- Email: [email protected]
γ-Al2O3 catalytic membrane in the partial hydrogenation of acetylene
Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
2687
Activity and Selectivity of a Pd/)r-A1203 Catalytic Membrane in the Partial Hydrogenation of Acetylene Christine Lambert, Matthew Vincent, Juan Hinestroza, Ning Sun, and Richard Gonzalez* Department of Chemical Engineering, Tulane University, New Orleans, LA 70118, USA Abstract
A flow dispersion model with Dankwert's boundary conditions describing a Pd/T-A1203 membrane reactor has been developed for the successive hydrogenation reactions of acetylene to ethylene to ethane. At 175 ~ a change in both transport and kinetics is observed. The Peclet number is seen to change dependence in temperature. This change is also seen in the Damk6hler number and the ratio of hydrogenation rates of acetylene to ethylene. It is suspected that a change in rate mechanism at low temperature is influenced by the neglect of the adsorption terms for acetylene and ethylene in the kinetic rate expressions. 1. Introduction
The use of membrane reactors in hydrogenation reactions has seen an increase in activity in recent years [1]. Several dense metal membranes have been used for the partial hydrogenation of acetylene at 100 ~ These include a Pd/Ni membrane [2] and Pd, Pd/Ni, Pd~u, and Pd/Ag membranes [3]. Both studies found that the permeate hydrogen was very active and ethylene was the main product. However, these involve metal membranes, which must be very thin (and thus brittle) in order to provide appropriate selectivity. Porous ceramic membranes can support highly dispersed metals on an inorganic oxide to combine the properties of higher permeability with thermal stability, which the dense metal membranes lack. This enables catalytic and separative properties to be developed at higher temperatures. For our system, a n Gt,-A]20 3 membrane is made catalytic by the deposition of Pd/7-AI203 layers. The ceramic tube purchased from U.S. Filter under the name Membralox is cleaned and then processed using the sol-gel method. The porous substrate is dipped into a solution of colloidal particles of the metal oxide or a precursor to the metal oxide [4]. Through capillary action, the particles concentrate on the substrate surface and form a gel layer. A catalytic membrane forms on the substrate surface by the addition of a water soluble precursor of the catalyst to the solution [5]. It becomes active upon calcination of the membrane. The details of this process were developed previously [6]. With repeated dips and calcinations, a desired thickness can be developed. With appropriate flow rates, it becomes a short contact time reactor. A diagram of this membrane is shown in figure 1. The layers are numbered and their pore size described. An SEM photo of the actual membrane is shown in figure 2. The thickness of the catalytic membrane is approximately 5 I~m. This value was both measured (from photo) and calculated from
*Corresponding Author; GrantingAgencies, BES.
2688
/1
2
ssssS. "" /'
Fig
....
C Mematal'~e Macropomus
thickness 5 um 15 mm ....
Support
r ~ ce.,~u~ofthe
I Alpha Alumina I Support from U.S. I I I
Filter
2
2OO nm
3
800 nm
4
Macro
/
Fig. 1. Diagram of Catalytic Membrane
Fig. 2. SEM photo of Pd/~'-A1203 Membrane
the bulk density of the same catalyst, prepared through traditional methods into its powdered form. A further description of the membrane is found in Section 2. This reactor is operated in a premixed mode with flow into the tube and out of the shell. A previous study by Lambert and Gonzalez [7] showed that this is the preferred orientation. As the results are promising in applications to selectivity in partial hydrogenation reactions, this study attempts to understand and begin to quantify the mechanisms of flow and reaction. Thus, a flow dispersion model has been developed for premixed flow of reactant gases through a cylindrical membrane reactor to direct future study. 2. Membrane Characterization and Reaction Analysis An r tube was used as a support for the membranes. The outer and inner diameter of the tube was 10 mm and 7 mm, respectively. The tube was dipped (for 10 s.) in the Pd solution (sol-gel method) with the outer surface covered by a polyethylene film so that only the inner surface was contacted with the solution. Between each dip-coat, the tube was allowed to dry for 24 h. at room temperature and then calcined in a furnace using a 1~ ramp (to 400~ The catalyst layer was characterized by drying the solution to form the gel and then using BET (Coulter Omnisorp Porosimeter), hydrogen chemisorption and metals analysis (ICP) to determine the physical properties of the Pd/y-A1203 materials. A comparison of the membrane catalyst to tradition methods is summarized in Table 1.
