2138
Ind. Eng. Chem. Res. 1991,30, 2138-2143
Kinetics of Isomerization of Maleic Acid Using Ammonium Bromide and Ammonium Peroxydisulfate as Catalyst Juan R.Gonziilez-Velasco,*Jose A. Gonziilez-Marcos, Miguel A. Gutierrez-Ortiz, and Joe6 I. Guti6rrez-Ortiz Department of Chemical Engineering, Universidad del Pals Vasco, Apartado 644, 48080 Bilbao, Spain
A kinetic model and proposed mechanism for the isomerization of maleic acid to fumaric acid using ammonium bromide and ammonium peroxydisulfate as catalyst have been developed. In an initial step, it has been proposed that the atomic bromine formed is added to the double bond, allowing rotation of the molecule to give fumaric acid. In a second step, sequential experimental design techniques have been used to maximize confidence in the resultant parameters of the kinetic model. Twenty-six experiments have been needed to estimate accurately the kinetic parameters. The confidence regions of estimated parameters are also reported.
Introduction Fumaric acid is a commercially valuable product that may be obtained by the isomerization of maleic acid at elevated temperature or in the presence of certain catalytic materials. The catalytic agent most used is thiourea, and this method is actually applied to the preparation of fumaric acid from dark-colored aqueous scrubber liquors that contain varying amounts of maleic acid and that are commercially obtainable from the phthalic anhydride process. Although this industrial process is well developed qualitatively (Katunin and Penkina, 1963;Reti et al., 1989),its mechanism remains unclear; nevertheless it is known that the catalytic action begins with an attack to the oxygen of the carboxylic group (Schliesser, 1962). Attention has been focused on the especially advantageous method for preparing fumaric acid employing a bromineproviding compound and an oxidizing agent with minimum oxidation potential of 1.23 V (Russell et al., 1968; Hoffman, 1981;Chen and Jwo, 1983). The catalytic action can be improved by adding a mineral action to the reaction medium. Recently, Gonzaez-Velasco et al. (1991)reported a techno-economic evaluation of the isomerization of maleic acid to fumaric acid using ammonium bromide as a soluble catalyst carried out in a batch stirred tank reactor, as in industrial practice. From a statistical analysis based on a central composite orthogonal design, they established the response surface equation for fumaric acid yield in relation to the operation conditions, Le., bromine and peroxydisulfate concentrations, and temperature. They showed a graphic chart that allows one to choose optimal combinations of those variables at which the process is carried out in the most economical-minimum cost-way. This work pretends to be a more detailed approach to the knowledge of the mechanism and kinetics of the isomerization of maleic acid to fumaric acid using ammonium bromide and ammonium persulfate as catalytic and oxidizing agents, respectively. The objective is to select a mechanism for the isomerization that fits our experimental data and then to estimate the kinetic parameters of the proposed model with the aid of sequential experimental techniques. Previous Kinetic Studies There is no general agreement among researchers on the mechanism of isomerization as the different nature of catalytic agents used in each case implies probably different kinetic mechanisms. Following is a brief revision on relevant work in literature on kinetics of maleic acid to fumaric acid isomerization.
