Ethylene epoxidation under the effect of gas phase thermal oxidation of methane

Article abstract of DOI:10.1134/S0965544111060041

The process of ethylene epoxidation under the effect of gas phase thermal oxidation of methane has been studied. It was shown that if methane oxidation is carried out in the first section of two-sectional reactor and ethylene is injected into the second section then epoxidation occurs as a result of interaction of ethylene and peroxy radicals generated by methane oxidation reaction. The dependence of ethylene oxide accumulation rate on methane/oxygen ratio in the first section of reactor as well as flow velocity and temperatures in the first and second sections has been studied. The results show that in the second section of the reactor the ethylene epoxidation takes place trough C ₂H₄ + RO^?₂ → C ₂H₄O + RO reaction. Pleiades Publishing, Ltd., 2011.

Full text of DOI:10.1134/S0965544111060041

ISSN 0965ꢀ5441, Petroleum Chemistry, 2011, Vol. 51, No. 6, pp. 448–453. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © R.R. Grigoryan, S.D. Arsentev, A.A. Mantashyan, 2011, published in Neftekhimiya, 2011, Vol. 51, No. 6, pp. 456–461.
Ethylene Epoxidation under the Effect of Gas Phase Thermal
Oxidation of Methane1
R. R. Grigoryan, S. D. Arsentev, and A. A. Mantashyan
Institute of Chemical Physics by A. B. Nalbandyan, National Academy of Sciences of Republic of Armenia, Yerevan
eꢀmail: arsentiev53@mail.ru
Received October 29, 2010
Abstract—The process of ethylene epoxidation under the effect of gas phase thermal oxidation of methane
has been studied. It was shown that if methane oxidation is carried out in the first section of twoꢀsectional
reactor and ethylene is injected into the second section then epoxidation occurs as a result of interaction of
ethylene and peroxy radicals generated by methane oxidation reaction. The dependence of ethylene oxide
accumulation rate on methane/oxygen ratio in the first section of reactor as well as flow velocity and temperꢀ
atures in the first and second sections has been studied. The results show that in the second section of the
i
reactor the ethylene epoxidation takes place trough C H +
→
C2H4O + RO reaction.
RO2
2
4
DOI: 10.1134/S0965544111060041
1
Olefin oxides particularly ethylene oxide [1, 2] are
Aforesaid brings to the conclusion that working up
the significant products of organic synthesis. Nonꢀ and studies of nonꢀcatalytic gas phase processes of oleꢀ
ionic surfaceꢀactive substances produced from ethylꢀ fin oxides production is topical task. Gas phase proꢀ
ene oxide are effectively used in petroleum and petroꢀ cess has indubitable advantages compared with cataꢀ
leumꢀrefining industries. Ethylene oxide polymerizaꢀ lytic one: simple apparatus, no need to use chlorine,
tion is the way for waxꢀlike and liquid polymers no special requirements for initial reagents purity and
production, which are used as plasticizers and lubriꢀ elimination of expensive catalyst use.
cants. Ethylene oxide itself [3, 4] as well as its mixtures
with other compounds [5, 6] can serve as motor fuel.
It is known that in the course of the process of simꢀ
ple olefin (ethylene and propylene) gas phase oxidaꢀ
i
At present only two methods are used for industrial
production of ethylene oxide – interaction of ethylꢀ
enchlorhydrin with alkaline [1] and ethylene oxidaꢀ
tion over silver catalyst [7]. The first method is accomꢀ
panied with noticeable amount of chlorine containing
wastes, which pollute environment. The catalytic
method of ethylene oxidation is more preferable from
ecological point of view.
