Ethylene unit design depends on the type of cracking feedstock. Ethylene plants generally fall into two categories, gas and liquid feeds. Ethane crackers are typically simpler designs, and consume less energy compared to naphta crackers. The products produced in the steam cracker largely depend on the composition of the feed, the hydrocarbon- to-steam ratio and the cracking temperature.
The role of steam
The dilution steam is injected into the furnace tubes to protect the pipes from fouling or coking formation. It works like a protection layer inside the coil. This is why the process is called steam cracking. The HC-to-steam ratio should be optimized to get the optimal flow condition. The flow rate of the steam depends on the severity of the feed stock. Steam can also be used for decoking the furnace so wide rangeability of the valve controlling the steam can also be required.
Metso Segment control valve.
Finetrol™ eccentric rotary plug valve provide a good solution for general dilution steam control with noise reduction Q-trim if needed, while RE-series V-port segment valve for high capacity applications and extremely high rangeability needs. Accurate control performance provided by Metso control valves ensures optimum product quality and yield with no additional energy requirements. Valve plays a significant role in control loop, especially when modern crackers are used and steam control needs to be optimized. A poor performance of the steam supply valve can lead to excessive fouling of the cracker. Better valve performance means increased profit to the cracker.
Decoking the furnaces
Since the steam cracking reaction occurs under high temperature, it also leads to the deposition of coke, a form of carbon, on the furnace tubes. This degrades the efficiency of the furnace, thus regular decoking is required for steam cracker furnaces, typically every 3-4 months/furnace for naphta crackers depending on the type of feed and furnace severity. Normally there is several furnaces in parallel in one steam cracking plant, while one furnace is under decoking others are still producing.
Metso Mapabloc™ installed at steam cracker furnace transfer line.
The decoking is typically done by steam/air decoking. When end of run conditions are reached, the hydrocarbon feed is discontinued, the furnace isolated, firing rate in burners adjusted, and controlled amounts of dilution steam and decoke air admitted. Decoking effluent is routed to decoke drums, where the coke fines are separated from the decoking effluent gases. On-line steam/ air decoking will allow the furnace and quench exchanger system to operate through several successive cycles. The transfer line valve (TLV) and decoking valve (DCV) have to withstand high piping forces due to high temperature variations. Downstream process must be protected from fire, high temperature steam and coke to ensure the plant operability and safety, therefore 100 % valve tightness is essential. Is it possible for such high demanding application valves like transfer line valve (TLV) / decoking valve (DCV) to work well over 10 years? What about the maintenance needs and value-adding services?
Another important application in steam cracking furnace is fuel gas control where accurate, reliable control valves play a significant role in reducing operating costs. Proper combustion maximizes heat transfer, which minimizes fuel gas consumption and related costs. Variations in fuel gas composition and different operating conditions at start up, normal operation and shut down mean that such valves must control various loads, which requires wide rangeability from the valve. This is typically solved by using a split range configuration with globe valves. Another method is to use rotary control valves with wide rangeability as high as 150:1, such as Neles® V-port segment valves. In this way, the wide rangeability allows accurate control with both small flows and large valve openings with a single valve solution instead of two globe valves.
In the steam cracking furnace, burners supply heat energy to thermal crack the hydrocarbon through burning the fuel. The burner fuel gas system valves consist of isolation ESD-valves, burner control valves and several burner shut-off valves. ESD valve automatically shuts off the supply of fuel when de-energized by a combustion safety control, safety limit control, or loss of actuating medium. It is required the use of one, two or three valves (redundancy). Gas flows into the burner through a double series of shut-off valves that have a vent (ESV) between them. The vent is typically about half size of the main line and is used to prevent pressure build up and flow through the second isolation valve when the system is isolated. Terminology often used for this is double isolation and bleed. Type approvals are becoming more and more typical requirement by the local authorities in different countries. Metso’s ESD and burner valves such as Jamesbury® ball valves provide a reliable solution with fire safe design, bubble tight long term tightness, low fugitive emissons and highest safety with 3rd party certified solutions, such as TÜV certified partial stroke safety device ValvGuard™ designed for ESD-applications and gas burner valve type approvals according to EN161/264.
Reliable valves play an important role in the whole ethylene production process availability, safety and environmentally sustainable production.
Oil & Gas Application Manager, Metso Flow Control