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Module 1: Introduction to Natural Gas

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Syngas Production from Natural Gas

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Hi friends, now we will discuss on the topic gaseous fuels, syngas production from naturalgas.In the previous class, we have seen that natural gas can be used conventionally for the heatapplication or it can be used in power plant for electricity production.We have also discussed that this can be converted to syngas and then syngas to different typesof chemicals including hydrogen.So if we can do it, then we can reduce emission level due to the utilization of syngas.In this class, we will be focusing on how the natural gas can be converted to syngas,later next we will discuss how the syngas can be converted to liquid fuel and also howwe can get hydrogen from natural gas.So the contents of this class is your reforming as a cleaner route for natural gas utilization;then types of reformers or reforming that is SMR, dry reforming, ATR, CPOX, bi-reformingtri-reforming, etc.; and then comparison of different types of reforming; and then reformingreactors.If we see the reforming, that means that it will be reformed the molecules, the methanewill be converted to other molecules.In this process, the syngas is produced, hydrogen and CO containing syngas by the thermal process,it may be exothermic or endothermic, both reactions take place in this and dependingupon the nature of reforming process, the del H value changes.Normally it is a catalytic process, but without catalyst also the reforming can take place,and if we use catalyst, we will get better performance.What is happening by this process, we can eliminate the oxygen from the initial fuelmixture that improves the overall system efficiency by minimizing energy losses from catalyticcombustion.This product which we are getting that syngas, this will be used for heat application, forsteam generation and for different types of chemical synthesis.So that is the major advantage of this process for the conversion of natural gas to syngasand then syngas to other chemicals.So this is our reforming process, CH4 will be reformed to CO+H2, that is called syngas.(Refer Slide time: 03:34)Then we can get different types of reforming process as you have mentioned and the mostimportant are steam reforming, then partial oxidation, auto thermal reforming, then dryreforming or combined reforming of methane that is called bi-reforming, then tri-reformingof methane, and reforming with membrane.So these are the different types of reforming processes available in literature and someof these are applied in commercial scale and some of these are still under developmentstage.The first 3 are mostly used in industry.Other these are under development and demonstration scale and these will be having better featurefor the control of carbon dioxide.Now we will see what SMR is, that is steam methane reforming.So in this case, we will be using methane and steam as a reacting agent and the reactionwill be CH4+H2O both remain in gas phase, it will give us CO and 3H2.So, this one SMR1.In this reaction, we can get 206.2 kilojoule/mol energy, this energy is required, that is endothermicprocess.Then another reaction may take place, CH4+2H2O that is CO2+4H2, so carbon monoxide can beproduced, carbon dioxide also can be produced and here 164.7 kilojoule/mol energy is requiredfor this reaction.Then we can get water-gas-shift reaction in the reforming reactor also, that is CO whichis produced here that can also react with H2O which is available in the reactor andthen converts to CO2+H2, so this reaction is exothermic.So overall, it is endothermic reaction.Then another important feature is that H2/CO ratio, this H2 and CO which is available insyngas, the ratio of these 2 components are very very important because this will decidethe suitability of the syngas for its downstream applications.Different downstream applications require different ratio of H2 is to CO.So in this case, we can get this ratio as 3-5, so the very high ratio we can get withrespect to other reforming and the temperature requirement is 700-850 degree centigrade andpressure is 15-30 bar.So, this is a commercial process and in this process catalyst is normally used.If we do not use catalyst, then also we will get some syngas formation, but basically nickelor on ceramic support, this catalyst nickel based catalyst on ceramic support with orwithout some promoter or stabilizers these catalysts have been used in the SMR process.The precious metals offers alternative to the nickel and supports may be strong, inert,thermally and chemically stable.So these are the catalyst for this reaction.It requires high geometric surface area, high heat transfer coefficient, low pressure drop,and less carbon deposition.So during this process, carbon deposition takes place and the catalyst should have thatproperty where the low carbon deposition will be achieved.This