Chen18904.qxd The Journal of Engineering Research Vol. 3, No. 1 (2006) 19-30 __________________________________________ *Corresponding author’s e-mail: anishf@ksu.edu.sa Sensitivity Studies for Propane Oxidative Dehydrogenation to Propylene in Circulating Fluidized Bed A.H. Fakeeha*, M.A. Soliman and A.A. Ibrahim Chemical Engineering Department, College of Engineering, King Saud University, P.O. Box 800, Riyadh 11421, Saudi Arabia Received 18 September 2004; accepted 25 April 2005 Abstract: In this paper, simulation and sensitivity studies for propane oxidative dehydrogenation to which were propylene in a circulating fluidized bed is undertaken using a previously developed model. Various experimental kinetics, obtained by several investigators for the reaction using different catalysts, are employed in this study. A comparison is made for the per- formance of certain catalysts when used in a circulating fluidized bed reactor. The effects of changing reaction temperature, solid circulation rate, feed composition, pressure, and gas superficial velocity on reactant conversion and product selectivi- ty and yields are studied. It was found that the catalyst prepared by Ramos et al. is superior to the others with respect to yield. Keywords: Circulating fluidized bed, Modeling, Oxidation, Propane and propylene Notation Ci,c = Concentration of the ith species in the core, mole/L Ci,a = Concentration of the ith species in the annulus, mole/L Dp = Mean particle diameter, m Dt = Riser diameter, m g = Gravitational acceleration constant, m/s2 Gs = Overall solids circulation rate, kg/(m2 s) k = Proportionality constant in the acceleration zone, m-1 kg = Gas mass-transfer coefficient from core to annulus, m/s kc = Reaction rate constant K = Equilibrium constant, mol/L Lacc = Length of the acceleration zone, m P = Total pressure, Pa ri = Rate of the ith reaction, mol/(g.s) rc = Core radius, m QGhO ™«‡ ~¡e ‘ Ú∏HhÈdG ¤G ¿ÉHhÈdG πjƒëàd ÚLhQ~«¡∏d áYRÉædG √~°ùcÓd á«°SÉ°ùM äÉ°SGQO ¬¡«µa .ì.CG@º«gGôHCG .CG.CG h ¿Éª«∏°S .CG.Ω , ::áá°°UUÓÓÿÿGGâe~îà°SG ~≤d .É≤HÉ°S √ôjƒ£J ” »°VÉjQ êPƒ‰ ΩG~îà°SÉH QGhO ™«‡ ~¡e ‘ Ú∏HhÈdG ¤G ¿ÉHhÈdG πjƒëàd ÚLhQ~«¡∏d áYRÉædG I~°ùc’G á°SGQO åëÑdG Gòg øª° àj ‘ É¡eG~îà°SG ~æY äGõØÙG √òg AGO’ áfQÉ≤à ΩÉ«≤dG ”h .äGõØÙG øe O~©d ÚãMÉH I~Y ¥ôW øe Égôjƒ£J ” »àdGh á«FÉ«ª«µdG á«côë∏d á«ÑjôŒ êPɉ I~Y á°SGQ~dG √òg ‘ .á«LÉàf’G h ájQÉ«ÿGh á∏jƒëàdG ≈∏Y RɨdG áYô°Sh §¨° dGh º«≤∏dG õ«côJh áÑ∏°üdG IOÉŸG ôjh~J ∫~©eh πYÉØàdG IQGôM áLQO øe πc ÒKÉJ á°SGQO â“ ~≤dh .QGh~dG ™«ªŸG ~¡ŸG .á«LÉàf’G ¢üîj ɪ«a èFÉàædG π° aG »£©j ¬bÉaQh ¢ùeGQ ᣰSGƒH ô° ÙG õØÙG ¿G ~Lhh ::áá««MMÉÉààØØŸŸGG ääGGOOôôØØŸŸGG.Ú∏HhôH ,¿ÉHhôH ,I~°ùcG ,áLò‰ ,IQGh~dG á©«ªŸG ~¡ŸG 20 The Journal of Engineering Research Vol. 3, No. 1 (2006) 19-30 1. Introduction Alkanes such as propane are available in larger quanti- ties than propylene since it is one of the components of natural or associated gas. The lower cost of propane, encourages further research to develop new catalysts and processes for propane utilization to produce many prod- ucts such as propylene. The reaction can be carried out either in a fixed bed or circulating fluidized bed reactor (CFB), which is shown schematically in Fig. 1. Several investigators studied oxidation reaction in CFB both theoretically and experimentally. The theoreti- cal simulation studies include the work of Fakeeha et al. (2001) for the ammoxidation of propane to acronitrile, Pugsley et al. (1992) and Fakeeha et al. (2000) for n- butane oxidation to maleic anhydride, Pugsley and Berruti (1996) and Pannek and Mleczko (1998) for oxidative cou- pling of methane, Pugsley and Malcus (1997) for the par- tial oxidation of methane to synthesis gas, Patience and Mills (1994) for partial oxidation of propylene to acrolein