Vapor-phase mixtures

Most of the vinyl acetate produced in the United States is made by the vapor-phase ethylene process. In this process, a vapor-phase mixture of ethylene, acetic acid, and oxygen is passed at elevated temperature and pressures over a fixed-bed catalyst consisting of supported palladium (85). Less than 70% oxygen, acetic acid, and ethylene conversion is realized per pass. Therefore, these components have to be recovered and returned to the reaction zone. The vinyl acetate yield using this process is typically in the 91—95% range (86). Vinyl acetate can be manufactured also from acetylene, acetaldehyde, and the hquid-phase ethylene process (see Vinyl polymers). 

Table 11.9 shows the composition of a four-component mixture to be separated by distillation. The K-values for each component at the bubble point temperature of this mixture are given. The liquid- and vapor-phase mixtures of these... 

Nitration of alkanes is successful only when conducted at high temperatures in the vapor phase. Mixtures of products are invariably obtained (Section 4-6) ... 

Wythoff, B., et al.. Computer-Assisted Infrared Identification of Vapor-Phase Mixture Components, J. Chem. Inf. Comput. Sci., 31, 392, 1991. 

Figure 11.3-3 shows the vapor-liquid and liquid-liquid equilibrium behavior computed for the system of methanol and n-hexane at various temperatures. Note that two liquid phases coexist in equilibrium to temperatures of about 43°C. Since liquids are relatively incompressible, the species liquid-phase fugacities are almost independent of pressure (see Illustrations 7.4-8 and 7.4-9), so that the liquid-liquid behavior is essentially independent of pressure, unless the pressure is very high, or low enough for the mixture to vaporize (this possibility will be considered shortly). The vapor-liquid equilibrium curves for this system at various pressures are also shown in the figure. Note that since the fugacity of a species in a vapor-phase mixture is directly proportional to pressure, the VLE curves are a function of pressure, even though the LLE curves are not. Also, since the methanol-hexane mixture is quite nonideal, and the pure component vapor pressures are similar in value, this system exhibits azeotropic behavior. 

Equation 25 was developed from an empirical representation of thg second virial coefficient correlation of Pitzer and Curl (I) parameter b was left unchanged at its classical value of 0.0866. Because of the substantial improvement in the prediction of and its temperature derivatives for nonsimple fluids, the Barner modification of the RK equation gave improved estimates of enthalpy deviations for nonpolar vapors and for vapor-phase mixtures of hydrocarbons. However, the new equation was unsuitable for fugacity calculations. 

Vapor-phase mixture rendered to be thermodynamically unstable leads to the formation of desired soUd material. Supersaturated vapour — Chemical super saturation ... 

Tsonopoulos, C, and J. M. Prausnitz. Fugacity coefficients in vapor-phase mixtures of water and carboxylic acids. Chem. Eng. J. l TTi-in (1970). 

When the vapor phase mixture behaves as a perfect gas, the partial pressure of the /th component is governed by Dalton s law as given by... 

Many important chemical processes involve the transition between the single-phase and two- phase regions. Consider the situation in the figure on the right in Figure 7.9. If we start from a pure liquid state and decrease the pressure on the system, thai at stxne point the liquid will begin to boil. This is termed the bubble point and is the location where the first drop of vapor is formed. If instead we initially have a vapor phase mixture and we inaease the pressure (or decxease the temperature), then liquid will frmn at the dew point. The dew point is defined as the location whae the first drop of liquid is formed. 

Vapor phase mixture properties at low pressures are calculated with the virial equation. Interaction coefficients are useful for nonpolar systems, important for polar ones, and essential where solvation occurs. 

Consider a vapor phase mixture of 2 moles propane (1) and 3 moles nitrogen (2) at 15°C and 12 bar. You wish to come up with the fugacity and fugacity coefficient for propane. Answer the following questions using the truncated virial equation of state, as follows ... 

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