Thermolysis of hexachlorocyclohexane in a flow-through packed reactor

Article abstract of DOI:10.1007/s11167-005-0063-z

Thermolysis of hexachlorocyclohexane in a flow-through packed system in an inert gas atmosphere was studied.

Full text of DOI:10.1007/s11167-005-0063-z

Russian Journal of Applied Chemistry, Vol. 77, No. 9, 2004, pp. 1518 1522. Translated from Zhurnal Prikladnoi Khimii, Vol. 77, No. 9,
004, pp. 1528 1532.
Original Russian Text Copyright
2
2004 by Kut’in, Gubanova, Zorin, Zanozina, Markova, Suprunova, Bykova.
ORGANIC SYNTHESIS
AND INDUSTRIAL ORGANIC CHEMISTRY
Thermolysis of Hexachlorocyclohexane
in a Flow-Through Packed Reactor
A. M. Kut’in, N. V. Gubanova, A. D. Zorin, V. F. Zanozina, M. L. Markova,
I. A. Suprunova, and E. A. Bykova
Lobachevsky State University, Nizhni Novgorod, Russia
Received March 12, 2004
Abstract Thermolysis of hexachlorocyclohexane in a flow-through packed system in an inert gas atmosphere
was studied.
The aim of the study is to analyze experimentally
the thermolysis of hexachlorocyclohexane in a labora-
tory flow-through packed reactor, elucidate the mech-
anism of the process, and determine its kinetic param-
eters using a macrokinetic model that allows modi-
fication of the process from a laboratory reactor to
an industrial installation.
The results of a chromatographic mass-spectro-
metric analysis (MD800/GC8060 instrument, HP 5MS
column 60 m long and 0.32 mm in diameter) were
used to identify the composition of products formed
in thermolysis of hexachlorocyclohexane that was
performed preliminarily under static conditions.
Five components were recorded under the chosen
chromatographic conditions: 1,2,4-trichlorobenzene,
1
,2,4,5-tetrachlorobenzene, 1,2,3,4,5-pentachlorocy-
EXPERIMENTAL
clohexene, and two of the possible eight stereoisomers
of hexachlorocyclohexane, -hexachlorocyclohexane
and -hexachlorocyclohexane.
The thermal decomposition was studied under
dynamic conditions by passing hexachlorocyclohex-
ane through a heated U-shaped reactor made of Pyrex
glass. A glass packing with a grain size of 0.4
The starting substances and the thermolysis prod-
ucts were analyzed under the following chromato-
graphic conditions: 100 0.4-cm glass column packed
with Inerton AW-DMCS with grain size of 0.25
0
.63 mm served to increase the reaction surface area.
The study was performed on a Tsvet-530 chromato-
graph. A heated reactor in a thermally insulated jacket
was placed in the thermostat of the chromatograph,
with the reactor inlet connected directly to the evap-
orator. Four packed reactors with lengths of 0.107,
0
.315 mm (impregnated with 5% SE-30); column
temperature in the temperature-programmed mode
varied from 100 (2 min) to 210 C (5 min) at a rate of
1
1
0 deg min ; temperature of the evaporator and
detector (FID) 200 C; flow rate of the carrier gas (ni-
0
.215, 0.322, and 0.427 m were used.
1
trogen) in the reactor and in the column 20 ml min ;
flow rates of hydrogen and air equal to 26 and
The temperature mode of the reactor was controlled
1
with a quartz furnace. The temperature was set and
maintained in the range from 250 to 500 C, with a
step of 50 C, using a Proterm-100 microprocessor
temperature controller. The temperature fluctuated
around the preset value in the course of thermostating
within 1 C.
3
00 ml min , respectively; motion velocity of the
1
chart paper 600 mm h . The substance was intro-
duced in the form of a melt on the needle of a micro-
syringe directly into the evaporator.
