Ind. Eng. Chem. Res. 2001, 40, 5487-5495
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KINETICS, CATALYSIS, AND REACTION ENGINEERING Parameter Estimation in a Fixed-Bed Reactor Operating under Unsteady State: Oxychlorination of Ethylene† Potharaju S. Sai Prasad,*,‡ Kuppili B. S. Prasad,*,‡ and M. S. Ananth§ Indian Institute of Chemical Technology, Hyderabad 500 007, India, and Indian Institute of Technologys Madras, Chennai 600 036, India
During the oxychlorination of ethylene, the composition of the catalyst, cupric chloride, is subject to change, and hence estimation of rate parameters considering the unsteady-state situation is more reliable than assuming steady state. In the present work, nonisothermal experimental data are collected in an unsteady-state fixed-bed reactor by changing the inlet temperature using a temperature-programmed preheater. The rate parameters are estimated by developing an unsteady-state model which takes into account not only the reaction temperature and gas-phase conversions but also changes in the solid-phase composition in the catalyst bed. The rate parameters estimated in this study are found to be different from the steady-state values reported in the literature. The disagreement in the rate parameters has been explained in terms of the properties associated with the solid phase. The most significant feature of this work is that the rate-temperature behavior, calculated using the present rate constants, corresponds closely with that derived from independent catalyst composition studies reported in the literature, thus indirectly validating the parameters estimated in this work. Introduction Oxychlorination of ethylene to produce dichloroethane (DCE) is an important intermediate step in the manufacture of poly(vinyl chloride) by the balanced vinyl chloride process. It involves reacting ethylene with hydrogen chloride and oxygen (or air) over a supported cupric chloride catalyst in the temperature range of 443-673 K.1-5 Current literature suggests that the overall reaction occurs through a sequence of simpler reactions.
C2H4 + 2CuCl2 f C2 H4Cl2 + 2CuCl 2CuCl + 1/2O2 f CuO-CuCl2 f CuO + CuCl2 CuO + 2HCl f CuCl2 + H2O C2H4 + 2HCl + 1/2O2 f C2H4Cl2 + H2O The overall reaction causes a change in the catalyst composition from the initial pure cupric chloride to a phase containing the three components, namely, cupric chloride, cuprous chloride, and copper oxide. Zernosek et al.6 initiated the kinetic treatment in terms of the component steps. Dmitrieva et al.7 confirmed the stepwise mechanism for additive oxychlorination from both the steady-state and unsteady-state experiments. How† IICT Communication No. 3650. * To whom correspondence should be addressed. E-mail:
[email protected]. Fax: +91-40-7170921. ‡ Indian Institute of Chemical Technology. § Indian Institute of TechnologysMadras.
ever, the composition of the catalyst was not one of the measured quantities in these experiments. Their conclusions were based on gas-phase conversions only. The ratio of Cu+/Cu2+ was measured by Conner et al.8 for oxychlorination of methane using X-ray photoelectron spectroscopy (XPS). They have reported that the catalyst undergoes a thermal transition at around 523 K when the reaction rate increases dramatically. In a study on low-temperature oxychlorination of ethylene, Rouco9 reported similar findings and concluded that the copper species on the catalyst becomes mobile at around 506 K, suggesting a rapid increase in the reaction rate. It may be argued that the rate constants must reflect such a behavior. Because the exothermic nature of the reaction does not permit a uniform temperature in the fixed-bed reactor, it is likely that