Liquid Liquid Equilibrium

Liquid-Liquid Equilibria of the Acetic Acid – Water – Ethyl Acetate Ternary System Ilao, Almajoy; Magboo, John Steven; Rica, Ariel Raye Department of Chemical Engineering University of the Philippines – Diliman ChE 124 Instructional Laboratory: FQRUV

  Abstract— The binodal curve for the system of acetic acid-waterethyl acetate at 25°C and 1 atm was constructed in a barycentric phase diagram using the ternary liquid-liquid equilibrium data obtained through cloud-point titration. Several mixtures of ethyl acetate and water, of known amounts, were titrated using glacial acetic acid. The endpoint was represented by the disappearance of the two distinct layers and the amount of glacial acetic acid used was recorded. The data obtained were compared to that acquired by Sohoni and Warhadpande at 30°C since the binodal curve is independent with temperature. The experimental data was found to be in agreement with the literature with an Rsquared value of 0.8971.

from the relative distances of a point to each of the vertices. Any point within the triangle represents a ternary mixture, but the diagram is divided into two regions by curve a-P-b, called the phase envelope or binodal curve. Inside this curve lies the two-phase region, where both extract and raffinate liquid phases coexist, while a mixture in the one-phase region does not separate into two distinct phases. For a point M lying in the two-phase region, the mixture will separate into two layers a and b, the raffinate and extract phases, which are in equilibrium on a tie line. The tie lines converge at the plait point P, where the two phases are identical.

Keywords-Liquid-liquid equilibria, ternary system, binodal curve, acetic acid, water, ethyl acetate

I.

INTRODUCTION

Acetic acid is the most widely used carboxylic acid. It is used in many reactions such as the synthesis of esters and more frequently as a solvent in the cellulose acetate industry and in the manufacture of pharmaceutical products [1]. Sources of acetic acid in dilute form are wood distillation and fermentation. Large quantities of aqueous acetic acid are obtained in these processes, and its recovery in more concentrated form is of economic significance [2]. The strong affinity of acetic acid for water makes the separation difficult. The boiling points of acetic acid and water are very close at 118°C and 100°C, respectively. Since the mixture is close-boiling, using conventional distillation would require a column with large number of stages and a high reflux ratio, the whole process being very expensive [3]. More economical methods that involve the addition of an auxiliary substance include extractive distillation, azeotropic distillation, and liquid-liquid extraction [4].

Figure 1. Ternary liquid-liquid phase diagram for partially miscible pairs A and B. Image from [6].

In liquid-liquid extraction, also referred to as solvent extraction, the solute is partitioned between two relatively insoluble liquid phases due to the differences in solubilities. The liquid phase rich in solute is called the extract phase, while the remaining phase being stripped of solute is the raffinate phase [5].

Ethyl acetate is commonly used as the extracting solvent for dilute aqueous acetic acid solutions. For the system acetic acid-water-ethyl acetate, water is the carrier, acetic acid is the solute, and ethyl acetate is the solvent. Knowledge of the liquid-liquid equilibria among these three components is essential for the design of extraction equipment. The solubility data for the phase envelope of this system at 30°C and 1 atm were obtained by Sohoni and Warhadpande [7] using cloudpoint titration. Experimental results by Garner and Ellis [8] for the same system at 30°C, 40°C, and 50°C revealed that the equilibrium data is almost independent with temperature changes.

Ternary liquid-liquid equilibrium data are often represented in a barycentric phase diagram shown in Fig. 1, where a pair of components A and B is partially miscible. The vertices represent pure components A (carrier), B (solute), or C (solvent), and the mass fraction of each component is read

The purpose of this experiment is to obtain ternary liquidliquid equilibrium data for the system acetic acid-water-ethyl acetate at 25°C and 1 atm using cloud-point titration and to construct the phase envelope on an equilateral triangular diagram similar to Fig. 1. The experimental data will be

compared to that obtained by Sohoni and Warhadpande [7] at 30°C since the binodal curve is only slightly dependent with temperature [8]. II.

