The Chemistry and Processing of Hydrocarbons PDF

The Chemistry and Processing of Hydrocarbons Energy Masters Section 4 Prof. K.R. Thampi, Hydrocarbon Processing, Masters in Energy 105 Two important refinery processes: 1. Hydro-cracking 2. Naphtha reforming* * Not to confuse with steam-reforming Prof. K.R. Thampi, Hydrocarbon Processing, Maste...

The Chemistry and Processing of Hydrocarbons Energy Masters Section 4 Prof. K.R. Thampi, Hydrocarbon Processing, Masters in Energy 105 Two important refinery processes: 1. Hydro-cracking 2. Naphtha reforming* * Not to confuse with steam-reforming Prof. K.R. Thampi, Hydrocarbon Processing, Masters in Energy 106 1. Hydrocracking This is cracking and hydrogenation combined into a single process. Low value gas oil containing a high % of polynuclear aromatics is simultaneously cracked and hydrogenated to get high-value low and middle distillates including gasoline and diesel fuel. The catalyst MUST be bi-functional: - acid sites to catalyse cracking reactions - metal sites to catalyse hydrogenation Prof. K.R. Thampi, Hydrocarbon Processing, Masters in Energy 107 Advantages 1) This process can be designed to crack polyaromatics selectively to gasoline, diesel fuel or jet fuel. In contrast, FCC is not as selective as hydrocracking. 2) Processed in higher pressures over a much broader temperature range (290 - 525 °C) than in FCC. 3) Possible to hydrorefine heavier cuts from crude distillation. 4) It increases alkane content in aromatic rich fuels improves the Cetane number (a measure of combustion efficiency) of diesel fuel. Prof. K.R. Thampi, Hydrocarbon Processing, Masters in Energy 108 Typical Hydrocracking Reactions Prof. K.R. Thampi, Hydrocarbon Processing, Masters in Energy 109 Disadvantages The high pressure of H2 (80 - 200 atm) causes high consumption of H2 and energy. High pressures are required to crack heavier feeds and avoid coking. Mild hydrotreating at low pressures is still favoured for removing metals, sulfur and nitrogen compounds normally found in heavier oil feeds. This is often done in combination with HDM/HDS on Co-Mo or Ni-W catalysts. Prof. K.R. Thampi, Hydrocarbon Processing, Masters in Energy 110 Catalysts Selection of catalysts depend on the nature of feed and the desired selectivity to product composition. For producing lubricants, diesel and middle or heavy distillates: Base metal oxides of Co, Mo, Ni or W supported on either acid-treated Al2O3, Al2O3-SiO2 or a zeolite. For producing gasoline from S and N-free feeds: Pt or Pd supported on shape-selective zeolites (Faujasite : 0.74nm; 0.78nm for modernite; 0.55nm for ZSM-5) Prof. K.R. Thampi, Hydrocarbon Processing, Masters in Energy 111 Process Dual stage fixed bed reactors at 375 - 425 °C and 100 - 170 atm. LHSV (liquid hourly space velocity) = 0.5 - 2 h-1. High conversion causes large heat releases from hydrogenation reactions. Hence, product stream is cooled by injection of cold high pressure H2 between the two fixed bed reactors. Catalysts survive for 1-3 years. Replacement by fresh or re-generated catalysts. Prof. K.R. Thampi, Hydrocarbon Processing, Masters in Energy 112 Hydrocracking process scheme Prof. K.R. Thampi, Hydrocarbon Processing, Masters in Energy 113 2. Naphtha reforming Primarily done for: 1) Increasing octane number of naphtha cut boiling at 70 - 200 °C (C5 - C10) 2) As a source of aromatics for petrochemicals (eg., polyester) Prof. K.R. Thampi, Hydrocarbon Processing, Masters in Energy 114 Octane numbers of pure HC Prof. K.R. Thampi, Hydrocarbon Processing, Masters in Energy 115 Composition of a naphtha feed for BTX* production * BTX = Benzene, Toluene, Xylene Prof. K.R. Thampi, Hydrocarbon Processing, Masters in Energy 116 Thermodynamic data Major reforming reactions are endothermic. Prof. K.R. Thampi, Hydrocarbon