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Chapter 1

Introduction - petroleum refining

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Chapter 1

  1. 1. EP422 Petroleum Refining Engineering Introduction
  2. 2. Oil History – A chronology http://www.sjgs.com/history.html 1814 First oil well in Caldwell, Ohio 1829 Oil discovered in Burkesville KY; (50,000 bbls) 1850 Samuel Kerr distilled oil shale to produce oil 1857 E. L. Drake hired to drill for industrial oil in Pennsylvania 1866 First “gusher” in Texas, PA oil was about $6 a barrel 1901 Lucas Spindletop “gusher” near Beaumont, Texas, and “Big Oil” began 1900s – 1940s, oil found in many places (Louisiana, California, Iran, Middle East) 1950s, oil and natural gas replaced coal due to the lower pollution and ease of use; natural gas predominated
  3. 3. Petroleum Pathway
  4. 4. Petroleum Products
  5. 5. Overview •Petroleum or crude oil is a complex hydrocarbon mixture (mostly gasoline) that is refined to get its constituents or feedstock for chemical transformations. •The typical fuels refinery has as a goal the conversion of as much of the barrel of crude oil into transportation fuels as is economically practical. •Although refineries produce many profitable products, the high-volume profitable products are the transportation fuels gasoline, diesel and turbine (jet) fuels, and the light heating oils, and etc.
  6. 6. Overview (continue) •Modern refinery operations are very complex processes. •Crude oils with low API gravities (high specific gravities) and high sulfur contents require additional hydrotreating equipment. •The quality of the product in refinery can be easily alter (to worsen quality) if sulfur contents and densities increase. •The greater densities will mean more of the crude oil will boil above 566°C (1050°F). •High-boiling material or residue has been used as heavy fuel oil but the demand for these heavy fuel oils has been decreasing because of stricter environmental requirements. • This will require refineries to process the entire barrel of crude rather than just the material boiling below 1050°F (566°C).
  7. 7. Processing Challenges •Sulfur restrictions on fuels (coke and heavy fuel oils) will require extensive refinery additions and modernization and the shift in market requirements among gasoline and reformulated fuels for transportation will challenge catalyst suppliers and refinery engineers to develop innovative solutions to these problems. •The environmental impacts of fuel preparation and consumption will require that a significant shift take place in product distribution (i.e., less conventional gasoline and more reformulated and alternative fuels). •This will have a major effect on refinery processing operations and will place a burden on refinery construction in addition to the need to provide increased capacity for high sulfur and heavier crude oils.
  8. 8. Petroleum Refineries Block Flow Diagram
  9. 9. Implementation of Technologies
  10. 10. Oil Refining •Crude oil contains many compounds; not homogenous. It may vary in appearance and composition from one oil field to another. •Refining separates the various compounds by evaporation temperature (fractional distillation). •Conversion causes chemical changes to make a different product by recombining the molecular chains. •The refining process uses chemicals, catalyst, heat and pressure to separate and combine the basic type of HC molecules naturally found in crude oil into group of similar molecules. •Octane rating is percentage of octane mixed with Heptane and determines pre-ignition point in a standard engine (knocking is bad for the engine)
  11. 11. The oil refining process starts with a fractional distillation column. The problem with crude oil is that it contains hundreds of different HC We have to separate the different types of hydrocarbons to have anything useful. Fortunately there is an easy way to separate things, and this is what oil refining is all about. Different hydrocarbon chain lengths all have progressively higher boiling points, so they can all be separated by distillation. This is what happens in an oil refinery - in one part of the process, crude oil is heated and the different chains are pulled out by their vaporization temperatures. Each different chain length has a different property that makes it useful in a different way. To understand the diversity contained in crude oil, and to understand why refining crude oil is so important in our society, look through the following list of products that come from crude oil:
  12. 12. Petroleum gas - used for heating, cooking, making plastics small alkanes (1 to 4 carbon atoms) commonly known by the names methane, ethane, propane, butane boiling range = less than 104 degrees Fahrenheit / 40 degrees Celsius often liquified under pressure to create LPG (liquified petroleum gas) Naphtha or Ligroin - intermediate that will be further processed to make gasoline mix of 5 to 9 carbon atom alkanes boiling range = 140 to 212 degrees Fahrenheit / 60 to 100 degrees Celsius
  13. 13. Gasoline - motor fuel-liquid mix of alkanes and cycloalkanes (5 to 12 carbon atoms) boiling range = 104 to 401 degrees Fahrenheit / 40 to 205 degrees Celsius Kerosene - fuel for jet engines and tractors; starting material for making other products-liquid mix of alkanes (10 to 18 carbons) and aromatics boiling range = 350 to 617 degrees Fahrenheit / 175 to 325 degrees Celsius Gas oil or Diesel distillate - used for diesel fuel and heating oil; starting material for making other products-liquid alkanes containing 12 or more carbon atoms boiling range = 482 to 662 degrees Fahrenheit / 250 to 350 degrees Celsius Lubricating oil - used for motor oil, grease, other lubricants liquid long chain (20 to 50 carbon atoms) alkanes, cycloalkanes, aromatics boiling range = 572 to 700 degrees Fahrenheit / 300 to 370 degrees Celsius
  14. 14. Heavy gas or Fuel oil - used for industrial fuel; starting material for making other products liquid long chain (20 to 70 carbon atoms) alkanes, cycloalkanes, aromatics boiling range = 700 to 1112 degrees Fahrenheit / 370 to 600 degrees Celsius Residuals - coke, asphalt, tar, waxes; starting material for making other products solid multiple-ringed compounds with 70 or more carbon atoms boiling range = greater than 1112 degrees Fahrenheit / 600 degrees Celsius You may have noticed that all of these products have different sizes and boiling ranges. Chemists take advantage of these properties when refining oil.
