August 2022

Catalysts

Hydroprocessing catalyst reload and restart best practices—Part 2

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.

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. 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 2 of 2) will discuss the activities associated with catalyst loading and the restart. Part 1 of this article, published in the July issue, discussed the activities associated with turnaround planning and shutdown, catalyst unloading and reactor inspection. The two articles cover the entire process across the full catalyst cycle and many of the best practices used to manage and mitigate the underlying risks for these units.

Catalyst loading

Catalyst is manufactured in the metal-oxide form, but it can also be provided in a presulfided or presulfurized form. Note: See the Startup section for an explanation of presulfided and presulfurized catalyst. The two latter forms are more expensive; however, they provide the advantage of reducing the time and complexity of the startup. These catalysts are loaded in an inert (nitrogen) atmosphere—which adds time and complexity to the catalyst loading—although they can be loaded in air, depending on temperature and catalyst storage conditions. An economic evaluation must be made to determine the benefits and risks of using presulfided or presulfurized catalysts over the conventional metal-oxide catalysts in each case.

Catalyst loading is performed using one of two techniques: sock loading or dense loading (shown in FIGS. 5 and 6, respectively). Sock loading is simpler and faster but carries a higher risk of non-uniform loading, leading to a higher probability of flow maldistribution during operation and a shorter run length.

A dense-loading machine spreads the catalyst evenly over the full cross-sectional area of the reactor, normally using shaped ports on a specially designed, variable-speed rotor so that it falls like a uniform rain of catalyst particles. This results in greater alignment of catalyst particles and provides a harder and more homogenous catalyst bed, more evenly distributed flow and a higher catalyst density. The result is 15%–25% higher density for extrudate catalysts and, therefore, a longer cycle, as the run length is proportional to the mass of catalyst loaded, in most cases. Consequently, dense loading is maximized unless pressure drop limitations are reached.

The first bed in a hydrotreating reactor is normally sock loaded, as it usually includes bulk physical filtering materials and/or a graded bed (layers of successively smaller catalyst) to limit pressure drop issues resulting from fine scale and other contaminants in the feed. The demetalization catalysts are also normally sock loaded. The hydrotreating catalyst in the first bed can be sock or dense loaded, or a combination, depending on pressure drop constraints. The bottom portion of the bed will be dense loaded if a combined sock-/dense-loading approach is proposed.

Numerous different proprietary dense-loading machines are used, most of which can provide excellent dense-loading results. It is important to ensure that the technician performing the dense loading is experienced with the machine being used, as the quality of loading will have a direct impact on refining margins for the unit over its subsequent catalyst cycle. Using the lowest cost option for dense loading will likely prove very costly.

Stab-in thermocouple assemblies are withdrawn for dense loading of the catalyst and replaced as the catalyst layer approaches the height of the nozzle. This is to avoid shadow effects in the dense-loaded catalyst bed.

Before loading begins, a final inspection of the reactor is required to ensure that the reactor, the distribution and other quench zone trays, and the catalyst support grids are clean and free of any fouling material that will affect liquid and gas distribution across the reactor. The catalyst dump nozzles are closed after the ceramic plug and locking plate are installed.

The bed depth of the layers of ceramics and catalyst is normally controlled by a combination of markings on the reactor wall and outage measurements to the ceramic/catalyst bed surface, after ensuring the bed is level. It is vital to confirm that the minimum bed depths for ceramic balls are adhered to, and a method to determine if these layers are uniform (i.e., a flat profile) should be used. This becomes more important as the inside diameter of the reactor increases. This guards against catalyst migration and ensures good liquid distribution during operation.

Equal preparation (or more) is required on the ground as in the reactor(s) for catalyst loading. Cranes must be located so that each lift is simplified to ensure that the catalyst supply to the top of the reactor can keep up with the loading rate of the dense-loading machine. Any unplanned interruptions to dense loading will affect the final loaded catalyst density.

Catalyst is delivered in catalyst bins, bulk bags or drums. Ceramic balls and catalyst should be brought to the site from the warehouse in catalyst bed lots, and catalyst bins/bags/drums must be clearly marked with the catalyst name and size to limit potential mistakes by loading the wrong catalyst in the wrong bed.

