Catalytic and Thermal Cracking Overview

Catalytic Cracking

• Two main characteristics determine catalytic cracking: acidity and morphology.
• Acidity is responsible for cracking activity.
• Mesoporous materials like MCM-41 and SBA-15 can be used as catalysts in cracking crude and used palm oil.
• Morphology determines selectivity via its pore accessibility.

Thermal Cracking

• Thermal cracking breaks larger hydrocarbons into smaller ones by heat alone.
• It occurs in the absence of oxygen and can be catalyzed.
• The process generates coke (solid), bio-oil (liquid), and biogas (gaseous).
• Temperature and residence time heavily influence product yield.
• Higher temperatures and longer residence times increase gas yield.
• Lower temperatures and longer residence times favor coke formation.
• Moderate temperatures and short residence times maximize liquid product yield, known as fast pyrolysis.
• Removing coke requires controlled burning in the reactor by feeding air instead of biomass.

Thermal Cracking Mechanism

• The process begins with the decomposition of the triglyceride molecule, forming heavy oxygenated hydrocarbons.
• It involves various sequential steps:
  • Initial cracking (thermolysis of the triglyceride molecule ester bond)
  • Decarboxylation/decarbonylation of long-chain oxygenated hydrocarbons
  • C-C bond cleavage of unsaturated oxygenated hydrocarbons
  • Decarboxylation/decarbonylation of short-chain oxygenated hydrocarbons
  • Isomerization, polymerization/dehydrogenation, cyclization to form dienes, acetylenes, cycloparaffins, and polyolefins
  • Dehydrogenations of cycloparaffins to form cyclo-olefins
  • Hydrogenations of cyclo-olefins to form cycloparaffins
  • Diels-Alder addition of dienes to olefins to form cyclo-olefins
  • Aromatization of cyclo-olefins to form aromatics and polyaromatics hydrocarbons
  • Coking from polyaromatics
  • Coking by polycondensation of oxygenated hydrocarbons
  • Coking by polycondensation of the triglyceride molecule
  • Polymerization of olefins to form coke
  • Direct route for C1-C5 hydrocarbon formation from the triglyceride molecule

Bio-oil Generation from Pyrolysis

• Pyrolysis is a thermochemical conversion process where biomass decomposes under high temperatures and no oxygen, producing condensable organic molecules, gases, and char.
• Fast pyrolysis aims to maximize bio-liquid yield.
• Key operating conditions for fast pyrolysis include temperature and vapor residence time.
• Fast pyrolysis is performed at approximately 500°C.
• Heat is transferred through convection, radiation, and conduction, breaking down biopolymers and releasing volatile components.
• The vapor residence time influences the composition of the bio-oil.
• Decarbonylation and decarboxylation reactions remove oxygen, forming CO and CO2.
• Double bonds and aromatics are saturated through hydrogenation.
• Hydrotreating involves high pressure and moderate temperatures, crucial for hydrogen solubility.

Upgrading Bio-oil

• Various upgrading methods exist:
  • Non-catalytic fast hydropyrolysis
  • Catalytic fast hydropyrolysis
  • Non-catalytic fast hydropyrolysis with ex-situ hydrotreating
  • Catalytic fast hydropyrolysis with ex-situ hydrotreating

Catalytic Cracking of Bio-oil

• Oxygen removal in catalytic cracking occurs through reactions like cracking, decarboxylation, decarbonylation, dehydration, and water gas shift.

• The process uses catalysts, primarily zeolites, under atmospheric pressure and temperatures between 300-600°C.

• The overall chemical reaction can be represented as:

  Bio-oil → CH<sub>1.2</sub> + CO<sub>2</sub> + CO + H<sub>2</sub>O 
  

  where "CH_1.2" represents an unspecified hydrocarbon product.

Zeolite Cracking of Bio-oil

• Bio-oil cracking over an acidic zeolite catalyst like H-ZSM-5 removes oxygen through cracking, forming CO and CO2.
• The resulting liquid product primarily consists of aromatics like benzene, toluene, xylenes, and naphthalene.
• The formation of aromatics is due to hydrogen deficiency in bio-oil.
• Compared to hydrotreating, catalytic cracking results in lower yields of liquid hydrocarbons because of carbon loss in the form of oxides.
• The process can be operated in either liquid or vapor phase.