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Mwaku W.M., Kornilov V.Y.
THEORETIC - EXPERIMENTAL FUNDAMENTALS OF THE CONSTRUCTION OF AN AUTOMATIC OIL PREPARATION AND TRANSPORT CONTROL SYSTEM
Очистка сырой нефти произошла благодаря успешному бурению первой нефтяной скважины в Титузвилле (штат Пенсильвания, США) в 1859. До того времени нефть была доступна только в очень небольших количествах от естественной утечки подповерхностной нефти в различных областях во всем мире. Однако такая малая доступность ограничила использование нефти в медицинских и других целях. С открытием «горной нефти» в северо-западной Пенсильвании сырая нефть стала доступной в достаточном количестве, чтобы вдохновить развитие обрабатывающих систем более широкого масштаба. Самые ранние очистительные заводы использовали простые единицы дистилляции или «кадры», чтобы отделить различные элементы нефти, нагревая смесь сырой нефти в судне и уплотняя проистекающие пары в жидкие фракции. Процесс первичной подготовки нефти перед подачей ее в магистральные трубопроводы обеспечивает получение товарной нефти с требуемыми качественными характеристиками. В ходе технологического процесса происходит удаление из эмульсии, поступающей со скважин, воды, песка и попутного газа. Ключевые слова: сырая нефть, подготовка и транспортировка нефти, система автоматического управления, завод углеводородной химии, технологический процесс, традиционная система измерений.
By 1970 the petroleum-refining industry had become well established throughout the world. Demand for refined petroleum products had reached almost 2.3 billion tons per year (40 million barrels per day), with major concentrations of refineries in most developed countries. As the world became aware of the impact of industrial pollution on the environment, however, the petroleum-refining industry was a primary focus for change. Refiners added hydro-treating units to extract sulphur compounds from their products and began to generate large quantities of elemental sulphur. Effluent water and atmospheric emission of hydrocarbons and combustion products also became a focus of increased technical attention. In addition, many refined products came under scrutiny. By the mid-1970s petroleum refiners in the United States were required to develop techniques for manufacturing high-quality gasoline without employing lead additives, and by 1990 they were required to take on substantial investments in the complete reformulation of transportation fuels in order to minimize environmental emissions. From an industry that produced a single product (kerosene) and disposed of unwanted by-product materials in any manner possible, petroleum refining had become one of the most stringently
regulated of all manufacturing industries, expending a major portion of its resources on the protection of the environment [3].
Saturated molecules
The simplest of the hydrocarbon molecules is methane (CH4), which has one carbon atom and four hydrogen atoms per molecule. The next simplest, ethane (C2H6), has two carbon atoms and six hydrogen atoms. A whole class of hydrocarbons can be defined by expanding upon the relationship between methane and ethane. Known as the paraffins, this is a family of chainlike molecules with the chemical formula CnH2n + 2. These molecules are also referred to as saturated, since each of the four valence electrons on a carbon atom that are available for bonding is taken up by a single hydrogen or carbon atom. Because these “single” bonds leave no valence electron available for sharing with another atom, paraffin molecules tend to be chemically stable.
Paraffins can be arranged either in straight chains (normal paraffins, such as butane; see figure) or branched chains (isoparaffins). Most of the paraffin compounds in naturally occurring crude oils are normal paraffins, while isoparaffins are frequently produced in refinery processes. The normal paraffins are uniquely poor as motor fuels, while isoparaffins have good engine-combustion characteristics. Longer-chain paraffins are major constituents of waxes.
Once a hydrocarbon molecule contains more than four carbon atoms, the carbon atoms may form not a branched or straight chain but a closed-ring structure known as a cyclo-compound. Saturated cyclo-compounds are called naphthenes. Naphthenic crudes tend to be poor raw materials for lubricant manufacture, but they are more easily converted into high-quality gasolines than are the paraffin compounds. The figure (fig. 1) below shows the hydrocarbon family [3].
