Ind. Eng. Chem. Res. 1988,27, 765-770
765
Methyl Acetate: Byproduct in the Terephthalic Acid Production Process. Mechanisms and Rates of Formation and Decomposition in Oxidation Paolo Raffia,*+ Pierangelo Calini,+fand Sergio Tontis Montedipe S.p.A., Research Unit of Bollate, Via S. Pietro 50, 20021 Bollate MI, Italy, and Montedipe S.p.A., Research Center of Porto Marghera, Via dell'Elettricit6 41, 30175 Porto Marghera V E , Italy
Methyl acetate is one of the main byproducts in the terephthalic acid process. This investigation has been aimed to shed more light on particular aspects concerning this byproduct: the relationship between the ester production and the process variables, methyl acetate behavior in the oxidation vessel, and recycle of the ester to the oxidation. The effect of some parameter variations on the ester formation is reported. Ester formation rate and the decomposition rate constant have been also calculated for operating conditions adopted in our oxidation. The ester decomposition preferably occurs through a hydrolysis reaction. The recycle of the ester to the oxidation appears then to be a valid solution to recover selectivelv acetic acid from this byproduct and to reduce solvent makeup, while CH30H decomposes to COX. The air oxidation process of p-xylene in acetic solution and in the presence of a Co-Mn-Br catalytic system is practically the only process applied on an industrial scale all over the world to terephthalic acid production (Hizicata, 1977; Roffia et al., 1980; Pohlmann et al., 1983). This process however has a drawback: part of the acetic acid used as solvent is decomposed to CO, CO,, and other byproducts (Nippon Chemical Consultants, Inc., 1980; Dermietzel et al., 1983). Methyl acetate is one of the main byproducts of this decomposition. It forms during the oxidation step. Unfortunately, its recovery from the mother liquor and its purification and use as such are not advantageous from an economical point of view. The companies involved in the production of terephthalic acid have claimed in their patents more suitable oxidation conditions to reduce the solvent decomposition to methyl acetate (Nippon Chemical Consultants, Inc., 1980). Namely, Mitsubishi has claimed in some patents (Hashizume and Izumisawa, 1979) that the recycle of such an ester to the oxidation reaction inhibits its formation. Following our previous work (Roffia et al., 1985/1986) on the continuous oxidation of p-xylene to terephthalic acid, concerning the optimization of the process variables and raw material consumptions, we went ahead with the purpose of facing the problem of the methyl acetate formation. Particular attention was paid to the following aspects: interdependence between ester production and process variables, methyl acetate behavior in the oxidation vessel, and advantages obtained from methyl acetate recycle to the oxidation.
Methyl Acetate Formation The methyl acetate formation can be schematically described as an oxidative decarboxylation of acetic acid 2CH3COOH + 1/202 CH3COOCH3+ H 2 0 + C02 (1) This reaction, occurring during p-xylene oxidation to terephthalic acid, is actually more complex; it can be in fact promoted by either the peroxy radicals or the catalytic system consisting of transition metals (Ariko et al., 1985).
-
Research Unit of Bollate. Present address: Montefluos S.p.A., Research Center of Bollate, Via S. Pietro 50, 20021 Bollate MI, Italy. *Research Center of Porto Marghera.