the catalyst composition is also not uniform in the bed. Further, sublimation and recondensation of cupric chloride4 also leads to further change in the local catalyst concentration. Therefore, in carrying out experiments aimed at evaluating reaction kinetics, due consideration must be given not only to the steady-state composition but also to the path followed to reach it because they are influenced by the rates of reaction on the surface. However, earlier studies have not given enough consideration to this aspect. In the present work, nonisothermal experimental data are obtained by changing the inlet temperature using a programmed preheater and the rate parameters are estimated by developing an unsteady-state model taking into account the change in catalyst composition, changes in temperature profiles, and exit gas-phase conversions with time. The rate constants thus estimated are expected to be more realistic. The objective of this paper is also to verify whether the experimentally observed
10.1021/ie010031i CCC: $20.00 © 2001 American Chemical Society Published on Web 10/13/2001
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Table 1. Steady-State Initial Temperature Profiles
Table 4. Experimental Observations of Temperature and Solid- and Fluid-Phase Conversions Obtained for Set S10a
temperature (K) set ID
ξ ) 0.125
ξ ) 0.375
ξ ) 0.625
ξ ) 0.875
ξ ) 1.000
S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 S14 S15
390 403 421 427 505 420 414 448 458 450 480 462 465 453 441
386 398 413 419 498 414 413 445 451 447 478 461 463 444 436
375 383 394 401 483 408 402 433 433 432 459 444 446 436 421
371 378 386 393 468 401 398 430 427 428 456 442 443 428 417
371 378 386 393 468 401 398 430 427 428 456 442 443 428 417
Table 2. Summary of the Experimental Settings time at inlet the end nitrogen flux ratio at the inlet temp of the run flux (s) set ID (kmol/m/s) C2H4/N2 O2/N2 HCl/N2 (K) S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 S14 S15
0.004 74 0.004 74 0.004 74 0.004 74 0.004 74 0.003 80 0.003 80 0.003 80 0.002 85 0.002 85 0.002 85 0.001 90 0.001 90 0.001 90 0.001 90
0.500 0.500 0.500 0.500 0.500 0.625 0.625 0.625 0.833 0.833 0.833 1.250 1.250 1.250 1.250
0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25
1.00 1.00 1.00 1.00 1.00 1.25 1.25 1.25 1.67 1.67 1.67 2.50 2.50 2.50 2.50
395 408 426 432 505 421 418 451 452 456 485 465 471 453 446
3840 4020 8760 5460 3720 3845 5160 4980 4980 4320 4020 4500 3600 3900 3600
Table 3. Experimental Observations of Temperature and Solid- and Fluid-Phase Conversions Obtained for Set S8a fluid-phase conversion (%)
temp (K) time (s)
ξ1
ξ2
ξ3
ξ4
x1
x2
x3
x4
x5
5 60 120 180 240 600 960 1980 4080 4980
488 464 487 498 505 524 528 529 531 534
445 460 482 497 505 524 533 536 541 545
433 445 468 494 492 518 531 534 545 554
430 443 466 492 491 521 536 547 570 584
2.5 3.1
2.6 3.4
2.2 3.2
2.0 2.6
0.2 0.4
CuCl2 conversion (%) CuCl formation (%) CuO formation (%)
4.4 5.1
4.8 5.4
5.1 5.4
4.0 4.2
fluid-phase conversion (%)
temp, K time (s)
ξ1
ξ2
ξ3
ξ4
5 60 120 180 240 600 1200 3000 4320
450 472 489 505 513 526 531 537 536
447 468 483 503 510 530 541 548 547
432 448 462 479 502 522 542 555 555
428 443 456 473 499 528 563 583 584
x1
x2
x3
x4
x5
2.5
2.8
2.4
2.1
0.2
2.9
3.3
3.2
2.5
0.4
3.8
4.1
4.1
3.2
0.4
4.8 4.8
5.4 5.4
5.3 5.3
4.1 4.1
0.5 0.5
solid-phase composition at time ) 4320 s
ξ1
ξ2
ξ3
ξ4
CuCl2 conversion (%) CuCl formation (%) CuO formation (%)
41.1 13.4 27.7
37.3 12.9 24.4
22.6 9.9 12.7
18.2 8.5 9.7
a Fractional bed length: ξ ) 0.125; ξ ) 0.375; ξ ) 0.625; ξ 1 2 3 4 ) 0.875. x1 ) ethylene conversion (%), x2 ) oxygen conversion (%), x3 ) HCl conversion (%), x4 ) DCE formation (%), x5 ) TCE formation (%).
