Ind. Eng. Chem. Res. 1994,33,1645-1653
1646
KINETICS, CATALYSIS, AND REACTION ENGINEERING Hydrogenation of m-Nitrochlorobenzene to m-Chloroaniline: Reaction Kinetics and Modeling of a Non-Isothermal Slurry Reactor? Chandrashekhar V. Rode and Raghunath V. Chaudhari' Chemical Engineering Division, National Chemical Laboratory, Pune 411 008, India
The kinetics of hydrogenation of rn-nitrochlorobenzene to rn-chloroaniline was investigated using sulfided Pt on carbon catalyst in a stirred slurry reactor in a temperature range of 313-363 K. The initial rate data were analyzed to ascertain the importance of mass-transfer effects, from which it was found that gas-liquid mass-transfer resistance was important a t 353 and 363 K. A LangmuirHinshelwood type rate model has been proposed on the basis of the data in the kinetic regime (313-333K). In order to verify the applicability of the kinetic model over a wide range of conditions, a semibatch reactor model under isothermal conditions was proposed and the predicted concentration vs time profiles were compared with the experimental results, which showed excellent agreement. A semibatch reactor model under non-isothermal conditions has also been developed, which can predict the temperature and the concentrations of reactant and product as a function time. These predictions were also verified with the experimentally observed temperature and concentration vs time profiles under non-isothermal conditions, which showed good agreement.
Introduction Catalytic hydrogenation of nitro compounds to amines is practiced in industry for the production of a variety of aromatic amines which have applications as intermediates and speciality products (Stratz, 1984). Hydrogenationof m-nitrochlorobenzene (MNCB) is one such industrial example for the production of rn-chloroaniline (MCA), which is an intermediate for dyes, drugs, and pesticides. An important application of MCA is also in the manufacture of antimalarial drugs such as chloroquine and amodiquin (Surrey and Hammer, 1946). The stoichiometric reaction involved in hydrogenation of MNCB is
In this process, it is desirable to achieve selective hydrogenation to MCA, without dehalogenation, and this selectivity depends on the type of catalyst used and the reaction conditions. Usually, the supported transition metal catalysts such as Pt/C or Pd/C have been suggested along with some inhibitors present in the catalyst or in the solution phase. The relevant literature on this subject is reviewed by Greenfield and Dove11 (1967),Bond (19691, and Kosak (1970,1980).It has been reported that sulfided Pt/C is highly selective catalyst for reaction 1 (Kosak, 1970). Hydrogenation of MNCB is usually carried out in a slurry reactor and is an example of a highly exothermic reaction with a heat of reaction of 5.64 X lo6 kJ/kmol of nitro group or about 1.88 X lo6 kJ/kmol of hydrogen (McNab, 1981). In order to understand the performance of a slurry reactor, knowledge of intrinsic kinetics and its coupled effects with mass- and heat-transfer processes is most essential. Most of the previous work on hydrogena-
* Author to whom correspondence should be addressed.
+ NCL Communication No. 5814.
tion of MNCB relates to catalytic activity and selectivity studies (Bavin, 1956;Kosak, 19801,and no reports on the reaction kinetics and engineering analysis of this reaction have been published. The previous work on the analysis of slurry reactors has mainly considered isothermal reactions, and the important developments have been reviewed by Ramachandran and Chaudhari (1983)and more recently by Mills et al. (1992). However, very few reports on the analysis of non-isothermal slurry reactors have been published. For exothermic reactions the implications of temperature variations on the reaction rate, mass transfer, and physicochemical parameters needs to be carefully incorporated. The objective of the present work was to investigate, first, the intrinsic kinetics of hydrogenation of MNCB using 1% Pt-S/C as a catalyst, under isothermal conditions, and then to obtain experimental data in a semibatch slurry reactor for both isothermal and non-isothermal conditions. A mathematical model has been developed for a semibatch nonisothermal reactor, and the experimental and theoretical results have been compared.
