Successfully reported this slideshow.
We use your LinkedIn profile and activity data to personalize ads and to show you more relevant ads. You can change your ad preferences anytime.
International Journal of Chemical Engineering and Applications, Vol. 1, No. 2, August 2010                                ...
International Journal of Chemical Engineering and Applications, Vol. 1, No. 2, August 2010                                ...
International Journal of Chemical Engineering and Applications, Vol. 1, No. 2, August 2010                                ...
International Journal of Chemical Engineering and Applications, Vol. 1, No. 2, August 2010                                ...
International Journal of Chemical Engineering and Applications, Vol. 1, No. 2, August 2010                                ...
International Journal of Chemical Engineering and Applications, Vol. 1, No. 2, August 2010                                ...
Upcoming SlideShare
Loading in …5
×

Dynamic modeling and simulation of catalytic

889 views

Published on

  • Be the first to comment

  • Be the first to like this

Dynamic modeling and simulation of catalytic

  1. 1. International Journal of Chemical Engineering and Applications, Vol. 1, No. 2, August 2010 ISSN: 2010-0221 Dynamic Modeling and Simulation of Catalytic Naphtha Reforming H.M.Arani1, S.Shokri2, M.Shirvani3 surface. Abstract—In this paper, according to the recent progresses in More recently Jorge [5] presented a twenty-four lumpsnaphtha reforming technology, dynamic modeling of catalytic model with 71 reactions and Taskar [6] proposed a morenaphtha reforming process is studied. The dynamic model is detailed thirty-five lumps model used for the optimization ofcomposed of the reforming reaction model, the heat exchangermodel, furnace model, which together, are capable to capture semi-regenerative naphtha catalytic reformers. Generallythe major dynamics that occur in this process system. Kinetic speaking, detailed models are theoretically sound but quitemodeling of the reactions of the fixed bed reactors connected in complicated, consequently difficult for application becauseseries form the most significant part of the overall simulation of very complex lumping scheme and parameter estimation toeffort. MATLAB software in SIMULINK mode is used for maintain a good balance between kinetics, simplicity anddynamic simulation. Hougen-Watson Langmuir-Hinshelwood applicability of the model [7].type reaction rate expressions are used to represent rate of eachreaction.The results show models are in fair agreement with the On the other hand, dynamic modeling was performed byactual operating data. Yongyou et al. [15] by a 17 lumps kinetic model and solution was performed after linearization. Actually, very limited Index Terms—Catalytic reforming, Dynamic modeling, attempt on dynamic simulation of this process have beenSimulation, Naphtha appeared in the literature. A modeling and dynamic simulation on the catalytic reforming process was carried out by Ansari [16] using Smiths kinetic scheme. I. INTRODUCTION A. Process Description Catalytic naphtha reforming is an important process forproducing high octane gasoline, aromatic feedstock and Catalytic naphtha reforming contains three reactors inhydrogen in petroleum-refining and petrochemical series, as shown in Fig. 1. It is generally carried out inindustries. three/four fixed or moving bed reformers. The catalyst used Advanced process control and process optimization in reformer is commonly a bifunctional bimetallic catalyststrategies are important to be considered in catalytic such as Pt-Re/Al2O3, providing the metal function and thereforming of naphtha due to its impacts on refineries profit. acid function. The product stream from the first reactor afterThese are