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Effect of catalyst concentration and simulation of precipitation processes on liquid-phase catalytic oxidation of p-xylene to terephthalic acid
Pergamon
('hemwal En~li,leerin9 5cWnce. Vol 52, Nos 21 22. pp 4205-4213. 1997 1997 Elsevier Science Lid All rights reserved Prmted in Great Britain P i l : S0009-2509(97)00263-7 o0o9 2509 97 $17.00 + 0.00
Effect of catalyst concentration and simulation of precipitation processes on liquid-phase catalytic oxidation ofp-xylene to terephthalic acid A. Cincotti, R. Orrfi, A. Broi and G. Cao* Dipartimento di Ingegneria Chimica e Materiali, Universita'degli Studi di Cagliari. Piazza d'Armi, 09123 Cagliari, Italy (Accepted 7 July 1997) Abstract---The influence of catalyst concentration, i.e. cobalt naphthenate, on product distribution and kinetic constants of the lumped kinetic scheme of liquid-phase p-xylene oxidation proposed in previous works (cf. Cao et al., 1994a, b) is investigated. The experiments involving various levels of catalyst concentrations (from 1.67 to 33.3 × 10-4 mol/kgi) are conducted in an isothermal semi-batch oxidation reactor where both the gas and the liquid phase are well mixed. The dependence of the kinetic constants of the lumped kinetic scheme on the catalyst concentration is examined. In addition, the interaction between the chemical reactions of the lumped kinetic scheme for p-xylene oxidation to terephthalic acid and the precipitation kinetics of both 4-carboxybenzaldehyde and terephthalic acid is analyzed theoretically. A semi-batch gas-liquid reactor model which incorporates the description of the above phenomena allows us to identify their interplay..(" 1997 Elsevier Science Ltd Keywords: p-xylene; oxidation; terephthalic acid; catalyst: precipitation.
INTRODUCTION
In the chemical industry, liquid-phase catalytic oxidation of organic compounds, which can be either homolytic or heterolytic depending upon the mechanism of oxygen activation (cf. Emanuel and Gal, 1986), constitutes a wide class of important processes. In particular, when addressing the study of homolytic oxidation processes, e.g. cyclohexanol from cyclohexane, terephthalic acid from p-xylene, isophthalic acid from m-xylene, emphasis was placed on the investigation of the catalytic system, i.e. catalyst and promoter concentration, nature of solvent, reaction temperature, etc., and its influence on the oxidation rate (Sheldon and Kochi, 1981; Raghavendrachar and Ramachandran, 1992). This type of approach typically accounts for the complex nature of the catalyst within the already complex chain elementary reaction schemes, which involve a very large number of radicals as well as molecular species (cf. Carr~ and Santacesaria, 1980). However, when simulating the gas-liquid reactors, the formulation of detailed kinetic models of oxidation processes requires a large computation effort for solving the diffusion-reaction equa-
*Corresponding author. Tel.: 39/70/675-5058: fax: 39.'70/675-5067: e-mail: [email protected].
tions in the film at the gas-liquid interface, coupled with the continuity equations for both gaseous and liquid bulk phases. This is due to the intrinsic difference between the space scale typical of molecular and radical species. In addition, the estimation of kinetic parameters of chain elementary reaction kinetic schemes may not be reliable due to the difficulty of monitoring all participating species, including highly reactive radicals, as a function of time in semi-batch reactors where specific experimental studies are typically conducted. The most common approach is to lump the detailed mechanism into a set of global reactions which involves only molecular species, whose concentration can be, in principle, easily monitored as a function of time. Without involving formal procedure of general validity but simply including the minimum number of reactions to describe the behavior of all the species of interest, various lumped kinetic schemes for homolytic oxidation processes have been developed in the literature, by Cavalieri d'Oro et al. (1980) for p-xylene oxidation, Chen et al. (1985) for o-xylene oxidation, Morbidelli et al. (1986) for ethyl-benzene autoxidation and Krzysztoforski et al. (1986) for cyclohexane oxidation. By accounting for the most important intermediates and final products of the process, i.e. p-tolualdehyde, p-tolualcohol, p-toluic acid,
4205
4206
A. Cincotti et al.
4-carboxybenzylalcohol, 4-carboxybenzaldehyde, terephthalic aldehyde and terephthalic acid, Cao et al. (1994a, bl have recently proposed the following lumped kinetic model for the liquid-phase oxidation of p-xylcne catalyzed by cobalt naphthenate:
both reactants and products. It was also shown that the model was able to describe the reactor behavior in any of the regimes which may prevail depending upon the operating conditions and the depletion of liquid reactants in time.
terephthalic aldehyde
+ CHO
/ p-xylene
p-tolualdehyde
CH~
CHO
CHO p-toluic acid
"~ 4-carboxybenzaldehyde
COOH
COOl I k5 ,.
CH.~
CH,
CH 3
COOH k~, ,,-
+
CHO
COOH
CH~ p-toluic alcohol
terephthalic acid
/k~
+
COOH
(I)
CH,OH 4-carboxybenzylalcohol
which may have important practical implications in the production of terephthalic acid. All the lumped reactions were assumed to be zeroth and first order with respect to the gaseous and the liquid reactant, respectively. The reliability of the proposed kinetic model was illustrated by comparison with suitable experimental data obtained in a semi-batch oxidation reactor where both the gas and the liquid phase are well mixed. The experiments included different values of the initial concentrations of the liquid reactants, two gaseous reactants (i.e. pure oxygen and air), temperature values in the range 80-130~C and were conducted at a unique value of the catalyst concentration (i.e. 1 × I0- 3 mol/kg~). However, catalyst concentration plays a fundamental rote upon the global rate of a wide variety of oxidation reactions such as the cobalt-catalyzed oxidation of toluene in acetic acid (cf. Scott and Chester, 1972; Kamiya and Kashima, 1972), the oxidation of alkyl-benzenes in acetic acid catalyzed by cobalt acetate and sodium bromine {cf. Kamiya, 1974), the durene oxidation in acetic acid catalyzed by cobalt salts (cf. Hanotier and HanotierBridoux, 1981), the catalytic oxidation of p-xylene (cf. Zaidi, 1986; Raghavendrachar and Ramachandran, 1992). The proposed lumped kinetic scheme was also incorporated (cf. Cao et al., 1994a) into semi-batch gas-liquid reactor model that properly accounts for inter- and intraphase mass transport processes of
The formulation of detailed kinetic models is even less desirable when the interaction among mass transfer resistances, chemical reactions and precipitation phenomena plays a fundamental role. This is particularly the case of liquid-phase p-xylene oxidation, where the formation of 4-carboxybenzaldehyde and terephthalic acid occurs through a reaction precipitation process, i.e. the latter compounds are formed by chemical reactions in concentration exceeding their solubility. Note that 4-carboxybenzaldehyde is probably one of the most deleterious contaminant of terephthalic acid, since its aldehyde group is unable to undergo condensation reactions with ethylene glycol during PET polymerization (cf. McElroy Brown and Myerson, 1989). Only few studies concerning reactive precipitation have been reported in the literature (cf. Garside and Shah, 1980; Aslund and Rasmunson, 1992 and references therein). This is particularly true when considering the simultaneous presence of gas-liquid mass transfer, chemical reaction and precipitation, which remain unexplored until recently (Wachi and Jones, 1990; Wachi and Jones, 1991). The present work consists in two parts. In the first one, the effect of cobalt naphthenate concentration on product distribution and kinetic constants of the lumped kinetic scheme (1) is established. In particular, the reactions leading to the formation of p-toluic acid are considered by operating at low values of p-xylene conversion. Various values of catalyst concentration
4207
Catalytic oxidation of p-xylene are investigated by running the experimental reactor used in previous works (cf. Cao e t al., 1994a, b) under the kinetic regime. In the second part, the interaction between chemical reactions of the lumped kinetic scheme (1) and the precipitation kinetics of both 4-carboxybenzaldehyde and terephthalic acid is analyzed theoretically. The latter phenomena are described by appropriate population balance equations incorporated in a semi-batch reactor model.
