The feedstock of a steam cracker can range from ethane, naphtha to vacuum gas oil, and more. Natural gas liquids (NGL) distilled from shale gas can also be used as a steam cracker’s feedstock. In recent years, the shale gas development in North America has led many petrochemical companies to increase their capacity to convert shale gas into hydrocarbons. Thus, many steam cracking plants are planning expansions or are under construction to utilize low-price, lighter raw materials which come from shale gas, especially ethane, to produce ethylene.
The initial process is cracking, which means that the feedstock is heated to the point that the energy transfer from heat is enough to “crack” the molecules into several smaller molecules. The feedstock is diverted to multiple furnaces and then further divided between multiple passes before entering the furnace. In the convection section, the feedstock is mixed with dilution steam to reduce coking and improve the end-product yield. This mixture is preheated using high-pressure steam before enteringthe radiant section. The radiant section has several burners which raise the temperature of the feed to 750–850 °C (1380–1560 °F), ensuring a high enough temperature for cracking to occur. It is important that the residence time is kept short to prevent further reactions from occurring, which would decrease desired product yield.
After exiting the furnace, the cracked gases quickly enter the transfer line exchanger, where they are indirectly cooled to prevent further reactions from occurring. Hereafter, the cooled gases are quenched with oil and/or water. This is followed by a series of compressors, dryers and product fractionation, which includes distillation and other processes for separating recoverable products.
This series of three posts discusses how flow control solutions can improve process control, reduce downtime and increase efficiency by:
- Field-proven, long-term tightness for safe and reliable furnace operation
- Savings in piping and valve costs with compact and lightweight valve solutions
- Meeting noise, emission and fire safety regulations set by local authorities
- Improving cracking furnace efficiency
- Eliminating the possibility of valves getting stuck due to piping forces, temperature variations or coke accumulation
The text has been updated in April 2022, due to the company name change to Valmet.
Part 2: Control valves for feedstock, dilution steam and fuel to furnaces
Written by Sari Aronen
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