Saturated Fluid - Boiling-point Diagrams

Boiling-point Diagrams

Binary mixture VLE data at a certain overall pressure, such as 1 atm, showing mole fraction vapor and liquid concentrations when boiling at various temperatures can be shown as a two-dimensional graph called a boiling-point diagram. The mole fraction of component 1 in the mixture can be represented by the symbol x₁. The mole fraction of component 2, represented by x₂, is related to x₁ in a binary mixture as follows:

x₁ + x₂ = 1

In multi-component mixtures in general with n components, this becomes:

x₁ + x₂ + ... + x_n = 1

The preceding equations are typically applied for each phase (liquid or vapor) individually. In a binary boiling-point diagram, temperature (T ) is graphed vs. x₁. At any given temperature where boiling is present, vapor with a certain mole fraction is in equilibrium with liquid with a certain mole fraction, often differing from the vapor. These vapor and liquid mole fractions are both on a horizontal isotherm (constant T ) line. When an entire range of boiling temperatures vs. vapor and liquid mole fractions is graphed, two (usually curved) lines are made. The lower one, representing boiling liquid mole fraction at various temperatures, is called a bubble point curve. The upper one, representing vapor mole fraction at corresponding temperatures, is called a dew point curve.

These two lines (or curves) meet where the mixture becomes purely one component, where x₁ = 0 (and x₂ = 1, pure component 2) or x₁ = 1 (and x₂ = 0, pure component 1). The temperatures at those two points correspond to the boiling points of the two pure components. In certain combinations of components, the two curves may also meet at a point somewhere in between x₁ = 0 and x₁ = 1. That point represents an azeotrope in that particular combination of components. That point has an azeotrope temperature and an azeotropic composition often represented as a mole fraction. There can be maximum-boiling azeotropes, where the azeotrope temperature is at a maximum in the boiling curves, or minimum-boiling azeotropes, where the azeotrope temperature is at a minimum in the boiling curves.

If one wants to represent a VLE data for a three-component mixture as a boiling point "diagram", a three-dimensional graph can be used. Two of the dimensions would be used to represent the composition mole fractions, and the third dimension would be the temperature. Using two dimensions, the composition can be represented as an equilateral triangle in which each corner representing one of the pure components. The edges of the triangle represent a mixture of the two components at each end of the edge. Any point inside the triangle represent the composition of a mixture of all three components. The mole fraction of each component would correspond to where a point lies along a line starting at that component's corner and perpendicular to the opposite edge. The bubble point and dew point data would become curved surfaces inside a triangular prism, which connect the three boiling points on the vertical temperature "axes". Each face of this triangular prism would represent a two-dimensional boiling-point diagram for the corresponding binary mixture. Due to their three-dimensional complexity, such boiling-point diagrams are rarely seen. Alternatively, the three-dimensional curved surfaces can be represented on a two-dimensional graph by the use of curved isotherm lines at graduated intervals, similar to iso-altitude lines on a map. Two sets of such isotherm lines are needed on such a two-dimensional graph: one set for the bubble point surface and another set for the dew point surface.