Indian Journal of Chemical Technology
Vol. 18, March 2011, pp. 91-98
Oxidation of vacuum residue by ozone and nitrous oxide: FTIR analysis
Mukesh Kumar Sahua, K Tewarib & A S K Sinhaa*
a
Department of Chemical Engineering & Technology, Institute of Technology,
Banaras Hindu University, Varanasi 221 005, India
b
Department of Chemical Engineering, National Institute of Technology, Rourkela 769 005, India
Received 1 December 2009; accepted 18 January 2011
Vacuum residue is crude oil fraction obtained at the bottom of vacuum distillation column having boiling point above
565°C. Vacuum residue is often used without further treatment as fuel oil or for the production of asphalt. Ozone (O3) is a
triatomic allotrope of oxygen and is an extremely strong oxidizing agent. It can cleave and oxidize benzene and other
aromatic structures including PAHs. Nitrous oxide is also a good oxidizing agent relate to a particular state of anion radical
oxygen species Oα- (called α-oxygen), which forms on α-sites from N2O. Liquid phase oxidation of vacuum residue at
temperature 100-150°C either ozone or nitrous oxide results into conversion of the residue to oxygenated hydrocarbons.
This process reduces concentration of aromatics and olefins which are known to be coke precursors in reforming
application. The reaction of oxidant with vacuum residue has been investigated in detail using FTIR.
Keywords: Vacuum residue, Partial oxidation, Ozone, Nitrous oxide, FTIR
Vacuum residues has a high molecular weight
(>>1000 amu), a very high viscosity and high
percentages of sulfur, nitrogen, heavy metals (nickel
and vanadium), and a low hydrogen/carbon ratio. The
composition of residue depends on both the original
oil and subsequent processing. Vacuum residue is
often used without further treatment as fuel oil or for
the production of asphalt1-4. With the development of
hydrogen fuel cell, and also hydrogen being termed as
future source of energy, there is a need for newer and
cheaper sources of hydrogen and therefore vacuum
residue can be utilized for the same5. The established
catalytic processes, for the conversion of hydrocarbon
feed to hydrogen, i.e., gas phase catalytic steam
reforming and catalytic partial oxidation, are not
suitable for the conversion of vacuum residue to
hydrogen because of extensive coke formation on the
surface of catalyst. The non-catalytic gasification is
possible at temperature in excess of 1300°C6. Because
of complexity, the gasification process does not
appear suitable for in-situ generation in small and
medium installations. Therefore, a different strategy
has to be adopted for the production of hydrogen from
vacuum residue in particular for in-situ generation at
small installations.
——————
*Corresponding author (E-mail: [email protected])
In the present work, a two-step process has been
developed. In the first step the vacuum residue has
been partially oxidized in the liquid phase. This
process reduces the concentration of aromatics and
olefins which are known to be coke precursors. In the
second step cracked and oxidized product is converted
to hydrogen by autothermal process using a catalyst.
This paper reports the results of our work on the first
step of the process. The oxidants used in the present
study were ozone and nitrous oxide.
Ozone (O3) is a triatomic allotrope of oxygen and
is an extremely strong oxidizing agent. It can cleave
and oxidize benzene and other aromatic structures
including PAHs. One pathway by which ozone breaks
down aromatic ring compounds is 1, 3-dipolar
cycloaddition across the double bonds. Another
ozone/PAH degradation pathway (in aqueous media)
involves highly reactive free radicals (e.g. HO2 and
HO) which are expected to be more effective and
require less ozone. Alkenes can also oxidize with
ozone to form alcohols, aldehydes or ketones, or
carboxylic acids through formation of molonozide7-9.
Nitrous oxide is also a good oxidizing agent. A
remarkable specificity of N2O is related to a particular
state of anion radical oxygen species Oα- (called αoxygen), which forms on α-sites from N2O, but
cannot form from O2. Among the specific features of
α-oxygen, its very high reactivity seems to be the
most remarkable one10-12.
