1. No category

CHEMICAL AND THERMAL MODIFICATION OF PETROLEUM COKE
M. Malekshahian and Josephine M. Hill
Department of Chemical and Petroleum Engineering
University of Calgary,
2500 University Drive, N.W., Calgary, Alberta, Canada. T2N 1N4
Abstract:
The influence of different pre-treatment methods, including devolatilization, chemical, and
thermal pre-treatment methods, on the properties of petroleum coke (petcoke) samples has
been investigated using N2 and CO2 adsorption, XRD, and FTIR techniques. After
characterization, a portion of each sample was impregnated with potassium. The effects of
both the pretreatments and the presence of K as a catalyst were investigated by testing the
reactivity of petcoke in air at 900 ºC using a thermogravimetric analysis instrument.
Characterization methods reveal that the modification methods influence the physical
properties, the crystal structure, and the surface functional groups of the petcoke samples. In
turn, the reactivity measurements show that modification of physical and chemical
properties of petcoke surface, as well as the presence of potassium enhances the reactivity of
the samples.
Keywords: petroleum coke, chemical pre-treatment, thermal pre-treatment, gasification,
catalyst
1.
INTRODUCTION
The production of petroleum coke (petcoke) is increasing as a by-product of heavy oil upgrading units. While some
of the produced petcoke in Canada is burnt in the boilers to produce steam and electricity, its economic utilization is
limited due to substantial sulphur concentration, unreactive forms of carbon, and relatively large quantities of
transition metals (Watkinson, et al. 1989). Since large quantities of petcoke are being stockpiled at plant sites due to
these constraints, petcoke has been considered as an attractive feedstock for gasification. The gasification reactivity
of petcoke, however, is low and high carbon conversions are only obtained at high temperatures and long residence
times (Furimsky 1985). Therefore, the addition of catalysts may be expedient to enhance the gasification rate at
lower temperatures (Zekel 2003). Using catalysts in gasification also significantly affects the product distribution
and product gas quality. The main difficulty with adding catalysts to petcoke arises from the fact that petcoke has a
low surface area such that there are insufficient anchoring sites for the active metal catalyst. Fortunately, factors
affecting metal sorption, including the surface area, acidic functional groups, and crystalline structure, can be
modified with various pre-treatment processes (Bautista-Toledo, et al. 1994; Ochoa, et al. 2001; Chingombe, et al.
2005). The objective of the present research is to investigate the effects of various pre-treatment methods on the
physical and chemical structure of petcoke using nitrogen and carbon monoxide adsorption, X-ray diffraction, and
Fourier transform Infrared spectroscopy. After characterization, the modified petcoke samples were reacted in a
thermogravimetric unit initially in an air atmosphere.
2.
EXPERIMENTAL
2.1. Preparation of petcoke samples
Petcoke, supplied by Suncor Energy, was ground and sieved to particle sizes smaller than 90 µm. The obtained
powder, referred to as raw petcoke, was used as the precursor for the various pre-treatment methods. The first
modified sample, referred to as char, was devolatilized at 800 ºC in flowing nitrogen to drive off volatile species.
The next two samples, referred to as chemically pretreated petcoke and thermally pretreated petcoke, were modified
with chemical and thermal pre-treatments, respectively, as follows. The chemical pre-treatment was performed by
placing a portion of the ground petcoke in a solution of HNO3 (20 v/v %) for 15 h at 90 ºC followed by washing
with distilled water (Strelko and Malik 2002). The sample was then dried at 110 ºC overnight. The thermal pretreatment was achieved by heating a portion of the ground petcoke to 450 ºC under flowing nitrogen in a fixed bed
reactor. The sample was kept at this temperature for 10 h with a flow of nitrogen in air (3:1 v/v) before being cooled
to room temperature in nitrogen (150 mL/min) (Serp 1999). The catalyst potassium (K) was loaded on raw and
modified petcoke samples using an impregnation method. Impregnated catalyst samples were prepared by wetting
the sample with an aqueous solution of potassium carbonate, stirring until the complete evaporation of solvent, and
oven drying at 110 ºC for 12 h. The catalyst target loading was 4 wt %.
2.2. Characterization
The characterization techniques used were physisorption, Fourier Transform Infrared (FTIR) spectroscopy, and Xray diffraction (XRD). Although nitrogen (N2) adsorption is a traditional technique for the characterization of porous
materials, CO2 adsorption has also been extensively used for microporous materials particularly carbonaceous
adsorbents because of the slow diffusion of N2 in small (< 2 nm) pores at 77 K (Marsh 1987; Mahajan 1991). As
such, nitrogen adsorption isotherms along with the CO2 adsorption isotherms yield helpful information about the
surface area and the pore structure of adsorbents. Nitrogen adsorption at 77 K and CO2 adsorption at 273 K were
performed with a Micromeritics micropore analyzer (Tristar 3000) to determine surface area, pore volume, and pore
size distribution. The Brunauer Emmett Teller (BET) method was used to calculate the surface area.
