- Email: [email protected]
Kinetics of isocyanuric acid pyrolysis
Journal of Analytical and Applied Pyrolysis, 28 (1994) 107-120
107
Elsevier Science Publishers B.V., Amsterdam
Kinetics of isocyanuric acid pyrolysis J. Mercadier, M. Pignon, F. Calabuig, J. Led& * and J. Villermaux Luboratoire des Sciences du GEnie Chimique, CNRS-ENSIC-INPL, B.P. 451, 54001 Nancy cedex (France)
1 rue Grandville,
(Received June 7, 1993; accepted June 21, 1993)
ABSTRACT The kinetics of isocyanuric acid pyrolysis are determined in a tubular continuous flow reactor under atmospheric pressure, for temperatures ranging from 698 to 798 K and with residence times from 1 to 73 s. The main product of the pyrolysis is isocyanic acid (HNCO), a monomer of isocyanuric acid. Isocyanic acid itself decomposes into different products. With correct choice of temperature and residence times, the yield of isocyanic acid reaches 85%. The Arrhenius parameters derived are compared with literature data. Isocyanic acid; isocyanuric acid; kinetics; plug flow reactor; pyrolysis.
INTRODUCTION
At ordinary temperature, isocyanuric acid (AIC) is a white, inert and non toxic crystalline solid which sublimes at Ts= 633 K [ 1,2]. Its general formula is
The commercial method for AIC preparation involves urea [3-51. Urea is first heated at 473-573 K to form biuret and several methods, including recycling of crude AIC, are employed to produce AIC. Trisubstituted organic derivatives (isocyanurates) can be formed from AIC. For industrial use, the greater part of AIC is chlorinated to form trichloroisocyanuric
* Corresponding
author.
01652370/94/$07.00 wnl
0 1994 - Elsevier Science Publishers B.V. All rights reserved
nifx_7~7nfcmm7h2-c
108
J. Mercadier
et al. / J. Anal. Appl. Pyrolysis
28 (1994) 107-120
acid ( C3ClsN303) [ 31 used in water treatment, particularly for swimming pools. The boiling point of HNCO is 296.5 K, but it can begin to polymerize at 273 K [2]. There is no industrial process to synthetize HNCO; on the laboratory scale it is generally produced from AIC thermal decomposition. As far as we know, little work has been devoted to the thermal behaviour of isocyanuric acid and no kinetics constants are available in the literature. East [6] pyrolysed AIC in a tubular continuous flow reactor between 723 and 923 K. The solid was introduced by means of an endless screw with a very low nitrogen flow rate in order to control the pressure. The AIC residence time is not reported, so no kinetic data can be derived from these experiments. The highest mass yield of HNCO is 84% and the main identified by product is HCN ( 10.1%). Several authors [ 7- 1l] used the pyrolysis of AIC to produce HNCO in order to study its chemical and physical properties, or to produce free radicals especially by photolysis of HNCO. Different by-products have been noticed - C02, HCN, CO, NH,, C2N2 - probably from HNCO decomposition. Details about processes are very scarce and no quantitative kinetic data are reported. Back and Childs [ 121.also studied HNCO pyrolysis in a quartz batch reactor at temperatures from 823 to 973 K, pressures from 670 to 2670 Pa and residence times of several minutes. CO*, CO, NZ, HCN and H2 were detected by gas chromatography. However, according to mass balances, the presence of NH3 and cyanamide (NH,CN) could not be excluded. According to these authors, the determination of quantitative kinetics of HNCO is quite difficult, particularly because of surface effects. Mertens et al. [ 131 and Kajimoto et al. [14] determined kinetic parameters in shock waves, thus under quite specific experimental conditions. Quantitative kinetic data were reported by Perry and Siebers [ 151 during the study of the reduction of nitrogen oxides in the presence of HNCO. The degradation reaction was carried out in a flow reactor (stainless steel). The temperature was 858 K, the pressure 68 x lo3 Pa and the residence time lower than 1 s. CO and CO* were detected as products as well. The pyrolysis of AIC produces several gases [ 7- 111. This may indicate that the reaction is followed by a series of consecutive reactions. Moreover, these gases are the same as in the case of the pyrolysis of HNCO [ 12-141. The purpose of this paper is to determine the kinetic parameters of AIC thermal decomposition performed in a continuous flow reactor, under atmospheric pressure and with gas residence times of a few seconds. A novelty of the apparatus is that decomposition occurs in a separate reactor downstream of a sublimation chamber of AIC. The reaction is monitored by the measurement of HNCO mass yield by high performance liquid chromatography (HPLC).
J. Mereadier
et al.
1J. Anal. Appl. Pyrolysis 28 (1994) 107-120
109
EXPERIMENTAL
Principle of the experiments A given amount of AIC is introduced into a first vessel, the sublimation chamber, where it rapidly sublimes in the presence of a nitrogen stream. The mixture of Nz and AIC vapour leaving the chamber enters a tubular reactor heated at the desired reaction temperature. At the exit, the gases flow through two chemical traps containing a mixture of acetonitrile and aniline. The isocyanic acid formed reacts with aniline to form phenylurea according to the overall reaction scheme: HNCO + C6HSNH2 + C6H,NH-CO-NH2
(1)
The contents of the two traps are separately analysed by HPLC. The whole experimental set-up is depicted in Fig. 1. Operation details The flowrate of nitrogen (U quality) entering the sublimation chamber is measured by means of a calibrated rotameter. The chamber is a stainless steel cylinder surrounded by an electric heating tape. Its temperature is controlled by a 2 x lop3 m chromel-alumel thermocouple. The temperature set point is higher than T, (about 653 K), except during preheating (423 K). About 5 x lop3 kg of AIC (Elf-Atochem) is introduced into the chamber at the beginning of each run. The flow of nitrogen and sublimed AIC immediately enters the stainless steel (NS 30) tubular reactor consisting of seven tubes (0.3 m each) in series connected by six double bends. The effective reactor length is 2.16 m and the inner tube diameter 8 x low3 m. Thus, the total volume of the reactor is 109 x 10e6 m3. The tubes are arranged inside a ceramic cylinder (diameter 5.4 x lo-’ m) in such a way that they are uniformly heated without screening effect. The whole system is placed in a furnace (Thermolyne type 1100 tube furnace) whose temperature is controlled by three 2 x lop3 m chromel-alumel thermocouples. Another thermocouple placed inside the last tube has shown that the temperatures in the furnace and in the reactor are very close. No important axial or radial temperature gradients are observed in this way. Each end of the furnace is insulated with quartz wool. The temperature is then assumed to be constant throughout the whole reactor (T). At the reactor outlet, the gases flow through two chemical traps (U type) containing a mixture of acetonitrile ( 150 x 10m6m3) and aniline (20 x 1O-6 m’). Very little phenylurea is formed in the second trap. The absorption of HNCO is then considered as complete in the first trap. The solubility of AIC in common solvents and, thus, in acetonitrile, is lower than 2 x 10e4 kg per kilogram of solvent [ 3,4,16] at 298 K. The determina-
J. Mercadier
110
[ll VI
Nitrogen
131
Sublimation chamber
141
electric heating tape
151
Reactor (7 tubes)
WI
Electric furnace
[71
Thermocouples
[8]
Chemical traps
[9]
gross section view of the reactor
m
Quartz wod
&I
Ceramic foam
q q
et al. /J. Anal. Appl. Pyrolysis
28 (1994) 107-120
_
Insulation Sdid AIC Solution of aniline and acetonitrile
Fig. 1. Scheme
of the whole experimental
set-up (dimensions
are in 1O-3 m).
tion of the quantity of unreacted AIC is then difficult and the conversion yield was not determined in our experiments. At the end of each run (where negligible quantity of AIC flows out of the sublimation chamber), the ambient temperature is re-established at every part of the -set-up. The chamber is opened and the unreacted AIC is
111
J. Mereadier et al. 1 J. Anal. Appl. Pyrolysis 28 (1994) 107-120
weighed. Each trap content is analysed by HPLC. The analytical procedure is as follows. The liquid volume of each trap is completed with acetonitrile up to 200 x 10e6 m3. 10 x 10T6 m3 of this solution are dissolved in 50 x lop6 m3 of acetonitrile, from which 100 x lOA m3 are dissolved in 25 x lop6 m3 of the chromatographic eluant from which 20 x 10e9 m3 are sampled out for analysis. The chromatograph assembly (Gilson) comprises a 802C type manometer, a 302 type pump and a 116 type W detector. The pressure is near 100 bar and UV detection is at 200 nm. The sample injection is performed with a Rheodyne valve connected to a 20 x 10e9 m3 sample loop. The Lichrocart RP 18-E column (Merck) has a 4 x 10e3 m diameter with a 5 x 10e6 m porosity. The eluant is a mixture of 80% water with 20% acetonitrile with a small amount of orthophosphoric acid (1 x lop3 m3 m-‘). Its flow rate is 1 x lop6 m3 min- I. The chromatogram is processed by an integrator (ICR lB, Intersmat). Under usual conditions, the mean retention times of the products are approximately: AIC, 2 min; phenylurea, 7 min; aniline, 2 min. The concentration of phenylurea is deduced from a calibration curve that is systematically redetermined. DATA INTERPRETATION
Average flow rate in the reactor
The total flow rate in the reactor is the result of four contributions: nitrogen, AIC, HNCO and unknown products yielded from HNCO decomposition. A mass m of AIC is sublimed in each experiment during a time tE. The mean AIC flow rate at the reactor inlet is then (2)
QAIC =&F;lE
conversion yield of AIC (X) and the mass yield of HNCO (Y) are identical if only the first reaction is taken into account and the decomposition of HNCO is negligible; The
AIC( g) + 3HNCO( g)
(3)
The relative increase of the overall flow rate due to the chemical expansion is [ 171: ,- I ,I
(QNZ+ QAIC) = QNZ+ QAIC
QT -
where Av O!=1+1
ax
=
cI y
,
i
!C
J. Mercadier
112
et al. / J. Anal. Appl. Pyrolysis
28 (1994) 107-120
TABLE 1 Exuerimental conditions and mass yield of HNCO for each experiment No.
T(K)
t(s)
b(S)
m x 103(kg)
YExp(%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35
673 700 700 699 698 698 697 698 724 723 724 726 723 723 723 723 723 722 725 747 747 748 761 773 773 773 775 774 773 773 733 773 773 773 798
33.5 6.8 10.9 18.0 30.4 29.7 42.8 56.2 6.9 7.8 10.5 16.5 22.0 27.0 29.2 52.0 50.1 68.6 72.8 2.73 6.6 11.2 2.64 0.99 1.31 1.71 2.09 2.55 3.01 4.47 6.4 12.8 17.3 33.8 1.00
1500 1020 3000 1440 1440 1620 1560 1500 2580 3600 1200 4020 5100 3000 4020 3600 2220 2400 2400 960 1380 2160 1680 900 1560 1980 1620 1740 2040 2580 900 2760 2280 2880 1560
0.86 2.57 1.78 1.73 1.02 1.29 1.34 0.96 1.56 3.82 1.98 1.96 1.56 2.98 2.60 0.89 0.90 0.48 0.50 3.38 1.8 4.62 3.92 4.95 5.00 5.00 3.74 5.00 3.46 4.47 1.23 3.34 2.71 2.52 4.90
58 33 63 54 64.5 62 70 73 57 74.5 74 85 84 83 78.5 73.5 74 70 65 58 64 75 85 51 46 54 75 61 50 55 57 39 31 6 70
QT is the total volumetric flow rate of nitrogen and other active species at the reactor outlet. Av is the difference between the stoichiometric coefficients of products and reactants (for eqn. (12), Av = 2). I is the ratio of the volumetric flow rate of inert carrier gas to sublimed AIC at the reactor inlet. In a first approximation, the gas residence time in the reactor is then calculated from an average value of the gas flow rate in the reactor (Table 1) for each experiment:
J. Mercadier
QM =
et al. 1 J. Anal. Appl. Pyrolysis
QT+ @IQ+ Q.d 2
28 (1994) 107-120
t=
v/Q,
113
(6)
All the experimental results are presented on Table 1 and Fig. 3. However, only the results obtained for residence times lower than 20 s will be used for the determination of the kinetic constants. Under these restricted conditions the extent of the decomposition reaction of HNCO may be neglected and the approximation X - Y is valid. In addition, QAIc is not usually larger than 0.05QNz SO that QM z QT z QN2. Flow regime in the reactor
For the extreme values of main flow rate and temperature investigated, the corresponding Reynolds numbers vary between 3 and 217 (Table 2). The calculations are based on nitrogen flow rate only. However, this does not change the nature of the flow regime which is therefore laminar. The calculation of the kinetic constant k, must theoretically take into account the radial distribution velocity in the reactor. Simplified calculations are still possible if the reactor behaves globally as a plug flow reactor. Such an assumption can be made depending on the values of Peclet and Damkijhler II numbers in two different approaches. Laminar jlow without chemical reaction
Applying the axially dispersed plug flow model, the flow regime is governed by a Peclet number defined on the reactor length: P = u,LIDA
(7)
The axial dispersion DA is a function of molecular diffusivity (D) and average velocity U, given in laminar flow by the Taylor and Aris relationship [17]: o,=D+g
(8)
According to Villermaux [ 171, a reactor can be assimilated to a plug flow reactor when P is greater than 100. Table 2 shows that this is always the case in our conditions where P roughly ranges from 300 to 1900. These values of P have been confirmed by experimental measurement of residence time distribution in the reactor [ 1,181. Laminar Jlow with chemical reaction
Chemical and flow regimes are now governed by two dimensionless numbers a and Pe. “a” is a Damkohler II number which compares chemical reaction rate to radial molecular diffusion rate: a = kld2/4D
(9)
114
J. Mercadier
et al. 1 J. Anal. Appl. Pyrolysis
28 (1994) 107-120
TABLE 2 Values of Reynolds, Peclet and Damkiihler II numbers for each experiment No.
Q&,( x lo6 m3 s-i)
Re
Da( x lo5 m2 s-i)
P
a
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35
1.25 6.05 4.00 2.20 1.29 1.30 0.80 0.61 6.29 5.32 3.65 2.50 1.90 1.30 1.30 0.77 0.75 0.48 0.54 14.6 6.05 3.11 15.1 40.0 30.7 23.5 19.0 15.3 13.3 8.90 6.07 2.92 2.10 1.05 38.9
7.6 34 23 13 7.4 7.5 4.6 3.5 35 30 21 14 11 7.3 7.3 4.4 4.4 2.9 2.9 81 33 17 83 217 167 127 103 83 72 48 33 16 12 5.6 206
8.12 8.67 8.67 8.65 8.63 8.63 8.61 8.63 9.17 9.15 9.17 9.21 9.15 9.15 9.15 9.15 9.15 9.13 9.19 9.66 9.66 9.68 9.97 10.23 10.23 10.23 10.27 10.25 10.23 10.23 10.23 10.23 10.23 10.23 10.79
1302 1537 1820 1753 1309 1324 895 700 1524 1660 1857 1809 1633 1299 1299 848 828 533 617 805 1572 1867 790 313 407 528 645 788 894 1241 1588 1850 1676 1059 330
1.3 x 2.7 x 2.7 x 2.7 x 2.6 x 2.6 x 2.5 x 2.6 x 5.1 x 5.0 x 5.1 x 5.4 x 5.0 x 5.0 x 5.0 x 5.0 x 5.0 x 4.8 x 5.3 x 8.9 x 8.9 x 9.1 x 0.12 0.16 0.16 0.16 0.17 0.16 0.16 0.16 0.16 0.16 0.16 0.16 0.27
Pe 10-2 1O-2 1O-2 1O-2 1O-2 1O-2 1O-2 1O-2 IO-2 10-2 10-2 10-2 10-2 10-2 10-2 10-2 IO-2 1O-2 IO-2 1O-2 1O-2 10-2
5.6 26 17 9.5 5.6 5.7 3.5 2.6 26 22 15 11 8.0 5.5 5.5 3.3 3.2 2.0 2.3 60 25 13 61 160 13 94 76 61 53 36 24 11 8.0 4.2 152
a D is calculated from the formula of Fuller, Schettler and Giddings, given by Perry and Green [ 231.
The new Peclet number (Pe) is based on the reactor diameter: Pe = u,d/2D
(10)
With anticipated values of k,, the Damkiihler numbers, are always lower than 0.27 (Table 2). This means that molecular diffusion is faster than chemical reaction and smoothes out the radial concentration profile in spite
J, Mercadier
et al. 1 J. Anal. Appl. Pyrolysis
28 (1994) 107-120
115
of the existence of a radial velocity profile. According to L&de and Villermaux [ 191 the plug flow assumption is then correct if a second condition is simultaneously verified (negligeable axial diffusion) : Pe2 >>4a
(11)
Table 2 shows that this condition is always verified. Moreover, it is likely that the laminar flow profile is not fully developed: the reactor is made up of seven tubular sections connected by six double bends, and these singularities induce perturbations in the flow. For all these reasons, the calculations below are made under the plug flow assumption. Pressure drop in the reactor
Calculations show that the pressure drop in the reactor is always lower than 150 Pa for residence times greater than 1 s [ 11. The pressure in the reactor will then be assumed constant and equal to atmospheric pressure. Conversion yield and yield of HNCO
As in the case of triphenylisocyanurate pyrolysis [20] and in the absence of any other information, the kinetics of AIC and HNCO pyrolysis will be supposed first order. The expression of the conversion yield of AIC is given by [ 171 X = [ 1 - exp( -k, yt)]
(12)
Unfortunately, the analysis of AIC has not been possible because of its poor solubility, and only mass yields of HNCO (Y) have been experimentally determined. We assume that HNCO undergoes a further first-order decomposition. In the case of two consecutive first-order reactions (kinetic constants k, and k,), Y is then given by the well-known expression [ 171 Y=
&
kxp( -4 0 - exp(-k201
Y is the ratio of the mass of HNCO collected in the traps to the mass of AIC (m) sublimed and injected at the reactor inlet during an experiment. RESULTS
AND DISCUSSION
Nature of products
From the samples taken in the traps, the only product detected by HPLC is phenylurea (issued from reaction ( 1)). Table 1 shows that yields of up to 85% may be observed. Unreacted AIC is also present in the gas stream but its concentration cannot be determined for the reasons previously mentioned.
116
J, Mereadier
et al. /J. Anal. Appl. Pyrolysis
28 (1994) 107-120
Gas chromatography/mass spectrometry analyses have shown that the main products of HNCO pyrolysis are COZ, CO, N2, HCN and H2 [ 11,121. The presence of cyanamide (NH2CN) can also be suspected although it has never definitively been proved. Measurement of the kinetic constants Measurements have been made at six different temperatures. Table 3 reports the values of the two first-order kinetic constants (k, and kJ calculated for four of these temperatures (698, 723, 748 and 773 K) by a two-parameter optimization software program ( FLExIPLEX). Figure 2 shows the variation of In(k) vs. l/T for both constants k, and k2 as well as a comparison with literature data [ 15,201. The corresponding frequency factors and activation energies are then calculated from the Arrhenius plot (Table 4). In Fig. 3 the experimental data and those calculated with these parameters are compared. TABLE 3 Kinetic constants of the thermal decomposition temperature
of AIC and HNCO as a function of reaction
T(K)
k, (s-7
k,(s-‘)
698 723 748 773
7.05 x 10-2 0.143 0.313 0.498
1.76 5.30 4.13 8.86
x x x x
lo-* 10-3 10-z 10-2
Fig. 2. Variations of the first-order kinetic constants of AIC and HNCO thermal decomposition as a function of reaction temperature in an Arrhenius representation. Comparison with the literature.
J. Mercadier
et al. / J. Anal. Appl. Pyrolysis
28 (1994) 107-120
117
TABLE 4 Frequency factors and activation energies of the first-order kinetic constants of thermal decomposition of AIC and HNCO. Correlation coefficients of the corresponding Arrhenius plots A, (s-l)
E,(Jmol-‘)
r
A,(s-‘)
E2(
J mol-‘)
6.3 x lo7
119 x 103
0.99
1.1 x 107
121 x 103
r
0.52
Figure 3 clearly shows that the pyrolysis of AIC is followed by consecutive reactions as assumed from literature. The maximum yield YH deduced from eqn. ( 13) is given by [ 171 (14) Y, only depends on the ratio kJkl and thus moderately depends on T if the values of the corresponding activation energies are similar. Table 4 shows that E 1 z E2. The results presented in Fig. 3 show that, between 698 and 773 K, experimental values of YH range between 73 and 85%. The same Figure exhibits good agreement between computed and experimental values of Y, even for residence times higher than 20 s. For AIC thermal decomposition, the frequency factor and activation energy are lower than those generally observed in the case of the pyrolysis of cyclic compounds [21,22] or for similar molecules like triphenylisocyanurate (triester of AIC) [20]. A comparison between the results from the present work and values of the kinetic constant of triphenylisocyanurate is shown in Fig. 2. For triphenylisocyanurate, the values of the frequency factor and activation energy are ATPIC= 5.85 x lo’* s-’ and ETpIc= 252 x lo3 J mol-’ respectively. Figure 2 also shows good agreement with the result of Perry and Siebers [ 151 (k2 = 8.6 s-l) obtained at a higher temperature (858 K), under reduced pressure (67 700 Pa) and with a residence time of 0.30 s in a stainless steel continuous flow reactor. These results are, however, difficult to compare with the second-order rate constant of reaction HNCO + Ar +products, obtained by other authors [ 13,141 under quite different conditions of very high temperatures (18303340 K) and in shock wave tubes. CONCLUSION
The kinetics of the thermal detrimerization of AIC have been measured in a continuous tubular flow reactor under atmospheric pressure, with reaction times of a few seconds and with temperatures from 673 to 798 K. The Arrhenius parameters (A, = 6.3 x 10’s_‘, El = 119 x lo3 J mol-‘) have
118
J. Mercadier
et al. / J. Anal. Appl. Pyrolysis
28 (1994) 107-120
0.2 0 0
“I
20
40
60
60
100
0
I 20
-
I 40
*
I 60
tw . 80
.
0
2;
4;
6b
f
0
10
20
30
40
0
5
10
15
20
Fig. 3. Variations in the conversion yield of AIC (X) and mass yield of HNCO (Y) as a function of the residence time (t) of gases in the reactor for different temperatures: ( -) computed results; (0) experimental (Ye_,) values of Y.
been determined. The only identified product of this pyrolysis is HNCO. However, one may suspect that some gases (COz, CO, HCN, . . .) are produced by consecutive HNCO decomposition. The highest mass yield of HNCO obtained in our experiments is 85%. The global kinetics of HNCO thermal decomposition were also determined but with less accuracy: a more complex decomposition scheme and possible surface effects [ 121 may occur for this reaction. ACKNOWLEDGEMENTS
The financial support of Elf-Atochem
is gratefully acknowledged.
J. Mercadier
et al. 1 J. Anal. Appl. Pyrolysis
28 (1994) 107-120
119
LIST OF SYMBOLS
A TPIC Al A2
AIC ; DA
d ETPIC El E*
HPLC I k k, kz L h4AK m mHNCO
P P, Pe AIC Em QM Q N2
Q0 QT R Re r T TS t tE U,
V x
frequency factor of triphenylisocyanurate pyrolysis (s-l) frequency factor of AIC pyrolysis (s-r) frequency factor of HNCO pyrolysis (s-l) isocyanuric acid Damkiihler II number ( = k, d2/4D) molecular diffusivity ( m2 s- ‘) axial dispersion (m’ s-l) reactor diameter (m) activation energy of triphenylisocyanurate pyrolysis (J mol-‘) activation energy of AIC pyrolysis (J mol-‘) activation energy of HNCO pyrolysis (J mol-‘) high performance liquid chromatography ratio of molar amount of inert carrier gas to active species vapours first-order kinetic constant (s-l) first-order kinetic constant of AIC pyrolysis (s-l) first-order kinetic constant of HNCO pyrolysis (s-l) reactor length (m) molecular weight of AIC (kg mol-‘) mass of AIC sublimed and injected into the reactor during an experiment (kg) mass of HNCO chemically trapped by aniline (kg) Peclet number determined on the reactor length ( =u,L/D,) pressure in the reactor (Pa) Peclet number determined on the reactor diameter ( =u,d/2D) carrier gas mass flow rate (kg s-l) AIC flow rate at the reactor inlet at T (m3 s-l) average volumetric flow rate in the reactor at T (m3 s-l) carrier gas volumetric flow rate at T (m3 s-‘) carrier gas volumetric flow rate at 293 K (m’ s-l) carrier gas and active species vapours volumetric flow rate at T at the reactor outlet (m3 s-l) gas constant (J mol-’ K-l) Reynolds number ( = 4Q,/zpd) correlation coefficient reaction temperature (K) temperature of sublimation of AIC (K) residence time of gases in the reactor ( = V/Q,)(s) experience duration (s) gas velocity through the reactor at T (m s-i) volume of the reactor (m’) conversion yield of AIC ( =mass of unreacted AIC at the reactor outlet/m)
120
Y
Y-P YH
J. Mercadier
et al. / J. Anal. Appl. Pyrolysis
28 (1994) 107-120
mass (or molar) yield of HNCO ( =mHNCo/m) experimental mass yield of HNCO maximum mass yield of HNCO
Greek letters a
p Av
Chemical expansion factor Dynamic viscosity of nitrogen at T(Pa s) Difference between product and reactant efficients
stoichiometric
co-
REFERENCES 1 J. Mercadier, These INPL, Institut National Polytechnique de Lorraine, Nancy, France, 1992. 2 E.M. Smolin and L. Rapoport, in Arnold Weissberger (Ed.), The chemistry of heterocyclic compounds, 2nd edn., Vol. 13, Wiley-Interscience, New York, 1967, pp. 17-48. 3 R.N. Mesiah, in A. Standen (Ed.), Kirk Othmer encyclopedia of Industrial chemistry, 2nd edn., Vol. 20, Interscience, New York, 1969, pp. 662-671. 4 N. Kriebitzsch and H. Klenk, in W. Gerhartz (Ed.), Ullman’s encyclopedia of industrial chemistry, 5th edn., Vol. A8, VCH, Weinheim, 1987, pp. 191-200. 5 A.N. Gandhi and L.V. Keshav, in J.J. McKetta and M. Delker (Eds.), Encyclopedia of chemical processing and design, Vol. 14, INC, New York, 1982, pp. 52-60. 6 R.C. East, Brevet d’invention francais, 1.328.696, 1962. 7 F.W. Hoover, H.B. Stenvenson and H.S. Rothrock, J. Org. Chem., 28 (1963) 1825-1830. 8 G. Herzberg and C. Reid, Discuss. Faraday Sot., 9 (1950) 92-99. 9 S.R. Smith and H.B. Jonassen, J. Inorg. Nucl. Chem., 29 (1967) 860-862. 10 J.Y.P. Mui and R.A. Back, Can. J. Chem., 41 (1963) 826-833. 11 J.N. Bradley, J.R. Gilbert and P. Svedja, Trans. Faraday Sot., 46 (1968) 911-918. 12 R.A. Back and J. Childs, Can. J. Chem., 46 (1968) 1023-1024. 13 J.D. Mertens, A.Y. Chang, R.K. Hanson and C.T. Bowman, Int. J. Chem. Kinet., 21 (1989) 1049-1067. 14 0. Kajimoto, 0. Kondo, K. Okda, J. Fujikane and T. Fueno, Bull. Chem. Sot. Jpn, 58 (1985) 3469-3474. 15 R.A. Perry and D.L. Siebers, Nature, 324 (1986) 657-658. 16 J.V. Burakevich in M. Grayson, Kirk-Othmer concise encyclopedia of chemical technology, Wiley-Interscience, New York, 1985, pp. 339. 17 J. Villermaux, Genie de la Reaction Chimique. Conception et dtveloppement des reacteurs, Technique et Documentation, Lavoisier, Paris, 1985. 18 M. Pignon, Diplome d’Etude Approfondies, Institut National Polytechnique de Lorraine, Nancy, France, 1992. 19 J. L&de and J. Villermaux, J. Chim. Phys., 74 (1977) 459-467. 20 J. LcdC, J. Mercadier, M. Coste and J. Villermaux, J. Anal. Appl. Pyrolysis, 24 (1992) 179-189. 21 S.W. Benson, Thermochemical Kinetics. Methods for the estimation of thermochemical data and rate parameters, 2nd edn., Wiley, New York, 1976. 22 C.H. Bamford and C.F.H. Tipper, Comprehensive Chemical Kinetics, Vol. 5, Elsevier, Amsterdam, 1972. 23 R.H. Perry and D. Green, Chemical Engineer’s Handbook, 6th edn., McGraw-Hill, New York, 3.285-3.287, 1984.
107
Elsevier Science Publishers B.V., Amsterdam
Kinetics of isocyanuric acid pyrolysis J. Mercadier, M. Pignon, F. Calabuig, J. Led& * and J. Villermaux Luboratoire des Sciences du GEnie Chimique, CNRS-ENSIC-INPL, B.P. 451, 54001 Nancy cedex (France)
1 rue Grandville,
(Received June 7, 1993; accepted June 21, 1993)
ABSTRACT The kinetics of isocyanuric acid pyrolysis are determined in a tubular continuous flow reactor under atmospheric pressure, for temperatures ranging from 698 to 798 K and with residence times from 1 to 73 s. The main product of the pyrolysis is isocyanic acid (HNCO), a monomer of isocyanuric acid. Isocyanic acid itself decomposes into different products. With correct choice of temperature and residence times, the yield of isocyanic acid reaches 85%. The Arrhenius parameters derived are compared with literature data. Isocyanic acid; isocyanuric acid; kinetics; plug flow reactor; pyrolysis.
INTRODUCTION
At ordinary temperature, isocyanuric acid (AIC) is a white, inert and non toxic crystalline solid which sublimes at Ts= 633 K [ 1,2]. Its general formula is
The commercial method for AIC preparation involves urea [3-51. Urea is first heated at 473-573 K to form biuret and several methods, including recycling of crude AIC, are employed to produce AIC. Trisubstituted organic derivatives (isocyanurates) can be formed from AIC. For industrial use, the greater part of AIC is chlorinated to form trichloroisocyanuric
* Corresponding
author.
01652370/94/$07.00 wnl
0 1994 - Elsevier Science Publishers B.V. All rights reserved
nifx_7~7nfcmm7h2-c
108
J. Mercadier
et al. / J. Anal. Appl. Pyrolysis
28 (1994) 107-120
acid ( C3ClsN303) [ 31 used in water treatment, particularly for swimming pools. The boiling point of HNCO is 296.5 K, but it can begin to polymerize at 273 K [2]. There is no industrial process to synthetize HNCO; on the laboratory scale it is generally produced from AIC thermal decomposition. As far as we know, little work has been devoted to the thermal behaviour of isocyanuric acid and no kinetics constants are available in the literature. East [6] pyrolysed AIC in a tubular continuous flow reactor between 723 and 923 K. The solid was introduced by means of an endless screw with a very low nitrogen flow rate in order to control the pressure. The AIC residence time is not reported, so no kinetic data can be derived from these experiments. The highest mass yield of HNCO is 84% and the main identified by product is HCN ( 10.1%). Several authors [ 7- 1l] used the pyrolysis of AIC to produce HNCO in order to study its chemical and physical properties, or to produce free radicals especially by photolysis of HNCO. Different by-products have been noticed - C02, HCN, CO, NH,, C2N2 - probably from HNCO decomposition. Details about processes are very scarce and no quantitative kinetic data are reported. Back and Childs [ 121.also studied HNCO pyrolysis in a quartz batch reactor at temperatures from 823 to 973 K, pressures from 670 to 2670 Pa and residence times of several minutes. CO*, CO, NZ, HCN and H2 were detected by gas chromatography. However, according to mass balances, the presence of NH3 and cyanamide (NH,CN) could not be excluded. According to these authors, the determination of quantitative kinetics of HNCO is quite difficult, particularly because of surface effects. Mertens et al. [ 131 and Kajimoto et al. [14] determined kinetic parameters in shock waves, thus under quite specific experimental conditions. Quantitative kinetic data were reported by Perry and Siebers [ 151 during the study of the reduction of nitrogen oxides in the presence of HNCO. The degradation reaction was carried out in a flow reactor (stainless steel). The temperature was 858 K, the pressure 68 x lo3 Pa and the residence time lower than 1 s. CO and CO* were detected as products as well. The pyrolysis of AIC produces several gases [ 7- 111. This may indicate that the reaction is followed by a series of consecutive reactions. Moreover, these gases are the same as in the case of the pyrolysis of HNCO [ 12-141. The purpose of this paper is to determine the kinetic parameters of AIC thermal decomposition performed in a continuous flow reactor, under atmospheric pressure and with gas residence times of a few seconds. A novelty of the apparatus is that decomposition occurs in a separate reactor downstream of a sublimation chamber of AIC. The reaction is monitored by the measurement of HNCO mass yield by high performance liquid chromatography (HPLC).
J. Mereadier
et al.
1J. Anal. Appl. Pyrolysis 28 (1994) 107-120
109
EXPERIMENTAL
Principle of the experiments A given amount of AIC is introduced into a first vessel, the sublimation chamber, where it rapidly sublimes in the presence of a nitrogen stream. The mixture of Nz and AIC vapour leaving the chamber enters a tubular reactor heated at the desired reaction temperature. At the exit, the gases flow through two chemical traps containing a mixture of acetonitrile and aniline. The isocyanic acid formed reacts with aniline to form phenylurea according to the overall reaction scheme: HNCO + C6HSNH2 + C6H,NH-CO-NH2
(1)
The contents of the two traps are separately analysed by HPLC. The whole experimental set-up is depicted in Fig. 1. Operation details The flowrate of nitrogen (U quality) entering the sublimation chamber is measured by means of a calibrated rotameter. The chamber is a stainless steel cylinder surrounded by an electric heating tape. Its temperature is controlled by a 2 x lop3 m chromel-alumel thermocouple. The temperature set point is higher than T, (about 653 K), except during preheating (423 K). About 5 x lop3 kg of AIC (Elf-Atochem) is introduced into the chamber at the beginning of each run. The flow of nitrogen and sublimed AIC immediately enters the stainless steel (NS 30) tubular reactor consisting of seven tubes (0.3 m each) in series connected by six double bends. The effective reactor length is 2.16 m and the inner tube diameter 8 x low3 m. Thus, the total volume of the reactor is 109 x 10e6 m3. The tubes are arranged inside a ceramic cylinder (diameter 5.4 x lo-’ m) in such a way that they are uniformly heated without screening effect. The whole system is placed in a furnace (Thermolyne type 1100 tube furnace) whose temperature is controlled by three 2 x lop3 m chromel-alumel thermocouples. Another thermocouple placed inside the last tube has shown that the temperatures in the furnace and in the reactor are very close. No important axial or radial temperature gradients are observed in this way. Each end of the furnace is insulated with quartz wool. The temperature is then assumed to be constant throughout the whole reactor (T). At the reactor outlet, the gases flow through two chemical traps (U type) containing a mixture of acetonitrile ( 150 x 10m6m3) and aniline (20 x 1O-6 m’). Very little phenylurea is formed in the second trap. The absorption of HNCO is then considered as complete in the first trap. The solubility of AIC in common solvents and, thus, in acetonitrile, is lower than 2 x 10e4 kg per kilogram of solvent [ 3,4,16] at 298 K. The determina-
J. Mercadier
110
[ll VI
Nitrogen
131
Sublimation chamber
141
electric heating tape
151
Reactor (7 tubes)
WI
Electric furnace
[71
Thermocouples
[8]
Chemical traps
[9]
gross section view of the reactor
m
Quartz wod
&I
Ceramic foam
q q
et al. /J. Anal. Appl. Pyrolysis
28 (1994) 107-120
_
Insulation Sdid AIC Solution of aniline and acetonitrile
Fig. 1. Scheme
of the whole experimental
set-up (dimensions
are in 1O-3 m).
tion of the quantity of unreacted AIC is then difficult and the conversion yield was not determined in our experiments. At the end of each run (where negligible quantity of AIC flows out of the sublimation chamber), the ambient temperature is re-established at every part of the -set-up. The chamber is opened and the unreacted AIC is
111
J. Mereadier et al. 1 J. Anal. Appl. Pyrolysis 28 (1994) 107-120
weighed. Each trap content is analysed by HPLC. The analytical procedure is as follows. The liquid volume of each trap is completed with acetonitrile up to 200 x 10e6 m3. 10 x 10T6 m3 of this solution are dissolved in 50 x lop6 m3 of acetonitrile, from which 100 x lOA m3 are dissolved in 25 x lop6 m3 of the chromatographic eluant from which 20 x 10e9 m3 are sampled out for analysis. The chromatograph assembly (Gilson) comprises a 802C type manometer, a 302 type pump and a 116 type W detector. The pressure is near 100 bar and UV detection is at 200 nm. The sample injection is performed with a Rheodyne valve connected to a 20 x 10e9 m3 sample loop. The Lichrocart RP 18-E column (Merck) has a 4 x 10e3 m diameter with a 5 x 10e6 m porosity. The eluant is a mixture of 80% water with 20% acetonitrile with a small amount of orthophosphoric acid (1 x lop3 m3 m-‘). Its flow rate is 1 x lop6 m3 min- I. The chromatogram is processed by an integrator (ICR lB, Intersmat). Under usual conditions, the mean retention times of the products are approximately: AIC, 2 min; phenylurea, 7 min; aniline, 2 min. The concentration of phenylurea is deduced from a calibration curve that is systematically redetermined. DATA INTERPRETATION
Average flow rate in the reactor
The total flow rate in the reactor is the result of four contributions: nitrogen, AIC, HNCO and unknown products yielded from HNCO decomposition. A mass m of AIC is sublimed in each experiment during a time tE. The mean AIC flow rate at the reactor inlet is then (2)
QAIC =&F;lE
conversion yield of AIC (X) and the mass yield of HNCO (Y) are identical if only the first reaction is taken into account and the decomposition of HNCO is negligible; The
AIC( g) + 3HNCO( g)
(3)
The relative increase of the overall flow rate due to the chemical expansion is [ 171: ,- I ,I
(QNZ+ QAIC) = QNZ+ QAIC
QT -
where Av O!=1+1
ax
=
cI y
,
i
!C
J. Mercadier
112
et al. / J. Anal. Appl. Pyrolysis
28 (1994) 107-120
TABLE 1 Exuerimental conditions and mass yield of HNCO for each experiment No.
T(K)
t(s)
b(S)
m x 103(kg)
YExp(%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35
673 700 700 699 698 698 697 698 724 723 724 726 723 723 723 723 723 722 725 747 747 748 761 773 773 773 775 774 773 773 733 773 773 773 798
33.5 6.8 10.9 18.0 30.4 29.7 42.8 56.2 6.9 7.8 10.5 16.5 22.0 27.0 29.2 52.0 50.1 68.6 72.8 2.73 6.6 11.2 2.64 0.99 1.31 1.71 2.09 2.55 3.01 4.47 6.4 12.8 17.3 33.8 1.00
1500 1020 3000 1440 1440 1620 1560 1500 2580 3600 1200 4020 5100 3000 4020 3600 2220 2400 2400 960 1380 2160 1680 900 1560 1980 1620 1740 2040 2580 900 2760 2280 2880 1560
0.86 2.57 1.78 1.73 1.02 1.29 1.34 0.96 1.56 3.82 1.98 1.96 1.56 2.98 2.60 0.89 0.90 0.48 0.50 3.38 1.8 4.62 3.92 4.95 5.00 5.00 3.74 5.00 3.46 4.47 1.23 3.34 2.71 2.52 4.90
58 33 63 54 64.5 62 70 73 57 74.5 74 85 84 83 78.5 73.5 74 70 65 58 64 75 85 51 46 54 75 61 50 55 57 39 31 6 70
QT is the total volumetric flow rate of nitrogen and other active species at the reactor outlet. Av is the difference between the stoichiometric coefficients of products and reactants (for eqn. (12), Av = 2). I is the ratio of the volumetric flow rate of inert carrier gas to sublimed AIC at the reactor inlet. In a first approximation, the gas residence time in the reactor is then calculated from an average value of the gas flow rate in the reactor (Table 1) for each experiment:
J. Mercadier
QM =
et al. 1 J. Anal. Appl. Pyrolysis
QT+ @IQ+ Q.d 2
28 (1994) 107-120
t=
v/Q,
113
(6)
All the experimental results are presented on Table 1 and Fig. 3. However, only the results obtained for residence times lower than 20 s will be used for the determination of the kinetic constants. Under these restricted conditions the extent of the decomposition reaction of HNCO may be neglected and the approximation X - Y is valid. In addition, QAIc is not usually larger than 0.05QNz SO that QM z QT z QN2. Flow regime in the reactor
For the extreme values of main flow rate and temperature investigated, the corresponding Reynolds numbers vary between 3 and 217 (Table 2). The calculations are based on nitrogen flow rate only. However, this does not change the nature of the flow regime which is therefore laminar. The calculation of the kinetic constant k, must theoretically take into account the radial distribution velocity in the reactor. Simplified calculations are still possible if the reactor behaves globally as a plug flow reactor. Such an assumption can be made depending on the values of Peclet and Damkijhler II numbers in two different approaches. Laminar jlow without chemical reaction
Applying the axially dispersed plug flow model, the flow regime is governed by a Peclet number defined on the reactor length: P = u,LIDA
(7)
The axial dispersion DA is a function of molecular diffusivity (D) and average velocity U, given in laminar flow by the Taylor and Aris relationship [17]: o,=D+g
(8)
According to Villermaux [ 171, a reactor can be assimilated to a plug flow reactor when P is greater than 100. Table 2 shows that this is always the case in our conditions where P roughly ranges from 300 to 1900. These values of P have been confirmed by experimental measurement of residence time distribution in the reactor [ 1,181. Laminar Jlow with chemical reaction
Chemical and flow regimes are now governed by two dimensionless numbers a and Pe. “a” is a Damkohler II number which compares chemical reaction rate to radial molecular diffusion rate: a = kld2/4D
(9)
114
J. Mercadier
et al. 1 J. Anal. Appl. Pyrolysis
28 (1994) 107-120
TABLE 2 Values of Reynolds, Peclet and Damkiihler II numbers for each experiment No.
Q&,( x lo6 m3 s-i)
Re
Da( x lo5 m2 s-i)
P
a
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35
1.25 6.05 4.00 2.20 1.29 1.30 0.80 0.61 6.29 5.32 3.65 2.50 1.90 1.30 1.30 0.77 0.75 0.48 0.54 14.6 6.05 3.11 15.1 40.0 30.7 23.5 19.0 15.3 13.3 8.90 6.07 2.92 2.10 1.05 38.9
7.6 34 23 13 7.4 7.5 4.6 3.5 35 30 21 14 11 7.3 7.3 4.4 4.4 2.9 2.9 81 33 17 83 217 167 127 103 83 72 48 33 16 12 5.6 206
8.12 8.67 8.67 8.65 8.63 8.63 8.61 8.63 9.17 9.15 9.17 9.21 9.15 9.15 9.15 9.15 9.15 9.13 9.19 9.66 9.66 9.68 9.97 10.23 10.23 10.23 10.27 10.25 10.23 10.23 10.23 10.23 10.23 10.23 10.79
1302 1537 1820 1753 1309 1324 895 700 1524 1660 1857 1809 1633 1299 1299 848 828 533 617 805 1572 1867 790 313 407 528 645 788 894 1241 1588 1850 1676 1059 330
1.3 x 2.7 x 2.7 x 2.7 x 2.6 x 2.6 x 2.5 x 2.6 x 5.1 x 5.0 x 5.1 x 5.4 x 5.0 x 5.0 x 5.0 x 5.0 x 5.0 x 4.8 x 5.3 x 8.9 x 8.9 x 9.1 x 0.12 0.16 0.16 0.16 0.17 0.16 0.16 0.16 0.16 0.16 0.16 0.16 0.27
Pe 10-2 1O-2 1O-2 1O-2 1O-2 1O-2 1O-2 1O-2 IO-2 10-2 10-2 10-2 10-2 10-2 10-2 10-2 IO-2 1O-2 IO-2 1O-2 1O-2 10-2
5.6 26 17 9.5 5.6 5.7 3.5 2.6 26 22 15 11 8.0 5.5 5.5 3.3 3.2 2.0 2.3 60 25 13 61 160 13 94 76 61 53 36 24 11 8.0 4.2 152
a D is calculated from the formula of Fuller, Schettler and Giddings, given by Perry and Green [ 231.
The new Peclet number (Pe) is based on the reactor diameter: Pe = u,d/2D
(10)
With anticipated values of k,, the Damkiihler numbers, are always lower than 0.27 (Table 2). This means that molecular diffusion is faster than chemical reaction and smoothes out the radial concentration profile in spite
J, Mercadier
et al. 1 J. Anal. Appl. Pyrolysis
28 (1994) 107-120
115
of the existence of a radial velocity profile. According to L&de and Villermaux [ 191 the plug flow assumption is then correct if a second condition is simultaneously verified (negligeable axial diffusion) : Pe2 >>4a
(11)
Table 2 shows that this condition is always verified. Moreover, it is likely that the laminar flow profile is not fully developed: the reactor is made up of seven tubular sections connected by six double bends, and these singularities induce perturbations in the flow. For all these reasons, the calculations below are made under the plug flow assumption. Pressure drop in the reactor
Calculations show that the pressure drop in the reactor is always lower than 150 Pa for residence times greater than 1 s [ 11. The pressure in the reactor will then be assumed constant and equal to atmospheric pressure. Conversion yield and yield of HNCO
As in the case of triphenylisocyanurate pyrolysis [20] and in the absence of any other information, the kinetics of AIC and HNCO pyrolysis will be supposed first order. The expression of the conversion yield of AIC is given by [ 171 X = [ 1 - exp( -k, yt)]
(12)
Unfortunately, the analysis of AIC has not been possible because of its poor solubility, and only mass yields of HNCO (Y) have been experimentally determined. We assume that HNCO undergoes a further first-order decomposition. In the case of two consecutive first-order reactions (kinetic constants k, and k,), Y is then given by the well-known expression [ 171 Y=
&
kxp( -4 0 - exp(-k201
Y is the ratio of the mass of HNCO collected in the traps to the mass of AIC (m) sublimed and injected at the reactor inlet during an experiment. RESULTS
AND DISCUSSION
Nature of products
From the samples taken in the traps, the only product detected by HPLC is phenylurea (issued from reaction ( 1)). Table 1 shows that yields of up to 85% may be observed. Unreacted AIC is also present in the gas stream but its concentration cannot be determined for the reasons previously mentioned.
116
J, Mereadier
et al. /J. Anal. Appl. Pyrolysis
28 (1994) 107-120
Gas chromatography/mass spectrometry analyses have shown that the main products of HNCO pyrolysis are COZ, CO, N2, HCN and H2 [ 11,121. The presence of cyanamide (NH2CN) can also be suspected although it has never definitively been proved. Measurement of the kinetic constants Measurements have been made at six different temperatures. Table 3 reports the values of the two first-order kinetic constants (k, and kJ calculated for four of these temperatures (698, 723, 748 and 773 K) by a two-parameter optimization software program ( FLExIPLEX). Figure 2 shows the variation of In(k) vs. l/T for both constants k, and k2 as well as a comparison with literature data [ 15,201. The corresponding frequency factors and activation energies are then calculated from the Arrhenius plot (Table 4). In Fig. 3 the experimental data and those calculated with these parameters are compared. TABLE 3 Kinetic constants of the thermal decomposition temperature
of AIC and HNCO as a function of reaction
T(K)
k, (s-7
k,(s-‘)
698 723 748 773
7.05 x 10-2 0.143 0.313 0.498
1.76 5.30 4.13 8.86
x x x x
lo-* 10-3 10-z 10-2
Fig. 2. Variations of the first-order kinetic constants of AIC and HNCO thermal decomposition as a function of reaction temperature in an Arrhenius representation. Comparison with the literature.
J. Mercadier
et al. / J. Anal. Appl. Pyrolysis
28 (1994) 107-120
117
TABLE 4 Frequency factors and activation energies of the first-order kinetic constants of thermal decomposition of AIC and HNCO. Correlation coefficients of the corresponding Arrhenius plots A, (s-l)
E,(Jmol-‘)
r
A,(s-‘)
E2(
J mol-‘)
6.3 x lo7
119 x 103
0.99
1.1 x 107
121 x 103
r
0.52
Figure 3 clearly shows that the pyrolysis of AIC is followed by consecutive reactions as assumed from literature. The maximum yield YH deduced from eqn. ( 13) is given by [ 171 (14) Y, only depends on the ratio kJkl and thus moderately depends on T if the values of the corresponding activation energies are similar. Table 4 shows that E 1 z E2. The results presented in Fig. 3 show that, between 698 and 773 K, experimental values of YH range between 73 and 85%. The same Figure exhibits good agreement between computed and experimental values of Y, even for residence times higher than 20 s. For AIC thermal decomposition, the frequency factor and activation energy are lower than those generally observed in the case of the pyrolysis of cyclic compounds [21,22] or for similar molecules like triphenylisocyanurate (triester of AIC) [20]. A comparison between the results from the present work and values of the kinetic constant of triphenylisocyanurate is shown in Fig. 2. For triphenylisocyanurate, the values of the frequency factor and activation energy are ATPIC= 5.85 x lo’* s-’ and ETpIc= 252 x lo3 J mol-’ respectively. Figure 2 also shows good agreement with the result of Perry and Siebers [ 151 (k2 = 8.6 s-l) obtained at a higher temperature (858 K), under reduced pressure (67 700 Pa) and with a residence time of 0.30 s in a stainless steel continuous flow reactor. These results are, however, difficult to compare with the second-order rate constant of reaction HNCO + Ar +products, obtained by other authors [ 13,141 under quite different conditions of very high temperatures (18303340 K) and in shock wave tubes. CONCLUSION
The kinetics of the thermal detrimerization of AIC have been measured in a continuous tubular flow reactor under atmospheric pressure, with reaction times of a few seconds and with temperatures from 673 to 798 K. The Arrhenius parameters (A, = 6.3 x 10’s_‘, El = 119 x lo3 J mol-‘) have
118
J. Mercadier
et al. / J. Anal. Appl. Pyrolysis
28 (1994) 107-120
0.2 0 0
“I
20
40
60
60
100
0
I 20
-
I 40
*
I 60
tw . 80
.
