Boiling range mixtures

Equations 12.62 and 12.63 are for boiling single component fluids for mixtures the coefficient will generally be lower than that predicted by these equations. The equations can be used for close boiling range mixtures, say less than 5°C and for wider boiling ranges with a suitable factor of safety (see Section 12.11.6). 

JFor boiling of mixtures, the saturation temperature (bubble point) of the final liquid phase (after the desired vaporization has taken place) is to be used to calculate the mean temperature difference. A narrow-boiling-range mixture is defined as one for which the difference between the bubble point of the incoming liquid and the bubble point of the exit liquid is less than the temperature difference between the exit hot stream and the bubble point of the exit boiling liquid. Wide-boiling-range mixtures require a case-by-case analysis and cannot be reliably estimated by these simple procedures. 

For wide boiling range mixtures, such as naphtha and kerosene, the temperature difference across portions of a distillation tower approximately corresponds to the degree of fractionation achieved. The ATs indicate that the amount of fractionation work done across the top five trays is about the same amount of fractionation work done in the top theoretical separation stage, which is represented by the partial condenser. The partial condenser consists of the air cooler and the hot drum, shown in Fig. 49.4. 

When applied to wide-boiling-range mixtures such as the customary reflnery products (and natural gasoline) the following general facts become apparent. 

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