The Chemistry and Processing of Hydrocarbons PDF

The Chemistry and Processing of Hydrocarbons Energy Masters Course Section 3 Prof.K.R. Thampi, Hydrocarbon Processing, Masters in Energy 71 An eye-view of important processes in crude oil processing Important catalytic processes are boxed or circled; rectangles denote base-metal catalysts; ovals...

The Chemistry and Processing of Hydrocarbons Energy Masters Course Section 3 Prof.K.R. Thampi, Hydrocarbon Processing, Masters in Energy 71 An eye-view of important processes in crude oil processing Important catalytic processes are boxed or circled; rectangles denote base-metal catalysts; ovals denote noble metal catalysts Prof.K.R. Thampi, Hydrocarbon Processing, Masters in Energy 72 Cracking • Cracking is the breaking down of large petroleum molecules into smaller hydrocarbons, primarily in the gasoline range. • Cracking can be performed both catalytically and non-catalytically. The catalysts can decrease the severity of reaction conditions, increases selectivity and yield of desired products. • Innovative shift from SiO2-Al2O3 catalysts to modern zeolite catalysts enforced the redesign of cracking process practiced 5-6 decades ago. Prof.K.R. Thampi, Hydrocarbon Processing, Masters in Energy 73 Design Changes • Instead of a large fluidized bed, the cracker is now a small tube. Catalyst particles are conveyed through it by rapidly flowing oil vapours, which stay in contact with the catalyst only about 5 s. • Cracking chemistry is well understood. Prof.K.R. Thampi, Hydrocarbon Processing, Masters in Energy 74 Importance of FCC (fluidized catalytic cracking) • • • • • • • Thermal cracking possibility was recognized in 1913. Cracking to get high octane fuels: 1928 Commercial (cyclic) fixed bed plant: 1936 Continuous fluid-bed cracking:1942 Zeolite based modern process:1962 ZSM-5 octane enhancer: 1986 Now, >1600 tons/day of FCC catalyst is consumed to process > 14 million/barrels of gas oil (21% of refinery capacity) Prof.K.R. Thampi, Hydrocarbon Processing, Masters in Energy 75 Reactions •O = gas oil; G = gasoline; •X = undesired products (light over-cracked products) Prof.K.R. Thampi, Hydrocarbon Processing, Masters in Energy 76 Principle Reactions in FCC Prof.K.R. Thampi, Hydrocarbon Processing, Masters in Energy 77 Typical reactions in FCC Prof.K.R. Thampi, Hydrocarbon Processing, Masters in Energy 78 At equilibrium…… • The main cracking reactions are not limited by equilibrium under industrial conditions. • At equilibrium, HC would totally degrade to graphite and H2 • Isomerizations, alkyl group rearrangement and dealkylation of aromatics go to a moderate extent. • Paraffin-olefin alkylation, aromatic hydrogenation and olefin polymerization (except ethylene polymerization) go little. Prof.K.R. Thampi, Hydrocarbon Processing, Masters in Energy 79 Energetics • Cracking reactions are much endothermic. • Isomerizations have very small heats of reaction • H-transfer reactions are exothermic • In cracking process, the endothermic reactions predominate • Magnitude of heat effect depends on the feedstock, catalyst and reactor conditions Prof.K.R. Thampi, Hydrocarbon Processing, Masters in Energy 80 Prof.K.R. Thampi, Hydrocarbon Processing, Masters in Energy 81 Cracking:thermal or catalytic? High yields of ethylene indicates thermal cracking, whereas high yields of propylene indicates catalytic cracking. Prof.K.R. Thampi, Hydrocarbon Processing, Masters in Energy 82 FCC Catalyst deactivation Because of the stability of polynuclear aromatic carbonium ions, it can continue to grow on catalyst surface before a termination reaction occurs. Cyclisation, aromatisation and polyarenes formation can go up to coke and tar formation. This deactivate the FCC catalyst. Coke may be removed in a fluidised bed regenerator. Prof.K.R. Thampi, Hydrocarbon