Nozaky and Ogg (1941)found a simple first-order potential equation to satisfy experimental data obtained in the isomerization of aqueous maleic acid solutions carried out without catalyst. The competition with the hydration reaction to give malic acid when the isomerization reaction is carried out in an aqueous medium was studied by Hughes and Adams (1960),who also proposed fmt-order kinetic equations for the isomerization as well as for the hydration reaction. These authors concluded that high temperature favors the hydration reaction versus the isomerization reaction. A second-order kinetic equation proposed by Davies and Evans (1955)suggested a more complex mechanism, later criticized by Cilento and Ferreira (1967). Pall& and Segarra (1973)also obtained a second-order kinetic equation for the reaction carried out in an organic medium. Kinetic modeling appears more complex when the isomerization is carried out in the presence of catalytic materials. Kinetic studies by Aurrecoechea et al. (1974) of the isomerization carried out in aqueous medium and with thiourea as catalyst indicated an important consumption of catalyst to give 2-amino-4-oxothiazole5-acetic acid in parallel with the isomerization:
HOC-N
A
A
The kinetic equations proposed by Aurrecoechea et al. (1974)were the following: isomerization reaction -rA = kaCT(CA - CA*) (2) where CA*is the maleic acid concentration at equilibrium and CT the thiourea concentration. reaction of thiourea consumption (eq 1)
= kTCTCA (3) Font et al. (1986,1988) studied the kinetics of the isomerization of concentrated maleic acid solutions carried out in the presence of nitric acid, resulting in a reaction order between 1.5 and 2. -rT
0888-5885/91/2630-2138$02.50/0 0 1991 American Chemical Society
Ind. Eng. Chem. Res., Vol. 30, No. 9,1991 2139 L
.-20 0.8P
>
)
-
0 0.6
-
0.4
-
C
0
0.2 -
Figure 1. Schematics of the experimental setup.
A reaction system similar to that studied in this paper was used by Jwo (1981), who employed a bromine salt as catalyst and Ce(1V) as oxidizing agent. The proposed mechanism resulted in the following (Jwo and Chen, 1982; Chen and Jwo, 1983; Jwo et al., 1983): Ce(N) + BrHOOC\,c=c,
COOH /
H
Ce(III) + Br* HOOC
+ Br* e
HOOC
'c-cf c-cd
H'\Br
H H
\Br
(W H H
HOOC,
\C+-
'C-CC H'
COOH
H'\Br
H
HOOC
(44
'COOH
H'
2Br*
HmC\ H
e
\er
(4c) 'COOH
H
/
+ Br* F=~\cooH
Br2
(4d) (44
Hoffman (1981) studied the role of bromine in the isomerization reaction by analyzing many different bromine compounds. The proposed mechanism follows the same steps shown above: initiation, propagation, and termination. The addition of the bromine activated atom to the double bond produces the rotation of the molecule to obtain the fumaric acid which presents higher thermodynamic stability. Experimental Section The kinetic experiments were carried out in a continuous stirred tank reactor, achematica of which is shown in Figure 1. The reactor was a cylindrical glass vessel with a volume of 71 mL and was mechanically agitated. The agitation speed was maintained constant at 300 rpm in all experiments, after checking that this speed was enough to consider perfect mix flow in the reactor. The feed flow was controlled by a valve and measured from the pressure drop through a capillary. The reactor temperature was controlled by means of a thermostat acting on the heating system of the water flowing into the jacket of the reactor. The fluid-dynamic behavior of the reactor was analyzed by the stimulus-response technique resulting in a dispersion number of 7, equivalent to 1.03 perfect-mixing tank reactor. The previous as well as a good concordance between the residence time obtained from the response curve and that obtained experimentally allowed us to consider a perfect-mixing flow in the reactor. Maleic acid was a pure chemical (PS grade, 99% minimum purity), and ammonium bromide and ammonium peroxydisulfate were extrapure chemicals (PRS grade, 99.5% minimum purity) of Panreac. All reactant solutions were freshly prepared and brought to the chosen temperature before mixing. Deionized water was distilled once before use. In each experiment, and after running the reaction for 7 times the residence time to ensure the steady state was practically reached (99.9% for perfect mix flow: Froment
