How can we minimise the time for catalyst to be fully sulphided?
Responses to a question in the Catalysis 2021 Q&A feature
Various from Advanced Refining Technologies, Arkema, CREALYST-Oil, Eurecat, MERYT Catalysts & Innovation, Porocel & Rector Resources.
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Stéphane Creton, Product Manager Refining Applications, Eurecat - email@example.com
While there may be unit specific opportunities to reduce the time required for catalyst sulphiding, catalyst manufacturers have optimised their procedures to minimise the time required without undue risk to catalyst performance. In many cases, excess time requirements for sulphiding are the result of external factors rather than the procedure itself. Mechanical failures, emission limitations, heat integration, special routing, and even staffing limits cause start-up to extend beyond plan. Even after sulphiding is complete, the catalyst needs to have a proper break-in to assure it will perform as expected, further increasing the time to return to normal operation.
The solution is to eliminate the in situ sulphiding step altogether by using fully activated catalyst. Eurecat can perform the entire sulphiding and activation process before the catalyst arrives at the refinery. Start-up is simply a matter of adding feedstock and bringing the reactor up to temperature, similar to a restart after a short shutdown. The break-in time before addition of cracked feedstock can also be eliminated to further reduce the time to normal operation.
Eurecat’s Totsucat is the most widely applied ex situ activation and has a stellar track record. It has been available for over 10 years and used to pre-activate thousands of tonnes of catalyst. Refinery feedstocks from naphtha to residue and units including ULSD, FCC-PT, lube oil, hydrocracking, tail gas, and renewable diesel have all used Totsucat. When Totsucat is combined with Cracked Feed Protection (CFP), cracked feedstocks can be added to the unit from feed-in, eliminating the headaches of a break-in period as well as capturing the additional margins from processing cracked feeds. For hydrocracking units, Totsucat can be combined with Hydrocracking Acidity Protection (HC-AP), ex situ conditioning to control passivation of zeolite acidic sites. HC-AP does not require additional passivation with nitrogen compounds during start-up. A risky hydrocracker start-up with in situ activation and passivation is made simpler and faster with Totsucat HC-AP. All types of Totsucat can also be treated by Eurecat’s AirSafe process to temper self-heating characteristics of the metal sulphides, allowing Totsucat catalyst to be handled and loaded in air without the inert atmosphere normally required.
Totsucat, CFP, HC AP, and AirSafe provide solutions to not only return units to operation much more quickly, but also to ensure the unit performs as expected. While well-controlled in situ sulphiding will deliver the expected catalyst performance, not all in situ sulphiding procedures proceed as expected. Ex situ sulphiding eliminates the risk of a less than ideal in situ sulphidation and a unit which performs below expectation or must be changed sooner than scheduled.
Akash Moradia, Technical Service Manager, Advanced Refining Technologies (ART) - firstname.lastname@example.org
Catalyst sulphiding and unit start-up can add significant time to a unit turnaround before normal feed can be processed. For many units, the time from introducing a hydrogen atmosphere into the reactors to normal operations can range from 2-7 days. There are several steps that can be done to minimise start-up time:
1. The most significant time saving is when the catalyst is pre-activated and has had cracked feed protection. Cracked feed protection eliminates the initial three-day period after sulphiding when only straight run feeds can be processed. Also, catalyst pre-activation eliminates the catalyst dry-out step, in addition to the low temperature and high temperature sulphiding hold steps. The catalyst dry-out can save approximately 4-12 hours, while removal of the catalyst sulphiding steps can save ~6-24 hours.
2. If the catalyst is not pre-activated, using a low-LOI catalyst (<10 wt% LOI) will provide significant time savings as well. Use of low LOI catalysts allows the refiner to heat up the reactor to ~350°F (177°C) prior to feed introduction, which allows the reactor to reach minimum pressurisation temperature (MPT) quicker while minimising the time for catalyst dry-out and feed introduction. This can save the refiner ~4-12 hours depending on the unit limitations. Comparatively, high LOI catalysts (>10 wt% LOI) limit the temperature before feed-in to 240-275°F (115-135°C).
