Production technology and progress of acetic acid

Production technology and progress of acetic acid

Throughout the development history of acetic acid industry, in 1911, the world's first set of acetaldehyde oxidation synthesis acetic acid industrial equipment was completed and put into production in Germany.

In 1960, Germany BASF Company developed the process of carbonylation of methanol into acetic acid at high pressure and high temperature with methanol as raw material and cobalt as catalyst, which realized industrialization. In 1983, Eastman Company in the United States built the industrial plant of acetic acid-acetic anhydride co-production technology.

In recent years, traditional methanol carbonylation and other processes have been improved, and new processes and technologies have emerged endlessly, thus upgrading and upgrading acetic acid production technology.

2.1 Acetic acid production method

In the early 1970s, in the United States and Western Europe, more than 50% of acetic acid was produced using acetaldehyde as raw material, and less than 15% of total acetic acid was produced using methanol as raw material. By the early 1990s, acetic acid from methanol accounted for 50% of the total production, and then continued to increase.

Among the acetic acid production processes, the methanol carbonyl synthesis process has the lowest cost, higher conversion rate and higher selectivity. In the world, the production of acetic acid by methanol carbonyl synthesis has increased from less than 50 kt/a in 1960 to more than 1.0Mt/a.

At present, the industrial production of acetic acid abroad mainly includes methanol carbonyl synthesis, acetaldehyde oxidation, butane (light oil) liquid phase oxidation. In terms of production capacity, about 60 per cent is by methanol carbonylation, 18 per cent by ethylene acetaldehyde oxidation, 10 per cent by ethanol acetaldehyde oxidation, 8 per cent by butane/naphtha oxidation and 4 per cent by other methods.

The proportion distribution of various acetic acid processes is shown in the figure below. Copyright 2015 6chem. All rights reserved

At present, the process of producing acetic acid is mainly carbonylation of methanol, that is, methanol reacts with carbon monoxide under the action of catalyst to produce acetic acid. This process accounts for more than 60% of the global share of acetic acid production processes and is growing. BP and Celanese have patented technologies for the process, called Cativa and AO Plus, respectively. The Japanese company Chiyoda has recently developed a new process called Acetica, which uses a" bubble column reactor ". Other commercial processes are: Wacker process, acetaldehyde oxidation to acetic acid, n-butane or naphtha liquid phase oxidation to acetic acid, etc.

More recent developments include the oxidation of ethylene, ethane, butene or methane to acetic acid: for example, the direct oxidation of ethylene (also known as one-step process) to acetic acid was commercialized by Showa Denko in Japan; Wacker developed a butene-based route; Sabic has developed an oxidation process using a special catalyst using ethane as a feedstock, which is generally considered feasible only where ethane is inexpensive. In addition, acetic acid is also produced by side during the production of PVA.

2.1.1 Methanol carbonyl synthesis process

Typical production processes of methanol carbonyl synthesis include Monsanto/BP and Halcon/Eastman. The rhodium catalyst was used for the former, and the non-precious metal catalyst system was used for the latter, namely nickel acetate/methyl iodine/tetraphenyl tin catalyst. Two new processes have emerged in recent years, namely Celanese AO Plus process (acid optimization process) and BP Chemicals Iridium-based Cativa process.

In 1970,Monsanto developed a rhodium/iodide catalyzed methanol carbonyl synthesis process, replacing the high pressure cobalt iodide catalyzed process pioneered by BASF in 1960. In 1986 BP acquired Monsanto's ownership of the technology and improved it (known as the Monsanto /BP process). The Monsanto /BP process has an acetic acid selectivity (in methanol) of more than 99%. The equation is as follows:

C0 + CH3OH CH3COOH + 33Kcal/mal

The rhodium-catalyzed methanol carbonyl synthesis process has high selectivity and can be operated under appropriate pressure (about 3.4MPa). However, due to the high price of rhodium and the complex recovery process, new rhodium and non-rhodium catalysts are constantly being developed. The M onsanto/BP process has been improved by Celanese (AOPlus Process) and BP (Cativa Process). Recently, Chiyoda (Japan) also developed a new acetic acid production process (Acetica process) based on methanol carbonyl synthesis technology. The catalyst systems of the four methanol carbonyl synthesis processes are shown in the following table.