  15. 15. Crude Oil is A Complex Mixture! http://www.energyquest.ca.gov/story/chapter08.html Crude oil range in consistency from water to tar-like solids, and in color from clear to black. A general crude oil may contains of 84% C, 14% H, 1-3% S and N, O, metals and salts @ <1%. Crude oils are classified as paraffinic, naphthenic or aromatic, based on the predominant proportion of similar HC molecules. Crude oils are defined @ API (American Petroleum Institute) gravity.  API, lighter the crude. Crude with less sulfur –Sweet. Other than that called as sour crude.
  16. 16. Heavier crude contains more polycyclic aromatics Lead to carbonaceous deposits called “coke”
  17. 17. Some crudes contain a lot of sulfur, which leads to processing considerations.
  18. 18. Classification of Petroleum The first crude oil classification is by the types of hydrocarbons (paraffins, naphthenes, and aromatics). This rating is important to the refinery since the value of the crude oil decreases from classification 1 to 6.
  19. 19. Refinery Products •Knowledge of the physical and chemical properties of the petroleum products is necessary for an understanding of the need for the various refinery processes. Major hydrocarbons in crude oil: •Paraffin: series of HC compound having general formula CnH2n+2, i.e. straight chains or branched chains. Paraffins from C1 to C40 usually appear in crude oil and represent up to 20% of crude by volume. •Since Paraffins are fully saturated (no double bond), they are stable and remain unchanged over long periods of geological time. •Examples: methane, ethane, butane, isobutane •Aromatics: unsaturated ring type (cyclic) compounds. Naphthalenes are double-ring aromatic compounds. >3 aromatic ring are found in heavier fractions of crude oil. • Some of the common aromatics found in petroleum and crude oils are benzene and its derivatives with attached methyl, ethyl, propyl, or higher alkyl groups.
  20. 20. Aromatics: Examples: benzene, methylbenzene Naphthenes: saturated hydrocarbon groupings (CnH2n), arranged in the form of closed ring (cyclic). Can found in all fractions of crude oil except the very lightest one. Thermodynamic studies show that naphthene rings with five and six carbon atoms are the most stable naphthenic hydrocarbons. Cycloalkanes(Naphthenes): CnH2n , ringed structures with one or more rings contain only single bonds between the carbon atoms. Examples: cyclohexane, methyl cyclopentane.
  21. 21. Low-boiling products •The compounds which are in the gas phase at ambient temperatures and pressures. •C1 is usually used as a refinery fuel, but can be used as a feedstock for hydrogen production by pyrolytic cracking and reaction with steam. •C2 can be used as refinery fuel or as a feedstock to produce hydrogen or ethylene, which are used in petrochemical processes. •C3 is frequently used as a refinery fuel but is also sold as a liquefied petroleum gas (LPG). •C4 in crude oils and produced by refinery processes are used as components of gasoline and in refinery processing as well as in LPG. nC4 has a lower vapor pressure than iC4, and is usually preferred for blending into gasoline to regulate its vapor pressure and promote better starting in cold weather. •On a volume basis, gasoline has a higher sales value than that of LPG, • thus, it is desirable from an economic viewpoint to blend as much normal butane as possible into gasoline.
