Equilibria with Ks Dependent on Composition

The liquid composition is known for a bubblepoint determination, but the temperature is not at the start, so that starting estimates must be made for both activity and fugacity coefficients. In the flow diagram, the starting values are proposed to be unity for all the variables. After a trial value of the temperature is chosen, subsequent calculations on the diagram can be made directly. The correct value of T has been chosen when E y,- = 1- 

Since the equations for fugacity and activity coefficients are complex, solution of this kind of problem is feasible only by computer. Reference is made in Example 13.3 to such programs. There also are given the results of such a calculation which reveals the magnitude of deviations from ideality of a common organic system at moderate pressure. 

For a mixture of ethane, n-butane, and n-pentane, the bubblepoint and dewpoint temperatures at lOOpsia, a flash at 100°F and lOOpsia, and an adiabatic flash at lOOpsia of a mixture initially liquid at 100°F will be determined. The overall composition 2, the coefficients A, B, and C of Eq. (13.22) and the coefficients a, b, c, and d of Eqs. (13.28) and (13.29) are tabulated  

Results for successive iterations for jS and the final phase compositions are 

Adiabatic flash calculation Liquid and vapor enthalpies off charts in the API data book are fitted with linear equations 

After P has been found by successive approximation, the phase compositions are obtained with 

A starting value of p = 1 always leads to a converged solution by this method. 

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Single-Stage Flash Calculations 375 Bubblepoint Temperature and Pressure 376 Dewpoint Temperature and Pressure 377 Flash at Fixed Temperature and Pressure 377 Flash at Fixed Enthalpy and Pressure 377 Equilibria with Ks Dependent on Composition 377... 

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