Table 2.1 Catalyst systems of four kinds of methanol carbonyl synthesis processes

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Monsanto /BP process is a traditional process to produce acetic acid. Acetic acid is synthesized by methanol and carbon monoxide in a stirred tank reactor at 175℃ and 2.8MPa. The liquid rhodium complex catalyst was dissolved in the reaction solution with iodide. The reaction conditions of Chiyoda's Acetica process are similar, except that the reactor is a bubble tower loop reactor and the catalyst is a suspended immobilized rhodium complex attached to the polymerized pellets. After the reaction, the products were purified by flash evaporation, dehydration and distillation. The acetic acid yield was more than 99% in methanol and 92% in CO.

In contrast to the M onsanto/B P process, where the concentration of the catalyst is lower due to its limited solubility, the Acetica process has the advantage of increasing the concentration of the catalyst, resulting in a 30-50% reduction in the reactor size and approximately 30% reduction in byproducts. Chiyoda expects the investment and operating costs of Acetica to be more than 20 percent lower than the traditional M onsanto/BP process.

2.1.2 Direct oxidation of ethylene

The indirect method can be divided into indirect method and direct method. The indirect method, namely the ethylene - acetaldehyde oxidation method, developed rapidly in the 1960s. However, with the development of Monsanto methanol carbonylation process, the proportion of ethylene - acetaldehyde method gradually decreased, because the method was inferior to the methanol carbonylation process in technical and economic indicators. At present, this technique is the main production method of acetic acid. The process uses acetaldehyde as raw material, and uses manganese acetate, cobalt acetate or copper acetate as liquid catalyst for oxidation reaction at 50 ~ 80 ℃, 0.6 ~ 0.8 MPa. The conversion rate of acetaldehyde is above 90%, and the selectivity of acetic acid is above 95%. All equipment used in the process must be made of stainless steel.

Direct gas phase catalytic oxidation of unsaturated hydrocarbons has always been considered as a feasible process for acetic acid production. Before 1997, however, only a multi-step acetic acid production process was available, in which ethylene was first oxidized to acetaldehyde, which was then oxidized to acetic acid with a precious metal catalyst. Showa Denko developed a one-step gas phase process for direct oxidation of ethylene to acetic acid (Showa Denko process), which was industrialised in 1997. This ethylene one-step gas phase process is economical for acetic acid plants with small production capacity (50-100kT /a) due to the relatively low investment cost (no infrastructure required for carbon monoxide production).

Showa Denko's one-step gas phase process is the highly selective preparation of acetic acid at 160 ~ 210 ℃ by a mixture of ethylene and oxygen with a supported palladium catalyst. The main side reactions are ethylene combustion and acetaldehyde formation. Under the reported reaction conditions, the one-way selectivity of acetic acid, acetaldehyde and carbon dioxide is 85.5%, 8.9% and 5.2%, respectively. The main byproduct, acetaldehyde, can be recycled to the reactor to reduce the combustion of ethylene and increase the total yield of acetic acid.

Acetic acid purification is an energy-intensive process because of the large amount of water generated during the reaction. To solve this problem, Showa Denko developed an energy-efficient process that combines extraction with distillation to effectively separate water from acetic acid. Showa Denko says the process is environmentally friendly because it produces only a small amount of wastewater.

Using the direct method, that is, the acetic acid production process without acetaldehyde, Showa Denki built an acetic acid plant with a production capacity of 100 kt/a in Chiba Plant in 1997. The plant used a new palladium catalyst and the reaction was carried out in a fixed-bed reactor at a temperature of about 150 ~ 160 ℃ and a pressure of about 0.9 MPa. The one-way conversion rate of ethylene was 7.4%, the selectivity of acetic acid, acetaldehyde and CO2 was 86.4%, 8.1% and 5.1%, respectively. Compared with similar scale methanol and acetaldehyde plants, the construction cost of the direct oxidation plant is significantly reduced, and the size of the plant can be designed according to user requirements. In addition, the process is very simple, the waste water discharge is reduced significantly, only one tenth of the acetaldehyde oxidation method.

2.1.3 Direct oxidation of ethane

SABIC has developed a new process for gas phase catalytic oxidation of ethane to acetic acid (SABIC process), which has attracted high attention throughout the petrochemical industry. a semi-industrial 30kt/a acetic acid production plant is under construction.

According to SABIC's patent, ethane oxidizes with pure oxygen or air to produce acetic acid at 150 ~ 450 ℃ and 0.1 ~ 5.0M Pa, with CO, CO2 and ethylene as byproducts. The new catalyst used in the SABIC process is calcined from a mixture of Mo, V, Nb, and Pd oxides, which helps to reduce the formation of byproducts while achieving higher selectivity and acetic acid yield. When using ethane and oxygen as raw materials, the selectivity of acetic acid is up to 71%, and the one-way conversion of ethane and oxygen is 13.6% and 100%, respectively. When ethane and air are used as raw materials, the selectivity of acetic acid is slightly lower, at 67%, but the one-way conversion of ethane is higher, at 49.6%, and oxygen conversion is 100%.