and Gainetto et al. (1990) for the ammoxidation of propy- lene into acrylonitrile and toluene to benzonitrile. On the contrary, Golbig and Werther (1997) carried out experi- mentally the partial oxidation of n-butane to maleic anhy- dride in CFB. In this paper simulation and sensitivity studies for propane oxidative dehydrogenation to propylene in CFB reactor will be carried out using three different kinetics based on Mg-V-Sb oxide catalysts (Michaels et al. 1996; Creaser and Anderson, 1996; Ramos et al. 2001) to deter- mine the most suitable catalyst to carry out the reaction in CFB. Sensitivity analysis of the operating conditions such as temperature, superficial velocity, pressure, solid circu- lation rate as well as propane and oxygen feed composi- tions will be performed. 2. Model Development The model consists of a couple of differential mass bal- ance equations for each component, one for the mass bal- ance in the core and the other for the mass balance in the R = Riser radius, m Re = Reynolds number = DpU0ρg/µg Rep = Particle Reynolds number = DpUtρg/µg T = Temperature, K U0 = Riser superficial gas velocity, m/s Us = Average solids velocity, m/s Usc = Core solids velocity, m/s Ut = Terminal settling velocity of a single solids particle, m/s x = Axial location in the riser, m Greek Letters Figure 1. Schematic diagram of a circulating fluidized bed as catalytic reactor 0.3m Riser Stripper Regenerator off Gas Regenerator Air Propane + Air 20 m Γ = Constant in acceleration zone εann = Annular voidage εavg. = Average axial voidage εb = Apparent voidage at riser bottom εc = Core voidage εmf = Voidage at mi nimum fluidization conditions ε∞ = Voidage at t he end of the acceleration zone ρg , ρs = Gas and solid particle densities, kg/m 3 21 The Journal of Engineering Research Vol. 3, No. 1 (2006) 19-30 annulus. This model requires knowledge of the reaction kinetics and the hydrodynamics, therefore the combined models of Berruti and Kalogerakis (1989) and Wong et al. (1992) is used for the description of the hydrodynamics. Accordingly, the hydrodynamic model equations are: Us = Gs / [ρs (1- εavg)] (1) εavg = (U0 ρs) / (2 Gs+U0ρ) (2) (3) (4) (5) where the apparent voidage at the riser bottom (εb) is cal- culated from a constant Γ by solving the quadratic equa- tion (6) with: εb = 0.714 (Gs/ρs U0)-0.02528 Dt-0.0794 Rep-0.12016 (7) Equation (6), being quadratic, has two roots. We choose the root which is less than one. Theoretical development leading to Eq. (9) requires Γ to be a fraction. In Eq. (6), k is given by: k = -ln (0.01)/Lacc (8) Finally we obtain the average voidage in the accelera- tion zone as (9) The CFB is assumed isothermal and reactions occur in core and annular regions. The gas input to the annulus is due solely to cross flow from the core. The following parameters and operating conditions are considered in the study which are catalyst properties: Dp = 75 µm ; ρs = 1500 kg/m3 ; εmf = 0.5 ; Ut = 2 m/s The simulation reactions require solution of two mass bal- ance equations for each species: A Core Region Mass Balance (10) B. Annular Region Mass Balance (11) Here the annular voidage, εann is assumed to be equal to the voidage at minimum fluidization conditions, εmf. The kg value is in the range of 0.015 - 0.094 m/s as used by Pugsley et al. (1992). 