*
0888-5885/88/2627-0765$01.50/0
The acetic acid decarboxylation initiated by the interaction of its carboxylic group with the peroxo species forming in the radical-chain oxidation process of the p xylene can be schematized according to the following equation: CH3COOH + RO2' ---* ROOH + CH3' COZ (2)
+
A similar reaction may occur through the interaction of the peroxy radicals with the CH, group of the acetic acid CH3COOH + RO2'
-
ROOH
+ 'CH2COOH
(3)
followed by the oxidative decomposition of the resulting radical 'CHZCOOH
+ 02
-+
CO, COP, CH,OH, CHZO, HCOOH (4)
Kinetic investigations (Denisov, 1980) show evidence that the carboxylic acids, including acetic acid, can decarboxylate in the presence of radical reactions through both of the above-specified modalities. Methyl radical and methanol are therefore intermediate species in the acetic acid decomposition. Both of these intermediates can give methyl acetate formation, the former through oxidation by the catalytic system (Serguchev and Beletskaya, 1980), the latter through esterification:
-
'CH3 + Mmfl + CH3COOH CH3COOCH3 + M"+ CH3OH
+ CH3COOH
CH3COOCH3
+ H+ (5)
+ HzO
(6)
Moreover, when the acetates of the transition metals Co and Mn, part of the catalytic system used in the reaction, are in a higher oxidation state in their redox cycle, they can discharge their oxidizing potential either on the p xylene and its oxidation intermediates (Hendriks et al., 1978) or directly on the binding acetate ion with methyl radical formation (Waters, 1968; Anderson and Kocki, 1970). The latter is oxidized to methyl acetate according to the following equation, which summarizes both stages (Hendriks et al., 1979): ZCo(CH3C00)3 2Co(CH3C00)2 + Cop + CH3COOCH3 (7)
-
Since the possible decomposition paths for acetic acid involve the formation of intermediate species prior to the 0 1988 American Chemical Society
766 Ind. Eng. Chem. Res., Vol. 27, No. 5, 1988
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10
12
14
16
8
15 .lo
.05
I
6
8
4 3
WATER
CONCENTRATION. X
WT
Figure 2. Three levels of operating conditions to produce terephthalic acid with an oxidation degree equivalent to 2500 ppm of 4-carboxybenzaldehyde. (A) 220 O C ; p-xylene oxidation rate, 3.8; stirring speed, 500 rpm. (B) 210 "C; p-xylene oxidation rate, 3.0; stirring speed, 500 rpm. ( C ) 200 O C ; p-xylene oxidation rate, 2.3; stirring speed, 500 rpm.
,
9
I
1
12
WATER CONCENTRATION. X
WT
Figure 1. Water concentration in the reaction medium (abscissa) and the methyl acetate produced (ordinate). (A) Three levels of p-xylene oxidation rate (mol/(kg-h)); Co2+,0.022%; 220 " C ; stirring speed, 500 rpm. (B) Three levels of CoZt concentration ( % wt); 220 "C;p-xylene oxidation rate, 3.8 stirring speed, 500 rpm. (C) Three levels of temperature; CoZt, 0.022% ; p-xylene oxidation rate, 3.8; stirring speed, 500 rpm.
formation of methyl acetate, and their kinetics can be modified by varying the reaction conditions, the methyl acetate formation was also expected to depend on given specific parameters of the oxidation process. In fact, during our investigation of the p-xylene oxidation reaction, we could verify that the methyl acetate formation is namely affected by variations in the following parameters: water concentration in the reaction medium, oxidation temperature, p-xylene oxidation rate, and catalytic system composition. Figure 1 obtained by using our mathematical model (Roffia et al., 198511986) of the oxidation reaction shows the trends of methyl acetate production as a function of these parameters. These results can lead to some considerations on the possible mechanisms of the ester formation. The positive role played by water in decreasing methyl ester formation is in fact imputable to various reasons: 1. The water can coordinate the metals of the catalytic system, thus shifting the acetate ions from the ligand field of the catalysts and causing the acetate decomposition to be less likely (Hendriks et al., 1979). 2. The water destabilizes the highest oxidation states of the metals responsible for the decarboxylation reaction. 3. The water is a reagent in the hydrolysis reaction of the ester to acetic acid and methanol. In any case, the methyl acetate formation does not always decrease with an increase in water concentration. Some trends of our mathematical model in fact show a minimum at water concentrations in the range 12-16%. This result could be explained with the possible mechanisms through which the p-xylene oxidation reaction
