Abnormal operation of the hydrocracking fractionation tower
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Since my unit was restarted after an emergency shutdown, the distillation process has not been operating properly. I urgently request advice from experts – it’s very urgent! ! ! ! ! ! Phenomena: The temperature at the bottom of Tower 1 is about 70 degrees lower than normal, and the extraction temperature of heavy diesel is also about 40–50 degrees lower than normal. The extraction valve is fully open, yet the temperature does not rise; the amount of heavy diesel extracted is significantly low, giving the impression that it cannot be extracted properly. There is a mixing of light and heavy diesel. The bottom of Tower 2 remains at full liquid level, with the liquid level continuing to rise. Some of the tail oil is discharged externally, but the liquid level still does not drop. Analysis of samples taken from the tail oil shows a large amount of heavy diesel present in it. (The level was raised significantly earlier, resulting in a sharp increase in the amount of naphtha.) There are unusual noises coming from the 3rd heavy diesel side column; intermittent flowing sounds can be heard. Additionally, my plant uses a distillation tower as the main tower, with two side columns for heavy diesel and light diesel; reflux is provided at the top of the tower, and all tail oil is recycled. Recently, the feedstock has been relatively light in composition, and the gas consumption has increased significantly compared to normal levels
It’s the first time I’ve heard of such an operational phenomenon； During the emergency shutdown of the fractionator, there were no serious issues such as tower flooding or water entrainment in the vapor stream at the bottom of the tower, right? From a practical perspective, if the feed temperature to the distillation column remains unchanged, it seems that no mass or heat transfer occurs once the heavy diesel fraction enters the column, with much of it ending up at the bottom of the column. Could there be an issue with the trays in the feed section up to the section where the heavy diesel is extracted? ； When the liquid level at the bottom of the tower is full and the lifting depth is increased, there is a high proportion of heavy naphtha components, indicating that after the back circulation, a large amount of heavy naphtha was produced as a result of the presence of large amounts of diesel components in the reaction mixture. It is advisable to discard more material, reduce the amount of material recycled back into the reaction, process more fresh raw materials, and maintain an appropriate conversion rate. The sound coming from the side tower of the heavy fuel oil unit is confusing; I’m not sure if steam stripping is being used. It’s likely that a large amount of light components vaporize when they come into contact with steam, so the presence of water isn’t possible. Since the reason cannot be determined, it’s better to cut off the feed again, let the distillation system cycle, and then restart it.
That’s right; currently, a large amount of heavy diesel components at the bottom of the tower fail to rise to the upper levels. We have reduced the amount of oil that is recycled back to the reaction zone, and we have also lowered the operating depth accordingly (the oil used for sampling analysis is very light; earlier it was suspected that the low density of this oil was responsible for the lack of heat absorption). The stripping steam in the stripping tower, the fractionation tower, and the heavy diesel side stream tower has all been removed (it was previously thought that water present in the fractionation feed was taking up a lot of heat). At present, the amount of naphtha has decreased while the amount of heavy diesel has increased. The temperature at the bottom of the fractionation tower has risen slightly, but not by much. The temperature at which the heavy diesel is extracted also falls short of the desired level by several degrees. There should be no problem with the heavy diesel extraction outlet, as we tried today to raise the liquid level in the distillation tower above the level of the heavy diesel extraction outlet, and as a result the amount of heavy diesel was very large. The problem now is that the heat at the bottom of the tower isn’t sufficient, which prevents heavy fuel from being vaporized properly. Shutting down and restarting is our last resort; we still hope to resolve this issue by adjusting the process conditions. We’re really anxious right now, as we’re concerned that it might affect the blending of the finished oil later on. I urgently need everyone’s advice; let’s give it a try! Thank you!
The phenomenon you're referring to is likely an insufficient vaporization rate. The feed temperature can be appropriately increased to reduce the reflux volume of slurry, lower the top pressure of the tower, and increase the steam supply at the bottom of the tower. Also, is it possible to measure the total pressure drop of the tower? If the pressure drop changes significantly, there should be a problem. In my opinion, if a tower has problems and doesn’t meet the standards no matter how it’s adjusted, it’s definitely an issue with the internal components of the tower; the tower should be shut down for inspection.
The operating parameters of the distillation tower are not provided in a comprehensive manner, making it difficult to make accurate judgments. However, one thing can be determined: your diesel components (light diesel and heavy diesel) are relatively light, while the tail oil contains a large amount of diesel components. As a result, the load within the tower decreases overall. In such a situation, it is necessary to reduce the internal reflux rate in the tower (mid-stage reflux) in order to increase the load within the tower and extract as much of the diesel components as possible. When making adjustments, it is important to focus on key parameters and make changes gradually; avoiding simultaneous adjustments to multiple parameters. This is because after adjusting one parameter, it takes some time for the various balances within the tower to be re-established. If adjustments are made too frequently over a short period, not only will the distillation tower fail to return to normal operation, but it will also be difficult to determine the true cause of any issues that arise.
I’ve only heard of this situation before; is superheated steam being used as the stripping steam? Ordinary steam tends to contain water. Also, it’s necessary to minimize the circulation of tail oil and allow it to be discharged externally. The liquid level at the bottom of the tower shouldn’t be too high, and it’s important to check whether there are any faults with the extraction valve or if it’s stuck
Agree with what was said above: increase the feed temperature appropriately, reduce the reflux volume of slurry, lower the pressure at the top of the tower, increase the steam supply at the bottom of the tower, and check the pressure drop. Also, check the amount of water condensation at the top of the tower to see if it has increased It has been confirmed whether the water content in the feed has increased.
It’s unlikely that there is water in the feed stream, as we are using superheated steam; moreover, all the steam was already removed earlier, but that didn’t have any effect. We also increased the feed temperature, but it didn’t have much effect; there was simply no heat at the bottom of the tower. The reflux flow rate is also being kept under control, but it cannot be reduced too much, as there is a fear that excessively high temperatures and too much naphtha, resulting in increased weight, could affect the reforming process. The problem now is that the depth is sufficient, and all the heat is carried to the top of the tower; due to the large amount of naphtha, there is no heat at the bottom of the tower, and we don’t have mid-stage reflux. We also carried out external venting of the residual oil earlier; it lasted for nearly four days, yet the liquid level didn’t drop. The process was quite strange – given the depth at that time and the amount of fluid vented, the liquid level should have dropped long ago, but it didn’t. Currently, there is still a large amount of heavy diesel in the bottom of the tower that hasn’t been vaporized. Actually, I feel that as long as the liquid level drops, the temperature at the bottom of the tower goes up as well. Now we’re going to reduce the depth to see if it makes a difference. What do you all think?
Is there a heating device at the bottom of the tower, and is it working properly? Common faults usually occur in the trays or the reboiler.
There is 380-degree superheated steam at the bottom of the tower; it was removed earlier and has been reintroduced now, but there is still no significant change.
Do people think that using too light of a raw material could cause this situation? The raw materials we’ve been using recently have been very light; I suspect this is the cause of the problem, and it has led to a vicious cycle inside the tower, preventing proper purification