Ind. Eng. Chem. Res. 2005, 44, 8837-8851
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On the Influence of Some Inorganic Salts on the Partitioning of Citric Acid between Water and Organic Solutions of Tri-n-octylamine. Part II: Toluene as Organic Solvent Andreas Schunk and Gerd Maurer* Chair of Applied Thermodynamics, Department of Mechanical and Process Engineering, University of Kaiserslautern, D-67653 Kaiserslautern, Germany
Reactive extraction is a suitable process for the recovery/purification of ionic species (e.g., heavy metals) and electrolytes (e.g., carboxylic acids) from aqueous solutions. The computer-aided basic design of such processes requires a thermodynamic model for the liquid-liquid equilibrium of multicomponent systems, for example, of the system (water + carboxylic acids + organic solvent + reactive extractant). However, as even very small amounts of a strong electrolyte (e.g., sodium chloride) can considerably reduce the amount of carboxylic acid extracted from the aqueous into the organic phase, such models should also be able to account for such effects as was demonstrated in Part I of this series, which dealt with the influence of sodium nitrate, sodium chloride, sodium sulfate, sodium citrate, and hydrochloric acid on the partitioning of citric acid to the coexisting aqueous/organic liquid phases of the system (water + methyl isobutyl ketone (organic solvent) + tri-n-octylamine (chemical extractant)) at 25 °C. In the present work, the organic solvent (methyl isobutyl ketone) is replaced by toluene. New experimental results for the influence of the mentioned strong electrolytes on the partitioning of citric acid to the coexisting aqueous/organic liquid phases are reported and successfully modeled/predicted by applying the previously developed thermodynamic framework. Introduction Liquid-liquid extraction is a commonly applied process for the recovery of carboxylic acids from aqueous solutions. Because the solubility of carboxylic acids in “physical” organic solvents is usually rather small, the recovery is often achieved by a “chemical” extraction, i.e., a reactive, hydrophobic component (e.g., an organic base) dissolved in a physical solvent is used for extracting carboxylic acids from the aqueous phase. Waterinsoluble organic amines (e.g., tri-n-octylamine) are typical reactive extractants for carboxylic acids. Such amines form complexes with carboxylic acids, which are insoluble in water and, thus, enable the removal of such acids from aqueous phases. The computer-assisted basic design of a separation process by reactive extraction requires a reliable thermodynamic model for the Gibbs energy of the liquid phases. However, such models can only be developed and tested when sufficient and reliable experimental data are available. In previous work, (1) the partitioning of some single carboxylic acids was determined experimentally and described by a thermodynamic model,1-6 (2) the research was extended to the competitive extraction of two carboxylic acids,2,7-9 and (3) the model was extended to describe the influence of some strong electrolytes on the distribution of citric acid on coexisting liquid phases, which are observed when the reactive extractant tri-n-octylamine (TnOA) is dissolved in an aqueous/organic two-phase system where the organic solvent is methyl isobutyl ketone (MIBK).10 That last investigation revealed a drastic decrease of the partition coefficient of a carboxylic acid * To whom correspondence should be addressed. Tel.: +49631-205-2410. Fax: +49-631-205-3835. E-mail: gmaurer@ rhrk.uni-kl.de.
