• Is there some way of reversing silicon poisoning in our hydrotreating catalyst?



  • Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com

    Silicon poisoning cannot be reversed. To stop or slow down its impact requires reducing the silicon content of the feed to the hydrotreater. Silicon in hydrotreaters comes from the thermal degradation of antifoams used in cokers or crude production. Silicon in coker products cannot be eliminated, but switching to a higher molecular weight antifoam can significantly reduce the amount of antifoam that ends up in the naphtha hydrotreater. Testing the virgin naphtha off of the crude tower can be used to tell if there is silicon coming in with the crude. If there is silicon in the crude, then additional testing of individual crudes being processed can be used to identify which crude(s) is causing the problem. A decision can then be made on whether it is worth removing the problem crude from the refinery feed slate or whether to live with the problem.

    When the unit is shut down to change catalyst, a layer of a ‘silicon trap’ can be added and/or a more silicon tolerant catalyst can be used to extend run length. If the problem is severe enough and the source cannot be eliminated, the installation of a guard reactor could be required to manage the problem.



  • Claus Brostrøm Nielsen, Haldor Topsoe, clbn@topsoe.com

    In oil refineries, silicon species are typically found in coker-derived products but can also be found in the crude oil where silicon is added during the drilling process. In the case of coker-derived products, the origin of silicon can typically be traced back to the silicone oil added to the heavy residue feed to the coker as an antifoaming additive. Silicone oil will crack or decompose to form modified silica gels and fragments when heated to high temperatures. These gels and fragments are mostly distilled in the naphtha range and are therefore carried to the downstream hydrotreaters together with the coker naphtha. However, silicon can also be found in other coker fractions. Silicon acts as a poison to hydrotreating catalysts, and poisoning of the catalyst reduces overall catalyst activity, meaning reduced activity for sulphur removal (HDS), nitrogen removal (HDN), and hydrogenation of unsaturated hydrocarbons. Silicon poisoning impacts and reduces in particular HDN activity. Therefore, the first sign of catalyst deactivation due to silicon poisoning is product nitrogen not meeting specifications due to reduced HDN activity.

    For these reasons, silicon will often be present in oil streams in the refinery, and it is therefore difficult to avoid silicon in the feed to hydrotreating catalysts. The best way to delay the negative effects is to use catalysts with the highest possible silicon pick-up capacity, as well as to include high-silicon pick-up capacity products as part of the catalyst grading in the top of the hydrotreating reactor to protect the downstream catalysts the most.

    During the hydrotreating process, silicon will deposit on the surface area of the hydrotreating catalyst. There are significant differences in silicon pick-up capacities when comparing the different available commercial hydrotreating catalysts. Haldor Topsoe has a strong and well-recognised series of catalysts, the TK-4xx SiliconTrap catalyst series, with very high silicon pick-up capacities. These specially developed catalysts and grading products can prolong the lifetime of the hydrotreating catalyst to the benefit of the refiner.

    When the maximum silicon pick-up capacity of the catalysts is reached, they are typically unloaded from the hydrotreating reactor and replaced with new catalysts. In general, most hydrotreating catalysts can be regenerated where the catalytic activity is partly regained.

    During regeneration, deposits of carbon on catalysts are burned off by exposing them to air at high temperatures. However, during regeneration only carbon and sulphur are removed from the catalysts, and any poisoning from metals, including silicon, is not removed. Silicon and other metals will stay on the catalyst surface and in the pore structure of the catalyst, and therefore catalytic activity is only partly regained. In many cases, poisoning is so severe that the catalyst cannot be reused and is therefore discarded. In other words, the silicon poisoning of hydrotreating catalysts is irreversible.