Petroleum Refinery Engineering

[1] Gary, J.H.; Handwerk, G.E. 2001. Petroleum refining: Technology and economics. 4th ed. Marcel Dekker, Inc. [2] Fahim, M.A., AlSahhaf, T.A. and Elkilani, A. 2010. Fundamentals of petroleum refining. Elsevier. [3] Parkash, S. 2003. Refining processes handbook. Gulf professional publishing, Elsevier. Singapore. [4] Wauquier, J.-P. (ed.). 1998. Petroleum refining: Separation processes. Vol. 2. Technip. [5] Meyers, R.A. 2004. Handbook of petroleum refining processes. 3rd ed. McGraw-Hill.
7. 7 Overall refinery flow [2] 8. 8 Crude oil distillation: Atmospheric
9. 9 Crude oil distillation: Vacuum
10. Approximate ranges of crude distillation products [2] 11. Conversion and separation processes Separation processes: Crude distillation (atmospheric distillation and vacuum distillation), solvent extraction, solvent deasphalting, solvent dewaxing, and clay treatment. Conversion processes: Catalytic reforming, hydrotreating, hydrocracking, catalytic cracking, alkylation, isomerization, delayed coking, flexicoking, and visbreaking. Refinery processes
12. 12 Separation processes Atmospheric distillation: Desalted crude oil is flashed in the atmospheric distillation unit and the crude oil is fractionated into various fractions. Light gases, light and heavy naphthas, kerosene, light and heavy gas oils, and atmospheric residuum may be the principal fractions. Vacuum distillation: The atmospheric residue is vacuum fractionated and vacuum gas oil and vacuum residuum may be the products. Solvent deasphalting: A solvent usually liquid propane is employed to remove asphaltenes from heavy crude fractions such as vacuum resid. Solvent dewaxing: Paraffins of high molecular weight (wax) are removed from the lube oil stock to adjust the pour point.
13. 13 Separation processes Solvent extraction: Lube oil stock is treated with a solvent and aromatics are removed. Clay treatment: Lube oil stocks are subjected to clay treatment to remove impurities to better stabilize and to improve the color.
14. 14 Conversion processes Alkylation: As an example, it is the addition of isobutane to butenes to produce high grade gasoline range product (alkylate). Hydrotreating: It is carried out to remove impurities such as sulfur, nitrogen, halides etc. Isomerization: For example, n-butane is isomerized to isobutane to feed the alkylation plant. n-hexane is isomerized to branched alkanes to produce a high octane rating product. Catalytic reforming: It is used to process low grade (octane number) fraction such as straight run gasoline and naphthas to produce high grade gasoline range products. Dehydrogenation, isomerization, and hydrocracking may occur during the course of catalytic reforming.
15. 15 Conversion processes Catalytic cracking: The catalytic cracking of heavy oil fractions to produce mainly of gasoline range products. Hydrocracking: The cracking of heavy oil fractions to produce low boiling products in the presence of hydrogen and catalyst. Coking: It is severe thermal cracking that results in light gases,, coker naphtha, and solid coke. Visbreaking: It stands for viscosity breaking. Liquid phase mild thermal cracking of heavy feedstocks.
16. 16 Conversion and separation processes • The crude oil is heated in a furnace and flash-charged to an atmospheric distillation tower (ADU). Here, it is separated into light wet gases, unstabilized light naphtha, heavy naphtha, kerosene, light and heavy atmospheric gas oils, and atmospheric reduced crude. • The atmospheric reduced crude enters the vacuum reduced distillation column (VDU) and separated into vacuum gas oil streams and vacuum reduced crude. • The vacuum reduced crude is sent to a coker where it is thermally cracked to produce wet gas, gasoline and gas oil range products and solid coke. • The gas oil ranged products from the ADU and VDU and gas oil from the coking process are subjected to catalytic and hydrocracking. The purpose is usually to produce products of gasoline and diesel range.
17. 17 Conversion and separation processes • “The light naphtha streams from the crude tower, coker and cracking units are sent to an isomerization unit to convert straight-chain paraffins to isomers that have higher octane numbers” [1]. • “The heavy naphtha streams from the crude tower, coker and cracking units are fed to the catalytic reformer to improve their octane numbers” [1]. • “The wet gases streams from the crude units, coker, and cracking units are separated in the vapor recovery section (gas plant) into fuel gas, liquefied petroleum gas (LPG), unsaturated hydrocarbons (propylene, butylenes, and pentenes), normal butane, and isobutane. The fuel gas is burned as a fuel in refinery furnaces and normal butane is blended into gasoline or LPG. The unsaturated hydrocarbons and isobutane are sent to the alkylation unit for processing” [1].
18. 18 Conversion and separation processes • “In some refineries, the heavy vacuum gas oil and reduced crude from paraffinic or naphthenic base crude oils are processed into lubricating oils” [1]. • “The vacuum gas oils and deasphalted stocks are first solvent- extracted to remove aromatic compounds and then dewaxed to improve the pour point. They are then treated with special clays or high-severity hydrotreating to improve their color and stability before being blended into lubricating oils” [1].
19. 19 Optimum refinery operation “Each refinery has its own unique processing scheme which is determined by the process equipment available, crude oil characteristics, operating costs, and product demand. The optimum flow pattern for any refinery is dictated by economic considerations and no two refineries are identical in their operation.” [1] 20. 20 Some crude oils of Pakistan (From thesis of my student Ahmad)
21. 21 Refining facility in Pakistan Byco 35,000 bbl/day ARL 40,000 bbl/day PRL 50,000 bbl/day NRL 65,000 bbl/day PARCO 100,000 bbl/day Byco has added nearly 120,000 bbl/day capacity in its new installation. In the past, further new installations were expected including KCR, Indus, and Trans Asia.
22. 22 General refinery products Refinery fuel gas Liquefied petroleum gas Solvent naphtha Gasoline Kerosene Jet fuel or gas turbine fuel Diesel fuel Fuel oil Residual fuel oil Lubricating oil or lube oil Wax Asphalt Petroleum coke
23. 23 Refinery products: Brief description[2] 24. 24 Tests on petroleum fraction Pour point: A lower pour point means paraffinic content is low. It is a measure of ease or difficulty of a fraction to be pumped in cold conditions. Viscosity: It is usually measured in centi Stokes or Saybolt seconds at 37.8 and 99 ºC. These two points are used to find viscosity index of the fraction. Aniline point: It is an indication of the amount of the aromatic content in a given fraction. Flash point: It is important for gasoline and naphtha. Octane number: Motor octane number (MON) is the test carried out at high speed (900 rpm). Research octane number (RON) is measured at low speed (600 rpm). PON (posted octane number is the arithmetic average of RON and MON).
25. 25 Tests on petroleum fraction Reid vapor pressure: Vapor pressure determined in a volume of air four times the liquid volume at 37.8 °C (100 °F). It indicates vapor lock characteristics and explosion hazards. Carbon residue: It indicates the soot forming characteristics of a fuel. Smoke point: It is a measure of the burning qualities of kerosene and jet fuels. It is measured in terms of the maximum height in mm of a smokeless flame of fuel. Refractive index: It is the ratio of the velocity of light in vacuum to the velocity of light in the oil. It is a used to characterize a petroleum fraction. Cetane number: It measures the ability for autoingnition in diesel (compression ignition) engines. It is the percentage of pure cetane (n- hexadecane) in a blend of cetane and alpha methyl naphthalene which corresponds to the ignition characterstics of a given diesel sample.
