CONTROLLING MECHANISMS I N THE CATALYTIC VAPOR-PHASE ETHYLATION OF ANILINE P R A K A S H
G O Y A L
A N D
L.
K .
D O R A I S W A M Y
N a t i o n a l Chemical Laboratory. P o o n a 8 , I n d i a
Several studies on the vapor-phase catalytic condensation of aniline and ethanol to monoethylaniline have been reported, but no information is available on the kinetics of this complex reaction and its possible mechanistic features. Mainly from thermodynamic considerations, i t was possible to speculate on the principal reactions of this complex system. Five reactions were considered; only three were found to be independent. Heterogeneous models are proposed on the basis of the initial rates for the first four reactions and a n empirical equation has been developed for the fifth reaction. The experimental results indicate that for the first three, chemical reaction is the controlling resistance in the lower temperature range, while in the higher temperature range pore diffusional resistance is also involved; for the fourth, chemical reaction appears to be the controlling resistance over the entire temperature range.
THEvapor-phase
catalytic condensation of aniline and ethanol is the method of choice for the industrial production of monoethylaniline (MEA). However, the liquidphase method, which is not selective to MEA but produces equal quantities of the diethylated product, is still in vogue. Several commercial procedures have been described (FIAT, 1937; CIOS, 1946; and other German reports). These processes differ from one another in the catalyst and ethylating agent used (Goyal, 1967). Many patents and publications are available on the vapor-phase processes, the only variations (as in the case of liquid-phase methods) being in the catalyst and ethylating agent used. Several combinations of catalyst and ethylating agent have been reported; in particular, alumina promoted with different metal oxides appears to have been widely employed in conjunction with ethyl alcohol as ethylating agent. Bauxite has also been reported to be a good catalyst for this reaction. The available data on different catalysts have been tabulated (Goyal, 1967). Several reactions are possible when aniline is ethylated with ethyl alcohol over a dehydrating catalyst (Table I ) . The kinetics of this complex reaction system has not so far been studied. The present work was undertaken to assess Indian bauxite as a possible catalyst for this reaction in the vapor phase, to propose plausible models for all the principal reactions, and to examine the controlling regimes in the ethylation of aniline. Basis of Kinetic Analysis
Eleven reactions are possible (excluding the reverse reactions) when ethyl alcohol and aniline are passed over an alumina catalyst a t an elevated temperature (Table I ) . The same reactions were also considered possible in a process development study reported earlier (Zollner and Marton, 1959), along with a few other reactions which involve ethylation in the ring. Under the temperature conditions employed in the present study, ring ethylation and formation of diethyl ether ( D E E ) by catalytic dehy26
dration of ethyl alcohol (Reactions d , e , f , and g ) are not possible, and therefore only seven reactions were considered in the kinetic analysis. Goyal (1967) found from the calculations of equilibrium constants for the different reactions that Reactions a , 6, c. and h are not reversible in the temperature range involved in the present study, and it is also very unlikely that Reactions c and ii occur to any appreciable extent. Taking these factors into consideration, the following scheme involving five reactions may be considered for kinetic analysis:
I:"
o t w
M t O
Of these, only three are independent reactions. This was established both by a trial-and-error procedure and by the use of matrices. Based on these procedures the following independent reactions were considered:
A M
+E +E
-M +W
D+W E-O+W
(i) (ii) (iii)
All the remaining reactions can be obtained by suitable algebraic manipulations of these three reactions. Since only three reactions are independent, the rates of the other two reactions (r4 and ri) can be expressed in terms of the independent reactions, and then, by suitable algebraic combinations, the following expressions can be obtained for the individual reaction rates in terms of the formation of ethylene and MEA and disappearance of ethanol (which can be experimentally determined) : Ind. Eng. Chem. Process Des. Develop., Vol. 9, No. 1, January 1970
1 REACTOR
Table I. List of Probable Reactions
2 THERMOWELL 3 THERMOSTATIC BLOCK I
CATALYST SUPPORT
5 PREHEATER 6 PACKING I VAPORIZERS
A
+
E
=
M
M
+
E
=
D
+
+
I CONDENSER
W J
W
i
9 OVERFLOW ARRANGEMENT 10 ANILINE FEED 11 ETHANOL FEED 12 COIL TRAPS
C2H5\
2C2H50H-
/O
+
C2H5
M
+
2E
DEE
+
W
)
+DEE
D
+
+w
J
C2H,
+
C2H50H
6
Figure 1. Experimental setup
o
E -
+
w
X M
J
-
D
J
+ 2 0 -
D
J
r,=(--+--8 4
8
A
+
O
M
+
O
A
-
r0
ri1
-
-
3r5
-18
- W F curves and then diff'erentiate these expressions. Since in the entire region of interest the two methods gave practically the same values, the graphical procedure (which was found more convenient) was adopted throughout. Experimental
(1)
r- =
i-8 -
ri =
rrJ ri (7 - 7)
(3)
ri=
(--3r0 + r4 + 8 8
(4)
4
-
if
-
3r0
rM 4
8
(2 1
(51
While rates Ti,, r s f , and ri were determined in all cases by graphical differentiation of the curves, an attempt was also made to develop analytical expressions for the Ind. Eng. Chern. Process Des. Develop., Vol. 9,No. 1, January 1970
