Ind. Eng. Chem. Process Des. Dev. 1981, 20, 82-85
82
Maximization of the Conversion to m -1oluenesulfonic Acid in Liquid-Phase Sulfonation Was S. Patwardhan and Roger E. Eckert' School of Chemical Engineering, Purdue University, West Lafayette, Indiana 47906
The ability to alter the distribution of positional isomer products of chemical reactions in a controlled manner offers great potential for selectivive synthesis of commerdaly useful compounds. Sulfonation of toluene Is representative of such processes. This reaction normally yields little m-toluenesulfonic acid. However, the conversion to the meta isomer can be increased manifold by proper selection of experimental conditions. The maximum conversion found in this study is about 31 % at a reaction temperature of 100 O C using 97.4% sulfuric acid. These experimental condiiis were found by following the path of steepest ascent and exploring the optimum region. Process variables investigated include acid strength, temperature of reaction, rate of sulfuric acid addiilon.
Sulfonation of toluene produces toluenesulfonic acids which are important as intermediates since the sulfonic acid group may be replaced many other groups. This reaction can be stoichiometrically represented as CGHbCH, + HzS04 CH, - CGH4 - SOBH + HzO Sulfonation of toluene is representative of chemical processes in which several positional isomers are formed. In general for organic compounds positional isomers vary in physical properties and chemical reactivity. Therefore synthesis routes and the properties of the product are considerably affected by the particular positional isomer employed. The utility of proper selection of the isomer is illustrated by several important applications of organic compounds. Solubility and chemical reactivity are of primary importance in both the agricultural chemical and the pharmaceutical fields. The properties of moleucles and the resultant biological activity in such applications are often dramatically adjusted by selection of the different positional isomers. Therefore, the importance of controlling chemical reactions to affect the distribution of positional isomers has a sound base for commercial application. The methyl group on the benzene ring is ortho and para directing and sulfonation of toluene is reported to result in an isomer distribution of 62% p-, 32% 0-,and only 6% m-toluene sulfonic acid (Morrison and Boyd, 1973). This isomer distribution can be qualitatively explained based on electronic and steric considerations (Broyles and Eckert, 1973). The effect of experimental conditions on the product distribution in the liquid-phase sulfonation of toluene has been studied previously by various workers (Holleman and Caland, 1911; Englund et al., 1953; Spryskov, 1960; Cerfontain et al., 1963a). Later a more extensive investigation was carried out by Broyles and Eckert (1973),who studied the effect of temperature, time of reaction, sulfuric acid concentration, mole ratio of reactants, and time of addition of sulfuric acid on the conversion to m-toluenesulfonic acid and also to total toluenesulfonic acids. Interest in the meta isomer was generated by the potential of increasing the amount of this low yield compound. They found that a higher concentration of sulfuric acid and a longer time of reaction increased the conversion to m-toluenesulfonicacid. However, the effect of time of reaction decreased considerably at higher acid concentrations. They put an upper limit of 97% on the acid strength used in their study. The conversion to m-toluenesulfonic acid went through a maxi-+
