Phase vapour—liquid

In drying a water-wet solid, free liquid water may or may not be present. If it is, there are in all three phases-vapour, liquid and solid- and three components, so F = 2. At constant... 

In most cases, a two-phase (vapour/ liquid or gas/ liquid) mixture is vented from a reactor emergency relief system. The relief system required for a two-phase mixture is very often larger (by, in some cases, several times the flow area) than for gas or vapour alone. It is therefore essential to take account of two-phase relief to size a relief system properly. J ... 

The most common industrial technique involves the creation of a second phase (vapour, liquid or solid) that is immiscible with the feed phase (Figure 2a). The creation is accomplished by energy transfer (heat and/or shaft work) to or from the process or by pressure reduction. 

Figure 1.20. Surf2u e defined by Equation 1.2 34 to characterize the gas and liquid states of a onc-component system in the coordinates P V-T (a). Surface defined by Equation 1.2-33 for the crystal and liquid states (6). ACD is the binodal of liquid-vapour phase equilibria BCC is the spinodal of liquid-vapour phase transition Be and Ff are fragments of the binodal of the crystal-liquid phase equilibria Kj is the spinodal of the crystal-liquid phase transition GAD is the straight line of three-phase (vapour-liquid-crystal) equilibrium at the triple point (Skripov and Koverda, 1984)...

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. 

Statistical mechanical theory and computer simulations provide a link between the equation of state and the interatomic potential energy functions. A fluid-solid transition at high density has been inferred from computer simulations of hard spheres. A vapour-liquid phase transition also appears when an attractive component is present hr the interatomic potential (e.g. atoms interacting tlirough a Leimard-Jones potential) provided the temperature lies below T, the critical temperature for this transition. This is illustrated in figure A2.3.2 where the critical point is a point of inflexion of tire critical isothemr in the P - Vplane. 

As the temperature of the liquid phase is increased, the system ultimately reaches a phase boundary, the bubble point at which the gas phase (vapour) begins to appear, with the composition shown at the left end of the horizontal two-phase tie-line . As the temperature rises more gas appears and the relative amounts of the two phases are detemiined by applying a lever-ami principle to the tie-line the ratio of the fractionof molecules in the gas phase to that hn the liquid phase is given by the inverse of the ratio of the distances from the phase boundary to the position of the overall mole fraction Xq of the system. 

The major chromatographic techniques can also be categorised according to tbe nature of the mobile phase used -vapour phase chromatography for when a gas is the mobile phase and liquid chromatography for when a liquid is the mobile phase. 

In contrast to vapour phase chromatography, the mobile phase in liquid chromatography is a liquid. In general, there are four main types of liquid chromatography adsorption, partition, ion-chromatography, and gel filtration. 

Vapour-phase reactor Liquid-phase reactor... 

Some of the important features of the flow of two-phase mixtures composed of a liquid together with a gas or vapour are discussed in this section. There are many applications in the chemical and process industries, ranging from the flow of mixtures of oil and gas from well heads to flow of vapour-liquid mixtures in boilers and evaporators. 

Equilibrium in multiphase and/or multireaction systems. If more than one phase is present in the system, a criterion of phase equilibria has to be satisfied together with the chemical equilibrium criterion. For instance, in a gas-liquid system components are in chemical equilibrium in the phase where the reaction occurs, but vapour-liquid equilibria between the gas and the liquid phases must also be taken into account. To determine the equilibrium composition of a reacting mixture in both phases, chemical equilibrium constants as well as data concerning vapour-liquid equilibria for all components of the reaction mixture should be known. In the equilibrium state ... 

Problems with the determination of chemical equilibria in multiphase systems are solved in practice by assuming that the reaction takes place in any phase and all components are also equilibrated between phases. Accordingly, for a single reaction any of Eqns. (5.4-32) and (5.4-33) must be solved, while the relationships of Eqn. (5.4-31) must also be fulfilled. Since vapour-liquid equilibrium coefficients are functions of the compositions of both phases, the search for the solution is an iterative procedure. Equilibrium compositions are assumed, vapour-liquid equilibrium coefficients are then estimated, and new equilibrium compositions are evaluated. If the new equilibrium compositions are close to those assumed initially one may consider the assumed values to be the solution of the problem. Otherwise the evaluated compiositions are taken as the start for repetition of the procedure until a reasonable agreement between tissumed and evaluated comfiositions has been reached. 

