Semicontinuous Studies on the Reaction Mechanism and Kinetics for

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Semicontinuous Studies on the Reaction Mechanism and Kinetics for the Liquid-Phase Oxidation of p-Xylene to Terephthalic Acid† Qinbo Wang,* Youwei Cheng, Lijun Wang, and Xi Li Department of Chemical Engineering, Zhejiang UniVersity, Hangzhou, 310027 Zhejiang, People’s Republic of China

The reaction mechanism and kinetics for the liquid-phase catalytic oxidation of p-xylene to terephthalic acid are briefly reviewed, which indicates that the available literature is not complete when one looks for the industrially applicable mechanism and kinetics. In this work, a detailed radical chain reaction mechanism for the liquid-phase catalytic oxidation of p-xylene to terephthalic acid is proposed. Using several assumptions, a simple fractional-like kinetic model is derived from the assumed reaction mechanism. Several semicontinuous oxidation experiments are carried out at different reactant feed rates and temperatures. The experimental results show that the measured concentrations of liquid-phase reactants increase approximately linearly with time, which indicates that the reaction rate for the liquid-phase reactants is approximately independent from its concentration. The derived kinetic model can explain and correlate these experimental results satisfactorily. By combining the results determined in batch oxidation experiments with the semicontinuous experimental results, the kinetic model parameters are determined by data fitting. The correlated results generally agree with both the semicontinuous and the batch experimental results satisfactorily. Introduction As one of the most important aromatic compounds, terephthalic acid (TA) is widely used in organic synthesis, particularly in the polyester industry. Worldwide production of TA has increased at an annual rate of >10% for the last 5 years, and the demands of TA have exceeded 30 million ton in the last year.1 Commercially, the majority of TA is produced by the air oxidation of p-xylene (PX) in acetic acid in the temperature range from 150 to 210 °C, catalyzed by cobalt and manganese salts and promoted by bromine.2-4 To gain a better insight into the reaction mechanism and identify the effects of different parameters on the progress of the oxidation process, it is essential to study the reaction kinetics. Further, the rational design, optimization, control, and analysis of the oxidation of PX to TA process also require the knowledge of the reaction kinetics.2-4 The liquid-phase catalytic oxidation of PX to TA belongs to the kind of classical free radical chain reaction. Researchers such as Kamiya,18 Hendriks,19 Jones,20-22 Hronec,23,24 Partenheimer,7,25-27 Suresh,6,28 Harustiak,29-31 Wang,4,8 Cheng,10,11,13 etc. had studied the mechanism of PX to TA for many years. In the oxidation of PX to TA, a mixture of cobaltic, manganic, and bromide salts are used as catalyst species. Until now it is commonly accepted that, under the reaction conditions employed, Mn(III) and Co(III) are not powerful enough to initiate the free radical reaction by electron-transfer mechanism. The combined effect of Co and Mn ions is only to decompose hydroperoxides to yield free radicals. The initiation mechanism is predominantly hydrogen abstraction from methyl groups by bromine atoms. The Mn and Co ions oxidize the bromine ions formed to bromine atoms, thus ensuring the availability of bromine atoms for initiation.5 Although different in some specific chain reaction, * Address correspondence to Dr. Wang. Phone: +86-571-87952210. Fax: +86-571-87951227. E-mail: [email protected]. † This paper is partly abbreviated from the Ph.D. dissertation of Qinbo Wang.

the following general free radical chain reactions involving the initiation, propagation, and termination steps are commonly accepted2-34

M3+ + Br- f M2+ + Br• Br• + RCH3 f RCH2• + H+ + BrRCH2• + O2 f RCH2O2• RCH2O2• + M2+ f RCHO + M3+ + OHRCHO + M3+ f RCO• + M2+ + H+ RCHO + Br• f RCO• + Br- + H+ RCO• + O2 f RCO3• RCO3• + RH f RCO3H + R• RCO3H + RCHO f 2RCOOH RCO3H + M2+ f RCO2• + M3+ + OHRCO2• + RH f RCOOH + R• Ii• + Ii• f IiIj where M denotes metal catalyst such as Co or Mn, R denotes aromatic reactants, and I denotes radicals produced. Here, the initial step is thought to be the generation of Br• radical and the hydrogen abstraction illustrated in the first two reactions. The propagation step consists of oxidation of the substrates by molecular oxygen, metal ion, and Br• radical, and reduction of radicals and hydroperoxides by metal ion. The termination step consists of reactions between two radicals. The formulation of detailed kinetic models of the complex process that describes all the involved radical chain reactions and all the intermediate products is not desirable in practice.

10.1021/ie0615584 CCC: $37.00 © 2007 American Chemical Society Published on Web 11/13/2007

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Figure 1. Lumped kinetic scheme for the oxidation of p-xylene to terephthalic acid.