temperature profile, the product distribution as a function of time, and the final catalyst composition for two typical experiments are given in Tables 3 and 4. Modeling The stoichiometry of the oxychlorination reaction network is given in Table 5. DCE was observed to be the major product. Finocchio et al.10 have reported that the uncovered alumina surface is responsible for the dehydrochlorination of DCE to the vinyl chloride monomer in the oxychlorination reactor. Vinyl chloride monomer, in turn, undergoes oxychlorination, leading to the formation of trichloroethane (TCE). However, because the TCE formation was found to be negligible and also for the sake of mathematical clarity, the original concept of TCE formation from ethylene was retained. Recent literature11-13 suggests that the rates of individual reactions may be assumed to be first order in both gas and solid concentrations and the rate expressions for reactions R-1-R-7 are given by the following equations:
0.4 0.6
r1 ) k1FSp(1)S(1)
(1)
r2 ) k2FSp(1)S(1)
(2)
ξ1
ξ2
ξ3
ξ4
r3 ) k3p(1)p(2)
(3)
34.3 12.0 22.3
30.5 9.84 20.7
30.6 8.29 22.3
25.5 6.29 19.2
r4 ) k4FSp(2)S(2)
(4)
r5 ) k5FSp(3)S(3)
(5)
r6 ) k6FS
(6)
r7 ) k7FS
(7)
ki ) Ai exp(-Ei/RT)
(8)
Fractional bed length: ξ1 ) 0.125; ξ2 ) 0.375; ξ3 ) 0.625; ξ4 ) 0.875. x1 ) ethylene conversion (%), x2 ) oxygen conversion (%), x3 ) HCl conversion (%), x4 ) DCE formation (%), x5 ) TCE formation (%). a
temperature and solid-phase composition behavior is in fact captured by the estimated rate parameters. Results The temperature profiles under no reaction conditions for each corresponding setting are presented in Table 1. Table 2 summarizes the experimental settings. The
where
The experimental rates are assumed to be diffusionfree considering the small size of the catalyst particles and the low gas-phase conversions ( 20) employed. It is further assumed that the axial mass dispersion is negligible and that the effect of radial dispersion of mass and heat is also not significant. However, the axial heat dispersion is included in the model. From the stoichiometry of the reaction network, it is clear that there are six gas-phase and three solid-phase primary observables. The gas-phase observables are ethylene conversion, oxygen conversion, HCl conversion, DCE formation, TCE formation, and fractional vaporization of CuCl2. The solid-phase observables are CuCl2 conversion and the formation of CuCl and CuO. Thus, there are 10 dependent variables, including temperature, for which different balance equations must be written. It is clear from the stoichiometry that a finite gas conversion results in a finite solid conversion and vice versa. The reaction system is intrinsically transient because the solid phase is stationary. A steady-state description of the system is therefore not adequate. The basic unsteady-state mass and energy balance equations are rendered dimensionless by defining dimensionless length (ξ) as the fraction of the full length of the reactor, dimensionless time (τ) as the time on stream relative to the feed residence time, and dimensionless temperature (θ) as the adiabatic temperature rise relative to the preheater temperature at zero time. The dimensionless equations are given as follows:
f (6) ) g(1) )
ht (1)
n C(I) ht
C(I)
(r2)
(16)
(r6 - r7)
(17)
ht
[2r4 + (1/2)r5 - 2r1 - 3r2 - r6 + r7] (18) FSS(1),0 g(2) )
ht FSS(1),0
g(3) )
[
(2r1 + 3r2 - 4r4)
(19)
[2r4 - (1/2)r5]
(20)
ht FSS(1),0
7
ht (
q ) - H(1+θ) +
(+∆Hi)ri) ∑ i)1
(T°-∞ - Tw)FfCp
]
(21)
where H ) 4hL/dtUFfCp. Clearly, in eqs 12-21 the rates, r, are functions of θ, through the Arrhenius relation (eq 8).
Initial conditions: At τ ) 0, x(i)(ξ,0) ) 0 y(j)(ξ,0) ) 0
i ) 1, ..., 6 j ) 1, ..., 3
θ(ξ,0) ) θ°(ξ)
(22) (23) (24)
Here θo(ξ) is given by the solution of the steady-state heat balance equation under no-reaction conditions.
Boundary conditions: δx(i) δx(i) ) + f (i) δτ δξ δy(j) ) g(j) δτ
i ) 1, ..., 6
j ) 1, ..., 3
δθ δ2θ δθ )β 2+q δτ δξ δξ
(9)
ht n(1)C(I)
f (2) )
(11)
(r1 + r2 + r3)
(12)
(3r3 + r4)
(13)
ht (2)
n C(I)
f (3) ) f (4) )
ht (3)
n C(I) ht n(1)C(I)
θξ)0 ) θ-∞(τ) + β
(10)
where x(i) and y(j) are gas- and solid-phase conversions, respectively, and f (i), g(j), and q are the source terms for the gas-phase component, i, the solid-phase component, j, and the heat defined by the following equations:
f (1) )
At ξ ) 0, x(i)(0,τ) ) 0
(r5)
(14)
(r3)
(15)
At ξ ) 1,
|
dθ dξ
(25)
|
dθ dξ
(26) ξ)0
)0
(27)
ξ)1
where θ-∞(τ) is given by the following experimentally determined inlet temperature policy function:
θ-∞(τ) ) a tanh(bτ)
(28)
Use of the above equation for θ-∞(τ) ensures that it is zero at τ ) 0, consistent with the definition of dimensionless temperature. Solution of Model Equations. Application of orthogonal collocation14 to the space derivatives in eq 9-11 reduces them to ordinary differential equations in time. The system of ordinary differential equations is integrated using the method proposed by Caillaud and Padmanabhan.15 The kinetic parameters of the model are estimated using the following objective function for the sum of the squared deviations.
