How new reactor internals and optimised catalyst selection can capture additional margin
SASREF, one of the world’s largest and most technologically advanced refineries, worked closely with Shell Global Solutions and Criterion Catalysts & Technologies (Criterion) on a hydrocracker revamp to replace conventional reactor internals with latest-generator generation reactor internals in all its four reactors.
The latest-generation reactor internals include:
- Shell high-dispersion (HD) trays that achieve near-perfect wetting of the catalyst right to the top of the bed, thus enabling ultra-uniform utilisation of the catalyst and minimising radial temperature differences;
- Shell scale-catching trays, Shell filter trays and Shell filter and sedimentation trays (FAST) – anti-fouling trays designed to reduce pressure drop build-up;
- Shell ultra-flat quench (UFQ) interbed internals for uniform process and quench mixing at the interbeds;
- Shell catalyst support grids;
- Shell bottom baskets; and
- reactor internal skirts to accommodate the elevations that may be required for minimisation of interbed spacing and maximisation of catalyst bed utilisation and volumes.
The opportunities from installing latest-generation reactor internals included:
- increasing catalyst volume by combining beds;
- improving catalyst utilisation with Shell HD trays;
- guarding against fouling and catalyst migration;
- upgrading thermometry to resolve temperature instabilities;
- reinstalling the liquid recycle;
- customising the catalyst system;
- fast removal and installation to help increase days on stream to be worth significant revenue; and
- less shutdown time and enhanced staff safety.
Increasing catalyst volume by combining beds
When replacing conventional reactor internals with latest-generation hardware, refiners can benefit from an increase in the volume of catalyst that can be loaded into the reactor. SASREF chose a moderate-activity catalyst with a higher middle distillates selectivity that could process the future higher feed rate for the entire interval between planned catalyst changeouts.
The Shell reactor internals portfolio offers the opportunity to combine beds because it provides enhanced vapour–liquid and thermal distribution throughout the bed and takes up less space than conventional reactor hardware. This enables an increase in the volume of catalyst that can be loaded into the reactor complemented by improved catalyst utilisation. At SASREF, beds were combined in the cracking and pretreatment reactors.
As a result of merging the beds and the hardware’s slimmer profile, the reactors’ catalyst volume increased substantially. The catalyst loads in the pretreatment reactors increased by 16%. In the cracking reactors, there was a 22% increase compared with the original loaded catalyst volume using conventional reactor hardware.
Improving catalyst utilisation
SASREF’s hydrocracker had been using conventional distribution trays that are notorious for low-uniformity of vapour–liquid distribution and undesirable radial temperature maldistribution. Conventional designs typically use about 80% of the catalyst, whereas Shell HD trays optimise catalyst utilisation by achieving enhanced vapour–liquid and thermal distribution. These trays can be extremely efficient: some enable nearly 100% of the catalyst inventory to be utilised and offer high feed rate flexibility.
The total catalyst utilisation improvement (catalyst loading plus wetting) was 21% for the pretreatment reactors and 52% for the cracking reactors.
Guarding against fouling and catalyst migration
SASREF reported that, before the revamp, fouling had affected the hydrocracker unit, so the revamp team reviewed the filter tray configuration, as fouling in a hydrocracker can be costly. The team discovered that the filter tray configuration and top bed filters were of first-generation, non-standard designs that required welding repairs during shutdown with the consequent risk of catalyst migration.
As a solution, they replaced the existing filter tray configuration with latest-generation filter elements, which have a reduced slit width to enable even the smallest catalyst particles to be used as filter media. They also have quick-to-open split keys to reduce shutdown times
The wire mesh catalyst support grids were also replaced with latest-generation catalyst support grids featuring a grid screen that utilises a wedge-wire construction to help prevent catalyst fall-through and that resists fouling. The key advantages of wedge wire, aside from guarding against fouling, are that it is self-cleaning and lasts up to five times longer than wire mesh.
