Production method of acetic acid

Acetic acid can be prepared by artificial synthesis and bacterial fermentation. Biosynthesis, the use of bacterial fermentation, only accounts for 10% of the world’s output, but it is still the most important method for the production of acetic acid, especially vinegar, because food safety regulations in many countries require that the vinegar in food must pass biological law Preparation, and fermentation method is divided into aerobic fermentation method and anaerobic fermentation method.

Aerobic fermentation

In the case of sufficient oxygen, bacteria of the genus Acetobacter can produce acetic acid from foods containing alcohol. Usually used is cider or wine mixed with grains, malt, rice or potatoes and fermented after mashing. The chemical equation for the fermentation reaction of these bacteria is:

C₂H5OH + O₂ →CH₃COOH + H₂O

The specific method is to inoculate the bacteria of the acetobacter in the diluted alcohol solution and keep it at a certain temperature, and place it in a ventilated place. Within a few months, it can be fermented and finally produce vinegar. The method of industrial production of vinegar speeds up the reaction process by providing sufficient oxygen. This method has been adopted in commercial production and is also known as the "quick method" or "German method". It was named after the first successful application in 1823 in Germany. . In this method, fermentation is carried out in a tower filled with wood chips or charcoal. Alcohol-containing raw materials drip in from the top of the tower, and fresh air naturally enters or forced convection from below. The enhanced air volume enables this process to be completed within a few weeks, greatly shortening the vinegar production time.

Otto Hromatka and Heinrich Ebner first produced vinegar from a liquid bacterial culture medium in 1949. In this method, alcohol is fermented into acetic acid during 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 Clostridium genus, can directly convert sugars into acetic acid without the need for ethanol as an intermediate. The overall reaction equation is as follows:

C6H12O6==3CH3COOH

In addition, many bacteria can produce acetic acid from compounds containing only a single carbon, such as methanol, carbon monoxide, or a mixture of carbon dioxide and hydrogen.

2CO2 + 4H2 → CH3COOH + 2H2O

2CO + 2H2 → CH3COOH

Clostridium has the ability to react with sugars, reducing costs, which means that these bacteria have the potential to produce acetic acid more efficiently than the ethanol oxidation method of Acetobacter bacteria. However, bacteria of the Clostridium genus are less acid-tolerant than bacteria of the Acetobacter genus. The bacteria of the genus Clostridium, which is the most acid-resistant, 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 vinegar. Therefore, although bacteria of the genus Clostridium have been discovered as early as 1940, its industrial application range is relatively narrow.

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

Methanol carbonylation

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

CH3OH + CO →CH3COOH

This process uses methyl iodide as an intermediate and is completed in three steps, and requires a multi-metal catalyst (in the second step)

⑴ CH₃OH + HI →CH₃I + H₂O

⑵ CH₃I + CO →CH₃COI

⑶ CH₃COI + H₂O →CH₃COOH + HI

By controlling the reaction conditions, acetic anhydride can also be produced through the same reaction. Because carbon monoxide and methanol are commonly used chemical raw materials, methyl carbonylation has always been favored. As early as 1925, the British Celanese company developed the first pilot plant for methyl carbonylation to produce acetic acid. However, due to the lack of containers that can withstand high pressure (200 atm or higher) and corrosion resistance, the application of this method has been limited. In 1963, the German BASF Chemical Company used cobalt as a catalyst and developed the first process suitable for industrial production of acetic acid. In 1968, the rhodium catalyst greatly reduced the difficulty of the reaction. Using a catalyst system composed of rhodium carbonyl compound and iodide, methanol and carbon monoxide are reacted in a water-acetic acid medium at 175°C and a pressure lower than 3 MPa to obtain an acetic acid product. Because the activity and selectivity of the catalyst are relatively high, there are few by-products of the reaction. The production of acetic acid by methanol low-pressure carbonylation has the advantages of cheap raw materials, mild operating conditions, high acetic acid yield, good product quality and simple process flow. However, the reaction medium is severely corrosive and requires special corrosion-resistant materials. In 1970, the American Monsanto Company built a device using this process, so rhodium-catalyzed methyl carbonylation to acetic acid gradually became the dominant Monsanto process. In the late 1990s, British Petroleum successfully commercialized the Cativa catalytic method. This method uses a ruthenium catalyst and uses ([Ir(CO)₂I₂]), which is greener and has higher efficiency than the Monsanto method.

Acetaldehyde oxidation method

Before Monsanto’s commercial production, most of the acetic acid was produced by the oxidation of acetaldehyde. Although it cannot be compared with methyl carbonylation, this method is still the second method of industrial production of acetic acid. The reaction equation is as follows:

2CH₃CHO+O₂→2CH₃COOH

Acetaldehyde can be produced by oxidizing butane or light naphtha, or it can be produced by hydrating ethylene.

Liquid Phase Oxidation of Low Carbon Alkanes

Using n-butane as the raw material, using acetic acid as the solvent, at 170°C-180°C, 5.5 MPa and the presence of a cobalt acetate catalyst, air is used as the oxidant for oxidation. At the same time, this method can also use liquefied petroleum gas or light oil as raw materials. This method has low raw material cost, but has a long process flow, serious corrosion, and low acetic acid yield. It is limited to areas where cheap isobutane or liquefied petroleum gas raw materials are readily available.

2C₄H₁₀ + 5O₂ →4CH₃COOH + 2H₂O

This reaction can be carried out at the highest temperature and pressure that can keep butane in a liquid state. The by-products include methyl ethyl ketone, ethyl acetate, formic acid and propionic acid. Because some of the by-products also have economic value, the reaction conditions can be adjusted so that more by-products are generated, but 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:

2 CH₃CHO + O₂ →2CH₃COOH

It can also be oxidized by copper hydroxide suspension:

2Cu(OH)₂+CH₃CHO→CH₃COOH+Cu₂O↓+2H₂O

Using a new type of catalyst, this reaction can obtain an acetic acid yield of more than 95%. The main by-products are ethyl acetate, formic acid and formaldehyde. Because the by-products have a lower boiling point than acetic acid, they are easily removed by distillation.

Ethylene Oxidation Method

Ethylene reacts with oxygen in the presence of a catalyst (the catalyst used is palladium chloride: PdCl₂, copper chloride: CuCl₂ and manganese acetate: (CH₃COO)₂Mn). This reaction can be regarded as the first oxidation of ethylene to acetaldehyde, which is then produced by the acetaldehyde oxidation method.

Topsoe

The Topsøe method uses a single natural gas or coal as a raw material. The first step: synthesis gas generates methanol and dimethyl ether under the catalyst; the second step: methanol and dimethyl ether (the two do not need to be purified) and CO carbonylation to generate acetic acid, this method is also called a two-step method.