Experimental Section Catalyst and Materials. The catalyst used for hydrogenation was sulfided 1 % Pt/C available commercially from M/s Engelhard, Cinderford, UK. The physical properties of the catalyst are average particle size (d& 1-6 pm; particle density, (1.8-2.0) X 103, kg/m3; pore volume, 4.5 X 1o-L m3/kg; porosity, 0.90;surface area, 8 X 106 m2/kg. The hydrogen gas was supplied by M/s Indian Oxygen Ltd., Bombay, with >99.98% purity and was used directly from the cylinder. The reactant m-nitrochlorobenzene (MNCB) was procured from M/s Fluka AG, Buchs, Switzerland, and methanol was used as a solvent. All the hydrogenation experiments were carried out in a 3 X 10-4 m3 capacity stirred reactor made of SS-316(Parr Instrument Co., Moline, USA). This reactor was provided with arrangements for automatic temperature control, variable
0888-5885/94/2633-1645$04.50/0 0 1994 American Chemical Society
1646 Ind. Eng. Chem. Res., Vol. 33, No. 7, 1994
9 10 MAIN GAS
WATER IN WATER OUT
1 INTERMEDIATE VESSEL FOR GAS
PRESSURE BOMB DIP TUBE TWO STAGE REGULATOR SAMPLING VALVE GAS INLET VALVE PRESSURE INDICATOR e STIRRER 9 GAS VENT 10 THERMOCOUPLE 11 COOLING LOOP 2 3 4 5 6 7
Figure 1. Schematic of experimental reactor setup.
stirrer speed, and sampling of the liquid and gaseous contents. The schematic of the experimental setup is shown in Figure 1. Kinetic Study. In a typical experiment for kinetic study, known quantities of the catalyst and the reactant rn-nitrochlorobenzene(MNCB) along with the solvent were charged into the reactor. The contents were first flushed with nitrogen and then with Ha. After a desired temperature was attained, the system was pressurized with hydrogen to a level required for the experiment. At this stage, the experiment was started by switching the stirrer on. In order to maintain constant pressure in the reactor, H2 was fed through a constant-pressure regulator from a reservoir vessel. The pressure drop in the reservoir vessel was recorded as a function of time by means of a pressure transducer. In each experiment, initial and final liquidphase samples were also analyzed for the concentrations of reactant (MNCB) and products to check the material balance. For this purpose, an H P 5840 gas chromatograph equipped with SS column, 2.5 m long and packed with 5% OV-17, on Chromosorbmaterial was used. The conditions of analysis were as follows: FID temperature, 350 OC; injection temperature, 250 "C; column temperature, 140 OC; carrier gas, N2, 2.0 X 10-5 m3/min. Initial rates of hydrogenation were calculated from the experimentally observed H2 pressure in the reservoir vessel vs time data. Experiments under Non-Isothermal Conditions. Experiments under non-isothermal conditions were carried out in a similar manner as described above, except that the circulation of water in the cooling coil of the reactor was stopped and the temperature outside the reactor wall was maintained constant a t a preset value by using a high circulation rate of fluid through the outer jacket. In these experiments, consumption of H2 (in reservoir vessel) as well as temperature of the liquid bulk were observed as a function of time. These data were obtained for different initial conditions. Results and Discussion The main objective of the present work was to study the followingproblems: (1)intrinsic kinetics of hydrogenation of MNCB using 1% Pt-S/C as a catalyst in a slurry reactor; and (2) performance of a semibatch slurry reactor under
Table 1. Range of Operating Conditions 1 catalyst loading 0.36-2.1 kg/ma 2 agitation speed 5-13.6 HZ 3 Hz press. 0.656 X 109-6.7 X 109 kPa 4 concn of MNCB 0.20-1.2 kmol/ms 5 concn of MCA 0.2-1.2kmol/ma 6 temp 313-363 K Table 2. Results on Catalyst Recycle temp, K no. of recycles sel of MCA, % 313 8 99.84 333 10 99.80 353 10 99.80
aniline formed, % 0.12 0.18 0.17
isothermal and non-isothermal conditions. For this purpose, the experimental data were obtained at different reaction conditions (see Table 1)to observe both initial rate of hydrogenation and the integral concentration-time profiles, under isothermal conditions. In order to study the non-isothermal behavior, the conditions were so chosen that both the concentration and temperature varied as a function of time. The results are discussed in the following sections. Preliminary Experiments. Some initial experiments on hydrogenation of MNCB showed that the material balance of reactants (H2 and MNCB) consumed and the product (MCA) formed agreed to the extent of 95-98% as per the stoichiometry given by eq 1. No hydrogenation took place without the catalyst, indicating the absence of any homogeneous reactions. Reproducibility of the rate measurement was found to be within 3-5% error as indicated by a few repeated experiments. From the process point of view, some experiments on catalyst recycle were carried out. The results are shown in Table 2. It was observed that at the highest temperature, even after 10 recycles, the selectivityof MCA was 99.8 % while the extent of aniline formed due to dehalogenation was always less than 0.2 % . It was also observed that the selectivity of MCA was >99.5%, under various operating conditions. The sulfur content in the reaction mixture was found to be negligible, indicating that there was no leaching of sulfur from the catalyst. The sulfur analysis was carried out using a sulfur analyzer (Model 286, ITT Barton, City of Industry, USA) which has a sensitivity of detection up to 0.08 ppm sulfur.