Upgrading thermometry to resolve temperature instabilities
SASREF’s thermometry upgrade replaced two old-fashion thermobars with multiple state-of-the-art flexible thermocouples. These thermocouples enable both radial and axial coverage that is in line with the latest design guidelines on tight hydrocracking bed temperature control and extensive temperature monitoring. In addition, the plant’s distributed control system and safety-instrumented system were upgraded to collect and process the information from these additional thermocouples. This has provided site operations and technologists with more-robust automatic temperature control and tools to identify early indications of temperature instability or maldistribution so they can take timely remedial action.
Reinstalling the liquid recycle
SASREF’s hydrocracker unit was originally built with a liquid recycle, but this was removed when the unit switched to once-through mode. By reinstalling the liquid cycle, SASREF has an inexpensive opportunity to increase middle distillates yield.
Customising the catalyst system
Technologists from SASREF, Shell Global Solutions and Criterion carried out a detailed analysis to select the most appropriate hydrocracking and pretreatment catalysts to meet SASREF’s needs. The study evaluated three different catalyst systems proposed by Criterion: Zeolyst Z-733 with Zeolyst Z-FX10, all Zeolyst Z-FX10, and Zeolyst Z-FX10 with Zeolyst Z-623. The analysis took into account the feedstock quality; the desired product slate; the design of the hydrocracker and its normal operating regime; the amount of hydrogen available; and the target cycle length.
The evaluations also took into account the liquid recycle option as a process parameter to boost middle distillate yield and to shift the yield more towards naphtha, when needed, by operating the hydrocracker in once-through mode only, thus switching off the liquid recycle. The focus was to have maximum middle distillate yields during the targeted four-year cycle length, taking into account the maximum draw-off capacities for kerosene and diesel. Criterion also advised SASREF to reuse part of the regenerated catalyst that was available on-site in order to reduce fill costs.
Following the study, SASREF opted to fill the pretreatment reactors with ASCENT® DN-3551 hydrocracking feed pretreatment catalyst and partly regenerated and fresh Zeolyst Z-503, a cracking catalyst that has high middle distillates selectivity.
The cracking reactors were filled with regenerated Zeolyst Z-733, a cracking catalyst designed to produce middle distillate range products with improved properties such as density, smoke point and cetane number, and Zeolyst Z-FX10, a flexible catalyst that can be used for high yields of middle distillates or naphtha.
Fast removal and installation to help maximise days on stream
SASREF’s hydrocracker unit’s normal turnaround window was extended from 18 to 38 days to enable the reactor internals and catalysts to be installed. For SASREF, it was economically imperative to ensure that the unit would come back on stream according this project timeline.
SASREF believes that the ways in which it mitigated the challenges were crucial to the success of the projects, including:
- widely communicating the common goal via senior management;
- adopting successful practices from other sites;
- employing high-quality people;
- selecting a highly experienced contractor;
- building reactor mock-ups so that contractors could practice assembly offline to ensure they were confident before starting the real assembly;
- developing a detailed execution plan;
- holding a scope optimisation workshop to ensure nothing had been missed;
- conducting a readiness assessment review;
- establishing very clear interfaces with the other activities and contractors that were working on the same area; and
- performing risk assessments for every critical activity.
Less shutdown time and enhanced staff safety
The new hardware means that SASREF’s future hydrocracker shutdowns will be some four days shorter than previously because the latest-generation reactor internals can be serviced much faster. This reduction in shutdown time is worth about $2.56 million per turnaround.1
There are also safety advantages such as less confined space working time because cutting and welding are not required and the reactor internals having the largest possible manways to enable people to move very quickly through the trays and quenches, which is particularly important during a serious incident
Value to SASREF
The capital expenditure on this project was low but it enhanced SASREF’s profitability by $10.5 million a year by increasing middle distillates yield by 4% and increasing the unit’s capacity and cycle length.2
In addition, adjusting the hydrocracker’s mode of operation by implementing the liquid recycle added flexibility, as it enables the product slate to swing from naphtha to middle distillates without any investment. Moreover, turnarounds will be shorter and there are safety advantages.
1Assumes a $80/t margin
2Assumes $14/bbl diesel–naphtha price differential, 7,500 t/d intake, 350 operating days a year and 4% shift from naphtha to diesel
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