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Catalytic CrackingCatalytic cracking process was developed in 1920 by Eugene Houdry for upgradation of residue and commercialized in 1930. Houdry process was based on cyclic fixed bed configuration.There has been continuous upgradation in catalytic in catalytic cracking process from its incept of fixed bed technology to latter fluidized bed catalytic cracking (FCC)Feed and ProductThe feed stock for catalytic cracking is normally light gas oil from vacuum distillation column. Catalytic cracking cracks low value high molecular weight hydrocarbons to more value added products (low molecular weight) like gasoline, LPG Diesel example of petrochemical feedstock like propylene, C4 gases like isobutylene, Isobutane, butane and butane.Kind of Reaction OccurMain reactions involved in catalytic cracking are Cracking Isomerisation Dehydrogenenation Hydrogen transfer Cyclization Condensation Alkylation and dealkylationMain Reaction

Fluid Catalytic CrackingFeed StockVacuum gas oil (VGO)Hydro-treated VGOHydro-cracker bottomCoker gas oil (CGO)Deasphalted oil (DAO)Reduced crude oil (RCO)Vacuum residue (VR)Typical feedstock consists of Vacuum and Atmosphere gas oil but may include otherheavy stream.Major contaminant in the feed includes carbon residue and metals.While FCC process feed containing up to 4% Conradson carbon MSCC can process all kinds of feed.Process Steps

Three basic functions in the catalytic cracking process are:Reaction - Feedstock reacts with catalyst and cracks into different hydrocarbons;Regeneration - Catalyst is reactivated by burning off coke; and recerculated to reactorFractionation - Cracked hydrocarbon stream is separated into various products like LPG and gasoline, like light cycle oil and heavy cycle oil are withdrawn as side streamReactionand Regenreation SectionThe feed to unit along with recycle streams is preheated to temperature of 365oC-370OCand enters the riser where it comes in contact with hot regenerated catalyst ( at atemperature of about 640-660oC. Finely divided catalyst is maintained in an aerated or fluidized state by the oil vapors.The catalyst section contains the reactor and regenerator & catalyst re circulates betweenthe two. Spent catalyst is regenerated to get rid of coke that collects on the catalyst during theprocess. Spent catalyst flows through the catalyst stripper to the regenerator, where most of the coke deposits burn off at the bottom where preheated air and spent catalyst are mixed. Fresh catalyst is added and worn-out catalyst removed to optimize the cracking process.Fractination Cracked hydrocarbon stream is separated into various products. LPG and gasile are removed overhead as vapour. Unconverted product like light cycle oil and heavy cycle oil arewithdrawn as side stream. Overhead product is sent to stabilsation section where stablised gasoline is separated from light products from which LPG is recovered.Opperating parameterRaw oil feed at heater inlet : 114 cubic meter /hourFurnace outlet temperature : 291oCReactor feed temperature : 371oCReactor Vapour temperature : 549oCProductLight gas -H2, C1, and C2sLPG C3s and C4s includes light olefinsGasoline C5+ high octane component for gasoline pool or light fuelLight cycle oil (LCO) blend component for diesel pool or light fuelHeavy cycle oil (HCO) Optional heavy cycle oil product for fuel oil or cutter stockClarified oil (CLO) or decant oil: slurry for fuel oilCoke by-product consumed in the regenerator to provide the reactor heat demandCatalystZeolitSpray dried micro-spherical powderReaction ProcessPreatreatmentThere are also varying amounts of contaminants such as sulphur, nitrogen and metals. To protect the catalyst, feed pre-treatment by hydrotreating is required in order to remove contaminants and improve cracking characteristics and yields.

The main reaction in the FCC is the catalytic cracking of paraffin, olefins, naphthenes and side chains in aromatics. The VGO undergoes the desired primary cracking into gasoline and LCO. A secondary reaction also occurs, which must be limited, such as a hydrogen transfer reaction which lowers the gasoline yield and causes the cycloaddition reaction. The latter could lead to coke formation (needed to provide heat for catalyst regeneration).

Primary ReactionsPrimary cracking occurs from the carbenium ion intermediates(a) Olefin is formed first by the mild thermal cracking of paraffin:

(b) Proton shift:

(c) Beta scission: Carboncarbon scission takes place at the carbon in the position beta to the carbenium ions and olefins.

New form of carbenium ion reacts with another paraffin molecule join the reaction

Primary Reactions17

Primary Reactions

1. Olefins smaller olefins

2. Alkylaromatics Dealkylation

3. Alkylaromatics Side chain cracking

Beside paraffins: 18

Secondary Reactions

IsomerisationThe final product is the transformation of paraffins and olefins to isoparaffins.The final result would be the cyclisation of olefins to naphthenes and can caused cyclisation to coke.Cyclisation19Process Description

riserSteam and VGO heated are fed to the bottom of the riser.The regenerated hot catalyst at is also fed to the bottom of the riser.316427 C649760 C endothermic reactions residence time =210 s

20Process Description

Steam is injected into the stripper section, and the oil is removed from the catalyst with the help of some baffles installed in the stripper. The oil is stripped in this way from the catalyst and the spent catalyst is sent to the regenerator. The coke in the spent catalyst is burned off in the regenerator by introducing excess airCO, CO2 , H2O, Air482538 C21Process Description

Hydrocyclones are installed to catch any solid particles carried out in the overheated stream. The product gases from the reactor are sent to the fractionator.The light gases are sent to the gas concentration unit. The filtered decant can be used as an aromatic solvent or recycled back to the riser with the HCO.22Advanteges of the ProsesChage lower production of hydrocarbons to the credit of higher yields of C3 and C4 hydrocarbons, which can be used for isoparaffins, gasoline, C7-C3 alkylates , ad LPG diesel/Lower cracking temperatures can reduce production ofdiolefins, that improving the oxidation stability of the gasoline fractionsHigher octane ratings of gasoline by increased formation of branched alkanes, naphthenes (cycloalkanes) and aromatics.Higher cracking reaction rates, make smaller dimensioning of plant equipment.The conversion reactions of oil distillates in the catalytic cracking process occur mainly in the vapor phase at elevated temperatures in the presenceof a cracking catalystThe cracking reactions occurring at the active sites of the catalystsproceed by a carbenium ion mechanism that predominantly effects theformation of olefins, isomeric components, and aromaticsThe formation of low boiling olefins, branched alkanes, and aromatics favors the production of gasolines with high octane levels