International Journal of Chemical Engineering Research. ISSN 0975-6442 Volume 9, Number 2 (2017), pp. 143-152 Research India Publications http://www.ripublication.com Fluidized Catalytic Cracking Riser Reactor Operating Process Variables Study and Performance Analysis P.K.Yadav a and Dr. Rajeev Kumar Garg b a Research Scholar, IKG Punjab Technical University, Kapurthala, Punjab (India) a Department of Chemical Engg. and Biotechnology, Beant College Of Engg. and Technology, Gurdaspur (Punjab-India) b Department of Chemical Engg., Shaheed Bhagat Singh State Technical Campus, Ferozpur (Punjab-India) Abstract The Fluid Catalytic Cracking (FCC) is one of the key process unit in modern oil refining. FCC process converts heavy distillates like gas oil or residue to gasoline and middle distillates using cracking catalyst.fcc consist of two interconnected gas solid fluidized bed reactor: Riser and Regenerator.FCC process is very complex and require good understanding of many factors such as kinetic models, reaction kinetic, fluid dynamics, feed and catalyst effects, temperature, residence time in the riser, catalyst deactivation and its regeneration. Keywords: FCC, Cracking Catalyst, Riser and Regenerator, Kinetic model. 1) INTRODUCTION: Fluid catalytic cracking units (FCCU) are used in most refineries to convert high molecular weight gas oils (Boiling range 280-545 o C)[1]or residuum charge stocks into more valuable lighter hydrocarbon products like gasoline or LPG inside a riser reactor in a few seconds using cracking catalyst[2]. The feeds used include molecules with carbon number from simple C7 to C8 molecules to complex structure of 100 or more carbon atoms mainly in form of paraffins(cnh2n+2),cycloparaffins(naphthenes) (CnH2n),aromatics(CnH2n-6) and olefins (CnH2n). Feedstock quality determine the value of product obtained from catalytic cracking. Main products obtained from unit
144 P.K. Yadav and Dr. Rajeev Kumar Garg are fuel gases (H2,C1,C2),Liquefied petroleum gas or LPG (C3 & C4) and gasoline range (C5-C12), diesel light cycle oil (LCO:C11 through C18),heavy cycle oil (HCO:C19+) considered as unconverted feedstock, sour gas (H2S) and solid Coke[24]. FCC contributes to the bottom up gradation far more than other processes. [3]. Fig. 1: Fluidized Catalytic Cracking Unit The FCC units are consist of two major operating parts, the reactor riser & the regenerator. The riser where almost all the endothermic reaction cracking reaction of the hydrocarbon feed and coke deposition takes place. The regenerator reactivates the catalyst by burning the accumulated coke on the catalyst in the riser reactor by use of air. The regenerator process provide heat required for endothermic cracking process.[4] 2. KINETIC MODEL In the first three lump kinetic model proposed by Weekman & Nace (1970),the charge stock & products divided into three components namely, the original feedstock, the gasoline, and the remaining C4 s (dry gas & Coke). Gasoline Gas oil k T1 1 k T3 k T2 Light gases&coke Fig 2: Three Lump kinetic Model Yen and Woei (1988) and Lee et al (1989) developed a four lump kinetic model by dividing the light gas plus coke lump into two different lumps C1-C4 gas and coke.
Fluidized Catalytic Cracking Riser Reactor Operating Process Variables Study.. 145 k 31 Light gases k 21 VGO (A) k 11 Gasoline (B) k 32 Coke k 22 Fig. 3: Four Lump kinetic Model A new reaction kinetic model was developed by Jacob et al by dividing the feed and products into a 10- lump reaction schemes including paraffins, napthenes, aromatic rings and aromatic substituent groups in light & heavy fuel oil fractions. [5,6]. Fig. 4: 10 lump kinetic model proposed by Jacob et al. 3. CATALYST Zeolite is the important ingredient of FCC catalyst. In the refinery, zeolite catalysts have been used for improvement in gasoline yield, octane number, production of cleaner fuels with enhanced performance properties. Zeolites as Fluid Catalytic Cracking catalysts increases the formation of desired cracking products in FCC unit compared to amorphous, silica-alumina catalyst. The zeolite catalyst are more active and more selective[7,8].
