Cracking

Summary

In the petroleum industry, cracking refers to the chemical conversion process following the distillation of crude oil, by which fractions and residue with long-chain hydrocarbon molecules are broken down into short-chain hydrocarbons. Cracking is accomplished under pressure by thermal or catalytic means and by injecting extra hydrogen. Cracking is done because short-chain hydrocarbons, such as gasoline, diesel, and jet fuel, are more commercially valuable than long-chain hydrocarbons, such as fuel and bunker oil. Steam cracking of light gases or light naphtha is used in the petrochemical industry to obtain lighter alkenes, which are important petrochemical raw products.

Definition and Basic Principles

Cracking is a key chemical conversion process in the petroleum and petrochemical industries. The process breaks down long-chain hydrocarbon molecules with high molecular weights and recombines them to form short-chain hydrocarbon molecules with lower molecular weights. This breaking apart, or cracking, is done by the application of heat and pressure and can be enhanced by catalysts and the addition of hydrogen. In general, products with short-chain hydrocarbons are more valuable. Cracking is a key process in obtaining the most valuable products from crude oil.

At a refinery, cracking follows the distillation of crude oil into fractions of hydrocarbons with different molecule chain lengths and the collection of the heavy residue. The heaviest fractions with the longest molecule chains and the residue are submitted to cracking. For petrochemical processes, steam cracking is used to convert light naphtha, light gases, or gas oil into short-chain hydrocarbons such as ethylene and propylene, crucial raw materials in the petrochemical industry.

Cracking may be done by various technological means, and the hydrocarbons cracked at a particular plant may differ. In general, the more sophisticated the cracking plant, the more valuable its end products will be but the more costly it will be to build. Being able to control and change a cracker's output to conform to changes in market demand has substantial economic benefits.

Background and History

By the end of the nineteenth century, demand rose for petroleum products with shorter hydrogen molecule chains, in particular, diesel and gasoline to fuel the new internal combustion engines. In Russia, engineer Vladimir Shukhov invented the thermal cracking process for hydrocarbon molecules and patented it on November 27, 1891. In the United States, the thermal cracking process was further developed and patented by William Merriam Burton on January 7, 1913. This doubled gasoline production at American refineries.

To enhance thermal cracking, engineers experimented with catalysts. American Almer McAfee was the first to demonstrate catalytic cracking in 1915. However, the catalyst he used was too expensive to justify industrial use. French mechanical engineer Eugene Jules Houdry is generally credited with inventing economically viable catalytic cracking in a process that started in a Paris laboratory in 1922 and ended in the Sun Oil Refinery in Pennsylvania in 1937. Visbreakinga noncatalytic thermal process that reduces fuel oil viscositywas invented in 1939. On May 25, 1942, the first industrial-sized fluid catalytic cracker started operation at Standard Oil's Baton Rouge, Louisiana, refinery.

Research into hydrocracking began in the 1920s, and the process became commercially viable in the early 1960s because of cheaper catalysts such as zeolite and increased demand for the high-octane gasoline that hydrocracking could yield. Into the twenty-first century, engineers and scientists worldwide sought to improve cracking processes by optimizing catalysts, using less energy and feedstock, and reducing pollution.

How It Works

Thermal Cracking. If hydrocarbon molecules are heated above 360 degrees Celsius, they begin to swing so vigorously that the bonds between the carbon atoms of the hydrocarbon molecule start to break apart. The higher the temperature and pressure, the more severe this breaking is. Breaking, or cracking, the molecules creates short-chain hydrocarbon molecules as well as some new molecule combinations. Thermal cracking—cracking by heat and pressure only—is the oldest form of cracking at a refinery. In modern refineries, it is usually used on the heaviest residues from distillation and to obtain petrochemical raw materials.

Modern thermal crackers can operate at temperatures between 440 and 870 degrees Celsius. Pressure can be set from 10 to about 750 pound-force per square inch (psi). Heat and pressure inside different thermal crackers and the exact design of the crackers vary considerably.

Typically, thermal crackers are fed with residues from the two distillation processes of the crude oil. Steam crackers are primarily fed with light naphtha and other light hydrocarbons. After their preheating, often feedstocks are sent through a rising tube into the cracker's furnace area. Furnace temperature and feedstock retention time are carefully set, depending on the desired outcome of the cracking process. Retention time varies from fractions of a second to some minutes.

After hydrocarbon molecules are cracked, the resulting vapor is either first cooled in a soaker or sent directly into the fractionator. There, the different fractions, or different products, are separated by distillation and extracted.

In cokers, severe thermal cracking creates an unwanted by-product. It is a solid mass of pure carbon, called coke. It is collected in coke drums, one of which supports the cracking process while the other is emptied of coke.

Catalytic Cracking. Because it yields more of the desired short-chain hydrocarbon products with less use of energy than thermal cracking, catalytic cracking has become the most common method of cracking. Feedstocks are relatively heavy vacuum distillate hydrocarbon fractions from crude oil. They are preheated before being injected into the catalytic cracking reactor, usually at the bottom of a riser pipe. There they react with the hot catalyst, commonly synthetic aluminum silicates called zeolites, which are typically kept in fluid form. In the reactor, temperatures are about 480 to 566 degrees Celsius and pressure is between 10 and 30 psi.