Table 1: Physical Characteristics of Pd/7-Al203 materials Material BET area (mZ/g) Ave. Pore Diameter (nm) Pore Volume (mUg) Pd (wt%) DHa(%) dmD ISGc IE MEM
366 172 334
3.6 8.5 3.6
0.31 0.48 0.37
0.80 0.59 0.72
32 41 29
3.1 2.4 3.4
dispersion as measured by hydrogen chemisorption, H:Pd surface = 1:1 b metal particle diameter (98.2/DH), assuming Pd density = 11.40 g/cm 3. c SG = sol-gel prepared catalyst, IE = ion-exchanged catalyst, MEM = catalytic membrane.
2689 All gases were UHP grade and the flow was 8 mL/min of H2 and 20 mL/min of 10% acetylene in argon at STP. The flow rates were controlled with Tylan mass flow controllers. Only the temperature was changed and 0.5 h was allowed for each new steady-state with pure helium flowing between runs. A Hewlett Packard gas chromatograph was used to measure the relative concentrations of outlet gas. In addition, each run was duplicated. Conversion is based on the mole difference of acetylene. We define selectivity as the moles of ethylene formed over the the moles of ethylene and ethane. 3. Theory
3.1 Dispersion Model A dispersion model for describing non-ideal reactors is employed to model the hydrogenation of acetylene to ethylene to ethane:
C2H2
t-t2 >C2H4
H2 ,~C2H6
Reaction 1,2
Originally, this model was developed by P.V. Dankwerts [8] in order to describe flow through a complex reactor. The one-dimensional equation with reaction at steady-state is shown without derivation: 1 02Ci ~ C i t - ~ +-r, Pe, a22 02 u
=0
(1)
Here, C is the concentration, r the reaction rate and Pe the Peclet number of species "i." Lambda is the dimensionless thickness through which a species flows. This equation is for an isothermal reaction with negligible pressure drop. It is also considered to be a constant volume, constant flow rate process. With the introduction of the ideal gas law, it can be written in terms of Xi, the mole fraction. The Peclet number is defined in terms of u, the superficial velocity, t the thickness and Di, the flow dispersion. ut Pe i = ~ Di (2) 3.2 The Kinetics The rate expressions for reactions I and 2 have been reported in the literature for a variety of conditions and supports for Pd on alumina. As a starting point, the rate expression is chosen from the literature in order to describe reactions within our system. From Shbib et al. [9,10], the hydrogenation of acetylene and ethylene are given in equations 3 and 4.
kacCacCH2 r~
(1+ (K,,,C,,~)~
3
rat =
ketCEt Ct.t2 (1 + (Kx2C•2)~
3
(3) (4)
Here, r' is the rate per weight of catalyst, k is the respective rate constant, K is an adsorption equilibrium constant and C is the respective molar concentration of either acetylene, ac; ethylene, et; or hydrogen, H2. These kinetics should be adequate as a first model of reaction. They were formulated over a variety of ethylene and acetylene concentrations on a Pd/a- A1203 commercial catalyst. They are rate controlled and exhibit normal Arrhenius behavior. They seem to
2690 roughly follow orders reported by Bond [ 11 ]. The denominator of the rate expressions also includes terms for the adsorption of acetylene, ethylene and ethane. However, under our reaction conditions, only inhibition by H2 adsorption is significant. Adsorption equilibrium constants for acetylene, ethylene and ethane earl be neglected with respect to the first two terms in the denominator as suggested [9-10 ]. In addition, Bond [11] reports that the rate selectivity of ethylene to ethane is proportional to the ratio of the ethylene concentration to acetylene concentration. This is observed in these expressions. When they are applied to equation 1 and the porosity(e) and density(pc) are considered, equation 5 is the result. In this equation X is the mole fraction of species i, and Da is a modified Damkohler number, that is defined in equation 6. 1 0 2X i
OX i
P e i O,;~,2
03,
D a i = Pc kit P z u RT
+ Da,
XiXH2
(I + (Kt.t2Ctt 2
)o.5)3 = 0
(5) (6)
3.3 Dankwert's Boundary Conditions Through any change of medium, there will be a corresponding change in flow characteristics (both from the change and the inherent properties of the geometry, velocity and diffusion). P.V. Dankwert realized this conceptually and developed the correct boundary conditions. The meaning of these boundary conditions and the Peclet and Damk6hler number have been the subject of a myriad of papers both recent and past. As expressed in Fogler [12], the boundary condition must satisfy a flux balance at the entrance and exit. For a continuous system, these state that the derivative is zero at the exit and the entrance concentration is known for a closed-closed system (equations 7 and 8).
