Why isn't "dangerous" vinyl chloride banned?

Recently, a train carrying hazardous chemicals derailed and caught fire in Ohio, USA, resulting in the leakage of hazardous chemical vinyl chloride. Due to the carcinogenicity and other properties of the related substances, the incident has attracted worldwide attention.

As I am engaged in chemical research, I am inevitably asked by friends about my views on this matter. Here I must explain that unfortunately, there is a lack of relevant test data, and we cannot give an accurate answer to the questions that everyone is concerned about, such as "how big is the harm?" Today, I mainly want to talk to you about two other questions. Since vinyl chloride is highly toxic, why is it still used in the chemical industry? Can't it be replaced by something else? What is the current situation of vinyl chloride production and use in my country?

Photos of the train accident scene on February 6, local time

(Photo source: People's Daily Overseas Edition)

Part 1

"High risk" warning ahead

What exactly is vinyl chloride? Why do people talk about it with fear? If we look at its structural formula C2H3Cl, we can see that it replaces one hydrogen atom in ethylene with a chlorine atom.

Although there is only one atom difference, its properties have changed dramatically.

Vinyl chloride

(Photo source: veer photo gallery)

Ethylene is an inert material and the cornerstone of chemical raw materials. Many high-value chemical products can be synthesized through ethylene.

Vinyl chloride is the basic raw material for the preparation of polyvinyl chloride plastic (PVC). It can also be polymerized with vinyl acetate, butadiene, etc. It can also be used as an extractor, refrigerant, etc. It has a wide range of applications in the chemical and chemical industry.

But at the same time, vinyl chloride is also a very dangerous and toxic chemical substance, which has been listed as a Class I carcinogen by the World Health Organization's International Cancer Agency. It can cause significant damage to human liver and lungs. In addition, burning vinyl chloride will produce more harmful toxic gases such as phosgene and hydrogen chloride. Phosgene can cause immediate death in certain circumstances.

Part 2

Vinyl chloride: a "dangerous" chemical and an important part of chlor-alkali chemical industry

If it is so dangerous, why do we have to make vinyl chloride? Is it okay to completely ban the use of vinyl chloride?

The answer is: Not possible at this stage.

In addition to its huge application background of polymer polyvinyl chloride plastic (PVC), it is also because it is included in chlor-alkali chemical industry. Based on my country's national conditions, a material that is so closely related to practical applications cannot be simply banned.

Chlor-alkali chemical industry refers to the process of producing sodium hydroxide and chlorine by electrolysis of sodium chloride in industry. It is the main industrial preparation method of caustic soda (sodium hydroxide).

China is the world's largest producer of caustic soda, accounting for more than 50% of the world's caustic soda production. The existing caustic soda preparation process is mainly the ion membrane method of electrolyzing brine. The main use of caustic soda is various pretreatments and absorption in industry, and its usage accounts for more than 50% of the total output. About 25% of caustic soda is used in the papermaking industry.

Therefore, chlor-alkali chemical industry plays an extremely important role.

However, this preparation method is not "perfect". When sodium chloride is electrolyzed, in addition to the desired product caustic soda, hydrogen and chlorine are also produced. These two unwanted products are often called "by-products".

Its overall reaction formula is as follows:

Among them, chlorine is an extremely strong oxidizing gas and is very corrosive. If chlorine leaks, it will produce highly toxic substances such as phosgene under the sunlight, which poses a huge safety hazard and is difficult to store in large quantities for a long time in industry.

If chlorine cannot be consumed or stored cheaply in time, it will seriously affect the production cost of chlor-alkali chemical industry. Considering that caustic soda is an extremely basic industrial raw material in the chemical industry, the increase in its production cost will continuously affect all aspects of the industry.

Imported liquid chlorine compressors and small liquid chlorine tanks are the most expensive equipment in a chlor-alkali plant.