L
00
2
4
6
8
Residence time, min Figure 2. Conversion vs residence time (temperature,343 K, maleic acid concentration, 0.345 M ammonium bromide concentration, 5.1 X lo4 M ammonium peroxydisulfate concentration, 4.38 X lo" M). Table I. Range of Variation for the Operational Conditions lower upper variable limit limit temperature, K 333 358 maleic acid concn, mol L-' 0.20 0.90 ammonium bromide concn, mol L-' 0.001 0.015 ammonium peroxydisulfate concn, mol L-I 0.001 0.015 residence time, min 1 8
and Bischoff (1990)),samples for analysis were drawn out of the system. These samples were quickly chilled, and the fumaric acid was removed from the reaction solution by filtration. The solid was washed with ice water to prevent redissolution of fumaric acid and then was dried at 90 O C for several hours and weighed to constant weight. Polarographic and mass spectra of the product exactly matched those of pure fumaric acid. M NaOH The residual liquid was titrated with 5 X solution to determine the amount of unreacted maleic acid and residual fumaric acid, and both components were distinguished by spectrometry, which allowed us to check that the material balance for each experiment was correctly closed (the error in the closure resulted less than 2 % for all experimental points). Thus, the conversion was calculated as the converted moles of maleic acid (or produced fumaric acid moles) per mole of maleic acid in the feed to the reactor. The results obtained in previous work (Godez-Velasco et al., 1991) and those obtained from a preliminary set of experiments designed to analyze the effect of the residence time on the conversion (Figure 2) allowed us to choose the adequate intervals for the operational variables with the aim of obtaining conversions in the largest range in order to achieve a precise estimation of the kinetic parameters. The lower and upper limib for each variable are shown in Table I. Kinetic Modeling Several attempts were made to fit the experimental data to simple models but not good results were obtained. The initiation period observed in Figure 2 made us think of the formation of an intermediate similar to that formed in the reaction scheme by Chen and Jwo (19831, after substitution of the oxidizing agent in the bromine formation initial step; i.e. (4a) becomes '/SZOs2-+ Br- * Br' + Sod2(5) This mechanism results in a very complex kinetic equation which can be simplified assuming that the propagation step corresponding to the rotation of the C4! bond is much more rapid than the addition of atomic
2140 Ind. Eng. Chem. Res., Vol. 30, No. 9, 1991
bromine, and the isomerization and termination steps are at equilibrium as was proved by Mathai (1958). With such assumptions the mechanism can be described by the following kinetics: reaction of bromine consumption -rB
=
(6)
k1cScB
isomerization reaction -?A = kZ'Cc(CA
- KCR)
(7)
Introduction of the material balance for B assuming perfect mix flow in the reactor -rB = c&xB/7 (8) and the conversion of ammonium bromide in (6) lead to
which can be rearranged to be solved for XB:
Table 11. Set of Experiments To Make the Initial Estimation of the Kinetic Parameters concn, "01 L-' kmp, cow % expt residence no. time, min CA CB Cs K expt calc 1 6 690 8.2 10.5 343 0.755 0.743 2 5 690 8.2 10.5 343 0.704 0.701 3 4 690 8.2 10.5 343 0.608 0.640 4 3 690 8.2 10.5 343 0.545 0.547 5 2 690 8.2 10.5 343 0.379 0.403 6 6 345 8.2 10.5 333 0.308 0.311 I 6 345 8.2 10.5 343 0.713 0.714 8 6 345 8.2 10.5 348 0.777 0.805 9 6 345 8.2 10.5 353 0.892 0.872 10 6 517 8.2 10.5 343 0.708 0.727 11 6 862 8.2 10.5 343 0.780 0.753 12 6 345 5.1 4.4 343 0.609 0.556
kinetic constants of the model and showed the adequacy of the model to the isomerization, as can be seen from a comparison of the two last columns of Table 11.
