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 distribu- tion 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.

I N T R O D U C T I O N

In the chemical industry, liquid-phase catalytic oxida- tion of organic compounds, which can be either homolytic or heterolytic depending upon the mecha- nism 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 cyclo- hexane, terephthalic acid from p-xylene, isophthalic acid from m-xylene, emphasis was placed on the in- vestigation 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 typi- cally accounts for the complex nature of the catalyst within the already complex chain elementary reaction schemes, which involve a very large number of rad- icals as well as molecular species (cf. Carr~ and San- tacesaria, 1980). However, when simulating the gas-liquid reactors, the formulation of detailed kinetic models of oxidation processes requires a large compu- tation effort for solving the diffusion-reaction equa-

*Corresponding author. Tel.: 39/70/675-5058: fax: 39.'70/675-5067: e-mail: cao@visnu.dicm.unica.it.

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 differ- ence 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 typi- cally conducted.

The most common approach is to lump the detailed mechanism into a set of global react ions which in- volves 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 litera- ture, 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 oxi- dation.

By accounting for the most important inter- mediates 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, both reactants and products. It was also shown that terephthalic aldehyde and terephthalic acid, Cao et al. the model was able to describe the reactor behavior in (1994a, bl have recently proposed the following lum- any of the regimes which may prevail depending upon ped kinetic model for the liquid-phase oxidation of the operating conditions and the depletion of liquid p-xylcne catalyzed by cobalt naphthenate: reactants in time.

terephthalic aldehyde

CHO + / CHO " ~

p-xylene p-tolualdehyde p-toluic acid

CH~ CHO COOH

CH.~ CH, CH 3

CH~ p-toluic alcohol

4-carboxybenzaldehyde

COOl I

k5 ,. +

CHO

COOH /k~

CH,OH 4-carboxybenzylalcohol

terephthalic acid

COOH

k~, ,,- +

COOH ( I )

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), tem- perature values in the range 80-130~C and were con- ducted at a unique value of the catalyst concentration (i.e. 1 × I0- 3 mol/kg~). However, catalyst concentra- tion 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 Hanotier- Bridoux, 1981), the catalytic oxidation of p-xylene (cf. Zaidi, 1986; Raghavendrachar and Ramachandran, 1992).

The proposed lumped kinetic scheme was also in- corporated (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 trans- fer resistances, chemical reactions and precipitation phenomena plays a fundamental role. This is parti- cularly the case of liquid-phase p-xylene oxidation, where the formation of 4-carboxybenzaldehyde and terephthalic acid occurs through a reaction precipita- tion process, i.e. the latter compounds are formed by chemical reactions in concentration exceeding their solubility. Note that 4-carboxybenzaldehyde is prob- ably 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 pre- cipitation, 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 lum- ped 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

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. Run

In the second part, the interaction between chem- ical 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 popu- lation balance equations incorporated in a semi-batch reactor model.

E X P E R I M E N T A L

The experimental set-up and procedure are de- scribed in detail by Cao e t al. (1994at. As sum- marized in Table 1, the experimental runs performed were carried out at three temperatures (80, 110 and 120~'C) and six levels of cobalt naphthenate concen- tration, while maintaining fixed initial p-xylene (4 mol/kgt) and p-tolualdehyde (0.11 mol,,"kg,) concen- trations.

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 per- formed at a stirring speed of 800 rpm, where the influence of this variable on the product distribution becomes negligible (cf. Cao e t 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 e t 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-tolualdehydc

CH~ CHO

CH~ CH~

k ~ CH:OH/k,

CH, p-toluie alcohol

p-toluic acid

C(X)H

IlL

CH~

(2)

4207

lable 1. Operating conditions for the experimental runs

T [ ('~ ('~,, {mol.kgh × 104

I 80 5.o 2 110 5.0 3 120 5.o 4 120 1.67 5 120 2.67 6 120 10.0 7 120 15.0 g 12(1 33.3

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 ~~ d t - ~" v j ' i r j i = 1, N c (3)

along with the initial conditions:

C i= ('~ at t =0. i = l . Nc (4)

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 re- actor 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-car- boxybenzaldehyde and terephthalic acid;

spherical crystals; growth rate independent of linear crystal di-

mension (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~ - ks (" .~ + k - ( ' - 4 k i o ( ' ~ - k , , ( " ~ - B ~ -.. G ~ { 5 )

dt

dC,, - - - = k ~ ( ' : - Bg - G~, (6) d t

where BI and GI, i = 5 and 6 are the mass-based nucleation and growth rate, respectively.

