July 2022

Catalysts

Hydroprocessing catalyst reload and restart best practices—Part 1

Hydroprocessing (hydrotreating and hydrocracking) units are high-pressure, high-temperature units that have multiple reactors, multiple beds per reactor and specialized metallurgy.

Hydroprocessing (hydrotreating and hydrocracking) units are high-pressure, high-temperature units that have multiple reactors, multiple beds per reactor and specialized metallurgy. Catalysts in these units are replaced on a 2 yr–5 yr cycle, depending on feed quality, unit design, catalyst selection, operational constraints and performance.

One of the key drivers for a catalyst change-out turnaround on these high-margin units is minimizing the time that the unit is offline—the cost of downtime is high, particularly when a turnaround is extended.

Managing a catalyst change-out turnaround is vastly different than normal refinery maintenance activities: the complexity is higher, the risks and consequences of unplanned events are much greater, and the required resources are significant but limited (FIG. 1). The experience of the key personnel involved is a significant factor during this activity; however, decades of experience are disappearing from refineries due to economic pressures and the age profile of operations/engineering/maintenance personnel.

The responsibilities of refinery process engineers change every 2 yr, so building and promoting experience and expertise in-house for hydroprocessing catalyst reloads is difficult. In the past, the catalyst vendor often provided much of the necessary technical expertise, specifically for the catalyst loading and unit restart activities. However, the impact of the COVID-19 pandemic and ongoing restrictions means that catalyst vendors are generally unable to provide this level of onsite support.

This situation leads to the question of how to best manage the complexities and risks associated with a catalyst change-out and restart for refinery hydroprocessing units, as well as hydroprocessing units for renewable fuels (e.g., vegetable oils and fatty acids).

This article (Part 1 of 2) will discuss the activities associated with turnaround planning and shutdown, catalyst unloading and reactor inspection. Part 2, which will appear in the August issue, will discuss the activities associated with catalyst loading and the restart. The two articles cover the entire process across the full catalyst cycle, as well as many of the best practices that are used to manage and mitigate the underlying risks for these units.

Turnaround planning for catalyst change-out

The best performers understand the risks associated with catalyst turnarounds and turnaround work, in general, compared to maintenance and inspection work during normal operation. For example, decommissioning and commissioning of a unit are the periods of maximum risk of unexpected events. Each time equipment is accessed or “opened up,” new risks are introduced. Safety incidents and labor disruptions have a greater impact during a turnaround than during normal operation. On startup, operators must have a high level of confidence that the unit will run the full cycle, without interruption.

For numerous reasons, the priorities for a turnaround should be safety, quality, time and cost—in that order. Too often, decisions are made in the planning and execution phases of a turnaround with an obsessive focus on minimizing costs. In many cases, this singular focus has resulted in massive losses because the unit had to be shut down again to rectify a problem resulting from these cost-minimizing decisions, or the performance of the unit was affected because shortcuts were taken during the catalyst reload, or the quality of the reload was compromised to save time or reduce cost. Two examples that illustrate this point are given here:

• Avoid compromising on the quality of the ceramic catalyst support material (normally ceramic balls). In some instances, trying to save $10,000–$20,000 on the cost of ceramic balls has cost millions of dollars because the cheap ceramic materials have broken or failed at the bottom of a large catalyst bed.
• Ensure a catalyst handling contractor has the experience, expertise, equipment and resources to complete the catalyst unload and reload. Saving $100,000–$200,000 on the cost of the catalyst handling contractor can easily result in a multi-day delay in completion of the turnaround, particularly during the unloading of the catalyst.

During a catalyst change-out turnaround, surprises and issues—impossible to predict—will arise that must be resolved. Managing the surprises is a critical success factor and begins with the planning phase.

Planning for a catalyst reload turnaround should begin soon after the completion of the last catalyst reload. The first item to address is the catalyst evaluation and selection for the next catalyst cycle, once data is available on how the current cycle is tracking against the predicted performance.

Ideally, the catalyst selection process should begin at least 18 mos–24 mos before the next catalyst change-out to provide sufficient time for all requisite tasks. Typically, catalyst lead time is 6 mos–12 mos, leaving the remaining time for planning, evaluation and internal processing. To put it simply: the earlier, the better.

Best practices

As a general practice, it is best to apply a multi-discipline approach to set the key catalyst requirement or strategy [e.g., longer run length or more difficult (cheaper) feedstocks]. Refinery management and the economics and scheduling departments will have input into the future catalyst strategies. Additionally, the refinery’s focal point (usually a unit process engineer) should also incorporate current operating issues (e.g., high reactor pressure drop, feed contamination levels) into the invitation to bid (ITB) so the catalyst supplier can properly address these problems in the next cycle.

