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Module 1: Impurities and Residues

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Mechanisms for Residue Upgradation

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Hi friends now we will start discussion on the topic Residue Upgradation.This is the part 2 of residue of gradation.In the part 1 we have discussed how the vacuum residue can be upgraded to liquid fuels, whichis and how to make the products more cleaner.And basically we have discussed on solvent deasphalting process and in this class wewill discuss on visbreaking, delayed coking and gasification.If we think the term visbreaking it itself indicates the viscosity breaking.So in this process will apply heat and the vacuum residue will be converted to lighterfractions with lower viscosity than the here.And it will also have another part that is more viscous part that the residue and thatcan be processed further through the delayed coking.And in this viscosity breaking process we will be applying moderate condition so thatthe asphaltenes and resins which are available in this will not convert into coke.So, moderate conditions is needed in this method as our target product is the liquidproduct or lighter products with lower viscosity.And this process can give us hydrocarbons like LPG, naphtha, gasoline, heavy gas oiletcetera and by this method we are getting some lighter from the residual part.So if I think about our fuel oil so the blending of fuel oil is reduced.The oil which is produced through this process thus that can be used as fuel oil and themild operating conditions as I have mentioned it is required because the materials whichare available in the residue is susceptible to heat.And in this method allowable conversion to gasoline is lower due to fuel oil stabilityrequirements.So this is about the basics of visbreaking process.Now we will see the mechanism what actually happens.As we have discussed that visbreaking is the thermal treatment.So in this case cracking will take place.So the mechanism will be based on free radical just like the thermal cracking we have discussedin the previous classes.So will be having here three steps: One is initiation another is propagation and thenwe have termination step.So for example we have one initiation step if say C6H14 so that will be producing C2H5dot and C4H9 dot.So these two radicals, free radicals are produced first one is lower molecular weight anotheris higher molecular weight with respect to this one.Now this is the initiation step then these free radical productions will propagate andmore number of free radicals we will get and then ultimately those will combine and itwill be terminated.So C2H5 which you are getting here that can again react with C6H14 and it can give usC2H6 hydrocarbon lower hydrocarbon and again C6H13 another bigger free radical.So this C4H9 free radical can give us again react with C6H14 and give us C4H10 + C6H13free radical.So we are getting basically lower molecular weight hydrocarbon and the higher free radicals.So those C4H9 this can give us C4H8 + H dot free radical.So this free radical again giving us some hydrocarbon and giving us some free radical.In this case H in this case C3H6 + CH3 dot and then C2H4 and C2H5 dot.So there are possibilities of production of number of hydrocarbons, types of hydrocarbonsand also the free radicals of lower size.So this similarly C6H13 can give us C4H8 + C2H5 dot plus many other products.Then in termination the free radicals can add with H dot and then it will give up C2H6so to another free radical C2H5 + CH3 dot it can give a C3H8 and it can CH3 dot + Hdot gives CH4.So we can get some gas also in this process.And these are all gas molecules in this.So LPG type of material we can get from the visbreaking process.Now we will see the process flow sheet of the visbreaking process.So what we need?We have to apply heat.So what will be happening?In the previous class we have discussed that Maltane phase and asphaltene phase, resinsand asphaltene are dispersed in the maltene phase.So by the application of heat the maltane phase will be broken and asphaltenes willsettle, so that is the mechanism.So you have to apply heat and we can apply heat in two mode.One for very small period and relatively high temperature and another is relatively lowertemperature and for a longer period.But whatever may be the mode we apply but basically we need one furnace.We have to heat it here and then we have to flash it.So you have to flash it means we have to use this in high pressure.So high pressure at furnace that the product pressure will be reduced and then it willgo to flash chamber and from flash chamber it will go to the fractionator and from fractionatorwe will get different products, gas and liquid products here and ultimately the tar willcome here for further distillation.And it is another vacuum fractionator you can it can come.So in the vacuum we will create some vacuum and we will get more heavy gas oil in thiscase.And this will be vacuum flashed tar and this will be the feedstock for delayed