H —c —c —c —c —H
H H H H
H H H H
butane
(paraffin)
H H cyclopentane (naphthene)
H
H
H
H—c—c=c
H H H
propylene
(olefin)
I benzene H (aromatic)
Fig. 1. Hydrocarbon family
1. Unsaturated molecules
Two other chemical families that are important in petroleum refining are composed of unsaturated molecules. In unsaturated molecules, not all the valence electrons on a carbon atom are bonded to separate carbon or hydrogen atoms; instead, two or three electrons may be taken up by one neighbouring carbon atom, thus forming a “double” or “triple” carbon-carbon bond. Like saturated compounds, unsaturated compounds can form either chain or ring molecules. Unsaturated chain molecules are known as olefins. Only small amounts of olefins are found in crude oils, but large volumes are produced in refining processes. Olefins are relatively reactive as chemicals and can be readily combined to form other longer-chain compounds.
The other family of unsaturated compounds is made up of ring molecules called aromatics. The simplest aromatic compound, benzene (C6H6), has double bonds linking every other carbon molecule (see figure). The double bonds in the benzene ring are very unstable and chemically reactive. Partly for this reason, benzene is a popular building block in the petrochemical industry.
Unsaturated hydrocarbons generally have good combustion characteristics, but their reactivity can lead to instability in storage and sometimes to environmental emission problems [3].
2. Types of crude oil
The above description of hydrocarbons refers to simpler members of each family, but crude oils are actually complex mixtures of very long-chain
compounds, some of which have not yet been identified. Because such complex mixtures cannot be readily identified by chemical composition, refiners customarily characterize crude oils by the type of hydrocarbon compound that is most prevalent in them: paraffins, naphthenes, and aromatics. Some crude oils, such as those in the original Pennsylvanian oil fields, consist mainly of paraffins. Others, such as the heavy Mexican and Venezuelan crudes, are predominantly naphthenic and are rich in bitumen (a high-boiling semisolid material frequently made into asphalt for road surfaces) [1].
The proportions of products that may be obtained by distillation of five typical crude oils, ranging from heavy Venezuelan Boscan to the light Bass Strait oil produced in Australia, are shown in the figure 2 below. Given the pattern of modern demand (which tends to be highest for transportation fuels such as gasoline), the market price of a crude oil generally rises with increasing yields of light products. It is possible to process heavier crudes more intensely in order to improve their yield of light products, but the capital and operating costs required to support such high conversion processes are much greater than those required to process lighter crudes into the same yield of products.
In addition to the hydrocarbons, compounds of sulfur, nitrogen, and oxygen are present in small amounts in crude oils. Also there are usually traces of vanadium, nickel, chlorine, sodium, and arsenic. These elements may affect the safety of oil-transport systems, the quality of refined products, and the effectiveness of processing units within a refinery. Minute traces can usually be tolerated, but crudes with larger amounts of these materials must be extensively treated in order to restrict their harmful effects [2].
3. Basic refinery processes
Each refinery is uniquely designed to process specific crude oils into selected products. In order to meet the business objectives of the refinery, the process designer selects from an array of basic processing units. In general, these units perform one of three functions: (1) separating the many types of hydrocarbon present in crude oils into fractions of more closely related properties, (2) chemically converting the separated hydrocarbons into more desirable reaction products, and (3) purifying the products of unwanted elements and compounds [3].
3.1. Separation
3.1.1. Fractional distillation
The primary process for separating the hydrocarbon components of crude oil is fractional distillation. Crude oil distillers separate crude oil into fractions
for subsequent processing in such units as catalytic reformers, cracking units, alkylation units, or cokers. In turn, each of these more complex processing units also incorporates a fractional distillation tower to separate its own reaction products.
120
100
80
60
40
20
0
naphtha
kerosene
middle distillates i fuel oils
Boscan Maya Arabian light W.Texas Bass Strait
Intermediate
Fig. 2. Crude oil product proportion by distillation
Modern crude oil distillation units operate continuously over long periods of time and are much larger than the fractional distillation units employed in chemical or other industries. Process rates are commonly delineated in American barrels; units capable of processing 100,000 barrels per day are commonplace, and the largest units are capable of charging more than 200,000 barrels per day [1].
