Olefin purification: Why selecting the right adsorbent matters
Removing difficult-to-detect contaminants helps minimise adsorbent reactivity while avoiding side reactions and excessive heat release.
Florence Pennetreau and Todd Burkes, Evonik Catalysts
Steffen Görlich, PCK Refinery
Viewed : 150
The global economic climate is volatile, and maintaining profitability is a constant challenge for operators in the downstream hydrocarbon processing industries. Staying competitive in a continually changing economic environment is no small task. The use of new process technologies may allow those in the hydrocarbon processing industries the operational flexibility and reliability to adapt to the changing economic environment while maintaining profitability.
Naturally, profitability challenges place even greater importance on optimising operational efficiencies to maximise productivity while keeping operating costs to a minimum. But crucially, this must be done without compromising safety and reliability levels, which are of the utmost importance.
For refiners, the selection of the proper adsorbent, or combination of adsorbents, is critical to ensure the removal of all contaminants. Failing to remove difficult-to-detect contaminants can result in system failures, unplanned downtime, and lost production, which can impact profitability. 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.
To increase their resilience within the market, refiners can invest in solutions to reduce these risks. Hybrid adsorbents offer a viable solution for businesses to remove contaminants reliably and safely and simultaneously maximise process efficiencies, therefore protecting profitability. Reliability and performance predictability of the adsorption unit are prerequisites, and adsorbent lifespan is another important element. A primary factor of success is minimising adsorbent reactivity to reduce or avoid side reactions, ensuring safe and effective contaminant removal while guaranteeing process safety.
Due to their high reactivity, the purification of olefins at an industrial scale has always been challenging in terms of efficiency, reliability, and operational safety. While typical adsorbent materials (promoted activated alumina and molecular sieves) can be used for olefin purification, a third class of adsorbent (hybrid) has been developed for their treatment. The selection of the proper class of adsorbent, or combination of adsorbents, must be dictated by the type of contaminants to be removed and the need for a high level of process safety:
• Promoted alumina: Promoted activated alumina adsorbents have the capacity for acid gases (H₂S, COS, and CO₂) in olefinic streams. These adsorbents are not a major safety concern as alumina-based adsorbents cause very low heat release when put in contact with olefins.
• Molecular sieves: Large pore diameter molecular sieves have the capacity for water and polar organic compounds (light alcohols, aldehydes, ketones, nitriles, mercaptans, and sulphides) removal from olefins. However, their use for olefins treatment can cause safety issues because they generate large amounts of heat (heat of adsorption) when put in contact with olefinic streams. If not properly controlled, the generated heat can lead to temperature excursion, causing olefin polymerisation and uncontrollable runaway reactions. When this type of product is used for unsaturated feed purification, particular care must therefore be taken. A pre-load step of dosing the reactive stream in inert media is highly recommended. Conversely, small pore diameter molecular sieves (3A-type) do not require a pre-load step but have capacity for water only.
• Hybrid (co-formed) adsorbent: Hybrid adsorbents contain both alumina and large molecular sieves. They, thus, have adsorption capacity for water, polar compounds, and acid gases removal from olefin streams. In addition, thanks to the presence of alumina, the overall heat of adsorption is lower than for large pore diameter molecular sieves alone. Three main companies offer hybrid adsorbents. Even if based on the same principle, the different hybrid products are not equivalent in efficiency and, more importantly, from a safety point of view. To benefit from the full potential of a hybrid adsorbent, it must guarantee the efficient removal of contaminants and the safest possible process.
R&D investments led to Dynocel 680, a hybrid adsorbent whose formulation has been specially tuned to provide efficient removal of contaminants and process safety. In pilot tests, safety performance was confirmed via the measurement of two different parameters: reactivity towards unsaturated streams and heat release when put in contact with unsaturated streams.
Œ Reactivity towards olefin streams: Reactivity tests were performed by charging the adsorbent and olefin in a sealed autoclave. The temperature was raised to 300°C, and pressure was monitored. In this type of test, reactivity can be detected by a drop in pressure during the temperature hold step, liquid formation due to oligomerisation of the olefin, and/or adsorbent colouration. Reactivity tests have been conducted on different co-formed products and with several olefinic streams. Reactivity tests were performed in harsh conditions (high temperature, high pressure, and sealed vessel). They, therefore, do not represent ‘normal’ operating conditions but rather show the difference in reactivity of the different products and what can happen in case of upset. Results obtained for propylene are shown in the left section of Figure 1.
While Evonik and Competitor A products showed low/no reactivity, pressure recorded during Competitor B product tests drastically dropped at around 200°C (pressure: 600 psig – 41 barg). This indicates that Competitor B product had reacted with propylene during the test. The yellow colour of the unloaded product confirmed the presence of oligomers at the adsorbent surface. As a reminder, oligomerisation is an exothermic reaction that can, on an industrial scale, generate an uncontrollable chain reaction with large heat release.
Heat release: Heat release (exothermic response) is measured with the Q value, which is recorded as the amount of heat generated by the adsorption of a compound at the surface of a solid. The lower the value, the safer the process. Q values have been measured for the adsorption of water and hexene at the surface of several classes of adsorbent (see Figure 1, right).
Alumina-based adsorbent showed little heat release during the test, whereas pure large pore diameter molecular sieves (13X type) showed a massive heat release, explaining the need for a pre-load step when used in olefinic processes. As expected, co-formed products showed Q values between pure alumina and pure molecular sieves data. However, Competitor A co-formed product still exhibited a fairly high Q value, justifying the need for a pre-load step for the latter.
Conversely, Dynocel 680 hybrid adsorbent showed a lower heat release, indicating that a pre-load step is not needed. These pilot tests confirmed that Evonik’s hybrid adsorbent exhibits both low reactivity towards unsaturated streams and low exothermic response. These important features have been further verified at an industrial level, as demonstrated in the following case study with PCK Refinery in Schwedt/Oder, Germany.
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