【2026 Distillation Technology】Specific phenomena of flooding in distillation columns and their causes
Thread Content
During the operation of a distillation tower, flooding is a serious abnormal operating condition in which, for various reasons, the liquid phase within the tower accumulates beyond its available space, disrupting the normal gas-liquid mass and heat transfer processes.
I. Liquid flooding phenomenon: When liquid flooding occurs, the hydrodynamic conditions and process parameters within the tower experience significant abnormalities, which are mainly manifested as follows: 1. A sharp increase in pressure difference – The pressure difference between the bottom and top of the tower increases suddenly (possibly by more than 30% or even several times the normal value), accompanied by severe fluctuations; this is the most direct indication.
2. Imbalanced temperature distribution: The temperature at the top of the tower usually decreases, while the temperature at the bottom of the tower continues to rise, resulting in a reduced temperature difference between the bottom and the top of the tower.
3. Abnormal liquid level: The liquid level in the reflux tank at the top of the tower decreases, while the liquid level and pressure at the bottom of the tower increase significantly.
4. Deterioration in product quality: The output of products at the top and bottom of the tower has decreased, and their quality is substandard; the separation efficiency has noticeably declined.
5. Severe liquid accumulation: In extreme cases, the foam layer inside the tower rises, connecting the upper and lower tray levels, and the liquid may even overflow from the top of the tower along with the gas.
II. Causes of flooding Flooding is primarily caused by three factors: operating load, equipment design, and the properties of the material: 1. Excessive gas-phase load An excessive amount of steam or high gas velocity leads to an increase in the pressure drop across the tray, thereby hindering the normal flow of liquid down through the downcomer； At the same time, excessive foam is entrained to the upper tray levels, causing the liquid layer on those trays to thicken, thus creating a vicious cycle.
2. Excessive liquid load: An excessive amount of feed or return flow causes the liquid flow velocity in the downcomer to exceed its capacity, resulting in the accumulation of liquid in the downcomer and its overflow onto the upper trays (downcomer flooding).
3. Defects in the equipment structure or blockages: An excessively small cross-sectional area of the downcomer, insufficient bottom clearance, or blockages in the tray/filler due to iron filings, slag, coking, polymers, etc., result in increased resistance in the gas-liquid phase flow channels.
4. Changes in material properties: Abnormal viscosity of the material or severe foaming can occur, making it difficult for the liquid to flow smoothly within the tower.
III. Handling Methods Once flooding occurs, the core principle is to rapidly reduce the gas-liquid load in the tower in order to restore equilibrium in the two-phase flow: 1. Emergency load reduction – Immediately decrease the feed rate (to 50%~70% of the design value), reduce the heating steam supply to the reboiler, and appropriately lower the reflux rate.