The equipment used for this technique are usually relatively simple when compared to those which utilise the static analytical technique. This is primarily due to the lack of any sampling mechanisms in the apparatus. The use of this apparatus is not only restricted to VLE measurement but can be effectively utilised in multi phase equilibria (Calderone et al., 2007), solid-liquid equilibria (Dohrn et al., 2007), critical curves of mixtures (Martin et al., 2007), cloud point determination (Akutsu et al., 2007) and gas hydrate formation (Beltrán and Servio, 2008). The general operating procedure requires the loading of known amounts of the necessary components into an equilibrium cell, followed by pressurisation of the contents until the bubble pressure is reached. If the method is visual, then the cell will either be fabricated from a transparent material such as sapphire crystal, or from a metal with transparent viewing ports. The phase transition can then be determined through observing the equilibrium mixture until the disappearance of the last bubble within the mixture, signifying the transition to a single phase. The bubble point can also be determined through the analyses of the discontinuities in the isothermal pressure–volume relationship (Williams-Wynn, 2016). This method is typically used to measure P-x data, but P-xy data can also be measured, but this would require the use of thermodynamic correlations such as the Gibb – Duhem equation, in order to determine the phase compositions (Van Ness, 2015). The visual static synthetic method was used in this work as a means of validating the data from the static analytic apparatus and as such only a few P-x points were to be measured. Table 3-2 highlights a non- exhaustive list of relatively modern visual static synthetic apparatus with different designs, including a brief synopsis of the respective apparatus. For more information regarding the various visual static synthetic apparatus found in the open literature, the reader is directed to the works of Fonseca et al. (2011) and Dohrn et al. (2012).
.
Equipment review
25 | P a g e Table 3-2: A summary of visual static synthetic apparatus.
Author Equipment description
(Dias et al., 2006)
The experimental setup used by Dias et al. to measure HPVLE data for carbon dioxide and PFC mixtures used a 30 cm3 variable volume cell in the form of a horizontal, hollow stainless steel cylinder which was fitted with a movable piston at one end and the other sealed with a sapphire window. The equipment was capable of operating in the temperature range of (273 - 507) K. A second sapphire window was installed on the wall of the steel cylinder to illuminate the cell contents. The contents of the cell were agitated by a magnetic stirrer and concurrently pressurised using the piston until the vapour phase completely disappeared. The bubble point pressure was measured when one small bubble was observed right before the disappearance of the vapour phase.
(Gil et al., 2008)
The rationale behind the work of Gil et al. was based on the need for phase equilibria data in the critical region, which was scarce in the open literature (Soo et al., 2010). This then led to the commissioning of an apparatus for the measurement of critical temperature and pressure of pure compounds and mixtures. The equipment utilised a flow apparatus. The maximum temperature and pressure constraints of the apparatus were 700 K and 20 MPa. The detection of the critical properties was achieved through critical opalescence. The equipment made use of a sapphire cell with an internal volume of 4 cm3, which was housed between two titanium flanges. The cell was loaded via two thermostatic chromatography syringe pumps, which provided a flow of constant composition.
(Lay, 2009)
This equipment made use of a 130 cm3 high pressure variable volume cell, which was fitted with viewing ports. The apparatus was designed to operate at a maximum pressure of 70 MPa and at temperatures not exceeding 473.15 K. The cell was pressured using a piston which was coupled with a displacement pump. The mixture in the cell was agitated by a magnetic mixer. The cell contents were pressurised until they completely liquefied, then the pressure was subsequently decreased until a bubble was observed, which signified the bubble pressure.
(Liu and Tomasko, 2007)
This work was focused on the study of the nature of carbon dioxide–polymer systems and utilised a visual static apparatus to measure the induced polymer swelling. The apparatus was designed to operate at a maximum pressure of 35 MPa and at temperatures not exceeding 523.15 K. The binary mixture was introduced into the cell in the form of a polymer melt drop which was saturated with carbon dioxide. The contents of the cell were then recorded through the use of two sapphire windows via a camera to determine the equilibrium interfacial tension.
Equipment review
26 | P a g e The review carried out by Dohrn et al. (2012), attests to the decline in the use of the analytic method in favour of synthetic type apparatus. This is arguably due to the challenges posed by sample analyses and the intensive labour inherent in the use of the analytic apparatus. However since the measurement of accurate phase equilibria data is dependent on the experimentalist as much as the equipment, the Thermodynamics Research Unit still uses mostly static analytic apparatus, due to the high-level experience within the group.
Equipment description
27 | P a g e