0
2;
4;
6b
f
0
10
20
30
40
0
5
10
15
20
Fig. 3. Variations in the conversion yield of AIC (X) and mass yield of HNCO (Y) as a function of the residence time (t) of gases in the reactor for different temperatures: ( -) computed results; (0) experimental (Ye_,) values of Y.
been determined. The only identified product of this pyrolysis is HNCO. However, one may suspect that some gases (COz, CO, HCN, . . .) are produced by consecutive HNCO decomposition. The highest mass yield of HNCO obtained in our experiments is 85%. The global kinetics of HNCO thermal decomposition were also determined but with less accuracy: a more complex decomposition scheme and possible surface effects [ 121 may occur for this reaction. ACKNOWLEDGEMENTS
The financial support of Elf-Atochem
is gratefully acknowledged.
J. Mercadier
et al. 1 J. Anal. Appl. Pyrolysis
28 (1994) 107-120
119
LIST OF SYMBOLS
A TPIC Al A2
AIC ; DA
d ETPIC El E*
HPLC I k k, kz L h4AK m mHNCO
P P, Pe AIC Em QM Q N2
Q0 QT R Re r T TS t tE U,
V x
frequency factor of triphenylisocyanurate pyrolysis (s-l) frequency factor of AIC pyrolysis (s-r) frequency factor of HNCO pyrolysis (s-l) isocyanuric acid Damkiihler II number ( = k, d2/4D) molecular diffusivity ( m2 s- ‘) axial dispersion (m’ s-l) reactor diameter (m) activation energy of triphenylisocyanurate pyrolysis (J mol-‘) activation energy of AIC pyrolysis (J mol-‘) activation energy of HNCO pyrolysis (J mol-‘) high performance liquid chromatography ratio of molar amount of inert carrier gas to active species vapours first-order kinetic constant (s-l) first-order kinetic constant of AIC pyrolysis (s-l) first-order kinetic constant of HNCO pyrolysis (s-l) reactor length (m) molecular weight of AIC (kg mol-‘) mass of AIC sublimed and injected into the reactor during an experiment (kg) mass of HNCO chemically trapped by aniline (kg) Peclet number determined on the reactor length ( =u,L/D,) pressure in the reactor (Pa) Peclet number determined on the reactor diameter ( =u,d/2D) carrier gas mass flow rate (kg s-l) AIC flow rate at the reactor inlet at T (m3 s-l) average volumetric flow rate in the reactor at T (m3 s-l) carrier gas volumetric flow rate at T (m3 s-‘) carrier gas volumetric flow rate at 293 K (m’ s-l) carrier gas and active species vapours volumetric flow rate at T at the reactor outlet (m3 s-l) gas constant (J mol-’ K-l) Reynolds number ( = 4Q,/zpd) correlation coefficient reaction temperature (K) temperature of sublimation of AIC (K) residence time of gases in the reactor ( = V/Q,)(s) experience duration (s) gas velocity through the reactor at T (m s-i) volume of the reactor (m’) conversion yield of AIC ( =mass of unreacted AIC at the reactor outlet/m)
120
Y
Y-P YH
J. Mercadier
et al. / J. Anal. Appl. Pyrolysis
28 (1994) 107-120
mass (or molar) yield of HNCO ( =mHNCo/m) experimental mass yield of HNCO maximum mass yield of HNCO
Greek letters a
p Av
Chemical expansion factor Dynamic viscosity of nitrogen at T(Pa s) Difference between product and reactant efficients
stoichiometric
co-
REFERENCES 1 J. Mercadier, These INPL, Institut National Polytechnique de Lorraine, Nancy, France, 1992. 2 E.M. Smolin and L. Rapoport, in Arnold Weissberger (Ed.), The chemistry of heterocyclic compounds, 2nd edn., Vol. 13, Wiley-Interscience, New York, 1967, pp. 17-48. 3 R.N. Mesiah, in A. Standen (Ed.), Kirk Othmer encyclopedia of Industrial chemistry, 2nd edn., Vol. 20, Interscience, New York, 1969, pp. 662-671. 4 N. Kriebitzsch and H. Klenk, in W. Gerhartz (Ed.), Ullman’s encyclopedia of industrial chemistry, 5th edn., Vol. A8, VCH, Weinheim, 1987, pp. 191-200. 5 A.N. Gandhi and L.V. Keshav, in J.J. McKetta and M. Delker (Eds.), Encyclopedia of chemical processing and design, Vol. 14, INC, New York, 1982, pp. 52-60. 6 R.C. East, Brevet d’invention francais, 1.328.696, 1962. 7 F.W. Hoover, H.B. Stenvenson and H.S. Rothrock, J. Org. Chem., 28 (1963) 1825-1830. 8 G. Herzberg and C. Reid, Discuss. Faraday Sot., 9 (1950) 92-99. 9 S.R. Smith and H.B. Jonassen, J. Inorg. Nucl. Chem., 29 (1967) 860-862. 10 J.Y.P. Mui and R.A. Back, Can. J. Chem., 41 (1963) 826-833. 11 J.N. Bradley, J.R. Gilbert and P. Svedja, Trans. Faraday Sot., 46 (1968) 911-918. 12 R.A. Back and J. Childs, Can. J. Chem., 46 (1968) 1023-1024. 13 J.D. Mertens, A.Y. Chang, R.K. Hanson and C.T. Bowman, Int. J. Chem. Kinet., 21 (1989) 1049-1067. 14 0. Kajimoto, 0. Kondo, K. Okda, J. Fujikane and T. Fueno, Bull. Chem. Sot. Jpn, 58 (1985) 3469-3474. 15 R.A. Perry and D.L. Siebers, Nature, 324 (1986) 657-658. 16 J.V. Burakevich in M. Grayson, Kirk-Othmer concise encyclopedia of chemical technology, Wiley-Interscience, New York, 1985, pp. 339. 17 J. Villermaux, Genie de la Reaction Chimique. Conception et dtveloppement des reacteurs, Technique et Documentation, Lavoisier, Paris, 1985. 18 M. Pignon, Diplome d’Etude Approfondies, Institut National Polytechnique de Lorraine, Nancy, France, 1992. 19 J. L&de and J. Villermaux, J. Chim. Phys., 74 (1977) 459-467. 20 J. LcdC, J. Mercadier, M. Coste and J. Villermaux, J. Anal. Appl. Pyrolysis, 24 (1992) 179-189. 21 S.W. Benson, Thermochemical Kinetics. Methods for the estimation of thermochemical data and rate parameters, 2nd edn., Wiley, New York, 1976. 22 C.H. Bamford and C.F.H. Tipper, Comprehensive Chemical Kinetics, Vol. 5, Elsevier, Amsterdam, 1972. 23 R.H. Perry and D. Green, Chemical Engineer’s Handbook, 6th edn., McGraw-Hill, New York, 3.285-3.287, 1984.