Sces2340 p3 hydrogen_synthesis_041218

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Sces2340 p3 hydrogen_synthesis_041218

  1. 1. P3Techniques for Hydrogen(Synthesis) Production• Treatment of Gas Mixtures• Decomposition of Hydrocarbons• Decomposition of Water
  2. 2. Techniques for Hydrogen ProductionA.Treatment of certain gas mixtures (side products)1. Catalytic Reforming of Naphtha2. Dehydrogenation reaction / process of alkanes (C1, C2, C4)3. Chloroalkali processB.Decomposition of hydrocarbons and other organic raw materials (coal, lignite, wood)1. Partial Oxidation• Partial Oxidation of Hydrocarbon(POX)• Gasificationi. From coalii. From Wood/Biomassiii. From Flash Pyrolysis2. Steam Treatment• Steam reformingC.Decomposition of Water1. Electrolysis2. Thermochemical cyclesWhat is synthetic gas (syngas) ?Synthesis gas (syngas) is a mixture of hydrogen, carbon monoxide and carbon dioxide invarious proportions
  3. 3. MethaneLPGNaphtaFuel OilVacuum residueAsphaltsCoalBiomassDesulfurization Steam reformingH2OPartial Oxidation(autothermal)Distillation H2OH2S absorptionShift ConversionH2OCO2DryingFinal PurificationHydrogenCO2 (and H2S)AbsorptionAirSulfur UnitSulfurMain Scheme for HydrogenProduction (Method 2)
  4. 4. Main Scheme for Hydrogen Production (Method 2)Operations (side)a) Conversion of CO with steam (shift conversion)b) Extraction of acid gases CO2 and H2S, supplemented(supplemented in the case of S-containing effluents bya Claus unit designed to prevent pollutant releasesinto the atmosphere)c) Final Purification designed to eliminate the last tracesof COPretreatment Processes :a) For steam reforming: desulphurization (to protectcatalyst)b) For partial oxidation with oxygen: air distillation
  5. 5. Steam Treatment(Steam Reforming Process)1. Thermodynamic & Kinetics of Reaction2. Catalyst and Process Conditions3. Process TechnologySteam reforming of natural gas is currently the least expensivemethod of producing hydrogenA large steam reformer which produces 100,000 tons of hydrogen ayear can supply roughly one million fuel cell cars with anannual average driving range of 16,000 kmNew processes are constantly being developed to increase the rateof production
  6. 6. Thermodynamic and Kinetic of ReactionsDefinition:Steam reforming is a process to reform hydrocarbons in the presence ofH20 to produce synthesis gas (SYNGAS) using catalyst (supported Ni-based) at a prescribed reaction conditions:“HC”(CH4, LPG, Naphtha) + H20 CO + 3H2Steam reforming is based essentially on the controlled oxidation ofmethane, by water, or more generally, hydrocarbons. Main reactionsare:CnHm + ¼ (4n – m)H2O 1/8 (4n + m)H2 + 1/8 (4n – m)COCH4 + H2 0 CO + 3H2 (steam reforming) ____(1)CO + H2 0 CO2 + H2 (water-gas shift reaction) ____(2)Reaction (1) is exothermic and complete between 400 & 600oC.Reaction (2) is endothermic and exentropic hence favored by lowtemperatures. However limited by equilibrium as shown in table.Nickel catalyst
  7. 7. Thermodynamic and Kinetic of Reactions(oC)CH4(mole %)H2O(mole %)CO(mole %)H2(mole %)427 42.6 42.6 3.7 11.1527 30.0 30.0 10.0 30.0627 14.5 14.5 17.5 52.5727 5.55 5.55 22.2 66.7827 1.80 1.8 24.1 72.3927 0.20 0.5 24.5 74.5Equilibrium Concentrations CH4 + H2O CO + 3H2
