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Isobutene hydration over Amberlyst-15 in a slurry reactor
Chemical Engineering and Processing 42 (2003) 985 /991 www.elsevier.com/locate/cep
Isobutene hydration over Amberlyst-15 in a slurry reactor C.M. Zhang, A.A. Adesina, M.S. Wainwright * School of Chemical Engineering and Industrial Chemistry, University of New South Wales, Sydney, NSW 2052, Australia Received 24 June 2002; received in revised form 30 October 2002; accepted 30 October 2002
Abstract The synthesis of tertiary butyl alcohol (TBA) via isobutene (iB) hydration was studied over Amberlyst-15 sulfonic acid catalyst particles using pure water and aqueous TBA solutions in a bubbling slurry reactor. Preliminary studies to investigate mass transfer effects showed that pore diffusion was present for catalyst particles greater than 165 mm in diameter. Therefore, intrinsic kinetic measurements were made using 90.5 mm catalyst particles and a catalyst loading of 10 kg m 3. The kinetic measurements revealed that iB hydration is a pseudo-first-order reaction with an activation energy of 69 kJ mol 1. Isobutene hydration experiments using TBA concentrations in water revealed a hindering effect of TBA, which indicates that separation of TBA formed by iB hydration in three-phase reactors using catalytic distillation is promising from a process design perspective. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Isobutene hydration; Bubbling slurry reactor; TBA effect; Amberlyst-15
1. Introduction Recent exhaust gas control regulations prohibit the use of heavy metal-based octane additives in gasoline production [1]. Tertiary butyl alcohol (TBA) produced from isobutene (iB) hydration is an alternative to lead additives and has similar properties to those of methyl tertiary butyl ether (MTBE). Because TBA is much cheaper than MTBE and is safe for the environment, it is an excellent octane additive [2]. Conventional ion exchange resins possessing sulfonic acid functional groups (Amberlyst-15 or AmberlystXN-1010) are considered to be good catalysts for iB hydration in three-phase reactors [2]. Both TBA and MTBE synthesis reactions in the gas phase suffer significant equilibrium limitations [3,4]. Therefore, catalytic distillation may provide significant improvements to TBA yields beyond the thermodynamic limits as has been reported for MTBE synthesis [5 /7]. However, there have been no reports of TBA synthesis using
* Corresponding author. Tel.: /61-2-9385-2700; fax: /61-2-93858008 E-mail address: [email protected] (M.S. Wainwright).
catalytic distillation [2]. As part of a study on the catalytic distillation process for TBA production [2], we used a bubbling slurry reactor to measure the reaction kinetics and mass transfer effects. The advantages of using a bubbling slurry reactor for such a study have been discussed in several reviews [8,9]. Recasens and co-workers [10 /13] studied the liquidphase hydration of iB in a trickle bed reactor and showed improvements beyond existing operations. Their findings suggested that the TBA /water /iB reaction system possesses interesting complex features which impact on the reactor design. For example, TBA enhances iB solubility, which can increase the rate of the iB hydration reaction. On the other hand, the reverse reaction, TBA dehydration, can be significant under the same reaction conditions. Furthermore, it was found that high TBA concentrations changed the structure of the Amberlyst-15 catalyst used in their study. Other studies [14,15] focused on iB hydration or TBA dehydration. Measurements of both intrinsic and apparent reaction kinetics are important since large catalyst particles are employed in a catalytic distillation column. Thus in this study, the effect of particle size on catalyst activity has been investigated.
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C.M. Zhang et al. / Chemical Engineering and Processing 42 (2003) 985 /991
2. Experimental Measuring the kinetics of three-phase reaction systems requires a judicious choice of reactor. In this study, we have used a bubbling batch slurry system in which the gas-phase reactant, iB, is bubbled through a slurry of aqueous TBA solution and Amberlyst-15 catalyst particles.
Shimadzu GC-8A gas chromatograph equipped with a flame ionization detector. Normal propanol (PA) was used as an internal standard in order to accurately determine the TBA concentration. Excellent separation of TBA and PA was obtained in less than 4 min using a 3 m /6 mm o.d. stainless steel column containing 15% Carbowax 20M on Chromosorb-AW operated at 383 K. 2.3. Catalyst activation
2.1. Apparatus Fig. 1 shows the experimental apparatus used. Flows of iB and N2 ( /99%), obtained from BOC Gases, Sydney, were delivered through two electronic mass flow controllers into a mixing chamber upstream of the reactor. The reactor was a stainless steel pressure vessel (inner diameter /70 mm, height /140 mm, and wall thickness /14.5 mm) equipped with a pressure gauge, safety valve, thermocouple, condenser, and a 20 mm gas sparger at the base. The temperature in the reactor was maintained constant using a temperature-controlled heating tape wound on the reactor wall. Prior to admitting iB to the reactor, the slurry phase was brought to reaction temperature using a heating plate. 2.2. Analysis In a semi-batch three-phase catalytic reactor such as the one employed in this study, the kinetics of reaction are determined by measuring the concentration of product as a function of time. Therefore, 1 cm3 samples of the liquid phase were removed from the reactor at times up to 50 min. The sample was analyzed using a
Amberlyst-15 catalyst (Rohm-Haas, USA) was usually pretreated as described by Leung [14]. The catalyst was washed in distilled water and dried in the oven at 373 K for 4 h, and then immersed in 10 wt.% sulfuric acid solution for 2 h and flushed with water until the pH of outlet solution was almost the same as the pure water. The catalyst was then dried in an oven at 373 K for 4 h, and then the catalyst was ready to use and carefully stored.
