Increased margin from updated internals and catalyst
A low cost hydrocracker revamp delivered extra value to a refinery by increasing diesel yield, cycle length and safety.
FAWZI AL-SOMALI, Saudi Aramco Shell Refinery Co.
EDWIN MAAS, Shell Global Solutions International B.V.
THEO VISSER, Criterion Catalysts & Technologies
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Despite the important role that reactor internals play, many refineries continue to use conventional or bubble-cap distribution trays in their hydrocrackers. In doing so, they may be failing to capture the maximum value from their units. For example, the latest generation reactor internals may enable the use of a higher selectivity catalyst to produce higher volumes of valuable middle distillates; this could be worth $3.5 million per year for a 200 t/h unit (based on a $14/bbl naphtha–diesel spread). The benefits for some refiners, such as SASREF, a joint venture between Saudi Aramco and Royal Dutch Shell, have been even greater.
Saudi Aramco Shell Refinery (SASREF) is one of the world’s largest and most technologically advanced refineries. It has a complex configuration that includes a hydrocracker, a visbreaker, a thermal gas oil unit and an aromatics section. This enables it to generate a high value product slate that includes diesel, kerosene, liquefied petroleum gas and naphtha, which it uses as a chemical feedstock and gasoline component.
The SASREF hydrocracker was initially designed to have a 6000 t/d capacity. Before the revamp, this had increased to 7500 t/d, which made it one of the largest such units in the world. When it was built in 1982, the hydrocracker was configured for maximum naphtha but, by 2012, the momentum in global dieselisation meant that middle distillates, not naphtha, was now the highest value product stream. So, keen to maintain its position as one of the world’s most competitive refineries, SASREF took bold, decisive action.
The refiner’s objectives were to retune the unit to maximise middle distillates and, to further enhance its profitability, increase the unit’s capacity and cycle length. However, it was crucial that any changes were implemented efficiently and in one turnaround in order to maximise the project economics.
To achieve these objectives, SASREF worked closely with â€¨Shell Global Solutions and â€¨Criterion Catalysts & Technologies (Criterion). Teamwork was a key factor in the success of this project; there was high quality interaction between the three parties that enabled the project to benefit from the refiner’s site specific knowledge and the licensor and catalyst supplier’s global operational and technical expertise.
The implementation phase was particularly successful. It is difficult to overstate the importance to SASREF of getting it right first time, safely and on schedule, but the refiner showed strong leadership during preparation and implementation was safely completed around eight days ahead of schedule.
The reactor internals
Over the years, some of the liquid distributors in SASREF’s hydrocracker had been upgraded but not replaced by latest generation reactor internals. This unit revamp, therefore, provided a major opportunity for improvement in this area.
The latest generation reactor internals include:
• High dispersion trays, which achieve near-perfect wetting of the catalyst right at the top of the bed, thus enabling an ultra uniform utilisation of the catalyst and minimising radial temperature differences
• Anti-fouling trays designed to reduce pressure drop build-up
• Quench internals for uniform process and quench mixing at the interbeds
• Robust catalyst support grid panels
• Ultra low bottom baskets
• Reactor internal skirts to accommodate the elevations that may be required for minimisation of interbed spacing and maximisation of catalyst bed utilisation and volumes.
When installing the latest generation reactor internals, there are often opportunities to capture additional value. For instance, depending on the axial temperature gradient, it could be possible to combine beds to increase catalyst uptake. And, when the quench internals, bottom baskets and filter trays are designed to occupy minimal reactor volume, they can enable more catalyst to be loaded into the reactor. This can be used to help extend the catalyst cycle life, lower the weighted average bed temperature, increase the throughput or process heavier, less expensive feedstock, which results in a higher margin. In some cases, the unit yields can be redefined for better economics.
In addition, some latest generation reactor internals are designed for fast removal and installation, which can help to increase days on stream and be worth significant revenue. There are also safety advantages, such as far less confined space working time because cutting and welding are not required, and large manways that enable fast entry and exit. In some cases, no physical entry is required to access the bed.
Increasing catalyst volume by combining beds
When replacing conventional reactor internals with latest generation hardware, refiners can typically benefit from an increase in the volume of catalyst that can be loaded into the reactor. If the same catalyst was used as in the previous cycle, this could help to extend the cycle length. Alternatively, a moderate activity catalyst could be used that has higher middle distillates selectivity. SASREF selected the second option.
SASREF’s revised unit objectives warranted a close look into the possibilities to combine beds in this way. Extra catalyst volume would be required in order to apply more middle distillate selective, moderate activity catalysts that still could process the future higher feed rate for the entire interval between planned catalyst change-outs. The latest middle distillate selective catalysts are a good fit because they limit the extent of secondary cracking to naphtha and lighter products.
To investigate the thermal behaviour of the existing system and the new options for the revamp, Shell Global Solutions conducted a thermal stability assessment for multiple bed combination scenarios. This took into account the proposed fresh feed intake, which SASREF wanted to increase from 7500 to 8000 t/d, the proposed catalyst system, the capacity of the existing interbed quench facilities and the hydrocracker’s operating mode. It verified that the rate of occurrence of temperature excursions could be adequately minimised and that excursions, should they happen, would be adequately moderated by the available operational handles.
At SASREF, the two pretreatment reactors had four beds each and the cracking reactors had six beds each. Ultimately, the thermal stability assessment led to a recommendation to merge beds 3 and 4 in the pretreatment reactor and beds 1 and 2 in the cracking reactors (see Figure 1).
As a result of merging the beds and the hardware’s slimmer profile, the reactors’ catalyst volume increased substantially. The catalyst load 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.
Reducing the number of beds has a positive impact on three main parameters:
• Less hardware is required, which reduces capital expenditure
• Less turnaround time is required in every future catalyst exchange, as there are fewer manways to open and close
• There is easier and faster catalyst loading due to longer beds.
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