Pyrolysis of Hydrocarbons Alkanes

Pyrolysis is described as converting the compound into smaller fragments in the absence of air with the application of heat, and it varies from combustion. It happens in the absence of air, and thus oxidation of compounds does not occur. In general, the Pyrolysis of alkanes is also named cracking.

• The decomposition of a compound using an application of heat is known as Pyrolysis.

• Pyrolysis of higher alkanes to produce a mixture of lower alkanes, alkenes are referred to as cracking. Usually, it is carried out by heating the higher alkanes to high temperatures under pressure ranging from 6-7 atm, either in the presence or absence of a catalyst. The chemical reaction for the same can be given as follows:

C6H14 → C6H12 + H2 + C4H8 + C2H6 + C3H6 + C2H4 + CH4

• Alkane’s Pyrolysis involves breaking C-H and C-C bonds and takes place by a free radical mechanism.

• The preparation of petrol gas from petrol and oil gas from kerosene oil is according to the process of Pyrolysis. For suppose, Dodecane, which is a constituent of kerosene oil, on heating up to 973 K in the presence of Pd, Ni, or Pt, forms a mixture of pentene and heptane along with other products. The chemical reaction for the same can be given as follows:

C12H26 → C7F116 + C5H10 + other products

Pyrolysis Process

When alkane vapors are passed in the absence of air through red-hot metal, it breaks down into smaller or simpler hydrocarbons. This specific process occurs at high pressure and high temperatures without a catalyst. At low pressure and temperatures, the presence of a catalyst such as palladium or platinum is needed for this reaction. Generally, large hydrocarbons are obtained during the fractional distillation of crude oil (such as petroleum). In cracking, the hydrocarbon molecules break into smaller hydrocarbon compounds randomly—a few compounds obtained from the cracking hold carbon-carbon double bonds.

The factors that are responsible for the formation of products during cracking are listed as follows:

• Temperature and pressure

• Nature of alkane

• Presence or absence of a catalyst

Pyrolysis of Alkanes

In contrast to combustion, the Pyrolysis rate increases with the increase in molecular weight and branching in an alkane. During the C-C bonds fission, alkenes and alkanes are produced, whereas the C-H bond fission results in hydrogen and alkene. The C-H bond fission occurs due to the catalytic action of V₂O₂, Cr₂O₃, MoO₃, and C-C bond fission that occurs under the presence of Al₂O₃, ZnO, and SiO₂.

The cracking of alkanes follows a mechanism, which is a free radical mechanism. It plays an essential role in the petroleum industry. The alkane’s higher molecules are transformed into the lower molecules (it means from petrol C6 to C11) by cracking.

For example, Dodecane (which is a component of kerosene oil) forms a mixture of pentane and heptane as Pyrolysis products, on heating it at a temperature of 973K under the catalytic action of palladium, nickel, or platinum.

C₁₂H₂₆ → pt/pd/ni →C₇H₁₆ + C₅H₁₀ + other products

Uses of Alkanes

• Methane in the form of natural gas can be used for running cars, scooters, buses, and more vehicles. LPG (which is a mixture of isobutane and butane) can be used as a fuel in homes and in the industry as well.

• Methane can also be used to form carbon black, which is used in the manufacturing of paint, automobile tyres, and printing inks.

• Catalytic oxidation of alkanes forms aldehydes, carboxylic acids, and alcohols.

• Higher alkanes in the form of kerosene oil, gasoline, lubricating oils, paraffin wax, and diesel are widely used.

• Methane can also be used in the manufacturing of halogen-containing compounds such as CHCL₃, CH2Cl2, CCI4, and more, which are used as solvents both in industry and in the laboratory.

Process of Cracking or Pyrolysis

Cracking: Alkanes, when subjected to high temperatures (ranging from 670-970 K) in the presence of a catalyst, are decomposed into very smaller molecules. The method of dissolving the less volatile higher hydrocarbons into different forms of more volatile lower hydrocarbons by heat application is referred to as Pyrolysis or cracking. The alkane’s cracking involves the cleavage of C-H and C-C bonds. 

For Example, Pyrolysis of alkanes is supposed to take place by the free radical mechanism. The preparation of petrol gas or oil gas petrol or from kerosene oil involves the Pyrolysis principle. For example, heating to 973K in the presence of Pd, Ni, Pt provides a mixture of pentene and heptane.

