DESCRIPTION OF EQUIPMENT USED IN THIS WORK
3.2 STATIC APPARATUS USED AT THE UNIVERSITY OF OLDENBURG, GERMANY
3.2.1 Appararatus of Gmehling and Kolbe (1985) with modifications
The general concepts of the static synthetic type of apparatus have been discussed in Chapter Two. The principle of static synthetic measurements is to introduce precisely known amounts of the pure components into a thermoregulated equilibrium cell. This is done in a stepwise process, allowing the system to equilibrate between steps. From this procedure the system pressure is then determined as a function of composition. In Chapter Two the development of this piece of apparatus was discussed. The progression from a glass still to a steel one to the present titanium cell with sapphire windows was outlined. The details of the equipment used in this work are given below and illustrated in Figure 3-1.
Equilibrium cell
The equilibrium cell (approximately 0.11 l) is constructed from two pieces of titanium. The bottom part houses two sapphire windows that are fitted fore and aft to allow the observation of the chemical mixture. The top part, which is essentially a fitted cap, is sealed to the bottom part with a recessed Teflon gasket. Pressure measurement is with a pressure gauge (digital dynamometer type 21000 combined with pressure block type 410 from Desgranges & Huot, France); the sensor is fitted in the top of the equilibrium cell. Temperature measurement is with a Pt. 100 (model 1560, Hart Scientific); the sensor is embedded in the wall of the cell and is accurate to within±0.03K. Specially constructed valves are fitted into the sides of the cell. The
such away as to eradicate any dead space. The contents of the cell are agitated by a magnetic stirrer. The stirrer itself is a bar magnet encased in stainless-steel and is driven by a rotating magnetic field which is external to the cell.
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'CO'
Figure 3-1: Experimental static apparatus of Kolbe and GmeWing (1985) with modifications (Fischer and GmeWing (1994) and Fischer and Wilken (2001».
Auxiliary Equipment
The equilibrium cell is kept at the equilibrium temperature by being housed in an air thermostat.
The air thermostat has a glass window and is lit inside so that the cell and its contents can be observed without disturbing the bath. CO2 is injected into the cell with a manual piston pump.
The volume change of the piston pump is read from an imbedded gauge. The displaced volume is a function of piston movement. The pump temperature and pressure are monitoredwithPt. 100 resistance thermometer (model 1560, Hart Scientific) and a pressure sensor (model PDCR 911, Druck) respectively. The determination of the amount ofCOz injected into the cell is discussed in Chapter Five.
3.2.2 The Equipment of Rarey and Gmehling (1993)
The development of the equipment of Rarey and GmeWing is discussed in Chapter Two. In principle this apparatus functions in the same way as the one described above. The most noticeable difference, however, is that the operation of this apparatus is completely automated.
The general outlay of the equipment is illustrated in Figure 3-2.
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vacuum pressure 'Sensor-Fi
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Figure 3-2: Schematic diagram of equipment from Rarey et a1. (1999).
Equilibrium Cell
Inprinciple the equilibrium cell (approximately 0.05 1)is very similar to the one described above.
The cell is constructed from Stainless-steel but has no windows. The contents of the cell are agitated by magnetic stirrer and the cell is immersed in a constant temperature water bath. The piston pumps are connected to the cell through automated valves.
Temperature and pressure range and measurement
The equipment is suitable for measurements between 278.15 to 368.15 K and 0 to 300 kPa.
Pressure (P) was measured inside the equilibrium cell using a Digiquartz pressure sensor (Model
measured using a Pt 100 resistance thermometer (Model 1506, Hart Scientific), which was positioned in the water bath close to the cell and is accurate to within±0.03K.
High-precision injection pump
The injection pump is based on the design of Gaube (1988). Figure 3-3 is a schematic diagram of the pump. The piston is moved by a stepper motor with the ratio of 1000 steps per rotation. The maximum injectable volume of the piston pump is roughly 0.032 / which equates to approximately 400 000 steps. The minimum injectable volume is roughly 0.00003 / with a precision of± 2%. The piston pump is kept at constant temperature and the temperature is measured with a Pt 100 resistance thermometer (Model 1506, Hart Scientific). The pressure inside the pump is measured with a CEREBAR PMCAR1M2A0520-sensor with modified sealing. The calculation of the amount of pure component injected into the cell is described in Chapter Five.
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Figure 3-3: Schematic diagram of high-precision injection pump of Gaube (1988).
Apparatus Automation
The most useful benefit from automation of the equipment is that it makes it possible to measure 40 to 60 VLE points for an isotherm in 3 to 4 days with only 3 to 4 hours of human labour required. Furthermore, computer control of the precision injection pump makes it possible to measure data in the dilute regions. A PC controls the automation of the apparatus. All experimental variables are fed to the PC and it controls the injection of pure component based on the pressure readings from the cell. The operation of the apparatus is discussed in Chapter Five.
3.3 EQUIPMENT PRESENTLY IN USE AT THE UNIVERSITY OF NATAL