Flash Drum Calculator

The vapor fraction is solved entirely from the K-values and feed composition you enter, not from temperature and pressure directly. In real vapor-liquid equilibrium, K-values themselves are functions of temperature and pressure, so this calculator expects you to already have K-values appropriate for your chosen operating point -- the temperature and pressure fields exist to document that operating point for reference, not to independently drive the calculation.

K-values (the equilibrium ratio K = y/x for each component) typically come from a process simulator using an appropriate thermodynamic model, published vapor- liquid equilibrium data or charts (like DePriester charts for light hydrocarbons), or a correlation suited to your specific chemical system at your actual operating temperature and pressure. Using K-values from the wrong system or conditions will produce a vapor fraction that does not reflect your real process.

A vapor fraction of 0 means the feed conditions do not support any vapor formation at these K-values -- the entire feed leaves as liquid -- while a fraction of 1 means the entire feed vaporizes and no liquid remains. The Rachford-Rice solution is clamped to the physically meaningful 0-to-1 range, so K-value combinations that would mathematically solve outside that range instead report the nearest boundary case.

For a two-phase flash to actually occur, at least one component's K-value must be above 1.0 (favoring vapor) and at least one below 1.0 (favoring liquid) -- if both components share the same preference, the mixture stays entirely in one phase regardless of composition. This calculator's default K-values (2.5 for the light component, 0.5 for the heavy one) reflect exactly that kind of genuine two-phase split.

Raising either component's K-value pushes more of the feed into the vapor phase, since a higher K-value means that component more strongly favors partitioning into vapor at equilibrium. Raising both K-values together compounds that effect, so a feed that was mostly liquid at lower K-values can shift toward being mostly vapor as both components' equilibrium ratios increase, for example as operating temperature rises or pressure drops.