achieved mostly via a reliable mathematical preheating enters the second reactor and similarly the productdynamic model. stream from the second reactor after preheating enters the The catalytic reforming process came into operation in third one. Thus, the reactor feeds are raised to the properrefineries in the late 1940s. The reforming of naphtha feed temperature by heaters located before the reactors, since thestocks over platinum catalysts was pioneered by Universal reactions are endothermic. The product stream from the finalOil Products (UOP), with the plat forming process introduced reactor enters a separator wherein hydrogen rich gas stream isin 1947. separated from the products which is then recycled back and Concerning the kinetic modeling of the naphtha processes mixed with the first reactor fresh feed. Reactors are ofSmith [1] firstly proposed a four lumps model by considering different sizes, with the smallest one located in the first stagenaphtha as being composed of naphthenes, paraffins, and the largest one in the last. Temperature profile of thearomatics and light hydrocarbons (<C5), lumps with average reactors is shown in Fig. 2.carbon number properties in 1959. However, it is not muchprecise because of its limited lumps. Kmak [2] developed adetailed model (Exxon model) including twenty-two lumps,which was refined by Froment [3] who developed a moredetailed twenty-eight lumps model with 81 reactions.Ramage [4] developed the Mobile thirteen lumps model thatconsidered the adsorption of chemical lumps on the catalyst 1,2 Research Institute of Petroleum Industry (RIPI), West Blvd. AzadiSport Complex, Tehran, Iran 3 Assistant Professor, School of Chem. Eng., Iran Univ. Sci. Tehran., Iran Corresponding author: Tel.:+982148252503; fax: +982144739713 Figure 1. Process flow scheme of the catalytic naphtha reformer E-mail address: Shokris@ripi.ir 159
  2. 2. International Journal of Chemical Engineering and Applications, Vol. 1, No. 2, August 2010 ISSN: 2010-0221 Where, Pi in the above equations is the partial pressure of the components and T is the temperature. As done by Hu et al. (2003) [9] and Taskar (1996) [6], all constants of adsorption terms were obtained by studying the reforming of C7 hydrocarbons. Thermodynamics of reactions are considered in another work [10]. B. Reactors Dynamic Model Figure 2.Temperature distribution in reactors length A dynamic mathematical model with the kinetics described above is developed by assembling the mass and energy balance on the system of reforming reactions. The II. DYNAMIC MODEL mass balance provides the variation of the concentrations of There are a number of different methods for modeling the the components along the reactors, and the energy balancecatalytic reforming process, and the main idea behind all of gives the variation of temperature. Mass and energy balancesthem is to determine the operating conditions of the for each reformer are both nonlinear partial differentialreforming unit and to predict the yield of reformate and the equations in space and time. By taking component materialtemperature profile accurately. balances as well as energy balanc on a differential element A model must predict the behavior not only within the “dw” of catalyst bed the following system of differentialreactor, but in the auxiliary areas of the unit as well. It should equations can be obtained.also consider the complex nature of the process and the Mass balance equations for an element of catalyst weight inreactions which take place during the process of reforming. a plug flow reactor (proved in appendix A) are: A. Reaction Kinetic Model Reactions of catalytic naphtha reforming are elementary dFi nr ε d ( ci ) − + ∑ γ i , j rj =and Hougen-Watson Langmuir- Hinshelwood type of dw j =1 ρ b dtreaction rate