EXPERIMENTAL SETUP AND PROCEDURE

CHAPTER 3 EXPERIMENTAL SETUP AND PROCEDURE 3.1 Determination of vapour-liquid equilibria Isobaric Vapour-Liquid Equilibria date have been obtained, using a Smith and Bonner [39] type still which is a modified design of Othmer still. It is very simple in design, construction and operation. The still design was further modified to suit the systems containing salt. A schematic diagram is shown in Figure 3.1. 3.1.1 Description of the still Figure 3.1 is the modified version of the Othmer still [40] and as such the essential features are retained in its design. The still was modified to prevent or minimise, as far as possible, the errors due to the different sources identified by Ellis [41]. The reboiler, A was made of 500 ml capacity round bottom flask (pyrex glass). A slight modification in the entry to the reboiler greatly reduced both the axial and radial concentration gradient in the bulk of the reboiler liquid as pointed out by Ellis [41] and provided for accurate equilibrium composition. The neck, B, through which vapour passes was heated externally by nichrome wire E, wound spirally and lagged with an asbestos-magnesia mixtjre, AL. Apart from this, a baffle-like arrangement was provided along the entire neck B, to prevent any entrainment of liquid droplets into the vapour. The heating element, H, of the reboiler is encased in a protecting pyrex tube P to prevent the coil from any corrosive action of the reboiler liquid. It was ensured that the entire heating element was immersed below the liquid level to avoid superheating of the vapour. The entire setup was completely lagged using asbestos-magnesia mixture to minimise the heat loss. A magnetic stirrer was used to maintain sufficient homogeneity of the liquid mixture to improve salt dissolution. At the same

21 -* TO ATMOSPHERE WATER OUT ICE COOLED WATER IN CONDENSATE CHAMBER C SAMPLE OUTLET p> (GRADUATED) FIG.3 1 VAPOUR-LIQUID EQUILIBRIUM STILL ( SMITH AND BONNER TYPE )

22 time it decreased the irregularities in the boiling behaviour such as blanketing, chugging and sputtering. This appartus is thus suited, not only for miscible systems, but also for partially miscible systems as it was possible to keep the partially miscible systems always in an emulsion form, with the help of the magnetic stirrer. The performance of this apparatus was ascertained by measuring the vapour liquid equilibrium data for the binary systems CCI^ - 2-propanol and n-butanol-water, a miscible system and a partially miscible system respectively. The data were compared with the literature data [42], [43] and there was good agreement. 3.1.2 Procedure- for determining raw VLE data The raw equilibrium data were obtained at atmospheric pressure (which remained constant at 760 mm ± 3 mmhg) using the modified Smith and Bonner type still [Fig. 3.1]. The still was charged with 300 ml of mixture of the desired composition containing dissolved salt at a particular weight percent. The reboiler electric voltage was adjusted in Such a way that approximately 5 to 8 ml of condensate were collected per minute. At this rate no entrainment was observed. The jacket heater was adjusted to give 2 to 3 C superheat. The equilibrium was attained within 45 minutes as indicated by the constancy of equilibrium vapour temperature recorded by the standard mecury thermometers. However the condensed vapour phase was circulated for 2\ to 3 hours before samples were drawn for analysis. The* vappyrphase, being free from salt, was analysed for its composition y?, by gas chromatographic analysis using established procedures [25]. The details of the analytical procedures adopted in this investigation are discussed in Appendix A-3.1. The liquid sample at the end of each run was analysed for its composition on a salt-free basis by first separating it quantitatively into salt and solvent fractions by a total distillation technique of evaporating to dryness. The equipment used for the evaporation is similar to the one used by Hashitani et al [44]. The liquid phase composition, x; was then analysed for composition [Appendix A-3.1] and the results were determined on salt-free basis.

23 3.2 Determination of liquid-liquid equilibria 3.2.1 Description of the apparatus used Figure 3.2 shows a schematic diagram of the apparatus used for the dertermination of the solubility (curve) data. It consists of a thermostat (water bath) (CT) which can be maintained at any desired temperature. In this case 30 C ± 0.1 C was maintained. The water can be circulated by means of a pump to both the glass flask (JMC) and to the burette (JB). The glass flask is provided with a jacket to facilitate the water flow. A tefloncoated iron piece (TS) placed inside the flask is actuated by the electrically operated magnetic stirrer (MS). The stirrer keeps the partially miscible system always in an emulsion form. 3.2.2 Procedure for determining the soluibility curve and Tie-line data The solubility data were determined by the following procedure. A known amount of mixture of partially miscible components, for example, ethyl acetate and water, were taken in the stoppered glass apparatus (JMC). This mixture was then titrated with the solute say, methanol, until a clear homogeneous liquid mixture is obtained. Methanol was added by means of micro-burette (JB). The end points were always sharp. The titration procedure is cumulative and for each amount of water added, a series of points were determined by titrating again with methanol. The amount of each component was weighed as it was added. The tie-lines were determined by Gas Chromatographic analysis of liquid samples drawn from the two liquid phases at equilibrium. The two phase liquid mixture of about 20 to 30 cc was shaken vigorously in a 50 cc separating funnel immersed in a water bath kept at constant tempeature. The mixture was placed in the temperature bath at 30 C, for a sufficiently long time to allow it to reach equilibrium. Frequently the mixture was shaken and finally allowed to separate into two layers. When the equilibration was complete the individual phases were separated. Two to three cc of samples were drawn from each layer with a preheated syringe and was immediately transferred

24 CONSTANT TEMPERATURE APPARATUS MIXING CHAMBER/ TITRATION CHAMBER ASSEMBLY Fig. 3 2 EXPERIMENTAL SETUP FOR LIQUID - LIQUID EQUILIBRIA.