lable 1. Operating conditions for the experimental runs Run I
2 3 4 5 6 7 g
T [ ('~
('~,, {mol.kgh × 104
80 110 120 120 120 120 120 12(1
5.o 5.0 5.o 1.67 2.67 10.0 15.0 33.3
EXPERIMENTAL
The experimental set-up and procedure are described in detail by Cao et al. (1994at. As summarized in Table 1, the experimental runs performed were carried out at three temperatures (80, 110 and 120~'C) and six levels of cobalt naphthenate concentration, while maintaining fixed initial p-xylene (4 mol/kgt) and p-tolualdehyde (0.11 mol,,"kg,) concentrations. The experiments were run in a semi-batch reactor continuously fed with pure oxygen where, after the temperature reached the desired value, suitablc amounts of catalyst (0.05 1 cm 3) and p-tolualdehyde (6 cm 3) as promoter were added. Methyl benzoate was used as solvent. All the experimental runs were performed at a stirring speed of 800 rpm, where the influence of this variable on the product distribution becomes negligible (cf. Cao et al., 1994a). Each run was carried out up to conversion values of p-xylene of about 14%. Thc reproducibility of the experimental runs was verified by repeating each of them at least twice. The reaction products were analyzed by HPLC using the technique described by Cao et al. (1994a, b) and Viola and Cao (19961. THE MODEL EQUATIONS At the p-xylene conversion levels considered in this work the lumped kinetic scheme(l) becomes (cf. Cao et al., 1994a):
p-xylene
p-toluic acid
p-tolualdehydc
CH~
C(X)H
CHO
IlL
CH~
CH~
CH~
(2) k~
CH:OH/k,
where each reaction involves the addition of I..2 0 2 and the corresponding kinetics are assumed to be zeroth and first order with respect to oxygen and the liquid reactant, respectively. The above scheme has been inserted in the model of a semi-batch gas-liquid reactor to simulate the experimental data. Since all experiments were run under the kinetic regime, the following mass balances are used: dCi dt
~" v j ' i r j
1, N c
(3)
i = l . Nc
(4)
i=
along with the initial conditions: C i = ('~
at t =0.
where C~ is the concentration of the ith component and the meaning of other symbols is reported in the Notation. The oxygen mass balance and the energy balance are not considered since all reactions are of zeroth order with respect to oxygen and the reactor has been operated under isothermal conditions. In order to simulate the precipitation phenomena of 4-carboxybenzaldehyde and terephthalic acid, a mathematical model of a semi-batch gas liquid reactor where the reactions of the lumped kinetic scheme (1) occur, is developed under the following assumptions: -- isothermal conditions: kinetic regime: ---neglibiblc agglomeration and disruption {cf. Wachi and Jones, 19911; precipitation due to supersaturation of 4-carboxybenzaldehyde and terephthalic acid; spherical crystals; growth rate independent of linear crystal dimension (cf. Garsidc and Shah, 1980). The mass balances for the non-precipitating species are identical to those reported in eq. (3), while those for precipitating species are as follows: dC~ dt dC,, --dt
CH, p-toluie alcohol
~~ -
-
ks(".~ + k - ( ' -
= k~(': -
4 kio('~
Bg - G~,
- k,,("~ -
B ~ -.. G ~ { 5 )
(6)
where BI and GI, i = 5 and 6 are the mass-based nucleation and growth rate, respectively.
4208
A. Cincotti et al.
The population balances of the precipitated particles are ~Ni G. ~Ni ?~-+ ' d L =0,
with respect to the lumped kinetic scheme (2): Uo2 = ½[C2 - C° + 2C3 + C,]
(15)
where Ci represents the experimentally measured concentration of the reaction products. A plot of the quantity Uo: as a function of time for where N~ is the population density of particles, G~ represents the linear growth rate, t is the time and L is the experimental runs 4 - 6 in Table 1, where different the coordinate of particle dimension. Equations (3) values of catalyst concentration have been considered, while maintaining constant the initial p-xylene and and (5)--(7) are subjected to the following initial and the p-tolualdehyde concentration, is shown in Fig. 1. boundary conditions: From the slopes of these curves evaluated at t = 0, the C ~ = C o at t=O, i = l , Nc (8) initial value of the oxygen uptake, R~2, was computed and plotted in Fig. 2 as a function of catalyst concenN~=0 at t =0, VL, i = 5 , 6 (9) tration. In the same figure the p-xylene conversion values reached in the corresponding experimental Ji Ni=~ a t L = L o . i , Vt, i = 5 , 6 (10) runs at t = 60 min are also reported. It is seen that as the catalyst concentration increases both quantities where J~ is the number nucleation rate and Lo.~ is the first increase and then reach a plateau. Note that this effective nucleic dimension. Note that the boundary behavior is consistent with that reported in the literacondition (I0) is used by several authors (cf. Garside, ture for p-xylene oxidation to p-toluic acid catalyzed 1985; Dirksen and Ring, 1991; Jones et al., 1992) and by cobalt acetate and sodium bromine in acetic acid may become the dominant factor of crystal distribu- as solvent (cf. Zaidi, 1986). In order to demonstrate tion. that all experiments were conducted under the kinetic The mass-based nucleation and growth rate equa- regime, we have performed the experimental runs 1 - 3, tions are as follows: where only the temperature values have been changed with respect to run 3. The corresponding initial values (I I) of oxygen uptake are shown in Fig. 3 as a function of B~ = gn ~)~.ijiL3.i, i = 5, 6 temperature. From the linear behavior of the Arrhenius-like plot shown in Fig. 3, it may be concluded G~= 7zGi~s. iL2Ni(L) dL, i = 5 , 6 (12) that the reactor operates in the chemically controlled regime for the specific catalyst concentration used for where n/6 and ~r are related to the spherical crystals, runs 1-3, and, consequently, for all catalyst concent~.~ is the crystal density, and the corresponding num- tration levels considered in this work, at which equal ber rate of nucleation and linear crystal growth rate or lower oxidation rates are found (cf. Fig. 2). The are expressed through the following equations (cf. apparent activation energy is equal to about Wachi and Jones, 1991): 18 kcal/mol, i.e. a typical value of oxidation reactions, which compares fairly well with those reported in the Ji = k,.i(Ci - C~')"', i = 5, 6 (13) literature for p-xylene oxidation in the presence of Gi = kg.i(Ci - C*) g', i = 5, 6 (14) water (cf. Hronec and Ilavsky, 1982; Hronec et al., 1985; Hronec and Hrabe, 1986). As expected, the where k,.~ and k~.~are the nucleation and growth rate apparent activation energy is also very similar to the constant, respectively, ni and 9i are the orders of one obtained by Cao et al. (1994a) under the operatnucleation and growth, respectively, and C* is the ing conditions corresponding to run 6 of the present equilibrium saturation concentration. work using the same experimental set-up and procedThe governing balance equations are in the form of ure. The values of the kinetic constants appearing in a system of ordinary and partial differential equa- the lumped kinetic scheme (2) are then estimated by tions. A backward finite-difference scheme was ad- fitting the time evolution of the experimental product opted for the spatial derivative appearing in the popu- composition of run 1-8 in Table 1 through a nonlation balances (7) in order to obtain a set of ordinary linear least-squares procedure using the model given differential equations which are solved as an initial by eqs (3) and (4). The comparison between model value problem using standard routines. During the results and experimental data is shown in Figs computations, the value of the finite-difference points 4(a)-(h), where it may be seen that the obtained agreewas generally kept as 600 and the integral appearing ment is generally satisfactory. This allows us to conin eq. (12) was solved numerically using standard clude that the lumped kinetic scheme proposed by routines. Cao et al. (1994a) retains a level of process description detailed enough to characterize the distribution of the RESULTS AND DISCUSSION most important products not only for a specific value The first goal of this work is to establish the behav- of the catalyst concentration, as demonstrated by Cao ior of the liquid-phase p-xylene oxidation as a func- et al. (1994a, b) by varying reaction temperature and tion of catalyst concentration. For this we consider initial concentration of liquid reactants, but also the overall oxygen uptake, Uo~, defined as follows when changing catalyst concentration levels within a i=5,6
(7)
Catalytic oxidation of p-xylene
17.2 kcal/mol which is in good agreement with those reported in the literature (cf. Bang and Chandalia,
05
0
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- -
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•
i•
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0 •
Run 5 Run 6
I
0 {I"
40
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140
Tmae[rn~] Fig. 1. Overall oxygen uptake, Uo2, as a function of time for different temperature values: runs 4-6 in Table 1.
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a~
C c . , t [rrlol/kl~ / ] x 1() ]
Fig 2. Initial uptake rate of oxygen, R°2, and p-xylene conversion at t = 60 min as a function of catalyst concentration: runs 3-8 in Table I.