process we will discuss in detail because we see here hydrogen is maximum.So if we want to get hydrogen from the syngas, the steam reforming may be one of the importantprocess or route and we will discuss in next class in more detail.Now we will see some aspect of the SMR reactor.So here is the SMR reactor schematic say.So we have these are the tubes that is called reformatives.So from this, we are putting natural gas and steam, both natural gas and steam.So, then high temperature will be provided in this reactor, these are the tubes through,inside of the tube we are having the hydrocarbon and steam, outside we are having high temperatureby burning some fuel gas.So here, the burners are there, so high temperature is generated, and there will be some reformingreactions inside the tube and we will get the product from the bottom of this tubularreactor or tubes of the reformer.The syngas we can get 850 degree centigrade, 20-30 bar, and 70% hydrogen in dry gas.So this is the steam reforming reactor and these are the reactions already we have discussed,endothermic and exothermic reactions, both are available during the process, and overallit is endothermic reaction.Next, we are coming to partial oxidation, that is POX, and catalytic partial oxidationCPOX.So these methods is also used largely and here as you see partial oxidation so you willbe providing less amount of oxygen.So CH4 will be converted to CO+2H2 that is the required in syngas.So then here we can get H2/CO ratio 2-1.5 and temperature is higher 1200-1400 degreecentigrade and then pressure 50-70 bar.So here, methane will be partially converted to this one, that is one reaction; anotherit is possible methane is reacting with 2O2, then it is CO2+H2O, then CO2+H2O further itcan give up CO+2H2.Other reactions which can take place here CH4 +H2O which is produced in this H2 canreact with the methane also and it can give CO+3H2 and CH4+CO2 which generated this reactionthat can also give a CO+H2 so that is why we can get H2/CO ratio as equal 2-2.5 andthis is exothermic process and faster than steam reforming and requires a smaller reactorvessels.So advantage of this, we do not need to give much external energy for this reactions.Then if we see the catalyst part, then also we will see that the first row of transitionmetals nickel and cobalt and precious metals ruthenium, rhodium, palladium, platinum, andiridium those are used as a catalyst for this reaction and the relative rates of carbondepositions are in the order, that is nickel greater than palladium greater than ruthenium,then it is greater than Ru, and then it is greater than Pt, so this is greater than iridiumokay.So carbon deposition is a major issue on nickel catalyst which may reduce the number of activesites.So heavy carbon formation may give increase in the pressure on the feed side that is thepores will be blocked and more pressure drop will get during the reactions and in the reactor.So we will see the reactor configurations.So we will be having 2 zones in this case.The first is your flame zone and another is your heat exchanger zone.So flame zone means here hydrocarbons, in our case methane, then oxygen, those willbe combusted and possibly low amount of steam react together.So here, some reaction takes place and then CO and H2 form here and then at high temperaturewe recover excess heat.So, this is the features of this, but this is for POX, but if we use CPOX, then we canhave some catalyst layer here at the first flame and then we will be having some catalysthere, so catalyst will help the reactions.So conversion of the process will be more than the POX.Then we are going to ATR, auto thermal reforming.So in this case, we use steam and oxygen, so steam and oxygen, so the reactions arethe methane plus steam then will be CO+3H2, it will give also CH4+1/2O2, then it is CO+2H2and then this is the heating value -36 megajoule per kilo mol and overall it is endothermicprocess.H2/CO ratio here we can get around 2 and temperature is 850-1000 degree centigrade and pressureis 20-100 bar.So what we see here are 2 types of reactions.So this reaction is basically for SMR and this reaction is for partial oxidation.So SMR plus partial oxidation basically gives us ATR and nickel based catalyst had beenused for this reforming process.If we see the reactor configurations, as you have discussed the first the hydrocarbon ormethane will be combusted and may use some oxygen and steam here and then some reformingreactions will take place in this zone.Now catalyst will be available here and then we will get syngas.So the reactions already we have discussed that CH4+O2 this will be there and then deformingwill be taking place CH4+ this H2 which is forming this and CH4+CO2, then we can getthis one.So this is the basics of the ATR process.So by proper adjustment of oxygen and steam to carbon ratios, the partial combustionsin the thermal zone supplies the heat for completing the subsequent endothermic steamand CO2 reforming reactions, that is 2 reactions which we are