Improve Ethylene Conversion: Critical Control Valves for Cracked Gas Processing

In this, the second part of a three part series, critical control valve applications for cracked gas processing will be discussed. The basic process configuration for cracked gas is similar; however, there are many variations in detailed design. Differences are found between plants processing light feedstocks (ethane, propane, butane) and those processing heavier feedstocks (naphtha, gas, oil). Processing cracked gas from liquid feedstocks becomes more complicated due to higher amounts of heavy hydrocarbons that must be removed.

Quench Tower Level Control Valve

Control Valve Considerations

• Spring and diaphragm actuator for increased stability
• Digital valve controller to ensure accurate response
• Rotary valve design providing streamlined flow passages
• Trim versatility allowing seal assembly replacement without removal of ball or actuator
• Erosion resistant trim materials

Compressor Antisurge Valve

Antisurge valves provide recycle flow to each stage of the multistage compression trains that are common to olefin production. During startup and commissioning, the valve provides throttling control to recycle a portion of the discharge flow as the compressor is brought to capacity. During the normal operation of the plant, the antisurge valve will remain closed or slightly open to allow for a small portion of the discharge to be recycled. When closed, it is important that the valve provides tight shutoff to prevent unwanted recycle flow.

The primary purpose of antisurge valves is to protect the most critical and expensive pieces of equipment in the plant, the compressors. During a surge event, the valve must respond quickly and accurately in order to recycle the discharge flow back to the suction side of the compressor. Failure of the valve to react quickly can result in severe damage to the impellers of the compressor.

Both the availability and efficiency of a plant’s compressors have a direct impact on the profitability of the facility. Cracked gas compressor downtime will result in lost production. Similarly, unexpected operational issues with any of the plant’s refrigerant loops will lead to reduced yields. Not only will damage to these assets cause lost production, it can also lead to very costly repairs.

Control Valve Considerations

• Fast stroking capability to quickly recycle flow and eliminate surge
• Digital valve controller providing specific antisurge control and tuning algorithms for accurate and stable response
• Easy-to-use tuning interface to reduce traditionally long setup times
• Noise attenuating trim for high pressure drop applications with noise requirements
• Customizable cages for precise control from startup to surge capacity
• Noise modeling per IEC 60534-8-3 to address both trim and valve body outlet noise

Rich Amine Letdown Valve

Before further processing, carbon dioxide (CO2) and hydrogen sulfide (H2S) are removed from the cracked gas. The presence of H2S can cause serious corrosion issues, and CO2 can freeze in heat exchange and fractionation equipment. Acid gas removal is accomplished by scrubbing with sodium hydroxide on a once-through basis or in combination with a regenerative solvent (amine). As cracked gas enters the bottom of the acid gas scrubber and flows upward, lean amine solution flowing countercurrent strips the gas of impurities.

The rich amine letdown valve serves two purposes. First, it regulates the level of rich amine solution that accumulates in the bottom of the contactor vessel. Second, it facilitates a pressure drop into the downstream flash tank, which liberates a portion of the acid gases entrained in the solution. If not addressed properly through detailed valve sizing and selection, this outgassing of the entrained gases can cause significant vibration and damage to the valve.

Control Valve Considerations

• Spring and diaphragm actuator for stability and shallow casing design to ensure quick response
• Digital valve controller with predictive diagnostics for detection of performance changes and allow for scheduled maintenance
• Valve sizing accounting for both liquid and vapor phases to ensure proper valve selection
• Trim options accounting for high pressure drops and outgassing services providing long life
• Corrosion resistant material offerings

Amine Pump Recirculation Valve

Maintaining the proper flow of lean amine to the acid gas absorber is necessary to ensure sufficient removal of carbon dioxide (CO2) and hydrogen sulfide (H2S) from the cracked gas stream. The lean amine pump ensures the stable flow of amine to the absorber.

The amine pump recirculation valve is most commonly used to facilitate startup and commissioning as the acid gas absorber is brought to capacity. This valve controls the pump discharge flow that is routed back to the suction side of the pump. When needed, the recycle flow increases the suction pressure to keep it above the vapor pressure of the amine. As a result, the amine pump recirculation valve must be responsive in order to protect the pump from cavitation damage.

Due to the high pressure differential from discharge back to suction, the valve trim must be capable of mitigating the potentially damaging effects of cavitation as it recycles flow. Any unplanned maintenance on these valves due to cavitation damage can bring the amine pumps down and reduce plant throughput.

Control Valve Considerations

• Spring and diaphragm actuator with appropriately sized accessories for accurate and rapid response to setpoint changes
• Accurately sized cavitation protection to effectively control or eliminate cavitation and potential valve damage
• Customizable trim options for a wide range of operating conditions
• Hardened trim material to provide wear resistance and long life

Gas-to-Flare Valve

Olefin production facilities have a flare system to safeguard against overpressure of critical assets within the plant and to dispose of any waste gas. Failure of the flare system to successfully relieve pressure from the process can lead to unexpected downtime or damage to costly pressure-retaining equipment.

Gas-to-flare valves are installed at numerous locations throughout the gas treatment and recovery section. They are used to control the flow of cracked gas or separated hydrocarbons to the flare stack for disposal. They are primarily used during plant startup, shutdown, or short-duration upset conditions. During these periods, flare valves will experience significant pressure differentials and high flow rates. If not addressed properly, these conditions can lead to excessive noise levels and even damaging vibration.

During the normal operation of the plant, gas-to-flare valves will remain closed. Because of this, it is important that these valves maintain tight, long-term shutoff in order to prevent loss of valuable product to the flare stack.

Control Valve Considerations

• Appropriately sized actuator and accessories for accurate and quick response
• Digital valve controller with advanced diagnostics to provide valve assembly health evaluation without shutting down the process
• Noise abatement trim to reduce effects of noise and vibration
• Trim designs capable of tight shutoff to reduce product loss to flare

Dryer Switching Valve

The valves responsible for switching dryer beds are exposed to high cycles between repairs requiring high reliability. Poorly performing switching valves can create bed disturbance and damage adsorption beads. Poor response to setpoint and overshoot are contributors to reduced throughput. Switching valve design should incorporate high-cycle and setpoint requirements to ensure high efficiency.

Control Valve Considerations

• High cycle capability for both valve and actuator combinations
• Low friction bearing and seal offerings to reduce friction and increase service life
• Tight shutoff capabilities to reduce leakage and increase dryer throughput
• Low emission packing for environmental compliance

Cracked gas processing must function successfully to achieve high plant efficiency. Control valves are a significant contributor to this success. A poorly operating control valve in these applications will allow impurities to pass downstream creating unnecessary rework in the fractionation section. In the final portion of the three part series, critical valve applications in the fractionation section will be reviewed.

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