With global hydrocracking unit capacity and feedstock diversity increasing, what strategies are available to prevent reactor runaway and increase overall unit safety?Jun-2023
Nasir Hussain, PARCO OIl refinery Pakistan, email@example.com
Temperature excursion or temperature runaway is the abnormal process condition at which catalyst bed temperatures go high beyond the normal operating limit on account of uncontrolled hydrotreating reactions. During a temperature runaway condition, highly exothermic cracking reactions may take place. Temperature runaway is a very serious, unsafe situation in hydroprocessing units since extremely high temperatures can be generated within a short period of time. Consequently, equipment may exceed the design temperature and can cause catastrophic damage to the catalyst and reactor.
Although hardware's installation and upgrading of Hydroprocessing reactors is increasing day by day but the most important parts emergency depressurization system, high temperature alarms.
Randy Alexander, Reactor Resources, firstname.lastname@example.org
Thanks for the excellent responses thus far. Andrew Layton mentioned the addition of nitrogen rich compounds to temper activity at startup. Our company, Reactor Resources, has carried out amine injection on at least 24 hydrocracker startups in conjunction with the sulfiding step. We use MDEA that quickly decomposes to ammonia upon enterring the reactor. This form of nitrogen temporarily neutralizes hyper-active acidic sites on the zeolitic catalyst support. The nitrogen can be driven off of the support by increasing the reactor temperature once more activity is desired.
We also have started utilizing an online pH Analyzer System to monitor water coming from the separator boot. Real-time data on the pH change of the water prevents over-injection of the amine. Over-injection wastes both amine and DMDS since ammonia will react with H2S to form ammonium sulfate. Sulfate salts can be problematic and excess nitrogen can reduce reactor activity (and yields) more than desired.
Amine injection is also utilized in renewable fuels units to temper the activity of the cracking beds.
Ricky Hsu, International Innotech, email@example.com
One of the most common problems with hydrocracking reactor runaway is the sudden reactor pressure rise. Hydrocracking feeds contain different sizes of solid particles that can form a cake layer on top of catalyst inside the reactor. When that happens, the pressure inside the reactor will rise suddenly, leading to serious problems. However, the proprietary magnetically induced Universal Filter filtration system can remove all types of solid particles down to 7 nm or less with much simpler operation.
This filtration system achieves a near-total prevention of solid particles in the liquid stream from entering the reactor, including FeO, FeS, Fe₂O₃, Ni, NiO, Co, and CoO. Furthermore, the need for expendable macropore filtration packings on top of the reactor and filter cartridges at the reactor feed entry is substantially eliminated, saving material and operational costs. These costs include loading/unloading and disposal of the spendable materials. More detailed information is available in a previously published article in PTQ Q1 2023 (p87-95), ‘Universal filter for ultra-cleaning of reactor streams’.
Andrew Layton, KBC (A Yokogawa Company), Andrew.firstname.lastname@example.org
There are various contributors that can generate a heat rise or even runaway. The following are several mitigations to address potential runaway situations:
Reactor internals Improved flow distribution technologies continue being developed to improve distribution of the feed and recycle gas. These advanced technologies prevent hot spots from developing and help quench distribution. Certain techniques are also available to ensure distribution trays function optimally. Additionally, thermowell design and reactor loading internal design help maintain sufficient flow distribution. It is also important to design the thermocouple location to best monitor temperature excursions or hot spots.
Reactor bed condition To improve catalyst flow distribution and minimise associated delta P (ΔP), it is important to practise proper catalyst loading and monitoring techniques. The reactor top bed catalyst and intergrading materials and design play a vital role in maintaining distribution and mitigating ΔP. Additionally, the design of dense loading machinery continues to improve. Note that the skill of the loading technician is still very important.
Filtration It is important to filter and treat the feed to prevent fouling in the bed, which can lead to hot spots. Knowing the fouling type and size helps filter selection.
Procedures and training Both standard and emergency operating procedures can be improved to prevent and mitigate temperature excursions:
• Standard procedures should be followed during start-up, which may include adding amine or ammonia to control excursions, especially if feed or recycle flow is low on sulphur or catalyst activity is extremely high
• Emergency procedures and feed type, such as loss of recycle gas flow, determines whether feed is, or is not, removed in addition to depressurisation
• Training operators on how all processes operate and how hot spots can develop
• HAZOP/MOC reviews should be conducted when changing catalysts. Some companies conduct exotherm speed potential tests to compare base catalysts against new catalysts with higher activity.
Control Several control systems can be used to help prevent and reduce temperature fluctuations. These controls include:
• Thermocouple monitoring can track maximum temperature, rate of temperature change above a certain point, radial temperature distribution, or shell temperatures. Alarms or even emergency depressurisation can be linked to these measurements
• Hydrocracking catalyst may have a few separate beds to control heat rise in one bed to mitigate heat rise before the next quench point. Note that in case of an excursion, gas quench is usually inadequate to control the heat rise and, therefore, controlled depressurisation is the better approach
• Temperature monitoring instrumentation can also trigger emergency depressurisation.
Jay Parekh, Advanced Refining Technologies, Jay.Parekh@grace.com
As refiners look for more opportunities to improve profitability and gross margin, feedstocks that are more refractory with higher aromaticity are processed in hydrocracking units to produce high-quality liquid fuels. The more difficult feedstock adds additional risk to refiners as the temperature exotherms in normal operation increase and requires additional quench between beds for reactor controllability.
Most modern hydrocracking units now have an auto-depressuring system (ADS), which will automatically trigger in the event of a major temperature excursion and bring the unit to a safe condition. The ADS will activate when specific reactor bed or reactor outlet temperatures exceed preset limits or when the recycle compressor fails. Loss of recycle flow is the leading cause of runaway incidents. ADS should also ensure that the reactor feed furnace burners have been tripped and the make-up hydrogen flow is reduced.
While the ADS provides a safeguard to avoid a catastrophic event, it is imperative for refiners with hydrocracking units to properly train operators to follow good operating practices and be able to institute emergency procedures quickly. The hydrocracking unit should always be in a posture where reactor heat can be rapidly removed from the unit (cut furnace fires) while ensuring adequate compressor capacity to provide additional emergency quench gas capability to cool reactor beds quickly.