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A Comparative Study of Different KineticLumps Models in the Fluid CatalyticCracking Unit Using COMSOL MultiphysicsD. Yousuoa & S. E. Ogbeideaa Department of Chemical Engineering, Faculty of Engineering,University of Benin, Benin City, NigeriaPublished online: 22 Dec 2014.

To cite this article: D. Yousuo & S. E. Ogbeide (2015) A Comparative Study of Different KineticLumps Models in the Fluid Catalytic Cracking Unit Using COMSOL Multiphysics, Petroleum Science andTechnology, 33:2, 159-169, DOI: 10.1080/10916466.2014.958237

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Petroleum Science and Technology, 33:159169, 2015Copyright C Taylor & Francis Group, LLCISSN: 1091-6466 print / 1532-2459 onlineDOI: 10.1080/10916466.2014.958237

A Comparative Study of Different Kinetic Lumps Modelsin the Fluid Catalytic Cracking Unit Using

COMSOL Multiphysics

D. Yousuo1 and S. E. Ogbeide11Department of Chemical Engineering, Faculty of Engineering, University of Benin,

Benin City, Nigeria

The COMSOL Multiphysics computational fluid dynamics software was used to simulate the fluidcatalytic cracking (FCC) riser reactor of the FCC unit. The 4-, 5-, 10-, 20-, and 35-lump kinetic modelswere used to describe the kinetics of the cracking reactions in the riser reactor. The results of the kineticlump models of the distributions of pressure, velocity, temperature, and yields of products were comparedusing practical values from the Port Harcourt Refinery Company plant. The results showed that the higherthe kinetic lumps the better the accuracy of the prediction of the product yields and that the 10-, 20-, and35-lump kinetic models could be used to predict the yield of various fractions of the riser reactor usingCOMSOL Multiphysics.

Keywords: Fluid catalytic cracking, FCC, computational fluid dynamics, CFD, Riser reactor, lumpingschemes, product yields

1. INTRODUCTION

The kinetics modeling of fluid catalytic cracking (FCC) has been traditionally based on using alumping strategy: chemical species with similar behaviors are grouped together forming a similarnumber of pseudo species. The lumping of species is important to make the kinetic modeling atraceable exercise. Detail work on kinetic lumping has been reported previously (Weekman and Nace,1970; Pitault et al, 1994; Gao et al., 1999; Gupta et al., 2005; Ahari et al., 2008; Hernandez-Barajaset al., 2009; Heydari et al., 2010; Jiang et al., 2013).

In this study, the COMSOL Multiphysics computational fluid dynamics (CFD) software was usedto simulate the FCC riser reactor of the FCC unit (FCCU). The 4-, 5-, 10-, 20- and 35-lump kineticmodels were used to describe the kinetics of the cracking reactions in the riser reactor to investigatethe kinetics of lumping scheme in the riser and to ascertain the comparative advantage of one lumpover the other.

Address correspondence to D. Yousuo, Department of Chemical Engineering, Faculty of Engineering, University ofBenin, P.M.B. 1154, Benin City, Nigeria. E-mail: digieneniyousuo@yahoo.com

Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/lpet.

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160 D. YOUSUO AND S. E. OGBEIDE

FIGURE 1 The FCC reactor.

2. METHODOLOGY

2.1 The Riser Kinetic Model

The modeling was based on the schematic flow diagrams of the riser reactor in the FCCU reactoras presented in Figure 1. The FCCU reactor consists of the riser reactor, reactor catalyst stripper,reactor separator or disengager, reactor cyclones, and other auxiliary parts. The riser reactor is 33 m

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DIFFERENT KINETIC LUMPS MODELS IN THE FCC UNIT 161

HFOPh

HFONh

HFOAh

HFORhLFONh

G

LFOAh

LFORh

LFOPh

C

K9

K6

K4

K12

K10

K7 K3

K5

K19

K11

K16

FIGURE 2 Ten-lump kinetic scheme.

long and the diameter is 0.8 m. The diagram for the 10-lump kinetic scheme of the FCCU is shown inFigure 2 while that of the other lumps have been shown elsewhere (Yousuo, 2014). The lumps for the10-lump kinetic scheme are the heavy fuel oils from the paraffins (HFOph), the heavy fuel oils fromthe naphtenes (HFONh), the heavy fuel oils from the aromatic substituent groups (HFOAh), the heavyfuel oils of the carbons among the aromatic rings (HFORh), the light fuel oils from the paraffins(LFOph), the light fuel oils from the naphthenes (LFONh), the light fuel oils from the aromaticsubstituent groups (LFOAh), the light fuel oils of the carbons among the aromatic rings (LFORh),gasoline (G), and COKE (C). In this model COKE (C) represents 50% coke and 50% C1-C4 gases.The rate expressions of the 10-lump kinetic scheme and other details are shown elsewhere (Jacobet al., 1976).

