Distillation column operation

If the pressure suddenly increases, leakage occurs; if it suddenly decreases, flooding takes place.

There is an excess of rising gas in the distillation column, which causes flooding; it could also be caused by the crude argon column – check whether the data from that column are normal

Is there a problem with a certain tray in the tower? Is it blocked?

It must be an operational issue that caused the flooding

It must be caused by the operation, which led to flooding.

It is approaching the point of flooding but has not yet reached it; the liquid load is high. Increase the amount taken from the top of the tower and reduce the reflux. During the adjustment process, lower the temperature first, and once things return to normal, adjust it back to its normal level. If it is ineffective, consider blockage in the gas rise channel.

This post was last edited by goldliyang on 2017-10-9 08:58. The pressure drop in a distillation column: First, it is necessary to understand how the pressure drop (i.e., the pressure difference) in a distillation column is generated The pressure drop in a distillation column consists of three aspects: the dry tray pressure drop, which is the pressure drop on the tray when there is no liquid layer present and only rising vapor is present; this value is determined by the inherent properties of the tray and cannot be changed during operation ； The pressure drop across the liquid layer, that is, the pressure drop generated as rising gas passes through the liquid layer on the plate; the liquid phase load in the tower has an impact on this, and feeding and reflux are factors that influence it during operation ； Overcoming the pressure drop caused by liquid surface tension is mainly related to the composition of the material on the plate. As long as, in daily operations, the causes of pressure drop in these three areas are identified, it is possible to control the pressure difference in the distillation tower. II. Impact of steam pressure fluctuations on distillation operations: When the distillation column is operating at low load or with minimal external influences, adjusting the temperature of the sensitive plate using the reflux ratio is generally sufficient to maintain good product quality. However, under high-load operating conditions, the steam pressure in the utility system often fluctuates significantly, which leads to uneven heat transfer in the reboiler at the bottom of the tower. This results in an imbalance between the gas and liquid phases in the distillation tower, causing larger temperature variations in the sensitive plates and affecting the quality of the product. When the external steam pressure suddenly increases, the amount of volatile components evaporating at the bottom of the tower rises, as does the temperature of the sensitive plates. It is necessary to increase the reflux rate in order to control the temperature of these plates and ensure the quality of the product at the top of the tower ； However, in practical operation, if the evaporation rate is too high, it will lead to foam entrainment, which in turn reduces the mass transfer between the gas and liquid phases, severely affecting the quality of the product ； In severe cases, flooding can also occur ； Furthermore, the variation in steam pressure is irregular, and there is a delay in controlling the temperature of the sensitive plate through the backflow rate; this leads to an increase in the content of poorly volatile substances in the product, or to the entry of readily volatile substances into the system, thereby affecting the quality of subsequent products. 2. When the external steam pressure drops suddenly, the amount of evaporation of the components that are difficult to vaporize in the tower bottom decreases, and the temperature of the sensitive plate falls. If the reflux rate is not reduced promptly, the temperature of the sensitive plate will drop significantly, allowing easily volatile components to enter the tower bottom and leading to a decline in the quality of the products produced later on ； On the other hand, in order to maintain the temperature of the sensitive plate, the reflux flow rate drops too rapidly, resulting in a lower reflux ratio and affecting the separation efficiency ； Similarly, if the evaporation rate is too low, the speed of the rising vapor decreases, resulting in liquid leakage inside the tower, which severely affects the separation efficiency. III. Adjusting steam flow to control the temperature of the sensitive plate. In view of the operational problems caused by fluctuations in steam pressure, we decided to change the fixed value of steam flow to a variable one; by manually adjusting the amount of steam supplied, we can maintain a relatively stable rate of evaporation in the bottom of the tower, thereby ensuring the appropriate temperature of the sensitive plate. Through careful operation and timely adjustments to the steam supply and return flow rates, we can maintain balance between the inflow and outflow of material in the distillation tower, control the temperature of the sensitive plate, and thus ensure product quality while preventing flooding phenomena ； Through adjustment and control, although there is some effect, many problems still remain. 1. It shows a relatively passive behavior during operation, with frequent actions that increase the labor intensity. 2. High special operational skills are required, but since the proficiency of each operator varies, the degree of adjustment differs as well; this makes it difficult to avoid sudden and significant fluctuations in the temperature of the sensitive plate. As a result, the temperature of the entire tower, as well as the frequency and magnitude of changes in the distribution curves of the material flow in and out, increase, which can easily lead to issues with product purity. 3. Since both the steam flow rate and the reflux flow rate are used to adjust the temperature of the sensitive plate, it is difficult to determine which factor is more important; improper operation can easily disrupt the normal progress of distillation and the vapor-liquid equilibrium, leading to a deterioration in the entire process. 4. It poses significant difficulties in the establishment and implementation of process parameters such as steam feed rate and reflux rate. IV. Automatic control of steam flow to ensure vapor-liquid equilibrium. During the continuous operation of a distillation tower, it is necessary to maintain material balance, vapor-liquid equilibrium, and heat balance; these three types of equilibrium influence one another and constrain each other. When the steam pressure changes suddenly, it directly affects the evaporation rate of the poorly volatile components in the bottom of the tower, causing an imbalance in heat within the tower and leading to a gas-liquid imbalance. Therefore, the key issue is how to automatically adjust the heat at the bottom of the tower according to the amount of steam fed in, so as to achieve relative stability and ensure heat balance within the tower. During the production process, the distillation column equipment has been determined; the evaporation rate at the bottom of the column is proportional to the gas flow rate, and the flow rate in turn is proportional to the pressure difference across the column. Therefore, by controlling the pressures at the top and bottom of the column, a certain level of evaporation can be ensured. In operation, the pressure at the top of the column can be adjusted stably using a pressure control system, or in many cases the column operates at atmospheric pressure. Hence, it is particularly important to maintain a stable pressure at the bottom of the column. Therefore, with the steam feed rate remaining constant, we compared the changes in steam pressure with those in the pressure at the bottom of the tower, and found that there is a direct proportional relationship between them, with an extremely small lag time (see Figure 2). Therefore, a cascade control system is used for the steam feed rate and the pressure at the bottom of the tower; the pressure signal from the bottom of the tower is sent to the steam flow control valve, which adjusts itself based on this pressure. By increasing or decreasing the steam feed rate accordingly, the pressure at the bottom of the tower is kept stable, thereby ensuring that the distillation process is not affected by fluctuations in external steam supply.