• VLE Calculation Problems:

    Low pressure VLE are often modelled with modified Raoult’s law (\(y_iP = \sum \gamma_i x_i P_i^{\text{sat}}\)). With the given \(x_i\) or \(y_i\) and temperature, finding of pressure and \(x_i\) or \(y_i\) is easier. Whereas estimating the temperature for a given \(P\) and \(x_i\) or \(y_i\) involves trial and error calculation.

    Problem Name Knowns Unknowns to find How to find?
    Bubble \(P\) \(T, x_i\) \(P, y_i\) Start from \(\sum y_i =1\)
    Dew \(P\) \(T, y_i\) \(P, x_i\) Start from \(\sum x_i =1\)
    Bubble \(T\) \(P, x_i\) \(T, y_i\) Start from \(\sum y_i =1\) and a guess value of \(T=\sum x_iT_i^{\text{sat}}\)
    Dew \(T\) \(P, y_i\) \(T, x_i\) Start from \(\sum x_i =1\) and a guess value of \(T=\sum y_iT_i^{\text{sat}}\)
    Flash \(T, P, z_i\) \(x_i, y_i, V/L\) Using Rachford-Rice equation given as \(\displaystyle \sum_i \frac{z_i(K_i-1)}{1+\frac{V}{F}(K_i-1)} = 0\) where \(K_i=\gamma_i P_i^{\text{sat}}/P\); and, \(\text{Feed } (F) = \text{Liquid }(L) + \text{Vapor } (V)\).