occurs. It preferably proceeds through a catalyzed interaction and rearrangement of the peroxy radicals on the catalytic system or through a typical radical mechanism (Sheldon and Kochi, 1981) at low and high water concentrations, respectively. In the first case, the ester formation is prevailingly determined by the solvent decomposition promoted by the catalytic system (reaction 7). When the water content is increased in the reaction medium, the catalytic system progressively loses its activity. Under these conditions, the interaction of the peroxy radicals with the acetic acid tends to be the preferred and less selective path (reactions 2-4) for acid decomposition and ester formation. An increase in p-xylene feeding rate equivalent to higher specific reactor productivity gives rise to decreased ester production in oxidations carried out at low water concentrations. This effect can be explained with an increased concentration, at steady-state operating conditions, of both p-xylene and its oxidation intermediate products caused by higher specific reactor productivity. In the interaction with the catalytic system and the peroxy radicals, these compounds result in competition with the acetic acid (Hendriks et al., 1978). The temperature appreciably affects the methyl ester formation at low water concentrations. This result can be explained by the high activation energy requested for the decarboxylation reactions catalyzed by transition metals in a high oxidation state (Waters, 1968). In the terephthalic acid production on an industrial scale, it is important to ensure a constant oxidation degree of the reaction medium. The latter is normally expressed as concentration of 4-carboxybenzaldehyde1an intermediate product which must be removed in the subsequent purification stage of the terephthalic acid. An oxidation degree equivalent to a prefixed 4carboxybenzaldehyde concentration is obtained through a suitable variation of certain process variables. An increased water concentration decreases the oxidation rate and increases the steady-state concentration of 4carboxybenzaldehyde. However it is possible to calculate the trends of the ester formation at different water concentrations and at a given oxidation degree by allowing the catalyst concentration to change in order to balance the negative water effect. The relationships reported in Figure 2 show that the ester production rate at a constant oxidation degree slows down considerablywhen an increased water concentration is attained; all this clearly evidences a strict link between the methyl acetate production and the water parameter determined by the hydrolysis reaction.
Ind. Eng. Chem. Res., Vol. 27, No. 5 , 1988 767 Table I. Tests Effected in a Semicontinuous Wayn MA' Co + Mn, w t ratio produced, oxidatn Br/(Co + g/lOO! g deg,d g/1000 g p-xylene water, 70 wt Mn) T.A. ppm 1.7 13 0.9 25 650 1.35 13 0.9 23 720 1 13 0.9 21.5 890 22 1.7 13 1.65 500 1.35 13 1.65 20.5 580 1 13 1.65 18 650 2.4 1.7 13 19.5 650 1.35 13 2.4 17.5 650 1 2.4 13 16 650 Oxidation conditions: 220 OC; 2.4 MPa; weight ratio of p-xylene/catalytic solution, 1:4. Terephthalic acid. Methyl acetate. 4-Carboxybenzaldehyde concentration in ppm.
The ester formation is also bound to another reaction parameter, that is, the composition of the catalytic system expressed as Br/(Co + Mn) weight ratio. Semibatch tests carried out to define the effect of such a parameter have given the results shown in Table I. As was observed, an increased Br/(Co + Mn) ratio involves a decrease in the ester produced by aboGt 25-30%. This result is imputable to the specific role played by the halogen as a promoter of the reaction. The bromide ion can coordinate to the transition metal, thus replacing the acetate ion in the electron-transfer process between the ligand and the metallic ion in its high valence state (Serguchev and Beletskaya, 1980). In this way, the formation of