mum a t 88 "C; thus temperature did not increase the conversion monotonically. The mole ratio of reactants and time of acid addition did not have appreciable effects. Their work indicated that a further increase in the conversion to m-toluenesulfonic acid may be achieved by increasing the sulfuric acid concentration beyond their upper limit of 97%. The present study was aimed at investigating the effect of higher concentrations of sulfuric acid, including oleum, along with the effect of other variables at this new acid concentration. Analytical Procedure Broyles and Eckert (1973) used a spectrophototmetric method for analysis of the products. This method was based on a linear resolution of the absorption spectrum of the product, in terms of the spectra of pure isomers of toluenesulfonic acid, 81.28% H2S04being used as the solvent. The para isomer is accurately estimated by this method. However, due to the great similarity in the shapea of the spectra of the ortho and meta isomers, these two are difficult to determine quantitatively in the analysis. A more accurate method for the estimation of the meta isomer has been described (Cerfontain et d,1963b),which is based on the disulfonation of the product. In weak oleum solution, 0- and p-toluenesulfonic acids are both converted into toluene-2,4-disulfonic acid while the meta isomer forms mixture of toluene-2,5- and toluene-3,5-disulfonic acid. The spectra in oleum of toluene-2,4-diusulfonic acid and of the mixture resulting after disulfonation of the m-toluenesulfonic acid show a fair difference in shape. Thus the spectrophotometric analysis in oleum gives a more accurate estimate of the mtoluenesulfonic acid. In our study, the para isomer was estimated by the former method and the meta isomer was estimated by the latter, more accurate method. Highpurity isomers were prepared and used to obtain spectra and verify this analytical procedure (Patwardhan, 1971). Experimental Section The reaction was conducted in a cylindrical roundbottomed glass vessel of 1.2 L capacity, with four equally spaced, deep, internal creases which acted as baffles. The reaction vessel was equipped with an agitator, a reflux condenser, a thermometer, and a dropping funnel. The reaction was carried out by preheating 1.5 g mol of toluene in the reaction vessel to the reaction temperature and adding sulfuric acid uniformly over a predetermined time. Vigorous agitation was used throughout the reaction, while
0796-4305/81/1120-0082$01.00/00 1980 American Chemical Society
Ind. Eng. Chem. Process Des. Dev., Vol. 20, No. 1, 1981 89 Table I. Levels of Independent Variables and Results % conversion to toluenesulfonic acid
acid concn
N
run no.
% acid
1
35.8
95.5
2
35.6
94.9
3
40.0
104.1
4
40.7
104.9
5 6 7
41.2 41.9 38.9
105.1 106.2 102.5
8
41.9
106.2
9
38.9
102.5
10
40.6
104.8
11
41.9
106.2
12
41.9
106.2
13
36.6
97.4
14
39.0
102.7
15
35.8
95.5
Experiments with
% free SO, temp, "C time, h 0mExperiments with R = 1.5, A = 20 min a8 2.7 15.8 24.8 3.7 14.9 22.5 95 2.7 11.3 30.4 3.7 13.9 33.1 17.8 88 2.7 14.9 33.8 3.7 23.0 27.9 21.8 95 2.7 14.6 32.2 3.7 12.5 29.0 24.4 24.2 22.7 97.7 3.2 25.7 23.0 27.6 99 3.2 24.6 26.1 11.1 96.3 2.7 22.1 28.0 3.7 18.5 30.3 27.6 96.3 2.7 29.9 21.0 3.7 11.1 29.6 99.0 2.7 20.9 3.7 19.4 28.8 21.9 29.6 21.3 100.3 2.7 3.7 22.4 27.4 27.6 93.7 2.7 21.0 25.6 3.7 19.1 28.4 27.6 aa 2.7 20.4 27.8 3.7 22.3 27.9 100 2.7 19.7 33.0 3.7 20.3 28.9 12.0 93.7 2.7 23.1 24.9 Experiments with R = 4.5, A = 87 min 88 2.7 19.9 26.7 3.7 16.0 29.5 variable R and A and Fixed C = 39.ON, T = 93.7 ' C , e = 2.7 h
P59.4 62.6 58.3 53.0 51.3 49.1 53.2 58.5 51.4 49.3 49.3 49.9 51.2 49.1 49.5 51.8 48.5 50.2 53.4 52.5 51.8 50.8 47.3 50.8 53.0 53.4 54.5
% conversion to toluenesulfuric acid
run no.