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S) M
U
Table 6. Transport Parameters
6
∑ ∑ ∑i (x(i)N,u,m - xφ,(i)N,u,m)2 + m v V
3
∑ ∑ ∑j m v
M
(y(j)V,U,m - yφ,(j)V,U,m)2 +
V
U
∑ ∑ ∑u (θV,U,m m v θφV,U,m)2 (29)
where M is the total number of experiments, U is the number of time nodes at which observations are made, and V is the number of space nodes at which observations are made. x(i) values are observed only at the reactor exit, and y(j) values are observed at the end of each experiment. Minimization of S in eq 29 has been carried out by the Marquardt method.16 Estimation of the Maximum Expected Error in Parameters. The estimated maximum error is calculated by the following:
e)
(M S- PPF
0.95(P,M-P)
λmax
)
1/2
v
(30)
where λmax is the largest eigenvalue of the modified precision matrix at convergence and v is its corresponding eigenvector.
transport parameter
calcd value
calcd from lit. correlation
1. wall heat-transfer coefficient (kJ/m2‚s‚K) 2. axial bed thermal conductivity [kJ/m2‚s‚(K/m)]
0.1960 × 101
0.167 × 101
0.1709 × 103
0.917 × 103
h is fairly good, the value of kz estimated using the present experiments is somewhat different from the calculated value. The difference may be attributed to the low value of the particle diameter used (0.5 mm), whereas the limiting axial heat Peclet number of 0.6 is generally applicable for larger particles of 5-6 mm.21 The values of kz and h estimated in this work are used in further calculations. Kinetic Parameters Obtained by Optimization. The final converged values are given in Table 7 in which literature values are also presented wherever such values are available. The corresponding maximum expected error, calculated by eq 30, is also given for each estimated parameter as a fractional value. The maximum expected error in all cases is very low, on the order of 1-5%. Figures 2-5 show the calculated and observed
Discussion Studying the changes in the solid composition has been the subject matter of several researchers in the recent past. The importance of formation of Cu+ species has been stressed. Friend et al.17 studied oxychlorination of ethylene in an aqueous medium. The reaction proceeded very well when some cuprous chloride was added to complex ethylene to bring it into solution. As a followup to this work, Specter et al.18 showed that the induction time in the heterogeneous reaction could be significantly reduced by adding cuprous chloride to cupric chloride. Conner et al.,8 by preparing the catalyst in nonaqueous media using silica as a support, studied the oxychlorination of methane. XPS spectra of the samples obtained after resolving Cu, CuCl, CuCl2, and CuO as standards for the peak shapes revealed that the activity of the reaction increases with an increase in the Cu+/Cu2+ ratio. Increases in CuCl levels with increases in CuCl2 conversion and gas-phase conversions, observed in the present investigation, are perfectly in conformity with the literature findings stated above. Heat-Transfer Parameters. The heat-transfer parameters, namely, the wall coefficient, h, and the bed thermal conductivity, kz, are estimated from independent steady-state experiments under no-reaction conditions using the analytical expression for θ°(ξ). The detailed procedure is given by Sai Prasad.19 Estimation of the wall coefficient in industrial fixed-bed reactors as given by Doraiswamy and Sharma20 is carried out by the expression
h ) 2.03Re, p0.8(kf/dt) exp(-6dp/dt)
Figure 1. Schematic of the reaction setup.
(31)
The bed thermal conductivity, kz, could be evaluated using the definition of the reciprocal of the axial heat Peclet number by substituting a value of 0.6 for the Peclet number, which is the asymptotic value for the Reynolds number greater than 20.21 For the conditions prevailing in the present experimental reactor, the calculated and estimated values of kz and h are compared in Table 6. While the agreement in the value of
Figure 2. Comparison of the observed temperature with the calculated temperature for set S8.
Ind. Eng. Chem. Res., Vol. 40, No. 23, 2001 5491 Table 7. Comparison of Kinetic Parameters frequency factor (kmol/kg of supp‚s‚MPa)
activation energy (kJ/g‚mol)
reaction step
estimated
literature
calcd error
estimated
literature
calcd error
1. reduction of CuCl2 to DCE 2. reduction of CuCl2 to TCE 3. C2H4 oxidation 4. CuCl oxidation 5. hydrochlorination of CuO 6. evaporation of CuCl2 7. recondensation of CuCl2
1.849 × 108
5.50 × 10-6 a (1.79 × 108 b)
0.206 × 10-4
97.72 × 103
0.456 × 10-3
0.200 × 10-5
111.30 × 103
79.42 × 103 a (84 × 103 b)
0.292 × 10-4 0.421 × 10-5 0.198 × 10-6
129.7 × 103 86.97 × 103 144.3 × 103
190.0 × 103 c 73.99 × 103 c 90.29 × 103 a
0.712 × 10-3 0.342 × 10-1 0.192 × 10-4
1.672 × 108 7.759 × 101 3.446 × 108 8.329 × 1013
1.796 × 10-1 a
9.95 × 103
0.215 × 10-2
65.00 × 103 d
9.546
0.170 × 10-1
65.00 × 103 d
a Reference 5. b Dmitrieva7 frequency factors recalculated as per the units of the present text. c Reference 11. the heat of sublimation.