This is mainly because the estimation of the kinetics parameters of the radical chain reactions by fitting of the experimental data cannot be performed when the concentrations of the participating components (that is, the radical species in the liquid phase) cannot be measured. In order to lower the computing efforts, the most common approach is to lump the detailed mechanism into a set of global reactions that involve only molecular species, whose concentration can be, in principle, easily monitored as a function of time.35-38 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 the liquid-phase catalytic oxidation process of PX to TA have been developed in the literature.2-16,35-40 By accounting for the most important intermediates and final products of the process, i.e., p-tolualdehyde (TALD), p-toluic acid (PT), 4-carboxybenzaldehyde (4CBA), and TA, Wang and co-workers recently proposed a lumped kinetic scheme shown in Figure 1 for the liquid-phase oxidation of PX catalyzed by cobalt acetate and manganese acetate, promoted by hydrogen bromide, which may have important practical applications in the production of TA,2,12,13,15,16 where the reactions of PX to TALD and PT to 4-CBA involve the addition of 1O2 and the removal of 1H2O, and the reactions of TALD to PT and 4-CBA to TA involve the addition of 1/2O2. By PX batch added into the oxidation system, the workgroup of Cao35-38 studied the air oxidation of PX in methyl benzoate in the temperature range from 80 to 130 °C, catalyzed by cobalt naphthenate, and promoted by TALD. On the basis of the assumption that all the reactions are zeroth order with respect to gaseous reactant and first order with respect to liquid reactants, a lumped kinetic model for the liquid-phase oxidation of PX to TA was presented. However, the experimental conditions differed from the currently adopted industrial operation condition greatly; furthermore, the kinetic models are completely empirical but not derived from the oxidation mechanism. From different simplified radical chain-reaction mechanisms, different kinetic models were derived by Scott,39 Kamiya,18 Hronec,23,24 Suresh,6 Jones20-22, Partenheimer,25-27,40 etc. However, the same as that of the workgroup of Cao,35-38 most of these studies were studied at conditions far away from the industrial operation condition. These defects made these experimental results unable to be used for practical use. Zhou41 and Wen42 studied the PX oxidation process at high temperatures. The experimental conditions covered the range that might be adopted in the high-temperature oxidation process. Also, based on the assumption that all the reactions are zeroth order with respect to gaseous reactant and first order with respect to liquid reactants, a lumped kinetic model for the liquid-phase oxidation of PX to TA was presented and the effects of temperature, catalyst concentration and ratio, and oxygen pressure on the reaction rate were determined. However, the established experimental technique for temperature controlling, sampling, and concentration determination for liquid-phase compounds was insufficient. As reported by Zhou41 and Wen,42 the experimental temperature fluctuated with the amplitude of (20 K, and some of the liquid reactant crystallized and some

of the solvent vaporized during sampling. These all directly affected the accuracy and credence of these experimental results. Furthermore, a first-order kinetic model, which is empirical but not derived from the reaction mechanism, was used to correlate the experimental results. From the above brief review, it can be said that, although many public reports dealt with the subject on the oxidation mechanism and kinetics of PX to TA, the available literature is still not complete when one looks for a detailed and complete mechanism and industrially applicable kinetics.5-7 There exists a scope and a need to further study the mechanism and kinetics of this reaction. In recent years, the workgroup of Li carried out some batch experiments on the oxidation of PX to TA under the industrial oxidation conditions. The experimental results showed that the reaction rate varied insignificantly with the concentration of liquid reactants, and the apparent reaction order was ,1, which was apparently inconsistent with the widely accepted first-order assumption.2-4,8-17,43-46 When the catalyst cycle and interaction of radicals were considered, the following 28-parameter kinetic model was proposed by Wang and Cheng based on a complicated radical chain-reaction mechanism3,4,45

rj )

kjcj (d1jc1 + d2jc2 + d3jc3 + d4jc4 + j)βj

j ) 1-4 (1)

where kj is the reaction rate constant and is affected by temperature, catalyst concentration and ratio, etc. The model parameter j represents a modification of the kinetics considering the effects of other side reactions and chain-termination reactions. When the kinetics model (eq 1) was used to simulate the industrial PX oxidation reactors at high temperature, the simulated results agreed well with the plant field data. However, the literature-reported first-order reaction kinetics completely failed to predict the special features of high conversion (>99.5%) and high yield (>97%) in an industrial continuous stirred tank reactor.2-4,45,47 By using a specially designed experimental technique, the oxidation of PX to TA was completely studied, and a great deal of experimental data was obtained.2-4,8-17,43-46 These batch studies disclosed the reaction mechanism and quantitatively determined the effects of various reaction conditions on the reaction rate, which was practically and theoretically valuable. Recently, a new semicontinuous oxidation apparatus was set up by Wang, and some oxidation experiments were performed.2 Comparison of the semicontinuous experimental results and the predicted results by eq 1 with the model parameters determined from batch experimental results is shown in Figure 2, in which apparent disagreement appears.2 It shows that the overly complex model eq 1 still fails to describe the semicontinuous system that it has not already been “fit” to, e.g., it appears to have no predictive powers at all. There might be two reasons. First, the model parameters in eq 1 were regressed from the batch experimental results. However, in batch experiments, the concentrations of liquid reactants were much higher than those in semicontinuous experiments. When the cumbersome kinetic model eq 1 determined at higher concentration was extrapolated to predict the semicontinuous experimental results, some uncertainty existed and discrepancy might occur. Second, the model parameters in eq 1 are too many to be uniquely determined by common data-regressing method, which resulted in a nonuniqueness in model-parameters determination.2 Because of this uncertainty and nonuniqueness, when eq 1 was used to simulate the industrial oxidation reactor in our previous

