Corrosion and crystallization issues in the butadiene plant
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This post was last edited by Butadiene outdoor operations on 2017-12-6 at 13:20. The butadiene plant where I work uses the acetonitrile method to extract butadiene from cracked C4 hydrocarbons, and the distillation system employs a process that first removes the heavier components and then the lighter ones. The second distillation column does not have a reflux drum; the overhead vapor from the column is directly used as the feed for weight removal via a pressure-controlled valve. Q1: In the past two years, corrosion has begun to occur at the welds of the pipelines in the degassing system, and this has gradually progressed to cracks appearing in the seals of the top condensers of the degassing towers. As a result, our plant has already replaced three such top condensers. After investigating the cause, it was determined that the second extraction stage lacked a reflux tank, which allowed a small amount of acetonitrile from the extraction system to enter the weight-removal system; subsequently, hydrolysis of the acetonitrile at the top of the weight-removal tower caused corrosion. Water samples were taken from the dehydration pack in the degassing top reflux tank for testing; the pH value was around 10, and the sample had a pungent odor, with a strong ammonia smell. The current solution is to use a flare on the material side of the condenser at the top of the degassing tower in each batch, with flaring continuing for 10 minutes; however, there is no significant improvement in performance. Q2: The solvent recovery system of this unit is responsible for collecting the wash water from various wash towers, thereby purifying the solvent acetonitrile. A 2%~5% sodium nitrite solution is added to the circulating solvent to kill the polymer seeds present in acetonitrile. The problem arises in the reflux tank of the solvent recovery unit; crystals often form on the pressure gauge at the top of the tank as well as on the upper part of the double-flange level gauge. These crystals are waxy in nature, and they can be dissolved by passing medium-pressure steam through them. The crystals were sent for analysis, and the conclusion was sodium carbonate. (I personally think it’s something like sodium acetate.) I now want to write a paper on device corrosion and crystallization; I earnestly request advice from experts in the dibutylene process! ! !
My post is about to get deleted; I need help!
Could adding a corrosion inhibitor be considered as a solution?
1. Corrosion at the weld joint may have resulted in alkali embrittlement. 2. Regarding the crystallization issue, consider reducing the water content in the solvent and controlling hydrolysis ； At the same time, by properly controlling the amount of sodium nitrite added, the crystalline substance could be *ao acid an.
Hello, thank you for your reply. Firstly, the water content in the solvent is used to lower the operating temperature of the system, as butadiene polymerizes at high temperatures. The water content of the solvent acetonitrile in our unit is generally maintained at 5–8%; if this water content is reduced, the operating temperature of the extraction distillation column increases. Secondly, for crystalline substances, I lean towards sodium acetate; when dealing with crystallizations in level gauges, the common method used is to dissolve them by steam heating. If it is the *ao acid an you mentioned, it will undergo explosive dissolution when exposed to heat. The sodium nitrite solution prepared by our system generally has a concentration of 3–5%, and it is injected into the system via a metering pump at a rate of no more than 5 kg per hour. You should also be aware that the role of sodium nitrite in the system is to remove oxygen and eliminate polymer seeds. Given that the specific circumstances are not clear, reducing the amount of sodium nitrite added – could this affect the formation of polymers within the system?
1. Ammonia has a strong smell; it is ammonia produced by hydrolysis. 2. We encountered cracks in Hankou; during welding to repair them, it was found that the cracks would extend forward in the direction of the welding. Later, experts involved in the design suggested that the alkali leaching phenomenon of carbon steel occurred (our temperature was slightly high and the alkalinity was high). Later, the issue was resolved by switching to 304 material. 3 The composition of the crystalline substance may include *ao acid an; its content and other details still need to be analyzed and confirmed. In the locations where crystallization occurs in the gas phase that I have encountered, Na+ should not be able to reach the gas phase and thus form crystals. 4. What other polymerization inhibitors do you add besides sodium hydride? If appropriate adjustments can be made, that’s fine; large-scale adjustments should be avoided.
I agree with the issue of alkaline corrosion; as you said, during equipment maintenance we observe that cracks tend to spread. However, we have a large number of pipelines and equipment, and their size makes it impossible to switch to white steel material. As for the crystalline substance, I also proposed *ao acid an, but the test results showed it to be sodium carbonate
Regarding the crystallization issue, considering reducing the water content in the solvent and controlling hydrolysis； At the same time, by properly controlling the amount of sodium nitrite added, the crystalline substance could be *ao acid an.
I have already responded to these suggestions you mentioned earlier; the water content in the system solvent cannot be adjusted simply as desired, as it affects the operating temperature of the extraction system. I have also specified the amount of sodium suboxide to be injected into our system; it should be reduced appropriately – by how much per hour? Your reply is too vague! What exactly the crystalline substance is still needs to be determined
Hello, I’ve encountered a similar problem recently. How was the sodium carbonate issue resolved in the end?