Processing, Masters in Energy 83 FCC reactor variables Single stage cracking: conversion to gas and coke high product flexibility low Two-stage cracking: low gas and coke, better product flexibility First stage: Riser reactor, short residence time, High T Separator: gas and gasoline products removal before stage II. Second stage: Fluidised bed reactor, low temperature. Regenerator: T= 650-760 °C, P = 3 atm., coke to be decreased from 2-5% to <0.1%. FCC catalyst is returned back to cracker along with fresh catalyst (< 1%/day). Prof.K.R. Thampi, Hydrocarbon Processing, Masters in Energy 84 FCC process flow diagram Prof.K.R. Thampi, Hydrocarbon Processing, Masters in Energy 85 Riser catalytic-cracking unit Prof.K.R. Thampi, Hydrocarbon Processing, Masters in Energy 86 FCC reaction sequence Reactor entrance: initiation, alkene desorption, isomerisation (leading mainly to C3 and C4 alkenes) Middle of the reactor: surface coverage of carbonium ions increases (H- transfer and oligomerisation becomes predominant; selectivity of alkene decreases, but that of alkane increases for a given C number) Reactor exit: C3=, iC4= and iC5 increased C4 products decreased Prof.K.R. Thampi, Hydrocarbon Processing, Masters in Energy 87 Operational Flow diagram of the CC process Prof.K.R. Thampi, Hydrocarbon Processing, Masters in Energy 88 Operating CC The refinery engineer should know: Quantitative comparisons of catalytic activity, selectivity and deactivation rate for various FCC catalysts to select 1) Best catalyst 2) Optimise the octane number 3) Model the effects of different catalysts using standard process models Prof.K.R. Thampi, Hydrocarbon Processing, Masters in Energy 89 FCC’s strategic importance FCC catalyst consumption per day (mostly for replenishment) : Processing per day : > 15 million barrels As a % of total refinery capacity : 21% Approx. Number of Refineries : 740 Prof.K.R. Thampi, Hydrocarbon Processing, Masters in Energy 1500 tons 90 Methanol Synthesis • • • • • Methanol is one of the top 10 most important sythetic organic chemicals. Methanol is a raw material for formaldehyde (40-50% of methanol production), chloromethanes, amines, acetic acid, methyl methacrylate and methyl-t-butyl ether (MTBE) manufacture. It is also a solvent. Until early 1900s, methanol was produced by destructive distillation of wood. In 1923, BASF developed catalytic methanol production using Zn/Cr2O3 catalyst (300-400°C, 300 bars) In 1966, ICI developed a better process (Cu/ZnO/Al2O3 catalysts, 220-300°C, 50-100 bars) Prof.K.R. Thampi, Hydrocarbon Processing, Masters in Energy ICI process advantages • Reduced compression power • Longer catalyst life • Larger capacity, single train converter designs • Productivity increase from 770 to 1120 tons of methanol per million M3 of NG • Globally > 30 million MT/year production • 50-2500 MT/day size plants • As MTBE is being phased out methanol demand is growing downwards from 1998. Prof.K.R. Thampi, Hydrocarbon Processing, Masters in Energy A combination of 2 exothermic equilibriums gives methanol • CO + H2O Þ H2 + CO2, DH298K = -41.2 kJ/mol • CO2 + 3H2 Þ CH3OH + H2O, DH298K = -49.5 kJ/mol • CO + 2H2 Þ CH3OH; • DH298K = -90.6 kJ/mol; DH600K = -100.5 kJ/mol • Note that in the final equation, CO2 is not shown. This has certain significance in the study of methanol synthesis. Reaction occurs at near equilibrium Conversion increases with decreasing temperature and increasing pressure • • Prof.K.R. Thampi, Hydrocarbon Processing, Masters in Energy Higher alcohol synthesis Because of their potential use as additives to gasoline and as chemical feedstocks, the synthesis of higher alcohols from NG via syngas is a promising technology. Na promoted Zn-Cr catalysts; 350-420°C, 1216 bars, GHSV 3000 - 15’000 h-1 Product composition: 