What makes methanol alkaline?

The free alkali exceeds the standard and is an amine substance. It is caused by the high partial pressure of nitrogen and hydrogen in the synthesis gas or the introduction of ammonia. It can be removed by increasing the reflux in the pre-column.

Generally speaking, there will be high alkalinity in combined alcohols. If it is a monoalcohol, it should not happen.

:o Isn’t crude methanol acidic? Isn’t it necessary to add alkali during distillation to neutralize it?

Check whether there is a problem with adding alkali to neutralize it; or there may be impurities containing alkaline content.

The reason why the free alkali in refined methanol exceeds the standard. Our company is a joint alcohol manufacturer. The quality of refined methanol products has always been very stable. Recently, the free alkali in refined methanol has exceeded the standard (high-quality product index ≤ 0.002‰). To address this phenomenon, we first investigate the source—crude methanol and desalted water. By sampling and analyzing the desalted water, the suspicion of desalted water was quickly ruled out. The compressor used by our company is a three-stage reciprocating compressor. The raw gas of the crude methanol synthesis tower is the outlet gas of the second stage. The post-alcohol gas enters the copper wash and is pressure-boosted in three stages before being removed from the ammonia synthesis tower. During the investigation, it was found that after the free alkali in the refined methanol exceeded the standard, the first reaction was that the compressor leaked air three times and one time, causing ammonia to enter one stage and then into the methanol tower to generate monomethylamine, dimethylamine, trimethylamine, etc. All three-way and one-way valves were inspected and no obvious internal leakage was found (similar situations occurred before and returned to normal after treatment). On March 2 and March 25, 2006, the three-circuit-one valves of four compressors were replaced by using the shutdown opportunity (the replaced valves were pressure tested and no leaks were found), but the quality of the refined methanol did not improve. Considering that the gas after hydrogen recovery also enters the compressor section, if the ammonia washing tower fails, ammonia may be brought into the methanol tower. On March 29, 2006, the hydrogen recovery gas was analyzed and it was found that ammonia was brought in. The ammonia washing tower was inspected and it was found that the packing had collapsed. After maintenance, it resumed operation on April 2. The hydrogen recovery gas was analyzed again and no ammonia was found. After tracking the quality of refined methanol products, it was found that the free base in refined methanol only dropped from 0.014‰ to 0.009‰, indicating that the main cause of the problem has not yet been found. After searching for information, it was found that the cause of excessive free alkali in refined methanol products was nothing more than the generation of methylamine substances, which was basically consistent with the company's judgment. However, our company has investigated the source and eliminated all possible places where ammonia could be introduced. Now that the source has been eliminated, the only way to find the cause is from the distillation system. 2 Solution Before the real reason for the excessive free alkali in the product was found, in order to improve the product quality, the methanol distillation operation was adjusted based on the experience of the external factory. Adjust the pre-tower top temperature index to 70~80℃ (other control indicators change with the tower top temperature), reduce the pre-tower condenser circulating water flow, and increase the vent temperature to 50~55℃. The quality of purified methanol cannot deteriorate for no reason. What is the real reason? When I was puzzled, I became suspicious of the methanol catalyst. Our company uses a dual-tower combined alcohol process. We have been using Nanhua's C207 catalyst in the past. In November and December 2005, we replaced it with another manufacturer's C207 catalyst. At that time, the reaction activation temperature was only high and the conversion rate was okay. The high alkalinity of purified methanol began to appear on February 16, 2006. It was once suspected that it was caused by the catalyst, but it could not be confirmed. So we started with the areas that were easy to improve and took some of the above measures. When replacing the catalyst of the 2# methanol tower in March 2006, it was recommended that the company resume the use of Nanhua's C207 catalyst. Since there is still one furnace of catalyst that has not been replaced, the alkalinity of the refined methanol product in April is still high. On May 11, the catalyst of the #1 methanol tower was replaced, and on the 13th, the alkalinity of the refined methanol met the requirements for high-quality products. After the 13th, the alkalinity of refined methanol products has been controlled at ≤0.002‰.

Methanol is synthesized to produce monomethylamine, dimethylamine, and trimethylamine. Increasing the amount of alkali will only make the finished product more acidic. The ester will be hydrolyzed, saponified in the first tower, and decomposed again at high temperatures in the second and third towers, and the acidic substances will evaporate to the top of the tower.

What are the free acids and free bases in crude methanol?

I think this phenomenon may not occur in the production of monools, but there are amines in the combined alcohols, which produce free bases.