While some refiners still rely on vendor estimations/predictions, pilot plant testing has become more popular as a tool to unveil or expose actual catalyst performance. This evaluation approach is particularly vital to a critical unit like a hydrocracker, where a slight difference in product yield can result in multi-million dollars of profit/loss per year over the whole cycle.

Comparing paper estimates/predictions from different catalyst vendors is not an apples-to-apples comparison, but this approach is nonetheless prevalent among refiners due to its simplicity. Catalyst vendors employ different design assumptions, feed characterization techniques, kinetic models and product property estimators (e.g., basic to non-basic nitrogen ratio, aromatics distribution). Consequently, it is fundamentally inaccurate to compare estimates/predictions between catalyst vendors at face value; unfortunately, many refiners are unaware of the errors inherent in this simplistic approach.

It is, however, acceptable to use a paper-based evaluation for less critical applications (e.g., for a naphtha or kerosene hydrotreater), although the best practice is still to have the catalysts tested before evaluating the options.

For refiners without an in-house pilot plant testing facility, several companies can provide an independent pilot plant testing service. Two primary methods are available: full pilot-scale and high throughput bench-scale approaches. Each method has its own advantages/disadvantages, and refiners must select the best independent pilot plant testing laboratory to suit their requirements and constraints. Based on experience, both approaches provide adequate essential information for hydrocracking catalyst evaluation and selection. It must be noted that some laboratories are more preferred by refiners than others. At least one independent laboratory requires 24 mos pre-booking before the actual test date. As such, a refiner should proactively contact these independent laboratories as soon as the new cycle starts.

The testing fee for each catalyst loading scheme can range between $45,000 and $95,000. So, who pays for the pilot plant testing? The refiner may have to pay the total cost, or the catalyst suppliers may agree to share this cost. In general, the willingness of the catalyst supplier to share the testing cost increases with the value of the catalyst package.

Accurately interpreting the results of the pilot plant testing is complex and requires experience and expertise.

A catalyst change-out turnaround is a large, complex logistical undertaking and a structured and disciplined approach to planning—along with attention to detail—is the foundation upon which good catalyst turnaround execution is based. A small, empowered, multi-discipline pre-shutdown team should be given the responsibility to conduct all planning and preparation activities and then provide the continuity throughout the turnaround execution phase. Established project management practices should be used in the planning phase and an integrated operations and maintenance schedule should form the basis of the turnaround plan.

Critical path activities should be subject to careful risk assessment and challenged and optimized aggressively throughout the planning and execution phases. Contingency plans should be prepared for the most likely and high-consequence events that may impact the turnaround (e.g., weather-sensitive work like catalyst loading, uncertain or risky activities, and critical materials or spares that may be required). Confirm the quantities of fresh catalyst onsite prior to the turnaround and ensure the catalyst is stored in a safe, dry, cool warehouse. Do not add additional maintenance or engineering work that could be conducted during normal operations just because the unit is down.

A detailed plan is required for the catalyst handling work, covering the spent catalyst unloading and fresh catalyst loading, along with a dedicated health, safety and environment (HSE) plan. Detailed procurement and logistics plans should involve the catalyst vendor and the catalyst handling contractor as early as possible.

The area around the reactor(s) must be a controlled space with only authorized entry due to the elevated level of activity involving people, cranes, vehicles, equipment and, consequently, the high risk attached to these activities happening in a small area. Clear planning and communication of how the space is to be used and controlled (crane locations, truck and forklift access ways, etc.) are required.

Utility requirements and availability, particularly air and nitrogen (N₂), must be understood and provided with a high degree of certainty. The N₂ requirement during catalyst unloading is often underestimated, especially when unloading from more than one reactor at a time, when N₂ usage is high elsewhere for purging vessels and columns of hydrocarbons and hydrogen (H₂). Ample N₂ supply and distribution capacity will reduce delays caused by high oxygen levels in the reactor(s) resulting from insufficient N₂ supply capacity. A dedicated air supply to the catalyst dense-loading machine is preferred to avoid fluctuating supply pressure caused by offtake of other users.

Do not assume the catalyst loading diagrams provided by the catalyst vendor are free of errors. Challenge any details that do not appear to be correct, based on your understanding. For example, if the vendor proposes to use 6-mm ceramic balls to directly support 1.7-mm extrudate hydrotreating catalyst it should be challenged, especially if the hydrotreating catalyst is to be sock loaded. In this case, 3-mm support material should be used under 1.7-mm extrudate catalysts (a maximum size factor of 3:1 is normally applied) to avoid catalyst migration.

A minimum 150-mm layer of large-diameter ceramic balls should be used above the outlet collector, and minimum 75-mm layers of finer ceramic balls are deemed appropriate to support the catalyst. Layers of less than 75 mm have an increased chance of allowing migration due to variations in the layer depth due to poor loading practices. Ceramic hold-down material is not used above 3-mm extrudate catalyst beds (i.e., more catalyst can be loaded). For smaller catalyst, the hold-down material is still used to guard against the potential for reverse flow.

