-liquid equilibria correlating all of the experimental data. Because only two compositions were experimentally determined for the quaternary system W + AA + TBP + H, the “solvent” (TBP/H) was considered as a pseudocomponent. Consequently, the equilibrium equation was obtained by using the ternary UNIQUAC model. The structural parameters (r and q) of H were assigned to this “solvent” because those of TBP are not available in the literature. Although this is not strictly correct, the simulated results are in agreement with the experimental ones. Table 4 lists experimental and simulated values with the corresponding standard deviation for the distribution coefficients. From Figure 4 and Table 4 it can be concluded that ChemCAD Process Flowsheet Simulator 4.0 is a useful tool to simulate the extraction process studied here. 4. Conclusions The equilibrium diagram for the ternary system W + trans-aconitic acid + TBP was obtained at 303.15 ( 0.05 K. From the experimental results and analysis of the distribution coefficients (0.8 e m e 13.5) and selectivities (12.2 e β e 254.9), we conclude that TBP
mc
∆ma
Feed: 20% trans-Aconitic Acid 15.9 0.8 13.6 14.6 12.1 0.9 11.6 9.8 8.6 1.2 9.5 5.8 5.3 1.7 7.1 2.8 2.4 2.9 4.4 0.9 0.6 6.5 1.8 0.2
0.9 1.2 1.6 2.5 4.7 9.6
8.51
Feed: 12% trans-Aconitic Acid 9.0 1.3 11.0 8.7 6.3 1.4 9.3 5.7 3.9 1.8 7.5 3.3 2.2 2.9 5.5 1.5 1.4 3.5 3.3 0.5
1.3 1.6 2.3 3.5 6.3
7.64
8.5 6.4 4.0
Feed: 6% 3.6 2.9 0.8
trans-Aconitic Acid 2.3 7.9 3.7 2.2 5.9 1.8 5.2 3.7 0.6
2.1 3.3 5.7
4.17
6.3 4.1
Feed: 3% trans-Aconitic Acid 2.9 2.2 5.3 1.9 0.4 9.7 3.6 0.4
2.9 10.1
2.43
stage
100w2E
1 2 3 4 5 6
12.1 11.2 10.7 9.1 7.1 3.6 11.3 9.1 7.3 6.3 4.9
1 2 3 4 5 1 2 3 1 2
ChemCAD value
100w2R
me
100w2E
100w2R
∆m ) 100{∑/k[(m ˜kwhere m ˜ k and mk are the calculated and experimental distribution coefficients, respectively, and M is the number of tie lines. a
mk)/mk]2/M}1/2,
appears to be a good solvent for extraction purposes when trans-aconitic acid needs to be extracted from its aqueous solutions. However, the viscosity of pure TBP and that of the extract phase are very high, which will surely lead to many problems in the extraction process such as high-energy consumption and difficulties in the separation of both phases. It was found that a mixed solvent, made of TBP and H with a volume ratio of 60/40, has distribution coefficients (0.6 e m e 3.4) and selectivity values (17.3 e β e 117.2) lower than those obtained with pure TBP but still appropriate to extract aconitic acid from its aqueous solutions, while maintaining a sufficiently low viscosity of the extract phase (2.34 e η e 5.07). Consequently, a cross-flow extraction process (lab scale) was made with this TBP/H ratio in order to determine the influence of the diluent on the extractive properties of TBP. A solvent ratio of 0.4 was found for feeds having 20% in mass of trans-aconitic acid in order to extract a percentage of acid higher than 25% in the first stage, while for feeds having 12, 6, and 3%, it reduced to 0.3. NTS obtained graphically was similar to the experimental value, which confirms that equilibrium in each stage was achieved and that the hypothesis of virtual immiscibility between the feed (W) and the extraction (TBP/H) solvents is correct. Additionally, eq 1 can be successfully used for this extraction process and system, because calculated and experimental values are in agreement. ChemCAD Process Flowsheet Simulator 4.0 for Windows proved to be useful to simulate the multistage cross-flow extraction process studied here, because the standard deviation among experimental and calculated distribution coefficients is small, as can be seen in Table 4. Acknowledgment We acknowledge financial support by the Consejo de Investigaciones de la Universidad Nacional de
Ind. Eng. Chem. Res., Vol. 39, No. 9, 2000 3369
Tucuma´nsArgentina (Grant CIUNT-26/E139). We thank Professors Francisco Ruı´z Bevia´ and Vicente Gomis Yagu¨es, Chemical Engineering Department of the University of Alicante, Spain, for ChemCAD Process Flowsheet Simulator 4.0 facilities and their valuable collaboration in this paper.