Figure 3. Comparison of the observed temperature with the calculated temperature for set S10.
dimensionless temperatures for four typical experiments. The points are evenly distributed about the diagonal, indicating that the parameters predict the temperature reasonably accurately. A comparison (Table 7) of the frequency factors, for the reduction of CuCl2 and hydrochlorination of CuO (R-1 and R-5), obtained in this work with those available in the literature reveals considerable differences in their values. Particularly, the very low values reported by Allen5 refer to experiments conducted with pure unsupported CuCl2 or CuO, wherein the dispersion of the solid reactant is virtually absent and the reaction occurs over a small surface area. On the other hand, the alumina support used for preparing the catalyst in this work has a high surface area resulting in good dispersion of the catalyst.22 The influence of the dispersion on the activity is also very well recognized by Sharpe and Vikerman.23 Dmietrieva et al.7 reported the values of the frequency factors as 3.585 × 108 and 4.0238 × 103 for the twocomponent reaction steps, namely, reduction of cupric chloride and its regeneration, respectively. The value reported by Dmietrieva et al.7 for R-1 is of the same order of magnitude as that obtained in the present
d
0.460 × 10-3
Assumed to be equal to
Figure 4. Comparison of the observed temperature with the calculated temperature for set S5.
investigation. However, the catalyst was fully saturated with HCl prior to the reaction. The difference in the value of the frequency factor for R-5 may be due to this reason. These authors have also reported that the frequency factor could vary with the nature of the support. The differences in the estimated and literature values are somewhat large in the cases of activation energies (R-1-R-5), but in no cases are the difference unreasonable. Different values for the activation energies arise perhaps because of the differences in the nature of the active sites. Dmietrieva et al.7 assumed the active site to be an adsorbed complex of cupric chloride with HCl. They were of the opinion that if the surface of the catalyst was not saturated with HCl, the active species would be different. Arcoya et al.24 also reported the significance of HCl pretreatment. They thought that in the case of CuCl2-KCl/γ-Al2O3 catalyst this would lead to an increased reoxidation rate of cuprous chloride. In the present investigation, no HCl pretreatment was carried out, and hence the active sites could be different. Temperature-Reaction Rate Behavior. For a complex reaction like oxychlorination of ethylene, the temperature-reaction rate behavior may be best de-
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λ1, λ2 ) -(a + b + c) ( [(a + b + c)2 - 2(ab + 2bc + 2ca)]0.5 2 (34)
Figure 5. Comparison of the observed temperature with the calculated temperature for set S15.
rived by the temperature dependence of the most significant eigenvalue of the rate constant matrix. To simplify the discussion, it is assumed that the change in catalyst composition occurs only because of three major component reactions: DCE formation (R-1), CuCl oxidation (R-4), and CuCl2 regeneration (R-5). Thus, by setting of the rates of sublimation and redeposition of CuCl2 and ethylene oxidation to zero, the rate constant matrix is obtained from eqs 18-20 as follows:
[
-a K- ) -a 0
b/2 -b b/2
c 0 -c
]
(32)
where a-c are given by
a ) 2k1n(1), b ) 4k4n(2), c ) (1/2)k5n(3)
(33)
This matrix, K-, obviously has two nonzero eigenvalues which are given by
The values of n(1) and n(3) in the present work vary as given in Table 2, whereas n(2), being the ratio of oxygen to nitrogen in air, is constant. Calculation was made for the two sets of extreme values of n(1) and n(3). Figure 6 shows the variation of the significant eigenvalue, λ2, with temperature. It is clear that around 500 K there is a dramatic change in its value irrespective of the values of molar ratios. In fact, |λ2| has changed from 0.10 at 475 K to 1.0 at 535 K over a short temperature range of 60 K, a 10-fold increase. From the reaction stoichiometry, it can be stated that an increase in rate constant k1 causes an increase in Cu+ at the expense of Cu2+. Similarly, an increase in k5 causes an increase in Cu2+ at the expense of CuO, and an increase in k4 causes an increase in CuO at the expense of Cu+. This suggests that Cu+ is proportional to the ratio (k1 + k5)/k4. Figure 7 shows variation of this ratio with respect to temperature, which also indicates behavior similar to Figure 6, with the ratio changing rapidly between 475 and 535 K. It may be recalled that Conner et al.8 and Rouco9 have reported a rapid change in the rate at temperatures falling within this range. Reports on commercial operation2 also advocated the hot-spot temperature to be preferably around 573 K ()300 °C). Thus, corroborative evidence is obtained for a rapid change in the reaction rate, from consideration of both the solid-phase composition and the reaction kinetics which are separate and independent studies. An observation of the relative rates of the three main reactions in the reaction network is worth mentioning. From the rate constants calculated for different temperatures and given in Table 8, it appears that, at low temperature, the overall oxychlorination rate is determined by either cupric chloride reduction (R-1) or cupric chloride regeneration (R-5) because both k1 and k5 corresponding to R-1 and R-5 are lower by several orders of magnitude than k4. Between R-1 and R-5, it is difficult to say which is rate controlling because both reaction rate constants are of the same order of magnitude.