92
INDIAN J. CHEM. TECHNOL., MARCH 2011
Experimental Procedure
In the present study, the vacuum residue of BPCL
refinery is used. The characteristics of vacuum
residue are summarized in Table 1. The vacuum
residue of BPCL refinery is having boiling point
above 565°C.
Experimental set-up
Figure 1 shows the set-up used in the present study
for the liquid phase oxidation of vacuum residue.
Vacuum residue was taken into a 500 mL Pyrex glass
round bottom flask. The flask was placed inside a
heating mantle kept at a desired temperature with the
help of an on-off controller. The reaction temperature
was measured with a thermocouple sliding inside a
thermocouple well provided in the flask. The desired
oxidant was bubbled continuously for the requisite
period of time after the liquid had attained the set
temperature.
Ozone was generated in-situ by subjecting air to
high intensity low wavelength UV radiation. Nitrous
oxide was fed from a cylinder and its flow was
regulated by a mass flow meter. Whenever
oxygenation of vacuum residue with N2O was studied,
a catalyst 2% Fe on Al2O3 was finely dispersed in the
liquid. The role of catalyst was to dissociate N2O.
Catalyst was prepared by wet impregnation of γalumina with aqueous solution of metal precursor
(FeSO4.5H2O) followed by drying at 110°C for 12 h
and calcining in air at 500°C for 4 h. Finally, the mass
was reduced in a tubular reactor by continuous flow
of hydrogen at 300°C for 3 h. The catalyst was
crushed and finely dispersed. Subsequent to the
oxidation reaction, the residue was vacuum distilled
(~ 10-2 bar). For 100 mL of vacuum residue, a 40 mL
clear distillate was obtained. Distillate without any
oxidation of residue was also collected and it was
used as a reference material (sample 1) for the
subsequent analysis. Ozone oxidized vacuum residue
samples at 100°C for 5 and 10 h and at 150°C for 5
and 10 h were assigned as sample 2, 3, 4 and 5
respectively. Nitrous oxide treated samples at 100°C
for 5 and 10 h and at 150°C for 5 and 10 h were
assigned as sample 6, 7, 8 and 9 respectively.
Analysis of product
Elemental analyses of the reference and oxidized
vacuum residue samples have been obtained by a
Perkin-Elmer CHN analyzer for C, H and N analysis
and Optim’X X-ray analyzer for Ni, V and S analysis.
The functional groups in oxidized samples (2-9) were
analyzed by FTIR (make Thermoelectron, USA
model Nicolet 5700) fitted with a multi bounce
attenuated total reflectance (ATR) accessory.
Analyses were carried out at room temperature in mid
IR range of 4000-400 cm-1 at a resolution of 4 cm-1.
Subtraction spectrum of a sample was obtained
through Omnic software by subtraction of spectrum of
distillate obtained from untreated residue from that of
the sample. From this subtraction spectrum it was
possible to find out additional peaks appearing on
various treatments of the residue.
Results and Discussion
Effect of treatment with ozone
The results of elemental analysis of reference and
ozone treated samples are shown in Table 2. The
weight percent of oxygen was obtained by difference.
It is observed from Table 2 that in ozone treated
samples (2-5) weight percentages of carbon, hydrogen,
nitrogen, sulfur, nickel and vanadium have decreased
and whereas that of oxygen has increase. Thus, increase
in oxygen wt% clearly shows that ozonation has resulted
into the partial oxidation of vacuum residue.
