Hydrocarbons, aptly named since they are a combination of just hydrogen and carbon atoms, are the simplest of organic compounds and are produced by processing natural gas or crude oil. As simple as their chemical structure may be, hydrocarbons are incredibly functional and used in a wide variety of processes and applications.
Natural gas is up to 90 percent pure methane and also contains varying quantities of ethane, butane, propane, carbon dioxide, nitrogen, helium and sulphur compounds. These associated higher hydrocarbons known as $quot;natural gas liquids$quot; (NGL's) are removed from the natural gas stream and have a variety of other uses. These include providing the raw material for refineries and petrochemical plants where they are sent for further processing into other hydrocarbons and petrochemical intermediates, then through a series of complicated reactions to produce common plastics and polymers used by all of us.
At the refinery, the NGL's are broken down into their constituents using a process known as fractionation. This works based on the different boiling points of individual hydrocarbons in the stream, and is essentially a process of boiling off each hydrocarbon one by one in a number of stages starting with the lightest first. Thus the deethaniser removes ethane, followed by the depropaniser, the debutanizer and finally the deisobutanisor which separates iso and normal butanes.
Within the industrial gases industry, hydrocarbons form an important part of the special gases and refrigeration areas of the business and have a bewildering variety of applications related to the chemical and physical properties of each product.
The method of naming them was defined in 1866 when the German chemist August Wilhelm von Hofmann proposed a system of nomenclature in which the suffixes -ane, -ene, -yne, -one, and -une are used to denote the hydrocarbons with 0, 2, 4, 6, and 8 fewer hydrogens than their parent alkane. The diagram shows how this system is still used today to name any hydrocarbon. However, old habits die hard and products such as ethylene and propylene are so called because in the mid-19th century, the suffix -ene (an Ancient Greek root added to the end of female names meaning $quot;daughter of$quot;) was widely used to refer to a molecule that contained one fewer hydrogen atom than the molecule being modified. Thus, ethylene (C2H4) was the $quot;daughter of ethyl$quot; (C2H5) and propylene (C3H6) the daughter of propyl (C3H7).
Ethane, ethylene (or ethene as we should call it), propane and butanes as well as other hydrocarbons are produced in vast quantities at refineries with world scale plants producing up to 1 million tonnes per year. These are used as raw materials for a huge variety of downstream products, as well as being supplied to the external fuel gas market for products such as propane and butane.
One of the major challenges for the industrial gas industry is to persuade these massive producers that they should supply these products in the volumes required, which are normally only a few tonne quantities. As well as the volume problem there is also a requirement for consistent quality better than that which may be required within the refinery. As a result gas companies need to invest in further processing and purification to remove critical impurities, such as moisture and sulphurs.
Methane is generally produced by the distillation of natural gas and can be purified up to 99.9999 percent (only 1ppm of total impurities) depending on the intended application. One of the major applications for high purity methane is actually to act as a balance gas in the manufacture of natural gas calibration standards. Here, highly accurate multi component mixtures containing a wide range of hydrocarbons and other components such as CO2, nitrogen and sulphurs are used as a benchmark against which real natural gas may be measured and the calorific value determined. These mixtures are vital to the entire natural gas industry which sells products by its calorific value. Therefore a few varying points on the measurement, multiplied by the massive volumes purchased and sold can have a profound effect on the total value of a contract.
More interesting applications for methane include its use by the UK nuclear power programme as a component in the coolant used to keep the graphite moderator from overheating.
The UK has the majority of the world's Advanced Gas Cooled nuclear reactors (AGR) which use carbon dioxide as the coolant, and methane is added in part per million quantities to ensure that the chemistry of the moderator is unaffected by the continual presence of the carbon dioxide. While the volume of methane in the CO2 is only at the ppm level, the UK is one of the largest markets for high purity methane due to the huge volumes of CO2 utilized. However as the AGR's are decommissioned this is reducing.
Since diamonds are a form of pure carbon it shouldn't be surprising that it is possible to artificially manufacture them given the right circumstances and a hydrocarbon gas such as methane. In this astonishing process, industrial grade diamonds are manufactured using a combination of ultra high purity methane and hydrogen at temperatures of 5000ºc and pressure of 55,000 atmospheres - almost 200 times greater than those in the highest pressure gas cylinder!