Ind. Eng. Chem. Res., Vol. 33, No. 7,1994 1647
I
H,
PRESSURE
: 0,656~10? kPa
TEMPERATURE,K
I
‘1
In
”, ‘m E
-g
AGITATION SPEED : 13.6 HZ
1
I
CATALYST LOADING 0 36 AGITATION SPEED 13 6 Hz
TEMPERATURE, K 8 313 o 323 h 333
2.0-
. I
mop a : z
1.6-
0 V
1.2-
LL LL
0
0.8-
w
I lP
; 5
0.4
Fw:, /
m-NITROCHLOROBENZENE
0.2
0.6
1.0
1.4
2.2
1.8
2.6
CONCENTRATION,kmollm’
Figure 4. Effect of m-nitrochlorobenzene concentration on initial rates of hydrogenation.
CATALYST LOADING, 0 ,k g / d
Figure 2. Effect of catalyst loading on rate of hydrogenation. 24, E
E
I
TEMPERATURE, K 0 313
‘n
X
I
1
20
0
323
,
,
I
CATALYST LOADING : 0.36, kq/m3 AGITATION SPEED : 13.6 HI MNCB CONC. : 0 , 8 2 5 , kmol/m3
x 333
CATALYST LOADING : 0 . 3 6 kgm/m3
I
J
T E M P E R ATUR E , K
SPEED : 1 3 . 6 H t
AGITATION
A 333
E x
I
z
r 16: k P o Figure 3. Effect of H2 pressure on rate of hydrogenation.
0.4 9
H2 P R E S S U R E x
Initial Rate Data. The effect of various reaction parameters like MNCB concentration, temperature, H2 pressure, and catalyst loading on the initial rate of hydrogenation was studied. The effects of individual parameters on the initial rate are discussed below. The effect of catalyst loading on the initial rate of hydrogenation of MNCB at 313,323,333,353, and 363 K is shown in Figure 2. The rate of hydrogenation was found to vary linearly with the catalyst concentration for 313333 K, while for 353 and 363 K the rates were observed to be independent of catalyst loading beyond 1.47 kg/m3, indicating that gas-liquid mass-transfer resistance is significant at 353 and 363 K. The rate of reaction was found to be first order with respect to H2 pressure at all the temperatures as shown in Figure 3. The effect of the concentration of m-nitrochlorobenzene on the rate of reaction is shown in Figure 4 for 313-363 K, which shows a zero order with MNCB except at very low concentrations < 0.28 kmol/ms. The results on the effect of m-chloroaniline (Figure 5) indicate that the product has no effect on the rate. The other product, water, ala0 showed no effect on the rate for the range of conditions studied in this work. The effect of agitation speed at the lowest as well as the highest catalyst loading in the temperature range of 313-363 K is shown in Figures 6 and 7. The rates
01
0.4 0.7 1.0 1.3 m-CHLOROANILINE CONCENTRATION,krnol/m3
Figure 5. Effect of m-chloroaniliieconcentration on initial rates of hydrogenation.
