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In continuation of my previous question regarding catalyst finalization for diesel hydrotreater, I would like to understand,

After hydrotreater feed specification is finalized, containing considerable Sulphur(1.5%) Nitrogen(400 ppm) and total aromatics in feed (30%).

if I choose, same volume of NiMo catalyst in Reactor 1 and same volume of CoMo catalyst in Reactor 2 for same feed, does both Reactor 1 and 2 outlet product gets same specification in terms of Sulphur/Nitrogen/Aromatics at same operating conditions??

If not why?

What changes needs to be done in NiMo loaded reactor and CoMo loaded reactor w.r.t volume of Catalyst/ Pressure / Temperature to obtain same product specification? Kindly guide.

Feb-2023

Answers

Marcio Wagner da Silva, Petrobras, marciows@petrobras.com.br

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.

Considering the hypothetic scenario mentioned is expected different compositions downstream of the reactors 1 and 2. The reactor containing NiMo catalysts tends to present lower contaminants concentration (Sulphur, Nitrogen, and aromatics) in the hydrotreated stream than the reactor filled with CoMo catalyst. As previously mentioned, the NiMo catalyst is chemically more active than CoMo and tends to present better performance for hard feedstock like the presented case.

To achieve similar performance in both reactors would be necessary to raise the severity in the reactor 2 including higher temperature, pressure and catalyst mass for this reason a blend of catalysts is normally carried out in industrial scale aiming to achieve hydrotreating goals with acceptable capital and operating costs.

Feb-2023
Marcio Wagner da Silva, Petrobras, marciows@petrobras.com.br

I believe that the question is regarding the desulfurization of heavy oils. The desulfurization of heavy fractions can be divided in two routes:

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 sulfur content of the light fractions.

The chemical characteristics of the sulphur compounds have a direct effect on its removal performance. Desulfurization of compounds that contain aliphatic sulfur, i.e. thiols and sulfides, is easier than desulfurization of compounds that contain aromatic sulphur, i.e. thiophenics. Due to this fact, the hydrodesulfurization of heavier fractions requires higher operating severity than the process units operating with lighter fractions.

Feb-2023
Marcio Wagner da Silva, Petrobras, marciows@petrobras.com.br

To elaborate this response I adopt as assumption that both hydrotreating units are operating with the same feedstock. In this case, the Axens processing unit which applies a combination of CoMo and NiMo catalysts operate under higher total pressure but lower H2/HC ratio considering the balance between the diferent catalysts. The NiMo catalyst is applied to promote harder hydrotreating reactions like the hydrodenitrogenation and aromatics saturation, in other words, heavier and high contaminants content feed stocks.

To promote the high severity reactions, the NiMo catalyst is highly active and an atmosphere with low hydrogen partial pressure tends to quickly coking the catalyst bed. For this reason, the processing unit designed by Haldor Topsoe which applies only NiMo as catalyst can operate under lower total pressure, but with higher H2/HC ratio in order to control the coke deposition and, consequently, the operating lifecycle of the processing unit.

In other words, the synergy between CoMo and NiMo catalysts in the Axens Processing unit allows operate under lower hydrogen partial pressure, but requires a higher total pressure to reach the same final product specification of the Haldor Topsoe processing unit which applies only NiMo catalyst which is highly active allowing operations with lower total pressure but with higher hydrogen partial pressure to put under control the coking rate of the catalyst bed.

Nov-2022
Marcelo Tagliabue, Air Liquide, marcelo.tagliabue@airliquide.com

In the first place, to elaborate this answer, I suppose that the two hydrotreatments have the same food, referring specifically to their composition.

In the past, the most used catalysts were CoMo, and almost all technologies used it. NiMo-based catalysts then burst in. The latter were more expensive than the former and were preferably used when the feed contained olefins or sulfur compounds that were difficult to hydrogenate, as in the case of Thiophene.

Hydrotreatments, in terms of their severity, respond to the concentration of H2 in the feed as well as to the working pressure. In other words, they respond to the partial pressure of H2. It is for this reason that we can find different pressures and different concentrations of H2 to achieve the same result.

The configuration of the reactor with respect to its dimensions, space velocity, and the use of a catalyst or a mixture of both respond exclusively to each particular technology and to the characteristics of the catalyst that each one has at the time of designing the system. Let's not forget that in this case almost ten years passed between one technology and the other, and that period for progress in the formulation of catalysts is a long time. It is probable that the most modern HTSA catalyst will meet or improve the performance of the mixture used by Axens.

Oct-2022
JON Mical, ADNOK, 1147-15pt@imco.edu.om

Thanks Jake. Also some fired heaters with BMS logic for each Pilot Burner and Main burner has flame scanner. If few burners losses their flame scanner it leads to trip the heater totally.

Mar-2022
Jake Gotham, InSite Technical Services, jake.gotham@insitetechnical.com

You are right to challenge this. I have seen this approach taken in several furnaces, but it is poorly thought through. The purpose of the pilot is to ensure a source of ignition remains in the firebox if the main burner flame is unstable. The worst thing you can do if the pilot is unstable (i.e. low pilot gas pressure) is to create an unstable main burner flame by tripping the main burner. If the main burner trip functions properly, the furnace will shutdown safely. But if the main burner valve doesn’t seat, the trip you describe has created a hazardous situation, not prevented one.

A better design trips the main burner on low main burner pressure, and trips the pilot on low pilot pressure. The only thing that should trip both is the emergency shutdown button.

You wouldn’t want to operate the furnace in the long-term without the pilots, but there is no need for an immediate trip. I have seen a furnace with a delayed trip — i.e. if the pilot gas pressure isn’t restored within a number of hours, the main burner trips. But most furnaces don’t trip the main burner at all if the pilot pressure is low, and that’s the solution I would advocate.

It is also worth bearing in mind that lighting a furnace is one of the most hazardous activities an operator has to do. Every time a furnace trips, somebody has to relight it. This is another reason to push back against unnecessary main burner trips.

Mar-2022