Shutdown and startup procedures should be reviewed in detail prior to the turnaround to build in changes in catalyst conditioning requirements, lessons learned from the last shutdown and startup, newly acquired best practices from other locations, and to consider specific requirements or opportunities for the coming turnaround.

The turnaround planning and execution phases should be a cooperative effort between engineering/maintenance and operations, with an integrated plan and schedule (starting with feed out) all the way to product rundown on grade to storage. This integrated plan should define when each piece of equipment will be handed over from operations to engineering/maintenance in a gas-free condition and when it is expected back for startup preparations.

Prior to the turnaround, a detailed discussion with the catalyst vendor and the catalyst handling contractor should finalize the detailed plans and contingencies for catalyst unloading and loading activities.

Throughout the catalyst change-out turnaround, the highest probability of delays and errors occur during handovers and transitions, such as the handover of equipment from operations to engineering/maintenance after shutting down the unit and gas-freeing individual systems and equipment, the handover between engineering/maintenance to the catalyst contractor, and when inspectors are requested to perform equipment inspection.

Safe work permit renewals for confined space entry, for example, occur at the start of every shift (2–3 times per day), so any delays in this process can add a considerable amount of time to the entire turnaround. All delays on each interface can add hours and sometimes days to the turnaround—by managing these interfaces and handovers efficiently, considerable time can be potentially saved during the turnaround.

Shutdown, catalyst unloading and reactor inspection

As discussed here, the process of shutting the unit down is one of the highest risk periods for hydroprocessing units. During this non-steady-state operation, conditions (temperature, pressure, composition, etc.) are fluctuating significantly and, in some steps, very close to engineering limits. An incident on shutdown can impact the scope of the turnaround (e.g., a heat exchanger leak or a heater tube failure).

In the case of a hydrocracker unit, the whole complex is likely to be shutting down at the same time, including the vacuum distillation unit, hydrogen plant and sour gas treating units. Particularly for a hydrocracker unit, ensure that the gasoil flush and hot hydrogen strip are performed thoroughly to remove as much heavy hydrocarbon from the catalyst as possible. If these steps are cut short, the probability of a delayed entry into the reactor will be higher due to lingering hydrocarbon on the catalyst [high hydrocarbon vapor (LEL) in the reactor]. Do not exceed maximum cooling rates of 28°C/hr for heavy wall equipment, especially the reactors, as it can lead to hydrogen stress cracking and overlay disbonding.

Cooling of the reactor and catalyst system at lower temperatures is the rate-controlling step that determines when the reactor can be opened. Ambient temperature is a key factor and some locations have experience with accelerated cooling using liquid nitrogen injection (as shown in FIG. 2); however, the metallurgy, piping design and equipment at and downstream of the injection point must be rated for the lower temperature operation. The composition of the recycle gas changes significantly during the accelerated cooling operation (nitrogen injection), and the impact on the recycle gas compressor must be evaluated prior to implementing this practice.

For most units, maintaining full hydrogen pressure for as long as possible down to the minimum pressurization pressure (MPT), as well as other cooling strategies, will allow for an acceptable cooling rate. Most refiners use the nominal MPT (to prevent the risk of brittle fracture or temper embrittlement) for their reactor system, which is determined by the bulk metal composition of the reactor base metal. It is worthwhile to request a specific MPT to be calculated by the reactor fabricator (J-factor calculation) for the reactor(s), based on impact testing (Charpy-V impact testing) of the base metal test blocks and welded coupons from the actual retained material used in the reactor fabrication. Most reactors have an actual MPT well below the nominal MPT of 66°C for post-1990 metallurgy (2¼ Cr – 1 Mo–0.25 V and 3 Cr – 1 Mo–0.25 V) (i.e., closer to ambient temperatures). Note: Naphtha hydrotreating (NHT) and kerosene hydrotreating (KHT) reactors (1¼ Cr – ½ Mo) do not have an MPT constraint above ambient temperatures.

Once the reactor(s) are cooled and blinded from the rest of the high-pressure circuit, the inlet elbow is removed and nitrogen purge connections are made to the reactor(s), the catalyst handling contractor can begin catalyst unloading. Strict control of reactor isolation (blinds), nitrogen connections and the inert/air atmosphere inside the reactor(s) must be maintained throughout the full process of unloading spent catalyst and reloading fresh catalyst. Fatalities have occurred during catalyst unloading, mostly due to nitrogen asphyxiation or to accidents in the inert atmosphere of the reactor. A detailed safety and rescue plan should be produced and agreed upon before work commences.