A diagrammatic sketch of the assembly is shown in Figure 1. The constant rate liquid feed systems employed were similar to that described by Rihani et al. (1965). Two separate (but similar) vaporizers were provided for vaporizing the two liquids. The reactor was of the conventional type, an integral glass reactor heated by a metallic thermostatic block. The reactor system consisted essentially of a preheater (30-mm. diameter x 380 mm. long, heated by electrical resistance wire, the heat input being controlled through a Variac) and a reactor. The reactor was heated separately for the top and bottom parts. Isothermicity of the reaction zone (lower part of the reactor) was accomplished by inserting the reactor inside a hole of diameter just greater than the reactor diameter bored into a stainless steel block, 60-mm. diameter x 100 mm. long, which was heated by an electrical resistance wire on its outer surface, the heat input being controlled through an autotransformer. The reaction being complex, several possible reactions taking place simultaneously resulted in an over-all heat effect that was only slightly exothermic. This fact, together with the use of a thermostatic block for heating, ensured nearisothermal conditions (to within + 2 " C . ) in the reaction zone. 27
I
TEMPERATURE : 3 5 0 ' C . PARTICLE SIZE ; - 3 6 + 60 6 . S S MESH ANILINE, ETHANOC- 1: 2
0.8 w/Fi
o - 20
- 40 $ - 60
0.7
9-80 0
- 100
a - 120 0.8
8 za
0.5
Q3
0.I
s
0.3
0.2
0.1
0
0.01
0.02
0 03
O.OL
0.05
0.06
0.07
LINEAR VELOCITY,
0.08
0.09
0.10
0.11
C I2
FT./SEC.
Figure 2. Aniline conversion as a function of linear velocity
T h e reaction products were collected from the bottom of the reactor in a coil trap through a n air and water condenser, and the uncondensed gas (mostly ethylene) was collected in a eudiometer tube. T h e liquid products were analyzed by vapor-phase chromatography (VPC) (Perkin-Elmer Vapor Fractometer 154 D and Aerograph A 350 B ) as described below. The products from the condenser consisted of aniline (unconverted), MEA, DEA, water, ethyl alcohol (unconverted), and traces of diethyl ether (the formation of which is negligible a t the reaction temperature). This formed two separate layers: a n oily layer containing aniline, MEA, and DEA; and an aqueous layer containing water, unconverted ethyl alcohol, and traces of diethyl ether, aniline, and MEA. The oily layer was separated from the aqueous layer, weighed, washed with 10' c brine solution t o remove any ethyl alcohol present. and finally dried over anhydrous sodium sulfate. 'The mixture after drying was analyzed by VPC. The weight of water formed was determined from stoichiometry (Reactions a, c . and h ) . Then, from the total weight of the aqueous layer, the amount of unconverted ethyl alcohol was determined. Catalyst. Alumina has been widely used for the ethylation of aniline t o MEA. But Heinemann and coworkers (1949) suggested that bauxite with varying proportions of FeiOi is superior to alumina for this reaction. I n view of' this fact, and also because bauxite is plentifully and cheaply available in India, it was decided to employ bauxite as catalyst in the present study. For this purpose, bauxite, specially brought from the Udgiri plateau in the Kolhapur district of Maharashtra (India), was used, since this variety of bauxite contains 6 . 2 ' ~of iron oxide, which is close to the proportion recommended by Heinemann and coworkers (5.65%). Since the results obtained with this bauxite far exceeded expectations (the amount of DEA formed, which constitutes the undesired
diethylated product, was less than 2 ' ~ under certain conditions), further work with different grades was not undertaken, and it was decided to use bauxite of this quality for the complete kinetics of this reaction. The best conditions for activating the bauxite were found to be: temperature, 400" C.; activation time, 5 hours. Diffusional Resistances
I n any kinetic study, the resistances due t o external film diffusion and pore diffusion must be eliminated to the maximum extent possible. The gas film resistance can be overcome by operating a t a high velocity and the resistance to pore diffusion by a proper choice of catalyst size. External Mass Transfer. Conversions were obtained as a function of W F I by varying FI for four weights (8, 12, 16, and 20 grams) of catalyst in the reactor. The four X - W FI plots were made, and from these plots secondary plots of X L'S. feed velocity a t different values of W F': were prepared (Figure 2 ) . By appropriate choice of the feed rate and catalyst weight (corresponding to the flat portions of the curves), it is possible to ensure a velocity higher than the minimum for any given run. This could be achieved in almost all the runs with a catalyst weight of 16 grams. The mass transfer effect was also estimated from a n independent series of calculations by the method of Yoshida et al. (1962). From their charts the partial pressure gradient (P P)was estimated to be negligible (less than 0.001). This seems to indicate that the mass transfer coefficient is high and as a consequence external difhsion offers negligible resistance. However, none of these methods can he considered conclusively indicative of the absence of the mass transfer effect. A yes-or-no decision based on the flatness of the conversion-velocity curve is questionable, as pointed out by Ford and Perlmutter (1963), who observed that mass Ind. Eng. Chem. Process Des. Develop., Val. 9, No. 1, January 1970
w/F,
0 0-03 Q .-20
G, G M / H R . C M ?