C12H26 → 973k → Pt/Pd/Ni → C7H16    +    C5 H10 + other products

Dodecane                                Heptane      Pentene  

Cracking is of Two Types

• Thermal Cracking: 

It can be carried out either in the liquid phase or in the vapor phase. It is quite difficult to control and gives rise to complex product mixtures.

• Catalytic Cracking: 

It can be carried out at a temperature ranging from 670-820 K using alumina and silica as a catalyst. Catalytic cracking can be useful as it forms gasoline having a higher octane number.

The products that are formed during cracking depends upon:

• The pressure applied,

• The structure of starting alkane,

• Use of a catalyst.

Applications of Pyrolysis

There are multiple applications of Pyrolysis, also it has a wide range of applications in the field of green technology. Pyrolysis can be used to extract materials from the items like tires or remove organic contaminants from oily sludges and soils, and make biofuel from crops and waste. Pyrolysis can assist in the breakdown of vehicle tires into some useful components on the other hand also reducing the environmental impact of disposal of tires.

Tires are a major source of landfill components in many areas, and when they are burned, they release PAHs and some heavy metals into the air and environment. When tires are pyrolyzed, however, they decompose into gas and oil (both of which can be used as fuel) as well as carbon black (usable as filler in rubber products, along with new tires, and in the form of activated charcoal in the filters and fuel cells). It also has the ability to remove organic contaminants like the synthetic hormones from the sewage sludge (semisolid materials that remain after wastewater is treated and the water content is reduced) and render heavy metals inert, allowing the use of sludge in the form of fertilizer.

More than that, pyrolyzing biomass (biological materials like wood and sugarcane) has a lot of potential for producing energy sources that could supplement or replace petroleum-based energy. Pyrolysis causes hemicellulose, cellulose, and a part of the lignin in biomass to break down into smaller molecules in a gaseous state. When those gasses cool, they condense into liquid bio-oil, leaving the rest of the original mass (mainly the remaining lignin) as solid biochar and non-condensable gasses.

Difference between Pyrolysis and Combustion

Combustion is a type of chemical reaction in which chemicals combine with oxygen to produce energy in the form of heat and light. Which only happens when there is enough oxygen present in the air. It also yields gaseous end products, which is more essential. 

Pyrolysis is an oxygen-free breakdown reaction in which organic molecules get decomposed. It happens when there is not enough oxygen present in the air. It also produces gaseous components as well as trace amounts of liquid and solid leftovers, unlike combustion. Pyrolysis and combustion are both thermochemical processes. There are, however, many other distinctions between combustion and Pyrolysis. The main distinction between combustion and Pyrolysis is that combustion takes place in the presence of oxygen, while Pyrolysis takes place in the absence (or near absence) of oxygen.

Difference between Pyrolysis and Gasification

Pyrolysis and gasification vary from each other. Pyrolysis and gasification are two key processes that are used in the decomposition processes.

The main difference between Pyrolysis and gasification is that Pyrolysis is the thermal conversion of organic matter in the absence of oxygen using a catalyst whereas Gasification is a thermochemical process that converts biomass into producer gas or a combustible gas (syngas). Pyrolysis and gasification differ in that Pyrolysis takes place in the absence of air, whereas gasification takes place in the presence of air. Furthermore, Pyrolysis produces heat, combustible liquid, and combustible gas, whereas gasification produces heat and combustible gas. As a result, there is a distinction between Pyrolysis and gasification.

Furthermore, Pyrolysis has applications in food manufacturing, such as caramelization, biomass fuel production, ethylene production, and the treatment of plastic waste, while gasification has applications in heat production, electricity generation, and other areas.

Conversion of Petroleum using Pyrolysis

Petroleum is a complex mixture of thousands of organic molecules, including straight-chain alkanes, cycloalkanes, alkenes, and aromatic hydrocarbons with four to several hundred carbon atoms that are pumped out of the ground. The components along with their identities and their relative abundances vary depending on the source - Texas crude oil differs from that of the Saudi Arabian crude oil. In fact, analyzing petroleum from several deposits can yield a "fingerprint" of each, which can be valuable in locating the sources of crude oil spills. For example, the crude oil of Texas is "sweet," which means that it includes a modest number of sulfur-containing molecules, but Saudi Arabian crude oil is "sour," containing a big amount of sulfur-containing molecules.