Revamping a hydrotreating unit for improved hydrogenation of diesel

Transcription

1 Revamping a hydrotreating unit for improved hydrogenation of diesel A key value driver for an EU refiner Refiners have enjoyed a period of high product demand and strong refining margins for the past few years Synopsis Preem AB has implemented the gasoil project, a major investment program to optimise conversion for middle distillate production at the refinery in Lysekil. After the start-up of this project, Preem AB recognised the need to reach a new level of operational excellence in order to maximise their return on investment. Shell Global Solutions was appointed to perform a Hydrocarbon Margin Improvement Program (HMIP), working closely with Preemraff Lysekil to identify, develop and deliver margin improving initiatives across the refinery. One of the margin improving opportunities identified by the HMIP was to increase the production of Swedish MK-1 diesel (Product Sulphur = 10ppm, Max T95 = 285 C, Max Density = 820 kg/m 3, Max Total Aromatics = 5vol%) from the 2-Stage Middle Distillate Hydrogenation (2-Stage MDH) unit operating at Preemraff Lysekil. Working closely with the Preemraff Lysekil technologists, the catalyst and reactor internals team from Shell Global Solutions and Criterion Catalysts & Technologies identified that the unit throughput could be increased by implementing a catalyst & reactor internals revamp on the 1st-stage pre-treat reactor (R1) of this unit. The expected increase unit throughput was 20 Sm 3 /h. With a refinery turnaround scheduled to happen within 7 months of identifying the opportunity, the project justification, design and confirmation (through pilot plant testing), of this solution had to be completed quickly and the project fasttracked for speedy implementation. The initial economic justification for the revamp project was based on recovering the cost of the reactor internals (a one-off cost Capital expenditure) and the on-going cost of extra catalyst, within the first 5-6 months of operation. Record production of Swedish Mk-1 diesel was reported soon after the unit started-up and the unit continues to perform at the level expected when the project objectives were set, with average throughput remaining 40 Sm 3 /hr higher than in the previous run. In the event, weaker than expected margins meant the break-even point on the project was pushed back by 3 months and the investment was recovered in the first 8-9 months of operation. After 20 months of an estimated 5 year cycle, Preem AB estimate the contribution will be close to 1.7 Million US$/annum on-going. At the heart of this success story is the fact that the ENCORE Centinel Gold catalyst in the first stage reduces not only the di+ aromatics but also the mono-aromatics in the feed to the 2nd stage (Reference 1). This allowed more feed to be treated through the second stage reactor, while still achieving the stringent < 5% wt aromatic specifications for Swedish MK-1 diesel. In this paper, we discuss how the ENCORE Centinel Gold catalyst and improved reactor volume and catalyst utilisation can help to promote chemistry that makes a significant difference to a refiner s profitability. Although specific reference is made to the work done on the Preemraff Lysekil project, it should be possible to use this approach to improve the efficiency of any hydroprocessing unit in a refinery where production is limited by initial process design, existing reactor hardware or catalyst. Furthermore, if there is surplus hydrogen in the refinery, this kind of revamp offers an excellent opportunity to fix hydrogen very selectively, increasing the volume of the middle distillate fractions that make up a refiners diesel pool. Additional benefits include higher feedrate, product density reduction, when processing cracked feeds and other product quality improvements that allow even greater flexibility to blend and upgrade the diesel pool; all of which add substantial value to a refiner s operations. Introduction Refiners have enjoyed a period of high product demand and strong refining margins for the past few years. During this time, maximising plant availability and throughput were key drivers to increasing margin. Refiners have focused on process units that make the biggest contributions to their profitability. Most typically these are the conversion units, such as Hydrocrackers and Fluid Catalytic Crackers (FCC s). Hydrotreating units on the other hand are regarded as pre-treatment or polishing units where hydrogen consumption only contributes to overall costs. There is a natural tendency for refiners to select catalysts that minimise hydrogen consumption in these units. I 6

