Managing Operating Lifecycle and Process Safety Issues of Hydroprocessing Reactors

Question 1 -

How to confirm the runaway reactions in Diesel and Vacuum Gas Oil hydrotreater units reactors? Is it localized phenomena at particular locations or gets spread over a wide area of the catalyst?

As the 1st bed hydrotreater bed exotherm for our unit already stays above 40degC during normal operation, I would like to understand how to confirm the run away phenomenon and take necessary actions like actuation of EDPS.

My Response-

This is one of the key points during the operation of hydrotreating units, especially those processing chemically unstable feeds like VGO or delayed coking gas oils and can reach a great part of the catalytic bed. The temperature runaway in hydrotreating reactors is a phenomena where the catalytic bed presents a sudden and uncontrolled temperature overshooting produced by exothermic reactions which can be provoked by the reasons below:

1 - Sudden flow rated reduction of the feed: This can led to a hot points in the catalytic bed and, in extreme cases, damage the catalyst due to sintering of the active phase;

2 - Inefficient control of the fired heater: The fired heater control needs to respond adequately in case of overheating of the reacting section;

3 - Change in the feed composition: A significant raise in the composition of chemical unstable compounds of the hydrotreating unit can lead to a temperature raising of the catalytic bed once the rate of highly exothermic reactions raises significantly. It's necessary to keep the feed composition as stable as possible;

4 - Failure or deficient quench gas flow: The quench injection is responsible for keep under control the temperature of the catalytic bed as well as supply additional hydrogen to the hydrotreating reactions, for this reason it's necessary to ensure that this system is well designed and operated as the design requirements aiming to minimize hot points in the catalytic bed which can led to temperature runaway of the reactors;

5 - Sudden Change in the Capacity of Recycle Compressor: This can led to a drastic reduction in the quench flow rate to the reactors and produce hot points in the catalytic bed, the change in the capacity needs to carried out in a smooth way to avoid sudden variations in gas flow rate through the catalytic bed;

6 - Methanation Reactions: This phenomena is a concern especially in processing units operating under high severity (hydrocracking units, for example) and is related with the dragging of CO2 and CO to the reactors which combined with the operating conditions (temperature and pressure) can favour methanation reactions which are highly exothermic and will produce temperature runaway in the catalytic bed;

The main characteristic of the temperature runaway of the catalytic bed is a sudden and abnormal raise in the reactor temperature. For this reason an adequate temperature monitoring of the catalytic beds is fundamental to identify the temperature runaway and allow mitigation actions in an adequate moment. One of the main side effect of the temperature runaway is the significant raise of coke laydown rate and the sintering of active phase of the catalyst which can produce a raise in the pressure drop in the reaction section, this can indicate that you have a problem with temperature runaway in the hydroprocessing unit. In summary, check if your processing unit is presenting a uniform temperature distribution through the catalytic bed and if the pressure drop is rising under abnormal rate and if you are facing some of the 6 reasons for temperature runaway above.

Question 2 -

What is the purpose of Hot H2 Stripping in Diesel and Vacuum gas oil hydrotreater units at 360degC, during shutdown steps for catalyst replacement work?

Significance of maintaining H2S and not maintaining H2S in the HP loop during Hot H2 stripping? What are the consequences?

My Response -

The use of hot hydrogen stripping in hydrotreating units is related to the cleaning the reactor internals previous the maintenance services (hydrocarbon removal) as well as the coking removal over the catalysts which also help the spent catalyst draining step. Another purpose of the hot hydrogen stripping is to reduce the H2S content in the reaction section, previously the access of the maintenance workforce, leading to safer work conditions.

The hot hydrogen stripping can also be used to minimize the pressure drop of the catalyst bed through the removal of "soft" coke which lay down over the catalyst, over the time this coke can be converted into "hard" coke which is more difficult to be removed, normally requiring a catalyst regeneration step.

