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Module 1: Impurities and Residues

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Impurities Removal from Liquid Fuels

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Hi friends, now we will discuss on the topic cleaner liquid fuel production from petroleumcrude. In the previous class we have discussed how to produce liquid fuel from petroleumcrude and now we will see how the liquid fuels can be produced more cleaner. Actually, wehave come to know that the petroleum crude contains different types of impurities, letus say salt content sulphur, nitrogen, oxygen etcetera.These impurities can create some problem and during the refining process, these can alsobe transferred to the liquid fuels and when the liquid fuel will be used as a transportfuel at that time pollutions will come. So, we need some cleaning options, the some methodsto remove those pollutants from the liquid fuels. Secondly, we have come to know thatthe residual part particularly the one vacuum residue contains high asphaltene, high sulphur,high metals etc.And this part is of less interest to the refiners and poorly managed and creates lot of environmentalproblems. So, up gradation of this vacuum residue is required and that can give us acleaner route or cleaner option to the overall refining process. And it will also help toincrease the economy of the refining process. Thirdly, we have come to know that graduallythe petroleum crude quality is decreasing its sulphur content is increasing, viscosityincreasing and degree API is reducing.So, the processing of this heavy crude through the conventional technology is not that effectivemay not be that effective. So, we need some alternative technology to use these feedstockheavy feedstocks for its refining. So, these are the topics or sub topics we can say weare going to discuss in this module.And in this class we will discuss basically on the impurities removal. And the contentsare impurities and need of their removal. Then process for impurities removal, likesulphur compound removal and then sulphur nitrogen and metal removal. So, sulphur canbe removed by sweetening process. And all Sulphur, nitrogen and metal can be removedby hydro treatment and we will make some more discussion on this topic. So, as you havediscussed that crude oil contains different types of impurities, salt and Sulphur, nitrogenoxygen etc.And desalting processes used to remove the salt from the crude oil, but that salt whichis dissolved in emulsified water or in the crystalline form. And basically we have seentheir sodium, calcium, magnesium, chloride are removed mostly apart from these. We alsohave some other, other metals. So, we will discuss how to remove those and sulphur canalso be present in this case, as inorganic sulphur and organic sulphur. So, we will discusshow the sulphurs can be removed.So, this sulphur removal process is called the sweetening process, sweetening of theliquid fuel and it is most important for the refiners to improve the quality of the liquidproducts. And this is done by alkali extractions and hydro treatment.The impurities removal are very important because it helps to improve the quality ofthe, of the product. Now, we will see in which form sulphur is present in crude oil. So,it can be an inorganic form or maybe an organic form. So, organic form basic compound is thiolsRSH, so, alcohols ROH so, this O is replaced by sulphur, then it is called thiols and mercaptans.So, this is the major compounds present in petroleum crude, and then sulfides will beRSR.So this two alkyl groups with sulphur that can also be available or disulfides, RSSRand may be cyclic sulphides as shown here and thiophene, Benzothiophene, from benzenering can also be available with the thiophene and then Dibenzothiophene this is the structureand naphtanobenzothiophene. So, these are the molecular structure of these compounds,which are present normally in petroleum crude.And if we think about the nitrogen as compound, so, those are some non basic type and somebasic type. So, non-basic type are pyrole, this is C4H5N and this is the structure andIndole C8H7N in this is the structure Carbazole then Benzo carbazole it may have basic typethat is Pyridine, Quiniline, and then Indoline and Benzo quinolone. So, Benzo quinolone.So, these are some compounds. Now we will see how these compounds can be removed, wewill discuss first how the sulphur can be removed and for the removal of sulphur compounds,number of methods have been reported and used. So, those are caustic wash, then Doctor treatment,then copper sweetening and Merox process that was the advanced one. So, we will discussabout all the processes. So, if we want to remove the sulphur, and if it is present issulphide, sulphide is acidic in nature.So, if we use alkaline solution, so that sulphides or H2S will be dissolved in this NaOH solution.And if we use this alkali solution that is caustic lye, so, that caustic lye and crudeoil or any other liquid oil will be mixing well. And then there will be two layer oneaqueous layer and another is your organic layer that is oil layer so, that all the impuritieslike sulphur mercaptans and sulfides will come to the lye solution.And reactions is represented here RSH plus NaOH that will give us NaSR + H2O. So, thisis the conversion which takes place during this process and the Mercaptans are made freefrom the sulphur. So, sulphur removal will take place like this way, and then it willcome to liquid phase and that sulphur recovery and lye recovery can take