Fluid Catalytic Cracking Process Engineering Essay

Fluid catalytic Cracking Process Engineering showINTRODUCTIONFluid catalytic cracking military operation, which is now more(prenominal) than 60 years old, is the cornerst angiotensin-converting enzyme of close to of the petroleum refineries. It has proven to be the most-efficient exhibit available for the variation of gunman oils and resi repayable into more valuable ignitor hydrocarbons. Many refiners consider the catalytic cracking process to be the highest net fake generating social unit of measurement in the entire refinery. In earlier times, Fluid shedalytic Cracking whole (FCCU) was ope dictated broadly in two modes, they argon maximum screw upoline modeMaximum distillate modeBut with the orgasm of Reformulated gasoline (RFG), these be now ope outrankd in maximum olefin mode. FCCU is a very sophisticated unit with many operators affecting individually separate and the oerall process. In tightlipped processes investigation of factors impact is d angiotensin- converting enzyme by changing champion factor at a time age kee bowling ping early(a) factors constant. In case of FCCU it is almost practically unfeasible to obtain a clear indication as, change in one single factor leads to change(s) in one or more other factors. This whole phenomenon is a natural consequence of the soup uping brace of FCCU. If the unit is to operate at steady state, then the unit has to be in lovingness residual condition. At this stage the vex requirement in the reactor is satisfied by burning cytosine in the regenerator and transferring the muscle to the reactor by circulating hot gun. Heat balance around the reactor-regenerator stinker be utilise to predict the effects of process changes although the hold degree of the changes whitethorn be difficult to establish. It is one step at a time thought process and rather difficult to pin down exact quashs without a c atomic number 18ful study of yields and incline laydown rates as affected by changing variables. In this work a plant data is distributen as reference and base on that, calculations have been done to find out the net passionateness of endothermic responses pass alongring in the riser pipeline pipe pipeline reactor, assuming that the unit is operating at steady state and that the riser is an isothermal one. Then as per the summations slate, a 7-lumped pattern is considered from various literatures and based on the energisings of responses, rate equations be formed and with the knowledge of available kinetic parameters the first derivative temperature drops along the crest of the riser argon calculated.PROCESS DESCRIPTIONMore than a 12 types of FCCU are operating worldwide. But the basic designs of all these type reside the same. FCCU comprises of two partsRiser reactor, in which catalytic cracking reactions occurRegenerator, in which burning of puff (deposited during cracking) from the catalytic sites is doneFigure 1 shows a schematic diagram of a typical FCCU. The course is pre affectionatenessed in a furnace and(Figure 1- Schematic Diagram of a typical FCCU)injected at the place of the riser along with a small amount of go. This move clean helps in dispersion of move over, good atomization and reduces shock formation by change magnitude the partial pressure of hydrocarbon dehydrations. The feed is subsequently vaporized when it comes in refer with the hot gas pedal from regenerator. The hydrocarbon vapors so formed undergo endothermic cracking reactions on their way up by means of the riser. The expansion of product vapours occurs through the length of the riser and the gas velocity increases with change magnitude gas density. Hot catalyst particles provide the sensible catch fire and latent arouse requirements for vaporizing the liquid feed and also endothermic passionateness of reaction for the cracking reactions. After a certain distance from the entry zone of the riser, the liquid feed is completely vaporized. Cracking reactions continue with the vapours miserable up in the riser and the temperature is dropped along the length of the riser due to endothermic nature of cracking. The catalytic cracking is