The principles of operation of a modern crude oil distillation unit are shown in the fig. 3. Crude oil is withdrawn from storage tanks at ambient temperature and pumped at a constant rate through a series of heat exchangers in order to reach a temperature of about 120°C (250°F). A controlled amount of fresh water is introduced, and the mixture is pumped into a desalting drum, where it passes through an electrical field and a saltwater phase is separated. (If the salt were not removed at this stage, it would be deposited later on the tubes of the furnace and cause plugging). The desalted crude oil passes through additional heat exchangers and then through steel alloy tubes in a furnace. There it is heated to a temperature between 315° and 400°C (600° and 750°F), depending on the type of crude oil and the end products desired. A mixture of vapour and unvaporized oil passes from the furnace into the fractionating column, a vertical cylindrical tower as much as 45 metres (150 feet) high containing 20 to
40 fractionating trays spaced at regular intervals.
The most common fractionating trays are of the sieve or valve type. Sieve trays are simple perforated plates with small holes about 5 to 6 millimetres (0.2 to 0.25 inch) in diameter. Valve trays are similar, except the perforations are covered by small metal disks that restrict the flow through the perforations under certain process conditions.
The oil vapours rise up through the column and are condensed to a liquid in a water- or air-cooled condenser at the top of the tower. A small amount of gas remains uncondensed and is piped into the refinery fuel-gas system. A pressure control valve on the fuel-gas line maintains fractionating column pressure at the desired figure, usually near atmospheric pressure (about 1 kilogram per square centimetre, or 15 pounds per square inch). Part of the condensed liquid, called reflux, is pumped back into the top of the column and descends from tray to tray, contacting rising vapours as they pass through the slots in the trays. The liquid progressively absorbs heavier constituents from the vapour and, in turn, gives up lighter constituents to the vapour phase. Condensation and reevaporation takes place on each tray. Eventually equilibrium is reached in which there is a continual gradation of temperature and oil properties throughout the column, with the lightest constituents on the top tray and the heaviest on the bottom. The use of reflux and vapour-liquid contacting trays distinguishes fractional distillation from simple distillation columns [3].
As shown in the figure 3, intermediate products, or «sidestreams», are withdrawn at several points from the column. In addition, modern crude distillation units employ intermediate reflux streams. Sidestreams are known as intermediate products because they have properties between those of the top or overhead product and those of products issuing from the base of the column. Typical boiling ranges for various streams are as follows: light straight-run naphtha (overhead), 20°-95°C (70°-200°F); heavy naphtha (top sidestream), 90°-165°C (195°-330°F); crude kerosene (second sidestream), 150°-245°C (300°-475°F); light gas oil (third sidestream), 215°-315°C (420°-600°F).
Unvaporized oil entering the column flows downward over a similar set of trays in the lower part of the column, called stripping trays, which act to remove any light constituents remaining in the liquid. Steam is injected into the bottom of the column in order to reduce the partial pressure of the hydrocarbons and assist in the separation. Typically a single sidestream is withdrawn from the stripping section: heavy gas oil, with a boiling range of 285°-370°C (545°-700°F). The residue that passes from the bottom of the column is suit-
able for blending into industrial fuels. Alternately, it may be further distilled under vacuum conditions to yield quantities of distilled oils for manufacture into lubricating oils or for use as a feedstock in a gas oil cracking process [1].
Fig. 3. Principles of operation of a modem crude oil distillation unit
3.1.2. Vacuum distillation
The principles of vacuum distillation resemble those of fractional distillation (commonly called atmospheric distillation to distinguish it from the vacuum method), except that larger-diameter columns are used to maintain comparable vapour velocities at reduced operating pressures. A vacuum of 50 to 100 millimetres of mercury absolute is produced by a vacuum pump or steam ejector [2].
The primary advantage of vacuum distillation is that it allows for distilling heavier materials at lower temperatures than those that would be required at atmospheric pressure, thus avoiding thermal cracking of the components. Firing conditions in the furnace are adjusted so that oil temperatures usually do not exceed 425°C (800°F). The residue remaining after vacuum distillation, called bitumen, may be further blended to produce road asphalt or residual fuel oil, or it may be used as a feedstock for thermal cracking or coking units. Vac-
uum distillation units are essential parts of the many processing schemes designed to produce lubricants.
3.1.3. Absorption
Absorption processes are employed to recover valuable light components such as propane/propylene and butane/butylene from the vapours that leave the top of crude-oil or process-unit fractionating columns within the refinery. These volatile gases are bubbled through an absorption fluid, such as kerosene or heavy naphtha, in equipment resembling a fractionating column. The light products dissolve in the oil while the dry gases - such as hydrogen, methane, ethane, and ethylene - pass through undissolved. Absorption is more effective under pressures of about 7 to 11 kilograms per square centimetre (100 to 150 pounds per square inch) than it is at atmospheric pressure.