2.2 Plug-flow Reactor Equations

The reactor model is an ideal plug-flow reactor, described by the mass balance in Eq. (1). Assumingconstant reactor cross section and flow velocity, the species concentration gradient as fraction ofresidence time ( ) is given in Eq. (2). The reaction rates are given by rf = KjCi and to account forthe different time scales, two different activity functions are used. For the non-coking reactions theactivity function is given in Eq. (3).

(1)

(2)(3)

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162 D. YOUSUO AND S. E. OGBEIDE

The reaction rates are modified by the activity according to Eq. (4). For the coking reactions, theactivity function is given by Eq. (5) where is a deactivation constant depending on the residencetime. The modified reaction rates are given by Eq. (6). The coke content is given by Eqs. (7) and (8).The values of a, b, , and are obtained from Gupta et al. (2005), Ahari et al. (2008), and Yousuo(2014) as shown in Eqs. (9) and (10).

(4)(5)

(6)

(7)

(8)(9)

(10)For the mass transport, the inlet and outlet concentrations are obtained from Eq. (11) and the velocityand pressure for ideal gases are obtained from Eqs. (12) and (13), respectively. The static head ofthe catalyst in the riser can be calculated using Eq. (14). The details on choosing the void fractionvariable, assumed gas velocity, slip factor, and the vaporization heat of the feed in the riser inlet havebeen shown elsewhere (Gupta et al., 2005; Yousuo, 2014).

Inlet : c = cin, Outlet : c = cout (11)

(12)

(13)

(14)For momentum transport, the inlet and outlet pressure are obtained from Eq. (15)

(15)For energy balance, neglecting pressure drop, the energy balance for an ideal reacting gas, as well asan incompressible reacting liquid is given by Eqs. (16) and (17). The inlet temperature is calculatedputting into consideration the energy balance of the components. Equation (18) is used in calculatingthe inlet temperature while Eq. (19) is used for calculating the outlet temperature.

(16)

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DIFFERENT KINETIC LUMPS MODELS IN THE FCC UNIT 163

(17)

At z = z0 = 0, ws = 0, Qext = 0, Eq. (16) and (17) becomes

i MiCp,idTdV

Q = 0

This implies that

That is

(18)

At z = h or z, ws = 0, Qext = 0, Eqs. (16) and (17) become

i MiCp,idTdV

= Q

That is,

This implies that

That is,

By our correlation

is

hence

Outlet: (19)

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164 D. YOUSUO AND S. E. OGBEIDE

TABLE 1Boundary Conditions

SETTING BOUNDARY 3 BOUNDARY 4 BOUNDARIES 1 and 2

TemperatureBoundary type Inlet outlet wallBoundary condition Temperature Temperature Thermal insulationValue T 0 T n

ConcentrationBoundary type Inlet outlet wallBoundary condition Concentration Concentration Insulation/SymmetryValue cin for all species cout for all species

Velocity and pressureBoundary type Inlet outlet wallBoundary condition Velocity Pressurre, no viscous stress No slipValue w0 = vs, uo = v0 = 0 P0 = Pn

2.3 Boundary Conditions

The boundary conditions for the riser reactor are shown in Table 1.

3. MATERIALS, MESH GENERATION AND SIMULATION

3.1 Materials

The average molecular weight, the thermodynamic properties of the feed, the plant operating condi-tions and the properties of the catalyst used in this study, the specific heat of different lumps, and thekinetic parameters for cracking reactions can be found elsewhere (Port Harcourt Refinery CompanyProject, 1987; Gupta et al., 2005; Ahari et al., 2008).