expressions are used to describe the rate of each (1)reaction. Rate expressions of this type explicitly account for the Where:interaction of chemical species with catalyst and contain ci Fi yi = nc = nc (2)denominators in which characteristic terms of adsorption ofreacting species are presented. The reaction rate coefficients ∑c i =1 i ∑F i =1 iobey the Arrhenius law. All reaction rate equations areshown in Table 1. m Ft = (3) M av TABLE 1: REACTION RATE EQUATIONS nc M av = ∑ yi M i (4) i =1 Fi = yi Ft (5) The energy balance equation for the reactor shown in Fig. 1 can be written (proved in appendix B) as: nc nc dT nr − ( ∑ Fi C pi ) − ∑ rj ( ∑ H iγ i , j ) dT i =1 dw j =1 i =1 (6) = dt ε nc ( C pcata + ∑ ci C pi ) ρb i =1 Where, ni is the number of moles of ith lump for element with dw of catalyst weight. Where, In the above table, adsorption terms for the metal function T(θ), and for the acid function (Γ), are defined as follows (Van. H i = H 0 + ∫ C pi dT (7)Trimpont, Marin and Froment, 1988 ) T0 In Hu et al. [9] catalyst heat capacity in the denominator of energy equation was ignored, while this has great influence on dynamic response of reactors. 160
  3. 3. International Journal of Chemical Engineering and Applications, Vol. 1, No. 2, August 2010 ISSN: 2010-0221 C. Furnace ِ Model: Assuming perfect mixing results in achieving a lumped Simple model of fired heater furnaces are used here. parameter model for the heat exchanger, as bellow: In the mathematical model of this system the followingassumptions are used: d (C p , cold M cold Tcold , o )1) Outlet gas temperature from the furnace is equal to tube dt (12) wall temperature = Fcold ( H cold ,i − H cold , o ) − q2) The temperature of the tube wall is equivalent over the length of tube d (C p , hot M hotThot , o )3) Variation of gas temperature inside tube is linear (13) dt We consider the tubes for heat transfer according to fig.3 = Fhot ( H hot ,i − H hot , o ) − q The heat transferred q is: (14) q = F U A Δ Tm Where, ΔTm is logarithm of mean temperature difference. (Thot ,i − Tcold ,o ) − (Thot ,o − Tcold ,i ) (15) ΔTm = Figure 3. Furnace tube. Thot ,i − Tcold ,o Unsteady state energy balance with the considered ln( )assumptions is as: Thot ,o − Tcold ,i d (mwC p wTw + m f C p f T f , m ) E. Octane Number: dt (8) Octane number of the product from naphtha reforming unit . = q − (T f , out − T f , in )C p f m f can be expressed by concentrations as follows [14]: n (16)Where, RON = b W ∑ r r (T f , in + T f , out ) r =1 T f ,m = (9) Where, br are coefficients and Wr are weight concentrations. 2 TABLE 2: GROUP DEFINITION AND REGRESSION COEFFICIENTS OF LINEAR Supposing linear variation of gas temperature, tube exit RON MODEL DEVELOPED BY WALSH ANDtemperature is equal to Tf,out and Tf,m. Substituting equation (9) COWORKERS.into (8) results in: • m f C p f d (T f ,in ) d (T f ,out ) q − (T f ,out − T f ,in )C p f m f − (10) = 2 dt dt mf Cp f (mwC p w + ) 2 In the above equations q is the amount of heat absorbedby tubes. If the amount of heat generated by fuel is denotedby Q and the efficiency of the furnace by η, then: q =η Q (11) D. Heat Exchanger Model: Low order lumped parameter model of heat exchangerswere used. Mathematical modeling plays an important rolenot only in designing and optimizing chemical engineeringprocesses but also in studying the dynamic characteristicsand controllability of existing equipment. Fig. 4 depicts the thermal shell balance scheme on heatexchanger for developing the required model. III. SIMULATION Level-1 M-files of S-function in MATLAB software are used in this paper for simulating the above equations. Distribution of nodes in the space of reactor plays an important role in converging to the solution when solving the equations. Nodes in beginning of reactor are very close together according to Fig. 5. Figure 4. Heat Exchanger. 161