To construct a calibration dependence for the sub-
strate, hexachlorocyclohexane solutions in acetone
were prepared. The calibration was made under condi-
tions similar to those in which products formed in
thermal decomposition of hexachlorocyclohexane
were analyzed, with the volume of a sample intro-
duced equal to 1 l.
A sample introduced was mixed and diluted with
an inert gas (N ) in the evaporator of the chromato-
2
graph. The carrier gas ensured that the reaction mix-
ture flowed within the heated zone of the reactor in
the plug mode. In the experimental regard, the study
performed is similar to a pulsed version of the chro-
matographic method.
The experimental dependences of the composition
(mole fraction) of hexachlorocyclohexane and conver-
1
070-4272/04/7709-1518 2004 MAIK Nauka/Interperiodica

THERMOLYSIS OF HEXACHLOROCYCLOHEXANE
a)
(b)
1519
(
(c)
(
d)
(
e)
(
f)
Fig. 1. Mole fractions x of products formed in thermolysis of hexachlorocyclohexane along the reactor length l at (a) 250,
b) 300, (c) 350, (d) 400, (e) 450, and (f) 500 C. Calculated values, symbols connected by lines; experimental data, the corre-
sponding open symbols. (1) 1,2,4,5-C H Cl , (2) 1,1,2,3,4,5,6-C H Cl , (3) 1,2,3,4,5-C H Cl , (4) 1,2,4-C H Cl , (5) C H Cl ,
(
6
2
4
6
5
7
6
7
5
6
3
3
6
6
6
(
6) 1,2- and 1,4-C H Cl , and (7) C H Cl ; (8) HCl.
6 4 2 6 5 5
sion products on the reactor length are shown in
Fig. 1.
where the stoichiometric coefficients of ith component
in jth reaction < 0 for reactants and > 0 for reac-
tion product, and I is number of components (sub-
stances) appearing in the equations of the chemical
reactions [Eqs. (2)].
ji
ji
For a steady-state mode of operation of a plug
reactor, the material balance for ith component along a
reactor length element dz can be represented as [1, 2]
.
1
.
Instead of the component flow rates n (mol s ),
i
.
dn /dz = S r ,
(1)
1
i
a i
reaction flow variables
(mol s ), reaction flow
rates, can be used. In this case, the component flow
rates n are expressed in terms of the reaction flow
j
where S is the surface area of the packing and reactor
.
a
.
walls per unit reactor length; n , component flow rate;
i
i
rates as
and r , component velocities for the overall reaction.
i
J
The kinetic scheme of the process is constituted by
J concurrent reactions:
.
.
.
n = n
+
,
1
i
I,
(3)
i
0i
ji j
j= 1
I
.
where n are the component flow rates at the reactor
0
=
Ai,
1
j
J,
(2)
0i
ji
i= 1
inlet (z = 0).
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 77 No. 9 2004

1
520
KUT’IN et al.
quantities used have, as functions of z, the form
The component rates r can be expressed in term
i
of the reaction rates R as
j
.
.
.
.
y = n /n,
n = n ,
(10)
(11)
(12)
(13)
i
i
i
J
.
.
.
0
ri =
ji
Rj,
1
i
I.
(4)
V = n RT/PP ,
v = V/S,
i
j= 1
.
.
c = n V,
i
i
Substitution of (3), (4) into Eq. (1) gives another,
equivalent to (1), form of material balance equations
for J independent reactions:
0
P = c RT/P ,
P = Pi,
i
i
i
.
.
d /dz = S R , 1
j
J,
(5)
where y is the mole fraction; n , total gas flow rate in
j
a
j
i
i
.
the second of formulas (1); V and v, space velocity and
linear flow velocity [formulas (11)]; S, cross-sectional
area of the reactor; c , component concentrations in
The change in enthalpy of the components in the
reactions, which is due to the flux of heat across the
reactor walls along its length element dz [expres-
sion (6)] gives an additional, with respect to rela-
tion (5), differential equation (7) for the temperature
of the reaction mixture:
i
the flow; P and P, partial and total pressures (atm)
i
0
(
P = 101325 Pa).