R
16 17
1.5 4.5 1.5 4.5
ia
19
A, % 12.5 12.5 100.0 100.0
the temperature was maintained constant to f0.5 "C. At the end of the reaction time about 50 g of the product was withdrawn and poured into about 100 g of cold water to quench the reaction. This was extracted twice with about 50 mL of cyclohexane. The water layer was separated, filtered, and put under vacuum to remove the dissolved cyclohexane. It was then analyzed by the two separate methods mentioned before. Design of Experiments The aim of this study was to maximize the conversion to m-toluenesulfonic acid in the liquid phase sulfonation of toluene using sulfuric acid stronger than 97 % , including oleum. Initially it was decided to investigate the effect of the acid concentration, temperature, and reaction time. These are the three factors which were previously found to have a significant effect on the conversion to m-toluensulfonic acid (Broyles and Eckert, 1973). The time of acid addition was fixed at about 20 min. Thus addition of acid was completed soon enough to give the reactants sufficient time to react completely, and slowly enough to keep the temperature under control within f0.5 "Cin spite of the highly exothermic reaction. The acid to toluene mole ratio was fixed at 1.5 to ensure complete reaction in reasonable reaction time. The time of reaction was varied between 2.7 and 3.7 h. These levels of time coincided with those used by Broyles and Ekkert (1973). The temperature was varied between 88 and 100 "C.At temperatures much above 100 "C the oxidation reaction becomes more and more important, resulting in colored products. The con-
0-
23.1 24.3 24.8 27.1
P-
m24.9 21.0 24.0 23.3
53.0 54.7 51.2 49.6
::
IN
35 "
Temperature 1.t)
Figure 1. Yield of m-toluenesulfonic acid and level curves of the regression equation.
centration of sulfuric acid was varied between 95% sulfuric acid and 27.6% oleum. The calculations for a few initial experiments such as run no. 1at a lower acid strength and temperature showed overall conversions above 98%. Therefore complete conversion of toluene was assumed in this study. The method of optimization by use of the path of steepest ascent (Box et al., 1963) starts with a two-level factorial experiment to determine the direction of this path. Runs are conducted along the path and further experimentation is conducted around the best value obtained. In this study runs 1-4 form a two-level fractional design in the three factors, temperature, concentration, and
84
Ind. Eng. Chem. Process Des. Dev., Vol. 20, No. 1, 1981
Table 11. Regression Analysis for Runs 1-14. Model: Y = b , t b,C + b,T + b,CT b0 bl b,
b,
coefficient -560.71 14.75 6.331 -0.1586
std error 704.0 5.19 2.17 0.0552
F 7.56 8.09 8.51 8.25
Fn.95
4.32 4.32 4.32 4.32
time (See Table I and Figure 1). The low conversion to meta in run 1 at low temperature and concentration directed the path of steepest ascent (runs 5 and 6) toward higher temperature and concentration while time did not have a significant effect. Conversions to meta decreased along the path indicating a maximum had been passed or the response surface was more complicated such as a saddle point or ridge. More runs in the region of this path (runs 7-10) confirmed that better conversions are in the region toward the original factorial. Finally, runs 11-14 were added to expand the range of the design and particularly to provide information on high acid strength with low temperature and on low acid strength with high temperature. Analysis of these data to obtain the response surface of Figure 1 is discussed in the Results section. Other runs were conducted to establish the analytical technique comparison and investigate two other factors that had not appreciably affected conversion to meta in Broyles and Eckert’s study. Run no. 15 was so designed that the experimental conditions were close to those used in two of the experiments reported by Broyles and Eckert (1973), so that the analytical procedures could be compared. In runs 1 to 14, the reactant ratio and the time of acid addition were held constant because they were not expected to have appreciable effects. Runs 16 to 18 were designed to give some idea about the effect of these two variables at the high acid concentrations used in this study. Results In runs 1to 14, the conversion to m-toluenesulfonic acid (Y) depends upon the sulfuric acid concentration ( C ) ,the reaction temperature ( T ) and the time of reaction (6). A theoretical form of the relationship between these variables is not known. As a first approximation, a full-quadratic model in the three variables C, T, and 6,including linear, square, and cross-product terms was employed in a stepwise regression procedure to fit the data. Taking the significant terms from this model, a satisfactory relationship is expressed by the equation Y = -0560.7 + 14.75C + 6.3317‘ - 0.1586CT (1) Note that 0 does not appear in the equation showing the conversion to m-toluenesulfonic acid is not significantly altered by changing time of reaction from 2.7 to 3.7 h within the experimental ranges of the other variables. Broyles and Eckert (1973) had found the effect of time of reaction in the range of 2.7 to 3.7 h decrease considerably at high acid concentrations of 97 wt %. The lack of significant effect of time observed at these still higher acidities in this work is therefore consistent with the previously noted trend. In stepwise regression the significance of terms is judged relative to the residual mean square. The terms of eq 1 are significant as shown in Table 11, where the F values listed for each term are determined with that term in the last position in the equation. Figure 1 shows the level curves obtained from eq 1along with experimental results. It is seen from Figure 1that Y does not change very much with C and T. In fact the scatter of the data masks the small changes in Y. However, the figure does show the uniformly high values of Y that have been obtained in this