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'
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i,
it..
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•
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i
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,
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.
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.
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[ x 10 3
Fig. 3. Initial uptake rate of oxygen, Rg:. as a function of temperature: runs 1-3 in Table 1.
relatively wide range. This variable was, in fact, assumed to be included in the apparent rate constants of the lumped reactions. Moreover, from the Arrhenius plot of the absolute value of the rate constant k~ shown in Fig. 5(a) a straight line is obtained, whose slope provides an apparent activation energy value of
1974 for methyl-p-toluate oxidation; Chen et al., 1985 for o-xylene oxidation). Similar plots are obtained for the reactivity ratios pj = kfikl, j = 2-4, as shown in Fig. 51b). As expected, the results reported in Figs 5(a) and (b/are also similar to those obtained by Cao et al. (1994a) for the same system at a catalyst concentration equal to 1 × I 0 3 mol/kgl. Thc estimated values of k~ and the reactivity ratios pj, j = 2 4 at each temperature and cobalt naphthenate concentration value are summarized in Tables 2 and 3, respectively, together with the average percentage error, r/, arising from the fitting procedure. The parameter values corresponding to C~a, = 1 x 10 3 mol:kgt compare very well with those obtained by Cao et al. 11994aL From the values of the kinetic constants ka, k2 and k4 rcported in Table 3, it may be seen that a similar behavior to that reported in F i g 2 for the case of initial oxygen uptakc is found. This is also true for the kinetic constant k 3 only for a certain range of catalyst concentration. Thus, the kinetic of reaction of the lumped kinetic scheme 121 may be assumed to be not only zeroth and first order with respect to oxygen and the liquid reactant, respectively, as previously demonstrated {cf. Cao et al., 1994a, b), but also as dependent upon catalyst concentration through a function to be evaluated, i.e. rj = kJ)[tC~,JC, exp{ - E / R T I , where C~ is the concentration of the ith component in the ith reaction. Although at this stage an empirical function. f(C~,,L can be obtained, it should be noted that the assumed expression of the reaction rate is in good agreement with the corresponding expressions available in the literature for the consumption rate of substrate and oxygen during cobalt catalyzed oxidation of toluene in acetic acid (of. Hanotier and Hanotier-Bridoux. 1981), wherc the complex radical chain mechanism, typical of homolytic oxidations, has been accounted for. Note that the function f(C,,,I has been also assumed, in the literature, to contain cobaltic and the cobaltous ion concentration, according to the presence of a m o n o m e r - d i m e r equilibrium of cobaltic acetate. However, since detailed radical chain mechanisms cannot be used for simulating gas liquid reactors, as discussed in the introduction, the established dependence of the rate of each reaction of the lumped kinetic scheme (2J on catalyst concentration may have a clear practical value. Let us now consider the behavior of a semi-batch gas-liquid reactor where the p-xylene oxidation to terephthalic acid occurs together with co-precipitation of the latter one and the 4-carboxybenzaldehyde. The model equations (3)-I 14) contain a large number of physico-chemical parameters, which, unfortunately, are not available in the literature for our reacting system with the exception of the kinetic constants of the lumped kinetic scheme I1) which are taken from Cao et al. (1994bt and correspond to a temperature of 1 2 0 C and a catalyst concentration equal to 1 × 10 3 mol/kg~. To overcome this difficulty and with the aim of theoretically demonstrating that the
A. Cincotti et al.
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p4o~c p401~ moddfit
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p-xylene conversion p-mlmldehyde
n ~
r~
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~
n
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|
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p-x'yla~conv~Aon
(b)
p-toluicackl
/ ^ /
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p-xylene conv~sion p-to~aklehyde pltOluic acid
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Fig. 4. Ca••u•atedvsexperimenta•va•ues•fpr•ductc•ncentrati•nandp-xylenec•nversi•nasafuncti•n•f time for various temperatures and catalyst concentrations: runs I(a)--8(h) in Table 1.
IOO
120
Catalytic oxidation of p-xylene
Table 4. Parameter values for the model given by eqs 13)-(14)
(a~
n
-'~
~ ~
_E -
"
~
~ I 2 55
I 260
I 265
I 270
t
I 2.75
i
I 280
,
I 2.85
I/TIK.I] x 103
'
=.~ •"
O •
P:
0
p~ p.~
• • ........~ I
I
I
I
260
265
270
,
6 x 1(1-6 mol..'kg~) 6 x 10 -~' mol:'kg~* 4.0 mol.'kg~ 0 . 1 1 mol kg[ 2.0* 1.0" 2.118x10 ~. s ' 8.56x 10-4. s ' 4 . 9 4 × 10 -4 s ' 1.6x 10 5.'s' 7.5 x 10-%s' 1.83 x 10-4.s * 6.68 x 10 ' a s' 1.8 x 10 ""s' 4.83 x 10 -~' s' 1.65 x 10-'L's ' 8.0 x 10 " ~* 8,0 x 10 ~o. 6.0 x 10'~* 5.0 x 104. 1.0x 10-:m~* 1.0 × 10 "" m,* 9360 mol.'m3: 9350 mol..m~
qs ~-h, k~ k: k3
k~ k,~ kto kq.s kq.~, k,.5 k,,.6 Lt).5 Lo.~> fi~. s fi~..
•
2 55
('~ C* C'?
k,
(b) 11-----41. O----------&.....
50
Value
k4 ks k~,
no~
.~ ,~ "
Parameter
C °
~,
250
4211
I
I
I
2.75
2 8O
:.85
l[rlK-q x 103 Fig. 5. Arrhenius plots for the reaction rate constant k~ (a) and reactivity ratios Ps, P3 and P4 (b) of the lumped kinetic scheme (2) at C,~, = 5.0 x 1 0 " mo ..'kg~.
*See text for c o m m e n t s . 'From Cao et al. 11994b). :From Lange 11973). 'From Perry and Green 11984). m o d e l d e v e l o p e d p r o p e r l y a c c o u n t s for the interaction b e t w e e n chemical r e a c t i o n a n d p r e c i p i t a t i o n phen o m e n a , we c o n s i d e r the m o d e l p a r a m e t e r s s u m -
Table 2. Numerical the reactivity ratios scheme (1) at C:,, kt Reaction
values of the Arrhenius parameters of for the reactions of the lumped kinetic = 5 x 10 4 mol/kgl, pj = kj,.'kl, where = .4~ e x p ( - E ~ . R T )* pj (110 Ct t
AEj = E j - E~ {kcal/mol)
2
46.27
0.83
3 4
34.46 ('1.871
4.67 6.21
• At = 3,620.000/min; E~ = 17.2 kcal/mol, "pflT ) = p~ 1110 C) exp[ - AEi(1.'T - 1/383); R].
m a r i z e d in Table 4. N o t e that s o m e of them, i.e. • C5, C o* , Lo.5 a n d Lo 6, are t a k e n from the literature (cf. W a c h i a n d Jones, 1991) even if they d o n o t refer specifically to o u r system. F o r the p a r a m e t e r s related to the linear g r o w t h and n u m b e r n u c l e a t i o n rates, we select the values r e p o r t e d in T a b l e 4 so that the q u a n t i t i e s a b o v e fall within the quite large range rep o r t e d in the literature for r e a c t i o n p r e c i p i t a t i o n systems (cf. G a r s i d e , 1980). T h e time b e h a v i o r of the species p-xylene, p-toluic alcohol, p - t o l u a l d e h y d e , t e r e p h t h a l i c aldehyde, p-toluic acid a n d 4 - c a r b o x y b e n z y l a l c o h o l , is not r e p o r t e d since they d o not u n d e r g o p r e c i p i t a t i o n p h e n o m e n a
Table 3. Estimated values of the kinetic parameters of the lumped kinetic scheme (21 as a function of catalyst concentration C,~, (mol/kg~) x 104 1.67 2.67 5.0 10 15 33.3
kl (per min) x 10'*
P:
Ps
P4
~1(%1
4.38 7.04 11.3 12.7 ll.l 11.0
71.2 69.2 45.7 39.2 41.8 44.7
59.8 71.4 35.0 27.4 41.6 45.0
1.6 1.8 1.1 0.9 1.1 1.1
6.5 4.2 1.1 2.7 2.2 4.6
4212
A. Cincotti et al.
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f:.