acyloxy radicals is hindered, while the formation of the bromine radical (which is the actual promoter of the oxidation reaction) is fostered. Methyl Acetate Behavior during Oxidation The methyl acetate is not a stable compound at the conditions adopted for p-xylene oxidation. Therefore, its production through oxidation must be determined by the difference between the values of formation and decomposition rates. Our investigation was carried out to verify the extent and the mechanism of this decomposition. In fact, according to the reported methyl acetate formation mechanims (reactions 2 and 4-7), the recycle of the ester itself or the presence of other esters in oxidation is not expected to inhibit its formation. As a matter of fact, feeding such esters as ethyl acetate and sec-butyl acetate during oxidation (Roffia and Tonti, 1983) does not affect the methyl acetate formation rate. But, as could be observed, the ethyl acetate fed during the reaction undergoes decomposition not involving an increased COX formation (COX = C 0 2 + CO). Table 11. Tests Effected in a Semicontinuous Way" MA recycled, CO, g/IOOO g HzO, 9/1000 g MA concn,b p-xylene 70 wt p-xylene 70 wt 0 0.27 0.34 5 0.34 5 40 0.49 0.34 5 200 1.19 0.34 10 0 0.24 0.34 10 40 0.5 0.34 10 91 0.9 0.4 15 0 0.16 30 0.30 0.4 15 0.4 15 50 0.42
Feeding either ethyl acetate or ethanol to the oxidation reaction evidenced that the ethyl alcohol concentration in the reaction medium is ruled by the following equilibrium CH,COOCH,CH, + HzO e CB3COOH CH3CH2OH (8) and the decomposition selectively occurs through oxidation of the alcoholic part to acetic acid. Therefore, also, methyl acetate, like ethyl acetate and isobutyl acetate, can decompose through the following reactions: hydrolysis of the ester to acetic acid and methanol, followed by oxidation of the latter to COX (Hobbs and Van't Hof, 1978); radicalic decomposition of the methyl acetate. In this case we must expect a partial recovery of the acetic acid contained in the ester molecule, owing to the low selectivity of this reaction, which could determine a complete combustion of the R radical (reaction 10) without acetic acid formation. These two paths can be schematically represented as follows:
+
CHSCOOCH~
+
H20
= CH30H +
CH3COOH
(9)
I
The hydrolysis equilibrium of the methyl acc 9 9) was ascertained by feeding methanol instead r to the oxidation. The alcohol/ester concentration )dY determined by the reaction conditions. The )1 is partially oxidized to COX,and the remaining is Lfied to methyl acetate. On this basis, it has been verified that the decon. ,sition reaction of the methyl acetate preferablji occwre through a hydrolysis reaction. For this purpose, tests were carried out with and without methyl acetate recycle, at different water concentrations in the reaction medium. This procedure does not affect the activity of the catalytic system. The adding of an amount of ester (3- or 4-fold) that was produced in a test without recycle causes a negligible variation in the oxiJation degree. This means that, under normal operating conditions, the concentration of the oxidation intermediates, namely 4-carboxybenzaldehyde, remains constant. In the tests involving recycle, the methyl acetate produced results from the difference between ester amounts at the reactor outlet and inlet. This value can also be negative, which indicates that all the methyl acetate produced and part of the methyl acetate fed had decomposed. On these experiments, the carbon balance with respect to the formation of COX and low boiling compounds such as methane, formaldehyde, formic acid, and methanol was evaluated.
.
MA produced, 9/1000 g p-xylene 41.2 35 16 27 16 5.2 20.7 4.54 4.62
COX: mol of C/lOOO g p-xylene 4.63 4.75 5.04 4.37 4.56 4.73 4.37 4.93 4.78
COX + MA, mol of C/lOOO g p-xylene 6.3 6.17 5.69 5.46 5.2 5.2 5.2 5.2 4.78
"Oxidation conditions: 220 "C; 2.4 MPa; weight ratio of Co/Mn, 1:3; Br/(Co + Mn); 1.2:l. *In the reaction solution. 'COX includes CO, COz, CH,, CHzO, and HCOOH. 4-Carboxybenzaldehyde concentrations, 0.14% wt f 0.005, 0.20% wt f 0.005, and 0.22% wt A 0.005, respectively at 5%, lo%, and 15% wt water; p-xylene/solvent weight ratio, 1:3; contact time, 60 min.
