Crude oil Treatment process Hydrotreatment Amine recovery Sulfur recovery

Hydrotreater BP-UK

Hydrotreatment What is hydrotreatment? It is a process to catalytically stabilize and/or remove undesirable elements from products or feedstocks by reacting them with hydrogen.  Stabilization usually involves converting unsaturated hydrocarbons such as olefins to paraffins. Undesirable elements removed by hydrotreating include sulfur, nitrogen, oxygen, halides, and metals.

Difference between Hydrotreating and Hydrocracking Hydrotreating and hydrocracking both can be called Hydroprocessing

PROCESS VARIABLES The principal operating variables are temperature, hydrogen partial pressure, and space velocity. Liquid Hourly Space Velocity (LHSV)= Volumetric flow rate/catalyst volume Weight Hourly Space Velocity (WHSV)= Mass flow rate/catalyst mass Increasing temperature and hydrogen partial pressure increases sulfur and nitrogen removal and hydrogen consumption. Increasing hydrogen partial 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

Hydrotreatent Reactions Convert Organic S to Hydrogen Sulfide Convert Organic N to Ammonia Convert Organic O to Water Convert Organic halides to Hydrogen Halides Saturate Olefins Saturate Aromatics Remove Metals

Desulfurization Increasing Difficulty

Denitrogenation Denitrogenation is usually more complex than desulfurization.

Oxygen Removal

Olefin Saturation Olefins are unstable components and they polymerize in presence of oxygen Olefin saturation is very rapid and exothermic reaction

Aromatic Saturation Aromatics present as 1, 2, 3 or more rings structure Aromatic saturation is a stepwise reaction Aromatic saturation is an exothermic reaction

Aromatic Saturation

Halide Removal

Metals Removal Lead, mercury, arsenic, silicon, nickel, vanadium, sodium Compounds that contain metals decompose in reactor and deactivate catalyst The metal deposited on 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 hydrotreatment reactions

Guard Reactor

Hydrotreatment Catalyst Solid consisting of a base alumina impregnated with metal oxides Pellets shaped as cylindrical or pills For Desulfurization, Co-Mb catalyst is used For Desulfurization, Denitrogenation and Aromatic saturation, Ni-Mb catalyst is used

Sulfur recovery Hydrogen sulfide created from hydrotreating is a toxic gas that needs further treatment Solvent extraction: using diethanolamine (DEA) dissolved in water separates the H2S gas from the process stream (DEA should be recovered) 34

H2S separation_ Amine Unit 35

DEA Chemistry DEA: weak organic base to absorb acid gases of H2S and CO2 from gas streams. Chemical Structure: DEA = [HO(CH2)2]2 - NH C2H4OH \ N - H / The reaction of DEA with H2S and CO2 are equilibrium reactions: H2S: R2NH + H2S ⇋ R2NH2SH CO2: 2R2NH + H2O + CO2 ⇋ (R2NH2)2CO3 where R = HO(CH2)2 Absorption Absorption is favoured by high pressures and low temperatures 36

Amine Regeneration Regeneration is favoured: low P and high T Acid gas components are boiled out of the amine solution The required heat input to the reboiler: Sensible heat required to bring the rich amine from the inlet temperature (~95°C) to the column base temperature (~127°C)  Heat of reaction to desorb the acid gas components.   Heat of vapourisation to generate sufficient water vapour within the column to strip the acid gas components in the amine Need to ensure that temperatures are maintained below 130°C to prevent amine degradation and subsequent amine corrosion 37

DEA Properties Amine Strength: Amine Loading: Ideally 25-27wt% DEA Higher amine concentrations = higher regenerator boiling temperatures and increased corrosion rates Lower amine concentrations = higher circulation rates and thus increase energy requirements for pumping and regeneration Amine Loading: Amine Loading = mol (H2S) + mol (CO2) mol (DEA) Target is an amine loading of 0.3 – 0.5 mol/mol 38