  8. 8. Thermodynamic and Kinetic of ReactionsRaising the proportion of steam in the reaction mixture cannot makepossibly complete conversionCan only be done by secondary reforming or post combustion(resembles POX in presence of catalyst) – mostly used forammonia synthesisHigh T makes CO conversion to H2 difficult therefore requires aseparate operation to convert the CO by low T steamSteam is needed not only for reaction, but also to prevent theconversion of :2CO CO2 + C (Boudouard’s equilibrium rxn)Which is replaced by the action of steam on CO :CO + H2O CO2 + H2 (water-gas shift reaction)
  9. 9. Thermodynamic and Kinetic of ReactionsProduct distribution are determined by:1. Thermodynamics of reaction (1) and (2); steam reforming & water-gas shift reaction2. Activity of the catalyst usedReactions (3-6) leading to carbon formation (undesirable reactions)CO + H2 C + H2O (3)2CO C + CO2 (4)CH4 C + 2H2 (5)2CO CO2 + C (6) Boudouard’s Equilibriumto prevent rxn (6), then add excess H2OCO + H2O CO2 + H2 (7)It is critical to keep catalyst surface free from carbon to preventdeactivationBuild up of carbon due to :Cracking polymerization / dehydrogenation rxnsCan be minimised by :1. Use excess steam to reverse rxn (3)2. Choice of catalyst support3. Presence of Alkali to promote rxn (3)
  10. 10. Steam ReformingFeed Gas(C2 – C6 Hydrocarbons)IntermediatesCH4, Alkenes, H2Oxygenated speciesEnd ProductCH4, CO, CO2, H2H2OProduct GasCH4, CO, CO2, H2CARBONBuild – up of carbonEquilibrationSteam ReformingThermal and CatalyticCrackingPolymerizationDehydrogenationCrackingC + H2O CO + H2to remove C
  11. 11. The catalyst and its conditions of useCatalystFor primary steam reformingNi/Al2O3Ni/Al2O3-K (to slow down carbon formation, K is added to helpaction of steam on CO)Ni/Al2O3-Ca (use for naphtha feedstock)Mg/SiO2-Al2O3-K (use for naphtha feedstock)Ni/Al2O3-U (use for naphtha feedstock)Typical Operating ConditionsSteam : HC = 2 to 4 (2-3 X higher than the stoichiometry)T = 850 – 940 oCP =1.5-2.5 x106 to 4 x 106 Pa absoluteFeed= CH4, Ethane, Naphtha (free from S) to preventdeactivation of catalyst due to poisoningAlthough thermodynamically, steam reforming reactions are favored atlow pressure, but to obtain high H2 purity and save cost oncompression, the process is normally carried out at high pressure
  12. 12. Steam reforming furnace section
  13. 13. Steam ReformingB. Reactor used for steam reforming (steam reformer):1. Dimension of reactorType : Tubular reactor100 – 1000 tubesInternal diameter : 10cmExternal diameter : 12cmLength : 50mWidth : >10m ,Height : >20m2. Operating conditionTemperature : 950oCPressure : 15 – 40 bar3. Catalyst employedNickel on alumina support
  14. 14. Steam ReformingC. Process flow diagram of steam reformingDesulphurizerSteamGeneratorSteamReformerNatural Gas Steam:NG = 1.6 – 4 H2:CO = 3 – 4T = 950oCP = 15 – 40bars
  15. 15. Partial Oxidation ProcessesA. Thermodynamic and Reaction KineticsB. Technological Aspects– three groups depending on raw material :1. Partial Oxidation of Petroleum Cuts2. Coal Gasification3. Conversion of Lignocellulose Wastesa. Gasification of biomass (wood)b. Flash pyrolysis
