Thermal Cracking

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Thermal cracking, the first downstream process that changed the petroleum industry, permitted - by the use of high temperature and pressure - the heavy, low-value feedstock to be broken into lighter, higher-value heating oil, diesel, and gasoline. Thermal cracking was followed by other developments in the 1920s and 1930s. Polymerization (oligomerization) yields high-octane gasoline from olefins produced as byproducts in thermal cracking units. Vacuum distillation takes the residual oil, which is left at the bottom of the column after atmospheric distillation, and allows its further separation. Using thermal action, the visbreaking unit can reduce the viscosity of the residues from the distillation columns, allowing a much easier flow and processing of the product. Coking uses the products left from atmospheric and vacuum distillation and produces, via thermal treatment at high temperature, gasoline, heavy oil, fuel gas and petroleum coke, an almost pure carbon residue.

During World War II, the petroleum industry shifted to products that were essential for the war effort, and especially advanced aviation fuels. This resulted in the development of the alkylation process in which a catalyst (usually sulfuric acid or hydrofluoric acid) is used to combine a branched alkane with an olefin (alkene) to produce high-octane compounds for use as high-quality gasoline components. Nowadays, this process is one of the most important steps in the production of high-octane gasoline for motor cars. Other advances during that period included catalytic cracking which uses a catalyst to accelerate the cracking process, and iso-merization converting straight-chain alkanes into branched ones having a much higher octane number. Catalytic reforming produces higher-octane components for gasoline from lower-octane naphtha feedstock recovered in the distillation process.

Over time, these processes have been continuously improved and new schemes such as dehydrogenation (to produce useful alkenes from alkanes) and hydro-cracking have been added. Without these it would be impossible to produce economically the large amounts of valuable lighter fractions from the intermediate and heavy compounds that constitute most of the crude oils.

Figure 6.3 BP Grangemouth refinery, UK (© BP p.l.c.).

Crude oil is a complex mixture of hydrocarbons. Depending on its source, it varies (among other things) in terms of its color, viscosity, and content of sulfur, nitrogen and other impurities. Most commonly, crude oils are generally classified by their density and sulfur content. Less dense, or lighter, crude oils have a higher share of the more valuable light hydrocarbons that can be recovered by simple distillation. Denser or heavier crude oils contain more heavy hydrocarbons of lower value and require additional processing steps to produce the desired range of products. Some crude oils also contain significant amounts of sulfur and heavy metals which are detrimental as they act as contaminants for most refining processes and finished products; they are also pollutants, necessitating additional purification steps. Because the quality of oil varies so widely, refineries differ in their complexity depending on the type of crude oil to be processed as well as the range of products desired. To allow the most flexibility, modern refineries (Fig. 6.3) however are designed to process various blends of different crude oils.

The processing of crude oil opened up the route to petrochemicals, because cracking produces, beside fuels, also unsaturated hydrocarbons containing one or more C=C double bonds, in particular ethylene, propylene, butylene, and butadiene. These compounds are called olefins and, unlike paraffins (the main saturated components of oil), can be readily used and further transformed by chemical reactions. They constitute the basic building blocks for numerous products and are produced in very large quantities. Yearly, some 100 million tons of ethylene and 60 million tons of propylene are manufactured worldwide. They are used mainly for the production of synthetic polymers, and also for many other products. Ethylene, for example, is the starting material for polyethylene, propylene for polypropylene, and synthetic rubber can be produced from butadiene. Besides olefins, aromatic compounds - principally benzene, toluene and xylenes - are also obtained during crude oil refining. These aromatic compounds are important starting materials for synthetic products such as polystyrene, nylon, polyurethane, or polyesters. In total, about 6% of crude oil is used today to produce petrochem icals. Crude oil, together with natural gas, are the sources for some 95% of organic chemicals, yielding products such as lubricants, detergents, solvents, waxes, rubbers, insulation materials, insecticides, herbicides, synthetic fibers for clothing, plastics, fertilizers, and many others. The advance of chemistry in the 20th century has depended - and still depends to a large extent today - on the availability of petrochemical building blocks.

The history of petrochemistry started around the 1900s, at which time the demand for natural rubber collected from Hevea trees began to surpass the supply when new applications such as motor car tires were introduced. Replacement materials were needed, and this led to the invention of synthetic rubbers; the process began with the polymerization of butadiene, which turned out to be superior to the natural products.

