Improve Ethylene Conversion: Critical Control Valves for Fractionation

In this, the final part of our three-part series on improving ethylene conversion, critical control valve application for fractionation processing will be discussed. Currently, there are three processing methods that have become commercially important. These processes differ in the sequencing of fractionation and hydrotreating steps but provide similar capabilities. The three processing routes include:

• Demethanizer first with tail-end hydrogenation
• Deethanizer first with front-end hydrogenation
• Depropanizer first with front-end hydrogenation

Regardless of the processing route utilized, several critical valve applications are associated with each distillation column in the fractionation train.

Cracked Gas Chilling

Before being fed to any of the distillation columns, cracked gas is cooled and partially condensed. Cracked gas is cooled by the refrigerant system and low-temperature recycle streams. Refrigeration in ethylene plants is essential and costly. Typically, ethylene and propene refrigeration systems are utilized.

Refrigeration loops require compression, and as a result, have associated antisurge protection systems. Antisurge valves provide recycle flow in order to protect the compressors. Failure of the valve to react quickly can result in severe damage to the impellers of the compressor.

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
• Noise modeling per IEC 60534-8-3 to address both trim and valve body outlet noise
• Customizable cages for precise control from startup to surge capacity

Column Operation

Distillation columns contain large amounts of mass and energy that must be balanced for high efficiency. Reboiler heat applied to the bottom of the column is balanced by cooling occurring near the top of the column in the overhead condenser. Control valves play an important role in stabilizing column operation. A single underperforming valve can impact overall column operation.

Feed Valve

This valve controls the feed going into the distillation column. A problem valve will often cause the feed flow to oscillate. As a result, the column will alternate between too little and too much reboil heat. Depending on the size and number of trays in the column, the effect of a swing in the feed will take anywhere from several minutes to more than an hour to reach the ends of the column. Sometimes, the reboil and reflux controls will amplify the swings. As a result, it is difficult to meet product purity targets.

Control Valve Considerations

• Spring and diaphragm actuator for stability
• Sliding stem valve for lower flow rates and precise throttling capabilities
• Customizable trim options for a wide range of operating conditions
• Hardened trim material to provide wear resistance and long life
• Digital valve controller to ensure accurate response

Reflux Valve

The reflux valve is typically either a flow or column temperature control loop. It is used to adjust the purity of the overhead product. The higher the reflux rate, the purer the overhead product will become. However, raising the reflux rate also will require more reboil heat and raise energy costs.

A poorly operating reflux valve will have the same effects as a bad feed valve. Product purities will oscillate and the column will be difficult to balance. This valve has direct impact on the efficiency of the column.

Control Valve Considerations

• Sliding stem design with rugged guiding to reduce vibration and provide stability
• Customizable trim options for a wide range of operating conditions
• Digital valve controller to provide accurate response and diagnostics to detect changes in valve assembly performance
• Low emission packing for environmental compliance

Pressure Control Valves

The pressure control valves are used to control column pressure. Higher column pressures will yield better product purities but require more energy to operate. Normal operating procedure is to minimize the pressure to lower energy costs while maintaining product specifications. A low limit exists because lower pressures reduce the amount of vapor/liquid traffic the column can handle and can make it more likely to flood.

Sizing of this valve is critical. If the valve is too large, a small valve movement will cause a large pressure swing. If the valve is too small, the pressure response will be very sluggish. It is likely that a valve that is too small will operate from completely closed to completely open. In either scenario, oscillating column pressure and reduced column control result. A sticking pressure control valve presents the same problem. A sticking valve is a common concern on vent gas service because the valve packing is normally tight to prevent fugitive emissions. Many columns use tray temperature to control overhead composition, thus stable pressure is required to ensure that temperature changes reflect composition changes, not pressure changes.

Control Valve Considerations

• Segmented ball valve for higher flow rates and wide rangeability
• Spring and diaphragm actuator on a splined shaft to reduce lost motion
• Digital valve controller with advanced diagnostics to provide valve assembly health evaluation without shutting down the process
• Low emission packing for environmental compliance

Reboil Valve

The reboil valve controls the amount of heat put into the column by the reboiler. In many cases, steam is used as a heat source. The service is very clean, and fugitive emissions are not a concern. Steam valves are usually very reliable. However, a problematic valve will make the column difficult to control precisely. This will be especially true if the column feed is subject to frequent changes.

Not all reboilers use steam as a heat source. To save energy, some plants have integrated their units so that higher-temperature process streams are used to provide heat for lower-temperature processes. In these cases, the reboil valve will foul more easily and might create fugitive emission concerns.

This valve is important because it drives the vapor back up through the column. Vapor through the column affects column efficiency. Reboiler control will have a direct effect on overhead reflux flow.

Control Valve Considerations

• Spring and diaphragm actuator with shallow casing design to ensure quick response
• Sliding stem designs for precise throttling capability with guiding design depending on heating fluid properties
• Trim materials with hardfacing for improved wear resistance
• Digital valve controller to ensure fast and accurate response

Conclusion

Fractionation operation is a constant balancing act. Each loop needs to be evaluated for specific needs. Efficiency and purity will suffer if control valve application needs are not properly addressed. Control valve performance is a critical piece to product recovery and to ensure overall product quality and column efficiency.