Cracking Process: Liquid vs Vapor Phase

Combustible Substances

• Combustible substances contain carbon as a main constituent and produce a large amount of heat when burned properly.
• They can be solid, liquid, or gas and are used economically for domestic and industrial purposes.
• Examples include wood, charcoal, coal, kerosene, petrol, diesel, producer gas, and oil gas.

Combustion

• Combustion is a vigorous reaction with oxygen, accompanied by the evolution of heat and light.
• It is an exothermic process, releasing heat energy.
• The final products of complete combustion of hydrocarbons are carbon dioxide and water vapor.

Incomplete Combustion

• Incomplete combustion results in a mixture of carbon dioxide and carbon monoxide with water.
• This occurs when there is not enough oxygen available for complete combustion.

Properties of Fuels

Density

• Density is the ratio of the mass of a fuel to its volume at a reference temperature of 15°C.
• It is measured by a hydrometer.
• Density is useful for quantity calculations and assessing ignition quality.
• It is measured in kg/m3.

Specific Gravity

• Specific gravity is the ratio of the weight of a given volume of oil to the weight of the same volume of water at a given temperature.
• It is measured by a hydrometer.
• Specific gravity is used in calculations involving weights and volumes.
• It has no units, as it is a ratio.

Asphalt

• Asphalt is obtained by the oxidation of residual heavy oil in the presence of air at higher temperatures.
• It can also be obtained through the deep vacuum distillation of residual heavy oil.
• Asphalt is available in liquid, semi-solid, and solid forms.
• It is used for road making, waterproofing roofs, and manufacturing waterproofing concrete and paints.

Cracking

• Cracking is the decomposition of high boiling hydrocarbons of high molecular weight into smaller, low boiling hydrocarbons of low molecular weight.
• Straight chain alkanes are converted into branched chain hydrocarbons during cracking.
• Saturated hydrocarbons are converted into a mixture of saturated and unsaturated hydrocarbons.
• Aliphatic alkanes are converted into cyclic alkanes.
• All hydrocarbons obtained by cracking have a lower boiling point than the parent hydrocarbons.

Types of Cracking

• Thermal Cracking: carried out at higher temperature and pressure without a catalyst.
• Catalytic Cracking: carried out at lower temperature and pressure in the presence of a suitable catalyst.

Synthesis of Petrol

• Polymerization: thermal and catalytic polymerization.
• Hydrogenation of Coal: Bergius Process or Direct Process, and Fisher Tropsch Process or Indirect Process.
• Alkylation.

Knocking and Octane Number

• Knocking occurs when the fuel-air mixture is heated to a temperature greater than its ignition temperature, resulting in spontaneous combustion.
• This leads to a sudden, badly controlled burning and explosion, producing a characteristic metallic or rattling sound from the engine.

Natural Gas

• Natural Gas is always found above the oil in oil wells and is also called Marsh gas.
• It consists of methane and other saturated hydrocarbons.
• Calorific value varies from 12,000 to 14,000 kcal/m3.
• Constituents: methane, ethane, propane, butane, and pentane.
• Uses: domestic and industrial fuel, raw material for the manufacture of carbon black and hydrogen, and generation of electricity by using it in fuel cells.

Water or Blue Gas

• Water or Blue Gas is a mixture of combustible gases like CO, H2, with a little non-combustible gases like CO2 and N2.
• Calorific value is 2800 kcal/m3.
• Composition: H2 = 48%, CO = 44%, and CO2, N2, and CH4 = rest.
• Uses: manufacture of ammonia by Haber's process, and generation of electricity by burning it to raise steam.

Combustion Efficiency

• N2 is a temperature-reducing diluent that must be present to obtain the O2 required for combustion.
• N2 reduces combustion efficiency by absorbing heat from the combustion of fuels and diluting the flue gases.
• This reduces the heat available for transfer and increases the volume of combustion by-products.