25 to 5 cc tablet tubes. The samples were then analysed by a gas chromatograph connected to a microprocessor. The analytical and equipment details are given in Appendix A-3.1. 3.3 Determination of heat of mixing d/fcfa.. 3.3.1 Description of the apparatus The schematic diagram of the calorimeter used in the experimental setup is shown in Figure 3.3. The calorimeter used for measurement of heat of mixing in the present investigation was similar to the one used by Tsao and Smith [45] and Viswanath et al [46]. The calorimeter (1) consisted of a cylindrical Dewar flask of 12 cm height and 5.5 cm internal diameter. The 3 total volume of the calorimeter was 215 cm. The vapour space in the calorimeter was kept small to minimise losses due to vaporization. The calorimeter was provided with inlets at the top, one for a nozzle from the jacketed burette, one for a heating element, the other two for resistance thermometer and Beckmann thermometer, respectively. A teflon coated magnetic piece (5) was actuated by a magnetic stirrer (6) to stir the liquid contents inside the flask. The heating element consisted of 16 gauge nichrome wire wound as a coil encased in a thin walled glass tube. It was placed inside the liquid just over the stirring magnetic piece. A resistance thermometer was used to detect the changes in temperature from the set experimental values, due to the heat effects accompanying the mixing of the liquids. The heat of mixing values are registered by a microprocessor based unit, 'MIPROC' [47]. It has a built-in programme to supply and measure the equivalent heat energy to the liquid system to bring it back to the original temperature conditions and to measure the values of enthalpy of mixing and display them directly (in cals/moles). Other details about 'MIPROC' are given in Appendix A-3.2. 3.3.2 Experimental procedure About 100 cc of one of the components was introduced from a calibrated burette into the calorimeter and the second component was placed in the jacketed burette. The whole assembly was placed in a constant temperature bath, whose temperature was controlled by a special relay circuit and water

26 0» 1 3 5 DEWAR PLASK 2 HEATING ELEMENT 4 MAGNETIC PIECE 6 * 7 CONSTANT TEMPERATURE JACKETED BURETTE BECKMANN THERMOMETER MAGNETIC STIRRER WATER BATH FIG. 3-3. SCHEMATIC DIAGRAM OF THE EXPERIMENTAL SETUP FOR HEAT OF MIXING.

27 from this bath was circulated through the jacket of the burette. Sufficient time (1 to 14 Hrs) was allowed for the liquid in the burette to attain the desired temperature. By operating the heating element the temperature of the liquid in the calorimeter was also brought to the same value. When both the components were at the desired temperature, a known amount of the liquid from the burette was then added slowly, with the stirrer in action, and the drop or rise in temperature was recorded in the digital display of the 'MIPROC' calorimeter, or registered in the Beckmann thermometer. Afterwards the heater circuit in MIPROC' was turned on, in order to bring the temperature back to the original experimental temperature. During the course of a run, about 4-5 readings were taken after each successive addition of the component from the burette. The heats of mixing for the whole range of composition were determined by taking the second component in the calorimeter and adding the first, from the burette, following the same procedure. 3A Material used All the solvents used in this investigation were obtained from M/s. E.Merck (India) Ltd., and also from British Drug House Laboratories (India) Ltd., Bombay. The hydrocarbons were assayed by gas chromatography and exceeded 99.5 percent purity and some of them were used after further purification. Anhydrous Reagent grade salts, Sodium Chloride, Calcium Chloride and Zinc Chloride have been purchased from BDH (Glaxo) Laboratories and E. Merck (India) Ltd., Bombay. Some of them have been imported from Retdel*de Haen AG. Germany. All salts were dried before use. 3.5 Details of the systems studied under the present investigation 3.5.1 Liquid-liquid equilibria (Temp. 30 C) 1. Ethyl Acetate (1) - 2-Propanol (2) - Water (3) 2. Benzene (1) - 2-Propanol (2) - Water (3) 3. Benzene (1) - Pyridine (2) - Water (3) 4. Benzene (1) - Methanol (2) - Water (3) 5. Ethyl Acetate (1) - Methanol (2) - Water (3)

28 3.5.2 Vapour-liquid equilibria (ls(q?aric) 1. 2-Propanol (1) - Water (2) 2. Ethyl Acetate (1) - 2-Propanol (2) 3. Ethyl Acetate (1) - Water (2) 4. Benzene (1) - 2-Propanol (2) 5. Benzene (1) - Water (2) 6. Benzene (1) - Pyridine (2) 7. Water (1) - Pyridine (2) 8. Methanol (1) - Water (2) 9. Methanol (1) - Benzene (2) 10. Methanol (1) - Ethyl Acetate (2) 3.5.3 Heat of mixing 1. Methanol (1) - Water (2) 2. Ethyl Acetate (2) - 2-Propanol (1) 3. Methanol (1) - Benzene (2) 4. 2-Propanol (1) - Benzene (2) 5. Methanol (1) - Ethyl Acetate (2) 6. Pyridine (1) - Water (2) 7. Benzene (1) - Pyridine (2) 8. 2-Propanol (1) - Water (2) Temperature : 30 C ± 1 C 3.5.4 Salts employed 1. Sodium Chloride (Dried) Anhydrous 2. Calcium Chloride (Fused) Anhydrous 3. Zinc Chloride (Powdered) Anhydrous 3.5.4.1 Concentrations : 5, 10, 15, 20, 25 percent by weight and saturated depending upon the solubility in each case.