(117a) Composition Proxies in Refinery Distillation

Classical distillation calculations focus on separating specific components. Concentrations of specific chemicals define the product compositions. Refinery distillations have wide boiling ranges and relatively dull separations, considerable amounts molecules with the same boiling point may split between the different products. Due to the number of components, refinery distillation calculations use lumped pseudocomponents defined with vapor pressure and thermodynamic properties defined by standardized methods.

In place of component purities, the refinery uses four major proxies for compositions in fractionation calculations. All of these proxies use a measure of vapor pressure that is not directly a measure of pure component properties. These are:

1. Distillation curves (often called vaporization curves)
2. Reid vapor pressure (RVP)
3. Flash point
4. Noack volatility

Since these proxies are related to vapor pressure, they do have a fundamental link to composition changes. However, they have complex behaviors and use prediction methods that may be based on limited data leading to uncertainties and difficulty in interpretation. This paper examines the inter-relationships between these purity proxies and how calculation methods and correlation accuracy can affect analysis and design of refinery distillation units.