A dissertation submitted in fulfillment of the academic requirements for the degree of Doctor of Philosophy in the School of Chemical Engineering at the University of KwaZulu-Natal, Durban. The main criteria in the design of the equipment were expediency, operational efficiency and versatility in obtaining reliable ELO data.
APPENDIX A
NOTATION
The non-randomness parameter for the NRTL equation, the relative volatility constant or function in the T-K Wilson equation, a simplified expression in the WSMR and TCMR models. Constant or function in the T-K Wilson equation A simplified expression for the WSMR and TCMR models.
CHAPTER ONE INTRODUCTION
Consequently, using the design and associated shortcomings of the Harris (2004) equipment as a benchmark for necessary modifications, we have developed a new device in our laboratories that allows measurement of vapor pressure and vapor-liquid. Isobaric and isothermal measurements were obtained for corresponding systems in low and moderate pressure regions to demonstrate the versatility of the apparatus.
CHAPTER TWO
A REVIEW OF THE CLASSIFICATION AND
DEVELOPMENT OF VAPOUR-LIQUID EQUILIBRIUM EQUIPMENT
Introduction
More emphasis will be placed on the development of dynamic recirculating DLE stills for low and moderate pressure, which encompasses the author's area of interest. For an in-depth overview of the classification and development of the different types of HPVLE measurement methods, Appendix A can be consulted.
Classification of HPVLE equipment
In this review chapter, a brief treatment of HPVLE methods will be presented with the aim of establishing theoretical and experimental considerations common to HPVLE and LPVLE methods.
HPVLE EQUIPMENT
DYNAMIC RECIRCULATING
COMBINED
I SYNTHETIC I I ANALYTICAL I
- Common features of Analytical HPVLE equipment
- Classification of experimental HPVLE variables
- Experimental challenges in the determination of HPVLE
- Classification and development of LPVLE Recirculation Methods
- Classification of LPVLE Recirculating equipment
A means of effective agitation of the contents of the equilibrium cell to facilitate the attainment of equilibrium. This is to ensure that it remains representative of the equilibrium state throughout the analytical procedure.
LPVLE EQUIPMENT
RECIRCULATION
CONDENSATE
Features of typical LPVLE Recirculating equipment
The Cottrell tube was historically designed to enter the side of the balance chamber (as will be seen later in many illustrations of early plans). This point proved to be a major flaw in the equipment design of Harris (2004). The device contained three separate temperature-controlled zones in the form of a residue chamber (A), a steam line (C) and a flash boiler (G).
Two of the heating elements were placed in the vapor jacket and the third in the evaporator. During the operation of the still, the mixture was charged into the contactor and evaporator of the still. Two major disadvantages of the still were the long equilibration times (up to four hours) and the large initial charge required (200 cm3 liquid sample).
The most important parts of the apparatus are the cooking chamber (A) with an external and internal heater, the Cottrell tube (B), liquid trap (F), condensate trap (E) and the condenser (D). In either passage (if the vapor-liquid contact and equilibration interface is inefficient), continued circulation will not improve the approach to equilibrium (by smoothing out any erroneous local equilibria). The backflow of liquid on the bottom corner occurred as a result of the accumulation of the returning condensate in the heated section.
T. 100 BULB
- Development of LPVLE methods for higher pressures and temperatures
- Designs for elevated pressures
- Designs for elevated temperatures
- Conclusion
The design of the Raal stall, as described in the text by Raal and Muhlbauer (1998), is shown in Figure 2.25. The vacuum jacket (H) formed in the annular space between the outermost and middle concentric tube provides additional insulation of the balance chamber. To compensate for any heat loss that may occur in the walls of the equilibrium chamber ie.
The trap was connected to a surge tank via a valve located in the trap's vent line. All parts of the equipment were heavily insulated, except the ballast ship and the vacuum system. A baffle system was employed in the boiler to promote good mixing of the liquid contents.
During normal operation of the boiler, the boiler was filled with sufficient material (minimum 150 cm3).
THEORETICAL TREATMENT OF VAPOUR PRESSURE AND VAPOUR-LIQUID EQUILIBRIUM DATA
Introduction
Accurate phase equilibrium data are of unparalleled importance for the design of the separation step in industrial processes; using incorrect data can severely compromise the operational efficiency and economic feasibility of the entire process. In case of availability of a complete set of P-T-x-y data, an effective and rigorous assessment of the accuracy of experimental VLE data in the form of thermodynamic consistency testing is a worthwhile exercise. Several methods have been proposed for verifying the consistency of VLE data sets, in addition to meeting the basic thermodynamic constraints of the Gibbs-Duhem equation as a fundamental criterion to ensure the thermodynamic consistency of a data set.