started and also completed in a very short period of time inside the riser reactor in which the catalyst is pushed upward by incorporating steam at various fixtures along the length of the riser and hydrocarbon vapours. Mixture of catalyst and hydrocarbon vapour travels up in the riser into the reactors. Steams injected at different locations in the riser are as fol confuseds,Fluffing steam at the bottom of the riser dispersion steam along with cherubic feed injectorsRiser dilution steam in a higher place the sweet-flavored feed injectorsDispersion steam along with recycle blend rate injectorsAeration steam into the riser J bend to fluidize the catalystAlong with this some other locations are there where steam is injected. They are as followsplayed out catalys t standpipe aeration steamRegenerated catalyst standpipe aeration steam reactor quench steamReactor dome steamPost riser quench steamdiscovery steam into strippersMixture of catalyst and hydrocarbon vapour is discharged from the riser to the riser cyclone assembly. The bulk of the spent catalyst is separated from product vapours in the cyclone assembly. If necessary the vapours leaving the riser cyclones are routed into auxiliary cyclone assembly located inside the reactor vessel. Separated catalysts flow through each cyclone declination leg into the stripper. Product vapours leave the reactor cyclones and flow into the main fractionator through the reactor overhead vapour line. abate steam is injected inside the reactor vessel to reduce the temperature, so as to minimize post riser thermal cracking reactions and carbon formation. Reactor dome steam is provided to sweep hydrocarbons and avoid dead areas on sneak of the reactor vessel that may lead to thermal cracking and coking in that area. The separated catalyst from the riser and reactor cyclone assemblies enters the catalyst stripper.As the catalyst flows down the stripper, it gets stripped off the entrained hydrocarbon vapours by the up sleek steam. Stripping enhances the product recovery and reduces the carryover of hydrocarbon to the regenerator along with the spent catalyst thereof. Fluffing steam fancys the fluidization of the circulating catalyst. Stripped catalyst from the stripper flows into the regenerator weighty derriere through the spent catalyst standpipe (SCSP). gas level in the stripper is kept up(p) by spent catalyst slide valve (SCSV). Aeration steam is provided in the SCSP to ensure proper flow and fluidization of spent catalyst.Coke adsorbed on the spent catalyst during cracking reaction is been removed in the regenerator by burning off the coke with demarcation. childs play is supplied from the air blower to the regenerator through multiple distributors. line of descent i s also introduced at different locations of the regenerator, they are as followsT-grid airRegenerated catalyst standpipe (RCSP) hopper aeration airRCSP aeration airRegenerator fluffing air at the bottom near the J bendThe regenerator canful be operated in two modes overtone burning modeComplete combustion modeFor partial combustion mode, a CO boiler is needed to convert CO to carbon dioxide. The authentic discussion is for complete combustion mode regenerator. trematode worm gas from the regenerated dense bed flows to the two stage regenerator cyclone assembly. Here the entrained catalyst is separated from the trematode gas. The separated catalyst flows back to the dense bed through cyclone dip legs. Flue gas from the cyclone flows out from pinch of the regenerator through a labial pipe gas line. Total air flow to the regenerator is regulated based on the desired level of group O in labial pipe gas. Too low O2 concentration forget defecate coke build up on regenerated cat alyst and CO release from regenerator. Too high O2 concentration pass on lead to regenerator cooling. So, regenerator flue pipe gas is regularly examined for O2, CO, CO2, NO2, SO2 analysis.FEED CHARACTERIZATIONThe precisely constant in FCC operation is the frequent change in feedstock quality. Thats wherefore two feeds with similar simmering luff ranges can exhibit abundant differences in cracking performance and product yields. feast