The three equilibria in distillation column operation
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In the operation of a distillation tower, three types of equilibrium need to be maintained: material balance, gas-liquid phase equilibrium, and heat balance； Material balance means that the amount of material entering the tower per unit time is equal to the sum of all the materials leaving the tower ； Gas-liquid phase equilibrium is primarily related to the quality of the product and any losses that occur ； Heat balance refers to the equilibrium between the heat entering the tower and the heat leaving it.
There should also be some pressure balance; pressure is very important too!
What is the relationship among the three balances~
How is pressure balance reflected? Is the pressure stable?
There is a relationship of mutual restraint and mutual promotion. For example, in the case of atmospheric and vacuum distillation: 1) if the temperature at the heat exchange terminal is higher than the designed value, less heat is lost at lower temperatures; this could be due to the properties of the crude oil or to a low yield of light oil. 2) if the difference between the feed at the bottom of the tower and the temperature there is small, it is either because of the properties of the crude oil or because the vapor-liquid equilibrium within the tower is not proper. 3) if there is an overlap in the masses of the material at the top of the atmospheric column and that of the first stage of the vacuum column, then the vapor-liquid equilibrium is not adequate. Ultimately, all issues related to tower operation can be resolved by examining these three types of equilibrium – if the amount of material taken from a certain stream is insufficient or excessive. In short, any problem with tower operation can be addressed by looking at these three equilibria