As well as being a basic building block for a huge variety of downstream products including high density polyethylene and linear low density polyethylene (HDPE and LLDPE), ethylene has a variety of interesting applications; from medical to food industries. Since ether was initially used as anaesthetic in 1842, surgeons had been seeking a more reliable alternative which was easier to dispense. In 1923 ethylene was used in the operating theatre mixed with 15 percent oxygen and continued to be used as an anaesthetic until the 1950's along with other hydrocarbons such as cyclopropane. However, despite its excellent properties the flammability of the mixture and the introduction of safer non-flammable products caused it to be discontinued
Despite its flammable properties, ethylene is used in other applications which have found that the advantages outweigh the disadvantages, particularly if strict safety procedures are followed. One of the more interesting examples of this is its use in ripening fruit. Fruit and vegetables naturally produce tiny quantities of ethylene gas during the ripening process, where it plays a regulatory role in many processes of plant growth, development and eventually death. It is quite common for someone to place an unripe piece of fruit in a sealed paper bag with an apple, which produces a larger amount of ethylene, and hasten the ripening of the fruit.
As is often the case, the role of ethylene in the ripening process was discovered by accident. Lemon growers would store newly harvested green lemons in sheds which were warmed by kerosene heaters until they turned yellow. However, some growers invested in modern heating systems which took significantly longer than the original systems to ripen the fruit. After much research it was discovered that ethylene gas was being emitted by burning kerosene, and a new industry was born. Today, vast quantities of fruit (particularly bananas) are ripened in large ripening rooms under atmospheres of ethylene and nitrogen mixtures.
In countries like the Philippines, ethylene is mixed with water through a dispensing nozzle and sprayed over pineapples so they may be sent to market at the peak of their condition.
Other industries use hydrocarbons in many non-fuel applications and the refrigeration industry is one of the best examples, particularly because hydrocarbons, as a naturally occurring product, are $quot;green$quot; and have zero ozone depletion levels. Although flammable, hydrocarbons have been used in refrigeration applications since 1867, it was only the invention of CFC's in 1926 which stopped their common use. The realization that CFC's were damaging the ozone layer lead to their removal from production by the mid 90's, and the scene was set for hydrocarbons to resume application in commercial and domestic refrigeration.
Today, outside of the USA, 90 percent of all new domestic refrigerators use isobutane (called R600a per the refrigerant nomenclature), while propane (R290), propylene (R1270) and ethane (R170) are also used in major industrial scale refrigeration applications. Ethane, which is a low temperature refrigerant, is the chosen product for the enormous accelerators at CERN in Switzerland, and others around the world, who conduct particle experiments at the forefront of modern physics.
Aerosols and Foam
Similarly until the 90's, CFC's were extensively used in the aerosol and foam blowing industries, until the banning of their use lead to their replacement by hydrocarbons in these applications. In aerosols, the propellant needs to be a liquid with a vapour pressure slightly higher than atmospheric and with a boiling point lower than room temperature. Fortunately, hydrocarbons such as propane, and butanes may be mixed together to achieve specific vapour pressures and boiling points making them ideal in these circumstances. The only downside is their flammability which is not a significant issue given the small volumes in the can.
Polyurethane foams are manufactured by adding blowing agents such as pentane to the liquid reaction mixture. Pentane is an excellent agent since it is volatile and has an ideal boiling point and vapour pressure. Since the polymerization process is exothermic the blowing agent volatizes into a gas during the reaction process which fills and expands the cells created during the mixing process, thus creating a solid foam.
Looking to more sophisticated applications, the semiconductor industry in its quest to follow Moore's Law, formulated by Gordon Moore of Intel in 1965, is employing hydrocarbons which allow them to produce smaller, faster devices. For this, thin amorphous carbon films are deposited using methane as the carbon precursor via a chemical vapour phase deposition (CVD) process. However other hydrocarbons are becoming increasingly more popular, including ultra high purity propylene particularly in DRAM (memory) applications. As geometries continue to shrink, work on other hydrocarbons in these applications is expected to continue.
The applications of hydrocarbons are many and varied and whilst there are sufficient volumes of oil and natural gas to provide them, we can expect them to continue to have a profound effect on our daily lives. From the tyres used on our cars to the semiconductor chips in our cellphones and computers, the utility of hydrocarbons may be seen all around us.