Greek Letters
Nomenclature
(1) Behr, A. Ber. Deutsch. Chem. Ges. 1877, 10, 351. (2) Yoder, P. A. Notes on the Determination of Acids in Sugar Cane Juice. Ind. Eng. Chem. 1911, 3, 640. (3) McCalip, M. A.; Seibert, A. H. Aconitic Acid from Sugar Cane Products. Ind. Eng. Chem. 1941, 33, 637. (4) Balch H. B.; Brong, C. N.; Ambler, J. A. Aconitic acid in Sugar Cane Products. Sugar 1945, 10, 32. (5) Regna, E. A.; Bruins, P. F. Recovery of Aconitic Acid from Molasses. Ind. Eng. Chem. 1956, 48, 1268. (6) Azzam, A. M.; Radwan, M. H. Separation of Aconitic Acid from Molasses by Solvent Extraction. Fette, Seifen, Anstrichm. 1986, 88, 97; Chem. Abstr. 1986, 104, 205674. (7) Barnes, N.; Gramajo de Doz, M. B.; So´limo, H. N. Aqueous Phase Diagrams Containing t-Aconitic Acid at 303.15 K. Fluid Phase Equilib. 2000, 168, 217. EQUIFASE 99, Vigo, Spain, June 1999. (8) Treybal, R. E. Liquid Extraction, 2nd ed.; McGraw-Hill: New York, 1963. (9) Ullmann’s Encyclopedia of Industrial Chemistry; Gerhartz, E., Ed.; VCH Verlagsgesellschaft mbH: Weinheim, Germany, 1988. (10) Perry, R. S. Manual del Ingeniero Quı´mico, 6th Spanish ed.; McGraw-Hill: New York, 1992; Vol. IV. (11) ChemCAD Process Flowsheet Simulator 4.0; Chemstation Inc.: Houston, TX, 1998. (12) Abrams, D. S.; Prausnitz, J. M. Statistical Thermodynamics of Liquid Mixtures: A New Expresion for the Excess Gibbs Energy of Partly or Completely Miscible Systems. AIChE J. 1975, 21, 116.
AA ) trans-aconitic acid (component 2) E ) extract phase F′ ) mass of the feed solvent H ) hexane (diluent) K ) distribution coefficient in Bancroft’s coordinates L ) liquid m ) distribution coefficient M ) number of tie lines NTS ) number of theoretical stages R ) raffinate phase S ) solid S′ ) mass of the extraction solvent Sr ) solvent ratio TBP ) tributyl phosphate (component 3) W ) water (component 1) wF ) mass fraction of AA to W in the feed wiE, wiR ) mass fraction of component i in the extract (E) and raffinate (R) phases, respectively X ) concentration of AA in the raffinate phase, in Bancroft’s coordinates Xi ) mass ratio of AA to TBP/H in the ith raffinate product, in Bancroft’s coordinates XF ) mass ratio of AA to W in the feed, in Bancroft’s coordinates XN ) mass ratio of AA to TBP/H in the final raffinate product, in Bancroft’s coordinates Y ) concentration of AA in the extract phase, in Bancroft’s coordinates Yi ) mass ratio of AA to TBP/H in the ith extract product, in Bancroft’s coordinates
β ) selectivity η ) viscosity (mPa s) F ) density (kg m-3)
Literature Cited
Received for review January 25, 2000 Revised manuscript received May 24, 2000 Accepted June 16, 2000 IE000103Q
