I SYNTHETIC I I ANALYTICAL I
2.5 Experimental challenges in the determination of HPVLE
The challenges encountered by experimenters in the field of VLE have been reviewed by Raal and Muhlbauer (1994, 1998) and can be summarised below as the following:
(a) Criteria for the establishment of equilibrium
The establishment of a true equilibrium condition for a non-reacting system necessitates that there are no thermal or material exchanges with the environment and that there be no internal macroscopic changes due to any gradients (pressure, temperature, compositions and densities) within the system, with respect to time. In other words, a steady state condition with
dt dx
= 0,where x is a measurable system variable, is required.
The attainment of the equilibrium condition is indeed adversely affected by the cumulative effect of the minor fluctuations of experimental parameters (pressure, temperature, volume, etc.), which with even attempts at strict control, often cannot be perfectly maintained at constant values (within the detectable limits of the system), coupled with the limitations of the experimenter and the methods employed. An example of an experimental limitation for ensuring an equilibrium condition is the effect of the incorporation of stirring or agitation in the equilibrium vessel. Although, stirring or agitation improves contact between the phases to speed up the attainment of equilibrium, the effects of "fluid friction" and the subsequent thermal gradients generated in the liquid phase are not negligible. Consequently, a true equilibrium is probably never reached or maintained (Raal and Muhlbauer, 1998) due to the unceasing small changes in the surroundings, difficulties in experimental control and "retarding resistances" in fluid flow and interfacial contact.
The steady state condition is 'judged" to have been attained by the experimenter when the system temperature, pressure and phase compositions (densities, refractive index, infrared spectra, etc.) remain stable i.e. there is no detected change in these system properties for an appreciable time interval.
Experimenters such as Rigas et al. (1958) and Fredenslund et al. (1973) based their criterion for the establishment of a state of equilibrium as being a constant system pressure. Fredenslund
Chapter 2. A Review of the Classification and Development of Vapour-Liquid Equilibrium Equipment
et al. (1973) in this regard, considered a change in pressure of less than 0.05% in 30 minutes as being sufficient to serve as an indication of equilibrium.
The experimental "assumption" of an attainment of equilibrium is therefore dependent upon the measurement accuracies and capabilities of the thermal and pressure sensors together with the analytical measuring devices (densitometers, gas chromatographs) for phase compositions.
(b) Control of experimental variables
As mentioned in the previous paragraph, in practice it is almost impossible to maintain system variables of pressure, temperature and volume at constant values throughout the course of the experiment. Usually, a "setpoint" or predetermined value for a system variable, with as Iowa tolerance (dead-band) as possible is the common control strategy. The greater the deviations of the system pressure or temperature from the control value, the greater the difficulty in obtaining reliable phase equilibrium data, due to the intimate relationship between system variables such as temperature and pressure.
Pressure control can be achieved through the use of electronic shut-off valves (on-off) or control valves (regulating). Electromagnetic valves such as solenoid valves have found widespread use in pressure control systems and with the advent of proportional solenoid valves, economically constructed pressure controllers are now possible. Shut-off valves are able to control the system pressure i.e. maintain a "setpoint" by opening to a lower or higher pressure source to stabilise the system pressure, which is between these two limits. The opening of the valves can be fine- tuned by pulse-width modulation and the use of manual control valves in conjunction with the solenoid valves. Proportional valves are able to regulate the system pressure through the control of the flow rate of an inert gas into the system, where the flow characteristics of the valve are proportional to the current delivered to the valve. The control of pressure is thus dependent upon the limitations of the response time of the pressure feedback and control unit and the pulsation of the valve. Leaks in the equipment would be detrimental to pressure control and welds, fittings and junctions in the equipment are the major sources of leaks in this regard.
Temperature control is commonly achieved through the use of a thermostat in the form of an isothermal fluid bath, where the temperature of the fluid is regulated through the use of a feedback system; the temperature of the system is monitored by a thermal sensor and then relayed to a control unit, which then adjusts the heating element current. Internal and external vacuum jacketing of the equilibrium cell (Rogalski et al., 1980) is another effective means for
The use of external insulation material (Fibrefax®, polyurethane foam, etc.) is also employed to insulate the equilibrium cell from thermal contact with its environment. The loss or gain of heat from the surroundings is a major impediment in obtaining isothermal conditions for the equilibrium cell. There should therefore be no gradients or paths for any conductive or radiative heat transfer (AC heaters) to or from the equilibrium cell. Strategies for accurate temperature monitoring (such as multiple temperature sensors) and control in the equilibrium cell have traditionally been the preserve of those experimenters employing static methods.
(c)Accurate measurementof temperatureand pressure
With the availability of highly accurate, linear and robust pressure and temperature sensors, the accuracy of the variable readings are often limited by the resolution (number of decimal places) of the respective instrument display units or multimeter readings. Consequently, display units, the proper mounting of the sensor, good signal conditioning and noise interferences,etc. and not the sensor itself, often limit the accuracy of pressure and temperature readings. Additionally, the calibration of the thermal and pressure sensors are also quite often a limiting factor for the accuracy of the respective measurements.
(d) Withdrawal and handling of samples prior to analysis
Difficulty is experienced for the withdrawal of samples from a system, which is either at a very high pressure or low pressure. For the analysis of samples in an HPVLE determination, the equilibrium state attained for the sample in the cell (pressure, temperature, compositions) differs from its initial state in its entry to an analytical measuring device e.g. gas chromatograph. Care must be taken to preserve the integrity of the withdrawn sample. This is to ensure that it remains representative of the equilibrium condition throughout the analytical procedure.
To minimize the effects of a large pressure drop and change in the interior volume of the cell during sampling, different strategies were employed. Some experimenters (Aroyan and Katz, 1951) employed controlled injections of compressed mercury into a pressure control cylinder or buffer tank coupled to the equilibrium cell during sampling. Sampling through capillaries (Heintz and Streett, 1983) and specialized valves (Lauretet aI., 1994) to ensure minimization of sample sizes and hence changes in cell volume were also employed.
The problem of ensuring representative samples is also exacerbated for mixtures of components with widely varying volatilities or volatile/non-volatile mixtures, as there is preferential flashing of the more volatile components during sampling and also partial condensation of the heavier components in the sample lines. To this end, complex homogenisation techniques employed by
Chapter 2. A Review of the Classification and Development of Yapour-Liquid Equilibrium Equipment
Rogers and Prausnitz (1970), Ng and Robinson (1978) and Wagner and Wichterle (1987) have been reported in the literature.
(d) Analysis of withdrawn phases
A wide variety of analytical techniques are available for the quantification of the equilibrium phases. These techniques include refractometry, densitometry, spectroscopy, mass spectrometry and chromatographic methods. The latter of these methods, gas chromatography, is the most popular.It is however, characterised by the need to calibrate the detector (thermal conductivity or flame ionisation detectors) as the detector response varies with column effluent composition, and the nature of substance. In particular, the proper analysis of high-pressure gas or gas-liquid mixtures presents a great problem for researchers, as the detector response factor varies greatly across the mole fraction range for the gas. To this end, a precision volumetric calibration device has been developed in our research group (Raal and Muhlbauer, 1998) for the preparation of highly-accurate standard gas or gas-liquid mixtures.