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Defend against silicon poisoning

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Hydrocarbon Engineering,

Over the past several years, silicon has become much more prevalent in the oil and gas industry. Before, refineries relied on silicon compounds in the delayed coking unit to prevent foaming in coker drums. Now, upstream companies are starting to favour silicon for its lubricating ability, which allows companies to move oil quickly through pipelines, as well as load and unload ships faster.

While pipeline lubricant applications benefit upstream companies by improving short-term profits and efficiencies, the use of silicon compounds also comes with inherent performance risks for refineries downstream.

Silicon is a contaminant that, when found in crude oil, can work its way into previously unexpected places in the refinery, adversely affecting the units it enters. The result is shorter catalyst cycle lengths, which lead to more frequent reloads and more overall downtime for the unit – all of which lead to increased expenses and decreased profits.

Silicon: origin and destination in the refinery

Silicon-containing compounds, like polydimethylsiloxane (PDMS), are great anti-foam agents in the coking unit’s coker drums, and refineries have known how to effectively plan for its impact on downstream processing. With increased upstream usage, however, silicon is now entering refineries in the crude oil, affecting many more units and types of hydrotreaters, refinery-wide. The presence of silicon in crude oil negatively impacts the whole refinery.

Figure 1 highlights the historical source of silicon in the refinery (the coker), as well as the negative impact of refinery-wide silicon presence. Unless the units (particularly hydrotreaters and hydrocracking units) can be properly defended, catalysts will lose their activity and see a decreased life span.

Figure 1. Silicon origins and destinations in the refinery.

The chemistry of silicon

Silicon compounds interact with the alumina base of hydrotreating catalyst. The reaction creates a strong chemical bond, which is difficult to break. This is why silicon is considered a permanent poison.

Instead of interacting with the active metal sites on the hydrotreating catalyst directly, over time the buildup of silicon impedes and prevents access to these active sites. This is what causes a permanent loss in the catalyst’s ability to remove sulfur and nitrogen.

Written by Peter Piotrowski, Honeywell UOP, USA, and Austin Schneider, Crystaphase.

This article was originally published in the September 2017 issue of Hydrocarbon Engineering. To read the complete article, please sign in or register for a free trial subscription.

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Downstream news Oil refinery news