were found to be independent of the agitation speed (see Figure 6) at 313-333 K even for the highest catalyst loading used. However, at 353 and 363 K, the rate of reaction increased with the increase in agitation speed even at the lowest catalyst loading used (see Figure 7). These observations indicate that gas to liquid mass-transfer resistance is significant at 353 and 363 K. Analysis of Mass-TransferEffects. For the purpose of kinetic study, it is important to ensure that the rate data are obtained under the kinetically controlled regime or the contribution of the mass transfer is suitable incorporated. In a three-phase slurry reactor, gas-liquid, liquid-solid and intraparticle diffusional resistances are likely to exist. In order to analyze the contribution of these mass-transfer steps, quantitative criteria suggested by Ramchandran and Chaudhari (1983) were followed. For this purpose, factors al,a2,and +elp were calculated (for details see the Appedix) which are defined aa the ratios of the observed rate of reaction to the maximum rates of gas-liquid, liquid-solid, and intraparticle mass-transfer rates, respectively. This analysis showed that the values of a 2 and for the data at all temperatures (313-363 K), were below 0.03 and 0.08, respectively. The values of
1648 Ind. Eng. Chem. Res., Vol. 33, No. 7, 1994
H p PRESSURE
:0,656X103,kPa
CATALYST LOADING
'
0.36 kg/m3 (*) 2.10 kg/m3 (01
1 z t
1-3K.;
313K
L
I
0
2
I
I
4
8 10 AGITATION SPEED ,Hz
6
I
I
12
14
Figure 6. Effect of agitation speed on rate of hydrogenation at 313 and 333 K, for different catalyst loadings.
where F is the objective function to be minimized (&in), representing the s u m s of squares of the difference between the observed and predicted rates: N is the number of experimental data; Rk' and FA^ represent predicted and experimental rates, respectively. The values of rate parameters and 4minare presented in Table 3. For the purpose of model discrimination, other criteria suggested by Froment and Bischoff (1979) were also considered. It can be seen from Table 3 that only for rate equation R3 were the values of the rate parameters positive for all temperatures and were found to represent the data satisfactorily. The residuals of the squares of the difference between the observed and predicted rates for rate equation R3 were plotted against H2 pressure at all temperatures (313-333 K), which is shown in Figure 8. it was found that there was no specific trend exhibited in such residual plots. For other models in Table 3, the values of the rate parameters were less than 0 for some conditions and hence were rejected. The rate equation R3 is also consistent with the following reaction scheme: KB
'
B + s == (B),
Is E
2.51
0 656
CATALYST LOADING
0 36 kg/m3
X
i
lo3, kPo
H p PRESSURE
4
m"
Q
d
!
_I_.j/
W
j
/
a 1.01
(iii) KF
(iv)
(F), + A + (E), + H,O KB
(E), + E i
4
0.5
L
0
2
4
6 a 10 AGITATION SPEED, H z
12
14
Figure 7. Effect of agitation speed on reaction rate at 353 ( X ) and 363 ( 0 )K.
+s
where I and F represent the rn-nitrosochlorobenzene and m-chlorohydroxylamine intermediates and E represents the product (MCA). Assuming reaction ii (which represents reaction of adsorbed B with A), as rate controlling, the following rate equation can be derived. ~ ~ K B [[BIL AI
al for the data at 313-333 K were below 0.09, but those
at 353 and 363 K were in a range of 0.3-0.6. This clearly indicates that the rate data at 313,323, and 333 K are in the kinetic regime,and gas-liquid mass-transfer resistance is important at 353 and 363 K. Also, the liquid-solid and the intraparticle diffusion effects are negligible for all the conditions. Kinetic Model. The initial rate data in the kinetic regime (313-333 K) were fitted to several forms of rate equations, some purely empirical and some LangmuirHinshelwood type of models. A power law model of the type r A = wkAmBn was also considered to represent the data. It was found that the reaction order with respect to the second reactant (B) varied for various temperatures, indicating the nonsuitability of the power law model. The list of other equations considered along with the best parameters obtained is presented in Table 3. In order to select a suitable rate equation, a nonlinear least squares regression analysis was used for each rate equation to obtain the best values of the parameters. For this purpose, an optimization program based on Marquardt's method was used. The objective function was chosen as follows: N. _