3. Results and discussion In order to obtain reliable kinetic data, iB hydration was studied in pure water or at very low TBA concentrations. The conversions of iB in all reaction runs were less than 3% in order to measure initial rates of reaction and avoid the interference of TBA on the reaction. Preliminary experiments showed that a total gas flow rate in excess of 5.45 /10 6 m3 s 1 was sufficient to ensure good mixing (i.e. uniform liquid composition throughout the reactor). 3.1. Effect of particle size Fig. 2 shows the rates of iB hydration in pure water as a function of particle size. The data show that the rates
Fig. 1. The schematic illustration for bubble slurry reactor. 1 */N2 and isobutene cylinders ; 2 */mass flow controllers; 3 */condenser; 4 */gas mixer; 5 */reactor; 6 */gas distributor; 7 */sampling port; 8 */pressure gauge; 9 */valves; 10 */gas chromatograph; 11 */temperature controller with readout.
Fig. 2. The influence of particle size on the rate of reaction. Reaction conditions: T /343 K; iB /3.0 cm3 s 1; N2 /2.45 cm3 s 1; catalyst loading/10 kg m 3; and water volume/400 cm3.
C.M. Zhang et al. / Chemical Engineering and Processing 42 (2003) 985 /991
of reaction were constant for particles less than 165 mm in diameter indicating that no significant internal mass transfer limitation existed. The Thiele modulus fexp may be obtained from the following equation [16]: f2exp
(rexp )dp2 4CiB;L Deff
;
(1)
where /rexp is the observed reaction rate in kmol m 3 s 1 and Deff the effective diffusivity, which can be estimated using the empirical relationship in Eq. (2) which was reported by Caceres et al. [10] for the hydration of iB to TBA using a similar catalyst: 3690 Deff exp 9:520:552CTBA : (2) T Using Eq. (2) at very low TBA concentrations and a temperature of 343 K, Deff /1.56 /109 m2 s1. From Fig. 2, for a particle diameter dp /165 mm, /rexp / 7.86 /106 kmol m 3 s1. The liquid-phase concentration, CiB,L, has been obtained in previous solubility research [2]. At 343 K, CiB,L /1.12 /103 kmol m 3. From Eq. (1), fexp /0.162 which is close to the accepted limit of 0.15 for the absence of significant mass transfer resistance [16]. Therefore, the overall mass transfer resistance can be considered negligible if the particle diameter is less than 165 mm. Therefore, catalyst particles of 90.5 mm diameter were used to measure the intrinsic reaction kinetics of iB hydration in the slurry reactor. Fig. 2 shows that the reaction rates are proportional to the reciprocal particle diameter for catalyst particle diameters between 165 and 322 mm. This is typical of internal diffusion control where pore diffusion controls the rate of reaction [17 /19]. The reaction rates for the particle diameters between 165 and 322 mm fit the empirical relationship: rTBA 2109 dp1 4106 :
(3)
For particle diameters between 322 and 680 mm, external mass transfer (between primary particles) takes effect. Fig. 2 shows that for particles larger than 322 mm, the catalyst activities decreased very slowly with increasing catalyst particle diameter. The correlation for these larger particles is given by rTBA 1107 dp0:5 4106 :
(4)
A correlation that can be used for modeling the rate of reaction for all catalyst particle diameters used and hence all three regimes of rate controlling resistances can be obtained from Eqs. (3) and (4). This correlation which is useful for future studies of iB hydration using catalytic distillation over the same catalyst is given by rTBA [1y(ax104x d x b)]r0 ;
(5)
where a and b are constant values of 2.5 and /1.426,
987
respectively, y and x are the parameters indicating the existence and which type of the mass transfer resistance. If dp B/165 mm, y /0, the reaction is operated without any catalyst mass transfer resistance. If dp /165 mm, y /1, mass transfer resistance exists. One is the internal diffusion resistance (165 mm B/dp B/322 mm), x /1; the other is the external diffusion resistance (dp /322 mm), x /0.5. Fig. 3 is a plot of predicted versus measured values of reaction rates. The linear regression constant (R2-values) is in excess of 0.999 indicating that Eq. (5) accurately represents the data. Eq. (5) is therefore a useful empirical model, which accounts for the different diffusion resistances that apply for the different sizes of the same batch of Amberlyst-15 sulfonic catalyst. It should be noted that the catalyst particles have the same active site density regardless of particle size since they came from the same sample. Mass transfer resistances using similar catalysts have been discussed by Ihm et al. [20 /22]. They used two different types of resin catalysts, Amberlyst-15 and Amberlyst-XN-1010. Their studies were limited to two particle sizes for each type of Amberlyst catalyst; 251 and 349 mm for Amberlyst-15, and 241 and 336 mm for Amberlyst-XN-1010. They deduced that reaction rates could be influenced by the internal and the external diffusion resistances and active centers. 3.2. Influence of catalyst loading on the reaction rate Catalyst loading can be a major factor in a threephase semi-batch reactor such as the one used in this study to measure reaction kinetics. Visual studies in a glass column of similar dimensions to the stainless steel reactor used in this study showed that catalyst loading of 5 /20 kg m 3 was able to be supported with perfect mixings using a total gas flow rate of 5.45 cm3 s 1 [2]. Using the correlation of Chaudhari and Ramachandran [23], Zhang [2] was able to confirm this visual result. The influence of catalyst loading is reported in Table 1 and Fig. 4 using the standard reaction conditions of this study: particle diameter /90.5 mm; temperature/ 343 K; iB flow /3.0 cm 3 s 1; N2 flow /2.45 cm3 s 1; water volume /400 cm3; and catalyst loading from 5 to 20 kg m 3. The data in Fig. 4 show linear relationships between CTBA and time. These were used to calculate the rates of reaction expressed as kmol m 3 s 1 in Table 1. The results in Table 1 show that the average reaction rate is 8.4 kmol kg 1 s 1. Within experimental error, the data show that the rate of reaction is independent of catalyst loading. Therefore, in measuring the rates of reaction to investigate intrinsic reaction kinetics, a loading of 10 kg m 3 was used. The influence of catalyst loading on the rate of reaction in a bubbling slurry reactor can be described by the following equation [17 /19]:
C.M. Zhang et al. / Chemical Engineering and Processing 42 (2003) 985 /991
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Fig. 3. Comparisons of the experimental and predicted values of reaction rates with different catalyst particle diameters (y/x , R2 /0.999).