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Ind. Eng. Chem. Res., Vol. 46, No. 26, 2007 k3

Mn(III) + Br- 98 Mn(II) + Br•

Figure 2. Comparison of the predicted results by eq 1 and the semicontinuous experimental results: scatter, semicontinuous experimental results; line, predicted results by eq 1 with with model parameters determined from batch experimental results.

works, some minor modifications were made for different oxidation processes.2 A more simple and practical kinetic model is required to be derived from the radical chain-reaction mechanism, and the model parameters also needed to be uniquely redetermined by using both the batch and semicontinuous experimental results. This is just the aim of the present work. As an extension of our kinetic studies on the oxidation of PX to TA in the aqueous acetic acid system, the present work consists of two parts. In the first part, a radical chain-reaction mechanism will be proposed and compared with the previously proposed oxidation mechanism in detail. On the basis of the proposed mechanism, a simpler kinetics model than eq 1 will be derived. In the second part, semicontinuous studies on the oxidation process will be discussed. On the basis of the presently obtained semicontinuous experimental results and the previously obtained batch experimental results, the model parameters in the proposed kinetics will be determined by data fitting. Reaction Mechanism and Kinetic Model Reaction Mechanism. Elucidation of the reaction mechanism helps in selecting more efficient catalysts, optimizing the reaction intermediates, selecting reaction conditions, and designing a commercial reactor. This process also generates ideas for further research and development.5 The oxidation of hydrocarbons is usually explained in terms of the classical free radical chain mechanism involving the initiation, propagation, and termination steps.5 Catalyzed oxidation of PX by cobalt salts is characterized by an induction period in which cobalt(II) ions are oxidized to Co(III). Monomeric Co(III) ion is a powerful oxidizing agent when it is surrounded by O-donor ligands such as water, OHions, or RCOO- ions.5 When Mn and Br are added to the catalytic system, Mn(III) and Co(III) are not powerful enough to initiate the free radical reaction by electron-transfer mechanism. The combined effect of Co and Mn ions is only to decompose hydroperoxides to yield free radicals. The initiation mechanism is predominantly hydrogen abstract from methyl groups by Br• radical. The Mn and Co ions oxidize the bromine ions formed to Br• radical, thus ensuring the availability of bromine atoms for initiation.5 These reactions can be illustrated by the following:3,4,7,45 k1

Co(III) + Mn(II) 98 Co(II) + Mn(III) k2

Co(III) + Br- 98 Co(II) + Br•

(2) (3)

(4)

Br• radical is the reaction initiator, and the chain propagation and termination reactions from PX to TA can be illustrated by the following reactions. To simplify the treatment, some assumptions are necessary:23 (a) there is no inclusion of RCH2O• and •OH radicals in the propagation and termination steps; (b) chain termination takes places by reactions of Br• radical and other active radicals, and others radicals are not allowed to undergo termination reactions. Oxidation of PX to TALD. Br• radical initiates the oxidation of PX by hydrogen abstraction from PX, which can be shown as3,4,7,45 k4

Br• + CH3-Ph-CH3 98 CH3-Ph-CH2• + H+ + Br-

(5)

where Ph denotes the benzene ring. The generated CH3-PhCH2• radical has high reactivity and can be combined with molecular oxygen quickly to generate CH3-Ph-CH2O2• radical by the reaction k5

CH3-Ph-CH2• + O2 98 CH3-Ph-CH2O2•

(6)

and then the following six parallel radical reactions may occur: k6

CH3-Ph-CH2O2• + Co(II) + H+ 98 CH3-Ph-CHO + Co(III) + H2O (7) k7

CH3-Ph-CH2O2• + Mn(II) + H+ 98 CH3-Ph-CHO + Mn(III) + H2O (8) CH3-Ph-CH2O2• + Co(II) + H+ f CH3-Ph-CH2OOH + Co(III) (9) CH3-Ph-CH2O2• + Mn(II) + H+ f CH3-Ph-CH2OOH + Mn(III) (10) CH3-Ph-CH2O2• + RCH3 f CH3-Ph-CH2OOH + RCH2• (11) CH3-Ph-CH2O2• + HAC f CH3-Ph-CH2OOH + (AC)• (12) The hydroperoxide CH3-Ph-RCH2OOH generated by reactions 9-12 is unstable and may be decomposed by Co(II) or Mn(II) through the following two reactions

CH3-Ph-CH2OOH + Co(II) + H+ f CH3-Ph-CH2O• + Co(III) + H2O (13) CH3-Ph-CH2OOH + Mn(II) + H+ f CH3-Ph-CH2O• + Mn(III) + H2O (14) Partenheimer reported that the rates of reactions 7 and 8 were much faster than those of reactions 9-12, which resulted in most of the methyl group being oxidized into aldehyde group directly, and the radical reactions 9-14 could be neglected7,25-27 when only the main reactions were considered. In this sense, the oxidation of PX to TALD can be expressed by the radical