Figure 3. External partial pressure gradient as a function of total mass velocity
rates a t which IP P approaches zero should be used for each value of W F ! . Although enticipated in the earlier discussion in a qualitative manner. b y this method it has been more rigorously established. Thus, the mass transfer effect is not significant if appropriate values of G are used (Figure 3), and the data can be used for correlating reaction rates in terms of specific reaction models. Preliminary Evaluation of Pore Diffusional Resistance. A convenient method of estimating the effectiveness factor of a catalyst for any reaction, particularly a complex reaction for which the values of the rate constants may not be known, is t o plot the time factor ( W FI) as a function of particle diameter (d,) with conversion ( X ) as parameter. When this method is applied to the present reaction, conversion should be based on the over-all disappearance of aniline, since this would also account for the disappearance of ethanol. A small fraction (usually less than 10';) of ethanol undergoes dehydration t o ethylene and would not be accounted for in the present analysis. A representative plot a t 375.C. is shown in Figure 4. Each curve can then be extrapolated to d , = O? which represents a situation where pore diffusion must be absent. The ratio of W FI a t d, = 0 to that a t any value of d, for a given conversion represents the effectiveness factor of the catalyst a t that particular value of d,. Thus, t =
transfer effects which have been found negligible by these two methods could lead to values of A P P as high as 15OcC (as against zero in the absence of mass transfer), when this effect is computed by a more rigorous procedure. By equating the rates of mass transfer and chemical reaction under conditions of steady state, they derived the equation
d In r' LP _ -- d l n G P b where b is the exponent of the well-known dimensionless mass transfer correlation,
NSU= const. ( N , c ) " ( N k e ) b ('7) In gas-solid reactions, b equals 0.59 for N k , > 350 (Garner and Suckling, 1958) and 0.49 for N k e < 350 (Wilke and Hougen, 1945). In the present case, since Nhe has been found to be always less than 360, Equation 6 becomes
d In r'
AP -- d I n G _ P
0.49
Plots of In r' (where r' has units of G) us In G were made a t different values of W F , ; the slope d In r ' / d In G (and therefore the ratio AP Pj approaches zero asymptotically as the mass rate is increased. I P P was calculated from Equation 8 for a series of flow rates and plots prepared to show the variation of this ratio with the mass rate. This is shown in Figure 3 for two values of W F,(20.0 and 40.0). This figure shows quantitatively the magnitude of the mass transfer effect a t different flow rates. At high values of G, AP P approaches zero. T o ensure the absence of mass transfer resistance, flow Ind. Eng. Chem.
Process Des. Develop.,Vol.
9,No. 1, January 1970
(W/Fi)d = ( I (WIFJd = d
(91
The calculations can be repeated a t different conversion values for a given d,, and the average of these taken as a representative effectiveness factor for that particular size. Similar calculations can be made for other diameters. The values of t estimated by this procedure were plotted (Goyal, 1967) as a function of d, a t four different temperatures; a typical plot a t 375°C. is shown in Figure 6. The resemblance of this curve to the typical effectiveness factor curves (Hougen and Watson, 1947; Levenspiel, 1962) for a first-order reaction may be noted. The usual t - @ curve can be divided into three regions: (1) where remains nearly unity; ( 2 ) transition region; and (3) where c is inversely proportional to Q. In the present case, it was found that region 1 is operative in the d, range of 0.1324 to 0.4064 mm., region 2 in the d, range of 0.4064 to 0.8124 mm., and region 3 has not been covered. In region 1 the value of c varies from 0.9 to 1.0 and for all practical purposes may be taken as unity. For region 2 the following equation (in terms of temperature and particle size) has been developed, which is accurate to within 3'~. t
= 3.235 x
10 [T
(10)
To plot the usual t curve for this reaction, a knowledge of the rate constant (hj is necessary. This being a complex reaction, such an analysis can be carried out only for the individual reactions involved. This is attempted below. The results presented a t this stage, however, show that pore diffusion is likely to be an important factor in this complex reaction for particle diameters greater than about 0.40 mm. Formulation of Rate Models
Considering Reaction i, it can be easily shown that the stoichiometric number for each of the elementary steps involved in the Hougen-Watson model is unity. Thus 29
A N I L I N E : ETHANOL
- 1:2
TEMPERATURE
: 375'C
CONVERSION (ANILINE)
-
- 0.1 - 0.2 0 - 0.3 0
e
0 - 0.4 0
6o
- 0.5
t
I
O'
0,008
O&
0.dlZ
0 . k
O.&O
PARTICLE
O&4
0,:28
0.1h
0.636
C
LO
OIA , IN.