The most part of ethylene oxide for industrial needs
is produced catalytically [8–11]. Silver and its comꢀ
pounds are the best catalysts of ethylene epoxidation
which provide purpose compound production with
selectivity exceeding 70% [12]. Nevertheless silver
usage brings to several complex problems. It is known
that technologies of catalyst preparation are multiꢀ
stage and complicated. Moreover in consequence of
ageing and poisoning that take place during the
exploitation, activity of catalyst is reduced so it has to
be periodically regenerated. Another factor – specific
requirements for initial hydrocarbon containing gases
purity. The most undesirable are acetylene and sulfur
compounds, which are ingredients of natural gas; the
presence of other hydrocarbons as well as hydrogen
and carbon oxides is unwanted to.
tion alkylperoxy radicals
R
which concentration
O2,
exceeds 1014 part/cm3 are formed [13–16]. At that,
depending on process conditions (pressure and temꢀ
perature) and hydrocarbon type, peroxy radicals
i
i
i
i
СН
3O2,
С Н
5O2,
С Н or their mixture can be
Н
7O2, O2
2
3
accumulated.
Using radical freezing method combined with
EPRꢀspectroscopy [17], authors of [18–24] have
shown that at oxidation of mixtures, containing unsatꢀ
urated hydrocarbons, there is a linear relationship
between the rate of olefin oxide accumulation and
product of peroxy radicals and olefin concentrations:
W
=
k
[R i ] [C H ].
O2
(1)
2
4
Mentioned relationship indicates that epoxidation
reaction takes place
•
•
>C
,
C<
+ RO2
>C C<
O
+ RO
leading to olefin oxide and more active alcoxy radical
formation.
Ethylene and propylene epoxidation constants,
calculated on the base of experimental data can be
expressed as following:
1
The article was translated by the authors.
448

ETHYLENE EPOXIDATION UNDER THE EFFECT
449
1
7
4
5
6
3
2
Fig. 1. Scheme of reactor:
1
⎯
sealing device for methaneꢀoxygen mixture feed;
2
– reactor;
3
– capillary for ethylene feed; 4 ⎯
movable packet; – first section furnace;
5
6
⎯
second section furnace; 7 ⎯ sealing device for sampling.
1011
1011
×
10–13760/RT cm3/mol s
10–11750/RT cm3/mol s
.
packet 4, consisting of quartz tubes (d = 5 mm, l =
= 0.97
= 1.86
×
kC H O
2
4
10 mm) divides reactor for two sections. Packet moveꢀ
ment along the reactor changes the ratio of residence
times in sections. Reactor is heated using two indeꢀ
pendent electric furnaces 5 and 6, what enables to have
different temperatures in reactor sections. To perform
analysis of gas phase products a sample was picked out
×
×
k
C3H6O
The constant of ethylene epoxidation experimenꢀ
tally obtained in our study is in a good agreement with
calculated value [25]. Epoxidation constants for
i
acetylperoxy radicals RC cited in [26, 27] are conꢀ
O3,
through sealing device 7.
siderably higher because of their higher epoxidation
ability [26–30] in comparison with alkylperoxy radiꢀ
cals.
To determine formaldehyde concentration, waste
gases were bubbled trough distilled water for a fixed
time. Bubbler was filled with fine glass breakage to
increase contact surface between gas and water and, in
a number of cases in order to check completeness of
formaldehyde absorption, after main bubbler an addiꢀ
tional one was placed.
As is well known, the highest concentrations of
alkylperoxy radicals were detected at paraffinic hydroꢀ
carbons oxidation [31–34]. Apparently in the course
of conjugated oxidation of paraffinic and olefinic
hydrocarbons the favorable conditions for epoxy comꢀ
pounds formation can be realized. At this the main
part of peroxy radicals is formed from paraffinic but
not from olefinic hydrocarbon, what increases selecꢀ
tivity of olefin conversion into its oxide. This was conꢀ
firmed by experimental data obtained at conjugated
oxidation of olefinic and paraffinic hydrocarbons with
equal number of carbon atoms – ethylene with ethane
[35] and propylene with propane [22].
The revealed mechanism of olefin epoxidation
makes it possible to define conditions essential for
working up gas phase nonꢀcatalytic ways of epoxy
compounds production: peroxy radicals are to be genꢀ
erated by means of cheap and widespread hydrocarꢀ
bon containing gas oxidation; to avoid olefin conꢀ
sumption at the earlier stages of the process it must be
fed into reaction mixture at the moment when conꢀ
centration of peroxy radicals is maximal.