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chain reactions 2-8 and the major radicals are Br•, CH3-PhRCH2•, and CH3-Ph-RCH2O2•. The major chain-termination reactions are those between two radicals.19,24 Because Br• radical acts the most important role in chain initiation and propagation, here only the following reactions between Br• radical and other radicals are considered as the chain-termination reactions: k8

Br• + CH3-Ph-CH2• 98 CH3-Ph-CH2Br

(15)

k9

Br• + CH3-Ph-CH2O2• 98 CH3-Ph-CH2O2Br (16) Oxidation of TALD to PT. The aromatic aldehyde group also can be oxidized to the corresponding peroxide radical RCO3• in the same manner as that of the methyl group, and it then reacts with divalent metal ion to generate aromatic peracid RCOOOH.3,4,45 The specific reactions may be as follows:

k17

Br• + HOOC-Ph-CH3 98 HOOC-Ph-CH2• + H+ + Br- (24) k18

HOOC-Ph-CH2• + O2 98 HOOC-Ph-CH2O2• k19

HOOC-Ph-CH2O2• + Co(II) + H+ 98 HOOC-Ph-CHO + Co(III) + H2O (26) k20

HOOC-Ph-CH2O2• + Mn(II) + H+ 98 HOOC-Ph-CHO + Mn(III) + H2O (27) k21

Br• + HOOC-Ph-CH2• 98HOOC-Ph-CH2Br

(28)

k22

Br• + HOOC-Ph-CH2O2• 98

k10

HOOC-Ph-CH2O2Br (29)

Br• + CH3-Ph-CHO 98 CH3-Ph-CO• + H+ + Br- (17) k11

CH3-Ph-CO• + O2 98 CH3-Ph-CO3•

(18)

k12

CH3-Ph-CO3• + Co(II) + H+ 98 CH3-Ph-COOOH +Co(III) (19) k13

CH3-Ph-O3• + Mn(II) + H+ 98 CH3-Ph-COOOH + Mn(III) (20) Different from the oxidation of the methyl group, no reactive C-H bond exists in the aldehyde group, which makes the direct reduction of peroxide radical CH3-Ph-CO3• by metal ions to the corresponding aromatic acid impossible. It indicates that similar reactions such as 6 and 7 will not occur in the oxidation of the aldehyde group. However, the reducibility of the aldehyde group is strong and the following self-catalyzed reaction between aldehyde group and aromatic peracid may occur to generate aromatic acid:32,48 k14

CH3-Ph-COOOH + CH3-Ph-CHO 98 2CH3-Ph-COOH (21) The oxidation of TALD to PT can be expressed by the radical chain reactions 17-21, and the major radicals and peracid in the oxidation of TALD to PT are Br•, CH3-Ph-RCO•, CH3Ph-CO3•, and CH3-Ph-COOOH. Here also, only the following reactions between Br• radical and other radicals are considered as the chain-termination reactions: •

(25)

•

k15

Br + CH3-Ph-CO 98 CH3-Ph-COBr k16

Br• + CH3-Ph-CO3• 98 CH3-Ph-CO3Br

(22) (23)

Oxidation of PT to 4-CBA. Similar to the oxidation of PX to TALD, the oxidation of PT to 4-CBA also belongs to the oxidation of methyl group to aldehyde group, and the following radical chain reactions can be determined according to that of reactions 2-8, 15, and 16:

Oxidation of 4-CBA to TA. Similar to the oxidation of TALD to PT, the oxidation of 4-CBA to TA also belongs to the oxidation of aldehyde group to carboxy group, and the following radical chain reactions can be determined according to that of reactions 17-23 k23

Br• + HOOC-Ph-CHO 98 HOOC-Ph-CO• + H+ + Br- (30) k24

HOOC-Ph-CO• + O2 98HOOC-Ph-CO3•

(31)

k25

HOOC-Ph-CO3• + Co(II) + H+ 98 HOOC-Ph-COOOH + Co(III) (32) k26

HOOC-Ph-CO3• + Mn(II) + H+ 98 HOOC-Ph-COOOH + Mn(III) (33) k27

HOOC-Ph-COOOH + HOOC-Ph-CHO 98 2HOOC-Ph-COOH (34) k28

Br• + HOOC-Ph-CO• 98HOOC-Ph-COBr (35) k29

Br• + HOOC-Ph-CO3• 98HOOC-Ph-CO3Br

(36)

There must be many other chain reactions that are not considered in the above mechanism, but the radical chain reactions 2-36 might be expected to approximately represent the reaction mechanism for the oxidation of PX to TA. Kinetic Model. The oxidation of PX to TA is a typical consecutive reaction; coupling effects exist among reactants, radicals, and catalyst. Rounded consideration of these interactions is necessary for the mechanistic kinetic model establishment. According to the mechanism illustrated by reactions 2-36, the reaction rates for the major reactants PX, TALD, PT, and 4-CBA may be determined as

