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A dynamic kinetic model for methanol synthesis on deactivated catalyst
them. EngngVol. 22, Suppl., pp. S675-S678, 1998 Q 1998 Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain SOO98-1354(98)00122-7 0098-1354/98 $19.00 + 0.00 Computers
Pergamon PII:
A DYNAMIC KINETIC MODEL FOR METHANOL SYNTHESIS ON DEACTIVATED CATALYST M. R. RAHIMPOUR’, P. BAHR12, J. FATHI KALJAHI’ A. JAHANMIRI’, A. ROMAGNOL13 1. Department of Chemical Engineering, Shiraz University, Shiraz, Iran, [email protected] 2. School of Engineering, Murdoch University, Murdoch WA 615O,Australia, [email protected] 3. Department of Chemical Engineering, Sydney University, NSW 2006, Australia, [email protected] ABSTRACT
A kinetic model for methanol synthesis and the deactivation of catalyst has been proposed. This model is a LangmuirHinshelwood-Hougen- Watson type, which has been obtainedfiom an active-site balance and describes the dynamic behavior of the active sites of a deactivated Cu-ZnO commercial catalyst. The parameters of the proposed kinetic model have been determined byfirting the experimentally obtained moleflactions of all components for a wide varieq of conditions to the transient mass conservation equation in a gradientless differential reactor. By using this model, the rates of reactions on deactivated catalyst have been obtained as a function of time. The results show that catalyst deactivation depends on the composition of CO, and water. Also, it is found that by enriching synthesis gas with CO, through a dynamic feeding strategy, the conversion of the reactor will be increased. 0 1998 Published by Elsevier Science Ltd. All rights reserved. Keywords: Kinetic model, Catalyst deactivation, and Dynamic parameter estimation, Methanol synthesis INTRODUCTION
Decay of catalyst activity is very common in petroleum and petrochemical industries. Unfortunately the deactivation of catalyst in any process will force the system into unsteady operation, unless steady operation is maintained by changing the operating conditions. The transient behavior of catalyst activity can be due to many factors, such as: ?? Impurities in the feed which gradually reduce active sites by physical adsorption. ?? Gradual occupation of some sites by the reactants or poisons. The drop of rate of CO and CO, hydrogenation reactions with time during methanol synthesis is believed to be due to the occupation of some catalyst active sites by CO and CO, (Kuechen and Hoffmann, 1993), but they do not propose any kinetic equations to describe the deactivation. Opposite to this phenomenon has also been noticed in some heterogeneous reactions that is the activity of the catalyst increases with time. (Fathi kaljahi and Wills, 1980; Fathi kaljahi, 1978; Gangwal et al., 1977). Ladebeck (1993) investigated the causes of catalyst deactivation in methanol synthesis. He stated that the deactivation factor of a catalyst depends on the CO, partial pressure. In petrochemical complex at Shiraz, the flow rate of methanol production decreases with time. Therefore, the ratio of CO, to CO in the feed must be varied with time to maintain a suitable production rate, but as yet, no feeding strategy has been developed. At present time no quantitative work has been reported on the transient rate of methanol synthesis with respect to the dynamic behavior of catalyst’s active sites. Therefore, it is the aim of this research to develop a
kinetic model based on the recent mechanistic and experimental findings. Fitting the mole fraction of alI components from experiments at different times to dynamic conservation equations then tests the validity of such a model. PROPOSED KINETIC MODEL
