Ethylene is a basic building block of the chemical industry, and it is the link between chemical companies and petroleum refiners. An ethylene plant, also well known as a steam cracker, is often and correctly called an olefin plant because of the fact that the end products are mainly olefins.
The feedstock of a steam cracker could range from ethane and naptha to vacuum gas oil, etc. In recent years the shale gas development in North America has led many petrochemical companies to increase their capacity to convert shale gas into petrochemicals, thus many steam cracking plants are under expansions or construction in order to utilize the low price, lighter raw materials that come from shale gas, especially ethane, to produce ethylene.
U.S. ethane pricing versus naphtha pricing. Courtesy of Nexant prospectus.
Nowadays such ethane-based North American ethylene producers are much more cost competitive than Northeast Asian and West European naphtha-based ethylene producers and have a cost benefit position even approaching Mid-East ethylene producers. Such enhanced competitiveness could be seen in downstream chemical export trends of the United States, where the country has gone from a major importer of ethylene based polymer products to an exporter today.
As already mentioned, nowadays many ethylene producers have already started to utilize lighter raw materials which come from shale gas, and this has influence on other type of petrochemical process as well. Actually such shift to lighter steam cracker feedstocks with relatively lower propylene yields has created an imbalance of supply and demand for propylene, thus on-purpose production methods such as propane dehydrogenation is increasing significantly these years, especially in the United States and China.
If you want to know more about propane dehydrogenation, you can read further in series of three postings that explain the propane dehydrogenation market, process and valves here.
What does ‘cracking’ mean?
In steam cracker, the initial process of course is cracking, which means that the feedstock is heated to the point that the energy transfer from heat is enough to break carbon-carbon or carbon-hydrogen chemical bonds within hydrocarbon molecules. This creates other molecules, some of which, like ethylene or propylene, are the ones desired. The ethylene steam cracker raises the temperature of the feed stock to around 800 °C, and adds steam to reduce coking and improve the yield of end product. The cracking is done during a short residence time and under a high temperature, followed by a sudden quench to stop the reaction in a transfer line heat exchanger or inside a quenching header using quench oil. The cracking is followed by ethylene gas compression, caustic washing, drying and product fractionation, that includes cold distillation (up to -100 °C) and other processes for separating recoverable products.
Ethylene cracking furnace, Courtesy of Jilin Petrochemical plant, China.
The steam cracking process has high yields with high temperature in the cracking furnaces, and typically there is several furnaces in parallel in one steam cracking plant. The ethylene industry is continuously aware of furnace-related safety incidents as a result of equipment failures, design problems and operator errors. A key safety system is the high-quality burner control/start-up instrumentation scheme to avoid startup ignition risks. Steam cracking furnace valves play an important role in ensuring proper ethylene process performance. Reliable and accurate valve performance is vital during startup, normal production and frequent furnace decoking operations to ensure process productivity and safety.
In the next posting, we’ll discuss more about steam cracking furnace and what role does ‘steam’ play in a steam cracker and how to add value into decoking and fuel gas system with valves.
Oil & Gas Application Manager, Metso Flow Control