As feedstock vaporizes and cracks during its seconds-long journey through the riser pipe, feedstock vapors are separated from the catalyst at cyclones at the top of the reactor and fed into a fractionator. There, different fractions condense and are extracted as separate products. The catalyst becomes inactive as coke builds on its surface. Spent catalyst is collected and fed into the regenerator, where coke deposits are burned off. The regenerated catalyst is recycled into the reactor at 650 to 815 degrees Celsius.

Hydrocracking. The most sophisticated and flexible cracking process, hydrocracking delivers the highest value products. It combines catalytic cracking with the insertion of hydrogen. Extra hydrogen is needed to form valuable hydrocarbon molecules with a low boiling point that have more hydrogen atoms per carbon atom than the less valuable, higher boiling point hydrocarbon molecules that are cracked. All hydrocracking involves high temperatures from 400 to 815 degrees Celsius and extremely high pressure from 1,000 to 2,000 psi, but each hydrocracker is basically custom designed.

In general, hydrocracking feedstock consists of middle distillates (gas oils), light and heavy cycle oils from the catalytic cracker, and coker distillates. Feedstock may also be contaminated by sulfur and nitrogen. Feedstocks are preheated and mixed with hydrogen in the first stage reactor. There, excess hydrogen and catalysts convert the contaminants sulfur and nitrogen into hydrogen sulfide and ammonia. Some initial hydrogenating cracking occurs before the hydrocarbon vapors leave the reactor. Vapors are cooled, and liquefied products are separated from gaseous ones and hydrogen in the hydrocarbon separator. Liquefied products are sent to the fractionator, where desired products can be extracted.

The remaining feedstock at the bottom of the fractionator is sent into a second-stage hydrocracking reactor with even higher temperatures and pressures. Desired hydrocracked products are extracted through repetition of the hydrocracking process. Unwanted residues can go through the second stage again.

Applications and Products

There are three modern applications of thermal cracking: visbreaking, steam cracking, and coking.

Visbreaking. Visbreaking is the mildest form of thermal cracking. It is applied to lower the viscosity (increasing the fluidity) of the heavy residue, usually from the first distillation of crude oil. In a visbreaker, the residue is heated no higher than 430 degrees Celsius. Visbreaking yields about 2 percent light gases such as butane, 5 percent gasoline products, 15 percent gas oil that can be catalytically cracked, and 78 percent tar.

Steam Cracking. Steam cracking is used to turn light naphtha and other light gases into valuable petrochemical raw materials such as ethene, propene, or butane. These are raw materials for solvents, detergents, plastics, and synthetic fibers. Because light feed gases are cracked at very high temperatures between 730 and 900 degrees Celsius, steam is added before they enter the furnace to prevent their coking. The mix remains in the furnace for only 0.2 to 0.4 second before being cooled and fractionated to extract the cracked products.

Coking. Coking is the hottest form of cracking distillation residue. Because it leaves no residue, it has all but replaced conventional thermal cracking. Cracking at about 500 degrees Celsius also forms coke. Delayed coking moves completion of the cracking process, during which coke is created, out of the furnace area. To start cracking, feedstock stays in the furnace for only a few seconds before flowing into a coking drum, where cracking can take as long as one day.

In the coke drum, about 30 percent of feedstock is turned into coke deposits, while the valuable rest is sent to the fractionator. In addition to coke, delayed coking yields about 7 percent light gas, 20 percent light and heavy coker naphtha from which gasoline and gasoline products can be created, and 50 percent light and heavy gas oils. Gas oils are further processed through hydrocracking, hydrotreating, or subsequent fluid catalytic cracking, or used as heavy fuel oil. The coke drum has to be emptied of coke every half or full day. To ensure uninterrupted cracking, at least two are used. The coke comes in three kinds, in descending order of value: needle coke, used in electrodes; sponge coke, for part of anodes; and shot coke, primarily used as fuel in power plants.

Flexicoking. Flexicoking, continuous coking, and fluid coking are technological attempts to recycle coke as fuel in the cracking process. Although these cokers are more efficient, they are more expensive to build.

Fluid Catalytic Cracking. Because of its high conversion rate of vacuum wax distillate into far more valuable gasoline and lighter, olefinic gases, fluid catalytic crackers (FCCs) are the most important crackers at a refinery. FCCs are essential to meet the gasoline demand. Design of individual FCCs, while following the basic principle of fluid catalytic cracking, varies considerably, as engineers and scientists continuously seek to improve efficiency and lessen the environmental impact.

The typical products derived from vacuum distillate conversion in an FCC are about 21 percent olefinic gases, often called liquefied petroleum gas; 47 percent gasoline of high quality; 20 percent light cycle (or gas) oil, often called middle distillate; 7 percent heavy cycle (gas) oil, often called heavy fuel oil; and 5 percent coke. Gasoline is generally the most valuable. Light cycle oil is blended into heating oil and diesel, with the highest demand for these blends in winter. The more an FCC can change the percentages of its outcome, the higher its economic advantage.

