What are other useful petrochemical products
Lexicon> Letter E> Petroleum Refinery
Definition: a technical installation for the processing of crude oil into petroleum products
English: oil refinery
Category: Basic Concepts
Author: Dr. Rüdiger Paschotta
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Original creation: January 23, 2013; last change: 05.02.2021
Petroleum is mainly a mixture of different hydrocarbons. It is first obtained as just roughly purified crude oil and then closed with tankers or pipelines Petroleum refineries transported. Various methods are used there to produce a range of petroleum products such as B. gasoline, diesel fuel, heating oil, liquid gas, lubricating oils, bitumen and petroleum coke are processed. However, for example, fuels in the refinery are usually not yet produced in their final composition. Rather, various raw products are created there (Blends), which are used by fuel suppliers in what are known as Blending stations then appropriately blended to achieve the desired properties of the fuels sold.
The refineries are mostly operated in the vicinity of large consumption centers. The greatest transport distances are therefore for the crude oil (which is usually delivered either by ship or through a pipeline), while the end products are only transported over shorter distances. This is more economical than transporting many different products over long distances. For the consumer countries it can also be beneficial that a larger part of the added value is generated by them - although the profits are not necessarily taxed there as well.
Refinery locations near large chemical plants are cheap because a large number of intermediate products that this industry needs can then be supplied directly. (Refineries and certain chemical companies are so closely interwoven that the Petrochemicals speaks.) The use of waste heat can also be made easier if other industrial companies are in the immediate vicinity.
Many oil refineries are operated by large oil corporations (often several jointly), others by companies owned by oil-producing countries or by independent companies.
Technical process steps in refineries
The crude oil must first be freed of salts that could otherwise attack various parts of the plant and would also be undesirable in the end products. Desalination takes place by making an emulsion from crude oil and water, whereby the salts dissolve in the water. (The solubility of salts is much higher in water than in oil.) Finally, the emulsion is separated again by the application of an electric field. The salty wastewater, which is also contaminated with certain amounts of petroleum components, is sent to a special sewage treatment plant.
The distillation of the crude oil is the central process step and is called Primary processing designated. This is about breaking the crude into several Factions to be separated: light fractions, which mainly consist of smaller molecules and have low evaporation temperatures, and heavier fractions with correspondingly higher evaporation temperatures.
The desalinated crude oil is first heated to z. B. 400 ° C and then introduced into the lower part of a dozen of meters high rectification column. There can also be a Sump evaporator located, d. H. a heat exchanger that z. B. is charged with high pressure steam and drives the evaporation of the crude oil. The column contains several compartments which are separated from one another by horizontal walls. Overflow openings allow lighter fractions to flow into the higher compartments, and a drain is used to discharge liquid products from each compartment. The temperature decreases steadily towards the top. One obtains z. B. the following political groups:
- The lightest parts of the crude oil reach the topmost compartment and remain there in gaseous form. These Head fraction is used for the production of liquid gas.
- The liquid drain from the top compartment (light petrol) is used to produce petrol.
- Below that you get kerosene, which already has higher boiling temperatures.
- The next fraction is used to produce light heating oil and diesel fuel.
- Then come heavy oils, the z. B. can be used as marine diesel.
- A residue remains in the lowest part, which does not evaporate despite the high temperature.
The residue is usually a second distillation column for the Vacuum distillation fed, which is operated with greatly reduced pressure (although not really a vacuum). This also allows heavier constituents to evaporate. Stronger heating would not be an alternative to this, as a number of substances would then decompose.
The high distillation columns and the complex tube systems (see Figure 1) contribute significantly to the external appearance of refineries. Gas flares, where undesired by-products are flared, are also often conspicuous. However, flaring is now being reduced as much as possible and almost all of the crude oil is used in one way or another.
Almost all refined products undergo further processing, which can have various aspects described below:Various chemical reactions take place in various refinement processes in order to better adapt the products obtained to requirements.
- Various unwanted substances can be included Hydrotreating eliminated, i.e. the reaction with the addition of hydrogen gas. For example, sulfur components can be used as gaseous hydrogen sulfide (H.2S) are separated and amines (nitrogen compounds) become ammonia (NH3) implemented. Alcohols are also broken down.
- A catalytic reforming takes place at an elevated temperature of e.g. B. 500 ° C and increased pressure. Here linear alkanes are converted into cycloalkanes and aromatics (with ring-shaped molecules), and in some cases isomerization (conversion into branched molecules) also takes place. Both of these increase the knock resistance, which is desirable for gasoline. At lower temperatures, only the isomerization takes place mainly. Hydrotreating (see above) can be used when the hydrogen produced during reforming is carried out.
