CHAPTER THREE
CHAPTER 3 CHAPTER 3 VAPOUR-LIQU ID EQUILIBRIUM
measures only iSOlhermal dala. the circulation method has been adopted. Circulation methods have been known to produce results of high accuracy in a rapid and simple manner (Joseph [2001]). This method operates by circulating vapour through a system and bringing it into repeated contact with the liquid until no further change in the composition of either phase takes place. Joseph [2001]
provides an excellent review on the development of the circulation method and the various types of apparatus used in this method.
A block diagram of the experimental set-up is shown in Figure 3-5. The various components of the experimental set-up include a VLE still. Schott 10 I and Pyrex 5 I ballast nasks. a TECHNE cold finger. a Fischer VKH 100 pressure controller. a LABOTEC water bath with glycol - water mix and twO pumps.
To atmosphere V2
SF
sel·polnt
BF Schott 10' and Pyrex 5 I ballast flasks CF TECHNE cold linger
OS Dynamic VlE still (Raal modification)
FPT Fischer Vakuum·Konslanthalter VKH 100 pressure controller P1 Pump 1 (vacuum pump)
P2 Pump 2 (water recirculation pump) PS Flscher pressure sensor
QC Pyrex glass condenser
TO Eurotherm temperature display TS PT 100 temperature sensor V1 Solenoid valve
V2 Needle valve
WB lABOTEC water bath with glycol-water mix
Figure 3-5: Schematic diagram
or
the VLE apparatus set-up39
CHAPTER 3 VAPOUR-LIQUID EQUILIBRIUM
3.3.1 Vapour-Liquid Equilibrium Still
A highly refined dynamic VLE 51i11 (Figure 3-6), based on the Yerazunis et al [1964] still has been designed by Raal (Raal & MLihlbauer [1998]). The still is constructed from specially blown glass and is suitable for low-pressure measurements. Harris [2001] and Joseph et a1 [2001] have described Ihe still in detail. It is worth nCling the advantages of Ihe design:
;.... Correct positioning of lemperature sensors provide for accurale temperature measurement.
;;.. The use of high-vacuum valves provide constant and stable pressure control.
;.... Both liquid and vapour samples are extracted easily and do not affect equilibrium operating conditions,
,. The use of packing in the equilibrium chamber ensures rapid attainment of equilibrium due to intimate conI act between the vapour and the liquid and the expansion of interfacial surface area.
:,... Several vacuum jackets and lagging streams prevent both heat losses and super heating from occurring.
;;.. Stirring in Ihe condensate receiver eliminales temperature and concentralion gradients. This ensures high reproducibility of Ihe vapour sample concentralions.
;.... Stirring in the boiling chamber which ensures thorough mixing of the returning condensate before evaporation. This prevents "nashing" and produces smooth boiling.
,. A syslem of internal and external heaters in the boiling chambers. This fealure ensures rapid boiling, permits precise control of the circulation rate and provides nucleation sites for smooth boiling.
CHAPTER 3
ss WIre, Mesh I'~ck,nl'
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VAPOUR-LIQUID EQUILIBRIUM
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r
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D13111 _ _ _ _ _ _ __ _ __ _ _ 1 V:oh..,
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Figure 3-6: Dynamic VLE still of Raal (Raal & Miihlbauer [19981)
41
CHAPTER 3 VAPOUR-LIQUID EQUILIBRIUM
3.3.2 Temperature and pressure measurement
A Eurothcrm temperature display was used to indicate the resistance of the PT-lOO tempermure sensor embedded in the equilibrium chamber. This accurately measured the equilibrium temperature. Before experimentation. however. the PT-lOO temperature sensor has to be calibrated.
This is discussed in the section below. The accuracy of temperature measurement is estimated to be within ±O.02 K.
The pressure was monitored with a Fisher pressure transducer. Accuracy of the pressure measurement is estimated to be within ±0.03 kPa.
3.3.3 Pressure control
For isobaric operation the pressure is maintained at a constant set-point that is controlled by a Fischer pressure controller which either vents to atmosphere through needle valve Y2 (Figure 5-1) or connects to the vacuum pump through solenoid valve VI (Figure 5-1). By actuation of the solenoid valve. leading to a vacuum pump and atmosphere. the pressure in the still can be controlled. Control of the still pressure is estimated to be within :t 0.1 % of the set pressure.
However. before the pressure controller can be used it is necessary to calibrate it to determine the actual pressures to which it is controlling. This is discussed in Section 3.4.2.
3.3.4 Composition analysis
The equilibrium compositions of the samples were determined using a gas chromatograph (GC). Gas ChrOm<.llography is a method of separating and identifying certain substances. For this project two gas chromatographs was used. For the cyclohex.me/cthanol system a Shimadzu. model GC 17A gas chromatograph was used. This GC could not be used for the carboxylic acids. as the column in the Shimadzu could not separate the carboxylic acids. Therefore. for the carboxylic acids a Yarian.