(Photo credit: Photo taken by the author)

In the 1990s, there was a shortage of caustic soda supply and a surge in prices in China due to a decrease in chlorine consumption, which caused serious losses to many chemical companies. In addition, the prices of corresponding products increased, which also had a corresponding impact on the national economy.

Therefore, in order to stabilize the chlor-alkali industry, reduce the storage of chlorine and save corporate costs, it is necessary to optimize the use of chlorine. Chlorine is widely used in the chemical, agricultural, and medical and health industries. Its main uses include the production of hydrochloric acid, organic solvents, pesticides, disinfectants, inorganic salts, and chemical reagents, etc. These applications consume about two-thirds of chlorine production.

Are you getting impatient after reading this? We have talked so much, what does it have to do with vinyl chloride? Don't worry, everything in the world is often not as simple as we see on the surface. Things that seem to be unwilling to do may actually be inextricably linked, not to mention the seemingly related "chlorine" and "vinyl chloride"! Next, vinyl chloride comes on the scene.

Another cheap way to store large amounts of chlorine is to synthesize vinyl chloride monomer and produce polyvinyl chloride plastic.

Polyvinyl chloride has good mechanical properties and heat resistance. It can be processed into pipe and cable insulation materials and is widely used. It is a commodity that can be mass-produced. In 2014, the national caustic soda production was 31.8 million tons, and the chlorine production was 28.22 million tons, of which the chlorine used for polyvinyl chloride plastics reached about 10 million tons.

Therefore, the use of chlorine to prepare vinyl chloride is crucial to the development of the chlor-alkali industry. Instead of completely banning it, we should further study the production technology related to it to promote its development.

Part 3

Not only do it, but also find your own way to do it

At present, there are two main methods for preparing vinyl chloride, one is ethylene oxychlorination and the other is acetylene hydrochlorination.

The ethylene oxychlorination method uses high-concentration ethylene and chlorine as raw materials, and eliminates some of the hydrogen atoms by introducing oxygen and replacing them with chlorine atoms. This method is more suitable for countries and regions with more shale gas, such as the United States.

First, ethylene reacts with chlorine to generate ethylene dichloride (Formula 1), then ethylene dichloride is thermally cracked into vinyl chloride and hydrogen chloride (Formula 2), and finally ethylene, oxygen and hydrogen chloride obtain ethylene dichloride (Formula 3). The overall reaction is shown in Formula 4.

The ethylene oxychlorination process achieves complete utilization of chlorine, and the process does not involve highly toxic mercury-based catalysts.

However, in my country, the prices of ethylene and ethane are relatively high, while coal resources are relatively abundant, so we can look for other raw materials that can replace ethylene and acetylene.

In coal-rich areas, coal and calcium oxide (CaO) are sintered in electric furnaces to form calcium carbide CaC2. It is then transported to chlor-alkali enterprises and reacts with water to produce acetylene gas (Formula 5). The purified acetylene reacts with hydrogen chloride to produce vinyl chloride (Formula 6).

This makes the cost of acetylene hydrochlorination much lower than that of ethylene oxychlorination, and it also has the advantages of low investment and high output. Therefore, China's vinyl chloride monomer process route is still mainly achieved through the acetylene method.

In China, about 13 million tons of polyvinyl chloride plastics are produced by the acetylene process each year, accounting for more than 80% of the total production of polyvinyl chloride plastics.

So, having solved the cost and yield issues, does this method become “perfect”?

The answer is still no.

What is different from the preparation of caustic soda is that the trouble this time does not come from the "by-products" but from the famous "catalyst".

At present, the industrial acetylene hydrochlorination process requires the use of mercuric chloride catalysts , and tens of thousands of tons of mercury are used each year. Even with strict control, a small amount of mercury still flows into the external environment. As we all know, mercury can cause serious damage to the environment and ecology. Therefore, the development of new catalysts and new catalytic processes that can replace mercury is urgently needed.