3. Numerical Algorithm The height of the CFB riser (20 m) is divided into ele- ments of a length of 0.2 m. Testing the base-case with smaller elements length does not improve the accuracy of the simulation results. At each step, the hydrodynamic model is first solved to determine the length of accelera- tion zone, axial voidage profile, core porosity and core radius of elements. The concentration gradient δc/δx is discretized in the spatial direction and a forward implicit finite difference is used to solve the mass balance Eqs. (10) and (11). Since there are variations in gas volume due to the reaction at each element, the increase of gas velocity is determined and the hydrodynamic model is solved at the end of each element. The system of non-linear algebraic equations resulting from discretization is difficult to solve. To provide a good initial guess for the solution of the non-linear system of equations the nonlinear kinetics is linearized around the inlet conditions and the system of approximate linear equations is solved for every step in the spatial direction. Then the system of non-linear algebraic equations are solved for the concentrations of components in the annu- lus and core regions at each step using the solution obtained from the linearized model as initial guess. Having obtained the concentration of different compo- nents of reactants and axial conversion, selectivity and yield of products are calculated. 4. Kinetics of Oxidative Dehydrogenation of Propane to Propylene Many investigators studied the kinetics and mechanism of oxidative dehydrogenation of propane to propylene on different catalysts (Ramos, et al. 2001; Grabowski et al. 2003; Creaser et al. 2000; Chen et al. 2000; Barsan and Thyrion, 2003). In this paper the comparison of the per- ( ) ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎣ ⎡ −⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ ε⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ ρ ε−ρ ⎥ ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎢ ⎣ ⎡ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ −+ −=ε t 2 cc o 2r s tmfs 2r s c U r RU R c U1 R c 1G 1 mfc mfav 2r R c ε−ε ε−ε =⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ 1 1 1 1 2− − = + −∞ ε ε b sU k g Γ Γ( ) )kx(exp1 1( 1 ) avg −Γ− − −= ∞ ε ε 0)CC( r k2 r)1()CU( x a,ic,ic g icsc,io =−−−+− ερδ δ 0)CC( rR rk2 r)1( a,ic,i2 c 2 cg ianns =− − +− ερ97.028.0 g0s 21.1 gs 76.0 pt 8 t acc Re)U/G1()/( )D/D(1092.7 D L −− − + ×=⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ ρρρ 22 The Journal of Engineering Research Vol. 3, No. 1 (2006) 19-30 formance of three fore mentioned catalysts in oxidation of propane based on their kinetics will be performed. Michaels et al. (1996) studied the kinetics and mechanism of propane oxidation on Mg2V2SbOx. The temperature range used was 723-803 K. The network of reaction is shown in Fig. 2. This catalyst will be called M catalyst in the subsequent work. It is noticed that the oxidative dehy- drogenation and deep oxidation of the hydrocarbon occur by two distinctly different mechanisms. (1) Mars-Van Krevelen mechanism fast, reversible (12) rate-determining (13) fast (14) where: ads denotes an adsorbed species, σHC is hydrocarbon adsorption site, Oo and Vo = are lattice oxy- gen and lattice, oxygen vacancy, respectively. (2) Langmuir-Hinshelwood mechanism fast,reversible (15) fast, reversible (16) rate-determining (17) where σo is oxygen adsorption site. Rate expressions for oxidative dehydrogenation and deep oxidation of propane are given next. (18) (19) (20) where: r = reaction rate, k = rate constant, x = mole fraction, K = adsorption equilibrium constant and (21) where pro stands for propane and the entropy of adsorp- tion value are 0.1 e.u. and -6.3 kJ/mol respectively as shown in Table 1. Creaser and Anderson (1996) studied the kinetics of propane oxidation on V-Mg-O catalyst with 60 wt % MgO and 40 wt % V2O5. The temperature range used was 783- 823K. This catalyst will be called C catalyst in the subse- quent work. The mechanism of the reaction is as follows. 