Petroleum provides the largest fraction of primary energy supply in the U.S. and in the world [Figure 1.1,eia1]. Resource consumption patterns shown in Figure 1.1 reflect major epochs in human history, such as The Industrial Revolution, ushering in the rapid increase in coal consumption. Petroleum trace, for example, marks the mass production of automobiles with the introduction of Model T by Ford, world wars, supply crises of 1973 and 1979 and the economic recession in 2008. Transportation of people and goods in many parts of the world depends almost completely on petroleum fuels, such as gasoline, jet fuel, diesel fuel, and marine fuel. Apart from the fuels, materials that are necessary for operating the combustion engines of cars, trucks, planes, and trains also come from petroleum. These materials include lubricating oils (motor oils), greases, tires on the wheels of the vehicles, and asphalt to pave the roads for smooth rides in transportation vehicles. All petroleum fuels and many materials are produced by the processing of crude oil in petroleum refineries. Petroleum refineries also supply feedstock to the petrochemicals and chemical industry for producing all consumer goods from rubber and plastics (polymers) to cosmetics and medicine. Only ten percent of petroleum consumption, the portion that is not used for transportation or other energy outlets, is sufficient to manufacture all the materials used in human economy, with the exception of those derived from wood or minerals.
The petroleum industry consists of two separate operations: Upstream and Downstream Operations. Upstream operations involve exploration of new oil reserves, development of oil fields, constructing the well-head and crude oil production facilities. Downstream operations cover processing of crude oil in petroleum refineries to produce liquid and gaseous fuels and materials for the market. This course addresses petroleum refining to review how a variety of physical processes and chemical reactions in separate refinery units are integrated to process compliant fuels and materials.
By the end of this lesson, you will be able to:
This lesson will take us one week to complete. Please refer to the Course Syllabus for specific time frames and due dates. Specific directions for the assignments below can be found on the Assignment page within this lesson.
Readings | J. H. Gary, G. E. Handwerk, Mark J. Kaiser, Chapter 1, pp. 1-12; Chapter 3, pp. 62-65 |
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Assignments | For your information, review the most recent supply of petroleum fuels from the data given at U.S. Energy Information Administration [3] (eia.gov) and research how petroleum refining addresses the environmental concerns from the combustion of petroleum fuels in internal combustion engines. |
If you have any questions, please post them to our Help Discussion (not email), located in Canvas. I will check that discussion forum daily to respond. While you are there, feel free to post your own responses if you, too, are able to help out a classmate.
Markets and demand for refinery products depend on the dynamics of a global economy. It is generally agreed that oil and gas will continue to be the primary energy resource in the U.S. and world economies for decades to come. Because of the projected increase in the production of oil in tight formations, the United States is expected to become an exporter of petroleum products and crude oil after decades of being an importer (Figure 1.2, EIA Annual 2013, eia.gov). Petroleum fuels will continue to dominate the transportation sector, but the following trends should be noted:
Competitive forces in the global economy lead to joint ventures and mergers and shutting down of inefficient refineries, or shutting down of processing units with low efficiency within refineries. Figure 1.3 shows the changes in the refinery capacity and number of refineries in the U.S. since 2000. The increasing refining capacity, with the decreasing number of refineries, results in the closing down of small inefficient refineries while expanding the large refineries.
Regarding the global competition, the technological advancement addresses the degrading quality of crude oils to produce cleaner and higher quality petroleum fuels. On the supply side, there is the increasing abundance of natural gas liquids (ethane, propane, n-butane, and isobutane) due to increased shale gas production in the U.S. and elsewhere. These liquids enter refineries as new feedstock in addition to crude oil supply.
Refineries need process improvements to advance their capabilities to deal with the changing crude oil base and changing environmental regulations. These improvements in refinery processes would need to create and use, for example:
Concerns for efficiency include running a refinery efficiently and producing fuels that will burn efficiently in the combustion engines, as follows:
Considering the market drivers just reviewed along the small profit margins that are often usually associated with petroleum refinery products, refineries should carefully select the crude oil feedstock and configure the refinery processes such that they produce the desirable petroleum products at the lowest cost.
