Thoughts on boiling range

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Range — an oil product specification commonly seen in petroleum refining. For pure compounds, there is a boiling point at a certain external pressure; for example, the boiling point of water at 1 atmosphere is 100°C. Oil is a complex mixture of hydrocarbons, and compounds with different molecular structures exhibit a continuous range of boiling points from low to high, known as the boiling range. The temperature range obtained by distilling the oil under specified conditions constitutes the boiling range of that particular oil. During distillation range determination, the temperature recorded at the moment the first drop of condensate falls from the end of the condenser is called the initial boiling point. As distillation proceeds, the temperatures at which the condenser collects 10%, 50%, and 90% by volume of the distillate are key parameters for the oil product and need to be recorded. Continuing with the distillation, the temperature observed at the moment when the last drop of liquid at the lowest point of the distillation flask vaporizes is called the dry point. Although there is no liquid left in the bottle at this point, you will find that the temperature of the thermometer inserted at the top of the distillation flask continues to rise, before eventually starting to drop; the highest temperature reached before this drop corresponds to the final boiling point. Due to the volatility of light oils, the amount of oil collected in the end is generally less than that of the sample, which we refer to as loss. When distilling heavy oil, residues remain in the distillation flask, and decomposition occurs; that temperature point is known as the decomposition point. Thus, after the boiling range test is completed, we generally record the initial boiling point, the distillation temperatures at 10%, 50%, and 90%, the final boiling point (or dry point), the residue amount, and the loss amount. Well, that’s all for the introduction to the boiling range. So, what does the temperature at the top of the distillation flask when the first drop of condensate appears, that is, the initial boiling point, actually represent? The liquid in the distillation flask vaporizes more rapidly as it is heated; as the oil heats up and starts to vaporize, the oil vapor passes through the neck of the flask at the top, where a thermometer is placed, and enters the condenser from the side. There, water mist is formed on the walls of the condenser; as this mist cools and grows larger, it drops down along the condenser, and it is at this moment that we read the temperature indicated by the thermometer. In fact, even before this point, the liquid in the distillation flask has already begun to vaporize partially; the vaporized light components must reach a certain concentration in the gas phase before they can condense back into a liquid. Due to this lag, the initial boiling point indicated by the thermometer is actually slightly higher than the boiling point of the lightest component in the liquid. So, when two different oils are mixed together, how does their boiling range change? The answer is that the initial boiling point of oils with a lower density is higher, while the final boiling point of oils with a higher density is lower. Although it is obvious, the reasoning is not simple. Henry enters the scene. In a distillation system where the pressure is at atmospheric level, the relationship between the equilibrium concentration of the gas phase above the liquid surface in the system and the molar fraction of that gas phase in the liquid phase follows P=Ex, where E is the Henry’s constant. After the two oils are mixed, the concentration of the light components in the blended oil is necessarily lower than that in the oil with a higher content of light components before mixing; that is, x becomes smaller and P also becomes smaller. As a result, the time required for vapor condensation at the initial boiling point is prolonged, which manifests macroscopically as an increase in the initial boiling point. The mixed oil is lighter than the heavy oil before mixing; during distillation, the vapor pressure of the heavier components decreases, making it easier for them to evaporate, which is manifested macroscopically as a lower final boiling point.