x,l +o: Xo
<7)
c~
(8)
s
=0 2--,I
4. Computational Model By also knowing the exit concentration, equation 5 can be written for acetylene, ethylene and hydrogen. Because of the pore size of our system, 3.6 to 4.0 nm, it is reasonable to assume a Knudsen diffusion mechanism. This has been shown [7]. By relating the rate constant of acetylene to ethylene, k, all three equations can be written in terms of a general Pe ,Da numbers and k referenced to acetylene. There are three equations and three unknowns. An algorithm was written in Mathematica 3.0, to solve these three equations for a range of Pe and Da numbers. Initial guesses were determined from the flow rate the thickness and Knudsen diffusion. The result is a locus of minima for each equation (Figure 3). The error is on the z axis (upward). Using another algorithm, these minima are abstracted and plotted. Where they converge for these three species is a solution (Figure 4). 5. Results and Discussion Figure 5 is a graph of the results of hydrogenation of acetylene in premixed mode versus temperature for two different runs. It passes through a maximum of both conversion (99.5 %)
2691
9
9
9
15.8
9
9
9
9
9
9
15.6 9 9 9 9
15.4
9 9
9
]5.2
2.56~er2.57~m~8. 12859 Fig. 4. Solution at 175 ~
Fig. 3.3D error solution for H2 @ 175 ~
k = 0.15
Peclet Number Solutions
Conversion and Selectivity of C=H= Hydrogenation with a Pd/~-AI=O3 Membrane Reactor
75
A v
60
100% X
80%
conv~ I II Sell
.i
60%
"~ 45
t
3O
i
40%
~
tl
20% 50
= 150
15 0
250
350
Temperature (*C)
Fig. 5. Experimental results of hydrogenation of C2H2 (See section 2) in premixed mode.
50
150 Temperature
250
350
(*C)
Fig. 6. Peclet number solutions from the model.
and selectivity (89.5%) at 300 ~ There is also a well defined change in the selectivity at 175 ~ indicating a possible change in mechanism at this temperature. In figure 6, above 175 ~ the Pe increases linearly with temperature. This would suggest a bulk flow mechanism. Below 175 ~ the mechanism appears to depend less strongly dependent on temperature. If it related to 89 order dependence in temperature, then diffusion would be expected to follow more of a Knudsen mechanism. In figure 7, the Damk6hler number for acetylene is seen to be fairly linear on the log plot above 175 ~ It may also be linear below 175 ~ C, but more data is needed. This would suggest a change in the kinetics. In figure 8, the ratio of the rate of acetylene to ethylene hydrogenation, k, is shown. The value of k decreases with temperature as expected. There is a definite pivot point at 175 ~ Considering the selectivity to ethylene, the change in the slope of "k" is reasonable. A model describing the selective hydrogenation of acetylene to ethylene with premixed flow in a catalytic membrane reactor on a ceramic support has been developed. The most general observation is that the mechanism of reaction changes through an increase in temperature at 175 ~ Figure 6, showed a possible change in transport mechanism and figure 7 and 8 showed a definite change in the kinetics. It is postulated that
2692
Damktihler Number Solutions
"k" Solutions 1.5
1.00E+13
1.2
i
0.9
zL
1.00E+11 I
3
1.1~E~09
0
0.6
1.00E+07
9
,,1r
0.3
E 1.00E+05 t3
0 50
150 250 Temperature (*C)
Fig. 7. "k" solutions from model.
