Dehydrogenation of 1-Butene into Butadiene. Kinetics, Catalyst Coking, and Reactor Design Francis J. Dumez and Gilbert F. Froment* Laboratorium
voor
Petrochemische Techniek, Rijksuniversiteit, Gent, Belgium
Ind. Eng. Chem. Proc. Des. Dev. 1976.15:291-301. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 08/21/18. For personal use only.
The kinetics of 1-butene dehydrogenation over a CrzOa-AlaOg catalyst between 490 and 600 °C were determined in a differential reactor. The discrimination between rival Langmuir-Hinshelwood models was based on a sequentially designed experimental program. The kinetics of coking from butene and butadiene and the deactivation functions for coking and for the main reaction were determined with a thermobalance. The equations derived from differential reactor results gave excellent predictions of the performance of an experimental integral reactor. The effect of internal transport limitation was investigated. An industrial reactor was simulated and optimized.
I. Introduction An important fraction of the world butadiene production is obtained by dehydrogenation of n-butane or n-butene. This reaction is accompanied by side reactions leading to carbonaceous deposits which rapidly deactivate the catalyst. The coking tendency may be limited up to a certain extent either by diluting the feed with steam or by operating under reduced pressure. The second solution has been favored. The butadiene production using vacuum processes amounted to 700 000 T in 1971 (Hydrocarbon Process., 1971). With vacuum processes operating with butene as feed, the on-stream time is limited to 7-15 min, after which regeneration of the catalyst by burning off the coke is required. With such cycle times the heat given off by the regeneration compensates for the heat requirements of the adiabatic dehydrogenation (Hornaday et al., 1961; Thomas, 1970).
Vacuum processes are based upon Cr203-Al203 catalysts. Fundamental properties of such catalysts were studied by Burwell et al. (1969), Poole and Maclver (1967), Marcilly and Delmon (1972), Masson and Delmon (1972), Traynard et al. (1971, 1973). These authors found that the catalytic activity was represented by surface Cr3+ and O2ions which are incompletely coordinated; ions of - 203AI2O3 solid solutions were found to be the most active. Aspects of the kinetics of butene dehydrogenation on such catalysts were investigated by Forni et al. (1969), Happel et al. (1966), and Timoshenko and Buyanov (1972). Although the surface reaction was generally found to be rate determining, there is little more agreement between the results. Further, none of these studies was carried out with particle sizes used in industrial operation. So far, no quantitative treatment of the deactivation of the catalyst by coke deposition has been published. Yet, without such information no rigorous optimization of industrial operation is possible. This paper reports on a detailed study of the kinetics of the dehydrogenation, of the coke deposition, and of the associated catalyst decay. The effect of internal transport limitations is investigated. Industrial operation is simulated and optimized.
II. Kinetics of the Main Reaction II. 1. Experimental Procedure and Range of Operating Variables. The catalyst used in this investigation was a Cr203-Al203 catalyst containing 20 wt % O2O3 and having a surface area of 57 m2/g. Experimental checks on the
absence of partial pressure and temperature gradients in the film surrounding the particle and of temperature gradients inside the particle were performed. Also, preliminary runs were carried out in order to determine the catalyst particle size which permits neglecting internal transport limitation. The kinetics of butene dehydrogenation were determined in a quartz tube inserted in an electrical furnace. The catalyst particles, diluted with quartz particles, were supported by a stainless steel gauze. The temperature was controlled by two thermocouples, one in the center of the catalyst section and one near the wall. The feed stream was calibrated and dried in the classical way. The outlet gases were analyzed by gas chromatography on a 20% propylene carbonate/chromosorb column. Experiments were performed at 4 temperatures: 490, 525, 560, and 600 °C. The butene pressure ranged from 0.02 to 0.27 atm, the hydrogen pressure from 0 to 0.10 atm, and the butadiene pressure from 0 to 0.10 atm. Although only 1butene was fed, the outlet gases always contained a mixture of 1-butene, cis-2-butene, and irons-2-butene close to the equilibrium composition. Therefore, the dehydrogenation equilibrium could be referred to butene equilibrium mixtures. In all these experiments, the conversion was kept below 2% by adjusting the amount of catalyst and the gas flow rates. Therefore, the reactor was considered to be differential. Due to coke deposition the dehydrogenation rate was found to decrease with time. To determine the rate of the main reaction in the absence of coke required extrapolation to zero time. Since the first analysis was taken after 2 min, while a run extended to 30 min, the extrapolation was no problem. II.2. Kinetic Analysis. Five possible reaction schemes, shown in Table I, were derived for the main reaction. For each of these mechanisms several rate equations may be derived, depending upon the postulated rate-determining step. Fifteen possible rate equations were retained. They are listed in Table II. The experimental program was designed to discriminate in an optimal way between the rival models. Sequential procedures for optimal discrimination have been introduced by Box and Hill (1967) and by Hunter and Reiner (1965). The methods have been applied to experimental data, but only a posteriori, for illustrative purposes (Froment and Mezaki, 1970). The present work is probably the first in which the experiments were actually and exclusively designed on the basis of a sequential discrimination project Eng. Chem., Process Des. Dev., Vol. 15, No. 2, 1976
