Vapour-liquid equilibrium diagrams

Figure 5.1 presents the behaviour of a pure species that can exist as solid, liquid or vapour in a pressure-temperature diagram. We may have three types of two-phase equilibrium solid/liquid, vapour/liquid and solid/vapour. There is a point where all three phases coexist, designated by the triple point. Here the phase rule gives F=C+2-P= +2-3=Q degrees of freedom. Neither pressure nor temperature can be used to modify the equilibrium. If only two phases can be found at equilibrium F=l+2-2=l, and either pressure or temperature can vary. The most important equilibrium in process engineering is vapour-liquid equilibrium, abbreviate as VLE. It may be observed that the two phases will coexist up to a point where it is difficult to make a distinction between vapour and liquid. This is the critical point, a fundamental physical property characterised by critical parameters and. Above the critical point the state... 

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...

The disadvantage of such a course of action is that water builds up in the residue and will be present in the vapour leaving the still. For an immiscible solvent the distillate will separate into two phases after condensing and because of the shape of the vapour-liquid equilibrium (VLB) diagram (Fig. 5.4) no fractionating column is needed. However, a water-miscible solvent will have to be freed of water by fractionation or some other means. Further, there are only two solvents in this class that do not form azeotropes with water—methanol and acetone. The latter is difficult to separate from water by fractionation below a level of about 1.5% w/w water so that only methanol can be mixed with water without a... 

If the Dechema series is unavailable, this single volume covers a great many common industrial mixtures together with vapour/hquid equilibrium diagrams. Maczynski, A. and Bilinski, A. (1974-1985) Verified Vapour-Liquid Equilibrium Data, Polish Academy of Sciences, Warsaw. 

When oil and gas are produced simultaneously into a separator a certain amount (mass fraction) of each component (e.g. butane) will be in the vapour phase and the rest in the liquid phase. This can be described using phase diagrams (such as those described in section 4.2) which describe the behaviour of multi-component mixtures at various temperatures and pressures. However to determine how much of each component goes into the gas or liquid phase the equilibrium constants (or equilibrium vapour liquid ratios) K must be known. 

The composition of the vapour in equilibrium with a liquid of given composition is determined experimentally using an equilibrium still. The results are conveniently shown on a temperature-composition diagram as shown in Figure 11.3. In the normal case shown in Figure 11.3a, the curve ABC shows the composition of the liquid which boils at any... 

The (liquid 4- liquid) equilibria diagram for (cyclohexane + methanol) was taken from D. C. Jones and S. Amstell, The Critical Solution Temperature of the System Methyl Alcohol-Cyclohexane as a Means of Detecting and Estimating Water in Methyl Alcohol , J. Chem. Soc., 1930, 1316-1323 (1930). The G results were calculated from the (vapor 4- liquid) results of K. Strubl, V. Svoboda, R. Holub, and J. Pick, Liquid-Vapour Equilibrium. XIV. Isothermal Equilibrium and Calculation of Excess Functions in the Systems Methanol -Cyclohexane and Cyclohexane-Propanol , Collect. Czech. Chem. Commun., 35, 3004-3019 (1970). The results are from M. Dai and J.-P.Chao, Studies on Thermodynamic Properties of Binary Systems Containing Alcohols. II. Excess Enthalpies of C to C5 Normal Alcohols + 1,4-Dioxane , Fluid Phase Equilib., 23, 321-326 (1985). 

These data are represented5 m the pressure-temperature diagram (fig. 42) by the fusion curve AB, which is steep, but curved towards the abscissa,6 as the results in the last column of the above table clearly demand. This curve represents the equilibrium between ordinary ice or ice I and water, the triple point A representing the condition of equilibrium of water-vapour, liquid water, and ice I. Under a pressure of 2200 kilograms, corresponding to the point B in the figure, there is a break in the fusion curve, a new form of ice appearing, known as ice III,... 