Pressure can also be a sought-for function of the
coordinate, if the hydrodynamic resistance is taken
into account. However, the pressure is considered
in this study to be constant along the reactor.
I
.
d(n H ) = SS (TS
T)dz,
(6)
i
i
i= 1
The system of differential equations is supple-
mented with boundary conditions at the reactor inlet
(z = 0)
SS (TS
T)
Sa i riHi
dT
=
,
(7)
.
dz
n C
i pi
i
T(0) = T0, TS(0) = TS0 (for the forward flow),
(
14)
where H is the molar enthalpy of ith component;
.
i
(
0) = 0 (1
j
J).
,
heat-transfer coefficient; S , lateral heat-exchange
S
surface area per unit reactor length; T , temperature of
the heat-carrying agent in the jacket; and C , molar
S
The coordinate at the outlet is equal to the reactor
pi
length (z = L).
specific heats of the components.
It is noteworthy that, when solving the system
numerically, its composition can be determined both
from Eqs. (1) and from Eqs. (5). The software im-
plementation of the method is preceded by a proce-
dure in which a change to dimensionless variables is
performed. The system of ordinary nonlinear differen-
tial equations was solved using the fourth-order
Runge Kutta method with automatic choice of the
step interval.
In this case, the sum over the components in the
numerator of (7) can be expressed in terms of the rates
I
Rj and enthalpies H =
H of the reactions:
i
j
ji
i= 1
r H = Rj Hj.
(8)
i
i
i
j
An equation similar to (7) describes the variation
of the temperature of the heat-carrying agent in the
temperature-control jacket:
In terms of the given model, a heterogeneous
chemical process is considered to be defined if its fol-
lowing characteristics are defined and prescribed:
kinetic scheme (2) of chemical reactions at the sur-
face; block of thermodynamic data represented by
dT
dz
SS (TS
T)
=
,
(9)
.
n C
quantities X , which determine the direction and extent
S
pS
j
of jth reaction:
.
where the flow rate of the heat-carrying agent n > 0 if
S
0
its flow direction coincides with that of the flow in the
lnX =
G/RT =
g =
[g (T, P) + lnx ]
j
r
r
ji
i
i
.
i
reactor (forward flow) and n < 0 otherwise (counter-
S
flow).
x)
=
lnK (
+
lnxi,
(15)
j
ji
i
Additional expressions that relate and define the
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 77 No. 9 2004

THERMOLYSIS OF HEXACHLOROCYCLOHEXANE
g (T, P) = g (T) + ilnP, (16) mula k = (T*)exp[(E /R)(1/T* 1/T)], which fol-
where x is the mole fraction of ith component, and
1521
0
0
i
i
j
ij
j
j
j
lows from this equality, into the first of expres-
sions (19) yields, after obvious rearrangements, the
relation
i
0
g are the standard reduced chemical potentials of
i
the components.
1
1
(T)
ij
The chain of equalities (17), derived from (15) with
account of expressions that relate the equilibrium
constants, demonstrates the invariance of the quanti-
1
=
1 +
exp[ E R (1/T 1/T*)] . (20)
j j
*
(Tj*)
ij
ij
ij
ties X expressed on different concentration scales:
j
Neglecting the considerably weaker temperature
dependence of the diffusion mass-transfer coefficient
(which is commonly proportional to T ), compared
to the exponential dependence of the co-factor con-
(
x)
0
(c)
Xj = exp gj =
r
xi ij /K
j
=
P ij /K = ci ij /K , (17)
i
j
j
1/2
i
i
i
K (j x) = K (P ) j,
0
K (j c) = K (RT/P ) j,
0
0
(18)
is the change in the number of
ij
taining E , gives the following relations for the effec-
tive mass-transfer coefficient:
j
j
j
where
=
j
i
1
0
0
moles in jth reaction, and K = exp[ g (T)] is the
standard equilibrium constant.
