Contemporary trends in the field of phase equilibrium thermodynamics indicate a burgeoning interest or concentration of efforts in the development of predictive thermodynamic models (Gmehling, 2003; Jaubert et al., 2004; Vetere, 2004) and molecular simulation (Deiters et al., 1999; Sandler, 2003) to enable the acquisition of quantitative estimates of the phase equilibrium properties of a system; most notably vapour-liquid equilibria. This aversion to experimental measurements can be attributed to high costs, long time periods, undesirable phenomena and numerous experimental difficulties (Van't Hoffet al., 2004) which have plagued researchers over the course of the last century; coupled with advances in computational efficiency and the sheer magnitude of chemical compounds and combinations thereof requiring thermodynamic characterization. However, not only does the measurement of vapour-liquid equilibria serve as the framework for the development, validation and continual modification of predictive or computational thermodynamics, it remains an integral part of the successful design of a large number of industrial thermal separation processes (distillation, extractive distillation, azeotropic distillation and flash operations).
It is widely acknowledged that the greatest capital investment and operating costs in the oil refining, petrochemical, chemical and related industries (gas processing, pharmaceutical, etc.) are frequently incurred in the design of the separation step. Despite the high-energy consumption of distillation, the latter accounts for 90% of all thermal separation processes in the chemical industry due to the numerous advantages of distillation over other methods (Gmehling et al., 1999). Consequently, the optimal design of separation sequences, control strategies and more significantly the separation equipment itself (as in the sizing of distillation columns) for an industrial separation process is a task of tremendous importance for the process engineer as its successful completion weighs heavily upon the economics of the entire chemical plant. In addition to transport, physical and thermo-chemical data of the components in the mixture to be separated, a vital ingredient for the proper realization of the separation step is a reliable knowledge of the phase equilibrium properties (vapour-liquid equilibrium) of the system. The ideal scenario is the availability of a thermodynamically consistent set of vapour-liquid equilibrium data at the pressure, temperature and composition range for the operating or working conditions of the process. Since this is infrequently the case, an alternative approach has been the use of the correlative approach (if a minimal amount of experimental data is
available) or predictive thermodynamic models (in the complete absence of any experimental data) in process simulators.
Critical evaluations and commentary on the use of predictive thermodynamic models in process simulators for the design of industrial separation processes (Wakeham et aI., 2000; Barnicki, 2002; Chen et aI., 2004) has yielded that a "blindfolded approach" to the use of the above i.e.
neglecting small errors between predicted and experimental values can translate into significant errors in distillation column design, which can severely compromise the economy and the efficiency of the separation process. It is therefore of the utmost importance to ensure the availability of a sufficiently large database of reliable experimental data, not only as a direct means for the design of separation equipment, but to improve the predicted results and confidence in the predictive capability (Chen et al., 2004) of underlying models of process simulators, such as the UNIFAC (Dortmund) approach (Kato and Gmehling, 2005), to allow for the accurate design of separation processes.
Traditional low-pressure dynamic recirculating stills possess considerable advantages over other vapour-liquid equilibrium measurement techniques (e.g. static methods), in their respective pressure ranges, such as low investment costs, simple operation and expediency in furnishing a full VLE data set of P-T-x-y measurements; an example of which is the highly successful contemporary low-pressure VLE glass still design of Raal (Raal and Muhlbauer, 1998). A survey of vapour-liquid equilibrium that has been published in open literature and data from a variety of reliable literature sources (Dortmund Data Bank, 1999; Hirata et aI., 1975) yields a relative void in the availability of published vapour-liquid equilibrium data in the moderate- pressure range (0.1 MPa < P ::; 1 MPa) for industrially relevant systems. The above pressure range is particularly applicable to the design of separation equipment as there are a number of distillation sequences whose operating conditions fall within this range.
There have been very few effective designs based on traditional low-pressure VLE stills that have been developed to address the above deficiency in the current phase equilibrium data pool.
The work of Harris (2004) represented an initial attempt at such a design in our laboratories at the Thermodynamics Research Unit at the University of KwaZulu-Natal, where the equipment was to measure VLE data at pressures up to 30 MPa and temperatures up to 700 K. However, as a result of an overambitious pressure range and an oversight of key design and operational considerations, there were serious concerns over the operational efficiency and quality of data produced. Consequently using the design and associated flaws of the equipment of Harris (2004) as a benchmark for the necessary modifications, a novel apparatus has been developed in our laboratories to allow for the measurement of vapour pressures and vapour-liquid
Chapter 1. Introduction
equilibrium data for pressures up to 750 kPa and temperatures up to 600 K. Despite an unavoidable diminution of the operating pressure range (as a result of thinner stainless steel vessel walls and the inclusion of glass inserts), this design represents a significant improvement to that of its predecessor. As with the design of Harris, the design of the novel equipment has been based on the successful design of the low-pressure liquid and vapour condensate recirculating still of Raal (Raal and Muhlbauer, 1998). An initial test of the performance of the equipment was conducted through the measurement of the pure component vapour pressures of cyclohexane, n-heptane, n-octane, ethanol and I-propanol.
The investigation of the vapour-liquid equilibria of mixtures containing hydrocarbons and alkanols is of considerable importance as such systems are industrially relevant for the design of separation processes and from a theoretical point of view, for the development and validation of liquid phase thermodynamic models (Peleteiro et al., 2001; Darwish and AI-Anber, 1997). In this work, the types of binary mixtures investigated were of the following combinations:
cycloalkane + alkanol, alkanol + n-alkane and alkanol + alkanol systems. The chemical components corresponding to the above that were studied are the systems of cyclohexane + ethanol, I-propanol + 2-butanol, I-propanol + n-dodecane and 2-butanol + n-dodecane. Both isobaric and isothermal measurements were obtained for the relevant systems in the low and moderate-pressure regions to demonstrate the versatility of the apparatus. The alkanol
+
n- dodecane systems studied, for which no VLE data is currently available in open literature, proved particularly challenging to measure (especially for the vapour phase composition) due to the very high relative volatility of the system. Consequently, only experimental P-T-x data were reliably obtained and the vapour phase composition (y) was computed in a BUBL P calculation with the Wilson equation. The latter served to highlight that the traditional difficulties experienced in the measurement of these types of systems with conventional dynamic recirculating VLE stills are indeed quite challenging to address, as these could not be circumvented despite the innovative novel features that were incorporated in the VLE apparatus presented in this study.The raw experimental vapour pressure and vapour-liquid equilibrium data were subjected to theoretical treatment in the form of data correlation and thermodynamic consistency testing.
The data reduction for the vapour-liquid equilibrium data was achieved through both the indirect (gamma-phi) and direct (phi-phi) approaches where a variety of activity coefficient models, equations of state and mixing rules were employed to correlate the data.
Thermodynamic consistency testing was performed on the P-T-x-y experimental data with the Direct Test of Van Ness (1995) to allow for a rigourous assessment of the quality of the data
The content of the research presented in this work allows for invaluable insight into the design principles and considerations inherent in the quite challenging task of designing phase equilibrium measurement equipment (assisted by providing critical commentary in tracing the origins of the early VLE still designs) and provides an extensive coverage of the theoretical aspects of VLE treatment to provide a holistic view on the contemporary state of vapour-liquid equilibrium measurements in the low and moderate-pressure regimes.
Chapter 2. A Review of the Classification and Development of Vapour-Liquid Equilibrium Equipment