Problem Solving Manual for Operating Issues of Hydroprocessing Catalysts

Question 1 - Why does NiMo catalyst has more hydrogenation potential than CoMo? What nature of it is enabling it to do so??

Why does Nitrogen molecules in hydrotreating units takes the path of hydrogenation followed by hydrogenolysis instead of direct hydrogenolysis unlike sulphur molecules which gets hydrogenolysis directly??

Between CoMo and NiMo, Which catalyst has more deactivation rate and why?

My Response:

The behaviour of NiMo and CoMo catalysts is strictly related to the chemical interaction between the metals and carrier (Type I and Type II catalysts) in the catalyst. The hydroprocessing reactions take place in the active sites of the catalyst which is generally accepted to be located in the sulfur vacancies on the edges of MoS2 crystallites, these vacancies are significantly increased when the catalyst is promoted with Co or/and Ni. The Co-Mo-S phase is similar to MoS2 structures with promoter atoms located in the edges of a tetragonal pyramidal geometry at the edge planes of the MoS2 while to Ni promoted catalysts, Ni can be present in three forms after the sulfidation: Ni3S2 crystallites over the support, nickel atoms on the edges of MoS2 structures, and nickel cations at octahedral or tetrahedral sites in the alumina. These different arrangement and interaction between the promoters (Ni and Co) with the MoS2 structures and the support leads to the different behaviour for CoMo and NiMo for hydrotreating reactions, being the CoMo more selective for sulfur removal under relatively low hydrogen consumption while the NiMo catalyst is more selective for hydrogenation and hydrodenitrogenation under higher hydrogen consumption rates.

The reactivity of sulfur compounds to the hydrotreating reactions tend to be higher than the nitrogen compounds once nitrogen in generally concentrated in the cracked and heavier fractions of the crude oil and great part of these nitrogen compounds have six or five pyridinic ring which are unsaturated, for remove nitrogen from these heterocyclic compounds it's necessary to hydrogenate the ring containing the nitrogen before to broke the carbon-nitrogen bond (hydrogenolysis), this is necessary due to the high energy of the carbon-nitrogen bonds in these rings. In the sulfur compounds case, the most part of the sulfur atoms are concentrated in thiophenic molecules that present relatively low energy bonds to carbon-sulfur and can directly result in sulfur removal without necessity to saturate the heteroatom ring. By this reason, in hydroprocessing units treating heavier feeds which can concentrated refractory sulfur compounds like dimethyldibenzothiophene, the catalyst blending requires to rely on NiMo bed aiming to promote the hydrogenation function of the catalyst in order to minimize the steric hindrance of the sulfur molecules and improve the reactivity and consequently the efficiency of the hydrotreatment.

Related to the deactivation rate, this depends on the feed quality and severity of the processing unit, but is expected than NiMo catalysts tends to have a higher deactivation rate than the CoMo catalysts once this catalyst (NiMo) is applied to treat heavier and cracked feeds which is notable refractories to hydroprocessing reactions.

Question 2 - Does sulphiding of hydrotreater CoMo and NiMo catalyst possible with Straight run feed containing Sulphur instead of DMDS?

If so, how exactly Sulphur containing Straight run feed can sulfide the catalyst?

As in the later case, DMDS decompose at the operating temperature of above 190 C and provides required H2S for initiation of sulphiding process, but how does Sulphur containing straight run feed can provide H2S?

My Response:

This is not recommended once the sulfiding process requires an adequate concentration of sulphur capable to promote the conversion of metal oxides into metal sulphides which is the active phase of the hydrotreating catalysts. It's difficult to ensure and control the concentration of sulphur in the catalytic bed with only the available sulphur in the feed, this can lead to the permanent deactivation of the catalyst due to the metal reduction.

By this reason the sulfiding process applying a sulfiding agent (DMDS or TBPS, for example) with carefully controlled procedure, especially related to the temperature control. The sulphur is heated in the presence of hydrogen generating H2S which is able to carry out the sulfiding reactions of the catalyst metals and generating the active phases (MoS2, CoS, NiS, and WS2).

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 - What is the difference between type1/type2/brim/Hybrim catalysts?

What is Direct and Indirect desulphurization route in HDS reaction in hydrotreaters, what factors affects the routes or pathways?

Is there any relation for catalyst selection and route preference for HDS? How does route or pathway makes any difference in final product s specification?

My Response:

This classification is related to the Mo-S2 in hydrotreating catalysts. In Type I structures there is a strong interaction between the active phase and the carrier (Al2O3) mainly the interaction between the Mo and Oxygen from the support.

In Type II structure there are only weak interactions between the active phase of the catalyst with the carrier, the literature describes that Type II structure tends to present higher catalytic activity than Type I structure once the strong interaction with oxygen raises the required energy to promote the desulfurization reactions in the Type I catalysts.

The BRIM catalyst family was introduced by Haldor Topsoe company in 2003 and, among other improvements, presents higher dispersion of the active phase over the catalyst carrier leading to higher catalytic activity. The HyBRIM catalysts is a improvement of the BRIM catalyst where the interaction between the active phase and the carrier is optimized leading to a higher catalytic performance according to the licensor.

Regarding the desulfurization route. 1 - Direct Desulfurization - The whole atmospheric residue (or the hydrotreating feed) is fed to a hydrodesulfurization unit and the sulphur compounds are treated according to hydrodesulfurization reactions.

2 - Indirect Desulfurization - The heavier fraction is separated from the atmospheric residue (or another stream which is the goal of the desulfurization process) from a separation process like vacuum distillation unit or through carbon rejection routes like Solvent Deasphalting (SDA). Once the sulphur and other heteroatoms tend to concentrate in the heavier fractions of the crude oil, this process indirectly reduces the sulphur content of the light fractions.

Question 5 - 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 on Crude Oil Refining Industry based in S?o 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).