In the U.S. refineries, a principal focus is on the production of gasoline because of high demand. Diesel fuel is the principal refinery product in most other parts of the world. Figure 1.4 shows a typical distribution of products from a barrel of crude oil in a U.S. refinery. Distillation process separates the crude oil into boiling point fractions. The liquefied petroleum gas (LPG) constitutes the lowest boiling point (most volatile) product from a refinery and higher boiling fractions lead to most desirable distillate liquids, such as gasoline, jet fuel, diesel fuel, and fuel oil in the increasing order of boiling points, while asphalt is made from the residual fraction remaining after distillation.
The following animation shows a refinery flow chart indicating some of the major refinery processes and refinery products. Note that the distillation process (Fractionation Tower) separates crude oil into a number of distillate fractions that are sent as feedstocks to different processes, some of which are interconnected. It is also important to recognize that petroleum refining not only produces transportation fuels and fuels for space heating or industrial furnaces, but also produces materials needed for the operation of the combustion engines and paving the roads for vehicles to travel on.
Figure 1.5 indicates that chemical constitution and physical properties of crude oils are important parameters that guide the refinery configurations. The refining processes can be divided into four groups, as indicated. While the separation processes involve just physical phenomena, the conversion, finishing, and support processes require chemical changes, i.e., breaking chemical bonds to modify the molecular structure of the feedstocks. These changes are necessary to produce the fuels and materials in accordance with industrial/commercial specifications.
Figure 1.6 (progressive image, 25 seconds) shows a more detailed refinery block diagram to show how different processes are integrated for producing the desired fuels and materials.
Separation processes, such as distillation, dewaxing, and deasphalting make use of the differences in the physical properties of crude oil components to separate groups of hydrocarbon compounds or inorganic impurities, whereas conversion processes cause chemical changes in the hydrocarbon composition of crude oils. For example, Fluid Catalytic Cracking process breaks chemical bonds in long-chain alkanes to produce shorter chain alkanes to produce gasoline from higher boiling gas oil fractions. Finishing processes involve hydrotreating to remove heteroatoms (S, N, and metals) and product blending to produce fuels and materials with desired specifications and in compliance with environmental and government regulations. Finally, supporting processes provide the recovery of the removed heteroatoms or heteroatom compounds, production of the hydrogen necessary for conversion and hydrotreating processes, and effluent water treatment systems.
Why is diesel fuel preferred over gasoline in many countries in the world?
ANSWER: Diesel fuel powers the engines in buses, locomotives, and ships used for public transport. Passenger vehicles fueled by gasoline are the most widely used means of transportation in the U.S.
Crude oil contains organic compounds, heteroatom compounds (S,N,O), hydrocarbons (C, H), metals and organic (Ni, V, Fe) and inorganic (Na+, Ca++, Cl-) compounds as listed in Figure 1.7. Compounds that contain only elements of carbon and hydrogen are called hydrocarbons and constitute the largest group of organic compounds found in petroleum. There might be as many as several thousand different hydrocarbon compounds in crude oil. Hydrocarbon compounds have a general formula of CxHy, where x and y are integer numbers.
Hydrocarbons are generally divided into four groups: (1) paraffins, (2) olefins, (3) naphthenes, and (4) aromatics (Figure 1.8). Among these groups, paraffins, olefins, and naphthenes are sometimes called aliphatic compounds, as different from aromatic compounds. The lightest hydrocarbon found as a dissolved gas is methane (CH4), the main component of natural gas. Olefins are not usually found in crude oils, but produced in a number of refining processes.
Aromatic hydrocarbons are an important series of hydrocarbons found in almost every petroleum mixture from any part of the world. Aromatics are cyclic but unsaturated hydrocarbons with alternating double bonds (Figure 1.12). The simplest aromatic hydrocarbon is benzene (C6H6). The name “aromatic” refers to the fact that such hydrocarbons are commonly fragrant compounds. Although benzene has three carbon-carbon double bonds, it has a unique arrangement of electrons with resonance structures of the double bonds (aromaticity) that allow benzene to be relatively stable. However, benzene is known to be a cancer-inducing compound. For this reason, the amount of benzene allowed in petroleum products such as gasoline or fuel oil is limited by government regulations in many countries. Under standard conditions, benzene, toluene, and xylene are in liquid form whereas higher aromatics such as naphthalene occur as solids in isolation, but dissolve to form a liquid solution with simple aromatics.