2 Since the introduction of ultra-clean/low aromatic Swedish Mk-1 diesel in 1994, Preem AB has operated 2-Stage Middle Distillate Hydrogenation units (2-stage MDH) at both their refineries in Lysekil (1994) and Gothenburg (1997). A 2-stage MDH can be operated in block mode to produce either Ultra Low Sulphur Diesel (ULSD) or the low-aromatic Swedish Mk-1 diesel (a very clean, low aromatic diesel). Ahead of the EU diesel specifications changes in 2005, the pre-treat reactor (R1) on the 2-stage MDH unit in Gothenburg was revamped to maximise the production of ULSD grade Mk-3 diesel (Product Sulphur = 10ppm, Max Density = 845 kg/m 3, Max T95 = 360 C). The pretreat reactor, R1 was designed to help with Centinel Gold catalyst technology from Criterion and reactor internal technology from Shell Global Solutions, designed to maximise reactor catalyst inventory and improve catalyst bed utilisation. Their robustness to a wide range of operating conditions, combined with their modular ergonomic design has made them reactor internals of choice for well over 350 reactor revamp projects and newly licensed grass root units world-wide (Reference 2). One of the unforeseen benefits of the revamp was the higher than expected Aromatics saturation activity in R1. Regular and detailed monitoring of the unit allowed the team to recognise that the Centinel Gold NiMo catalyst was saturating all the di+ aromatics and more importantly a substantial amount of the mono-aromatics both the LAGO and full range Gasoil feeds. When Shell Global Solutions was appointed by Preem AB to do a Hydrocarbon Margin Improvement study at Preemraff Lysekil, one of the drivers identified to improve margin was to increase the production of Swedish MK-1 diesel. To achieve this, it was recognised that the aromatic saturation capability of the 2-stage unit would have to be increased. Building on the successful revamp and switching from a conventional NiMo to a Type-II catalyst in the 2-stage MDH unit in Gothenburg (Reference 3) with the resultant increase in the hydrogenation activity, Criterion Catalysts & Technologies and Shell Global Solutions proposed a similar reactor internals + catalyst revamp for reactor R1 for the unit in Preemraff Lysekil. To avoid any loss of MK-1 production and refinery margin, it became clear that the modifications required could only be completed during the down-time at a major refinery turn-around. With the planned refinery turn-around only seven months away the project had to be fast tracked and managed if all the design, manufacturing and quality control steps were to be implemented in time. A project team comprising the unit engineer, a reactor internals technician and a catalyst technician was assembled and empowered to ensure that all the modifications, including installation of new reactor thermocouples would be completed in time for the turn-around. All aspects of the reactor re-engineering and project delivery (including subcontracted work) were project managed by Shell Global Solutions. Strong team work with high customer involvement allowed the project to go from the initial conceptual phase through all the steps of design, manufacture and quality checks to installation and completion in 7 months. The catalyst loaded in reactor R1 of the 2-stage MDH unit in Lysekil was mainly a revitalised ENCORE Centinel Gold catalyst that had been previously installed in the 2-Stage MDH unit in Preemraff Gothenburg. Since the revamp, the MK-1 production has increased by 40 Sm 3 /hr, compared to the previous cycle, substantially in excess of the original expectation to increase the throughput by 20 Sm 3 /h. By installing state-of-the-art reactor internals and a very specific type of catalyst the Project Team was able to successfully address the constraint limiting the production of the Swedish MK-1 diesel in the 2-stage MDH unit in Preemraff Lysekil. The key to success was excellence in all aspects of project management, fast-track project execution and a flexible team approach on all sides, especially during the final delivery phase of the project. A Hydrocarbon Margin Improvement Program identifies the need to increase production of MK-1 diesel The refinery wide Hydrocarbon Margin Improvement Program (HMIP) in Preemraff Lysekil was carried out in close collaboration between Preem and Shell Global Solutions experts. A recommendation from the HMIP program, which became a high priority objective, was to identify opportunities to increase production of Swedish MK-1 diesel and to implement the changes needed to achieve this. Background Information on the Refinery, Middle Distillate Feeds and Products Preemraff Lysekil is one of two refineries in the Preem AB group. Both refineries are based in Sweden. The Lysekil Refinery is a 220,000 Barrel/day fully complex refinery processing mainly a blend Urals and occasionally low-sulphur North Sea crude. The Gasoil Hydrotreating unit was revamped to a 2-stage Middle Distillate Hydrogenation unit (2-Stage MDH) in 1994 to produce two types of diesel products; a Swedish 7 I

