What is a flash calculation and how is it performed?

Flash calculation determines the amounts and compositions of vapor and liquid phases when a mixture is brought to specified T and P. For PT-flash: given feed composition z, T, P, find vapor fraction V/F, vapor composition y, and liquid composition x. Rachford-Rice equation: sum(z_i*(K_i-1)/(1+V/F*(K_i-1))) = 0, solved iteratively for V/F. K-values (y/x) from VLE correlation or equation of state. Material balance gives compositions. Used for separator design, flash drums, and any phase separation modeling.