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d[PX] ) -r1 ) -k4[PX][Br•] dt d[TALD] ) r1 - r2 ) r1 - [TALD](k10[Br•] + dt k14[CH3-Ph-COOOH]) d[PT] ) r2 - r3 ) r2 - k17[PT][Br•] dt

When the steady-state approximation is used for radicals and aromatic peracids produced in the radical chain reactions, the concentrations for these radicals and peracids from the mechanism presented above lead to the expressions

(k2[Co3+] + k3[Mn3+])[Br-] ) (k4[PX] + k10[TALD] + k17[PT] + k23[4-CBA] + [I•])[Br•] k4[PX][Br•] ) (k5[O2] + k8[Br•])[CH3-Ph-CH2•] +

•

•

k7[Mn ][H ] + k9[Br ])[CH3-Ph-CH2O2 ] k10[TALD][Br•] ) (k11[O2] + k15[Br•])[CH3-Ph-CO•] k11[O2][CH3-Ph-CO•] ) (k12[Co2+][H+] + k13[Mn2+][H+] + k16[Br•])[CH3-Ph-CO3•] +

•

+

(k12[Co ][H ] + k13[Mn ][H ])[CH3-Ph-CO3 ] ) k14[CH3-Ph-CO3H][TALD] 2+

k4[PX][Br•] k5[O2]

•

[CH3-Ph-CO ] )

k18[O2][HOOC-Ph-CH2•] ) (k19[Co2+][H+] +

[CH3-Ph-CO3•] )

k11[O2] k10[TALD][Br•] k12[Co2+][H+] + k13[Mn2+][H+]

[HOOC-Ph-CH2•] )

k17[PT][Br•] k18[O2] k17[PT][Br•]

•

[HOOC-Ph-CH2O2 ] )

[CH3-Ph-CO3H] )

k19[Co2+][H+] + k20[Mn2+][H+]

k23[4-CBA][Br•] k24[O2]

[HOOC-Ph-CO3•] )

k23[4-CBA][Br•] k25[Co2+][H+] + k26[Mn2+][H+]

k10[Br•] k14

[HOOC-Ph-CO3H] )

k20[Mn2+][H+] + k22[Br•])[HOOC-Ph-CH2O2•] •

k6[Co2+][H+] + k7[Mn2+][H+]

k23[Br•] k27

(39)

It is shown in eq 39 that the concentrations of all the radicals and peracids are linearly related to the concentration of Br• radical, and the total concentration of radicals can be expressed as

k17[PT][Br•] ) (k18[O2] + k21[Br•])[HOOC-Ph-CH2•]

•

k4[PX][Br•]

k10[TALD][Br•]

[HOOC-Ph-CO•] )

k5[O2][CH3-Ph-CH2•] ) (k6[Co2+][H+] +

2+

[CH3-Ph-CH2•] )

[CH3-Ph-CH2O2•] )

d[4-CBA] ) r3 - r4 ) r3 - [4-CBA](k23[Br•] + dt k27[HOOC-Ph-COOOH]) (37)

2+

negligible. By this assumption, the items representing termination reactions in eq 38 can be neglected. By using these two assumptions, eq 38 becomes

•

k23[4-CBA][Br ] ) (k24[O2] + k28[Br ])[HOOC-Ph-CO ] k24[O2][HOOC-Ph-CO•] ) (k25[Co2+][H+] + k26[Mn2+][H+] + k29[Br•])[HOOC-Ph-CO3•] (k25[Co2+][H+] + k26[Mn2+][H+])[HOOC-Ph-CO3•] ) k27[HOOC-Ph-CO3H][4-CBA] (38) where [I•] ) ∑ ki[Ii•] represents the linear assembled total concentration of all the reactive radicals that are generated in the oxidation process. Theoretically, by coupling eq 38 with eq 37, the detailed mechanistic kinetic models can be obtained. However, the kinetic models obtained in this manner are too complex and difficult to be used practically. Some simplifications are necessary, and the following assumptions are made: (a) Since oxidation operates at high oxygen pressure, the concentration of oxygen in the liquid may be much higher than that for Br• radical, i.e., [O2] . [Br•]. (b) The products of chain-termination reactions 14-15, 2122, 27-28, and 34-35 are side products or byproducts, which are microcontent compounds referring to the main reaction products. The concentrations of byproducts are assumed to be

[I•] )

4

fjcj[Br•] ∑ ki[Ii•] ) ∑ j)1

(40)

By inserting eq 40 into the first equation in eq 38, the expression for the concentration of Br• radical can be obtained in the following form,

[Br•] )

1 λ1c1 + λ2c2 + λ3c3 + λ4c4 + 

(41)

where  is an approximate modification of [Br•]. By inserting eq 41 into eq 37, the simplified kinetic model for PX liquidphase catalytic oxidation to TA can be

ri ) kici[Br•] )

kici λ1c1 + λ2c2 + λ3c3 + λ4c4 + 

(42)

where ki is the reaction rate constant, and factors affecting ki are temperature, water content, and catalyst concentration and ratio. Comparing model eq 42 with the previously obtained model eq 1, we find that the number of model parameters needed to be regressed are greatly decreased from 28 to 9 at a certain operation condition, and the nine model parameters can easily be uniquely determined by data fitting.2 Mechanistic model eq