3 MK-1 grade (MK-1) from a Light Atmospheric Gasoil (LAGO) and a full range Ultra Low Sulphur Diesel (ULSD) manufactured to EN-590 specification from a full range gasoil. The properties of these products are described below: Name Description Specification MK-1 ULSD Swedish Environmental Class 1 Diesel Standard European ULSD Diesel Table 1: The specification of both MK-1 Diesel and ULSD Product Sulphur = 10ppm Max T95 = 285 C Max Density = 820 kg/m 3 Max Total Aromatics = 5vol% Product Sulphur = 10ppm Max Density = 845 kg/m 3 Max T95 = 360 C As there has been a strong demand for Swedish MK-1 the unit is operated primarily with the LAGO feed. All MK-1 produced at the refinery is sold in the Swedish market. The growing diesel market in Sweden was a strong driver to further increase production of MK-1 diesel from this unit the environmental benefit of MK-1 compared with Standard European ULSD (EN 590) is 10ppm Sulphur for reduced SOx emissions, 20% lower particulate emissions, 5% lower NOx emissions and it also has improved cold flow properties. Identifying constraints to the production of Swedish Mk-1 Diesel The production unit that came under scrutiny to achieve the objective identified in HMIP was the 2-stage MDH unit in Lysekil. This unit consists of a co-current 60-Bar Hydrodesulphurisation (HDS) reactor, R1 (containing a Base metal catalyst), an inter-stage stripper and a counter-current Hydrodearomitisation (HDA) reactor, R2 (containing noble metal catalyst), see Figure 1. The original design capacity was 200 Sm 3 /h when operating in MK-1 mode and 310 Sm 3 /h in HDS mode. As the unit already contained Criterion s catalysts there was already good understanding of the chemistry and operating parameters that were constraining the unit throughput in MK-1 mode. Unit Configuration and Process Flows The 2-stage MDH unit was initially designed to allow operation in two different feed modes, depending on whether MK-1 or ULSD is production is required. Feeds to the unit come from the Atmospheric unit via intermediate storage tanks. When producing the MK-1, the feed to the unit is configured as shown in figure 1, with R1 in series with R2. In view of the strong demand for MK-1, this has been the main operating mode for the unit in Preemraff Lysekil. Chemistry limiting production of MK-1 diesel Having monitored the performance of this unit using Criterion s CatCheckSM tools, it was recognised that production of MK-1 from this unit was limited by the overall capacity of the unit to reduce Aromatics in the feed. To increase throughput and production of MK-1, a stronger hydrogenating catalyst was needed for either R1 or R2. A combination of Centinel Gold catalyst technology and HDS reactor internals replacement was considered the best option to meet Preem s future objectives A state-of the art high activity Aromatic Saturation catalyst had been loaded to R2 in the previous cycle and to achieve further improvement, the focus shifted to increasing the hydrogenation activity in R1. Based on the experience from the Gothenburg unit, it was recognised that a Centinel Gold catalyst would not only increase the HDS and HDN activity in R1 (which would improve the hydrogenation activity in R2), but it would also substantially reduce the poly and some of the mono-aromatics in the feed. This in turn would relieve some of the HDA duty in R2 and allow a higher unit throughput to be achieved. MK-1 Feed LAGO HDS Reactor Hydrogen Two-Stage Unit Interstage Stripper HDA Reactor Hydrogen Sulphur MK-1 Product Total Aromatics 10 ppm 5 vol% Figure 1: MK-1 Mode Feed Sulphur and Nitrogen is removed in the Hydrodesulphurisation (HDS) Reactor and Aromatics Hydrogenation is achieved in the 2nd Hydrodearomitisation (HDA) reactor A solution based on the revamp of R1, that would allow improved utilisation of a more active hydrogenation catalyst was contemplated. This would involve a complete overhaul of all R1 reactor internals, as well as an upgrade from thermobars to flexible thermocouples, and the installation of Centinel Gold catalyst technology. These changes were geared to increasing the catalyst inventory, improving catalyst utilisation of a more active NiMo hydrogenation catalyst and better monitoring of distribution in R1. The resultant benefit was higher HDS/HDN and HDA in the 1st stage of the process. This allowed higher overall throughputs to be achieved in the 1st and 2nd-stage reactors without compromising product specification of < 5% vol Total Aromatics. I 8