when a strong inorganic electrolyte is present in the aqueous feed. That behavior is caused by the chemical loading of the extractant by the inorganic acid, i.e., both acids (the carboxylic acid as well as the strong inorganic acid from the additionally dissolved electrolyte) compete for the cations of the strong electrolyte (in the aqueous phase) and for the amine (in the organic phase). In phase equilibrium, the amine is loaded with the inorganic acid while some of the carboxylic acid remains in the aqueous phase. That behavior was described by a thermodynamic framework that is able to predict the complex liquid-liquid equilibrium from information determined exclusively from investigations on subsystems. In the present work, that investigation is extended to systems where the previously studied organic solvent (MIBK) is replaced by toluene. New experimental results for the liquid-liquid equilibrium of the system (citric acid + water + toluene + TnOA) in the presence of each of several sodium salts (sodium nitrate, sodium chloride, sodium sulfate, and sodium citrate) as well as of hydrochloric acid at 25 °C are presented, discussed, and compared with predictions from the previously developed thermodynamic model. Experimental Section Materials. Citric acid monohydrate (min. 99.8%), sodium nitrate, sodium chloride, sodium sulfate (min. 99.5%), and toluene (min 99.7% GC) were purchased from Riedel-de Hae¨n, Seelze, Germany. Hydrochloric acid (Fixanal 1.0 mol) was purchased from Merck AG, Darmstadt, Germany. These chemicals were used without further purification. Tri-n-octylamine (TnOA, min. 99%) was from BASF AG, Ludwigshafen a. Rh., Germany. Before use, TnOA was washed with deionized
10.1021/ie050735i CCC: $30.25 © 2005 American Chemical Society Published on Web 10/12/2005
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water to remove traces of water-soluble amines. Deionized water was used in all experiments. Liquid-Liquid-Phase Equilibrium Experiments. The details of the phase equilibrium measurements (including the experimental uncertainties) have been described before10 and are, therefore, only summarized here. Equal volumes (∼25 cm3) of an aqueous solution containing both citric acid and the inorganic salt or hydrochloric acid, respectively, and a solution of TnOA in toluene were mixed in thermostated (25 ( 0.1 °C) glass flasks and stirred for 2 h to achieve equilibrium. After equilibration, the coexisting phases were separated by centrifugation. Both phases were analyzed. A few experiments were also performed without citric acid and TnOA. Organic Phase. The concentration of water in the organic phase was determined by Karl Fischer titration with a relative uncertainty of (3%. The concentration of the protons in the organic phase was determined with a relative uncertainty of (4% by backextracting the acid into an aqueous solution of sodium hydroxide and titrating the excess amount of sodium hydroxide with hydrochloric acid. The concentration of inorganic anions (nitrate, chloride, and sulfate, respectively) was determined by analyzing that aqueous solution by ion chromatography. The “total” concentration of citric acid in the organic phase was calculated from these results as described before.10 Aqueous Phase. The concentration of the protons in the aqueous phase was determined by acid-base titration. The concentration of toluene in the aqueous phase was determined with a relative experimental uncertainty of 12% by gas chromatography, applying the internal standard procedure. Acetone was used as the internal standard. The concentrations of the ions, nitrate, chloride, and sulfate, were determined by ion chromatography, applying the internal standard procedure. The relative experimental uncertainty is (4% for the concentration of nitrate and chloride and (6% for the concentration of sulfate. The “total” concentration of citric acid (i.e., the total mass of citrate ions present either as citric acid or as sodium citrate) was calculated for the systems with hydrochloric acid (as the added strong electrolyte) from these results and for the systems with sodium salts (as the added strong electrolyte) by applying a mass balance (using also the experimental results for the composition of the organic phase).10 The masses of both phases which were required in these calculations were not directly measured but were again calculated from material balances.10 For these calculations, the (very low) aqueous phase amine concentration was neglected. Additionally, the pH of the aqueous phase was determined with an experimental uncertainty of (0.1 pH units. Experimental Results. Table 1 gives a survey of the experimental investigations. The mass fraction of TnOA in the organic feed solution was 28.5% (the stoichiometric molality was m ˜ (org)0 TnOA ) 1.13 mol/(kg of toluene)). The molality of the inorganic salt or hydrochloric acid in the aqueous feed solution varied between 0.01 and 1.0 mol/kg. The experimental results are presented in Table 2 (for the experiments with sodium chloride), Table 3 (sodium nitrate), Table 4 (sodium sulfate), Table 5 (sodium citrate), and Table 6 (hydrochloric acid). In Tables 2-4 and 6 the stoichiometric molalities of all citric acid species (i.e., neutral citric acid as well as (aq) (org) and m ˜ Cit.tot. ) are citrate ions) in both phases (m ˜ Cit.tot.
Table 1. Survey of Liquid-Liquid-Phase Equilibrium Experimentsa) electrolyte
(aq)0 ξH 3Cit mass %
m ˜ (aq)0 anion mol/kg
ξ(org)0 TnOA mass %
sodium chloride sodium nitrate sodium sulfate sodium citrate hydrochloric acid
0.5-13 0.7-8 0.7-8 1-14 0.6-7
0.01-1.0 0.01-0.1 0.01-0.2 0.01-0.1 0.02-1.0
∼28.5 ∼28.5 ∼28.5 ∼28.5 ∼28.5
(aq)0 feed ) mass fraction of citric acid, ξH ; stoichio3Cit metric molality of inorganic salt (or hydrochloric acid), m ˜ (aq)0 anion; (org)0 solvent feed ) mass fraction of TnOA, ξTnOA.