Table 1 Reaction rates using different catalyst loadings Catalyst load- rTBA ing (kg m 3) ( 106 kmol m 3 s 1)
rTBA ( 107 kmol kgcat1 s 1)
5 7.5 10 15 20
8.3 8.11 7.76 9.4 8.45
4.15 6.08 7.76 14.1 16.9
Fig. 4. The influence of catalyst loading on the rate of reaction. Reaction conditions: T /343 K; iB /3.0 cm3 s 1; N2 /2.45 cm3 s 1; dp /90.5 mm; and water volume/400 cm3.
Ci r
1 KL a
1=KS a 1=hk m
:
(6)
Eq. (6) combines various resistances. 1/KLa is the gas absorption resistance whilst 1/KSa the iB diffusion resistance of liquid to solid and 1/hk the chemical reaction resistance. In the case of iB hydration in a bubbling slurry reactor, it is possible that the absorption of iB in water could be a controlling resistance. The data in Table 1 are plotted according to Eq. (6) in Fig. 5, and a linear relationship is observed. The gas absorption resistance (KLa) 1 is /3.8 s whilst the combined reaction / diffusion resistance 1/KSa/1/hk is 1404 s. The small negative value of /3.8 shows that the gas /liquid diffusion resistance is very small and several orders of magnitude less than the chemical reaction resistance. Indeed, the correction for a zero intercept model (i.e. zero gas absorption resistance) was CiB,L/r/1376/m with an R2-value of 0.9923 compared with the interrupt
Fig. 5. Linearized plot of Eq. (6) for the influence of catalyst loading on the rate of reaction. Reaction conditions: T/343 K; iB/3.0 cm3 s 1; N2 /2.45 cm3 s 1; dp /90.5 mm; and water volume/400 cm3.
C.M. Zhang et al. / Chemical Engineering and Processing 42 (2003) 985 /991
model CiB,L/r/1404/m/3.8 with an R2-value of 0.9928. We can therefore conclude that absorption of iB in water is a rapid process compared with the rate of reaction on the catalyst surface. The results in Figs. 2 and 5 show that chemical reaction rate data for iB hydration can be obtained in the bubbling slurry reactor in a regime where intrinsic reaction kinetics were controlled using catalyst particle diameters less than 165 mm and a catalyst loading of 20 kg m 3 or less.
3.3. Influence of iB partial pressure on the rate of reaction The effect of the iB partial pressure on reaction rate was investigated using 90.5 mm diameter catalyst particles and a catalyst loading of 10 kg m3 in pure water. The experimental results are shown in Fig. 6. The data in Fig. 6 show good linearity between the reaction rates and the iB partial pressures PiB, thus: rTBA 1:34107 PiB :
(7)
In order to compare with other experimental data, we need to substitute the iB partial pressure with the iB equilibrium concentration in the liquid phase. The value of Henry’s constant at 343 K is 5.41 /104 (kPa m3 kmol 1) [2], and therefore the correlation for the rate of TBA formation with the liquid-phase iB concentration can be written as rTBA 7:26103 CiB;L :
(8)
The reverse reaction was negligible in pure water for low conversions of iB to TBA and the low concentration of TBA in solution also makes the absorption term negligible. The reaction kinetics under these conditions can therefore be simplified from that published by Velo et al. [11]:
Fig. 6. The influence of iB partial pressure on the reaction rate. Reaction conditions: T/343 K; total gas flow rate/5.45 cm3 s 1; catalyst loading/10 kg m 3; dp /90.5 mm; and water volume/400 cm3.
rTBA
kf (CiB Cw CTBA =Ke ) (1 kr CTBA )n
989
(9)
to rTBA k?CiB;L kƒPiB ;
(10)
which confirms a pseudo-first-order reaction with respect to iB partial pressure. 3.4. Influence of temperature on the reaction rate Initial rates of iB hydration were measured in pure water at temperatures in the range 323 /353 K. Fig. 7 shows the Arrhenius plot for those initial rate data. The activation energy from this plot is 69 kJ mol 1, which is close to the value of 67 kJ mol 1 reported by Leung et al. [24] for iB hydration over Amberlyst-15 catalyst. Ihm et al. [21] calculated a value of the activation energy at the surface of Amberlyst-15 and Amberlyst-XN-1010 to be 80 kJ mol 1 and internally to be 61 kJ mol1. That is an average of 70 kJ mol 1, which is again consistent with our result. At low conversions of iB in pure water, the pseudofirst-order reaction can therefore be expressed by rTBA 2:07106 e8323=T CiB;L :
(11)
3.5. The influence of TBA on the rate of iB hydration Previous studies of iB hydration have shown that the product TBA can limit the rate of the forward reaction [15,25]. Eq. (9) is typical of adsorption models, which describe a reaction that is limited by both the forward and the reverse reaction due to the chemical equilibrium of the reaction and strong adsorption of the product on the active sites of the catalyst surface. At low concen-
Fig. 7. Arrhenius plot for iB hydration in pure water.