Processing, Masters in Energy 117 Octane number vs C number for important groups in reforming Prof. K.R. Thampi, Hydrocarbon Processing, Masters in Energy 118 Prof. K.R. Thampi, Hydrocarbon Processing, Masters in Energy 119 Catalyst Typically, Pt or Pt-Re supported on g-Al2O3 (BET area = 200 m2/g) Pt - Re (0.35% Pt - 0.5% Re) g-Al2O3 is acidic. This is further improved by adding Cl- to it. Acididc function helps isomerisation reactions. Pt or Pt-Re helps hydrogenation and de-hydrogenation functions. Re is pre-sulfided using 10-20ppm H2S to improve the catalyst life. It breaks up Pt particles into smaller ensembles and prevent coking. Extrudates or pellets (1.5 - 6 mm dia) Prof. K.R. Thampi, Hydrocarbon Processing, Masters in Energy 120 Process - 1 Pre-treatment: hydrotreat to remove thiophene, pyridines, phenols Design should accommodate a variety of reactions with different relative activites and equilibrium limitations. Therefore, a series of separate reactors are used: The front end is preferentially dehydrogenation (endothermic and favours high operating temperatures). H2 is added in large excess to prevent excessive de-hydrogenation to coke). Prof. K.R. Thampi, Hydrocarbon Processing, Masters in Energy 121 Process - 2 Reforming processes are of three types: 1) Semi-regenerative (small units) - periodic shut down and catalyst regeneration 2) Cyclic (fully regenerative; large scale units) - swinging only one of 4-6 reactors off-line for catalyst regeneration 3) Continuously regenerative (moving bed) - small quantities of catalysts are withdrawn continuously, regenerated and returned to the top of the reactor system, which consists of 4-5 reactors stacked vertically. Prof. K.R. Thampi, Hydrocarbon Processing, Masters in Energy 122 Process - 3 1) Entry: 500 °C, 14-17 atm., LHSV = 1-3 h-1. 2) Reactor 1: 5% of the total catalyst, naphthenes to aromatics, high space velocity, kinetics easy, endothermic, exit cools to 420 °C. 3) Reactor 2: prior reheating to 500 °C, 15% of the total catalyst; slower kinetics, dehydrogenation + isomerization; Texit = 450 °C 4) Reactors 3 - 4: prior reheating to 500 °C, 20 & 60% bed volume; (smaller LHSV); dehydrocylisation. Overall process produces H2, a part of which is recycled to keep Prof. K.R. Thampi, Hydrocarbon 123 coke formation to a minimum. Processing, Masters in Energy Semi-regenerative reforming process Prof. K.R. Thampi, Hydrocarbon Processing, Masters in Energy 124 Cyclic regenerative reforming Prof. K.R. Thampi, Hydrocarbon Processing, Masters in Energy 125 Exercise 4 1. Compare briefly thermal cracking, FCC and hydrocracking. 2. What are the roles of H2 in hydrocracking and petroleum reforming? 3. What are the bi-functional characters of a (a) hydrocracking catalyst, (b) reforming catalyst? 4. What are the modes of deactivation of FCC, hydrocracking and Pt-reforming catalysts? 5. List important points in the re-activation of these catalysts. Prof. K.R. Thampi, Hydrocarbon Processing, Masters in Energy 126 Exercise 4 6. Why is hydrocracking activity greatly decreased by the presence of quinoline in higher amounts? How could the hydrocracking process be designed to minimise this effect? 7. Why acid sites and metal sites are provided for performing reforming reactions? 8. What are the main purposes of hydrocracking and catalytic reforming? 9. What is the role of Re in Pt-Re/Al2O3 catalyst used for reforming process? Prof. K.R. Thampi, Hydrocarbon Processing, Masters in Energy 127 Exercise 4 10. A refinery received a gas oil supply containing high amounts of metals, sulfur and nitrogen compounds. The refinery wants to maximise the production of gasoline and minimise the formation of light gases. As a refinery manager, what will you do? Prof. K.R. Thampi, Hydrocarbon Processing, Masters in Energy 128