A static hopper is installed above the top reactor manway (FIG. 7A) to facilitate catalyst delivery into the reactor. Catalyst bins and bulk bags are the most efficient means of storage and handling for catalyst and can be lifted to the top of the reactor to supply catalyst to the static hopper and, from there, into the reactor. Catalyst and ceramic balls delivered in drums or small bags must be decanted into a mobile hopper (FIG. 7B) for transporting between the ground and the top of the reactor. All hoppers should have coarse mesh screens installed to avoid ingress of any extraneous material or clumps of agglomerated catalyst (FIG. 8) that may block the loading pipe and dense-loading machine. Setting up the static hopper, installing the loading pipe, adjusting the scaffold platforms and weather protection, and finally setting up the dense-loading machine can take several hours.

When loading ceramic balls into the bottom of the reactor, avoid broken or cracked balls in the region around and on top of the outlet collector—the use of spiral inserts, like those shown in FIG. 9, in the top of the loading pipe or the use of a proprietary loading system^a, for example, is recommended.

Weather protection will be required in most locations. Most catalysts, especially hydrotreating catalysts, are sensitive to moisture and must be protected from any rain. The hoppers should have fitted lids and the whole area around the reactor manhole and the static hopper should have weather protection. If a drum decanting platform is being used on the ground, it should have its own weather protection, the same for all catalysts stored onsite (bulk bags, drums or bins).

The fresh catalyst sampling protocol must be clearly communicated to the catalyst handling contractor and sample containers provided. The catalyst loading must have a robust quality assurance/quality control (QA/QC) process that should include duplicate identification and the counting of catalyst bins/bags/drums loaded into the designated reactor/bed. Each bin/bag/drum number, as well as the catalyst batch number, should be recorded against the respective reactor/bed. This prevents any errors and provides important information if a catalyst performance issue occurs after startup.

The dense-loading technician will stop loading at intervals to check the bed profile and the catalyst density to ensure the bed is loading consistently flat and not dished or mounded. A dished or mounded profile will affect liquid distribution through the bed during operation. If the bed must be levelled, it is important that a uniform layer of the bed is disturbed and levelled, resulting in a uniform layer of the equivalent of sock-loaded catalyst. Just pushing the mounded catalyst out or filling the dished section into the middle of the bed will cause distribution issues.

The dense-loading machine can only load effectively up to 300 mm–500 mm from the bottom of the distribution tray. The final part of the bed must be sock loaded.

The catalyst bed must be levelled and confirmed as level at each catalyst layer transition and at the top of the bed. This should be verified by physical inspection or, at the very least, a video inspection.

When the catalyst bed is accepted as completed, the distribution tray and other quench zone trays are cleaned again and closed, along with the catalyst support grid for the bed above. The closing of each tray should be witnessed. The process of loading the next bed is then repeated.

Physical filtration materials may be used in the very top of the lead reactor, per stage (i.e., the last layer loaded). This material is often awkward to load and spread in the confined space between the top of a catalyst bed and the distribution tray. Vendors provide loading guidelines and catalyst-handling contractors have developed innovative ways for loading this material.

Restart

Standard preparations for startup are commenced after the top elbow(s) are reinstalled on the reactor(s) and the reactor circuit is put into its startup configuration. All equipment opened or worked on should be handed back from engineering or maintenance following documented close-up and hand-over procedures, and a detailed (piping and instrumentation diagram) P&ID-based check of the condition of the unit is performed, including a cross-check that all blinds are in their correct position.

The reactor circuit is freed of oxygen—either by evacuation or nitrogen purging—prior to rolling the recycle gas compressor. When recycle gas circulation has been established, the heater is commissioned and the system pressure is raised to 25% of normal operating pressure, with hydrogen (H₂), until the minimum pressurization temperature (MPT) is reached.

During this heating/hold phase, when the mass of the reactor and catalyst is being brought to the MPT, the catalyst is being dried out. Any moisture the catalyst has adsorbed during transport, storage and loading is slowly driven off in this drying step. Normally, by the time the reactor has reached MPT, the catalyst dry-out has been completed; however, if this step is incomplete, the catalyst temperatures must be held to complete the dry-out.

Once all reactor skin temperatures are above the MPT, the system can be pressured to close to normal operating pressure and temperatures can be adjusted (as determined by the catalyst vendor) for catalyst sulfiding. During this process, pressure testing and leak detection steps are completed to ensure the circuit is tight before introducing liquid feed and/or hydrogen sulfide (H₂S).

