Chapter 6

Chapter 60: Catalytic Cracking Process

Introduction to Catalytic Cracking

  • Definition: Catalytic cracking is a vital and widely used refinery process, exceeding 1 million tons/day.

  • Historical Context: Became significant in petroleum refining post-World War II.

  • Objective: To produce high yields of high-octane gasoline from high-boiling stocks through catalysts.

  • Comparison to Thermal Cracking: Unlike thermal cracking, which relies on heat alone, catalytic cracking uses catalysts to enhance the process.

Process Overview

  • Goal: Break complex hydrocarbons into simpler molecules.

  • Benefits: Increases quality and quantity of lighter, desirable products while reducing residuals.

  • Output Products: Converts heavy hydrocarbon feedstocks into lighter fractions like kerosene, gasoline, LPG, heating oil, and petrochemical feedstocks.

  • Operating Efficiency: Catalysts permit better yields under milder conditions compared to thermal cracking.

Comparison: Thermal vs. Catalytic Cracking (Table 6.1)

  • Thermal Cracking Outputs: 32% gasoline; 50% residual oil from vacuum residue.

  • Catalytic Cracking Outputs: 47% gasoline; 32% light cycle oil from vacuum gas oil.

Feed to Cracking Processes

  • FCC Process: Produces high-octane gasoline from straight-run atmospheric gas oil and light vacuum gas oil (LVGO).

  • Hydrocracking: Upgrades heavier fractions (HVGO and VDR) by introducing hydrogen and utilizing bi-functional catalyst systems.

  • Coking Issues: Heavy fractions may cause issues due to coking on catalysts, necessitating careful control during hydrocracking.

Crackability of Feedstocks

  • Watson Factor: Indicator of feedstocks' crackability.


    • 12: High (Paraffinic)

    • 11.5 – 11.6: Intermediate (Naphthenic)

    • < 11.3: Low (Aromatic)

  • Definition: Crackability measures a feedstock's potential to generate valuable products.

Chemistry of Catalytic Cracking

  • Mechanism: Formation of ionic species on catalytic surfaces creates branched-chain alkanes (iso-alkanes) from long straight-chain alkanes.

  • Outcome: Produces high-octane gasoline, leading to the replacement of thermal cracking.

Products from Cracking

  • Products Overview:

    • Thermal Cracking Products: Paraffins, naphthenes, aromatics, olefins.

    • Catalytic Cracking Products: Higher yield of paraffins and aromatics.

Cracking Products and By-products

  • Light Cycle Oil (LCO): Used for diesel fuel.

  • Light Gases: Includes undesired LPG (containing light olefins).

  • C5+ Gasoline: High octane fuel.

  • Heavy Cycle Oil (HCO): Used for fuel oil.

  • Coke: A byproduct burned in the regenerator.

Catalytic Cracking Processes

Fixed-Bed Configuration

  • Systems: Utilizes fixed-bed natural alumina/silica pellets as catalysts.

  • Swing Reactors: Staggered usage of reactors to manage rapid coking.

  • Regeneration: Involves burning off coke using hot air, taking ~5 minutes for both regeneration and steam stripping.

Moving-Bed Configuration - Thermafor Catalytic Cracking (TCC)

  • Integration: Combines endothermic cracking and exothermic catalyst regeneration.

  • Process Flow: Catalysts and feed enter from the top, with steam injection facilitating product transport.

  • Heat Efficiency: While efficient, heat loss occurs during catalyst movement.

Fluidized Catalytic Cracking (FCC)

  • Mechanism: The charge combines with a recycle stream to vaporize and raise temperature via hot catalyst.

  • Operation: Cracking occurs in the riser; cyclones separate oil vapors from catalyst.

  • Regeneration: Spent catalyst is regenerated to remove coke; fresh catalyst is added for optimization.

Catalyst Types and Advancements

  • Inclusions of Zeolite Catalysts: Significant for selectivity and flexibility in FCC processes.

    • Properties of Zeolites: High surface area, acid sites for reaction.

Catalyst Deactivation and Regeneration

  • Coke Formation: Results from cracking reactions; reduces catalyst activity.

  • Regeneration Process: Continuous catalyst transfer between reactor and regenerator is essential.

  • Temperature Management: Reactor temperatures are kept optimal to minimize coke formation while maximizing conversion.

Conversion Factors and Efficiency

  • Activity: The ability of the catalyst to crack feedstock into lower boiling fractions.

  • Selectivity and Efficiency Metrics: Ratio of desirable product yields versus undesirable products; efficiency ratios based on gasoline yield and conversion.

  • Key Operating Conditions:

    • Maximum regeneration temperature: 1200-1500°F (650-820°C)

    • Reactor temperature: 850-950°F (450-510°C)

    • Pressure limitations: typically 15 to 20 psig to manage coke formation risks.

Summary of Catalytic Cracking Process

  • Catalytic cracking demonstrates significant advancements in catalyst technology and operational efficiency, essential for producing gasoline and other valuable light products from crude oil. Continuous improvement and adaptations to feed stock types enhance yield and reduce waste.