Crude oil Treatment process Hydrotreatment Sulfur recovery Amine recovery
Hydrotreatment Catalyst Solid consisting of a base alumina impregnated with metal oxides Pellets shaped as cylindrical or pills HDS: Co-Mb (hydrodesulfurization (HDS ) HDS, HDN and aromatics: Ni-Mb (Hydrodenitrogenation (HDN )
Metals Removal Lead, mercury, arsenic, silicon, nickel, vanadium, sodium Decompose in reactor and deactivate catalyst Cannot be removed by regeneration
Lead poisoning effect on catalyst activity
Arsenic poisoning effect on catalyst activity
Sodium poisoning effect on catalyst activity
Ease of hydroprocessing reactions
PROCESS VARIABLES The principal operating variables are temperature, hydrogen partial pressure, and space velocity. Increasing temperature and hydrogen partial pressure increases sulfur and nitrogen removal and hydrogen consumption. Increasing pressure also increases hydrogen saturation and reduces coke formation. Increasing space velocity reduces conversion, hydrogen consumption, and coke formation. Although increasing temperature improves sulfur and nitrogen removal, excessive temperatures must be avoided because of the increased coke formation.
PROCESS VARIABLES Typical ranges of process variables in hydrotreating operations are
Sulfur recovery Hydrogen sulphide (H2S) created from hydrotreating is a toxic gas that needs further treatment The usual process involves two steps: 1. Removal of the hydrogen sulphide gas from the hydrocarbon stream. 2. Conversion of hydrogen sulphide to elemental sulphur. Solvent extraction: using diethanolamine (DEA) dissolved in water separates the H2S gas from the process stream (DEA should be recover) The DEA and hydrogen mixture is then heated at a low pressure and the dissolved H2S is released as a concentrated gas stream which is sent to another plant for conversion into sulphur (SRU)
Chemistry Conversion of the concentrated H2S gas into sulphur occurs in two stages: Thermal reaction in furnace Catalyst reaction in the reactor As the reaction products are cooled the sulphur drops out of the reaction vessel in a molten state. Sulphur can be stored and shipped in either a molten or solid state.
The Claus Process The overall reaction that takes place in the SRU is known as the Claus Reaction and can be represented by the following equation: 1. Thermal Reaction: 1/3 of the H2S in the acid gas stream is partially oxidised to sulphur dioxide in the Muffle Furnace to produce a 2:1 molar ratio of H2S to SO2. 3H2S + 3/2 O2 SO2 + H2O + 2H2S DH = - 174 kJ/mol H2S This reaction occurs at ~1100oC and the energy released is used to generate steam in a waste heat boiler. 2. Catalytic Reaction: The remaining 2/3 of the H2S is reacted over a catalyst with SO2 produced in (1) to form elemental sulphur. 2H2S + SO2 3S + 2H2O DH = -31 kJ/mol H2S Thermal and catalytic reactions are Reversible reactions Thermodynamic Equilibrium
Thermodynamic Equilibrium There are 2 regions of high conversion of H2S to elemental sulphur High Temperatures – thermal reaction Low Temperatures – catalytic reaction Often the Claus process is kinetically limited due to residence time and mixing issues, especially in the burner in the Muffle Furnace Activated alumina (Al2O3) – (Co-Mb) Average life in a refinery SRU is 3-5 years
Incineration or Further Treatment Claus Process H2S O2 Muffle Furnace 925 – 1300 C Thermal Reaction Waste Heat Boiler HP Steam – 3400kPa Catalytic Reaction Reactor)s) 170 – 350 C Sulphur Condenser LP Steam – 345kPag Liquid Sulphur Incineration or Further Treatment
Claus Process Acid Gas Produced in the Amine Regenerators (Major components of acid gas: H2S, CO2, light HC
COS and CS2 Formation The presence of CO2 and HC in the feed gases to the Claus plant gives rise to carbonyl sulphide (COS) and carbon disulphide (CS2) in the Muffle Furnace and Waste Heat Boiler If these compounds are not hydrolysed back to H2S they will proceed through the catalytic reactors unreacted This results in reduced sulphur conversion and decreased SRU efficiency.
COS and CS2 COS and CS2 Destruction The hydrolysis reactions can be written as: COS + H2O CO2 + H2S CS2 + 2H2O CO2 + 2H2S Hydrolysis is favoured by high temperatures (>310C) The ability of the catalyst to carry out the COS/CS2 hydrolysis reactions depends on: 1. the catalyst formulation 2. the operating temperatures in the first reactor 3. the state of deactivation of the catalyst Destruction The hydrolysis reactions can be written as: COS + H2O CO2 + H2S CS2 + 2H2O CO2 + 2H2S Hydrolysis is favoured by high temperatures (>310C) The ability of the catalyst to carry out the COS/CS2 hydrolysis reactions depends on: 1. the catalyst formulation 2. the operating temperatures in the first reactor 3. the state of deactivation of the catalyst Alternative: SCOT Process
SRU Efficiency Sulphur recovery efficiency is a measure of the percentage of sulphur present in SRU feed as H2S, that is converted and recovered as liquid elemental sulphur. Recovery = SPRODUCED/SFEED(as H2S)
End of lecture