Jan-2022

Monitoring technology for ethylene crackers and SMRs

Digital solutions and infrared technologies help steam cracker and SMR operators improve temperature homogeneity and fuel efficiency.

James Cross
Ametek Land

Article Summary

Bold strategies to reduce carbon emissions are being released with growing regularity, often with aggressive targets and recommendations. For example, in August 2021, the UK government released a low carbon hydrogen strategy, built around blue and green hydrogen, to meet what it terms legally binding commitments to net-zero emissions. The US government is now proposing a 52% reduction in US greenhouse gas emissions from 2005 levels by 2030, far sooner than previous pledges. The new economic and technical regulatory environment means that refineries and petrochemical industries must radically change in the long term and make significant improvements in efficiency in the short term. A sensible place to start is with the optimisation of their most carbon-intensive fired heater processes.

Two of the largest and most carbon-intensive refinery and petrochemical fired heaters are steam methane reformers (SMR), which largely supply hydrogen to refineries for ammonia/methanol production, and steam crackers for ethylene production. Steam methane reformers emit around 800 million tonnes of carbon dioxide (CO2) per year, and steam crackers are estimated to produce 260 million tonnes of CO2 emissions per year.¹Whilst much is written around the potential to reduce CO2 emissions from these fired heaters by upgrading burners, changing tube/coil/refractory materials, and improving combustion efficiency, little attention is paid to the importance of temperature monitoring and control and the role it can play in improving process efficiency.

Steam methane reforming and blue hydrogen production
Over 95% of the world’s hydrogen is produced using steam methane reforming, generally utilising desulphurised natural gas, refinery off-gas, liquefied petroleum gas (LPG), or naphtha as a feedstock. The feed is preheated and mixed with steam before entering the primary reformer, where the mixtures pass over a catalyst to produce hydrogen, carbon monoxide (CO) and CO2. CO is shifted with steam to additional hydrogen and CO2, before pressure swing adsorption (PSA) is used to separate hydrogen. Without carbon capture, usage and storage (CCUS), this is known as grey hydrogen, but when CCUS is used blue hydrogen is produced.

There are two main sources of CO2 emissions from SMRs - the CO2 produced alongside hydrogen in the reforming reaction in the primary reaction furnace and the CO2 produced by combustion of the fuel. The process of capturing CO2 is relatively simple and low cost for the reformed gas, but capturing CO2 post-combustion is more expensive because it needs to be separated from nitrogen. Traditional grey hydrogen production may capture CO2 from one of these sources, whereas blue hydrogen production is assumed to capture CO2 from both. SMRs can achieve a conversion efficiency of 74% (HHV) and a CO2 capture rate of up to 90%.

Autothermal reformers (ATR) can also produce blue hydrogen when used together with CCUS, and technology licensors claim conversion efficiencies of 84% (HHV) with CO2 capture rates of 95%. The natural gas feed is combusted to produce heat for the reforming reaction, so no separate fuel source is needed, as with SMRs. This process means ATRs can achieve higher conversion efficiencies and CO2 capture rates than SMRs as there is only a single CO2 stream. In the new, highly competitive landscape where efficiency and CO2 capture are critical to a plant’s profitability, SMR operators must improve their efficiency to stay competitive.²

Steam cracking for ethylene production
The cracking reaction in a steam cracker takes place in sets of tubes (known as coils) that hang in huge fired radiant sections, usually in a single or twin cell layout with a common convection section. A wide variety of fuels and feedstocks can be utilised, including naphtha, butane, propane, and ethane. Flue gas temperatures are recorded continuously from contact thermocouples, whilst thermocouples also provide coil outlet temperatures (COTs) that indicate cracking severity. Tube metal temperatures (TMTs) are gathered periodically and manually using an infrared pyrometer, and a process engineer analyses the data.

Coke is deposited on the inside surface of coils which can cause plugging, overheating, and ultimately failure. This is a well-understood phenomenon, but it is not closely monitored in many cases because decoking (where coke is gasified by passing steam and/or air through the radiant coils) is scheduled on frequent, planned intervals. Excessive decoking cycles can lead to a loss in ethylene production, reduced tube life due to thermal cycling, high maintenance spending, and increased particulate release to the atmosphere. Visual inspections and temperature data collection are periodic, so these deposits may go unnoticed and unplanned decoking may be required. The layer of coke forms a layer of insulation between the hot furnace atmosphere and the comparatively cooler reaction gas, which impairs heat transfer. Therefore, the prevention and reduction of coke formation is a key priority from a maintenance, throughput, energy efficiency, and environmental perspective. Reducing coke formation also reduces energy input and potentially increases the availability of the furnace by extending run lengths.

Temperature measurement and furnace monitoring
To achieve a reduction in CO2 emissions, process adjustments are being made, including reducing oxygen setpoints and increasing the hydrogen content in fuel stocks. These trends increase the need for closer visual and temperature monitoring inside both a steam cracker and SMR. Flue gas and surface temperatures may be hotter, new flame behaviours may be observed, and burner nozzles, tiles, and insulation could deteriorate faster. Careful attention must also be paid to subsequent potential increases in NOx emissions and other changes in fired heater characteristics. These trends create new uncertainty, an increased risk of material failures and potentially unsafe conditions.³

Traditional temperature monitoring is performed by pyrometers such as the AMETEK Land Cyclops L portable handheld pyrometer (see Figure 2), used throughout the ethylene, hydrogen, and syngas industry. Given new challenges and demands, other technologies can be used with the Cyclops L to provide more accurate and comprehensive temperature data for the tubes, burners, burner tiles, and refractory. These technologies include the AMETEK Land Gold Cup pyrometer, as well as both fixed and portable borescope thermal imaging systems.

On an SMR, depending on furnace design, if temperatures approach or exceed the tube’s design limits as measured by the pyrometer, firing can be reduced, burners can be gagged, or burners may be shut off in extreme cases. This method is labour intensive and reactive, and potentially concerning conditions will only be acted upon if observed and recorded by the inspection team. In terms of optimising temperature homogeneity, reactive, single-point, manual data collection using a pyrometer will not provide the same volume of data as thermal imaging systems that employ automated, continuous monitoring of a measurement array. Whilst fixed monitoring systems provide this comprehensive data continuously in real-time, portable systems can also be valuable inspection and thermal survey tools (see Figure 3).

The cost of adjustments designed to improve efficiency in reduced reliability may outweigh the benefit, especially if process temperatures are not accurately collected, recorded, and acted upon. Temperatures of 20°C (36°F) above design temperatures may halve the lifetime of the tube. Because of this, many plants operate conservatively to reduce the risk of material failures. However, running 10°C (18°F) below design temperatures, for example, results in a 1% productivity loss on an SMR. Similarly, a steam cracker running significantly under design temperatures will not crack enough hydrocarbon feed to produce a valuable product. Operating in the optimum surface temperature window is critical to an efficient process and material lifetime.⁴