What constitutes the white crystals of moth balls?
ANSWER: Napthalene! Naphthalene is an effective moth killer because it sublimes (forms a vapor from a solid without going through a liquid state) at room temperature.
Some of the common aromatics found in crude oil and petroleum products are benzene derivatives with attached methyl, ethyl, propyl, or higher alkyl groups. This series of aromatics is called alkylbenzenes, and compounds in this homologous group of hydrocarbons have the general formula of CnH2n-6 (where n ≥ 6). Generally, an aromatic series with only one benzene ring is also called mono- aromatics or mononuclear aromatics. However, heavy petroleum fractions and residues contain unsaturated multirings with many benzene and naphthene rings attached to each other. Such aromatics that exist as solids in isolation are also called polyaromatic hydrocarbons (PAHs) or polynuclear aromatics (PNAs) (Figure 1.13). Heavy crude oils usually contain more aromatics than light crudes. It is common to have compounds with naphthenic and aromatic rings side by side (hydroaromatics, or naphthenoaromatics, Figure 1.13) especially in heavy fractions.
Figure 1.13 shows examples of PAHs, such as anthracene, phenathrene, and pyrene. The configuration of rings in PAHs strongly influences the physical and chemical properties of these compounds. For example, three-ring aromatics anthracene and phenanthrene have significantly different properties. In petroleum, PAHs exist mostly as alkyl substituted ring systems such that the substitutent alkyl groups (e.g., methyl, ethyl) replace (substitute for) the hydrogen atoms on the rings.
Normally, high-molecular-weight polyaromatics contain several heteroatoms such as sulfur, nitrogen, or oxygen, but these compounds are still called aromatic compounds because their electronic configurations maintain the aromatic character.
Sulfur is the most important heteroatom found in crude oil and refinery products petroleum, and it can be found in cyclic (e.g., thiophenes) and noncyclic compounds such as mercaptans (R-S-H) and sulfides (R-S- R′), where R and R′ are alkyl groups. Sulfur in natural gas is usually found in the form of hydrogen sulfide (H2S). Figure 1.14 shows the types of sulfur compounds in crude oils. The amount of sulfur in a crude oil may vary from 0.05 to 6 % by weight. The presence of sulfur in finished petroleum products is not desirable. For example, the presence of sulfur in gasoline can promote corrosion of engine parts and produce sulfur oxides upon combustion, contributing to air pollution.
Normally, the concentration of the other heteroatom compounds (nitrogen, oxygen, and metals) in crude oils is usually lower than that of the sulfur compounds. Figure 1.15 shows the nitrogen compounds that may be found in crude oils.
Generally, in heavier crude oils the proportions of carbon, sulfur, nitrogen, and oxygen compounds are higher at the expense of hydrogen content. Heavier crude oils also contain organometallic compounds of common nickel and vanadium (Figure 1.16). These compounds are highly corrosive and toxic and should be removed in the refinery. Nickel, vanadium, and copper can also severely affect the activities of catalysts and result in lower quality products. Organometallic compounds tend to concentrate in heavy, or residual fractions of crude oils.
What is the principal type of air pollution caused by the emission of sulfur oxides into the atmosphere?
ANSWER: Acid rain, caused by the formation of sulfuric acid through reactions of the sulfur oxides in the atmosphere.
Paraffins are also called alkanes and have the general formula of CnH2n+2, where n is the number of carbon atoms in a given molecule. Paraffins are divided into two groups of normal and isoparaffins. Normal paraffins or normal alkanes are simply written as n-paraffins or n-alkanes, and they are open, straight-chain saturated hydrocarbons. The second group of paraffins is called isoparaffins, which are branched-type hydrocarbons, and they begin with isobutane (also called methylpropane), which has the same closed formula as n-butane (C4H10). Compounds of different structures with the same closed formula are called isomers (Figure 1.9). For example, the open formula for n-butane, n-C4, can be shown as CH3-CH2-CH2-CH3, based on the quadrivalency of the carbon atom, and for simplicity, only the carbon-carbon bonds are drawn and most C-H bonds are omitted, as shown in Figure 1.7 and 1.8 on the previous page. Paraffins are the largest series of hydrocarbons found in petroleum and beginning with the simplest compound, methane.