350
-~
1.00E+03 50
150
250
350
Temperature (*C)
Fig. 8. DamkOhler solutions from model.
the apparent change in transport mechanisms (thus errant model) may have been induced by the change in kinetic dependence. The increase in flow also increases the number of wall collisions. Because of the small pore diameter, these collisions are important. Thus, below 175 ~ the adsorption of ethylene is important to the kinetics. With kinetics that include adsorption, the observed Pe might then correct to follow the trend above 175 ~ Otherwise, the transport may be having an effect. 6. Conclusion and Future Studies
A short contact time reactor can greatly aid selectivity in a catalytic membrane reactor. The possible influence of transport on the rate of reactions has been shown, but a more detailed investigation is needed. For future studies, a kinetic expression for our relative concentrations, temperatures and pressures will be developed. This is needed to overcome the difference in support material and the suspected role of adsorption of acetylene and ethylene as rate determining. In addition, the diffusive behavior of the reactants must be determined. Finally, a better model would include the changes in volume and flow rate. Thanks to the BES for its support. References
[1] J.N. Armor, Appl. Catal., 49 (1989) 1. [2] V.M. Gryaznov, M.G. Slin'ko, Discuss. Faraday Sot., 72 (1982) 73. [3] N. Itoh, W.C. Xu, A.M. Sathe, Ind. Eng. Chem. Res., 32 (1993) 2614. [4] C.J. Brinker, G.C. Frye, A.J. Hurd, C.S. Ashley, Thin Solid Films, 201 (1991) 97. [5] M. Chai, M. Machida, K. Eguchi and H. Arai, J. Membr. Sci. 97 (1994) 199. [6] C.K. Lambert and R.D. Gonzalez, J. Mater. Sci., 34 (1999) 3109. [7] C.K. Lambert and R.D. Gonzalez, Catalysis Let. 57 (1999) 1. [8] P.V. Dankwerts, Chem. Eng. Sci. 2 (1953) 1. [9] N.S. Schbib, M.A. Garcia, et al., Ind. Eng. Chem. Res. 35 (1996) 1496. [ 10] N.S. Schbib, M.A. Garcia, et al., Ind. Eng. Chem. Res. 36 (1997) 4014. [11] G.C. Bond, Heteroeneous Catalysis: Principles and Applications, 2 nd ed. (Clarendon Press, Oxford, 1987). [ 12] H.S. Fogler, Elements of Reactor Engineering, 2nd. ed. (Prentice Hall, Inc., 1992).
2687
Activity and Selectivity of a Pd/)r-A1203 Catalytic Membrane in the Partial Hydrogenation of Acetylene Christine Lambert, Matthew Vincent, Juan Hinestroza, Ning Sun, and Richard Gonzalez* Department of Chemical Engineering, Tulane University, New Orleans, LA 70118, USA Abstract
A flow dispersion model with Dankwert's boundary conditions describing a Pd/T-A1203 membrane reactor has been developed for the successive hydrogenation reactions of acetylene to ethylene to ethane. At 175 ~ a change in both transport and kinetics is observed. The Peclet number is seen to change dependence in temperature. This change is also seen in the Damk6hler number and the ratio of hydrogenation rates of acetylene to ethylene. It is suspected that a change in rate mechanism at low temperature is influenced by the neglect of the adsorption terms for acetylene and ethylene in the kinetic rate expressions. 1. Introduction
The use of membrane reactors in hydrogenation reactions has seen an increase in activity in recent years [1]. Several dense metal membranes have been used for the partial hydrogenation of acetylene at 100 ~ These include a Pd/Ni membrane [2] and Pd, Pd/Ni, Pd~u, and Pd/Ag membranes [3]. Both studies found that the permeate hydrogen was very active and ethylene was the main product. However, these involve metal membranes, which must be very thin (and thus brittle) in order to provide appropriate selectivity. Porous ceramic membranes can support highly dispersed metals on an inorganic oxide to combine the properties of higher permeability with thermal stability, which the dense metal membranes lack. This enables catalytic and separative properties to be developed at higher temperatures. For our system, a n Gt,-A]20 3 membrane is made catalytic by the deposition of Pd/7-AI203 layers. The ceramic tube purchased from U.S. Filter under the name Membralox is cleaned and then processed using the sol-gel method. The porous substrate is dipped into a solution of colloidal particles of the metal oxide or a precursor to the metal oxide [4]. Through capillary action, the particles concentrate on the substrate surface and form a gel layer. A catalytic membrane forms on the substrate surface by the addition of a water soluble precursor of the catalyst to the solution [5]. It becomes active upon calcination of the membrane. The details of this process were developed previously [6]. With repeated dips and calcinations, a desired thickness can be developed. With appropriate flow rates, it becomes a short contact time reactor. A diagram of this membrane is shown in figure 1. The layers are numbered and their pore size described. An SEM photo of the actual membrane is shown in figure 2. The thickness of the catalytic membrane is approximately 5 I~m. This value was both measured (from photo) and calculated from
*Corresponding Author; GrantingAgencies, BES.