The enriched absorption fluid is heated and passed into a stripping column, where the light product vapours pass upward and are condensed for recovery as liquefied petroleum gas (LPG). The unvaporized absorption fluid passes from the base of the stripping column and is reused in the absorption tower [2].
3.1.4. Solvent extraction
Solvent extraction processes are employed primarily for the removal of constituents that would have an adverse effect on the performance of the product in use. An important application is the removal of heavy aromatic compounds from lubricating oils. Removal improves the viscosity-temperature relationship of the product, extending the temperature range over which satisfactory lubrication is obtained. The usual solvents for extraction of lubricating oil are phenol and furfural.
3.1.5. Adsorption
Certain highly porous solid materials have the ability to select and adsorb specific types of molecules, thus separating them from other materials. Silica gel is used in this way to separate aromatics from other hydrocarbons, and activated charcoal is used to remove liquid components from gases. Adsorption is thus somewhat analogous to the process of absorption with oil, although the principles are different. Layers of adsorbed material only a few molecules thick are formed on the extensive interior surface of the adsorbent; the interior surface may amount to several hectares per kilogram of material [3].
Molecular sieves are a special form of adsorbent. Such sieves are produced by the dehydration of naturally occurring or synthetic zeolites (crystalline alkali-metal aluminosilicates). The dehydration leaves intercrystalline
cavities that have pore openings of definite size, depending on the alkali metal of the zeolite. Under adsorptive conditions, normal paraffin molecules can enter the crystalline lattice and be selectively retained, whereas all other molecules are excluded. This principle is used commercially for the removal of normal paraffins from gasoline fuels, thus improving their combustion properties. The use of molecular sieves is also extensive in the manufacture of high-purity solvents.
3.1.6. Crystallization
The crystallization of wax from lubricating oil fractions is essential to make oils suitable for use. A solvent (often a mixture of benzene and methyl ethyl ketone) is first added to the oil, and the solution is chilled to about -20°C ( -5°F). The function of the benzene is to keep the oil in solution and maintain its fluidity at low temperatures, whereas the methyl ethyl ketone acts as a wax precipitant. Rotary filters deposit the wax crystals on a specially woven cloth stretched over a perforated cylindrical drum. A vacuum is maintained within the drum to draw the oil through the perforations. The wax crystals are removed from the cloth by metal scrapers, after washing with solvent to remove traces of oil. The solvents are later distilled from the oil and reused.
3.2. Conversion
The separation processes described above are based on differences in physical properties of the components of crude oil. All petroleum refineries throughout the world employ at least crude oil distillation units to separate naturally occurring fractions for further use, but those which employ distillation alone are limited in their yield of valuable transportation fuels. By adding more complex conversion processes that chemically change the molecular structure of naturally occurring components of crude oil, it is possible to increase the yield of valuable hydrocarbon compounds.
3.2.1. Naphtha reforming
The most widespread process for rearranging hydrocarbon molecules is naphtha reforming. The initial process, thermal reforming, was developed in the late 1920s. Thermal reforming employed temperatures of 510°-565°C (950°-1,050°F) at moderate pressures (about 43 kilograms per square centimetre, or 600 pounds per square inch) to obtain gasolines with octane numbers of 70 to 80 from heavy naphthas with octane numbers of less than 40. The product yield, although of a higher octane level, included olefins, diolefins, and aromatic compounds. It was therefore inherently unstable in storage and tended to form heavy polymers and gums, which caused combustion problems.
By 1950 a reforming process was introduced that employed a catalyst to improve the yield of the most desirable gasoline components while minimizing the formation of unwanted heavy products and coke. (A catalyst is a substance that promotes a chemical reaction but does not take part in it.) In catalytic reforming, as in thermal reforming, a naphtha-type material serves as the feedstock, but the reactions are carried out in the presence of hydrogen, which inhibits the formation of unstable unsaturated compounds that polymerize into higher-boiling materials [1].