MATERIALS AND METHODOLOGY

Analytical grade ethyl acetate, glacial acetic acid, and a laboratory source of distilled water were used. These were assumed of high purity. Indicated amounts of water and ethyl acetate were mixed in 100-mL volumetric flasks for a total of 9 different mixtures. Each mixture was titrated with glacial acetic acid as the titrant. The end point, or the cloud point, was ascertained by the disappearance of the two distinct layers and the formation of a hazy, turbid solution. Data points for the binodal curve were obtained from the known amounts of water and ethyl acetate and the amount of acetic acid titrant required to reach the end point. The experiment was performed under ambient conditions (assumed 25°C and 1 atm) and near sunlight exposure to confirm the turbidity of the solutions. III.

data is insensitive to small temperature changes. Hence, the comparison made was acceptable. Based on Fig. 2, the composition of acetic acid increases with the ethyl acetate composition up to a certain point. Then, it decreases as the composition of ethyl acetate is further increased. Hence, it can be deduced that there is a maximum composition of acetic acid wherein the system can have a twophase region. Deviations of the experimental data in determining the binodal curve can be accounted to the judgment of the experimenter on the disappearance of the two layers and the turbidity of the mixture. Also, when the solution was mixed during titration, the organic ethyl acetate (extract) layer developed small bubbles which affected the turbidity of the mixture. This resulted to overtitration. Moreover, hydrolysis of acetic acid may have occurred during titration producing ions which altered the interaction of the molecules in the system [7]. IV.

RESULTS AND DISCUSSION

The binodal curve for acetic acid-water-ethyl acetate ternary system at ambient conditions was determined using cloud-point titration. Ethyl acetate and water are immiscible with each other. Thus, their mixtures form two distinct layers. Ethyl acetate, which is less dense than water, is the top layer. The dropwise addition of glacial acetic acid in the mixture decreases the immiscibility of the two species resulting to a single-phase mixture, i.e. disappearance of the two layers. The known amounts of ethyl acetate and water, together with the amount of glacial acetic acid used in titration, were converted to mass. Then, the compositions by mass of each species in the titrated mixtures served as the data points to construct the binodal curve. The experimental data was compared to that obtained by Sohoni and Warhadpande [7] in Figure 2.

CONCLUSION AND RECOMMENDATIONS

Cloud-point titration was performed in several ethyl acetate-water mixtures using glacial acetic acid as the titrant. The known amounts of ethyl acetate and water, and the amount of glacial acetic acid used in titration were used as data points to construct a phase envelope for the mixture. The experimental data obtained is close to the literature value [7] with an R-squared value of 0.8971. The use of a spectrometer will be helpful in obtaining more accurate results. Also, it is important to titrate the solution in an ice bath because at low temperature, hydrolysis of acetic acid is minimized. Other data such as the plait point of the mixture can also be determined by gathering equilibrium data and generating tie lines. Effects of temperature and presence of inorganic salts can also be investigated in the future experiments. Also, for additional comparison, the equilibrium data for this system may be modeled using NRTL, UNIQUAC, UNIFAC, etc. REFERENCES [1] [2]

[3] [4] [5] [6] Figure 2. Comparison of the experimental data and literature data [7] in a ternary diagram

The experimental data is in agreement with the data found in literature [7] with an R-squared value equal to 0.8971. The experiment was performed at a room temperature of 25°C while the literature data was gathered at 30°C. But [8] showed that for the acetic acid-water-ethyl acetate system, equilibrium

[7] [8]

Y. S. Park, K. Toda, M. Fukaya, H. Okumura, and Y. Kawamura. Applied Microbiology and Biotechnology, vol. 35, pp. 149-153, 1991. F. H. Garner, S. R. M. Ellis, and U. N. G. Roy. “Extraction of acetic acid from water. 1 – benzene-acetic acid-water.” Chemical Engineering Science, Pergamon Press Ltd., vol. 2, pp. 14-17, 1958. E. Sebastiani and L. Lacquaniti. Chemical Engineering Science, vol. 22, pp. 1155-1162, 1967. F. W. Further and A. R. Cook. Journal of Heat and Mass Transfer, vol. 10, pp. 23-37, 1967. A. S. Foust, et al. Principles of Unit Operations, 2nd ed. John Wiley and Sons, Inc., Singapore, 1980. C. J. Geankoplis. Principles of Transport Processes and Separation Processes, 4th ed. NJ: Prentice Hall, 2003. V. R. Sohoni and U. R. Warhadpande. Ind. Eng. Chem. 1428, vol. 44, no. 6, 1952. F. H. Garner and S. R. M. Ellis. “Extraction of acetic acid from water. 2 – ethyl acetate-acetic acid-water.” Chemical Engineering Science, Pergamon Press Ltd., vol. 2, pp. 18-26, 1958.

