Adsorbent choices help refiners find safer solutions
For refiners, selection of the proper adsorbent, or combination of adsorbents, is critical to ensure the removal of all contaminants during olefin (such as ethylene and propylene) purification.
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However, choosing the correct adsorbent is about more than production efficiencies and effectiveness. Reliability and safety (including heat release and reactivity) should also be key considerations.
Three main adsorbent types may be used to purify olefins:
1. Molecular sieves: have capacity for water, light alcohols, aldehydes and ketones, nitriles, as well as for mercaptans and sulphides. They are not suitable for H₂S, CO₂ or COS in olefin streams.
2. Promoted alumina: has capacity for H₂S, COS and CO₂, but little to no affinity for water, light alcohols, aldehydes and ketones, and nitriles in olefin streams.
3. Alumina/molecular sieve hybrid (co-formed): has capacity for water, light alcohols, aldehydes and ketones, nitriles, R-SH, sulphides and H₂S, CO₂ or COS in olefin streams, and this combines the properties of 1 and 2 efficiently.
Case study: safety matters
Several years ago, a European refiner operating a propylene recovery unit (PRU) – using a 3A molecular sieve to remove water and a co-formed adsorbent to remove NH₃ – suffered an incident with a fresh adsorbent, and it took nearly a decade to resolve the situation satisfactorily.
During start-up, a mechanical blockage occurred due to inert balls plugged in a pipe. The result was that the reactor heated up without any gas flowing out, and the temperature in the vessel increased. Due to its fresh activity, the adsorbent actually catalysed the polymerisation of propylene, leading to a runaway reaction. The vessel burst, and propylene ignited.
Following the incident, the refiner used a complete load of 3A molecular sieve for several years, but when ammonia (NH₃) related issues occurred, such as corrosion and equipment blockage, it needed to find a more suitable solution.
Initially, it tried reverting to a previous configuration, but with the addition of pressure release valves to the flare and multiple thermocouples. It also introduced a different type of co-formed catalyst and a preload step.
However, over the next few years, this new approach turned out to be insufficient. An issue was identified in the heating process, and the gas entering the vessel was far too hot. There was a higher temperature in the bed than in the vessel inlet. Although the thermocouples and release valves meant there was no immediate safety issue, the refiner tendered for a different co-formed catalyst to ensure problems did not arise.
The result was that despite strong management of change policy, the refinery selected the proprietary Dynocel 680 thanks to the higher level of safety it offers. It is a spherical adsorbent for the purification of vapour or liquid phase olefinic hydrocarbon streams and falls under Category 3 - a hybrid adsorbent. It is particularly useful for the removal of polar compounds such as water, alcohols, ammonia, ethers, ketones and mercaptans, and acidic compounds such as H₂S, COS, and CO₂.
Dynocel 680 exhibits low reactivity, no heat release concerns, and does not require a preload step. Safety is paramount in these facilities, and Evonik is committed to the highest safety standards for its products. Therefore, Dynocel 680 has been in service in this refinery for more than five years providing safe, reliable, and predictable unit operations.
Selection of the proper adsorbent, or combination of adsorbents, is critical to ensure the removal of all contaminants. However, on top of that, the impact of operational requirements and adsorbent properties should be considered from a safety, reliability, and predictability standpoint to protect people, profits, and the environment.
This short case study originally appeared in PTQ's Technology In Action Feature - Q3 2022 Issue
For more information: Todd Burkes
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