In 1907, the first fully synthetic plastic - named "bakelite" - was created by the reaction of phenol and formaldehyde. This new liquid resin, when hardened, took the shape of the vessel in which it was formed. Unlike earlier plastics such as celluloid, it could not be remelted, it retained its shape under any circumstances, and it would not readily burn, melt or decompose in common acids or solvents. Bakelite is still used today as an electric insulator. During the next decade, cellophane, the first clear, flexible and waterproof packaging material was developed. The 1920s and 1930s witnessed the introduction of petrochemical solvents and the discoveries of numerous new plastics and polymers including nylon, polyvinyl chloride (PVC), Teflon, polyesters, and polyethylene. The petrochemical industry grew especially rapidly during the 1940s when, during World War II, the demand for synthetic materials to replace costly and often difficult to obtain, less-efficient natural products led the industry to develop into what would become a major factor in today's technological society. During that time, many other synthetic materials such as acrylics, neoprene, styrene-butadiene rubber (SBR) and others went into use, taking the place of dwindling natural material supplies. Among other applications, Nylon was used to make parachutes and to reinforce tires, besides its latter use for synthetic fibers, especially for nylon stockings. Plexiglas was initially introduced during World War II for airplane windows. Lightweight polyethylene insulation made it possible to mount otherwise too-heavy radar units on airplanes. From then on, petrochemical products - and especially polymers - moved into an astonishing variety of areas. Together with oil- and natural gas-based fuels, they touch our daily lives in countless ways. In fact, we are so used to them that we no longer notice their unique nature!

Today, our households are full of products derived from hydrocarbons. In the bathrooms, shampoo and shower gel are composed of synthetic soap formulations and their bottles made out of polyethylene, polypropylene or PVC which have the advantage of being unbreakable. The toothbrushes, hair blow dryers, combs, shower curtains and toilet brushes are all made of plastics. In the kitchen, the refrigerators, coffee machines, toasters, microwaves and other appliances are all composed in part from synthetic polymers. Without proper insulation with polyurethane foam, refrigerators and freezers would consume much more energy. Teflon coating led to non-stick cookware, and plastic packaging of food allows for better conservation and protection against contamination. Plastic bottles and containers for all kinds of beverages, mostly made from polyethylene terephthalate (PET), are safer and lighter than glass bottles. Waste-disposal garbage bags and many other uses of plastics have become essential. In the bedroom, from the "linens" to the alarm clock which wakes us up in the morning, all are made using polymers. A large part of our clothes hanging in the closet or folded in drawers are based on synthetic fibers such as polyesters, polyacrylics, or rayon. To wash our clothes and other fabrics, we use detergents or dry-cleaning solvents, both made from hydrocarbons. In the living rooms, carpeting, furniture and its coverings, televisions, video recorders, home entertainment systems, DVD and CD players together with their remote controls, would not exist without plastics. From the outdated vinyl records to audio and video tapes or the modern CDs and DVDs, all are made using polymers. The electric cables to power all the appliances and equipment would be difficult to run safely throughout the house without plastic insulation. Even the utility lines which bring natural gas or water into our homes are nowadays made out of plastics such as PVC. PVC is also increasingly the material of choice to replace more maintenance-demanding wood for windows, doors and other construction applications. Heating and cooling our homes and buildings uses natural gas, heating oil, and electricity which, for a large part, comes from the combustion of fossil fuels. In our gardens, we relax on weatherproof plastic chairs and let the sprinklers attached to the underlying network of PVC pipes water the lawns treated with synthetic fertilizers to keep them attractive.

Once we step into our motor car, we are literally surrounded by hydrocarbon products. The seats, head and arm rests are made of synthetic fibers, and the upholstery cushioning from urethane foams. The dashboard, steering wheel, door panels, floor mats and almost all the apparent inner parts use different plastics with specific properties. Security features such as bumpers, baby seats and life-saving airbags are also manufactured with polymeric materials. Even structural steel and aluminum frames are increasingly partly replaced by new generations of high-performance composite plastics. The versatility, durability and cost-saving properties of plastics have given them much advantage in the automotive industry. Furthermore, their light weight - especially compared to steel - allows a better fuel efficiency to be achieved. Considering the engine and drive-train, all of the fluids necessary for their proper operation - motor oil, transmission, cooling and steering fluid - are hydrocarbon-based, as are the tires. Our vehicles run on roads which are asphalted, and are powered by gasoline or diesel fuels. Oxygenated and other petrochemical-based additives are added to gasoline in order to improve engine performances and reduce air pollution.

Our furniture, carpeting, computers, printers, telephones, pagers, and mobile phones to ball point pens are also made, at least in part, from plastics. Most of our outdoor activities involve varied hydrocarbon-derived products present in many sport articles, from inline skating, skis and ski boots, snowboards, golf equipment, basket and tennis balls, canoes and boats, bicycle helmets, swimsuits or even the elastic cords for bungee-jumping.