Figure 4. Representative plot for empirical determination of effectiveness factor
" 0.7
-
0 6-
oO "L
TEMPERATURE
O d W
O.bO8
O.dU
Odl6
PARTICLE
0.b20
d14
- 315.C
Od18
Od32
OA6
OIA , I N .
that none of the models holds good for this reaction. However, in a complex system like this, the number of constants involved is so large that they cannot be meaningfully estimated, although the number can be cut down by reducing the equation t o initial conditions (products are not formed). By this procedure most of the equations can be easily linearized, and the values of the slopes and the intercepts of the resulting linearized plots can yield acceptable values of the reaction rate constant and adsorption constants of the reactants. Using these constants in the original equation, the adsorption constants of the products can be estimated by the usual least squares method. Thus, considering surface reaction controlling (with all the components adsorbed), the model equation is represented by
Figure 5 . Experimental effectiveness factor as a function of particle diameter for over-all reaction a t 375"C.
for this reaction (and the other four reactions), a preliminary clue to the possible controlling elementary step cannot be obtained. I n the present study the stoichiometric niimber was therefore taken as unity for all the reactions, and the rate and adsorption constants of the models were determined by the usual Hougen-Watson linearization method. In a highly complex reaction of this type, where the determination of the individual rate is subject to unavoidable errors, the use of nonlinear techniques does not seem to be justified. Reaction i. From the values of the equilibrium constant for this reaction (Goyal, 1967) ( K = 1 to 2 ) , the reverse reaction should also be taken into account. All the models gave one or more negative constants, which would indicate
30
which reduces to Equation 12 for the initial conditions.
Equation 1 2 is linear, and a representative plot a t 375" C. (for -36 +60 B.S.S. mesh catalyst) is shown in Figure 6. From a knowledge of the slope and intercept of this Ind. Eng. Chem. Process Des. Develop., Vol. 9,No. 1, January 1970
20
conditions, and based on these equations the surface reaction model given below was found acceptable.
a + E
-8
-12
A
M
+
TEMPERATURE
W
: 315.C.
P A R T I C L E SIZE : - 3 6 160 0.S.S. MESH
I
I
I
I
I
I
I
A representative plot of Equation 14 a t initial conditions a t 375°C. (for -36 +60 B.S.S. mesh catalyst) is shown in Figure 7. The slope of this line represents l / k l and intercept K E I k J . Thus KE and k J were calculated. By substituting the values of KE and k3 in the original singlesite surface-controlling equation (with all the products and reactants adsorbed), the values of the remaining constants (KOand K W )were determined from the usual least squares analysis. These calculations were made a t all the experimental conditions, and the rate and adsorption constants obtained are shown in Figure 12 and Table 11. Reaction iv. By an analysis similar t o that for Reaction ii, constants were determined for several models; the model with desorption of ethylene as the controlling mechanism gave positive values of all the constants, the corresponding equation being
Figure 6. Initial rate plot for Reaction i
line, and of KE(calculated from Reaction iii as described below), k , and Ka can be evaluated. Then, by substituting the value of k l , Ka, and K f in Equation 11, constants KM . and KW may be calculated by the usual least squares method [using the equilibrium constants estimated by Goyal (196711. Calculations were made a t all the temperatures and particle sizes studied, and positive constants were obtained, thus vindicating the model represented by Equation 11. All other models were found unacceptable. The rate constants for all the conditions studied are shown in Figure 10, while the adsorption constants appear in Table 11. The kinetic term alone changes with particle size and the adsorption constants are independent of the diffusional effects. Reaction ii. T h e magnitude of the equilibrium constant for this reaction (Goyal, 1967), indicates that the reverse reaction can be neglected. However, it was not possible to consider the model equations a t initial conditions for this reaction (unlike Reaction i), because the partial pressure of MEA a t the initial condition could not be determined. Therefore the following method was adopted. From a knowledge of KE, K M , and K W (determined from Reactions i and iii), different models were tried and the surface reaction model (with all the products and reactants adsorbed) gave positive values of the remaining constants ( k 2 and K D ) . The corresponding equation is
Calculations were made a t all the experimental conditions; the rate constants obtained are shown in Figure 11, while the adsorption constants are listed in Table 11. Reaction iii. An analysis similar to that for Reaction i was made after it was found that the usual least squares analysis of the rate data over the entire experimental range yielded one or more negative constants for all the models. The model equations were then reduced t o initial Ind. Eng. Chern. Process Des. Develop., Vol. 9, No. 1, January 1970
The rate and adsorption constants for all the experimental conditions are given in Figure 13 and Table 11. Reaction v. All possible mechanisms were tried for this reaction, but none yielded all positive constants, which shows that these models do not hold good for this particular reaction. The rate data were then correlated in terms of a simple power equation,
rj = kip:& The values of a , 8, and of least squares. Thus
kj
(16)
were determined by the method
= 1.1987 113 = 1.0120
LY
TEMPERATURE PARTICLE SIZE
-
3'15.C -36 * 60 B S S MESH
e4
'0
2-
s 0
'/PE
0
Figure 7. Initial rate plot for Reaction iii
31
Table II. Adsorption Constants Calculated from Model Equations
Temp.,
S . NO.
a
c.
