Catalytic pyrolysis of propane and butane over silica surface

Article abstract of DOI:10.1134/S1070363208100137

Pyrolysis of raw propane-butane mixture at 500-800°C in helium atmosphere in a quartz flow reactor was studied in the presence and in the absence of disperse silica packing. Under the conditions excluding radical chain propagation, the process was found to occur as catalytic decomposition of the initial hydrocarbons over silica surface.

Full text of DOI:10.1134/S1070363208100137

ISSN 1070-3632, Russian Journal of General Chemistry, 2008, Vol. 78, No. 10, pp. 1900–1901. © Pleiades Publishing, Ltd., 2008.
Original Russian Text © Yu.A. Aleksandrov, V.M. Shekunova, I.I. Didenkulova, I.A. Pishchurova, 2008, published in Zhurnal Obshchei Khimii, 2008,
Vol. 78, No. 10, pp. 1662–1664.
Catalytic Pyrolysis of Propane and Butane over Silica Surface
a
b
b
a
Yu. A. Aleksandrov , V. M. Shekunova , I. I. Didenkulova , and I. A. Pishchurova
a
Lobachevskii Nizhnii Novgorod State University,
pr. Gagarina 23, Nizhnii Novgorod, 603005 Russia
e-mail: chicoma@mail.ru
b
Research Institute of Chemistry, Lobachevskii Nizhnii Novgorod State University, Nizhnii Novgorod, Russia
Received April 21, 2008
Abstract—Pyrolysis of raw propane–butane mixture at 500–800°C in helium atmosphere in a quartz flow
reactor was studied in the presence and in the absence of disperse silica packing. Under the conditions
excluding radical chain propagation, the process was found to occur as catalytic decomposition of the initial
hydrocarbons over silica surface.
DOI: 10.1134/S1070363208100137
One of the main methods for large-scale prepara-
tion of ethylene [1] is based on pyrolysis of raw
propane–butane mixture. This process is performed in
a steel flow reactor, and its significant disadvantage is
profound pyrolysis of a considerable amount of the
initial hydrocarbon mixture to give coke. As a result,
periodical process shutdown is necessary to remove
coke. Various protective coatings are applied to reactor
walls to reduce amount of coke. Among these, the best
chain reaction. Typical experimental results are
collected in Table 1. It is seen that the yield and ratio
of the pyrolysis products depend on both contact time
and contact surface. On the basis of the temperature
dependence of the yields of pyrolysis products we
calculated the energies of activation of the overall
decomposition process and formation of particular
products (Table 2).
It should be noted that the effective activation
energies for decomposition of propane and isomeric
results were obtained with the use of SiO and,
2
probably, SiO2–x(OH) [2, 3].
–1
x
butanes in an empty quartz reactor (179–303 kJ mol )
The goal of the present work was to estimate the
effect of silica surface on the pyrolysis of raw
propane–butane mixture in helium medium, i.e., under
the conditions which suppress radical chain process
and coking.
are smaller than the energy of the weakest C–C bond
–
1
in their molecules (up to 326 kJ mol [5]); the dif-
ference is comparable with the heat of physical
adsorption of hydrocarbons by silica [6]. Presumably,
adsorption of propane and butane on silica surface
precedes their homolytic decomposition, and the
subsequent pyrolysis process occurs mainly on the
silica surface. This assumption is consistent with
published data [7].
Detailed study on the pyrolysis of propane in a
quartz reactor [4] showed that the process in the gas
phase follows radical chain mechanism, formalistically
along two pathways:
C H + CH
4
2
4
EXPERIMENTAL
C3H8
The kinetics of pyrolysis of raw propane–butane
mixture in helium medium were studied using a setup
consisting of a flow quartz tube reactor (0.46–0.5 cm
i.d.) equipped with a pulse batcher (the hydrocarbon
mixture was supplied in 7.5×10 -mol portions) and a
sampler. Propane–butane–helium mixture was supplied
to the pulse batcher from a special rough-vacuum tank.
The amount of the supplied hydrocarbons was
H + C H
3 6
2
No participation of the reactor surface in the
process.
–
5
We used a raw propane–butane mixture strongly
diluted with helium which is incapable of participating
in chain propagation; therefore, the pyrolysis process
in an inert gas may be considered to not involve radical
1900