A new developed process PGSS (Particles from Gas Saturated Solutions) was applied for generation of powder from polyethyleneglycols. Principle of PGSS process is described and phase equilibrium data for the binary systems PEG-CO2 for the vapour-liquid and the solid-liquid range are presented in a master diagram . The influence of the process parameters on particle size, particle size distribution, shape, bulk density and crystallinity is discussed. 

The constant pressure diagram for this system is shown schematically in fig. 21.14. The boiling point of the mixture is independent of composition as shown by the horizontal dotted line at except when the second component disappears when, of course, the boiling point rises abruptly to that of the pure component T or T ), The line T E gives the composition of the vapour in equilibrium with pure liquid 1 as a function of temperature. The equilibrium temperature is lower than the boiling point of 1 as its partial pressure in the vapour phase is lower than total pressure. Similarly T E gives the composition of T mixed vapour in equilibrium with p liquid 2. At the eutectic point we have co-existence of the two liquid phases and vapour. The lines T E and T E are given by equations like (18.23) and (18.23 ). 

When one passes to the consideration of the equilibrium relations which exist in the case of two components which are not completely miscible in the liquid state, the additional complexity is introduced that at a certain value of the temperature and composition two liquid phases are formed. Since, at this point, there now coexist four phases, solid, two liquid phases, and vapour, the system is invariant. Not only the temperature, therefore, but also the composition of the two liquid phases must have definite values. If the solid phase is allowed to be absent, then the system becomes univariant, and the composition of the two liquid phases will alter with the temperature. Into the ordinary equilibrium diagram, such as is represented in Figs. 33 and 37 (pp. 103 and 109), there will be introduced a curve for the relation between the temperature and the composition of the two liquid phases. 

Figure J.2J. Stale diagram of a one-component system in the coordinates F-V ACD is the binodal of liquid-vapour phase equilibria, Ee and Ff are portions of the binodal of crystal-liquid phase equilibria, CL and DM are portions of the binodal of crystal-vapour phase equilibria (no corresponding surface defined by Equation 1.2-33 is shown in Figure 1.20), BCC is the spinodal of the liquid-vapour phase transition, Kj is the spinodal of the crystal-liquid phase transition, GAD is the straight line of three-phase (vapour-liquid-cryslal) equilibrium at the triple point (Kirilin ct al., 1983 Skripov and Koverda, 1984)...

If the equilibrium constants Kj arc known for all components, then the composition of a vapour in equilibrium with a liquid of a given composition can be computed relatively simply with the aid of eq. (28). The equilibrium constant Aj is a function of the total pressure P and the temperature T for each component and thus can be read from diagrams like those developed by Brown and Sauders for hydrocarbons. Figs. 20 and 21 show examples of these diagrams for methane and pentane. 

The composition of the mixtures is given either in mole or mass fractions. X( is the ratio of the number of moles of the i-th component to the number of moles of all the components of the mixture. Corrcspondinglj nii is the ratio of the mass of the t-th component to the total mass. The liquid at the point of saturation is denoted by the vapour in equilibrium with it is denoted by In the diagrams the liquid is represented by solid lines, the vapour by dotted lines. 

A one-component system (C = 1) has two independent state variabies (T and p). At the tripie point three phases (soiid, iiquid, vapour) coexist at equiiibrium, so P = 3. From the phase ruie f = 0, so that at the tripie point, T and p are fixed - neither is free but both are uniqueiy determined. If T is free but p depends on T (a sloping line on the phase diagram) then f = 1 and P = 2 that is, two phases, solid and liquid, say, co-exist at equilibrium. If both p and T are free (an area on the phase diagram) F = 2 and P = 1 only one phase exists at equilibrium (see Fig. A1.18). 

Fig. 5. The essential form of the phase diagram for the mesogen GB(3.0, 5.0, 2, 1) the open circles indicate the approximate coexistence lines and the solid circles show the density of the isotropic liquid in equilibrium with the vapour phase...

The situation for any plate n, with liquid composition x corresponding to an equilibrium vapour composition y , but with actual vapour composition y , is represented on a small section of the McCabe-Thiele diagram in Fig. 3.62. 

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