Fig. 1—Schematic diagram of experimental set-up
Table 1–Characteristics of vacuum residue
Characteristics
Feed cut point TBP(°C)
o
API
S (wt%)
N (wt%)
Viscosity at 100°C,cSt
Ni (ppm)
V (ppm)
Asphaltene (C7-insoluble) (wt%)
CCR* (wt%)
*CCR=Conradson carbon residue
Value
565+
4.53
2.0
1.9
5955
346
2000
21.0
22.3
SAHU et al.: OXIDATION OF VACUM RESIDUE BY OZONE AND NITROUS OXIDE
93
Table 2–Elemental analyses of vacuum residue and ozone treated vacuum residue
Sample
1
2
3
4
5
Element
C
(wt%)
H
(wt%)
N
(wt%)
S
(wt%)
Ni
(ppm)
V
(ppm)
O
(wt%)
83.41
78.68
77.58
78.60
78.44
12.5
10.68
10.08
10.58
10.42
1.9
0.24
0.16
0.22
0.17
2
1.33
1.33
1.17
1.17
375
153
153
153
116
2000
375
375
350
350
0.16
9.06
10.84
9.42
9.79
The FTIR spectrum of vacuum residue (reference)
and ozone oxidized vacuum residue at 100°C for 5 h
(sample 2) and their subtraction spectrum are shown
in Figs 2-4. Figures 2 and 3 are similar in nature
except few small bands and also it is very difficult to
identify the difference among them. Figure 4 is the
subtraction spectrum of the sample 2, and has number
of peaks. These peaks arise due to the treatment of the
residue with ozone. In other words, it means that a
number of functional groups are present in the ozone
treated samples which otherwise were absent before
treatment. The subtraction spectra of the sample 3-5
are shown in Figs 5-7, respectively.
It is also seen that all the spectra are similar. The
observed distinct broad band in the region 3382 to
3448 cm-1 attributes to O-H stretching. The O-H inplane bending vibration is observed in the region
1398 to 1413 cm-1. The bands centered in the region
2979 to 2872 cm-1 correspond to the stretching mode
of asymmetrical CH3 and CH2 vibrations mainly of
aliphatic carbonyl containing functional groups. The
band centered at 2794 cm-1 corresponds to the
stretching mode of symmetrical CH vibrations of
cyclic ether. The absorption bands observed in the
region 1500-1800 cm-1 are for the over-lapping of the
ν (C=C) stretching vibration mode of the aromatic
ring, lactone and quinone with the ν (>C=O)
absorption bands of free carboxylic, ester, lactone and
carbonyl groups. The most remarkable feature of the
spectra is the appearance of a broad carbonyl >C=O
band centered in the region 1709 to 1712 cm-1 which
is asymmetrical to higher frequencies, and displays a
shoulder at 1770 cm-1. The band in the region 1709 to
1712 cm-1 is not exclusively associated to the carbonyl
groups; it may also be due to carboxyl group. The
strong band at about 1165 cm-1 is compatible with
C–O single-bond stretching vibration from γ and δ
lactone groups, aromatic and aliphatic ether, and
phenols and epoxide structures. The band in the
region 1026 to 1041 cm-1 is associated over lapping of
S=O of sulphoxide and C–O–C of ether13-15. The C-S
stretching of sulphoxide present in the region 750 to
Table 3–Summary of functional groups assigned from FTIR studies
Wave number (cm-1)
Functional group assignment
3328 - 3448
2794 - 2993
O-H stretching vibration
asymmetrical CH3 and CH2 vibrations
of aliphatic carbonyl containing
functional groups
>C=O
OH bending vibration of carboxyl and
hydroxyl groups
C–O single-bond stretching vibration of
γ and δ lactone
overlapping vibration due to S=O of
sulphoxide and C–O–C of ether
C-S stretching sulphoxide
1709 - 1712
1398 - 1413
1193 - 1248
1026 - 1041
750 - 790
790 cm-1. The various bands and corresponding
function groups that appeared on treatment of vacuum
residue with ozone are summarized in Table 3.
It is also seen from the above figures that increasing
the ozonation temperature from 100°C to 150°C results
into changes in the intensities of the observed peaks.
Significant change in the relative intensities of peaks
at 3374 (due to hydroxyl group) and 1712 cm-1 (due to
carbonyl group) is observed. On increasing the reaction
temperature from 100 to 150°C the broad absorption
peak at 3374 gets shifted to 3449 cm-1 with a much
diminished intensity. It indicates a significantly
decreased content of hydroxyl groups. On the other
hand, absorption peak at 1712 gets shifted to 1709 cm-1
with an enhanced intensity. The increase in the
intensity of the peak indicates a significantly increased
content of carbonyl groups in the samples which were
treated with ozone at a higher temperature of 150°C.
