The Chemistry Behind Hydroprocessing Units

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 - What is the main role of support material of the hydrotreater catalyst CoMo/NiMo? Does Support material participate in the hydrogenation reactions for HDS/HDN/HDA?

Hydrotreater catalyst at times said to be acidic and at times to be neutral as per the documents, which is true? Acidic nature for the catalyst is due to catalyst material or due to support material? Kindly guide.

My Response :

The catalyst carrier or support offers mechanical resistance, high supercial area aiming to ensure an adequate distribution of the active phase (metals), and it's responsible to control the acid function of the catalyst which is desired to be low in the hydrotreating units. The support normally don't have catalytic activity for hydrogenation reactions which is essentially carried out in the metal sites. Another function of the support in hydrotreating catalysts is to ensure an adequate pore distribution aiming to minimize the catalysts plugging due to coke or metals deposition which can lead to short operating lifecycle of the hydroprocessing units, this is a especial concern in residue hydrotreating units.

The catalysts applied in most severe services normally present acid and hydrogenation characteristics especially those apllied in residue hydrotreating or hydrocracking processes. Catalysts applied in hydrocracking processes can be amorphous (alumina and silica-alumina) and crystallines (zeolites) and have bifunctional characteristics more pronunciated, once it's desired that the cracking reactions (in the acid sites) and hydrogenation (in the metals sites) occurs simultaneously. The active metals used to this process are normally Ni, Co, Mo and W in combination with noble metals like Pt and Pd.

It’s necessary a synergic effect between the catalyst and the hydrogen because the cracking reactions are endothermic  and the hydrogenation reactions are exothermic, so the reaction is conducted under high partial hydrogen pressures and the temperature is controlled in the minimum necessary to achieve the desired convertion of the feed stream. Despite these characteristic, the hydrocracking global process is highly exothermic and the reaction temperature control is normally made through cold hydrogen injection between the catalytic beds.

As described above, the acid function in hydrocracking catalysts is take place in the acidic support which can be amorphous silica-alumina (ASA) and/or a zeolitic material while the hydrogenation reactions are carried out in the metal sites.

Question 3 - What is the importance of analyzing basic nitrogen and non basic nitrogen in Hydrotreater and Hydrocracking feeds. How does they affects the process and catalysts?

My Response:

Nitrogen compounds are knowed as strong inhibitors of activity of hydrotreating catalysts. Basic nitrogen compounds are the main concern related to the catalyst activity, but the non basic nitrogen compounds can lead to inhibition reactions during the hydrotreating processes. Another relevant impact of the non basic nitrogen compounds are poor performance of hydrodesulfurization due to the competitive adsortion over the active sites of the catalyst. The most part of nitrogen compounds are aromatics with great chemical stability, being hard to be hydrotreated. To overcome this challenge the nitrogen content in residue hydrotreating/hydrocraking units are controlled using adequate catalyst grading, in some cases with contaminants traps.

In fixed bed hydrocracking units which operates with high nitrogen content feeds, it's common to use a hydrotreating section upstream to the hydrocracking section aiming to protect the hydrocracking catalyst which are normally expensive, furthermore are apllied separation vessels between the sections to reduce the nitrogen concentration in the hydrocracking section as presented in Figure 1.

Figure 1 - Process Arrangement for a two stage fixed bed hydrocracking unit relying on gas separation vessel upstream to hydrocrackers reators to control the nitrogen content.

Question 4 - What is the purpose of Multi-catalyst Bed philosophy in hydrotreating of Diesel and Vacuum gas oil cuts?

Besides this, for Multi - Catalyst bed configuration, some x or y % of HDS / HDN / HDA happens in each bed, before getting admitted into next catalyst bed. How and who (factors) controls that x or y%?

My Response :

The main purpose of the multi-catalyst bed in hydrotreating units is to ensure a volume swell in the reactor leading to the optimization of the processing unit.

Normally, the grading of hydrotreating catalysts involves the use of guard beds in the top of the reactor aiming to control the contaminants concentration using macro porous catalysts which have the function to retain fouling agents like corrosion products, metals, organo-metallic compounds, and diolefins which tends to raise the pressure drop in the reactors and reduce de operational lifecycle of the hydrotreating unit. The following regions of the hydrotreating reactor is filled aiming to maximize the hydrogen uptake, a major part of the HDS and polyaromatic hydrogenation reactions are carried out in the region immediately bellow of the guard bed.

The following zone is normally dedicated to promoting the HDN and monoaromatic saturation reactions, and the following catalyst region is dedicated to promoting hydrogenation reactions once the limitation by nitrogen content tends to be minimized in this section. The percentages of HDS/HDN/HDA in each region relies on the characteristics of the processing unit like the total and partial hydrogen pressure, temperature, quench strategy, the characteristics of the employed catalysts, and from the characteristics of the feedstock.

Question 5 - What exactly causes coking on Diesel and Vacuum gas Oil hydrotreating catalysts (treating Crack and Straight run feeds)?

My Response :

The coking process in hydrotreating units generally occurs due to the cracking and dehydrogenation reactions in the catalyst beds that are favored by high temperature and due to the feedstock quality, hydrotreating units processing heavier feeds with high concentration of olefinics, polyaromatics, and asphaltenic compounds tends to present higher coke laydown rates. The hydroprocessing units operates under high hydrogen excess aiming to overcome the hydrogen diffusion limitations, once the hydrogenation reactions are exothermic, there is a temperature raising in the catalyst bed which favours the cracking and another side reactions (dehydrogenation) leading to the production of lower added value derivatives and coking deposition over the catalyst surface.

By this reason, for hydrotreating units processing unstable feeds (cracked feeds and residue) its fundamental an adequate design and operation of the quench and temperature control system of the hydrotreating reactors in order to ensure an adequate temperature control through the catalyst bed without hot points.

Dr. Marcio Wagner da Silva is Process Engineering and Optimization Manager at a 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).