Steam distillation is a process whereby organic liquids may be separated at temperatures sufficiently low to prevent their thermal decomposition or whereby azeotropes may be broken. Fats or perfume production are examples of applications of this technique. The vapour-liquid equilibria of the three-phase system is simplified by the usual assumption of complete immiscibility of the liquid phases and the validity of the Raoult and Dalton laws. Systems containing more than one volatile component are characterised by complex dynamics (e.g., boiling point is not constant). 

It should be noted that in a vapour phase the liquid layer on the surface of a sensitive element of the sensor (zinc oxide) must be sufficiently thin, so that it would not produce any influence on the diffusion flux of oxygen through this layer. Possible lack of the film continuity (the presence of voids) does not prevent determination of concentration of oxygen in the bulk of the cell by the vapour - gas method. In this case, one deals with a semi-dry method. On the contrary, the presence of a thick liquid layer causes considerable errors in measuring t, because of different distribution of oxygen in a system gas - liquid layer -semiconductor film (this distribution is close to that in the system semiconductor film - liquid), in addition to substantial slowing down of oxygen diffusion in such systems. 

Note that in weakly polar (.e < 5) and nonpolar solvents, it is impossible to blow off oxygen adsorbed on ZnO film with an inert gas hydrogen, nitrogen, etc.), similar to the case of gas or saturated vapour phase (polar liquid at any e) at room temperature, i.e., under the conditions, where f = 1, and no liquid layer is condensed on the film. 

Figure 7.10 Typical optical arrangements employed for detection of (a) vapour phase (b) liquid phase and (c) solid chromatographic phases. After White [167], Reprinted from R. White (ed.), Chromatography/Fourier Transform Infrared Spectroscopy and Its Applications Marcel Dekker Inc., New York, NY (1990), by courtesy of Marcel Dekker Inc.

Given the temperature and pressure, the concentration of any component in the vapour phase can be obtained from the concentration in the liquid phase, from the vapour-liquid equilibrium data for the system. 

This example illustrates the use of phase equilibrium relationships (vapour-liquid) in material balance calculations. 

The NRTL equation developed by Renon and Prausnitz overcomes the disadvantage of the Wilson equation in that it is applicable to immiscible systems. If it can be used to predict phase compositions for vapour-liquid and liquid-liquid systems. 

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). 

Using the classification given in Table 8.10, Table 8.11 can be used to select the appropriate vapour-liquid and liquid-liquid phase equilibria method. 

Unless liquid phase activity coefficients have been used, it is best to use the same equation of state for excess enthalpy that was selected for the vapour-liquid equilibria. If liquid-phase activity coefficients have been specified, then a correlation appropriate for the activity coefficient method should be used. 

H = specific enthalpy vapour phase, h = specific enthalpy liquid phase, hf = specific enthalpy feed (vapour + liquid). 

Nielsen, C. J. etal., J. Mol. Struct., 1987, 16(1-2), 41-56 Precautions appropriate to a potentially explosive compound were observed during synthesis and handling. Electron diffraction studies on the vapour, and IR, NMR and Raman spectral studies on the vapour, liquid and solid (—183°C) phases were effected without incident. 

Vrabec, J. Loth, A. Fischer, J., Vapour liquid equilibria of Lennard-Jones model mixtures from the NPT plus test particle method, Fluid Phase Equil. 1995,112, 173-197... 

The non-random two-liquid segment activity coefficient model is a recent development of Chen and Song at Aspen Technology, Inc., [1], It is derived from the polymer NRTL model of Chen [26], which in turn is developed from the original NRTL model of Renon and Prausznitz [27]. The NRTL-SAC model is proposed in support of pharmaceutical and fine chemicals process and product design, for the qualitative tasks of solvent selection and the first approximation of phase equilibrium behavior in vapour liquid and liquid systems, where dissolved or solid phase pharmaceutical solutes are present. The application of NRTL-SAC is demonstrated here with a case study on the active pharmaceutical intermediate Cimetidine, and the design of a suitable crystallization process. 

At present there are two fundamentally different approaches available for calculating phase equilibria, one utilising activity coefficients and the other an equation of state. In the case of vapour-liquid equilibrium (VLE), the first method is an extension of Raoult s Law. For binary systems it requires typically three Antoine parameters for each component and two parameters for the activity coefficients to describe the pure-component vapour pressure and the phase equilibrium. Further parameters are needed to represent the temperature dependence of the activity coefficients, therebly allowing the heat of mixing to be calculated. 

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