In this model the effect of CO, CO,, H,O and high temperature on the reduction of catalyst active sites will be determined experimentally. Since a stoichiometric amount of water is formed in the presence of CO, , the effects of CO, and H,O can not be separated .The proposed deactivation model is based on the assumption that carbon monoxide and carbon dioxide are simultaneously involved in catalyst deactivation but on different copper sites and through different pathways. In developing this model, one of the reaction mechanisms developed by Coteron and Hayhurst (1994) will be used. Based on isotopic labeling experiments and the fact that both the reaction and catalyst deactivation would take place on the same sites, they developed a kinetic model with two non-separable mechanisms. According to this, the two different pathways that lead to methanol synthesis and catalyst deactivation are: ?? CO hydrogenation There is a general agreement on the dissociative adsorption of hydrogen on Zn sites (Dennison et al., 1989), and adsorption of CO on copper sites . The Zn sites are denoted with ** and the copper sites are denoted with *. Since the effect of CO] on catalyst deactivation has been reported (Ladebeck, 1993) we propose the deactivation of first type active sites by this component as well as H,O. This
S675
S616
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deactivation maybe a result of either destruction or occupation of sites by CO, or water:
co, +. c) co,
(1)
The rate of deactivation of * sites by CO, according to eq. (1) is given by :
de’, = kd,x PC:, xeed2 -
kd, x 0’;
(2)
dt
where 8* is the total vacant sites of type * available for adsorption , e*_
is the fraction of total vacant sites of
type * occupieyor destroyed by CO, at each time. Also, k,, and kd2 are forward and backward rate constant for this occupation. Since k d, and k dz aretemperature dependent and obviously changing with time, the thermal destruction and sintering of catalyst is also included in these two parameters. The most strongly supported mechanism for CO hydrogenation (Chinchen et al., 1987b) consists of successive additions of adsorbed hydrogen atoms to an adsorbed carbon monoxide molecule. These successive additions can be combined to give the rate of reaction for CO hydrogenation (Cotreon and Hayhurst, 1994): *M = kf2K&,2KcHP&%Z The site balance can be expressed as: e* +e;7, +e;IIco
+e:,
(3)
28*
=I
(4)
Substituting the expressions for surface coverage and rearranging the above equation gives: (
0’ = I+ K,P,
1- eY+
1
(5)
+ K,K;2K,P,_.oP;;2)
By Inserting the expression for 8 * from eq. (5) into eq. (3), the rate of production of methanol from hydrogenation of carbon monoxide will be as follows: q(t) = Initially
“f G&JW’coP;, ’ 1+ K co Pco + K co K”‘ HK
(1 - 8: CH
de*; dt
= kd3 PCodl
(6)
8 *_ is zero for fresh catalyst but at times co2
greater than zero, 0*= can be obtained by combining eqs. (2) and (5):
X (e
***)d2 -kd,
destroyed or occupied by CO at each time and k,
kd,P$, (1 - e’&d2
(7)
(9)
and
k, are forward and backward rate constants for the above occupation. In this case k 63 and k, arealso temperature dependent and thermal destruction and sintering of catalyst are included in these two parameters. As for CO hydrogenation, the most strongly supported mechanism for CO, hydrogenation (Chinchen et al., 1987b) consists of successive additions of adsorbed hydrogen atoms onto adsorbed carbon dioxide molecules. The rate of methanol production by CO* hydrogenation can be considered as follows: (Coteron and Hayhurst, 1994) ‘% =kf2Kco2K~K~co2PC0,PH
2 xeh)i* (10)
The site balance can be expressed as:
e*+* +eTG2+ eHco2 + e*z=i ?? ???
(11)
By inserting the expressions for surface coverage, and solving for e * * * gives: (1-e” 1 e***= (12) 1+ 40, 40, f Kco2i?; ‘&o&co,P;/2’ By Inserting the expression for 8”’ from the above equation into eq. (lo), the rate of production of methanol from hydrogenation of carbon dioxide will be as follows:
kf~Kco,KHKHcozPcozPH, (I-e*;1 ‘2 0)
(13)
=
+ Kco2K~2K~co+-'co2~~~
Initially 8 y
is zero and for times greater than that 8 r co can be obtzed by inserting eq. (12) into eq. (9): ._*** do_ Co
=-
kd,“zd‘
dt kd, Pcod’(1- 0;)”
de’ zq = -kd2etd3+ co1 dt
X(e;)d’
where 8’: is the fraction of total vacant sites of type ***
l+Kco2Pco2
)
PCo P3’ 2) H,
1993) we propose occupation of third type catalyst sites on Cu by CO. This occupation may also lead to catalyst destruction or reversible occupation: -..I co+***#co (8) The rate of deactivation of *** sites by CO according to eq. (8) is given by:
(14)
(1+ GoI 40, + Go> KH”‘KHCO, 40, ‘H, “2 Id’ Finally, the total rate of formation of methanol can be obtained from the sum of the contributions of CO and CO, to methanol production.
