Types of Cracking - Thermal Cracking and Catalytic Cracking
Cracking is a process by which complex high molecular weight organic compounds are broken down into smaller fragments of molecules. The complex high molecular weight organic compounds are generally long chained hydrocarbons such as petroleum. This process is used extensively in the petroleum industry for the commercial preparation of low molecular weight hydrocarbon fuels such as gasoline and diesel from high molecular weight long chain hydrocarbons and kerogens. This process basically involves the breakage of carbon-carbon (C–C) single bonds and is carried out at a very high temperature and pressure, and may involve the presence of certain specific catalysts.
Cracking of petroleum which is carried out on a large scale commercially yields a range of varied types of oils and other materials – such as light oils (e.g., gasoline), medium ranged oils, heavy oils, carbon residues (e.g., coke) and a mixture of various gases (including methane, butylene, ethane, propane, ethylene and propylene). Depending on the molecular weight and utility of the oils produced in the first step of cracking, they can either be extracted directly to be later refined for use or they can be resent to undergo another cracking step to produce smaller fragments of organic compounds, which can be used commercially or industrially.
The history of cracking process dates back to the year 1913. An American chemist, named William Merriam Burton, invented a novel method to produce gasoline from petroleum and other large non-volatile high molecular weight hydrocarbons. He used heat or thermal decomposition technique to break down the large molecular weight hydrocarbons into gasoline. Later, this process was more refined and became popular in the petroleum industry with the name, cracking, that we know today. Gradually, with time, more improvements were done to this process to make it more efficient by incorporating the use of certain other chemicals or catalysts in it.
Cracking is generally of two types, based on the types of procedures involved – namely, Thermal Cracking and Catalytic Cracking. As indicated by their names, thermal cracking procedure involves the use of heat as a source to break the bonds of high molecular weight compounds into low molecular weight molecules where catalytic cracking involves the use of certain catalysts which help in breaking these bonds. The thermal and catalytic cracking processes are further sub-classified into different types, based on the types of variations done to the original processes to make them more efficient, which are discussed as follows:
Thermal Methods of Cracking
Thermal technique of cracking is sub-classified into two types:
• Modern thermal cracking: Unlike conventional thermal cracking technique, the modern thermal cracking methods employ the use of high pressure along with high temperatures to carry out the degradation of large molecular weight hydrocarbons into smaller fragments. The modern technique of high pressure coupled thermal cracking involves reaching absolute pressures as high as 7000kPa. The process involves homolytic fission of the carbon bonds where each fragment of the high molecular weight hydrocarbon compound retains one electron on each side which then couple together or condense to lead to the formation of small molecules of lesser molecular weight like alkenes. These reactions are also important industrially for the production of certain polymers which involve alkynes as their basic structural units; the most common example is one of the most popular plastic – polythene whose basic structural unit is the smallest alkyne that exists namely ethylene. Thermal cracking has been refined and improved greatly since it earlier process in the early 1900. It is now used to produce industrially useful small molecular fragments obtained from the crude large hydrocarbons. The lighter fractions of these hydrocarbons produced are used as burner fuels. As one of the process variants, thermal cracking is sometimes carried out at a relatively milder temperature (about 500oC, unlike normal operating temperature which ranges from about 750oC to 900oC). This process is also known as delayed coking. This is done to obtain a fine carbon-rich solid compound known as needle coke, which is a highly crystalline form of coke obtained from petroleum and is used in the production of carbon electrodes used in the aluminium and iron or steel industry.
• Steam Cracking: As the name suggests, this type of thermal cracking is carried out by using heat energy from the steam. This technique is also known as pyrolysis. This method of thermal cracking is more efficient and productive as compared to the conventional heat-based thermal cracking method. This is because steam possesses more latent heat of energy as compared to the normal heat source. Steam cracking is a very useful technique industrially as it is currently the most widely used source of production of low molecular weight alkenes (also known as the olefins). The most useful alkene obtained from this method is ethene (or ethylene) which is widely used in the polymer industry and is one of the most common basic structural units of plastics. Another such useful alkene produced is propene (also commonly known as propylene). In the process of steam cracking, raw materials such as liquid petroleum gas, naphtha, butane, propane and ethane are pumped into the source which is then supplied with high energy steam to produce light and small molecular weight hydrocarbons. The yield of the molecules produced in this process is generally dependent upon the ratio of raw materials and steam present in the chamber and also on the initial composition of the raw materials used. This process is usually carried out in the absence of oxygen and the temperature of the actual reaction recorded is approximately about 800°C to 850oC. In the modern-day variants of this technique, the speed at which the steam is passed through the raw materials is increased thereby resulting in the decrease in the total time of the reaction.
Catalytic Methods of Cracking
The catalytic technique of cracking is sub-classified into two types:
• Fluid Catalytic Cracking: This is one of the most commonly used methods of cracking in the oil refineries these days. The early developed fluid catalytic cracking process during the 1940s involved the use of alumina as a catalyst. The particles of the catalyst are suspended in the air during the process with the help of the fluidized bed present in the reactor. Alumina is still used in some small scale procedures done for research purposes at University level. It is basically comprised of small particles of aluminium oxide and silica obtained from the pumice stone. However, in the industries, alumina has now been replaced by zeolite based catalysts which are more effective and productive as compared to alumina. Use of zeolite catalysts has shown to increase the yield of the cracking reaction. In the newer version of this process, the raw materials are pre-heated and sprayed onto the base of the reactor which contained hot and fluidized zeolite catalyst being blown up in the air by the fluidizer. The reaction is operated at a temperature of about 650 to 750oC. When the reactant comes in contact with the catalysts, the reaction proceeds faster with cracking of the high molecular weight hydrocarbons such as oils into lighter fragments such as gasoline and diesel. After the completion of the reaction and obtainment of low molecular weight hydrocarbons, the catalyst is separated from the product mixture with the help of cyclone mechanism of the fluidized bed reactor. The catalyst-separated product mixture is then redirected to the fractionator of the reactor for the separation of individual products.
• Hydrocracking: It is a kind of catalytic cracking which employs the use of hydrogen gas as a catalyst. The hydrogen gas is used to break the carbon-carbon single bond. The products obtained in this process are saturated hydrocarbons (such as alkanes), instead of unsaturated hydrocarbons (such as alkenes), which were usually obtained in almost all the other cracking methods discussed above. The type of product obtained in this type of reaction is dependent upon the various parameters of the reaction conditions such as the temperature, pressure and the activity of the catalyst. Hydrogen, apart from acting as a catalyst in this reaction, also performs several other functions such as prevention of the formation of polycyclic aromatic compounds such as naphthalene, reducing the formation of tar, reduction of impurities present in the raw material, prevention of the build-up of coke on the surface of the reaction mixture, conversion of nitrogen and sulphur elements present in the raw materials into ammonia gas and hydrogen sulphide, respectively and achieving a highly efficient fuel as a product. The main products formed in this reaction are jet fuel and diesel. However, some minor amounts of other compounds such as liquid petroleum gas and naphtha fractions are also produced. This process is more popular in the industries of the countries where diesel is more commonly used fuel.