coking.Now as I told that there will be two mode of operation one is very relatively longerperiod and the lower temperature.So in this case furnace and fractionator we put one soaker drum.So soaker gives some time at relatively lower temperature for the conversion process.So initially we preheat the feed here the initial ambient temperature to some preheatedtemperature.Then it is coming to furnace so it is heated further and then for a very small time inthe furnace it, it stays and then it goes to soaker.And relatively more longer period it stays here and then it is goes to flash drum fractionatorand in this case the soaker drum is optional otherwise we can use here some coil visbreaking.There is no soaker coil visbreaking is there from furnace it is through the furnace coilfor small span of time relatively higher temperature.So that way also some fractionations will be there and we will get different productsbut will may, the products may vary in their quality and the relative amount may changeto some extent.So in this case the furnace Inlet temperature is normally 305 to 325 degree centigrade hereand then pressure is 15 to 40 bar and then outlet temperature it is coming here it is480 to 500 degree centigrade and pressure reduction is there 2 to 10 bar.So residence time is 2 to 5 minutes and steam injection is 1%.So 1% steam, steam is injected in this furnace.So this is the visbreaking process and as you have discussed that soaker drum helpsto reduce the time in the furnace.So temperature and pressure maintained at soaker unit is around 440 degree to 460 degreecentigrade and 5 to 15 bar with 20 to 30 minutes residence time and preheating of the feedstockthat 335 degree centigrade is done by using visbreaking tar.So this preheating which we are that is done basically this visbreaking tar which is obtainedwith some part of it is used for heating this.And as I told that there are two mode one is soaker mode another is your we have yourcoil mode.So soaker mode visbreaking process is relatively low temperature process with high residencetime.So here we have told that; 20 to 30 minutes.So it is around 1200 to 1800 second whereas in coil mode this is 100 to 300 seconds.So in the soaker mode of visbreaking 2 stages conversion we get, one minor degree of conversionof short period in the furnace and majority conversion takes place in the soaker unitfor predetermined extended period that 20 to 30 minute.As we are able to perform the conversion at lower temperature so we can save fuel consumption.However, decoking in soaker unit require more equipment and handling cost so one way weare using less energy but we are also passing other problem that decoking in soaker requiresmore equipment and handling cost.So this is that drawback and this drawback the coke formations in the unit that can bereduced by using some internals.So if there is some back mixing in the soaker then there will be more coke formations.But if you can reduce the back mixing then the coke formations will be reduced so thatway internals has been used.So internal helps to reduce the coke formations in the soaker.And application of soaker internals satisfies these conditions and control stability ofproducts.Now we will compare the different soaker with internal and without internal.What will be the difference between these two processes?So if we compare the temperature then we see 420 420.In both the cases temperature is similar but if we are seeing about the conversion thenwe get 2.90% of the liquid fractions at 150 degree centigrade.So the liquid with the boiling point of 150 degree centigrade is 2.90% whether it is forinternal it is 4.12 percent.So more liquid we are getting here in case of soaker internals, soaker with internals.And then kinetic viscosity if we take, then, it is 248.43 cSt whereas it is 230.67 cSt.So viscosity is lower in case of soaker internal because it is it is preventing the coke formationsand back mixing in a soaker unit.And then stability, both are stable.So that weight is very clear to us that if we use the soaker internals then the qualityof the product will be improved.Next we will discuss on the delayed coking.So the term indicates that coking will take place and it will be delayed.That means there will be some arrangement some more time will be given for the cokeformation.So as we have discussed in the visbreaking part that application of it breaks the maltenephase and then we get the separation of the asphaltenes from the colloidal suspensionof the vacuum residue or residual part.So here also the same mechanism will work if we apply heat then the Maltene phase willbe broken and again the asphaltenes and resins will precipitate and they will get the coke.If we get sufficient time so that part will give us coke.Another type may also be possible that when we are getting the at higher temperature ifwe increase the temperature we will be getting more vapors from it also and then that vaporsmay if contains more aromatics etcetera so those will be polymerized.And if some condition is maintained it can also give us coke.So coke