4208

The population balances of the precipitated par- ticles are

~Ni G. ~Ni ?~-+ ' d L =0, i = 5 , 6 (7)

where N~ is the population density of particles, G~ represents the linear growth rate, t is the time and L is the coordinate of particle dimension. Equations (3) and (5)--(7) are subjected to the following initial and boundary conditions:

C~=C o at t=O, i = l , Nc (8)

N~=0 at t =0, VL, i = 5 , 6 (9)

Ji N i = ~ a t L = L o . i , Vt, i = 5 , 6 (10)

where J~ is the number nucleation rate and Lo.~ is the effective nucleic dimension. Note that the boundary condition (I0) is used by several authors (cf. Garside, 1985; Dirksen and Ring, 1991; Jones et al., 1992) and may become the dominant factor of crystal distribu- tion.

The mass-based nucleation and growth rate equa- tions are as follows:

n ~)~.ijiL3.i, i = 5, 6 (I I) B~ = g

G~= 7zGi~s. iL2Ni(L) dL, i = 5 , 6 (12)

where n/6 and ~r are related to the spherical crystals, t~.~ is the crystal density, and the corresponding num- ber rate of nucleation and linear crystal growth rate are expressed through the following equations (cf. Wachi and Jones, 1991):

Ji = k,.i(Ci - C~')"', i = 5, 6 (13)

Gi = kg.i(Ci - C*) g', i = 5, 6 (14)

where k,.~ and k~.~ are the nucleation and growth rate constant, respectively, ni and 9i are the orders of nucleation and growth, respectively, and C* is the equilibrium saturation concentration.

The governing balance equations are in the form of a system of ordinary and partial differential equa- tions. A backward finite-difference scheme was ad- opted for the spatial derivative appearing in the popu- lation balances (7) in order to obtain a set of ordinary differential equations which are solved as an initial value problem using standard routines. During the computations, the value of the finite-difference points was generally kept as 600 and the integral appearing in eq. (12) was solved numerically using standard routines.

RESULTS AND DISCUSSION

The first goal of this work is to establish the behav- ior of the liquid-phase p-xylene oxidation as a func- tion of catalyst concentration. For this we consider the overall oxygen uptake, Uo~, defined as follows

A. Cincotti et al.

with respect to the lumped kinetic scheme (2):

Uo2 = ½[C2 - C° + 2C3 + C,] (15)

where Ci represents the experimentally measured con- centration of the reaction products.

A plot of the quantity Uo: as a function of time for the experimental runs 4 -6 in Table 1, where different values of catalyst concentration have been considered, while maintaining constant the initial p-xylene and the p-tolualdehyde concentration, is shown in Fig. 1. From the slopes of these curves evaluated at t = 0, the initial value of the oxygen uptake, R~2, was computed and plotted in Fig. 2 as a function of catalyst concen- tration. In the same figure the p-xylene conversion values reached in the corresponding experimental runs at t = 60 min are also reported. It is seen that as the catalyst concentration increases both quantities first increase and then reach a plateau. Note that this behavior is consistent with that reported in the litera- ture for p-xylene oxidation to p-toluic acid catalyzed by cobalt acetate and sodium bromine in acetic acid as solvent (cf. Zaidi, 1986). In order to demonstrate that all experiments were conducted under the kinetic regime, we have performed the experimental runs 1 - 3, where only the temperature values have been changed with respect to run 3. The corresponding initial values of oxygen uptake are shown in Fig. 3 as a function of temperature. From the linear behavior of the Ar- rhenius-like plot shown in Fig. 3, it may be concluded that the reactor operates in the chemically controlled regime for the specific catalyst concentration used for runs 1-3, and, consequently, for all catalyst concen- tration levels considered in this work, at which equal or lower oxidation rates are found (cf. Fig. 2). The apparent activation energy is equal to about 18 kcal/mol, i.e. a typical value of oxidation reactions, which compares fairly well with those reported in the literature for p-xylene oxidation in the presence of water (cf. Hronec and Ilavsky, 1982; Hronec et al., 1985; Hronec and Hrabe, 1986). As expected, the apparent activation energy is also very similar to the one obtained by Cao et al. (1994a) under the operat- ing conditions corresponding to run 6 of the present work using the same experimental set-up and proced- ure. The values of the kinetic constants appearing in the lumped kinetic scheme (2) are then estimated by fitting the time evolution of the experimental product composition of run 1-8 in Table 1 through a non- linear least-squares procedure using the model given by eqs (3) and (4). The comparison between model results and experimental data is shown in Figs 4(a)-(h), where it may be seen that the obtained agree- ment is generally satisfactory. This allows us to con- clude that the lumped kinetic scheme proposed by Cao et al. (1994a) retains a level of process description detailed enough to characterize the distribution of the most important products not only for a specific value of the catalyst concentration, as demonstrated by Cao et al. (1994a, b) by varying reaction temperature and initial concentration of liquid reactants, but also when changing catalyst concentration levels within a