(I+ KcoPco + KCOKH3’2KCHPcoPH,3’2)dz
CO, hydrogenation In CO, hydrogenation, hydrogen is considered to adsorb dissociatively on zinc sites as in CO hydrogenation, and CO, is considered to adsorb on copper sites. The copper sites participating in CO2 adsorption are denoted with *** notation. Since the effect of CO on catalyst deactivation has been proven (Kuechen and Hoffmann, ??
MODEL PARAMETERS ESTIMATION AND VALIDATION Dynamic experimental data on mole fractions of all components were used to estimate values of kinetic parameters. These data were obtained by Kuchen and HofTmam~(1993) using an internal recycling reactor with a fured commercial catalyst of Cu/ZnO operated at a variety
European
Symposium
on Computer
of operating conditions. The mass balance equations were used to build the model for estimation of kinetic parameters. For each component i we can write: F"yoi -Fyi = W(a ,p q(t) + a *,xr2(f)) (15) where r, and r, are the rates of methanol production given by eqs. (6) and (13) and oli and aziare the stoichiometric coefficients of component i in CO and CO, hydrogenation reactions, respectively. Eq. (15) for each component, coupled with eqs. (6), (13) for the reaction rate equations and eqs.(9) and (14) for the rate of occupation of active sites form a system of differential equations with the following initial conditions:
Pi(t)=POi ,
ez=O
,
O’, =O
Aided Process
Engineering-8
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Simulating the operation of a gradientless differential reactor has checked the validity of the kinetic model and of the kinetic parameters calculated in this way, as shown in Fig. (1). It can be noticed that the methanol conversion drops because of catalyst deactivation.
4
(16)
The estimation problem is solved using SPEEDUP flowsheeting package. In this way all temperaturedependent parameters in the kinetic model were assumed to be of the Arrhenius type resulting in 14 parameters and 3 occupation exponents. The dynamic measurements for outlet mole fractions of all components have been grouped together in time ascending order in ESTIMATION section of the program. The number of measurements for space velocity of 2.9 kg/s. mol is 118 and for space velocity 3.1 kg/mol.s is 48. The parameter estimation problem seeks to determine the values of the parameters which solve the following minimization problem: (17)
100
200 Time(hr)
300
400
Fig. 1 Comparison of the experimental data (points) and the model results (solid lines) for methanol conversion, I% Mpa, I=513 K, W/F=o.O00861 Kg.hr/mol, y&.7,
yco=.24,y,-,@3.06
,
RESULTS AND DISCUSSION From a comparison between the experimental and simulation results, the important effect of deactivation on methanol production can be observed. Using the parameters given in Table 1 for a catalyst similar to the one used by Kuchen and HofIlnamt (1993), a dynamic simulation has been done using SPEEDUP package. Fig. (2) shows the results of computer simulation, for the fraction of active sites deactivated at constant operating conditions.
where M is the total number of dynamic experiments and N is the number of components. The general expressions for the model parameters obtained in this way are listed in Table 1. Table 1 The values for kinetic parameters k,,
1 1.475x103exp(-3023.1/T)
&o
5.58 x 10e2exp(907.6/T)
L
1.56x10-2exp(1050/Z’)
KCH
2.68x 10e2exp(2087.21T)
kfZ
3.7773x105exp(-3949.8/T)
Ka
1 2.17~10~~ exp(1084.5iT)
0
KHCQ
3.35 x exp(93.46iT)
kdl
4.536 x 10-l exp(- 3268.49/T)
kd2
6.153~10~“exp(-6220.1/T)
k >.
10.827exp(-5892.75/Z’)
k d4
4.563 x lO’exp(- 6889.9/T)
d..d.