1) Propane reacts directly with surface oxygen according to (22) 2) The absorbed propylene reacts further with surface oxygen to give carbon oxides. (23) The rate equations derived take the form (24) (25) (26) )ads(HCHC 63HC83 ⇔σ+ VoHCO)ads(HC 63o83 +→+ Oo2Vo2O 2 →+ → is irreversible, ⇔ is reversible and equilibrium, CO x is carbon oxides )ads(HCHC 83HC83 ⇔σ+ )ads(O22O o2 ⇔σ+ x83 CO)ads(O)ads(HC →+ propanepropane propanepropane propane xK1 xk r + = propro 2/1 2OproCO CO xK1 xxk r + = propro 2/1 2Opro2CO 2CO xK1 xxk r + = Temperature, oC 450 474 500 530 108 )s/mol(k propane× 78 173.3 286.7 438.3 108 )s/mol(k CO× 113.3 181.7 245 435 104 )s/mol(k 2CO× 250 390 500 796.7 proK 2.58 3.55 3.27 2.37 E (activation energy) for partial oxidative dehydrogenation 103.3 kJ /mol E (activation energy) for d eep oxidation CO 79.1 kJ/mol E (activation energy) for d eep oxidation CO 2 65.3 kJ/mol Enthalpy -6.3 kJ/mol Entropy 0.1 e.u. Table 1. Kinetic model parameters (Michaels et al. 1996) where n = 1 or 2, O = surface oxygen site, = free surface site. 08H3C11 Pkr θ= 06H3C22 kr θθ= where the fractional cove rages oθ , 6H3Cθ and the adsorption equilibrium constant for oxygen are given below respectively ; the kinetic parameters are given in Table 2. 22 22 OO OO 0 PK1 PK + =θ Figure 2. Reaction network for propane oxidation (Michaels et al. 1996) propro propro propane xK1 xk r + = ( ) ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ ∆− ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ ∆ = RT H exp R S xpeadsorptionK adsadspro Propane Propylene CO, CO2 23 The Journal of Engineering Research Vol. 3, No. 1 (2006) 19-30 (27) (28) where : Tm = reference temperature, r = reaction rate, k = rate constant, K = adsorption equilibrium constant at temperature T, K0 = adsorption equilibrium constant at temperature Tm. Ramos et al. (2001) studied the kinetics of propane oxi- dation on V-Mg-O with 24 wt% V2O5 and 76 wt% MgO. The range of temperature was 450- 550 K. This catalyst will be called R catalyst in the subsequent work. The reaction scheme suggested is given in Fig.3 below: The power law model takes the form: i = 1, 2, 5., (29) (30) where: Tm = 773.15 K, r = reaction rate, k0 = rate con- stant at reference temperature. The kinetic parameters for Ramos et al. (2001) are pre- sented in Table 3, where: Ea = activation energy, RC = regression coefficient. The reactor model and its numerical solutions used in this work were presented by Fakeeha et al. (2000, 2001) before. 5. Results and Discussion The base case parameters used in this study are shown in Table 4. The results for the base case for different cata- lysts at the end of the riser height of 20 m are shown in Table 5. This case will be used in the sensitivity analysis as a basis for comparison. Changing one parameter of the base case is performed each time while keeping the other parameters constant in order to carry out the sensitivity study. The effects of changing temperature, solid circulation rate, feed compo- sition, pressure and superficial gas velocity are studied. The variation of the mass transfer coefficient kg in the range of 0.015-0.094 ms-1 was found to have a very slight effect on conversion. 5.1 Effect of Temperature For catalysts C and M, as temperature increases, the conversion increases linearly; while using catalyst R, the conversion along the riser increased initially at a sharp rate, then the rate of conversion increase becomes slow. However R catalyst gives the highest conversion, e.g. at a temperature of 823 K, the conversion with R catalyst could reach about 12%, C catalyst reaches about 5.5% and the M catalyst reaches only 1.0% (Fig. 4). On the other hand, the selectivity with M catalyst is the highest and constant along the riser length, attaining values in the range of 97-98% at highest temperature. For catalyst