Question
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What are the symptoms of Silicone (Si) poisoning of a hydrotreating catalyst? How is it best avoided?
Mar-2021
Answers
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Steven Zink, Honeywell UOP, Steven.Zink@honeywell.com
Silica poisoning of hydrotreating catalysts is ordinarily incurred by the processing of streams that are laced with siloxanes, derived from the thermal decomposition of polysiloxanes. Polysiloxanes are routinely injected to improve crude oil recovery rates and to moderate foaming in the delayed coking process. Thermal cracking of polysiloxanes renders the majority into the naphtha, then diesel, then VGO boiling ranges. The siloxanes are catalytically hydrotreated, leading to silicon tying up the hydrotreating catalyst support’s surface hydroxyls, accumulating over time, and eventually obstructing adsorption at the active metal sulphides. The most effective catalysts have a combination of high surface area and porosity that is compatible with the size of the siloxane species. Silica poisoning manifests itself as a relatively steeper deactivation rate, particularly of the hydrodenitrogenation function, and a relatively steeper pressure differential rise rate. At half of their represented silicon capacity, the activity of high Si capacity naphtha hydrotreating catalysts will be approximately half vs fresh, tracked indirectly via bed-by-bed exotherm reduction rates.
Silica poisoning is best managed with a comprehensive strategy: 1) managing foaming in the delayed coking process according to industry best practices; 2) using only the most thermally stable polysiloxanes for the lowest polysiloxane dosing; 3) regularly monitoring elemental silicon concentrations in the hydrotreating feedstock components and product streams; 4) carefully specifying silica adsorbents and hydrotreating catalysts appropriate for the expected siloxane species; and 5) operating the silica adsorbents and hydrotreating catalysts in a temperature range known to be especially effective for silica adsorption, from start-of-cycle. Cycle-by-cycle vacuum unloading can provide a record of catalyst capacities and inform decisions related to catalyst reuse. Regular application of sectioned catalyst baskets will likewise help to inform future catalyst selections. The strategy may even include dedicated lead/lag or swing guard beds upstream of the hydrotreater and/or mid-cycle reactor skims. Phosphorus may likewise be included in the management strategy, as phospholipid and/or phosphatide anti-wear/lubricant products are also commonly injected to improve crude oil recovery rates. Such compounds will thermally decompose and can lead to hydrotreating catalyst poisoning downstream, and these are managed in a similar manner to siloxanes.
Mar-2021
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