2
4
Time(hr) Fig. 2 Dynamic variation of active sites and total rate of reaction for constant operating conditions, P=SMpa, T =513 K, W/F=O.O0087 Kg.hr/mol, y,=O.7,y,=O.15,y,,=O15
According to this figure one may notice that deactivated
I’ 1.6x 1O-2
i=l k=i
1000 2000 3000 4000
_ increase with time and the site tractions, 8 *G and 8 ?? cq probability of adsorption of CO and CO, on Copper sites is thus reduced. Therefore, for constant inlet composition and temperature, the total rate of reaction decreases to a constant lower limit. Fig. (3) shows the effects of changing feed composition on the rates of site deactivation and reaction. In this simulation, the mole fraction of CO has been decreased with 5% amplitude and the mole fraction of CO, has been increased with 5% amplitude, so that the sum of mole fractions of these two components remains constant. As we can see from this figure, variation of CO,
European Symposium on Computer Aided Process Engineering-8
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composition in feed does not reverse the process of deactivation of first type active sites by CO, and catalyst deactivation depends irreversibly on the CO, partial pressure. Regardless of reaction pathway, a relatively high 1.4
1.2 1
Rate of reaction,mol/(kg catalyst . hr) Temperature, K Time, hr Weight of catalyst, Kg Mole fraction Number of components N A4 Number of experimental measurements r T t W Y
0’ Total vacant sites of type * 8 * Total vacant sites of type ** 8 *** Total vacant sites of type ***
0.8 ,m 0.6 u 0.4 0.2 0
.g
??
Fraction of sites( *) occupied with CO,
8’ cs 0
1000
2000
0 y Fraction of sites(***) occupied with CO co
3000
Steochometric
Time(hr)
coefficient
Lbsrias Fig. 3 Variation of occupied active sites and rate of reaction for
0
cycling of CO and CO, when the total feed flow rate is constant,
j
Zero time conditions, Reactor inlet conditions Component number Reaction number
T=513 K, P=5 Mpa, W&O.OOO87Kg.hr/mol, yH2=0.7, y~+y~2=0.3
j
amount of water is formed in the presence of CO,. One can assume that water leads to active site blocking or may affect the catalytic system in another irreversible physical or chemical way. In this way CO, increases the rate of methanol production as a reactant and deactivates catalyst sites as a deactivator. The effect of changing CO composition in feed on the deactivation of active sites is also shown in Fig. (3). As we can see, CO deactivates active sites reversibly, and by varying CO mole fraction in the feed, the rate of deactivation changes. Therefore, the ratio of CO to CO, in the feed could be helpful in decreasing the rate of catalyst deactivation and increasing total methanol production.
REFERENCES
CONCLUSION A dynamic kinetic model has been proposed for the prediction of the rate of catalyst deactivation during methanol production. It has then been validated using a variety of experimental data. The simulation results using the proposed model show that the ratio of methanol production by CO, to that by CO increases with time and confirm that methanol is mainly produced from CO, over a deactivated catalyst. Variation of feed composition with time shows that the irreversible deactivation of catalyst depends on the partial pressure of CO, and water formed and catalyst aging. It also shows nicely that changing the feed composition with time results in an increase in total methanol production with minimum CO, consumption.
1. Chinchen, G. C., C. M. Hay, H. D. Vandervall, and K. C. Waugh , 1987b, The measurement of copper surface areas by reactive frontal chromatography. J. Caiul.103, 79-86. 2. Coteron, A and A.N. Hayhurst, 1994, “Kinetics of the synthesis of methanol from CO+H, and CO+CO,+H, over copper-based Amorphous catalysts.” C&m. Engng. Sci. 49, No.2,209-22 1. 3. Dennison, P. R., K. J. Packer, and M. S. Spencer, 1989, IH and 13C nuclear magnetic resonance investigations on the CU/ZnlAl oxide methanolsynthesis catalyst. J. Chem. Sot. Faraday Trans. 85, 3537-3560.
Fathi kalajahi, J. and G. B. Wills, 1980, Effects of ammonia upon propylene metathesis over a WO,-SiO, catalyst. Journal of Molecular Catalysis, 8, 127-l 34. Fathi Kalajahi, J., 1978, Break-in behavior of a tungsten oxide on silica catalyst during propylene disproportion. Ph.D. thesis, Blacksburg, Virginia.
Gangwal, S. K., J. Fathi kalajahi, and G. B. Wills, 1977, Break-in behavior of a Tungsten on Silica Catalyst in Propylene Disproportionation. Ind. Eng. 7.
