Improve Ethylene Conversion Part One: The Furnace

Feedstocks into the pyrolysis furnace can be ethane, propane, butane, gas oil, or naphtha, but no matter the source, the feed rate must be balanced with dilution steam to lower hydrocarbon partial pressure and increase olefin yield. An improper ratio reduces efficiency of the furnace and will impact ethylene conversion. Valves in the furnace section play a critical role in maximizing ethylene production and throughout.

Hydrocarbon Feed Control Valve

A number of valves in parallel are commonly used to control the flow of the feedstock into the furnace. Reliability is the foremost consideration of inlet feed valves. Unexpected maintenance or surprise failures can stop production. In addition, these valves must be able to provide precise, stable control through a wide range of flow rates from startup and commissioning through full rated output of the plant.

These valves must operate with minimal variability to help ensure stable and predictable performance of all process units downstream. Depending on the operating pressure of the pipeline, these valves can also experience a significant pressure drop. If not addressed properly, damaging noise and vibration may occur.

Control Valve Considerations

Spring and diaphragm actuator for increased stability
Digital valve controller to ensure accurate response
Sliding stem design for precise control
Noise reducing trim to minimize aerodynamic noise
Hardened trim material to provide wear resistance and long life

Dilution Steam Ratio Control Valve

The steam dilution ratio control valve maintains proper balance of steam supplied with hydrocarbon feed. An improper feed-to-steam ratio reduces efficiency of the cracker and can result in the need for additional decoking cycles, therefore reducing furnace uptime. With the feed-to-steam ratio directly impacting olefin yields, it is important to maintain precise control of that ratio.

Steam is added to the feed stock to lower hydrocarbon partial pressure and increase olefin yield. The quantity of steam used varies with feedstock, cracking severity, and design of cracking coils. Steam also assists to reduce coking deposits by reacting with coke to form carbon dioxide (CO2), carbon monoxide (CO), and hydrogen (H2).

Control Valve Considerations

Spring and diaphragm actuator for stability and shallow casing design to ensure quick response
Digital valve controller to provide accurate response and diagnostics to detect changes in valve assembly performance
Sliding stem design for precise control with interchangeable trim for simplified maintenance
Customizable trim options to meet a wide range of operating conditions

Burner Fuel Control Valve

Accurate and precise temperature is needed to maintain optimum furnace performance and prevents undesirable side reactions. Temperature profiles applied along the cracking coil are designed to avoid long residence times at low temperatures. Low temperatures favor reactions involved in the formation of secondary products, thereby reducing olefin yields.

Control Valve Considerations

High temperature options for valve, actuator, and accessories with exposure to high ambient temperatures
Low emission and low friction packing to minimize stem leakage and provide responsive and stable control
Sliding stem design with customizable and interchangeable trim options to provide accurate throttling through various process conditions
Cage style trim options allowing inspection of parts without removal of the valve body from the pipe

Boiler Feedwater Regulator and Startup Valve

From steam dilution to heat exchange, steam is an essential part of olefin production. The feedwater regulator control valve provides flow to the boiler during normal plant operation when the boiler is under pressure. During this time, the pressure drops are small and cavitation is not a concern. Stable, reliable throttling is most important for efficient boiler operation.

The feedwater startup and regulator applications are often combined into one valve. Combined startup and regulation can eliminate cross-over points and ease operation. During startup, the feedwater regulator control valve experiences low flow rates with high differential pressure, which can cause severe cavitation damage.

Control Valve Considerations

Spring and diaphragm actuator with molded diaphragm to provide consistent linearity between loading pressure and travel
Digital valve controller with advanced diagnostics to provide valve assembly health evaluation without shutting down the process
Accurately sized cavitation protection to effectively control or eliminate cavitation and potential valve damage

Boiler Feedwater Pump Recirculation Valve

The boiler feedwater pump gets its feed from the deaerator at low pressure, and discharges at high pressure above the main steam pressure. During boiler startup of low load conditions, flow may not meet the minimum requirements of the boiler feedpump.

The boiler feedwater pump recirculation valve protects the feedpump by ensuring that adequate flow is passing through the pump at all times. A modulating boiler feedwater pump recirculation valve provides an efficient method to prevent cavitation from occurring in the pump. The high pressure drops experienced by the recirculation valve can cause severe cavitation that must be accounted for to ensure long valve trim life. Cavitation abatement trim allows for protection over a range of conditions from boiler startup to full load.

Control Valve Considerations

Spring and diaphragm actuator with accessory options to provide quick response
Digital valve controller with predictive diagnostics for detection of performance changes and allow for scheduled maintenance
Accurately sized cavitation protection to effectively control or eliminate cavitation and potential valve damage
Dirty service trim options for applications with potential for entrained particulate

Steam Vent Valve

Boiler startup requires a gradual temperature and pressure increase in the steam distribution system in order to minimize stress on equipment. A steam vent is useful to vent low temperature and pressure steam containing a large amount of moisture. This allows for a gradual warm up of the steam system. In the event of a pressure surge, the vent system prevents overpressure of critical assets.

Vent valve applications have potential for noise and vibration due to the high flow and pressure drops experienced. Noise attenuating trim can reduce noise and vibration to acceptable levels. A vent diffuser can also be used for additional noise abatement and allow the valve to operate at a lower pressure drop ratio. Valve shutoff is important as any leakage results in wasted energy.

Control Valve Considerations

Noise abatement trim to reduce effects of noise and vibration
Trim materials engineered for high temperature service
Sealing technologies to provide tight shutoff preventing valuable leakage
Accessories to facilitate fast stroking requirements

Control valves related to furnace operation have a significant impact on overall plant performance. Therefore, certain considerations should be made when selecting control valve assemblies. After passing through the furnace, feedstock is converted to a cracked gas and must be further processed. Cracked gas treatment and associated critical valve applications will be discussed in the second portion of this three part series.

Blake Coleman is chemical industry sales engineer at Emerson Process Management. Contact him at Blake.Coleman@Emerson.com