formation by both ways by the first one that is cracking of the maltanes.And then then the breaking of the emulsions and settling of the asphaltanes as well asthe production of aromatics vapours and then the polymerization of those and then condensationand then then also give us some coke.And these two cokes which we can get by these two mechanisms the quality will be differentthe asphaltene and resins, which will produce the coke that will be amorphous in nature.And not of that high quality but, when the aromatics are condensed and polymerized andcondensed and gives this product, the coke that is crystalline in nature and having somegood properties and it is considered as premium grade coke, coke or needle coke.Now we will see how to get it.So this this delayed coking technology particularly, for the needle coke production is an, it isan art and the combination of art and technology.And already this delayed coke technology is an open art, it is known to everybody.But companies do license technology emphasizing coke furnace, special processing modes andoperations.So these are the area when the company varies from one to others among the technology licenses.And they have more focus on these areas and in this case generally two types of reactionscan take place one is your primary reactions and another is your secondary reactions.So primary reactions is your decomposition of large molecules into smaller moleculesand then active compounds from the primary cracking reactions further crack or reactto form other compounds or polymerized to give heavy products, already we have discussed.And two types of mechanism also we have discussed that is one mechanism was for asphaltene precipitationsand another was polymerization and condensation of parametric compounds.So it is very clear to us if our feedstocks has more aromatics then it will be givingbetter quality coke and this type of coke is called needle coke.This term applies because the particles appear in the form of needle in the; it is the crystalstructure it has and the crystals are appeared like needles so that is why it is called needlecoke.And we see what are the typical feedstock for this vacuum residue, delayed coking?So for delayed coking the typical feedstock is vacuum residue.Atmospheric residue can also be used in some cases and typical feed composition if we see6% sulphur it contains and 1000 ppm metals and CCR 20 to 30%.The CCR, Conradson carbon ratio higher the CCR value obviously we will get higher coke.And what type of feed we will use that will influence the product quality.And what type of products we can get for through these delayed coking?We can get gaseous products, we can get liquid product and we can get solid product.So gas products which we can get that is a Coker gas that contains C3, C4 and this isrecovered as LPG and lighter can be used as fuel gas.And then liquid; liquid part can contain naphtha or heavy liquid products gas oil.So naphtha and gas oil, these two products we can get from this delayed coking and thisnaphtha contains high olefin content and usually sent to the hydrotreating units in the refineryfor stabilizations.In the previous slides, we have discussed the mechanism and we have seen that olefinsare formed.So those olefins are present in it.So it requires more hydrogenation and sent to hydrotreating units in the refinery forits upgradation.And heavy liquid products that is gas oil we are getting from it that that is sent tocatalytic cracking units where the valuable transportation fuels like gasoline, dieseland jet fuel can be produced.So, this is our liquid product and solid product we can get different types of coke.So coke may be a sponge coke it may be of shot coke and it may be of needle coke.So here, we see needle coke.So this is a crystalline structure whereas this shot cook and your sponge coke we arenot getting any crystal structure.Here this is a spongy sponge coke spongy appearance and this very small particles we are gettinghere for shot coke.So these among these three types of coke through these two cokes that is your sponge coke andshort coke those are not of that high quality.And those can be used for the production of electricity in combustion or in gasificationplant.And then this needle coke is superior quality premiere grade coke and it apart from thesethermal applications it has more attractive applications that is as electrode.So it can be used as electrode because of its some specific properties.So you see the characteristics of needle coke.So volatile matter is less than 0.50 percent and then ash content is less than 4% and sulfuris less than 0.8 percent and density is greater than 2 the densities around 2.1 gram per ml.But this is the property which is most important for needle coke that is CTE coefficient ofthermal expansion into 10 to the minus 6 per degree centigrade it is 1.25 so that is 1.25x 10 to the power of minus six per degree centigrade, because of this low CTE valueit is used as electrode.And needle coke production whether the needle coke will be produced or other type of coalwill produce will be