In liquid-phase sulfiding, the startup oil—normally a low endpoint (< 370°C) diesel—is introduced at 105°C–110°C. During the introduction of the startup oil, called pre-wetting (FIG. 10), there will be an exotherm in each catalyst bed caused by the heat of adsorption as the liquid contacts and adsorbs onto the surface of the catalyst. Type-II hydrotreating catalysts must be kept below 140°C–150°C until the catalyst is fully wetted to retain the hydrotreating catalyst structure and activity. The temperature limit for hydrocracking catalyst (prior to the introduction of H₂S) is 205°C to prevent reduction of the metal oxides on the catalyst to their base metallic (zero valence) state in the H₂-rich conditions. Any reduced metal sites cannot be regenerated and result in permanent catalyst deactivation.

Traditionally, no quench has been required to maintain the catalyst temperature within the 140°C–150°C and 205°C limits mentioned above; however, as the hydrocracker catalyst activity has increased in recent years (with the inclusion of zeolitic base material to hydrocracking catalysts), the heat of adsorption exotherms have increased. In addition, if the startup has been delayed and the catalyst subjected to an extended period (several days) of recycle gas-only circulation without any liquid feed, the wetting exotherm can be very high. In gas-phase sulfiding, the catalyst will be extremely dry by the end of the sulfiding step and pre-wetting exotherms can be extreme and must be proactively controlled with quench.

Therefore, the wetting exotherm can be significant, > 40°C for hydrotreating catalyst and > 100°C for hydrocracking catalyst, hence the low oil cut-in temperature. An additional factor that must be understood is that the wetting exotherm is extremely fast in its increase and subsequent decrease—much faster than the onset of a reactor catalyst temperature runaway during normal operation. If the reactor(s) has temperature rate-of-change alarm/trip functionality, a high probability exists of it being activated during catalyst pre-wetting. This very severe exotherm must be controlled proactively. If the operator waits until the exotherm starts in a particular catalyst bed, it is too late to control it with quench. Therefore, it is imperative to start the quench flow a few minutes before the liquid reaches the top of the respective reactor/bed. For example, in a multi-bed reactor, as the exotherm reaches the first thermocouples in the bottom of the first bed, the quench between the first and second beds should be opened (to at least 50% of the quench valve capacity) and the quench to the current bed decreased and closed. This process should be repeated for each bed (FIG. 10).

After pre-wetting and catalyst flushing are complete, the catalyst temperatures can be increased for low-temperature catalyst sulfiding (normally 230°C). At this time, the H₂ purity in the recycle gas should be above 90 vol%, ideally, and there should be no wash water or amine circulation.

Catalyst conditioning

This consists of sulfiding of the active metal sites on hydrotreating and hydrocracking catalysts and is the most important and most sensitive step in the startup procedure. Most hydrotreating and hydrocracking catalysts have metals—nickel, molybdenum, cobalt and tungsten are most prevalent—that are in the oxide state when manufactured and must be converted to the sulfide state to make them effective and stable for hydrotreating and hydrocracking reactions. The sulfiding reactions include the following (Eqs. 1–3):

3NiO + H₂ + 2H₂S →Ni₃S₂ + 3H₂O + heat        (1)

MoO₃ + H₂ + 2H₂S →MoS₂ + 3H₂O + heat       (2)

WO₃ + H₂ + 2H₂S →WS₂ + 3H₂O + heat          (3)

If the catalyst has been presulfided or pre-sulfurized, the startup process is much simpler, using a procedure provided by the vendor. For presulfided catalysts, the above reactions have been completed using a carefully controlled ex-situ procedure following manufacture; therefore, the active metals are already in their sulfided form. Startup comprises heating in a controlled manner in a H₂ environment.

Presulfurized catalyst have a sulfur-rich compound (normally a polysulfide) bound to the catalyst that decomposes at a known temperature and provides sulfur at the active metal sites during the startup. This eliminates the need to inject a sulfur compound, such as dimethyl disulfide (DMDS), into the reactor circuit. For presulfurized catalyst, the startup procedure is like that for catalyst, where the active metal sites are in the oxide form but simpler and so a little quicker. The startup of pre-sulfurized catalyst is vulnerable where the startup is interrupted shortly after the decomposition of the sulfur-rich compound. If the reactor circuit must be depressured before the high-temperature soak, a loss of sulfur may require replacement by DMDS injected into the high-pressure circuit.