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42 is a radical competition model and reveals the following three aspects of mechanism. Br•

(1) Radical is a key factor to initiate the oxidation process, and its concentration determines the reaction rate directly. When the concentration of reactants increases, from eq 41 we can see that the concentration of radical Br• decreases, which restricts the acceleration of the reaction rate. When the concentration of reactants decreases, from eq 41 we can see that the concentration of radical Br• increases, which restricts the deceleration of the reaction rate. Because of the restriction effect of radical Br•, the change of apparent reaction rate is not sensitive with the change of reactants concentration. This unique characteristic is consistent with the large quantity of experimental data in our group.2-4,8-17,43-46 Further, based on model eq 42 and the determined model parameters, the simulation results for an industrial oxidation reactor agree well with the plant field data.2 (2) The oxidation rate is approximately independent with the oxygen concentration in the liquid, and the reaction is zeroth order with respect to the gaseous reactant. Only when the oxygen concentration in the liquid phase is low enough to influence the assumption of [O2] . [Br•] will the oxidation rate be affected by the oxygen concentration. This is consistent with most of the reported experimental results.14,35-38,49 (3) When the reactants concentration is low enough to be negligible compared with the model parameter  in eq 42, the kinetic model will be reduced to a first-order kinetic model, which is widely assumed in most of the research.35-38,41,42 Comparison of the Present Oxidation Mechanism and Kinetics with our Previous One. Our previous reaction mechanism reveals our previous comprehension and understanding on the PX oxidation process. Through several years of further study on this process, in this work, our previously proposed mechanism was complemented and perfected in the following aspects. (1) Effect of Metal Catalyst on the Initiation of the Oxidation. Initiation of the oxidation of alkyl aromatic hydrocarbons can be explained in terms of two mechanisms: (i) the electron-transfer mechanism in which electron transfer occurs from arene to a metal catalyst complex, producing an arene radical cation that in turn forms a benzyl radical by proton loss, and (ii) abstraction mechanism in which benzylic hydrogen is abstracted by bromine atoms, RO• radicals, or ROO• radicals. Mechanism I is more applicable to the oxidation process with no presence of promoter Br, and mechanism II is important in the Co/Mn/Br catalytic oxidation of PX.5 In the oxidation of PX to TA, mechanism I can be represented by the following reactions:

Co(III) + CH3-Ph-CH3 f Co(II) + CH3-Ph-CH2• + H+ Co(III) + HOOC-Ph-CH3 f Co(II) + HOOC-Ph-CH2• + H+ Mn(III) + CH3-Ph-CH3 f Mn(II) + CH3-Ph-CH2• + H+ Mn(III) + HOOC-Ph-CH3 f Mn(II) + HOOC-Ph-CH2• + H+

Co(III) + CH3-Ph-CHO f Co(II) + CH3-Ph-CO• + H+ Co(III) + HOOC-Ph-CHO f Co(II) + HOOC-Ph-CO• + H+ Mn(III) + CH3-Ph-CHO f Mn(II) + CH3-Ph-CO• + H+ Mn(III) + HOOC-Ph-CHO f Mn(II) + HOOC-Ph-CO• + H+ (43) In our previously proposed mechanism, besides mechanism II (represented by eqs 5, 17, 24, and 30), mechanism I (represented by eq 43) was also considered.8,45 In this work, eq 43 was omitted, which made the reaction mechanism simpler and more reasonable. (2) Generation and Decomposition of Peroxide. The peroxide CH3-Ph-RCH2OOH generated by eqs 9-12 is unstable and may be decomposed in two ways. One is the decomposition by Co(II) and Mn(II) through eqs 13 and 14, the other is the self-decomposition through the following reaction

CH3-Ph-CH2OOH f CH3-Ph-CH2O• + •OH (44)

In our previously proposed mechanism, the two decomposition ways have been considered. Furthermore, the formation and decomposition of HOOC-Ph-CH2OOH are also considered. Partenheimer7,25-27 and Sheldon and Kochi32 reported that the generating rates of CH3-Ph-CH2OOH and HOOCPh-CH2OOH were much faster than their decomposition rates, which resulted in most of the methyl group being oxidized into aldehyde group directly, and the decomposition reactions for CH3-Ph-CH2OOH and HOOC-Ph-CH2OOH could be neglected when only the main reactions were considered. In this sense, the oxidation of methyl group to aldehyde group can be substantially simplified, and also the number of radical species can be reduced. This simplification is also reasonable. (3) Radical Chain Termination. The major chain-termination reactions are those between two radicals.19,24 In our previously proposed mechanism, all the reactions between any two radicals are considered, which results in all kinds of byproducts. In this wok, the main reactions are concerned. Because Br• radical acts the most important role in chain initiation and propagation, here only the following reactions between Br• radical and other radicals are considered as the chain-termination reactions. This simplification also seems reasonable. Additionally, considering reaction 43, the above generation and decomposition reactions of peroxide, and the radical chain termination due to the reactions between any two radicals, we previously obtained the following kinetic model for the oxidation of methyl group and aldehyde group separately:

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Ind. Eng. Chem. Res., Vol. 46, No. 26, 2007 4

bici ∑ i)1

rm ) km

4

m ) 1, 3

cm

4

∑ ∑ai,jcicj i)1 j)1 4

4

(45)

4

bici + ∑∑ci,jcicj ∑ i)1 i)1 j)1

r a ) ka

4

4

ca

a ) 2, 4

∑ ∑ai,jcicj i)1 j)1 The kinetic model eq 45 is cumbersome to be used directly. Furthermore, the parameters in the model are too many to be uniquely determined by data fitting. In our previous work, we simplified eq 45 into eq 1, in which there are still as many as 28 model parameters that need to be determined. In this project, the reaction mechanism is further simplified, which directly results in the simpler kinetic model eq 42, in which only 9 model parameters need to be determined. The liquid-phase catalytic oxidation of PX to TA involves many complex radical chain reactions, and many uncertainties exist when dealing with these reactions. It is impossible to consider all the radical chain reactions that occurred in the oxidation process, and only the most important reactions are considered in the presently proposed mechanism. Although many assumptions are made during the derivation of the kinetic model eq 42, we find most of the experimental results can be well-explained by the model eq 42,2-4,8-17,43-46 which will be discussed in detail in the following sections. Lab Reactor Model In the reactor for the manufacturing of TA, PX is oxidized to TA in the liquid phase. Owing to the low solubility of TA in aqueous acetic acid, most of it precipitates as it forms. When TA is crystallized, the impurity 4-CBA will also incorporate into TA crystals. In this sense, the reactor for the manufacturing of TA has two functions: reaction and crystallization. In the present manuscript, we are just concerned with the reaction, and in a following manuscript, we will be concerned with the crystallization. The crystallization of TA has no effect on the reaction process; however, the incorporation of 4-CBA will affect the concentration of 4-CBA in solution, which will directly influence the reaction process. By taking into account only the formation of the molecular species that represents the most important intermediate and final product, the lumped kinetic scheme as shown in Figure 1 and the kinetic model of eq 42 is used. The description of the diffusion and reaction processes at the gas-liquid interface is neglected because of the elimination of mass transfer influence. Since all experiments are performed under the kinetic regime, the following mass balances are used,

dcj dt

4

)

νi,jrj + Fj/Mj - NLS,j ∑ i)1

j)1∼4

(46)

along with the initial conditions

cj ) cj,0

j ) 1-4

(47)

where cj and Fj are the concentration and feed rate of the jth component, and for batch operation, Fj ) 0. NLS,j is the

incorporation rate of the jth component and will be discussed in detail in a following paper. The meaning of other symbols is reported in the notation. The oxygen mass balance and the energy balance are not considered here since the reactor has been operated under isothermal conditions and all reactions are of zero order with respect to oxygen, because at the experimental oxygen partial pressure, no obvious effect of oxygen partial pressure on the reaction was found. The kinetic parameters are determined in a nonlinear optimization, minimizing the difference between the simulated and the experimental time evolution of the product composition of experimental runs 3-8 and that of previously obtained results in batch experiments. The fourth-order Runge-Kutta method is used to solve eq 46, and the simplex method is used in the nonlinear optimizations. The method is implemented in the Matlab Optimization Toolbox (The Mathworks). The simulation is written in Matlab to use its optimization routine. Experimental Setup and Procedure Apparatus and Procedure. The industrial reactor is a continuous reactor, into which the solvent, reactants of PX, and air are continuously fed and the reaction slurry, including TA product, is continuously discharged out. The operation of continuous reactor is steady state, and the concentrations of reactant PX, intermediates TALD, PT, and 4-CBA and the product TA in solution are very low. In our previous works, we studied the PX oxidation in a batch reactor, in which the liquid reactant PX was batch-added into the reactor, while the gaseous reactant air was continuously fed through the reactor. In this work, we further studied the PX oxidation in a semicontinuous reactor, in which the liquid reactant PX and the gaseous reactant air were all continuously fed into the reactor. However, in the batch and semicontinuous experiments, no slurry was discharged. The apparatus employed in this work appears in Figure 3. The 2.0 L reactor was constructed from titanium to resist corrosion and was designed for temperatures up to 533 K and pressures up to 60 atm. A paddle-type agitator with a turbine impeller is used for agitation. The system was equipped with three condensers in order to ensure complete condensation and recycle of the evaporated compounds. The pressure was controlled by a back-pressure valve to be 18 atm. A Pt100 thermal resistance thermometer was inserted into the vessel for the measurement of temperature, and the thermometers had an uncertainty of (0.1 K. In each experiment, TA seeds, solvents acetic acid and water, and catalysts Co/Mn/Br were batchdeposited in the reactor at room temperature. The slurry in the vessel was heated at 10 K/min to the experimental temperature. The temperature was maintained within (0.5 K of the desired temperature by a proportional-integral-derivative (PID) controlling system shown in Figure 3. When the temperature was higher than the desired point, the computer program would start the peristaltic pump, and cooling oil would flow through the spiral cooling coil in the vessel to cool the content in the reactor. When the temperature was lower than the desired point, the computer program would start the heating controlling circuit, and the reactor wall would be electrically heated. When the slurry temperature was heated to the experimental point, the liquid reactant PX and the gaseous reactant air were continuously fed into the reactor, and the oxidation began. The reaction slurry was sampled every 10 min. Each oxidation experiment lasted 90 min. To simultaneously determine the components content in the solution and solid, we separated the solution from the slurry at the experimental temperature by using the specially