C.M. Zhang et al. / Chemical Engineering and Processing 42 (2003) 985 /991
990
trations of the product, we have shown the reaction to be pseudo-first-order (Fig. 6). Experiments were carried out in aqueous TBA solutions with concentrations in the range 0 /0.027 kmol m 3. A plot of the rates of TBA formation versus TBA concentrations is presented in Fig. 8. The data in Fig. 8 show a linear decrease in the rate of TBA with the concentration of TBA in the reaction medium. The data are correlated by rTBA 7:8106 2:7104 CTBA :
(12)
Eq. (12) applies to low concentrations of TBA so that the adsorption term for TBA in Eq. (9) is small. Therefore, the overall rate of iB hydration is approximated by rTBA kf CiB;L kr CTBA :
(13)
The rate constants can be obtained as follows. The rate of the forward reaction is obtained from the rate in the absence of TBA in the reactant mixture. From Eq. (12), rf /7.8 /106. Using the Henry’s Law constant in pure water at 343 K [2], CiB,L /1.12 /103 kmol m 3. Hence, kf /7.0 /103 s1. From Eq. (13), kr /2.7 / 104 s1. Thus, equilibrium constant, K , at 343 K is
4. Conclusions The influence of mass transfer effects and the kinetics of iB hydration over particles of Amberlyst-15 catalyst have been studied in a bubbling slurry reactor. The study has shown that chemical reaction resistance controls for particles less than 165 mm in diameter and that for particles between 165 and 322 mm pore diffusion are the controlling mechanism. For particles in excess of 322 mm, external mass transfer is in the limiting resistance. The kinetics in the chemical reaction-controlled regime have shown that the reaction is pseudofirst-order in iB concentration and that product TBA inhibits the rate of the overall reaction due to the significant reverse reaction resulting from an equilibrium constant of 26 at 343 K. The activation energy for the reaction when chemical resistance controlled the reaction was found to be 69 kJ mol1. The data presented in the paper indicate that a catalytic distillation reactor will have the ability to enhance TBA yield in this catalytic system.
Acknowledgements K
kf kr
26:
The K -value is within the range of equilibrium constants of 20/118 reported elsewhere [26].
The support of the Australian Research Council is gratefully acknowledged. Thanks are also due to Dr. Deyan Guang and Mr. J. Starling for their advice during the study.
Fig. 8. The influence of TBA on the rate of iB hydration. Reaction conditions: T/343 K; iB /3.0 cm3 s 1; N2 /2.45 cm3 s 1; catalyst loading/10 kg m 3; dp /90.5 mm; and water volume/400 cm3.
C.M. Zhang et al. / Chemical Engineering and Processing 42 (2003) 985 /991
Appendix A: Nomenclature a, b Ci CiB,L dp Deff Ea H k , k?, k?? kf ko kr Ke K La KSa m PiB r rexp rmea ro rpre rTBA R t T x XTBA y
constants concentration of species i for water and TBA (kmol m 3) iB equilibrium concentrations in the liquid phase (kmol m 3) the particle diameter (m) effective diffusivity (m2 s 1) activation energy (kJ mol 1) Henry’s constant (kPa m 3 kmol 1) reaction rate constants (s 1) forward reaction constant (s 1) Arrhenius pre-exponential constant (s 1) reverse reaction constant (s 1) equilibrium constant gas to liquid mass transfer coefficient (s 1) liquid to solid mass transfer coefficient (s1) catalyst loading (kg m 3) iB partial pressure (kPa) reaction rate (kmol m 3 s 1) the reaction rate determined by experiment (kmol m 3 s1) the reaction rate measured by experiment (kmol m 3 s1) intrinsic reaction rate (kmol m 3 s1) the reaction rate predicted by modeling formation rate of TBA (kmol m 3 s 1) universal gas constant (J mol 1 K 1) time (s) absolute temperature (K) variable for the type of mass transfer resistance the fraction conversion of iB to TBA variable for the mass transfer resistance existence
Greek symbols h effectiveness factor 8exp Thiele modulus
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[5] K. Sunmacher, U.H. Gerd Uhde, Multiple reactions in catalytic distillation processes for the production of the fuel oxygenates MTBE and TAME: analysis by rigorous model and experimental validation, Chem. Eng. Sci. 54 (1999) 2839 /2847. [6] M.F. Doherty, G. Buzad, Reactive distillation by design, Trans. Inst. Chem. Eng. A 70 (1992) 448. [7] M. Matouq, A.T. Quitain, K. Takahashi, S. Goto, Reactive distillation for synthesizing ethyl tert -butyl ether from low-grade alcohol catalyzed by potassium hydrogen sulfate, Ind. Eng. Chem. Res. 35 (3) (1996) 982 /984. [8] S. Schlueter, A. Steiff, P.M. Weinspach, Modeling and simulation of bubble column reactors, Chem. Eng. Process. 31 (2) (1992) 97 / 117. [9] M.A. Kohler, Comparison of mechanically agitated and bubble slurry column slurry rectors, Appl. Catal. 22 (1) (1986) 21 /53. [10] E. Caceres, L. Puigjaner, F. Recasens, A trickle-bed process for hydration of isobutene to tert -butyl alcohol: a study of reactor performance, Chem. Eng. J. 37 (1) (1988) 43 /52. [11] E. Velo, L. Puigjaner, F. Recasens, Inhibition by product in the liquid-phase hydration of isobutene to tert -butyl alcohol: kinetics and equilibrium studies, Ind. Eng. Chem. Res. 27 (12) (1988) 2224 /2231. [12] E. Velo, L. Puigjaner, F. Recasens, Intraparticle mass transfer in the liquid-phase hydration of isobutene: effects of liquid viscosity and excess product, Ind. Eng. Chem. Res. 29 (7) (1990) 1485 / 1492. [13] E. Velo, F. Recasens, Reaction and diffusion on Amberlyst-15 ion-exchange polymer, Makromol. Chem. Macromol. Symp. 75 (1993) 217 /222. [14] P. Leung, Hydration of isobutene in trickle bed reactors, Ph.D. Thesis, University of California, Davis, CA, 1986. [15] B.C. Gates, J.S. Wisnouskas, H.W. Heath, Jr., Dehydration of tert -butyl alcohol catalyzed by sulfonic acid resin, J. Catal. 