I
KENNETH A. KOBE1 and JOHN T. FORTMANL
University of Texas, Austin, Tex.
Nitration of Ortho-
ononitrotoluene
With proper reactor system design, rates of reaction in commercial nitration can be greatly increased NITRATION of an aromatic hydrocarbon and its mono and dinitro derivatives is carried out commercially with a nitric-sulfuric acid mixture. Because accurate prediction of reaction rates is essential to proper reactor design, the study described below was undertaken to provide data which would show the relative importance of the major process variables, including the composition of the mixed acid, the phase volume ratio, space velocity, and reaction temperature. Very high reaction rates were observed a t all conversions for the nitration of onitrotoluene to dinitrotoluene over a range of the above variables overlapping normal commercial practice. Mass transfer effects often limit commercial reaction rates but were eliminated from this study, since their magnitude is solely dependent on .physical equipment design. In order to eliminate the mass transfer effect, a reactor with an effective volume of only 1.26 ml. was used. I t was stirred a t 16,000 r.p.m. The data collected, therefore, represent absolute chemical reaction rates, unaffected by mass transfer. Under these conditions, acid mixture composition was shown to have the most pronounced effect on reaction rate while temperature was least significant in the range studied. Plotted rate and conversion data permit graphical interpolation of the results for each of the variables. The nitration of o-nitrotoluene showed high rates of reaction in the acid concentration range a t which dinitration supposedly would not occur because: the lack of the presence of the nitronium ion claimed by Chtdin ( 4 ) , and the high water-sulfuric acid ratio claimed by Bennett (7) and Hetherington and Masson ( 7 7 ) . However, Bonner (2) also was able to obtain appreciable rates of dinitration in weak nitrating acids. The rate based on the volume of the acid phase was not independent of the ratio
-
-
Deceased. Present address, Universal Oil Products Co., Des Plaines, 111. 2
of the phase volumes a t the same fresh acid strength. This is a t variance with the results of McKinley and White (17) for the nitration of toluene. The rate and conversion data were plotted to allow visual interpolation over the entire range of the variables studied.
Experimental
Mechanism. The nitronium ion concept was first introduced by Euler ( 5 ) . I t has since become the accepted aromatic nitration mechanism theory. Ingold and coworkers (6, 7, 9, 70, 72) are largely responsible for the present acceptance of this theory. These authorities believe that the mechanism is an electrophilic displacement reaction. The nitronium ion, NOz+, attaches itself to a carbon atom in the aromatic nucleus by displacing a hydrogen atom, expelling the latter in the form of a proton. The existence of the nitronium ion was conclusively demonstrated in 1946 ( 4 ) . Kinetic studies of aromatic nitration reaction order indicate that the nitronium ion is the attaching agent.
These four steps are essential to nitration with nitric acid (13): HA
+
"03
HzNO,+
$ HzNOz+ --+
+
+ A-
N O % + HzO
+ ArH ArHNOs+ + A NOz+
-.f
+
ArHNOz+
ArN02
+ HA
(1) (2)
(3) (4)
Ar represents the aromatic nucleus. The acid, HA, is the strongest acid present. Nitration is not only dependent upon the attack of the nitronium ion on the aromatic nucleus, but also on the effective removal of the proton by a capable proton acceptor. The rate of nitration thus is dependent on the ability of the solvent medium to form the nitronium ion and act as a proton acceptor. The nitronium ion and the ion from which it is formed, the nitric acidium ion, are the only agents capable of nitrating those aromatic compounds which are relatively difficult to nitrate. The nitronium ion nitration is the more significant of the two. Equipment and Techniques. The purity of the o-mononitrotoluene (MNT) used was determined by infrared spectrophotometry to be 99.9% pure. T h e mixed acids were prepared from C.P. sulfuric and nitric acids.
DINITROTOLUENE FACTS Preparation Nitration of nitrotoluene with hot nitrosulfuric acid
Purifkatio n
Crysta I I iza tion Uses Organic syntheses intermediate Preparation of: toluidines dyes explosives VOL. 53, NO. 4
APRIL 1961
269
A flow reactor used previously (3, 75): was modified for the nitration of onitrotoluene to dinitrotoluene (DNT). Since dinitrotoluene is a solid at room temperature, the major modification required in the reactor was the separation of the cooling medium and the product quench water.