characterization is one of the most important activities in monitoring the FCC process. turn tail characterization is the process of determining physical and chemical properties of the feed. Understanding feed properties and also crafty their impact on units performance is an essential thing. Trouble shooting, catalyst selection, unit optimization and subsequent process evaluation, all depend on feedstock. Feed characterization relates product yields and qualities to feed quality. Analytical techniques like mass spectrum analysis are sophisti cated and not practical for determining complete topic of FCC feedstock. Simpler empirical correlations are often used. They are as followsoAPI gloominess and UOP KBoiling rangeAverage stewing point light speed eternal sleepMetalsSulphur, Nitrogen and OxygenoAPI gravity and UOP KIt is a specific gravity relating the density of oil to the density of water. The empirical formula for this isoAPI 131.5 (3.1)Feed to an FCC can range from 15o to 45o API. If the API gravity increases the charge stock go forth crack more readily and for the same reaction temperature there volition be great conversion. Secondly at a constant conversion level, there will be greater gasoline yield with slightly lower octane.A rough indication of the quantities of paraffin present is a characterization factor which relates stewing point to specific gravity, is called the UOP K factor. This is presumption by(3.2)WhereCABP = brick-shaped sightly boiling point, oRSG = specific gravity at 60 oFHigher th e UOP K prise more is the paraffinic nature of the feedstock.Boiling padThe boiling range of FCC feed varies from an initial point of 500oF to an endpoint of virtually speed of light0oF. thither are two boiling point ranges which are used to describe the lighter material in the feed. They arePer cent over 430oFPer cent over 650oFThe first quantifies the amount of gasoline in the feed. The randomness one quantifies the light fuel oil in the charge.Average boiling pointAverage boiling point of the FCC feed depends on the average molecular(a) weight. An increase in average boiling point and molecular weight will typically eccentric the followingThe charge will crack more readily, so at constant reactor temperature conversion will increaseAt constant conversion, yield of C4 and lighter will decreaseOlefinic content of the product will decreaseRegenerator temperature will tilt to riseAt constant conversion, the gasoline yield will increase about 1% for an increase in the molecular weight of 20.Carbon residueThe carbon residue of a feedstock is an indirect measure of its coke producing nature. Values may be determined by either Conradson or Ramsbottom methods. The carbon residue can be a useful number for determining possible contamination in storage. Entrainment in vacuum newspaper column is a common cause of increased carbon residue. Colour may be used to approximately evaluate the carbon content of the feedstock. Darker stocks tilt to have higher carbon residues.MetalsOrganometallic elementlic com digs in the FCC feed can cause serious overcracking if the metals deposit on the catalyst. The cleanliness of a chargestock is inclined by a metals factorFm = Fe + V + 10 (Ni + Cu) (3.3)WhereFm = Metals componentFe = Iron concentrationV = Vanadium concentrationNi = Nickel concentrationCu = Copper concentration on the whole metal concentrations are ppm by weight in the feed. A factor of 1.0 is considered safe, over 3.0 indicate a danger of poisoning of catalyst .Sulfur, Nitrogen, OxygenSulfur is as undesirable in FCC feed as it is in the feed to most of the refining units, causing corrosion of the equipment and increased difficulty in treating products. At 50% conversion about 35% sulfur charged is reborn to H2S, and at 70% conversion the figure will rise to 50%. Nitrogen produces NH3 and CN- in the reactors, and NOx and trace quantities of NH3 in the regenerator. These NH3 and CN- cause plugging and corrosion, plot of land the NOx and NH3 in the flue gas cause environmental problems. bodge oil will absorb oxygen in storage unless the tanks are gas blanketed. This oxygen will combine with the com confiscates in the oil at about 450oF to form gum, which fouls heat exchangers.FCC REACTION CHEMISTRYCracking reactions are predominantly catalytic, but some non-selective thermal cracking reactions do take place. The two processes proceed via different chemistry. The occurrence of both the reactions is confirmed by distribution of products. Ca talytic cracking proceeds mainly via carbenium ion intermediates. There are three dominant reactions in cracking are catalytic cracking, isomerization, total heat transfer. The idealized reaction classes are tabled below with specific reactions to support them.