24 (2) (1972) 320 /327. [16] O. Levenspiel, Chemical Reaction Engineering, 2nd ed., Wiley, New York, 1982. [17] R.V. Chaudhari, P.A. Ramachandran, Three phase slurry reactors, AIChE J. 26 (2) (1980) 77 /201. [18] S.T. Warna, Dynamic modeling of catalytic three phase reactors, Comput. Chem. Eng. 20 (1) (1996) 39 /47. [19] M.P. Dudukovic, Trends in catalytic reaction engineering, Catal. Today 48 (1 /4) (1999) 5 /15. [20] S.K. Ihm, S. Sung-Sup, O.H. In-Hawan, Reaction and mass transfer in a macroreticular resin catalyst, J. Chem. Eng. Jpn. 15 (3) (1982) 206 /210. [21] S.K. Ihm, M.J. Chung, K.Y. Park, Activity difference between the internal and external sulfonic groups of macroreticular ionexchange resin catalysts in isobutylene hydration, Ind. Eng. Chem. Res. 27 (1) (1988) 41 /45. [22] S.K. Ihm, A. Jou-Hyeon, Jo. Young-Do, Interaction of reaction and mass transfer in ion-exchange resin catalysts, Ind. Eng. Chem. Res. 35 (1996) 2946 /2954. [23] R.V. Chaudhari, P.A. Ramachandran, Three Phase Catalytic Reactors, Gordon and Breach, New York, 1983, pp. 313 /316. [24] P. Leung, C. Zorilla, F. Recasens, Hydration of isobutene in liquid-full and trickle-bed reactors, AIChE J. 32 (11) (1986) 1839 / 1847. [25] M.T. Vicent Huguet, A. Martinez Andreu, J.M. Paris Molina, Kinetic study of the catalytic dehydration of tert-butanol, Ing. Quim. 18 (208) (1986) 165 /168. [26] A. Delion, B. Tork, M. Hellin, Equilibrium constant for the liquid-phase hydration of isobutene over ion-exchange resin, Ind. Eng. Chem. Process Des. Dev. 25 (4) (1986) 289 /893.
Isobutene hydration over Amberlyst-15 in a slurry reactor C.M. Zhang, A.A. Adesina, M.S. Wainwright * School of Chemical Engineering and Industrial Chemistry, University of New South Wales, Sydney, NSW 2052, Australia Received 24 June 2002; received in revised form 30 October 2002; accepted 30 October 2002
Abstract The synthesis of tertiary butyl alcohol (TBA) via isobutene (iB) hydration was studied over Amberlyst-15 sulfonic acid catalyst particles using pure water and aqueous TBA solutions in a bubbling slurry reactor. Preliminary studies to investigate mass transfer effects showed that pore diffusion was present for catalyst particles greater than 165 mm in diameter. Therefore, intrinsic kinetic measurements were made using 90.5 mm catalyst particles and a catalyst loading of 10 kg m 3. The kinetic measurements revealed that iB hydration is a pseudo-first-order reaction with an activation energy of 69 kJ mol 1. Isobutene hydration experiments using TBA concentrations in water revealed a hindering effect of TBA, which indicates that separation of TBA formed by iB hydration in three-phase reactors using catalytic distillation is promising from a process design perspective. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Isobutene hydration; Bubbling slurry reactor; TBA effect; Amberlyst-15
1. Introduction Recent exhaust gas control regulations prohibit the use of heavy metal-based octane additives in gasoline production [1]. Tertiary butyl alcohol (TBA) produced from isobutene (iB) hydration is an alternative to lead additives and has similar properties to those of methyl tertiary butyl ether (MTBE). Because TBA is much cheaper than MTBE and is safe for the environment, it is an excellent octane additive [2]. Conventional ion exchange resins possessing sulfonic acid functional groups (Amberlyst-15 or AmberlystXN-1010) are considered to be good catalysts for iB hydration in three-phase reactors [2]. Both TBA and MTBE synthesis reactions in the gas phase suffer significant equilibrium limitations [3,4]. Therefore, catalytic distillation may provide significant improvements to TBA yields beyond the thermodynamic limits as has been reported for MTBE synthesis [5 /7]. However, there have been no reports of TBA synthesis using
* Corresponding author. Tel.: /61-2-9385-2700; fax: /61-2-93858008 E-mail address: [email protected] (M.S. Wainwright).
catalytic distillation [2]. As part of a study on the catalytic distillation process for TBA production [2], we used a bubbling slurry reactor to measure the reaction kinetics and mass transfer effects. The advantages of using a bubbling slurry reactor for such a study have been discussed in several reviews [8,9]. Recasens and co-workers [10 /13] studied the liquidphase hydration of iB in a trickle bed reactor and showed improvements beyond existing operations. Their findings suggested that the TBA /water /iB reaction system possesses interesting complex features which impact on the reactor design. For example, TBA enhances iB solubility, which can increase the rate of the iB hydration reaction. On the other hand, the reverse reaction, TBA dehydration, can be significant under the same reaction conditions. Furthermore, it was found that high TBA concentrations changed the structure of the Amberlyst-15 catalyst used in their study. Other studies [14,15] focused on iB hydration or TBA dehydration. Measurements of both intrinsic and apparent reaction kinetics are important since large catalyst particles are employed in a catalytic distillation column. Thus in this study, the effect of particle size on catalyst activity has been investigated.
0255-2701/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 5 - 2 7 0 1 ( 0 2 ) 0 0 1 6 9 - 1
986
C.M. Zhang et al. / Chemical Engineering and Processing 42 (2003) 985 /991
2. Experimental Measuring the kinetics of three-phase reaction systems requires a judicious choice of reactor. In this study, we have used a bubbling batch slurry system in which the gas-phase reactant, iB, is bubbled through a slurry of aqueous TBA solution and Amberlyst-15 catalyst particles.