146 P.K. Yadav and Dr. Rajeev Kumar Garg Table 1: Physical and Chemical properties of commercial equilibrium FCC catalyst Chemical Composition Al2O3 % wt 40.2 Na2O % wt 0.13 Re2O3 % wt 1.9 Fe % wt 0.56 V ppm 25 Ni ppm 195 Physical Properties Average Particle Size µm 81 Average Bulk density g/cm 3 0.88 BET Matrix surface Area m 2 /gm 56 BET surface area m 2 /gm 165 (Source: Ivelina Shishkova et al,2011) 4. CATALYST DEACTIVATION FCC catalyst gets deactivated by the deposition of coke on the catalyst surface, during the cracking reactions. The catalyst deactivation also occur by deposition of nitrogen on active sites which neutralize the catalytic activity or by deposition of metals (nickel, vanadium, sodium) which can change or destroy the activity. The blocking of pore by deposition of coke or metals, reduces the mass transfer. Most of the popular theories on the deactivation are based on the time on stream concept. 5. MODELING OF RISER REACTOR 5.1 Riser Kinetics: The feed to the FCC process consist of long chains paraffins, single & multiple ring cycloalkanes, and large aromatic compounds. The catalytic cracking begins with the formations of carbenium ions by the interaction of olefin molecules with the acidic site on the catalyst followed by the beta scission of the carbenium ion. In the beta scission reaction,the β bond of the carbenium ion break to form an olefin and a new carbeniumion.the carbanium ion formed by beta scission
Fluidized Catalytic Cracking Riser Reactor Operating Process Variables Study.. 147 can undergo further cracking reaction. The olefin can also be cracked further after being converted to a carbenium ion through hydrogen addition.. Figure 5: Catalytic Cracking of petroleum hydrocarbons 5.2 Riser Hydrodynamics: The riser reactor of a FCC unit can be divided into equal size compartments along the axis. The entry part of riser reactor consists of three phases catalyst(solid phase),hydrocarbons vapors and atomizing steam(gas phase ),and hydrocarbons liquid.in the feed injection zone at the bottom of the riser, the catalyst particles and liquid drops are accelerated upwards, the gas velocity also increases because of feed vaporization, lowers the density of the flowing system due to formation of lower molecular weight products on cracking of gas oil [9]. In the feed injection zone the hydrocarbon feed dispersed in the form of droplets by the feed nozzles system comes in contact with the hot regenerated catalyst. The liquid drops vaporize due to intimate contact between liquid drops and hot catalyst. The flow of feed oil, atomizing steam and hot generated catalyst results in velocity, temperature and concentration gradients of high magnitude in this zone [10]. Catalyst temperature decreases due to heat requirement for increasing sensible heat of feed, vaporization and cracking reactions. Catalyst activity also decreases due to deposition of coke on catalyst surface.
148 P.K. Yadav and Dr. Rajeev Kumar Garg Fig. 6: Schematic of the riser flow. The feed injection design is important to control the flow of hydrocarbons at plug flow conditions for minimizing the temperature gradients in the inlet zone that causes undesirable cracking reactions. Increased number of nozzles at the bottom promotes proper contact of the feed oil with the catalyst particles. Feed nozzle design is also important which affect the performance of FCCU. In the middle and upper section,hydrocarbons vapors and solid catalyst are present since all the feed droplets vaporizes after travelling 2-4 m up from the feed inlet. Fig. 7: Entrance (a) and Exit (b) of a riser Initial decline in gas velocity occur because of the sharp increases in the gas void fraction, due to increase in the moles of the gas as a result of cracking.after this initial decline, the gas velocity starts increasing as the cracking reactions along the
Fluidized Catalytic Cracking Riser Reactor Operating Process Variables Study.. 149 riser height continues to increase.the initial sharp increase in the catalyst velocity is due to the sharp fall in solid volume and drag exerted by the gas. After this initial sharp increase, catalyst velocity keeps on increasing gradually all along riser height. High value of slip factor are predicted in the riser entry zone which gradually decreases along the riser height.(gupta et. Al,2001)[11] Core annulus structure exists in FCC risers. At the bottom, generally there is higher concentration of solids and flow is highly nonhomogeneous. (Lopes,G.C et al,2011)[12] 5.3 Effect of catalyst to oil ratio: The variation of catalyst to oil ratio in riser mainly effect various products yield. Flow rate of catalyst in the reactor increases with increase in catalyst to oil ratio. Due to more catalyst flow rate,the number of active sites increases, cause more cracking, increases conversion of gas oil and yield of fuel gases and coke increases. Higher rate of reaction by increase of catalyst to oil ration also produces more coke. The activity of the catalyst decreases due to deactivation of catalyst by deposition of coke and drop in gasoline yield occur. 5.4 Effect of Temperature: The temperature inside the FCC riser reactor decreases because of endothermic reactions. Catalyst temperature at the inlet of riser falls sharply, because sensible heat of catalyst coming from the regenerator is utilized in providing heat for raising the sensible heat of feed, for vaporizing the feed and for further heating of the vaporized feed. The decrease in reaction mixture temperature and catalyst activity along the riser height cause a decline in the reaction rate hence the temperature gradient falls appreciably with increasing riser height. (Gupta et. Al,2001)[11]. 5.5 Effect of Feedstock: Feedstock qualities is measured in term of crack abilities parameter. Crack ability is a function of the relative proportions of paraffinic, naphthenic, and aromatic species in the feed. Crack ability of FCC feedstock s correlated with the UOP characterization factor K as : K= 3 TR/s where TR is the molal average boiling point of the feedstock, R; and s is its specific gravity. Sulfur compounds do not affect crack ability.