768 Ind. Eng. Chem. Res., Vol. 27, No. 5, 1988 ; 12,
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20
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16
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48
16
12 6
-2
2,6
7.1
11.7
C
A 0 1 - METHYL ACETATE INLET CbNCENTRATION
80
9 : METHYL ACETATE RECYCLED
Figure 3. Relationship between the steady-state concentration of the methyl ester in the reactor inlet and outlet liquid streams. Experimental steady concentrations (g/L): IA(,3.5,6.2,11.5, 11.46; lAol -1.375, 3.765, 11.96, 12.14. Methyl acetate/methanol weight ratio, 50:l. The experimental point with a negative value of lAol corresponds to the test without ester recycle. In this case, lAOl takes into account the amount of ester which escapes from the system through the off-gases and the condensate withdrawn to keep the water concentration at a wanted value. Oxidation conditions: 220 "C; 2.4 MPa; Co-Mn-Br, components of the catalytic system; p-xylene/ acetic acid ratio, 1:4; air as oxidant; water concentration in the mother liquor, 11%; 4-carboxybenzaldehyde concentration, 0.21 "70 wt f 0.01; contact time, 40 min.
The results of Table I1 were obtained in a semicontinuous way. They evidenced what follows: (1)An increase in the amount of recycled methyl acetate involves an almost linear decrease in the ester produced. (2) The methyl acetate decomposition rate increases by increasing the water concentration; less recycle is required, at higher water concentrations, to decompose the same ester amount. (3) The carbon balance indicates that the increased material losses as COX, COX including combustion gases, methanol, formaldehyde, and formic acid, are justified by the decomposition of the methanol corresponding to the unproduced methyl acetate. Both the water effect and the selectivity of the methanol oxidation to COXsuggest that the methyl acetate decomposition occurs through the hydrolysis of the ester bond rather than the less selective ester decomposition promoted by the peroxy radicals forming in the p-xylene oxidation. Experiments with methyl acetate recycle have been repeated in a continuous way on a micropilot plant by feeding increasing ester amounts to the oxidation. The results obtained are reported in Figures 3 and 4. As clearly appears, an increased concentration !Ao[ of the methyl acetate in the feeding solvent, and then of the amount of the recycled ester, involves a linear increase in the ester concentration IAI in the reactor and a simultaneous linear decrease in its production. From this result, we can infer that, in our reaction system, the methyl acetate forms at a rate u strictly bound to the oxidation conditions and simultaneously decomposes to COX and acetic acid at a rate that is a function of its concentration. As far as the liquid phase is concerned, the material balance for the methyl acetate in our oxidation reactor, which must be considered as back-mixing under steadystate conditions, must be expressed by taking into account the ester formation and decomposition rates. This correlation can be written as follows: Q1Aol - Q1Al = KVlAl - VU
64
(11)
where Q = solvent flow rate (L/h), lAol and IAI = inlet and outlet methyl acetate concentrations (g/L), V = solution
G/lOOo
G P-XYLENE
Figure 4. Relationship between the methyl acetate produced and the amount of ester recycled. Oxidation conditions are the same as given in Figure 3.
volume in the reactor (L),K = constant for methyl acetate decomposition rate (h-l), and u = methyl acetate formation rate (g/(L.h)). From eq 11 one obtains
where 1 / r = Q/V, the reciprocal of the contact time. The straight line reported in Figure 3 is well expressed by such an equation. Consequently, the rate of methyl acetate formation ( u ) and its apparent decomposition constant ( K ) can be calculated from the slope of the straight line and its intercept on the ordinate axis. The values obtained were 10.32 g/(L-h) and 0.975 (h-l), respectively. It should be noted that both values refer to the operating conditions adopted during oxidation and to the features of our reactor. Methyl Acetate Recycle in an Industrial Plant Our previous investigation aiming at the optimization of the process parameters in p-xylene oxidation to terephthalic acid, evidenced the importance of operating at low water concentrations in order to improve p-xylene selectivity to terephthalic acid and to increase the activity of the catalytic system. Under these conditions, the acetic acid losses through methyl acetate formation are not negligible. The recycle of methyl acetate in the plant does not affect the oxidation reaction, while it speeds up the ester molecule decomposition to acetic acid and COX in a fairly selective way. The choice of recycling the ester could prove to be a good solution for an exploitment of this byproduct. Anyway, as it appears from the results obtained, the validity of such an operation is strictly bound to the efficiency in ester recovery. The equation of the straight line in Figure 4, representing the correlation between the methyl acetate produced and the amount of recycled ester, can be written as follows: methyl acetate produced = @ - aR
(13)
where 8 is the ester amount forming when no recycle is carried out and a is the slope of the straight line. In our experiments they were equal to 18.9 and 0.31, respectively, at the operating conditions specified in Figure 4.
