VAPOUR-LIQUID EQUILIBRIUM IN SYSTEMS WITH ISOBUTYL ACETATE , ACETIC ACID AND METHYL ETHYL KETONE

The experimental temperature dependence of the vapour pressures of isobutyl acetate, acetic acid and methyl ethyl ketone were determined, and isobaric vapour–liquid equilibrium data of binary systems of methyl ethyl ketone + isobutyl acetate and methyl ethyl ketone + acetic acid were obtained. The experimental data were processed using the Antoine and Riedel equations and the NRTL and Wilson local composition equations, respectively. Comparison of the experimental and calculated data confirmed the adequacy of the vapour–liquid equilibrium mathematical simulation.


Introduction
Reliable experimental data on the vapour-liquid equilibrium (VLE) of binary systems are necessary to obtain adequate mathematical parameters, which enable one to investigate the phase equilibrium and estimate optimal operation parameters of single distillation columns and the entire flow sheet.
The ternary system of isobutyl acetate (IBA) + acetic acid (AA) + methyl ethyl ketone (MEK) is notably interesting, because it includes the binary IBA+AA system with two binary azeotropes [1,2].Its separability in the presence of MEK by extractive distillation has been argued by Chelyuskina et al. [3].
In the present article, the IBA+AA+MEK ternary system was selected as an object of phase-diagram study.There are no experimental VLE data of the binary MEK+IBA system [4,5].
The investigation of VLE of the ternary IBA+AA+MEK system is necessary to determine the NRTL and Wilson binary interaction parameters and to design the separation processes.

Experimental (Chemicals, Apparatus and Procedure)
In the first step of experimental research, MEK (Hungarian manufacturer, qualification: "for electrotechnical purpose"), IBA (Belgian manufacturer, 98% mass), and AA (Russian manufacturer, qualification: "chemically pure") were purified, because there were relatively small amounts of impurities (up to 1% by weight in MEK and IBA and up to 1.5% in AA).The impurities were identified using gas-liquid chromatography.
MEK was dried on molecular sieves (3 Å), which were heated to 693.15 K.The concentration of MEK after purification was 99.9% by mass.
IBA was purified from AA impurities by adding dry alkali (KOH).IBA was boiled at reflux for 6 hours.The splitting distillate was selected, its organic phase (IBA) was dried on molecular sieves, and the water phase was used.The purity of IBA was more than 99.9% by mass.
AA was purified using a packed batch distillation column, which was filled with a Fenske glass nozzle.The efficiency of the packed column was approximately 15 theoretical stages.The concentration of AA after purification was 99.95% by mass.
For the AA component and the MEK+AA mixture, the conditions of the GC analysis were as follows: packed column, 10% polyphenylmethylsiloxane on Polychrom-1; column material, glass; column length, 3.0 m; internal diameter, 4 mm; carrier gas, helium; carrier gas volume rate, 100 mL/min; thermal conductivity detector; t co l = 90°C; t ev = 250°C; t d = 150°C; i d = 150 mA.
The outlet order of the components on the chromatograms was MEK-IBA and then MEK-AA.In the method of internal normalization [6], MEK was used as the linking component (its correction coefficient is 1).The correction coefficients for the other components of the binary mixture were 0.7826 for IBA and 0.8012 for AA.The uncertainty in mass fraction was less than 3%.
The physical-chemical properties of the components are provided in Table 1.Experimental investigation of the VLE was performed using a modified Sventoslavsky's [7] still.The construction of the still enables one to pick a sample of vapour using a 1-2 mkL chromatographic syringe without interrupting the experiment.
The VLE measurements were performed at 101.33 kPa azeotropes; the pressure was maintained using an electronic system with an accuracy of ±0.5 kPa.The temperature was measured using a calibrated thermocouple, which was inserted in a thermometric sleeve filled with glycerol; the accuracy was 0.1°C.The samples of vapour condensate were analysed using the GC method.The vapour pressure was determined using a modified Sventoslavsky's still.

Determination of the vapour pressures of the components
The experimental values of vapour pressure of the pure components are provided in Table 2.The experimental dependences of the vapour pressure on temperature were described using Antoine (1) and Riedel (2) equations: where P s is in mm Hg and T is in K.
The coefficients of the equations were evaluated using experimental data and are provided in Table 3.
Because the units of pressure in the experiment and equations ( 1) and ( 2) are mm Hg, the values of the component vapour pressures in Table 2 have the same units.
The comparison of the calculated and experimental vapour pressures (Table 2) demonstrates the satisfactory description of experimental data using both equations.The average deviations of the calculated values obtained using both equations are similar.However, there is an increase in the relative deviation in the series of components of ketone-ester-acid, which may be related to the peculiarity of a molecular interaction.