produced during delayed coking process that will depend upon somefactors.One is your feed quality and other is your operating parameters.So more the aromatic contents in the feed we it is expected that the needle coke willget and the quality will also improve.And operating condition is also very, very important as I told that delayed coking isalso a blend of art and technology.So in this case also, needle coke can, quality needle coke quality can be improved by thepractice or maintaining the operating parameters.Now we will see the delayed coking process flows sheet.So here we have some feed so you are giving some feed here.So this feed it is going to heater it is heated up so it will be taking the cracking willtake place here where we are putting heat here.So heat it up then it is going to coke drum.So in this heater also we add some amount of steam.So steam is also added here.So steam and a small amount of steam and then residue this is equal to VR so VR and thisit is heated and then it is going to coke drum.So, high temperature here and here pressure is applied and then longer period the cokestays here that is it is called coke drum, so when it is after heating the VR and steamit is going to this the coke drum.It is remaining here under high pressure at high temperature, then, vapours are goingout then it is going for the fractionation.So after fractionation we will get different products we can get gas, we can get unsaturatednaphtha here and we can get some middle product also.So gas oil, stripper and then we will get steam and we like we will send steam herefor its heating and then we will get gas oil.So that we are getting gas oil here and we are getting some Fresh feed also we are addingin this case.So fresh feed and this vapour which is coming it is fractionated and the bottom part isagain taken here.So this is the feed mixed feed actually it is taken and sent into the heater and thenit is going to Coke drum.And then it is the vapour pass and it is condensed and then we are getting different products.This is for liquid and gas recovery from this but we getting coke from this coke drum willget the coke after certain time will collect the coke from the coke drum.So this is the process for the delayed coking where we can get coke we can get gas and wecan get different types of liquid.Now how we can change the distribution of this product?Because unlike visbreaking, in visbreaking our main focus was to get more liquid andhere our focus is also positively to get liquid and gas as well as that coke also of premierequality coke that is of needle coke.If we can get needle coke then this process will be more economic so that is the one ofthe main objective of the delayed coking unit.So what are the factors that will influence the quality of different products and itsrelative distribution of the products?That is the some operating variables are transfer of coke drum temperature, what is the cokedrum temperature?I am maintaining that will influence how much gas is produced how much coke is producedhow much liquid is produced and what is the pressure that also influence.And recycle ratio that means that I have shown you that we are adding some free fresh feedand then we are taking this one so fresh feed and VR how we are taking that composite feedand primary feed that is the ratio is called your recycle ratio so that also influencethe quality of the products and the reaction time.How much time I am giving in the furnace and how much steam is added.It has been shown that if we increase the drum temperature then, coke yield decreases;Because more cracking takes place and the more lighter parts goes off and if temperatureis too low then coking reaction do not happen.So we have to select an optimum temperature in coke drum.And then very high temperature makes the coke removal difficult from the drum and temperaturecontrols the volatile combustible metal content of the coke products.And then if we change the pressure of the coke drum.If we increase the pressure then, we may get more condensation of the vapours into thecoke, so coke yield will increase but we reduce the pressure then we will get more liquidproduct.So decreasing coke drum pressure reduces coke yield.So some data is given here each 0.5 kg per centimetre square pressure reduction in drumincreases liquid yield by 1.3% by volume and cut in Coke or coke yield by 1.0 weight percentof fresh resid feed.So this is some data which is available in literature.Then for making the premium grade coke, the coke drum pressure should be high.It improves coke quality and CTE and electrical resistivity.So, when we are increasing the pressure one way we are getting the more coke as well asthe quality of premium grade coke is also better.And increasing recycle ratio increases coke make and improves properties because it lowersthe asphaltene in the feed.More fresh feed we are using that means the asphaltene concentration is reduced.So that is why quality of the product improves it also reduces coke deposition in coil.Now we use some steam.What is the role of steam?Steam in the