K Ex IO'
KA
KW
KM
1 2
225 260
8.370 8.150
0.075 0.120
13.910 6.060
3
275
9.630
0.250
5.700
4
325
5.800
0.881
1.740
5
350
5.708
3.910
0.652
6
375
6.805
8.960
0.567
7
400
7.400
13.820
2.470
25.270 16.390" 6.600' 10.000" 15.000b 8.640" 13,950' 2.860" 7.900' 1.205" 8.300' 0.421" 6.910b
K Dx IO'
KMD
K o x 10'
0.500 0.200
1.406 2.500
5.870 5.530
0.080
7.090
5.730
0.052
9.225
5.410
0.042
47.180
5.090
0.040
176.000
6.220
...
...
6.030
For Reaction i. 'For Reactiom ii and iii.
These figures were rounded off to 1.2 and 1.0 without significant loss of accuracy, leading to the final rate equation, 12
r5 = ksp,po
10
(17)
For a given particle size the rate constant shows an optimum value. This may be explained on the basis of the fact dhat k5 is an empirical constant which apparently includes the effect of the equilibrium constant as well (Table 111). Validity of Models. Simple rate equations could not be fitted for the individual reactions comprising the system. Attempts to formulate such expressions led to errors of the order of about 50%; thus the individual reactions cannot be expressed in terms of a simple rate law and recourse to Hougen-Watson models is essential. The reaction models for the five different steps obtained in the present study should be regarded only as plausible mathematical models which represent the data satisfactorily. The validity of these models can be established by three methods: Method 1. By comparing the experimental and calculated reaction rates for all the individual reactions. Method 2 . By comparing the experimental and calculated rates of disappearance-formation of the principal components of the reaction. Method 3. By comparing the experimental X - W F plots with the calculated plots (determined by integration of the rate equations).
shown (Goyal, 1967) that the average error is around 18%, which compares favorably with the errors normally obtained in fitting models of this type for simple reactions. I n accordance with Method 2 for the comparison of rates, the rates of formation-disappearance of ethylene, MEA, and ethanol were calculated by using the computed values of the individual reaction rates and compared with the experimental over-all rates. A representative plot is shown in Figure 8 at 375°C. (for a particle size of -72 +lo0 B.S.S. mesh). The agreement between the experimental and calculated values is remarkably good, the average error being 13 to 15%. The best method of comparing the experimental and calculated values (Method 3) is through X - W / F plots. The value of W / F ,for obtaining a given conversion X M may be determined from
Equation 18 was solved graphically for all the conditions used in this study. A representative plot for MEA formation appears in Figure 9 a t 325°C. and for a particle size of -36 +60 B.S.S. mesh. The agreement here seems far better than the other methods (10 to 12% deviation). Similarly, X O- W/F,and X E- W/ FP plots can be compared from the following equations:
Method 1 is straightforward and the results have clearly and Table 111. Rate Constants for Reaction V
R.S.S. Mesh -16 i 3 6
-36 i 6 0
Temp., C. 225 260 275 325 350 375 400
32
-72 +IO0
-120 i l 5 O
k5 x 10'
... ... ...
2.163 2.874 1.i60 0.644
...
...
...
...
2.175 4.376 2.455 1.023
1.208 2.385 3.132 1.942 0.645
...
1.116 1.268
...
2.045 2.809
... ...
I n these cases the agreement also was good (Goyal, 1967), the average deviation being 10 to 12%. The proposed mathematical models represent the data satisfactorily. Another reason for accepting the validity of these models is that, although the same value of the adsorption constant for a given component is used in all the reactions in which this component appears, the model equations represent the data with a high degree Ind. Eng. Chem. Process Des. Develop., Vol. 9, No. 1, January 1970
0
20 I
40 I
60
80
I
700
n
0 x L=
0-CALCULATED
- -EXPERIMENTAL
Fiaure 8. ComDarison of rates of formation-disappearance at 375" C. for catalyst particle size of -72 i o 0 B.S.S. mesh
+
a
b C
of accuracy. Even in the case of water, except a t the higher temperatures, the values of K W calculated do not differ greatly. Also, K W has one value (at a given temperature) for all the reactions which do not involve aniline, and another value in which aniline is a reaction partner. If the average value of K W is taken at a given temperature for all the reactions and the rates are recalculated, an error of 30 to 35% is obtained, against 15 to 2 0 5 using the specific values of K W for the reactions involved. Probably the adsorption constants determined by kinetic methods for a complex system like the present one are not true adsorption constants of the components on the catalyst used, and it is more appropriate to regard them as empirical constants which satisfy the data, especially since some of the adsorption constants do not show the expected trend with temperature (Table 11). Temperature Dependence of Model Constants. I t was observed from the values of the adsorption constants that some of the constants ( K E ,K.v, K w , K D ,and K M Dshow ) a definite trend with temperature, while the others ( K A and K O ) are almost insensitive to temperature. Thus, if the data presented in this study are t o be extrapolated outside the experimental range, the following average Ind. Eng. Chem. Process Des. Develop., Vol. 9, No. 1, January 1970
Ethylene MEA Ethanol
temperature- independent values can be used for constants K A and K O :
K A (average) = 7.41 KO (average) = 0.57 All the remaining constants have been plotted as functions of temperature (Arrhenius plots) by Goyal (1967), the corresponding equation being
K
= AeE RT
(21)
The values of Arrhenius parameters A and E calculated for each of these adsorption constants are presented in Table IV. A more rigorous determination of these values is discussed below. The temperature dependence of the four rate constants h l , h2, h3, and h4 has been expressed through the usual Arrhenius equation (Figures 10 to 13). The activation energies for the different reactions are given in Table V. The plots as well as the tabulated values of E show that the activation energy has different values in the high and low temperature ranges for Reactions i, ii, and iii, while for Reaction iv it is independent of the temperature range.