aAqueous
reported. Furthermore, the experimental results for the other solute species in the aqueous phase (stoichiometric molalities of the inorganic anion (m ˜ (aq) anion), of toluene ( (aq) (aq) m ˜ tol ), and of sodium (m ˜ Na )), for the other solute species in the organic phase (stoichiometric molalities (org) ˜H ), and TnOA of the inorganic anion (m ˜ (org) anion), water (m 2O (org) (m ˜ TnOA)), and for the aqueous phase pH are reported. The partitioning of both acids is described by the (org) partition coefficients of citric acid (PCit.tot. () m ˜ Cit.tot. / (aq) m ˜ Cit.tot.)) and of the inorganic acid (Panion () m ˜ (org) anion/ m ˜ (aq) anion)). In Table 5, a somewhat different presentation is chosen. Because sodium citrate is not extracted but rather shifts the chemical reaction equilibrium for the dissociations of citric acid in the aqueous phase (cf. Modeling Section below), the aqueous phase is treated as a solution of citric acid and sodium citrate. The (aq) stoichiometric molalities of citric acid (m ˜H ) and of 3Cit (aq) sodium citrate (m ˜ Na3Cit) in the aqueous phase are, therefore, given. Consequently, the partitioning of citric acid is described by the partition coefficient PH3Cit () (org) (aq) /m ˜H ). m ˜H 3Cit 3Cit The quality of the experimental results was checked by a mass balance for citric acid
j Cit0 - m ∆m j Cit m j anal Cit ) m j Cit m j 0
(1)
Cit
0 where m j Cit is the mass of the citrate groups (Cit3-) in anal is the mass of the the aqueous feed solution and m j Cit citrate group found during the analyses. Numbers for ∆m j Cit/m j Cit are given together with the other experimental results in Tables 2-6. In accordance to the experimental uncertainties given before, ∆m j Cit/m j Cit is typically about (2%. However, when the concentration of citric acid in the aqueous feed is small, that difference increases to nearly (10%. As was expected, the influence of sodium chloride on the phase equilibrium in the (citric acid + water + toluene + TnOA) system is very similar to that in the system with MIBK.10 As is shown in Figure 1, already very small concentrations of sodium chloride in the aqueous feed solution result in a drastic decrease of the partition coefficient of citric acid when the molar ratio of citric acid to sodium chloride in the feed is below ∼1. For the chloride-free as well as for the chloride-containing systems, the partition coefficient PCit.tot. increases at first with increasing “total” molality of citric acid (aq) m ˜ Cit.tot. in the aqueous phase, runs through a maximum (where nearly all TnOA is loaded with citric acid), and then decreases. This behavior was explained before,
Ind. Eng. Chem. Res., Vol. 44, No. 23, 2005 8839 Table 2. Experimental Results for the Liquid-Liquid Equilibrium of the System (Citric Acid + Water + Toluene + TnOA + NaCl) at 298.15 K aqueous phase
organic phase
(aq) m ˜ Cit.tot. mol/kg
m ˜ (aq) Cl mol/kg
m ˜ (aq) tol mol/kg
m ˜ (aq) Na mol/kg