heated tubes prevents coke formation, so coke formation does not take place.Otherwise you have to shut down the plant very frequently so that way the steam helps.But steam does not help to get good quality coke it has some negative impact on it.It has a bad effect on the coke yield and quality as the stripping action that steamhas on the residual oil on the coke.So we can summarize that high-density feed then it will give us high carbon by hydrogenratio, it will give high CCR, it will give high coke, less distillate and less gas andcoke yield is proportional to CCR.Now delayed coking if we think that first it is heated in the furnace then you go tocoke drum and then coke drum to vapour goes for fractionation the upper part and bottompart is we are getting them from the coke drum as a coke.So this is a cycle coke drum once it will fill then we have to replace the coke fromthe drum and again new drum has to be added.So if we think about the whole process it is it will require some time.And in one drum cycle the time distribution is provided in this table.You see it has steam to fractionator then steam to blow down then depressurize thenwater quench and fill and drain unhead top and bottom and cutting coke and rehead, steamtest, purge, drum warm-up.So we see the drum warm-up requires around 4:00 hour out of 16 hour.Then major part is here.Depressurize water quench and fill filling of the coke drum.And the gradually it fills at a time the coke drum is not filled, it takes some time tofill.Then, we give some time for cutting of the coke.Coke cutting also takes some time and then other times as already say drain times alsotakes some two hours and others are given.Now, there are some empirical relationship to determine the coke yield.So if we know the properties of the feedstock then we can guess what will be the coke, amountof coke produced during the delayed coking process.So coke yield in percentage basis is equal to 1.6 into CCR value in weight percent whatis the CCR then 1.6 into that that will be the coke yield and gas that will be thereis LPG, the 7.80 plus 0.144 into CCR value and gasoline we can get this one 11.29 plus0.343 into CCR value in weight percent and gas oil will be 100 minus this coke minusgas minus gasoline.That way we can calculate by mass balance.Another formula gasoline, how much we can get 186.5 by 131.5 plus degree API into weightpercent of gasoline and then gas oil 155.5 divided by 131.5 plus degree API into wt.percentage of gas oil.So, these are some expressions empirical expressions which are available in literature for thepredictions of different types of products of the delayed coking unit.Now we are coming to gasification.Already you have discussed many thing about the gasification in the previous classes andwe have we have discussed that there are four types of gasifier basically 3, one is fluidizedbased and then fixed bed and entrained bed and the transfer reactor in between entrainedand fledged weight.We have already discussed and we have also discussed that entrained bed gasifier is moresuitable for feedstocks which is less reactive.So what pet coke residue all those are not of that high reactive because it is comingthe bottom product after high thermal treatment.So these products if we want to gasify we need a reactor like entrained bed gasifiersto handle this less reactive feedstock.So now we see here VR and Pet coke can be used to produce in syngas through gasificationand in entrained bed gasifiers will be most, more preferable and IGCC concept will be moreattractive for the utilisation of this coke produced through the delayed coking unit.Now we will see some example of gasification plants based on this VR and pet coke.So around the world there are number of plants.But these these technology gasification technology is not pretty old one for pet coke.You see it is started from 1995 and 2005 like this and most of the 5, 6 like this 1995.And around the globe in many countries the gasification of vacuum residue and pet cokeis implemented and applied.And if we see that all the reactors are basically Texaco and shell.So Texaco and shell means entrain bed reactors for all the cases.Here only Lurgi MGP used but here it is oil and slurry.So Oil and slurry it is relatively reactive than the pet coke and residue.So that is your Lurgi reactor is applied but all other cases Texaco and Shell.And here we see electricity, ammonia, hydrogen, steam, methanol, synthesis all have been implementedand different types of syngas cleaning rectisol, selexol, MDEAs are used and Rectisol is mostlyused.So, here else you see all the pet coke so pet coke has been used widely for the productionof electricity.But those are of sponge coke and shot coke type of coke.But needle coke is not processed through this route.It has better applications as electrode so that way in this part we have seen how thevacuum residue can be converted to different liquid parts and the solid part also thatwe will be having more value than the conventional petroleum coke so up to this in this class.Thank you very much for your patience.