In the petrochemical world, refractory ceramic fiber (RCF) — also known as aluminosilicate wool (ASW) — has long ruled as the material for lining fired heaters. It is not hard to understand why: The material possesses several attributes that ensure it performs well in the harsh environment. They include:
- A low thermal conductivity and heat capacity.
- Superior thermal-shock resistance.
- The ability to withstand extreme temperatures.
On the operation side, refractory ceramic fiber is lauded for its excellent “handle ability” and longevity in service. In addition, it has good resistance to pollutants. In all, these characteristics mean refractory ceramic fiber can extend the life of fired heaters, increase the energy efficiency and, ultimately, improve a furnace’s reliability.
Despite its benefits, however, refractory ceramic fiber comes with drawbacks. Notably, the material produces crystalline silica as a byproduct when fired. This can have an adverse effect on health when overexposure occurs. European Union regulations are now in place to address the health concerns of refractory ceramic fiber and crystalline silica, requiring materials manufacturers to search for alternatives to protect workers.
Under the EU Carcinogen Directive, businesses are required to use substitutes to refractory ceramic fiber where it is technically and economically feasible to do so. Until recently, however, no material has been able to match the insulating performance and high melting point of refractory ceramic fiber for use in units such as reformers and cracking heaters.
The Economics vs. Ethics vs. Regulation Argument
For many oil refinery operators, replacing refractory ceramic fiber with alternatives will negatively affect furnace uptime. For that reason is an unpopular and costly option.
A typical large furnace can generate product revenues of $125,000 (€110,000) or more per day. Many operators are having to weigh up the pros and cons between putting furnaces into extended periods of downtime — affecting the bottom line — and complying with EU regulations designed to further protect worker’s safety.
At the same time, operators do value their ethical approach. In a number of cases, refractory ceramic fiber linings have been maintained beyond their expected lifespan rather than replacing them. This is both for monetary reasons and in the hope that a viable alternative will be made available. (Once replaced, the lining typically will be in use for another 15 to 20 years.) However, this conscious decision may be taken at the expense of an efficient and high-performing furnace.
Additionally, from 2020 onwards, producers and users of refractory ceramic fiber in Europe will have to keep the amount of fibers in the workplace to less than 0.3 fibers/ml. Many other countries around the world have imposed similar stringent controls. Such regulations increase the cost of using refractory ceramic fiber. Disposal of refractory ceramic fiber is also costly, requiring special landfill sites due to its classification as hazardous waste.
All of this is compounded by the increasing commitment of major industrial companies and trade associations to improve sustainability standards. This places the onus on the fiber industry to find viable alternatives that match or even exceed the performance of refractory ceramic fiber. Key characteristics make it a difficult material to replace. For instance, the fiber has been more resistant to attack by alkali-based pollutants than the existing low biopersistence fiber compositions. This has prevented the replacement of refractory ceramic fiber in many applications.
The material meets the American Petroleum Institute’s (API) standards for the petrochemical industry.
The Case for Operational Improvement
Clearly, replacing refractory ceramic fiber is easier said than done; instead, there is a fine balancing act between economics, ethics and compliance. Switching out refractory ceramic fiber, however, provides benefits that should not be overlooked.
The first benefit that operators could realize is minimized downtime. When furnaces are scheduled for downtime, it usually takes at least 24 hours before the furnace is cool enough for engineers to enter and inspect it. Also, due to the crystalline silica in refractory ceramic fiber, before any work in or around a fired heater with a refractory ceramic fiber lining can happen, workers must ensure that they wear a full set of appropriate personal protective equipment (PPE). This is required by guidelines for occupational exposure limits (OELs) but it also protects against other hazards.
Wearing personal protective equipment increases the time it takes to prep for entering the furnace. Also, when undertaking bigger jobs like repairing linings or metal tubes in the furnace, there can be additional complications. Due to occupational exposure-limit restrictions and the need for extra protective gear, engineers cannot work extended periods of time in the furnace. Especially in warmer climates, engineers must stop and vacate the furnace at set intervals.
Finally, if any emergency repairs are needed, then operators need to call in specialists and order materials to repair the lining. Quite quickly, what looked like one day for planned downtime can spiral into days or even weeks of unplanned extended downtime where the furnace and revenue-generating abilities are out of action.
Arguably, low biopersistence linings would, to an extent, prevent these scenarios from happening for operators.
The Issue of Hazardous Waste
Refractory ceramic fiber linings do not just pose operational problems during their working life. When decommissioned, they also require specialist disposal that increases costs.