68-72% methanol, 2-3% ethanol, 3-5% C3, 10-15% C4 and 7-12% C5 alcohols. Prof.K.R. Thampi, Hydrocarbon Processing, Masters in Energy F-T synthesis • Production of liquid HC from syn-gas • Possibility to use NG, biomass and coal • Enormous potential for this technology as we face dwindling oil reserves • Seen as a gas-to-liquid (GTL) technology suitable to beneficially use under-utilized or flared NG to a premium quality S-free diesel fuel. Prof.K.R. Thampi, Hydrocarbon Processing, Masters in Energy 4 steps in F-T synthesis • Starting from biomass, coal, NG (BTL, CTL and GTL, respectively) • 1) Production of syngas • 2) Syngas purification • 3) FTS • 4) Separation and upgrading of products Prof.K.R. Thampi, Hydrocarbon Processing, Masters in Energy From coal Prof.K.R. Thampi, Hydrocarbon Processing, Masters in Energy From NG Prof.K.R. Thampi, Hydrocarbon Processing, Masters in Energy Relevance of F-T synthesis • 1902-1928: Sabatier reaction leads to BASF’s Co-catalyzed liquid HC synthesis at severe conditions. Then, Fischer and Tropsch invented oxygenated hydrocarbon synthesis. This led them to perfect the presently known F-T synthesis over Co-Fe catalysts at <300°C and 1 bar pressure in 1925. The commercial development took place in Germany during WW II, due to lack of petroleum resources in Germany. USA and UK then keenly followed the technology and established F-T plants in USA. After 1957, due to cheap petroleum coming from the Middle-East F-T technology became redundant. However, it continued (1955 - 1994) in South Africa (SA) in a major way due to the apartheid embargo. The SA process is called SASOL. Prof.K.R. Thampi, Hydrocarbon Processing, Masters in Energy F-T technology today • The ‘oil embargo’ of 1973 stimulated interest in F-T synthesis again. Many process improvements happened in 1975-1990. SASOL and FTS Diesel processes resulted as a result. The modern GTL process (gas to liquid) is the latest in the F-T series. It is now gaining prominence due to the price rise of oil again….. Co and Fe catalysts are used along with tube-shell, fixed bed or fluidised bed reactors for GTL process. Prof.K.R. Thampi, Hydrocarbon Processing, Masters in Energy Exercise 3 1. What are the key differences you can observe between thermal and catalytic cracking? 2. What are the features of a reaction model governing thermal cracking? 3. In catalytic cracking, a tertiary carbon readily donates a H- to a primary or secondary carbonium ion; other transfers are slower. True or false? Prof.K.R. Thampi, Hydrocarbon Processing, Masters in Energy 101 Exercise 3 4. Formation of carbonium ions allow branching polymerisation reactions. Why? 5. Why FCC favours branched chain and isomerised products than thermal cracking? 6. What is the benefit of FCC when compared to thermal cracking process? Prof.K.R. Thampi, Hydrocarbon Processing, Masters in Energy 102 Exercise 3 7. Even though cracking reactions are endothermic, the equilibrium conversions in cracking may be high. What does this imply about the entropy changes in cracking reactions? 8. Examine the routes to the formation of carbonium ions from a long paraffin molecule. What are the most important routes to form carbonium ions in cracking reactions? 9. Why do we use low residence times in FCC process? 10. How FCC catalyst gets deactivated? Prof.K.R. Thampi, Hydrocarbon Processing, Masters in Energy 103 Exercise 3 11. What is the benefit in choosing zeolite based catalysts for FCC? 12. Why do we add ZSM-5 type additives to FCC catalysts? 13. What is b scission rule and why is it critical in cracking? 14. In zeolite based FCC catalysts, we find both Brønsted and Lewis acid sites. Which one is more important in FCC Reactions and why (from a reaction and molecular point of view? Prof.K.R. Thampi, Hydrocarbon Processing, Masters in Energy 104