Cox chart used in distillation design (especially for petroleum hydrocarbons): which variables are plotted?

Introduction / Context:
Cox charts are convenient nomographs for estimating vapor pressures and boiling points, widely used in petroleum fractionation design and quick column sizing. They provide near-linear relationships over practical ranges for homologous series, simplifying interpolation without full Antoine parameters.

Given Data / Assumptions:

• Hydrocarbon or similar organic compounds in the relevant temperature range.
• Chart is constructed so that data align approximately as straight lines.
• Used for boiling-point–vapor-pressure correlation and component comparison.

Concept / Approach:
On a Cox chart, the ordinate is typically log10 of vapor pressure, and the abscissa is temperature (often °C). For many petroleum hydrocarbons, plotting log(vapor pressure) against temperature yields roughly straight lines, enabling interpolation/extrapolation of boiling temperatures at specified pressures or vice versa. This is distinct from the Clausius-type plots of log P vs. 1/T, which are common but not the Cox construction referenced in refinery practice.

Step-by-Step Solution:

Verification / Alternative check:
Comparing with standard refinery references shows Cox charts plotted as log P versus T, with families of curves for related compounds that are approximately linear across relevant ranges.

Why Other Options Are Wrong:

• Vapor pressure vs. temperature without the log scale yields curved plots, less linear.
• log(temperature) variants are not standard Cox constructions.

Common Pitfalls:
Confusing Cox charts with Clausius–Clapeyron (log P vs. 1/T) or Antoine-based linearizations; the Cox chart’s linearity advantage for hydrocarbons lies in log P versus T.

Final Answer:
temperature vs. log(vapor pressure)