Acetylene, Coke, and calcium carbide are no longer part of the vocabulary of chemists and chemical engineers working at most modern industrial sites. But in China, these materials, dating back to the early days of the chemical industry, are still widely used to manufacture polyvinyl chloride.
Making an anachronism contemporary, Chinese firms are building new, larger scale facilities to produce vinyl from calcium carbide, a process that most of the world replaced with a petrochemical one decades ago. The method has been widely faulted for using too much energy and creating too much waste, but Chinese firms are confident they will be able to modernize it.
China has valid economic reasons for sticking with the calcium carbide route to PVC, industry observers say. The country is endowed with the vast coal and lime resources necessary for the production of calcium carbide. In contrast, it does not have the abundant supply of ethylene required for making PVC via the petrochemical route that the rest of the world uses.
China is a major producer and consumer of PVC. According to figures C&EN obtained from a major international PVC producer, the country consumed nearly 10 million metric tons of PVC in 2009. That year, it had a production capacity of 19 million metric tons, about 80% of which was based on the carbide route. Chemical Market Associates Inc. (CMAI), a market research firm, estimates that China represents one-third of the world’s PVC capacity.
“If you look at the expansion of facilities in China in the past four to five years, it’s been mostly plants featuring the carbide route,” says Eddie Kok, Asian director for chlor-alkali and vinyls at CMAI. “There’s little ethylene from the Chinese crackers available for making PVC because it’s often allocated to more profitable products.”
PVC, one of the world’s most widely used plastics, can basically be made in two ways. The petrochemical route involves the chlorination of ethylene to yield ethylene dichloride, which is then cracked to generate vinyl chloride. Vinyl chloride is polymerized into PVC. Implementing this method in an economically viable fashion typically requires that a chlorine plant be set up in the vicinity of an ethylene cracker.
The calcium carbide route used in China involves heating lime and coal-derived coke in an electric furnace at a temperature of 2,000 °C to obtain calcium carbide. Acetylene is generated by the hydrolysis of this calcium carbide. This early part of the process is labor intensive, requires a lot of energy, and generates vast quantities of a watery calcium hydroxide slag. With the use of a catalyst that is usually based on mercuric chloride, the acetylene is then reacted with anhydrous hydrogen chloride to produce vinyl chloride.
Both methods for making PVC were invented in the early-20th century. Calcium carbide even dominated until the 1960s, when ethylene became the preferred starting material in most of the world.
The calcium carbide route has a number of flaws that explain why it has mostly been replaced. Yet because China is such a big business partner, chemical industry executives refuse to be quoted speaking critically of the practice.
An executive at a major producer of PVC, who requested anonymity because China is a big market for his company, tells C&EN that PVC made by the carbide route is of inferior quality. “There are a lot of impurities,” he says. “It can be used in construction materials, such as in pipes, but you can’t make film from it.” Others say the poor quality is the result of inferior polymerization techniques and is unrelated to the carbide process.
Calcium carbide production requires huge amounts of power, the executive further explains, but that issue has been largely overcome in China. “They can make cheap electricity with all the coal they have,” he says. Indeed, most of the PVC plants in China that employ the calcium carbide route are located near coal mines, and some house large on-site generators.
But by far the biggest flaw in the carbide route is its negative environmental impact. First of all, there is all the calcium hydroxide slag to deal with. “It’s the biggest problem,” the PVC executive says. His Chinese competitors, he says, endeavor to use the slag in cement production and are encouraged to do so by government policies. But the amount of slag yielded in the process is huge—as much as the calcium carbide generated—and it’s unlikely that all of it can be turned into cement.
An additional environmental issue with the calcium carbide route is the use of mercuric chloride catalysts to turn acetylene into vinyl. There is much concern worldwide over the release of mercury compounds into the environment. Representatives of environmental protection bureaus from several countries have met repeatedly with Chinese government officials and executives at Chinese vinyl-producing firms in recent years to discuss their concerns.
One official at a non-Chinese environmental protection agency, who asked not to be named, tells C&EN that “the Chinese vinyl industry may well be doing a good job of recovering the residue of mercuric chloride remaining within the depleted catalyst.”
The environmental official estimates that mercuric chloride represents up to 10% of the fresh catalyst by weight; the rest of the catalyst is mainly carbon. Inside a reactor vessel, he explains, heat causes a gradual vaporization of mercuric chloride. Some of the compound goes into the raw vinyl, from which it must be removed along with other impurities. After a period of roughly six months, the official says, the catalyst becomes depleted of mercuric chloride and must be replaced.
The responsibility for recovering depleted catalyst lies with catalyst suppliers, according to Kok. “The producers say that whoever supplied the catalyst has to take it back,” he says. Regulatory enforcement of this aspect of PVC production in China is in the process of being strengthened, he believes. None of the four Chinese PVC producers contacted by C&EN responded to requests for comment.
When they appear at events where the fate of mercury is discussed, top managers in the Chinese vinyl industry acknowledge that even if spent mercury catalyst is properly recovered, it would be better not to use it in the first place. Speaking at a conference two years ago, Li Jun, a representative from the China Chlor-Alkali Industry Association, noted the existence of research on nonmercury catalysts. The British catalyst maker Johnson Matthey has started commercialization of such catalysts in tandem with the engineering firm Akers Solutions. Neither firm responded to C&EN’s requests for comment before press time.
Between 2009 and 2013, Chinese producers of PVC will boost their capacity 20%, to 24 million metric tons per year, mostly by building more plants that will employ the carbide route, according to figures C&EN obtained from a major international resin manufacturer. Kok says the new plants will likely be able to hold their own against foreign producers in terms of price.
“It’s the price of ethylene that really determines whether calcium carbide-route PVC is competitive,” he says. “As long as oil prices are high, ethylene prices are high—and the carbide route has a position in the industry.”
Even if many engineers consider it an oddity, Kok says, China’s devotion to PVC via the calcium carbide route is here to stay. “They’re building new plants,” he points out. “I don’t see it going away for at least 20 years.”