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Ind. Eng. Chem. Res. 1998, 37, 4528-4530
RESEARCH NOTES On the Purification of β-Naphthalenesulfonic Acid from Dilute Aqueous Solutions Containing Sulfuric Acid Stefano Brandani, Vincenzo Brandani,* and Francesco Veglio` Dipartimento di Chimica, Ingegneria Chimica e Materiali, Universita` de L’Aquila, Monteluco di Roio, I-67040 L’Aquila, Italy
In the development of a biological process for the production of β-naphthol starting from β-naphthalenesulfonic acid, the purification of the acid from dilute aqueous solutions containing sulfuric acid is of relevant importance. We investigate the feasibility of carrying out such a purification using the secondary amine Amberlite LA-2 dissolved in toluene. Experimental measurements were performed with aqueous solutions of β-naphthalenesulfonic acid (NSA) in the concentration range 0.01-0.5 m. The organic phase comprised 0.5 m of Amberlite in the diluent toluene. Experimental results were modeled to determine the Nernst distribution coefficients of NSA and sulfuric acid and the infinite dilution selectivity. The obtained value of this last parameter shows that separation is thermodynamically favored. Introduction
Materials and Methods
Current production of β-naphthol involves a process which is not sustainable under current environmental regulations. Alternative production routes are therefore sought; one of these could be the biological conversion starting from NSA β-naphthalenesulfonic acid (NSA).1 NSA is a naphthalene molecule with an SO3H group in the β position (MW 208). NSA is produced as a mixture which contains approximately 30% of sulfuric acid. In the development of the biotechnological process, it is necessary to reduce the content of sulfuric acid, which tends to be metabolized preferentially. It is, therefore, necessary to introduce a separation unit in order to purify NSA before the reactor unit. It is known that aliphatic secondary and tertiary amines dissolved in an organic solvent are powerful extractants for carboxylic acids.2-4 The amine binds the acid through reversible complexation, forming reverse micelles in the organic phase. In a recent study5 it was found that the commercial secondary amine Amberlite LA-2 dissolved in toluene showed a high thermodynamic selectivity for H2SO4 with respect to HCl. Since NSA is a monoprotic acid like HCl, it is to be expected that Amberlite LA-2 could be a feasible extractant for the mixture considered in this study. Therefore, we have investigated the distribution of NSA between the aqueous and organic phases in the concentration range 0.01-0.5 m. The aim of this study is to determine the value of β∞, i.e., the selectivity at infinite dilution of NSA and sulfuric acid, which can be considered the true thermodynamic quantity in the evaluation of affinity and, therefore, may be used as an indication of the feasibility of a separation process.
NaOH Normex (1, 0.1 and 0.01 N) was purchased from Merck. Toluene (g99.8% GC) and perchloric acid (0.1019 N) in a solution of glacial acetic acid were purchased from Aldrich. Amberlite LA-2 (MW 377) and acetic acid (g99.8%) were purchased from Fluka. NSA sodium salt (90%) was purchased from Aldrich. NSA was obtained (98%) by repeated ion exchange with the resin Amberlite IR-120 PLUS purchased from Sigma. Bidistilled deionized water was used. The liquid-liquid equilibrium measurements were carried out by adding equal volumes of an aqueous NSA solution and an organic solution of Amberlite LA-2 (0.5 m) in a stirred glass flask in a temperature-controlled water bath at 25 ( 0.1 °C. Runs at different total contacting times confirmed that after 1 h complete equilibrium was achieved. After equilibration, the two phases were separated by centrifugation. The concentration of the acid in the aqueous phase was determined by titration with aqueous sodium hydroxide (relative uncertainty 1%). The free amine in the organic phase was determined by titration with perchloric acid in glacial acetic acid (relative uncertainty 1%). The amount of water in the organic phase was determined by Karl Fischer titration with a relative uncertainty of 3%. Modeling the Liquid-Liquid Equilibrium. NSA is a strong monoprotic acid which in aqueous solutions dissociates according to
* To whom correspondence should be addressed. Telephone: (+39) 862 434219. Fax: (+39) 862 434203. E-mail:
[email protected].