  22. 22. Low-boiling products (continue) •Regulations promulgated by the Environmental Protection Agency (EPA) to reduce hydrocarbon emissions during refueling operations and evaporation from hot engines after ignition turn-off have greatly reduced the allowable Reid vapor pressure of gasolines during summer months. •This resulted in two major impacts on the industry. The first was the increased availability of n-butane during the summer months and the second was the necessity to provide another method of providing the pool octane lost by the removal of the excessive n-butane. •The pool octane is the average octane of the total gasoline production of the refinery if the regular, mid-premium, and super-premium gasolines are blended together.
  23. 23. Gasoline •Most refiners produce gasoline in two or three grades, unleaded regular, premium, and super-premium, and in addition supply a regular gasoline to meet the needs of farm equipment and pre-1972. •The principal difference between the regular and premium fuels is the antiknock performance. •Gasolines are complex mixtures of hydrocarbons having typical boiling ranges from 100 to 400°F (38 to 205°C) as determined by the ASTM method •Components are blended to promote high antiknock quality, ease of starting, quick warm-up, low tendency to vapor lock, and low engine deposits.
  24. 24. Refinery Feedstocks •The basic raw material for refineries is petroleum or crude oil, even though in some areas synthetic crude oils from other sources (Gilsonite, tar sands, etc.) and natural gas liquids are included in the refinery feedstocks. The elementary composition of crude oil usually falls within the following ranges. •The U.S. Bureau of Mines has developed a system which classifies the crude according to two key fractions obtained in distillation: –No. 1 482 to 527°F (250 to 275°C) at atmospheric pressure and –No. 2 from 527 to 572°F (275 to 300°C) at 40 mmHg pressure.
  25. 25. Petroleum Standards There are a number of international standard organizations that recommend specific characteristics or standard measuring techniques for various petroleum products. Some of these organizations are as follows: 1. ASTM (American Society for Testing and Materials) in the United States 2. ISO (International Organization for Standardization), which is at the international level 3. IP (Institute of Petroleum) in the United Kingdom 4. API (American Petroleum Institute) in the United States
  26. 26. API Gravity •specific gravity and API gravity refer to the weight per unit volume (density) at 60°F and 1 atm as compared to water at 60°F and 1 atm. •Crude oil gravity may range from less than 10°API to over 50°API but most crudes fall in the 20 to 45°API range. •Generally, a crude with the API gravity of less than 20-22 is called heavy crude and with API gravity of greater than 33-40 is called light crude. Similarly, if the sulfur content of a crude is less than 0.5 wt% it is called a sweet oil.
  27. 27. Sulfur Content, wt% •Sulfur content and API gravity are two properties which have had the greatest influence on the value of crude oil. 0.1% < S >5%. •Sour crude (>0.5%) require more extensive processing. Pour Point, °F (°C) The pour point of a liquid is the temperature at which it becomes semi solid and loses its flow characteristics. •The pour point of the crude oil, in °F or °C, is a rough indicator of the relative paraffinicity and aromaticity of the crude. •The lower the pour point, the lower the paraffin content and the greater the content of aromatics. Carbon Residue, wt% •Carbon residue is determined by distillation to a coke residue in the absence of air. •Related to the asphalt content of the crude and to the quantity of the lubricating oil fraction that can be recovered. •In most cases the lower the carbon residue, the more valuable the crude.
  28. 28. Salt Content, lb/1000 bbl •If the salt content of the crude, when expressed as NaCl, is greater than 10 lb/1000 bbl, it is generally necessary to desalt the crude before processing. •If the salt is not removed, severe corrosion problems may be encountered. If residue are processed catalytically, desalting is desirable at even lower salt contents of the crude. Characterization Factors •Correlations between yield and the aromaticity and paraffinicity of crude oils. •UOP or Watson ‘‘characterization factor’’ (KW) and the U.S. Bureau of Mines ‘‘correlation index’’ (CI).
  29. 29. •The Watson characterization factor ranges from less than 10 for highly •aromatic materials to almost 15 for highly paraffinic compounds. Crude oils show a narrower range of KW and vary from 10.5 for a highly naphthenic crude to 12.9 for a paraffinic base crude. •The correlation index is useful in evaluating individual fractions from crude oils. •The CI scale is based upon straight-chain paraffins having a CI value of 0 and benzene having a CI value of 100. •The CI values are not quantitative, but the lower the CI value, the greater the concentrations of paraffin hydrocarbons in the fraction; and the higher the CI value, the greater the concentrations of naphthenes and aromatics.