Parameter Estimation For intrinsically nonlinear rate models, one can find the parameter values minimizing the sum of squares function: On the other hand, since active bromine is at equilibrium
with Br2 and step (4e) is displaced to the right side, the atomic bromine concentration can be considered proportional to the converted bromide: CC = BCBoXB (11) and inclusion of XA,(111,and the material balance for A in (7) lead to CAJA/T e k&BoXB[CAo(1 - XA) - KCR] (12) where /3 is included in the kinetic constant k2. The fumaric acid concentration in (12) refers to the concentration in the medium of reaction, so that it corresponds to the converted maleic acid, i.e., C d A ,provided this amount is below the solubility of the acid in the medium. Above the solubility, CR corresponds to the saturation concentration. Thus,rearrangement of (12) permits calculation of conversion XA:
The saturation concentration of fumaric acid and the conversion at equilibrium were determined experimentally by polarographic and mass spectrometry analysis. The solubility of fumaric acid in the reaction medium was not practically affected by the presence of maleic acid, ammonium bromide, and ammonium peroxydisulfate. Thus, the following equations may be used to estimate CRoand K with temperature: CR' = 11.53 - 7.68 X 10-2T + 1.28 X 10-4P (15)
K = exp(103.3 - 0.59T + 8.24
10-4P) (16) Taking into account the intervals shown in Table I, an initial set of experiments was designed at different levels for each variable, attending to the influence deduced from the previous work (Gonzilez-Velasco et al. (1991) and Figure 1). Table I1 shows the designed experiments and the obtained experimental conversion. A regression analysis of the experimental data to the proposed kinetic model allowed us a first estimation of the X
N
S(K) = C (yu - XAJ' u-1
(17)
by different optimization techniques. The nonlinear least-squares technique used here is from Marquardt (1963) and is a combination of steepest descent and Gaussian procedures. The parameters to be estimated in this work are the preexponential factors and the activation energies corresponding to the rate constants kl and kz. The first estimation of the kinetic parameters was made from the 12 experiments shown in Table 11, resulting in kl = exp(55.8 - 149.1/RT) (18) k2 = exp(44.9 - 113.6/RT)
(19)
To determine more accurately the kinetic parameters and the effect of temperature, a sequential design of experiments has been applied. This methodology is wellknown (Box and Hunter, 1965; Hosten, 1974; Froment, 1975; Godez-Velasco et al., 1991) and can be summarized in the diagram shown in Figure 3. Assuming the model based on the scheme by Chen and Jwo (1983) to be valid and the experimental errors uniformly distributed and independent of the variable values, a choice of experimental settings for the Nth experiment, which maximizes the determinant lDTDl (D is the matrix of the partial derivatives of XA with respect to any parameter of the model), will minimize the volume of the joint confidence region of the parameter estimates. Hence, the conditions at which the 13th experiment should be made resulted in 'I 4 min C A=~ 0.8 M Cso = 1.1 X M Cs, = 1.5 X M T = 358 K Table I11 shows the experiments deduced by iteration of the previous procedure as well as the obtained conversion and the expected reduction of the confidence region after each experiment was carried out. On the other hand, Figure 4 shows the successive estimated values of the preexponential factors and activation energy of kl (10) and kz (12) after each experiment. The experimental design was stopped with experiment 26 because for the next experimental point the expected reduction of the confidence region was only 990, and the
Ind. Eng. Chem. Res., Vol. 30,No. 9,1991 2141 KImyowLToBEm
I
I
O'
12 14 16 18 20 22 24 26
Experiments