place.Now, this process we use the lye solution of lower concentration not high sodium hydroxideconcentration, which is 5 to 20% weight by weight are used at 20 to 45 degrees centigradeand 5 to 40 psi pressure. If we use the high concentration of NaOH and high temperature,then it will have some drawback that it may produce some colour body as well as the productmay lose the stability.That is why this condition is maintained and when the lye solution we are using that itis not possible that all the lye or NaOH solution is consumed by sulfides. So, if it is around65% sodium hydroxide is consumed by the sulfides then we call it as a Spent lye solution. ThisSpent lye solution can be used further for the removal of hydrogen sulphide, light mercaptans,organic acids or mineral acids.Lye solution that is Spent, as far as hydrogen sulphide is concerned, may still be used toremove mineral or organic acids from petroleum products. Now we will see what is the doctortreatment. So, Doctor treatment in this process Mercaptans is removed by the use of sodiumplumbite and elemental sulphur.So, the mechanism is like this, this Mercaptans will react with sodium plumbite and this PbSR2will form and then sodium hydroxide will be formed. And this PbSR2 which is produced herethat will react with sulphur it will react to sulphur and then it will give us RSSR +RSxR, x is a number say 2345 like this, + 2 PbS, so, + 2PbS and then PbSR2 also canbe oxidized to some extent to produce RSSR + PbO.These are the chemical reactions which take place for the conversion of Mercaptans tothese products. Now, these processes have some disadvantages like say we are using excesssulphur. So, if we can make some mistake and more sulphur is added then that sulphur willproduce more polysulphides, which is not desirable in terms of corrosive nature of the fuel.So, it may fail the copper strip corrosion test.And when more Sulphur is used then that PbSR2 will be more and that can also be convertedto PbO. So that PbO conversion can be more and more PbO it will create some other problemthat will be called water tolerance test. So, that water tolerance will be reduced.So, these are the two major disadvantage of Doctor Process, and then you will see coppersweetening process.Another process which was used to remove the mercaptans was the use of copper chloride.So, if we use copper chloride, so, mercaptans will react with this copper chloride so, Cupricchloride, so, 2CuSR + 4HCL + RSSR. This is the reaction and then if we have Cuprous chloride,then the reaction will be this. So, CuSR which is produced here, it can again the react withCuCl2 and give us RSSR + 4 CuCl and then CuSR can also be oxidized to 2CuO and RSSR.So, these processes also have some drawback. So, what will happen in this case, this cuprousmercaptide that is oil soluble cuprous marcaptide captide remains in treated oil. And then copperchloride reacts with naphthenic acids, phenols and other polar compounds to form oil solublecopper compounds. The oil soluble cuprous mercaptide oxidation to copper this reactionwill make this product to fail the water tolerance test. It will prevent the product to passthe water tolerance test.Then we are coming to Merox process.So Merox process is the advanced one. So it replacesthe previous processes. And in this case, we use some catalyst and this process is developedby UOP in the year 1958 and it completely replaced the older processes and it is verysimple with respect to others. And it can be performed by two ways. One is light molecularweight mercaptans from the LPG and light straight run naphtha, those can be separated by usingsome alkali solution, then that can be catalytically oxidized.So, we have say LPG or straight run naphtha. So, this will be reacting with some alkalisolution and it will give us the mercaptans will come here in the solution alkali solutionand then the solution will go for catalytic oxidation. So, this is a mechanism throughwhich we can do or directly we can also handle high molecular weight mercaptans present inheavy naphtha, FCC gasoline ATF and kerosene into disulfides. So, this is the two waysof processes for two different type of feedstock.And the basic reactions are RSH + NaOH Oil phase with Aqueous phase it is giving us NaSR+ H2O and then NaSR produced here that will be oxidized and then 2H2O gives NaOH + 2RSSRagain oil phase we are getting insoluble in caustics. So, this is insoluble in causticsand will be getting it and separate it. So, 4RSH + O2 is 2RSSR + 2H2O. So these are thereactions which take place in this Merox process for mercaptans removal.Now, this reaction one this is very mostly applicable for low molecular weight mercaptansand at low temperature and high caustic concentration. Now we will see how the merox process work.So, this say we are putting our feed then we need one caustic prewash, so then, afterwatching some mercaptans is going out so rest is coming here and here extractor were givingsuction time, this place basically inorganic sulfides are removed and then it is comingthe mercaptans removal here in this extractor and then the mercaptans rich alkali solutionis coming in here. And it is going to second stage for air addition.And this is our oxidizer, catalyst are used then disulfides forms here. So disulfide iscoming and here it is separated so disulfide separation spent air is going out and thendisulphide. We are having a two phase disulfide phase oil phase and here Aqueous phase. Aqueousphase your alkali phase and Oil phase. So this is insoluble in water so or alkali