Increasing Ethylene Conversion and Plant Reliability

A look at critical control points and effective use of control valves

OLEFIN production plants are unique in the fact that they can accommodate a variety of feedstocks to produce ethylene, propylene, and butadiene. Yields of these three main products will vary depending on the feedstock used. An olefins plant consists of three main areas: a hot section, where the reaction takes place; a compressor section, that provides the necessary pressure to move feedstock through the plant and provide refrigeration; and the recovery section, where product is distilled at low cryogenic temperatures can be found. Regardless of feedstock or technology used, there are several key control valve points that are an essential part of plant operations.

Furnace critical control points

Furnace performance is key to achieving high yields and controlling energy costs. Furnaces tend to foul, leaving a layer of coke on the internals of the cracking tubes. This results in reduced throughput or significant downtime while the furnace is cleaned. Feedstock flow control valves and steam feed valves are critical control points that have a direct impact on furnace performance.

The feed rate must be balanced with dilution steam to lower hydrocarbon partial pressure and increase olefin yield. Several valves are commonly used to control the flow of 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.

Steam use 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). 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.

Hydrocarbon feed and steam dilution control valve considerations

• spring-and-diaphragm actuators increase stability
• digital valve controllers with online diagnostics monitor real-time performance changes
• sliding-stem body designs offer precise control
• noise-reducing trims minimise aerodynamic noise
• customisable trim options meet a wide range of operating conditions
• hardened trim materials provide wear resistance and long life

Burner fuel, boiler feedwater, boiler feedwater pump recirculation, and steam vent valves are additional critical control points impacting furnace operation and overall plant performance. Therefore, careful considerations should be made when selecting control valve assemblies in association with the furnace and hot section of the plant.

Cracked gas critical control points

Maximum ethylene production requires several conditions: a highly-saturated feedstock, high coil outlet temperature, low hydrocarbon partial pressure, short residence time in the radiant coil, and rapid quenching of the cracked gases. Out of these, cracked gas quenching is the only condition occurring downstream of the furnace and is one of several steps that must occur before hydrocarbon fractionation occurs.

Cracked gas leaving the furnace must be cooled immediately to preserve the composition and avoid formation of undesirable products in secondary reactions. The quench tower level control valve provides sufficient contact time between cracked gas and quenching medium. Depending upon the feedstock, water or oil is used as the quenching medium, with oil commonly used for heavier feedstocks.

Control valve considerations:

• spring-and-diaphragm actuators increase stability
• rotary valve designs provide streamlined flow passages
• trim versatility allows seal replacement without removal of ball or actuator
• trim materials should resist erosion

 An ethylene plant’s most critical pieces of equipment are the cracked gas compressor and turbine. If the compressor or turbine trips due to excess vibration, malfunctioning instruments or surge, it can bring the entire operation to a halt for multiple days. Since this valuable turbomachinery doesn’t have a backup, it must run reliably for 2–7 years so operators can efficiently and safely maximise production to meet market demands.

Compressors must be protected against surge, when gas flow is reversed in the unit. Anti-surge control valves recycle gas from the outlet to the inlet of the compressor to ensure a high enough rate of flow. These valves must respond quickly and accurately to prevent severe damage to 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 millions of dollars in lost revenue. 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.

 Anti-surge control valve considerations:

• fast stroking capabilities will quickly recycle flow and eliminate surge
• digital valve controllers provide specific anti-surge control, minimal overshoot, and accurate and stable response
• easy-to-use tuning interfaces can reduce traditionally long setup times
• noise-attenuating trims are good for high pressure drop applications with noise requirements
• customisable cages aid precise control from startup to surge capacity
• noise modeling per IEC 60534-8-3 will address both trim and valve body outlet noise

 Additional critical control points in cracked gas treatment include: rich amine letdown, amine pump recirculation, gas-to-flare, and dryer switching. 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 the applications above will allow impurities to pass downstream creating unnecessary rework in the fractionation section.

Recovery section critical control points

The fractionation process receives the compressed cracked gas for separation into various products at specified quality levels. Over the years, numerous methods have been developed for cracked gas separation. 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:

• demethaniser first with tail-end hydrogenation
• deethaniser first with front-end hydrogenation
• depropaniser first with front-end hydrogenation

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

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 are an important piece for stable column operation. A single underperforming valve can impact overall column operation. In turn, large safety constraints are implemented with yield and throughput suffering due to poor composition control.

 A problem column feed 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, and energy costs increase. With distillation columns accounting for up to 40% of total plant energy costs, optimising their performance can produce significant cost savings.

Control valve considerations:

• spring-and-diaphragm actuator for stability
• sliding-stem valves offer lower flow rates and precise throttling capabilities
• customisable trim options suit a wide range of operating conditions
• segmented ball valves for wide rangeability needs
• hardened trim materials provide wear resistance and long life
• digital valve controllers with online diagnostics monitor real-time performance changes

 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 in product recovery to ensure overall product quality and column efficiency.

Prioritising critical valve applications as discussed above is a foundation for maintenance planning and is crucial piece to process availability. Combining a high-performing control valve solution with real-time diagnostic capabilities can provide the information necessary to make an educated decision on repair and operational needs. Knowing what valves need attention and at what time can mean the difference in unplanned downtime and millions in profit. A reliability programme combining these elements along with key performance measures will reduce the risk of shutdowns and lead to a high-performing plant.