At the 2025 AIChE (American Institute of Chemical Engineers) Spring Meeting, Alina Green presented WIKA’s unique R&D evaluation of critical ethylene steam cracker coil outlet temperature (COT) sensor designs and installation best practices. The following is an abridged version of the paper* “Improving Olefin Production Efficiency: Evaluating sensor accuracy for common surface-measurement and in-process designs in coil outlet temperature measurement.”
Abstract: In the olefin production industry, optimizing steam cracker operations is crucial to achieving increased run times, consistent conversion rates, and efficient burner fuel usage. A key parameter is the coil outlet temperature, which directly impacts coke formation, the frequency of de-coking cycles, and the production of unreacted feedstock and undesirable by-products. Accurate measurement of this temperature is essential, and various sensor designs are utilized to achieve this. These designs include skin thermocouples—either as replaceable sensors or sensors permanently welded to the tubes—and in-process thermowell measurements.
This paper explores the performance and accuracy of the common sensor designs used in the industry today, presenting the findings from tests conducted in the process unit at WIKA’s R&D Center in Pasadena, Texas. This facility comprises a fired heater and fixed-bed reactor capable of simulating environments similar to those found in industrial heaters and reactors, which allows us to test the performance of temperature sensors. This study highlights how different sensor designs impact measurement accuracy.
Ethylene production and the importance of coil outlet temperature accuracy
Olefins are essential base materials in the manufacturing of endless everyday goods. These downstream products range from detergents and disinfectants to textiles and synthetic rubbers. Of the olefins, ethylene (C2H4) is the most widely produced as it is used in so many different areas of life. Polyethylene, the most common type of plastic, is a lightweight polymer used in making grocery bags, cling film, milk jugs, and much more. Other ethylene-derived compounds are ethylene oxide / ethylene glycol, ethylene dichloride, and styrene. In fact, according to The Essential Chemical Industry (ECI) – online, “ethane (ethylene) is the most important organic chemical, by tonnage, that is manufactured.”
Petrochemical plants extract ethylene from oil or natural gas in three broad stages:
- Steam (thermal) cracking and quenching (rapid cooling) of the hydrocarbons
- Multi-stage compression, gas scrubbing, and drying
- Separation and storage
Steam cracking in ethylene production
Ethylene production normally begins by cracking, or breaking, the long carbon chains of ethane (C2H6) and other hydrocarbons using superheated steam. To achieve a consistently high throughput with a consistently high quality via pyrolysis, ethylene plants must keep a close eye on process parameters—namely pressure, temperature, flow rate, and fluid level of cracking furnaces. To achieve optimal furnace performance, coil outlet temperature (COT) is particularly important.
Cracking furnace with pressure (P) measuring points on the fireboxes and electronic temperature sensors (T) on the radiant coils.
Depending on the facility, a typical cracking furnace has one or more rectangular fireboxes with vertical radiant coils in the center between two radiant refractory walls. These radiant coils superheat the hydrocarbon feedstock and the steam stream, with the reaction products in the tubes reaching temperatures of up to 950°C (1742°F).
The COT of an ethane cracker has a direct relationship to the conversion efficiency of feedstock to ethylene: up to a certain point, the hotter the heater, the more olefin is produced. One study of a cracking furnace found that increasing the temperature from 600°C to 650°C (1112°F to 1202°F) increased the yield by 4%, from 13% to 17% (Che, Hao, et al., 2018). But raising the temperature also burns more fuel. Pyrolysis is energy-intensive: one ton of ethylene consumes 23.5 gigajoules of energy, 47% of which is used in steam cracking (Worrell, Phylipsen, et al., 2000). Furthermore, Gál and Lakatos (2008) found that increasing the temperature above 845°C (1553°F) did not net a considerable increase in the yield. In other words, above this optimal temperature, the energy expenditure would exceed the income that the additional produced ethylene would bring. Thus, finding and maintaining the optimal cracking zone is crucial for ethylene plants to achieve maximum yields while balancing fuel usage.
The coking conundrum
Another challenge of ethylene production is to balance higher yields while minimizing coking, a type of fouling that occurs when the molecules formed during cracking collect inside radiant coils and then polymerize. Gál and Lakatos (2008) found that raising the temperature from 830°C to 845°C (1526°F to 1553°F) resulted in a 5% increase in ethylene yield (from 30% to 35%)—but coke formation surged from 7% to 23%.
The problem with coke is that it acts as a thermal insulator, clogging the insides of the coil and creating a pressure drop. To compensate, operators increase the process temperature, which not only consumes more energy, but also shortens the tube’s lifespan if it rises above the tube wall temperature (TWT).
The more coke that’s formed, the more frequently a cracking unit has to be decoked – a time-consuming and energy-intensive operation. First, the furnace has to be shut down. Operators then feed high-pressure air and steam into the unit while heating it up to about 900°C (1652°F) or even 1110°C (2012°F). This combination burns off the deposited coke, which can also be removed mechanically or with a pressure washer. Finally, the furnace is fired up again. The entire decoking process takes 20 to 40 hours (Gholami, Gholami, et al., 2021).
The amount of coke that forms inside a coil is a function of several factors:
- Intensity of the cracking
- Unfired residence time of the feed
- Final boiling point of the heaviest molecules in the feed
- Temperature to which the cracked gas is cooled in the transfer line exchanger
- Temperature of the transfer line exchanger cooling
- Unbalanced distribution of feedstock flow
Several solutions exist for each of these contributors to coking. For example, a Venturi nozzle installed at the inlet of each radiant coil ensures that the feedstock flows evenly. And accurate temperature sensors at the coil outlet can fine-tune the intensity of the cracking and, thus, reduce coking.
The high cost of inaccurate COT measurement
Coke formation is inevitable in ethylene processing, but operators can – and should – slow it down. A common way to minimize coking is by monitoring the coil outlet temperature (COT). A too-high COT can lead to increased coke production, which calls for more frequent cycles of decoking. As mentioned above, decoking requires shutting down production. In a typical 800,000 MT/year ethylene plant, each day of outage leads to a loss exceeding $2.2 million in revenue alone.
A too-low coil outlet temperature is also problematic, resulting in unreacted feedstock and undesirable byproducts. Insufficient conversion ratios cost petrochemical companies around $100,000 per day.
To maximize yields while minimizing coking and energy expenditure, operators need to monitor the coil outlet temperature and keep it within the optimal temperature range. This is possible only through accurate temperature measurement. As shown above, if measurement inaccuracies are off by just 15°C (27°F), coke formation increases by 16% at some temperatures. The preferred thermocouple for COT applications is Type N. (While Type K is popular due to its durability, reliability, and affordability, this thermocouple is susceptible to grain growth, aging, and drift at high temperatures.)
* Authors: Adam DeLancey, Manager, R&D Center, Electrical Temperature Measurement, WIKA USA; Alina Green, Market Segment Specialist, Chemical, WIKA USA; Kyle Wilkinson, Global Project Manager, Electrical Temperature Measurement, Engineered Products, WIKA Canada; Michael Tier, Senior Application Engineer, R&D Center, WIKA USA; Tim Schaefer, Product Engineer II, P.E., R&D Center, WIKA USA
This series continues next week with details on various COT sensor designs and how WIKA set up the test.