Figure 6. Variation of significant eigenvalue with temperature: 9, n(1) ) 0.5, n(3) ) 1.0; b, n(1) ) 1.25, n(3) ) 2.5.
Ind. Eng. Chem. Res., Vol. 40, No. 23, 2001 5493
Figure 7. Variation of the rate constant ratio with temperature. Table 8. Relative Rates of Reaction Steps temp (K)
k1
k4
k5
443 473 573
5.6 × 10-4 3.0 × 10-3 0.23
0.019 0.087 4.100
8.2 × 10-4 9.8 × 10-3 5.9
However, the situation is dramatically changed at high temperature when only R-1 appears to determine the overall rate, because both k4 and k5 corresponding to R-4 and R-5 are at least 1 order of magnitude higher than k1, which corresponds to cupric chloride reduction or DCE formation. This qualitative observation is in very good agreement with the reported experience of some earlier investigators.9,11 Conclusions 1. On the basis of a series of experiments carried out in a fixed-bed rector, it has been concluded that the supported CuCl2 catalyst in oxychlorination of ethylene undergoes considerable change in composition during the reaction and that this composition change is a strong function of the reaction conditions. 2. The kinetic parameters for the component reactions obtained in this study are somewhat different from those reported in the literature. Because change in the catalyst composition is also considered in this work along with the temperature and gas-phase conversions in the model to describe the system unlike previous literature, these parameters may be more realistic. Further, variation in the dispersion of CuCl2 on Al2O3 and a possible change in the nature of the active sites may also be contributing to the variation in the values of the kinetic parameters. 3. The overall reaction appears to have three distinctly different kinetic regimes: (i) below 475 K, (ii) between 475 and 535 K, and (iii) above 535 K. The reaction appears to occur with no clear rate-determining step between 475 and 535 K. Below 475 K, either reduction of CuCl2 or regeneration of CuCl2 controls the rate of the overall reaction, whereas above 535 K, reduction of CuCl2 alone governs the rate of the overall reaction. This observation based on reaction kinetics is
in conformity with inferences from measurement of catalyst species on the surface by other workers. Acknowledgment The authors are extremely grateful to Dr. P. Kanta Rao, Former Head, Catalysis & Physical Chemistry Division, for his keen interest in this study. The authors thank Dr. K. V. Raghavan, Director, Indian Institute of Chemical Technology, Hyderabad, India, for permitting us to publish this work. Financial assistance by M/s Indian Petrochemical Corp. Ltd., Vadodara, India, for the catalyst development project is greatly acknowledged. Thanks are also due to Dr. T. S. R. Prasada Rao, Former Director, Indian Institute of Petroleum, Dehradun, India, for his encouragement. Appendix: Experimental Section Commercial γ-Al2O3 (Harshaw, Al-111-61E, crushed and sieved to 18/25 BSS mesh) was impregnated with an aqueous mixture of CuCl2‚2H2O and KCl (BDH, analar grade) adapting the incipient wetness method such that the finished catalyst contained 18 wt % of CuCl2 and 2 wt % of KCl. The catalyst, dried at 393 K for 8 h in air, had a BET surface area of 111 m2/g. A schematic representation of the experimental setup is given in Figure 1. The glass reactor (20 mm i.d. and 50 mm length) with an axial thermowell was provided with a silver-coated, evacuated, and sealed jacket to achieve near-adiabatic conditions. The bottom of the reactor was detachable to enable quick withdrawal of the catalyst from the reaction zone. The glass-coiled preheater, which was inserted in an electrical furnace, with the reactor portion extending just outside the furnace, was heated, controlling the temperature by a PID controller with an accuracy of (0.5 K. Interspersed by very thin layers of quartz wool, the catalyst was loaded in the reactor as four equal parts. Six K-type, fine thermocouples were positioned: one at the inlet, one at the outlet, and the remaining four at the midpoints of the four parts of the bed. Ethylene (99.95% pure) and anhydrous hydrogen chloride (99.5% pure; supplied by M/s Matheson, Secaucus, NJ) were used
