Table of Contents
Role of Petroleum in the Chemical Industry
Petroleum (crude oil) is a complex mixture of hydrocarbons and small amounts of heteroatom-containing compounds (S, N, O, metals). In chemical engineering it is treated not as a single substance, but as a multi‑component raw material that can be separated and transformed into:
- Energy carriers: gasoline, kerosene/jet fuel, diesel, heating oil, fuel oil, LPG.
- Chemical feedstocks: light alkanes, alkenes, aromatics, naphtha and gas oil fractions, from which plastics, solvents, detergents, synthetic fibers, additives, and many other products are made.
The central task of petroleum processing is therefore twofold:
- To produce fuels that satisfy strict property specifications (boiling range, octane/cetane number, sulfur content, vapor pressure, etc.).
- To generate petrochemical raw materials in the types and quantities demanded by the chemical industry.
Composition and Classification of Crude Oils
Crude oils differ considerably in composition and quality. Important classification criteria include:
- Density / API gravity
- Light crudes: rich in low‑boiling components, desirable for gasoline and naphtha.
- Heavy crudes: richer in long‑chain, high‑boiling components and residues.
- Sulfur content
- Sweet crude: low sulfur, easier to refine, produces cleaner fuels.
- Sour crude: high sulfur, requires intensive desulfurization and yields more sulfur by‑products.
- Proportion of hydrocarbon types
- Paraffinic (alkane‑rich) crude: good for high‑octane gasoline components and waxes.
- Naphthenic (cycloalkane‑rich) crude: good lubricating oil base stocks.
- Aromatic‑rich crude: higher density and often higher carbon residue, influences processing route.
Because of this variability, each refinery is designed and operated to handle certain crude “slates” and to transform them into the product mix demanded by the market.
Overview of a Refinery: From Crude to Product
A refinery is an integrated network of unit operations and processes arranged in process chains. The rough sequence is:
- Crude desalting and pretreatment
- Primary distillation: atmospheric distillation
- Secondary distillation: vacuum distillation
- Conversion processes (thermal and catalytic) to adjust molecular size and structure
- Finishing processes (purification, property adjustment, blending)
The core logic is:
- First, separate by boiling point (fractionation).
- Then, convert and upgrade heavy, less valuable fractions into lighter, more valuable products.
- Finally, remove impurities and tailor properties to specifications.
Pre‑Treatment of Crude Oil
Before distillation, crude oil must be made compatible with refinery equipment and catalysts:
- Desalting
- Crude contains water with dissolved salts (e.g. NaCl, MgCl₂), solids (sand, clay), and metals.
- In an electrostatic desalter, wash water is mixed with the crude to dissolve salts; an electric field promotes coalescence and separation of the aqueous and oil phases.
- This prevents corrosion, fouling, and catalyst poisoning downstream.
- Dehydration
- Mechanical separation and heating remove free water and some emulsified water.
The pre‑treated crude, now with minimized salts and water contents, enters the atmospheric distillation column.
Atmospheric Distillation – Primary Separation
Atmospheric distillation is a fractional distillation under ambient pressure. The crude is heated (without cracking) to about 350–380 °C and fed into a large column with multiple trays or structured packing.
Along the column, a temperature gradient is maintained:
- Hotter at the bottom
- Cooler at the top
Components separate mainly according to boiling range. Typical fractions are:
- Gases (refinery gas, LPG)
- Overhead products with the lowest boiling points; mainly C₁–C₄ hydrocarbons (methane, ethane, propane, butanes).
- Used as fuel gas in the refinery or as feedstock for steam cracking (petrochemicals).
- Light naphtha
- Low‑boiling C₅–C₆ fraction; part can be used as petrochemical feedstock, part goes to gasoline blending or isomerization.
- Heavy naphtha
- C₆–C₉ fraction; major gasoline component after further upgrading (e.g. reforming).
- Kerosene fraction
- Intermediate boiling; used for jet fuel, some heating fuels, and kerosene solvents after treatment.
- Gas oil (light and heavy gas oils)
- Higher boiling than kerosene; used as diesel fuel and heating oil after further treatment, or as feed for cracking units.