solution.So we are getting the disulphide oil and Aqueous layer containing alkali.It is recycled back to this column extractor for further extraction and which goes herethat is coming to caustic settler and then sand filter and then treated product. Basically,if it is a LPG stream if feed is the LPG stream than that will be gas will come here and wewill pass it through the sand filter. So, other impurities if something from here, so,that will be removed and gas will go here.So, this is a flowsheet for the merox process which represents to LPG, then in this process,we see one is your caustic prewash, another your oxidizer these two very important stepsapart from these extractor.So, what we do in prewash our objective is to remove the inorganic sulfides just I havementioned. So, H2S + NaOH, it will form Na2S + 2H2O and Na2S + H2S 2NaHS. So, this typeof reaction takes place in the caustic prewash and in the oxidizer which is happening Na2Swhich is available in the caustic solution, then it will come in a oxidizer it will reactwith oxygen and then Na2S2O3 will form.And these will be NaOH so these are regenerated our alkali solution we are getting so thatis recycle. NaSH will react with 2O2 gives Na2S2O3 + H2O and again Na2S + 2O2 that givesus Na2SO4. So, these are the reactions which take place in oxidizer.Now, in this oxidizer we supply air so what will be the air requirement? Obviously, wehave to provide excess air than stoichiometric requirement and typical values given here0.84 normal meter cube per kg of RSH-sulphur. This is a stoichiometric requirement and excessair is 100 to 200% is provided. If we increase the air, then it causes rapid and completecaustic regeneration. But if the air supply is very high then some disadvantage we mayget.That is dissolved oxygen will be more and that can cause sweetening in extractor, becausewe are, we are we are recycling this in this. And spent air will have higher oxygen leadingto increased corrosion and hazards. So, that is a one important drawback if we use veryhigh oxygen air or air here and setting of catalyst may also take place. Now, how canyou quantify the air requirement?There are some formula it is provided here in case of LPG the air requirement is 1.5into 0.84 into F into SG into RSH by 1000. So, AR injection rate is normal meter cubeper hour. F is the LPG flow rate meter cube per hour and is the specific gravity of LPGat 15.5 degrees centigrade and RSH is the mercaptans sulphur in ppmw weight basis.So, now, this oxidizer or oxidation unit of Merox process is a catalytic process and UOPhave developed two catalyst. One is Merox WS and another is merox FB. So Merox WS, WSis it totally water soluble and specially formulated catalyst for using extractive orliquid-liquid sweetening. So it is the liquid process, liquid solution in this form, thecatalyst is available and specific gravity 1.16 +- 0.01 and freezing point 0 degree centigradeand active ingredient is 1 kg per gallon.Now Merox FB catalyst, this is developed for in-situ impregnation of the fixed bed sweetening,version of the UOP Merox process. So some fixed bed is used in this case, and Meroxcatalyst is impregnated on it as water slurry ready for use in the fixed bed Merox reactorimpregnation step. So, this is two type of catalyst they have developed on applicationpoint of view and here it is also liquid dispersion.Specific gravity 1.15 +- 0.01 and Freezing point 0 degree centigrade and Active ingredient2.5 Kg for 2.5 gallon. Now, examples of applications of the Merox process so, different type offeedstock we can use, different types of feedstock can be cleaned through this process.And these are the applicable treatment. So, how, what the people have done for differenttype of feedstock what type options of the Merox process have been implemented is providedhere. See it if it is a LPG mostly extraction based process and the light straight run naphtha-extraction or liquid-liquid sweetening; Visbreaking gasoline- fixed bed sweetening and FCC gasoline-fixed bed sweetening.And then, Full boiling range straight run naphtha, liquid-liquid sweetening, fixed bedsweetening and both can be used and then kerosene, aviation turbine fuel we are getting fixedbed sweetening. So, these are the preferred configurations of Merox process for differenttype of feedstock. Now, we will discuss the hydro treatment. Now, how the hydrogen isused to remove the impurities?So, in terms of hydro treatment, we can term it as hydro processing. So this hydro processingis thermal conversion process in which hydrogen is used to accomplish the objectives of therefiner. So these hydro processing can we have two types: one is hydrotreating and anotherhydrocracking. So hydro hydrotreating means we provide hydrogen under such conditionsfor a certain duration that only sulphur and nitrogen those compounds will be separatedfrom this.So, if this is the process in which hydrogen is used to convert heteroatom constituentsinto their heteroatom hydrogen analogues and hydrocarbons, so, this is R S R1 + H2. So,RH R2 H + H2S so that this HS is removed by hydrogen to produce S2S so that sulphur isremoved. There is no cracking or molecular chain in this compound and hydro crackingis a thermal decomposition.Its extensive and hydrogen assist in the removal of the heteroatoms as well as mitigating thecoke formation which normally accompanies thermal cracking of high molecular weightpolar constituents. So, if we go for thermal cracking, then what happens there is a cokeformations on the catalyst. But if we use hydro hydrocracking, so, that