i o~ ~ 2
Fig. 6. Comparison between theoretical 4-carboxybenzaldehyde and terephthalic acid concentration time profiles with (solid line) or without (dashed line) precipitation. and depend only upon chemical reaction on the basis of the kinetic scheme (1). As expected, all these species, except for p-xylene, which is continuously consumed, go through a maximum and eventually disappear from the solution. On the other hand, the effect of precipitation phenomena involving 4-carboxybenzaldehyde and terephthalic acid may be clearly seen in Fig. 6 which shows the time profiles of these two species if their precipitation is neglected, i.e. B'I = G~ = 0 in eqs (5) and (6), respectively, or taken into account. In general, it may be noted that the time profile of the two species when precipitation is accounted for by the model lies below those corresponding to the case when precipitation is not considered. In particular, 4-carboxybenzaldehye displays the same qualitative behavior in both cases. This is because it behaves as an intermediate species and thus it is expected to first increase up to a maximum value and then decrease when consumption rate prevails. The latter one is clearly enhanced when precipitation also occurs. When considering the terephthalic acid behavior, it is seen that its concentration increases, eventually reaching a plateau if precipitation phenomena are not accounted for, since it is the only non-reacting species according to the kinetic scheme (1). On the other hand, by taking precipitation into account in our model, terephthalic acid is not only formed by chemical reaction but also precipitates simultaneously, when its solution concentration exceeds the corresponding saturation value, so that the concentration profile shows a maximum in time. Although the proposed model is able to properly simulate the time course of the crystal size distribution of 4-carboxybenzaldehyde and terephthalic acid, we do not report the corr~ponding results in this work for lack of knowledge of the appropriate model parameters rclated to the precipitation phenomena involved in our system. Suitable experimental studies are certainly needed in order to evaluate these parameters. CONCLUDING
In the experimental part, the effect of catalyst concentration on product distribution and kinetic constants of the lumped kinetic scheme proposed by Cao et a/. (1994a) is investigated. It is concluded that this lumped kinetic scheme retains a level of process description detailed enough to characterize the distribution of the most important products when changing catalyst concentration levels within a relatively wide range. Experimental reaction studies to demonstrate the effect of varying the catalyst to promoter, i.e. ptolualdehyde, ratio are currently in progress. In the theoretical part, a semi-batch reactor model, which accounts for the presence of the reactions of the lumped kinetic scheme proposed by Cao et al. (1994b) and precipitation phenomena of 4-carboxybenzaldehyde and terephthalic acid, is developed, it is shown that the time profiles of these species display a maximum whose value depends upon the relative magnitude of the rates of reactions where these species are involved and the precipitation parameters. The model may provide a useful tool for the simulation of gas-liquid oxidation reactors of p-xylene to terephthalic acid. The problem of accounting for intra- and interphase mass transfer resistances when simulating precipitation phenomena occurring in complex reaction systems of the type considered in this work is currently being investigated.
NOTATION
B~ (?'c~t C, C*
Ej 9~ G~ G~ ko.i kj k,.~ j~ L Lo.~ ~,~
REMARKS
In this paper, we present both experimental and theoretical work related to the catalytic liquid-phase oxidation of p-xylene.
N~ NR
mass-based nucleation rate of the ith precipitating component, mol kg i ~ s- 1 catalyst concentration, mol kg i concentration of the ith component, mol kgf t equilibrium solubility of the ith precipitating component, mol kg7 t activation energy of the jth reaction, cal mol- t order of growth of the ith precipitating component linear growth rate of the ith precipitating component, m, s- 1 mass-based growth rate of the ith precipitating component, mol kg/~ sgrowth rate constant of the ith precipitating component rate constant for the jth reaction, min- t nucleation rate constant of the ith precipitating component number nucleation rate of the ith precipitating component, kg~- t s coordinate of particle dimension, ms effective nucleic dimension of the ith precipitating component, m, order of nucleation of the ith precipitating component number of components in the system population density of particles of the ith precipitating component, kg~- 1 m~- t number of reactions in the system
Catalytic oxidation of p-xylenc pj r~ R{)): t T Uo:
reactivity ratio, k~,.'k~ rate of the jth reaction, mol kg{ ~min overall initial oxidation rate, molkgi j rain time, s temperature, K overall oxygen uptake, tool kg[
Greek letters AEj E~ - Et, cal mol~1 average percentage error rj., stoichiometric coefficient of the ith component in the jth reaction t~.~ crystal density of the ith precipitating component, mol m~ ~ r dimensionless time, t k~ Subscripts i component index j reaction index I liquid phase s solid Superscript 0 initial conditions REFERENCES
Aslund, B. L. and Rasmuson, A. C. (1992) Semibatch reaction crystallization of benzoic acid, A.I.Ch.E.J., 38, 328. Bang, S. R. and Chandalia, S. B. (1974) Liquid phase oxidation of methyl-p-toluate to monomethyl terephtalate. Ind. Chem. Enyn9, 16, T92. Cao, G., Servida, A., Pisu, M. and Morbidelli, M. (1994a) Kinetics of p-xylene liquid-phase catalytic oxidation. A.I.Ch.E.J. 40, 1156. Cao, G., Pisu, M. and Morbidelli, M. (1994b) A lumped kinetic model for liquid-phase catalytic oxidation of p-xylene to terephthalic acid. Chem. Engng Sci., 49, 5775. Carr'a, S. and Santacesaria, E., (1980) Engineering aspect of gas--liquid catalytic reactions. Catal. Ret'.Sci. Enyng, 22, 75. Cavalieri d'Oro, P., Danoczy, E. and Roflia, P. (1980) On the low temperature oxidation of p-xylene. Oxidation Commun., 1, 153. ('hen, Z., I,i, G., Li, W., Dai, L., Niu, Y,, Hu, J., Gu, J., Mao, X. and Li, M. (1985) The kinetics of the oxidation of o-xylene. Int. Chem. Engng, 2:5, 738. Dirksen, J. A. and Ring, T. A. (1991) Fundamentals of crystallization: kinetic effects on particle size distributions and morphology. Chem. Engng Sci., 46, 2389. Emanuel, N. M. and Gal, D. (1986) Modelling of Oxidation Processes. Akademiai Kiado, Budapest Garside, J. and Shah, M. B. (1980) Crystallization kinetics from MSMPR crystallizers. Ind. Engng Chem. Process Des. Dev., 19, 509. Garside, J. (1985) Industrial crystallization from solution. ('hen1. Engng Sc/. 40, 3.
4213
Hanotier. J. and Hanotier-Bridoux, M. (19811 Mechanism of the liquid phase homogeneous oxidation of alkylaromatic hydrocarbons by cobalt salts. J. Mol. Catal. 12, 133. Hronec, M. and llavsky, J. (1982) Oxidation of polyalkylaromatic hydrocarbons. 12. Technological aspects of p-xylene oxidation to terephthalic acid in water. Ind. Engng Chem. Prod. Res. Dev.. 21. 455. Hronec, M.. Cvengrosova, Z. and llavsky, J. (1985) Kinetics and mechanism of cobalt-catalyzed oxidation of p-xylene in the presence of water, hld. Engng Chenl. Proc. Des. Det'., 24. 787. Hronec, M. and Hrabe, Z. (1986) Liquid-phase oxidation of p-xylene catalyzed by metal oxides. Ind. Engng Chent. Prod. Res. Dev., 25, 257. Jones, A. G.. Hostomsky, J. and Li, Z. (1992) On the effect of liquid mixing rate on primary crystal size during the gas liquid precipitation of calcium carbonate. Chem. Engng Sci., 47, 3817. Kamiya, Y. and Kashima, M. (1972) The autoxidation of aromatic hydrocarbons catalyzed with cobaltic acetate in acetic acid solution, d. Catal. 25, 326. Kamiya, Y. (1974) Catalysis by cobalt and bromine ions in the autoxidation of alkylbenzenes in acetic acid. d. Catul. 33. 480. Krzysztoforski. A., Wojcik, Z., Pohorecki, R. and Baldyga, J. (1986) Industrial contribution to the reaction engineering of cyclohexane oxidation. Ind. Engng Chem. Process Des. Dev., 2:5, 894. Lange, N. A. 11973) Lange's Handbook of ('hemistry, ed. J. A. Dean. McGraw-Hill, New York. U.S.A. McElroy Brown, P. and Myerson, A. S. (1989) The growth, dissolution and aging of terephthalic acid crystals. A.I.Ch.E.J., 35, 1749. Morbidelli, M., Paludetto, R. and Carra. S. (1986) Gas liquid autoxidation reactors. Chem. Engng Sci.. 41, 2299. Perry. R. H. and Green, D. {1984) Perry's Chemical Engineers" ltandhm)k. McGraw-Hill, New York, U.S.A. Raghavendrachar, P. and Ramachandran, S. (1992) Liquid-phase oxidation of p-xylene. Ind. Engng Chem. Res. 31,453. Scott, E. J. Y. and Chester, A. W. (1972) Kinetics ofthe cobalt-catalyzed autoxidation of toluene in acetic acid. The role of cobalt. J. Phys. Chem., 76, 1520. Sheldon. R. A. and Kochi. J. K. (1981) Metal Catalyzed Oxidation of Organic Compounds. Academic Press. New York, U.S.A. Viola. A. and Cao, G. (1996) A rapid direct analysis of p-xylene oxidation products by reverse phase HPLC. J. Chromatogr. Sci., 34, 27. Wachi, S. and Jones, A. G. (1990) Model calculations of precipitation with gas-liquid rapid chemical reaction. Industrial Crystallization 90, ed. A. Mersmann, pp. 229 235. GVC-VDI, Dfisseldorf. Wachi, S. and Jones, A. G. {1991) Mass transfer with chemical reaction and precipitation. Chem. Enqng Sci., 46, 1027. Zaidi, S. A. H. (1986) Liquid phase oxidation of pxylene to p-toluic acid. Appl. Catal.. 27, 99.