Shimadzu GC-8A gas chromatograph equipped with a flame ionization detector. Normal propanol (PA) was used as an internal standard in order to accurately determine the TBA concentration. Excellent separation of TBA and PA was obtained in less than 4 min using a 3 m /6 mm o.d. stainless steel column containing 15% Carbowax 20M on Chromosorb-AW operated at 383 K. 2.3. Catalyst activation
2.1. Apparatus Fig. 1 shows the experimental apparatus used. Flows of iB and N2 ( /99%), obtained from BOC Gases, Sydney, were delivered through two electronic mass flow controllers into a mixing chamber upstream of the reactor. The reactor was a stainless steel pressure vessel (inner diameter /70 mm, height /140 mm, and wall thickness /14.5 mm) equipped with a pressure gauge, safety valve, thermocouple, condenser, and a 20 mm gas sparger at the base. The temperature in the reactor was maintained constant using a temperature-controlled heating tape wound on the reactor wall. Prior to admitting iB to the reactor, the slurry phase was brought to reaction temperature using a heating plate. 2.2. Analysis In a semi-batch three-phase catalytic reactor such as the one employed in this study, the kinetics of reaction are determined by measuring the concentration of product as a function of time. Therefore, 1 cm3 samples of the liquid phase were removed from the reactor at times up to 50 min. The sample was analyzed using a
Amberlyst-15 catalyst (Rohm-Haas, USA) was usually pretreated as described by Leung [14]. The catalyst was washed in distilled water and dried in the oven at 373 K for 4 h, and then immersed in 10 wt.% sulfuric acid solution for 2 h and flushed with water until the pH of outlet solution was almost the same as the pure water. The catalyst was then dried in an oven at 373 K for 4 h, and then the catalyst was ready to use and carefully stored.
3. Results and discussion In order to obtain reliable kinetic data, iB hydration was studied in pure water or at very low TBA concentrations. The conversions of iB in all reaction runs were less than 3% in order to measure initial rates of reaction and avoid the interference of TBA on the reaction. Preliminary experiments showed that a total gas flow rate in excess of 5.45 /10 6 m3 s 1 was sufficient to ensure good mixing (i.e. uniform liquid composition throughout the reactor). 3.1. Effect of particle size Fig. 2 shows the rates of iB hydration in pure water as a function of particle size. The data show that the rates
Fig. 1. The schematic illustration for bubble slurry reactor. 1 */N2 and isobutene cylinders ; 2 */mass flow controllers; 3 */condenser; 4 */gas mixer; 5 */reactor; 6 */gas distributor; 7 */sampling port; 8 */pressure gauge; 9 */valves; 10 */gas chromatograph; 11 */temperature controller with readout.
Fig. 2. The influence of particle size on the rate of reaction. Reaction conditions: T /343 K; iB /3.0 cm3 s 1; N2 /2.45 cm3 s 1; catalyst loading/10 kg m 3; and water volume/400 cm3.
C.M. Zhang et al. / Chemical Engineering and Processing 42 (2003) 985 /991
of reaction were constant for particles less than 165 mm in diameter indicating that no significant internal mass transfer limitation existed. The Thiele modulus fexp may be obtained from the following equation [16]: f2exp
(rexp )dp2 4CiB;L Deff
;
(1)
where /rexp is the observed reaction rate in kmol m 3 s 1 and Deff the effective diffusivity, which can be estimated using the empirical relationship in Eq. (2) which was reported by Caceres et al. [10] for the hydration of iB to TBA using a similar catalyst: 3690 Deff exp 9:520:552CTBA : (2) T Using Eq. (2) at very low TBA concentrations and a temperature of 343 K, Deff /1.56 /109 m2 s1. From Fig. 2, for a particle diameter dp /165 mm, /rexp / 7.86 /106 kmol m 3 s1. The liquid-phase concentration, CiB,L, has been obtained in previous solubility research [2]. At 343 K, CiB,L /1.12 /103 kmol m 3. From Eq. (1), fexp /0.162 which is close to the accepted limit of 0.15 for the absence of significant mass transfer resistance [16]. Therefore, the overall mass transfer resistance can be considered negligible if the particle diameter is less than 165 mm. Therefore, catalyst particles of 90.5 mm diameter were used to measure the intrinsic reaction kinetics of iB hydration in the slurry reactor. Fig. 2 shows that the reaction rates are proportional to the reciprocal particle diameter for catalyst particle diameters between 165 and 322 mm. This is typical of internal diffusion control where pore diffusion controls the rate of reaction [17 /19]. The reaction rates for the particle diameters between 165 and 322 mm fit the empirical relationship: rTBA 2109 dp1 4106 :
(3)
For particle diameters between 322 and 680 mm, external mass transfer (between primary particles) takes effect. Fig. 2 shows that for particles larger than 322 mm, the catalyst activities decreased very slowly with increasing catalyst particle diameter. The correlation for these larger particles is given by rTBA 1107 dp0:5 4106 :
(4)
A correlation that can be used for modeling the rate of reaction for all catalyst particle diameters used and hence all three regimes of rate controlling resistances can be obtained from Eqs. (3) and (4). This correlation which is useful for future studies of iB hydration using catalytic distillation over the same catalyst is given by rTBA [1y(ax104x d x b)]r0 ;
(5)
where a and b are constant values of 2.5 and /1.426,
987