coke formationwill be reduced, because those will be converted by this addition of hydrogen. So, coke willnot be deposited. So, that is called as hydrocracking.So, hydrocracking will reduce it will break the bigger molecules and it will convert itinto lower hydrocarbons rather than deposition of coke unlike thermal cracking process. So,major differences between hydro treating and hydro cracking are the time at which the feedstockremains at reaction temperature and the extent of the decompositions of non hetero constituentand products.How long we are providing this feedstock in contact with hydrogen and what is the conditionof the reactor, so those will differentiate this low less, ok. So now we will see thecomparison of these the processes.And before that, we will see the reactions mechanism which take place during hydro treatmentor hydro processing. So, sulphur compound when hydrogen is added RSH is converted toRH + H2S. And then RSR + 2H2 2RH + H2S. RSSR + 3 H2 2 RH + 2 H2 and H2S is combusted toCO2 + SO2, so these are the reactions which take place during the hydro processing orhydro treatment of the feedstock. And then for nitrogen removal, when we add hydrogen,the nitrogen is also removed and this is mechanism.We have one hydrogen containing compound. So, that is we are using some catalyst andH2 this catalyst is basically acid base pair + H2 that converts this one to this and thereare to two reaction states. Two routes that is one and another is 2. So, this intermediateproducts forms in this route and that is converted to this one and again finally converted tothis product and we get ultimately this one typical presentation of this phenomena.And other route we get this intermediate product. So, here it is coming to this one almost similar.So substitution one and two reactions, and then it is converting to this one. Ultimately,this one nitrogen is removed from the compound. Now substitution reactions leading to Denitrogenation,these were the compound.So H2S is present. So, this N will be replaced by S. That can also be possible by the ammoniarelease. So, these are different type of reactions which may take place during the denitrificationprocess denitrogenation process. And here the catalyst which are used those catalystare basically tungsten and molybdenum sulphides on alumina, supported on alumina and theirproperties are modified by adding some cobalt and nickel.So, those are because of hydrogenation promoters, so, cobalt and nickel. So, cobalt molybdenumand nickel molybdenum, two active ingredients we are getting for the nitrogen removal sulphurand nitrogen removal. So out of these two, these ones Ni/Molybdenum is less active thannickel molybdenum for nitrogen removal, because the nickel molybdenum catalyst have higherhydrogenation activity than cobalt molybdenum catalysts.Now we will see the metal removal, how the metal can be removed from the product or thefeedstock.So, as you have seen that in in crude oil different types of metals are available, somemetals are available in water, in emulsified water, the calcium, magnesium, sodium etceteraand some metals are also their that is vanadium, manganese, tin, lead, cobalt, titanium, goldetcetera. Those are in less amount and typically vanadium and nickel, those are mostly availablein crude and the concentrations are vanadium varies from 0.1 to as high as 1200 ppm.And nickel commonly varies from trace to 150 ppm. So nickel and vanadium these are thetwo measure metals are present in petroleum crude and which needs removal from it to gethigh quality liquid product. In which form these vanadium and nickel exist? There aretwo thoughts. One is told that that is your porphyrinic and another is your nonporphyrinic.So, for porphyrinic state or nonporphyrinic state in which form they are existing. So,most of the literature prefers the porphyrinic state and more information is available onthis. Other one is not widely reported and this may be the molecular weight of this typeof compounds 420 to 520 and molecular formula C27 N4 to C33N4. For this is the representationof these porphyrinic metallic compounds. So, we have to remove these metal.So, for the removal of these metals, different types of methods are applied, physical andchemical methods.So, physical methods basically solvent extraction, so, we add some solvent. So, lighter partsfrom the vacuum residue it comes to the solvent and heavy asphaltenes etcetera where the porphyriniccompounds those go at the bottom. So it is separated from the, from the oil. So, thatway it can be done or thermal process also used, there is the visbreaking etcetera.And the chemical concept of the demetallization is to selectively remove the metal from theorganic moeity with minimal conversion of the remaining petroleum. The demetallizationof metalloporphyrins by acid is a reversible reaction. So, this is the reactions it isshown that PM + HX porphyrinic metal and acid HX. So, that H may react with metal chloridewe can get and that will be the compound porphyrinic compound without metals.So, that way the addition of acid can convert the metal into their salt, respective salt.So, then catalytic hydro processing, these are the chemical and physical methods. Apartfrom this, we can go for hydro processing steps using some catalyst. In that way also,these metals can be removed along with nitrogen, sulphur and oxygen.So, these are the different routes through which the metals sulphur and nitrogen areremoved from the liquid oil to make it more cleaner, transport fuel. Thank you very muchfor your patience.