('hemwal En~li,leerin9 5cWnce. Vol 52, Nos 21 22. pp 4205-4213. 1997 1997 Elsevier Science Lid All rights reserved Prmted in Great Britain P i l : S0009-2509(97)00263-7 o0o9 2509 97 $17.00 + 0.00
Effect of catalyst concentration and simulation of precipitation processes on liquid-phase catalytic oxidation ofp-xylene to terephthalic acid A. Cincotti, R. Orrfi, A. Broi and G. Cao* Dipartimento di Ingegneria Chimica e Materiali, Universita'degli Studi di Cagliari. Piazza d'Armi, 09123 Cagliari, Italy (Accepted 7 July 1997) Abstract---The influence of catalyst concentration, i.e. cobalt naphthenate, on product distribution and kinetic constants of the lumped kinetic scheme of liquid-phase p-xylene oxidation proposed in previous works (cf. Cao et al., 1994a, b) is investigated. The experiments involving various levels of catalyst concentrations (from 1.67 to 33.3 × 10-4 mol/kgi) are conducted in an isothermal semi-batch oxidation reactor where both the gas and the liquid phase are well mixed. The dependence of the kinetic constants of the lumped kinetic scheme on the catalyst concentration is examined. In addition, the interaction between the chemical reactions of the lumped kinetic scheme for p-xylene oxidation to terephthalic acid and the precipitation kinetics of both 4-carboxybenzaldehyde and terephthalic acid is analyzed theoretically. A semi-batch gas-liquid reactor model which incorporates the description of the above phenomena allows us to identify their interplay..(" 1997 Elsevier Science Ltd Keywords: p-xylene; oxidation; terephthalic acid; catalyst: precipitation.
INTRODUCTION
In the chemical industry, liquid-phase catalytic oxidation of organic compounds, which can be either homolytic or heterolytic depending upon the mechanism of oxygen activation (cf. Emanuel and Gal, 1986), constitutes a wide class of important processes. In particular, when addressing the study of homolytic oxidation processes, e.g. cyclohexanol from cyclohexane, terephthalic acid from p-xylene, isophthalic acid from m-xylene, emphasis was placed on the investigation of the catalytic system, i.e. catalyst and promoter concentration, nature of solvent, reaction temperature, etc., and its influence on the oxidation rate (Sheldon and Kochi, 1981; Raghavendrachar and Ramachandran, 1992). This type of approach typically accounts for the complex nature of the catalyst within the already complex chain elementary reaction schemes, which involve a very large number of radicals as well as molecular species (cf. Carr~ and Santacesaria, 1980). However, when simulating the gas-liquid reactors, the formulation of detailed kinetic models of oxidation processes requires a large computation effort for solving the diffusion-reaction equa-
*Corresponding author. Tel.: 39/70/675-5058: fax: 39.'70/675-5067: e-mail: [email protected].
tions in the film at the gas-liquid interface, coupled with the continuity equations for both gaseous and liquid bulk phases. This is due to the intrinsic difference between the space scale typical of molecular and radical species. In addition, the estimation of kinetic parameters of chain elementary reaction kinetic schemes may not be reliable due to the difficulty of monitoring all participating species, including highly reactive radicals, as a function of time in semi-batch reactors where specific experimental studies are typically conducted. The most common approach is to lump the detailed mechanism into a set of global reactions which involves only molecular species, whose concentration can be, in principle, easily monitored as a function of time. Without involving formal procedure of general validity but simply including the minimum number of reactions to describe the behavior of all the species of interest, various lumped kinetic schemes for homolytic oxidation processes have been developed in the literature, by Cavalieri d'Oro et al. (1980) for p-xylene oxidation, Chen et al. (1985) for o-xylene oxidation, Morbidelli et al. (1986) for ethyl-benzene autoxidation and Krzysztoforski et al. (1986) for cyclohexane oxidation. By accounting for the most important intermediates and final products of the process, i.e. p-tolualdehyde, p-tolualcohol, p-toluic acid,
4205
4206
A. Cincotti et al.
4-carboxybenzylalcohol, 4-carboxybenzaldehyde, terephthalic aldehyde and terephthalic acid, Cao et al. (1994a, bl have recently proposed the following lumped kinetic model for the liquid-phase oxidation of p-xylcne catalyzed by cobalt naphthenate:
both reactants and products. It was also shown that the model was able to describe the reactor behavior in any of the regimes which may prevail depending upon the operating conditions and the depletion of liquid reactants in time.
terephthalic aldehyde
+ CHO
/ p-xylene
p-tolualdehyde
CH~
CHO
CHO p-toluic acid
"~ 4-carboxybenzaldehyde
COOH
COOl I k5 ,.
CH.~
CH,
CH 3
COOH k~, ,,-
+
CHO
COOH
CH~ p-toluic alcohol
terephthalic acid
/k~
+
COOH
(I)
CH,OH 4-carboxybenzylalcohol
which may have important practical implications in the production of terephthalic acid. All the lumped reactions were assumed to be zeroth and first order with respect to the gaseous and the liquid reactant, respectively. The reliability of the proposed kinetic model was illustrated by comparison with suitable experimental data obtained in a semi-batch oxidation reactor where both the gas and the liquid phase are well mixed. The experiments included different values of the initial concentrations of the liquid reactants, two gaseous reactants (i.e. pure oxygen and air), temperature values in the range 80-130~C and were conducted at a unique value of the catalyst concentration (i.e. 1 × I0- 3 mol/kg~). However, catalyst concentration plays a fundamental rote upon the global rate of a wide variety of oxidation reactions such as the cobalt-catalyzed oxidation of toluene in acetic acid (cf. Scott and Chester, 1972; Kamiya and Kashima, 1972), the oxidation of alkyl-benzenes in acetic acid catalyzed by cobalt acetate and sodium bromine {cf. Kamiya, 1974), the durene oxidation in acetic acid catalyzed by cobalt salts (cf. Hanotier and HanotierBridoux, 1981), the catalytic oxidation of p-xylene (cf. Zaidi, 1986; Raghavendrachar and Ramachandran, 1992). The proposed lumped kinetic scheme was also incorporated (cf. Cao et al., 1994a) into semi-batch gas-liquid reactor model that properly accounts for inter- and intraphase mass transport processes of
The formulation of detailed kinetic models is even less desirable when the interaction among mass transfer resistances, chemical reactions and precipitation phenomena plays a fundamental role. This is particularly the case of liquid-phase p-xylene oxidation, where the formation of 4-carboxybenzaldehyde and terephthalic acid occurs through a reaction precipitation process, i.e. the latter compounds are formed by chemical reactions in concentration exceeding their solubility. Note that 4-carboxybenzaldehyde is probably one of the most deleterious contaminant of terephthalic acid, since its aldehyde group is unable to undergo condensation reactions with ethylene glycol during PET polymerization (cf. McElroy Brown and Myerson, 1989). Only few studies concerning reactive precipitation have been reported in the literature (cf. Garside and Shah, 1980; Aslund and Rasmunson, 1992 and references therein). This is particularly true when considering the simultaneous presence of gas-liquid mass transfer, chemical reaction and precipitation, which remain unexplored until recently (Wachi and Jones, 1990; Wachi and Jones, 1991). The present work consists in two parts. In the first one, the effect of cobalt naphthenate concentration on product distribution and kinetic constants of the lumped kinetic scheme (1) is established. In particular, the reactions leading to the formation of p-toluic acid are considered by operating at low values of p-xylene conversion. Various values of catalyst concentration
4207
Catalytic oxidation of p-xylene are investigated by running the experimental reactor used in previous works (cf. Cao e t al., 1994a, b) under the kinetic regime. In the second part, the interaction between chemical reactions of the lumped kinetic scheme (1) and the precipitation kinetics of both 4-carboxybenzaldehyde and terephthalic acid is analyzed theoretically. The latter phenomena are described by appropriate population balance equations incorporated in a semi-batch reactor model.