8. NOTATION d,, d2 , d,
F k, K k P P
Occupation exponent Total molar flow rate, mol/hr Occupation rate constant Equilibrium constants, libar Reaction rate constant moUhr.Kg Partial pressure , bar Total pressure, bar
Chem., Prod. Res. Dev. 16, No. 3,237-241. Kuechen,C.,1992,Reaktionstechnische Untersuchungen zur simultanen Umsetzung von Kohlenmonoxide und Kohlendioxid mit Wasserstoff: Dissertation, TU
Clausthal. Kuechen, C. and U. Hoffmann, 1993, Investigation of simultaneous reaction of carbon monoxide and carbon dioxide with hydrogen on a commercial copper/zinc oxide catalysts. Chem. Engng. Sci. 48, No. 22, 37673776.
9.
Ladebeck, J., 1993, Improving methanol synthesis. Hydrocarbon Processing, 89-9 1.
Pergamon PII:
A DYNAMIC KINETIC MODEL FOR METHANOL SYNTHESIS ON DEACTIVATED CATALYST M. R. RAHIMPOUR’, P. BAHR12, J. FATHI KALJAHI’ A. JAHANMIRI’, A. ROMAGNOL13 1. Department of Chemical Engineering, Shiraz University, Shiraz, Iran, [email protected] 2. School of Engineering, Murdoch University, Murdoch WA 615O,Australia, [email protected] 3. Department of Chemical Engineering, Sydney University, NSW 2006, Australia, [email protected] ABSTRACT
A kinetic model for methanol synthesis and the deactivation of catalyst has been proposed. This model is a LangmuirHinshelwood-Hougen- Watson type, which has been obtainedfiom an active-site balance and describes the dynamic behavior of the active sites of a deactivated Cu-ZnO commercial catalyst. The parameters of the proposed kinetic model have been determined byfirting the experimentally obtained moleflactions of all components for a wide varieq of conditions to the transient mass conservation equation in a gradientless differential reactor. By using this model, the rates of reactions on deactivated catalyst have been obtained as a function of time. The results show that catalyst deactivation depends on the composition of CO, and water. Also, it is found that by enriching synthesis gas with CO, through a dynamic feeding strategy, the conversion of the reactor will be increased. 0 1998 Published by Elsevier Science Ltd. All rights reserved. Keywords: Kinetic model, Catalyst deactivation, and Dynamic parameter estimation, Methanol synthesis INTRODUCTION
Decay of catalyst activity is very common in petroleum and petrochemical industries. Unfortunately the deactivation of catalyst in any process will force the system into unsteady operation, unless steady operation is maintained by changing the operating conditions. The transient behavior of catalyst activity can be due to many factors, such as: ?? Impurities in the feed which gradually reduce active sites by physical adsorption. ?? Gradual occupation of some sites by the reactants or poisons. The drop of rate of CO and CO, hydrogenation reactions with time during methanol synthesis is believed to be due to the occupation of some catalyst active sites by CO and CO, (Kuechen and Hoffmann, 1993), but they do not propose any kinetic equations to describe the deactivation. Opposite to this phenomenon has also been noticed in some heterogeneous reactions that is the activity of the catalyst increases with time. (Fathi kaljahi and Wills, 1980; Fathi kaljahi, 1978; Gangwal et al., 1977). Ladebeck (1993) investigated the causes of catalyst deactivation in methanol synthesis. He stated that the deactivation factor of a catalyst depends on the CO, partial pressure. In petrochemical complex at Shiraz, the flow rate of methanol production decreases with time. Therefore, the ratio of CO, to CO in the feed must be varied with time to maintain a suitable production rate, but as yet, no feeding strategy has been developed. At present time no quantitative work has been reported on the transient rate of methanol synthesis with respect to the dynamic behavior of catalyst’s active sites. Therefore, it is the aim of this research to develop a
kinetic model based on the recent mechanistic and experimental findings. Fitting the mole fraction of alI components from experiments at different times to dynamic conservation equations then tests the validity of such a model. PROPOSED KINETIC MODEL
