High-pressure Vapour-Liquid Equilibrium

Plocker, U., Knapp, H. and Prausnitz, J. (1978) Ind. Eng. Chem. Proc. Des. and Dev. 17, 243. Calculation of high-pressure vapour-liquid equilibria from a corresponding-states correlation with emphasis... 

Prausnitz, J. M. and Chueh, P. L. (1968) Computer Calculations for High-pressure Vapour-liquid-equilibria (Prentice-Hall). 

A purely phenomenological description of the effects occurring in high-pressure vapour-liquid equilibria was given by Wichterle [83b] in the first part of a series of papers. 

Shibata S.K., Sandler S.L, Critical evaluation of equation of state mixing rules for the prediction of high-pressure vapour-liquid equilibria, Ind. Eng. Chem. Res. 1989, (28), 1893-1898... 

P. L. Chuch and J. M. Prausnitz Calculation of high-pressure vapour-liquid equilibria. Ind. Eng. 

There are two principal methods of obtaining high-pressure vapour-liquid equilibrium data, the flow method and static method. These methods are extensions of the principles used at, or near, atmospheric pressure. However, the details and materials of construction of the apparatus are often very different. We will not be concerned with the details of standard high-pressure equipment as reviews and books on this subject are available. 

Figare 9 High-pressure vapour-liquid equilibrium cell used by Kirk and Ziegler... 

At pressures above a few atmospheres, the deviations from ideal behaviour in the gas phase will be significant and must be taken into account in process design. The effect of pressure on the liquid-phase activity coefficientmustalso be considered. A discussion of the methods used to correlate and estimate vapour-liquid equilibrium data at high pressures is beyond the scope of this book. The reader should refer to the texts by Null (1970) or Prausnitz and Chueh (1968). 

Ohe, S. (1990) Vapour-Liquid Equilibrium at High Pressure (Elsevier). 

At high pressures, especially approaching the critical pressure, the accuracy of the vapour-liquid equilibrium data are questionable. Direct measurement is not easy prediction with equations of state is risky [1],... 

Dr. Denis Doizi of CEA presented about the HIx Vapour Liquid Equilibrium (VLE) measurement. A collection of accurate thermodynamic data for the HIx section is critical for robust modelling of the S-I process. CEA has developed experimental techniques to obtain such data at the high temperatures and pressures expected in a full-scale plant. A tantalum microautoclave was used to collect data, with FTIR spectrometry used to analyse HI and H20 concentrations. UV-visible spectroscopy was used for... 

Fugacity is a thermodynamic property intensively used in chemical engineering, the most important being chemical equilibrium of gases at high pressures, and vapour-liquid equilibrium. The first subject is not covered here, but second topic will be discussed in detail in Chapter 6. Therefore, the problem is how to calculate fugacities... 

The study of critical points and vapour-liquid equilibrium at high pressures and temperatures has attracted interest since before the turn of the century. The first studies, beginning with the classic work of Andrews, and greatly influenced by van der Waals, were mainly of an exploratory nature. The second phase was the study of systems of practical importance, such as hydrocarbon and cryogenic mixtures, and was almost exclusively undertaken by chemical engineers. Major studies were undertaken by Sage and co-workers, Kay and co-workers, and Hiza and co-workers. ... 

Figure 8 Block diagram of flow system used by Hiza and co-workers for the recirculating vapour technique of studying vapour-liquid equilibrium at high pressures...

New solvents or solvent formulations are required for the capture of CO2 from natural gas power plants, the flue gases from which typically contain low concentrations of CO2 coupled with relatively high concentrations of O2. Information concerning vapour-liquid equilibrium, in particular CO2 loading capacity, is important for the selection of solvents to be employed for absorption. A static-analytic apparatus has been developed to acquire such data at temperatures between (313 and 393) K and various CO2 partial pressures. A tertiary amine, 2-dimethylaminoethanol (DMAE) and its blended solutions with piperazine (PZ) were studied. The density and viscosity of these amine solutions were also measured to yield a better understanding of then-performance in mass transfer and fluid dynamics. 

Membrane distillation is one of the membrane processes in which the membrane is not directly involved in separation. The only function of the membrane is to act as a barrier between the two phases. Selectivity is completely determined by the vapour-liquid equilibrium involved. This means that the component with the highest partial pressure will show the highest permeation rate. Thus, in the case of an etbanol/water mixture where the membrane is not wetted at low ethanol concentrations, both components will be transported through the membrane but the permeation rate of ethanol will always be relatively higher. With salt solutions, for example NaCl in water, only water has a vapour pressure, i.e. the vapour pressure of NaCl can be neglected, which means that only water will permeate through the membrane and consequently very high selectivities are obtained. 