3.2 Mesh Generation and Simulation

The extra fine mesh generator of the COMSOL Multiphysics software was used to produce gridrefinement in the riser reactor. The riser reactor was meshed into 77,358 triangular elements. Figure 3shows the computational grid used to represent the computational domain of the riser reactor. Thesimulations in this work used the three-dimensional model of the COMSOL multiphysics CFDsoftware in a Windows Vista Home Premium HP Pavilion dv 6500 Notebook PC (processor: IntelCore 2 Duo CPU T5450 @ 1.661.67 GHz; memory [RAM]: 250 GB; and type: 32-bit operatingsystem).

4. RESULTS AND DISCUSSION

Figure 4 shows the temperature in the reactor riser. Gas oil/heavy diesel oil, medium pressure steam,and fresh catalyst enter the reactor riser at a temperature of 505, 464, and 1004 K, respectively. Themedium pressure steam atomizes the gas oil/heavy diesel oil as they travel up along the reactor riserincreasing catalysis and the rate of reaction. The hydrocarbons and catalyst mixture travel upward

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DIFFERENT KINETIC LUMPS MODELS IN THE FCC UNIT 165

FIGURE 3 Computational domain and grid.

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166 D. YOUSUO AND S. E. OGBEIDE

FIGURE 4 The temperature in the riser reactor.

and the temperature inside the FCC riser decreases because of the endothermic cracking reactions.The mixture temperature of the riser falls sharply to 803 K for the Port Harcourt Refinery Company(PHRC) plant because sensible heat of catalyst coming from the regenerator is utilized in providingheat for raising the sensible heat of feed, for vaporizing the feed, and for further heating of thevaporized feed.

Figure 5 shows the yield in the reactor riser of the PHRC plant of the 10-lump kinetic model.HFOPh, HFONh, HFOAh, and HFORh are broken down and as a result their weight fraction de-creased along the riser from the inlet to the outlet. LFOP i, LFONi, LFOAi, and LFORi lumps wereformed and later broken down to G and C lumps. The gasoline (G) yield was 51% and coke (C)

FIGURE 5 The yield in the riser reactor.

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DIFFERENT KINETIC LUMPS MODELS IN THE FCC UNIT 167

TABLE 2Kinetic Lumps, Predicted Values, and Deviation of Predicted Values From the Practical Values

Predicted Value

PHRC Plant Data Practical Value 4-lump 5-lump 10-lump 20-lump

Gasoline yield, wt% 49.50 67 60 51 50.5Coke yield, wt% 5.90 6 6 6.25 7Outlet temperature, K 805 829 821 803 803

Deviation of Predicted Values From the Practical Values

Gasoline yield, wt% 49.50 17.5 10.5 1.5 1Coke yield, wt% 5.90 0.1 0.1 0.35 1.1Outlet temperature, K 805 24 16 2 2

yield was 12.50%. For the 10-lump kinetic model, coke (C) represents 50% coke and 50% C1C4gases and therefore the actual coke yield was 6.25%.

Table 2 shows the predicted values of the 4-, 5-, 10-, 20-, and 35-lump kinetic models, whichwere compared with the PHRC plant practical values and the deviations of the predicted valuesfrom the PHRC plant values. From the tables it is observed that with four lumps, the deviation ofthe predicted values from the PHRC plant values is 17.5% for gasoline yield and 0.1% for cokeyield, and outlet temperature is 24 K; with five lumps, the deviation is 10.5% gasoline yield and0.1% for coke yield, and the outlet temperature is 16 K; with 10 lumps, the deviation is 1.5% forgasoline yield and 0.35% for coke yield, and outlet temperature is 2 K; with 20 and 35 lumps, thedeviation is 1% for gasoline yield and 1.1% for coke yield, and outlet temperature is 2 K. Thisimplies that the predicted values and the practical values from the PHRC plant become closer asthe lumps increases and because the difference in the deviation of the predicted values of gasoline,coke, and the temperature for 10, 20, and 35 lumps are very small, and any from the 10-lump kineticscheme could be used to predict the yield of gasoline, coke, and temperature of the riser reactorusing COMSOL Multiphysics.

5. CONCLUSION

The 4-, 5-, 10-, 20-, and 35- kinetic schemes were effectively used to describe the kinetics of thecracking reactions in the FCCU. The results showed that the predicted values and the practicalvalues from the PHRC plant become closer as the lumps increased. The results also showed that forgasoline, coke, and the temperature the difference in the deviation of the predicted values for 10, 20,and 35 lumps are very small and this could mean that any from the 10-lump kinetic scheme couldbe used to predict the yield of gasoline, coke, and temperature of the riser reactor using COMSOLMultiphysics.