Experimental VLE data and thermodynamical consistency test
The experimental VLE data for the MEK+IBA (18 points) and MEK+AA (12 points) systems are provided in Table 4.
In the following stage, the experimental VLE data were verified for thermodynamic consistency.The test was performed using the Herringtone and Redlich-Kister criteria [5,8].The vapour pressure was calculated using the Riedel equation ( 2).The graphical processing of the results for the MEK+IBA system is shown in Figure 1.The dependence of the logarithm of the ratio of activity coefficients was approximated in Microsoft Excel using a cubic polynomial.By integrating this curve in a mole fraction range (0-1) we determined the value of the Herringtone and Redlich-Kister criterion to be -0.0013,which is in the range of permissible values ±0.02 [5].Thus, the obtained experimental VLE data for the MEK+IBA system are thermodynamically consistent.
For the MEK+AA system, the thermodynamic consistency test was conducted regarding the non-ideality of the vapour phase in the virial equation.The liquid mole volumes were calculated from the density data [9] at an average temperature of 99.1°C for the experimental data (Table 4).This value was 0.1012 L/mol for MEK and -0.0626 L/mol for AA.
The second virial coefficients of AA were graphically interpolated from the experimental data in [5].The second virial coefficients for MEK and cross coefficients were calculated using the Pitzer and Curl method [9].The results for the MEK+AA system are shown in Figure 2.
The dependence of Lg(γ 1 /γ 2 ) was approximated in Microsoft Excel using a 5th-order polynomial.
By integrating the curve in Figure 2 we obtain a Herringtone and Redlich-Kister criterion of 0.008.This value is in the permissible range ±0.02; therefore, the experimental VLE data for the MEK+AA system are thermodynamically consistent.

Mathematical simulation of the vapour-liquid equilibrium
The vapour-liquid equilibrium in binary systems was modelled using NRTL and Wilson models as follows: NRTL [10]: ; ; ; .
Wilson [11]: where The binary interaction parameters of the Wilson and NRTL models were obtained using experimental VLE data (Tables 5 and 6).
The boiling points and equilibrium vapour compositions in the considered systems were calculated on the basis of two local-composition equations and two vapour pressure equations.The calculations were performed with the use of the Flash unit operation in Aspen Plus.The considered pressure was 101.33 kPa.
Thus, for each binary system, four sets of calculated  7).
The VLE data are well described by both Wilson and NRTL equations.For the MEK+IBA system, the lowest average relative deviations are observed with the combination of the Wilson and Antoine equations.For this system, the Antoine and Riedel equation can be used to calculate the vapour pressure.For the MEK+AA system, the equation combinations Wilson+Riedel, NRTL+Antoine, NRTL+Riedel can be used.All combinations give a satisfactory correspondence between experimental and calculated VLE data.However, the average deviations in this case are higher than in previous cases, which may be related to the nonideality of the vapour phase.It should be noted that the NRTL binary-interaction parameters, which were obtained on the basis of the presented experimental data, may be useful for the simulation of the phase equilibrium of multicomponent multiphase systems, which contain the binary mixtures MEK+IBA, MEK+AA and water, for example.
The calculated VLE data for the equation combinations, which correspond to set 1 for the MEK+IBA system and sets 6-7 for the MEK+AA system, are characterized by lower average deviations, as shown in Tables 8-10.

Conclusion
Experimental data were obtained for the temperature dependence of the vapour pressure of isobutyl acetate, methyl ethyl ketone, acetic acid and vapour-liquid equilibrium data for the binary MEK+IBA and MEK+AA systems at 101.33 kPa.By combining the Wilson and NRTL equations with the Antoine and Riedel equations, eight sets of the calculated VLE data for the binary MEK+IBA and MEK+AA systems were obtained.All calculated data are consistent with the experimental data.

Fig. 1 .
Fig. 1.Dependence of the logarithm of the ratio of activity coefficients of MEK (1) and IBA (2) on the composition.

Fig. 2 .
Fig. 2. Dependence of the logarithm of the ratio of activity coefficients of MEK (1) and AA (2) on the composition.

Table 1 .
Physical-chemical properties of the components

Table 2 .
Vapour pressure of the components (mm Hg)

Table 3 .
Coefficients of the equations used to calculate the vapour pressures of the components

Table 4 .
Experimental VLE data for binary systems at 101.33 kPa

Table 7 .
Comparative analysis of the calculated VLE data at 101.33 kPa

Table 9 .
Calculated VLE data for the MEK (1) + AA (2) system at 101.33 kPa.Wilson + Riedel equations The lowest average relative deviations of the vapour composition and boiling point description are observed in the following cases: the MEK+IBA system with the Wilson and Antoine equations (Δy 1 , Δt); the MEK+AA system with the Wilson and Riedel equations (Δy 1 ,) and the NRTL and Antoine equations (Δt).