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without any further treatment, while the air line was provided with a battery of driers. The reactor was first kept at the required temperature by passing nitrogen gas at the prescribed flow rate, which was within the limits of the experimental flow rates proposed. Half-an-hour after the system had attained steady state, the temperature profile under noreaction conditions was recorded along with the temperature of the jacket. The procedure was repeated at the beginning of each experimental run to give the initial temperature profile for the estimation of heattransfer parameters. Experimental Procedure. After the initial steadystate temperature profile was noted, the nitrogen flow was diverted to vent by means of a four-way stopcock while simultaneously admitting the reactant mixture of ethylene, hydrogen chloride, and air into the reactor. This changeover did not create any appreciable temperature fluctuations. At this point of time, the inlet temperature was varied at a fixed rate by changing the rate of heating in the preheater to obtain the inlet temperature policy. The temperature profile was recorded at every 30 s initially for about 15 min and thereafter less frequently. Simultaneously, the unreacted HCl was analyzed by titrating it against a standard NaOH solution. Both the condensable product (collected by absorption in cooled o-xylene) and the uncondensed gas mixture were analyzed in a gas chromatograph equipped with a thermal conductivity detector. An SE-30 on chromosorb P column was used for organic gases and liquids, whereas a molecular sieve 5A column was used to separate the inorganic gases.19 At the end of the run, the reactant feed was stopped, admitting nitrogen flow into the reactor simultaneously and cutting off the heat supply to the preheater. The catalyst was allowed to cool to room temperature in the inert medium. The individual portions of the catalyst bed were then quickly transferred into separate receivers containing known quantities of a ferric ammonium sulfate solution to analyze the solid-phase composition. Chemical analyses for the cuprous and cupric chloride contents were carried out following the procedures described by Vogel.25 XPS was used to confirm the presence of CuCl2, CuCl, and CuO in a few carefully collected used catalysts by following the procedure reported by Kaushik26 and employed by Sai Prasad19 earlier. Notations a ) constant in eq 28 Ai ) frequency factor for reaction i b ) constant in eq 28 C ) concentration of species i (kmol/m3) Cp ) specific heat (kJ/kg‚K) dp ) diameter of the particle (m) dt ) diameter of the tube (m) ) bed voidage Ej ) activation energy of reaction j (kJ/kmol) f (i) ) source term for the ith gas-phase component g(j) ) source term for the jth solid-phase component h ) heat-transfer coefficient (kJ/m2‚K‚s) kj ) rate constant for reaction j (kmol/m3‚s‚MPa) kz ) axial bed thermal conductivity (kJ‚m2‚s(K/m)) n(i) ) molar feed ratio of species i to that of nitrogen at the inlet p(i) ) partial pressure of species i (MPa) q ) source term for heat as defined in eq 21 ri ) rate of reaction i (kmol/m3‚s)
R ) gas constant (MPa‚m3/kmol‚K) S(j) ) concentration of solid species (kmol/kg of the support) S(1),0 ) initial concentration of cupric chloride (kmol/kg of the support) t ) time on stream (s) t ) residence time of the feed (s) T ) fluid temperature (K) Tw ) temperature of the reactor wall (K) Tφ-∞ ) temperature of the preheater (K) x(i) ) conversion of the ith gaseous species y(j) ) conversion of the jth solid species β ) reciprocal of the heat Peclet number ∆H ) heat of reaction (kJ/kmol) e ) ratio of the maximum numerical error to the maximum tolerance ζ ) fraction of the reactor length relative to the full length θ ) adiabatic temperature rise relative to the preheater temperature at zero time Ff ) density of the fluid (kg/m3) Fg ) bulk density of the catalyst support (kg/m3) τ ) time on stream relative to the feed residence time
