There is currently a growing need to hydroprocess heavier and tougher crude oils with increased nitrogen content. Therefore, hydrodenitrogenation (HDN) has become a critical hydroprocessing reaction, making it essential to gain insight into which nitrogen-containing compounds are the most difficult to treat. In the present article, we describe the identification of nitrogen compounds in severely pretreated feed for hydrocracking (HC). The nitrogen compounds in the N-slip to the hydrocracker are isolated and concentrated on solid-phase extraction (SPE) columns and identified by gas chromatography mass spectrometry (GC-MS), gas chromatography with atomic emission detection (GC-AED), and nuclear magnetic resonance (NMR) spectroscopy. Density functional theory (DFT) calculations support the structural identification and are further used to investigate the reactivity. We find that the most refractory organic nitrogen compounds in the N-slip belong to the family of 4,8,9,10-tetrahydrocyclohepta[def]carbazoles. These molecules are slightly more basic than other carbazoles and thus are likely to have an impact on the performance of the downstream catalysts; however, their very low reactivities make them extremely difficult to remove under normal hydrotreating conditions.
In this work, a model for a trickle-bed reactor for catalytic hydrotreating (HDT) of oil fractions is developed and simulations are performed to investigate its behavior. The model considers dynamic, one-dimensional plug-flow to describe a heterogeneous, adiabatic trickle-bed reactor. It takes into consideration the main reactions present in the HDT process: hydrodesulfurization (HDS), hydrodenitrogenation (HDN), and hydrodearomatization (HDA) with a reconstructed petroleum feedstock using a practical approach of generation of pseudo-components by dividing the boiling point curves of the feedstock. The model is solved using the method of lines with a finite difference scheme for discretization in the axial direction and simulations are performed for an industrial hydrotreating unit to evaluate the behavior of the system under different conditions and assumptions e. g. related to the linear gas velocity. A study of the dynamics is carried out to investigate the behavior of the system with a change in the sulfur compound concentration of the feed. In addition, a sensitivity analysis of the most relevant model parameters is performed.
Hydrogen production processes, e.g., steam reforming and partial oxidation, begin with a pretreatment section where a sulfur removal method is employed to prepare the feedstock. The characteristics of possible reforming fuels, i.e., liquefied natural gas, liquefied petroleum gas, naphtha and diesel are discussed in a refinery context. Sulfur compounds present in crude oil fractions are divided into different classes. Some exhibit a very low reactivity, which makes them more difficult to desulfurize, and which significantly impacts the achievement of ultra‐low sulfur levels. The improved design of efficient hydrodesulfurization processes is based on the understanding of the process chemistry. In the case of a feedstock with low sulfur content, removal of sulfur can be achieved by absorption processes, e.g., absorption on a zinc oxide catalyst bed. In the case of heavier feedstocks or feedstocks with a high sulfur content, a first step consisting of a hydrodesulfurization process becomes necessary. The process conditions, i.e., temperature, pressure, space velocities, catalysts, etc. are determined by the type of feedstock to be treated. The catalysts most generally used for hydrotreatment consist of cobalt and molybdenum oxides or nickel/molybdenum compounds, both supported over alumina. CoMo and NiMo catalysts desulfurize the most refractory compounds through different desulfurization routes, thus the choice of catalyst will be a function of the type of feedstock and process conditions. It is required that the hydrogen sulfide produced by the desulfurization reactions is removed. Options used to remove H 2 S from the system include the use of zinc oxide absorption, amine wash treatment or both.
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