4 Centinel Gold technology catalyst to increase aromatics saturation activity in the first reactor and Use of ENCORE Revitalised catalyst In order to develop the most cost-effective catalyst option, it was proposed to revitalise approximately 200mT of Centinel Gold NiMo catalyst that would become available after the first successful cycle in Preemraff Gothenburg. ENCORE is a Criterion proprietary revitalisation process for recovering the activity of Type-II catalysts, close to fresh activity (Reference 4). This part of the project was coordinated by a team of Criterion and Porocel Adsorbents, Catalysts and Services to ensure that the correct quality and quantity of catalyst could be delivered to Preemraff in a timely manner. This catalyst solution was evaluated to confirm that the main project goal of increasing the unit throughput by 20 Sm 3 /hr could be achieved. Pilot Plant testing confirmed that the proposed system would increase the aromatic saturation capability in the 1st stage, by up to 17%wt compared to conventional NiMo hydrotreating catalysts (Reference 1) and this would significantly reduce the duty required in the second reactor, allowing more feed to be processed through the unit. Back to back testing of Centinel Gold catalyst technology compared to a conventional NiMo catalyst indicated that conversion of monoaromatics could be increased by up to 17% with the Centinel Gold catalyst. It was estimated that this in combination with higher reactor volume and catalyst utilisation (Reference 5), would lead to a substantial improvement in overall performance of the unit; overcoming the chemistry limitation and allowing a higher throughput to be achieved. The Shell Global Solutions reactor internals, comprising the Shell HD tray (HD tray) Ultra-flat quench (UFQ) and bottom basket were installed to increase the catalyst volume by 34%. The actual increase in effective catalyst volume utilisation was higher than this, estimated to be close to 53% (Reference 5) because inherent in the design of the HD tray and the UFQ is a highly efficient gas/liquid mixing device that serves to maximise contact with the catalyst. The full reactor internals modifications are shown in Figure 2, below. The above mentioned modifications and modular design, through ease of handling, will deliver major safety improvements and cost reductions in future shutdowns. Based on the experience with Shell Reactor internals, installed in the MHC unit in 2003, the time required to open and close man-ways as a result of the boltless man-ways is expected drastically reduce the overall shutdown time and at the same time result in great improvement with regards to safety during the shutdown (Reference 6). The enhanced dispersion and ultra uniform distribution of the liquid over the full catalyst 9 I bed is also expected to reduce the risk of localised coking and catalyst plugging which can lead to considerable delay in unit turn-arounds. All of these are re-occurring cost savings for each turn-around. Project Management Excellence, a dedicated team and a flexible approach on all sides was key to successful delivery Part of Shell Global Solutions philosophy and quality system is assuring that all new internals are mounted correctly and in accordance to the high tolerances required to achieve an optimum performance of the unit. Prior to the installation of the new reactor internals all design drawings and quality documents from the reactor internals manufacturer were reviewed and commented by Shell Global Solutions. The total design and production phase is monitored. Upon completion of the products a comprehensive mock-up inspection is performed to ensure all individual items are correct and fit the required set out by Shell Global Solutions as well as all interfaces with the reactor and the existing supports inside the reactor. In addition to replacement of the reactor internals, the scope of the project included replacement of the existing thermowell in the reactor with new Gayesco Flex-R thermocouples. During the turn-around, installation of the reactor internals and the thermocouples was managed by Preemraff Maintenance Engineering team with 24-hour on-site support from the Shell Global Reactor Internals team. Strong teamwork and a flexible approach allowed this team to manage the unexpected problems encountered in dismantling the existing reactor internals and installing the new thermocouples. As a result the revamp of the reactor stayed off the critical path during the turn-around. Preem s objectives achieved! The combination of the high activity of the ENCORE Centinel Gold catalyst and Shell Global Solutions reactor internal modifications to the 2-stage MDH unit in Preemraff Lysekil have yielded the following results: MK-1 Throughput and Production (1) MK-1 H2 Consumption (Nm 3 /m 3 