26. 26 Tests on petroleum fraction Freezing point: It is the temperature at which the hydrocarbon liquid solidifies at atmospheric pressure. It is one of the important property specifications for kerosene and jet fuels due to the low temperatures encountered at high altitudes in jet planes. Sediments: These are solid materials that are not soluble in the hydrocarbon or water and can be comprised of sand, drilling mud, rock or minerals, particles from erosion of metal pipes, tanks, and other process equipments.
27. 27 Separation processes Generally, separation processes may be classified as either mechanical-physical separation processes or mass transfer operations. o Mechanical-physical separation processes (do not require a mass transfer gradient for the separation) o Mass transfer operations (based on diffusion and require a mass transfer gradient for the separation)
28. 28 Separation processes Examples of mechanical-physical separation processes are: Size reduction Size enlargement (not crystallization) Size separation (screening, etc.) Filtration Some and not all membrane separation processes Sedimentation (Thickening and clarification) Elutriation Floatation Centrifugation
29. 29 Separation processes Examples of mass transfer operations are: Distillation Drying Liquid-liquid extraction Leaching or lixiviation Gas absorption Membrane separation (Not all membrane separation processes) Humidification Adsorption
30. 30 Atmospheric distillation unit (ADU) What is distillation? Why do we need distillation of a crude oil?
31. 31 Atmospheric distillation unit (ADU)[3] 32. 32 Atmospheric distillation unit (ADU) The column operates generally at a pressure greater than atmospheric pressure. This may be done for pressure drop considerations, to flow the vapors from one location to the desired location, and/or cooling water to be used for the overhead condenser. A higher pressure increases the bubble point. What is the criteria for setting pressure in the overhead condenser (column)?
33. 33 Atmospheric distillation unit (ADU) Nearly 50–60% of the crude oil is vaporized in the flash zone of the tower [3]. A preflash tower is sometimes added before the atmospheric column, if the crude oil contains appreciable amounts of lighter products. How is this advantageous? The bottom temperature is bounded in the range of 700- 750 °F [3]. This is done avoid cracking. The superheated steam required to boil off the crude bottoms is usually at about 600 °F [3]. The steam consumption is usually 5–10 lb/bbl of stripped product [3].
34. 34 Vacuum distillation unit (VDU) Overflash: 5 to 10% of the bottoms that acts as an internal reflux to better fractionate the few trays above the flashzone. Pumparound reflux is used to remove heat from the column. A stream at a higher temperature in a column is taken out of the column, exchanges heat with the crude oil feed and heats it, and then returns back to the column at some higher position in the column (lower temperature).
35. 35 Atmospheric distillation unit (ADU) Most atmospheric towers contain 25–35 trays between the flash zone and the top tower. The allowable pressure drop for trays is approximately 0.1–0.2 psi per tray. Generally a pressure drop of 5 psi is allowed between the flash zone and the top tower. Ref.: 3
36. 36 Atmospheric distillation unit (ADU)[3] 37. 37 Vacuum distillation unit (VDU)[1] 38. 38 Vacuum distillation unit (VDU) The temperature required at the furnace outlet for atmospheric distillation unit (ADU) will be excessive if the heavier fractions will be distilled in the ADU. This will be resulted in thermal cracking and loss of the product and fouling of the equipment. This necessitates the use of vacuum distillation column where the distillation occurs under sub-atmospheric conditions. Decreasing pressure of a component decreases the boiling point and vice versa.
39. 39 Vacuum distillation unit (VDU) •Furnace outlet temperatures are usually in range of 730– 850 °F (388–454 °C) [1]. •The pressure in the flash zone is around 25–40 mm Hg [1]. •The effective pressure for hydrocarbon vaporization is reduced by using stripping steam in the furnace as well as at the tower bottom. The steam added to the furnace increases the velocity of the fluid and decreasing the coke formation. •The steam consumption is usually 10–50 lb/bbl of feed [1]. •The lower pressure in the tower increases the diameter of column. A higher pressure increases the boiling temperatures and difficulty of separation. Vacuum distillation columns have large diameters and the diameter may easily reach 40 ft [1].
40. 40 Vacuum distillation unit (VDU)[3] 41. 41 Vacuum distillation unit (VDU)[3] 42. 42 Vacuum distillation unit (VDU) •The gases produced at the top of the column are sent to the fired heater and burned to release heat. •The gases separated from the sour water in foul water stripper are sent to the flare system. Part of the water recovered in the stripper is sent to the desalters. •Vacuum gas oil produced may be sent to hydrocracking, catalytic cracking, or lube oil processing. What for vacuum residue?
43. 43 Vacuum distillation unit (VDU) “It is essential to design the fractionator tower, overhead lines, and condenser to minimize the pressure drop between the vacuum-inducing device and the flashzone. A few millimeters decrease in pressure drop will save many dollars” [1]. Structured packings may be employed in the column as having high HETP and offer low pressure drop. “The desired operating pressure in column is maintained using steam ejectors and barometric condensers or vacuum pimps and surface condensers. For a flash zone pressure of 25 mmHg, three ejector stages are usually required” [1]. “The vacuum produced is limited to the vapor pressure of the water used in the condensers”[1].
44. 44 Vacuum distillation unit (VDU): Examples of structured packings [7] 45. 45 Vacuum distillation unit (VDU): Steam ejector [8] 46. 46 Vacuum distillation unit (VDU): Barometric condenser [8] 47. 47 Separation processes (Lube oil processing) What is a lubricating (lube) oil? What is its function?
48. 48 Separation processes (Lube oil processing) What should be the important properties of lube oils? 1. Viscosity 2. Viscosity change with temperature (viscosity index) 3. Pour point 4. Oxidation resistance 5. Flash point 6. Boiling temperature 7. Acidity (neutralization number) 8. Thermal stability 8. Color See Chapter 14 of Ref. 1.
49. 49 Separation processes (Lube oil processing) Viscosity: Higher the viscosity, the thicker the film of the oil. However, too high a viscosity may cause undesirable friction and heat. Higher the boiling range of the fraction, usually greater the viscosity. What boiling range should be selected for a lube oil that requires a high viscosity? Viscosity index: Higher the viscosity index, the smaller the change in viscosity with temperature. It may a negative or even greater than 100. Additives such as polyisobutylenes and polymethacrylic acid esters are added to improve the viscosity index characteristics.
50. 50 Separation processes (Lube oil processing) Pour point: For motor oils, a low pour point is very important to obtain ease of starting and proper start up lubrication on cold days. Oxidation resistance: At high temperatures such as in an internal combustion engine can cause rapid oxidation. Oxidation causes the coke formation and the formation of asphaltic type materials. These products can injure the metal surfaces and can block the flow lines. Additives such as phenolic compounds and zinc dithiophosphates are added to inhibit the oxidation reactions.
51. 51 Separation processes (Lube oil processing) Flash point: It may indicate the type of hydrocarbons are present in the lube oil and determines the volatility of the oil and the possible emissions it may cause. Boiling point: It tells the type of the hydrocarbons present and gives an idea of the viscosity of the oil. Acidity: Organic acids formed by the oxidation of oil or that formed as byproducts in the combustion of oil may cause corrosion and therefore needed to be reduced. Lube oil blending stocks from paraffinic crude oils have excellent thermal and oxidation stability and exhibit lower acidity than do the oils from the naphthenic crude oils. Acidity characteristics are determined by neutralization number. Can you define acid number, base number, and neutralization number?
52. 52 Separation processes (Lube oil processing) Thermal stability: It ensures that the lubricating oil is stable under the conditions of operation and not cracked or reactive. Color: Reasonably unacceptable color may a sign of the presence of olefins, metal complexes, and heteroatoms such as nitrogen.