getting, this heat generated,that is used in the second case.Next we are coming to DRM, that is dry reforming of methane.So dry reforming means there will be no steam use, obviously carbon dioxide is used forthe reforming purpose.So reaction is methane + carbon dioxide 2CO+2H2 in gaseous phase and this is our del H 247kilojoule/mol, so endothermic reaction again, and your temperature 630-850 degree centigradeand pressure 1-20 bar and here the H2/CO ratio is less, 0.4-1.5 and here we can get nickelbased catalysts are used widely.The presence of CO2 gives rise to more chance of carbon formation, so as we are using carbondioxide as the reactant for the reforming process, the carbon deposition is more inthis type of reforming process and that is the major disadvantage of dry reforming andthat is the more challenging part also and that is why it has not got industrial applicationyet.Then efforts are on to improve this process, how to reduce the carbon deposition in DRM.Development of active catalyst materials but with a very low coke formation rate, eitheron the catalyst or in the cold zones of the reactor is the main challenge just we havediscussed.Then we are coming to bi-reforming, then in this case we will be using 2 reforming agents,so here methane plus steam and then carbon monoxide.So here, we will be having methane plus steam CO+3H2, del H is equal to 206.2 kilojoule/mol,then CH4 + steam, this reaction can also take place and then the dell H is this one andCO+H2O that can give us CO2+H2 and other reactions which is taking place here CH4+CO2 it is givingus 2CO+2H2.So in this case, we can use carbon dioxide and steam as a reactant for the reformingprocess.Overall is -41.2 kilojoule/mol and H2/CO ratio is 2 and temperature 700-950 degree centigradeand pressure 10-30 bar.So one of the important advantage of this process is that it can use carbon dioxideas a reacting agent.So it helps to reduce the carbon emission and increase the carbon footprint.Here also, nickel based catalysts are used and nano-composite that is nickel zirconiumoxide is superior to conventional nickel zirconium oxide, that means efforts are on to increasesuperior quality catalyst using nano-particles for this particular process.Then we will discuss the tri-reforming.So here, we will be using 3 agents; oxygen, carbon dioxide, and steam for the conversionof the methane, that is why it is tri-reforming.So major reactions are here; CH4+1/2O2, CO+2H2O, this is your del H value and then CH4+CO2it will give us 2CO+2H2 and then this is del H value, and CH4+H2, it will give H2O, itwill give CO+3H2 with this heating value and CH4+2O2, CO2+2H2O, heating value is this one.So overall heating value 166 kilojoule/mol and temperature 1000 degree centigrade, andpressure 20 bar, hydrogen to carbon monoxide ratio 1.2-1.5.So here, we are getting tri-reforming, that is carbon dioxide reforming, steam reforming,and methane oxidation reactions.So these are the features of tri-reforming and catalysts are basically nickel-based catalystand some examples are given here, nickel, calcium oxide, zirconium; nickel, cerium oxide,lanthanum oxide and aluminum and corderite; nickel ceria and then lanthanum doping.So these are some example of this catalyst used in this reaction.Now we will compare different type of reforming processes.So steam reforming, partial oxidation and then bi-reforming and ATR we will discuss.So here are reforming reactants as we have discussed, operating conditions we have discussed,methane conversions we will see here how it changes and all of these are having some advantageand some disadvantage, so that part we will see here.So steam reforming gives us 65-95% of methane conversion, partial oxidation 95-100%, andthen it gives auto thermal 95-100%, and dry reforming 70-96%.So these are different methane conversions reported in literature.Advantage as you have discussed that the steam reforming can give us maximum H2/CO ratio,so when we are interested to get hydrogen from the syngas, then this process will prefer.Steam as you are using steam, it may induce some corrosion problems also and that is itsdisadvantage.Similarly for partial oxidation, high cost of air separation units, we have to separatethe oxygen from the air, so that increases the cost for the process and no direct heatexchanger is needed and more compact than steam reforming and low reformer vessel cost,so these are the advantage of this partial oxidation.Similarly auto thermal process, we have high cost of air separation unit.Advantage is all heat needed is supplied by partial oxidation and then that heat is usedfor reforming, so more compact than steam reformer.Then dry reforming as you have discussed that it uses carbon dioxide, so that is the majoradvantage of this process and conversion of greenhouse gas into fuel or to syngas.Specific ratio of methane, steam, and carbon monoxide 3:2:1 can produce a gas mixture withessentially a 2:1 ratio of hydrogen and CO2, so this is required for liquid fuel synthesis,methanol synthesis, etc.So