APPENDIX A. Equilibrium Data for the Binodal Curve at 30°C by Sohoni and Warhadpande [7] TABLE I.

COMPOSITIONS AT CLOUD POINT FROM SOHONI AND WARHADPANDE (1952)

Mass % Ethyl Acetate 90.80

5.30

77.40

9.60

13.00

64.70

17.90

17.40

55.30

24.60

20.10

44.80

33.10

22.10

37.00

41.00

22.00

Experimental y values

29.30

48.90

21.80

37.00

41.25

21.75

22.00

57.00

21.00

15.25

67.50

17.25

8.00

80.30

11.70

7.10

92.90

0.00

96.50

3.50

0.00

8.38 19.55 21.25 22.02 22.58 20.40 19.40 15.74 11.84 R2

EXPERIMENTAL DATA FOR THE COMPOSITIONS AT CLOUD POINT

Mixture A B C D E F G H I

Ethyl acetate Mass (g) Mass Percent 1.83885 9.186327 3.7674 18.98059 5.5614 28.88664 7.3554 38.60819 9.1494 48.02759 10.9434 57.52206 12.69255 65.19381 14.48655 74.94859 16.19085 83.03575

Mass (g) 16.5 12.2 9.6 7.5 5.6 4.2 3 1.8 1

Water Mass Percent 82.42891 61.46498 49.86365 39.36719 29.39586 22.07656 15.40915 9.312601 5.12856

Acetic Acid Mass Mass (g) Percent 1.6784 8.384768 3.8813 19.55443 4.0911 21.24971 4.196 22.02463 4.3009 22.57655 3.8813 20.40137 3.7764 19.39704 3.0421 15.73881 2.3078 11.83569

C. Sample calculations for Mixture A

0.897𝑔 = 1.83885   ± 0.05  𝑔  𝐸𝑡𝑂𝐴𝑐   𝑚𝐿 !! 16.5 ± 0.05  𝑚𝐿  𝐻! 𝑂 = 16.5   ± 0.05  𝑔  𝐻! 𝑂

2.05 ± 0.05  𝑚𝐿  𝐸𝑡𝑂𝐴𝑐

1.6 ± 0.05  𝑚𝐿  𝐻𝑂𝐴𝑐

!" !.!"#  ! !"

= 1.6784 ± 0.05  𝑔  𝐻𝑂𝐴𝑐  

1.83885 %  𝐸𝑡𝑂𝐴𝑐 = ×100% = 9.186327 ± 0.25% 1.83885 + 16.5 + 1.6784 16.5 %  𝐻! 𝑂   = ×100% = 82.42891 ± 0.44  %   1.83885 + 16.5 + 1.6784 1.6784   %  𝐻𝑂𝐴𝑐 = ×100% = 8.384768 ± 0.25% 1.83885 + 16.5 + 1.6784

D. R2 Goodness-of-fit calculation The best-fit 6th-degree polynomial equation for the literature equilibrium data is given by Equation (1):

(1)  

where x is the mass percent ethyl acetate and y is the mass percent acetic acid in the mixture. Given that Equation 1 provided a good representation of the literature data with an R2 value close to 1, the y values obtained from this equation were compared to the obtained experimental data for the same mass percentages of ethyl acetate given in Table II.

Mass % Acetic Acid 3.90

Mass % Water

B. Experimental masses and mass percents of ethyl acetate, water and acetic acid for Mixtures A - I TABLE II.

y  =  -­‐1.54343E-­‐09x6  +  5.20399E-­‐07x5  -­‐  7.15685E-­‐05x4  +  5.08674E-­‐ 03x3  –  0.202013x2  +  4.46149  –  22.9879   R²  =0.  996774    

TABLE III.

   

COMPARISON OF THE EXPERIMENTAL AND LITERATURE DATA y values based on Equation (1) 10.4165 18.4758 21.2358 22.0992 21.6255 20.414 18.5579 14.3354 7.51894 0.8971