0 5

- - 0 2

0 {I"

I I •

i •

I

• 0

0

Catalytic oxidation of p-xylene

• Run4 0 Run 5 • Run 6

40 6 0 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.

O 0

0

• • • •

0 0 0 0

• R ° o :

O p - x y l m ~ c t m v e r ~ o n

i r , i . t . i . i • i 0 t l~ 0 5 i c 15 2 0 2 5 ~ o a ~

C c . , t [ r r lo l /k l~ / ] x 1() ]

0 o5

=

5

Fig 2. Initial uptake rate of oxygen, R°2, and p-xylene conversion at t = 60 min as a function of catalyst concentra-

tion: runs 3-8 in Table I.

D " [ " , ' , ' , " ,

i,

it. .

\

• L . i . l . i . , . , . i

2 .~o 2~5 2.6o 26s 2.70 2 7~ 2,~o 2s~ i / r [ K I [ x 1 0 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, as- sumed 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

4209

17.2 kcal/mol which is in good agreement with those reported in the literature (cf. Bang and Chandalia, 1974 for methyl-p-toluate oxidation; Chen et al., 1985 for o-xylene oxidation). Similar plots are obtained for the reactivity ratios pj = k f ik l , 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 concentra- tion 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 cor- responding 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 ig 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 demon- strated {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 avail- able in the literature for the consumption rate of substrate and oxygen during cobalt catalyzed oxida- tion 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 cobal- tic and the cobaltous ion concentration, according to the presence of a monomer-d imer equilibrium of co- baltic acetate. However, since detailed radical chain mechanisms cannot be used for simulating gas liquid reactors, as discussed in the introduction, the estab- lished 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-precipita- tion of the latter one and the 4-carboxybenzaldehyde. The model equations (3)-I 14) contain a large number of physico-chemical parameters, which, unfortunate- ly, 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 120C and a catalyst concentration equal to 1 × 10 3 mol/kg~. To overcome this difficulty and with the aim of theoretically demonstrat ing that the

4210 A. Cincott i et al.

020

O~

O~

~°~ ~ ° D I

O 08

0.04

0 02,

0 00~

' ' " ' ' ' " ( "a) " '

o p-xylene conversion [] p-mlmldehyde O p4o~c A p401~

moddfit

n ~ r ~ ~ ~ n ~ [ ]

¢ ¢

l . I , I , I , i

~0 DO ~ 200 2 ~ ~

Tune {mini

1 os [- O p-x'yla~conv~Aon (b)

| o / I 0 p-toluicackl ^ /

~°~

Ol ^

0 . 0 4(I bO gO ~ [20 J40

TLme [ram]

05

0.4

~ 0 2

0

040

O35

{ ) 30

025

~ 020

" o~ ~ o~ r

005

05

04

" i 0.2

0c ~

, | ! , i

0 p -xy k n e convm~ion (c)

o p-totualdd~yde / A 0 p-tOhaic acid A p-tOhfic ~ / . ~

20 30 40 5O 6O

Time [mini

i , w , 1 , i i

O p-xylene oonversi(m (e) o pqo~kid~yd~ / 0 p-tOl~C~td

I0 20 30 40 ~) 60

Tune [rain]

i i i i

0 p-xylene o~version (g) 0 p - t o h ~ d e 0 p-toluic acid ~ /X p-tOhfic aloohol

modd fit

I ) 20 30 40 50 60

Tm-,e lminl

0 5

04

i 02

Ol

o o:

