LIU Qisong (劉其松), YAO Shun (姚舜), ZHU Tangfeng (朱堂峰), ZENG Hong (曾紅) and SONG Hang (宋航)

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Vapor Pressure Measurement and Correlation of 2-Methyl-ButanolAcetate Containing Calcium Chloride

LIU Qisong (劉其松), YAO Shun (姚舜), ZHU Tangfeng (朱堂峰), ZENG Hong (曾紅) and SONG Hang (宋航)*

School of Chemical Engineering, Sichuan University, Chengdu 610065, China

The CaCl2solubility in 2-methyl-butanol acetate and the vapor pressure of 2-methyl-butanol acetate containing CaCl2were measured in the range of 90-135 °C and from very low salt concentration to saturation. The experimental data were correlated with two equations, a modified Antoine equation with the dissolved salt taken into account and a nonrandom two liquid-electrolyte (e-NRTL) model. Both models are in good agreement with the experimental data. This study provides essential physical data for further investigation of vapor-liquid equilibrium system containing salt.

vapor pressure, 2-methyl-butanol acetate, calcium chloride, Antoine equation, e-NRTL

1 INTRODUCTION

2-Methyl-butanol acetate is routinely obtained as a byproduct in lipase-catalyzed enantioselective esterification of (±)-2-methyl-butanol [1]. 2-methyl-butanol and 2-methyl-butanol acetate form a minimum-boiling azeotrope at a mole fraction of 2-methyl-butanol of 0.78 at 127.9 °C and atmospheric pressure [2]. Special processes are required for separating 2-methyl-butanol acetate from 2-methyl-butanol. The extractive distillation with salts may be used, since a soluble salt may change the relative volatility of azeotrope components [3-6]. Phase equilibrium data of the system are necessary to build VLE models to develop the separation process. To find the parameters in the models, experimental data of pure components and binary subsystems are generally needed. When calcium chloride is added to perform an extractive distillation, VLEs of subsystems 2-methyl-butanol + 2-methyl-butanol acetate, 2-methyl-butanol + calcium chloride, and 2-methyl- butanol acetate + calcium chloride are needed. Among them, 2-methyl-butanol acetate + calcium chloride data are not available, while others have been reported [2, 7-10]. Therefore, this work focuses on the investigation of binary system 2-methyl-butanol acetate + calcium chloride at different salt concentrations ranging from a very low value to saturation (mass fractions from 0.018 to 0.065). The solubilities of CaCl2in 2-methyl-butanol acetate at different temperatures (from 90 to 135°C) are determined. The experimental binary VLE data,.. vapor pressure of 2-methyl-butanol acetate containing salt calcium chloride (without salt in vapor), are correlated with two models: an empirical Antoine equation in the presence of the dissolved salt and a thermodynamic model, nonrandom two liquid- electrolyte (e-NRTL) model based on the concept of local composition.

2 EXPERIMENTAL

2.1 Chemicals

2-Methyl-1-butanol acetate (mass fraction purity≥0.99) was purchased from Aldrich. Anhydrouschloride calcium (mass fraction purity ≥0.99, from Chengdu Hangjia Biological and Pharmaceutical Tech. Ltd) was dried at 130 °C in the vacuum drying chamber.

2.2 Apparatus and procedures

The vapor pressure was measured by the boiling point method [8-12], which is valid for the system containing a volatile species and a non-volatile species. It is a fast measurement method and is usually adopted in a vacuum system. The existence of inert gases has no effect on the result [13].

The schematic diagram of the apparatus similar to the literature [8] is shown in Fig. 1. This apparatus was primarily consisted of an equilibrium vessel (1000 cm3), an oil bath, a condenser cooled by water, a Pt100 digital thermometer with the uncertainty of ±0.1 °C, a U-tube mercury manometer with the minimum calibration of 1 mm, a magnetic stirrer, and a vacuum system. The atmospheric pressure was measured with a calibrated barometer with minimum calibration of 0.1 kPa, whose accuracy is comparable with that of the U-tube mercury manometer. The pressure of the system was determined with the U-tube mercury manometer and the atmospheric barometer.

A solution at a desired concentration was preparedwith an electronic balance with an uncertainty of ±0.001 g. 600 cm3of the solution was placed in the equilibrium vessel and evacuated to a proper vacuum pressure, which was controlled by the operation of the vacuum control valve and the vacuum pump. A pressure buffer was used to suppress pressure fluctuations. The sample solution was then heated in the oil bath and stirred well by the magnetic stirrer to prevent superheating.