In most catalytic reforming processes, platinum is the active catalyst; it is distributed on the surface of an aluminum oxide carrier. Small amounts of rhenium, chlorine, and fluorine act as catalyst promoters. In spite of the high cost of platinum, the process is economical because of the long life of the catalyst and the high quality and yield of the products obtained. The principal reactions involve the breaking down of long-chain hydrocarbons into smaller saturated chains and the formation of isoparaffins, made up of branched-chain molecules. Formation of ring compounds (technically, the cyclization of paraffins into naphthenes) also takes place, and the naphthenes are then dehydrogenated into aromatic compounds (ring-shaped unsaturated compounds with fewer hydrogen atoms bonded to the carbon). The hydrogen liberated in this process forms a valuable by-product of catalytic reforming. The desirable end products are isoparaffins and aromatics, both having high octane numbers.
In a typical reforming unit the naphtha charge is first passed over a catalyst bed in the presence of hydrogen to remove any sulfur impurities. The desulfurized feed is then mixed with hydrogen (about five molecules of hydrogen to one of hydrocarbon) and heated to a temperature of 500°-540°C (930°-1,000°F). The gaseous mixture passes downward through catalyst pellets in a series of three or more reactor vessels. Early reactors were designed to operate at about 25 kilograms per square centimetre (350 pounds per square inch), but current units frequently operate at less than 7 kilograms per square centimetre (100 pounds per square inch). Because heat is absorbed in reforming reactions, the mixture must be reheated in intermediate furnaces between the reactors.
After leaving the final reactor, the product is condensed to a liquid, separated from the hydrogen stream, and passed to a fractionating column, where the light hydrocarbons produced in the reactors are removed by distillation. The reformate product is then available for blending into gasoline without further treatment. The hydrogen leaving the product separator is compressed and returned to the reactor system.
Operating conditions are set to obtain the required octane level, usually between 90 and 100. At the higher octane levels, product yields are smaller, and more frequent catalyst regenerations are required. During the course of the reforming process, minute amounts of carbon are deposited on the catalyst, causing a gradual deterioration of the product yield pattern. Some units are semi regenerative facilities—that is, they must be removed from service periodically (once or twice annually) to burn off the carbon and rejuvenate the catalyst system—but increased demand for high-octane fuels has also led to the development of continuous regeneration systems, which avoid the periodic unit shutdowns and maximize the yield of high-octane reformate. Continuous regeneration employs a moving bed of catalyst particles that is gradually withdrawn from the reactor system and passed through a regenerator vessel, where the carbon is removed and the catalyst rejuvenated for reintroduction to the reactor system [3].
3.2.2. Catalytic cracking
The use of thermal cracking units to convert gas oils into naphtha dates from before 1920. These units produced small quantities of unstable naphthas and large amounts of by- product coke. While they succeeded in providing a small increase in gasoline yields, it was the commercialization of the fluid catalytic cracking process in 1942 that really established the foundation of modern petroleum refining. The process not only provided a highly efficient means of converting high-boiling gas oils into naphtha to meet the rising demand for high-octane gasoline, but it also represented a breakthrough in catalyst technology.
The thermal cracking process functioned largely in accordance with the free-radical theory of molecular transformation. Under conditions of extreme heat, the electron bond between carbon atoms in a hydrocarbon molecule can be broken, thus generating a hydrocarbon group with an unpaired electron. This negatively charged molecule, called a free radical, enters into reactions with other hydrocarbons, continually producing other free radicals via the transfer of negatively charged hydride ions (H-). Thus a chain reaction is established that leads to a reduction in molecular size, or “cracking,” of components of the original feedstock [2].
Table 1. Principal Reactions in Fluid Catalytic Cracking
Use of a catalyst in the cracking reaction increases the yield of high-quality products under much less severe operating conditions than in thermal cracking. Several complex reactions are involved, but the principal mechanism by which long-chain hydrocarbons are cracked into lighter products can be explained by the carbonium ion theory. According to this theory, a catalyst promotes the removal of a negatively charged hydride ion from a paraffin com-
pound or the addition of a positively charged proton (H+) to an olefin compound. This results in the formation of a carbonium ion, a positively charged molecule that has only a very short life as an intermediate compound which transfers the positive charge through the hydrocarbon. Carbonium transfer continues as hydrocarbon compounds come into contact with active sites on the surface of the catalyst that promote the continued addition of protons or removal of hydride ions. The result is a weakening of carbon-carbon bonds in many of the hydrocarbon molecules and a consequent cracking into smaller compounds.