Hydrocracking. Hydrocrackers are the most flexible and efficient cracker, but they have high building and operation costs. High temperatures and pressure require significant energy, and the steel wall of a hydrocracker reactor can be as thick as 15 centimeters. Its use of hydrogen in the conversion process often requires a separate hydrogen generation plant. Hydrocrackers can accept a wide variety of feedstock ranging from crude oil distillates (gas oils or middle distillate) to light and heavy cycle oils from the FCC to coker distillates.

Hydrocracker output typically falls into flexible ranges for each product. Liquefied petroleum gas and other gases can make up 7 to 18 percent. Gasoline products, particularly jet fuel, one of the prime products of the hydrocracker, can be from 28 to 55 percent. Middle distillates, especially diesel and kerosene, can make up from 15 to 56 percent. Heavy distillates and residuum can range from 11 to 12 percent.

Hydrocracking produces no coke but does have high carbon dioxide emissions. Its products have very low nitrogen and sulfur content.

Careers and Course Work

Cracking is a key refining process, so good job opportunities should continue in this field. High school students interested in designing, constructing, operating, and optimizing a cracker at a refinery or petrochemical plant should take science courses, particularly chemistry and physics. The same is true for those who want to pursue research in the field. There also are many opportunities for technicians to build, operate, and maintain a cracker.

A bachelor of science degree in engineering, particularly chemical, electrical, computer, or mechanical engineering, is excellent job preparation in this field. A bachelor of science or arts degree in a major such as chemistry, physics, computer science, environmental science, or mathematics is also a good foundation. An additional science minor is useful.

For an advanced career, any master of engineering degree is suitable. A doctorate in chemistry or chemical or mechanical engineering is needed if the student wants a top research position, either with a corporation or at a research facility. Postdoctoral work in materials science (engaged in activities such as searching for new catalysts) is also advantageous.

Because cracking is closely related to selecting crude oil for purchase by the refinery, there are also business or business administration positions. The same is true for members of the medical profession with an emphasis on occupational health and safety. Technical writers with an undergraduate degree in English or communications may also find employment at oil and engineering companies. As cracking is a global business, career advancement in the industry often requires a willingness to work abroad.

Social Context and Future Prospects

As fossilized hydrocarbons are a finite resource, they must be used as efficiently as possible. To this end, cracking at a refinery seeks to create the most valuable products out of its feedstocks derived from crude oil. This is not limited to fuels such as gasoline. The steam crackers of the petrochemical industry create raw materials for many extremely valuable and useful products such as pharmaceuticals, plastics, solvents, detergents, and adhesives, stretching the use of hydrocarbons for consumers.

At the same time, international concern with the negative environmental impact of some hydrocarbon processes is increasing. Traditionally, crackers such as cokers or even hydrocrackers released a large amount of carbon dioxide or, in the case of cokers, other airborne pollutants as well. To make cracking more environmentally friendly, to save energy, and to convert feedstock efficiently are concerns shared by the public and the petroleum and petrochemical industries. This is especially so for companies when there are commercial rewards for efficient, clean operations.

The rise of alternative fuels, replacing some hydrocarbon-based fuels such as gasoline and diesel, may challenge refineries to adjust the output of their crackers. The more flexible hydrocrackers are best suited to meet this challenge. Exploration of cracking in biofuel production is underway.

Bibliography

Burdick, Donald. Petrochemicals in Nontechnical Language. 4th ed., PennWell, 2010.

Conaway, Charles. The Petroleum Industry: A Nontechnical Guide. Penn Well, 1999.

"Cracking and Related Refinery Processes." The Essential Chemical Industry Online, University of York Centre for Industry Education Collaboration, 7 Sept. 2014, www.essentialchemicalindustry.org/processes/cracking-isomerisation-and-reforming.html. Accessed 19 Mar. 2018.

Kaiser, Mark J., et al. Petroleum Refining: Technology, Economics and Markets. 6th ed., CRC Press Taylor & Fracis Group, 2020.

Leffler, William L. Petroleum Refining in Nontechnical Language. 5th ed., PennWell, 2020.

Meyers, Robert A. Handbook of Petroleum Refining Processes. 4th ed., McGraw-Hill Education, 2016.

Sadeghbeigi, Reza. Fluid Catalytic Cracking Handbook. 4th ed., Elsevier Health Sciences Division, 2019.

Naji, Samah Zaki, et al. "State of the Art of Vegetable Oil Transformation into Biofuels Using Catalytic Cracking Technology: Recent Trends and Future Perspectives." Process Biochemistry, vol. 109, Oct. 2021, pp. 148–168. doi:10.1016/j.procbio.2021.06.020. Accessed 20 May 2024.

Sharma, Kabita. "Cracking Crude Oil-Petroleum Refining: Types, Processes, Importance." Science Info, 10 June 2023, scienceinfo.com/cracking-crude-oil-petroleum-refining. Accessed 20 May 2024.

Speight, James G. Handbook of Petrochemical Processes. CRC Press, 2021.

Uner, Deniz. Advances in Refining Catalysis. CRC Press, 2017.