CATALYTIC PYROLYSIS OF PROPANE AND BUTANE OVER SILICA SURFACE
1901
a
Table 1. Pyrolysis of raw propane–butane mixture
Yield, mol % (calculated on the reacted hydrocarbons)
Temperature,
Conversion,
%
Contact
°C
methane
ethane
ethylene
propylene
Empty reactor
700
725
43.0
69.6
81.2
87.7
87.2
43.5
64.6
82.3
94.7
95.7
13.7
22.9
28.0
34.9
37.5
23.4
31.5
37.4
41.1
41.9
–
–
–
–
1.1
2.6
4.1
5.2
5.3
5.5
20.0
37.5
45.5
50.9
47.6
17.5
29.0
39.7
46.3
46.5
9.3
9.2
7.7
1.9
1.0
(
τ 6.6 s)
7
7
8
50
80
00
Empty reactor
700
725
–
–
–
(
τ 9.0 s)
7
7
8
50
80
00
2.0
1.8
b
Silica
700
725
31.2
50.0
78.5
89.5
98.8
6.9
1.0
3.0
3.0
4.0
5.0
12.7
22.1
27.9
34.4
48.0
10.6
13.7
9.7
8.1
7.5
(
τ 0.75 s)
11.2
37.9
43.0
38.3
7
7
8
50
80
00
b
a
No coking was observed. The reactor was filled with silica (grain size 0.3–0.5 mm).
Table 2. Energies of activation for the overall decomposition process of propane and butane and formation of methane and
ethylene in the pyrolysis of raw propane–butane mixture in helium medium
–
1
а
Е , kJ mol
Contact
Contact time
τ, s
СН4
С Н
С Н
4 10
∑С Н
2
4
3
8
Empty reactor
Empty reactor
Silica
6.6
9.0
0.75
62.6±3.9
37.8±3.2
35.9±2.7
85.1±16.5
77.0±8.6
109.4±14.8
211.9±9.2
179.0±14.9
211.6±20.7
302.9±19.2
287.6±16.3
196.6±23.0
determined from the pressure reduction and
composition of the mixture.
REFERENCES
1
. Khimicheskaya entsiklopediya (Chemical Encyclopedia),
Moscow: Bol’shaya Rossiiskaya Entsiklopediya, 1998,
vol. 5, p. 983.
In serial experiments the hydrocarbon mixture was
supplied to the reactor from the batcher with a stream
of helium (carrier gas flow rate 30 ml/min). The
calculated time of contact τ was 6.6–9.0 s in empty
reactor and about 0.75 s in the reactor filled with silica.
The initial propane–butane mixture was produced by
the Sibur-Neftekhim joint-stock company, and it
contained 7.3 mol % of methane, 4.2 mol % of ethane,
2
3
. US Patent no. 3536776, 1970; Ref. Zh., Khim., 1971,
no. 15P168P.
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4
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7
5
5.2 mol % of propane, 7.7 mol % of isobutane, and
.6 mol % of butane. The selectivity of pyrolysis of the
no. 6, p. 340.
5
. Mortimer, C.T., Reaction Heats and Bond Strengths.
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reaktsii i prochnost’svyazei, Moscow: Mir, 1964, p. 16.
hydrocarbon mixture in a stream of helium was studied
in the temperature range from 500 to 800°C. The gas
mixture was analyzed for methane, ethane, ethylene,
propane, propylene, isobutane, and n-butane using a
Tsvet 100M chromatograph equipped with a thermal
conductivity detector and a 2-m column packed with
6
. Kiselev, A.V. and Yashin, Ya.I., Adsorbtsionnaya
gazovaya i zhidkostnaya khromatografiya (Adsorption
Gas and Liquid Chromatography), Moscow: Khimiya,
1979, p. 55.
1
0% of KOH on Al O . The product concentrations
2 3
were determined by the absolute calibration technique
using standard hydrocarbon samples.
7. Kazanskii, V.B., Kinet. Katal., 1977, vol. 18, no. 1, p. 43.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 78 No. 10 2008