In this model the effect of CO, CO,, H,O and high temperature on the reduction of catalyst active sites will be determined experimentally. Since a stoichiometric amount of water is formed in the presence of CO, , the effects of CO, and H,O can not be separated .The proposed deactivation model is based on the assumption that carbon monoxide and carbon dioxide are simultaneously involved in catalyst deactivation but on different copper sites and through different pathways. In developing this model, one of the reaction mechanisms developed by Coteron and Hayhurst (1994) will be used. Based on isotopic labeling experiments and the fact that both the reaction and catalyst deactivation would take place on the same sites, they developed a kinetic model with two non-separable mechanisms. According to this, the two different pathways that lead to methanol synthesis and catalyst deactivation are: ?? CO hydrogenation There is a general agreement on the dissociative adsorption of hydrogen on Zn sites (Dennison et al., 1989), and adsorption of CO on copper sites . The Zn sites are denoted with ** and the copper sites are denoted with *. Since the effect of CO] on catalyst deactivation has been reported (Ladebeck, 1993) we propose the deactivation of first type active sites by this component as well as H,O. This
S675
S616
European Symposium on
Computer Aided Process Engineering--d
deactivation maybe a result of either destruction or occupation of sites by CO, or water:
co, +. c) co,
(1)
The rate of deactivation of * sites by CO, according to eq. (1) is given by :
de’, = kd,x PC:, xeed2 -
kd, x 0’;
(2)
dt
where 8* is the total vacant sites of type * available for adsorption , e*_
is the fraction of total vacant sites of
type * occupieyor destroyed by CO, at each time. Also, k,, and kd2 are forward and backward rate constant for this occupation. Since k d, and k dz aretemperature dependent and obviously changing with time, the thermal destruction and sintering of catalyst is also included in these two parameters. The most strongly supported mechanism for CO hydrogenation (Chinchen et al., 1987b) consists of successive additions of adsorbed hydrogen atoms to an adsorbed carbon monoxide molecule. These successive additions can be combined to give the rate of reaction for CO hydrogenation (Cotreon and Hayhurst, 1994): *M = kf2K&,2KcHP&%Z The site balance can be expressed as: e* +e;7, +e;IIco
+e:,
(3)
28*
=I
(4)
Substituting the expressions for surface coverage and rearranging the above equation gives: (
0’ = I+ K,P,
1- eY+
1
(5)
+ K,K;2K,P,_.oP;;2)
By Inserting the expression for 8 * from eq. (5) into eq. (3), the rate of production of methanol from hydrogenation of carbon monoxide will be as follows: q(t) = Initially
“f G&JW’coP;, ’ 1+ K co Pco + K co K”‘ HK
(1 - 8: CH
de*; dt
= kd3 PCodl
(6)
8 *_ is zero for fresh catalyst but at times co2
greater than zero, 0*= can be obtained by combining eqs. (2) and (5):
X (e
***)d2 -kd,
destroyed or occupied by CO at each time and k,
kd,P$, (1 - e’&d2
(7)
(9)
and
k, are forward and backward rate constants for the above occupation. In this case k 63 and k, arealso temperature dependent and thermal destruction and sintering of catalyst are included in these two parameters. As for CO hydrogenation, the most strongly supported mechanism for CO, hydrogenation (Chinchen et al., 1987b) consists of successive additions of adsorbed hydrogen atoms onto adsorbed carbon dioxide molecules. The rate of methanol production by CO* hydrogenation can be considered as follows: (Coteron and Hayhurst, 1994) ‘% =kf2Kco2K~K~co2PC0,PH
2 xeh)i* (10)
The site balance can be expressed as:
e*+* +eTG2+ eHco2 + e*z=i ?? ???
(11)
By inserting the expressions for surface coverage, and solving for e * * * gives: (1-e” 1 e***= (12) 1+ 40, 40, f Kco2i?; ‘&o&co,P;/2’ By Inserting the expression for 8”’ from the above equation into eq. (lo), the rate of production of methanol from hydrogenation of carbon dioxide will be as follows:
kf~Kco,KHKHcozPcozPH, (I-e*;1 ‘2 0)
(13)
=
+ Kco2K~2K~co+-'co2~~~
Initially 8 y
is zero and for times greater than that 8 r co can be obtzed by inserting eq. (12) into eq. (9): ._*** do_ Co
=-
kd,“zd‘
dt kd, Pcod’(1- 0;)”
de’ zq = -kd2etd3+ co1 dt
X(e;)d’
where 8’: is the fraction of total vacant sites of type ***
l+Kco2Pco2
)
PCo P3’ 2) H,
1993) we propose occupation of third type catalyst sites on Cu by CO. This occupation may also lead to catalyst destruction or reversible occupation: -..I co+***#co (8) The rate of deactivation of *** sites by CO according to eq. (8) is given by:
(14)
(1+ GoI 40, + Go> KH”‘KHCO, 40, ‘H, “2 Id’ Finally, the total rate of formation of methanol can be obtained from the sum of the contributions of CO and CO, to methanol production.