150 P.K. Yadav and Dr. Rajeev Kumar Garg Table 3: Range of K Relative Crackability Feedstock Type Sr.No K value Feedstock Type 1 >12.0 High Paraffinic 2 11.5-11.6 Intermediate Naphthenic 3 <11.3 Refractory Aromatic The organometallic compounds decompose on the circulating catalyst, with the metals remaining irreversibly deposited on the catalyst. These deposited metals effect the process in two ways: i. They affect the product distribution, causing more light gases, especially hydrogen, to be formed; ii. They have a serious deactivating effect on the catalyst. With increase in end point of feedstock, organomettalic compounds content increases dramatically causing process problem. These compound can be removed by hydro treatment of feedstock in form of H2S and NH3. 5.6 Other Process Variables. Pressure and reaction time are some other process variables which affect conversion. High conversion and coke yield are favored by higher pressure. However, conversion is not affected by pressure since a substantial increase in pressure is required to significantly increase conversion. Conversion also increases with increase in reaction time for cracking. Reaction time can be varied by making variation in feed rate, steam rate, recycle rate, and pressure. Conversion can be increased by a decrease in the rate of injection of fresh feed. Due to this residence time increases, which cause over cracking of gasoline to liquefied petroleum gas and dry gas. Selective cracking may be promoted by addition of steam to lower hydrocarbon partial pressure. Residence time can also be decrease by lowering reactor pressure or increase the recycle rate. These variables needs to be optimized to achieve optimum conversion corresponding to given feed rate, feed quality, processing objectives and other unit constraints(reactor temperature, regenerator temperature, catalyst circulation etc.)
Fluidized Catalytic Cracking Riser Reactor Operating Process Variables Study.. 151 6. CONCLUSIONS 1. Kinetic modeling of the riser reactor is based either on the lumping scheme or on the single events approach. The kinetic constants evaluated using the lumping scheme are empirical in nature and are too much feed and plant specific. 2. FCC riser reactor model are dependent on the values of cracking reactions rate constant, which can easily be obtained with the help of kinetic model for different characteristics of the feedstock, type of catalyst, activity of catalyst and operating parameters. 3. In FCC riser the gas and solids feed flow rates influence the flow pattern. A different interaction between the phases changes the way in which heat is transferred and consequently the yield of the reactions. Thus, it is important to know the non uniformities of the flow in order to correctly predict the reaction yield. It is necessary to maintain a more uniform radial dispersion to get a high yield of gasoline. 4. Zeolite and different modified form of zeolites are mainly used as FCC catalyst for improvement in gasoline yield, octane number enhancement, production of cleaner fuels and olefins productions. 5. Pressure, reaction time, reactor temperature and catalyst to oil ratio are some important process variable which affect conversion in riser. ACKNOWLEDGEMENT The author gratefully acknowledge I K Gujral Punjab Technical University, Kapurthala (India) for providing guidance, support and all other required facilities. REFERENCES [1] G.M.Bollas,A.A.Lappas,D.K.Iatridis,I.A.Vasalos, 2007, Five Llump Kinetic Model with selective catalyst deactivation for the prediction of the product selectivity in the fluid catalyst cracking process, Catalyst today,vol 127,pp 31-43. [2] In-Su han,chang-bock Chung,James B Riggs,2000, Modeling of a Fluidized Catalytic Cracking Process, Computer and Chemical Engineering,vol 24, pp 1681-1687. [3] Kiran Pashikanti and Y.A.Liu,2011, Predictive Modeling of Large Scale Integrated Refinery Reaction and fractionation systems from plant data.part 2:Fluid Catalytic Cracking (FCC) process,energy Fuels, vol 25, pp 5298-5319.
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