33
0.8
0.6
2 050.4-
0 30-EXPERIMENTAL
0
- CALCULATED
TEMPERATURE PARTICLE
20
30
60
50
W/F
70
: 325.C
S I Z E -72 r l O O B S S M E S H
80
90
*
I
Figure 9. Comparison of experimental values of W/F1 with those calculated by graphical integration of rate equation for MEA formation
Table IV. Arrhenius Parameters of Adsorption Constants
S . No. 1 2 3 4 5 6
Adsorption Constant KE KM Kb KbW KD
KMMD
E , Activation Energy, Kcal., Gram-Mole Graphical Nonlinear est. est. -23.910 +13.590 +12.850 +5.150 1-13.110 -17.210
-21.410 +9.020 +13.180 +4.290 +11.520 -20.910
A 7.488 X 10’ 1.705 x 0.646 X lo-‘ 1.475 x lo-‘ 9.600 x 4.535 x l o - -
‘ F o r Reaction i. For Reactions ii and iii.
For Reaction v, an optimum value exists for h6; thus no attempt has been made to examine this constant through an Arrhenius correlation. The existence of an optimum is attributable t o the reversible nature of the reaction, which has not been accounted for in developing the rate equation because of the high values of the equilibrium constant estimated from generalized procedures. Apparently the estimated values of K for this reaction are in error. Controlling Regimes in the Ethylation of Aniline
The behavior of a complex system under conditions of pore diffusion control is difficult to predict, since each of the reactions comprising the complex system might behave differently under given conditions of particle size and temperature. Goyal (1967), using the method of Satterfield and co34
workers (Knudsen et al., 1966; Roberts and Satterfield, 1966), has shown that the effect of adsorption in estimating the effectiveness factor is negligible, and therefore the simple analysis based on power law kinetics can be employed. Thus the activation energy approaches E / 2 as the temperature is raised (corresponding to diffusion control). As can be seen (Figures 10 to 13), the rate constants have been obtained a t relatively low temperatures for the smallest particle size used, since under these conditions the controlling influence would be chemical reaction. Nonlinear Estimation of Constants. T o obtain a firm value of the activation energy in the chemical control regime, it is desirable to subject the values of the constants determined a t different temperatures in the lower temperature range for the smallest particle size to nonlinear estimation. Peterson (1962) has successfully used a reparameterized form of the Arrhenius equation for obtaining a nonlinear estimate of E . This equation is given by
where
and l / T , the mean reciprocal temperature, is defined as
To expose any trend that might exist in the value of E , it was estimated over different temperature intervalsInd. Eng. Chem. Process Des. Develop., Vol. 9,No. 1, January 1970
100
-
T,
FOR -72.100
B S S MESH
1
0 - -16 +36 E S
0 10
i
I
1
Figure 10. Temperature dependence of rate constant for Reaction i
1w
-
_-__
____
-
e- -36 r 6 0 B S S M E S H
1
-72 rlOO E S S M E S H ~ - - 1 2 0 + 1 5 0 E S S MESH
0-
Tt
-. Z
1
!
x
FOR -16 +36 E S S M E S H
I
I
1
Figure 1 1. Temperature dependence of rate constant for Reaction ii
225" to 260°, 260" to 325', and 325" to 350"C.-by solving the two simultaneous equations in each of the ranges by nonlinear estimation. This was done in an IBM 1620 computer for all the four reactions. For Reactions i, ii, and iii, the E values for the ranges 225" to 260" and 260" to 325" C. were almost equal and considerably higher than for the range 325" to 350°C. Thus, a second estimation was made for the temperature ranges 225' to 325' and 325" to 350°C. (Table VI). The values of E for the range 225" to 325°C. obviously correspond to chemical control and are independent of particle size. For Reaction iv, the activation energies obtained for all the three temperature ranges were practically identical. Therefore E was re-estimated for the entire temperature range 225" to 350°C. (Table V I , column 2). Ind. Eng. Chem. Process Des. Develop., Vol. 9,No. 1, January 1970
From the nonlinear estimates presented in Table VI it is clear that for Reactions i, ii, and iii there is a transition a t a temperature of about 325°C. for even the smallest particle size used. T o obtain firm values of the adsorption constants also, the nonlinear estimation procedure described above was used and the values of E thus obtained (over the entire temperature range) are included in Table IV. These values differ somewhat from the values determined by the usual graphical procedure listed in Table IV. Probable Shift of Controlling Regime. When a catalyst of progressively increasing size is used, obviously the influence of pore diffusion commences a t corresponding lower temperatures (Figures 10 to 13). Thus for Reactions i, ii, and iii the transition occurs a t a certain temperature 35
I
._
I
I
I
0 --72 r l O O E
O--120+150
B
I
. , *
S S MESH S S MESH
8.