- At elevated temperatures - with or without a catalyst, and possibly with the addition of steam or hydrogen - this can happen Cracking (Splitting) take place, d. H. the splitting of larger molecules into smaller ones. This is desirable, for example, for the heavy residues from distillation; smaller molecules are suitable for fuels because they do not only allow a liquid consistency at high temperatures. Part of the residue can also become petroleum coke, which is e.g. B. is used in steel production and can also be processed into electrodes, as required in aluminum electrolysis. Because of the production of coke, plants are often called Koker designated.
- Conversely, in the context of the Polymerization Lighter molecules are combined with the help of catalysts to form larger ones (especially isoparaffins) in order to avoid, for example, an excessively large proportion of liquefied gas. Something similar happens with the Alkylation, a reaction between olefins and paraffins. For this purpose, the starting materials are mixed with sulfuric acid or hydrofluoric acid, and the mixture is then separated again.
Such processes are selected and combined with one another depending on the nature of the crude oil and depending on the need for different end products. A suitable mixing of components is also part of the refinement. Many products such as gasoline are also made with special additives (Additives) in order to control certain properties in a targeted manner. A variable composition of the individual components can be balanced out via variable mixing ratios.
The energy consumption for processing crude oil in the refinery depends to a large extent on the extent to which processes such as cracking, reforming and alkylation are necessary in order to produce the end products in the desired proportions from the crude oil used. It is advantageous if the naturally occurring proportions (without expensive additional processes) correspond approximately to the requirements, and if the crude oil is of high quality.
Despite the use of cracking, reforming, alkylation and the like, it is often difficult to obtain all of the products in the desired quantities; For example, at locations remote from the coast, a surplus of heavy fuel oil can arise, which can then only be converted into electricity. The problem in Europe is that, due to the high proportion of diesel in the vehicle fleet, diesel fuel tends to be scarce, while there is a surplus of petrol. This development, also called Dieselization called, was tightened by the reduced taxation of diesel fuel and by the addition of bioethanol. As a result, gasoline has to be shipped from Europe to the USA, while middle distillates are shipped to Europe as raw materials for diesel fuel and heating oil. By adapting the processes in the refineries, the diesel share can only be increased to a limited extent. As the US import demand is likely to decrease further, the problem is likely to increase. As a result, the benefit of the higher energy efficiency of diesel drives is effectively diminished, and of course higher costs arise.
Influence of oil quality
Refineries can process crude oil of different qualities, but this has an impact on the proportions of the products received and the effort required for certain process steps. For example, it is helpful to use low-sulfur grades, which however tend to be more expensive. When using lighter types of crude oil, a higher proportion of gasoline can be achieved, which is desirable in the USA, for example, due to the relatively high demand for gasoline compared to diesel fuel.
In earlier times, oil refineries caused massive environmental pollution through improperly disposed of residues, inadequately treated wastewater and harmful exhaust gases. However, the technology has been greatly optimized over many decades, not least due to the influence of increasingly stricter environmental legislation. Most of the oil refineries have become much more environmentally friendly. Nevertheless, various burdens remain:
- Refineries have significant CO2Emissions. However, these only make up a few percent of the CO2-Emissions that arise later when the products are burned.
- The exhaust gases still contain certain amounts of pollutants such as sulfur dioxide and nitrogen oxides, despite complex exhaust gas cleaning systems.
- During storage and decanting, hydrocarbons can escape into the air. (This can be greatly reduced with gas shuttle processes or the supply of such gases to incineration plants.) Unpleasant odors can also occur.
- Even after the usual cleaning, the wastewater still contains certain residual amounts of petroleum constituents and extraction agents. In the event of accidents, wastewater pollution can be considerably higher.
- The soil of the site can be contaminated by substances leaking in the event of accidents. The groundwater can also be contaminated. Please note that very large amounts of substances hazardous to water are converted in refineries.
- Refineries have significant water consumption, both for cooling water and for process water. But that depends very much on the system; In old systems, the water consumption can be a dozen times higher than the amount of crude oil converted, while in newer systems it may only be roughly as high as the oil throughput.
- Wastewater and cooling towers can give off significant amounts of waste heat, which can have local effects.
- Depending on the means of transport for the crude oil and the products produced, the environment can be polluted with the effects of the transport.
The environmental pollution from petroleum refineries can be mathematically assigned to the products produced, even if this assignment is not necessarily unambiguous due to the complex process. So z. B. gasoline and diesel fuels are provided with additional environmental pollution points.
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See also: petroleum, hydrocarbons, gasoline, diesel fuel, heating oil, liquefied petroleum gas
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