Flash distillation equilibrium equations

In this study, the phase equilibrium in the binary mixtures that are expected to be found in the flash distillation was modeled with the Predictive Soave-Redlich-Kwong (PSRK) equation of state [4], using modified molecular parameters r and q. Five binary ethanol + congener mixtures were considered for new yield values for parameters r and q. The congeners considered were acetic acid, acetaldehyde, furfural, methanol, and 1-pentanol. Subsequently, the model was validated with the water + ethanol binary system, and the 1 -pentanol + ethanol + water, 1-propanol + ethanol + water, and furfural + ethanol + water ternary systems. 

McCabe-Thiele calculations are easiest to do on spreadsheets if the y versus x VLE data are expressed in an equation. The form y = f(x) is most convenient for flash distillation and for distillation columns (see Chapter 4) if stepping off stages from the bottom of the column up. The form x = g(y) is most convenient for distillation columns if stepping off stages from the top down. Built-in functions in Excel will determine polynomials that fit the data, aldiough the fit will usually not be perfect. This will be illustrated for fitting the ethanol-water equilibrium data in Table 2-1 in the form y = f (x ). (Note An additional data point Xg = 0.5079, = 0.6564 was added to the numbers in the table.) Enter the data in the... 

In the previous section we saw that the RT curve took the place of the equilibrium curve in binary flash distillation. For multiconponent flash distillation the y-x equilibrium curve is replaced by equilibrium expressions of the form y = K X and the combination of the operating equations and equilibrium are summed using Hxj = 1, Sy = 1, or Zy - ZX = 0. Since the operating equations for well-mixed permeators are similar to the operating equations for flash distillation fHoffman. 20031. if we can write the rate expression in the form y = Xj, then the mathematics to solve the permeation problem will be very... 

In modern separation design, a significant part of many phase-equilibrium calculations is the mathematical representation of pure-component and mixture enthalpies. Enthalpy estimates are important not only for determination of heat loads, but also for adiabatic flash and distillation computations. Further, mixture enthalpy data, when available, are useful for extending vapor-liquid equilibria to higher (or lower) temperatures, through the Gibbs-Helmholtz equation. ... 

This chapter considers the vapor-liquid equilibrium of mixtures, conditions for bubble and dew points of gaseous mixtures, isothermal equilibrium flash calculations, the design of distillation towers with valve trays, packed tower design. Smoker s equation for estimating the number of plates in a binary mixture, and finally, the computation of multi-component recovery and minimum trays in distillation columns. 

Thus, the minimum number of equilibrium stages depends on the degree of separation of the two key components and their relative volatility, but is independent of feed-phase condition. Equation (12-12) in combination with (12-13) is exact for two minimum stages. For one stage, it is equivalent to the equilibrium flash equation. In practice, distillation columns are designed for separations corresponding to as many as 150 minimum equilibrium stages. 

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