53. 53 Separation processes (Lube oil processing) Undesirable characteristics of raw lube oils [1]: 1. High cloud and pour points 2. Large viscosity change with temperature, i.e., low viscosity index 3. Poor oxidation resistance 4. Poor color 6. High organic acidity 7. High coke forming and sludge forming ability
54. 54 Definitions of lube oils Neutral oil: Straight run distillation fraction suitable for lube oil products. Bright stock: Deasphlated vacuum residue suitable for lube oil. Finished lube oil: The blended lube oil product of final desired properties. Paraffinic lube oils: “These are all grades of lube oils, from both neutral and bright stocks, that have a finished viscosity index greater than 75” [6] Naphthenic lube oils: Lube oils with a finished viscosity index of less than 75 [6].
55. 55 Separation processes (Lube oil processing) The processes used to change the above mentioned undesirable properties of the raw lube oils are as follows: 1. Solvent deasphalting 2. Solvent extraction or Hydrocracking 3. Solvent dewaxing or selective hydrocracking (hydrodewaxing) 4. Clay treatment or Hydrotreating The modern trend is to use hydroprocesses, however, we will go through separation processes here and similar hydroprocesses will be discussed in rather detail in the final term.
56. 56 Separation processes (Lube oil processing)
57. 57 Atomic H/C ratios of various species [9] 58. 58 Solvent deasphalting The purpose of deasphalting is to remove asphaltenes and resins and that it is required to reduce metallic contents and coke and sludge formation tendencies. For maximizing fuel (in fuel refinery) or when the vacuum fraction is not suitable for lube oil production, the deasphalting is carried out to prepare feed for the subsequent conversion processes. So, it is not always carried out for preparing lube oil stock. Owing to the high carbon residues and metal contents (sulfur contents may well be) of the asphaltene and resin fraction, removal of asphaltenes and resins reduces these contents in the feed for catalytic conversion units. Catalysts involved in the conversion processes may be damaged and greatly deactivated in the presence of metals and high carbon residue. Severe fouling of the process equipment may also be a reason.
59. 59 Solvent deasphalting Removal of sulfur, metal, and nitrogen from a vacuum residue is usually less expansive by deasphalting than by hydrotreating. The desulfurized, demetallized, etc., residue may be used as a blending stock for the feed for hydrocracking and catalytic cracking. The light lube oil fractions, if any, can avoid deasphalting and directly treated in solvent extraction.
60. 60 Solvent deasphalting From:
61. 61 Asphaltenes (Solvent deasphalting) From: The shell bitumen industrial handbook, p. 53.
62. 62 Resins (Solvent deasphalting) From: The shell bitumen industrial handbook, p. 53.
63. 63 Propane deasphalting [1] 64. 64 Rotating disc contactor (RDC) [5] From:
65. 65 Rotating disc contactor (RDC) What would be the name of extractor if the moving rotor is made to stop in an RDC?
66. 66 Propane deasphalting [1] o Feed is mixed with a small amount of solvent to increase the fluidity of the feed. o The feedstock is usually treated with 4 to 8 volumes of liquid propane. o The extractor is usually a baffle column or rotating disc contactor. Name the other types of column extractors. o The extract phase contains from 15 to 20% by weight of oil with the remainder solvent. The raffinate phase contains from 30 to 50% propane by volume. o The heavier the feedstock, the higher the ratio of propane to oil required.
67. 67 Propane deasphalting o “The propane deasphalting tower is operated at a pressure sufficiently high to maintain the solvent in the liquid phase. This is usually about 500 psig (3448 kPa).” [1]. o The critical temperature of propane is 96.8 °C so the upper limit may have to be set below this temperature (gases cannot be liquefied above critical temperature). The temperature is usually limited to 82 °C [9]. o Solvent to oil ratio is a function of feedstock properties and the required product specifications. An increase in the solvent to oil ratio increases the product quality.
68. 68 Selection of solvent for liquid-liquid extraction: Class input 1. Start…
69. 69 Selection of solvent for liquid-liquid extraction 1. High solubility of the solute in the solvent but low solubility or preferably immiscibility with feed solvent 2. Should not be reactive with the feed 3. Phases should have high density difference. 4. Solute and solvent should be economically separable. 5. High distribution coefficients 6. Should be stable and non-volatile under the conditions of operation 7. Should have low cost 8. Easy and regular availability (no inventory problems) 9. Non-corrosive, non-toxic, and environmental friendly.
70. 70 Selection of solvent for liquid-liquid extraction Do you have an idea of ternary phase diagrams used in liquid-liquid extraction?
71. 71 Propane deasphalting https://books.google.com.pk/ books?id=VTIvBQAAQBAJ& pg=PA149&dq=propane+de asphalting+ternary+diagram &hl=en&sa=X&ved=0CBoQ6 AEwAGoVChMIxYzExdGKx wIVSF0sCh0ZFgbL#v=onep age&q&f=true Butterworth- Heinemann. 1994.
72. 72 Propane deasphalting How does the solvent to feed ratio affect the design and performance of a liquid-liquid extractor? What are dispersed phase, continuous phase, flooding, dispersed phase holdup, mass transfer efficiency (overall mass transfer coefficient or overall height of transfer unit), interfacial tension, drop diameter, and drop distribution in relation to a continuous liquid-liquid extractor?
73. 73 Propane deasphalting “Propane, usually, is used as the solvent in deasphalting but it may also be used with ethane or butane in order to obtain the desired solvent properties. Propane has unusual solvent properties in that from 100 to 140 °F (40 to 60 °C) paraffins are very soluble in propane, but the solubility decreases with an increase temperature until at the critical temperature of propane [206 °F (96.8 °C)] all hydrocarbons become insoluble. In the range of 100 to 206 °F (40 to 96.8 °C) the high molecular weight asphaltenes and resins are largely insoluble in propane” [1] 74. 74 Propane deasphalting As the metal, sulfur, and nitrogen are generally concentrated in the larger molecules, the metal, sulfur, and nitrogen content of deasphalted oil is considerably reduced as shown in the next slide [3]. Asphalt may be burned to produce energy, but due to fluidity problems and stack gas issues (high cost for gas cleaning) it is commonly used for road paving, water proofing, and insulation. Asphalt may be air treated (asphalt blowing) to improve its properties. Residue oxidation or asphalt blowing is carried out to increase softening point by removing aromatics and polar aromatics.
75. 75 Propane deasphalting [3] 76. 76 Propane deasphalting [9] 77. 77 Propane deasphalting [3] 78. 78 Propane deasphalting [10] Generally deasphalting: 1. decreases asphaltene and resin content 2. decreases metal amounts 3. decreases carbon residue 4. increases pour point? 5. increases API gravity (decreases specific gravity) 6. improves color 7. decreases aromatic content 8. decreases nitrogen and sulfur content 9. decreases viscosity
79. 79 Propane deasphalting [10] Macrcel-Dekker. 1994. For more details and design consideration. pp. 53-80 (Chapter 4) Available at: https://books.google.com.pk/boo ks?id=TAzJvJ1HmFQC&printsec =frontcover#v=onepage&q&f=fals e (accessed on 02-Aug-2015)
80. 80 Propane deasphalting [11] 81. 81 Propane deasphalting [11] [accessed on: 17-Aug-2015] 82. 82 Characteristics of various families in lube oil stock [4] 83. 83 Characteristics of various families in lube oil stock [1] 84. 84 Solvent extraction: Purposes (lube oil extraction) [1] The purpose of solvent extraction is to remove the aromatics from the lube oil fraction and in doing so to improve the: 1. Viscosity index (VI) 2. Oxidation resistance 3. Color 4. Coke forming tendency, and 5. Sludge forming tendency.