C, the activation energy for the product k2KC3H6 , for CO and CO2 formation is lower than that for k1 for propylene for- mation, thus, we obtain higher selectivity for propylene at higher temperature (Fig. 5). While for catalyst R, the changes of selectivity in the first 0-1m length of the riser are too small to be significant. Moreover, the activation energies for the formation of CO and CO2 from propylene (r4, r5) are higher than that for propylene formation (r1). Thus we obtain lower selectivity for propylene at higher temperature. In general the R catalyst gives the highest yield among the catalysts used due to its higher values of conversion and selectivity. 5.2 Effect of Solid Circulation Rate As solid circulation rate increases, conversion increas- es for all catalysts due to exposure to more catalyst. The increase in conversion has similar trend as that of the tem- Parameter Value Confidence interval (95%) Units 10k 2.50 × 10 -4 ±1.42 × 10-5 mol s-1 (g cat) -1bar-1 20k 2.35 × 10 -5 ±1.37 × 10-5 mol s-1 (g cat)-1 1aE 1.23 × 10 1 ±1.25 × 102 kJ mol-1 2aE 2.34 × 10 1 ±1.31 × 102 kJ mol-1 63 HC H∆ -2.29 × 10 2 ±1.53 × 102 kJ mol-1 2O H∆ 3.56 × 10 2 ±3.59 × 102 kJ mol-1 0 2O K 3.58 × 10 0 ±7.18 × 100 Table 2. Kinetic parameters of Creaser and Anderson (1996) 6363 6363 63 HCHC HCHC HC PK1 PK + =θ ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ − ∆− = m O0 OO T 1 T 1 R H expKK 222 Figure 3. Reaction network for propane oxidation (Ramos et al. 2001) b HC 2 i P a OPkir = ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ − − = m 0ii T 1 T 1 R E expkk 24 The Journal of Engineering Research Vol. 3, No. 1 (2006) 19-30 perature. The results are shown in Figs. 6 and 7, for con- version and selectivity respectively. The highest conver- sion obtained for catalyst R is 5.5% at 783oK. For M and C catalysts selectivity increase is very slight and nearly constant with increase of solid circulation rate. The selectivity of propylene did not change apparently because the contribution of the deep oxidation of propy- lene is small due to the lower conversion of propane to propylene. However for R it decreases substantially for solid circulation rate. The yield for M and R catalysts increases with increase in circulation rate. For the C cata- lyst, the yield increases up to 600 kg/m2s but then decreas- es at 800 kg/m2s. 5.3 Effect of Feed Composition For M and C catalysts, the increase in propane mole fraction does not affect the conversion, because the reac- tion is first order with respect to propane. For R catalyst, as propane mole fraction increases, conversion increases slightly because of propane partial pressure in the kinetic expression is having an exponent of 1.11. For all cata- lysts, there is no change in selectivity due to changes in propane mole fraction. The non-variance of selectivity on propane mole fraction indicates the same order of depend- ence on propane mole fraction for propylene, CO and CO2 formation. The selectivity for R and C catalysts decreases substantially along the riser length. The yield for M cata- lyst increases slightly and that for C catalysts is not affected by propane mole fraction as may be inferred from Figs. 8 and 9, since there are no appreciable changes in their selectivities and conversions. While for R catalyst the yield also increases slightly with the increase in mole fraction. For C and R catalysts in Fig. 10, conversion increases a little with oxygen mole fraction while for M catalyst conversion does not change because the propane rate of reaction to propylene does not depend on the oxygen mole fraction. All catalysts show a decrease in selectivity with the increase in oxygen mole fraction due to formation of carbon oxides as shown in Fig. 11. The yield for M cata- lyst is not affected by the increase in oxygen mole fraction but those for C and R catalysts increase. 