Due to the low production cost of ethane, direct oxidation of ethane to produce acetic acid can compete economically with carbonylation of methanol. The technology includes catalyst production, new oxidation reactor design, integrated process flow and basic process design. SABIC will continue to improve the technology and is considering building a 200kt/a acetic acid production unit.

2.1.4 Other process methods

The two processes using n-butane or light oil as feedstock are basically similar. Using light oil in the range of C5 ~ C7 as raw material, the catalyst of cobalt acetate, chromium acetate, vanadium acetate or manganese acetate was used to react at 170 ~ 200℃ and pressure of 1.0 ~ 5.0MPa. The final products were formic acid, propionic acid and acetic acid products. The ratio of acetic acid, formic acid and propionic acid was 1:0.25.

In addition, the ethanol acetaldehyde oxidation method mainly includes two processes: the oxidative dehydrogenation of ethanol to acetaldehyde and the oxidation of acetaldehyde to acetic acid. At present, some developing countries still maintain this production technology, but due to poor technical and economic indicators, most of them are in a state of stoppage or half-stoppage.

2.2 Comparison of several acetic acid production process routes

The raw material consumption of different production processes is shown in Table 2.2.

Table 2.2 Comparison table of consumption of products of different acetic acid production processes per ton

Nexant/Chem Systems conducted an evaluation of all acetic acid production processes and the evaluation results are shown in Table 2.3.

Table 2.3 Comparison of economic indexes of acetic acid production routes of three different raw materials

project

Direct oxidation of ethane

Direct oxidation of ethylene

Methanol carbonylation

Traditional BP process

Celanese AO process

BP Amoco Cativa process

As can be seen from Table 2.3, the Monsanto/BP process and SABIC direct air (non-circulating) ethane oxidation process have the higher total investment cost of $130.4 million and $166.1 million respectively among the three processes for acetic acid production scale of 200kt/a. The total investment cost of the direct oxidation process of ethylene was lower at $124.1 million. Of the two processes with acetic acid production scale of 500kt/a, the total investment cost of Celanese AO Plus process is lower than that of BP Cativa process. The Celanese AO Plus and B PCativa processes have the lowest investment costs in terms of unit investment costs, i.e., the total investment costs per kilogram of acetic acid per year, while the Monsanto/BP processes and SABIC direct air ethane oxidation processes have the higher investment costs.

In addition, it can be seen from Table 2.3 that:

(1) The newly developed Celanese AO process and BP Amoco Cativa process, due to the relatively advanced technology, the production capacity of the newly built or modified acetic acid plants is above 500 kt/a, thus significantly reducing the unit investment cost and production cost. The cost of a 500 kt/a plant at Celanese is US $116.7 million, lower than the 200kt/a direct ethane oxidation plant, and the total production cost is US $0.326 /kg, the lowest of all processes.

(2) The direct oxidation process of ethylene developed by Showa Denko, Japan, at the same 200 kt/a scale, the investment cost is lower than the traditional BP-Monsanto process, $124.1 million, $42 million and $6.3 million lower than the direct oxidation of ethane and traditional BP-Monsanto process, respectively. However, the production cost of $0.605 /kg is $0.132 /kg higher than that of the traditional BP-Monsanto process, because the price of ethylene raw materials is relatively high. Although the production cost is better than that of the traditional ethylene acetaldehyde oxidation method, it is not much better than that of the methanol carbonylation method and the ethane oxidation method.

(3) Compared with the direct oxidation of ethylene and the traditional BP-Monsanto process, the direct oxidation of ethane has certain advantages in terms of production cost due to the relatively low raw material price. The total production cost was US $0.449, which was US $0.156 /kg and US $0.224 /kg lower than the ethylene direct oxidation method, respectively. This is particularly beneficial for ethane rich regions such as the Middle East.

2.3 Progress of acetic acid technology

In recent years, the important trends of acetic acid production technology are the emergence of the "Acetica" process in Chiyoda, Japan based on bolstered catalyst, Hoechst and SABIC processes based on ethane, and various acetic acid processes based on syngas.

2.3.1 Direct oxidation of ethane

2.3.1.1 Cealnese company

Cealnese Company invented the gas phase catalytic oxidation process of ethane to acetic acid. The general formula of catalyst used in this process is MoaPdbXcYd, where X and Y are different metal elements. Typical reaction conditions are: reaction