85. 85 Solvents commonly applied 1. Furfural 2. Phenol 3. N-methyl-2-pyrrolidone (NMP)
86. 86 Comparison between the three solvents [4] 87. 87 Phenol extraction [1] Phenol being toxic is not studied further here.
88. 88 Comparison between the NMP and furfural NMP Furfural
89. 89 NMP advantages over furfural [4] • Better stability • Better oxidation resistance • Less carryover in the raffinate and the extract • Higher solvent power towards aromatics • Lower process temperature • Less toxicity
90. 90 NMP advantages over furfural [4] o Since NMP is much more stable than furfural, the pretreatment (deaeration) section is unnecessary and the feed can directly be let into the extraction tower. o Solvent ratios in an NMP unit are significantly lower than for furfural, i.e., it has higher solvent power. A smaller plant size is therefore required. o Solvent injection temperatures are lower by 10 to 20 °C for the same final viscosity index (VI) and an identical raffinate yield. o Furfural is sensitive to oxidation and to the presence of water, which significantly lower extraction performance.
91. 91 NMP disadvantages over furfural [4] • Lower specific gravity • Less selectivity • Higher boiling points NMP’s higher boiling point and heat of vaporization requires higher temperatures and energy consumption is also greater than for furfural recovery.
92. 92 Simplified furfural extraction process [6] 93. 93 Rather detailed furfural extraction process [12] 94. 94 Furfural extraction • The furfural is fed at the top while the oil flows countercurrently from the bottom. • The extractor is commonly a packed column with Raschig rings or an RDC [1]. The main advantage of using RDC is that by varying the speed of rotor a wide range of throughputs can be handled [10]. • The oil behaves as a continuous phase while furfural acts as a dispersed phase. • Furfural to oil ratio ranges between 2:1 for light stocks and 4.5:1 for heavy stocks [1].
95. 95 Factors for furfural extraction process • Extraction temperature depends upon the feed characteristics and adjust the viscosity of the oil-furfural mixture and miscibility of the furfural with oil. • A part of the extract phase may be recycled and may affect the efficiency of the extraction. • The extraction column operates between 50 and 200 psig [6] and at a top temperature of 105 to 150 °C [1]. The temperature gradient betweeen top and bottom of the extractor is between 30 to 50 °C [1].
96. 96 Capacity curves for RDC [10] 97. 97 Furfural extraction [6] 98. 98 Furfural extraction [11] 99. 99 NMP extraction [4] (Not for the exam.)
100. 100 NMP extraction [4] (Not for the exam.)
101. 101 NMP extraction: Water addition [4] NMP solvent power is very high towards aromatics, and also considerable towards paraffins, which consequently lowers the raffinate yield. To attenuate the solvent power of NMP, a small amount of water (0.8 to 3.0 wt%) is added and the solvent that circulates in the unit is a mixture of NMP and water. The amount of water required in the solvent depends on the target level of the extraction.
102. 102 NMP extraction: Drying section [4] After cooling to approximately 50°C, NMP containing some 30 wt% water is temporarily stored in a tank. When the tank has become full enough, the dehydration section is started up.
103. 103 Modification of the existing plant As mentioned before phenol is toxic and no more required for the extraction process in the existing or new installations. Also some refiners prefer to replace furfural with NMP and modify the plant accordingly. Reasons may include [10]:  Significant increase in refining capacity  Significant reduction in energy costs  Use of marginal quality crudes  Reduced maintenance costs  Reduced solvent toxicity  Reduced solvent losses
104. 104 Modification of the existing plant Give your suggestions to modify an existing phenol extraction plant to be replaced by an NMP extraction plant.
105. 105 NMP extraction [11] 106. 106 Dewaxing [1, 4] The straight chain and slightly branched paraffinic compounds tend to crystallize under ordinary temperature conditions. However, at low temperature such as –20 °C, the oil needs to be in a liquid state in the engine crankcase. If crystallize or solidify, it will cause flow problems. The dewaxing unit is required to lower the cloud and pour points of the oil by eliminating the compounds that crystallize at relatively higher temperature.
107. 107 Dewaxing [10] “Although the cold settling-pressure filtration processes and centrifuge dewaxing processes have for the most part been replaced by solvent dewaxing, these older processes are still used to a limited degree.” [10] 108. 108 Solvent dewaxing • The solvent reduces the viscosity of the oil and facilitate in pumping and filtration. • “The most common dewaxing process is based on crystallization with a solvent that modifies the conditions of thermodynamic equilibrium solely by its presence in the liquid. The ideal solvent should dissolve the oil and precipitate all the wax. Additionally, in precipitating the wax crystal structure should be loose so that oil can be filtered through the wax”. [6] 109. 109 Dewaxing: Solvents [1, 4] Mixture of methylethylketone (MEK) and toluene Methylisobutylketone (MIBK) Dichloromethane Trichloroethylene Propane Mixture of ethylene chloride and benzene Mixture of acetone and benzene
110. 110 Dewaxing: Solvents [1, 4] MEK displays low solvent power for paraffinic compounds (and therefore good selectivity) Toluene has excellent solvent power for base stocks The proportions in the mixture of these two solvents can be optimized.
111. 111 Dewaxing: Solvents [12] 112. 112 Simplified dewaxing PFD [4] Slack wax
113. 113 Simplified dewaxing process [4] • Contacting the solvent with oil • Crystallization in the presence of the solvent • Filtration to remove wax from dewaxed oil • Separation of the solvent from the dewaxed oil and the wax by distillation.
114. 114 Simplified dewaxing process [4] • The feedstock is mixed with the solvent such as mixture of MEK and toluene. • The mixture is heated to make a mixed of oil and solvent. • The mixture is then cooled and chilled in heat exchanger and chiller system. The chiller is usually a refrigeration system operating at propane. • The mixture of crystals, oil, and solvent flows to the rotary filters. The cakes are washed with solvent to purify the wax from oil. • The solvent is recovered in wax and dewaxed oil distillation columns.
115. 115 Rotary drum vacuum filter [13] com/?q=content/rotary-drum-filter [Accessed on: 17-Aug-2015] 116. 116 Simplified dewaxing process [10] Dewaxing process may be affected by:  Nature of the feedstock  Type of solvent  Solvent to feed ratio  Chilling temperature  Filtration process (use of filter aids, washing, etc.)  Solvent recovery method The final desired pour point
117. 117 Simplified dewaxing process [10] What is filter aid? Types of filter aids used in solvent dewaxing
118. 118 Dewaxing Case Study [12] Feed
119. 119 Clay treatment [1, 4] Clay treatment is an adsorption process which is used to remove • Colored compounds • Organic acids • Oxidizable hydrocarbons Organic nitrogen compounds importantly affect the color and color stability oil.
120. 120 Clay treatment [1, 4] Clay treatment may either be carried out by: • Contact process (mixing) • Percolation technique such as flow through a packed particle (adsorbent) bed
121. 121 Clay treatment: Contact process [10] 122. 122 Clay treatment: Contact process [1, 4] In the contact process the oil and clay are mixed, heated, agitated, and filtered. The process is affected by: clay type, clay quantity, and treating temperatures (300-700 oF) [10].
123. 123 Clay treatment: Percolation method [10] 124. 124 Percolation adsorption cycle [10] 125. 125 Clay treatment: Clays [10] Two common types of clays used are: Attapulgus and Porocel. Attapulgus is a hydrous magnesium-aluminum silicate (Fuller’s earth) while Porocel is primarily hydrated aluminum oxide (bauxite). Both the clays are activated by heat treatment. Attapulgus clay is tempered at 260-427oC with a residence time of 15-30 min. Porocel clay is tempered for 15-30 min at 371-482oC. Properties of clays
126. 126 Clay treatment: Clays [1, 4] Spent clay disposal issues may be one of the major reasons for replacing clay treatment with an increasingly popular hydrogen treatment (hydrofinishing).