Figure 1 Vapor pressure determination apparatus

1—Pt100 digital thermometer; 2—heater; 3—salt solution; 4—mercury manometer; 5—magnetic stirrer; 6—molecular sieve sorbent; 7—pressure-controller vessel; 8—vacuum pump; 9—condensing tube; 10—oil bath; a, b—two-way valve; c—three-way valve; d-solenoid valve

It took about 30 min to achieve constant temperature and the temperature was kept for about 40 min to insure equilibrium. Then the temperature and pressure were measured. The amount of salt in each sample was determined by weighing the sample when it was taken from the reboiler and after the solvent was completely evaporated. Each measurement was carried out three times. The estimated concentration error was 0.2%, which is the standard deviation of the differences in the duplicate determinations of concentration.

2.3 Validity of the method

To validate the method used in this work, the vapor pressures of pure water and the methanol + sodium acetate system with 0.0952 mass fraction of sodium acetate were measured. The results were accorded with those reported [14, 15] very well. The average relative deviations are 0.33% and 0.15% in vapor pressures for water and methanol + sodium acetate, respectively.

3 EXPERIMENTAL RESULTS

3.1 Solubility and correlation

The solubility of CaCl2in 2-methyl-1-butanol acetate at different temperatures is listed in Table 1. The solubility (in mass fraction) and temperature (in °C) can be correlated with Eq. (1)

Table 1 The CaCl2 solubility in 2-methyl-1-butanolacetate at different temperatures

Figure 2 Vapor pressure of the 2-methyl-1-butanol acetate + CaCl2system at different temperatures

3.2 The vapor pressure of 2-methyl-1-butanol acetate and empirical correlation

The vapor pressure-temperature data of 2-methyl- 1-butanol acetate with different saline concentrations are shown in Fig. 2. As expected, the vapor pressure decreases slightly as salt concentration increases.

For data correlation, a modified Antoine equation, Eq. (2a), is applied [16]. Parameters are used to take into account the salt concentration effect, as represented by Eq. (2b).

wheresalis the salt concentration (in mass fraction) andis a parameter in the Antoine equation. Parameters,andfor each salt concentration are determined by using a regression method with the following objective function

Table 2 gives parameters A, B, and C for different salt concentrations. The coefficients in Eq. (2b) for parameters A, B and C are depicted in Table 3.

Table 3 Cubic equation coefficients for 2-methyl-1-butanol acetate + CaCl2 system

With the model Eq. (2) and parameters in Table 3, the vapor pressure of the system containing salt can be calculated in the range of temperature and salt concentration in this study . The experimental and calculated values are given in Table 4. The standard deviation between the experimental and the calculated values for the whole data set is 0.20 kPa. Thus the modified Antoine equation is not only quite simple but also suitable to correlate vapor pressure of organic solvent containing salt.

Table 4 Experimental and calculated( by means of the modified Antoine equation and the e-NRTL model ) vapor pressure of 2-methyl-1-butanol acetate + CaCl2 system①

3.3 Correlation of the vapor pressure with e-NRTL model

When the e-NRTL [17] model is used for systems with a solvent and a salt, the equilibrium condition is for solvent only, since solvent is the only species present in both phases. For pressures equal to or less than atmospheric pressure, the following expression applies

where subscripts ca, c and a represent the salt, cation and anion, respectively. The model is applied to experimental data by regression method with the objective function by Eq. (3).

The equilibrium vapor pressure is calculated with Eq. (4). The values of the adjustable equation parameters for 2-methyl-1-butanol acetate + calcium chloride system at different temperatures are given in Table 5. With these three model parameters and Eq. (5), the activity coefficient can be calculated for any composition of the liquid phase. The vapor pressure is calculated by Eq. (4) at the operating temperature. From the comparison of the experimental vapor pressures with those calculated by the e-NRTL model, it is found that the standard deviation of all the data is 0.37 kPa (Table 4). This is an acceptable deviation, so the model is in good agreement with experimental data.

Table 5 Parameters of the e-NRTL model

①Dimensionless.②In units of J·mol-1.

4 CONCLUSIONS

The solubility of calcium chloride in solvent 2-methyl-butanol acetate and the vapor pressure of the solvent containing calcium chloride, in temperature range of 90-135 °C at different salt concentrations from a very low one to saturation (mass fractions from 0.018 to 0.065), were experimentally determined. The solubility and temperature are satisfactorily correlated by an empirical correlation. The vapor pressure can be correlated by the modified Antoine equation and the e-NRTL model with the standard deviations of 0.20 kPa and 0.37 kPa, respectively.

NOMENCLATURE

,,Antoine equation constants

parameter of Antoine equation

number of experimental points

mass fraction of salt in liquid

Subscripts

anion

cation

ca salt

cal calculated with Antoine equation or e-NRTL model

exp experimental

component

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* To whom correspondence should be addressed. E-mail: hangsong@vip.sina.com

2010-07-14,

2010-10-19.