- Atmospheric residue
- The non‑vaporized remainder at the bottom; too high boiling to be distilled at atmospheric pressure without decomposition.
- Feed for vacuum distillation and heavy‑residue conversion units.
This step provides the first rough sorting of crude into useful boiling ranges but does not yet yield final product quality.
Vacuum Distillation – Separating High-Boiling Fractions
Atmospheric residue is introduced into a vacuum distillation column, where pressure is reduced significantly (e.g. 20–50 mbar). This lowers boiling points and allows remaining high‑boiling components to be distilled without thermal cracking.
Typical products of vacuum distillation are:
- Light vacuum gas oil (LVGO) and heavy vacuum gas oil (HVGO)
- Feedstocks for catalytic cracking and hydrocracking to make gasoline, diesel, and petrochemical feedstocks.
- Vacuum residue
- Still heavier fraction; feed for residue conversion (coking, visbreaking, residue hydrocracking) and for asphalt/bitumen production.
Vacuum distillation extends the fractionation concept to higher boiling ranges and maximizes the yield of material that can be upgraded.
Conversion Processes: Upgrading Heavy to Light
Distillation alone cannot match the high demand for light products (gasoline, jet fuel, diesel) relative to heavier fractions naturally present in crude. Conversion processes change the molecular structure, allowing:
- Breaking down large molecules into smaller ones (cracking).
- Recombining and rearranging structures (reforming, alkylation).
- Removing heteroatoms (desulfurization, denitrogenation).
These processes often use high temperatures and pressures, and many use catalysts.
Thermal Cracking and Visbreaking
Thermal cracking uses heat (and sometimes pressure) without catalysts to break C–C bonds in heavy fractions.
- Visbreaking
- Mild thermal cracking of heavy residues to reduce viscosity and pour point.
- Produces some lighter distillates (e.g. gas oils) and reduces the amount of unmanageable heavy fuel oil.
- Delayed coking and other coking processes
- More severe thermal cracking of vacuum residue.
- Products: gas, naphtha, gas oils, and solid carbonaceous material called petroleum coke.
- Coke can be used as fuel or in metallurgical processes, depending on quality.
Thermal processes are robust and can handle very heavy feedstocks but often yield products with lower quality (e.g. unstable, olefin‑rich liquids) that need further treatment.
Catalytic Cracking (Fluid Catalytic Cracking, FCC)
Fluid Catalytic Cracking (FCC) is a central refinery process for converting heavy gas oils into lighter, high‑value products using a solid acid catalyst (typically zeolite based).
Characteristics:
- Feed: atmospheric and/or vacuum gas oils.
- Conditions: high temperatures (~500 °C), moderate pressures, short contact times.
- Catalyst circulates in fluidized form between reactor and regenerator.
Main products:
- FCC gasoline
- High‑octane component for motor gasoline blending, rich in branched and aromatic hydrocarbons.
- Light cycle oil (LCO)
- Used as diesel blending component or fuel oil component, often desulfurized.
- Heavy cycle oil (HCO)
- Recycled as FCC feed or used as fuel oil component.
- LPG and light olefins (propene, butenes)
- Valuable feedstocks for petrochemical processes and alkylation.
FCC thus transforms relatively low‑value gas oils into high‑octane gasoline and petrochemical feedstocks.
Hydrocracking
Hydrocracking combines cracking with hydrogenation using a bifunctional catalyst (acidic support plus metal, e.g. Ni–Mo, Co–Mo, Ni–W).
Key features:
- Feed: heavier gas oils (often vacuum gas oils) or residues; can also process FCC cycle oils.
- Conditions: elevated pressures (often 80–200 bar) and temperatures (350–450 °C), presence of hydrogen.
- Catalyst: simultaneously promotes C–C bond breaking and saturation of olefins and aromatics.
Main advantages:
- Produces clean, low‑sulfur middle distillates (diesel, jet fuel) and some naphtha.
- Superior product quality (high cetane diesel, good jet fuel) due to saturation and impurity removal.