The environment inside the reactor is non-life supporting (almost 100% nitrogen) and risks associated with the potential presence of hydrocarbons and pyrophoric metal sulfides must be managed, as well. If oxygen ingress cannot be controlled, the carbon on the catalyst will oxidize to form carbon monoxide CO, carbon dioxide (CO₂) and potentially nickel carbonyl (an extremely poisonous gas), along with generating excess heat. These risks must be actively mitigated, including continuous monitoring of the reactor(s) for temperature, oxygen and other contaminants mentioned above.

Once the top distribution tray is opened and any physical filter material is removed by the inert entry team, the graded bed material and any demetalization catalyst layers are normally removed by vacuum (FIG. 3). Sampling of the spent catalyst in these top layers, including the first hydrotreating bed, is important to confirm the level of feed contaminants (V, Ni, As, Si, Na, etc.) during the previous cycle. The top hydrotreating catalyst bed in a hydrocracker unit is often very hard and difficult to remove; this is caused by a very hard crust or agglomerated catalyst. It is common for this bed to require the use of jack hammers to break it up before the catalyst can be removed.

The other beds in the reactor system are normally gravity-dumped from dump nozzles at the base of each catalyst bed (FIG. 4). Only 65%–75% of the catalyst will flow from the bed under gravity. The inert entry team then enters the reactor and “chases” the remaining catalyst and ceramic balls from the bed manually, although robots are being developed for some of this work. The manway sections are then removed from the catalyst support grid and the quench zone trays to access the next catalyst bed, where the process is repeated until the reactor is empty.

During catalyst removal in the bottom of the reactor, it is possible that catalyst and small-diameter ceramic balls fall through the outlet collector and into the reactor outlet elbow. This debris must be removed prior to catalyst loading or it will be pushed through to a downstream reactor or feed/effluent heat exchanger on startup.

The inert entry crew should take extreme care of multi-point thermocouple arrays in the top and bottom of the catalyst beds during unloading of the spent catalyst, as these instruments are easily damaged.

After removing all catalyst and support material from the reactor(s), the internal, austenitic stainless-steel surfaces must be neutralized with an aqueous soda ash solution to protect against polythionic acid stress corrosion cracking (PASCC).^1,2 This failure mechanism is caused by polythionic acids that are formed by the reaction of sulfide corrosion products [sulfur, hydrogen sulfide (H₂S), metal sulfides] with oxygen and water. Normally, TP-347 and TP-321 stainless steel grades are used for the weld overlay and internals of hydroprocessing reactors. Austenitic stainless steels are sensitized for PASCC by welding and/or high temperatures (370°C–815°C). Newer, chemically stabilized grades of stainless steels resist sensitization for some time and therefore are less susceptible to PASCC (e.g., TP-347AP). Soda ash neutralization applies to all sensitized austenitic stainless-steel surfaces, not just reactor internals, that are exposed to the mix of sulfur, air and moisture (e.g., heater tubes and some feed/effluent heat exchangers).

When the soda ash wash is complete, the reactor can be turned over to an air atmosphere. All nitrogen connections to the reactor(s) must be positively isolated (blinded or physically disconnected).

Prior to loading, the reactor must be cleaned of all loose scale, debris and other deposits fouling the internals. Of particular importance are the outlet collector, distribution trays, other quench zone trays and the catalyst support grids (profile wire or mesh screens), which must be 90%–95% clear to ensure good distribution in the bottom of the catalyst beds.

The inspection requirements of the reactor and its internals should be detailed in the inspection plan and will include:

• General cleanliness and condition of the reactor, nozzles and internals
• Reactor internal overlay cracking: spot dye penetrant testing is utilized to detect cracking in the weld overlay, especially in areas adjacent to nozzles and at load-bearing, high-stress areas
• All reactor seams and support skirt-to-shell seams should be inspected on a set frequency.

The authors emphasize that the shutdown of the unit and the catalyst unloading are the phases of greatest uncertainty and hold the highest risk of unplanned events. Therefore, the completeness of the planning, attention to detail and the presence of refinery and contract people with experience and expertise to deal with these potentially schedule-destroying events cannot be overemphasized.

Part 2

Part 2 of this article (August 2022) will address the activities associated with catalyst loading and the restart of the unit. HP

ACKNOWLEDGEMENT

The authors want to thank Trevor Penny from CR International for the images and helpful comments.

LITERATURE CITED

1. The National Association of Corrosion Engineers (NACE) NACE Standard RP0170-2004, “Protection of austenitic stainless steels and other austenitic alloys from polythionic acid stress corrosion cracking during shutdown of refinery equipment,” 2004.
2. American Petroleum Institute (API) Standard 571, “Corrosion and materials: (Section 5.1.2.1) Polythionic acid stress corrosion cracking (PASCC),” September 2010.

The Authors

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