Crude oil- Occurence and CompositionOcuurence -Earth’s crust, more homogeneous than coal mostly in sedimentary rocks.Colour – Light brown to dark brown or black exhibiting luminescence in some casesComposition – Mixture of Hydrocarbons of homologous series namely paraffins, naphthenes and aromatics.

Main elements :C (84%-86%)H2(11-14%)O2, N2, S (rarely constitute 8%)Metals-Traces

Origin and formation Activity of acetylene seriesCaC2 + 2H2O C2H2 + Ca(OH)2Al4C3 + 12 H2O 3 CH4 + 4 Al(OH)3

Medeleeff and Berthelot explained that these carbides present in the earth’s crust are the origin for petroleum

V.D.Sokolov hypothesis: HC vapors present in cosmic clouds lead to precipitation of these clouds under favourable conditions.HCs adsorbed and entrapped by earth’s crust.

Composition of crude oil

General composition: Carbon and hydrogen in varying

proportions along with organic compounds containing

sulphur, oxygen, nitrogen and metals such as vanadium,

nickel, iron and copper.

Based on type of carbon-carbon bonds present HC

classification in crude:

Saturated hydrocarbons: Acyclic-Paraffins / Alkanes;

cyclic-naphthenes.

Unsaturated hydrocarbons: Alkenes, alkynes, alkadienes.

Aromatic hydrocarbons

Composition of crude oilSulphur compounds: Sulphur content: less than 0.05 to more

than 10 wt% but generally in the range 1-4 wt%.

Crude with less than 1 wt% sulphur – sweet. Mercaptans(R-

SH), sulfides and disulfides (R-S-R and R-S-S-R’),

Thiophenes (Polynuclear aromatics replaces one or more

carbon atoms in the aromatic ring).

Oxygen compounds: Alcohols, ethers, carboxylic acids,

anhydrides, carboxylic acid esters, Ketones, Furans (5

member oxygenated rings), Benzofuran.

Composition of crude oilNitrogen compounds: pyridines (C2H5N), quinoline

(C9H7N), isoquinoline (C9H7N). Non-basic nitrogen compounds – pyrroles (5 membered heteroaromatic compounds), pyrrole is converted to the polycyclic heteroatomic compounds indole and carbozole.

Metallic compounds: Chlorides and sulphates of sodium, potassium, magnesium and calcium. Removed in desalting operations. Zinc, titanium, calcium and magnesium appear in the form of organometallic soaps. Vanadium, nickel, copper and iron are present as oil-soluble compounds.

Composition of crude oilAsphaltenes and Resins:

Dark brown friable solids that have no definite m.p.usually leave carbonaceous residue on heating. made up of condensed polynuclear aromatic layers linked by saturated links. Layers are folded creating a solid structure called micelle. (MW- few hundred to several million). Aphaltenes are separated in lab using non-polar solvents such as pentane and n-heptane. Resins are polar molecules (MW:500-1000), insoluble in liquid propane but soluble in n-heptane. Resins are responsible for dissolving and stabilizing the solid asphaltene molecules.

Flow sheet of Modern Refinery

Major refining processes in modern refineries

Crude distillation products

Petroleum Products

High Conversion Refinery

Refining ProcessesPhysical Separation Processes:

Crude Distillation: Crude first desalted >Atmospheric distillation column(with steam)>Vacuum distillation column (50 mm Hg).

Solvent Deasphalting: Only physical process where carbon is rejected from heavy petroleum fraction such as vacuum residue. Propane in liquid form (at moderate pressure) is used to dissolve the whole oil, leaving asphaltene to precipitate.This low sulfur and low metal DAO is called “Bright Stock” and is used as feed stock for lube oil plant. Also sent to cracking units to increase light oil production.

Solvent Extraction: Lube oil stock is treated by a solvent, such a s N-methyl pyrrolidone(NMP), which can separate aromatics into the extract phase. Solvent is then removed from both the phases and the raffinate is dewaxed.

