High Performance Materials: Reducing Heat Loss and Lowering Energy Consumption in an Ethylene Cracker
Furnaces and fired heaters are used throughout the petrochemical, hydrocarbon and chemical processing industries in applications ranging from distillation, coking and alkylation to various forms of cracking. They lie at the heart of these processes, yet they can – if not carefully managed – become a liability. All heaters use large amounts of fuel, and the resulting energy costs can represent a substantial proportion of the cost of running a refinery or petrochemical plant.
Therefore, it is clearly in operators’ best interests to optimise the functioning and management of their furnaces and other heaters, not least because savings made in these areas can have a major impact on revenue.
Fortunately, companies such as Morgan Advanced Materials are developing their increasing range of thermal insulation options, including lightweight and highly flexible solutions to minimise heat loss and increase durability and reliability in refinery and petrochemical heaters. This is important because time has revealed problems with traditional solutions that modern products can overcome.
Traditionally used to line heater floors, hydraulically bonded refractory castables are supplied in powder form and mixed with water before application, which can be carried out in a fabrication shop or at the plant itself. Castables have a limited shelf life, and while they can be strong, in practice their performance relies heavily on the installer’s expertise. Other factors, such as start-up procedure can also affect their functionality, and initial heat up must be carefully controlled since if it is too fast, explosive spalling may result.
Increasingly, operators are now lining heater floors with a ‘dry’ solution consisting of high-duty firebrick backed by insulating firebrick (IFB) and a structurally stable proprietary block insulation. The use of IFB is cost-effective and provides excellent thermal efficiency, low levels of heat loss and structural strength. These “dry floors” are much more efficient, due in large part to the excellent thermal conductivity of the IFB (25-50% better than castable). In addition, no mixing or special dry-out is required, which can reduce the time of an installation.
Another option is to line heater floors with fibre logs made from one of the newer, more thermally efficient monolithic high temperature insulation fibres, such as Pyro-Log. These are easy to install as Pyro-Log is made with a proprietary lubricant that burns out at moderate temperatures, leaving the fibre logs strong enough to stand on – and have a long life cycle.
High- temperature IFB backed with mineral wool block has often been used to protect the lower walls of floor fired units. However, these mineral wool-based blocks tend to degrade, causing long term hot spots.
The use of modern microporous materials, which boast ultra-low thermal conductivity, allows users to avoid these problems, and can convey multiple benefits – as has been shown in the case study below.
Ceramic fibre linings such as layered blanket or modules with vacuum formed fibre peepsites are traditionally found lining the upper walls of floor fired units, but vacuum formed pieces can be expensive and may prove to be fragile in operation. It can also be difficult to combine vacuum formed shapes with the lining that surrounds them.
The use of high temperature insulation fibre modules overcomes these issues. Not only do they offer equal or better thermal conductivity, they also avoid the issue of broken peepsites and combining different materials. High temperature insulation fibre modules are extremely durable in use, last longer than the traditional options and can be easily cut for use as peepsites. High temperature insulation fibre modules also compress in all directions, delivering a better fibre-to-fibre and fibre-to-casing fit.
Furthermore, the use of high density insulation fibre modules can provide key benefits in terms of reduced heat flow and considerable energy savings.
One recent example saw a key petrochemical company contact the Thermal Ceramics business of Morgan regarding six ethylene furnaces.
Cracking is the conversion of complex organic molecules into more simple molecules, by breaking the carbon bonds in the feedstock. To crack ethane (the feedstock) into ethylene, the ethane is heated to an extremely high temperature so that the molecular bonds are broken and ethylene results.
When the client in this case consulted Morgan, the client’s furnaces had been in operation for 10 years, and were lined to a depth of 200mm (8 inches) with refractory ceramic fibre (RCF) 1,430°C (2,606°F) modules (175mm or 7 inches) and RCF 1260°C (2,300°F) blanket (25mm or 1 inch). Besides being inefficient, these units had such high skin temperatures that personnel protection had become an issue. The furnace linings also showed extensive deterioration.
The client asked Morgan to quantify the level of degradation in the lining, and in light of this to develop a new lining solution which would reduce the casing temperature and improve efficiency. Crucially, the depth of the new lining was not to increase the thickness of that already in place.
Analysis by Morgan revealed that the furnace linings had deteriorated by around 35-40 per cent, and that the average temperature was X03 = 123°C (253°F).
Morgan therefore recommended complete re-lining of the furnaces, and presented the following solutions for the client to choose from:
Solution A: RCF 1430°C (2,606°F) Pyro-Bloc® modules 240 kg/m3 (15 lb/ ft3) 175mm (7”) and RCF 1260°C (2,300°F) Cerablanket® 128 kg/m3 25mm (1”).
Solution B: RCF 1430°C (2,606°F) Pyro-Bloc® modules 240 kg/m3 (15 lb/ ft3) 175mm (7”) and microporous board 1000°C (1,832°F) at a thickness of 25mm (1”).
Morgan developed a choice of solutions because the client was, at least initially, sceptical about using microporous materials to line the furnaces. It was decided, therefore, that at first just two furnaces would be lined, one of these with microporous insulation, and that the design for the remaining units would be informed by the in-field results from the first two furnaces.
Both of the proposed solutions maintained the lining thickness at 200mm (8”), as per the customer’s requirements. In the field, for Solution A the average temperature in area X03 = 104°C (219°F) while for Solution B, the average temperature in area X03 = 82°C (180°F).
In light of this, the client decided to re-line all of the remaining furnaces using microporous products, because this reduced the casing temperature by an average of 41°C (106°F). In time, they used this technique to re-line all of their crackers.
It is not just in furnace insulation that modern materials have a role to play. They can also help to overcome issues in areas that have, traditionally, been hard to insulate. Side walls and burner blocks, for example, have often been lined with IFB and refractory castables, but the latter present thermal shock issues, amongst other problems.
One solution is to replace the IFB with fibre – however, when this is done, the castable burner blocks can cause issues due to thermal shock and excessive weight to the surrounding lining. This can be solved using the high density (240kg/m3 or 15 lb/ ft3) high temperature fibre (2600F grade) as burner blocks. This practice was once assumed to be too risky but the flame pattern of the flat flame burner in fact lends itself well to a fibre burner block. With like fibre materials, the surrounding wall is much easier to design and avoid possible hot spots.
Ceramic fibre modules offer a good solution here: although once considered risky in this setting, in practice high density monolithic ceramic fibre modules effectively avoid the problems associated with castables, and transmit substantially less heat than castables or brick. They can also be retro-fitted to many burners.
High density monolithic modules 192-240 kg/m3 (12-15 lb/ ft3) which compress in all four directions, can also be used to line difficult areas such as arches and corners. They are suitable for use in convection sections, as long as tubes are not steam cleaned - which can damage the lining.
Calculating the Benefits
The development of lightweight and more convenient heater linings has clearly benefited the petrochemical and chemical processing sectors. However, with these advances a new challenge has arisen – that of comparing thermal insulation options available, and making a choice from amongst them that will optimise the company’s efficiency, safety profile, productivity and revenue.
The case study presented here clearly shows how the use of modern thermal insulating materials can transform older, thinner insulation designs to meet today’s requirements for energy savings and lining longevity, resulting in improved furnace reliability, a safer working environment and improved bottom line.
Products such as IFB, monolithic fibre logs and lightweight microporous insulation can all help to transform outcomes, both financial and operational. However, in any furnace or heater re-lining project, it is important to consider all options carefully and take factors such as operating conditions, heat loss, temperature and energy consumption into account, in order to make the best choice of insulation and ensure the best possible results alongside optimised furnace reliability.