1
1
1.4
1.5
1.8
1,7
1.6
1.9
-
2.0
llTl o 3
Figure 12. Temperature dependence of rate constant for Reaction iii
15
0-
-16 +36
B
0-
-36.60
B S S MESH
S S MESH
0-
- 7 2 + 1 W B S S MESH
0-
-1201150 B S S MESH
16
17
ilT
18
19
20
lo3
Figure 13. Temperature dependence of rate constant for Reaction iv
( T J to the chemical control regime. For the smallest catalyst size used in this study, the transition temperature is approximately 325°C. As the particle size is progressively increased, T , also decreases correspondingly. A plot of the approximate transition temperature ( T i ) as a function of particle size is shown in Figure 14 for all three reactions. This figure provides an approximate value of the temperature a t which chemical reaction apparently becomes the sole controlling factor for a catalyst of given size. In the region where pore diffusion is also operative (it can never be the sole controlling resistance), the activation energy is independent of particle size for each of these reactions. This would suggest that pore diffusion plays a predominant role in this regime. 36
For Reaction iv, in the entire temperature range studied, the effect of particle size seems negligible, and it may be assumed that this reaction is chemically controlled throughout. The magnitude of the activation energy (about 22 kcal. per gram-mole) seems to uphold this conclusion. Let us consider any temperature, say 280" C., in Figure 14. From the curves for the three reactions involved, it is evident that the transition to chemical control for Reaction iii occurs a t a particle size of 0.0124 inch, for Reaction ii a t 0.0087 inch, and for Reaction i a t 0.0067 inch. This clearly suggests that the influence of pore diffusion in these three reactions is in the order:
Reaction i
> Reaction ii > Reaction iii
Ind. Eng. Chem. Process Des. Develop., Vol. 9, No. 1, Jonuary 1970
'--I
Table V. Activation Energies (from Arrhenius Plots)
-E, Activation Energy, Kcal. Gram-Mole Chemical control region
Reaction A+E
k
M+W
M + E $ D + W
Diffusioncum-chemical control region"
17.190
12.312
19.300
13.270
16.770
10.122
21.770
..
+-REACTION 0 - REACTION 0- REACTION
I
ii iil
.: F
k,
E - O i W
D
k,
M+O
e c w
a Nature of resista,lce in this region should be regarded as speculative at this stage
Thus, if Reaction i is to be relatively curtailed, the use of a larger catalyst size is indicated. But since this is the principal reaction involved, as small a particle size as practicable should be used. This is particularly true since in the high temperature range, which is the range of practical interest, pore diffusion-cum-reaction appears to be controlling in all three reactions. I n the light of the treatment presented above, and the conclusion reached earlier that pore diffusion cannot be neglected, the reaction rate models presented should be used for the particular size of catalyst for which the rate constants are given. If the rate constants for particle sizes outside the range investigated are to be used, the value of h can be obtained by extrapolation of the given k values. Another method would be to assume the effectiveness factor to be unity for the smallest particle size used (-120 + E O B.S.S. mesh), and estimate the value of c for larger sizes from the empirical equation developed earlier (Equation 10). The treatment given here is primarily of theoretical interest, but it does give a qualitative interpretation of the effect of particle size and temperature on the reaction, which should be of assistance in a rational choice of the operating conditions.
Conclusions
Indian bauxite is a good catalyst for the selective ethylation of aniline to monoethylaniline ( M E A ) , only small quantities of the diethylated product and ethylene being formed as by-products. Five main reactions were considered; of these only three
2001 0
0 004
S . "V'o.
Constant
1 2 3
hi kl hr hi
4
-E, Actiuation Energ?, Kcal. Gram-Mole DifusionChemical cum-chemical control region control region 14.27 (225-325' C.) 15.52 (225-325" C.) 13.96 (225-325" C.) 20.12 (225-350" C.)
10.60 (325-350" C.) 10.02 (325-350" C.)
9.18 (325-35O'C.)
...