feed) (2) 1. MK-1 throughput and Swedish Class 1 Diesel production has increased by 40 Sm 3 /hr, mainly due to hydrogenation of aromatic compounds in the HDS reactor, relieving some of the duty of Original situation Thermowell (3x) Ceramic balls Quench distributor Quench box, tray Distributive packing Outlet collector and ceramic balls Figure 2: Hardware changes to improve reactor volume and catalyst utilisation Before Revamp After Revamp Actual Operation Base Base + 20 Sm 3 /hr Base + 40 Sm 3 /hr Base Base Base After internals revamp Inlet device and predistributor (1) HD Tray Elevation skirt Top bed grading (2) new quench distributor (gas or liquid only) (2) HD tray, new skirt Remove horizontal thermobars (3) new ultra flat bottom basket and ceramic balls

5 the HDA reactor (slight reduction in Sulphur and Nitrogen to the HDA reactor also has an impact, due to reduced poisoning, but this is less significant). See Graph 1, below. 2. Following the revamp of R1 with Shell reactor internals and installation of Centinel Gold catalyst, the level of hydrogen consumption in R1 increased from 50% of the total consumption to 64%. This confirmed the increased hydrogenation activity of R1. Within the accuracy of measuring the hydrogen mass balance over the whole unit and possible variations in feedstock quality, the overall chemical hydrogen consumption on a Nm 3 /Sm 3 feed remained unchanged. This is in line with expectation as under the mild operating conditions in the 2-stage MDH unit, the level of hydrogen consumption will depend primarily on the difference between feed aromatics and product specification. The higher hydrogenation activity in R1, reduced the duty required in R2, and this allowed the unit to be debottlenecked and more feed to be processed through the unit. Diesel Production during MK-1 Mode Cycle After Revamp Cycle Before Revamp 300 Average after revamp Sm 3 /hr Average before revamp Sm 3 /hr Days on stream Graph 1: MK-1 Swedish Class 1 Diesel production has increased by 40 Sm 3 /hr In summary, an opportunity identified from the Shell Global Solutions Hydrocarbon Margin Improvement Program at Preemraff Lysekil led to the revamp of an existing the 2-Stage Middle Distillate Hydrogenation unit to increase production of the highly valuable Swedish MK-1 Environmental Diesel. Working closely with Criterion Catalysts & Technologies and Shell Global Solutions, a solution combining market leading hydrotreating catalyst and reactor internals was designed, developed and implemented within the space of 7 months from initial conceptualisation to installation and production, without incurring any extra time loss for making the reactor modifications. This project has allowed Preemraff Lysekil to overcome the constraints limiting middle distillate production, in particular for Swedish Mk-1 grade. And this will make a significant on-going contribution to increased revenue and delivering on one of many initiatives identified in the original Hydrocarbon Margin Improvement Program. The on-going sustained improvement in unit performance has exceeded the original expectation from the program and the premise on which the revamp project was recommended and approved. Acknowledgement The authors would like to acknowledge the contributions of the following people during the Preemraff Study and implantation of the revamp project to maximise middle distillate production at Preemraff Lysekil refinery: Mats Hornfelt Preemraff Lysekil Morgan Svanberg Preemraff Lysekil Bernard Poussin Crealyst catalyst logistics support Volker Dreessen Buchen catalyst loading and management Carl van der Grift Porocel Adsorbents, Catalysts & Services Tom Smith Gayesco Gabe Garza Gayesco Ronald Klute CRI/Criterion Liz Allen Shell Global Solutions John Nickson Shell Global Solutions Jan Stolwijk Shell Global Solutions References 1. Bordogna, E., HDA activity of SC-50 vs. SC-1 under Preemraff Lysekil conditions, Confidential Report, April Schouten E.,Stolwijk, J.,Ouwerkerk, E., et al., Choosing Optimal Quench Interbed Technology:Shell Ultra Flat Quench, BBTC, Annual Meeting, September Egby, C., and R. Larsson, Leading-Edge Technology Combined with Team Approach Achieves Winning ULSD Solution for Preemraff, ERTC 11th Annual Meeting, Berlin, November Torrisi, S.P.,Street, R.D., DiCamillo, D.J., Smegal, J.A., Bhan, O.K., A. Et al., Catalyst advancements to increase reliability and value of ULSD assets, NPRA Annual Meeting, San Fransisco, California, March Maas, E., Bordogna, E., Part A Technical Proposal for the revamp of R-2851 in Preemraff Lysekil, Confidential Report, February Davé, D Axeborne, L., private communication between Shell Global Solutions and Preemraff Lysekil on experience with Shell reactor internals installed in the MHC unit in I 10