Photocolorimetric analysis of obtained solution
was performed using chromotropic acid. At this the
concentration of formaldehyde in bubbler was deterꢀ
mined. Formaldehyde concentration inside the reacꢀ
tor was calculated by formula
—formaldehyde concentration inside the reactor
(mol/cm3), Cb—concentration of solution from bubꢀ
bler (mol/cm3), —water volume in bubbler (cm3),
—rate of gas flow through the reactor cm3/s), tb
C = CbVTk/QtbTr where
C
V
Q
—
duration of bubbling (s), Tk and Tr—room and reactor
temperatures correspondingly (K). Partial pressure
can be calculated by expression
P = PаCNATr/TnNL,
where —partial pressure (kPa), Tn—normal temꢀ
P
perature (298 K), Pа—normal pressure (101.308 kPa),
NA—Avogadro number, NL—Loshmidt number
(
2.68
×
1019 part/cm3).
The purpose of present work is to realize the proꢀ
cess of ethylene epoxidation under the effect of therꢀ
mal gas phase reaction of methane oxidation.
Determination of gaseous products concentration
was performed by means of chromatography. Methaꢀ
nol, ethanol, acetaldehyde and ethylene oxide were
separated using chromatographic column filled with
EXPERIMENTAL
Polisorbꢀ1 (
30 cm3/min, carrier gas—helium). Hydrocarbons
C1⎯C4 were separated on Siliporꢀ600 ( = 3 m,
3 mm, col = 363 K,
= 24 cm3/min, carrier gas—
helium). Hydrogen, oxygen, methane and CO were
separated on molecular sieves CaA ( = 2 m, = 3 mm,
col = 363 K,
= 24 cm3/min, carrier gas—argon).
l = 3 m, d = 3 mm, Tcol = 378 K, Q =
Experiments were carried out on installation which
is depicted in Fig. 1. The mixture of methane and oxyꢀ
gen is injected trough sealing device
tion of twoꢀsectional quartz reactor
drical part – 20 cm, diameter of reaction zone – 2 cm)
where oxidation of methane occurs providing peroxy
radicals formation. Ethylene is fed into the second
l
d =
1
into the first secꢀ
(length of cylinꢀ
T
Q
2
l
d
T
Q
section through capillary 3 (d = 2 mm). Movable Katharometer was used as detector in all cases.
PETROLEUM CHEMISTRY Vol. 51
No. 6
2011

450
GRIGORYAN et al.
Table 1. The effect of residence times in 1ꢀst and 2ꢀnd sections of the reactor on yield of reaction products;
2 = 778 K; CH4 : O2 = 3.8; = 86.7 kPa
T1 = 983 K;
T
P
Mixture feed
Ethylene feed
into 1 section into 2 section
Partial pressures of products at reactor outlet, kPa
of reactor, cm3/s of reactor, cm3/s
Q1
Q2
CH3OH
CH3CHO
C2H4O
HCHO
CO
1.00
1.40
1.60
1.80
2.20
2.70
3.30
0.63
0.351
0.214
0.196
0.180
0.161
0.093
0.057
0.298
0.317
0.326
0.348
0.306
0.194
0.098
0.056
1.784
2.311
2.423
1.848
1.432
0.661
0.124
0.199
0.311
0.582
0.450
0.318
0.283
3.650
3.380
2.860
2.080
1.630
0.980
0.240
0.88
1.00
1.13
1.38
1.69
2.07
Note:
Q
and
Q
are fixed so as to have the ratio
τ
/
τ
the same in all experiments.