lable 1. Operating conditions for the experimental runs Run I
2 3 4 5 6 7 g
T [ ('~
('~,, {mol.kgh × 104
80 110 120 120 120 120 120 12(1
5.o 5.0 5.o 1.67 2.67 10.0 15.0 33.3
EXPERIMENTAL
The experimental set-up and procedure are described in detail by Cao et al. (1994at. As summarized in Table 1, the experimental runs performed were carried out at three temperatures (80, 110 and 120~'C) and six levels of cobalt naphthenate concentration, while maintaining fixed initial p-xylene (4 mol/kgt) and p-tolualdehyde (0.11 mol,,"kg,) concentrations. The experiments were run in a semi-batch reactor continuously fed with pure oxygen where, after the temperature reached the desired value, suitablc amounts of catalyst (0.05 1 cm 3) and p-tolualdehyde (6 cm 3) as promoter were added. Methyl benzoate was used as solvent. All the experimental runs were performed at a stirring speed of 800 rpm, where the influence of this variable on the product distribution becomes negligible (cf. Cao et al., 1994a). Each run was carried out up to conversion values of p-xylene of about 14%. Thc reproducibility of the experimental runs was verified by repeating each of them at least twice. The reaction products were analyzed by HPLC using the technique described by Cao et al. (1994a, b) and Viola and Cao (19961. THE MODEL EQUATIONS At the p-xylene conversion levels considered in this work the lumped kinetic scheme(l) becomes (cf. Cao et al., 1994a):
p-xylene
p-toluic acid
p-tolualdehydc
CH~
C(X)H
CHO
IlL
CH~
CH~
CH~
(2) k~
CH:OH/k,
where each reaction involves the addition of I..2 0 2 and the corresponding kinetics are assumed to be zeroth and first order with respect to oxygen and the liquid reactant, respectively. The above scheme has been inserted in the model of a semi-batch gas-liquid reactor to simulate the experimental data. Since all experiments were run under the kinetic regime, the following mass balances are used: dCi dt
~" v j ' i r j
1, N c
(3)
i = l . Nc
(4)
i=
along with the initial conditions: C i = ('~
at t =0.
where C~ is the concentration of the ith component and the meaning of other symbols is reported in the Notation. The oxygen mass balance and the energy balance are not considered since all reactions are of zeroth order with respect to oxygen and the reactor has been operated under isothermal conditions. In order to simulate the precipitation phenomena of 4-carboxybenzaldehyde and terephthalic acid, a mathematical model of a semi-batch gas liquid reactor where the reactions of the lumped kinetic scheme (1) occur, is developed under the following assumptions: -- isothermal conditions: kinetic regime: ---neglibiblc agglomeration and disruption {cf. Wachi and Jones, 19911; precipitation due to supersaturation of 4-carboxybenzaldehyde and terephthalic acid; spherical crystals; growth rate independent of linear crystal dimension (cf. Garsidc and Shah, 1980). The mass balances for the non-precipitating species are identical to those reported in eq. (3), while those for precipitating species are as follows: dC~ dt dC,, --dt
CH, p-toluie alcohol
~~ -
-
ks(".~ + k - ( ' -
= k~(': -
4 kio('~
Bg - G~,
- k,,("~ -
B ~ -.. G ~ { 5 )
(6)
where BI and GI, i = 5 and 6 are the mass-based nucleation and growth rate, respectively.
4208
A. Cincotti et al.
The population balances of the precipitated particles are ~Ni G. ~Ni ?~-+ ' d L =0,
with respect to the lumped kinetic scheme (2): Uo2 = ½[C2 - C° + 2C3 + C,]
(15)
where Ci represents the experimentally measured concentration of the reaction products. A plot of the quantity Uo: as a function of time for where N~ is the population density of particles, G~ represents the linear growth rate, t is the time and L is the experimental runs 4 - 6 in Table 1, where different the coordinate of particle dimension. Equations (3) values of catalyst concentration have been considered, while maintaining constant the initial p-xylene and and (5)--(7) are subjected to the following initial and the p-tolualdehyde concentration, is shown in Fig. 1. boundary conditions: From the slopes of these curves evaluated at t = 0, the C ~ = C o at t=O, i = l , Nc (8) initial value of the oxygen uptake, R~2, was computed and plotted in Fig. 2 as a function of catalyst concenN~=0 at t =0, VL, i = 5 , 6 (9) tration. In the same figure the p-xylene conversion values reached in the corresponding experimental Ji Ni=~ a t L = L o . i , Vt, i = 5 , 6 (10) runs at t = 60 min are also reported. It is seen that as the catalyst concentration increases both quantities where J~ is the number nucleation rate and Lo.~ is the first increase and then reach a plateau. Note that this effective nucleic dimension. Note that the boundary behavior is consistent with that reported in the literacondition (I0) is used by several authors (cf. Garside, ture for p-xylene oxidation to p-toluic acid catalyzed 1985; Dirksen and Ring, 1991; Jones et al., 1992) and by cobalt acetate and sodium bromine in acetic acid may become the dominant factor of crystal distribu- as solvent (cf. Zaidi, 1986). In order to demonstrate tion. that all experiments were conducted under the kinetic The mass-based nucleation and growth rate equa- regime, we have performed the experimental runs 1 - 3, tions are as follows: where only the temperature values have been changed with respect to run 3. The corresponding initial values (I I) of oxygen uptake are shown in Fig. 3 as a function of B~ = gn ~)~.ijiL3.i, i = 5, 6 temperature. From the linear behavior of the Arrhenius-like plot shown in Fig. 3, it may be concluded G~= 7zGi~s. iL2Ni(L) dL, i = 5 , 6 (12) that the reactor operates in the chemically controlled regime for the specific catalyst concentration used for where n/6 and ~r are related to the spherical crystals, runs 1-3, and, consequently, for all catalyst concent~.~ is the crystal density, and the corresponding num- tration levels considered in this work, at which equal ber rate of nucleation and linear crystal growth rate or lower oxidation rates are found (cf. Fig. 2). The are expressed through the following equations (cf. apparent activation energy is equal to about Wachi and Jones, 1991): 18 kcal/mol, i.e. a typical value of oxidation reactions, which compares fairly well with those reported in the Ji = k,.i(Ci - C~')"', i = 5, 6 (13) literature for p-xylene oxidation in the presence of Gi = kg.i(Ci - C*) g', i = 5, 6 (14) water (cf. Hronec and Ilavsky, 1982; Hronec et al., 1985; Hronec and Hrabe, 1986). As expected, the where k,.~ and k~.~are the nucleation and growth rate apparent activation energy is also very similar to the constant, respectively, ni and 9i are the orders of one obtained by Cao et al. (1994a) under the operatnucleation and growth, respectively, and C* is the ing conditions corresponding to run 6 of the present equilibrium saturation concentration. work using the same experimental set-up and procedThe governing balance equations are in the form of ure. The values of the kinetic constants appearing in a system of ordinary and partial differential equa- the lumped kinetic scheme (2) are then estimated by tions. A backward finite-difference scheme was ad- fitting the time evolution of the experimental product opted for the spatial derivative appearing in the popu- composition of run 1-8 in Table 1 through a nonlation balances (7) in order to obtain a set of ordinary linear least-squares procedure using the model given differential equations which are solved as an initial by eqs (3) and (4). The comparison between model value problem using standard routines. During the results and experimental data is shown in Figs computations, the value of the finite-difference points 4(a)-(h), where it may be seen that the obtained agreewas generally kept as 600 and the integral appearing ment is generally satisfactory. This allows us to conin eq. (12) was solved numerically using standard clude that the lumped kinetic scheme proposed by routines. Cao et al. (1994a) retains a level of process description detailed enough to characterize the distribution of the RESULTS AND DISCUSSION most important products not only for a specific value The first goal of this work is to establish the behav- of the catalyst concentration, as demonstrated by Cao ior of the liquid-phase p-xylene oxidation as a func- et al. (1994a, b) by varying reaction temperature and tion of catalyst concentration. For this we consider initial concentration of liquid reactants, but also the overall oxygen uptake, Uo~, defined as follows when changing catalyst concentration levels within a i=5,6
(7)
Catalytic oxidation of p-xylene
17.2 kcal/mol which is in good agreement with those reported in the literature (cf. Bang and Chandalia,
05
0
• 0 0 2
- -
II
•
i•
•
Run4
0 •
Run 5 Run 6
I
0 {I"
40
60
8(I
ID0
[20
140
Tmae[rn~] Fig. 1. Overall oxygen uptake, Uo2, as a function of time for different temperature values: runs 4-6 in Table 1.
