Vapor-liquid system

Vapor-Uquid systems are encountered frequently in industrial processing. The composition difference between tbe 

Using the equality of fugacity of a species in vapor (y = v) and liquid phases (J = Z), 

The Gibbs phase rule for a closed nonreactive system of j phases made up of n species states that, at equilibrium, the number of degrees of freedom F is given by 

Once the intensive variables F are specified, the system is compietely defined. For example, in a binary system ( = 2) having vapor-Uquid equiUbrium (7 = 2), the degrees of freedom available are 2 (= F). Once the values of any two intensive variables are fixed, the values of aU other intensive variables become fixed. If values of pressure and temperature are specified, the compositions of the two phases of the system are determined. Similarly, if the pressure and Uquid mole fraction are specified, the composition of the vapor phase and the value of the tem-peramre become fixed. Alternatively, if the temperature and the vapor phase mole firaction are given, the pressure and the liquid phase mole fraction wiU be fixed automatically from the solution of the governing equations. 

Before we present briefly the methodology for such calculations, it is worthwhile iUustrating the basic types of vapor-liquid equiUbrium (VLB) behavior frequently encountered. The variety in VLB arises primarily from different types of nonideal behavior, as indicated, for example, by the Uquid-phase activity coefficients. Minimumboiling azeotropes and maximum-boiling azeotropes are two such types of nonideal behavior. To become fatniUar with such behavior, we start with the ideal solution behavior. 

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At pressures to a few bars, the vapor phase is at a relatively low density, i.e., on the average, the molecules interact with one another less strongly than do the molecules in the much denser liquid phase. It is therefore a common simplification to assume that all the nonideality in vapor-liquid systems exist in the liquid phase and that the vapor phase can be treated as an ideal gas. This leads to the simple result that the fugacity of component i is given by its partial pressure, i.e. the product of y, the mole fraction of i in the vapor, and P, the total pressure. A somewhat less restrictive simplification is the Lewis fugacity rule which sets the fugacity of i in the vapor mixture proportional to its mole fraction in the vapor phase the constant of proportionality is the fugacity of pure i vapor at the temperature and pressure of the mixture. These simplifications are attractive because they make the calculation of vapor-liquid equilibria much easier the K factors = i i ... 

Relative volatility is the volatility separation factor in a vapor-liquid system, i.e., the volatility of one component divided by the volatility of the other. It is the tendency for one component in a liquid mixture to separate upon distillation from the other. The term is expressed as fhe ratio of vapor pressure of the more volatile to the less volatile in the liquid mixture, and therefore g is always equal to 1.0 or greater, g means the relationship of the more volatile or low boiler to the less volatile or high boiler at a constant specific temperature. The greater the value of a, the easier will be the desired separation. Relative volatility can be calculated between any two components in a mixture, binary or multicomponent. One of the substances is chosen as the reference to which the other component is compared. 

Vapor—Liquid Systems. The vapor-liquid region of a pure substance is contained within the phase or saturation envelope on a P-V diagram (see Figure 2-80), A vapor, whether it exists alone or in a mixture of gases, is said to be saturated if its partial pressure (P.) equals its equilibrium vapor pressure (P, ) at the system temperature T. This temperature is called the saturation temperature or dew point T ... 

The onset of flow instability in a heated capillary with vaporizing meniscus is considered in Chap 11. The behavior of a vapor/liquid system undergoing small perturbations is analyzed by linear approximation, in the frame work of a onedimensional model of capillary flow with a distinct interface. The effect of the physical properties of both phases, the wall heat flux and the capillary sizes on the flow stability is studied. A scenario of a possible process at small and moderate Peclet number is considered. The boundaries of stability separating the domains of stable and unstable flow are outlined and the values of the geometrical and operating parameters corresponding to the transition are estimated. 

Let us look now at vapor-liquid systems with more than one component. A liquid stream at high temperature and pressure is flashed into a drum, i.e., its pressure is reduced as it flows through a restriction (valve) at the inlet of the drum. This sudden expansion is irreversible and occurs at constant enthalpy. If it were a reversible expansion, entropy (not enthalpy) would be conserved. If the drum pressure is lower than the bubblepoint pressure of the feed at the feed temperature, some of the liquid feed will vaporize. 