2
1
2
1
Table 2. The effect of methane/oxygen ratio in mixture injected into 1ꢀst section of the reactor on yield of reaction prodꢀ
ucts; 1 = 983 K; 2 = 778 K; = 86.7 kP
1 = 1.80 cm3/s; 2 = 1.13 cm3/s;
T
T
Q
Q
P
Mixture composition
inside 1 section
Partial pressures of products at reactor outlet, kPa
CH4 : O2
CH3OH
CH3CHO
C2H4O
HCHO
CO
8.0
6.2
4.5
3.8
3.0
2.5
1.0
0.3
0.207
0.195
0.189
0.180
0.176
0.170
0.154
0.091
0.106
0.235
0.342
0.348
0.337
0.297
0.164
0.103
0.356
1.584
2.341
2.423
2.316
2.043
0.810
0.422
0.410
0.475
0.551
0.582
0.516
0.473
0.460
0.397
0.910
1.120
1.500
2.080
2.090
3.120
3.600
4.530
RESULTS AND DISCUSSION
branching nature of methane oxidation, which leads
to appearance of maximum on kinetic curve of peroxy
radicals while residence time in the first section
increases. As long as ethylene oxide accumulation rate
is determined by expression (1), and ethylene concenꢀ
tration under conditions of Table 1 was kept the same,
then the existence of maximum on the kinetic curve of
peroxy radicals will cause the appearance of maximum
Concentrations of main reaction products, meaꢀ
sured for different residence times in 1ꢀst and 2ꢀnd
sections of the reactor are given in Table 1, and those
obtained for different equivalence ratio of reaction
mixture injected into the first reactor section are given
in Table 2. Dependences of average rate of ethylene
oxide accumulation on residence time in the first secꢀ
tion of the reactor, as well as on СН4/О2 ratio are
shown in Figs. 2a and 2b. Calculation was performed
on the dependence of
first section.
on residence time in the
W
C2H4O
by formula
=
, where
2
—average
WC H O
4
W
C2H4O
P
C2H4O
τ
As it can be seen from Table 2 and Fig. 2b
the dependence of on [СН4]/[О2] has a maxiꢀ
mum – the increase of [СН4]/[О2] from 0.3 to
3.8 leads to increase of ethylene oxide accumulation
rate 5.7 times. At this change in hydrocarbon/oxygen
2
rate of ethylene oxide accumulation (kPa/s),
—
P
C2H4O
WC H O
2 4
partial pressure of ethylene oxide at reactor outlet
(kPa), – residence time of reaction mixture in the
τ
2
second section (s).
Figure 2a shows that ethylene oxide accumulation ratio has an influence on the time of maximal concenꢀ
rate is maximal at residence time in the first section
1 ~ 5 s; increase or decrease of residence time results
in sharp decrease of accumulation rate. Apparently,
tration of peroxy radicals reach. The shape of curve in
Fig. 2b can be explained by extremal dependence of
hydrocarbon oxidation intensity on hydrocarbon/oxyꢀ
τ
observed relationship is conditioned by degenerate gen ratio [36]. Consequently the dependence of perꢀ
PETROLEUM CHEMISTRY Vol. 51
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2011

ETHYLENE EPOXIDATION UNDER THE EFFECT
451
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
WC H O
2 4
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
WC H O
(b)
2
4
(а)
2
4
6
8
10
2
4
6
8
10
1, s
τ
CH4/O2
Fig. 2. Dependence of average rate of ethylene oxide accumulation on: a ꢀ residence time in the first section of the reactor at
3
3
СН /О = 3.8, b ꢀ ratio methane/oxygen.
Q = 1.8 cm /s, Q = 1.13 cm /s, T = 983 K, T = 778 K, P = 86.7 kPa.
4
2
1 2 1 2
oxy radicals maximal concentration on [СН4]/[О2
has to be a curve passed over the maximum. As a result
]
generated in methane oxidation in 1ꢀst section of
reactor.
the dependence of WC2H O on [CH4]/[O2] will have a
The influence of temperature in both sections of
reactor on average ethylene oxide accumulation rate
as well as concentrations of main reaction products
was studied (Tables 3 and 4). It can be seen, that averꢀ
age ethylene oxide accumulation rate depends on
temperature in the first section and passes over the
maximum. Obviously it is due to degenerate branching
4
maximum, what can be seen in Fig. 2b.