NSA f H+ + NSATherefore, the liquid-liquid equilibrium model can be derived in the same manner as that for HCl.5 The equilibrium distribution equation is given by
m′′H+ m′′NSAmNSA ′2 -
)
γ′(2 2 γ′′ (
H′NSA γ′(2 ) K H′′NSA γ′′2 N,NSA
10.1021/ie980088d CCC: $15.00 © 1998 American Chemical Society Published on Web 09/24/1998
(
(1)
Ind. Eng. Chem. Res., Vol. 37, No. 11, 1998 4529
where KN,NSA is the Nernst distribution coefficient of NSA between the aqueous phase (single prime) and the organic phase (double prime). Taking into account the equilibrium of protonation of the amine and electroneutrality of the organic phase
mNSA ′′2 mNSA ′2 -
)
γ′(2 2 γ′′ (
KN,NSA(1 + Qγm′′A)
(2)
where
Qγ )
m′′AH+ γ′′A γ′′H+ )K m′′A m′′H+ γ′′AH+
(3)
and the equilibrium constant for the amine protonation is
K)
m′′AH+ γ′′AH+ m′′A m′′H+ γ′′A m′′H+
(4)
In the limit to infinite dilution we may derive two useful relationships
lim
m′′H+ m′′NSA-
m NSA ′ - f0
mNSA ′2 ‚-
) KN,NSA
(5)
and
m′′NSA/ 1/2 lim ) [KN,NSA(1 + Ka′′ A )] m NSA ′ - f0 m′NSA-
(6)
/ where a′′ A is the activity of free amine in the initial organic phase. An alternative way to describe the equilibrium may be obtained by considering a simplified physical picture of the contacted mixtures. The water pools in the reverse micelles may be seen as “adsorption” sites. This leads to a langmuirian type of equilibrium isotherm.6 This approach yields a simple expression which may be used to correlate the experimental results
m′′NSA- )
bm′NSA1 + bcm′NSA-
(7)
where
b)
lim
m NSA ′ - f0
m′′NSAm′NSA-
(8)
From appropriate plots of the experimental values, it is therefore possible to determine the Nernst constant. Using the experimental values determined for H2SO45, it is possible to evaluate the infinite dilution selectivity, defined as
β∞ )
m′′NSA-/m′NSA) m Anion ′ f0 m′′ SO42-/m′SO42lim
(KN,NSA)1/2 / 1/6 (4KN,H2SO4)1/3 (1 + Ka′′ A )
(9)
Results and Discussion The experimental results obtained for the system containing NSA are reported in Table 1. The experimentally measured quantities were all the initial com-
Figure 1. Determination of the Nernst distribution coefficient for NSA. Table 1. Experimental Results for the Liquid-Liquid Equilibrium in the System Naphthalenesulfonic Acid (NSA)/Water/Toluene + Amberlite LA-2 organic phase aqueous phase m′NSA- (mol/kg)
m′′NSA(mol/kg)
1.50 × 10-5 2.30 × 10-5 2.80 × 10-5 3.60 × 10-5 4.30 × 10-5 5.30 × 10-5 6.40 × 10-5 9.60 × 10-5 5.10 × 10-4 1.28 × 10-3 1.80 × 10-3
2.78 × 10-2 4.29 × 10-2 5.41 × 10-2 7.08 × 10-2 8.30 × 10-2 9.88 × 10-2 1.12 × 10-1 1.22 × 10-1 1.70 × 10-1 1.98 × 10-1 2.77 × 10-1
m′′H+ m′′A m′′AH+ w H 2O (mol/kg) (mol/kg) (mol/kg) (g/kg) 0.0120 0.0127 0.0120 0.0143 0.0142 0.0154 0.0165 0.0172 0.0550 0.0644 0.1399
0.469 0.455 0.443 0.429 0.416 0.402 0.389 0.380 0.370 0.352 0.348
0.016 0.030 0.042 0.057 0.069 0.083 0.096 0.105 0.115 0.133 0.137
1.10 1.14 1.13 1.25 1.82 2.01 2.18 2.41 2.76 2.91 3.11
positions, the free amine in the organic phase, the amount of water in the organic phase, and the final acid composition in the aqueous phase. All other quantities are obtained from mass balances and electroneutrality relationships. The values of the Nernst constant for NSA is obtained from an extrapolation to zero concentration in the plot shown in Figure 1. The experimental data are also correlated using eq 7, and Figure 2 shows the comparison between calculated and experimental concentrations. The values of the parameters obtained are reported in Table 2. The simple langmuirian type isotherm is able to represent the equilibrium behavior in the concentration range which we investigated. Using the activity of the amine in toluene,5 evaluated from the Flory-Huggins model for activity coefficients,7 a value for the amine protonation equilibrium constant of 1.0 is found. This compares well with the value of 1.2 previously found5 for the systems HCl and H2SO4. The most important quantity that we want to evaluate is the infinite dilution selectivity. Since this is necessarily obtained indirectly, we also report in Table 2 the standard deviations and show in Figure 3 the 95% confidence ellipse. The estimated value for β∞ is 1.7 ( 0.1. This result shows the thermodynamic feasibility of the separation of NSA from dilute aqueous solutions containing H2SO4.