F = z[RA,' - REIj2 p i
(3)
where KB, KI, KF, and KE are the respective adsorption equilibrium constants for steps i, iii, iv, and v kl is the reaction rate constant for step ii, and L is the total number of sites available. If we assume that I is rapidly converted to E and the sites occupied by I and other intermediates are negligible, then eq 3 can be simplified as
rA =
k,[AI [Bl 1 + KB[Bl
(4)
The above assumption is justified since the intermediates like nitroso and hydroxylamines were not detected in the analysis of liquid products. The overall rate of reaction, RA,expressed as kmol/(rn%) is then given as (5)
Equation 5 is the same as eq R3 in Table 3. Therefore, model R3 was considered the best model for representing the kinetics of hydrogenation of MNCB to MCA using the 1 ?6 Pt-S/C catalyst. The experimental and predicted
Ind. Eng. Chem. Res., Vol. 33, No. 7, 1994 1649 Table 3. Rate Eauations Used and Parameters Obtained
(1+ KAA + K&)'
R2
R3
R4
wklA*Bl (1+ KAA + KBBJ
wk,A*B, (1+ KBBJ
wkl-1 (1+ KAA + KBB,)
wk1-1
(1+ KAA + K&
I
1.6316 X 109
7.4 x
10-12
0.83326
7.8 x
10-13
4.183
2.3 x
10-13
4.36132
3.7 x
10-13
3.7601
8.2 X
4.61888
4.8 x
10-14
4.21480
4.8 x
10-14
3.7 x
10-13
4.810
333
0.2078
0.3812
313
0.133
1.862
323
0.18694
-0.78266
333
0.29927
1.93273
313 323
0.1334 0.18869
333
0.27964
3.86400
X
-1.344
X
10'
313
-3.0801
X
10-9
3.052 X 108
-3.820
323
-1.5640
X
10-9
8.537 X 105
-4.585 x 102
1.206 x 1o-e 1.300 X 10-11
333
-0.14
-5.044
7.900 x
2.676
X
105
3.840 X 102
4.0220 X 10-9
313
R5
108
323
323
5.4000 x 10'
3.612
333
3.0130
7.900 x 10'
I
X
10'
X
X
X
102 103
-14.80
108
10-13
1.40 X
10-11
10-12
-2.300
X
10'
1.04 X
-1.240
X
10'
3.110
X
10-12
I
0 313 K 0
0
0
0
0
-4.QE-005
.
323 K 333 K
0
0
v,
0
J
a 3 2 v, w
a -0GE -005
0 0 0
0.11
0
- 1.2E -004
I
2
0
I
2.7
I
I
4
6
3.1 !/T x
0 1
I
2.9
w
H2 PRESSURE x l G 3 , kPa
Figure 8. Plot of residuals va Hz pressure at various temperatures.
io3, i'
Figure 9. Temperature dependence of rate parameters. Table 4. Values of 4, KB,and %a at Higher Temperatures ~
rates with model R3 (in Table 3) were found to agree within &5-7% error. The temperature dependence of the rate constants is shown in Figure 9, from which the activation energy was calculated as 3.06 X 104 kJ/kmol. The adsorption coefficient obtained for the above modelshowed the usually expected trend of decrease in the values of adsorption coefficients with the temperature (Table 3, model R3). The heat of adsorption calculated from the Arrhenius plot of In KBva 1/T was found to be 9.40 X lo3 kJ/kmol. As discussed earlier, the rate data at 353 and 363 K were found to be under conditions wherein gas-liquid masstransfer resistance was significant. In order to evaluate the kinetic parameters for these temperatures, it was necesssaryto account for the external mass-transfer effecta. It was noticed that only gas-liquid mass transfer is important, so under such conditions an overall rate of reaction can be expressed as (Chaudhari and Ramachandran, 1980)
353 363
0.52 0.70
3.15 2.80
~~
0.124 0.127
Equation 6 was then used to simulate the initial rate data at 353 and 363 K using the optimization routine to obtain values of k2 and KB,which are presented in Table 4. The value of kLa, the gas-liquid mass-transfer coefficient, was used from the work of Chaudhari et al. (1987), and was determinedfor equipmentsimilar to that used in this work. Semibatch Reactor Model Isothermal. In order to verify the applicability of the kinetic model under integral conditions, experimental data on the liquid-phase concentrations of MNCB and MCA as a function time were also obtained. The variation of the concentration of MNCB and MCA can be represented by the following mass balance equations, for conditions of constant H2 pressure in the reactor:
1650 Ind. Eng. Chem. Res., Vol. 33, No. 7, 1994
"E
x
EXPERIMENTAL m-CHLORONITROBENZENE EXPERIMENTAL m - C H LO ROANlL I N €
with the initial conditions H 2 PRESSURE
t = 0,
B, = B,,,,
E, = 0
(9)
For the data at a constant Hp pressure, the equations were solved numerically by using the RungeKutta method to obtain the concentration of MNCB and MCA as a function of time. For this purpose, the intrinsic rate parameters determined from the initial rate data were used. The comparison of the experimental and the predicted results for some cases are presented in Figures 10 and 11, which show an excellent agreement. It is interesting to note in Figure 11that the data at 353 K are well represented by the rate model, after incorporating the mass-transfer effects, while the predictions do not match if the data are assumed to be in the kinetic regime (dotted curves in Figure 11). These results indicate that the rate model proposed on the basis of the initial rate data is also applicable over a wide range of conditions and can be used for design and scale-up purposes. Semibatch Reactor Model: Non-Isothermal. Hydrogenation of nitro compounds to amines is an example of a highly exothermic reaction with a heat of reaction of about 1.88 X lo5 kJ/kmol. For such a case, a significant temperature increase can be expected in commercial reactors, and hence in addition to multiphase mass transfer and chemical reaction, the influence of exothermicity of the reaction needs to be considered. In addition to the features of isothermal batch reaction, reaction rate, solubility, and mass-transfer parameters will also change with time due to temperature variation. This would affect the relative rates of the mass transfer and chemical reaction at different times, and hence this factor needs to be considered in the analysis of a non-isothermal slurry reactor. The various complexities due to the exothermic reaction are as follows: 1. Temperature will vary with time affecting rates, selectivity, and deactivation of the catalyst. 2. Vaporization of the liquid reactants/solvent results in reduction of the partial pressure of hydrogen, but is useful in temperature control. 3. Under certain conditions, multiple steady states can be observed (Marchan et al., 1986). 4. Parameters He, f"*, kLa,kp,and K Bvary with change in temperature and hence with time. In order to understand the performance of a nonisothermal slurry reactor, experiments were carried out under conditions where the temperature as well as the reactant concentration (Bl)varied with time of the reaction. The procedure followed in these experiments is described earlier. In these experiments, change in the temperature as well as Hp consumption (in the reservoir vessel) were observed as a function of time. For the present case, the following assumptions were made in deriving the batch reactor model: 1. The intraparticle heat effects are assumed to be negligible,which means the temperature at the surface of the catalyst is representative throughout the particle. In order to justify this assumption the following criterion given by Mears (1971) was used. yP
< 0.05
(10)
where
P = A*[-A",ID,IA,T~
(11)
0 656 XIO3 kPo
L
-.
n
E
1
9.6-
-MODEL PREDICTIONS INCORPORATING MASS TRANSFER EFFECTS _-__MODEL PREDICTIONS ASSUMING KINETIC REGIME I
1
T I M E , t x16z,SEC
Figure 11. Liquid reactant ( 0 )and product ( X ) concentration profile for 353 K. Table 5. Values of Thermicity Parameter for Various Temmraturesa temp, "C lor0 Y l@YS 40 8.80 11.77 1.0 50 7.80 11.40 0.87 90 6.20 10.12 0.63 a A, = 1.7 X 10-4 kJ/(s.m.OC);-AHA =: 1.88 X lob kJ/kmol; E = 3.0632 X 104 kJ/kmol;R = 8.314 kJ/kmol.
