Refinery Process

This weblog gives you brief information regarding Various Refinery Process with its detail description, including its process sequence and detail in P&ID’s. For any process or chemical Engineers, this basic information is required for his design and understanding the process layout.

Further in refinery process below process schematics are described in this blog,

• Crude Oil Pre-treatment (Desalting)
• Crude Oil Distillation
• Atmospheric Distillation
• Vacuum Distillation
• Solvent Extraction and De-waxing
• Aromatics Extraction
• Thermal Cracking
• Visbreaking
• Delayed Coking
• Catalytic Cracking
• Fluid Catalytic Cracking
• Hydrocracking
• Catalytic Reforming
• Catalytic Hydrotreating
• Distillate Hydrodesulphurisation
• Isomerisation
• C₅ and C₆ Isomerisation
• Polymerisation
• Alkylation
• Sweetening and Treating Processes etc

Layout and Description of Processes

 

Typical Refinery Processing Scheme

Crude Oil

Complex mixtures containing many different hydrocarbon compounds

Composition vary from field to field

• High reactive sulphur = sour crude
• Low reactive sulphur = sweet crude

Density in API (American Petroleum Institute) gravity

• High API gravity = light crude
• Low API gravity = heavy crude

PONA (Parafins, Olefins, Naphtenes, Aromatics)

Hydrocarbons: Typical usage by carbon number

• C1 : fuel gas, methane reforming
• C2 : fuel gas, ethane cracking & polymerisation
• C3 : polypropylene, LPG, catalytic polymerisation
• C4 : LPG, catalytic polymerisation, C4 Isomerisation
• C5 : C5/C6 Isomerisation, TAME, pentene
• C6 : Hexene extraction, C5/C6 Isomerisation
• C4-C11 : petrol
• C11-C12 : paraffin, kerosene (IP)
• C12-C20+ : diesel
• Heavier fractions : fuel oil, pitch, coke

Hydrocarbon Chemistry

Paraffins

• Saturated hydrocarbons, straight chain or branched
• C_nH_2n+2

Methane:  CH₄

Butane:  C₄H₁₀                       

Isobutane:  C₄H₁₀               

Alkenes

• Mono Olefin hydrocarbons, straight chain or branched
• C_nH_2n

Ethylene:  C₂H₄         

    

1-Butene:  C₄H₈          

_{CH2 = CH = CH2 = CH3} 

2-Butene:  C₄H₈          

_{CH3 = CH = CH = CH3}                 

Isobutene:  C₄H₈       

              

Dienes and Alkynes

• Dienes (Diolefins): Two carbon double bonds, straight chain or branched
• Alkynes: Three Carbon double bonds
• C_nH_2n-2

Acetylene:  C₂H₂                       H-CΞC – H

1,2-Butadiene:  C₄H₆           CH₂=C=CH=CH₃

1,3-Butadiene:  C₄H₆           CH₂=CH=CH=CH₂

Dienes are “nasty components” as they easily polymerise to sticky gluey polymers at higher temperatures (>80 deg C). Dienes are thus hydrogenated to olefins and parafins.

Aromatics

• Unsaturated ring-type (cyclic) compounds
• All aromatics have at least one benzene ring

Benzene:  C₆H₆

Toluene (methyl benzene):

Naphthalene:    

Naphthenes

• Saturated ring-type (cyclic) compounds
• C_nH_2n

Cyclohexane:  C₆H₁₂      

                               

Methyl Cyclopentane:

Crude Oil Pre-treatment (Desalting)

• Remove water, inorganic salts, suspended solids and water-soluble trace materials
• Chemical and Electrostatic separation
• Use hot water as extracting agent

Crude Oil Distillation

• First step in refining process
• Separation of crude into various fractions or straight run cuts
• Main fractions or “cuts” have specific boiling point ranges
• Atmospheric Distillation
• Vacuum Distillation
• Other Distillation Towers (Columns)

 Atmospheric Distillation

 

Vacuum Distillation

• Further distillation of residuum
• Reduced pressure to prevent thermal cracking
• Separate catalytic cracking feedstock from surplus residuum
• Vacuum columns usually larger in diameter to maintain required vapour velocities

Solvent Extraction and De-waxing

• Method of refining lubricating oils and other refinery stocks
• Solvent extraction and solvent de-waxing to remove undesirables (sulphur, nitrogen, oxygen, salts, dissolved metals)
• Prevent corrosion, protect catalyst, and improve finished products.
• Most widely used extraction solvents: phenol, furfural and cresylic acid.
• De-waxing to remove wax from distillate or residual base stocks.
• Solvents for de-waxing: toluene for dissolving oil, methyl ethyl ketone (MEK) for dissolving wax.