2688
/1
2
ssssS. "" /'
Fig
....
C Mematal'~e Macropomus
thickness 5 um 15 mm ....
Support
r ~ ce.,~u~ofthe
I Alpha Alumina I Support from U.S. I I I
Filter
2
2OO nm
3
800 nm
4
Macro
/
Fig. 1. Diagram of Catalytic Membrane
Fig. 2. SEM photo of Pd/~'-A1203 Membrane
the bulk density of the same catalyst, prepared through traditional methods into its powdered form. A further description of the membrane is found in Section 2. This reactor is operated in a premixed mode with flow into the tube and out of the shell. A previous study by Lambert and Gonzalez [7] showed that this is the preferred orientation. As the results are promising in applications to selectivity in partial hydrogenation reactions, this study attempts to understand and begin to quantify the mechanisms of flow and reaction. Thus, a flow dispersion model has been developed for premixed flow of reactant gases through a cylindrical membrane reactor to direct future study. 2. Membrane Characterization and Reaction Analysis An r tube was used as a support for the membranes. The outer and inner diameter of the tube was 10 mm and 7 mm, respectively. The tube was dipped (for 10 s.) in the Pd solution (sol-gel method) with the outer surface covered by a polyethylene film so that only the inner surface was contacted with the solution. Between each dip-coat, the tube was allowed to dry for 24 h. at room temperature and then calcined in a furnace using a 1~ ramp (to 400~ The catalyst layer was characterized by drying the solution to form the gel and then using BET (Coulter Omnisorp Porosimeter), hydrogen chemisorption and metals analysis (ICP) to determine the physical properties of the Pd/y-A1203 materials. A comparison of the membrane catalyst to tradition methods is summarized in Table 1.
Table 1: Physical Characteristics of Pd/7-Al203 materials Material BET area (mZ/g) Ave. Pore Diameter (nm) Pore Volume (mUg) Pd (wt%) DHa(%) dmD ISGc IE MEM
366 172 334
3.6 8.5 3.6
0.31 0.48 0.37
0.80 0.59 0.72
32 41 29
3.1 2.4 3.4
dispersion as measured by hydrogen chemisorption, H:Pd surface = 1:1 b metal particle diameter (98.2/DH), assuming Pd density = 11.40 g/cm 3. c SG = sol-gel prepared catalyst, IE = ion-exchanged catalyst, MEM = catalytic membrane.
2689 All gases were UHP grade and the flow was 8 mL/min of H2 and 20 mL/min of 10% acetylene in argon at STP. The flow rates were controlled with Tylan mass flow controllers. Only the temperature was changed and 0.5 h was allowed for each new steady-state with pure helium flowing between runs. A Hewlett Packard gas chromatograph was used to measure the relative concentrations of outlet gas. In addition, each run was duplicated. Conversion is based on the mole difference of acetylene. We define selectivity as the moles of ethylene formed over the the moles of ethylene and ethane. 3. Theory
3.1 Dispersion Model A dispersion model for describing non-ideal reactors is employed to model the hydrogenation of acetylene to ethylene to ethane:
C2H2
t-t2 >C2H4
H2 ,~C2H6
Reaction 1,2
Originally, this model was developed by P.V. Dankwerts [8] in order to describe flow through a complex reactor. The one-dimensional equation with reaction at steady-state is shown without derivation: 1 02Ci ~ C i t - ~ +-r, Pe, a22 02 u
=0
(1)
Here, C is the concentration, r the reaction rate and Pe the Peclet number of species "i." Lambda is the dimensionless thickness through which a species flows. This equation is for an isothermal reaction with negligible pressure drop. It is also considered to be a constant volume, constant flow rate process. With the introduction of the ideal gas law, it can be written in terms of Xi, the mole fraction. The Peclet number is defined in terms of u, the superficial velocity, t the thickness and Di, the flow dispersion. ut Pe i = ~ Di (2) 3.2 The Kinetics The rate expressions for reactions I and 2 have been reported in the literature for a variety of conditions and supports for Pd on alumina. As a starting point, the rate expression is chosen from the literature in order to describe reactions within our system. From Shbib et al. [9,10], the hydrogenation of acetylene and ethylene are given in equations 3 and 4.
