Multicomponent Equilibrium Flash Calculation

When a multicomponent liquid stream undergoes a sudden decrease in pressure (by, for example, flowing through a valve), part of the feed vaporizes.  The vapor product is richer in the more volatile components (i.e. those with a higher equilibrium constant) than the liquid product. This is known as an equilibrium flash, and is the basis of many unit operations in the process industries, including distillation.

This Excel spreadsheet implements an isothermal multicomponent equilibrium flash calculation. The spreadsheet includes a set of chemical components as the input feed (i.e. the molar amount in the feed stream, together with their K-values), but these can be easily modified.

We then use the Rachford-Rice equation to calculate the fraction of each component in the product streams. The equation is

where z_i is the mole fraction of component i in the liquid feed, K_i is the equilibrium constant (at the appropriate temperature and pressure) and β is the fraction of feed that is vaporised. Obviously, β is between 0 and 1.

We then solve the Rachford-Rice equation iteratively. All you need to do is find the value of β that makes the Rachford-Rice equation equal to zero using Excel's Solver function.  See the screengrab below for specific instructions.

The principles demonstrated in the spreadsheet are the basis of many other calculations that involve vapor-liquid equilibria, and are used in petroleum refineries, chemical plants and natural gas plants,

Download Excel spreadsheet for Multicomponent Equilibrium Flash