127. 127 Characteristics of some commercial grade lube oils [12] 128. 128 National Refinery Limited: A lube oil refinery
129. 129 National Refinery Limited: A lube oil refinery [Accessed on: 16-Aug-2015] 130. 130 Concept of pseudocomponents A crude oil or its fraction can be divided into a number of fractions each having a narrow boiling range and called as pseudocomponent. The narrow boiling range of the crude allows one to define an average boiling point of a pseudocomponent as (IBP+EP)/2, which is then called as NBP if the TBP data is at normal pressure. Here, IBP and EP are that of a pseudocomponent. Why do we need pseudocomponents? Bring to mind the discussion on undefined and complex mixtures in the class. Read through Fahim or else.
131. 131 Concept of pseudocomponents [1] 132. 132 Concept of pseudocomponents [6] 133. 133 Concept of pseudocomponents [6] 134. 134 Concept of pseudocomponents If TBP curve is available, it is further divided into narrow boiling fractions, called pseudocomponents. For these pseudo- components, the average boiling point can be estimated as either mid-boiling point or mid-percent boiling point. The TBP curve is divided into an arbitrary number of pseudo- components or narrow boiling cuts. How will you decide about the number of pseudo-components? Since the boiling range is small, both mid points (mid-boiling point and mid-percent boiling point) are close to each other and can be considered as the MeABP for that pseudo-component.
135. 135 Concept of pseudocomponents Each pseudocomponent is characterized by an average normal boiling point, specific gravity, and molecular weight. The first two properties are obtained experimentally from the TBP curve and gravity versus volume percent curve. In some cases, only the overall specific gravity is measured. In such cases, characterization factor of whole crude or given fraction is considered as the characterization factor of each pseudocomponent and specific gravity of each is then measured. The molecular weight is usually calculated through a correlation. Other physico-chemical properties are then calculated.
136. 136 Concept of pseudocomponents [2] 137. 137 Concept of pseudocomponents Divide the TBP curve of the petroleum cut as calculated into 20 pseudo-components. Calculate the liquid volume percentage of each pseudo-component. The TBP data is shown below:
138. 138 Concept of pseudocomponents The TBP data is drawn in excel and extended by curve fitting to 100% distilled as it is 95% distilled in the TBP apparatus. When using Excel make sure you have reasonable number of significant figures. Try to have nearly 8-10 digits after decimal as shown in the figure below.
139. 139 Concept of pseudocomponents Apart from any arbitrary curve fitting (least square), say a polynomial fit, what are the other methods by which a certain TBP curve can be fitted and extended on each side?
140. 140 Concept of pseudocomponents The end point is 218.2 and IBP is –5.4 °C. The TBP data is drawn in excel and extended by curve fitting to 100% distilled as it is 95% distilled in the given problem. As there are 20 pseudocomponents, so each pseudocomponent has a temperature interval of (218.2–(–5.4))/2 or 11.2 °C. The EBP of the first component is IBP + 11.2 or 5.8 °C. The average boiling point of the first component is (–5.4+5.8)/2 or 0.2 °C. The volume percent for the first component is 2.84%. Note: If not to find directly from the curve by viewing, you may need trial-and-error solution for vol%. You can use Excel Solver (or Goal seek) for finding vol%.
141. 141 Concept of pseudo-components [2] 142. 142 Concept of pseudocomponents Calculate the specific gravity and molecular weights of each pseudocomponent of the previous example. The characterization factor is 11.94. Use following correlation for molecular weight in which Tb is the mean average boiling point.
143. 143 Use of Aspen Hysys (Oil Manager) to divide a given TBP curve and to get the cut distribution For procedural steps, please see class notes. See the book of Fahim [2]. For tutorial on Aspen HYSYS, see the following website. distillation-of-crude-oil-hysys#watch-the-tutorial For exam: You may be asked the questions about the steps, etc., to show how you can use Aspen Hysys for the above mentioned subject.
144. 144 Use of Aspen Hysys (Oil Manager) to divide a given TBP curve and to get the cut distribution For procedural steps, please see class notes. See the book of Fahim [2]. For tutorial on Aspen HYSYS, see the following website. distillation-of-crude-oil-hysys#watch-the-tutorial For exam: You may be asked the questions about the steps, etc., to show how you can use Aspen Hysys for the above mentioned subject.
145. 145 Course outline 1.Simplified overall crude oil refinery picture 2. Major refinery products and tests: Brief description 3.Separation process: Atmospheric and vacuum distillations, lube oil extraction, dewaxing, deasphalting, and clay treatment. 4.Catalysts used in refinery operations 5. Conversion processes: Description of alkylation, polymerization, isomerization of light paraffins, hydrotreating, catalytic reforming, catalytic cracking, hydrocracking, visbreaking of resids, and coking. 5.Material and energy balances for refinery processes (Aspen HYSYS applications) 6.Design guidelines for the selected refinery equipment
146. 146 Overall refinery flow [2] 147. 147 Conversion processes We will discuss the following conversion processes: • Alkylation, polymerization, and isomerization • Hydrotreating (hydrodesulfurization) • Catalytic reforming • Catalytic cracking (fluid catalytic cracking) • Hydrocracking • Coking (delayed coking) • Visbreaking
148. 148 Alkylation Alkylation in the petroleum refinery is the reaction of a low molecular weight olefin with an isoparaffin to produce a higher molecular weight isoparaffin. Usually isobutylene and propylene are used as olefins while isobutane is used as isoparaffin. Continuous mixed flow reactors are used. Hydrofluoric acid or sulfuric acid catalyst is employed. Temperature of 5 to 21 oC or lower is used for sulfuric acid process and 38oC or lower for anhydrous hydrofluoric acid process. Enough pressure is maintained to keep the hydrocarbons in the liquid state.
149. 149 Alkylation reactions
150. 150 Alkylation The volume of catalyst (acid) and liquid hydrocarbon feed are used in equal amounts and isoparaffin to olefin ratios are: 4:1 to 15:1. The major sources of olefins are catalytic cracking and coking operations. Olefins can also be produced by the dehydrogenation of paraffins and isobutane may well be cracked commercially to provide alkylation unit feed. Isobutane is produced in hydrocrackers and catalytic crackers, catalytic reformers, crude distillation, and natural gas processing. In few cases, normal butane is isomerized to produce additional isobutane for the alkylation unit.
151. 151 Polymerization Propylenes and butylenes may be polymerized to from a gasoline range product having high octane number. The product itself is an olefin having octane number of 97. Polymerization reactions are of the following type [1]:
152. 152 Polymerization The most widely used catalyst is phosphoric acid on an inert support. This can be in the form of phosphoric acid mixed with kieselguhr (a natural clay) or a film of liquid phosphoric acid on crushed quartz (a natural mineral). Feed enters the reactor at around 204 oC. The reaction is highly exothermic and temperature is required to be controlled. Reactor pressure is 3450 kPa. The polymerized product may need to be hydrogenated before it goes to gasoline pool. Why?
153. 153 Polymerization [14] 154. 154 Isomerization Benzene is toxic and therefore we do not want benzene in a gasoline. If present in gasoline, the spillage of gasoline and incomplete combustion of gasoline in an engine may add benzene to the atmosphere we breath in. For a naphtha feed, how can we reduce the concentration of benzene in a gasoline while virtually keeping the same octane rating?