5.4 Effect of Pressure and Superficial Velocity All catalysts show increase in conversion with increase in pressure as depicted in Fig. 12. However, the increase for M catalyst is fairly small. Selectivity to propylene decreases with increase in pressure due to the formation of carbon oxides as shown in Fig. 13. In all cases the yield increases with the increase in pressure, while M catalyst gives small increase. When the superficial velocity increases, conversion decreases in all cases due to lower residence time. For M catalyst, as velocity increases, selectivity decreases. For C and R catalysts, the selectivity initially decreases with the increase in velocity, but after a short distance along the riser it increases with increase in velocity. In all cases the yield decreases with increase in velocity. Reaction rates 6 0 10×ik (mol/g.s.bar a+b) aE (kJ/mol) a b RC 1r 2.30±0.11 157±5 0.05±0.04 1.11±0.06 0.987 2r 0.51±0.12 151±23 0.55±0.33 1.13±0.28 0.973 3r 1.27±0.13 97.5±11 0.63±0.19 1.03±0.14 0.962 4r 6.23±0.49 241±13 0.27±0.04 1.27±0.11 0.980 5r 11.8±0.48 202±6.7 0.21±0.02 1.14±0.06 0.985 Table 3. Kinetic parameters of (Ramos et al. 2001) Parameters Values Pressure, atm 1.0 Mass transfer coefficient, kg m s -1 0.017 Temperature , K 783.0 Superficial gas velocity, U 0, m s -1 2.0 Riser length , m 20.0 Riser diameter, m 0.3 Solid circulation rate, G, kg m-2 s-1 400.0 Feed composition (%) C3H8 5.0 O2 2.0 Table 4. The base case feed condition Catalyst Type % 63 HC Conversion % 63 HC Selectivity % 63 HC Yield C 1.46 35.35 0.52 M 0.73 98.23 0.72 R 4.57 73.25 3.35 Table 5. The results for the simulation of the base case at the exit of the reactor 25 The Journal of Engineering Research Vol.3, No. 1 (2006) 19-30 Figure 4. Effect of temperature on propane conversion Figure 5. Effect of temperature on propylene selectivity 26 The Journal of Engineering Research Vol.3, No. 1 (2006) 19-30 Figure 6. Effect of solid circulation rate on propane conversion Figure 7. Effect of solid circulation rate on propylene selectivity 27 The Journal of Engineering Research Vol.3, No. 1 (2006) 19-30 Figure 8. Effect of propane concentration on propane conversion Figure 9. Effect of propane concentration on propylene selectivity 28 The Journal of Engineering Research Vol.3, No. 1 (2006) 19-30 Figure 10. Effect of oxygen concentration on propane conversion Figure 11. Effect of oxygen concentration on propylene selectivity 29 The Journal of Engineering Research Vol.3, No. 1 (2006) 19-30 Figure 12. Effect of pressure on propane conversion Figure 13. Effect of pressure on propylene selectivity 30 The Journal of Engineering Research Vol. 3, No. 1 (2006) 19-30 6. Conclusions Sensitivity studies of three different catalysts for oxida- tive dehydrogenation of propane to propylene were car- ried out using a circulating fluidized bed reactor. The fol- lowing conclusions are made. Increasing temperature, solid circulation rate, and pressure, and decreasing the gas velocity increase the conversion. However, the catalysts studied behave differently with respect to selectivity. R catalyst is, in general, superior in terms of yield with respect to the other catalyst although its selectivity towards propylene formation is not the best. References Barsan, M.M. and Thyrion, F.C., 2003, "Kinetic Study of Oxidative Dehydrogenation of Propane Over Ni-Co, Olybdate Catalyst," Catal. Today., Vol. 81, p. 159. Berruti, F. and Kalogerakis, N., 1989, "Modeling the Internal Flow Structure of Circulating Fluidized Beds," Can. J. Chem. Eng., Vol. 67, p. 1010. 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