What are the symptoms of silica poisoning of a hydrotreating catalyst? How is it best avoided?
Responses to a question in the Catalysis 2021 Q&A feature
Various from Advanced Refining Technologies, Haldor Topsoe, Porocel, Honeywell UOP, Albemarle, Catalyst Intelligence & Unicat / G. W. Aru.
Viewed : 194
Ashok Monteiro, Technical Service Manager, Advanced Refining Technologies LLC (ART) - Ashok.Monteiro@Grace.com
In order to understand the impact and options to mitigate silica poisoning, it is important to understand the mechanism of silica poisoning. The silica does not poison the active metal sites directly, but rather restricts the pore structure, and eventually blocks access to the active sites. This phenomenon is referred to as pore mouth plugging and is associated with a characteristic deactivation pattern.
The main source of silica is from delayed coker operation which uses an anti-foam agent based on polydimethylsiloxane to suppress foaming in the coker drums. Lower molecular weight silica gels are formed as the siloxane complex breaks down in the coking process. Catalysts with a high alumina surface area are typically used to trap silicon efficiently from coker naphtha containing modified silica gels. As the silicon content in the catalyst increases, the surface area is progressively used up which affects the hydrotreating activity of the catalyst. The impact of silica poisoning does not become evident in the initial stages as there is a period of modest activity loss followed by more rapid deactivation as the available surface area is plugged with silica. As a result, silicon contamination is a major concern in units treating coker naphtha. In these units, the rate of silicon deposition on the catalyst is usually what determines run length rather than coke deactivation. In extreme cases of contamination, cycle lengths can be as short as three to six months.
A prominent aspect of catalyst deactivation by silica contamination is the relative rates of deactivation for HDS and HDN. The loss in HDN activity tends to be more rapid than for HDS activity even though pore mouth plugging should affect them equally. This is a reflection of the different reaction mechanisms for HDS and HDN. This phenomenon can be useful to the refiner because nitrogen breakthrough typically precedes silica breakthrough. It is much easier to analyse for nitrogen in the product than it is to analyse for silicon. This gives the refiner a way to anticipate silicon breakthrough and prevent contamination of the downstream noble metal reforming catalyst.
It is critical to monitor the operation of the unit and optimise the bed operating temperature as the silicon pickup is a temperature dependent catalytic reaction. At start of run (SOR), most of the reaction occurs near the top of the catalyst bed. Over 50% of the delta temperature occurs between the top and middle of the catalyst bed. As the cycle progresses, and the catalyst at the top of the bed is poisoned, the exotherm shifts to lower in the catalyst bed. It is often possible to monitor how the exotherm moves through the catalyst bed as silicon poisoning occurs provided there are enough thermocouples and/or there are multiple beds in the unit. This is a useful way to track the progression of silicon through the reactor.
Besides the silicon encountered in naphtha from cokers, silicon can also be found in synthetic crudes because the process of making synthetic crude often involves a coking step. Many crude suppliers have also used additives containing silicon in drilling processes, and pipeline companies are using silicon containing additives injected into the crudes for both flow enhancing performance and foaming issues. As a result, even refiners who do n0t have coking capabilities may run into Si contamination issues.
Refinery economics often dictate that opportunity crudes be processed to maximise profitability. Given this limitation, many refiners opt to have a coker unit in their configuration which makes it hard to avoid silica poisoning. Better monitoring and optimisation of the coker unit is one way to reduce the impact of silica poisoning. Another option is to use high surface area catalyst that is effective at trapping Si to help mitigate some of the poisoning issues.
Per Zeuthen, Senior Director, Europe, Haldor Topsoe - email@example.com
Silicon species are typically found in coker-derived products, and they act as poisons to the hydrotreating catalysts. The origin of silica can typically be traced back to the silicone oil added to the heavy residue feed to the coker. However, silicone oil is added in other streams as well. 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. Silicon poisoning of the catalyst in the hydrotreaters reduces the overall catalyst activity, however in particular the silicon poisoning impacts and reduces the HDN activity. As mentioned, the silicon poisoning impacts all catalyst functionalities, such as HDS, HDN, as well as saturation activities, but in particular the HDN activity. Reduction in HDN activity is therefore the first sign of catalyst deactivation due to silica poisoning.