For example, in Europe, disposal of waste materials in EU Member States is controlled by the implementation of a number of directives. Wastes containing more than 0.1 wt% of refractory ceramic fiber are classified hazardous under Directive 91/689/EC. Refractory ceramic fiber wastes from manufacture and use are required to be handled and disposed of by a licensed waste contractor in an appropriately licensed hazardous-waste landfill.
In practice, many refractory ceramic fiber users have experienced significantly increased costs because local waste disposal sites are not licensed to or prepared to accept hazardous wastes.
An Alternative to Refractory Ceramic Fiber
While there are many reasons to switch from refractory ceramic fiber linings to a low biopersistence alternative, as alluded to earlier, existing low biopersistence fiber compositions do not perform as well from a thermal perspective. Also, they do not resist pollutants as well in comparison.
To address these concerns, one refractories manufacturer sought to develop a viable alternative to refractory ceramic fiber by revisiting RFC itself. The goal was to produce a new fiber with:
- The performance attributes of refractory ceramic fiber.
- Low biopersistence.
- A composition that does not form crystalline silica during use.
The result of this research is a fiber — dubbed Superwool Xtra — that does not form crystalline silica. Crucially, in terms of its effect on environmental, health and safety (EHS) risks, the material is exonerated from any carcinogenic classification under Nota Q of Directive 97/69EC.
The material also meets the American Petroleum Institute’s (API) standards for the petrochemical industry. The API’s classification temperature which is to be used for insulation outlines an obligatory 150°C (302°F) overtemperature capability on the fiber within furnaces. Critical furnaces within the oil refinery and petrochemicals industry run between 2192 and 2282°F (1200 and 1250°C) and therefore require materials with a 2552°F (1400°C) minimum classification rating. With a classification rating of 2642°F (1450°C), the new refractory fiber offers a performance equal to or exceeding refractory ceramic fiber.
Critical furnaces within the oil refinery and petrochemicals industry run between 2192 and 2282°F (1200 and 1250°C) and therefore require materials with a 2552°F (1400°C) minimum classification rating.
Making the Switch
For operators looking to switch away from refractory ceramic fiber, it is important that due consideration is given. Identifying the need for a new furnace lining is not an easy task. Testing and running trials of new materials can be costly per trial — not to mention furnace downtime. Ensuring lining quality is essential for protecting personnel, minimizing heat loss and maintaining operational reliability.
Furnace linings that may have developed breaches over time are prone to increased thermal loss; some may not be visible from the outside. To minimize costs, it is recommended that testing coincides with the scheduled downtime, so fabricators can swap-out and install the new lining.
Clearly, replacing refractory ceramic fiber is easier said than done; instead, there is a fine balancing act between economics, ethics and compliance.
One technique for identifying thermal losses lies in the use of infrared thermography scans. By using these scans, engineers can keep the furnace in operation while conducting an analysis. If a hot spot is found, it is faster and more cost-effective for the repair to be done on-line if possible.
To achieve maximum efficiency and longevity for the materials specified during the furnace relining process, it is critical to ensure the engineering design is appropriate.
Not only must the materials have enough studs to hold them in place, they also require sufficient joints for expansion or shrinkage. If a brick lining is installed without adequate expansion joints, the brick can grow so large that it pushes the entire lining off the furnace wall. This will lead to further inefficiency, requiring the entire process to be repeated.
Over long periods of time at temperature, fiber modules degrade, resulting in shrinkage gaps between the modules. These normally need filling with more fiber during scheduled maintenance shutdown periods. The new fiber expands when heated to high temperatures, which has several benefits.
First, potential shrinkage gaps that maybe visible at cooldown will close up during operation. Second, expansion properties allow the refractory fiber to be used in critical applications when mating dissimilar lining materials. These properties and design considerations can result in reduced downtime, minimizing labor costs.
In conclusion, a newly installed furnace lining that uses appropriate materials, is designed to requirement and installed correctly, can last for as long as 20 years. With the range of furnace lining products available, each requires unique installation methods. It is important, therefore, that the personnel employed to carry out the work are highly skilled and experienced. Failing to do so can lead to complications and, ultimately, result in large sums of money being lost.
Additionally, degradation of furnace insulation can result in development of hot spots on the casing that can damage equipment and cause an unsafe condition for personnel. It also can disrupt the process as operators compensate for the higher heat loss. In turn, if these hot spots are located near the tubes within the furnace, they can be extremely dangerous. (The typical hydrocarbon process materials in these tubes are highly flammable.) If the tubes break, there is a significantly heightened risk of an explosion.