in this work entailed loading of known amounts of the required binary components through the action of weighing. However, slightly intricate steps were included to obtain the most accurate and precise results possible. Since sample analysis is not required when using the synthetic technique, calibrations were performed for only the temperature probes and the pressure transmitter, in the exact manner as was explained for the analytical equipment. The apparatus was thus operated as per the following steps:
i. The upper chamber within the equilibrium cell (which is located above the piston), was first charged with moderate amounts of fluid from the syringe pump. This fluid was used as a buffer so as to prevent the contact of the piston with the top flange when the cell was pressurised. The non-rotating needle valve (Parker, 10V series) which was installed on the
Experimental procedures
50 | P a g e cell – syringe pump line was thereafter closed, only to be opened at the commencement of the actual measurements.
ii. The cell and its housing were then thoroughly dried of any and all fluid on the external surfaces through the use of compressed air. This action also resulted in the expulsion of all foreign material which would have adhered to the cell and housing structure. This was done to ascertain that no foreign material interfered with the weight of the structure.
iii. Thereafter the cell was then placed under vacuum by connecting a two stage vacuum pump (Edwards RV3) onto the loading line. This process was usually carried out overnight to ensure proper evacuation.
iv. Once the cell was sufficiently evacuated to pressures of approximately -0.01 MPa, the non- rotating needle valve (Parker, 10V series), which was also installed on the loading line was sufficiently closed. The cell housing and the connected pressure transmitter were then removed from the support frame for the purpose of weighing the structure.
v. Following the initial weighing, all possible traces of air within the cell were dispelled through the load-purge technique, which was carried out using the gaseous component.
After sufficient load-purge cycles, the cell was filled with relatively large quantities of the gas. The pressure within the cell was then measured for the purpose of effectively reducing it to a slightly positive value of approximately 0.14 MPa, via the careful opening of the needle valve. The pressure was never allowed to reach atmospheric levels, as this would introduce air into the cell. After the pressure reduction, the cell and housing were again weighed.
vi. This was then followed by the loading of the liquid component. Prior to being charged into the cell, the liquid was degassed in a Büchner flask which was connected to the vacuum pump. The subsequent loading was carried out using a syringe, which was fitted with a nut and a rubber seal. Since only a slight positive pressure of the gas was present within the cell, loading via a syringe was possible. After the successful loading of the liquid component and the closure of the valve, the vacuum pump was again used to remove any residual liquid from the valve, as it would interfere with the mass during the weighing of the liquid infused cell and its housing.
vii. Further charging of the gaseous component was undertaken in order to increase the volume of the mixture, viz., increase the mass of both components, which in turn increased the
Experimental procedures
51 | P a g e probability of obtaining the most accurate results. Before charging, the housing structure was inverted, in order to allow the depression of the piston upon entry of the gas, resulting in the loading of larger amounts of the lighter component. During the charging of the gas, the mixer was activated to enhance the absorption rate of the gas into the liquid. The housing structure was thereafter weighed and its mass recorded for the last time.
viii. The housing structure was then returned onto its frame, the syringe pump connected and the structure was then submerged in the bath liquid. The pressure within the two chambers in the cell was allowed to equilibrate, and the stirring mechanism activated for approximately 10 minutes before the next step commenced.
ix. The binary mixture which was being rigorously agitated was then compressed through the action of the syringe pump and the piston at a rate of (10 µl/min) to obtain the bubble point.
x. Two complementary techniques were employed in attaining the bubble point; these were by visual observation and the pressure technique. For the former, as the name implies, the mixture was observed up until the transition to a single phase occurred, and this was characterised by the disappearance of the last bubble within the mixture.
xi. The pressure technique entailed monitoring of the pressure within the cell, and thus the bubble point was characterised by a sudden spike in pressure, with the pressure reading just before the increase being the desired bubble point. Figure 5-1 depicts the aforementioned phenomenon.
Experimental procedures
52 | P a g e Figure 5-1: Pressure vs time chart for the carbon dioxide (1) + 1,1,2,2-tetrafluoroethyl 2,2,3,3- tetrafluoropropyl ether (2) system. (■) mixture exists as two phases, (■) mixture exists as a single phase, (●) bubble point.
3,48 3,48 3,49 3,49 3,50 3,50 3,51 3,51 3,52 3,52 3,53 3,53
0 20 40 60 80 100 120 140
Pressure (MPa)
Time (sec)
Theoretical analysis of phase equilibria data
53 | P a g e