In-situ sulfiding can be done in a gas-phase procedure or in a procedure that uses liquid feed to the unit, normally on full recycle mode. To enable an efficient catalyst sulfiding step, a sulfur compound is injected into the high-pressure circuit, either directly or with the liquid feed, to provide the sulfur for the sulfiding reactions. The most common sulfiding compounds are dimethyl disulfide or DMDS (CH₃SSCH₃) and Sulfrzol 54 (C₄H₉S₄C₄H₉), shown in Eqs. 4 and 5:

DMDS: CH₃SSCH₃ + 3H_{2 →}2CH₄ + 2H₂S                    (4)

Sulfrzol 54: C₄H₉S₄C₄H₉ + 11H₂ → 8CH₄ + 4H₂S        (5)

DMDS is preferred due to its high-sulfur content, low decomposition temperature and low vapor pressure.

Gas-phase sulfiding has been replaced by liquid-phase sulfiding, especially since the development of Type-II hydrotreating catalysts. As mentioned above, the high-activity Type-II hydrotreating catalysts have a temperature limit of 140°C–150°C prior to being fully wetted. To allow gas-phase sulfiding, a protective compound must be applied to the catalyst before delivery and loading to allow the catalyst to be fully sulfided at temperatures up to 315°C–320°C before liquid feed is cut in.

At 200°C, the injection of the sulfiding agent can be started. Increasing the injection rate should be done carefully and in a step-wise manner to avoid excessive exotherms from the decomposition of the sulfiding agent and the initial sulfur laydown. The injection rate can be kept at the maximum (set by the catalyst vender) until H₂S breakthrough is achieved, unless excessive catalyst exotherms are experienced, at which time the injection rate must be reduced until the exotherms return to within acceptable limits.

Regular sampling and composition testing of the recycle gas, low-pressure separator and fractionator off-gasses, and liquid streams should commence with the start of catalyst sulfiding to monitor the progress and allow the calculation of a sulfur balance. Shortly after sulfiding is begun, water will begin to accumulate in the cold high-pressure separator and will continue until sulfiding is complete.

Low-temperature sulfiding conditions should be maintained until H₂S breakthrough is achieved and at least 70% of the theoretical sulfur up-take has been injected in the form of the sulfiding agent. When these criteria have been met, the catalyst temperatures can be increased for high-temperature sulfiding.

Through the catalyst sulfiding process, recycle gas H₂ purity decreases as H₂ is consumed by the decomposition of the sulfiding agent and the only H₂ make-up is to compensate for solution losses. If the H₂ purity drops to 70%–75%, the high-pressure bleed should be opened to purge some recycle gas and allow a higher H₂ make-up. This will result in H₂S loss from the recycle gas and should be minimized.

As the H₂ purity drops and the methane (CH₄) content increases through the sulfiding process, the density of the recycle gas increases significantly and the load on the recycle gas compressor driver also increases. For an electric motor-driven recycle gas compressor, the load on the motor (amps) should be monitored and the H₂ purity increased, if necessary—at the expense of H₂S loss—to ensure that the compressor does not trip on motor overload.

After H₂S breakthrough has occurred and before catalyst temperatures are increased, the injection rate of the sulfiding agent should be decreased by about 30%–50% to keep the H₂S content in the recycle gas in the 1 vol%–1.5 vol% range. If the H₂S content goes too high, the H₂S losses resulting from solution losses become excessive. As the catalyst temperatures increase, the rate of sulfur laydown on the catalyst increases again and the injection rate of the sulfiding agent can be increased. The high-temperature sulfiding soak for hydrotreating catalyst is normally 315°C–320°C and 290°C for the hydrocracking catalyst.

When increasing catalyst temperatures from the low-temperature sulfiding step in the transition to high-temperature sulfiding, an additional precaution is required for highly active hydrocracking catalysts (higher zeolyte content). An injection of a nitrogen (N)-based modifier stream (ammonia or high-N liquid feed) is required before the catalyst reaches 260° to avoid the impact of excessive cracking of the circulating liquid and coking of the fresh, hyperactive hydrocracking catalyst. Production of significant quantities of liquefied petroleum gas (LPG) is a sign that hydrocracking reactions are occurring: the temperature increase should be stopped and the injection rate of the nitrogen-based inhibitor increased.