At present, mercury-free catalysts suitable for acetylene hydrochlorination process mainly include precious metal catalysts, non-precious metal catalysts and metal-free catalysts. Precious metal catalysts mainly use gold (Au) as a catalyst, which has excellent performance and stability. An industrial demonstration project with a duration of 8000+ h has been carried out in the enterprise with stable operation. However, precious metals are expensive and the chlor-alkali industry itself has low profits, so large-scale industrial applications are limited. Typical non-precious metal catalysts, such as copper-based catalysts, still need to be further optimized in terms of stability.

Is it possible to do without metal?

The answer is yes.

Different catalyst types for acetylene hydrochlorination

(Image source: produced by the author)

Part 4

Cheap and easy-to-use catalyst, really good!

The team led by Academician Bao Xinhe and Researcher Pan Xiulian from the Dalian Institute of Chemical Physics, Chinese Academy of Sciences, has developed a new type of metal-free catalyst based on silicon carbide (SiC)-derived nitrogen-carbon material that can catalyze the hydrochlorination of acetylene to produce vinyl chloride.

This method uses chlorine-containing gas and ammonia to act on SiC at high temperature, so that the Si atoms in it generate SiCl4 gas, and the remaining C atoms react with ammonia in situ to form a nitrogen-containing carbon layer. Throughout the process, the good thermal conductivity and easy forming characteristics of SiC are cleverly utilized to support nitrogen-carbon materials.

This catalyst achieved an acetylene conversion rate of about 80% and a selectivity of 98% at a speed close to that of industrial space velocity, and its stability can last for several hundred hours.

Schematic diagram of SiC@NC material preparation

(Image source: Nat. Commun., 2014, 5, 3688.)

Based on the above research results, the team developed a new porous boron nitride material and found that it has excellent ability to catalyze acetylene hydrochlorination reaction.

Porous boron nitride materials have high specific surface area, rich pore structure (micropores and mesopores), and abundant defects and boundaries.

Surprisingly, under optimal conditions, the activity can reach 99%, the selectivity 99%, and the continuous reaction can last for about 1,000 hours! Not only that, the activity of this material is maintained for a long time, and the process of losing activity is relatively slow. The overall performance is better than that of carbon-nitrogen materials. This also provides a new opportunity for metal-free catalytic systems.

Acetylene hydrochlorination catalyzed by porous boron nitride

(Image source: Acs Catal., 2017, 7, 8572-8577.)

Although the metal-free catalysts such as the above-mentioned carbon-nitrogen materials and boron nitride materials cannot compare with precious metal catalysts in terms of yield, their catalyst materials only need to use conventional elements such as carbon, nitrogen and boron. In theory, the cost can be reduced to a very low level, thus providing a new path for the mercury-free production of acetylene hydrochlorination, and is expected to be further developed and promoted to industrial applications.

This system has also been highly recognized by domestic and foreign peers and is considered to be an innovative achievement with great potential.

Expert review in the field

(Image source: ACS Sustainable Chem. Eng. 2019, 7, 17979)

Part.5

I believe that the problems caused by technology can be solved by better technology.

In recent years, when similar accidents involving hazardous chemicals occur, they often have a relatively large impact. Whenever this happens, a question may arise in our minds: from a safety perspective, does the preparation of hazardous chemicals need to be banned?

In this regard, I would like to say that the chemical industry occupies an important position in the national economy, and our lives are inseparable from chemical products. Banning them is not a solution to the problem. Chemical production is a chain reaction, and changes in key links may even affect the development of the entire national economy. The problems brought about by technology can ultimately be solved by technology. Adopting efficient protection measures and developing and optimizing alternative strategies to achieve safer and more efficient goals while creating higher value is the direction scientists are working towards, and it is also the meaning of science and technology making life better.

Produced by: Science Popularization China

Author: Fang Guangzong and Li Xueyang (Dalian Institute of Chemical Physics, Chinese Academy of Sciences)

Producer: China Science Expo

The article only represents the author's views and does not represent the position of China Science Expo

This article was first published in China Science Expo (kepubolan)

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