(Table 1 idealized reactions of importance in FCCU)Reaction classesSpecific reactionsCrackingn-C10H22 n-C7H16 + C3H6 1-C8H16 2C4H8 henry transfer4C6H12 3C6H14 + C6H6 cyclo-C6H12 + 3 1-C5H10 3n-C5H12 + C6H6Isomerization1-C4H8 trans-2-C4H8 n-C6H10 iso-C4H10 o-C6H4(CH3)2 m- C6H4(CH3)2TransalkylationC6H6 + m- C6H4(CH3)2 2C6H5CH3Cyclization1-C7H14 CH3-cyclo-C6H11DealkylationIso-C3H7-C6H5 C6H6 + C3H6Dehydrogenationn-C6H14 1-C6H12 + H2Polymerization3C2H4 1-C6H12Paraffin alkylation1-C4H8 + iso-C4H10 iso-C8H18Some of the reactions are endothermic in nature and some are exothermic in nature. Each reaction has a heat of reaction associated with it. The overall heat of reaction is the combination of both the types of heat of react ions. Though there are a number of exothermic reactions, then also the net reaction is endothermic. It is apparent that the type and magnitude of reactions have an impact on the heat balance of the unit. If the catalyst is with less hydrogen transfer characteristics, it will cause the net heat of reaction to be more endothermic. This in turn results in higher catalyst circulation and possibly a higher coke yield to maintain the heat balance.FCC UNIT MATERIAL BALANCEFor this, a complete set of commercial plant data is used. The data is given in subsequent tables belowFEEDSTOCK(Table 2 Properties of feed components)FeedUnitHydrotreatedVGOUn-hydrotreatedVGOLight Coker NaphthaQuantity,TMTPA3200800170% of total feedwt%76.7419.184.08Density 15oCgm/cc0.8940.9320.6762CCRwt%0.11.2Sulfurwt%0.13.320.434enthalpy contentwt%13Ni + Vwppm16.38Nitrogenwppm500159430ASTM Distillation, vol.%D-1160, oCD-1160, oCD-86, oCIBP3663493653743791038539443304204354950443468577048550865905455567595576573FBP6206 0986Bromine no.107.86Paraffinsvol.%46.7Olefinsvol.%43.38Naphthenesvol.%7.25Aromaticsvol.%2.68RON, clear79.4Diene value5.31WATSON K12.436MW82.001PRODUCT YIELDS(Table 3- product yields, Ex-reactor and Perfect fractionator basis)Productswt %Weight (lbs. /hr.)H2S0.394309Hydrogen0.041606Methane1.0611710ethane1.5417010ethylene1.7619442Dry gas4.40148768Propane2.8631592Propylene9.66106708n-butane1.6918668i-butane5.5260976butenes7.4782516LPG27.2300460LCN14.50160174MCN23.40257978HCN3.9043082LCO16.45181713CLO4.75153347 degree centigrade5.01-OPERATING CONDITIONS(Table 4- operate conditions for the Unit)Riser-ReactorUnitValue reinvigorated heavy feed rate (VGO)m3/hr.533.4Fresh light feed rate (Coker naphtha)m3/hr.30.2CLO recyclem3/hr.46Riser top temperatureoC540Riser top pressureKg/cm21.5Feed preheat temperatureoC350Regenerator atmosphere to regenerator (dry basis)Nm3/hr.310717Regenerator pressureKg/cm21.9Dense bed temperatureoC640Dilute bed temperatureoC654Flue gas temperatureoC657Blower disch arge temperatureoC226StripperStripping steam rateKg/hr.5000Stripping steam temperatureoC290Stripping steam pressureKg/cm210.5Base temperatureoC0Ambient temperatureoC35Flue gas writingMW= 30.6O2vol. %2.49COvol. %0.005CO2vol. %15.58N2vol. %81.83SO2vol. %0.085SO3vol. %0.01Now using the above data, amount of oxygen that was consumed by burning the hydrogen in coke is estimated. All the gas calculations are based upon 100 moles of flue gas. The oxygen consumed for H2O is given by the expressionO2 consumed = * (vol. % of N2 in flue gas) 2 * (vol. % of O2 in flue gas) 2 * (vol. % of CO2 in flue gas) (vol. % of CO in flue gas) (5.1)So, O2 consumed = * (81.83) 2 * (2.49) 2 * (15.58) (0.005)= 7.36The weight of the hydrogen and carbon in the coke are calculatedWeight = 2.016 * (7.36) + 12.01 * (15.58+0.005)= 202.01The temperature differentials are calculated (oF