3. A refiner can also save 2-4 hours once the feed has been introduced. Once the feed has gone through the rectors and reached the separators, the reactor temperatures can be raised while the unit is still being flushed. This allows the unit to be close to or at low temperature sulphiding temperatures when the feed is put into a recycle loop.
4. Finally, a refiner can save time during the sulphiding process by ensuring all necessary equipment is on hand and ready to use. This includes having enough DMDS or other sulphiding compound (with contingency), having enough Draeger tubes, and having qualified personnel to take samples and analyse additional samples in the lab in an expedited manner.
Guillaume Vincent, Business Segment Manager HPC, Porocel - email@example.com
Hydrotreating catalysts with metals in the oxide state are inactive for removing sulphur or nitrogen from hydrocarbon feedstocks. These metal oxides must be converted to metal sulphides to maximise activity according to one of the following options:
• In situ sulphiding, which requires sulphiding agents (for instance DMDS), heat, and hydrogen during the activation
• Ex situ preactivation (UltraCAT technology) which is fully preactivated and ‘ready-to-use’
Legacy Porocel, which is now part of Evonik, specifically developed UltraCAT preactivation to provide a solution to convert oxidic catalysts in their sulphide state while offering the following benefits:
• Provide full activity for temperature limited units (<600°F/<315°C)
• Avoid temperature excursions and metal reduction during start-up
• Minimise H2S generation for downstream units sensitive to sulphur (noble metal catalysts)
• Avoid the use of sulphiding agents, long dry-out steps, and complicated in situ activation
• Provide cracked feed protection, which allows the introduction of cracked feed immediately after start-up
For in situ sulphiding, the typical dry-out step under hydrogen may last 24 hours and even up to 72 hours. This time is necessary to efficiently dry out the oxidic catalyst prior to ramping up the unit temperature and starting the DMDS injection, which typically lasts for an additional 24 hours.
By using UltraCAT technology, your catalyst is delivered to your site fully preactivated and ready to use once the unit reaches the start of cycle temperature. With UltraCAT technology, there are no temperature holds and no DMDS injection, which results in faster start-ups with maintained optimal catalyst performances. The technology offers a cracked feed protection which allows refiners to gradually introduce the cracked feedstocks immediately after start-up, whereas a three-day break-in period is typically required for in situ sulphiding. A typical time savings comparison between in situ sulphiding and UltraCAT preactivation is shown in Figure 1.
The catalyst manufacturer has a prescribed procedure to sulphide the catalyst. Any deviation from this procedure should be reviewed with the catalyst manufacturer and approved.
Spiked feed sulphiding of a metal oxide hydrotreating catalyst is a process where a spiking agent such as dimethyl disulphide (DMDS) in the presence of H2 converts to hydrogen sulphide (H2S) in-situ, which then reacts with the metal oxide to generate an active metal sulphide catalyst. This process being exothermic has to be controlled by appropriate removal of heat to maintain temperatures prescribed by the catalyst manufacturer. Dimethyl disulphide (DMDS) is one of the most efficient sulphiding agents. With a high active sulphur content (68 wt%), it decomposes at relatively low temperatures to generate H2S that reaches stoichiometric levels in the DMDS decomposition at around 240°C.
All hydrotreating catalysts require a stoichiometric amount of sulphur to be activated. The process of catalyst activation involves sulphiding coupled with controlled metals reduction. Knowing that the whole process is exothermic, the rate at which the sulphiding agent is introduced into the feed oil has to be controlled. Both the sulphiding reaction and the reduction reaction are accelerated by increasing temperature. One needs to be cognissant of the fact that there is a time delay between the increase in DMDS flow rate and the concomitant increase in reactor outlet temperature.