* = wj, wj =
. (21)
j
i
ij
ij
1
1
+ exp[ E R (1/T 1/T*)]
j j
The validity of Eqs. (17) and (18) is confirmed
The resulting expression for the rate of jth hetero-
geneous reaction [2]
using the relations between the partial pressures Pi
(
atm) of the components, mole fractions x , and
i
3
volume concentrations c (mol m ) of ith compo-
R = [(1 X )w ][ ij( ij ci) 1
+ Xj i ij( ij ci) 1
i
] 1
ij
i
j
j
j
ij
nents: P = x P = c RT/P .
i
i
i
i
i
0
(
22)
The diffusion-kinetic model used in the study is
based on the known law of summation of the diffu-
sion and kinetic resistances [1]:
contains a factor 0 < w < 1, which has the form of
j
a switch that turns on the diffusion-controlled mode
of the reaction at temperatures T > T*and switches
1
1
1
j
=
+
,
k = A exp( E /RT). (19)
it into the kinetically controlled mode at T < T *. The
j
j
j
j
*
kj
ij
ij
range of temperatures close to the transition tempera-
ture T*corresponds to a mixed diffusion-kinetic con-
j
The effective coefficient *i j of mass transfer of
ith component to the reaction surface is determined by
the smallest of the following two quantities: ij, the
initial diffusion mass-transfer (mass-supply) coef-
trol, and at T = T*the shares of each of the limiting
j
modes are equal (w = 0.5). The simultaneously high
j
activation energy E (which exceeds its estimate based
j
on bond dissociation energies) and T*(above the tem-
j
ficient for ith component, and k , the coefficient
perature range under study) give wj
0, which in-
j
describing the efficiency of a chemical event of
transfer of a component from the category of reac-
tants to that of products of jth reaction. In the latter
dicates that this equation is to be excluded from the
kinetic scheme. Thus, the trial, but insignificant with-
in the experimental error, equations were rejected in
the stage of experimental data processing.
case, the actual rate constant k of the heterogeneous
j
reaction is represented in expression (19) by an
The information data block for solving the macro-
kinetic problem included the following factors: stan-
dard thermodynamic functions [3], which were evalu-
ated for unstudied substances by the group contribu-
tion method [4]; transfer coefficients (viscosity, heat
conductivity) calculated using the procedures de-
scribed in [4]; and heat- and mass-transfer coefficients
in the form of criterial relations for a grainy bed
from [5 7].
Arrhenius term with an activation energy E and a
j
pre-exponential factor A , which also determines the
j
dimension of the rate constant of the heterogeneous
1
reaction (m s ).
The constant A can be conveniently redefined and
j
expressed in terms of the known, and more illustrative
parameter T *, the temperature at which the reaction
j
passes from the kinetic to the diffusion control. In
doing so, a natural condition of its definition is the
Raw experimental data (areas of chromatographic
peaks), converted to mole fractions, were subjected to
a smoothing approximation processing. The kinetic
equality k =
ponents in expression (19). Substitution of the for-
of the diffusion and kinetic com-
j
ij
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 77 No. 9 2004

1522
KUT’IN et al.
Activation energies E and characteristic temperatures T*[formulas (19) (21)] of the reactions that constitute the kinetic
j
j
scheme [Eqs. (2)] of hexachlorocyclohexane thermolysis
Reaction no. (j )
Reaction equation
C H Cl + HCl
T *, K
Ej, J mol 1
j
1
2
3
4
5
C H Cl
10257.5
16118.2
822.729
2136.06
35802.6
28240
151652
34282.2
6
6
6
5
6
5
5
C H Cl
C H Cl + 2HCl
6
5
6 3 3
2C H Cl
C H Cl + C H Cl
7
6
6
6
6
7
5
6
5
C H Cl
C H Cl + 3HCl
6
6
7
5
6 2 4
C H Cl
C H Cl + 3HCl
6 4 2
6
7
parameters of the reactions were found by solving
the inverse problem (see table). The average relative
deviation of the calculated curves from the experi-
mental values was 5.7%.
chlorobenzene, 1,2-dichlorobenzene, and 1,4-dichloro-
benzene, is due to the concurrent bimolecular reaction
of exchange of hydrogen and chlorine atoms in the
substrate; dilution of the system with an inert com-
ponent must make higher the yield of 1,2,4-trichloro-
benzene.