basis)TRR = (Regenerator dense bed temperature Riser outlet temperature) (5.2)= 1184 1004TRR = 180TRB = (Regenerator fluegas temperature Blower discharge temperature) (5.3)= 1215 439TRB = 776TRS = (Riser outlet temperature Stripping steam temperature) (5.4)= 1004 554TRS = 450The weight combined feed ratio is calculated as(Flow rate)CLO * (Density)CLO * 2.204CFR = (5.5)(Flow rate)Fresh feed * (density) uninfected feed * 2.204=CFR = 0.074The stripping steam and dull gases carried to the reactor by the regenerated catalyst are calculated on a weight per pound fresh feed basisSteam = (5.6)Steam = 0.01 impersonal gases = (5.7)Inert gases = 0.007The amount of hydrogen in the coke is calculated asHydrogen in Coke, wt % = 2.016 * 7.36 / 202.01 * 100 %= 7.35 wt. %The air to coke ratio isAir to coke, wt/wt = (2897/202.01) * (81.83/79)Air to coke, wt/wt = 14.85 lbs air / lb cokeWhere2897 is the molecular weight of air work out by 100 (basis of 100 moles of flue gas)The weight of coke per mo may be calculated asWeight of coke, lbs/hr. = (4591) * 193.23 / 14.85= 59738.6 lbs/hr.Where(310717 Nm3/hr. = 5178.62 Nm3 /min. = 193.23 MSCFM4591 = air rate conversion factor from MSCFM to lbs/hr.)So, weight % of coke is thenwt. % coke = * 100%= (59738.6 / 1104941.7) * 100 %wt. % coke = 5.41In the product yield table, the coke wt. % is indicted as 5.01 wt%. But it is calculated as 5.41 wt. %. Now the overall weight balance is as followsOVERALL WEIGHT BALANCEINPUT-= Fresh feed + Coker naphtha + CLO recycle= (533.4 * 0.8 * 894 * 2.204) + (533.4 * 0.2 * 932 * 2.204) + (30.2 * 676.2 * 2.204) + (46 * 808 * 2.204)= 1186860.1 lbs. / hr.OUTPUT-= Total product yields + coke= 1149831 + 59738.6= 1209569.6 lbs. / hr.So, error in weight balance is calculated as= INPUT OUTPUT= (1186860.1 1209596.6) lbs. / hr.= 22736.5 lbs. / hr.= 1.88 wt. %= 98.12 % solventNow combustion heat of coke is determined as follows (at hottest temperature = flue gas temperature = 1215oF)Hcomb = (X) (vol. % of CO in flue gas) + (Y) (vol. % of CO2 in flue gas) + (Z) (vol. % of O2 consumed) / (weight if hydrogen and carbon in coke) (5.8 )= (48000) * (0.005) + (169743) * (15.58) + (106472) * (7.36) / 202.01Hcomb = 16971.8 Btu / lb cokeWhereX = heat of combustion of CO at 1215oFY = heat of combustion of CO2 at 1215oFZ = heat of combustion of H2O at 1215oFThere is field factor for the hydrogen in coke, this is given as field of study factor, C = 1133 (134.6) (wt. % hydrogen) (5.9)= 1133 (134.6) (7.35)= 143.7The net heat of combustion after using the correction factor is-HC = 16971.8 + 143.7 Btu / lb coke-HC = 17115.5 Btu / lb cokeAt this point the reactor and regenerator heat balances are calculated. The catalyst supplies the heat to the reactor. The regenerator heat balance is calculated first using a basis of one pound of coke at the hottest regenerator temperature. The reactor heat balance is based on one pound of fresh feed.HEAT BALANCEREGENERATOR HEAT(Figure 2- Regenerator heat In Out scheme)HEATREG = HCOMB. HCOKE HAIR HRADIATION LOSS (6.1)Now, HCOKE = heat necessary to erect coke to combustion temperatu re= (0.4) * (TRR) (6.2)HAIR = heat required to raise air to combustion temperature= (lb air / lb coke) * (0.26) * (TRB) (6.3)HRADIATION LOSS = 250 Btu / lb cokeSo, HEATREG = 17115.5 (0.4) * (180) (14.85) * (0.26) * (776) 250HEATREGHEATREG = 13797.4 Btu / lb coke-HCSo, regenerator efficiency = *100% (6.4)= 80.6REACTOR HEAT(Figure 3- Reactor heat In Out scheme)HEATRX = HFRESH FEED + HRECYCLE + HSTRIPPING STEAM + HREACTION + HRADIATION LOSS + HINERTS (6.5)HFRESH FEED, HRECYCLE = heat required to raise fresh feed recycle to reactor temperatureHSTRIPPING STEAM = heat required to raise steam to reactor temperature= TRS * (0.485) * (lb steam / lb fresh feed) (6.6)HRADIATION LOSS = 2 Btu / lb fresh feedHINERTS = heat of inert gases carried from regenerator to reactor by regenerated catalyst= TRR * (-0.275) * (lb inerts / lb fresh feed) (6.7)HEATRX = (enthalpy of fresh feed at riser outlet temperature enthalpy of fresh feed at preheat temperature) + CFR (enthalpy of recycle feed at ris er outlet temperature enthalpy of recycle feed) + TRS * (0.485) * (lb steam / lb fresh