If you cannot avoid silicon in the feed to the hydrotreating catalysts, then the best way to delay the negative effects is to use catalysts with the highest possible silicon capacity, as well as to include high-silicon capacity products as part of the catalyst grading in the top of the unit to protect the downstream catalysts the most. There are significant differences in silicon capacities comparing the different available commercial hydrotreating catalysts. Haldor Topsoe has a very strong and well-recognised series of catalysts, our TK-4xx catalyst series, with very high silicon capacities.
Guillaume Vincent, Business Segment Manager HPC, Porocel - firstname.lastname@example.org
Silicon is the most common contaminant for hydrotreating units. Silicon contamination is a major concern for coker naphtha, but small quantities can be found in the diesel and kerosene fractions. The use of anti-foaming agents (mainly polydimethylsiloxane, PDMS) in delayed coker operations can lead to about 80% of silicon ending up in the coker naphtha fractions. Typically, PDMS is thermally decomposed as cyclic siloxanes, which induce pore mouth plugging and deactivation of the main catalyst bed. Up to 6°C of activity loss for each 1.0 wt% of Si on the hydrotreating catalyst is possible due to decreased accessibility of active sites leading to higher weighted average bed temperature (WABT) to achieve the desired specifications in the product. This deactivation phenomenon will result in shorter cycle length, underperformance of the unit, and shutdown of the unit for catalyst change-out.
In order to maximise the HDS/HDN reactor performance, Legacy Porocel, now part of Evonik, has specially designed CatGuard Si21 as a silicon trap to:
• Gently saturate the olefins and mitigate the delta pressure build-up
• Trap silicon due to its high surface area (silicon capacity ≥ 20 wt%)
• Maximise volumetric retention
• Minimise diffusional limitations due to its optimised pore size structure
CatGuard Si21 silicon trap is a key part of Legacy Porocel’s hydroprocessing product line. Legacy Porocel’s technology portfolio enables refiners to meet their product quality and cycle length targets in hydroprocessing applications for a fraction of the cost of conventional catalysts. CatGuard Si21, as a part of the top bed grading supply, will allow refiners to:
• Maximise unit performance
• Protect against poisoning
• Increase life cycle
Steven Zink, Principal R&D Engineer/Scientist, 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.
Chad A. Perrott, Business Advisor, Albemarle - email@example.com
The first indications of a silicon (Si) poisoning event within a hydrotreating catalyst system are usually detected in the feed, which when sampled shows increases in the average Si levels exceeding the design of the system.
The most common kinetic symptoms of Si poisoning are a general loss of hydrogenation activity (including hydrodenitrogenation, HDN) and reduced olefin saturation activity. For heavier feed units with more sterically hindered heteroatoms, the hydrogenation losses can also affect the hydrodesulphurisation (HDS) reactions. Eventually, Si slippage to the product may become evident. All the aforementioned can result in reduced cycle length.
The level of Si poisoning should be determined after the cycle is completed and before regeneration to avoid formation of mixed silicon-molybdenum oxidic phases during regeneration. This is important if catalyst regeneration is considered since mixed silicon-molybdenum oxidic phases cannot be sulphided and will result in lower than expected activity when reloaded.
Even if the catalyst is not to be regenerated, knowing the Si poison level after unloading is important to enable the planning of future cycles. The knowledge from these samples can help optimise the Si traps for future loads. The benefit of optimisation is that future cycles can be extended and/or opex can be reduced when spent catalyst is considered for regeneration.
X-ray diffraction (XRF) is used for simple quantitative analysis. Scanning electron microscopy with energy dispersive X-ray analysis (SEM-EDX) is a quantitative method with a qualitative aspect (see Figure 1). Both methods are useful to determine the Si poisoning on spent catalyst.
There are multiple steps to avoid silicon poisoning in hydrotreating catalyst. These include proper planning and execution, monitoring the running unit, and analysing spent catalyst to inform the next planning cycle.
During planning, it is important to communicate your typical feed contaminant levels with catalyst vendors. This critical information, in turn, provides vendors an opportunity to enhance metals pickup capability and ensure management of Si poisoning. Technology like Albemarle’s STAX enables refiners to meet increasingly challenging feeds and processing goals in this way. Then during turnaround execution, it is crucial to load the catalysts as designed, and then maximise catalyst activation.
When the unit is running, regular monitoring of contaminants in the feed can help identify unexpected changes that may harm the catalyst. This data is important for determining errant sources of Si, including overuse of anti-foaming agents such as organo-polysiloxanes from delayed coker units, identifying off-spec import barges or, more recently, changes in tight oil qualities.