•
•
•
•
0
0
0
0
O0
=
4209
0 •
5
R°o:
0 o5
i
r
05
ic
,
i
.
15
O
p-xylm~
ctmver~on
t
.
.
i
20
i
25
•
i
~o
0 tl~
a~
C c . , t [rrlol/kl~ / ] x 1() ]
Fig 2. Initial uptake rate of oxygen, R°2, and p-xylene conversion at t = 60 min as a function of catalyst concentration: runs 3-8 in Table I.
D"
[
"
,
'
,
'
,
"
,
i,
it..
\
•
2.~o
L
2~5
.
i
2.6o
.
l
.
26s i/r
i
.
2.70 [KI
,
27~
.
,
2,~o
.
i
2s~
[ x 10 3
Fig. 3. Initial uptake rate of oxygen, Rg:. as a function of temperature: runs 1-3 in Table 1.
relatively wide range. This variable was, in fact, assumed to be included in the apparent rate constants of the lumped reactions. Moreover, from the Arrhenius plot of the absolute value of the rate constant k~ shown in Fig. 5(a) a straight line is obtained, whose slope provides an apparent activation energy value of
1974 for methyl-p-toluate oxidation; Chen et al., 1985 for o-xylene oxidation). Similar plots are obtained for the reactivity ratios pj = kfikl, j = 2-4, as shown in Fig. 51b). As expected, the results reported in Figs 5(a) and (b/are also similar to those obtained by Cao et al. (1994a) for the same system at a catalyst concentration equal to 1 × I 0 3 mol/kgl. Thc estimated values of k~ and the reactivity ratios pj, j = 2 4 at each temperature and cobalt naphthenate concentration value are summarized in Tables 2 and 3, respectively, together with the average percentage error, r/, arising from the fitting procedure. The parameter values corresponding to C~a, = 1 x 10 3 mol:kgt compare very well with those obtained by Cao et al. 11994aL From the values of the kinetic constants ka, k2 and k4 rcported in Table 3, it may be seen that a similar behavior to that reported in F i g 2 for the case of initial oxygen uptakc is found. This is also true for the kinetic constant k 3 only for a certain range of catalyst concentration. Thus, the kinetic of reaction of the lumped kinetic scheme 121 may be assumed to be not only zeroth and first order with respect to oxygen and the liquid reactant, respectively, as previously demonstrated {cf. Cao et al., 1994a, b), but also as dependent upon catalyst concentration through a function to be evaluated, i.e. rj = kJ)[tC~,JC, exp{ - E / R T I , where C~ is the concentration of the ith component in the ith reaction. Although at this stage an empirical function. f(C~,,L can be obtained, it should be noted that the assumed expression of the reaction rate is in good agreement with the corresponding expressions available in the literature for the consumption rate of substrate and oxygen during cobalt catalyzed oxidation of toluene in acetic acid (of. Hanotier and Hanotier-Bridoux. 1981), wherc the complex radical chain mechanism, typical of homolytic oxidations, has been accounted for. Note that the function f(C,,,I has been also assumed, in the literature, to contain cobaltic and the cobaltous ion concentration, according to the presence of a m o n o m e r - d i m e r equilibrium of cobaltic acetate. However, since detailed radical chain mechanisms cannot be used for simulating gas liquid reactors, as discussed in the introduction, the established dependence of the rate of each reaction of the lumped kinetic scheme (2J on catalyst concentration may have a clear practical value. Let us now consider the behavior of a semi-batch gas-liquid reactor where the p-xylene oxidation to terephthalic acid occurs together with co-precipitation of the latter one and the 4-carboxybenzaldehyde. The model equations (3)-I 14) contain a large number of physico-chemical parameters, which, unfortunately, are not available in the literature for our reacting system with the exception of the kinetic constants of the lumped kinetic scheme I1) which are taken from Cao et al. (1994bt and correspond to a temperature of 1 2 0 C and a catalyst concentration equal to 1 × 10 3 mol/kg~. To overcome this difficulty and with the aim of theoretically demonstrating that the
A. Cincotti et al.
4210 020 '
'
o [] O
O~ O~
~°~ °D I
'
'
'
"
("a)
1 os [-
'
"
p4o~c p401~ moddfit
A
~
"
p-xylene conversion p-mlmldehyde
n ~
r~
~
~
n
~
O
|
o
I
0
p-x'yla~conv~Aon
(b)
p-toluicackl
/ ^ /
[]
~°~
O08
¢¢
0.04
Ol
^
0 02, 0 00~
.
l
I
~0
,
I
DO
I
,
~
,
i
200
0
2~ ~
.0
4(I
05
0.4
,
0 o 0
A
~
|
~
[20
J40
,
,
TLme[ram]
!
,
•
i
p -xy k n e convm~ion
(c)
p-totualdd~yde p-tOhaicacid p-tOhfic~
i
o o 0
05
/A
.
i
-
i
,
i
p-xylene conv~sion p-to~aklehyde pltOluic acid
(d)
/
04
/
.
~
02
i 0
02 Ol
o o: 30
20
40
5O
20
6O
40
040 O35 {)30
i
,
w
,
O
p-xylene oonversi(m
o
pqo~kid~yd~ p-tOl~C~td
0
1
80
60
1OO
RO
¼0
Time [mini
Time [mini ,
i
i
(e)
/
0
p-Xylene oonv~rsion
n
p4oluddehyde
0 A
p -toluic acid pqolt~ak~ho4
(f) O///
,
025
~
gO
bO
Tune {mini
020
"~ 02
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~
o~ r 005 I0
30
20
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40
~)
IO
60
20
05
i
i
i
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LO
i
p-xylene o~version p-toh~de 0 p-toluic acid /X p-tOhficaloohol modd fit 0
04
40
30
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Tune [rain] i
(g)
'
0
~
~
,
,
0
p -x-ykne cmvo'sion
o s ~-
D
p-tolualddlyde
I
o ~
p-ta,~ p-ta~aoa~
[
•
,
'
,
'
(h)
J
/
0.2
6
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20
30
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40
50
60
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20
40
60
80
Tune [mini
Fig. 4. Ca••u•atedvsexperimenta•va•ues•fpr•ductc•ncentrati•nandp-xylenec•nversi•nasafuncti•n•f time for various temperatures and catalyst concentrations: runs I(a)--8(h) in Table 1.
IOO
120
Catalytic oxidation of p-xylene
Table 4. Parameter values for the model given by eqs 13)-(14)
(a~
n
-'~
~ ~
_E -
"
~
~ I 2 55
I 260
I 265
I 270
t
I 2.75
i
I 280
,
I 2.85
I/TIK.I] x 103
'
=.~ •"
O •
P:
0
p~ p.~
• • ........~ I
I
I
I
260
265
270
,
6 x 1(1-6 mol..'kg~) 6 x 10 -~' mol:'kg~* 4.0 mol.'kg~ 0 . 1 1 mol kg[ 2.0* 1.0" 2.118x10 ~. s ' 8.56x 10-4. s ' 4 . 9 4 × 10 -4 s ' 1.6x 10 5.'s' 7.5 x 10-%s' 1.83 x 10-4.s * 6.68 x 10 ' a s' 1.8 x 10 ""s' 4.83 x 10 -~' s' 1.65 x 10-'L's ' 8.0 x 10 " ~* 8,0 x 10 ~o. 6.0 x 10'~* 5.0 x 104. 1.0x 10-:m~* 1.0 × 10 "" m,* 9360 mol.'m3: 9350 mol..m~
qs ~-h, k~ k: k3
k~ k,~ kto kq.s kq.~, k,.5 k,,.6 Lt).5 Lo.~> fi~. s fi~..