155. 155 Isomerization
156. 156 Isomerization Isomerization is used to improve the octane number of the n- paraffinic feeds by converting them into isoparaffins. As an example, n-pentane has RON (research octane number) of 61.7 while isopentane has a rating of 92.3. Light straight run naphtha is used as a feed for the isomerization and the following reactor conditions are maintained. Reaction temperature = 95–205 oC Reaction pressure = 1725–3450 kPa. Hydrogen to hydrocarbon molar ratio = 0.05:1. Catalyst is usually platinum supported on an acidic catalyst such as zeolite or chlorinated metal oxide support.
157. 157 Isomerization o At elevated temperatures cracking reactions become important and cracked products may increase exponentially and the isomerized products thus become intermediates, i.e., first increase and then decrease with increase in temperature. o A high hydrogen pressure can lead to reduced coke formation. o A balance between metal and acidic function may produce better iso/normal ratios.
158. 158 Isomerization
159. 159 Isomerization Hydrogen is added to the feed to avoid the deposition of coke on the catalyst surface and consumption of hydrogen is negligible. Due to hydrogen presence in the feed, the process is frequently called as hydroisomerization. A typical feed and product composition of an isomerization unit is given below [1]:
160. 160 Isomerization How can you work out the reaction equilibrium constant and equilibrium conversion of a reaction, if experimental values are lacking? o Using Gibbs free energy of formation and equilibrium constant o Using Gibbs free energy minimization
161. 161 Equilibrium conversion of n-pentane Let’s workout equilibrium conversions and product distribution of n-butane, n- pentane, and n-hexane. See class notes.
162. 162 Equilibrium composition of isopentane
163. 163 n-hexane equilibrium conversion Workout equilibrium conversions for n- hexane isomerization
164. 164 Isomerization AspenHysys can be used to workout equilibrium conversions whether the stoichiometry is known (using equilibrium reactor) or not known (using Gibbs reactor, a Gibbs free energy minimization problem). Standard thermodynamics books give strategy for solving such problems. Let’s proceed to AspenHysys.
165. 165 Mechanistic steps of isomerization process in the presence of a bifunctional catalyst See also slides regarding cracking and hydrocracking.
166. 166 Isomerization Some refineries do not have hydrocracking facility to supply isobutane for alkylation unit. The required isobutane can be obtained from n-butane using isomerization.
167. 167 Isomerization Develop process flow diagram (PFD) for the hydroisomerization process. (Class activity)
168. 168 Simplified PFD of a once-through (O-T) isomerization process [5] 169. 169 Typical performance of O-T process [5] 170. 170 Typical performance of O-T process (Continued …..) [5] For detail of isomerization process, see ref [5] 171. 171 Isomerization plant at PRL
172. 172 Hydrotreating Hydrotreating is the removal of impurities from a petroleum fraction. “Hydrotreating refers to a relatively mild operation whose primary purpose is to saturate olefins and/or reduce the sulfur and/or nitrogen content (and not to change the boiling range) of the feed” [1]. The process is used to stabilize (converting unsaturated hydrocarbons such as olefins and diolefins to paraffins) a petroleum fraction and to remove sulfur, nitrogen, oxygen, halides, and trace metals from the petroleum fraction. Hydrotreating is applied to a wide variety of feedstocks. Examples may include: • Naphtha • Kerosene • Diesel (Gas oil) • Atmospheric resids
173. 173 Hydrotreating [2] 174. 174 Hydrotreating The process is generally carried out at moderately high temperature and pressure and in the presence of a catalyst. Typical temperature and pressure are 270–340oC and 690–20700 kPag respectively [1]. Nickel/Cobalt and molybdenum oxides on alumina are widely used catalysts.
175. 175 Hydrotreating catalysts [2] “In the case of a guard reactor, which is used to protect the main catalyst from metal deposition, catalysts with wide pores are chosen and are generally plugged by metal deposition” [Fahim, 2010].
176. 176 Hydrotreating reactions [2] 177. 177 Hydrotreating reactions [2] 178. 178 Hydrotreating (hydrodesulfurization) [1] 179. 179 Hydrotreating process parameters [2] 180. 180 Thermodynamics of hydrotreating Hydrotreating reaction are reversible and highly exothermic. “The logarithmic equilibrium constants for several hydrodesulfurization reactions are positive (Gibbs free energy change is negative), indicating that the reaction can virtually proceed to completion if hydrogen is present in the stoichiometric quantity” [Fahim, 2010]. “Although equilibrium conversions decrease with temperature rise, commercial hydrotreating is run at rather high temperatures to promote the kinetics of these reactions ” [Fahim, 2010].
181. 181 How can you find heat of reaction? Thermodynamics of hydrotreating [2] Heats of reactions of some hydrotreating reactions:
182. 182 Thermodynamics of hydrotreating [2] 183. 183 Thermodynamics of hydrotreating Olefin hydrogenation reaction usually goes to completion. “An aromatic hydrogenation reaction can reach equilibrium under certain conditions, as in kerosene and gas oil hydrogenation” [2]. “ Hydrodesulfurization can be carried out easier than denitrogenation, while aromatic saturation is the most difficult reaction” [2].
184. 184 Kinetics of hydrotreating If the rate of a hydrotreating reaction follows nth order mechanism: The n = 1 is obtained for the narrow cuts (naphtha and kerosene) [2]. n >1 is for gas oil and n = 2.0 for vacuum gas oil (VGO) or residue [2].
185. 185 Kinetics of hydrotreating [2] 186. 186 Makeup hydrogen for a hydrotreating process [2] 187. 187 Hydrotreating correlations: Naphtha and gas oil correlations [2] 188. 188 Hydrotreating correlations: Naphtha and gas oil correlations [2] 189. 189 Naphtha hydrodesulfurization The hydrotreating process that is specifically employed for the removal of sulfur is called hydrodesulfurization. The principle impurities to be removed in the naphtha are: • Sulfur • Nitrogen • Oxygen • Olefins • Metals
190. 190 Sulfur impurities The sulfur containing compounds are mainly: • Mercaptans • Sulfides • Disulfiudes • Polysulfides • Thiophenes. The thiophenes are more difficult to eliminate than most other types.
191. 191 Example desulphurization reactions [3] 192. 192 Desulphurization flow diagram [3] 193. 193 Desulphurization operating conditions
194. 194 Desulphurization reactor [3] 195. 195 Desulphurization results [3] 196. 196 Desulphurization yields [3] 197. 197 Simulation of a hydrotreating process that is accompanied by cracking reaction [2] 198. 198 Simulation of a hydrotreating process [2] 199. 199 Simulation of hydrotreating of ARDS: Solution [2] 200. 200 Simulation of hydrotreating of ARDS: Solution [2] 201. 201 Catalytic reforming The objective of catalytic reforming is to convert hydrocarbons of lower octane number to higher octane number gasoline (blend). The fact that isoparaffins and aromatics have high octane numbers is the basis of catalytic reforming process.
202. 202 Octane numbers of various pure compounds [2] 203. 203 Octane numbers of various pure compounds [2] 204. 204 Catalytic reforming Several technologies are developed by various commercial organizations. Platforming by UOP Powerforming by Exxon Magna forming by Engelhard Catalytic eforming by IFP Rheniforming by Chevron Ultraforming by Amoco. 345–2415 kPa 3–8 mol H2/mol feed Reactor inlet: 925–975oF
205. 205 Catalytic reforming The feed, generally (C7-C10) to the catalytic reformer is straight run naphtha or hydrocracker naphtha or coker naphtha that may have an IBP 82 oC and final boiling point of 190 oC. The feed is the hydrotreated naphtha (through a hydrotreater) to remove sulfur, nitrogen, and other impurities which can poison the reforming catalyst.
