Oil and gas
Refining Oil and Gas
07/08/2010
Refining is a key step before hydrocarbons are put on the market. It involves converting crude into a range of petroleum products to be used by consumers and industry. It consists of a series of operations to upgrade the features and quality of crude oil.
© Keblow
Why Does Oil Need to Be Refined Before Use?
The gasoline or diesel we put in our cars, the fuel we burn to heat our homes in winter, and the natural gas we use for cooking are all energy sources that come from oil and gas deposits. However, these products cannot be used directly after extraction: before using them, they need to be refined. Refining purifies the crude and converts it into various types of finished products suitable for a range of uses.
Crude oil could be used directly if special engines and boilers were developed.
Therefore, there are many different types of crude. Some are black and viscous and contain a lot of heavy molecules, others are brown and more liquid and therefore lighter. Moreover, they all contain a certain quantity of dissolved gas and sulfur or acid products, which are highly corrosive for metals.
It is not possible to build a universal corrosion-resistant engine or boiler that could operate with all these types of crude- this is why oil needs to be refined.
Refining to Meet Consumer Demand
Demand for petroleum products has changed since the mid-20th century. Road and air transport development has accelerated in industrialized countries, leading to higher demand for light products (fuel). At the same time, the heavy fuel consumption has fallen because it is used less and less for heating or to produce electricity. Other energy sources -such as nuclear- are now preferred.
Nowadays, global demand for petroleum products is distributed as follows:
• 40% light products (fuel)
• 40% mid-range products (heating fuel, diesel)
• 20% heavy products
The only unrefined oil that more or less complies with these proportions is Algerian Sahara light crude. All the other crude oil extracted worldwide contains more heavy products and so must be converted into light products before being placed on the market. These light products are therefore more expensive than crude oil because their price includes the conversion costs.
Specifically designed production plants with innovative refining techniques have been designed so as to obtain oil suitable for new consumer and industry needs. In 2009, Total started to upgrade its Normandy refinery to produce more diesel, a fuel type for which demand has regularly increased1. Industrialized countries also encourage the development of these new refineries, which ensure that they have an independent refining policy to meet their set standards. For example, in May 2010, the European Investment Bank granted Hellenic Petroleum a loan of €400 million to upgrade the Elefsina refinery in Greece so that it could produce more environmentally-friendly fuel2.
The Three Stages of Refining
In a refinery, crude is converted into finished products using a specific process that involves three types of operation (separation, conversion, upgrading).
• The first stage involves distillation to separate heavy and light molecules. This process involves heating the oil so that it gradually evaporates, like with a kettle. As the water reaches 100°C little bubbles of dissolved gas can be seen escaping. At over 100°C, the water boils and turns into steam. Afterwards, esidual white salt remains at the bottom of the kettle. It is vaporized when heated to very high temperatures.
• Distilling oil follows the same principle. It is heated in a 60m-high still tower, also known as a topping or atmospheric distillation tower (because the pressure inside the tower is close to atmospheric pressure). When it reaches 350-400°C, the oil partly evaporates and starts rising inside the tower, while the heavier molecules or residue remain at the bottom. As the vapor rises, it partially condenses into liquid due to the drop in temperature. It continues to rise to the top of the tower, where the temperature is 150°C. This is where the last uncondensed vapor - petroleum gas - is recovered. Plates placed at different heights all along the tower are used to recover around a dozen products of varying weight called petroleum cuts - from bitumen (mixtures of highly viscous hydrocarbons) to gases.
• The heavy residue from this distillation contains many mid-density products. This residue is placed in another tower and distilled again to recover mid-range products (heavy fuel and diesel).
• After these separation operations, there are still many heavy hydrocarbon molecules. To meet demand for light products, these are broken up, i.e. the atoms they are composed of are separated to obtain lighter molecules. This conversion process, carried out at 500°C, is also known as catalytic cracking because it uses a catalyst (a substance that accelerates and facilitates chemical reactions). 75% of heavy products subjected to conversion are converted into gas, gasoline, and diesel this way. The result can be improved by adding hydrogen (hydrocracking) or using carbon extraction methods to recover more light molecules (deep conversion). So, all heavy hydrocarbons can be converted into light hydrocarbons, but this operation can be costly and uses a lot of energy.
• The next phase is upgrading. This involves removing the corrosive or environmentally harmful molecules - particularly sulfur - from the products of the distillation and conversion processes. The EU has very strict standards in terms of sulfur emissions- since 2008, diesel containing over 0.1% sulfur has been banned in Europe3. The purpose of these measures is to limit atmospheric sulfur pollution. Sulfur is a respiratory irritant that also impoverishes soil and vegetation. Diesel is desulfurized at 370°C at pressure of 60 bars and in the presence of hydrogen. Under these physical conditions, the sulfur atoms break off from the hydrocarbon molecules and combine with hydrogen atoms to form hydrogen sulfide (H2S). The hydrogen sulfide is treated to obtain liquid sulfur, which is reused in industry. Similarly, kerosene and butane and propane gas are washed in soda to remove the mercaptans they contain. Mercaptans are molecules of nauseating, corrosive alcohol containing one or more sulfur atoms instead of oxygen atoms. This process is called sweetening.
• Finally, before the gasoline can be sold at the pumps, the octane rating has to be increased - this is not high enough for direct use in an engine. The octane rating -from 1 to 100- provides information on a fuel's resistance to auto-ignition. If the octane rating is not high enough, the gasoline will tend to auto-ignite, which over time will damage the engine. To avoid this, the octane rating needs to be raised to 95 or 98 so that the gasoline is compatible with vehicle engines. A chemical reaction called catalytic reforming is carried out at 500°C at pressure of 10 bars using platinum as a catalyst. This reaction restructures the skeleton of the hydrocarbon molecules to reduce their tendency to auto-ignite. There are also other chemical reactions, such as alkylation, which also improves the gasoline's resistance to auto-ignition.
What Are The Uses of Refined Petroleum Products?
Each refined petroleum product made from crude oil is used for a specific purpose:
• Butane and propane gas are used for domestic needs
• Liquefied petroleum gas (LPG) is used in gas-fueled vehicles
• Gasoline and diesel are used in motor vehicles
• Kerosene is used as aviation fuel
• Heating oil is used to heat homes
• Oil is used to make lubricants
• Naphtha is a raw material used in the petrochemicals industry
• Bitumen is used in road surfacing
Apart from these features and basic uses, each refinery markets its own finished products, which are obtained by mixing various products and additives. Thus, oil companies can market a wide range of fuels that are more environmentally-friendly, and that ensure a longer life engine life.
[1] Source: Total
[2] Source: European Investment Bank
[3] Source: Europa.eu