  4. 4. International Journal of Chemical Engineering and Applications, Vol. 1, No. 2, August 2010 ISSN: 2010-0221 714.5 Output temp. of first reactor 714 713.5 713 Figure 5. Node distribution in reactor length. 712.5 0 2000 4000 6000 8000 10000 12000 time (sec) Derivative terms in the right hand side of partial Figure 7. Comparison of plant data with simulated model for 3 o Cdifferential equations are fitted by second order polynomial. temperature rise in reactor 1. 744.7 y = cx 2 + bx + a (17) 744.6 Output temp. of second reactor 744.5 Slope is calculated from the equation: 744.4 744.3 dy (18) = 2cx + b 744.2 dx 744.1 Constants b and c can be calculated using the values of 744three consecutive nodes. 743.9 0 2000 4000 6000 8000 10000 12000 Fig. 6 shows a flow diagram of the system simulated via time (sec)MATLAB SIMULINK. Figure 8. Comparison of plant data with simulated model for 3 o C temperature rise in reactor 2. 763.8 763.7 Output temp. of third reactor 763.6 763.5 763.4 763.3 763.2 763.1Figure 6. Dynamic model and simulation of model by MATLAB software. 763 0 2000 4000 6000 8000 10000 12000 time (sec) Figure 9. Comparison of plant data with simulated model for 3 o C In the SIMULINK mode all variables can be plotted versus temperature rise in reactor 3.time. The Solver type used in simulation is ode23tb with SATISFACTORY FITTING between PLANT AND MODEL ISvariable steps. DETECTED FROM THE FIGURES. IV. RESULTS AND DISCUSSION Results of simulated model are compared with V. CONCLUSIONexperimental data obtained from the operation data sheets of In this paper, simulation of the dynamic model of thea refinery plant in Iran and are shown in Fig. 7. In order to catalytic naphtha reformer process by MATLAB software isvalidate the dynamic model, output temperatures of the presented. The kinetic parameters of model are based on thereactors for input temperatures of three degrees centigrade steady state condition and are obtained from a commercialare compared with the data of a refinery in Iran. Plant data are plant data furnished by a domestic petroleum refinery. Thiscollected at every 600 seconds. The results of simulation and dynamic model is in form of partial deferential equations.comparison are shown in figures 7, 8 and 9. The nonlinear model of the system was solved numerically by MATLAB software. The simulation results reveals of satisfactory fitness between data and the model. 162
  5. 5. International Journal of Chemical Engineering and Applications, Vol. 1, No. 2, August 2010 ISSN: 2010-0221 NOMENCLATURE nc d (∑ Fi H i ) nc dH i − i =1 dw = −(∑ Fi )dw (24)Ac Catalyst bed area dw i =1 dwci (kmol / m3 ) Particle i mole concentration nc dFi Catalyst diameter − (∑ H i )dwd p( m ) i =1 dwE Aj Activation energyG( kg / m 2 .s ) Total mass flux Internal energy is related to enthalpy and by expansion ofε Porosity of catalyst bed the right hand side of equation (20) we have: ncμ( pa.s ) Viscosity d ( Ecata + ∑ ni ( H i − PVi )) dEsysri (kmol / kg cat.s) Reaction Rate = i =1 dt dtR Gas constant dEcata nc dH i = dw + ∑ niρ s ( kg / m 3 ) Density of catalyst particle dt i =1 dt (25) ncρ f ( kg / m )3 dn d ( PV ) Fluid Density + ∑ Hi i − i =1 dt dtΩ( m2 ) Catalyst bed Area = dEcata nc dw + ∑ ni dH i dt i =1 dtMav (kg / kmol) Average molecular weight nc εdw dci d ( PV ) + ∑ Hi ( ) − i =1 ρ b dt dt APPENDIX A Where V is the volume of catalyst element upon which the Mass balance equation for element of catalyst weight in a balance is performed.plug flow reactor is: ε V = dw (26) nr ρb Fi.w − Fi , w + dw + ∑ γ i , j rj dw ni is the number of ith lump mole in the element with dw j =1 (19) of catalyst weight. d (ni ) d (ci ) ε By equations (19) and (23) the following equation is = = dw dt dt ρb obtained: dE sys dE cata nc dH i dt = dt dw + ∑n i dt + (27) In the above equation ni is as bellow: i =1 nc dFi nr d (P) ε ε ∑H i (− + ∑γ i, j jr ) dw − dt ρ b dw ni = ci dw (20) i =1 dw j =1 ρb By equation (7), (20), (23), (25) and (27), and by ignoring And in final step the following equation is attained: pressure effects, we reach to the following equation: nc dT dT ε nc dT dFi nr ε d (ci ) − (∑ Fi C pi ) = C pcata + ∑c C − + ∑ γ i , j rj = (21) dw dt ρ b i pi dt (28) ρb dt i =1 i =1 dw j =1 nr nc + ∑ r j (∑ H i γ i , j ) j =1 i =1 APPENDIX B Energy balance equation for an element of catalyst weight And the resulted final equation is:in a plug flow reactor is: nc nc dT nr − ( ∑ Fi C pi ) − ∑ rj ( ∑ H iγ i , j ) dT i =1 dw j =1 i =1 (29) nc nc dEsys = ∑ FH − ∑ Fi H i ε nc = dt i =1 i i w i =1 w + dw dt (22) ( C pcata + ρb ∑c C i =1 i pi ) Differential form of the above equation is in the form: nc REFERENCES d (∑ Fi H i ) [1] Smith, R.B. (1959). Kinetic analysis of naphtha reforming dEsys − i =1 dw = (23) withplatinum catalyst. Chem. Eng. Pm~ e s s5,5 . 6,76. dw dt [2] Kmak, W.S., Stuckey, A.N. (1973). Powerforming process studies with a kinetic simulation model. AlChE National Meetmg, New Orleans, The following equation is attained by expansion of the left March, PaperNo.56a.hand side of the above equation: [3] Froment, GF.(IY87). Thc Kinetic of Complex Catalytic Reactions. Chem. Eng. Sci., 42,5, 1073. 163
  6. 6. International Journal of Chemical Engineering and Applications, Vol. 1, No. 2, August 2010 ISSN: 2010-0221[4] Ramagc, M.P., Graiani, K.R.,K rambeck, F.l.(1980). Developmcnt of Mobils kinetic reforming model. Chem. Eng. Sci., 35, I, 41[5] lorge Ancheyta~luarezE, duardo Villafuene-Macias. (2000). Kinetic Modeling of Naphtha Catalytic Reforming Reactions. Energy & Fuels, 14, 5, 1032.[6] Taskar, U,, Riggs, J.B. (1997). Modeling and optimization of asemiregenerative catalytic naphtharefarmer. AIChEf., 43, 3, 740.[7] Hu. Y., Su, H., Chu, 1. (2003). Modeling. Simulation andOptimization of Commercial Naphtha Catalytic Reforming Process. Pmceedinxs "/the 42"* IEEE Cofifwmce on Decaion and Coniml, 6206-6211,Dec. 9~12,Hawuii,USA.[8] Rahimpour, M.R., Esmaili, S., Bagheri, GN.A., “Kinetic and deactivation model for industrial catalytic naphtha reforming”, Iran. J. Sci. Tech., Trans. B: Tech., 27, 279- 290(2003).[9] Hu, Y.Y., Su, H.Y., Chu, J., “The research summarize of catalytic reforming unit simulation”, Contr. Instrum. Chem. Id.2,9 (2), 19-23(2002). (In Chinese)[10] H. M. Arani, M. Shirvani2, K. Safdarian and E. Dorostkar," Lumping procedure for a kinetic model of catalytic naphtha reforming" Braz. J. Chem. Eng. vol.26 no.4 Oct./Dec. 2009[11] HOU Weifen, SU Hongye, H, U Yongyou, CHU Jian, Zhejiang University, "Modeling, Simulation and Optimization of a Whole Industrial Catalytic Naphtha Reforming Process on Aspen Plus Platform", Chinese J. Chem. Eng., 14(5) 584-591 (2006).[12] M. R. Rahimpour, "Operability of an Industrial Catalytic Naphtha Reformer in the Presence of Catalyst Deactivation",Chem. Eng. Technol., 29, No. 5,(2006).[13] Yongyou Hu, Weihua Xu, Hongye Su, Jian Chu, Zhejiang university,"Modeling, Simulation and Optimization of Commercial Naphtha Catalytic Reforming Process", IEEE, (2004).[14] George J. fintos, abdullah M. aitan,"Catalytic naphtha reforming, second edition, revised an expanded", MARCELD EKKER, INC, (2004).[15] Yongyou Hu, Weihua Xu, Hongye Su, Jian Chu, Zhejiang university, " A Dynamic Model for Naphtha Catalytic Reformers", IEEE, Taiwan, September 2-4,2004[16] R.M. Ansari, M.O. Tade, "Constrained nonlinear multivariable control of a catalytic reforming process", Control Engineering Practice 6 (1998) 695-706.[17] Anderson, P.C.; Sharkey, J.M.; Walsh, R.P. J. Inst. Petr. 1972, 58 (560), 83. 164

×