Figure 3. Design of experiments for parameter estimation. Table 111. Experiments in the Sequential Design and Expected Volume Reduction of the Confidence Region concn, expected expt residence mmol L-' temn conv. reduction, no. time.min C. C= ca K-. % . % 4.0 800 1.1 15.0 358 0.692 13 63.8 14 1.0 200 3.0 2.0 358 0.344 34.0 7.0 600 13.1 6.0 333 0.363 30.4 15 26.3 16 4.0 800 1.1 15.0 358 0.692 17 2.0 200 2.0 1.0 358 0.358 16.0 8.0 700 6.0 15.0 338 0.619 20.3 18 20.0 19 3.5 800 1.0 15.0 358 0.667 20 6.0 700 13.0 6.0 333 0.320 16.9 21 2.5 200 2.0 1.0 358 0.479 19.7 22 8.0 800 1.0 15.0 343 0.434 13.5 23 3.5 800 1.0 15.0 358 0.667 13.6 24 8.0 700 10.0 5.0 333 0.301 12.7 14.2 25 1.0 200 4.0 2.0 358 0.377 26 8.0 9.4 800 1.0 15.0 345 0.482 ~
~~
Table IV. Obtained Kinetic Parameters and Standard Deviations parameter optimal value std dev AI, L mol-' min" 50.21 0.26 El, kJ mol-' 134.6 0.37 A2, L mol-' min-' 46.21 1.80 E2,kJ mol-' 118.3 2.62
values of Al, A2,El, and E2were maintained practically constant during the last steps of the design. The multiple regression coefficient of the fit resulted in 99.3%, and the calculated F test resulted in a value of 388, superior to that tabulated for a probability level of 99% (F= 4.43), which validates the significance of the proposed model. Repetition of experiment 1five times leads to a mean square of the experimental error of 3.23 X 10"'. A comparison between this value and the mean square of lack of fit square sum (5.60 X 10"') indicated no significance of the lack of fit (Fdc = 1.71 < 5.80 = Fteb,95%). The estimates of the preexponential factors and activation energy are reported in Table IV, together with the standard deviations for each parameter calculated from the matrix DTD.The analysis of the coefficients of the correlation matrix showed the existence of multicollinearity between Al and El (correlation coefficient = 0,9991) and A2 and E2(correlation coefficient = 0.9993). This effect
Figure 4. Successive values of the preexponential factors and activation energies estimates after each experiment in the design arid their confidence intervals for 95%.
is probably due to the narrow interval inside which the temperature can be varied-333-358 K-a good separation of the influence corresponding to the frequency factor tuid activation energies for each kinetic constant not being possible. Although elimination of one constant will lead to a similar fit of the experimental data to the model, we have preferred to maintain all of them so that relationships of kinetic constant with temperature follow the Arrheniuv expression. The facts analyzed above make the confidence intervals calculated as tsk (skis the last column of Table IV) to be underestimated since independence between parameteis is being assumed. Thus, following is the calculation of the real confidence regions of the parameters.
Confidence Regions of Estimated Parameters The functional form of some models is such that the parameter estimates are sometimes highly correlated and poorly estimated. The least-squares minimum may not be sufficiently informative to allow much confidence to be placed in the parameter estimates obtained. Consequently, one must frequently turn to procedures that indicate the size of the region within which the true parameter values might be expected to lie (Mezaki and Kittrell, 1967). A confidence region of the parameter estimates may be approximately calculated by
To apply this equation, one must calculate the sum of squares of enough sets of parameter values until a locus of all parameter values possessing a contour level of magnitude S(K) is obtained. Then a plot of the locus of these parameter values provides a region within which orre can state with l O O ( 1 - a)% confidence that the true parameter values lie. For one, two, or three parameters in the rate equation, of course, graphical representations of the confidence regions are easily presentable. However, for more dimensions such representations are not possible. Thus, here (four parameters to be estimated) we have determined the confidence region for a probability level of 95%, resultiq in the upper and lower limits for each parameter thst tire shown in Figure 4.
2142 Ind. Eng. Chem. Res., Vol. 30, No. 9,1991
n
< +5
X
I x
0
W
-5
. , ,
-101,
-150
0.2
0.4
0.6
, 0.8
j 1.0
Conversion, X A Figure 6. Percentage of deviation of the experimental and predicted conversion values of maleic acid ( 0 , initial points; 0,designed points).
