What type of catalyst do we need for low pressure hydrotreating of middle distillates?
Responses to a question in the Q1 2021 issues Q&A Feature
Various from Shell Catalysts & Technologies & Axens
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Ihsan Raad, Senior Technical Services Engineer, Shell Catalysts & Technologies - Ihsan.Raad@shell.com
There is no single catalyst suitable for every hydrotreating unit. Selecting the optimum catalyst or combination of catalysts for a given application is a skilled exercise requiring in-depth understanding of the unit key performance indicators, constraints, and reaction kinetics. A typical range for a low pressure hydrotreater is 300-600 psig. When selecting a catalyst system for a low pressure unit, the following factors need to be considered:
- Feed properties such as nitrogen and tail end distillation (D2887 preferred).
- Cracked stock content and aromaticity due to the increase in difficult sulphur and nitrogen containing compounds
- Operating conditions such as reactor outlet H2 partial pressure, LHSV, treat gas to oil ratio, and treat gas quality
- Operating objectives, such as product sulphur, density, cetane, aromatics, T-95, hydrogen consumption limitations, and cold flow properties cycle life requirements.
Shell Catalysts & Technologies currently has two principle technology platforms: Ascent and Centera GT. The highest activity possible is achieved with our Type II Centera GT catalysts, which have built upon the fundamentals of the original Centera platform to construct highly dispersed MoS2 particles with step-out intrinsic activity for hydrodesulphurisation (HDS), hydrodenitrification (HDN), and aromatic saturation (HDA).
Ascent catalysts benefit from a balance of both Type I and Type II active sites, which provide reaction pathways for both direct and indirect desulphurisation. They enable hydrogen consumption management throughout the cycle and sustained performance at end-of-run conditions when indirect desulphurisation becomes difficult. This balance proves unusually effective at low-to-medium operating pressures, especially for units with limited hydrogen availability.
Shell Catalysts & Technologies offers both CoMo and NiMo Ascent and Centera GT catalysts, which can be utilised to design catalyst systems that meet the processing objectives for low pressure units. CoMo catalysts are ideal for the hydrodesulphurisation of low-severity feeds under hydrogen-constrained conditions, and NiMo catalysts, with increased hydrodenitrification and hydrogenation activity, provide clear advantages when faced with more challenging feeds. The higher hydrogenation activity of the NiMo catalysts inevitably comes at the expense of greater hydrogen consumption. Experience has shown that sandwich systems containing both CoMo and NiMo catalysts can be designed to provide significant activity benefits and temper the consumption of hydrogen.
All these factors and objectives must be considered to design the optimum catalyst system for any hydrotreating unit.
ASCENT and CENTERA GT are marks of Shell.
Gregory Lapisardi, Technologist, Middle distillates HDT, Axens - firstname.lastname@example.org
When it comes to selecting the right catalyst for the hydrotreatment of a middle distillates fraction at low pressure, a distinction has to be made between light (kerosene) and heavy (diesel) cuts.
In a low pressure kerosene hydrotreater, nickel-molybdenum (NiMo) catalysts are in most cases the best choice. When the heaviness of the feed is increasing toward a light diesel cut, a cobalt-molybdenum (CoMo) catalyst can also be considered (see Figure 1).
Low to medium pressure diesel hydrotreaters are the playground of CoMo catalysts. Thanks to a higher selectivity towards hydrodesulphurisation reaction, in comparison with a NiMo catalyst, a CoMo catalyst will exhibit a higher hydrodesulphurisation rate for a limited hydrogen consumption.
However, the latest challenges appearing in refineries are questioning the domination of CoMo catalysts in low to medium pressure diesel hydrotreaters. Co-processing of lipids (vegetable oils, animal fats, UCO…) is one of them. Renewable feedstocks, like vegetable oils, are mainly composed of triglycerides as well as glycerides and free fatty acids.
The desired reaction for converting vegetable oils to commercial fuel is the deoxygenation of these molecules in the presence of hydrogen to form linear paraffins, propane, CO2, and water (see Figure 2). The water gas shift equilibrium will lead to the presence of carbon monoxide in the recycle gas. The higher the incorporation rate of vegetable oil, the higher will be the concentration of carbon monoxide in the recycle gas.
CoMo catalysts are strongly inhibited by carbon monoxide partial pressure while inhibition for NiMo catalyst is very limited. For this particular case, in a low pressure unit, it is recommended to load NiMo catalysts or at least a stack bed of NiMo/CoMo catalysts when the unit has hydrogen availability constraints (see Figure 3).
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