The intensity ratios of bands –O–H to >C=O are 1.66
and 0.95 respectively for 5 and 10 h treated samples at
100°C. The respective values are 0.64 and 0.33 for
5 and 10 h treated samples at 150°C. Therefore, it is
concluded that on ozonation of vacuum residue,
hydroxyl groups are first formed which subsequently
converted to carbonyl groups.
A similar effect as described above is observed
when instead of temperature, greater time is
increased. A comparison of absorption peaks also
reveals that the ratio of intensities of bands >C=O to
94
INDIAN J. CHEM. TECHNOL., MARCH 2011
Fig. 2—FTIR spectrum of vacuum residue
Fig. 3—FTIR spectrum of ozone treated vacuum residue at 100°C for 5 h
Fig. 4—FTIR subtraction spectrum of ozone treated vacuum residue at 100°C for 5 h
SAHU et al.: OXIDATION OF VACUM RESIDUE BY OZONE AND NITROUS OXIDE
Fig. 5—FTIR subtraction spectrum of ozone treated vacuum residue at 100°C for 10 h
Fig. 6—FTIR subtraction spectrum of ozone treated vacuum residue at 150°C for 5 h
Fig. 7—FTIR subtraction spectrum of ozone treated vacuum residue at 150°C for 10 h
95
INDIAN J. CHEM. TECHNOL., MARCH 2011
96
C–H in sample treated at 100°C for 5 and 10 h are
0.77 and 0.81 respectively. Whereas, the values are
0.97 and 1.58 for the samples (4 and 5), which are
ozonated at 150°C for 5 and 10 h respectively. It is,
therefore, concluded that by the ozonation of vacuum
residue the hydrocarbons are first converted to
compounds having hydroxyl groups and finally to
those having carbonyl groups.
Effect of treatment with N2O
The vacuum residue was treated with nitrous oxide
at 100°C and 150°C for 5 as well as 10 h. The results
of elemental analysis of reference and nitrous oxide
treated samples are shown in Table 4.
It is observed from Table 4 that in nitrous oxide
treated samples (6-9) weight percentages of carbon,
hydrogen, nitrogen, sulfur, nickel and vanadium have
decreased and whereas that of oxygen has increase as
with ozone. Thus, increase in oxygen wt% clearly
shows that nitrous oxide treatment has also resulted
into the partial oxidation of vacuum residue.
The subtraction spectra of the nitrous oxide treated
sample (5-9) are shown in Figs 8-11, respectively. All
the spectra are seen to be similar in nature. The
distinct small broad band in the region 3361 to
3377 cm-1 attributes to -O-H stretching. The different
bands centered in the region 2829 to 2978 correspond
to the stretching mode of asymmetrical CH3 and CH2
vibrations mainly due to aliphatic carbonyl containing
functional groups. The band centered at 2794 cm-1
corresponds to the stretching mode of symmetrical
Fig. 8—FTIR subtraction spectrum of nitrous oxide treated vacuum residue at 100°C for 5 h
Fig. 9—FTIR subtraction spectrum of nitrous oxide treated vacuum residue at 100°C for 10 h
SAHU et al.: OXIDATION OF VACUM RESIDUE BY OZONE AND NITROUS OXIDE
97
Fig. 10—FTIR subtraction spectrum of nitrous oxide treated vacuum residue at 150°C for 5 h
Fig. 11—FTIR subtraction spectrum of nitrous oxide treated vacuum residue at 150°C for 10 h
Table 4–Elemental analyses of vacuum residue and nitrous oxide treated vacuum residue
C
(wt%)
H
(wt%)
N
(wt%)
Element
S
(wt%)
Ni
(ppm)
V
(ppm)
O
(wt%)
83.41
80.53
80.32
80.37
80.23
12.5
10.71
10.51
10.63
10.39
1.9
0.75
0.67
0.64
0.61
2
1.33
1.33
1.33
1.33
375
116
116
116
116
2000
375
375
375
375
0.16
6.67
7.16
7.02
7.43
Sample
1
6
7
8
9
CH vibrations mainly due to cyclic ether. The
absorption bands observed in the region 1500-1800 cm-1,
are for the over-lapping of the ν (>C=C<) stretching
vibration mode of the aromatic ring, lactone and
quinone with the ν (>C=O) absorption bands of free
carboxylic, ester, lactone and carbonyl groups.
