Acetic acid's preparation method

Preparation method of acetic acid

Acetic acid can be prepared by artificial synthesis and bacterial fermentation. Biosynthesis, i.e. fermentation by bacteria, accounts for only 10% of the world's output, but it is still the most important method to produce acetic acid, especially vinegar, because the food safety regulations of many countries stipulate that vinegar in food must be prepared by biological methods, and fermentation methods are divided into aerobic fermentation and anaerobic fermentation.

Aerobic fermentation

Under the condition of sufficient oxygen, Acetobacter bacteria can produce acetic acid from food containing alcohol. Cider or wine is usually fermented by mashing it with grain, malt, rice or potato. These substances can be fermented into acetic acid under the action of catalytic enzymes and oxygen.

The specific method is to inoculate the bacteria of the genus Acetobacter in the diluted alcohol solution, keep a certain temperature, and place it in a ventilated place. It can be fermented within a few months and finally produce vinegar. The method of industrial production of vinegar accelerates the reaction process by providing sufficient oxygen. This method has been adopted in commercial production. It is also known as "fast method" or "German method". It is named because it was successfully applied in Germany for the first time in 1823. In this method, the fermentation is carried out in a tower filled with wood chips or charcoal. The raw materials containing alcohol drop in from the top of the tower, and the fresh air naturally enters or forcibly convection from the bottom. The enhanced air volume enables this process to be completed within a few weeks, greatly reducing the time of vinegar production.

Otto hromatka and Heinrich Ebner first proposed the preparation of vinegar by liquid bacterial culture medium in 1949. In this method, alcohol is fermented into acetic acid under continuous stirring, and air is filled into the solution in the form of bubbles. By this method, vinegar containing 15% acetic acid can be prepared in two to three days.

Anaerobic fermentation

Some anaerobic bacteria, including some members of the genus Clostridium, can directly convert sugars to acetic acid without requiring ethanol as an intermediate. Sucrose can be fermented into acetic acid in the absence of oxygen.

In addition, many bacteria are able to produce acetic acid from compounds containing only single carbon, such as methanol, carbon monoxide, or mixtures of carbon dioxide and hydrogen.

Clostridium has the ability to react sugars, reducing the cost, which means that these bacteria have the potential to produce acetic acid more efficiently than Acetobacter by ethanol oxidation. However, Clostridium bacteria are less acid tolerant than Acetobacter bacteria. Clostridium bacteria with the highest acid resistance can only produce less than 10% acetic acid, while some acetic acid bacteria can produce 20% acetic acid. Using acetic acid bacteria to make vinegar is still more economical than using Clostridium bacteria to make concentrated vinegar. Therefore, although Clostridium bacteria have been discovered as early as 1940, its industrial application scope is narrow.

In addition to the above biological methods, industrial acetic acid is mainly synthesized by the following methods:

Methanol carbonylation method

Most acetic acid is synthesized by methyl carbonylation. In this reaction, methanol and carbon monoxide react to form acetic acid, and the equation is as follows

This process is completed in three steps with iodomethane as the intermediate, and requires catalysts with multi-metallic components (in the second step)

Acetic anhydride can also be produced by the same reaction by controlling the reaction conditions. Because carbon monoxide and methanol are common chemical raw materials, methyl carbonylation has always been favored.

Acetaldehyde oxidation

Before the commercial production of Monsanto process, most of acetic acid was produced by oxidation of acetaldehyde. Although not comparable to methyl carbonylation, this process is still the second industrial process for producing acetic acid.

Alkane liquid oxidation method

N-butane was used as raw material, acetic acid was used as solvent, and air was used as oxidant in the presence of 170 ℃ - 180 ℃, 5.5 MPa and cobalt acetate catalyst. At the same time, liquefied petroleum gas or light oil can also be used as raw materials. This method has low raw material cost, but the process flow is long, the corrosion is serious, and the acetic acid yield is not high. It is only used in areas where cheap isobutane or liquefied petroleum gas raw materials are easily available.

This reaction can be carried out at the highest temperature and pressure that can keep butane in liquid state. The by-products include butanone, ethyl acetate, formic acid and propionic acid. Because some by-products also have economic value, the reaction conditions can be adjusted to generate more by-products. However, the separation of acetic acid and by-products increases the cost of the reaction.

Under similar conditions, using the above catalyst, acetaldehyde can be oxidized by oxygen in the air to produce acetic acid.

Acetaldehyde can also be oxidized by copper hydroxide.

Using new catalyst, the yield of acetic acid can be over 95%. The main by-products are ethyl acetate, formic acid and formaldehyde. Since the boiling point of the by-products is lower than that of acetic acid, they are easily removed by distillation.

Ethylene oxidation process

It is produced by the reaction of ethylene with oxygen in the presence of a catalyst (palladium chloride: PdCl 2, copper chloride: CuCl 2 and manganese acetate: (CH ₃ COO) 2 Mn). This reaction can be regarded as the oxidation of ethylene to acetaldehyde and then the oxidation of acetaldehyde.

Topso method

Topso process uses single natural gas or coal as raw material. Step 1: synthesis gas generates methanol and dimethyl ether under the catalyst; Part II: carbonylation of methanol and dimethyl ether (both do not need purification) and CO to produce acetic acid. This method is also called two-step method.