Everyone, come and take a look! The distillation off-gas condenser often freezes, resulting in an excessive level of acetylene monomer
Thread Content
During the past week, the operation of the low-boiling-point column in the distillation system has been unstable. The temperature at the bottom of the column needs to be maintained between 38–40 degrees, while the temperature at the top of the column (before it enters the condenser) should be kept between 35–37 degrees in order for the acetylene monomer content to be within acceptable levels. Otherwise, the acetylene content exceeds the allowed limits significantly. The exhaust gas condenser often freezes, and the temperature at the top of the column remains unstable, resulting in inconsistent quality of the monomer. Could anyone tell me what might be causing this issue? How did everyone maintain stable control? Let’s all come and discuss it!
The original poster is really amazing; the sun and the moon are in your hands. Regarding the low-boiling-point tower, some points weren’t clarified. The main purpose of this tower is to remove the low-boiling substances from the monomer (acetylene and other gases). I’m not sure what the original control parameters were, but I think it’s more likely to be an issue related to the operation; we should first look for reasons in the operational aspects. For exhaust gas condensers, saline at -35 degrees is generally used as the cooling medium; therefore, it’s normal for the equipment to freeze. In the design, two exhaust gas condensers are provided, so that one can operate while the other is shut down to allow the saline inside it to thaw, with these two units alternating in use. I’m not sure what the situation is like in your case
The low-boiling tower needs maintenance; let’s take it apart and have a look
During this tower maintenance work, we found a large amount of self-polymer inside; in particular, the feed section had blocked some of the downcomers and bubble caps, which prevented proper flow of liquid and vapor downward, resulting in an excessive level of acetylene in the monomer. We have three exhaust condensers; if one freezes, we switch to using another. We analyze that freezing occurs due, first, to the high moisture content in the monomer, and second, to the high temperature control in the tower bottom and the low temperature control in the tail cooler, which causes some VCM monomer to vaporize and reach the tail cooler, leading to the liquefaction and freezing of VCM.
Reply to 2# yangwenhua0404: Hehe! Happy New Year! It’s quite humorous! Let’s be friends! During the New Year period, the acetylene content in our monomer exceeded the allowable limits significantly, making it impossible to carry out production. We decided to shut down the plant and disassemble the tower for maintenance; it was found that there was a large amount of self-polymer inside, especially in the feed section, where some of the downcomers and bubble caps were blocked. This led to poor flow of liquid and vapor, resulting in an excessive acetylene content in the monomer. We have three exhaust condensers; if one freezes, we switch to using another. We analyze that freezing occurs due, first, to the high moisture content in the monomer, and second, to the high temperature control in the tower bottom and the low temperature control in the tail cooler, which causes some VCM monomer to vaporize and reach the tail cooler, leading to the liquefaction and freezing of VCM. What range are the parameter controls for the operation of your high and low towers?
This post was last edited and replied to by yangwenhua0404 on 2011-2-21 at 18:58. Reply 5# sunmoon2010: It seems that the process control parameters vary among different manufacturers; this is for reference only. Pressures and temperatures of the two towers (Hebei University of Technology): High tower: Top pressure 0.33 MPa, reactor pressure 0.35 MPa; top temperature ≈ 32°C, reactor temperature ≈ 40°C. Low tower: Top pressure 0.53 MPa, reactor pressure 0.55 MPa; top temperature ≈ 42°C, reactor temperature ≈ 45°C℃
Top temperature ≈ 42°C, reactor temperature ≈ 45°C. Excessive temperature control leads to high evaporation rates; it is sufficient for the low tower to remove acetylene. Meanwhile, freezing in the tail cooler is caused by poor temperature control – the key factor is the temperature during the micro-cooled feeding process.
1. The function of the distillation tower is indeed to purify the material and to ensure control over the acetylene content in the monomer; however, it is also necessary to strengthen the control of the ratio between hydrogen chloride and acetylene. Otherwise, the purification process via the distillation tower will lead to increased loss of material. 2. The issue of self-polymerization in low-tower and feed pipelines was discussed in the forum; I believe it is mainly related to abnormalities such as the presence of water, oxygen, and iron ions in the crude vinyl chloride monomer. Please conduct corresponding sampling analyses. 3. The frequent freezing of the tail cooler is mainly related to water content in the system; we have also encountered situations where the control valve for the tail cooler froze up (as verified through disassembly and inspection). The long-term solution is to address the issue of excessive water content, while in the short term, the frequency of switching the tail cooler should be increased.
Reply to 2# yangwenhua0404 Hi! Long time no see! How are the profits of your factory these days? Are you in charge of vinyl chloride technology or polymerization technology?
Reply to 6# yangwenhua0404: The temperature control at the tops of the high and low towers is set too high. Let’s check what your conversion rate is like – could it be due to an excessive amount of acetylene in the system or an improper ratio???