Sulfur Recovery Unit_Flowsheet Based on Claus Process Muffle furnace Catalytic reaction Thermal reaction

Equipment Roles_ Muffle Furnace H2S Air Thermal Reaction Muffle Furnace 925 – 1300 C Muffle Furnace: 1/3 of the H2S in the acid gas stream is partially oxidised to sulphur dioxide 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 and the first sulphur condenser. 40

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. 41

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 (>310C) 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 (>310C) 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 42

Equipment Roles_ Waste Heat Boiler HP Steam – 3.4 MPa Waste Heat Boiler: Cools process stream Recovers energy in a useful form (HP Steam) for use in the preheaters Reactions also take place in the WHB 43

Equipment Roles_ Reactors Catalytic Reaction 170 – 350 C The Two Reactors Reactor 1: COS and CS2 destruction / hydrolysis at temps > 310 C Operation at elevated temperatures compromises Claus reaction equilibrium Reactor 2: Maximise Claus Reaction Operate at as lower temperature as practicable above the sulphur dewpoint to prevent dropping sulphur onto the catalyst bed Catalytic Reaction: The remaining 2/3 of the H2S is reacted over a catalyst with SO2 produced in thermal reaction to form elemental sulphur. 2H2S + SO2  3S + 2H2O DH = -31 kJ/mol H2S 44

Equipment Roles_ Condensers Sulphur Condensers LP Steam – 345 kPa Liquid Sulphur Sulphur Condensers: Recover produced sulphur 45

Equipment Roles_ Incinerator Incineration or Further Treatment Incinerator: Combust unreacted H2S to SO2 prior to emission of tail gas to the environment 46

The Claus Process_ Thermodynamics Equilibrium 1.  Thermal Reaction: 3H2S + 3/2 O2  SO2 + H2O + 2H2S DH = - 174 kJ/mol H2S 2. Catalytic Reaction: 2H2S + SO2  3S + 2H2O DH = -31 kJ/mol H2S Thermal and catalytic reactions are Reversible reactions Thermodynamic Equilibrium 47

The Claus Process_ Thermodynamics 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 the Muffle Furnace 48

SRU Catalyst Standard Catalyst Promoted Catalyst Activated alumina (Al2O3) – (Co-Mb) Average life in a refinery SRU is 3-5 years Important catalyst properties Surface area: 320 – 390 m2/g Pore volume distribution: to ensure bulk diffusion to internal surface area Physical and mechanical properties: crush strength Promoted Catalyst Titanium dioxide (TiO2) COS / CS2 hydrolysis, O2 scavenger, deactivation resistance Poor crush strength Expensive 49

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. SRU recovery efficiency is: dependent upon all significant operating conditions a direct measure of sulphur plant performance is independent of plant size can be measured accurately 50

SRU Efficiency It can be represented by the following equation: Recovery = SPRODUCED/SFEED(as H2S) The efficiency of a conventional Claus plant is limited by: the efficiency of COS/CS2 hydrolysis in the first catalytic reactor the equilibrium conversion of H2S and SO2 to elemental sulphur at the minimum safe operating temperature the saturation vapour pressure of sulphur at the minimum safe condenser operating temperature. 51

To Maximise SRU Efficiency Minimise impurity level in the feed Impurities all reduce the partial pressures of the Claus Reactants CO2 acts as a ‘fire extinguisher’ in the Muffle Furnace decreasing temperatures, decreases plant capacity and leads to the production of COS Natural Gas/HC leads to the production of CS2, increases air demand, and cause possible downstream contamination H2O is a product of the Claus Reaction NH3 increases air demand, and can lead to plugging from ammonium salts 52

To Maximise SRU Efficiency Maximise product removal at earliest opportunity Use 3 catalytic stages with active catalyst Maximise Reactor 1 temperatures to maximise COS and CS2 destruction Optimal air control H2S and O2 should be delivered to the muffle furnace in the correct stoichiometric ratio Excess O2 can result in sulphation of the catalyst = deactivation 53

End of Chapter Nine