(I+ KcoPco + KCOKH3’2KCHPcoPH,3’2)dz
CO, hydrogenation In CO, hydrogenation, hydrogen is considered to adsorb dissociatively on zinc sites as in CO hydrogenation, and CO, is considered to adsorb on copper sites. The copper sites participating in CO2 adsorption are denoted with *** notation. Since the effect of CO on catalyst deactivation has been proven (Kuechen and Hoffmann, ??
MODEL PARAMETERS ESTIMATION AND VALIDATION Dynamic experimental data on mole fractions of all components were used to estimate values of kinetic parameters. These data were obtained by Kuchen and HofTmam~(1993) using an internal recycling reactor with a fured commercial catalyst of Cu/ZnO operated at a variety
European
Symposium
on Computer
of operating conditions. The mass balance equations were used to build the model for estimation of kinetic parameters. For each component i we can write: F"yoi -Fyi = W(a ,p q(t) + a *,xr2(f)) (15) where r, and r, are the rates of methanol production given by eqs. (6) and (13) and oli and aziare the stoichiometric coefficients of component i in CO and CO, hydrogenation reactions, respectively. Eq. (15) for each component, coupled with eqs. (6), (13) for the reaction rate equations and eqs.(9) and (14) for the rate of occupation of active sites form a system of differential equations with the following initial conditions:
Pi(t)=POi ,
ez=O
,
O’, =O
Aided Process
Engineering-8
S671
Simulating the operation of a gradientless differential reactor has checked the validity of the kinetic model and of the kinetic parameters calculated in this way, as shown in Fig. (1). It can be noticed that the methanol conversion drops because of catalyst deactivation.
4
(16)
The estimation problem is solved using SPEEDUP flowsheeting package. In this way all temperaturedependent parameters in the kinetic model were assumed to be of the Arrhenius type resulting in 14 parameters and 3 occupation exponents. The dynamic measurements for outlet mole fractions of all components have been grouped together in time ascending order in ESTIMATION section of the program. The number of measurements for space velocity of 2.9 kg/s. mol is 118 and for space velocity 3.1 kg/mol.s is 48. The parameter estimation problem seeks to determine the values of the parameters which solve the following minimization problem: (17)
100
200 Time(hr)
300
400
Fig. 1 Comparison of the experimental data (points) and the model results (solid lines) for methanol conversion, I% Mpa, I=513 K, W/F=o.O00861 Kg.hr/mol, y&.7,
yco=.24,y,-,@3.06
,
RESULTS AND DISCUSSION From a comparison between the experimental and simulation results, the important effect of deactivation on methanol production can be observed. Using the parameters given in Table 1 for a catalyst similar to the one used by Kuchen and HofIlnamt (1993), a dynamic simulation has been done using SPEEDUP package. Fig. (2) shows the results of computer simulation, for the fraction of active sites deactivated at constant operating conditions.
where M is the total number of dynamic experiments and N is the number of components. The general expressions for the model parameters obtained in this way are listed in Table 1. Table 1 The values for kinetic parameters k,,
1 1.475x103exp(-3023.1/T)
&o
5.58 x 10e2exp(907.6/T)
L
1.56x10-2exp(1050/Z’)
KCH
2.68x 10e2exp(2087.21T)
kfZ
3.7773x105exp(-3949.8/T)
Ka
1 2.17~10~~ exp(1084.5iT)
0
KHCQ
3.35 x exp(93.46iT)
kdl
4.536 x 10-l exp(- 3268.49/T)
kd2
6.153~10~“exp(-6220.1/T)
k >.
10.827exp(-5892.75/Z’)
k d4
4.563 x lO’exp(- 6889.9/T)
d..d.