Petrochemicals also contribute to the amazing progress of healthcare and hygiene. For many years, plastic medical products, from flexible intravenous solution and blood bags to disposable syringes to artificial heart valves, have helped doctors and nurses to save numerous lives. Seriously injured or handicapped persons can recover a considerable level of mobility thanks to plastics and resins for artificial joints and limbs. Various plastics are used in medical applications because of their clarity, transparency, flexibility, sterilizability, and ease of processing. Today, an increasing number of tailor-made polymers with very specific properties are being widely developed and used. The crucial factor for plastics placed into the human body for extended periods is their biocompatibility and stability. Petrochemicals are also the basis of synthetic pharmaceutical products. Phenol, for example, has long been used as a starting material for aspirin. Other basic chemicals are the source for medicines aimed at reducing cholesterol, reducing blood pressure, and curing skin diseases. On the prevention side, with the use of condoms made from elastic plastics, the risk of spreading sexually transmitta-ble illnesses such as AIDS can be dramatically reduced. Condoms can also play a major role in population control.

Hydrocarbons also play an essential role in increasing the production of food and other crops. By introducing modern agricultural equipment such as tractors and harvesters fueled by gasoline or diesel fuel, many agricultural operations have been mechanized, reducing dramatically the former need for back-breaking human or animal labor. To increase the yield of crops and to avoid the exhaustion of necessary plant nutriments from the soil, fertilizers are needed. Potassium, phosphorus, calcium, magnesium, iron, and other minerals used as fertilizers can be relatively easily extracted from natural sources, but sources of nitrogen - perhaps the most essential element - are rare in nature. During the 19th century, the main source of nitrogen was "guano", a bird fecal matter collected on islands near the coast of Chile and shipped to Europe or other distant destinations. However, at the start of the 20th century, two chemists - Fritz Haber and Carl Bosch - developed a revolutionary method to produce ammonia (NH3) from nitrogen contained in the air and hydrogen obtained from methane. Ammonia remains the basis of nitrogenous fertilizers used today, including urea. The use of pesticides synthesized from petrochemical building blocks is also essential to avoid crops losses from diseases and insects, and be able to continue the efficient provision of food for the growing world population.

As we can see, hydrocarbons and their products extracted from fossil resources, together with advances in chemistry, have made our lives better, longer, safer, and more comfortable. They are essential for our society and have transformed or even revolutionized transportation, information, communication, entertainment, medicine, and agriculture, and essentially the way in which we live and work. Further progress will allow future generations not only to enjoy the same quality of life, but most probably also to improve upon it.

In the future, as our non-renewable resources decrease, we will need to replace fossil fuels with different other sources in order to fulfill our energy needs. The demand for derived hydrocarbon products (transportation fuels, plastics, synthetic fabrics, elastomers, rubbers, paints and innumerable other products) is unlikely to regress, however, and it will be necessary to identify sustainable synthetic hydrocarbon sources to produce these commodities and products. In the short term, we can still rely on the available oil and natural gas sources, as well as more extensive coal resources. Using syn-gas from natural gas and coal for Fischer-Tropsch processes allows the production of synthetic hydrocarbons on an industrial scale, as shown during World War II in Germany and in the 1960s in South Africa. At present, Qatar is involved in developing a large-scale Fischer-Tropsch-based industry to produce diesel fuel from abundant natural gas resources. But Fischer-Tropsch chemistry, besides being highly energy-demanding, capital intensive and environmentally polluting, is also based on our diminishing fossil fuel resources. Hence, in the longer term new solutions based on renewable sources from biomass or significantly through methanol obtained by recycling of CO2 are needed (see Chapters 10-14).

Assuming that mankind can solve its energy needs by using alternative sources and atomic energy, there will still be a need for synthetic hydrocarbons and their products, for convenient transportation fuels, and for various derived materials and products (Figs. 6.4 and 6.5). It has been frequently suggested that agricultural or other bio-sources might be used in this way. Whilst agricultural ethanol, produced by the fermentation of sugar cane, corn or other crops, can be produced in significant quantities and used as a fuel additive, its large-scale use via dehydration to ethylene and subsequently to synthetic hydrocarbons and products, is highly questionable (this was first proposed by Lenin in the 1920s, but soon abandoned). The amount of ethanol required would be staggering, and the production of crops would in turn still require large quantities of hydrocarbon fuels, fertilizers, and pesticides. One reasonable new approach to overcome this problem is based on methanol, which can be produced from a variety of sources and -most importantly - from carbon dioxide and hydrogen (obtained by the electrolysis of water using any form of renewable and atomic energy) to serve as a convenient feedstock for synthetic hydrocarbon and products; this is described in the proposed "Methanol Economy".

Thermal Cracking Scheme
Figure 6.4 Petroleum products and uses in Enegry Profile 1999, Energy Information Ad-the United States (% refinery yields of finished ministration (U.S.). products in 1997). Source: Petroleum - An
Crude Oil Cracking Butadiene
Figure 6.5 Crude oil destination. Source: Petroleum: An Energy Profile 1999, Energy Information Administration (U.S.).

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Responses

  • Stefan
    How oil made a fractional distillation cracking of hydrocarbons?
    8 years ago
  • danny
    How crude oil cracking works?
    8 years ago
  • DANIELA ROTHSTEIN
    What are the economic importance of thermal and catalytic cracking.?
    8 months ago

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