206. 206 Catalytic reforming The feed contains four major groups of hydrocarbons: Paraffins, Olefins, Naphthenes, and Aromatics (PONA). What is the source of olefins?
207. 207 Catalytic reforming Typical feed and product PONA composition in vol% is:
208. 208 Catalytic reforming catalyst The catalyst for the reforming process is a bifunctional catalyst Pt/Al2O3 with Pt content usually of 0.2 to 0.6% by weight. A bimetallic catalyst using rhenium, tin, or iridium is usually required. For example, rhenium is added along with Pt for stabilization. The surface area of a typical γ-alumina support is 210 m2/g.
209. 209 Catalytic reforming reactions There are four major reactions involved in catalytic reforming: • Dehydrogenation of naphthenes to aromatics • Dehydrocyclization of paraffins to aromatics • Isomerization • Hydrocracking The product contains aromatics and isoparaffins.
210. 210 Reforming reactions [Parkash, 2003] 16 4
211. 211 Reforming reactions [Parkash, 2003] 212. 212 An example reforming process [2] 213. 213 Reforming thermodynamics and kinetics [2] 214. 214 Reforming thermodynamics and kinetics [5] 215. 215 Effect of pressure A reduction in pressure leads to increased production of hydrogen and reformate yield and therefore the required temperature for a quality product is decreased, however, coking rate is increased. Why is CCR (to be discussed later) useful over fixed bed operation?
216. 216 An example reforming process [2] 217. 217 Catalytic reforming: Coke deposition Due to the coke deposited on the active surface, the catalyst is required to be regenerated. The catalyst is regenerated by burning off the coke from the catalyst surface. This is called regeneration of the catalyst. Catalyst regeneration is an exothermic reaction. A good control is desired.
218. 218 Reforming processes Depending upon the catalyst regeneration processing, reforming processes may be: • Semi-regenerative • Cyclic • Continuous 95% of new installations are continuous [5].
219. 219 Reforming processes Semi-generative process requires shutdown of the process and then regeneration of the catalyst. Cyclic process is a modification of the semi- regenerative process in which an extra reactor is installed and regeneration is carried out in parallel so that the shut down is avoided. Initial cost is higher. Continuous regeneration is licensed by UOP under the name UOP CCR platforming. The catalyst flows (in a moving bed fashion) from one reactor to the other under gravity and continuously regenerated in the regenerator. Initial cost is higher. Rather frequent shutdown is avoided so more production.
220. 220 Semi-regenerative catalytic reforming [1] 221. 221 Why hydrogen is required in the feed? Remember a lot of hydrogen is produced in catalytic reforming and reformer is a principal source of hydrogen in a refinery. Catalytic reforming
222. 222 Catalytic reforming is highly endothermic reaction that is why it requires interstage heating. Catalytic reforming
223. 223 Continuous catalyst regeneration (CCR) [2] 224. 224 Continuous catalytic reforming (CCR) [2] 225. 225 Reforming correlations [2] 226. 226 Example [2, p. 108] 227. 227 Example [2, p. 108] 228. 228 Thermodynamics of reforming [2] 229. 229 Thermodynamics of reforming [2] 230. 230 Catalytic reforming: Simulation Simulation of catalytic reforming using a few model components. Go to Fahim et al., 2010 [2] Chapter No. 5 and see Example 5.5 for more rigorous simulation of a reforming unit.
231. 231 Catalytic cracking Catalytic cracking is used to convert heavy fractions such as vacuum gas oil to gasoline range and lighter products. Catalytic cracking has advantages over thermal cracking as more gasoline having higher octane rating and less heavy fuel oils and light gases are produced. Also light gases contain more olefins.
232. 232 Catalytic cracking Cracking is generally carried out in either a 1. Moving bed reactor, or 2. Fluidized bed reactor The fluid catalytic cracking (FCC) is a fluidized bed cracking system and is widely acceptable more than moving bed reactor system.
233. 233 Catalytic cracking The role of catalytic cracking is to take heavy desulfurized feedstock and crack it into lighter, mainly high octane gasoline. The FCC also produces olefins and LPG.
234. 234 Catalytic cracking Feed to FCC: The gas oil from vacuum column and conversion processes boiling between 316 oC and 566 oC is used [2]. The gas oil can be considered a mixture of aromatics, naphthenic, and paraffinic hydrocarbons. Atmospheric residue and vacuum residue may also be used. FCC products: Gasoline and lighter products, i.e., FCC is used to produce gasoline, olefins, and LPG. Olefins produced can be used for alkylation and polymerization.
235. 235 Catalytic cracking reactions
236. 236 Catalytic cracking thermodynamics [2] Overall catalytic cracking is quite endothermic and the heat of reaction is provided by combustion of coke in the regenerator.
237. 237 Catalytic cracking catalyst The FCC catalyst is a powder with an average particle size of 75 μm. It is zeolite (crystalline aluminosilicates) type catalyst. Y-zeolite and ZSM-5 zeolite are common in catalytic cracking.
238. 238 Catalytic cracking catalyst Zeolites are aluminosilicate crystalline structures. They consist of SiO4 and AlO4 ‒ tetrahedra that are linked through common oxygen atoms and give a three dimensional network. In the interior of the structure there are water molecules and mobile alkali metal ions such as of sodium that are capable of being ion exchanged with other cations such as that of Pt.
239. 239 Sodalite unit (β-cage) [15] 240. 240 Zeolite Y structure [15] Y-zeolite contains a faujasite structure and has a large pore size and allows large size molecules.
241. 241 Zeolite ZSM-5 In the cracking of long chain paraffins, ZSM-5 is used. ZSM-5 allows normal alkane such as n-heptane and isomerizes to improve octane rating. Branched and cyclic compounds are not allowed to enter and hence are not cracked.
242. 242 Zeolite ZSM structure: Pentasil unit [15] 243. 243 Shape selectivity of ZSM-5 [2] 244. 244 Fluid catalytic cracking The FCC unit mainly depends on circulating a zeolite catalyst with the vapor of the feed into a riser-reactor for a few seconds. The cracked products are disengaged from the solids and taken out to a distillation column for the separation of the desired products. The catalyst is circulated back into the regenerator where coke is burned and the catalyst is regenerated. The combustion of the coke generates the heat required to carry out the generally endothermic reaction in the riser.
245. 245 Fluid catalytic cracking Two basic types of FCC units are used today. 1. Side-by-side, 2. Stacked
246. 246 Fluid catalytic cracking: Operating conditions “The high volume of products caused by the cracking of larger molecules requires low operating pressure (1-5 bar). The high endothermic nature of cracking reactions requires that the reactor operates at high temperatures 480-550 oC” [2] 247. 247 Fluid catalytic cracking Reactor and regenerator operating conditions
248. 248 Fluid catalytic cracking (FCC) [1] 249. 249 Fluid catalytic cracking (FCC) [1] 250. 250 Catalytic cracking: Simulation Go to Fahim et al., 2010 Chapter No. 8 and see Example 8.5 of simulation of a catalytic cracking unit.
251. 251 Hydrocracking Hydrocracking is a catalytic hydrogenation process in which high molecular weight feedstocks are converted and hydrogenated to lower molecular weight products. Cracking will break bonds, and the resulting unsaturated products are consequently hydrogenated into stable compounds. Hydrogenation also removes impurities such as sulfur, nitrogen, and metals in the feed.
252. 252 Hydrocracking It is mainly used to produce kerosene and diesel range products. Vacuum gas oil is the feedstock for hydrocrackers, however a variety of feedstocks may be applied and variety of products may be obtained. The catalyst used in hydrocracking is a bifunctional catalyst.