Olefins crack more readily than paraffins, since their double carboncarbon bonds are more friable under reaction conditions. Isoparaffins and naphthenes crack more readily than normal paraffins, which in turn crack faster than aromatics. In fact, aromatic ring compounds are very resistant to cracking, since they readily deactivate fluid cracking catalysts by blocking the active sites of the catalyst. The Table 1 illustrates many of the principal reactions that are believed to occur in fluid catalytic cracking unit reactors. The reactions postulated for olefin compounds apply principally to intermediate products within the reactor system, since the olefin content of catalytic cracking feedstock is usually very low [1].
3.3. Purification
Before petroleum products can be marketed, certain impurities must be removed or made less obnoxious. The most common impurities are sulfur compounds such as hydrogen sulfide (H2S) or the mercaptans (“R”SH)—the latter being a series of complex organic compounds having as many as six carbon atoms in the hydrocarbon radical (“R”). Apart from their foul odour, sulfur compounds are technically undesirable. In motor and aviation fuels they reduce the effectiveness of antiknock additives and interfere with the operation of ex-haust-treatment systems. In diesel fuel they cause engine corrosion and complicate exhaust-treatment systems. Also, many major residual and industrial fuel consumers are located in developed areas and are subject to restrictions on sulfurous emissions.
Most crude oils contain small amounts of hydrogen sulfide, but these levels may be increased by the decomposition of heavier sulfur compounds (such as the mercaptans) during refinery processing. The bulk of the hydrogen sulfide is contained in process-unit overhead gases, which are ultimately consumed in the refinery fuel system. In order to minimize noxious emissions, most refinery fuel gases are desulfurized.
Other undesirable components include nitrogen compounds, which poison catalyst systems, and oxygenated compounds, which can lead to colour formation and product instability [2].
4. Bulk transportation
Large oceangoing tankers have sharply reduced the cost of transporting crude oil, making it practical to locate refineries near major market areas rather than adjacent to oil fields. To receive these large carriers, deepwater ports have been constructed in such cities as Rotterdam (Neth.), Singapore, and Houston (Tex.). Major refining centres are connected to these ports by pipelines.
Countries having navigable rivers or canals afford many opportunities for using barges, a very inexpensive method of transportation. The Mississippi River in the United States and the Rhine and Seine rivers in Europe are especially suited to barges of more than 5,000 tons (37,000 barrels). Each barge may be divided into several compartments so that a variety of products may be carried.
Transport by railcar is still widely practiced, especially for specialty products such as LPG, lubricants, or asphalt. Cars have capacities exceeding 100 tons (800 barrels), depending on the product carried. The final stage of product delivery to the majority of customers throughout the world continues to be the familiar tanker truck, whose carrying capacity is about 150 to 200 barrels.
The most efficient mode of bulk transport for petroleum is the network of pipelines that are now found all over the world. Most crude-oil-producing areas are connected by pipeline either to refining centres or to a maritime loading port. In addition, many major crude-oil-receiving ports have extensive pipeline distribution networks to inland refineries. Centrifugal pumps usually provide the pumping power, with booster stations installed along the line as necessary. Most of the major product lines have been converted to fully automated operation, with the opening and closing of valves carried out by automatic sequence controls initiated from remote control centres [2].
5. Conclusion
The oil and gas industry has the resources to meet the global future demands. This is evident from the growing application of automatic control systems in the chemistry of crude oil preparation and transportation. I believe that automation will always play a fundamental role in this energy sector and it’s our job as control system engineers to enhance this development.
References
1. URL: http://www.britannica.com/EBchecked/topic/454440/petroleum-refining.
2. Venkat Venkatasubramanian, Raghunathan Rengaswamy, Surya N. Kavuri, Kewen Yin. A review of process fault detection and diagnosis Part III: Quantitative model-based methods. Computers and Chemical Engineering, 2003. V. 27. № 3. P. 327-346.
3. Equipment and automatic control systems for automation of objects for oil extraction, transport and preparation / Catalogue. Ufa: MOAO «Neftavtomatika», 2006. P. 120-125.
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