Ind. Eng. Chem. Res., Vol. 46, No. 26, 2007 8987

Figure 3. Experimental apparatus for the liquid-phase catalytic oxidation of p-xylene to terephthalic acid: 1, nitrogen cylinder; 2, air cylinder; 3, mass flowmeter; 4, nitrogen/air switch valve 1; 5, air buffer vessel; 6, thermal resistance thermometer; 7, aging vessel; 8, vessel wall heating control circuit; 9, cooling coil; 10, cooling oil tank; 11, peristaltic pump; 12, agitator; 13, condenser; 14, back-pressure valve; 15, gas-liquid separator; 16, flowrator; 17, vent washing tank; 18, dewatering cotton; 19, nitrogen sweeping valve; 20, sampling valve; 21, atmospheric valve; 22, solid-phase sample collector; 23, sintered metallic filter; 24, liquid-phase sample collector; 25, plunger-type pump; 26, reactant tank. Table 1. Operating Conditions for the Experimental Runs run

T /°C

N/rpm

FPX/(g/min)

run

T /°C

N/rpm

FPX/(g/min)

1 2 3 4

160 160 160 160

600 900 750 750

3.76 3.76 4.24 3.76

5 6 7 8

160 160 156 164

750 750 750 750

3.36 2.77 3.76 3.76

designed sampling system shown in Figure 3. When the sampling valve (valve 20) was open, the slurry was transferred from the reactor into a titanium pipe between the atmospheric valve (valve 21) and nitrogen sweeping valve (valve 19). The titanium pipe was electric heated around the wall, and the wall temperature was controlled to be equal to the slurry temperature. After this was done, the sampling valve was closed, and the atmospheric valve and the nitrogen sweeping valve were opened. The slurry was then pressed toward a porous sintered metallic filter with an internal aperture size of 1 µm. The filtered solution was then collected and cooled in a liquid sample collector, and the components’ concentrations in the solution were determined by the chromatograph method. The filtered crystals in the solidphase sample collector were taken out. The concentration of 4-CBA in the solid was determined by the chromatograph method, and the size distribution of TA crystals was measured by Malvern 2000S laser particle size analyzer. About 10 mL of saturated solution was sampled each time. The experimental runs’ reproducibility was verified by repeating each of them at least twice.2,17 The experimental conditions were listed in Table 1, while fixing the reaction pressure at 1.8 MPa, the oxygen volume fraction in the vent at 4%, the initial mass percent of water at 3%, the catalyst mass fraction at w(Co) ) 1.9 × 10-3, the catalyst ratio (molar) at Co/Mn/Br ) 26/1/18, and the mass of crude TA seeds at 50 g, in which the mass content of 4-CBA is 4500 ppm and the mass content of PT is 7000 ppm. Analytical Techniques. The liquid reactant PX, liquid intermediates TALD, PT, and 4-CBA, and product TA in the solution were analyzed by high-performance liquid chromatography (HPLC), and the solvent of acetic acid was analyzed by gas chromatography (GC). The internal standard method was used in the analysis, and toluene was used as the internal standard substance to correlate the data obtained from GC and

HPLC analyses. The mass ratio of liquid component to the internal standard substances in the liquid was determined by HPLC. Gradient elution was used for complete separation of the analytes at room temperature. The mobile phase consisted of three eluents (i.e., water + acetonitrile + methanol), and the following three-component gradient evolution program was adopted: at 0 min, 5 mass % acetonitrile and 95% water; from 0 to 5 min, the mixture composition changed linearly with time to become 85 mass % water, 5 mass % methanol, and 10 mass % acetonitrile; from 5 to 8 min, the mixture composition changed linearly with time to become 55 mass % water, 10 mass % methanol, and 35 mass % acetonitrile; from 8 to 12 min, the mixture composition changed linearly with time to become 15 mass % water, 10 mass % methanol, and 75 mass % acetonitrile; from 12 to 14 min, the mixture composition changed linearly with time to become pure acetonitrile; and from 14 min on, it became 100 mass % acetonitrile. Each analysis took ∼20 min. The mass ratio of solvent HAc to the internal standard substances in the solution was determined by a Shimadzu GC9A GC with a hydrogen flame ionization detector. The SE-54 (30 m) capillary chromatographic column was used. To verify the uncertainty of the concentration measurement, two TA + 4-CBA + PT + TALD + PX + toluene + HAc solutions of known concentration were analyzed. Compared with the known concentration, the uncertainty of concentration was