  16. 16. Partial OxidationHydrocarbon Fractions
  17. 17. Thermodynamics & Reactions KineticTransformation Considered :a. Combustion reactionCH4 + 3/2 O2 CO + 2H2O ___(1)b. Carbon monoxide equilibrium reaction due to presence ofwater formed during combustion, or added by steaminjectionCO + H2O CO2 + H2 (water-gas shift reaction) __(2)c. Hydrocarbon decomposition reactionCH4 C + 2H2 (side reaction) ___(3)Reaction (1) is exothermic & exentropic and takes placeadiabatically
  18. 18. Enthalphy and Entrophy Variations in ReactionsAssociated with the Partial Oxidation of MethaneReactions ∆Ho298 (kJ / mol) ∆So298 u.e.1. CH4 + H2O CO + 3H2 206.225 214.832. CO + H2O CO2 + H2 – 41.178 42.423. CH4 + 2H2-O CO2 + 4H2Reaction (1 + 2)165.047 172.414. CH4 C(g) + 2H2 74.874 75.015. 2CO C + CO2Reactions (4 + 2 – 1)– 172.528 –176.546. C(g) + H2O CO + H2Reactions (1 – 4)131.350 134.107. CH4 + CO2 2CO + 2H2 247.402 257.258. CH4 + 3/2 O2 CO + 2H2O – 519.515 81.62
  19. 19. Thermodynamics & Reactions KineticTo shift the equilibrium of reaction (2) to form the most H2Use excess water, low reaction temperaturePresence of CO2 and water helps to eliminate side reactions (rxn(3)) which occurs at high temperature by means of :CO2 + C 2COC + H2O CO + H2Production of Carbon increases with decrease in “HC” ratio in feedRequires presence of steam because not sufficiently formed duringcombustionIncrease in pressure at fixed temperature would result in:larger water requirementdecrease in O2 requirementincrease in residual methane contentCan be offset by raising the temperature
  20. 20. Technological aspects : POX ofpetroleum cutsGenerally thermal and use burners e.g. Texaco & ShellSome use contact masses but not favorable (high temperaturesemployed and danger of carbon deposit on the contact masses)Flow sheet comprisesa) A burner in which O2 and preheated steam are injected with HCb) Heat recovery sectionc) Carbon black removal section (by washing or filtration)Next two figures show Texaco and Shell POX unit, whose specialfeatures are :Shell :Recover carbon by washing with water then extract thesludgeExtract is homogenized with feed then sent to POX reactorTexacoStripping the fuel/crude oil (in presence of heavier HC) thenseparate and recycle the naphtha
  21. 21. Hydrogen Production – Partial OxidationPartialOxidationUnitCooler CondenserAbsorberFlashNatural gasH2OOxygenSteamRecycledCO2MDEAC. Process flow diagram of POX H2, CO
  22. 22. Process TechnologyA. Steps in hydrogen production via partial oxidation (POX):1. Natural gas, oxidant (such as O2) and moderating agent (steam)enter the POX unit to be combusted and reacted2. Reaction that takes placeCH4 + 1.5O2 = CO + 2H20 (POX) (3)CO + H2 0 = CO2 + H2 (water-gas shift reaction) (4)3. Synthesis gas leaves the POX unit and enters a cooler. Some ofthe water vapour in the gas is condensed and removed.4. Cooled synthesis gas is now conveyed to an absorber toseparate CO2 from the mixture. Absorbent normally used is anamine solvent. In this case methyl diethanolamine solution(MDEA ) is used.5. Absorbed CO2 in the amine solution is later passed to a flash,where CO2 is removed from the stream and MDEA isregenerated.