Ind. Eng. Chem. Res., Vol. 27, No. 5, 1988 769 Table I11 r 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
R 0 2.03 4.4 7.15 10.44 14.4 19.35 25.59 33.75 44.88 60.97
P
% dec of MA Droduced
18.9 18.3 17.5 16.7 15.7 14.4 12.9 10.97 8.44 4.98 0
0 3.2 7.4 11.7 17.1 23.6 31.7 42.0 44.6 73.6 IIO0
Under steady-state conditions, our oxidation system with reference to the methyl acetate cycle can be schematically represented as follows:
t' reactor
where R and U indicate the methyl acetate amounts (8) at the reactor inlet and outlet for every 10oO g of p-xylene to be oxidized, while P gives the ester losses through offgases and the unrecycled mother liquor solutions. Under steady-state conditions, the P loss corresponds to the methyl acetate amount produced in the reaction: P = U - R = methyl acetate produced = p - aR (14) Calling that r is the ester recovery yield, r = R / U (its value ranging from 0 to 1)and applying it to eq 14, one obtains
With a and p being known, and setting r values, one can determine the recycle amounts, R , and the amounts of methyl acetate produced, P. These data are reported in Table 111. As clearly appears, high recoveries of methyl acetate are necessary to decompose a significant percentage of the ester produced and to recover the acetic acid present in the molecule of this byproduct.
Conclusions The methyl acetate formation during p-xylene oxidation to terephthalic acid is strictly bound to some reaction parameters. Conditions favorable to p-xylene oxidation affect also ester formation that is minimized by recycling the ester during oxidation. The recycle, while it does not modify the ester formation rate, affects its decomposition rate. Such decomposition preferably occurs through a hydrolysis reaction. This allows recovery, in an almost quantitative way, of the acetic acid present in the byproduct, while the carbon corresponding to the methanol is lost as COXand formic acid. The addition of other esters does not affect the methyl acetate formation rate: such esters as ethyl acetate and see-butyl acetate are oxidized in a fairly selective way to acetic acid and can be fed to restore the acetic acid lost through combustion. The recycle of the methyl acetate in an industrial plant does not give rise to any trouble as far as the p-xylene oxidation reaction is concerned. This operation causes an appreciably higher steady ester concentration. Only an effective recovery of the ester to be recycled allows its production to decrease to an appreciable extent and renders the operation valid from an economic point of view.