 

Aromatics Extraction

 

Thermal Cracking

• Alter molecular structure of hydrocarbons from crude distillation to suit marker requirements
• Breaks or cracks heavier hydrocarbons into more valuable products: gasoline, fuel oil and gas oils
• Two types: Thermal cracking and catalytic cracking
  • Visbreaking: mild thermal cracking
  • Steam Cracking: Produce olefins (e.g. ethylene)
  • Coking Processes: Severe thermal cracking
    • Delayed coking
    • Continuous (contact of fluid) coking

Visbreaking

 

Delayed Coking

 

Catalytic Cracking

• Breaks complex hydrocarbons into simpler molecules
• Rearrange molecular structure to convert heavy hydrocarbon feedstock into lighter fractions such as kerosene, gasoline, LPG, heating oil and petrochemical feedstock
• Compared to thermal cracking: Better yield at less severe operating conditions, flexible process
• Three functions in catalytic cracking process:
  • Hydrocarbon Reaction
  • Catalyst Regeneration
  • Fractionation
• Three types of CC:
  • Fluidised Catalytic Cracking (FCC)
  • Moving bed Catalytic Cracking
  • Thermofor Catalytic Cracking (TCC)
• Other Activities Involved:
  • dehydrogenation
  • hydrogenation
  • isomerisation

 Fluid Catalytic Cracking

Hydrocracking

• Two stage process combining catalytic cracking and hydrogenation
• Heavier feedstock cracked in presence of hydrogen
• Used for feedstock difficult to treat with catalytic cracking or reforming
• Feedstock are characterised by high polycyclic aromatic content and high concentrations of two principle catalyst poisons, sulphur and nitrogen compounds
• High pressure (70 – 140 bar) and high T (400°C – 800°C)
• On feed with high paraffin content hydrogen prevent formation of polycyclic aromatics
• Hydrogen prevent tar formation and coke build-up on catalyst
• Hydrocracking produces relatively large amounts of isobutane for alkylation feedstock
• Hydrocracking also performs pour-point control and smoke-point control, important for high-quality jet fuel

Two-Stage Hydrocracking

 

Catalytic Reforming

• Convert low-octane naphtha’s into high-octane gasoline blending components called reformates
• Represents numerous reactions:
  • cracking
  • polymerisation
  • dehydrogenation
  • isomerisation
• Reformates can be high in toluene, benzene, xylene and other aromatics
• Hydrogen is a significant by-product and is separated for recycling and use in other processes.
• Typical catalytic reforming processes:
  • Platforming
  • Powerforming
  • Ultraforming
  • Thermofor catalytic reforming

Catalytic Reforming Process

 

Catalytic Hydrotreating

• Hydrogenation process to remove about 90% of contaminants such as nitrogen, sulphur, oxygen and metals
• Done upstream of processes such as catalytic reforming to prevent catalyst poisoning
• Upgrade middle-distillate petroleum fractions into finished kerosene, diesel fuel and heating oils
• Converts olefins and aromatics to saturated compounds
• Hydrotreating to remove sulphur = hydrodesulphurisation
• Other hydrotreating processes:
  • Improve burning characteristics of kerosene by converting aromatics to naphthenes
  • Lube-oil hydrotreating
  • Improve quality of pyrolysis gasoline (pygas) by hydrogenation of diolefins to mono-olefins

Distillate Hydrodesulphurisation

Isomerisation

• Converts n-butane, n-pentane and n-hexane into their respective isoparaffins with substantially higher octane number
• Iso-butane is an important feedstock for alkylation units
• Two distinct isomerisation processes
  • C4 Isomerisation
  • C5/C6 Isomerisation

C₅ and C₆ Isomerisation

 

Polymerisation

• Process of converting light olefin gases into hydrocarbons of higher molecular weight and higher octane number
  • Same olefin molecules combined = polymerisation
  • Different olefin molecules combined = copolymerisation
• Accomplished thermally or in presence of a catalyst at lower temperature
• Only two (dimer) or three (trimer) molecules combined.

 

Alkylation

• Alkylation combines low-molecular-weight olefins (primary a mix of propylene and butylene) with isobutene in the presence of a catalyst, either sulphuric acid or hydrofluoric acid
• Product = alkylate. A mixture of high octane, branched-chain paraffinic hydrocarbons
• Alkylates is a premium blending stock
  • Exeptional antiknock properties
  • Clean burning
• Hydrofluoric acid extremely hazardous and phased out. Newer solid catalyst processes implemented

Sulphuric Acid Alkylation

Hydrogen Fluoride Acid Alkylation

 

Sweetening and Treating Processes

• Removal of contaminants:
  • Sulphur
  • Nitrogen
  • Oxygen
  • Dissolved metals
  • Inorganic salts
  • Soluble salts dissolved in emulsified water
• Can be intermediate step or just before sending product to storage
• Processes
  • Acid, Caustic or Clay Treating
  • Drying and Sweetening
  • Sulphur recovery
  • Hydrogen Sulphide Scrubbing