Weekly Topic: What are the reasons for unsatisfactory solubility in water, water content, and oxidizing properties of pure methanol? 2016.06.06--06.12
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For the methanol section’s weekly topics, we encourage all members to participate actively. The purpose of this activity is to review the knowledge we have already learned; through answering questions, we can improve our knowledge, broaden our understanding, reinforce what we already know, help us recall what we have forgotten, and deepen discussions on controversial topics, thereby progressing together. We would appreciate it if you could share any opinions or suggestions you might have; they will help us to improve this methanol section together and to create a better platform for communication for everyone! ! There will be a reward for everyone who answers; great content is available upon reply. Weekly Topic: What are the reasons for unsatisfactory solubility in water, water content, and oxidizing properties of pure methanol? 2016.06.06--06.12 1. What are the reasons for the product’s poor water solubility? How to handle it? Answer: Water solubility is also known as turbidity. When pure methanol contains organic impurities that are insoluble or poorly soluble in water, upon addition of water, these impurities precipitate as gel-like particles, resulting in turbidity. 1) Water extraction distillation in the pre-column is the best method for removing these impurities; the amount of water added generally should not exceed 20% of the feed volume. Adding more water certainly helps to remove organic impurities, but it reduces the production capacity of the pre-column, so it must be done in an appropriate manner. 2) The control of the pre-tower top temperature also has a significant impact on the water solubility of the product; it is generally maintained at 30–40°C. As the catalyst in the synthesis tower is used for an extended period, the top temperature of the tower also needs to be adjusted accordingly. 3) Ensure proper control of the main tower’s operation; based on the results of sampling and analysis, it is necessary to remove fusel oils, thereby preventing them from ending up in the product and affecting its clarity. 2. What are the reasons for the excessive water content in the methanol product? Answer: 1) From a process perspective, excessive control of the bottom temperature and a low reflux ratio can lead to the upward movement of heavier components, thereby causing the water content to exceed the specified limit. 2) From the perspective of the equipment, leaks in the water heat exchanger can cause the water content in the product to exceed the specified level. 3) Damage to the internals of the main distillation tower and a reduced separation efficiency can also lead to an increase in water content. 3. What are the reasons for the unsatisfactory oxidizing property of methanol in the product, and what are the solutions? Answer: Oxidizing power and stability. It is a standard for measuring the level of reducing impurities (potassium permanganate value) in pure methanol. A high value for potassium permanganate indicates a low content of reducing impurities in the product, and it also means that the level of other organic impurities is necessarily low as well. The main methods to increase oxidizing capacity include: controlling the temperature at the top of the distillation column, employing extractive distillation, and increasing the reflux ratio of the main reactor
1) Improper pretreatment of the tower resulted in some alkanes that form azeotropes with methanol being carried over into the product, affecting its water solubility. Treatment measure: Adjust the amount of extraction water appropriately based on the quality of crude methanol, and remove alkane impurities through extraction. Control the pre-column condensation temperature to enable light components to be discharged from the system smoothly. (2) Improper treatment in the main column results in heavy components being carried into the product, affecting its water solubility. Countermeasures: Strictly control the operating conditions inside the tower; by adjusting the heat load and reflux rate, maintain an appropriate temperature of the sensitive plate to prevent the upward movement of heavier components. Components that affect water solubility are generally esters, aldehydes, ketones, ethers, etc., whose boiling points are similar to that of methanol, as well as most compounds in the C5–C10 range. These substances form azeotrope components with methanol and end up in the extracted product, resulting in unsatisfactory water solubility levels. The main reason is related to the operation of the pre-tower, but the influence of the main tower cannot be completely ruled out. Generally, it might be due to the following reasons: 1. The amount of water used for extraction is not appropriate. 2. The temperature of the pre-tower condenser is not controlled properly. 3. Fusel oils were not removed in appropriate amounts. 4. Collect a small amount of the initial distillate from the pre-cooling reflux liquid. 5. The operation is unstable, and the system experiences fluctuations.
The reason for the unsatisfactory water solubility of pure methanol is as follows: 1. Improper pretreatment in the tower resulted in some alkanes that form azeotropes with methanol entering the product, thereby affecting its water solubility. 2. Improper treatment of the main tower results in the introduction of heavy components into the product, affecting its water solubility. The reason for the unsatisfactory oxidizing property of pure methanol is: 1. The pre-distillation process does not remove light components such as ethers thoroughly ； 2. The low amount of alkali added to the pre-distillation tower results in incomplete decomposition of the carbonyl compounds that are prone to decomposition in crude methanol ； 3. The content of organic Reorganization components in crude methanol is too high, resulting in a low yield of fusel oils in the atmospheric pressure tower ； 4. An excessively low reflux ratio in the pressurized tower or atmospheric pressure tower causes the heavier components to move upward ； 5. The steam supply to the reboiler of the pressure tower is low, resulting in a cold tower.
Our plant has encountered problems with the water solubility of the product obtained from the atmospheric pressure column; this is mainly due to excessive impurities in the crude methanol, as well as contamination of the catalyst – in particular, the presence of iron which leads to the formation of carbonyl compounds. There are also many alkane compounds in the crude methanol, and even after pre-treatment in the column, a large portion of these alkanes remains combined with the crude methanol to form azeotropic components, resulting in poor water solubility of the final product. The capacity for adjustment and handling in distillation is limited.
One of the reasons in our company is that a tank containing crude methanol was left there for a long time, causing the methanol in it to form an azeotrope with various alkane impurities. When this crude methanol was added to the purified methanol, it resulted in the purified methanol not meeting the required water tolerance standards.
The impact of the amount of alkali added, whether the conditions in the pre-tower are normal during operation, the quality of the crude alcohol, and the amount of fusel oils recovered.