Experimental Section Materials. p-Xylene, acetic acid, methyl acetate, methanol, ethanol, ethyl acetate, and isobutyl acetate were of analytical grade and used as supplied. Cobalt, manganese, and bromine, used as the catalytic system, were added to the oxidation tests as cobalt acetate tetrahydrate, manganese acetate tetrahydrate, and 48% aqueous hydrogen bromide solution. Oxidation Procedure Some trends of methyl acetate formation with respect to some process parameters have been defined by means of a mathematical model already described in our previous work (Roffia et al., 1985/1986). In the same work is given the description of the equipment used to investigate the methyl acetate behavior during oxidation. The oxidation tests have been carried out both in a semicontinuous and a continuous way. Semicontinuous oxidation tests to determine the effect of the Br/(Co + Mn) parameter and water concentration on ester formation were effected by putting the solvent and the catalyst into the stirred reactor and feeding p-xylene or p-xylene + methyl acetate in the prefixed amounts a t a constant rate by a metering pump and air as oxidant. The continuous tests were carried out feeding the reagents, the solvent, and the catalyst and discharging the reaction product as a terephthalic acid slurry. The water concentration was kept to a prefixed value by withdrawing part of the vapor-phase condensate. Analytical Procedure The reaction solutions and the off-gases were analyzed through gas chromatography to determine methyl acetate and its decomposition products. The chromatograph used was a Fractovap 2300, C. Erba, supplied with thermoconductivity detector and glass column (length 1 m, diameter 6 X 3) filled with Porapak, 80-100 mesh. The carrier gas was helium, the flow rate 35 cm3/min, and oven temperature 180 "C. The off-gases composition was determined in a continuous way by using analyzers for CO, COz, and oxygen directly connected to the oxidation equipment or through sampling and subsequent analysis on a Fractovap 2300 chromatograph, C. Erba. For a correct determination of carbon balance, the lowboiling organic compounds escaping from the condenser were also determined. Nomenclature IAol, IAl = inlet and outlet methyl acetate concentration,g/L K = constant for methyl acetate decomposition rate, h-' Q = solvent flow rate, L/h V = solution volume in the reactor, L u = methyl acetate formation rate, g/(L.h) Literature Cited Anderson, J. M.; Kocki, J. K. J. Am. Chem. SOC.1970, 92, 2450. Ariko, N. G.; Samtsevich, V. S.; Mitskevich, N. I. Zh. Prikl. Khim. (Leningrad) 1985, 58(4), 954. Denisov, E. T. Compr. Chem. Kinet. 1980, 16, 195. Dermietzel, J.; Wienhold, C.; Grundmann, H.; Staschok, A.; Koch, J.; Bordes, E. ZfZ-Mitt. 1983, 71, 85. Hashizume, H.; Izumisawa, Y. (Mihubishi Chemical Industries), G.B. Patents 1554 487, 1554 488, 1554 489, 1979. Hendriks, C. F.; Van Beek, H. C. A.; yeertjes, P. M. Ind. Eng. Chem. Prod. Res. Deu. 1978, 17(3), 256. Hendriks, C. F.; Van Beek, H. C. A.; Heertjes, P. M. Ind Eng. Chem. Prod. Res. Deu. 1979, 18(1), 43. Hizicata, M. CEER, Chem. Econ. Eng. Reu. 1977, 9, 32.
Ind. Eng. Chem. Res. 1988,27, 770-774
770
Hobbs, C.; Van't Hof, H. (Celanese Corporation) Can. Patent 1038 403, 1978. Nippon Chemical Consultants, Inc. Report 15, Jan 1980. Pohlmann, H. P.; Meyer, D. H.; Leipold, H. A. Prepr-Am. Chem. SOC.,Diu. Pet. Chem. 1983, 28, 1085. Roffia, P.; Tonti, S. Montedipe, Internal Report, 1983; Bollate, Italy. Roffia, P.; Calini, P.; Tonti, S. Oxid. Commun. 1985/1986, 8(1-2), 167. Roffia, P.; Tonti, S.; Tancorra, R. Chem. Znd. (Milan)1980,62, 500.
Serguchev, Yu. A,; Beletskaya, I. P. Russ. Chem. Rev. (Engl. Transl.) 1980, 1119. Sheldon, R. A.; Kochi, J. K. Metal Catalyzed Oxidations of Organic Compounds; Academic: New York, 1981. Waters, W. A. Discuss. Faraday SOC.1968,46, 158.