- Flexible operation: can be tuned toward more naphtha or more middle distillates.
Hydrocracking is particularly important in refineries oriented toward diesel and jet fuel production.
Processes for Adjusting Molecular Structure
Besides changing molecular size, refineries also adjust molecular structure to achieve desired fuel properties and to create chemical feedstocks.
Catalytic Reforming
Catalytic reforming converts low‑octane naphtha into high‑octane reformate, which is a key gasoline component and source of aromatics.
Main transformations:
- Dehydrogenation of naphthenes to aromatics.
- Isomerization and dehydrocyclization of paraffins to aromatics.
- Some hydrocracking and isomerization.
Characteristics:
- Feed: heavy naphtha fraction (C₆–C₁₀) of appropriate composition.
- Products:
- Reformate (aromatic‑rich, high octane number).
- Hydrogen as a valuable by‑product used in hydrodesulfurization and hydrocracking units.
Reforming thus links fuel production and hydrogen supply, and provides aromatic feedstocks for the petrochemical industry (benzene, toluene, xylenes after separation).
Isomerization
Isomerization rearranges straight‑chain alkanes to branched isomers under mild conditions with an acid catalyst.
- Typical feed: light naphtha (C₄–C₆).
- Main goal: raise octane number without changing carbon number.
Example: conversion of $n$‑pentane and $n$‑hexane into isopentane and isohexane, which burn more smoothly in engines (higher octane).
Isomerization is especially important in unleaded gasoline formulations, where high octane must be achieved without lead additives.
Alkylation and Polymerization
Alkylation and polymerization combine light olefins with isoparaffins or with each other to form high‑octane gasoline components.
- Alkylation:
- Reactants: isobutane plus C₃–C₄ olefins (e.g. propene, butenes from FCC).
- Catalyst: strong acid (e.g. sulfuric or hydrofluoric acid in conventional units).
- Product: “alkylate” – a highly branched isoparaffin mixture with excellent octane, low vapor pressure, and low sulfur.
- Polymerization:
- Reactants: light olefins (mainly propene and butenes).
- Product: higher olefins (“polymer gasoline”), usually hydrogenated afterward.
These processes convert gaseous by‑products into valuable liquid fuel components and help meet gasoline blending requirements.
Removing Impurities: Hydrotreating and Desulfurization
To comply with environmental regulations and protect catalysts, refineries must remove sulfur, nitrogen, metals, and unsaturated compounds from intermediate streams.
Hydrotreating (hydrodesulfurization, HDS):
- Feed: naphtha, kerosene, gas oil fractions, FCC streams, etc.
- Conditions: hydrogen atmosphere, moderate temperatures and pressures, Co–Mo or Ni–Mo catalysts.
- Reactions:
- Sulfur compounds + H₂ → H₂S + hydrocarbons.
- Nitrogen compounds + H₂ → NH₃ + hydrocarbons.
- Olefins + H₂ → paraffins (saturation).
Hydrotreating is central to achieving:
- Ultra‑low sulfur gasoline and diesel.
- Stable fuels with good storage properties (low gum formation).
- Clean feedstocks for sensitive catalytic processes.
Hydrogen sulfide produced is processed further (e.g. Claus process) to elemental sulfur, an important industrial product (e.g. in sulfuric acid manufacture).
Product Finishing and Blending
Individual process units produce intermediate streams with specific compositions and properties. These streams are combined to final products that meet detailed specifications (composition, volatility, flash point, cold flow properties, etc.).
Gasoline Blending
Gasoline is not a pure substance but a blend of various refinery streams:
- Reformate from catalytic reforming (high octane, aromatic‑rich).
- FCC gasoline (high octane, olefin‑ and aromatic‑containing).
- Alkylate (high octane, paraffinic, low sulfur).
- Isomerate (high octane, paraffinic).
- Hydrotreated naphthas and others.
Blending aims to achieve:
- Required octane number (research octane, motor octane).
- Acceptable vapor pressure (starting and drivability).
- Limited content of aromatics, olefins, benzene, sulfur, and other regulated components.