• i . i - i , i , ,

o p-xylene conv~sion (d) - o p-to~aklehyde / 0 pltOluic acid

20 40 60 80 1OO RO ¼0

Time [mini

,

"~ 02

o~

0 p-Xylene oonv~rsion (f) n p4oluddehyde 0 p -toluic acid O/ / / A pqolt~ak~ho4

IO 20 30 40 N) 6U

"rum [nunl

LO i , , • , ' , '

' 0 p -x-y kne cmvo'sion (h)

o s ~- D p-tolualddlyde J I o p - t a , ~

~ [ ~ p - t a ~ a o a ~ /

6

o 2 0 40 60 80 IOO 120

Tune [mini

Fig. 4. Ca••u•atedvsexperimenta•va•ues•fpr•ductc•ncentrati•nandp-xylenec•nversi•nasafuncti•n•f t ime for var ious t empera tu res and catalyst concentra t ions: runs I(a)--8(h) in Table 1.

Catalytic oxidation of p-xylene 4211

Table 4. Parameter values for the model given by eqs 13)-(14)

n (a~

- ' ~ Parameter Value

('~ 6 x 1(1-6 mol..'kg~ ) ~ C* 6 x 10 -~' mol:'kg~*

_E ~ C'? 4.0 mol.'kg~ - " C ° 0 . 1 1 mol kg[

~ , qs 2.0* ~ ~ ~-h, 1.0"

k~ 2.118x10 . s ~ ' k: 8.56x 10-4. s '

I I I I t I i I , I 250 2 55 260 265 270 2.75 2 8 0 2.85 k 3 4 . 9 4 × 10 -4 s '

I/TIK.I] x 103 k 4 1.6x 10 5. 's ' ks 7.5 x 10-%s' k~, 1.83 x 10-4.s *

' k , 6.68 x 10 ' a s ' no~ (b) k~ 1.8 x 10 ""s'

k,~ 4.83 x 10 -~' s ' 11- - - - -41 . • O----------& ..... kto 1.65 x 10-'L's '

=.~ O kq.s 8.0 x 10 " ~* • " kq.~, 8,0 x 10 ~o.

• P: k,.5 6.0 x 10 '~* .~ 0 p~ k,,.6 5.0 x 104. ,~ • p.~ Lt).5 1.0x 10-:m~* " Lo.~> 1.0 × 10 "" m,*

• ........ ~ fi~. s 9360 mol.'m 3: fi~.. 9350 mol..m~

I I I I , I I I

50 2 55 260 265 270 2.75 2 8O :.85 *See text for c o m m e n t s .

l[rlK-q x 103 'From Cao et al. 11994b). :From Lange 11973).

Fig. 5. Arrhenius plots for the reaction rate constant k~ (a) 'From Perry and Green 11984). and reactivity ratios Ps, P3 and P4 (b) of the lumped kinetic

scheme (2) at C,~, = 5.0 x 1 0 " mo ..'kg~. mode l deve loped p roper ly accoun t s for the interac-

t ion be tween chemical reac t ion and p rec ip i t a t ion phe-

n o m e n a , we cons ide r the mode l pa r ame te r s sum-

Table 2. Numerical values of the Arrhenius parameters of mar ized in Table 4. N o t e that some of them, i.e. • * and Lo 6, are taken f rom the l i tera ture the reactivity ratios for the reactions of the lumped kinetic C5, Co , Lo.5

scheme (1) at C:,, = 5 x 10 4 mol/kgl, pj = kj,.'kl, where (cf. Wachi and Jones , 1991) even if they do no t refer

k t = .4~ e x p ( - E ~ . R T )* specifically to our system. F o r the pa rame te r s re la ted to the l inear g rowt h and n u m b e r nuc lea t ion rates, we

Reaction pj (110 Ct t AEj = E j - E~ select the values r epor ted in Table 4 so that the

{kcal/mol) quant i t ies a b o v e fall wi thin the qui te large range re-

2 46.27 0.83 po r t ed in the l i tera ture for reac t ion prec ip i ta t ion sys- tems (cf. Gars ide , 1980). 3 34.46 4.67