Below 240°C, the reduction reaction is not pronounced and thus as we increase the sulphiding agent flow rate in the temperature range of 220-240°C, we are accelerating mainly the sulphiding reaction. For faster sulphiding, one needs to have higher H2S concentrations in the reactor while controlling the temperature. This can be done by independent control of the liquid feed rate into the reactor if feasible, and the sulphiding agent flow rate into the liquid feed. Sulphiding is carried out in two phases: primary and secondary. Primary sulphiding is done at temperatures below 250°C and secondary sulphiding is carried out from above 250°C to >300°C or start of run temperatures (SOR) (>300-350°C). Different sulphiding agents decompose differently to H2S as a function of temperature. DMDS is used extensively as a sulphiding agent because it starts to generate H2S at temperatures as low as 190°C and decomposes exclusively to H2S and methane at temperatures as low as 240°C. Maximising DMDS flow rate above 220°C, ascertaining a controlled exotherm by intermittent changes in DMDS flow rate, while continuing oil feed, will allow for a quick breakthrough of H2S (3000 ppm in the recycle gas). At this point after breakthrough, DMDS feed needs to be continued to establish a H2S concentration significantly higher than 3000 ppm in the recycle gas at a temperature of about 240°C. A vent of the recycle gas can then be taken, to purge the CH4 and some of the H2S*. Fresh hydrogen is then introduced to restore at least 60% hydrogen purity of the recycle gas (the minimum value prescribed by the catalyst manufacturer). The H2S concentration should still be significantly higher than 3000 ppm.
After H2S breakthrough, the DMDS flow rate is adjusted to maintain H2S, typically at 1.5-2.0% in the recycle gas, while the temperature is increased by 15-20°C per hour (or the catalyst manufacturer’s prescribed increase). Remember, as the temperature is increased, H2S is consumed to sulphide the catalyst. DMDS flow is continued at a rate that maintains the H2S concentration well above 3000 ppm. Once there is no more consumption of H2S and the temperature is >300°C, the catalyst is deemed to be completely sulphided and DMDS feed is stopped. The catalyst is sometimes soaked in recycle gas with high H2S concentrations at this high temperature for about an hour or more. The recycle gas is then partially purged*, fresh H2 is brought in, a value of H2S >5000 ppm in the recycle gas ascertained, and the unit is prepared to accept sour feed in the production mode.
*All venting and purges require the amines gas treating unit to be on-line.
Alain Rouquier, Consultant Chemical expert, CREALYST-Oil - ARouquier@crealyst.fr
The metals contained in hydrotreating catalysts are typically molybdenum with either nickel, cobalt, or tungsten as promoters. The metal oxides are not active forms, they must be activated by converting them to metal sulphides under a stable phase Co-Mo-S with the active species MoS2. The catalyst is sulphided by reaction of its active metal sites with hydrogen sulphide produced, either in situ from a reaction between available sulphur in the feed stream and hydrogen, or more commonly from sulphur in a sulphiding spiking agent, usually DMDS or DMS. The introduction of such a chemical is done with a volumetric pump to control the precise stoichiometric quantity of H2S generated.
Another method of presulphiding hydrotreating catalyst suggests contacting a hydrotreating catalyst with elemental sulphur at a temperature below the melting point of sulphur – not often used industrially.
For catalytic reforming, it is known that hydrocracking activity at start-up can be diminished if the catalyst is sulphided prior to contact with the charge stock.
CREALYST-Oil, using its experience in solid particle handling and precisely in dense loading catalysts, suggests refiners improve the sulphiding step, in efficiency and in time, by bringing homogeneity and uniformity to the catalyst bed to facilitate and increase contact between H2S and the catalyst metallic phase by avoiding any empty dead spaces around the solid. This process, using the technique of Homogeneous Dense Loading, reduces the void volume within the catalyst pellets to a minimum and gives a homogeneous uniform fixed bed with increased packed density. The approach of H2S reactant to the catalyst surface is greatly improved and the number of metallic sites in contact can be maximised. This allows a more complete, efficient, and shorter sulphiding step with reduced consumption of spiking agent, leading to fully active catalysts.
Meritxell Vila, General Manager, MERYT Catalysts & Innovation - firstname.lastname@example.org
There are three ways to presulphide a catalyst in situ: liquid phase sulphiding with a sulphur spiking agent, liquid phase sulphiding with the unit feed, and gas phase sulphiding.
