The distillation tower won’t start
Thread Content
A distillation tower cannot be operated after a single maintenance session. Under normal circumstances； Top pressure: 400 KPA; a slight flooding of the tower is not a problem. Depending on the quality, a small amount may be returned or extracted. The normal pressure difference should be below 50 KPA. Steam flow rate: 65%–75%. In case of abnormalities ； The top pressure is between 200 and 300 KPA. Foaming occurs at a steam flow rate of 35%, and as a result the liquid level drops to 0; the pressure difference gradually increases. Treatment is required ； The steam level drops to 30%. The liquid level gradually rises; over time it will drop again. Continue to reduce the steam supply to 28%, maintaining the liquid level at 70%–90% as the normal range. The pressure difference is around 50 KPA, while the pressure at the top of the tower is around 300 KPA. Analyze the data ； A small amount of non-condensable gas N₂, along with heavier components; 93% is the product. Please help resolve the problems with this distillation tower ； 1. Why does flooding occur at half the steam opening? What is the reason? 2. Why is there a large pressure difference between the bottom and top of the tower when it is flooded? It’s nearly 100 KPA. 3. After steam is introduced, the pressure at the top of the tower never reaches the expected value of 400 KPA; in reality, it’s only 300 KPA
You have ignored a very important parameter: temperature!
1) Flooding occurs at a very low steam flow rate, indicating that the maximum gas-liquid load that the tower can handle has decreased. Given that it has just been overhauled, it is unclear whether the internal components of the tower have been restored, such as the distributor in the packed tower, the levelness of the tray in the plate tower, and the downcomers. Also, I don’t know if there have been any changes in the feed rate or steam pressure. 2) When flooding occurs, the liquid holdup in the tower increases, which in turn raises the resistance to the upward movement of gas; as a result, the pressure difference increases, and this is also an indication of flooding ； 3) The low estimation of the top pressure is related to the control of the top pressure; if the evaporation rate is low while the cooling amount remains the same, the top pressure will definitely be low.
1. The tower bottom vessel of the poster might be relatively small, resulting in a too short residence time for the liquid. Plus, the temperature rises too quickly, and it rises under low pressure conditions. This causes the liquid at the bottom of the tower to vaporize too quickly. So when the pressure is below 400 KPa, there is no liquid in the tower bottom. The poster might as well increase the temperature while removing non-condensable gases under high pressure. 2. Due to severe flooding of the tower by the operator, the liquid level in the tower bottom exceeded that indicated by the pressure gauge; as a result, the pressure difference was excessive, caused by the pressure at the tower bottom plus the hydrostatic pressure of the liquid above the gauge (ρgh) ； 3. The fact that the pressure at the top of the tower never reaches 400 KPa indicates that the amount of steam supplied is too low. Increasing the steam flow or reducing the cooling load at the tower top can raise the pressure at the tower top.
According to the poster, it seems that no flow meter is installed for the steam; relying solely on the scale may not provide an accurate indication of the actual flow rate. Please check whether the control valve is functioning properly.
1. Conduct a process inspection to ensure that the internal components of the tower are assembled correctly after maintenance. 2. First, start the vacuum system and then gradually increase the temperature; the boiling point of the material varies at different pressures, and starting the vacuum after heating can cause bumping. 3. Tower flooding occurs due to an excessive amount of vapor and liquid.
1. Please have the instrumentation team check whether there are any issues with the pressure gauges and steam control valves. 2. The process and equipment teams should verify whether there are any problems with the installation after the tower has been repaired. 3. Analyze the composition of the feed, as well as that of the liquid at the bottom of the tower; make adjustments based on any abnormalities detected. 4. Operate slowly, pay attention to the exhaust gas at the top of the tower, and check whether the reflux at the top and the circulation at the bottom are functioning properly. 5. Generally, there is a relationship between temperature and pressure; if the pressure is abnormal, the temperature will also be abnormal. With unchanged feed conditions, check for changes in temperature. Since there is no flow meter for steam, is it regulated using condensate, or by the liquid level or the temperature of a sensitive plate? It should be operated manually during startup
It seems that all three points mentioned by the original poster indicate the problem of flooding.
Abnormalities after maintenance: It appears that the tower’s processing capacity has significantly decreased (the reboiler is set at a low level, and the tower pressure is low). There seems to be an issue with the installation of the tower internals; it is recommended to consult the person who carried out the installation.
It is obvious that the reflux pipe at the top of the tower is blocked and not functioning properly; the plant has been shut down to repair the reflux pipe
Based on the poster’s data, I judge that the problem with your tower is not a blockage in the downcomer inside the tower. The reason is that the flow area of the downcomer accounts for a large proportion of the tray area, so blockage of the downcomer is generally not considered. I analyze that the main reason might lie in operational issues. The feed composition of your tower has exceeded the process requirements for that tower, namely the non-condensables and heavy components listed in your analysis data. These two substances affect the two ends of your tower operation respectively. First, N2 occupies the top of the tower, reducing the efficiency of the condenser and causing the tower pressure to get out of control. So what happens is that when a slight amount of steam is introduced, the tower experiences flooding; this is actually due to the components having too low a density and the empty-tower velocity being too high. In other words, it’s all about attacking the tower. A typical example of this is that the Recombined components is also carried to the top of the tower. Of course, by reducing the steam and addressing the root cause, you will quickly break this situation and return to normal gas and liquid flow rates. This is the answer to your first question. There is a measuring instrument between the last tray from the top of the tower to the bottom, namely the tower pressure difference. In the case of flooding, the liquid level at the bottom rises and submerges the lower interface of this measuring instrument, resulting in a large pressure difference between the bottom and the top of the tower. Or it will also flood this interface when flooding occurs, for the same reason. After steam is introduced, the pressure at the top of the tower never reaches the expected value of 400 KPA; in fact, it’s only 300 KPA. That’s because you’ve set your steam valve to a low opening degree. You can increase the pressure in the tower to a very high level (just kidding). A distillation tower is quite picky; its feed materials cannot be added casually. The presence of substances that should not be there, such as non-condensable gases, or the entry of heavier components, can upset the tower’s operation. Therefore, strict control over the feed materials supplied to the distillation tower is a crucial condition for ensuring its stable operation. When non-condensable gas enters the system, the only way is to remove it.