Literature Cited (1) Magistro, A. J.; Cowfer, J. A. Oxychlorination of ethylene. J. Chem. Educ. 1986, 63, 1056. (2) Naworski, J. S.; Velez, E. S. Oxychlorination of ethylene. Appl. Ind. Catal. 1983, 1, 239. (3) Mallikarjunan, M. M.; Hussain, S. Z. Oxychlorination of some lower hydrocarbons. J. Sci. Ind. Res. 1983, 42, 209. (4) Mallikarjunan, M. M.; Hussain, S. Z. Direct oxychlorination of ethylene to vinyl chloride monomer. J. Sci. Ind. Res. 1984, 43, 94. (5) Allen, J. A. Energetic criteria for oxychlorination catalysts. J. Appl. Chem. (London) 1962, 12, 406. (6) Zhernosek, V. M.; Vasileva, J. B.; Avetisov, A. K.; Gelbstein, A. J. Kinetics and mechanism of the catalytic oxidative chlorination of ethylene. Kinet. Catal. 1971, 12, 407. (7) Dmitrieva, M. P.; Bakshi, Yu. M.; Gel’bshtein, A. I. Effect of the support of the copper salt catalyst on the kinetic parameters of the individual reaction steps. Kinet. Katal. 1985, 26, 1359. (8) Conner, Wm. C.; Pieters, W. J. M.; Signorelli, A. J. The oxychlorination of methane on fumed silica based Cu, K, La catalysts: III. Bulk and surface analysis. Appl. Catal. 1984, 11, 59. (9) Rouco, A. J. Low-temperature ethylene oxychlorination: effects of supports and promoters on the mobilities of the active species in CuCl2 catalysts. J. Catal. 1995, 157, 380. (10) Finocchio, E.; Rossi, N.; Busca, G.; Padovan, M.; Leofanti, G.; Cremaschi, B.; Marsella, A.; Carmello, D. Characterization and catalytic activity of CuCl2-Al2O3 ethylene oxychlorination catalysts. J. Catal. 1998, 179, 606. (11) Allen, J. A.; Clark, A. J. Oxychlorination catalysts. Pure Appl. Chem. 1971, 21, 145. (12) Skudaev, V. I.; Gertsen, P. P.; Solomonov, A. B.; Morozovskii, A. I. Kinetics of oxidation of CuCl with oxygen in the zinc chloride-cuprous chloride melt. Kinet. Katal. 1978, 19, 1448. (13) Cavaterra, E. Catalysts to make dichloroethane. Hydrocarbon Process. 1988, Dec, 63. (14) Villadsen, J. V.; Stewart, W. E. Solution of boundary valued problems by orthogonal collocation. Chem. Eng. Sci. 1967, 22, 1483. (15) Caillaud, J. B.; Padmanabhan, L. Improved semi-implicit Runge-Kutta method for stiff systems. Chem. Eng. J. 1971, 2, 227. (16) Draper, N. R.; Smith, H. Applied Regression Analysis; John Wiley and Sons: New York, 1966. (17) Friend, L.; Wender, C.; Yerze, J. C. Liquid-phase oxychlorination of ethylene to produce vinyl chloride. Adv. Chem. Ser. 1968, 70, 168. (18) Specter, M. L.; Heinemann, V. H.; Miller K. D. Olefin chlorination in homogeneous aqueous copper chloride solution. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1966, 11940, D15-D20.
Ind. Eng. Chem. Res., Vol. 40, No. 23, 2001 5495 (19) Sai Prasad, P. S. Thesis, Indian Institute of Technology, Madras, India, 1993, submitted. (20) Doraiswamy, L. K. D.; Sharma, M. M. Heterogeneous Reactions: Analysis, Examples, and Reactor Design; 1984; p 216. (21) Carberry, J. J. Chemical and Catalytic Reaction Engineering; McGraw-Hill: London, 1976. (22) Sai Prasad, P. S.; Kanta Rao, P. Low-temperature ethylene chemisorption (LTEC), A novel technique for the characterization of CuCl2/KCl/Al2O3 oxychlorination catalysts. J. Chem. Soc., Chem. Commun. 1987, 951. (23) Sharpe, P. K.; Vicherman, J. C. Proc. VI ICC (Imperial College, London) 1976, 225. (24) Arcoya, A.; Cortes, A.; Seoane, X. L. Optimization of copper
chloride based catalysts for ethylene oxyhydrochlorination. Can. J. Chem. Eng. 1982, 60, 55. (25) Vogel, A. I. Quantitative Inorganic Analysis; Longmans: London, 1961; p 323. (26) Kaushik, V. K. Identification of oxidation states of copper in mixed oxides and chlorides using ESCA. Spectrochim. Acta 1989, 44B, 581.
Received for review January 2, 2001 Revised manuscript received July 16, 2001 Accepted July 27, 2001 IE010031I