0.00842 0.00836 0.0137 0.0131 0.0211 0.0220 0.0274 0.0280 0.0375 0.0376 0.0522 0.0506 0.0927 0.0937 0.0182 0.0181 0.0250 0.0250 0.0378 0.0383 0.0466 0.0467 0.0622 0.0618 0.0997 0.101 0.141 0.134 0.0280 0.0284 0.0515 0.0512 0.0661 0.0665 0.0857 0.0867 0.127 0.127 0.182 0.180 0.126 0.123 0.177 0.177 0.209 0.216 0.295 0.274 0.430 0.420 0.253 0.263 0.324 0.320 0.430 0.431 0.489 0.487 0.651 0.625
0.00529 0.00529 0.00340 0.00368 0.00184 0.00190 0.00128 0.00133 0.00088 0.00108 0.00103 0.00094 0.00112 0.00078 0.0392 0.0384 0.0307 0.0305 0.0189 0.0182 0.0132 0.0131 0.0100 0.0093 0.0061 0.0063 0.0053 0.0066 0.0678 0.0684 0.0449 0.0452 0.0341 0.0350 0.0281 0.0275 0.0203 0.0203 0.0183 0.0166 0.297 0.304 0.246 0.244 0.235 0.216 0.184 0.183 0.127 0.128 0.583 0.555 0.534 0.529 0.505 0.493 0.484 0.477 0.460 0.460
0.0056 0.0043 0.0039 0.0051 0.0031 0.0041 0.0039 0.0024 0.0035 0.0043 0.0058 0.0056 0.0049 0.0063 0.0048 0.0050 0.0046 0.0035 0.0039 0.0048 0.0031 0.0056 0.0057 0.0060 0.0063 0.0061 0.0047 0.0050 0.00547 0.0023 0.0023 0.0022 0.0028 0.0023 0.0031 0.0024 0.0025 0.0028 0.0037 0.0038 0.0031 0.0048 0.0033 0.0039 0.0028 0.0031 0.0029 0.0030 0.0062 0.0064
0.0101 0.0101 0.0101 0.0101 0.0102 0.0102 0.0101 0.0101 0.0101 0.0101 0.0103 0.0103 0.0102 0.0102 0.0501 0.0501 0.0501 0.0501 0.0502 0.0502 0.0505 0.0505 0.0508 0.0508 0.0512 0.0514 0.0513 0.0513 0.100 0.100 0.101 0.101 0.101 0.101 0.102 0.102 0.101 0.101 0.102 0.102 0.508 0.507 0.509 0.510 0.512 0.513 0.517 0.515 0.519 0.517 1.04 1.04 1.05 1.05 1.05 1.06 1.06 1.06 1.06 1.06
0.0017 0.0017 0.0020 0.0022 0.0030 0.0041 0.0033 0.0060 0.0062
pH
(org) m ˜ Cit.tot. mol/kg
m ˜ (org) Cl mol/kg
(org) m ˜H 2O mol/kg
m ˜ (org) TnOA mol/kg
PCit.tot.
PCl
∆m j Cit/m j Cit %
3.50 3.55 3.27 3.31 3.15 3.15 3.00 3.01 2.79 2.76 2.72 2.73 2.45 2.44 3.93 3.94 3.83 3.78 3.62 3.60 3.54 3.53 3.25 3.30 2.97 3.01 2.56 2.76 4.04 4.04 3.89 3.88 3.77 3.78 3.57 3.58 3.23 3.20 2.95 2.91 4.18 4.22 4.06 4.07 3.88 3.87 3.50 3.59 3.20 3.18 4.19 4.24 3.95 4.03 3.64 3.64 3.51 3.51 3.12 3.12
0.0293 0.0291 0.0775 0.0820 0.247 0.246 0.413 0.429 0.586 0.567 0.668 0.667 0.720 0.726 0.0145 0.0138 0.0462 0.0465 0.183 0.183 0.385 0.385 0.559 0.567 0.680 0.702 0.746 0.748 0.0157 0.0158 0.141 0.142 0.283 0.285 0.460 0.464 0.510 0.535 0.596 0.628 0.0279 0.0212 0.0716 0.0879 0.227 0.221 0.343 0.349 0.383 0.378 0.0321 0.0271 0.129 0.133 0.207 0.218 0.222 0.232 0.239 0.318
0.00787 0.00786 0.0111 0.0106 0.0135 0.0134 0.0141 0.0140 0.0144 0.0144 0.0143 0.0145 0.0141 0.0147 0.0179 0.0194 0.0320 0.0321 0.0505 0.0513 0.0594 0.0597 0.0636 0.0646 0.0690 0.0686 0.0699 0.0676 0.0528 0.0518 0.0890 0.0893 0.106 0.105 0.115 0.116 0.124 0.124 0.126 0.129 0.330 0.318 0.404 0.408 0.419 0.447 0.487 0.489 0.555 0.556 0.663 0.703 0.731 0.740 0.769 0.778 0.787 0.796 0.806 0.810
