Number of distillation tray stages
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Hello everyone, are the theoretical plate number of a distillation column, the required theoretical plate number, and the theoretical plate number in terms of overall plate efficiency the same? The information provided, such as principles of chemical engineering, is confusing: in the diagrams and step-by-step calculations for tray numbers, the number of trays includes the reboiler, whereas the theoretical number of trays required in the examples does not include the reboiler. Does the formula for calculating the overall tray efficiency include the reboiler in the count of theoretical trays? That is, we need everyone’s help to clarify the following questions: In the calculation of distillation columns, does the number of theoretical plates required include the reboiler? Is the reboiler included in the overall plate efficiency formula? Does the theoretical number of plates in a distillation column include the reboiler?
Reply to 1# lrn20061221: I think the concepts of the theoretical number of plates in a distillation column, the required theoretical number of plates, and the theoretical number of plates based on overall plate efficiency are all the same. As for whether a reboiler (top condenser) is included or not, the key lies in the presence of these two devices. If it exists, it is included; if not, it is not included.
The theoretical number of plates is a concept that corresponds to the actual number of plates required; in packed columns, it is described in terms of equal plate heights and the number of mass transfer units, with the height of one mass transfer unit being equivalent to that of one theoretical plate. A typical distillation unit has a reboiler at the bottom of the tower and a condenser at the top. The gas-liquid equilibrium at the reboiler level is equivalent to that of a single tray, while the composition in the top condenser is the same as that of the gas phase in the first tray; therefore, there is no gas-liquid equilibrium relationship, and it cannot be considered as a single tray. This is how I understand it; I’m not sure if it’s correct ? ?
Reply to 2# liqc840426: Logically, it should be the same, but based on the examples and the solutions to various *type problems, it seems different, which leads to confusion.
Reply to 1# lrn20061221: Which senior member can provide a better explanation and introduction? ? ?
In a distillation unit, there is generally a reboiler at the bottom of the tower, which counts as one theoretical plate. If the top of the tower is equipped with a total condenser, there is no gas-liquid equilibrium, and it does not count as one theoretical plate; whereas if it is a fractional condenser, with gas-liquid equilibrium present, it counts as one theoretical plate. This is how I understand it; I’m not sure if it’s correct
Reply to 6# yyp33871: I understand it the same way, but in the examples related to chemical engineering principles, the theoretical plates sometimes include the reboiler and sometimes do not, which leaves me confused. Also, when calculating based on the overall plate efficiency, is the reboiler included in the theoretical plates? This result affects the actual number of plates required. Everything is different everywhere.
Reboilers and condensers should not be included when calculating efficiency, as the purpose of calculating tray efficiency is to determine the height of the tower
When calculating the theoretical number of plates, both the reboiler and the separator are considered as part of the theoretical calculation, and their values should be deducted when determining the tower height. The total condenser is not considered a theoretical tray