Table 111. Analysis of Variance for Runs 1-14. Y = bo t b,C t b,T t b,CT source d.f. sumssq. meansq. F regression residual LOF error
3 21 10 11
61.44 152.95 107.19 45.76
22.48 7.28 10.72 4.16
Model: F,
o1
5.40
3.59
2.58
2.85
Table IV. Effects of Ratio of Acid to Toluene ( R )and Acid Addition Time ( A )in Runs No. 14, 16, 17, and 1 8 term (R 1 (A 1 (RA 1
mean sq. 5.20 0.49 2.56
F 1.24 0.12 0.61
FO - 9 5 4.84 4.84 4.84
Table V. Comparison of Analytical Procedures present study run15 run15 C, % H,SO,
T,“C 0, h A,%ofe
Y, %
95.5 88 2.7 53.5 26.7
95.5 88 3.7 39.2 29.5
Broyles and Eckert (1963) 96.95 88 3.1 100.0 19.1
96.95 88 3.7 100.0 19.6
study. The experimental points and the level curves obtained from eq 1both show the same general trends in the variation of Y. For example, higher values of Yare obtained at high temperature and low acid concentration. However, the particular form of eq 1 is relatively unimportant; the unexpectedly high meta isomer content which has been achieved is the major result. Estimate of Experimental Error. Runs 1and 14 do not contain any duplicates. Still, it is possible to estimate the experimental error from these runs. Equation 1shows that time of reaction (6)does not have a significant effect on the conversion to m-toluenesulfonicacid (Y). Therefore the two values of Y obtained at different times of reaction (6) but at the same values of C and T can be regarded as duplicates. Eleven such pairs are found in runs 1 to 14. In the analysis of variance of the model, Table 111, the estimate of experimental error is based on these pairs of observations. The total regression model is clearly significant while the “lack of fit” (LOF) is not. Effect of Rand A . Runs 14,16,17, and 18 have the same values of C , T, and 6. These four runs constitute a 22 factorial design in R and A. A regression analysis of the results, shown in Table IV, indicates that none of the effects or the interaction are significant. Comparison of Analytical Procedures. The experimental conditions for run 15 fall close to those of two runs reported by Broyles and Eckert (1973) as shown in Table V. Under these conditions Y is rather insensitive to C, 0, and A , and the small differences in the experimental conditions are not likely to be important. It is seen that Y obtained in this study is considerably higher. The analytical procedure used in this study is more accurate than the one used by Broyles and Eckert (1973). However, since it is based upon carrying out one more reaction step (that of disulfonation), it is less precise as reflected in the standard deviation of Y. In the present study the standard deviation of Y was estimated to be 2.05% while Broyles and Eckert (1973) estimated theirs to be 1.13%. Conclusions The following conclusions apply to the ranges of variables previously given. 1. The maximum conversion to m-toluenesulfonic acid achieved in this study was about 31% at a reaction temperature of 100 O C using 97.4% sulfuric acid.
Ind. Eng. Chem. Process Des. Dev. 1981, 20,85-90
2. The use of oleum instead of concentrated sulfuric acid does not increase the conversion to m-toluenesulfonicacid appreciably. 3. If the acid concentration and the reaction temperature are both high, the conversion decreases. 4. The reactant ratio and the time of acid addition do not significantly affect the conversion to m-toluenesulfonic acid. Nomenclature A = acid addition time, min b = regression equation constants to be estimated C = feed acid concentration expressed in normality N = normality, mol of hydrogen ion/L R = mole ratio of acid to toluene T = reaction temperature, “C
85
Y = conversion to m-toluenesulfonic acid, % 8 = time of reaction, h
Literature Cited Box, G. E. P. et ai. “Deslgn and Analysis of Industrial Experiments”, 0. L. Davies, Ed., Hafner Publishing Co.: New York, 1963. Broyles, A. R.; Eckert, R. E. Ind. Eng. Chem. Rocess Des. Dev. 1973, 72, 295. Cerfontain. H.; Slxma, F. L. J.; Voiibracht. L. Recl. Trav. Chem. Pays-8as 1963a, 82, 659. Cerfontain, H.; Duin, H. G. J.; Voiibracht, L. Anal. Chem. 1083b, 35 1005. Engiund, S. W.; Aries, R. S.; Othmer, D. F. Ind. Eng. Chem. 1953, 45, 189. Holleman, A. F.; Caland, P. Ber. 1911, 44, 250. Morrison, R. T.; Boyd, R. N. “Organic Chemistry”. Aiiyn and Bacon, Inc.: Boston, 1973; pp 339-341. Patwardhan, V. S. M.S. Thesis, Purdue University, West Lafayette, IN, 1971. Spryskov, A. A. Zh. Obshch. Khlm. 1960, 30, 2449.