2
4
Time(hr) Fig. 2 Dynamic variation of active sites and total rate of reaction for constant operating conditions, P=SMpa, T =513 K, W/F=O.O0087 Kg.hr/mol, y,=O.7,y,=O.15,y,,=O15
According to this figure one may notice that deactivated
I’ 1.6x 1O-2
i=l k=i
1000 2000 3000 4000
_ increase with time and the site tractions, 8 *G and 8 ?? cq probability of adsorption of CO and CO, on Copper sites is thus reduced. Therefore, for constant inlet composition and temperature, the total rate of reaction decreases to a constant lower limit. Fig. (3) shows the effects of changing feed composition on the rates of site deactivation and reaction. In this simulation, the mole fraction of CO has been decreased with 5% amplitude and the mole fraction of CO, has been increased with 5% amplitude, so that the sum of mole fractions of these two components remains constant. As we can see from this figure, variation of CO,
European Symposium on Computer Aided Process Engineering-8
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composition in feed does not reverse the process of deactivation of first type active sites by CO, and catalyst deactivation depends irreversibly on the CO, partial pressure. Regardless of reaction pathway, a relatively high 1.4
1.2 1
Rate of reaction,mol/(kg catalyst . hr) Temperature, K Time, hr Weight of catalyst, Kg Mole fraction Number of components N A4 Number of experimental measurements r T t W Y
0’ Total vacant sites of type * 8 * Total vacant sites of type ** 8 *** Total vacant sites of type ***
0.8 ,m 0.6 u 0.4 0.2 0
.g
??
Fraction of sites( *) occupied with CO,
8’ cs 0
1000
2000
0 y Fraction of sites(***) occupied with CO co
3000
Steochometric
Time(hr)
coefficient
Lbsrias Fig. 3 Variation of occupied active sites and rate of reaction for
0
cycling of CO and CO, when the total feed flow rate is constant,
j
Zero time conditions, Reactor inlet conditions Component number Reaction number
T=513 K, P=5 Mpa, W&O.OOO87Kg.hr/mol, yH2=0.7, y~+y~2=0.3
j
amount of water is formed in the presence of CO,. One can assume that water leads to active site blocking or may affect the catalytic system in another irreversible physical or chemical way. In this way CO, increases the rate of methanol production as a reactant and deactivates catalyst sites as a deactivator. The effect of changing CO composition in feed on the deactivation of active sites is also shown in Fig. (3). As we can see, CO deactivates active sites reversibly, and by varying CO mole fraction in the feed, the rate of deactivation changes. Therefore, the ratio of CO to CO, in the feed could be helpful in decreasing the rate of catalyst deactivation and increasing total methanol production.
REFERENCES
CONCLUSION A dynamic kinetic model has been proposed for the prediction of the rate of catalyst deactivation during methanol production. It has then been validated using a variety of experimental data. The simulation results using the proposed model show that the ratio of methanol production by CO, to that by CO increases with time and confirm that methanol is mainly produced from CO, over a deactivated catalyst. Variation of feed composition with time shows that the irreversible deactivation of catalyst depends on the partial pressure of CO, and water formed and catalyst aging. It also shows nicely that changing the feed composition with time results in an increase in total methanol production with minimum CO, consumption.
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Gangwal, S. K., J. Fathi kalajahi, and G. B. Wills, 1977, Break-in behavior of a Tungsten on Silica Catalyst in Propylene Disproportionation. Ind. Eng. 7.
8. NOTATION d,, d2 , d,
F k, K k P P
Occupation exponent Total molar flow rate, mol/hr Occupation rate constant Equilibrium constants, libar Reaction rate constant moUhr.Kg Partial pressure , bar Total pressure, bar
Chem., Prod. Res. Dev. 16, No. 3,237-241. Kuechen,C.,1992,Reaktionstechnische Untersuchungen zur simultanen Umsetzung von Kohlenmonoxide und Kohlendioxid mit Wasserstoff: Dissertation, TU
Clausthal. Kuechen, C. and U. Hoffmann, 1993, Investigation of simultaneous reaction of carbon monoxide and carbon dioxide with hydrogen on a commercial copper/zinc oxide catalysts. Chem. Engng. Sci. 48, No. 22, 37673776.
9.
Ladebeck, J., 1993, Improving methanol synthesis. Hydrocarbon Processing, 89-9 1.