【Q&A Question 077】2016.07.06

There should be sufficient space for the gas and liquid phases, but it must not be too large, as that would result in a tall tower and increased equipment costs. The general spacing ensures that people can pass through during maintenance. I’ve seen ones that are 400mm or 600mm

There is a certain distance between the tower plates, which allows the large liquid droplets carried by the airflow to have sufficient time to settle. This helps reduce the amount of mist carried along, ensuring effective separation at the tower plates

The size of the tray spacing directly affects the number of liquid droplets entrained in the upward airflow; if the tray spacing is large, the droplets have more time to settle within the upward airflow, and as a result, the amount of mist entrained decreases. If the plate spacing is too small, it becomes difficult for the liquid on the upper tray to flow smoothly through the downcomer to the lower tray; in severe cases, this can even lead to a tower flooding accident.

It provides gas exchange at the interface between the two phases, enabling heat and mass transfer between them

There is a certain distance between the tower plates, which provides enough time for the large liquid droplets carried by the air flow to settle. This helps reduce the amount of mist carried along, ensuring effective separation at the tower plates.

The spacing between the trays determines the height of the entire distillation column; therefore, it is desirable for this spacing to be as small as possible. The minimum plate spacing is limited by two conditions: 1) to avoid flooding. That is, the plate spacing must be greater than the minimum spacing required to ensure that the liquid flows smoothly from one tray to the next. It is calculated based on the flow resistance of the return liquid through the overflow hopper. Obviously, it is closely related to the structural form of the overflow hopper, as well as to the height of the baffle at the inlet of the tray ； 2) It is necessary to ensure no mist entrainment. On the tray, vapor passes through the sieve holes and rises as it bubbles through the liquid layer. The mass transfer area on the tray consists essentially of the following components: a thin static liquid layer that lies right against the tray surface ； And then there is the bubble layer ； Above it is a foam layer with a honeycomb structure. Due to the bursting of the foam and the action of steam jets, along with splashing liquid droplets, a mist layer is formed. If vapor carries liquid droplets upward to the upper tray, mist entrainment is formed. To ensure the proper operation of the distillation process and to maintain a minimum tray spacing that prevents foam entrainment, it should be equal to the height of the foam layer on the tray plus the gas-liquid separation space. Under normal conditions, when the empty tower velocity in the lower tower is Wn≤0.1 m/s and that in the upper tower is Wn≤0.3 m/s, the separation distance is 15–20 mm. The actual plate spacing should be greater than the spacing at which flooding does not occur, and also greater than the spacing at which no foam entrainment takes place. Operational flexibility should also be considered during design, typically verified using a design load of ±20%. For the convenience of manufacturing. The plate spacing in a distillation column should be uniform and standardized. The plate spacing of domestically produced medium and large-scale air separation towers is generally 90 mm, 110 mm, 130 mm, and 150 mm, with a difference of 20 mm between each level.

The spacing between the trays determines the height of the entire distillation column; therefore, it is desirable for this spacing to be as small as possible. The minimum plate spacing is limited by two conditions: 1) to avoid flooding. That is, the plate spacing must be greater than the minimum spacing required to ensure that the liquid flows smoothly from one tray to the next. It is calculated based on the flow resistance of the return liquid through the overflow hopper. Obviously, it is closely related to the structural form of the overflow hopper, as well as to the height of the baffle at the inlet of the tray ； 2) It is necessary to ensure no mist entrainment. On the tray, vapor passes through the sieve holes and rises as it bubbles through the liquid layer. The mass transfer area on the tray consists essentially of the following components: a thin static liquid layer that lies right against the tray surface ； And then there is the bubble layer ； Above it is a foam layer with a honeycomb structure. Due to the bursting of the foam and the action of steam jets, along with splashing liquid droplets, a mist layer is formed. If vapor carries liquid droplets upward to the upper tray, mist entrainment is formed. To ensure the proper operation of the distillation process and to maintain a minimum tray spacing that prevents foam entrainment, it should be equal to the height of the foam layer on the tray plus the gas-liquid separation space. Under normal conditions, when the empty tower velocity in the lower tower is Wn≤0.1 m/s and that in the upper tower is Wn≤0.3 m/s, the separation distance is 15–20 mm. The actual plate spacing should be greater than the spacing at which flooding does not occur, and also greater than the spacing at which no foam entrainment takes place. Operational flexibility should also be considered during design, typically verified using a design load of ±20%. For the convenience of manufacturing. The plate spacing in a distillation column should be uniform and standardized. The plate spacing of domestically produced medium and large-scale air separation towers is generally 90 mm, 110 mm, 130 mm, and 150 mm, with a difference of 20 mm between each level.