Increasing hydrocracker reactor heater temperature is a typical strategy when upgrading heavy feedstocks but should be balanced against higher energy costs and emissions considerations. Further complicating matters is fouling and corrosion of hydrocracker unit heat exchangers by unconverted oils (UCOs). What trends do you see in resolving increased fouling from UCOs?Dec-2022
Ole Frej Alkilde, Topsoe, firstname.lastname@example.org:
When the severity of a hydrocracking unit is increased by increasing the reactor temperatures and/or processing heavier feedstocks, the fouling tendency of the UCO increases, and ultimately the UCO can become unstable, and a solid phase will precipitate out. The reason for this is an increased content of HPNA. The conventional way to control the HPNAs is to limit overall conversion by drawing a UCO bleed stream from the unit, typically 2-5% of the feed rate to the unit.
Other strategies are to reduce the reactor temperatures by using a more active hydrocracking catalyst, control the endpoint of the VGO feed to the hydrocracking unit, or revamp the unit by installing a hot separator, which removes the heavy product from the reactor effluent air cooler and thereby reduces the fouling. It is also possible to selectively remove HPNA by various industrially proven technologies like carbon bed absorption or advanced separation like Topsoe’s proprietary HPNA-Trim. This can reduce the UCO bleed rate from the unit by 60-80% and thereby increase overall conversion without reducing unit cycle length.
Dipankar Mitra, KBC (A Yokogawa Company), Dipankar.Mitra@kbc.global
In the current market that rewards efficient operations, many refiners are looking closely at furnace performance. In many cases, a detailed economic analysis based on yield gain and energy cost within emission limits is the appropriate, cost-effective approach to pursue. Typically, modern hydrocracker units have proper heat integration with high furnace efficiencies and effective recovery of high-level heat.
Therefore, increased conversion will likely result in less duty and emissions than upgrading lower-value heavy materials to distillates. While the increase in furnace outlet temperature directly affects fuel consumption and emissions, its effects extend much further. As cracking levels increase, hydrogen requirements and fractionation duties also increase. Even so, increasing conversion can be beneficial unless carbon costs are exceptionally high or upgrade margins are exceptionally low. To determine an appropriate conversion level, a comprehensive analysis should be completed.
A key aspect of the hydrocracking process involves converting crude oil into valuable components, such as fuels, lubricants, and chemicals. In order to increase the hydrocracker conversion rate, the cracking bed temperature of the unit can be increased, the nitrogen slip from the pretreat reactor can be reduced, or both can be done. Furthermore, reducing the nitrogen slip from the pretreat reactor will lead to fewer chances of forming incremental heavy polynuclear aromatics (HPNAs), which are known to foul process equipment and shorten catalyst life during conversion. While a variety of methods can be used to achieve high conversion, where each method offers a different yield/economic impact, the rate of HPNA formation increases with higher conversion and heavier feedstocks.
By recycling unconverted oils (UCO), HPNA formation and the per pass conversion are reduced for constant overall conversion. Following are some of the ways industry can manage HPNA issues:
- Limit feed FBP <600ÂºC, especially for HCGO (crack feed)
- Monitor the UCO colour as it changes from white to yellow to orange to red as HPNA concentrations rise and increase bleed as much as possible
- Use bulk metal pretreat catalysts to saturate/remove HPNA precursors
- Saturate HPNAs by using superior hydrogenation function catalysts in the second-stage cracking reactor
- Employ hot, high-pressure separator design to avoid HPNA deposits on the reactor effluent air cooler (REAC), the main location where HPNAs cause problems
- Adjust the unit design, such as installing liquid recycle filters, designing the separator with trays, and exporting unconverted oil with high HPNA content from the bottom while recycling material from the upper tray.
Marcello Ferrara, ITW Technologies, email@example.com
It is quite common for UCO heat exchangers to foul, given the highly waxy nature of the feed. Despite the operating solutions that can be found and developed on a case-by-case basis when trying to adjust a physical parameter, it should be noted that fouling deposition is not a physical matter and, by evidence, it will continue forming despite operating/equipment adjustments.
Antifoulants can sometimes be used to mitigate this problem, but they do not eliminate fouling deposition, and operators frequently struggle to run until the next turnaround in a ‘run-to-death’ mode which involves losses. ITW Online Cleaning technology can solubilise UCO fouling using a tailor-made and proprietary chemical. The chemical is specifically designed to dissolve and stabilise UCO fouling. An entire system can be cleaned in 24 hours on a feed-out/feed-in basis. After Online Cleaning, the unit can immediately resume production without the need for equipment opening and waste generation.
Matthew Stephens, Imubit, firstname.lastname@example.org
From a process optimisation perspective, we see other ways to optimise the yields of a hydrocracker aside from simply increasing conversion/severity in response to heavy feeds. In a VGO hydrocracker, profitability can be boosted at a fixed UCO yield by manipulating conversion between first and second stages. We have seen significant value in this approach without increasing conversion, and thereby a higher UCO flow can be maintained, keeping overall energy, fouling, and corrosion concerns at a lower level. Doing this, of course, requires a very accurate process model that can dynamically adjust to the varying feed quality and catalyst condition of the unit as well as market prices.
Roberto Tomotaki, Becht, email@example.com
For existing exchangers, the most immediate performance improvement is to clean them with the most effective cleaning methods. Advancements in cleaning technologies such as Ultrasonics and Thermal cleaning have allowed the return of the exchanger’s performance to near clean design conditions.
Exchanger fouling can be reduced by redesigning the bundles with new technology. Becht’s Bundle Technology Upgrade (BTU) programme can help identify the bundle technology best suited for the service. Recent advancements in thin film antifouling exchanger coatings have led to reports of significant fouling reduction.
Chad Perrott, Albemarle Corporation, firstname.lastname@example.org
UCO heat exchanger fouling is common in conversion hydrocracking units with an insufficient purge. This leads to a build-up of heavy polynuclear aromatics (HPNAs). The HPNAs are most often associated with coronene and ovalene, which have a red/orange colour. These HPNAs are increasingly hard to convert when recycled through the hydrocracker because their ring count will typically increase. Historically, the only way to manage them was with increased purging of HPNAs to prevent additional condensation reactions or the addition of adsorbents.
Currently, bulk metal catalysts (BMC) such as the proprietary Nebula or the proprietary Celestia are advanced formulations with up to 200% higher hydrodearomatisation (HDA) activity and hydrodesulphurisation (HDS) activity capable of saturating HPNA precursors like coronene and ovalene. Saturating these multi-ring aromatics can minimise fouling in hydrocracker UCO streams, leading to reduced UCO purge rates.
In addition, overall reactor HDS and HDN activity will also increase because of the loaded BMC. Higher HDN activity results in lower reactor WABTs for equal nitrogen (N) slip to the hydrocracking catalyst. Lower hydrocracker pre-treat WABTs increase the cycle time spent in the kinetic aromatic saturation mode, thus avoiding the thermodynamically controlled regime where dehydrogenation is present and more HPNAs are produced. Therefore, the trend towards increased use of BMCs in hydrocrackers can increase profitability through less UCO purge and reliability through fewer fouling events.