kacCacCH2 r~
(1+ (K,,,C,,~)~
3
rat =
ketCEt Ct.t2 (1 + (Kx2C•2)~
3
(3) (4)
Here, r' is the rate per weight of catalyst, k is the respective rate constant, K is an adsorption equilibrium constant and C is the respective molar concentration of either acetylene, ac; ethylene, et; or hydrogen, H2. These kinetics should be adequate as a first model of reaction. They were formulated over a variety of ethylene and acetylene concentrations on a Pd/a- A1203 commercial catalyst. They are rate controlled and exhibit normal Arrhenius behavior. They seem to
2690 roughly follow orders reported by Bond [ 11 ]. The denominator of the rate expressions also includes terms for the adsorption of acetylene, ethylene and ethane. However, under our reaction conditions, only inhibition by H2 adsorption is significant. Adsorption equilibrium constants for acetylene, ethylene and ethane earl be neglected with respect to the first two terms in the denominator as suggested [9-10 ]. In addition, Bond [11] reports that the rate selectivity of ethylene to ethane is proportional to the ratio of the ethylene concentration to acetylene concentration. This is observed in these expressions. When they are applied to equation 1 and the porosity(e) and density(pc) are considered, equation 5 is the result. In this equation X is the mole fraction of species i, and Da is a modified Damkohler number, that is defined in equation 6. 1 0 2X i
OX i
P e i O,;~,2
03,
D a i = Pc kit P z u RT
+ Da,
XiXH2
(I + (Kt.t2Ctt 2
)o.5)3 = 0
(5) (6)
3.3 Dankwert's Boundary Conditions Through any change of medium, there will be a corresponding change in flow characteristics (both from the change and the inherent properties of the geometry, velocity and diffusion). P.V. Dankwert realized this conceptually and developed the correct boundary conditions. The meaning of these boundary conditions and the Peclet and Damk6hler number have been the subject of a myriad of papers both recent and past. As expressed in Fogler [12], the boundary condition must satisfy a flux balance at the entrance and exit. For a continuous system, these state that the derivative is zero at the exit and the entrance concentration is known for a closed-closed system (equations 7 and 8).
x,l +o: Xo
<7)
c~
(8)
s
=0 2--,I
4. Computational Model By also knowing the exit concentration, equation 5 can be written for acetylene, ethylene and hydrogen. Because of the pore size of our system, 3.6 to 4.0 nm, it is reasonable to assume a Knudsen diffusion mechanism. This has been shown [7]. By relating the rate constant of acetylene to ethylene, k, all three equations can be written in terms of a general Pe ,Da numbers and k referenced to acetylene. There are three equations and three unknowns. An algorithm was written in Mathematica 3.0, to solve these three equations for a range of Pe and Da numbers. Initial guesses were determined from the flow rate the thickness and Knudsen diffusion. The result is a locus of minima for each equation (Figure 3). The error is on the z axis (upward). Using another algorithm, these minima are abstracted and plotted. Where they converge for these three species is a solution (Figure 4). 5. Results and Discussion Figure 5 is a graph of the results of hydrogenation of acetylene in premixed mode versus temperature for two different runs. It passes through a maximum of both conversion (99.5 %)
2691
9
9
9
15.8
9
9
9
9
9
9
15.6 9 9 9 9
15.4
9 9
9
]5.2
2.56~er2.57~m~8. 12859 Fig. 4. Solution at 175 ~
Fig. 3.3D error solution for H2 @ 175 ~
k = 0.15
Peclet Number Solutions
Conversion and Selectivity of C=H= Hydrogenation with a Pd/~-AI=O3 Membrane Reactor
75
A v
60
100% X
80%
conv~ I II Sell
.i
60%
"~ 45
t
3O
i
40%
~
tl
20% 50
= 150
15 0
250
350
Temperature (*C)
Fig. 5. Experimental results of hydrogenation of C2H2 (See section 2) in premixed mode.
