Enthalpy of mixing and heat of vaporization of ethyl acetate with benzene and toluene at 298.15 k and 308.15 k

THERMODYNAMICS AND SEPARATION PROCESSES

Enthalpy of mixing and heat of vaporization of ethyl acetate with benzene and toluene at 298.15 k and 308.15 k

K. L. Shivabasappa^I,*; P. Nirguna Babu^I; Y. Jagannadha Rao^II

^IDepartment of Chemical Engineering, Siddaganga Institute of Technology, Phone: [phone], Fax: [phone], Tumkur, 572103, India

^IIDepartment of Chemical Engineering, MVJ College of Engineering, Bangalore - 560067, India E-mail: [e-mail]

ABSTRACT

The present work was carried out in two phases. First, enthalpy of mixing was measured and then the heat of vaporization for the same mixtures was obtained. The data are useful in the design of separation equipments. From the various designs available for the experimental determination of enthalpy of mixing, and heat of vaporization, the apparatus was selected, modified and constructed. The apparatus of enthalpy of mixing was tested with a known system Benzene – i-Butyl Alcohol and the data obtained was in very good agreement with literature values. Experiments were then conducted for mixtures of Ethyl Acetate with Benzene and Toluene. The experimental data was fitted to the standard correlations and the constants were evaluated. Heat of vaporization data were obtained from a static apparatus and tested for accuracy by conducting experiments with a known system Benzene – n-Hexane and the data obtained were found to be in agreement with literature values. Experiments were then conducted to measure heat of vaporization for the mixtures of Ethyl Acetate with Benzene and Toluene. Using experimental data of enthalpy of mixing from the first phase, and heat capacity data, the heat of vaporization were calculated.

Keywords: Enthalpy of Mixing; Heats of Vaporization; Ethyl Acetate; Benzene and Toluene.

INTRODUCTION

Enthalpy of mixing refers to the change in the enthalpy per mole of solution formed when pure components are mixed at the same temperature and pressure. Enthalpy effects on mixing of liquids are quite large and of considerable importance especially for the design of absorption and distillation columns. In the present work a static type calorimeter incorporating the design features of Tanaka et.al. (1972), Patel (1974), Rao and Viswanath (1973) and Nirguna Babu (2003) was designed and used to measure enthalpy of mixing at 298.15 K and 308.15 K.

Heat of Vaporization is the amount of heat required to change a unit amount of liquid at bubble point (saturated liquid) to a vapor at its dew point (saturated vapor). Heat of vaporization data is useful for heat load determination in distillation columns. The design features of Dana (1925) were incorporated in the construction of the calorimeter adopted for the present study.

EXPERIMENTAL

Apparatus

In the present work two types of setups were used to generate data for enthalpy of mixing and heat of vaporization, as described below.

The schematic diagram of the calorimeter used for the measurement of enthalpy of mixing is shown in Figure 1. It consists of a cylindrical Dewar flask of inner diameter 55 mm and 113.7 mm height. The space between the wall of the calorimeter and the jacket is evacuated to 10 ^{– 4} mm Hg. The total volume is 270 cc. The top section of the calorimeter has provisions for inserting transducer AD590 (temperature sensing device), nichrome heater, a stirrer and two openings to feed two liquid components. The stirrer used is of paddle type. There are two paddles fixed equidistant and placed at 15 mm from the bottom as suggested by Uhl and Gray (1967) and Robert H Perry et al (1998). The stirrer passes through teflon joint with O-ring seal. Transducer AD590 and nichrome heater are inserted through B-14 ground glass joints. Two B-14 joints are provided on the sides of the calorimeter to feed the two liquid components that are in the jacketed burettes. A capillary tube of 85 mm height and 0.1 mm diameter is fused to the top section of the calorimeter as shown in Figure 1, to eliminate the pressure build up in the calorimeter and conduct the experiment at atmospheric pressure. A transducer AD590 with an accuracy of ± 0.1 ºC embedded in the glass tube is used as the temperature-sensing device. A digital multi meter with accuracy of 0.001mA is connected to the transducer and used for temperature measurement. The entire unit is kept in a water bath, whose temperature is maintained by a thermostat with 0.1 ºC accuracy. The heater consists of a nichrome wire coil of 1.5 W resistance fused to enameled copper wire ends. A power pack supplies controlled power to the heater. A 0.01 sec. accuracy electronic digital watch is used to measure the time.

A fractional h.p. motor is used for stirring through a speed regulator. Water from the constant temperature water bath is circulated through the jackets of the burettes fixed on the calorimeter with the help of another fractional h.p. pump.

Figure 2 shows the diagram of the apparatus used to measure heat of vaporization by Rao and Viswanath (1973) with few modifications. The calorimeter is enclosed in a jacket and the annular space is evacuated to 10 ^{– 4} mm Hg. The heating element inside the calorimeter is a nichrome wire fused to tungsten wire leads. The vapors from the calorimeter are condensed and collected in the liquid meter. When the liquid meter is filled with the condensate, the liquid is automatically siphoned into the pre heater. This siphoning device permits a close cycle of operation. The vapor line between the calorimeter and the condenser is heated 1ºC above the boiling point to prevent any partial condensation and downward flow of condensate. The pre heater consists of a three-liter flask and an external heater. The liquid in the pre heater is well stirred by means of a stirrer introduced through an O-ring seal. The black dots in the diagram represent heating elements. The liquid from the pre heater flows by gravity into the calorimeter and the flask surrounding it.

