Comprehensive Guide to Crude Oil Composition, Natural Gas, and Thermal Cracking Processes

Posted on Aug 5, 2024 in Chemistry

Crude Oil Composition

  • -84% C, 14% H,
  • -Paraffinic/naphthenic/aromatic
  • -Higher API, lighter crude, low specific gravity, usually rich in paraffins
  • -More sulfur: sour, less: sweet
  • -Sulfur undesirable, hard to remove in thio/benzothiophenes
  • -More polycyclic aromatics: heavy, less useful products, PNA -> coke
  • -Asphaltenes clog up well/lines, resins too

Constituents of Natural Gas (>85% Methane), Dissolved/Associated

  • -Associated has more easily liquefied components

World Energy Consumption (Oil > NG > Coal > Nuclear > Hydro)

  • 1 barrel = 160L
  • H/C ratio: Coal 1
  • 1 ton coal (30GJ), oil (45GJ), gas (55GJ)
  • Mass energy density: H2 >> methane > rest
  • Calorific value: propane highest, H2 lowest

Flash Point and Vapor Pressure

  • Flash point (FP): lowest temperature to form ignitable mixture (Diesel >62°C, Jet Fuel >38°C)
  • Vapor pressure (VP): pressure exerted by vapor, high VP = volatile

Solvent Extraction (BTX), Benzene/Toluene/Xylenes

  • Naphtha -> catalytic reforming -> Solvent extraction to get BTX
  • Toluene -> hydro-dealkylation -> benzene
  • Naphtha, Gas oil -> cracking for lower alkenes -> pyrolysis gasoline -> hydrogenation and extraction -> BTX, higher aromatics

Units: bcm, Mtoe, TW, kWh

  • 1 bcm = 0.9 Mtoe = 1.0467 * 10^10 kWh = 10.467 TW

Thermal Cracking Units and Operations (Crude Oil and NG)

  • Crude -> refinery -> 1. ethane/propane, 2. butane, 3. naphtha
  • 1. Steam cracking -> ethene/propene
  • 2. Dehydrogenation -> butadiene
  • 3. Steam cracking -> ethene/propene/butenes/butadienes
  • NG -> separation -> 1. ethane/propane, 2. butane, 3. condensate

Solvent Extraction (Propane Deasphalting)

  • -Reduce coke tendencies by removing “asphaltenic” materials, propane solvent, asphalt residue sent to thermal processes (visbreaking, delayed coking, flexicoking)

Main Thermal Cracking Mechanisms

  • -Break petroleum into molecules and introducing functional groups like olefin for chemical reactions
  • -Steam cracking: cracked into molecules with double bonds to form H2

Carbenium Formation

  • Ionization -> addition to pi bond -> organic halide and Lewis acid catalyst -> hydrogen abstraction at Lewis acid site -> SN1 mechanism -> acid-catalyzed dehydration -> formation of carbonium

Free Radical Reaction: Initiation, Addition, Chain Transfer, Termination

Catalytic Cracking in Fluidized Bed Catalytic (FCC) Reactor

Objective of Catalytic Reforming

  • -To convert heavy naphtha into high-octane reformate and produce H2, product is low in sulfur and blending for gasoline

Alkylation Unit

  • Combining, Catalytic, unite olefins & isoparaffins
  • Feed: tower isobutane/cracker olefin
  • Product: Iso-octane (alkylate)

Zeolite Catalyst (Aperture/Acidic Sites)

  • -Various pore size (4-13A), size of ring (4-18 T-atoms) determine pore aperture
  • -Larger Si/Al ratio: more acidic, decrease unit cell parameters, window size, number of cations, free space, increase hydrophobicity

Aim of Desalting – Remove Water, Inorganic Salts, Suspended Solids, Water-Soluble Metals from Crude Oil

Atmospheric Distillation Fraction (from Low Temp/°C), Refinery Gas, Petrol, Naphtha, Kerosene, Diesel Oil, Residue (Ships/Lube/Road)

Vacuum Distillation – Low Pressure to Prevent Thermal Cracking (15-30mmHG)

  • Residuals from atmospheric tower, separated into gas oil (catalytic cracker), lubricants (hydrotreating/solvent), residual (deasphalter/visbreaker/coker)

Cycling Reflux – To Preheat Crude (Driving Force), and Remove Heat from Column

Flash Drum – A Vapor-Liquid Separator

Stripper – Remove Component from Liquid by a Vapor

Trickle/Fluidized Bed Reactors

  • Trickle – suitable since evaporation not possible, L (heavy HC part) and G (H2, evap) flow downward cocurrently
  • Fluidized bed – G and L flow up and keep catalyst particles in suspension, solve catalyst deactivation since can swap

Poisoning/Rejuvenation of Catalyst – Accumulation of Metals in Feedstock on Catalyst (Pore Plugging)

  • -HYCON process to replace catalyst, or catalytic hydrogenation

Venturi Scrubber – To Remove Dust/Coke from Gas Stream

Hydrotreating (HT) Process Objectives – Remove Heteroatoms (S, N, O), Metals & Aromatics by Hydrogenation

Weight Hourly Space Velocity (WHSV = Mass Flow/Catalyst Mass)

Hydrocracking (HC System)

  • Reactions: 1. Hydrogenation, dehydrogenation (metal), 2. Protonation, Cracking (C-C), isomerisation (acid) via carbenium ions
  • Catalyst: Bifunctional metal / zeolite (NiMo(W)S or Pt, Ni on USY/ZSM-5)

Octane/Cetane Ratings and Hydrocarbon Structures

  • -Higher octane number, more fuel compression before igniting, high octane rating fuel for high compression ratio engine
  • -Compression raises P/T of air-fuel mixture, heptane ON is 0 (BP 371), 2,3,3-trimethylpentane is 100 (BP 372)
  • -Low octane number but high cetane ideal for diesel because they do not compress fuel but rather air

Cetane Number: Measure of Combustion Quality under Compression

  • -The time between injection of fuel into the combustion chamber and the start of combustion of fuel charge

B-Scission of Carbenium Species

-alkyl-substituted cycloparaffins decompose by means of scission of alkyl chain to produce an olefin and a methyl/ethyl cyclohexane