-I
INCH-
This miniature jacketed kettle provided absolute chemical reaction rate data The reaction chamber, A, i s separated from the outer chamber, 5, b y the water jacket. The cap, C, contains two vertical holes, one for the thermocouple, D , and the other for the turbine agitator shaft, E. The Teflon gasket, F, seals the water jacket from the reaction chamber. The discharge line, G, i s separated from the water jacket b y a plug through which the reactant exit tube, H, extends a short distance. Water entering the discharge tube near the
end o f the reactant exit tube i s used to cool the products and ensures quenching o f the reaction b y dilution at the point o f discharge from the reactor. The reactants enter the reactor by tubing, I, which pass through the water jacket at opposite ends o f the reactor. The cooling water entrance and exit lines are adjacent t o each other. Therefore, a baffle is located between them across the bottom o f the reactor in the annulus between the outer chamber and the reactor. The thermocouple enters the reactor through a hole in the cap, C. The ironconstantan thermocouple i s covered with raw, unset Teflon, thermally set and machined to fit the well. The effective volLme o f the reactor with the turbine agitator in place was 1.26 ml.
Reactor System. Because the subject reaction was more difficult to control than the mononitrations previously studied, several modifications were made in the reaction system (figure below). An electric motor was used to drive the turbine agitator. With this motor and the new, rigid, easily adjustable suspension and alignment system, reproducible nitrations were carried out over a range of agitator speeds between 9000 and 17,000 r.p.m. Because flow rates could be controlled better at higher agitator speeds, lG,,OOO r.p.m. was used. The reaction temperature indicating system cnnsisted of a Teflon-covered, ironconstantan thermocouple connected to a recording potentiometer. The constant temperature bath supplying water to the reactor jacket was adjusted manually
for flow rate and temperature level to give the desired reaction temperature reading on the potentiometer. Cooling water temperatures were adjusted to within 5 O C. of the reaction temperature in order to avoid temperature gradients in the reacting medium. Sufficient quench Jvater was supplied to the discharge line to stop the reaction at the point of discharge from the reactor. At a high pzrcentage of conversion the reaction products solidified a t normal quench water temperatures. To prevent plugging of the discharge line, the quench water was heated to 50' C . Displacement of at least 10 reactor volumes of steady-state operation was allowed to ensure that equilibrium was attained prior to collecting a sample. About 50 rnl. of reaction products was collected. The system was then adjusted to the conditions selected for the next nirration. Because of the difficulties encountered in obtaining steady-state conditions a t start-up, all nitrations were made in sets of five in which only the space velocities were changed. This procedure lessened the number of start-ups required and decreased the possibility of inconsistent results within each set of nitrations. Criteria for Successful Operation. The reaction temperature, being the most sensitive variable, was not allowed TO vary by more than k0.3' C. from the desired temperature. The reactant flow rates were not allowed to vary by more than &0.5yoof the acid flow range studied. Any difficulty in controlling steady-state conditions was a basis for discarding the sample.
AUTOTRANSFORMER
GITATOR MOTOR
THERMOMETER
TOLUENE
MOUNTING RACK AND ALIGNMENT MECHANISM
The continuous flow system effected the nitration o f o-nitrotoluene
270
INDUSTRIAL AND ENGINEERING CHEMISTRY
TEMPERATURE SYSTEM
0-MON ON I T R O T O L UENE I36
Previous Work
I34
For the most part, the literature on the nitration of toluene and its derivatives is concerned with the mononitration reaction. There is little information on the dinitration reaction except for the distribution of the isomers produced by nitrating the monoisomers. Some kinetic information on dinitration has been obtained as a secondary result of various investigations on mononitration. Gillespie a n d Millen (8) have reviewed the important work concerned with the modern theories of aromatic nitration. The following are the conclusions of various investigators:
I32
I30 I ea I
\
(3
126
i
E
124
z
122 I20 I18 I16
The products of the nitration of o-nitrotoluene are 2,4- and 2,6dinitrotoluene in the ratio of 2 to 1, respectively, with only trace quantities of the other possible isomers formed (18, 19). These conclusions were verified as a secondary result of this work. 0 The mechanism of dinitration is the same as that for mononitration; the nitronium ion is the nitrating agent.
At least 90%, a n d perhaps all, of the nitration reaction takes place in the acid phase of mixed acid, two-phase nitrations (16,17). Ktration rates are independent of the ratio of the hase' volumes when based on tge volume of the acid phase (17). T h e rate of dinitration approaches zero when the mole ratio HgO to HzS04 approaches unity (1, 16). Some active aromatic compounds have exhibited considerable nitration rates beyond this acid ratio limit ( 2 ) .
The final acceptability test for any data obtained was consistency with the other data of the set with which they were taken. T h e conversion analysis of the set of five nitrations was plotted against space velocity. I n this testing of the data, more weight was given to the points indicating a higher conversion. This was justified by the possibility that undetected cavitation had occurred, which would result in less than anticipated conversion. Many series of nitrations were repeated to obtain consistency with the above criteria for successful operation. Ranges of t h e Variables Studied. T h e ranges of the variables studied were as follows :
1. Acid compositions, designated by mole per cent sulfuric and nitric acids;
I14
I121
o
I
IO
I
20
I
40
30
50
60
70
CONVERSION, Co. MOLE 'A
Figure 1 .
H2SOcHNOa,
ao
90
100
DNT
Conversions were measured by means of density
Mole %
HzSOcHNO Mole yo
30-15 35-15 40-15 45-15 50-15
40-5 40-10 40- 15 40-20 40-25
a,
2. Temperatures, O C.; 50, 60, and 70. 3. Acid to organic volume ratios, ( A / O ) ; 1, 2, and 3. 4. Space velocities, ( F / V ) and the corresponding total volumetric flow rates, ( F = A 0);
+
F / V , Min.-'
F , RiIl./Min.
1.59 3.18 4.76 6.35 7.94
2 4 9 8
10
Each set of nitrations represented five different space velocities; all other variables were held constant. For each of the acid compositions, 35-15, 4015, 45-15, and 50-15, three acid to organic ratios ( A / O ) were studied at three temperatures. The remaining acid compositions were studied at an ( A / O ) of 1 and at 50" C. Product Analyses. Dinitrotoluene composition of the samples was obtained from a density-composition correlation curve. This method had the advantage of not requiring separation of the compounds, since mono- and dinitrotoluene are not separable by any simple process. This procedure is a modification of the analytical schemes used by McKinley and White (77) and James (14).
A glass pycnometer of approximately 10-ml. capacity was used to determine the densities of the dry, acid-free reaction products. A 60° C. (=tO.0lo C.) constant temperature water bath was used to maintain the temperatures of the reaction products and the pycnometer to a constant temperature. T h e density us. composition curve of the o-MNT-DN?' system was established by density determinations with corresponding infrared spectrophotometric analyses obtained from Picatinney Arsenal, Dover, N . J., and Joliet Arsenal, Joliet, Ill. Results of these analyses are shown in Figure 1. Because the analyses from Joliet Arsenal are comparatively more consistent, these data were weighted more heavily in establishing the density-conversion calibration curve. Density determinations were reproducible within 0.0001 gram per ml., which accounted for the maximum deviation of less than 0.1% of conversion. All the data in the lower half of the conversion range, except one point, were within 1% conversion. Since the one point deviated 570 conversion from the remaining data, it was discarded. All the data in the upper half of the conversion range, except two points, were within 2.5% conversion. Since the two points deviated by 4 and 5% conversion from the remaining data, they also were discarded. T h e accuracy uncertainty of the two halves of the conversion range are, therefore, 1% for the lower half and 3yo for the upper half. Conversions, CO,in units of mole per cent DNT, of the nitration products were VOL. 53, NO. 4
APRiL 1961
271
60 0
3000
500
2000
400
300
Ra Xrn
1000 900
200
800 700 600
500 IO0 90 80 70 60
400
300 0
5
IO
15
20
25
30
35
40
45
50
55
co
200
For 35-15 acid
I50 0
10
20
50 60 70 80 90 100
30 40
co For 40-1 5 acid 7000 6000
5000 4000
20,0001
1
1
1
I
1
1
1
'
1
I
1
3000 10,000 8000
2000
6000 5000 4000
3
3000
Xm
IO00 900 800 700 600 500
2000
1000
400 300 25 30 35 4 0 45 50 55 60 65 70 75 80 85 9 0 95 100
800 600 500 4001
LEGEND.
-1
35 40
1
45
co
5OOC. 6OOC.
........ 7OOC.
50
55
60 65
70
75
8 0 85
90
I
95 100
CO
For 45-1 5 acid
For 50-1 5 acid
Figure 2. Reaction rate, conversion, space velocity, acid-organic phase volume ratio, temperature, and acid composition are interrelated
determined by using Figure 1 from the determined densities of the samples for each nitration. The resulting conversion data were used in calculating rates of nitration reaction.
Results Nitrations were made in a homogeneous two-phase, or perfectly stirred, reactor, with the assumption that the compositions of the reactants in the reacting media are identical to the compositions of the products in the reactor effluent stream. Because of the reactor design and water dilution of the products, it was not possible to analyze
272
the acid phase of the product stream. Consequently the resulting data were correlated on the basis of the concentration of the entering acid phase. The perfectly stirred reactor eliminates the mass transfer variable. I t has been determined experimentally that (for the particular design of agitator and reactor used in this study) for any agitator speed over 9000 r.p.m., the conversion, and hence the rate of reaction, is constant. The rates reported are absolute rates of chemical reaction. Internal Consistency. The plotted data representing the 35-15 and 40-15 acids showed all conversions within 0.670. The plotted data representing the 40-5,
INDUSTRIAL AND ENGINEERING CHEMISTRY
40-10, 40-15, 40-20, and 40-25 acids were consistent with respect to the curves within 0.2570 in conversion. The nitrations fromwhich thesedatawere obtained were easily controlled. The percentage conversions, reaction rates, and resulting heats of reaction were low enough to allow the reactor cooling medium temperature to be within a few degrees of the reaction temperature. A temperature difference of more than a few degrees is not desirable in the study of reaction rates, since temperature gradients could be present in the reaction zone even under conditions of extreme turbulence. The plotted data representing the 45-
olMONONITROTOLUENE 15 and 50-15 acids showed the lowest degree of consistency, and were within 2 and 470 conversion, respectively. Nitrations from which these data were obtained were not easily controlled. The resulting heats of reaction, corresponding to nitrations with high percentage conversion and rate of reaction, required the temperature differential between the reaction zone and the cooling water medium to be more than a few degrees. Ordinarily, rate data would not have been collected under the undesirable conditions described. However, the 45-1 5 and 50-1 5 acid strengths are close to those acids now being used in the manufacture of D N T ; therefore, data were collected for these acids. The dispersions of all the data were within the limits of accuracy of the density-conversion correlation curve for the range of conversion which the data exhibit. Calculation of Rates. Reaction rates were calculated for each of the nitrations from the conversion analyses, space velocities, and volume ratios of the acidto-organic phases. Several investigators (71, 77) have shown that nitration occurs almost exclusively in the acid phase. Since the rates were most effectively correlated with this assumption, the reaction rates based on the volurnr of the acid phase, R,, have been reported. In order that the reaction rates used for
comparison would have a common basis, the rates R,, were divided by the mole fraction of M N T in the organic phase, X,. This manipulation suggests that the value of R,/X,,, is the rate which would be realized if the organic phase were pure o-nitrotoluene. This rate is independent of the organic phase composition and may be plotted against C,, with parameters of temperature, space velocity, and acid-to-organic volume ratio. This method of plotting nitration rate data is accepted as the most effective (3, 75, 77). The rate of reaction, R,/X,, was very sensitive to X , when the conversion exceeded 9570. Because the densityconversion correlation curve was only accurate to within 3% in the higher range of conversion, the rates calculated from the conversions over 95% are not reliable. These rates are given here only to indicate trends and are not to be mistaken for absolute values. Method of Plotting. The data were plotted in such a manner as to present as much information as possible on each plot. In Figures 2 and 3, the reaction rates were plotted on a log grid against the percentage conversion, to show the effect of the variables on the rate. This method also exhibited the best correlation. In Figure 2, all of the experimental data were plotted for the 35-
15, 40-15, 45-15, and 50-15 acids, respectively. These plots show the effect of all the variables except the acid compositions. The experimental data points are not shown on the plots, which have three parameters; A / O , F / V , and temperature. The relationship among thew variables is shown. Each surface represents a temperature. The parameters which exhibit a negative slope are lines of constant, A / O , and those of a positive slope are lines of constant, F/ V. Figure 2, showing the 45-15 acid, exhibits the values of each of the parameters. Figures 3 and 4 of the series exhibit the values of A / O on the 50' C. surface and the values of F/ V on the 70' C. surface. The units are consistent with those used throughout this study. Values of R J X , over 18,000 are not shown since they were inherently erroneous. The effects of nitric and sulfuric acid compositions were plotted in Figure 3. These plots show the effects of varying acid concentrations on the rate and the conversion, with space velocity as a parameter. The effects of temperature of reaction and A / O on the rate are shown in Figure 4. The reaction rate was plotted on a log grid against temperature for various sulfuric acid compositions with d / O as a parameter.
3000 2000
IO00 800 60 0 500 400 300 Ra -
200
Xrn
IO0
80
60 50
40 30 20
0
5
IO
15
20
25
30
co Effect of HNOa composition
Figure 3.
35
40 7 5 5 0
m
foi =
1.0
5OOC.
IO 5
IO
15
20
25 co
30 35
40
45
50 5 5
Effect of HzSO4 composition
Reaction rates and conversions are strongly influenced by acid composition
VOL. 53, NO. 4
e
APRIL 1961
273
ACID
1000
Maximum Percentage Error in Reported Rates Percentage Error in Rate at
4
Ra/Xm 50-15
Co
10,000
I
1,000 500
600
100
5 50%
..
4.5 3.8 1.6
Co
2 50% 20 12 10
..
A/ 0
C,
DNT DSTT = dinitrotoluene F = total volumetric flow rate, ml./ min.
F/ V
MNT
R,
40-15
300
35-15
200
X,,
35-15 35-15
100 80 601
I TEMP.,
O C
Figure 4. Temperature exhibited less effect on reaction rate than did the other variables
Discussion
By using the curves and surfaces shown in Figures 2 through 4, any conditions of the variables may be interpolated visually over the entire range of the variables studied. Nitric Acid Concentration. T h e rate, R,/X,, is more sensitive to a n increase in nitric acid composition in the lower concentration range. T h e conversion, C,, is more sensitive to a n increase in nitric acid composition in the higher concentration range. Figure 3 (left) shows the relative nitrating strengths of several compositions of nitric acid over the range of 5 to 25 mole yo nitric acid. Sulfuric Acid Concentration. The effect of sulfuric acid concentration over the range from 30 to 50 mole yo is similar to the effect of nitric acid over the 5 to 25 mole yo range. T h e rate and conversion are slightly less sensitive to an increase in nitric acid concentration. In general an inSpace Velocity. crease in space velocity, F/V, increases the rate, R,/X,, and decreases the conversion, C,. T h e rate is extremely sensitive to a n increase in space velocity, particularly in the range of high conversions where the spent acid composition varies significantly from that of the fresh acid. T h e conversion is much less sensitive to changes in space velocity. An increase in space velocity decreases the conversion more slowly a t lower rates, which correspond to weaker acids, lower A / O , and lower temperatures.
Acid-to-Organic Phase Volume Ratio. McKinley and White (77) nitrated tol-
274
INDUSTRIAL AND ENGINEERING
uene and correlated their data on the volume of the acid phase, indcpcndcnt of the ratio of the phase volumes. The rate data collected in this study of the nitration of o-nitrotoluene do exhibit the ratio of the phase volumes, A / O , as a significant variable, even though the rates have been calculated and correlated in a manner similar to that used by McKinley and White. In general a n increase in A/O increased both the rate and the conversion Temperature. Temperature has the least effect of any of the variables on the rate of reaction, R,/X,, and the conversion, C,. T h e rate and conversion are both increased by a n increase in temperature, but are less sensitive in the range of lower rates and conversions corresponding to lower A I 0 and F / V . Figure 4 shows that an increase in A / O does not influence the temperature effect on the rate of reaction. Reliability of Results. T h e rate data collected in this study were calculated from conversions. T h e accuracy with which the conversions could be determined depended on the accuracy of the analyses from which the density-conversion correlation was made, temperature control of the reaction, and reactant flow rates. The accuracy limits of 1 and 3Yc (corresponding to the lower and higher halves of the conversion range, respectively) did not include the probable errors incurred in controlling the reaction temperature and metering the reactantb. Including the latter two sources of error, the maximum errors in the conversion were 1.7 and 3.7% in the low and high ranges of conversion, respectively. The above table approximates the maximum error possible for srveral values of the rate, R,/X,, corresponding to both the high and low ranges of conversion. The error indicated in the table accounts for all the known possible sources of error, assuming that all errors possible are additive.
Nomenclature
40-15 = composition of the mixed acids in mole percentages; the first figure representing sulfuric
CHEMISTRY
acid and the second nitric acid = volume ratio of acid to organic phases = per cent MNT converted to
= space velocity, min.-l = mononitrotoluene = rate of reaction, gram moles of
DNT formed per hour per liter of acid phase = mole fraction of MNT in the organic phase
Literature Cited
(1) Bennett, G. M.: Brand, J. C. D., James, D. M., Saunders, T. G., Williams, G., J . Chem. SOC.1947, pp. 1185590. (2) Bonner, T. G., James, M. E., Lowen, A. M., Williams, G., Nature 163, 95-5 (1949). (3) Brennecke, H. M., Kobe, K. .4., IND. ENG.CIIEU.48, 1298 (1956). (4) ChCdin, J., Mkm. fioudres 28, 7--42 (1938). (5) Euler, Hans, Ann. 330, 280-91 (1903). ( 6 ) Gillespie, R. J., Graham, J., Hughes, E. D., Ingold, C. K., Peeling, E. R. A., Nature 158, 480 (1946). (7) Gillespie, R. J., Graham, J.? Hughes, E. D., Ingold, C. K., Peeling, E. R. A , , J . Chrm. SOC.1950. n. 2504. (8) Gillespie, R. J., Millen, D. J., Quart. Reo. (London) 2, 277-306 (1948). (9) Goddard, D. R., Hughes, E. D., Ingold, C. K., Nature 158, 480 (1946). (10) Goddard, D. R., Hughes, E. D., I n gold. C. K.. J . Chin. SOC.1950., ID. 2559. (11) Hetherington, J. A., Masson, I., Ibid., 1933, pp. 105-14. (12) Hiighes, E. D., Ingold, C. K., Reed, R. I., Ibid., 1950, p. 2400. (13) Ingold, C. K., “Structure and Mechanism in Organic Chemistry,” p. 269, Cornel1 University Press, Ithaca, N. Y . , 1953. (14) James, C. M., M.S. thesis, University of Texas: Austin, Tex., 1956. (15) Lakemeyer, ,J. L., Zbid., 1956. (16) Lewis, W. K., Suen, T. J., IND.ENG. CHEM.32, 1095-1101 (1940). (1’7) McKinley, C., White, R. R., Trans. ilm. Insf. Chem. Engrs. 40, 143-75 (1944). (18) Smith, E. L.: Hansknecht, C., “ T N T Manufacture Operatin%Manual.” U. S. Rubber Co., Joliet Arsenal-KNK, Joliet, Ill.? 1953. (19) Watkins, J. D., M.A. thesis, University of Texas, Austin, Tex., 1957. I
I
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