and y = EIRT,
(12)
The values of all these parameters are given in Table 5, from which it is clearly seen that for the temperatures studied in this work y/3 is well below 0.05, and hence the intraparticle heat transfer transfer can be safely assumed to be negligible. When the parameter /3 is calculated, a knowledge of the effective thermal conductivity (A,) of the catalyst particle is essential. No experimental data are available on the thermal conductivity, A,, but Sehr's (1958) value of A, = 1.7 x lo4 kJ/(s.m"C), for activated carbon powder in air,
Ind. Eng. Chem. Res., Vol. 33, No. 7,1994 1651 can be taken for the order of magnitude calculations. This can be justified as the thermal conductivity of a liquid or vapor under reaction conditions will usually be much greater (10-100 times) than that of air at room temperature. Therefore, the thermicity parameter @, calculated on the basis of the X values in air, would give a conservative measurement of the intraparticle heat effects. 2. The liquid-solid and intraparticle diffusion effects were assumed to be negligible. 3. The rate equation proposed in the previous section is applicable and the variation of He,kLa, kz,KB and PH* with temperature has been considered. The change in the partial pressure of hydrogen due to the vaporization of the solvent is also accounted for. The parameters He, A*, kz,KB,and k ~ can a be represented a t any temperature T, in terms of initial temperature TO,as follows:
350
I
I
I
I
I
I
0,D EXPERIMENTAL
340
1
I
w
I
,
0 50
-0.36k g l d
PREDICTED
" 1 /./ ' 3
0
12
0
24
o
36
O
48 i
,
TIME, t x G2,SEC
P-P,
Figure 12. Temperature and Hz consumption profiles under nonisothermal conditione at initial temperature = 323 K.
W T 0
circulating liquid in the outer jacket of the reactor. Cpe, Cpl, and , C are the specific heats of the gas, liquid, and bulk solid, respectively, expressed as kJ/(kg K). U, is the overall wall heat-transfer coefficient, kJ/(m2-s.K). A, is the wall heat transfer area, m2, eg is the gas holdup, and (-AHA)is the heat of reaction, kJ/kmol. VRis the volume of the liquid phase, m3. In order to predict the concentration and temperature profile in a batch reactor, it is required to solve only eqs 19 and 21, simultaneously incorporating eqs 13-18. Once B1 and T profiles are known, the amount of hydrogen consumed and the concentration of the product E1 a t any time t can be calculated by the following relationships:
(A*), = -
(kLa),= (kLalT0e ~ p [ T / T ~ l ~ ~ ~(17) The temperature dependence of the vapor pressure for the solvent methanol used in this work was calculated as (P,)T
= u + b/T
(18)
For the present system the values of a and b were (Weast and Astle, 1976) a = -4.99 X 10-6 and b = 3 X 10-8. On the basis of these assumptions, the variation in the concentration of the liquid-phase reactant (MNCB) and the product (MCA) as a function of time can be expressed as follows for non-isothermal conditions:
The variation of temperature Tis given by the following heat balance equation for a batch slurry reactor operated under non-isothermal conditions (Ramachandran and Chaudhari, 1983): [ c , V R ~ ~+CVR(l~ ~ cg - W/pP)plCp1 + VRWC,] dT/dt = (-AHA)RAVR - U,,A,(T- T,) - QgppCpg(T-Tfi) (21)
where
The initial conditions are t = 0, Bl = Bio, and T = To. The gas flow rate Qg in this case is the same as the rate of uptake of HZas the reaction was carried out in a dead end pressure reactor. Hence, Qe was taken as (2.24 X ~O-~RAVR) for all the calculations. In eq 21, T i s the reaction temperature, Tfi is the inlet gas temperature, and T, is the temperature of the
Hz = 3(Bb - B,)
(23)
El=Bb-Bi
(24)
and Equations 19 and 21 were solved numericallyusing RungeKutta method. For this purpose, the kinetic parameters obtained using initial rate data were used. The values of C = 13.83 kJ/(kg K), Cpl = other parameters used are , 2.55 kJ/(kg K), Cr = 7.1 kJ/(kg K), pg = 9 X 102 kg/m3, -AHA = 1.88 X 10 kJ/kmol, VR = 1.5 X 10-4 ms, pi = 9.03 X lo2 kg/m3, and pa = 2.3 X lo3 kg/m3. The temperature dependence of these parameters was assumed to be negligible. The overall heat-transfer parameter, U,, is specific to the equipment configuration and the system. For this case, reliable correlations are not available and hence uncertainty in its values exists. Therefore, it was evaluated by simulation of the concentration and temperature profiles at one of the temperatures (323 K). The experimental data agreed very well with the model predictions (Figure 12) for U, = 0.08 kJ/(m2.s-K). This value of heat transfer coefficient, U,,was then used to predict the results at other conditions. The value of U, obtained as 0.08 kJ/(m%.K) was then used for other temperatures. A comparison of the predicted results with the experimental data is shown in Figures 13 and 14 for 333 and 353 K, which show a good agreement. The non-isothermal batch reactor model proposed here can thus be useful for predicting the maximum temperature rise for a given set of initial conditions. The present study also confirms the applicability of the kinetic model for integral conditions.