•
2 55
('~ C* C'?
k,
(b) 11-----41. O----------&.....
50
Value
k4 ks k~,
no~
.~ ,~ "
Parameter
C °
~,
250
4211
I
I
I
2.75
2 8O
:.85
l[rlK-q x 103 Fig. 5. Arrhenius plots for the reaction rate constant k~ (a) and reactivity ratios Ps, P3 and P4 (b) of the lumped kinetic scheme (2) at C,~, = 5.0 x 1 0 " mo ..'kg~.
*See text for c o m m e n t s . 'From Cao et al. 11994b). :From Lange 11973). 'From Perry and Green 11984). m o d e l d e v e l o p e d p r o p e r l y a c c o u n t s for the interaction b e t w e e n chemical r e a c t i o n a n d p r e c i p i t a t i o n phen o m e n a , we c o n s i d e r the m o d e l p a r a m e t e r s s u m -
Table 2. Numerical the reactivity ratios scheme (1) at C:,, kt Reaction
values of the Arrhenius parameters of for the reactions of the lumped kinetic = 5 x 10 4 mol/kgl, pj = kj,.'kl, where = .4~ e x p ( - E ~ . R T )* pj (110 Ct t
AEj = E j - E~ {kcal/mol)
2
46.27
0.83
3 4
34.46 ('1.871
4.67 6.21
• At = 3,620.000/min; E~ = 17.2 kcal/mol, "pflT ) = p~ 1110 C) exp[ - AEi(1.'T - 1/383); R].
m a r i z e d in Table 4. N o t e that s o m e of them, i.e. • C5, C o* , Lo.5 a n d Lo 6, are t a k e n from the literature (cf. W a c h i a n d Jones, 1991) even if they d o n o t refer specifically to o u r system. F o r the p a r a m e t e r s related to the linear g r o w t h and n u m b e r n u c l e a t i o n rates, we select the values r e p o r t e d in T a b l e 4 so that the q u a n t i t i e s a b o v e fall within the quite large range rep o r t e d in the literature for r e a c t i o n p r e c i p i t a t i o n systems (cf. G a r s i d e , 1980). T h e time b e h a v i o r of the species p-xylene, p-toluic alcohol, p - t o l u a l d e h y d e , t e r e p h t h a l i c aldehyde, p-toluic acid a n d 4 - c a r b o x y b e n z y l a l c o h o l , is not r e p o r t e d since they d o not u n d e r g o p r e c i p i t a t i o n p h e n o m e n a
Table 3. Estimated values of the kinetic parameters of the lumped kinetic scheme (21 as a function of catalyst concentration C,~, (mol/kg~) x 104 1.67 2.67 5.0 10 15 33.3
kl (per min) x 10'*
P:
Ps
P4
~1(%1
4.38 7.04 11.3 12.7 ll.l 11.0
71.2 69.2 45.7 39.2 41.8 44.7
59.8 71.4 35.0 27.4 41.6 45.0
1.6 1.8 1.1 0.9 1.1 1.1
6.5 4.2 1.1 2.7 2.2 4.6
4212
A. Cincotti et al.
"L
,,"~..
..-
•
-
010~"
o_ =
,.'
8
.
;,
,
• L
,
O05
f:.
i o~ ~ 2
Fig. 6. Comparison between theoretical 4-carboxybenzaldehyde and terephthalic acid concentration time profiles with (solid line) or without (dashed line) precipitation. and depend only upon chemical reaction on the basis of the kinetic scheme (1). As expected, all these species, except for p-xylene, which is continuously consumed, go through a maximum and eventually disappear from the solution. On the other hand, the effect of precipitation phenomena involving 4-carboxybenzaldehyde and terephthalic acid may be clearly seen in Fig. 6 which shows the time profiles of these two species if their precipitation is neglected, i.e. B'I = G~ = 0 in eqs (5) and (6), respectively, or taken into account. In general, it may be noted that the time profile of the two species when precipitation is accounted for by the model lies below those corresponding to the case when precipitation is not considered. In particular, 4-carboxybenzaldehye displays the same qualitative behavior in both cases. This is because it behaves as an intermediate species and thus it is expected to first increase up to a maximum value and then decrease when consumption rate prevails. The latter one is clearly enhanced when precipitation also occurs. When considering the terephthalic acid behavior, it is seen that its concentration increases, eventually reaching a plateau if precipitation phenomena are not accounted for, since it is the only non-reacting species according to the kinetic scheme (1). On the other hand, by taking precipitation into account in our model, terephthalic acid is not only formed by chemical reaction but also precipitates simultaneously, when its solution concentration exceeds the corresponding saturation value, so that the concentration profile shows a maximum in time. Although the proposed model is able to properly simulate the time course of the crystal size distribution of 4-carboxybenzaldehyde and terephthalic acid, we do not report the corr~ponding results in this work for lack of knowledge of the appropriate model parameters rclated to the precipitation phenomena involved in our system. Suitable experimental studies are certainly needed in order to evaluate these parameters. CONCLUDING
In the experimental part, the effect of catalyst concentration on product distribution and kinetic constants of the lumped kinetic scheme proposed by Cao et a/. (1994a) is investigated. It is concluded that this lumped kinetic scheme retains a level of process description detailed enough to characterize the distribution of the most important products when changing catalyst concentration levels within a relatively wide range. Experimental reaction studies to demonstrate the effect of varying the catalyst to promoter, i.e. ptolualdehyde, ratio are currently in progress. In the theoretical part, a semi-batch reactor model, which accounts for the presence of the reactions of the lumped kinetic scheme proposed by Cao et al. (1994b) and precipitation phenomena of 4-carboxybenzaldehyde and terephthalic acid, is developed, it is shown that the time profiles of these species display a maximum whose value depends upon the relative magnitude of the rates of reactions where these species are involved and the precipitation parameters. The model may provide a useful tool for the simulation of gas-liquid oxidation reactors of p-xylene to terephthalic acid. The problem of accounting for intra- and interphase mass transfer resistances when simulating precipitation phenomena occurring in complex reaction systems of the type considered in this work is currently being investigated.
NOTATION
B~ (?'c~t C, C*
Ej 9~ G~ G~ ko.i kj k,.~ j~ L Lo.~ ~,~
REMARKS
In this paper, we present both experimental and theoretical work related to the catalytic liquid-phase oxidation of p-xylene.
N~ NR
mass-based nucleation rate of the ith precipitating component, mol kg i ~ s- 1 catalyst concentration, mol kg i concentration of the ith component, mol kgf t equilibrium solubility of the ith precipitating component, mol kg7 t activation energy of the jth reaction, cal mol- t order of growth of the ith precipitating component linear growth rate of the ith precipitating component, m, s- 1 mass-based growth rate of the ith precipitating component, mol kg/~ sgrowth rate constant of the ith precipitating component rate constant for the jth reaction, min- t nucleation rate constant of the ith precipitating component number nucleation rate of the ith precipitating component, kg~- t s coordinate of particle dimension, ms effective nucleic dimension of the ith precipitating component, m, order of nucleation of the ith precipitating component number of components in the system population density of particles of the ith precipitating component, kg~- 1 m~- t number of reactions in the system
Catalytic oxidation of p-xylenc pj r~ R{)): t T Uo:
reactivity ratio, k~,.'k~ rate of the jth reaction, mol kg{ ~min overall initial oxidation rate, molkgi j rain time, s temperature, K overall oxygen uptake, tool kg[
Greek letters AEj E~ - Et, cal mol~1 average percentage error rj., stoichiometric coefficient of the ith component in the jth reaction t~.~ crystal density of the ith precipitating component, mol m~ ~ r dimensionless time, t k~ Subscripts i component index j reaction index I liquid phase s solid Superscript 0 initial conditions REFERENCES
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4213
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