respectively, y and x are the parameters indicating the existence and which type of the mass transfer resistance. If dp B/165 mm, y /0, the reaction is operated without any catalyst mass transfer resistance. If dp /165 mm, y /1, mass transfer resistance exists. One is the internal diffusion resistance (165 mm B/dp B/322 mm), x /1; the other is the external diffusion resistance (dp /322 mm), x /0.5. Fig. 3 is a plot of predicted versus measured values of reaction rates. The linear regression constant (R2-values) is in excess of 0.999 indicating that Eq. (5) accurately represents the data. Eq. (5) is therefore a useful empirical model, which accounts for the different diffusion resistances that apply for the different sizes of the same batch of Amberlyst-15 sulfonic catalyst. It should be noted that the catalyst particles have the same active site density regardless of particle size since they came from the same sample. Mass transfer resistances using similar catalysts have been discussed by Ihm et al. [20 /22]. They used two different types of resin catalysts, Amberlyst-15 and Amberlyst-XN-1010. Their studies were limited to two particle sizes for each type of Amberlyst catalyst; 251 and 349 mm for Amberlyst-15, and 241 and 336 mm for Amberlyst-XN-1010. They deduced that reaction rates could be influenced by the internal and the external diffusion resistances and active centers. 3.2. Influence of catalyst loading on the reaction rate Catalyst loading can be a major factor in a threephase semi-batch reactor such as the one used in this study to measure reaction kinetics. Visual studies in a glass column of similar dimensions to the stainless steel reactor used in this study showed that catalyst loading of 5 /20 kg m 3 was able to be supported with perfect mixings using a total gas flow rate of 5.45 cm3 s 1 [2]. Using the correlation of Chaudhari and Ramachandran [23], Zhang [2] was able to confirm this visual result. The influence of catalyst loading is reported in Table 1 and Fig. 4 using the standard reaction conditions of this study: particle diameter /90.5 mm; temperature/ 343 K; iB flow /3.0 cm 3 s 1; N2 flow /2.45 cm3 s 1; water volume /400 cm3; and catalyst loading from 5 to 20 kg m 3. The data in Fig. 4 show linear relationships between CTBA and time. These were used to calculate the rates of reaction expressed as kmol m 3 s 1 in Table 1. The results in Table 1 show that the average reaction rate is 8.4 kmol kg 1 s 1. Within experimental error, the data show that the rate of reaction is independent of catalyst loading. Therefore, in measuring the rates of reaction to investigate intrinsic reaction kinetics, a loading of 10 kg m 3 was used. The influence of catalyst loading on the rate of reaction in a bubbling slurry reactor can be described by the following equation [17 /19]:
C.M. Zhang et al. / Chemical Engineering and Processing 42 (2003) 985 /991
988
Fig. 3. Comparisons of the experimental and predicted values of reaction rates with different catalyst particle diameters (y/x , R2 /0.999).
Table 1 Reaction rates using different catalyst loadings Catalyst load- rTBA ing (kg m 3) ( 106 kmol m 3 s 1)
rTBA ( 107 kmol kgcat1 s 1)
5 7.5 10 15 20
8.3 8.11 7.76 9.4 8.45
4.15 6.08 7.76 14.1 16.9
Fig. 4. The influence of catalyst loading on the rate of reaction. Reaction conditions: T /343 K; iB /3.0 cm3 s 1; N2 /2.45 cm3 s 1; dp /90.5 mm; and water volume/400 cm3.
Ci r
1 KL a
1=KS a 1=hk m
:
(6)
Eq. (6) combines various resistances. 1/KLa is the gas absorption resistance whilst 1/KSa the iB diffusion resistance of liquid to solid and 1/hk the chemical reaction resistance. In the case of iB hydration in a bubbling slurry reactor, it is possible that the absorption of iB in water could be a controlling resistance. The data in Table 1 are plotted according to Eq. (6) in Fig. 5, and a linear relationship is observed. The gas absorption resistance (KLa) 1 is /3.8 s whilst the combined reaction / diffusion resistance 1/KSa/1/hk is 1404 s. The small negative value of /3.8 shows that the gas /liquid diffusion resistance is very small and several orders of magnitude less than the chemical reaction resistance. Indeed, the correction for a zero intercept model (i.e. zero gas absorption resistance) was CiB,L/r/1376/m with an R2-value of 0.9923 compared with the interrupt
Fig. 5. Linearized plot of Eq. (6) for the influence of catalyst loading on the rate of reaction. Reaction conditions: T/343 K; iB/3.0 cm3 s 1; N2 /2.45 cm3 s 1; dp /90.5 mm; and water volume/400 cm3.
C.M. Zhang et al. / Chemical Engineering and Processing 42 (2003) 985 /991
model CiB,L/r/1404/m/3.8 with an R2-value of 0.9928. We can therefore conclude that absorption of iB in water is a rapid process compared with the rate of reaction on the catalyst surface. The results in Figs. 2 and 5 show that chemical reaction rate data for iB hydration can be obtained in the bubbling slurry reactor in a regime where intrinsic reaction kinetics were controlled using catalyst particle diameters less than 165 mm and a catalyst loading of 20 kg m 3 or less.
3.3. Influence of iB partial pressure on the rate of reaction The effect of the iB partial pressure on reaction rate was investigated using 90.5 mm diameter catalyst particles and a catalyst loading of 10 kg m3 in pure water. The experimental results are shown in Fig. 6. The data in Fig. 6 show good linearity between the reaction rates and the iB partial pressures PiB, thus: rTBA 1:34107 PiB :
(7)
In order to compare with other experimental data, we need to substitute the iB partial pressure with the iB equilibrium concentration in the liquid phase. The value of Henry’s constant at 343 K is 5.41 /104 (kPa m3 kmol 1) [2], and therefore the correlation for the rate of TBA formation with the liquid-phase iB concentration can be written as rTBA 7:26103 CiB;L :
(8)
The reverse reaction was negligible in pure water for low conversions of iB to TBA and the low concentration of TBA in solution also makes the absorption term negligible. The reaction kinetics under these conditions can therefore be simplified from that published by Velo et al. [11]:
Fig. 6. The influence of iB partial pressure on the reaction rate. Reaction conditions: T/343 K; total gas flow rate/5.45 cm3 s 1; catalyst loading/10 kg m 3; dp /90.5 mm; and water volume/400 cm3.