This Chapter deals with the vapour + liquid equilibrium of pure substances over the range of pressure 0.1 kPa to their critical pressures - which seldom exceed 5000 kPa. It therefore excludes consideration of all methods for the determination of very low vapour pressures, such as that of Knudsen (also excluded are consideration of measurements at temperatures greater than 700 K, and more than passing reference to measurements at temperatures below ambient), and it deals primarily with the determination at high accuracy of the vapour pressures of organic liquids, and with the subsequent treatment of the results obtained. The subject has been well covered in two chapters in Volume 1 of the series Physical Methods of Organic Chemistry , and matters dealt with in those two chapters are included in the pages which follow only when they are necessary for the development of the discussion, or when they are matters the writer wishes to emphasize. Another source of relevant information is the book by Hala, Pick, Fried, and Vilim. ... 

Supercritical fluids represent a different type of alternative solvent to the others discussed in this book since they are not in the liquid state. A SCF is defined as a substance above its critical temperature (Tc) and pressure (Pc)1, but below the pressure required for condensation to a solid, see Figure 6.1 [1], The last requirement is often omitted since the pressure needed for condensation to occur is usually unpractically high. The critical point represents the highest temperature and pressure at which the substance can exist as a vapour and liquid in equilibrium. Hence, in a closed system, as the boiling point curve is ascended, increasing both temperature and pressure, the liquid becomes less dense due to thermal expansion and the gas becomes denser as the pressure rises. The densities of both phases thus converge until they become identical at the critical point. At this point, the two phases become indistinguishable and a SCF is obtained. 

The wide varity of the properties of chemical compounds does not enable the use of a universal apparatus for the measurement of thermodynamic properties for pure components and mixtures at high pressure. In the case of two-phase equilibria like vapour-liquid equilibria, the typical set of data to be determined is the pressure, the temperature, and the composition of the two phases at equilibrium. Some experimental apparatus also allows the... 

Alkene hydration to alcohols is a reaction of some industrial importance, although there have been few fundamental investigations in recent years. Beranek and Kraus have pointed out that the reaction equilibrium for the vapour phase process, though more favoured by low temperatures, still favours dehydration even at room temperature. Consequently, when high temperatures are employed to give more rapid reaction, high pressures must also be employed and even then the maximum attainable conversion may be low. Matters are improved somewhat by use of a three phase system (solid catalyst, liquid water, and gaseous alkene), for which conversion is improved by virtue of the alcohol solubility in water. 

However in many heat and mass transfer processes in fluids, condensing or boiling at a solid surface play a decisive role. In thermal power plants water at high pressure is vaporized in the boiler and the steam produced is expanded in a turbine, and then liquified again in a condenser. In compression or absorption plants and heat pumps, boilers and condensers are important pieces of equipment in the plant. In the separation of mixtures, the different composition of vapours in equilibrium with their liquids is used. Boiling and condensing are, therefore, characteristic for many separation processes in chemical engineering. As examples of these types of processes, the evaporation, condensation, distillation, rectification and absorption of a fluid should all be mentioned. 

The distinction is best made clear by means of an example Take the case of the chemical reaction which occurs between water and sulphuric acid Let us think of an apparatus similar to that indicated in Fig s In one vessel, A, there is a quantity of liquid water, and m contact with it some saturated vapour at pressure p0 The vapour fills the space on the left-hand side of the tap C In the vessel B there is some concentrated sulphuric acid, that is acid containing a little water, and above this acid is some vapour in equilibrium with the water in this sulphuric acid mixture The partial pressure of the water vapour is here p, where p is much less than pa This water vapour at low pressure (along with some sulphuric acid vapour which does not come into the calculation) occupies the space on the right of the tap C If we simply open the tap, water vapour would stream from left to right, that is from the region of high pressure p0 to that of low pressure p If a piston were placed in the tube it would be driven at a speed not by any means infinitely slowly, and the pressure difference on the two sides of the piston would be finite, 1 e (p0 - p ) This process, which is the spontaneous one, is an irreversible one, since the piston is not made to move infinitely slowly with infinitely small pressure difference on the two sides... 

Section 8.2.1 was concerned with equilibrium between a condensed phase and the vapour. It is often necessary, however, to estimate the effect of pressure on equilibria between two condensed phases. For example, the melting point of sodium at one atmosphere pressure is 97.6°C. Can it be used as a liquid heat transfer medium at 100°C, at a pressure of 100 atm, or will it solidify It is known that the liquid is less dense than the solid, and this argues that high pressures will encourage solidification. This is another aspect of Le Chatelier s work, which we can now quantify. This and similar problems may be solved by the Clapeyron equation, which we shall now derive. 

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