Petroleum Refining Processes and Chemistry
Crude Oil Composition
– 84% C, 14% H, <1% others (N, O, metals)
– 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 product, 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, crude = 2, gas > 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
FP: Lowest temperature to form ignitable mixture (D>62C, JF >38C)
VP: Pressure exerted by vapor, high VP = volatile
Solvent Extraction (BTX), Benzene/Toluene/Xylenes
Naphtha -> catalytic reforming -> Solvent extraction 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
- Steam cracking -> ethene/propene
- Dehydrogenation -> butadiene
- 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 P 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)
Reaction: 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