Our fixed-bed reactor is clogged. . .
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Our company has a fixed-bed reactor used for alkylation reactions. However, over the past year, the flow rate of this reactor has not been able to reach its full capacity. Since the market has not been very good lately, there is no need for us to run this setup at full capacity. So, they didn’t touch him either. However, with changes in the market, it is now necessary to increase the output of our equipment. The problem with this device becomes apparent. This reactor is filled with a resin catalyst and has a straight-tube shape. There is a cover on the bottom discharge port to shield the catalyst. This catalyst has been in use for 4 years (with a service life of 10 years), and the conversion rate of the reactants is still quite satisfactory at present. After discussing it, it seems that this reactor is blocked, and the possible reasons are as follows: 1. The catalyst has broken down, resulting in small particles blocking the bottom outlet ； 2. Some of the materials in the reactor coking at high temperatures, increasing viscosity and causing the catalyst to agglomerate. It blocked the reactor outlet. In response to this, the measure we plan to take for now is to perform a backflush of the reactor, without considering catalyst replacement for the time being; after all, the catalyst has not yet reached its expected service life, and the conversion rate of the reactants is still quite satisfactory. So, the method to be used for recoil is the focus of our discussion at present; since we have not carried out a process similar to this before, we would appreciate it if everyone could offer some suggestions and help us identify any gaps. 1. Use nitrogen for back-purging: First, add the reaction solution to the reactor and immerse the resin in it; then introduce nitrogen at the bottom while creating a vacuum at the top. Bubbling is carried out in this way. The advantage is that the recoil is relatively gentle, and the bubbles can help to sort the packing layer. The disadvantage is 1) the reactor needs to be cooled (cold nitrogen is introduced directly, and there is a concern that this may cause the resin catalyst to crack). 2) There is concern that the agglomeration of particles will be washed to the upper layer, leaving all the small particles in the lower layer and causing even more severe clogging of the reactor. 3) The pressure generated by the material inside the reactor will cause the material to enter nitrogen (nitrogen pressure: 5 kilograms). 2. Use the reaction mixture for backwashing – backwashing can be carried out directly using the material in the intermediate tank of the reaction mixture, without the need to cool the reactor, as one of the components in the reaction mixture has a high freezing point. The downside is that there are no other facilities similar to intermediate tanks connected to the top of our reactor; if the reaction mixture needs to be backflushed, it becomes quite troublesome. It is likely that the pump used for backflushing will have to be stopped as soon as it is started, as there is concern that the backflushing process will not be effective. Also, can backblowing be used to empty the material in the reactor first?
Personally, I think it’s caused by the coking mentioned in your second point. As for the solution, simply blowing it out may not be sufficient; by studying the components of the coking, one could consider using light oil or your product to clean it for about a week
Our reaction temperature is not high, around 100°C, so the likelihood of coking should be low. I wonder if the moderator has ever performed a back-purge on the reactor? What methods are generally used?
I’m not quite sure what specific process you’re referring to. We have previously carried out flushing of the reactors in aromatic compounds processing units, but we have never done pure purging operations. Due to the high temperature of 520°C, if that’s the temperature in question, it’s likely there is an excess of dust. Moreover, if there is a cover at the bottom, the backflow pressure might be too high, which could cause damage; this aspect still needs to be evaluated. We are concerned that the high backflow pressure might create a large pressure difference that damages the equipment inside the reactor, leading to catalyst leakage, which would be problematic
This post was last edited by liaifeng on 2018-6-3 21:14. If cryogenic nitrogen isn’t effective, consider using thermal nitrogen to avoid excessive temperature differences that could affect the catalyst and materials! Generally, such a design should take into account the possibility of the catalyst outlet getting clogged by debris; therefore, a Y-type filter should be added at the outlet to facilitate disassembly and cleaning! And a point for back-blowing nitrogen can be added at the filter!
It is recommended to consult the catalyst supplier on how to handle this
Thank you for the reminder; I will ask our equipment and process personnel about it.
I talked to the front-line operators again today and learned about a new development. It’s that when our reactor stops operating, although there is a display showing the liquid level of the material, this level disappears quite quickly. (The liquid level sensor for our material is located above the reactor; when the liquid level drops below the threshold set by the gauge, the corresponding reading on the DCS disappears.) Does this situation mean that my catalyst contains a large number of bubbles? Bubbles should also affect my flow rate. So the question arises: why are there so many bubbles in our reactor? Is there any way to solve this? Because our catalyst particles are very small. It’s estimated to be around 1–2 mm; if the bubble is located in the lower layer of the catalyst, it’s difficult to remove. And if that’s the reason, then using nitrogen for purging definitely won’t work.
President Huang, regarding the issues with the reactors, my assessment at that time was that the catalysts had become clogged due to the adhesion of by-products, which increased the pressure drop in the bed layers and thus affected the processing capacity. This problem was most evident in the first and third reactors. It would be useful to analyze the axial temperature distribution within these bed layers, to observe how the hot spots (the areas with the highest temperatures) have changed over the past year, thereby assessing any changes in the catalyst’s activity.
If it really is as you say, then it’s easy to solve. However, it’s necessary to stop operations in order to adjust the baffle at the bottom of the fixed-bed from a horizontal position to one with a certain inclination, so as to facilitate the removal of gases from the catalyst. As for the exact angle, fluidized-bed theory suggests around 30 degrees; you will need to determine this through practical operation!
1. What is the pressure difference in the reactor? Is it beyond the design value? If the pressure difference is normal, the problems with processing volume and flow rate are likely not related to the reactor; if the pressure difference is indeed high, the reactor is very likely to be the source of the problem ； 2. Resin catalysts generally do not break down into particles. Given your reaction temperature, the possibility of sintering is very low; the following possibilities should be considered: 1) The reactor should be divided into several sections; if it consists of vertical cylinders, and the catalyst particles are small in size, this creates considerable resistance ； 2) During the initial filling of the reactor, the system is quite dirty, containing impurities such as rust and welding slag; these may accumulate inside the pipes as well as within the reactor, leading to blockages in certain areas ； 3) As for the possibility of blockage at the bottom of the reactor mentioned by the poster, I don’t think it’s very likely. Instead, it’s possible that the catalyst mesh at the bottom has too fine a mesh size, which hinders proper flow. One can check whether any resin-based catalysts have fallen out among the reaction products; usually, very small particles of catalyst may be present. If none are found, it suggests that the mesh might indeed be too fine ； 4) There are defects in the design itself; it’s possible that the size of the entrances and exits is too small. In any case, to resolve the issue, it’s necessary to understand its root cause. It is recommended to remove the catalyst during shutdown and reinstall it, thereby avoiding the cost of replacing it. It’s just a bit more work, and remember that resin catalysts must not come into contact with water. This is just my personal opinion; please feel free to correct me