Fluid Catalytic Cracking Process Engineering Essay

politic Catalytic wisecrack Process Engineering EssayINTRODUCTIONFluid catalytic grab edge, which is now to a greater extent than 60 years old, is the cornerstone of most of the petroleum refineries. It has proven to be the most-efficient process available for the renewal of gasconadeconade oils and residue into more valuable fainthearteder hydrocarbons. Many refiners consider the catalytic cracking process to be the highest profit generating unit in the entire refinery. In earlier times, Fluid Catalytic Cracking Unit (FCCU) was operated broadly in two modes, they atomic number 18Maximum gasoline modeMaximum distillate modeBut with the approach of Reformulated gasoline (RFG), these be now operated in maximum olefin mode. FCCU is a very sophisticated unit with many factors affecting each early(a) and the boilers suit process. In nearly processes investigation of factors impact is done by changing one factor at a time while kee gloamingg other factors constant. In case of FCCU it is almost practically impossible to obtain a clear indication as, dislodge in one single factor leads to change(s) in one or more other factors. This whole phenomenon is a natural consequence of the kindle balance of FCCU. If the unit is to operate at steady state, and so the unit has to be in passion balance condition. At this stage the fire up requirement in the reactor is satisfied by burning hundred in the regenerator and transferring the energy to the reactor done circulating live gas pedal. Heat balance around the reactor-regenerator can be utilise to predict the effects of process changes although the exact degree of the changes may be difficult to establish. It is one step at a time thought process and rather difficult to pin down exact numbers without a careful study of topics and ascorbic acid laydown rates as affected by changing variables. In this work a plant data is taken as reference and ground on that, calculations have been done to find out t he net heat of endothermic reactions occurring in the riser main pipe pipe reactor, assuming that the unit is operating at steady state and that the riser is an isothermal one. Then as per the harvest-feasts slate, a 7- accumulateed model is considered from various literatures and based on the kinetics of reactions, rate equations are formed and with the knowledge of available kinetic parameters the differential temperature drops along the height of the riser are calculated.PROCESS verbal descriptionMore than a dozen types of FCCU are operating worldwide. But the basic designs of all these type remain the same. FCCU comprises of two partsRiser reactor, in which catalytic cracking reactions occurRegenerator, in which burning of blast (deposited during cracking) from the catalytic sites is doneFigure 1 shows a schematic diagram of a typical FCCU. The lam is preheated in a furnace and(Figure 1- Schematic Diagram of a typical FCCU)injected at the layer of the riser along with a sm all amount of travel clean clean. This steam helps in dispersion of ease up, good atomization and reduces coke formation by decreasing the partial pressure of hydrocarbon dehydrations. The food is subsequently vaporized when it comes in contact with the hot catalyst from regenerator. The hydrocarbon vapours so formed undergo endothermic cracking reactions on their way up through with(predicate) the riser. The expansion of product vapours occurs through the length of the riser and the gas velocity increases with decreasing gas density. Hot catalyst particles provide the sensible heat and latent heat requirements for vaporizing the liquid incline and similarly endothermic heat of reaction for the cracking reactions. After a definite distance from the entry zone of the riser, the liquid feed is completely vaporized. Cracking reactions continue with the vapours moving up in the riser and the temperature is dropped along the length of the riser due to endothermic nature of cra cking. 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 locations 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 follows,Fluffing steam at the bottom of the riserDispersion steam along with fresh feed injectorsRiser dilution steam above the fresh feed injectorsDispersion steam along with recycle stream 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 followsSpent catalyst standpipe aeration steamRegenerated catalyst standpipe aeration steam reactor quench steam reactor dome steamPost riser quench steamStripping steam into strippersMixture of catalyst and hydrocarbon vapour is discharged from the ri ser to the riser cyclone assembly. The bulk of the spent catalyst is disjointed from product vapours in the cyclone assembly. If necessary the vapours leaving the riser cyclones are routed into secondary cyclone assembly located inside the reactor vessel. divide catalysts draw through each cyclone dip leg into the stripper. Product vapours leave the reactor cyclones and flow into the main fractionator through the reactor everyplacehead vapour line. Quench steam is injected inside the reactor vessel to reduce the temperature, so as to minimize post riser thermal cracking reactions and coke formation. Reactor dome steam is provided to sweep hydrocarbons and avoid dead areas on top 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 flowing steam. Strip ping enhances the product recovery and reduces the carryover of hydrocarbon to the regenerator along with the spent catalyst thereof. Fluffing steam ensures the fluidization of the circulating catalyst. Stripped catalyst from the stripper flows into the regenerator dense sleep together through the spent catalyst standpipe (SCSP). gas level in the stripper is maintained by spent catalyst slide valve (SCSV). Aeration steam is provided in the SCSP to ensure proper flow and fluidization of spent catalyst. snowfall adsor go to sleep on the spent catalyst during cracking reaction is been removed in the regenerator by burning off the coke with shine. Air is supplied from the mien blower to the regenerator through multiple distributors. Air is 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 can be operated in two modesPartial burning at the stake modeComplete combustion modeFor partial combustion mode, a CO boiler is needed to convert CO to CO2. The current discussion is for complete combustion mode regenerator.Flue gas from the regenerated dense bed flows to the two stage regenerator cyclone assembly. Here the entrained catalyst is separated from the good luck gas. The separated catalyst flows back to