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Table I. Reaction Schemes for Butene Dehydrogenation (a) Atomic Dehydrogenation; Surface Recombination of Hydrogen (al) (1) B + L- BL (a2) (2) BL + L^ ML+ HL (3) ML + L^ DL + HL (a3) (a4) (4) DL ^ D + L + (5) 2HL^ H2L L (6) H2L ^ H2 + L
n-butene; D butadiene; H2 hydrogen; intermediate complex) Atomic Dehydrogenation; Gas Phase Recombination of Hydrogen (1) B + L** BL (bl) (2) BL + L =» ML + HL (b2) + + (3) ML L^ ML HL (b3) (4) DL^ D + L (b4) (5) 2HL^ H2 + 2L Molecular Dehydrogenation (1) B + L^ BL (cl) (2) BL + L ^ DL + H2L (c2) (3) DL ^ D + L (c3) (4) H2L^ H2 + L Atomic Dehydrogenation; Intermediate Complex with Short Lifetime Surface Recombination of Hydrogen (1) B + L^ BL (dl) (2) BL + 2L ^ DL + 2HL (d2) (3) DL^ D + L (4) 2HL^ H2L + L (5) H2L^ H2 + L Atomic Dehydrogenation; Intermediate Complex with Short Lifetime; Gas Phase Hydrogen Recombination (el) (1) B + L^ BL (2) BL + 2L ^ DL + 2HL (e2) (3) DL^ D + L (4) 2HL^ H2 + 2L (where B =
(b)
(c)
(d)
(e)
=
=
=
an
cedure. The operating conditions for an experiment were selected on the basis of the design criterion. Then the experiment was carried out, the parameters of the models were estimated, and the current state of adequacy of the rival models was tested. With this information the next experiment was designed and so on, until the discrimination was achieved. Eventually, some further experiments were carried out to improve the significance of the parameters of the retained model(s). The sequential choice of experimental conditions for optimal discrimination between the rival models was based upon the following design criterion
D=tt |*Hi°-VI ¿=Ij=i 3
^
(!)
—
?
=
1=
(rHl°
~
(2)
fHl0)2
1
N at the four temperature levels. for all the data l 1,. This involves nonlinear regression. Indeed, the expression for th° according to model c2 is =
,„o
.
.
,
(3)
-_____
KbPb + -KhPh + -KdPd)2 in which the adsorption equilibrium constants Kb, Kb, and Kd are related to the equilibrium constants of the steps of the reaction in the following way (1 +
Kb
=
Ki; Kb
=
1/K3;
=
1
/K4
(4)
Statistical tests indicated that the adsorption equilibrium constants were not significantly temperature dependent. The rate coefficient hn0 obeyed the Arrhenius temperature dependence kB°
=
Ab° exp(-EH/RT)
(5)
Reparameterization according to
*
where rm0 represents the estimated value of the reaction rate according to model i, and D is the divergence between the predicted rates. The double summation ensures that 1 exeach model is taken in turn as a reference. Given periments the nth experiment was performed in the differential reactor at those values of pe, Ph, and pn which maximized D. A grid is selected for possible combinations of Pb, Ph, and pn within the operability region. From previous experience on constructed examples the criterion (1) was shown to lead to the same experiments as the Box-Hill criterion that accounts for the variances. The state of model adequacy was tested by means of a criterion proposed by Hosten and Froment (to be published). The underlying idea is that the minimum sum of squares of residuals divided by the appropriate number of degrees of freedom is an unbiased estimate of the experimental error variance for the correct mathematical model only. For all other models this quantity is biased due to a lack of fit of the model. The criterion for adequacy therefore consists in testing the homogeneity of the estimates of —
292
the experimental error variance obtained from each of the rival models. This is done by means of Bartlett’s 2 test (Bartlett, 1937). The details of the procedure, the designed operating conditions, and the evolution of the discrimination will be reported elsewhere (Dumez et al., to be published). Suffice it to mention that at 525 °C, e.g., a total of 14 experiments, 7 of which were preliminary, i.e., required to start the sequential design, allowed discarding all the models except a2, b2, c2, d2, and e2, all corresponding to surface reaction on dual sites as rate-determining step. The differences f#,° r# ·° between these models were smaller than the experimental error. The models a2, b2, and d2 were eliminated because they contained at least one parameter that was not significantly different from zero at the 95% confidence level. Model c2, corresponding to molecular dehydrogenation and the surface reaction on dual sites as rate determining step, led to a fit which was slightly superior to that of e2 and was finally retained. The same conclusion was reached at 490, 560, and 600 °C. It should be pointed out here how efficient sequential design procedures for model discrimination are. A classical experimental program, less conscious of the ultimate goal, would no doubt have involved a much more extensive experimental program. The parameters of model c2 were estimated by minimizing
Ind. Eng. Chem., Process Des. Dev., Vol. 15, No. 2, 1976
Ah°
=
Ah0' exp(EH/RTm)
(6)
with Tm the average temperature, facilitated the estimation. The values of the parameters and their standard deviations are given in Table III. The Arrhenius plot for fen0 is given in Figure 1. The dots represent the parameter values obtained from a treatment of the data per temperature.
III. Kinetics of Coking The kinetics of coking and the deactivation functions for coking and for the main reaction were determined by of a Cahn RH thermobalance. The catalyst was means placed in a stainless steel basket suspended at one balance arm. The temperature was measured in two positions by thermocouples placed just below the basket and between the basket and the quartz tube surrounding it. The temperature in the coking experiments ranged from 480 to 630 °C, the butene pressure from 0.02 to 0.25 atm, and the butadiene pressure from 0.02 to 0.15 atm. Individ-
Table II. Rate Equations for Butene Dehydrogenation
PhPdX
K ) eiCil^Pb PhPd pd'/ph Pd '/ph + + K2K3KtK3K6 K3KAfYT6 K