  23. 23. GasScrubbingStrippingSteamGenerationFueloilStrippingPartialOxidationCarbonSeparationBoilerFeed waterFuel oilOxygen or airFuel oil and carbonNaphthaNaphthaHP SteamWater and carbonWaterProduct GasesSteamHydrogen manufacture by partial oxidation. Texaco Process
  24. 24. PreheatingWater recycleCarbonFiltrationGasScrubbingPartialOxidationCarbonrecoveryOxygen or airBoilerFeedwaterFuel OilSteamGenerationBoiler Feed waterHP SteamNaphthaProduct GasesMake-upwaterWastewaterHydrogen manufacture by partial oxidation. Shell Process
  25. 25. Technological aspects : POX ofpetroleum cutsB. Reactor used for POX:Type: Fluidized bed reactorOperating Condition :Temperature = 1000oC – 1500oCPressure = 150atmCatalyst employed :• Composition of the synthesis gas produced using POX (molarbasis):30 – 50% hydrogen20 – 45% carbon monoxideabout 2 – 20% methaneabout 0.5 – 2% carbon dioxideless than about 0.5% higher hydrocarbons
  26. 26. Coal Gasification
  27. 27. HistoryAncient technique of producing hydrogen ~ since early 19thcentury1940s – growing availability of low-cost natural gas slowlysubsituted coal gasification process.Recently, diminishing sources of natural gas creates interest inproduction of gases from coal.However, operation cost is double the cost of producinghydrogen from natural gasCoal is heated up to 900oC with a catalyst and without air2 methods of coal gasificationi. Simple methoda) Coal heated in a retort in the absence of airb) Coal partially converted to gas with a residue of cokec) Technique introduced by a Scottish engineer ~ WilliamMurdockd) Pioneer to the commercial gasification of coal in 1792.ii. Complete conversion of coala) Coal is continuously reacted in a vertical retort with air andsteamb) Product is called producer gas
  28. 28. Technological aspects : Coal GasificationInitial activity : Crushing, drying and grinding of feedThree types of coal gasification installation :1. Moving (incorrectly called fixed) bed reactors – LurgiOperated in counter current flowHydrocarbon content (CH4, C2H6) high - require theirseparation from the gas produced and supplementary steamreforming2. Fluidized bed reactor – WinklerHydrocarbons other than methane are not formed3. Entrained-bed (dual flow) reactor – Koppers, TexacoMethane content is very low therefore does not requirespecific fractionationRemoval of ash and soot it vital where coal gasification technique isapplied
  29. 29. Typical Composition of a Dry Crude GasProduced by Partial Oxidation (% vol)Feedstock Fuel Oil CoalReactor Type Burner Entrained Bed Moving Bed Fluidized BedComponents :H2 47.3 34.7 38.1 40.0CO 46.7 52.4 21.0 35.0N2 + A 0.2 0.9 0.8 1.6CO2 4.4 10.3 29.0 21.0CH4, C2H6 0.6 0.1 9.0 2.0H2S + COS 0.8 1.6 1.4 0.4NH3 – – 0.7
  30. 30. Biomass/Wood
  31. 31. Technological aspects : Conversion oflignocellulose’s wastesA) The gasification of wood.Comprises of three stagesi. Drying between 100 and 300oCii. Pyrolysis between 200 and 500oC or higheriii. Reduction and oxidation which occur between O2, moisture,CO2, CO and C at temperature below 1000oCThree types of gasifiersFixed BedBed actually moving, with the fuel flowing by gravityAsh removed at the bottom of reactor by mobile gridsystem or in batchesGasses flow in parallel, co- or countercurrent contactor also perpendicular to each otherEntrained bedFluid bed
  32. 32. Technological aspects : Conversion oflignocellulose’s wastesb) Flash PyrolysisDeveloped by Garret Energy Research and Engineering,subsidiary of Occidental Petroleum and by BattelleColumbusWood drying using equipment with isolated gas transfersPyrolysis at 800 to 900oC using flue gas obtained bycombustion of residues formed – produced effluents withhigher heating valueHeat exchanger occurs on the biomass itself whichadvances by gravity from one section to the next or bymeans of solid heat transfer medium which retains tarsCleaned by combustion and recycled
  33. 33. Typical Compositions of Dry GasesProduced by Wood Gasification (%vol)Process Partial Oxidation Flash PyrolysisN2 0.3 –H-2 28.4 15.5CO 47.5 32.5CO2 17.2 38.0CH4 11.5Heavy 2.5Total 100.0 100.06.6
  34. 34. Biomass Gasification• The conversion of lignocellulose wastes, or dry biomass (wood) can beachieved after reducing feedstock to suitable particle size distribution• Proceed to– partial oxidation process, similar to coal– Flash pyrolysis• Gasification of Biomass (wood) Process– Drying @ 100-300 oC– Pyrolysis between 200-500 oC or higher– Reduction and oxidation, which occur between oxygen, mositure, carbon dioxide,carbon monoxide and carbon @ 1000 oC for wood– Licensors (technology owner)• Union Carbide• Flash Pyrolysis Process– Licensors (Garrett Energy Research and Engineering- Occidental Petroleum,Batelle Columbus– Pyrolysis between 800-900 oC

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