4 ('1.871 6.21 The t ime behav io r of the species p-xylene, p- toluic a lcohol , p - to lua ldehyde , t e reph tha l ic a ldehyde , p-tol-

• At = 3,620.000/min; E~ = 17.2 kcal/mol, uic acid and 4 -ca rboxybenzy la l coho l , is not r epo r t ed "pf lT ) = p~ 1110 C) exp[ - AEi(1.'T - 1/383); R]. since they do not unde rgo p rec ip i t a t ion 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 kl (per min) x 10'* P: Ps P4 ~1 (%1

1.67 4.38 71.2 59.8 1.6 6.5 2.67 7.04 69.2 71.4 1.8 4.2 5.0 11.3 45.7 35.0 1.1 1.1

10 12.7 39.2 27.4 0.9 2.7 15 l l . l 41.8 41.6 1.1 2.2 33.3 11.0 44.7 45.0 1.1 4.6

4212

" L

o_ =

8

f:.

, , " ~ . . . . - • -

,.' . •

;, , L ,

2

A. Cincotti et al.

010 ~"

O 0 5

i o ~ ~

Fig. 6. Comparison between theoretical 4-carboxybenz- aldehyde 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-carboxybenz- aldehyde 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 ac- counted for by the model lies below those correspond- ing to the case when precipitation is not considered. B~ In particular, 4-carboxybenzaldehye displays the same qualitative behavior in both cases. This is be- (?'c~t cause it behaves as an intermediate species and thus it C, is expected to first increase up to a maximum value and then decrease when consumption rate prevails. C*

The latter one is clearly enhanced when precipitation also occurs. When considering the terephthalic acid Ej behavior, it is seen that its concentration increases, eventually reaching a plateau if precipitation phe- 9~ nomena are not accounted for, since it is the only non-reacting species according to the kinetic scheme G~ (1). On the other hand, by taking precipitation into account in our model, terephthalic acid is not only G~ formed by chemical reaction but also precipitates simultaneously, when its solution concentration ex- ko.i ceeds the corresponding saturation value, so that the concentration profile shows a maximum in time. kj

Although the proposed model is able to properly k,.~ simulate the time course of the crystal size distribution of 4-carboxybenzaldehyde and terephthalic acid, we do j~ not report the corr~ponding results in this work for lack of knowledge of the appropriate model para- L meters rclated to the precipitation phenomena in- Lo.~ volved in our system. Suitable experimental studies are certainly needed in order to evaluate these parameters. ~,~

N~

NR

C O N C L U D I N G R E M A R K S

In this paper, we present both experimental and theoretical work related to the catalytic liquid-phase oxidation of p-xylene.

In the experimental part, the effect of catalyst con- centration on product distribution and kinetic con- stants 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 de- scription detailed enough to characterize the distribu- tion 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. p- tolualdehyde, 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-carboxybenzal- dehyde and terephthalic acid, is developed, it is shown that the time profiles of these species display a max- imum whose value depends upon the relative magni- tude 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 tereph- thalic acid. The problem of accounting for intra- and interphase mass transfer resistances when simulating precipitation phenomena occurring in complex reac- tion systems of the type considered in this work is currently being investigated.

N O T A T I O N

mass-based nucleation rate of the ith pre- cipitating 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 j th reaction, cal mol- t order of growth of the ith precipitating com- ponent linear growth rate of the ith precipitating component, m, s- 1 mass-based growth rate of the ith precipita- ting component, mol kg/~ s- growth rate constant of the ith precipitating component rate constant for the jth reaction, min- t

nucleation rate constant of the ith precipita- ting component number nucleation rate of the ith precipita- ting component, kg~- t s

coordinate of particle dimension, ms effective nucleic dimension of the ith precipi- tating 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 reactivity ratio, k~,.'k~ r~ rate of the jth reaction, mol kg{ ~ min R{)): overall initial oxidation rate, molkgi j

rain t time, s T temperature, K Uo: overall oxygen uptake, tool kg[

Greek letters AEj E~ - Et, cal mol- ~1 average percentage error rj., stoichiometric coefficient of the ith compon-

ent in the jth reaction t~.~ crystal density of the ith precipitating com-

ponent, mol m~ ~ r dimensionless time, t k~

Subscripts i component index j reaction index I liquid phase s solid

Superscript 0 initial conditions

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