After a decade of uncertainty, the ethylene industry is experiencing significant growth.
Driven by strong consumer demand for plastic and lower oil prices, which in turn means cheaper feedstocks, growth is projected by global consultancy company Wood Mackenzie to be 3.4% per year. Further research by IHS Markit suggests that ethylene demand is set to exceed 60 million t between 2018 and 2022.
Now, following a successful first ‘building wave’ of new ethylene plants, plans for the second wave are being finalised globally. Choosing the right sealing solution is essential to ensure that new installations are optimised for maximum safety and efficiency, as the harsh operational conditions of ethylene production can cause the deterioration of plant equipment, especially seals, leading to the risk of leaks, fires and costly downtime.
To ensure the highest levels of safety and efficient operations, there are some typical challenges of ethylene production to consider that will assist with the specification of the most suitable and effective sealing solution.
Tackle the safety risks of oxidation
Ethylene production involves steam cracking of hydrocarbon feedstocks at very high temperatures. Among the furnace’s key components of its radiant section, crossover piping, Transfer Line Exchangers (TLE) and quench cooler, temperatures range from approximately 650°C to 1000°C (1200°F to 1830°F), which is well above the auto-ignition temperature of the media.
Consequently, leakage arising from the use of inappropriate gaskets in these areas will result in spontaneous fires upon contact with the atmosphere.
Besides the high temperatures, the oxidising environment of ethylene production represents a big challenge for sealing materials. Despite the cracking reaction producing hydrogen as a by-product, conditions both within the radiant section of the furnace and the external environment around gasketed joints are oxidising rather than reducing in nature.
Superheated steam and external air, in combination with the high temperatures, create oxidising conditions even for high specification, oxidation inhibited, grades of graphite.
Traditional sealing materials have been shown to fail under these conditions, creating significant safety risks. It is paramount to use a sealing material that can both resist oxidation and remain gas tight over its lifetime as graphite, a commonly used gasket material, is susceptible to degradation. Being a form of carbon, graphite reacts with oxygen present in the external atmosphere or internal process media to form carbon dioxide, which results in mass loss in the gasket and leads to leakage.
In addition, TLE’s can be a highly challenging application to seal as they are exposed to high operating temperatures and frequent thermal cycling. Older designs can be particularly susceptible to reliability problems due to an absence of refractory lining.
Prevent decoking-related leaks and avoid unplanned shutdowns
Regardless of feedstock, formation of coke is an undesirable, yet unavoidable, side reaction of steam cracking. Although effective process control and feedstock selection can reduce coke build-up (and enhance olefin yield), the build-up of these hard carbon deposits on the inner walls of the furnace coils and TLEs is inevitable.
The periodic removal of coke deposits known as ‘decoking’, is essential to maintain acceptable heat transfer, reduce hot spots, and control pressure drop, ensuring maximum plant efficiency and equipment life.
The process of decoking is carried out by periodically stopping production, roughly every 30 - 60 days, and purging the system at elevated temperature, typically in excess of 800°C, with high pressure steam and/or air.
The steam/air mixture acts as a powerful oxidiser converting the solid coke deposits into gas (CO2) and thus cleaning the system.
The decoking procedure, combined with the high process temperatures, can lead to oxidation of graphite gaskets, shortening their life span considerably.
This damage can go unnoticed after every decoking cycle, posing a significant safety threat and risk of leaks, which can result in fires and unplanned maintenance events.
Choosing the right gasket technology can make a huge difference in protecting an ethylene plant from decoking-related damage. The specification and use of a gas tight, oxidation resistant sealing material should be standard practice under these conditions.
Flexitallic has worked with numerous plants to address these issues. For example, an ethylene plant in Canada was experiencing weekly fires through leakage on the furnace elbow flanged joint, this was due to the temperature gradient from the furnace to the transfer line exchanger and they had unsuccessfully tried several types of gasket material, before deciding upon our proprietary Thermiculite® as it offered effective sealing performance.
Thermiculite products are developed for use in high temperature processes in services up to 1000°C (1832°F) working in highly oxidising environments.
This material is made from chemically and thermally exfoliated vermiculite, with a similar structure to exfoliated graphite, ensuring it can endure a wide range of temperatures without compromising integrity as it is intrinsically resistant to oxidation.
Switching to Thermiculite gaskets has been shown to be beneficial in modern and older plants alike as it offers key safety and efficiency benefits through oxidation resistant technology. In a US plant, for instance, there were frequent occurrences of graphite spiral gaskets oxidising in less than six months, causing flash fires and regular maintenance shutdowns. The issue was resolved after installing Thermiculite.
The product can be integrated into a range of Flexitallic industrial gaskets, which includes, for applications in heat exchangers, Change™.
Improve exchanger and joint reliability during thermal cycling
As with any petrochemical manufacturing process, optimising product yield is paramount to maximising plant efficiency. Conversion rates at such high process temperatures are extremely fast, this means reaction times must be tightly controlled to prevent under or over conversion into unwanted by-products.
Termination of the conversion reaction at the correct time is achieved by rapid cooling or quenching of the cracked gas in a TLE. Immediately after cooling, the media stream consists of an ethylene rich gaseous mixture containing ethane, methane, propane and butane. Further processing, including additional cooling, compression and fractionation results in the desired purified ethylene as a compressed gas or cryogenic liquid.
Additional cooling is carried out in the cold section of the process train in a specially designed heat exchanger fabricated from braised aluminium (BAHX), commonly referred to as the cold box. This necessitates the use of a dissimilar metal, aluminium to steel, flanged connection.
Differences in thermal expansion between the aluminium and steel flanges can result in a reduction in load on a gasket leading to premature failure. As a solution, Change gasket technology has been employed in both hot and cold cycling services to maintain sealing during thermal changes including differential thermal expansion and contraction seen in heat exchangers.
As demand for ethylene increases, the importance of operating at high levels of productivity from plant operations is paramount. However, the operational challenges present in ethylene production cannot be underestimated. The high risk of safety complications, through the steam cracking of hydrocarbons that requires the equipment to witness temperatures of between 650°C and 1000°C (1200°F – 1830°F), temperatures that are well above the auto-ignition of the media.
Superheated steam and external air, in combination with the high temperatures create accelerated oxidising conditions for even the highest specification of graphite.
Thermal expansion and contraction of dissimilar metals of the TLE and heat exchanger’s connections due to the need to control the rapidly cooling or quenching requirements to maximising the production yield, calls for serious considerations when choosing sealing materials.
Selecting the right sealing material is the first step to eliminating leaks and eventual unplanned outages due to joint failure. It plays a significant part in the multi-faceted operations of an ethylene plant, which is integral to achieving safe and efficient transportation of product.
Written by Alex Lattimer, The Flexitallic Group.
Read the article online at: https://www.hydrocarbonengineering.com/special-reports/06042020/gaskets-for-ethylene-production/
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