  30. 30. Metals Content, ppm •The metals content of crude oils can vary from a few parts per million to more than 1000 ppm and, in spite of their relatively low concentrations, are of considerable importance. Reid vapor pressure (RVP) •is the absolute pressure exerted by a mixture at 100 oF (37.8 °C) as determined by the test method ASTM-D-323 and a vapor-to-liquid volume ratio of 4. The RVP is one of the important properties of gasolines and jet fuels. For a pure compound the freezing point is the temperature at which liquid solidifies at 1 atm pressure. Similarly the melting point is the temperature that a solid substance liquefies at 1 atm. A pure substance has the same freezing and melting points; however, for petroleum mixtures, there are ranges of melting and freezing points versus percent of the mixture melted or frozen.
  31. 31. Pour point of a petroleum fraction is the lowest temperature at which the oil will pour or flow when it is cooled without stirring under standard cooling conditions. Pour point represents the lowest temperature at which an oil can be stored and still capable of flowing under gravity. When temperature is less than pour point of a petroleum product it cannot be stored or transferred through a pipeline.
  32. 32. Cloud point is the lowest temperature at which wax crystals begin to form by a gradual cooling under standard conditions. At this temperature the oil becomes cloudy and the first particles of wax crystals are observed. Low cloud point products are desirable under low-temperature conditions. Wax crystals can plug the fuel system lines and filters, which could lead to stalling aircraft and diesel engines under cold conditions. Cloud points are measured for oils that contain paraffins in the form of wax and therefore for light fractions (naphtha or gasoline) no cloud point data are reported.
  33. 33. Flash point should not be mistaken with fire point, which is defined as the minimum temperature at which the hydrocarbon will continue to burn for at least 5 s after being ignited by a flame. Autoignition temperature is the minimum temperature at which hydrocarbon vapor when mixed with air can spontaneously ignite without the presence of any external source. Values of autoignition temperature are generally higher than flash point. This is particularly important from a safety point of view when hydrocarbons are compressed. Standard test is ASTM D 2155.
  34. 34. Nitrogen Content, wt% •A high nitrogen content is undesirable in crude oils because organic nitrogen compounds cause severe poisoning of catalysts used in processing and cause corrosion problems such as hydrogen blistering. Crudes containing nitrogen in amounts above 0.25% by weight require special processing to remove the nitrogen. Distillation Range •The boiling range of the crude gives an indication of the quantities of the various products present.
  35. 35. Octane number is a parameter defined to characterize antiknock characteristic of a fuel (gasoline and jet fuel) for spark ignition engines. Octane number is a measure of fuel's ability to resist auto-ignition during compression and prior to ignition. Higher octane number fuels have better engine performance. The octane number of a fuel is measured based on two reference hydrocarbons of n-heptane with an assigned octane number of zero and isooctane (2,2,4-trimethylpentane) with assigned octane number of 100.
  36. 36. There are two methods of measuring octane number of a fuel in the laboratory; motor octane number (MON) and research octane number (RON). The MON is indicative of high-speed performance (900 rpm) and is measured under heavy road conditions (ASTM D 357). The RON is indicative of normal road performance under low engine speed (600 rpm) city driving conditions (ASTM D 908). The arithmetic average value of RON and MON is known as posted octane number (PON). Isoparaffins and aromatics have high octane numbers while n-paraffins and olefins have low octane numbers.
  37. 37. Generally there are three kinds of gasolines: regular, intermediate, and premium with PON of 87, 90, and 93, respectively. Improving the octane number of fuel would result in reducing power loss of the engine, improving fuel economy, and a reduction in environmental pollutants and engine damage. There are a number of additives that can improve octane number of gasoline or jet fuels. These additives are tetra-ethyl lead (TEL), alcohols, and ethers.
  38. 38. For diesel engines, the fuel must have a characteristic that favors auto- ignition. The ignition delay period can be evaluated by the fuel characterization factor called cetane number (CN). The shorter the ignition delay period the higher CN value. The cetane number is defined as: CN = vol% n-cetane + 0.15(vo1% HMN) Where n-cetane (n-C16H34) has a CN of 100, and heptamethylnonane (HMN) has a CN of 15. The cetane number of a diesel fuel can be measured by the ASTM D 613 test method.
  39. 39. Higher cetane number fuels reduce combustion noise and permit improved control of combustion resulting in increased engine efficiency and power output. Higher cetane number fuels tend to result in easier starting and faster warm-up in cold weather and can cause reduction in air pollution. The product distributed in France and Europe have CN in the range of 48-55. In the United States and Canada the cetane number of diesel fuels are most often less than 50. Cetane number of diesel fuels can be improved by adding additives such as 2-ethyl-hexyl nitrate or other types of alkyl nitrates.