Despite the benefits, the hot hydrogen stripping needs to be carried out carefully once the process can lead to the reduction of the metals in active phases of the catalyst if a very low H2S concentration is applied in the reaction system. By this reason, the catalyst licensors recommend a minimum concentration of H2S in the recycle gas (normally above 1.000 ppm) during the hot hydrogen stripping process to minimize the risk of catalyst deactivation by metal reduction.

Question 3 -

Loading of catalyst in Hydrotreaters units follows Dense and Sock loading, despite having many advantages of Dense loading except high pressure drop.

It is observed in plants, that 1st bed grading and bulk catalyst to be in Sock loading and following beds top catalyst layers of little height in Sock loading followed by Dense loading for remaining bed height?

Q1. During what instances we choose to go for Sock loading?

Q2. How to choose inert balls size and quantity on catalyst bed support grid and on outlet collector?

My Response -

Normally, the dense loading is preferred once  minimize the void spaces in the catalytic bed leading allowing a better flow distribution as well as higher catalyst mass in the reactor leading to a better performance during the operating run.

The advantage of sock loading process is the lower pressure drop through the catalytic bed, this can be a decision factor in processing units which limitations in dynamic equipment, but even under this scenario this issue tends to be relevant in the end of run, not in the start of run. Under normal conditions, the dense loading is preferred than sock loading process.

Regarding the choice of inert balls size, bed support grid, and outlet collector these devices have great impact over the total pressure drop and performance of the reactor, the design needs to follow the recommendations of technology licensors considering the specificities of each processing unit allied with the best engineering practices once high pressure drop can lead to the collapse of the support grid, causing an unplanned shutdown of the processing unit.

Question 4 -

For Diesel hydrotreaters. How to finalize the catalyst from only CoMo & only NiMo and combination of NiMo/CoMo? Why is it said to be that NiMo catalyst consumes more H2 than CoMo catalyst?

My Response -

The catalyst grading of the diesel hydrotreater reactors relies on the feed stream quality, especially related to the contaminants content like sulfur and nitrogen as well as the participation of cracked streams like LCO, Coker Gas oil, etc. which are harder feeds to hydrotreating process. For feed streams with high content of these compounds it's applied a catalyst grading in the hydrotreating reactors with increased presence of high active catalysts like NiMo over alumina.

Once the CoMo is less active than NiMo catalysts, the first is applied to improve sulphur removal and olefins saturation while the NiMo catalyst is responsible for promoting nitrogen removal and aromatics saturation. The filling of the reactor (downflow reactors) normally starts with guard beds to protect the active catalysts against contaminants like metals (Ni and V) followed by the heteroatoms and unstable compounds saturation in the following beds in order to ensure an adequate temperature control in the catalyst beds. A relatively common configuration is to use a wide pore NiMo catalyst in the guard bed followed by a blending of CoMo and NiMo in the first catalytic bed aiming to promote sulfur removal and aromatics saturation followed by a NiMo bed aiming to promote the hydrodenitrogenation reactions followed by a last catalytic bed with a catalyst with high dehydrogenation performance (CoMo). Again, the catalyst grading configuration relies on the feed stream quality, design characteristics of the processing unit, and hydrotreating goals (specifications of the hydrotreated stream).

Regarding the higher hydrogen consumption of NiMo catalysts, as described above these catalysts are more chemically active than CoMo and are responsible for nitrogen removal and aromatics saturation which are more refractory contaminants, leading to a higher hydrogen consumption to achieve hydrotreating goals.

Dr. Marcio Wagner da Silva is Process Engineer and Stockpiling Manager at Crude Oil Refinery based in São José dos Campos, Brazil. Bachelor’s in chemical engineering from University of Maringa (UEM), Brazil and PhD. in Chemical Engineering from University of Campinas (UNICAMP), Brazil. Has extensive experience in research, design and construction to oil and gas industry including developing and coordinating projects to operational improvements and debottlenecking to bottom barrel units, moreover Dr. Marcio Wagner have MBA in Project Management from Federal University of Rio de Janeiro (UFRJ), in Digital Transformation at PUC/RS, and is certified in Business from Getulio Vargas Foundation (FGV).