Receiued for review April 12, 1979 Accepted October 20, 1980
Simulation of Solidification Temperature Profiles in the Polyester Process for Immobilization of Hazardous Wastes R. Mahallngam,” R. K. Biyanl, and J. T. Shah Deparfmnt of Chemical Engineering, Washington State University, Pullman, Washington 99 164
As part of the development of a process for immobilizing hazardous wastes in a polyester matrix, an analysis is provided here for the prediction of temperature profiles during curing of the emulsion, by consideration of reaction exotherms and polymerization kinetics. Such anaiyses should be helpful in the optimal design of burial containers.
Introduction The dominant feature of current industrial development is an increasing concern to prevent pollution of the environment by hazardous industrial wastes. Until the seventies the operating philosophy was basically to treat the waste as necessary to meet the operating conditions while not exceeding existing regulations. A basic approach in waste management is to develop processes for the conversion of hazardous residuals in the form of liquids or semi-solid sludges to solids for safe handling, transportation, and storage with minimal potential for contamination of the environment. Immobilization (Neilson, 1977) of the waste is primarily concerned with the incorporation of the waste into a solidification agent. The basic operations are waste collection, waste pretreatment, solidification-agent mixing, packaging, and waste package handling. Pretreatment is primarily directed toward reducing waste volume, dewatering sludges, and adding chemicals for pH adjustment and foam prevention. The solidification operation is the most important stage of the immobilization process. A monolithic free-standing solid is formed by using solidification agents such as bitumen, hydraulic cement, absorbents, and organic polymers. If 100% retention of a waste for its hazardous lifetime is the goal of shallow land disposal, better waste processing techniques must be adopted. Recently, the feasibility of immobilizing hazardous wastes in a polyester matrix has been demonstrated by our research team, both in the laboratory and on the pilot plant 0196-4305/81/1120-0085$01.00/0
(Subramanian and Raff, 1975; Juloori, 1976; Wu, 1978; Jain, 1978; Mahalingam et al., 1977; Subramanian et al., 1977; Biyani, 1978; Washington State University, 1977). By finely dispersing the waste solution, slurry, or solids in a water-extensible polyester resin, each waste particle or droplet is individually encapsulated inside a thin skin of the resin matrix. Addition of an initiator polymerizes this resin matrix to produce a rigid monolithic solid suitable for land burial. One of the objectives in our pilot plant studies has been to develop a computer model, based on thermal analyses and polyermization kinetics, to predict temperature profiles during the curing of the emulsion; this should enable one to arrive at an optimum LID ratio for burial containers of various sizes. Some discussion is available in the literature on problems of nonuniform reaction due to heat transfer and the reaction exotherm (Horn, 1960; Lee and Neville, 1967; Doyle, 1969; Hills, 1971). Mathematical analyses, however, are limited. More recently, Progelhof and Throne (1975) have carried out one-dimensional transient heat conduction analyses of reactive, unfilled polymers and epoxies, showing that isothermal heat generation rates underestimate the nonisothermal values by more than an order of magnitude. Broyer and Macosko (1976) showed through their one-dimensional transient analyses that for cyclic processes, such as thermoset injection molding and reaction injection molding, it may be more desirable to control the heat flux through the mold walls rather than the wall temperature. Adabbo et al. (1979) have expanded the above work to include, among 0 1980 American Chemical Society