Received for review December 9, 1986 Revised manuscript received June 25, 1987 Accepted November 24, 1987
Kinetics of Absorption of Nitric Oxide in Aqueous FeII-EDTA Solution Li Huasheng* and Fang Wenchi Chemical Engineering Department, Zheng Zhou Institute of Technology, HeNan Province, China
The rates of absorption of nitric oxide in aqueous solution of Fe"-EDTA chelate were measured by using a stirred vessel with a free flat gas-liquid interface. The experimental results were analyzed with the chemical absorption theory based on the film model. The second-order forward rate constants for the reaction between nitric oxide and Fe'LEDTA in aqueous solution were calculated and correlated as a function of temperature and ionic strength of the solution. The chemical equilibrium constants for the reaction were determined from the measurement of the total solubility of nitric oxide in aqueous Fe*'-EDTA solution. The exhaust gases emitted from nitric acid plants have caused concern because of the high NO, concentration and the great exhaust quantity. How to decrease NO, concentration in exhaust gases has become a problem that scientists have been trying to solve. So far, though many methods have been proposed, none can be considered perfect. The complexing absorption method of NO, in aqueous FeeEDTA solution is a promixing method because of very fast absorption. However, as absorption proceeds, the existence of a little O2 and NOz oxidizes a part of the Fen-EDTA into Fern-EDTA which is of lower complexing activity. So, how to reduce Fe"'-EDTA to Fe'I-EDTA is an important problem in such a NO, removal process. Many methods of reducing Fe"'-EDTA have been reported. Because they have their own disadvantages, there is along way to go before commercialization can be realized. Based on our previous research on removing H2Sin coal gases of ammonia plants by using Fe"'-EDTA solution, this paper presents that H2S can be used as a reducing agent for Fe"'-EDTA in solution after removing NO,. Thus, NO, and H2S are not only removed, but elemental S and NO are also recovered. Removing NO, and removing H2S form a closed, circulating system. The main reactions are
NO
+ FeII-EDTA
= Fe"(N0)EDTA
control this process balance becomes a key problem in realizing this combined proceM for removing H2Sand NO,. The purpose of this paper is to establish the kinetic equation of absorption of NO in aqueous Fe"-EDTA solution.
Chemical Absorption Mechanism The complexing reaction 1is a fast, reversible reaction, second order with respect to the forward reaction (first order in NO and FeLEDTA, respectively) and first order with respect to the reverse reaction (Kustin et al., 1966; Sada and Kumazawa, 1980). The complete analytical solution for the enhancement factor for this (2,1)-orderreaction can't be obtained. It is only possible to obtain a particular solution under a certain particular condition. Hatta (1957), Danckwerts (1970), and Teramoto et al. (1978) presented enhancement factor expressions for a similar system, but the scope of the use of these expressions was restricted by their assumed conditions. For a general, (2,1)-order, fast, reversible reaction (4) AM + 4)= E(l) based on the film model, the material balance equation for each component in the liquid film can be written as DA
(d2A/dX2)= K2AB - K-1E
(5)
DB
(d2B/dX2) = KzAB - K-IE
(6)
DE
(d2E/dX2) = K-IE - KQAB
(7)
(1)
2N02 + 2Fe"-EDTA = Fe"(N0)EDTA + FeIII-EDTA + NO,- (NO, removal process) (2) H2S + Fe"'-EDTA = Fe"-EDTA + S + 2H+ (H2Sremoval process) (3) Obviously, compared with processes for separately removing H2S and NO,, this combined method can omit two regeneration processes, which makes energy consumption, cost, and equipment investment decrease greatly. So, this combined method is no doubt the most economical. It is easy to imagine that, if the combined method were operated, considerable process coordination would be required. The two processes would influence each other, if either of them acts too fast. Thus, researching the kinetics of absorption of NO in aqueous Fe"-EDTA solution to 0888-5885/88/2627-0770$01.50/0
The boundary conditions are dB/dx = dE/dx = 0
x = 0; A = Ai, x = x,; -DA
(8)
(dA/dx) = (U - Xo)Ro, (9) E = Eo B = Bo, Furthermore, the chemical equilibrium of reaction 4 is established in the bulk of the liquid, so the following condition should be fulfilled (Zhu, 1981): K = Eo/A$o (10) For such fast reactions, the authors considered that the diffusion of component B into the liquid film from the bulk 0 1988 American Chemical Society