0.0828 0.0875 0.153 0.163 0.322 0.330 0.558 0.543 0.753 0.752 0.763 0.760 0.876 0.870 0.103 0.113 0.134 0.128 0.346 0.334 0.578 0.586 0.767 0.763 0.806 0.794 0.907 0.943 0.162 0.144 0.343 0.356 0.588 0.599 0.782 0.789 0.802 0.797 0.879 0.928 0.422 0.410 0.669 0.671 0.795 0.856 0.975 0.976 0.981 0.979 0.761 0.763 0.927 0.963 1.00 0.987 0.989 1.01 1.03 1.03
1.13 1.13 1.13 1.13 1.13 1.13 1.13 1.13 1.13 1.13 1.13 1.13 1.13 1.13 1.13 1.13 1.13 1.13 1.12 1.12 1.12 1.12 1.12 1.12 1.12 1.12 1.12 1.12 1.12 1.12 1.12 1.12 1.12 1.12 1.12 1.12 1.12 1.12 1.12 1.12 1.10 1.11 1.10 1.10 1.10 1.10 1.09 1.09 1.09 1.09 1.08 1.07 1.07 1.07 1.07 1.07 1.07 1.07 1.07 1.07
3.48 3.48 5.66 6.25 11.7 11.2 15.1 15.3 15.6 15.1 12.8 13.2 7.77 7.74 0.80 0.76 1.85 1.86 4.83 4.78 8.27 8.25 8.99 9.18 6.82 6.97 5.29 5.57 0.56 0.56 2.74 2.77 4.29 4.29 5.36 5.35 4.01 4.22 3.27 3.49 0.22 0.17 0.41 0.50 1.09 1.02 1.16 1.27 0.89 0.90 0.13 0.10 0.40 0.42 0.48 0.50 0.46 0.48 0.37 0.51
1.49 1.49 3.26 2.88 7.31 7.07 11.1 10.5 16.4 13.4 13.9 15.4 12.6 18.9 0.45 0.51 1.04 1.05 2.67 2.81 4.50 4.54 6.45 6.97 11.4 11.0 13.1 10.3 0.78 0.76 1.98 1.98 3.10 2.99 4.09 4.21 6.10 6.10 6.89 7.78 1.11 1.05 1.64 1.67 1.79 2.07 2.64 2.68 4.38 4.36 1.14 1.27 1.37 1.40 1.52 1.58 1.63 1.67 1.75 1.76
0.0 -0.6 -0.2 3.3 6.9 6.9 2.5 6.3 3.1 -2.0 4.0 3.2 < 0.1 0.5 1.7 -0.7 0.3 0.8 -7.1 -6.3 3.5 3.1 4.9 6.1 2.2 5.2 0.6 0.2 1.5 2.9 3.1 2.5 -2.3 -1.6 2.1 3.1 -1.6 1.7 1.5 5.3 2.6 -2.4 -2.5 1.9 1.4 2.1 1.8 -1.5 0.6 -1.3 1.4 3.4 -4.6 -4.9 -5.1 -3.5 -4.7 -4.0 -5.0 -1.6
by the competing effects of the dissociation of citric acid on one side and the extraction of citric acid by the amine on the other side.4-9 With increasing concentration of sodium chloride, the maximum number for the partition coefficient is shiftedsnearly along the curve for the saltfree aqueous feed solutionsto higher “total” citric acid molalities, resulting in lower maximum numbers for the partition coefficient. However, the decrease of the partition coefficient of citric acid is really drastic at very low citric acid concentrations, i.e., in the range where the
partition coefficient of citric acid increases with increasing “total” aqueous phase molality of citric acid. The decrease of the partition coefficient of citric acid may amount to >1 order of magnitude. For example, at (aq) ) 0.02, adding ∼0.05 mol sodium chloride/(kg m ˜ Cit.tot. of water) to the aqueous feed solution of an aqueous/ organic system (of equal volumes of the organic and aqueous phases, where the organic phase molality of TnOA is m ˜ (org)0 TnOA ) 1.13 mol/(kg of toluene)) reduces the
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Table 3. Experimental Results for the Liquid-Liquid Equilibrium of the System (Citric Acid + Water + Toluene + TnOA + NaNO3) at 298.15 K aqueous phase
organic phase
(aq) m ˜ Cit.tot. mol/kg
(aq) m ˜ NO 3 mol/kg
m ˜ (aq) tol mol/kg
m ˜ (aq) Na mol/kg
0.0130 0.0132 0.0176 0.0169 0.0225 0.0224 0.0489 0.0495 0.0328 0.0317 0.0408 0.0398 0.0482 0.0480 0.0489 0.0487 0.122 0.120 0.0427 0.0434 0.0618 0.0623 0.0749 0.0744 0.0908 0.0900 0.205 0.199
0.00162 0.00178 0.00130 0.00135 0.00081 0.00086
