Constant relative volatility systems vapor liquid equilibrium

Another issue is that the effect of pressure on the design is not explored here because we are assuming constant relative volatility systems. The column pressure is fixed at 8 bar in this work. Pressure is very important in reactive distillation because of the effect of temperature on both vapor-liquid equilibrium and reaction kinetics. For exothermic reactions, the optimum column pressure is affected by the competing effects of temperature on the specific reaction rates and the chemical equilibrium constant. 

The compositions of vapor and liquid phases of a binary system at equilibrium sometimes can be related by a constant relative volatility which is defined as... 

After heat recovery, via HXl and HX2, the reactor effluent is fed into a distillation column. The two reactants, A B, are light key (LK) and intermediate boiler (IK), respectively, while the product, X, is the heavy component (HK). The Antoine constants of the vapor pressure equation are chosen such that the relative volatilities of the components are ttA = 4, ttB = 2, and Oc=l for this equal molar overflow system (Table 1). Only one distillation column is sufficient to separate the product (C) from the unreacted reactants (A B). Ideal vapor-liquid equilibrium is assumed. Physical property data and kinetic data are given in Table 1. 

Using Equation 7-12, a curve can be established that shows the relationship between the liquid composition and the equilibrium vapor composition at a constant relative volatility. Figure 7-1 shows curves at a values of 1.4, 2.0, and 4.0, which represent separations of increasing ease. The equilibrium relationships between vapor and liquid compositions for some nonideal binary systems are shown in Figure 7-2. Curve I is a methanol/water system, and Curve II is a water/acetic acid system. Note that these curves no longer are symmetrical like those in Figure 7-1. 

If the temperature dependence of the vapor pressure of both componoits is the same, a Avill be independent of temperature. In other words, relative volatility is constant if the vapor pressiure lines are parallel in a In P versus 1/T plot. This is true for many components over a limited temperature range, particularly when the components are chemically similar. Distillation columns are frequently designed assuming constant relative volatility because it greatly simplifies the vapor—liquid equilibrium calculations. Relative volatilities usually decrease somewhat widi increasing temperature in most systems. 

In this simple distillation process, it is assumed that the vapor formed within a short period is in thermodynamic equilibrium with the liquid. Hence, the vapor composition xp is related to the hquid composition xb by an equiUbrium relation of the form xp = fixs)- The exact relationship for a particular mixture may be obtained from a thermodynamic analysis depending on temperature and pressure. For a system following the ideal behavior given by Raoult s law, the equilibrium relationship between the vapor composition y (or xp) and liquid composition x (or xb) of the more volatile component in a binary mixture can be approximated using the concept of constant relative volatility a), and is given by ... 

In the next three chapters we will explore various aspects of the ideal quaternary chemical system introduced in Chapter 1. This system has four components two reactants and two products. The effects of a number of kinetic, vapor-liquid equilibrium, and design parameters on steady-state design are explored in Chapter 2. Detailed economic comparisons of reactive distillation with conventional multiunit processes over a range of chemical equilibrium constants and relative volatilities are covered in Chapter 3. An economic comparison of neat versus excess-reactant reactive distillation designs is discussed in Chapter 4. 

Gas-liquid relationships, in the geochemical sense, should be considered liquid-solid-gas interactions in the subsurface. The subsurface gas phase is composed of a mixture of gases with various properties, usually found in the free pore spaces of the solid phase. Processes involved in the gas-liquid and gas-solid interface interactions are controlled by factors such as vapor pressure-volatilization, adsorption, solubility, pressure, and temperature. The solubility of a pure gas in a closed system containing water reaches an equilibrium concentration at a constant pressure and temperature. A gas-liquid equilibrium may be described by a partition coefficient, relative volatilization and Henry s law. 

If phase equilibrium is assumed between liquid and vapor, the right-hand side of Eq. (13-126) may be evaluated from the area under a curve of / y — x) versus x between the limits Xj and Xf. If the mixture is a binary system for which the relative volatility a can be approximated as a constant over the range considered, then the VLE relationship... 

A fundamental difference between the two flowsheets is the ability in the conventional process to adjust reactor temperature and distillation column temperamres completely independently, which is not possible in the reactive distillation process. In the conventional system, the reactor temperature can be set at an optimum value and distillation temperatures can be independently set at their optimum values by adjusting column pressures. In reactive distillation, these temperatures are not independent because only one pressure can be set in the vessel. Therefore, the design of a reactive distillation requires a tradeoff between temperatures conducive for reaction (kinetics and equilibrium constants) and temperatures favorable for vapor-liquid separation. The temperature dependency of the relative volatilities will illustrate this important difference between the two processes. 

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