Under standard conditions of temperature and pressure (STP), the first four members of the alkane series (methane, ethane, propane, and butane) are in gaseous form, and compounds starting from C5H12 (pentane) to n-heptadecane (C17H36) are liquids (constituting large fractions of hydrocarbons found in liquid fuels (e.g., gasoline, jet fuel, and diesel fuel), whereas n-octadecane (C18H38) or heavier compounds exist in isolation as wax-like solids at STP. These heavier paraffins are soluble in lighter paraffins or other hydrocarbons and can be found in diesel fuel and fuel oils. Paraffins from C1 to C40 usually appear in crude oil (heavier alkanes in liquid solution, not as solid particles) and represent up to 20% of crude by volume.
Figure 1.10 shows the statistically possible number of isomers of paraffins that increase exponentially with carbon number, starting with just one isomer for butane, reaching approximately 60,000 for C18 paraffins. Note that the branching in hydrocarbons causes significant changes in physical properties (e.g., boiling point and density, Figure 1.11) and chemical behavior (e.g., octane number, Figure 1.10) of paraffins with the same carbon number. Note in Figure 1.10 that the removal of an H atom from alkanes generates free radicals (reactive species containing unpaired electrons) that are called alkyl species (e.g., methyl formed from methane and ethyl formed from ethane by removing a hydrogen atom) also a radical with an unpaired electron. Also note the nomenclature using alkyl groups to specifically name isoalkanes (e.g., 2,2,4-trimethylpentane to designate a specific iso-octane).
Naphthenes or cycloalkanes are rings or cyclic saturated hydrocarbons with a general formula of CnH2n5H10), cyclohexane (C6H12), and their derivatives such as n-alkylcyclopentanes are normally found in crude oils.
Each week, you will be required to do assignments. The assignment for this week is:
For your information (no submission), review the most recent supply of petroleum fuels from the data given at U.S. Energy Information Administration [4] and research how petroleum refining addresses the environmental concerns from the combustion of petroleum fuels in internal combustion engines.
Take a few minutes to answer the five questions below. Use the arrow to go to the next question. When you are ready, click Check to see the solution.
Petroleum, the most important crude oil, consists of a mixture of hydrocarbon compounds including paraffinic, naphthenic, and aromatic hydrocarbons with small amounts of impurities including sulfur, nitrogen, oxygen, and metals. The first process in petroleum refining operations is the separation of crude oil into its major constituents, using distillation to separate the crude oil constituents into common boiling-point fractions. Other separation processes include deasphalting to remove the heaviest fraction of crude oil, asphalt, and dewaxing to remove long-chain n-paraffins called wax.
To meet the demands for high-octane gasoline, jet fuel, and diesel fuel, heavier components of crude oils are converted to gasolines and other distillate fuels. Among the conversion processes are cracking, coking, and visbreaking that are used to break large petroleum molecules into smaller ones. Polymerization and alkylation processes are used to combine molecules smaller than those in gasoline into larger ones to make more gasoline in the refinery. Isomerization and reforming processes are applied to rearrange and reform the structure of hydrocarbons to produce higher-value gasoline components of a similar molecular size.
Finishing processes in a refinery processes stabilize and upgrade petroleum products by hydrogenation and remove undesirable elements, such as sulfur and nitrogen, by hydrotreating processes. Blending of many product streams, to come up with commercial refinery products with the required specifications, also belong to the category of finishing processes.
You should now be able to:
You have reached the end of Lesson 1! Double-check the to-do list below to make sure you have completed all of the activities listed there before you begin Lesson 2. Please refer to the Course Syllabus for specific time frames and due dates. Specific directions for the assignment below can be found within this lesson.
Readings | J. H. Gary, G. E. Handwerk, Mark J. Kaiser, Chapter 1, pp. 1-12; Chapter 3, pp. 62-65 |
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Assignments | For your information, review the most recent supply of petroleum fuels from the data given at The U.S. Energy Information Administration website [3] (eia.gov) and research how petroleum refining addresses the environmental concerns from combustion of petroleum fuels in internal combustion engines. |
If you have any questions, please post them to our Help Discussion (not email), located in Canvas. I will check that discussion forum daily to respond. While you are there, feel free to post your own responses if you, too, are able to help out a classmate.