Typical phenomena of salt deposition on distillation column trays causing tower blockage during normal production (practical version)

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During the normal operation of a vacuum distillation tower, salt deposition, scaling, and blockages on the tray surfaces are common equipment-related problems. These issues are mainly caused by residual salts in crude oil, ammonium salts formed by the injection of ammonia, alkali scales resulting from the injection of alkalis, as well as the accumulation of sediment and carbonized substances, which block the openings in the trays and the downcomers. As a result, gas-liquid flow throughout the tower is impaired, and the separation efficiency drops significantly. Based on actual on-site operations, the typical phenomena are as follows: 1. The pressure drop inside the tower increases abnormally, with obvious signs of localized pressure buildup. After salt accumulation on the tower trays, the effective open area decreases, resulting in increased resistance to gas flow, and thus a significant rise in the pressure drop throughout the tower or in certain areas. By comparing the pressure difference between the bottom and top of the tower as well as the pressure changes in various sections, it is possible to roughly determine the location of the salt deposition blockage; the more severe the blockage, the higher the pressure drop. 2. Temperature regulation is sluggish and subject to large fluctuations, making it extremely difficult to maintain stability. When gas-liquid mass and heat transfer are severely disrupted, the temperature at the top of the tower as well as the distillation temperatures at various side streams experience irregular or periodic fluctuations. The most typical manifestation is that adjusting the tower top temperature using the reflux flow results in slow response and reduced sensitivity; whether the reflux flow is increased or decreased, the temperature does not change much, and the situation even becomes more chaotic as adjustments are made. At the same time, the pressure at the top of the tower changes frequently, fluctuating between high and low levels, resulting in a severely uneven distribution of the gas-phase load. 3. The distillation efficiency deteriorates significantly, and product quality declines. As the efficiency of the tray columns decreases, the separation of various components becomes poor, which is directly reflected in the product: – The boiling ranges of the oils from different side streams overlap severely ； Side-line products tend to become heavier ； In severe cases, black oil will appear in the side lines, the color will deteriorate, and the parameters will fail to meet the standards. 4. The flow of gas and liquid is hindered; in severe cases, signs of tower flooding and liquid overflow occur. If the downcomer is blocked by salt deposits, the downward flow of the liquid phase is impaired, which can lead to local flooding of the tower and liquid overflow. This is accompanied by abnormal noises inside the tower and sudden changes in load distribution, further exacerbating large fluctuations in pressure and temperature. In short: increased pressure drop, difficulty in controlling temperature, greater fluctuations, worse separation, heavier products, and even the appearance of black oil – these are all typical signs of salt deposition on the tower plates, which leads to blockages in the tower.