Mass Balances for Binary Vapor-Liquid Equilibrium (VLE)

P-x-y diagram

mole balance

number of moles added

5

add A

add B

This Demonstration illustrates how a binary mixture in vapor-liquid equilibrium (VLE) can maintain the same compositions (at constant temperature and pressure) in the liquid and vapor phases upon addition of a pure component. Here

x

A

is the mole fraction of A in the liquid and

y

A

is the mole fraction of A in the vapor. If enough of either pure component is added, the mixture can be transformed to a single phase. The initial mixture consists of 4 mol of A and 6 mol of B at a pressure of 130 kPa in vapor-liquid equilibrium. You can use the slider to add either pure A or pure B to the mixture, depending on whether "add A" or "add B" is selected. Adding a pure component changes the overall mole fraction of A in the mixture, represented by the black point in the

P

-

x

-

y

diagram. If the point stays within the phase envelope, the mole fractions of A in the vapor and liquid phases remain constant. Clicking the "mole balance" button shows how this is possible. The size of the rectangles is proportional to the amount, while their color distinguishes the liquid and vapor phases (blue for liquid and green for vapor). For example, adding A increases the vapor-to-liquid ratio (lever rule), so some of the initial liquid vaporizes, and this amount is determined by a mass balance. The phase of the added component does not matter because the system is isothermal. For example, if pure A were added as a liquid instead of a vapor, it would then vaporize to satisfy the mass balances.