Industrial Chemistry & Engineering
Beyond the Ethylene Steam Cracker
by Jeffrey S. Plotkin
October 17, 2016
Ethylene is the most-produced petrochemical building block. The global demand for ethylene in 2015 is estimated at 143 million tonnes.
Ethylene is the starting point for four very mature end products: polyethylene (three types: LDPE, LLDPE, and HDPE), ethylene oxide, ethylene dichloride (the precursor to vinyl chloride monomer), and ethylbenzene (the precursor to styrene). Smaller-volume, more specialized products include linear α-olefins, vinyl acetate monomer, and synthetic ethanol. The demand profile for the major end products is shown in Figure 1.
By far the predominant manufacturing route to ethylene is steam cracking gaseous feedstocks (ethane, propane, or butane) or liquid feedstocks (naphtha or gas oil). This noncatalytic cracking process is run at very high temperatures, up to 850 ºC. Ethylene is the intended product; but other valuable building-block molecules, such as propylene, butadiene, and benzene, are coproduced.
The yield of each coproduct is mostly a function of the feedstock used. Cracking ethane gives almost no coproducts; but cracking naphtha provides substantial amounts of propylene, butadiene, and benzene. Globally, steam cracking is the most important source of propylene and butadiene and the second leading source of benzene. (Catalytic reforming used in gasoline production is the primary source of benzene.) A simplified schematic of the steam cracking process is shown in Figure 2.
From a technical perspective, steam cracking is a mature process. Much of the technology developers’ current emphasis is to make steam crackers increasingly larger to realize economy of scale. Currently, the largest steam crackers have an annual ethylene capacity of 1.5 million tonnes.
Despite the process’ maturity, tweaks to steam cracking are being developed and practiced commercially. For example, in 2014 ExxonMobil announced that it was directly cracking crude oil in its Singapore-based steam cracker. Saudi Aramco and SABIC have reported that they too are working on this technique.
Ethylene from methanol
Perhaps more exciting is the recent commercialization of an entirely new way to make ethylene: methanol-to-olefins (MTO). In the mid-1980s, the first indication that methanol could be converted to olefins appeared in a process operated by Mobil in New Zealand that was called methanol-to-gasoline (MTG). Mobil’s researchers observed that in addition to the desired liquid hydrocarbons, relatively small amounts of ethylene and propylene were coproduced.
In the mid-1990s, UOP was the first company to recognize this method as a potential way to make olefins. It was not until 2010, however, that the first MTO plant came on stream commercially—but in China. This plant does not use UOP technology; it uses a similar process developed independently by Chinese R&D institutions.
In the years since, seven Chinese MTO units have been started up. Many more were in the planning stages, but the recent uncertainty in oil pricing put a temporary halt to this commercialization effort.
A different way to make ethylene, which while not new has gained momentum recently, is cracking fermentation-based ethanol to ethylene (see Figure 3).
The reason for the recent interest in this old route is the recognition that the ethylene produced can be viewed as “green” because it is made from a renewable resource. The Brazilian company Braskem built a commercial-scale plant (200,000 tonnes per year) to polymerize this ethylene to polyethylene, which it is marketing as green. This allows polyethylene-based packaging to be labeled “environmentally friendly”.
Another use of green ethylene is in the production of green monoethylene glycol (MEG), one of the monomers used to make PET plastics. Coca-Cola sells its Dasani brand water in PET bottles made with green MEG; it promotes it as the “Plant Bottle”.
The future of ethylene?
In addition to these commercial alternatives to steam cracking, newer routes are under development. A review of the patent literature over just the past 3 months turned up several developments:
- Siluria Technologies (San Francisco) and SABIC (Riyadh, Saudi Arabia) are developing catalysts and processes for oxidatively coupling methane to make ethylene (US Pat. Appl. 20160272557, Sept 22, 2016, and US Pat. Appl. 20160237003, Aug 18, 2016).
- Shell Oil (Houston) and a consortium of the Mexican Institute of Petroleum (Mexico City), Pemex Petrochemicals (Veracruz, Mexico), and the University of Valencia (Spain) are working on processes for oxidatively dehydrogenating ethane to ethylene (US Pat. Appl. 20160237005, Aug 18, 2016, and US Patent 9,409,156, Aug 9, 2016).
- Northwestern University (Evanston, IL) is working to develop palladium sulfide catalysts that can couple methane to form ethylene nonoxidatively (US Patent 9,416,070, Aug 16, 2016).
Cheaper ethylene (but there’s a hitch)
Thus, work continues on finding ways to make ethylene production more energy-efficient or to allow the use of less costly feedstocks such as natural gas. If these non–steam cracking alternatives proliferate, they will have important ramifications, not only for ethylene production economics, but also for the supply of the important building-block molecules propylene, butadiene, and benzene, which typically are coproduced in steam crackers.