Solvent Dewaxing: The raffinate is dissolved in a solvent (MEK) and the solution is gradually chilled, during which high molecular weight paraffin (wax) is crystallized, and the remaining solution is filtered. Extracted and dewaxed oil is called “lube oil”.

Refining ProcessesChemical Separation Processes:

Catalytic Reforming: Supported Pt catalyst used to reconstruct naphtha(C6-C10) into aromatics and isoparaffins. This produced naphtha reformate used in gasoline formulation and as a feedstock for aromatic production (BTX).

Hydrotreating: Hydrogen used to clean petroleum fractions from impurities such as sulphur, nitrogen, oxy-compounds, chloro-compounds, aromatics, waxes and metals using hydrogen (cobalt and molybdenum oxides on alumina).

Catalytic Hydrocracking: ARs and VGO are cracked in the presence of hydrogen to get light products.

Refining ProcessesChemical Separation Processes:

Catalytic Cracking: FCCU uses zeolite based catalyst to crack VGO to gasoline and small quantities of gas oil and refinery gases.

Alkylation: Process in which isobutane (collected mainly from FCC and delayed coker) reacts with olefins such as butylene (C4=).

Isomerization: Isomerization of light naphtha is the process in which low octane number hydrocarbons (C4, C5, C6) are transformed to a branched product with the carbon number. Main advantage of this process is to separate hexane (C6) before it enters thereformer.

Refining ProcessesChemical Separation Processes:Thermal Chemical Conversion Processes:Delayed coking: This process is based on the thermal cracking of vacuum residue by carbon rejection forming coke and lighter products such a s gases, gasoline, and gas oils.

Flexicoking: In this thermal process, most of the coke is gasified into fuel gas using steam and air. Products are gases, gasoline and gas oils with very little coke.

Visbreaking: This is a mild thermal cracking process used to break the high viscosity and pour points of vacuum residue. Products include gases, gasoline, gas oil and unconverted residue.

Refinery-Petrochemical Integration

Products CompositionLiquefied Petroleum Gas (LPG): They include ethane, ethylene,

propane, propylene, normal butane, butylene, isobutane and isobutylene.

Gasoline: Gasoline is classified by octane ratings (conventional, oxygenated and reformulated) into three grades: regular (octane rating: 85-88), midgrade (octane rating: 88-90) and premium (octane rating>90).

Kerosene: Max distillation temp. of 204 deg.C at the 10% recovery point, a final boiling point of 300 deg.C and a minimum flash point of 37.8 deg. C. ASTM D3699 (separate grades), ASTM D1655.

Jet fuel: Both gasoline and kerosene and meets specns. for use in aviation turbine power units

Products CompositionDiesel fuel: The quality of diesel fuels is expressed as cetane number or cetane index. The cetane number (CN) is expressed in terms of the volume percent of cetane (C16H34) which has high ignition (CN-100) in a mixture with alpha-methyl-naphthalene (C11H10) which has low ignition quality (CN=0).

Super diesel-cetane number 45 and it is used in high speed engines, trucks and buses. No. 2 diesel has 40 cetane number. Railroad diesel fuels are similar to he heavier automotive diesel fuels, but have higher boiling ranges upto 400 deg.C and lower cetane numbers (CN=30).

Products Composition

Fuel Oil: No.1 fuel oil is similar to kerosene and No. 2 fuel oil is very similar to No.2 diesel fuel. Heavier grades of No.3 and 4 are also available.

Residual Fuel oil: Vacuum residue. Critical specns. are viscosity and sulphur content. Low sulphur residues are in more demand in the market.

Lube oil: Paraffinic and naphthenic lubricants have a finished viscosity index of more than 75.Asphalt: Asphalt is an important product in the construction industry and comprises upto 20% of products. It can be produced only from crude containing asphaltenic material.Petroleum coke

Products Composition