REFERENCES

Ahari, J. S., Farshi, A., and Forsat, K. (2008). A mathematical modeling of the riser reactor in industrial FCC unit. Pet. Coal50:1524.

Gao, J., Xu, C., Lin, S., and Yang, G. (1999). Advanced model for turbulent gas-solid flow and reaction in FCC riser reactors.AIChE J. 45:1095.

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168 D. YOUSUO AND S. E. OGBEIDE

Gupta, R., Kumar, V., and Srivastava, V. K. (2005). Modeling and simulation of fluid catalytic cracking unit. Rev. Chem. Eng.21:95131.

Jacob, S. H., Gross, B., Voltz, S. E., and Weekman, V. M. (1976). A lumping and reaction scheme for catalytic cracking.AICHE J. 22:701713.

Jiang, L., Zheng-Hong, L., Xing-Ying, L., Chun-Ming, X., and Jin-Sen, G. (2013). Numerical simulation of the turbulentgas-solid flow and reaction in a polydisperse FCC riser reactor. Power Technol. 237:569580.

Hernandez-Barajas, J. R., Vazquez-Roman, R., and Felix-Flores, M. G. (2009). A comprehensive estimation of kineticparameters in lumped catalytic cracking reaction models. Fuel 88:169178.

Heydari, M., AleEbrahim, H., and Dabir, B. (2010). Study of seven-lump kinetic model in the fluid catalytic cracking unit.Am. J. Appl. Sci. 7:7176.

Pitault, I., Nevicato, D., Foressier, M., and Bernard, J.-R. (1994). Kinetic model based on a molecular description for catalyticcracking of vacuum gas oil. Chem. Eng. Sci. 49:42494262.

Port Harcourt Refinery Company Project. (1987). Nigerian National Petroleum Corporation Process. Project 9465A: Area 3FCCU 16.

Weekman, V. W., and Nace, D. M. (1970). Kinetics of catalytic cracking selectivity in fixed, moving and fluid-bed reactors.AICHE J. 16:397404.

Yousuo, D. (2014). Application of COMSOL multiphysics in the simulation of the fluid catalytic cracking riser reactor andcyclones. PhD thesis, Benin City, Nigeria: Department of Chemical Engineering, University of Benin.

NOMENCLATURE

c: Concentration, mol/m3E: Activation energy for

rate constant, J/molg: Acceleration due to

gravity, m/sec2P: The pressure of gases, paR, r: Rate expression valueT: Tempersature, Kt, : Residence time, secv: Volume, m3z: Axial distance from the

inlet, mCP cat (Cpcat): Specific heat of

catalyst, J/kgKCp ds(Cpds): Specific heat of steam,

J/kgKCpL GO (CPLgo): Specific heat of liquid

gas oil, J/kgKCpV GO (CPVgo): Specific heat of gaseous

gas oil, J/kgKCi: Species molar concen-

trations, mol/m3cin: Inlet concentration,

mol/m3cout: Outlet concentration,

mol/m3Kd: Deactivation constant

M go (Mgo): Mass flow rate of gas oil,kg/sec

M ds (Mds): Mass flow rate of steam,kg/sec

M ca (Mcat): M cat (Mcat): Mass flowrate of catalyst, kg/sec

Pin: Inlet pressure, paRg (Ru): Gas constant, J/(mol.K)Tcat: Temperature of the cata-

lyst, K: Void fractionTgo: Temperature of gas oil,

KTvap: Gas oil vapourization

temperature, Kv0: Outlet velocity, m/secTds: Temperature of the

steam, KV R, , V: Reactor volume, m3Ws: Additional work termQ: Heat due to chemical re-

action, J/m3.secQext: Heat added to the sys-

tem, J/m3.sec: Viscosity, N.S/m2: Density, kg/m3: Slip fact

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DIFFERENT KINETIC LUMPS MODELS IN THE FCC UNIT 169

Subscripts

j: Refers to lump j that is crackedi: Refers to lump i that is formedp (or s): Particle/solid

a (or f): Air/fluidcat: Catalystc: Coke content

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