Conducting normalised catalyst activity reviews during operation can assist in identifying unexpected catalyst deactivation. Maintaining a sum of the total Si imported to the unit can help enable rationalisation of opportunity feeds to the refinery while balancing Si poisoning effects on the cycle.
Finally, sampling spent catalyst at various catalyst layers and analysing via SEM-EDX or XRF for metals contamination can help identify necessary adjustments in metals traps for subsequent loadings.
Carl van der Grift, Director, Catalyst Intelligence - firstname.lastname@example.org
Silicon contamination of hydrotreating catalysts is caused by silicon compounds entering the hydrotreater with the feed. The silicones in the feed may originate from drilling fluids and flow improvers used in the upstream, or from antifoaming agents used in the refinery. A third source of silicon in the hydrotreater may be clay, usually found in oil sands. High silicon levels will reduce the cycle length due to catalyst deactivation and/or pressure drop build-up, or both. Low silicon levels may not noticeably affect the cycle length of a fresh catalyst in a first cycle, but accumulated silicon levels on a spent catalyst may render it unsuitable for regeneration/reactivation.
Silicon deactivation is best avoided by limiting silicones in the feedstock. This can be done by feedstock selection and by being careful with the use of anti-foaming agents in the refinery. In cases where the presence of silicon in the feedstock cannot be avoided, the next best solution is to use a suitably designed guard bed on top of the main hydrotreating catalysts. The silicon trap in the guard bed will capture a large part of the silicon, and thereby protect the main (high activity) hydrotreating catalyst from deactivation.
Most catalyst companies have silicon traps in their portfolio and they can supply the silicon trap as part of the full load. A catalyst supplier will guarantee a silicon pickup of the silicon trap and a cycle length of the unit in case their technical offer is accepted. In case you want to design your own guard bed, you can test several trap materials from different suppliers in a basket in the hydrotreater and investigate at the end of a cycle which trap shows the highest removal on a weight or volume basis.
Some companies offer guard materials at reduced prices. Obviously, if you design your own guard bed, you do not get a guarantee on the complete fill or a conditional guarantee based on silicon slip to main catalyst. After a first cycle with silicon in the feed it is important to check the silicon on the spent catalyst to be regenerated as silicon will cause deactivation upon regeneration. As a rule of thumb, a 1 wt% Si on regenerated catalyst will cause a 20% RVA loss on a conventional type 1 hydrotreating catalyst. Reactivation of a type 2 hydrotreating catalyst is only possible if the silicon level is low (typically below 1 wt%), as specified by the original supplier.
Tom Ventham, Sales & Technical, Europe and Africa, Unicat / G. W. Aru - email@example.com
Silica is a known issue for hydrotreating catalyst. A common source of silica are coking operations that use certain anti-foaming chemicals.1 In applications not fed by delayed cokers, silicon can still be found in the hydrotreater. This can derive from processing of synthetic crudes, which often employ coking steps in their production, or use of regular crudes from new or existing sources where drilling and flow improvement chemicals are injected into wells and pipelines to assist recovery. Once silicon enters the hydrotreater, it can bind with the alumina surface and an activity loss will be observed. A common rule of thumb is a 3-6°C loss in temperature for every 1 wt% Si deposited on the catalyst. This activity loss cannot be recovered through catalyst regeneration. When Si reaches 15-20 wt% on catalyst, only about 50-60% catalyst activity remains. Hydrotreating catalyst can be protected by installing a highly effective guard material at the reactor inlet to trap silicon and prevent it entering the catalyst bed. Unicat’s Active Filtration System-Silica Trap (AFS-ST) removes silicon and many other poisons found in process streams. As a specialised solution to remove silica, grading systems using correctly sized AFS disks can be implemented to help protect against poisoning issues. Using an efficient AFS-ST system allows the required grading outage to be minimised to maximise the amount of catalyst that can be loaded into the reactor.
An alternative approach is also available, which is a skid mounted system containing pre-loaded AFS baskets to condition the feed upstream of process units. The advantages of such a system include the flexibility to replace spent grading baskets while the downstream reactor remains online, avoiding costly downtime. Another major advantage is the ability to increase catalyst volume by moving guard material out of the main reactor.
The benefits of using the AFS advanced grading solution means run lengths can be increased, catalyst life can be maximised, shutdowns and change-outs can be delayed, and opportunity feeds can be explored whilst mitigating the dangers of catalyst poisoning.
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