Selective Hydrogenation of Diolefins in Gasoline
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Solution Overview
Problem
Conventional hydrodesulphurization processes for gasolines, particularly those from catalytic cracking, are non-selective, leading to a significant drop in octane number and high hydrogen consumption, and struggle to efficiently convert light sulphur compounds like dimethylsulphide and methyl-ethylsulphide, resulting in catalyst deactivation and reactor clogging.
Innovation Solution
A process involving a reactor with a catalyst containing Group VIb and Group VIII metals on a support, where a light gasoline cut is recycled to selectively hydrogenate diolefins at controlled temperatures and pressures, reducing the content of light sulphide compounds without severe temperature increases or reactor enlargement, and subsequent fractionation separates hydrocarbons by carbon atoms to retain octane number and prevent catalyst deactivation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If conventional hydrodesulphurization processes are used to reduce sulphur content, then sulphur content is reduced, but octane number drops significantly and hydrogen consumption increases
Solution Approach 1:
The process segments the sulphur removal task into two distinct stages: first, selective hydrogenation of diolefins to mono-olefins using a bifunctional catalyst at moderate conditions (60-150°C), and second, hydrodesulphurization of the treated stream. This segmentation allows each stage to optimize for its specific function, preventing the non-selective hydrogenation of olefins that occurs in conventional single-stage processes, thereby preserving octane number while removing sulphur.
Solution Approach 2:
The selective hydrogenation of diolefins is performed as a preliminary action before hydrodesulphurization. This preliminary step converts unstable polyunsaturated compounds into more stable mono-olefins, which have better octane numbers and are less prone to polymerization. This preliminary action prevents catalyst deactivation and reactor clogging that would otherwise occur during subsequent hydrodesulphurization, while also preserving the olefin content needed for high octane fuel.
2Productivity
If temperature conditions are made more severe to improve conversion of light sulphur compounds, then conversion efficiency increases, but catalyst deactivates prematurely and reactor clogs
Solution Approach 1:
The process changes the temperature parameter from severe conditions to moderate conditions (60-150°C) in the selective hydrogenation step. This parameter change is sufficient to convert diolefins to mono-olefins and improve light sulphur compound conversion without causing the excessive polymerization and coke formation that leads to catalyst deactivation and reactor clogging. The moderate temperature parameter maintains reliability while achieving adequate productivity.
Solution Approach 2:
The bifunctional catalyst acts as an intermediary that facilitates the selective hydrogenation of diolefins without causing severe side reactions. The catalyst's specific composition (Group VIb metal and Group VIII metal on porous support) enables it to mediate the reaction at moderate temperatures, converting unstable diolefins into more stable mono-olefins while preventing the polymerization that would otherwise require severe temperature conditions to achieve conversion.
3Productivity
If liquid hourly space velocity is reduced to improve sulphur compound conversion, then conversion efficiency increases, but reactor size and catalyst quantity must increase
Solution Approach 1:
The process changes the liquid hourly space velocity parameter to moderate values (1-10 h⁻¹) in the selective hydrogenation step. This parameter change, combined with the specific catalyst system, achieves effective conversion of light sulphur compounds without requiring excessively low velocities that would demand larger reactor volumes. The moderate LHSV range optimizes the balance between conversion efficiency and reactor size.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This process effectively reduces the content of light sulphide compounds in gasolines while maintaining octane number and extending catalyst life, avoiding premature deactivation and clogging, thus improving the efficiency and longevity of the hydrodesulphurization process.
Implementation Method 1
selectively hydrogenating the diolefins, while simultaneously transforming the saturated light sulphur-containing compounds into heavier compounds
Implementation Method 2
brought into contact in a reactor with catalyst A comprising at least one metal of group VIb and at least one non noble metal of group VIII deposited on a support
Implementation Method 3
By fractionating the gasoline originating from the selective hydrogenation step, a light desulphurized gasoline cut (or Light Cracked Naphtha (LCN)) is produced
Implementation Method 4
Another effect of the selective hydrogenation is to prevent the progressive deactivation of the selective hydrodesulphurization catalyst and/or to avoid a progressive clogging of the reactor due to the formation of polymerization gums on the surface of the catalysts or in the reactor. In fact, the polyunsaturated compounds are unstable and have tendency to form gums by polymerization.
Data Source
AI summary
A process reducing sulfides R1-S-R2, with R1 and R2 methyl or ethyl, in a gasoline containing diolefins, mono-olefins and sulphur:A) contacting gasoline in mixture with a light gasoline cut recycled from C) and hydrogen in a reactor with catalyst A at least one VIb metal and at least one non noble group VIII metal on a support, producing effluent having diolefins and sulfides R1-S-R2, with R1 and R2 methyl or ethyl radicals lower than that that of the starting gasoline;B) the effluent from A) is sent into a fractionating column separating at the top a light gasoline cut containing hydrocarbons having less than 6 carbon atoms per molecule and at the bottom a heavy gasoline cut containing hydrocarbons having 6 and more than 6 carbon atoms per molecule;C) recycling a part of the light gasoline from B) to the reactor of A) with a recycle ratio 0.1 to 0.7.