  40. 40. Aniline point for a hydrocarbon or a petroleum fraction is defined as the minimum temperature at which equal volumes of liquid hydrocarbon and aniline are miscible. The aniline point is important in characterization of petroleum fractions and analysis of molecular type. The aniline point is also used as a characterization parameter for the ignition quality of diesel fuels. It is measured by the ASTM D 611 test method. Aromatics have very low aniline points in comparison with paraffins, since aniline itself is an aromatic compound (C6H5-NH2) and it has better miscibility with aromatic hydrocarbons.
  41. 41. The smoke point (SP) is a maximum flame height at which a fuel can be burned in a standard wick-fed lamp without smoking. It is expressed in millimeters and a high smoke point indicates a fuel with low smoke- producing tendency. Measurement of smoke point is described under ASTM D 1322. Smoke point is a characteristic of aviation turbine fuels and kerosenes and indicates the tendency of a fuel to burn with a smoky flame. Higher amount of aromatics in a fuel causes a smoky characteristic for the flame and energy loss due to thermal radiation.
  42. 42. Crude Oil Assay •Distillation Analysis • Indicates the quality of the crude oil feedstock. • Based on the amount of material that boils in a particular temperature range. • Represents expected products from crude & vacuum distillation. • Amount of data depends on laboratory analysis. •Quality Measures • Gravity, API • Characterization factor • Sulfur content, wt% • Pour point, F (C) • Carbon residue, wt% • Salt content, lb/1000 bbl • Nitrogen content, wt% • Metal content, ppm
  43. 43. Importance of Crude Oil Characterization Crude grades vary considerably from each other - in yield and properties. Crude characterization is essential to estimate feedstock properties for refinery units, produce an optimal amount of finished products, meet product quality specifications and to provide an economic assessment for crude oils. Upstream Planning - to determine the economic viability of new fields / discoveries Supply Organizations - to assign crude value for individual grades Refinery Operations - to schedule crude receipts and determine product yields Model Engineers - to optimize refinery crude slates Research & Development - to design equipment and process planning
  44. 44. Crude Oil Distillation Ref: R. Smith, Chemical Process Design and Integration, Wiley, 2005. In the first stage of processing crude oil, it is distilled under conditions slightly above atmospheric pressure. A range of petroleum fractions are taken from the crude oil distillation. Designs are normally thermally coupled. Most configurations follow the thermally coupled indirect sequence as shown in Figure (a). However, rather than build the configuration in Figure (a), the configuration of Figure (b) is the one normally constructed. Notice that the two arrangements are equivalent.
  45. 45. Crude Oil Distillation
  46. 46. Crude Oil Distillation Unfortunately, a practical crude oil distillation cannot be operated in quite the way shown in Figure (b), because: • Extremely high temperature sources of heat would be required. Steam is usually not distributed for process heating at such high temperatures. • High temperatures in the reboilers would result in significant fouling of the reboilers from decomposition of the hydrocarbons to form coke. Therefore, in practice, some or all of the reboiling is substituted by the direct injection of steam into the distillation. The steam is condensed in the overhead and is separated in a decanter from the hydrocarbons.
  47. 47. Crude Oil Distillation Another problem with the arrangement in Figure (b) is that as the vapor rises up the main column, its flow rate increases significantly. This problem can be solved by removing heat from the main column at intermediate points by pumparound. This corresponds with introducing some condensation of the vapor at the top of intermediate columns.
  48. 48. Crude Oil Distillation Pumparound In a pumparound, liquid is taken from the column, sub cooled and returned to the column at a higher point. By choosing the most appropriate flow rate and temperature for the pumparound, the heat load to be removed can be adjusted to whatever is desired. The trays between the liquid draw and return in a pumparound have more to do with heat transfer than mass transfer. In addition to returning a sub cooled liquid to the column, mixing occurs as material is introduced to a higher point in the column.
  49. 49. Crude Oil Distillation Furnace The crude oil entering the main column needs to be preheated to around 400 ◦C. This is down by a furnace (fired heater). Note that this temperature is higher than decomposition limit, but a high temperature can be tolerated in the furnace if it is only for a short residence time. All of the material that needs to leave as product above the feed point must vaporize as it enters the column. In addition to this, some extra vapor over and above this flowrate must be created that will be condensed and flow back down through the column as reflux. This extra vaporization to create reflux is known as overflash.
  50. 50. Crude Oil Distillation The distillation of crude oil under conditions slightly above atmospheric pressure is limited by the maximum temperature that can be tolerated by the materials being distilled, otherwise there would be decomposition. The residue from the atmospheric crude oil distillation is usually reheated to a temperature around 400◦C or slightly higher and fed to a vacuum column, which operates under a high vacuum (about 50 mmHg) to allow further recovery of material from the atmospheric residue, as shown in the next Figure.
  51. 51. Crude Oil Distillation

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