Power is supplied to the calorimeter heater from a very well regulated DC power source of not more than 0.1% ripple. This DC power unit is connected to AC power supply through a voltage stabilizer of 3% ripple. The DC power unit supplies up to a maximum of 300 Volts and 5 amperes. The power is measured with calibrated voltmeter and ammeter.

Procedure

The procedures for the two types of setups used are given below.

a) Procedure for Measurement of Enthalpy of Mixing

The calorimeter is immersed in the constant temperature water bath. The liquids whose enthalpy of mixing is to be determined are taken in jacketed burettes. Long stemmed thermometers of range -10 to 110 ºC with 0.1ºC accuracy are used to measure the temperatures of the liquids in the jacketed burettes. The temperature of the water bath is maintained at the desired value at which the enthalpy of mixing is to be measured.

When the temperature of the liquids in both burettes equals that of the constant temperature bath, T_0, a known amount of liquid (1) from one of the burettes is run down into the calorimeter. Stirring is started and continued till the end of the experiment. Then, from the second burette, a known amount of liquid (2) is run down into calorimeter so that the total volume of both liquids is 270 ml. The temperature T₁ in the calorimeter is registered by means of transducer AD590. The liquid in the calorimeter is allowed to cool to the original temperature T_0, .and the heater is switched on and heating is continued till the liquid attains the temperature T₁. The current through the heater, the potential drop across it and the heating time of the mixture are all recorded. Finally, the liquids are emptied from the calorimeter.

The same procedure is repeated for different volume ratios of the two liquids, always keeping the total volume of 270 ml.

b) Procedure for Measurement of Heat of Vaporization Measure

About 2100 CC of liquid mixture of known composition is taken in the pre heater. The liquid is heated to about 2ºC below its bubble point. The calorimeter heater is switched on and the voltage is adjusted to about 7.26 volts. The vapor line heater-voltage is adjusted to prevent the re-condensation of saturated vapor from the calorimeter. Steady state is reached in about 3 to 4 hours depending on the boiling point of the mixture. At steady state, the composition of samples drawn from the liquid meter is constant and equal to that of the feed mixture. The time required to fill the meter with condensate is also constant.

Purification and Analysis

The products used in the present work were purified by the methods suggested by Riddick and Bunger (1970) and also Weissberger (1949). The purity was checked by specific gravity, refractive index and vapor phase chromatography.

RESULTS AND DISCUSSIONS

Initially the enthalpy of mixing of a known system (Benzene – i-Butyl Alcohol) was measured at 298.15 K and 308.15 K. All data were taken at the local atmospheric pressure of 689 mm Hg.

The enthalpy of mixing was calculated using the formula.

Obtained data were in close agreement with Perrin (1981) values found in literature with average absolute deviations of 0.0157 and 0.0189, and maximum deviations of 0.024683 and 0.03003 at 298.15 K and 308.15 K respectively.

The experiments were then conducted with the unknown systems, Ethyl Acetate with Benzene and Toluene, at 298.15 K and 308.15 K. All experiments were conducted at 298.15 K and 308.15 K since there was no appreciable change in the enthalpy of mixing values beyond 308.15 K. Also the liquids start vaporizing at higher temperatures. The experimental data obtained are given in Tables 1 and 2 and shown in Figures 3 and 4. That enthalpy of mixing data was then fitted to the standard equation of the type

where A₀, A₁, A₂, and A₃ are constants and evaluated using regression analysis.

Using the following relation percentage deviation was calculated.

The calculated values of enthalpy of mixing and the deviations obtained for the two systems are also given in Tables 1 and 2.

In a second phase, experiments were conducted following the procedure explained above to obtain the heat of vaporization data for Ethyl Acetate with Benzene and Toluene systems using the equation (4). The latent heat is given by

The heats of vaporization were calculated using equation (5). The data required in the equation for the calculation are VLE data, heat of vaporization data and enthalpy of mixing values at boiling point. Since there was no variation in DH_m values beyond 308.15 K, the DH_m data at 308.15 K were used instead of at boiling point in the present calculations. The calculated values are compared with the experimental values and are given in the Table 4 along with heat of vaporization values at dew point. The data required at dew point were calculated using Watson (1943) relation and the deviations were found to be less than 5% as shown in Figures 5 and 6.

CONCLUSIONS

The enthalpy of mixing and heat of vaporization were measured experimentally for the systems Ethyl Acetate with Benzene and Toluene and from the data obtained it can be concluded that the apparatus employed for both determinations can be used for any unknown systems.

ACKNOWLEDGEMENT

The authors thank the management of Siddaganga Institute of Technology – Tumkur - Karnataka -India, for their financial support

NOMENCLATURE

Ai

Constants in enthalpy equation

J. mole ^–1

A, B

Constants in density equation

(-)

Cpi

Ideal gas specific heat of pure component

J. mole ^–1 . K^–1

E

Potential drop across the heater

milli volts

I

Current

milli amperes

Li

Heat of vaporization of pure component

J. mole ^–1

M

Molecular weight

(-)

T

Time

seconds

T_b, T_d

Bubble point and dew point temperatures

K

V

Volume of components

m³

x

Mole fraction

DH_m

Enthalpy of mixing

J. mole ^–1

DH^V

Heat of vaporization

J. mole ^–1

r

Density

kg . m ^{– 3}

(Received: June 4, 2007 ; Accepted: November 5, 2007)

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*

To whom correspondence should be addressed

Publication Dates

• Publication in this collection
  28 Apr 2008
• Date of issue
  Mar 2008

History

• Accepted
  05 Nov 2007
• Received
  04 June 2007