Sulphiding with a sulphur spiking agent (this can be DMDS, DMS, DMSO, TBPS, TNPS, n-butyl mercaptan) includes the following steps: drying, wetting, first step sulphiding, H2S break-through, and second step sulphiding. Normally, the complete process can take 24-36 hours, depending on the catalyst and unit conditions.
Sulphiding with the unit feed will include the drying step, wetting, and sulphiding, and because the concentration of sulphur is lower, this could take up to 2-3 days to finish.
Depending on each catalyst (its quantity of active sites to be sulphided), the duration of these steps can vary. It is extremely important to follow the supplier’s instructions regarding the presulphiding procedure, specially referred to rates of temperature and flows, in order to assure complete activation of the catalyst and to avoid coking or undesirable reduction of metals.
Regarding sulphiding directly with H2S (gas phase), this would be the fastest way in situ, because the wetting step is avoided, and the active sulphidation agent, which is H2S, is directly injected into the system. However, this procedure is not normally used at industrial scale because H2S gas is not available and it seems that the activity of the catalyst presulphided by this method is lower than in the liquid phase.
There are also two ways to have the catalyst ex situ presulphided or activated that could save time in order to start up the unit. With in situ presulphided catalysts, the catalyst pores are filled with a sulphur rich liquid compound, which decomposes on start-up and converts metal oxides into sulphides. With in situ activated catalysts, the catalyst is already active, but the sites are passivated with a special compound that is released during start-up. This last way is the shortest way to start up a hydrotreatment unit.
Randy Alexander, Partner, Rector Resources - email@example.com
In situ sulphiding of a fresh catalyst load will typically require 18-30 hours. While ex situ activation can shave a few hours off of this process, the cost for ex situ treatment is typically 3-5 times higher than in situ sulphiding when all of the extra expenses are taken into consideration. These extra costs include bin rental for 3-6 months, delivery of empty bins to the processing site, return shipment of empty bins, and catalyst losses due to fines. Catalyst losses are caused by the additional handling of the material (loading into the furnace, transfer of the material to a screening station, and unloading from the screens into blanketed bins). Further, the sulphided catalyst is now softer and more fragile than oxide material. Loading this fragile material from the bins into the reactor results in additional catalyst erosion, shorter particles, and even more fines that can lead to pressure drop issues at start-up.
For in situ sulphiding, numerous steps can be taken to optimise the time required for completion of the process and to reduce the total cost. The first step is advance planning with the sulphiding service provider. This is critical for avoiding delays in the start-up once the unit is ready for sulphiding. Advance communication regarding the project details will ensure that the proper equipment is brought on-site and that the personnel and sulphur spiking agent arrive on time.
In addition, utilisation of the following equipment will minimise sulphiding time while the catalyst bed is thoroughly and accurately sulphided:
• DMDS/TBPS injection pump with web-connected telemetry: a web-connected pump (for instance, SmartSkid injection system) allows engineers and operators to continually monitor the critical parameters associated with catalyst sulphiding in near real time. These parameters should include injection flow rate, pressure, recycle gas concentration (see below), fluid temperature, and valve position. By viewing this data on a secure web page, the chemical injection rate can be optimised to ensure the proper levels of H2S are maintained in the system while the reactor temperature is adjusted to appropriate levels.
• Online H2S analyser: an online H2S analyser continually monitors the H2S level of the recycle gas. This technique is much safer than using Draeger tubes for measuring the H2S concentration, and real-time data from the analyser allows the second temperature ramp to proceed without delay as soon as H2S breakthrough occurs.
• Hydrogen purity analyser: a hydrogen purity analyser measures the gas concentration in real time. It is important to keep the hydrogen concentration above 70% during sulphiding in order to maximise the kinetic rate of the sulphiding reaction. Maintaining an adequate hydrogen level reduces the time required to complete sulphiding and also prevents premature coking in the catalyst bed.
Online monitoring of the sulphiding conditions aides in the decision-making process by minimising the time lag waiting for or debating the accuracy of lab results. Accurate control of the process also directly minimises chemical usage. This can result in savings in chemical costs of up to 20-30% since only the minimum amount of spiking agent will be injected into the unit.
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