An effective approach in the application of thermodynamic consistency testing has often been achieved through the systematic combination of different and complementary consistency tests in the overall evaluation of a data set (Moon et al., 1991). The content of this chapter is heavily biased towards the theoretical treatment of VLE data, as opposed to pure component vapor pressure, as the former is inherently more theoretically complex than the latter. Greater emphasis will also be placed on the use of correlative as opposed to predictive methods in VLE data treatment as this approach is of greater relevance to the author's field of study.
For an overview of the fundamental thermodynamic relationships presented in this review, the texts of Sandler (1999), Smith et al.
Correlation of Vapour Pressure data
- Overview
- Vapour Pressure correlations
Antoine (1888a, 1888b) proposed a simple empirical modification of the Clapeyron equation in the form of a three-parameter equation, shown below in equation (3.3). For a reference substance, Pref and Tref represent a precisely known point on the substance's vapor pressure curve, e.g. One of the most popular of these equations was the Frost-Kalkwarf equation (Malanowski and Anderko, 1992) or the Harlecher-Braun equation (Sandler, 1999).
Due to the form of the equation, the vapor pressure must be solved iteratively. The above equation was used extensively by the DIPPR Compilation Project for the correlation of the vapor pressures of many industrially relevant chemicals (Daubert andlones, 1990). They used the kinetic theory of polyatomic liquids, developed by Moelwyn-Hughes (1961), to formulate a vapor pressure equation (AMP) with the same algebraic form as the empirical equation proposed by Miller (1964).
Basically, there are two correlation strategies for fitting experimental vapor pressure data to vapor pressure equations, viz.
Thermodynamic Consistency Testing of Vapour Pressure data
Graphical analysis of such a graph shows whether a chemical substance decomposes or polymerizes at elevated temperatures. If the slope of the graph increases exponentially with increasing temperature, thermal decomposition is indicated (see Figure 3.1). A plot of In P versus (lIT) showing the onset of thermal decomposition at elevated temperatures. c) As for constrained fitting of the data (Malanowski and Anderko, 1992), constrained fitting is assessed by observing that the corresponding plot is M1v versus T or via.
An additional test (Daubert and Jones, 1990) is that for a line drawn on a graph of In P versus (1/T) between the melting point and the critical point, a positive deviation should be observed for all data points from the line. . In this study, the graphical test in (b) was used to test the vapor pressure data.
Prediction of Pure Component Vapour Pressures
- Vapour Pressure predictive models
However, the true essence or value of a prediction method lies in the fact that ideally only pure component properties of the chemicals of interest are required to predict physicochemical properties. Using the Gomez-Thodos method involves dividing compounds into the following classes: non-polar, polar non-associating, polar associating or hydrogen-bonded compounds. As previously mentioned, the constants of vapor pressure correlations can be generalized to allow for predictive power to be transferred to the original form of the correlation.
These can be obtained by using the following generalized relations based on the normal boiling point and critical conditions. Vetere concluded that the overall performance of the equation was superior to that of other methods in the literature; however, Vetere also acknowledged that the strong sensitivity of the Wagner equation with respect to the ab parameter was indeed a flaw in the predictive approach with the Wagner equation. Macknick and Prausnitz (1979) developed a group contribution scheme for estimating the parameters s and Eo, which in the predictive method represent the number of equivalent oscillators per molecule and the enthalpy of vaporization of the hypothetical liquid, respectively.
In predicting the vapor pressures of non-electrolytic organic compounds via the group contribution method, achievements have been made within our research group in the form of the work of Nannoolalet al.
Correlation of Vapour-Liquid Equilibria
- Overview
The expressions for the activity coefficients of the other GE models presented in this review are derived in the same way. For nonrandom mixtures, aij was related to the inverse of the coordination number (z), which was obtained from the lattice theory of Guggenheim (1952). A much simpler approach to the evaluation of the size (rj) and surface area (qi) parameters is achieved by obtaining the parameters as the sum of the contributions to the molecular structure (R and Q respectively) of the different functional groups present in the molecule, i.e.
For the treatment of partially miscible systems, the use of the empirically modified Wilson equation, which is shown in Equation (3.124), is undesirable. The exit condition for the inner loop is a convergence of the Yi values upon recalculation from the~~values i. The practical implementation of this method involves the use of ratios of the vapor pressures.
A discussion of the above two methods is given in Raal and Muhlbauer (1998). Forms of the cubic equations in terms of Z, together with expressions for the fugacity coefficient of a pure liquid. The introduction of the Redlich-Kwong (RK) equation of state in 1949 was an important milestone and represented a significant improvement over the complex virial equation and other equations of significantly simpler forms for use in applications such as distillation column process simulators (Palmer, 1987).