Summary and Conclusions The kinetics of the cis-trans isomerization of maleic acid to fumaric acid has been studied with ammonium bromide and ammonium peroxydisulfate as catalytic agents. A mechanism similar to that reported in literature by Jwo (1981) has been proved to be adequate for representing the process, and its kinetic model has been deduced. The conversion to fumaric acid, XA,can be calculated from (13)or (14) depending of whether the fumaric acid concentration is below or above its solubility in the reaction medium. In a second step, a more statistically significant estimation of the kinetic parameters has been made with the use of a sequential experimental design. Twenty-six experiments were needed to obtain the parameter estimates with high statistical significance. Relative deviations of the predicted conversion from the experimentally observed conversion for each experiment are shown in Figure 5, where a random distribution ranging from -10% to +lo% can be seen, which corroborates the validity of the proposed model and the kinetic parameter estimates. The cis-trans maleic-fumaric isomerization in presence of ammonium bromide and ammonium peroxydisulfate as catalyst can be described as = exp(46.2 1 1 8 , 3 / R n c & X ~ ( C-~KCR) (21) where K is the equilibrium constant estimated from (16) and XB is calculated from the rate equation of consumption of bromine: -rB = eXp(50.2 - 134.6/RT)C& (22)
'
Acknowledgment We gratefully acknowledge financial support by the Universidad del PGis Vasco and Direccidn General de Investigacidn Cientifica y T6cnica. Nomenclature CA, CT = concentration of maleic acid and thiourea in (2) and (3, mol L-' C, CS= concentration of ammonium bromide and ammonium peroxydisulfate, respectively, mol L-l Cc = concentration of atomic bromine, mol L-I CR = concentration of fumaric acid, mol L-' D = matrix of the partial derivatives of XAwith respect to any parameter of the model for every experiment El, E2= activation energies of the maleic acid isomerization and bromine oxidation, kJ mol-' K = vector of parameter estimates K, K = equilibrium constant
kl, k2 = kinetic constants of the maleic acid isomerizationand bromide oxidation, L mol-' min-' k,, kT = kinetic constants of isomerization and thiourea consumption, in (2) and (3), L mol-' min-' N = number of experimental points p = number of parameters R = gas constant, kJ mol- K-I -rA,-rB = rates of the isomerization and bromide oxidation, respectively, mol L-' min-' -rT = thiourea consumption rate, mol L-' m i d S(K) = sum of squares of residuals for a set of parameter values K sk = standard deviation for parameter k T = temperature, K t = Student function XA= conversion of maleic acid to fumaric acid, % XB = conversion of ammonium bromide in the oxidation reaction, % y = predicted value of XA Greek Symbols
fl = coefficient defined in (11) I = residence time, min Su bscriptslsuperscripts o = initial value in the feed * = value at equilibrium s = value at saturation T = transpose of a array u = number of experiment estimated value Registry No. Maleic acid, 110-16-7;ammonium bromide, 12124-97-9;ammonium peroxydisulfate,7727-54-0.
Literature Cited Aurrecoechea, A; P a U , L.; Roces, M. C. Cinetica e la iaomerizacibn catalltica del Bcido maleico en preaencia de tiourea. Afinidad ( E ) 1974,31,827-842. Box, G. E. P.; Hunter, W.G. Sequential design of experiments for nonlinear models. Proc. ZBM Sci. Comput. Symp. Stat. 1965, 113-137. Chen, Y. H.; Jwo,J. J. Isomerization of maleic acid to fumaric acid catalyzed by bromate ion and bromine. J.Chin. Chem. SOC.1983, 39,45-54. Cilento, G.; Ferreira, C.Note on kinetics of diamine-catalyzed geometrical isomerization of dimetil-maleate. J. Chim. Phys. 1967, 64 (lo),1547. Davies, M.; Evans, F. P. Kinetics of some cia-trans isomerization 1966,51,1506-1517. reactions in