Additives (detergents, corrosion inhibitors, antioxidants, metal deactivators, etc.) are incorporated in small amounts to improve engine performance and fuel stability.
Diesel and Jet Fuel Blending
Diesel and jet fuels are blended primarily from:
- Straight‑run gas oils from atmospheric distillation.
- Hydrocracked middle distillates.
- FCC light cycle oil (after severe hydrotreating, if used).
- Kerosene fractions for jet fuel.
Important target properties:
- Cetane number (ignition quality) for diesel.
- Cold flow properties (cloud point, pour point).
- Lubricity, energy content, stability.
- Flash point and freezing point for jet fuel.
- Very low sulfur content for environmental and engine-protection reasons.
Here too, additives like flow improvers, anti‑foaming agents, and corrosion inhibitors may be used.
Production of Petrochemical Feedstocks
Refineries supply the basic building blocks for the wider chemical industry. Major petrochemical feedstocks from petroleum processing include:
- Light alkanes and alkenes
- Ethane, propane, butanes, as well as ethene, propene, and butenes.
- Often separated from refinery gas and LPG streams and sent to steam crackers or on‑purpose olefin production units.
- Naphtha (light and heavy)
- Principal feedstock for steam cracking to produce lower olefins (ethene, propene) and aromatic streams.
- Aromatics (BTX: benzene, toluene, xylenes)
- Obtained from reformate and certain pyrolysis gasoline streams via extraction and separation.
- Key starting materials for polymers (e.g. polystyrene, polyesters, polyurethanes) and numerous chemicals.
- C₄ and C₅ streams
- Sources of butadiene, isoprene, and other components used to make synthetic rubbers and specialty chemicals.
Thus, petroleum processing is the primary upstream step that feeds large parts of organic and polymer chemistry.
Integration with Other Energy and Feedstock Sources
Modern refineries increasingly integrate with other processes to adapt to changing energy systems and environmental constraints:
- Integration with natural gas and NGL (natural gas liquids) processing**
- Joint production of LPG, condensates, and petrochemical feedstocks.
- Co‑processing of renewable feedstocks
- Some hydroprocessing units can partially convert bio‑oils or plant oils together with fossil feed, producing “drop‑in” renewable diesel and jet fuels.
- Hydrogen production and management
- Hydrogen from catalytic reforming and dedicated units (e.g. steam reforming of natural gas) is centrally managed to supply hydrotreaters and hydrocrackers.
Such integration allows a refinery–petrochemical complex to function as a flexible producer of both fuels and chemical raw materials.
Environmental and Efficiency Considerations in Petroleum Processing
Within petroleum processing, several strategies are used to reduce environmental impacts and improve efficiency:
- Deep desulfurization of fuels to lower SO₂ emissions.
- Catalyst optimization to improve selectivity and lower energy consumption.
- Energy integration within refineries (heat recovery between hot and cold streams through heat exchanger networks).
- Flare minimization and gas recovery to avoid burning valuable hydrocarbons and reduce emissions.
- Treatment of by‑products and waste streams (e.g. sulfur recovery from H₂S, wastewater treatment, controlling particulate and NOₓ emissions from furnaces).
Although a detailed treatment of environmental chemistry is given elsewhere, it is important here to note that modern refinery design strongly couples process choice and operating conditions to environmental regulation and energy efficiency goals.
Summary: From Crude Oil to Fuels and Chemical Raw Materials
In petroleum processing, crude oil is transformed through a series of separation, conversion, purification, and blending steps into:
- Marketable fuels: gasoline, diesel, jet fuel, heating oil, LPG, fuel oils.
- Fundamental chemical feedstocks: light alkanes and alkenes, naphtha for steam cracking, BTX aromatics, and specialty streams.
The overall strategy is to:
- Use distillation to separate components by volatility.
- Employ cracking and structural modification to convert heavy, low‑value fractions into lighter, high‑value products.
- Remove impurities to meet environmental and performance standards.
- Tailor mixtures through blending to meet detailed product specifications.
In this way, petroleum processing forms a key interface between the energy sector and the broader chemical industry.