Why is the final boiling point of the lowest side line higher than the feed temperature?

The final boiling point of the lowest side stream in a vacuum distillation unit is higher than the feed temperature for the following reasons: 1. During the distillation process, the feed passes through the tower under gradually decreasing pressure and heating, which allows the components to be separated based on their boiling points. Since the pressure in each fractionation stage is different, the boiling points of each stage also differ. Therefore, the final boiling point of the lowest side line is higher than the feed temperature. 2. In a atmospheric and vacuum distillation unit, the lowest side stream is usually caused by the reflux of low-boiling components. During the distillation process, low-boiling-point components are usually separated at higher levels and return to the bottom of the tower in liquid form. This portion of the returning liquid will mix into the lowest side stream, resulting in the final boiling point of that lowest side stream being higher than the feed temperature. 3. The feed temperature is only the temperature of a certain part of the entire system, while the lowest stream corresponds to the last part of the entire system. Throughout the distillation process, the volatility of the components increases as the temperature rises. Therefore, in the final part of the unit, the increase in volatility causes the final boiling point of the lowest side stream to be higher than the feed temperature. Generally speaking, the final boiling point of the lowest side stream in a atmospheric and vacuum distillation unit is higher than the feed temperature due to a combination of factors such as progressive pressure reduction and heating, the reflux of low-boiling-point components, and increased volatility. .

Could you be more detailed and specific? It seems that everything mentioned is true and factual, even to the point of being as straightforward as what’s in textbooks; however, it lacks specificity and doesn’t delve into an analysis of the issues. For example, in the atmospheric pressure tower, separation occurs gradually based on boiling points; yet this does not mean that the final boiling point of the third fraction is higher than the feed temperature. The third fraction is located 5 trays above the feed – what could cause the substances in this fraction to have a higher boiling point and thus vaporize at a temperature higher than that of the feed? The second low-boiling component flows downward layer by layer via reflux; even after reaching the outlet of the normal paraffins fraction, this can only indicate the initial boiling point of this fraction, not its final boiling point. Thirdly, the atmospheric tower in a vacuum distillation unit is not a complete distillation column; it has only a distillation section and no stripping section. The feed to the atmospheric tower enters at the bottom, and the third side stream is distilled off on the 5th tray above the feed section. The lowest side stream is not the final product of the entire unit – the final product is the residue at the bottom of the atmospheric tower. Volatility generally refers to pure substances and is not very applicable to mixtures. At which point is the final boiling point higher than the feed temperature, given that volatility increases as temperature rises? Could you give me more specific guidance?

Thanks for sharing:victory: