How to deal with excessive acidity in the purified methanol obtained through methanol distillation?

There’s a discussion on this topic in the forum; check there again. It’s mainly 1 ； Too much or too little alkali, 2 ； Crude methanol has a high acidity level, 3 ； The pre-tower condenser temperature is too low ； such as reasons

The high acidity of pure methanol generally stems from the following reasons: 1. The temperature of the non-condensable gases condensed in the secondary condenser of the pre-tower is too low, causing some light components at the top of the pre-tower to be carried over to the subsequent tower; it is important to control the temperature of these non-condensable gases properly. 2. If the pH value of the pre-tower is normal but the acidity of the pure methanol obtained later is not up to standard, it is because the addition of caustic soda leads to the hydrolysis of esters in both the pressurized tower and the atmospheric pressure tower, resulting in excessive acidity. The solution is to increase the amount of product taken off via the side stream

The reasons for excessive acid content in pure methanol are as follows: 1. If the acid level is too high in the liquid obtained from the pressure tower, it is caused by improper control of the temperature and pressure in that tower； If the outlet temperature of the pre-tower secondary cooler is too low (usually kept between 35-45 degrees), the light components are not removed ； 2. If the acid level in the product from the atmospheric pressure tower is too high, it is caused by improper control of the temperature and pressure in that tower, or by insufficient addition of extraction water, which results in azeotropes entering the atmospheric pressure tower ； 3. If the acidity levels of the acid obtained from the pressurized tower and the atmospheric pressure tower are both above the specified limits, it is caused by either a low amount of alkali added or excessive acidity in the crude methanol.

Could the person on the 4th floor, Haiyou, please elaborate further on your view?

1) I believe the cause of excessive acid levels in methanol is to first ensure that the pH in the pre-tower is kept at a slightly higher level, around 8-9. 2) It is necessary to control the temperature of the non-condensable gases condensed in the secondary condenser of the pre-tower. 3) As for the situation where the acid level in the product from the pressure tower is too high, the solution is to increase the amount of product taken off via the side stream and to control the temperature of the sensitivity plate properly.

1. Basic knowledge of distillation; make judgments based on the actual conditions of production. 2. The main issue lies in the synthesis reaction; it is necessary to control the inlet temperature of the synthesis tower in order to reduce side reactions. 3. For other operations, follow the steps mentioned on the 4th floor to determine the cause. 4. If the free acid level is still above the limit, do the opposite: try to keep the methanol pH below 7; I’ll explain the reason later.

For prognosis, try to keep the methanol pH below 7; this should prevent the hydrolysis of esters such as methyl acetate, allowing them to be extracted at higher boiling points.

It should be easy to solve; it just requires controlling the amount of alkali added and the extraction temperature of the sensitive plate based on the situation of methanol synthesis and output.

1. Study the basic principles of distillation and make judgments based on the actual conditions of one’s own production process. 2. The main issue lies in the synthesis reaction; it is necessary to control the inlet temperature of the synthesis tower in order to reduce side reactions. 3. For other operations, follow the steps mentioned on the 4th floor to determine the cause. 4. If the free acid level is still above the limit, do the opposite: try to keep the methanol pH below 7

Discussion on the issue of excessive acid value in methanol distillation. Many manufacturers of methanol encounter a problem: the acid value of the methanol products obtained through distillation is too high. Many side reactions occur during the synthesis of methanol, resulting in a complex mixture of products, including dimethyl ether, formaldehyde, monomethylamine, dimethylamine, trimethylamine, acetaldehyde, methyl formate, pentane, propanal, ethyl formate, hexane, ethanol, butane, propane, heptane, isobutanol, water, formic acid, isopentanol, butyric acid, acetic acid, octane, and pentanol. Many manufacturers of methanol encounter the problem of excessive acidity levels in the methanol product after distillation. Many side reactions occur during the synthesis of methanol, and the products are complex, including dimethyl ether, formaldehyde, monomethylamine, dimethylamine, trimethylamine, acetaldehyde, methyl formate, pentane, propionaldehyde, ethyl formate, hexane, ethanol, butane, propane, heptane, isobutanol, water, formic acid, isopentanol, butyric acid, acetic acid, octane, pentanol, nonane, isooctane, decane, and others. Among them, formic acid and acetic acid are the main factors affecting the acid value of crude methanol. Both formic acid and acetic acid are highly acidic; in particular, formic acid has a significant impact on the acidity value of methanol. Furthermore, methyl formate and methyl acetate, which are among the impurities, will also hydrolyze to yield corresponding formic acid and acetic acid, thereby affecting the acid value. However, their concentrations in methanol are very low, and since methyl formate and methyl acetate have lower boiling points than methanol, they can be removed directly in a flash tank (through vacuum evaporation to remove light components) or a pre-column (to remove light components). As a result, methyl formate and methyl acetate do not have a significant impact on the acid value of the final purified methanol. Following conventional reasoning, the amount of alkali should be increased to neutralize and reduce the acid value. However, after adjusting the pH value with alkali, the acid value of the distilled product increased, and it remained unqualified. On the contrary, if the amount of alkali added was reduced to lower the pH value in the pre-column, the distillation product unexpectedly met the specifications. How should such a situation be handled? It should be understood that the purpose of adding alkali during the distillation process is not to reduce the acid value of the distilled product, but rather to increase the pH value at the bottom of the tower, thereby preventing the high-temperature tower components from being corroded by acidic liquids. Understanding this, it should be possible to see that increasing the amount of alkali does not resolve the issue of the product’s excessive acid value. The main cause of corrosion inside the tower is the formation of organic acids and other substances as side reactions during synthesis. Some of these organic acids have a boiling point higher than that of methanol. Some of these organic acids arise from side reactions that occur during methanol synthesis, while others result from the hydrolysis of a small amount of esters produced during methanol synthesis. Therefore, adding alkali can neutralize some of the organic acids present in the bottom of the tower, but it also accelerates the hydrolysis of certain esters, resulting in a dynamic equilibrium. Acids with a low boiling point and a boiling point similar to that of methanol dissolve in the methanol product, ultimately resulting in the acid value of the pure methanol product exceeding the acceptable limits and rendering it unsuitable. So how can the pure methanol product meet the required standards while also preventing corrosion of the tower bottom? The following measures can be taken for comprehensive adjustment: 1. Control the operating parameters of methanol synthesis to avoid overheating, reduce the likelihood of side reactions, and address the acid value issue at its source ； 2. Adjust the location where alkali is added, and strictly control the pH value at the bottom of the tower; the pH value should not be too high, with a range of 7.5–8.5 being sufficient ； 3. Increase the amount of potassium permanganate added appropriately to oxidize some of the organic substances, but the amount should not be excessive, as this could have the opposite effect ； 4. Adjust the operations of the tower: increase the temperature and boost the venting, reduce the pressure slightly, and add more extraction water ； 5. Increase the recovery rate of fusel oils in the upper layer of the pre-condensation reflux tank. If the above methods still cannot solve the problem, replace the tower bottom with corrosion-resistant 304 stainless steel.

You explained it in great detail!:handshake

Leaks in the water cooler can also cause acidity to exceed limits

Leaks in the water cooler can also cause acidity to exceed limits

The circulating water level is 8-9.2; it might just meet the requirements.:D

In October, I spent a month trying to control the acidity level here; increasing it didn’t work, and reducing it either didn’t help. Since it was impossible to meet higher standards, I had to settle for producing the product in this way