50
150 Temperature
250
350
(*C)
Fig. 6. Peclet number solutions from the model.
and selectivity (89.5%) at 300 ~ There is also a well defined change in the selectivity at 175 ~ indicating a possible change in mechanism at this temperature. In figure 6, above 175 ~ the Pe increases linearly with temperature. This would suggest a bulk flow mechanism. Below 175 ~ the mechanism appears to depend less strongly dependent on temperature. If it related to 89 order dependence in temperature, then diffusion would be expected to follow more of a Knudsen mechanism. In figure 7, the Damk6hler number for acetylene is seen to be fairly linear on the log plot above 175 ~ It may also be linear below 175 ~ C, but more data is needed. This would suggest a change in the kinetics. In figure 8, the ratio of the rate of acetylene to ethylene hydrogenation, k, is shown. The value of k decreases with temperature as expected. There is a definite pivot point at 175 ~ Considering the selectivity to ethylene, the change in the slope of "k" is reasonable. A model describing the selective hydrogenation of acetylene to ethylene with premixed flow in a catalytic membrane reactor on a ceramic support has been developed. The most general observation is that the mechanism of reaction changes through an increase in temperature at 175 ~ Figure 6, showed a possible change in transport mechanism and figure 7 and 8 showed a definite change in the kinetics. It is postulated that
2692
Damktihler Number Solutions
"k" Solutions 1.5
1.00E+13
1.2
i
0.9
zL
1.00E+11 I
3
1.1~E~09
0
0.6
1.00E+07
9
,,1r
0.3
E 1.00E+05 t3
0 50
150 250 Temperature (*C)
Fig. 7. "k" solutions from model.
350
-~
1.00E+03 50
150
250
350
Temperature (*C)
Fig. 8. DamkOhler solutions from model.
the apparent change in transport mechanisms (thus errant model) may have been induced by the change in kinetic dependence. The increase in flow also increases the number of wall collisions. Because of the small pore diameter, these collisions are important. Thus, below 175 ~ the adsorption of ethylene is important to the kinetics. With kinetics that include adsorption, the observed Pe might then correct to follow the trend above 175 ~ Otherwise, the transport may be having an effect. 6. Conclusion and Future Studies
A short contact time reactor can greatly aid selectivity in a catalytic membrane reactor. The possible influence of transport on the rate of reactions has been shown, but a more detailed investigation is needed. For future studies, a kinetic expression for our relative concentrations, temperatures and pressures will be developed. This is needed to overcome the difference in support material and the suspected role of adsorption of acetylene and ethylene as rate determining. In addition, the diffusive behavior of the reactants must be determined. Finally, a better model would include the changes in volume and flow rate. Thanks to the BES for its support. References
[1] J.N. Armor, Appl. Catal., 49 (1989) 1. [2] V.M. Gryaznov, M.G. Slin'ko, Discuss. Faraday Sot., 72 (1982) 73. [3] N. Itoh, W.C. Xu, A.M. Sathe, Ind. Eng. Chem. Res., 32 (1993) 2614. [4] C.J. Brinker, G.C. Frye, A.J. Hurd, C.S. Ashley, Thin Solid Films, 201 (1991) 97. [5] M. Chai, M. Machida, K. Eguchi and H. Arai, J. Membr. Sci. 97 (1994) 199. [6] C.K. Lambert and R.D. Gonzalez, J. Mater. Sci., 34 (1999) 3109. [7] C.K. Lambert and R.D. Gonzalez, Catalysis Let. 57 (1999) 1. [8] P.V. Dankwerts, Chem. Eng. Sci. 2 (1953) 1. [9] N.S. Schbib, M.A. Garcia, et al., Ind. Eng. Chem. Res. 35 (1996) 1496. [ 10] N.S. Schbib, M.A. Garcia, et al., Ind. Eng. Chem. Res. 36 (1997) 4014. [11] G.C. Bond, Heteroeneous Catalysis: Principles and Applications, 2 nd ed. (Clarendon Press, Oxford, 1987). [ 12] H.S. Fogler, Elements of Reactor Engineering, 2nd. ed. (Prentice Hall, Inc., 1992).