In the mathematical processing, the concentrations
of by-product components: 1,2,4,5-tetrachlorobenzene,
1
,2-dichlorobenzene, and 1,4-dichlorobenzene, were
(3) The mechanism of the process, established in
the study, and the obtained kinetic parameters of the
occurring reactions, which account for the properties
of the packing and the heat-and-mass-exchange char-
acteristics of the process, enable a scaling transition
from a laboratory reactor to an industrial installation
for processing of outdated and prohibited pesticides
based on hexachlorocyclohexane.
taken into account in addition to the data for the
main substances (loss of hexachlorocyclohexane and
increase in the concentration of 1,2,4-trichloroben-
zene). However, the available data gave no way of
understanding in detail the mechanism of their forma-
tion. A numerical solution made it possible to specify
the parameters of the corresponding overall reaction
nos. 3 5, with the last of these occurring under purely
diffusion control.
REFERENCES
The kinetic scheme of thermal decomposition of
hexachlorocyclohexane is oriented toward the ob-
servable products and comprises a set of overall reac-
tions that only indirectly take into account a number
of possible intermediate species, including radicals.
For example, reaction no. 2, in which 1,2,4-trichloro-
benzene is formed, actually includes two successive
stages. As already mentioned, this refers to an even
greater extent to the compound reaction nos. 4 and 5.
1
2
3
. Frank-Kamenetskii, D.A., Diffuziya i teploperedacha v
khimicheskoi kinetike (Diffusion and Heat Transfer in
Chemical Kinetics), Moscow: Nauka, 1987.
. Kut’in, A.M., Pyadushkin, D.V., Zorin, A.D., and Kats-
nel’son, K.M., Matem. Model. Optim. Upravl.: Vestn.
Nizhegor. Gos. Univ., 1998, no. 1(18), pp. 96 104.
. Termodinamicheskie svoistva individual’nykh ve-
shchestv: Spravochnik (Thermodynamic Properties of
Individual Substances: Reference Book), Gurvich, L.V.,
Voits, I.V., Medvedev, V.A., et al., Eds., Moscow:
Khimiya, 1982.
In the temperature range studied, products of
deeper stages of thermolysis of hexachlorocyclohex-
ane, which include breakdown of the benzene ring,
were not observed, and, therefore, they are not re-
flected in the kinetic scheme.
4. Reid, R.G., Prausnitz, J.M., and Sherwood, T.K., The
Properties of Gases and Liquids, New York: McGraw-
Hill, 1977.
5
. Kutateladze, S.S., Teploperedacha i gidrodinamiche-
skoe soprotivlenie: Spravochnoe posobie (Heat Transfer
and Hydrodynamic Resistance: Reference Book), Mos-
cow: Energoizdat, 1990.
CONCLUSIONS
(
1) Thermolysis of hexachlorocyclohexane in its
conversion into trichlorobenzene occurs by the com-
monly accepted mechanism of successive elimination
of HCl in two stages, with an intermediate, penta-
chlorocyclohexene, formed in the first of these (which
is rate-limiting).
6
7
. Gol’dshtik, M.A., Protsessy perenosa v zernistom sloe
(
Transfer Processes in a Grainy Bed), Novosibirsk: Inst.
Teplofiz. Sib. Otd. Akad. Nauk SSSR, 1984.
. Bezobrazov, Yu.N. and Molchanov, A.V., Geksakhlo-
ran (Hexachlorocyclohexane), Moscow: Goskhimizdat,
(
2) The formation of by-products, 1,2,4,5-tetra-
1
949.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 77 No. 9 2004