Conclusion Hydrogenation of m-nitrochlorobenzeneusing 1 % PtS/C catalyst has been studied in a slurry reactor, over a
1652 Ind. Eng. Chem. Res., Vol. 33, No. 7, 1994
Nomenclature
0.50
400
0.40
380
-
;
5
Io-
0.30
360
9
3
+a
n W
a:
I
LL1
3
340
0.20
c
g
0 0
3
4
i 0,U
EXPERIMENTAL
w
_ PREDICTED -
:0.36
k g d
pH2 :0.65611O3,kPo B,, 2 o m k m 0 i i m 3
-
A = hydrogen A* = concentrationof A at the gas-liquid interface,kmol/m3 B1 = concentration of MNCB, kmol/m3 D = diffusivity of H2 in liquid, m2/s De = effective diffusivity of H2 in liquid-filled pores, m2/s dp = diameter of catalyst particle, m E = energy of activation, kJ/kmol E1 = product concentration in liquid phase, kmol/m3 KB = adsorption equilibrium constant, m3/kmol kl = rate constant, m3/(kg.s) k2 = rate constant, (m~/kg)(m3/(kmol~s)) kLa = volumetric gas-liquid mass-transfer coefficient, s-1 Qg= volumetric flow rate of gas, m3/s RA = overall rate of hydrogenation reaction, kmol/(m3.s) rA = rate of hydrogenation reaction, kmol/(kg.s) T = temperature, K t = time, s w = catalyst loading, kg/m3 Greek Letters "1 = parameter defined by eq 25 "2 = parameter defined by eq 26 B = parameter defined by eq 11 &p = parameter defined by eq 27 y = parameter defined by eq 12 h = effective thermal conductivity, kJ/(s-m°C) p~ = liquid density, kg/m3 = viscosity of liquid, kg/(m.s) Appendix
For ascertaining the contribution of various masstransfer resistances, the following criteria described by Ramachandran and Chaudhari (1983) were used.
Yo
3000
12
24
TIME, t
36
48
60
* 16',SEC
(a) absence of gas-liquid mass transfer if
Figure 14. Temperature and H2 consumption profiles under nonisothermal conditions with initial temperature = 353 K.
RA = -< 0.1 kLaA*
"1
temperature range of 313-363 K. It was found that the rate of reaction was first order with respect to H2 and was independent of MNCB concentration except at very low concentrations while the product has no effect on the rate under the present range of conditions. On the basis of these data a rate equation has been proposed and intrinsic kinetic parameters evaluated. The activation energy was found to be 3.06 X lo4kJ/kmol, and the heat of adsorption evaluated was 9.4 X lo3kJ/kmol. At higher temperatures (353,363K), gas-liquid mass-transfer resistance was found to be significant. For these conditions the kinetic parameters were evaluated incorporating the mass-transfer effects in the overall rate equation. In order to verify the applicability of the rate equation under integral reactor conditions, experimental data in a semibatch reactor were obtained. A theoretical model was developed to predict the concentration-time profile in a semibatch reactor. The predicted and observed results were found to agree very well. Some experiments were also carried out under nonisothermal conditions with different initial temperatures, where the H2 consumption and temperature vs time profiles were observed. A non-isothermal semibatch reactor model was developed based on the intrinsic kinetics already studied. The agreement between the model predictions and the experimental results was excellent, indicating the applicability of the kinetics and reactor model over a wide range of conditions.
(b) absence of liquid-solid mass transfer if "2
R A P ~ d< ~ o.l =6k,wA*
(c) absence of pore diffusion if
For the above calculations, the solubility data for the H2-methanol system as reported by Siedell (1940) were used, while the other parameters, De, kLa, and k, were calculated from the following correlations:
De = Dc/r
(Satterfield, 1970)
V d,