feed) + 2 Btu / lb fresh feed + TRR * (-0.275) * (lb inerts / lb fresh feed) + HREACTION= (745 460) + 0.074 * (745 460) + 450 * (0.485) * 0.01 + 2 + 180 * (-0.275) * 0.007 + HREACTIONHEATRX = 310 + HREACTIONNote-Enthalpies for the fresh feed and the recycle feed were calculated by taking several(prenominal) UOP K values, oAPIs and the temperatures from the API technical data book.Regenerator heat is calculated on a one lb of coke basis. This can be converted to one lb of fresh feed by use of weight % of coke term.So, HEATRX = HEATREG () (6.8)HREACTION + HEATRX = HEATREG () + HREACTION (6.9)HREACTION = HEATREG () + HREACTION + HEATRX (6.10)But HEATRX = + HREACTIONPutting this relation in equation (6.10), the equation changes toHREACTION = HEATREG () HREACTION = 13797.4 * 310HREACTION = 436.44 Btu / lb fresh feedSo, HEATRX = 310 + 436.44HEATRX = 746.44 Btu / lb fresh feed(0.275) (TRR)Cat / Oil (wt. / wt.) = HEATRX (6.11)Cat / Oil (wt. / wt.) = 15 lb Catalyst / lb OilCatalyst circulation rate = (Cat / Oil) * (lb fresh feed / hr.) (6.12)= 15 * 1104941.8CCR = 16574127 lbs. / hr.= 7524 MT/ hr.Overall heat flow scheme for the whole FCCU can be shown as below(Figure 4- Typical FCCU heat balance scheme)Now, the net total endothermic heat of reaction is calculated through empirical formulae. But we took the assumption as the riser is an isothermal one. Practically it is not isothermal. The temperature at the base of the riser is higher than what is at the top of the riser or at the riser outlet. This is because the cracking reactions occurring along the length of the riser is endothermic in nature. So heat is being absorbed during the reaction and causes the temperature at that particular location to decrease. Gradually the temperature decreases and at the riser outlet the temperature is dropped significantly. In this context we can estimate the riser base temperature using empir ical relations and hence can estimate the drop in temperature at the next differential element up in the riser DNS. But before this a multi-lumped model is to be considered along with possible reaction schemes and there kinetic parameters.SEVEN LUMP KINETIC MODELFor this purpose a viisome lump kinetic model proposed by Mehran Heydari et al. (2010) is used. They divided the model into seven lumps namely VGO/Coker Naphtha, Clarified Oil, Light Cycle Oil, gasoline (LCN, MCN, and HCN), LPG, Dry gas and Coke. The schematic flow diagram is as follows(Figure 5- Seven lump kinetic model in FCCU)In order to develop a mathematical model for this particular system, certain assumptions has to be taken, they are as followsThe riser is an one dimensional ideal plug flow reactor with no radial and axial dispersionReactor is an adiabatic riserFeed viscosity and heat capacities of all components are constantFluid flow is not affected by the coke deposition on the catalystFeed is vaporized instan taneously in the riser entranceAll cracking reactions are taking place in the riserThe model considers seven lumps and eighteen reactions and eighteen kinetic constants. Molecular weights of different lumps and boiling ranges are given DNS in the table below(Table 5- molecular weights and boiling ranges of lumps)jLumpMolecular weight(Kg/ Kmol)Boiling range(oC)1VGO418.7349 6202CLO291232 -5673LCO226170 3924 flatulency11430 2285LPG656DRY GAS307COKE12Values of kinetic constants and activation energies along with heat of reactions for each reaction are given in the table below (DNS, Mehran Heydari, Praveen ch. shishir sinha)(Table 6- reaction schemes with kinetic parameters)ReactionsRate constants(m3/ kg cat. hr.)Activation energy(KJ/Kmol)Heat of reaction(KJ/Kg)VGO CLO14.9350.7345.821VGO LCO5.7850.7379.213VGO GASOLINE11.6950.7392.335VGO LPG3.5916.15159.315VGO DRYGAS0.3516.15159.315VGO COKE11.5516.15159.315CLO LCO5.7850.7356.314CLO GASOLINE0.9446.24128.571CLO LPG0.13559.75455.185CLO D RYGAS0.013559.75455.185CLO COKE0.327259.75455.185LCO GASOLINE0.574246.2493.030LCO LPG0.008659.75704.93LCO DRYGAS0.000959.75704.93LCO COKE0.059659.75704.93GASOLINE LPG0.000378.49372.10GASO DRYGAS0.000178.49372.10LPG DRYGAS0.003359.7532.30The riser model is assumed to be a two class model