The main factor that has been responsible for the slow adoption of absorption heat pumps for heating and air conditioning duties has been their high capital cost compared with that of vapor compression equivalents. This is due largely to the cycle complexity, as shown in Figure 14, which displays the four principal cycle elements, all of which involve vapor-liquid systems ... 

Section 4.2 is focused on phase equilibrium-controlled vapor-liquid systems with kinetically or equihbrium-controlled chemical reactions. The feasible products are kinetic azeotropes or reactive azeotropes, respectively. 

There are three independent variables in coexisting equilibrium vapor/liquid systems, namely temperature, pressure, and fraction liquid (or vapor). If two of these are specified in a problem, the third is determined by the phase behavior of the system. There are seven types of vapor/liquid equilibria calculations in our program, as in Figure 1 under "Single Stage Calculation."... 

For a binary vapor-liquid system, the Gibbs-Duhem relations are... 

When an equilibrium reaction occurs in a vapor-liquid system, the phase compositions depend not only on the relative volatility of the components in the mixture, but also on the consumption (and production) of species. Thus, the condition for azeotropy in a nonreactive system y = Xi for all i) no longer holds true in a reactive system and must be modified to include reaction stoichiometry ... 

A vapor-recirculating equilibrium still similar to that described by Hipkin and Myers (1) was used to determine vapor-liquid equilibrium data for the system, water-MEK-THF. In this still shown schematically in Figure 1, a recirculating vapor is continuously contacted with a static liquid sample. The vapor-liquid system is enclosed by a jacket where... 

Prediction of Vapor Composition in Isobaric Vapor-Liquid Systems Containing Salts at Saturation... 

Interfacial polycondensations can also be carried out in vapor-liquid systems. Reaction takes place at the interface between an aqueous solution of a bifunctional active hydrogen compound and the vapor of diacid chloride. Interfacial condensation is commercially important in the synthesis of polycarbonates (1-52). Polymerizations based on diacids are always less expensive than those that use diacid chlorides. In the polycarbonate case, however, the parent reactant, carbonic acid, is not suitable and the derived acid chloride, phosgene (COCI2), must be used. 

It should also be noted that the symbol Ki is utilized for the mol fraction ratio y, /x, in comparing the permeate composition to the reject or raffinate composition, as will be developed in Example 19.4. This is the usual symbolism as used in phase equilibria, say that of the X-value or equilibrium vaporization ratio for correlating the behavior of vapor/liquid systems—and, ideally, reflects Raoult s law. The foregoing illustrates the general problem... 

Use UNIQUAC and UNIFAC v-l-e parameters for vapor-liquid systems and 1-1-e parameters for liquid-liquid systems. 

The reader should become familiar with the following definitions, which are pertinent to vapor-liquid systems (some specifically to air-water systems). 

Eew multicomponent systems exist for which completely generalized equilibrium data are available. The most widely available data are those for vapor-liquid systems, and these are frequently referred to as vapor-liquid equilibrium distribution coefficients or K value. The K values vary with temperature and pressure, and a selectivity that is equal to the ratio of the K values is used. Eor vapor-liquid systems, this is referred to as the relative volatility and is expressed for a binary system as... 

Various modeling procedures have been proposed in the literature to predict the phase behavior of vapor-liquid systems at high pressures. (The designation vapor will be used synonomously with supercritical fluid in this chapter.) Regardless of the modeling procedure, the following thermodynamic relationships, or their equivalent relationships in terms of chemical potentials, must be satisfied for two phases to be in equilibrium. 

Vapor-liquid systems, modeling, 110-113 Vaporization process, 138-139 Variable-volume view cell apparatus, 93 Venier, C. G, 325 Villard, P., 20 Vinyl chloride, 319 Viscosity effects, 325-326 Vitzthum, O., 296... 

A few blnaty systents whea cooled do not deposit one of the components in a totally pore state. Instead, behavior remmbles that of many vapor-liquid systems and the solid is a tree solution. Figure 11.2-3 depicts... 

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