rTBA
kf (CiB Cw CTBA =Ke ) (1 kr CTBA )n
989
(9)
to rTBA k?CiB;L kƒPiB ;
(10)
which confirms a pseudo-first-order reaction with respect to iB partial pressure. 3.4. Influence of temperature on the reaction rate Initial rates of iB hydration were measured in pure water at temperatures in the range 323 /353 K. Fig. 7 shows the Arrhenius plot for those initial rate data. The activation energy from this plot is 69 kJ mol 1, which is close to the value of 67 kJ mol 1 reported by Leung et al. [24] for iB hydration over Amberlyst-15 catalyst. Ihm et al. [21] calculated a value of the activation energy at the surface of Amberlyst-15 and Amberlyst-XN-1010 to be 80 kJ mol 1 and internally to be 61 kJ mol1. That is an average of 70 kJ mol 1, which is again consistent with our result. At low conversions of iB in pure water, the pseudofirst-order reaction can therefore be expressed by rTBA 2:07106 e8323=T CiB;L :
(11)
3.5. The influence of TBA on the rate of iB hydration Previous studies of iB hydration have shown that the product TBA can limit the rate of the forward reaction [15,25]. Eq. (9) is typical of adsorption models, which describe a reaction that is limited by both the forward and the reverse reaction due to the chemical equilibrium of the reaction and strong adsorption of the product on the active sites of the catalyst surface. At low concen-
Fig. 7. Arrhenius plot for iB hydration in pure water.
C.M. Zhang et al. / Chemical Engineering and Processing 42 (2003) 985 /991
990
trations of the product, we have shown the reaction to be pseudo-first-order (Fig. 6). Experiments were carried out in aqueous TBA solutions with concentrations in the range 0 /0.027 kmol m 3. A plot of the rates of TBA formation versus TBA concentrations is presented in Fig. 8. The data in Fig. 8 show a linear decrease in the rate of TBA with the concentration of TBA in the reaction medium. The data are correlated by rTBA 7:8106 2:7104 CTBA :
(12)
Eq. (12) applies to low concentrations of TBA so that the adsorption term for TBA in Eq. (9) is small. Therefore, the overall rate of iB hydration is approximated by rTBA kf CiB;L kr CTBA :
(13)
The rate constants can be obtained as follows. The rate of the forward reaction is obtained from the rate in the absence of TBA in the reactant mixture. From Eq. (12), rf /7.8 /106. Using the Henry’s Law constant in pure water at 343 K [2], CiB,L /1.12 /103 kmol m 3. Hence, kf /7.0 /103 s1. From Eq. (13), kr /2.7 / 104 s1. Thus, equilibrium constant, K , at 343 K is
4. Conclusions The influence of mass transfer effects and the kinetics of iB hydration over particles of Amberlyst-15 catalyst have been studied in a bubbling slurry reactor. The study has shown that chemical reaction resistance controls for particles less than 165 mm in diameter and that for particles between 165 and 322 mm pore diffusion are the controlling mechanism. For particles in excess of 322 mm, external mass transfer is in the limiting resistance. The kinetics in the chemical reaction-controlled regime have shown that the reaction is pseudofirst-order in iB concentration and that product TBA inhibits the rate of the overall reaction due to the significant reverse reaction resulting from an equilibrium constant of 26 at 343 K. The activation energy for the reaction when chemical resistance controlled the reaction was found to be 69 kJ mol1. The data presented in the paper indicate that a catalytic distillation reactor will have the ability to enhance TBA yield in this catalytic system.
Acknowledgements K
kf kr
26:
The K -value is within the range of equilibrium constants of 20/118 reported elsewhere [26].
The support of the Australian Research Council is gratefully acknowledged. Thanks are also due to Dr. Deyan Guang and Mr. J. Starling for their advice during the study.
Fig. 8. The influence of TBA on the rate of iB hydration. Reaction conditions: T/343 K; iB /3.0 cm3 s 1; N2 /2.45 cm3 s 1; catalyst loading/10 kg m 3; dp /90.5 mm; and water volume/400 cm3.
C.M. Zhang et al. / Chemical Engineering and Processing 42 (2003) 985 /991
Appendix A: Nomenclature a, b Ci CiB,L dp Deff Ea H k , k?, k?? kf ko kr Ke K La KSa m PiB r rexp rmea ro rpre rTBA R t T x XTBA y
constants concentration of species i for water and TBA (kmol m 3) iB equilibrium concentrations in the liquid phase (kmol m 3) the particle diameter (m) effective diffusivity (m2 s 1) activation energy (kJ mol 1) Henry’s constant (kPa m 3 kmol 1) reaction rate constants (s 1) forward reaction constant (s 1) Arrhenius pre-exponential constant (s 1) reverse reaction constant (s 1) equilibrium constant gas to liquid mass transfer coefficient (s 1) liquid to solid mass transfer coefficient (s1) catalyst loading (kg m 3) iB partial pressure (kPa) reaction rate (kmol m 3 s 1) the reaction rate determined by experiment (kmol m 3 s1) the reaction rate measured by experiment (kmol m 3 s1) intrinsic reaction rate (kmol m 3 s1) the reaction rate predicted by modeling formation rate of TBA (kmol m 3 s 1) universal gas constant (J mol 1 K 1) time (s) absolute temperature (K) variable for the type of mass transfer resistance the fraction conversion of iB to TBA variable for the mass transfer resistance existence
Greek symbols h effectiveness factor 8exp Thiele modulus
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