The high-temperature soak period (normally 4 hr–8 hr) commences after the catalyst temperatures reach the designated high-temperature sulfiding temperatures. The H₂S content in the recycle gas should be controlled in the 1 vol%–2 vol% range. When this period is complete and when catalyst bed temperatures have stabilized (no exotherm), no more water is being produced and the injection rate of the sulfiding agent has reduced to the level of the H₂S losses via solution losses (recycle gas H₂S content stable or increasing slowly), the catalyst sulfiding step is deemed to be complete. The sulfur balance should also confirm the completion of sulfiding.

A catalyst sulfiding process that is smooth and relatively fast normally creates the best results. Time wasted between the various startup steps results in a higher probability of unwanted outcomes, such as running out of sulfiding agent or having the catalyst at elevated temperatures (> 200°C) for too long (hr) before injecting the sulfiding agent.

After completing gas-phase sulfiding, the catalyst temperatures are reduced for the cut in of liquid feed (see the discussion above on catalyst pre-wetting). If liquid feed has already been established, normally on circulation, the catalyst temperatures should be reduced for the switch to normal feed quality: straight run vacuum gasoil (VGO) or diesel for a hydrotreating unit, or VGO for a hydrocracker. Normally, cracked or high endpoint feedstocks are not permitted for 3 d–7 d to allow the catalyst to stabilize without excessive coke laydown on the fresh catalyst.

After completing liquid-phase sulfiding in a hydrocracking unit, the circulating startup oil (diesel) will be completely desulfurized and switching to VGO will result in a significant and rapid temperature rise in the hydrotreating catalyst beds. VGO can be cut in over a period of 3 hr–5 hr at constant catalyst bed inlet temperatures (determined by the catalyst vendor) and in incremental tranches to control the transition.

During this transition there should be no hydrocracking conversion activity. The hydrocracking catalyst will be sensitive to the high organic N-slip from the hydrotreating catalyst until target hydrotreating temperatures and hydrodenitrogenation (HDN) activity are attained. The organic N-slip will temporarily passivate the hydrocracking catalyst, reducing its initial activity. The aim, once the liquid feed has been switched to 100% VGO, is to increase the hydrotreating catalyst temperatures quickly, within the constraints set by the catalyst vendor, to establish the design N-slip to the hydrocracking catalyst while keeping the hydrocracking catalyst temperatures as low as possible.

When design conditions have been reached at the outlet of the hydrotreating catalyst, the hydrocracking catalyst temperatures can be increased carefully. A descending temperature profile should be adopted in the hydrocracking catalyst beds (each catalyst bed inlet temperature lower than the previous bed inlet). Once conversion has been established, the feed rate can be increased to 70%–75% of design and the fractionation section lined out. Over time, during this period, the organic N will be stripped from the hydrocracking catalyst and its activity will slowly increase. This additional activity will allow a higher conversion, or a higher feed rate at constant conversion.

Two-stage hydrocracking unit

For a two-stage hydrocracking unit, two approaches to the operation of the second stage are available once VGO is introduced. The second stage of a two-stage unit normally operates in an almost N-free environment except for a small organic N-slip from the first stage (< 5 ppmw) and a quantity of ammonia in the recycle gas (higher if the unit does not have a recycle gas scrubber).

As mentioned above, the organic N-slip from the first stage will be high until the hydrotreating catalyst reaches its target temperature and HDN activity. If the hydrocracking catalyst can tolerate the high initial organic N-slip, the approach outlined above can be followed. However, if it is deemed inappropriate to subject the second-stage catalyst to high levels of organic N, then the feed can be removed and the catalyst temperatures reduced to < 260°C, until such time as the organic N-slip from the first stage is within specification (normally 12 hr–24 hr). When the second-stage feed (recycle oil) is nitrogen-free, it can be reintroduced and the second stage conversion established, as above.

FIG. 11 provides an example of the predicted behavior of the main process variables of a two-stage hydrocracker for a typical startup procedure (liquid-phase sulfiding), as described above.

The catalyst vendor will suggest temperature limits on the operation of the hydrotreating catalyst as well as the conversion levels for the first few weeks of operation. During this initial period of operation, it is beneficial to limit the severity of operation (no difficult refractory feeds or full conversion) until the catalyst has stabilized. These short-term limitations, for a few weeks, can add several months to the catalyst cycle. HP

NOTES

  ^a Petroval’s S-Load system

ACKNOWLEDGEMENT

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

The Authors

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