' F o r ,smallest particle size used (-120 +I50 B.S.S. mesh)
Ind. Eng. Chem. Process Des. Develop., Vol. 9,No. 1, January 1970
1
0 012
1
0016
PARTICLE
0 024
0 020
SIZE,
0 028
0 032
Reaction ii
>
Reaction iii
Since Reaction i is the main reaction giving MEA, as small a particle size as possible should be used. But if larger particle sizes are to be used, the effectiveness factor should be taken into account and calculated from the empirical equation developed for the over-all reaction as a function of temperature and particle size. Nomenclature a =
A = b = Table VI. Nonlinear Estimates of Activation Energies"
1
0 008
d, =
D = E =
exponent of mass transfer correlation in Equation 7 Arrhenius constant; also aniline exponent of mass transfer correlation in Equation 7 particle diameter, inches bulk diffusion coefficient, sq. cm./sec.; also diethylaniline activation energy, kcal.! gram-mole; also ethanol feed rate of aniline, gram-mole,;hour feed rate of ethanol, gram-mole!hour mass velocity, gramihour sq. cm. reaction rate constant, gram-mole hour (gram cat.) atm. thermal conductivity, cal./sq. cm. hour (" K.; cm.)
37
SUBSCRIPTS
h i , hr, hl,
hr, k ; = reaction rate constants of Hougen-Watson models for Reactions i, ii, iii, iv, and v, respectively K = equilibrium constant; also adsorption constant,atm. ‘ L = characteristic length, cm. KyJ = Yusselt number, kd,/ D 1VKe = Reynold number, d,G N,? = Schmidt number, p i p D A P P = partial pressure gradient P = partial pressure, atm. r = reaction rate, gram-mole: hour (gram cat.) r’ = reaction rate, grami hour sq. em. r l , r y ,r i , r4. rj = rate of Reactions i, ii, iii, iv, and v, respectively, gram-moleihour (gram cat.) r?: = rate of unconverted ethanol, gram-mole/ hour (gram cat.) r.w = rate of monoethylaniline formation, grammoleihour (gram cat.) ro = rate of ethylene formation. gram-mole.: hour (gram cat.) R = gas constant, cal./gram-mole$c K. t = temperature, C. T = temperature, K . T = mean temperature, K . T, = approximate temperature a t which transition from pore diffusion to chemical control occurs weight of catalyst, grams; also water fractional conversion xs = gram-mole ethanol unconverted/ gram-mole ethanol fed = gram-mole monoethylaniline formed’ grammole aniline fed xo = gram-mole ethylene formed/ gram-mole ethanol fed O
w = x =
x.,,
A = aniline D = diethylaniline E = ethanol M = monoethylaniline 0 = ethylene W = water Literature Cited
Combined Intelligence Objectives Subcommittee, C I 0 S Report XXVII-80 (1945). F I A T Re?. Ser. Sei., m:F Reel C-28, P.B. 14998, Frames 498-503, May 1937. Ford, F . E., Perlmutter, D. D., A.I.Ch.E.J . 9, 371 (1963). Garner, F. H., Suckling, R . D., A . I . C h . E . J . 4, 114 (1958). Goyal, P., Ph.D. thesis in chemical engineering, Lniversity of Bombay, August 1967. Heinemann, H., Wert, R . W., McCarter, W. S.W., Ind. Eng. Chem. 41,2928 (1949). Hougen, 0. A., Watson, K. M., “Chemical Process Principals,” Vol. 111, “Kinetics and Catalysis,” Wiley, New York, 1947. Knudsen, C . W., Roberts, G. W.. Satterfield, C. K.,Ind. Eng. Chem. Fundamentals 5, 325 (1966). Levenspiel. O., “Chemical Reaction Engineering,” Wiley, Kew York, 1962. Peterson, T. I., Chem. Eng. Sci. 17, 203 (1962). Rihani, D. N., Narayanan, T. K., Doraiswamy, L. K., IND.ENG.CHEM.PROC. DESIGNDEVELOP. 4,403 (1965). Roberts, G. W., Satterfield, C. N., Ind. Eng. Chem. Fundamentals 5, 317 (1966). Wilke, C. R., Hougen, 0. A., Trans. A m . Inst. Chem. Engrs. 41,445 (1945). Yoshida, F., Ramaswami, D., Hougen, 0. A,, A.I.Ch.E. J . 8, 3 (1962). Zollner, Gy, Marton, J., Acta Chim. Acad. Sci. Hung. 20, 321-9 (1959).
GREEKLETTERS = exponent of pa in Equation 16 3 = exponent of p o in Equation 16 A = reciprocal of adsorption term 01
t
p p @
effectiveness factor density, gramsicc. viscosity, gram/ hour em. summation = Thielemodulus, L ( h De) ’
= = = =
’
RECEIVED for review March 26, 1968 ACCEPTED April 28, 1969 Kational Chemical Laboratory CommunicationNo. 2003. For supplementary material, order NAPS Document 00488 from ASIS National Auxiliary Publications Service, c / o CCM Information Sciences, Inc., 22 West 34th St., New York, N.Y., 10001, remitting $1.00 for microfiche or $3.00 for photocopies.
Ind. Eng. Chem. Process Des. Develop., Vol. 9, No. 1, January 1970