solutions. Trans. Faraday SOC. Font, R.; Miiiana, A.; Gbmez, D.; Rubio, A. Isomerization del acid0 maleico. An. Quim. (E) Ser. A. 1986,82 (l),131-136. Font, R.; Gbmez, D.; Rubio, A. Kinetics of isomerization of maleic acid in concentrated solutions. Ind.Eng. Chem. Res. 1988,27(5), 774-779. Froment, G. F. Model discrimination and parameter estimation in heterogeneous catalysis. AZChE J. 1976,21,1041-1057. Froment, G. F.; Bischoff, K. B. Chemical Reactors Analysis and Design, 2nd ed.; Wiley: New York, 1990. Gonzilez-Velasco, J. R.; Gutihez-Ortiz, M. A.; GutMrrez-Ortiz, J. I.; Godez-Marcoe, J. A. Techno-economic optimization of isomerization of maleic acid to fumaric acid wing ammonium bromide as a soluble catalyst. Chem. Eng. Process. 1991,in press. Hoffman, V. F. Addition of bromine to maleic and fumaric acids, esters and salts. Ph.D. Thesis, University of Iowa, 1981. Hosten, L. H. Sequential experimental design procedure for precise parameter estimation based upon shape of joint confidence region. Chem. Eng. Sci. 1974,29,2247-2252. Hughes, M. F.; Adams, R. T. Kinetics of the catalytic vapor-phase oxidation of phthalic anhydride. J. Phys. Chem. 1960, 64, 781-784. Jwo, J. J. Bromine-catalyzed isomerization of maleic-acid to fumaric-acid. J. Chin. Chem. 1981,28 (11,35-41. Jwo, J. J.; Chen. Y. H. Kinetic-study of the bromine-catalyzed isomerization of maleic-acid in aqueous Ce(IV)-Br-H8O4medi-
Ind. Eng. Chem. Res. 1991,30, 2143-2147 um. J. Chin. Chem. 1982,29 (2), 71-80. Jwo, J. J.; Chen, Y. H.; Chang, E. F. Isomerization of maleic-acid to fumaric-acid catalyzed by cerium(IV) and N-bromo compounds. J . Chin. Chem. 1983,30 (2), 103-115. Katunin, V. Kh.; Penkina, V. I. Isomerization of maleic-acid with thiourea. Zh.Prikl. Khim. 1963,36 (lo), 2261-2265. Marquardt, D. W. An algorithm for least squares estimation of nonlinear parameters. SAM J . Appl. Math. 1963,II, 431-441. Mathai, I. M. Kinetics of the olefin-bromine reaction. X Influence of producta on the kinetics of bromine addition to unsaturated acids. J. Sci. Znd. Res. (India) 1958, I7B,145149. Mezaki, R.;Kittrell, J. R.Parametric sensitivity in fitting nonlinear kinetic models. Znd. Eng. Chem. 1967,59 (3), 63-69. Nozaki, K.; Ogg, R. Cis-trans isomerizations. The mechanism of a catalyzed isomerization of maleic acid to fumaric acid. J. Am. Chem. SOC. 1941,63,2583-2586.
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P W ,L.; Segarra, R.Cingtica de la isomerizacidn maleico-fum6rico. Afinidad (E) 1973,30, 13-21. Reti, T.; Kovacs, T.; Bognar, J.; Kolonics, Z.; Tuba, G.; Halas, G.; Bassa, R.;Legradi, L.; Horvath, 1.; Meszaros, P. Preparation of pure fumaric acid from maleic acid-containing industrial wastee. Br. U.K. Pat. Appl. GB2,207,915, 1989. Russell, J. L.; Ridgewood, N. J.; Olenberg, H. Procedure to manufacture fumaric acid of good color from maleic acid. US. Pat. 3,389,173, 1968. Schliesser, W. Catalytic activity of thiourea and ita analogs in the isomerization of maleic-acid to fumaric-acid. Angew. Chem. 1962, 74, 429-430. Received for review May 22, 1990 Revised manuscript received December 26, 1990 Accepted May 21,1991
Reactivation of Fly Ash and Ca(OH)2Mixtures for SO2 Removal of Flue Gas Juan C. Martinez, Jose F. Izquierdo, Fidel Cunill,* Javier Tejero, a n d Javier Querol Chemical Engineering Department, University of Barcelona, Mart; i Franques 1, 08028 Barcelona, Spain