Under the conditions of Table 1 the experiments
were carried out in which argon/oxygen mixture was
injected into the first section of the reactor instead
methane/oxygen; at this ethylene oxide accumulation
rate was ~ 9 times lower than that in the presence of
methane. Obtained result shows that ethylene epoxiꢀ
nature of methane oxidation. At
T < 970 K radical
concentration in 1ꢀst section does not reach its maxiꢀ
dation in the second section is initiated by radicals, mal value at fixed residence time and ethylene epoxiꢀ
Table 3. The effect of temperature in 1ꢀst section of the reactor on yield of reaction products and rate of ethylene oxide
accumulation; CH4 : O2 = 3.8;
T
2 = 793 K;
Q Q P = 86.7 kPa
1 = 1.8 cm3/s; 2 = 1.13 cm3/s;
Rate of ethylene
oxideaccumulation,
kPa/s
Temperature inside
1ꢀst section, K
Partial pressures of products at reactor outlet, kPa
W
T1
CH3OH
CH3CHO
C2H4O
HCHO
CO
C2H4O
910
927
0.026
0.032
0.075
0.114
0.122
0.132
0.131
0.129
0.106
0.215
0.296
0.312
0.334
0.348
0.360
0.345
0.325
0.308
0.427
0.664
1.532
2.193
2.370
2.439
1.821
0.746
0.330
0.522
0.521
0.506
0.502
0.500
0.493
0.487
0.486
0.458
1.060
1.130
1.660
1.760
1.790
1.900
1.910
1.940
2.300
0.106
0.165
0.380
0.544
0.588
0.605
0.452
0.185
0.082
940
965
980
993
1000
1016
1048
PETROLEUM CHEMISTRY Vol. 51
No. 6
2011

452
GRIGORYAN et al.
Table 4. The effect of temperature in 2ꢀnd section of the reactor on yield of reaction products and rate of ethylene oxide
accumulation
Rate of ethylene
oxide accumulaꢀ
tion, kPa/s
Temperature inside
2ꢀnd section, K
Partial pressures of products at reactor outlet, kPa
W
T2
CH3OH
CH3CHO
1 = 983 K;
C2H4O
HCHO
CO
C2H4O
CH4 : O2 = 3.8;
T
Q
1 = 1.80 cm3/s;
Q
2 = 1.13 cm3/s;
P
= 86.7 kPa
760
765
778
798
815
840
0.105
0.103
0.180
0.183
0.184
0.179
0.320
0.351
0.348
0.346
0.310
0.293
0.899
1.488
2.423
2.476
2.468
2.398
0.583
0.586
0.582
0.577
0.548
0.474
2.060
2.060
2.080
2.100
2.160
2.240
0.214
0.357
0.591
0.619
0.630
0.631
Ar : O2 = 3.8;
T
1 = 983 K;
Q
1 = 1.80 cm3/s;
Q
2 = 1.13 cm3/s;
P
= 86.7 kPa
762
765
778
801
811
846
0.041
0.045
0.050
0.052
0.059
0.068
0.124
0.146
0.168
0.173
0.211
0.293
0.143
0.182
0.213
0.225
0.261
0.307
0.127
0.184
0.215
0.259
0.273
0.298
0.036
0.043
0.052
0.054
0.062
0.073
0.024
0.034
0.052
0.056
0.060
0.062
dation rate is lower than maximum; at
T
> 1000 K section of reactor) as well as on flow rate and temperꢀ
ature in both sections, is evidence of epoxidation reacꢀ
tion С2Н4 + RO2 C2H4O + RO taking place (2ꢀnd
section). At that separation of epoxidizing agent (radꢀ
СН4 oxidation in 1ꢀst section is suppressed because of
initial reagent consumption and concentration of radꢀ
icals incoming into the second section is decreased. As
a result ethylene oxide accumulation rate is decreased
too, what leads to maximum appearance on the
→
i
i
i
icals СН
3O2,
С Н and others) generation
Н
5O2, O2
2
zone from ethylene epoxidation zone allows to reduce
expensive ethylene consumption into secondary prodꢀ
ucts.
dependence of
on temperature in 1ꢀst section.
W
C2H4O
Data in Table 4 show that temperature increase
inside the second section in the range 760–800 K
increases rate of ethylene oxide accumulation
almost 3 times. As conditions of methane oxidation in
1ꢀst section remain permanent i.e. concentration of
radicals incoming into the second section is the same,
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2011