253. 253 Hydrocracking reactions Hydrocracking involves hydrotreating and cracking (hydrocracking) reactions. The hydrocracking reactions produce lower molecular weight products. An example hydrocracking reaction is The reactions such hydrodealkylation, aromatics saturation, dehydrocyclization, etc., also occur during the coarse of hydrocracking.
254. 254 Heat of reactions of hydrocracking reactions [2] 255. 255 Hydrocracking reaction mechanism
256. 256 Hydrocracking operating conditions The hydrogen partial pressure ranges between 85-170 bar and reactor temperature 300-450 oC.
257. 257 Hydrocracking catalyst The catalyst used in hydrocracking is a bifunctional or dual function catalyst. It is composed of a metallic part, which promotes hydrogenation (and dehydrogenation), and an acid part (support), which promotes hydrocracking, hydroisomerization, and dehydrocyclization reactions.
258. 258 Hydrocracking catalyst [2] 259. 259 Hydrocracking catalyst [2] 260. 260 Hydrocracking catalyst The acidic function is provided by the support which is usually a amorphous oxide such as silica-alumina, zeolite. Silica-alumina is support is common. The metal function for hydrocracking may be obtained from a combination of Group VIA (Mo, W) and Group VIIIA (Co, Ni) metal sulfides.
261. 261 A single-stage with recycle hydrocracking process [2] 262. 262 Typical hydrocracking reactor configuration [6] 263. 263 Thermal cracking processes 1. Coking 2. Visbreaking
264. 264 Coking Coking is a severe thermal cracking process which is carried out to convert heavy fractions into solid coke and lighter hydrocarbons.
265. 265 Coking Coking is typically employed for treating mostly vacuum residues to prepare feed for the catalytic cracking/hydrocracking units and thereby decreasing the amount of low priced heavy fuel oils. “The deposited coke contains most of the asphaltenes, sulfur, and metals present in the feed, and the products are unsaturated gases (olefins) and highly aromatic liquids.” [2] 266. 266 Applications of petroleum coke Coke may be used • as a fuel • to make anodes for the electrolytic cells • to manufacture graphite, and • in the production of chemicals. Petroleum coke is different from coke from coal and not suitable for metallurgical processing.
267. 267 Coking The feed for the coking process is usually vacuum residue, however, FCC residue and visbreaker residue may also be subjected to a coking process. The products of a coker are typically light gases (e.g. unsaturarted C1-C4 and i-butane), coker naphtha (may be sent reforming or sent directly to gasoline blending unit), light and heavy gas oils. Light gas oil may be blended with kerosene [2].
268. 268 Coke formation
269. 269 Dehydrogenation of methylcyclohexane leading to coke formation
270. 270 Coking processes Delayed coking is the most widely used process to carry out a coking operation. A high velocity of oil in the furnace tubes prevents significant coking in the furnace even above the coking conditions. Sufficient time is then given in the coke drums for coking reactions to take place. That is why the process is called delayed coking. Fluid coking is a continuous process compared to delayed coking. Flexicoking also uses fluid coking process but a gasification reactor is also there.
271. 271 Delayed coking Steam is injected in the furnace to inhibit the coke formation. The feed is heated higher than 900 oF (482 oC) and a vapor-liquid mixture leaving the furnace flows to the coking drum. The coking reactions take place in the drum and the vapors issuing from the coking drum are quenched in the fractionator by the liquid feed to prevent further coking. Moreover, it simultaneously condense a part of heavy ends which are then recycled.
272. 272 Delayed coking: Effect of pressure “Increasing pressure will increase coke formation and slightly increase gas yield. However, refinery economics require operating at minimum coke formation. New units are built to work at 1 bar gauge (15 psig), while existing units work at 2.4 bar gauge (35 psig). In a case of production of needle coke, a pressure of 150 psig is required”. [Fahim] 273. 273 Delayed coking [1] 274. 274 Delayed coking When a coke drum is filled, the heater output is switched to the second drum and the coke is collected. Usually two coke drums are employed however, four drums are also used.
275. 275 Visbreaking Visbreaking is a mild thermal cracking carried out to reduce the viscosities and pour points of atmospheric and vacuum tower bottoms. The objective is to produce a fuel oil stock of improved viscosity characteristics. The visbreaking product yields are dependent upon the reaction temperature and the residence time.
276. 276 Visbreaking: Typical product yields [2] The products of visbreaking include light gases, naphtha (gasoline), gas oil, and residue (main product).
277. 277 Visbreaking: Kuwait oil product yields [2] 278. 278 Visbreaking types Visbreaking is either coil breaking when the resid is broken in the furnace coil and soaker breaking when soaked in the reactor for few minutes.
279. 279 Visbreaking [2] 280. 280 Coil visbreaking The feed residue feedstock is heated and then mildly cracked in the visbreaker furnace. Reaction temperature range from 850 to 900 oF and operating pressures vary from as low as 3 bar to as high as 10 bar. Coil furnace visbreaking is used and the visbroken products are immediately quenched to stop the cracking reaction. The quenching step is essential to prevent coking in the fractionation tower. The gas oil and the visbreaker residue are the most commonly used as quenching streams.
281. 281 Comparison of coil and soaker visbreakings “Coil cracking is described as a high temperature, short residence time route whereas soaker cracking is a low temperature, long residence time route. The yields achieved by both options are in principle the same, as are also the properties of the products. Both process configurations have their advantages and applications. Coil cracking yields a slightly more stable visbreaker products, which are important for some feedstocks and applications. It is generally more flexible and allows the production of heavy cuts, boiling in the vacuum gas oil range. Soaker cracking usually requires less capital investment, consumes less fuel and has longer on-stream times.” [2] 282. 282 Comparison of visbreaking and delayed coking [2] 283. 283 Temperature and pressure conditions of various conversion processes
284. 284 References [1] Gary, J.H.; Handwerk, G.E. 2001. Petroleum refining: Technology and economics. 4th ed. Marcel Dekker, Inc. [2] Fahim, M.A.; AlSahhaf, T.A.; Elkilani, A. 2010. Fundamentals of petroleum refining. Elsevier. [3] Parkash, S. 2003. Refining processes handbook. Gulf professional publishing, Elsevier. Singapore. [4] Wauquier, J.-P. (ed.). 1998. Petroleum refining: Separation processes. Vol. 2. Technip. [5] Myers, R.A. 2004. Handbook of petroleum refining processes. 3rd ed. McGraw-Hill. [6] Stan Jones, D.S.J.; Pujadó, P.R. 2006. Handbook of petroleum processing. Springer. [7] Seader, J.D.; Henley, E.J.; Roper, D.K. 2011. Separation process principles: Chemical and biochemical operations. 3rd ed., John Wiley & Sons, Inc. [8] Usman, M.R. 2015. Comprehensive dictionary of chemical engineering. Lulu Publishing. [9] Speight, J.G.; Ozum, B. 2002. Petroleum refining processes. Marcel Dekker, Inc. [10] Sequeiri, A. 1994. Lubricating base oil and wax processing. Marcel Dekker. [11] Refining Processes Handbook. Hydrocarbon Processing. [12] Parakash, S. 2010. Petroleum Fuels Manufacturing Handbook: Including specialty products and sustainable manufacturing techniques. McGraw-Hill. New York. [13] Brown, G.G.; Foust, A.S.; Katz, D.L.V.; Schneidewind, R.; White, R.R.; Wood, W.P.; Brown, G.M.; Brownell, L.E.; Martin, J.J.; Williams, G.B.; Banchero, J.T. 1950. Unit operations. John Wiley & Sons. [14] Ancheyta, [15] Hagen, J. 2006. Industrial catalysis. Wiley-VCH Verlag GmbH & Co. KGaA.