The lower trays of the PVC low-boiling point tower’s feed pipe are severely clogged; due to self-polymerization, black oily substances are present on the trays
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After the PVC low-boiling tower was disassembled, severe self-polymerization occurred on the tray below the feed pipe; the self-polymerized material was white with a yellowish tint, and there was black oily substance on the trays below this area (as shown in the image). Could anyone tell me what causes this phenomenon? Some people say it is due to too high a water content in the feed----
You can consult relevant rubber and plastic testing institutions such as Dongbiao Rubber and Plastic Testing Center, which is a testing facility specialized in various rubber and plastic products. This company has an excellent reputation; you can search for it on Baidu
In May, our company encountered such a problem; not to mention the self-polymerizing substance, there was also a black oily substance, which is likely impurities in the pipes. . For short-term operation of self-polymerization, we summarize it in two aspects: first, considering polymer recovery, whether only monomers recovered from polymerization are used as materials for distillation; second, the moisture content in the monomers.
The high water content causes the VCM monomer to readily absorb oxygen in the presence of water, resulting in low-molecular-weight peroxides with an average degree of polymerization of less than 10. The presence of iron ions and acids induces the autopolymerization of vinyl chloride; the chemical reaction is as follows: nCH2=CHCl + nO2 → (CH2 – CHCl-O-O)n. Such peroxides can trigger the autopolymerization of VCM, leading to the formation of peroxide chains within the PVC polymers, which lowers the decomposition temperature of PVC. PVC under such conditions is prone to decompose into hydrogen chloride, formaldehyde, and carbon monoxide during operation in distillation systems, and it also reduces the pH value of VCM. Excess hydrogen chloride content can disrupt the polyethylene glycol dispersion system and consume most of the dispersant, resulting in larger particle sizes and affecting the particle morphology of the product.
Reply to 4# redondo5: By controlling the feed to be alkaline, can self-polymerization be suppressed?
My views and suggestions: The main reasons are high moisture content in the monomer and an acidic pH value (there are various reasons for this). Additionally, there is a high level of residual monomer from polymerization. Suggestions: Add an inhibitor to VCM to improve its pH value. It is essential to deal with the residual water after corrosion occurs in the distillation equipment; although it can be removed from the system (that’s what everyone thinks), it has a fatal impact on the low-boiling towers. This is especially true when the system is restarted after being shut down – special attention must be paid to this
Reply to 5# txf001: The manufacturing process of those little Japanese companies is quite thorough. Solid-alkali drying serves two purposes: dehydration and filtering, while also helping to keep the pH value slightly alkaline at 0.2. However, it’s important to control the residence time and avoid letting it be too long
Reply to 7# redondo5: Do you have a picture of the firmware dryer? Give it to someone who wants to learn—txf001@hotmail.com. What should this alkalinity level be?