4530 Ind. Eng. Chem. Res., Vol. 37, No. 11, 1998
picture of the contacted solutions. The Nernst distribution coefficient was determined for NSA. The calcultate amine protonation equilibrium constant compared well with previously determined values. Finally the infinite dilution selectivity between NSA and H2SO4 clearly shows that Amberlite LA-2 is a potential extractant for the purification of NSA. Acknowledgment The authors thank G. Spagnoli for his help in the experimental measurements. Financial support from MURST is gratefully acknowledged. List of Symbols
Figure 2. Comparison between calculated and experimental NSA- concentrations in organic and aqueous phases. Table 2. Parameter Estimation of the Models (5) and (7) for the NSA Systema regressed parameters
Greek Letters
KN,NSA ( s.d. (×10-6)
b ( s.d.
c ( s.d.
Ka′′ A
4.0 ( 0.3
3070 ( 200
4.9 ( 0.1
0.8
a
a ) activity b ) distribution ratio at infinite dilution c ) constant in eq 7 H ) Henry’s constant K ) equilibrium constant of amine protonation KN ) Nernst distribution coefficient m ) molality Qγ ) defined in eq 3
/
s.d. ) standard deviation.
β∞ ) selectivity at infinite dilution γ( ) mean activity coefficient Superscripts and Subscripts A ) amine ′ ) aqueous phase ′′ ) organic phase
Literature Cited
Figure 3. 95% confidence ellipse for the parameters obtained for NSA.
Conclusions The liquid-liquid equilibrium of aqueous acid solutions containing NSA in contact with toluene and the secondary commercial amine Amberlite LA-2 has been investigated at 25 °C. The experimental equilibrium concentrations were correlated using a simple langmuirian type isotherm based on a simplified physical
(1) Zurrer, D.; Cook, A. M.; Leisinger, T. Microbial Desulfonation of Substituted Naphthalenesulfonic Acids and Benzenesulfonic Acids. Appl. Environ. Microbiol. 1987, 53, 1459-1463. (2) Yang, S. T.; White, S. A.; Hsu, S. T. Extraction of Carboxylic Acids with Tertiary and Quaternary Amines: Effect of pH. Ind. Eng. Chem. Res. 1991, 30, 1335-1342. (3) Ju, L. K.; Varma, A. Extraction of Anions with Tertiary Amine from Aqueous Solutions of Mixed Acid and Salt. Ind. Eng. Chem. Res. 1995, 34, 4479-4485. (4) Kirsch, T.; Maurer, G. Distribution of Oxalic Acid between Water and Organic Solutions of Tri-n-octylamine. Ind. Eng. Chem. Res. 1996, 35, 1722-1735. (5) Brandani, S.; Brandani, V.; Veglio`, F. Extraction of Anions from Aqueous Solutions Using Secondary Amines. Ind. Eng. Chem. Res. 1998, 37, 292-295. (6) Brandani, S.; Brandani, V.; Di Giacomo, G.; Spera, L. A Thermodynamic Model for Protein Partitioning in Reversed Micellar Systems. Chem. Eng. Sci. 1994, 49, 3681-3686. (7) Prausnitz, J. M.; Lichtenthaler, R. N.; de Azevedo, E. G. Molecular Thermodynamics of Fluid Phase Equilibria, 2nd ed.; Prentice-Hall: Englewood Cliffs, NJ, 1986.
Received for review February 11, 1998 Revised manuscript received July 27, 1998 Accepted August 11, 1998 IE980088D