that is the advantage of this process, but if we can develop suitable catalyst andcoke deposition problem can be reduced, so this process can be attractive, but this isthe challenge for the process.Now we will see the reforming reactors.So different types of reactor configurations or reactor types have been used, out of thosefixed bed tubular reactor this is mostly used in commercial scale for larger reformer units.We see here feed inlet, it goes here and then it goes through this tube as we have discussedin the previous slide, so these are the tubes through which the feed hydrocarbon and otherreactants goes through.Then after reforming, it is going for as a product.So in this case, the tubular reactors, in the tubes in this reactor, the reforming reactiontakes place and the temperature is given by the combustion of another fuel gas at theoutside of the tubes.So in commercial scale if we see, one furnace contains 500-600 tubes with inner diameterof 70-130 millimeter and length of 7-12 meter and conversion section methane and steam arepreheated to 500 degree centigrade.Then there are some radiant sections and catalytic sections and the catalyst used for the conversionsof the methane to syngas, it is top fired, it can be bottom fired, it can be terracewall, side fired.So, the feed can be at the top, bottom, or side of the wall, anywhere it can be done.Then we are coming to microchannel reactor.So because of the latest development on this technology on this reactor part, if we usethe microchannel, then the performance or economics of the process can be improved becauseit reduces the mass and heat transfer limitation.So if you can reduce the diameter of this, it is almost similar to say tubular reactor,but if we can reduce the diameter very, very small and then we can coat some catalyst innerpart of this tube, so very small diameter and the inner side coating or washcoatingof the catalyst, so that will increase the performance of the reactor and this is a microchannelreactor.Here, critical channel dimension we see 50 to 5000 micrometer.So, we had 70 millimeter, here we are having 50 to 5000 micrometer.So because of the reductions of this tube size, the performance increases because ofthe reduction in different mass and heat transfer losses.Here also, it is laminar flow regime and large amount of surface area per unit volume increasethe overall productivity per unit volume.So, you can get more productivity per unit volume in this reactor.Then another development is monolithic reactor.So, it is like say honeycomb structure we can say.These are the number of comb or circular or the square shaped channels are made and thisis a honeycomb like structure.So, it is also similar to the microchannel, here also we can put catalyst loading insideand washcoating and only the improvement in the design is available in this case and longparallel and usually straight channels and catalyst washcoated walls.Physical properties of monolith depend on specific requirements of process and goodthermal conductivity for rapid heating and of washcoat.Washcoat thermal expansion of same order of magnitude as the support.Cell density here, what is the cell density, where you see here in this case, so numberof cells are there, so that is CPSI that is channels per square inch, that is very importantthat will decide the dimensions of the channels also, so that influences the performance ofthe process and CPSI is 1 by d channel square in inch.So 3 modeling zone, the gas to the in interior of the channel, washcoat, and channel wall;so these are 3 parts, this one, this one and in between.Now, comparison of different reactor types.If we compare the fixed bed and tubular 700-950 degree centigrade, 10-25 bar, and regular/irregularpacking materials, raschig rings are used, high production capacity it has, less heattransfer efficiency as you are talking about the limited sense is reduced by the reductionsof the diameter of the tubes, and then larger footprint and catalyst deactivation startsat 700 degree centigrade okay.Then we see microchannel.The temperature is given here that is 15 bar and 860-900 degree centigrade, the washcoatedcatalyst, uniform temperature, low rate of excess air, reduces the cost of blower orcompressors, their requirement reduces, and then thermal losses below 5% for scaled-upplant, and then it requires advanced technology.This technology is not very matured, some plants have come up and it is still developmentstage and it is having small capacity also and many reactors can be arranged in parallelfor high throughput.The monolithic, also the similar operating conditions it is having and same type of catalystis also used.It has also similar properties, low pressure drop, good mass transfer interphase, goodthermal and mechanical properties, simpler scale up due to specific geometries.These are the disadvantages that the ceramic monoliths have poor thermal conductivity,hence metal monoliths are preferred.Up to this in this class.Thank you very much for your patience.