The band observed in the region 1711 to
1715 cm-1 has been assigned to >C=O stretching
INDIAN J. CHEM. TECHNOL., MARCH 2011
98
Table 5– Summary of functional groups assigned from FTIR studies
-1
Wave number (cm )
Functional group assignment
3361 - 3382
2829 - 2978
-O-H stretching vibration
asymmetrical CH3 and CH2 vibrations of
aliphatic carbonyl containing functional
groups
CH vibrations of cyclic ether
>C=O
bending vibration of -O-H
stretching vibration of epoxide groups and
phenol
overlapping
vibration
of
C–O–C
stretching of ether and S=O of sulphoxide
C-S stretching sulphoxide
2794
1711 - 1715
1392 - 1396
1180 - 1217
1032 - 1048
766 - 798
vibrations of carbonyl groups. The band in the region
1393 to 1396 cm-1 is corresponding to the -OH
bending vibration of carboxyl and hydroxyl groups.
The strong band in the region 1180 to 1217 cm-1 is
compatible with C-O-C stretching vibration of
phenols and epoxide structures. The band in the region
1032 to 1048 cm-1 are for the overlapping of C-O-C
stretching of ether and S=O stretching of sulphoxide.
The weak band in the region around 766 to 798 cm-1 is
attributed to C-S stretching of sulphoxide13-15. The
various bands and corresponding functional groups as
discussed above have been given in Table 5.
It is also observed from these spectra that on
increasing the reaction temperature from 100 to
150°C the broad absorption peak at 3382 cm-1 gets
shifted to 3366 cm-1 with a diminished intensity. It
indicates a significantly decreased content of
hydroxyl groups. On the other hand, absorption peak
at 1048 cm-1 gets shifted to 1032 cm-1 with an
enhanced intensity. The increase in the intensity of
the peak indicates a significantly increased content
of C-O-C groups in the samples which were treated
with nitrous oxide at a higher temperature of 150°C.
The intensity ratios of bands O–H to C-O-C are 1.
63 and 1.19 respectively for 5 and 10 h treated
samples at 100°C. The respective values are 1.15 and
0.96 for 5 and 10 h treated samples at 150°C.
Therefore, it is concluded that on nitrous oxide
oxidation of vacuum residue, hydroxyl groups are
first formed which subsequently convert to alkoxy
groups. A similar effect as described above is
observed when instead of temperature, oxidation time
is increased. It is in order to mention that oxidation
using ozone has resulted in formation of compounds
having carbonyl groups.
A comparison of absorption peaks also reveal that
the ratio of intensities of bands C-O-C to C-H in
sample treated at 100°C for 5 and 10 h are 0.36 and
0.71, respectively. Whereas. the values are 0.66 and
0.77 for the samples which were oxidized at 150°C
for 5 and 10 h, respectively. It is, therefore, concluded
that by the oxidation of vacuum residue using nitrous
oxide the hydrocarbons are first converted to
compounds having hydroxyl groups and finally to
those having alkoxy groups.
Conclusions
Liquid phase oxidation of vacuum residue at
temperature 100-150°C either ozone or nitrous oxide
results into conversion of the residue to oxygenated
hydrocarbons. Carbonyl groups are observed when
ozone was the oxidant. Whereas, in the case of nitrous
oxide alkoxy groups are formed. It is observed that in
the case of both the oxidant first hydroxyl groups are
formed which subsequently change to carbonyl or
alkoxy groups. These partially oxygenated products
can be used for hydrogen production by steam
reforming or autothermal reforming with much
reduced propensity for coke formation.
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