Mathematical modeling of the process of hydrotreating diesel fuel from organosulfur impurities

Article ID: 2837

Keywords: diesel fuel; hydrodesulfurization process; mathematical modeling; diesel feedstock; reaction rate constant; industrial reactor block

Abstract

Hydrotreating of diesel fuel is a major catalytic process for motor fuel production. This process aims to reduce the organosulfur content in the fuel to 10 parts per million (ppm) in order to meet environmental standards. However, this deep purification of diesel fuel requires the use of an expensive catalyst at hydrotreating plants, which have giant reactors with a capacity of 200–600 cubic meters. Such a volume of reactors is associated with the use of methods of classical kinetics of chemical reactions, when all the raw materials of the process are in the reactor until the required conversion depth is reached, while hydrotreating has its own specific features. All known mathematical models for diesel fuel hydrotreating take into account different nuances of the process, but they all have one common disadvantage: they use approximate, often crude, ideas about the composition of multicomponent raw materials, such as diesel oil fractions, which contain several dozen different organosulfur compounds with varying activity in hydrogenation reactions. Most often, these raw materials are represented in a mathematical model as a combination of two to six pseudo-components, or lumps, that combine sulfo-organic impurities from one or more homologous groups. Such a theoretical framework allows us to model the current state of hydrotreating technology, but it does not advance it. We propose a new approach to mathematical modeling of diesel fuel hydrotreatment, which better takes into account the actual features of the process. The structure of the mathematical model considers the composition of the raw material as a set of 10–20 narrow fractions. In each fraction, the set of hydrogenated organosulfur impurities is treated as a single pseudo-component. Another feature of the model is the use of different rate constants for different organosulfur impurities in the raw material, represented as a continuous kinetic characteristic that changes over time. This allows us to integrate the system of differential equations in the model and adapt the rate constant to the concentration of the hydrogenated organosulfur impurity at any given time during the process. The developed model has also made it possible to propose a new technology, hydrotreatment: separating the feedstock into two or three wide fractions, combining the corresponding narrow fractions, and then subjecting them to separate hydrogenation processes. This differential hydrotreatment technique will allow for a reduction of the catalyst load in the hydrotreatment unit by almost 50% while maintaining its efficiency or for doubling the efficiency while maintaining the same catalyst load with traditional technology.