the dense bed through cyclone dip legs. Flue gas from the cyclone flows out from top of the regenerator through a flue gas line. Total air flow to the regenerator is regulated based on the desired level of atomic number 8 in flue gas. similarly low O2 concentration allow for consume coke build up on regenerated catalyst and CO release from regenerator. Too high O2 concentration will lead to regenerator cooling. So, regenerator flue gas is regularly examined for O2, CO, CO2, NO2, SO2 analysis.FEED CHARACTERIZATIONThe only constant in FCC operation is the frequent change in feed r ailway line quality. Thats why two feeds with similar simmering point ranges can exhibit huge differences in cracking performance and product yields. Feed movie is one of the most important activities in monitoring the FCC process. Feed portrait is the process of find 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 picture relates product yields and qualities to feed quality. Analytical techniques like mass spectrometry are sophisticated and not practical for determining complete composition of FCC feedstock. Simpler empirical cor dealings are often used. They are as followsoAPI somberness and UOP KBoiling rangeAverage boiling pointCarbon residueMetalsSulphur, Nitrogen and OxygenoAPI gravity and UOP KIt is a ad hoc 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 will crack more readily and for the same reaction temperature there will be greater 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 boiling point to specific gravity, is called the UOP K factor. This is given by(3.2)WhereCABP = cubic average boiling point, oRSG = specific gravity at 60 oFHigher the UOP K value more is the paraffinic nature of the feedstock.Boiling RangeThe boiling range of FCC feed varies from an initial point of 500oF to an endpoint of about 1000oF. on that point 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 second one quantifies the light fuel oil in the charge.Average boiling pointAverage boiling point of the FCC feed depends on the average molecular cargo. An increase in average boiling point and molecular weight will typically cause 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 tend 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 tower is a common cause of increased carbon residue. Colour may be used to approximately evalua te the carbon content of the feedstock. Darker stocks tend to have higher carbon residues.MetalsOrganometallic compounds in the FCC feed can cause serious overcracking if the metals deposit on the catalyst. The cleanliness of a chargestock is given by a metals factorFm = Fe + V + 10 (Ni + Cu) (3.3)WhereFm = Metals FactorFe = Iron concentrationV = Vanadium concentrationNi = Nickel concentrationCu = Copper concentrationAll 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, ca development 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 nighttime and trace quantities of NH3 in the regenerator. These NH 3 and CN- cause plugging and corrosion, while the NOx and NH3 in the flue gas cause environmental problems. Gas oil will absorb atomic number 8 in storage unless the tanks are gas blanketed. This oxygen will combine with the compounds 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. Catalytic cracking bribe mainly via carbenium ion intermediates. There are three dominant reactions in cracking are catalytic cracking, isomerization, atomic number 1 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 2C4H8Hydrogen 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-C8H18 well-nigh 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 reactions. Though there are a number of exothermic reactions, then also the net reaction is endothermic. It is bare 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 m aintain 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%0.13.320.434Hydrogen contentwt%13Ni + Vwppm16.38Nitrogenwppm500159430ASTM Distillation, vol.%D-1160, oCD-1160, oCD-86, oCIBP3663493653743791038539443304204354950443468577048550865905455567595576573FBP62060986Bromine 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 base)Productswt %Weight (lbs. /hr.)H2S0.394309Hydrogen0.041606Methane1.0611710ethane1.5417010ethylene1.7619442Dry gas4.40148768Propane2.8631592Propylene9.66106708n-butane1.6918668i-butane5.5260976but enes7.4782516LPG27.2300460LCN14.50160174MCN23.40257978HCN3.9043082LCO16.45181713CLO4.75153347COKE5.01-OPERATING CONDITIONS(Table 4- Operating conditions for the Unit)Riser-ReactorUnitValueFresh 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 temperatureoC350RegeneratorAir to regenerator (dry basis)Nm3/hr.310717Regenerator pressureKg/cm21.9Dense bed temperatureoC640Dilute bed temperatureoC654Flue gas temperatureoC657Blower discharge temperatureoC226StripperStripping steam rateKg/hr.5000Stripping steam temperatureoC290Stripping steam pressureKg/cm210.5Base temperatureoC0Ambient temperatureoC35Flue gas compositionMW= 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 ox ygen 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 = clxxxTRB = (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)fresh feed * 2.204=CFR = 0.074The stripping steam and inert gases carried to the reactor by the regenerated catalyst are calculated on a weight per pound fresh f eed basisSteam = (5.6)Steam = 0.01Inert 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 multiplied by 100 (basis of 100 moles of flue gas)The weight of coke per hour 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 % closureNow 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 correction factor for the hydrogen in coke, this is given asCorrection 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. warmheartedness BALANCEREGENERATOR HEAT(Figure 2- Regenerator heat In Out organisation)HEATREG = HCOMB. HCOKE HAIR HRADIATION LOSS (6.1)Now, HCOKE = heat required to sharpen coke to combustion temperature= (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 + HRADIAT ION 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 riser 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 respective UOP K values, oAPIs and the temperatures from t he 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 / embrocate (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 empirical relations and therefore 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 seven 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 instantaneously 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 DRYGAS0.013559.75455.185CLO 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 phase model