Mixtures of fly ash and Ca(OH)2 were hydrated, characterized, and tested in laboratory-scale experiments. Hydrated mixtures developed a high total surface area, greater than the arithmetical addition of surface areas of initial solids before hydration. The relative surface area increment increased with temperature, time of hydration, and fly a ~ h / c a ( O H ratio, ) ~ the temperature effect being the most important. Tetracalcium aluminate monosulfate and tetracalcium aluminate were assumed to be responsible for these area increments. Only 5 % SO2 removal was observed with untreated fly ash after 1h of contact and at high humidity. High SO2capture occurred with hydrated mixtures. SO2removal correlates well with the relative surface area increment. The amount of SO2 capture increases strongly with the increasing relative humidity of the gas. Introduction Severe limits on emissions of sulfur dioxide from coal combustion plants have risen drastically for large, new plants. These limits are not as strict for retrofitting existing power plants. These regulatory changes have resulted in the development of a variety of processes for removing SO2,removal of sulfur from flue gases being the most widely used process (Merrick and Vernon, 1989). Flue gas desulfurization processes can be grouped into three broad categories, dry adsorption, wet scrubbing, and dry/wet processes, based on the mechanism of removing SOz. In wet scrubbing processes, the removal of sulfur is by chemical absorption using a limestone or slaked lime slurry in spray towers. Dry adsorption systems used physical or chemical sorption on charcoal or other sorbent in a typical fixed bed to remove SO2, but these systems are expensive and hardly used. In the last few years, dry/wet processes (Yoon et al., 1986; Statnick et al., 1987; FECSA, 1986; Merrick and Veron, 1989), which are based on the injection of a solid sorbent and water into the flue gas duct work, are being developed by several companies because of their inherent low capital cost. Dry/wet systems can be categorized, based on the way in which sorbent and water are introduced in the duct, into two groups: those that inject sorbent as an slurry, namely, Bechtel CZD (Abrams et al., 1985; Bechtel Corp., 1987), General Electric IDS (Yoon et al., 1986; Statnick et al., 1987; Shilling, 1986; Martinelli et al., 19871, and EPA E-SOX (Yoon et al., 1986; Sparks et al., 1985; Ponder, 1985); and those that inject dry sorbent and water separately, namely, Dravo HALT (Yoon et al., 1986; Statnick
et al., 1987; Babu et ai., 1986; Forsythe and Kaiser, 1985), EPRI (Yoon et al., 1986; McElroy, 1985; Hooper et al., 1985),and Consol Coolside (Yoon et al., 1986; Statnick et al., 1987; Yoon et al., 1985a,b; Conoco, 1985; Stouffer et al., 1989). Nearly all of these employ hydrated lime as the sorbent, and SO2removal, drying of sorbent particles, and gas humidification occur simultaneously. SO2is removed not only by wet sorbent particles in the duct but also when they have been dried and the gas humidified, which can take place in the last part of the duct and in the particulate collection system. The advantages of dry/wet processes over conventional wet methods are that a dry solid waste is produced and that the equipment is easier to set up in existing power plant. High SO2 removal is well-known to be carried out by hydrated sorbents in the presence of humidity. The amount of SO2captured at a given temperature increases as the adiabatic saturation temperature is approached (Yoon et al., 1985a,b; Stouffer et al., 1989). In an attempt to explain these findings, puzzolanic reaction between fly ash, Ca(OH)2,and water was hypothesized to be the main factor for SO2 capture (Jozewicz and Rochelle, 1986; Jozewicz and Chang, 1987). So, in the range of temperatures from 20 to 100 "C, hydrated calcium silicate (CaO*Si02H20), dicalcium silicate hydrate (2Ca0*SiO2-H2O), and tetracalcium aluminate hydrate (4Ca0.A1203.13H20)can be formed. The presence of sulfates in the fly ash can lead to the formation of gehlenite (2Ca0.Al2O3.SiO2-8HTO), ettringite (3Ca0-A1203-3CaS04.32H20), and tetracalcium aluminate monosulfate (3Ca0A1203.CaS04-12H20).These highly hydrated products are probably responsible for SO2 removal.
0888-588519112630-2143$02.50/0 0 1991 American Chemical Society