FTIR spectra were measured by transmission FTIR spectroscopy (Nicolet Avator 360) to determine the presence of
functional groups on the surface of the petcoke samples. For the FTIR analysis, a measured amount of sample was
added to KBr for a concentration of 1 wt%. The finely ground material was then massed and pressed into 25 mm
diameter disks. Spectra were recorded by co-adding 34 scans at a resolution of 4 cm-1 with the automatic baseline
correction. The XRD analysis (Rigaku Multiflex X-ray diffractometer) was conducted to investigate the structural
parameters of raw and modified petcoke. Cu/Kα1 radiation (λ= 1.54056 Å) at a 40 kV tube voltage and a 40 mA
tube current with a scanning speed of 2° /min was used.
2.3. Thermogravimetric analysis
Formatted: Bullets and Numbering
Reactivity measurements were conducted using a TherMax500 thermogravimetric system. In each experiment
approximately 2-3 mg of sample was placed in a crucible inside the furnace and heated under a N2 atmosphere up to
900 ºC with the heating rate of 20 ºC/min. When the reaction temperature was reached, N2 was replaced by air (100
ml/min) and the mass loss recorded.
3.
RESULTS AND DISCUSSION
3.1. Pre-treatment effects on the petcoke characterization
The surface areas of the petcoke samples are listed in Table 1. After devolatilization, the surface area based on CO2
adsorption decreased while the surface area based on N2 adsorption increased. Since CO2 and N2 are appropriate
probe molecules to investigate microporosity and mesoporosity, respectively, it is likely that mesopores are created
after devolatilization. The significant increase in the surface area obtained by N2 and CO2 adsorption suggests that
the thermal pre-treatment method enhanced both mesoporosity and microporosity. Modification of the petcoke using
the chemical pre-treatment resulted in relatively minor changes to the physical properties.
Table 1. Surface area of petcoke samples as measured by N2 and CO2 adsorption.
Material
Raw petcoke
Char
Thermally pretreated petcoke
Chemically pretreated petcoke
BET specific surface area (m2/g)
N2
CO2
2.7
93
3.6
28
57
176
2.9
107
The surface functional groups of the samples were identified by FTIR. Fig. 1 shows the FTIR spectra of the raw and
modified petcoke samples. The spectrum of raw petcoke was characterized by 1. asymmetric and symmetric –C–H
stretching vibrations in aliphatic fragments such as –CH3, =CH3, and –CH2CH3 (2914 cm-1 and 1377 cm-1), 2. –
C=C– aromatic bonds phenolic groups (1442 cm-1), 3. bending vibration of –C–H of aromatic rings (3044 cm-1), and
4. –O–H stretching vibration mode of hydroxyl functional groups (3450 cm-1) (Lee and Choi 2000; Jiang, et al.
2008). The FTIR spectra of chemically pretreated petcoke had a weak band appearing at 1720 cm-1, which can be
ascribed to the stretching vibrations of carboxyl groups. In addition, the alkyl group band at approximately 2900
cm-1 disappeared. Fig. 1 also illustrates the disappearance of IR bands in char samples, which suggests noticeable
reduction of surface functional groups after devolatilization.
0.7
char
Absorbance ( Arbitrary units)
0.6
C=O
0.5
chemically pretreated petcoke
thermally pretreated petcoke
0.4
C=C (ar)
0.3
0.2
0.1
O-H
C-H (al)
C-H (ar)
C-H (al)
raw petcoke
0
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber (cm-1)
Fig. 1. FTIR spectra of the raw and pretreated petcoke samples.
XRD analysis of the raw and modified petcoke samples was performed to investigate the crystal structure of the
samples. Table 2 lists the interlayer spacing, d002, and crystallite size, Lc, of the samples, calculated by the Bragg and
Scherer equations. The crystallite size decreased while the value of interlayer spacing increased after pre-treatments.
These values imply that disordered structures were produced. This result is consistent with observations reported
elsewhere (Chunlan, et al. 2005; Jiang, et al. 2008) in which petcoke was modified by chemical pre-treatment and
pre-carbonization to produce activated carbon.
Table 2. Microcrystalline structural parameters of the raw and pretreated petcoke samples
Sample
Raw petcoke
Char
Chemically pretreated coke
Thermally pretreated coke
Lc (nm)
18.0
14.4
13.6
8.9
D002 (nm)
0.342
0.343
0.355
0.348
3.2. Reactivity test
The conversion trends for the raw and pretreated petcoke samples as a function of time are shown in Fig. 2. By
definition, the observed rate (R) and conversion (X) of the reactant are calculated, respectively, by:
R
X
1
W0
W0
W0
dW
dt
W
W final
(1)
( 2)
where W0 is the initial weight of the raw or pretreated petcoke sample, dW
is the instantaneous weight loss rate,
dt
W is the weight at time t, and Wfinal is the weight at the end of the reaction. The conversion of petcoke increased with
the addition of catalyst either with or without pre-treatment. However, the addition of catalyst on the chemically and
thermally pretreated petcoke samples enhanced the reactivity more than on the raw petcoke and char. The reaction
rates of raw petcoke, char, chemically pretreated coke, and thermally pretreated coke increased from 0.016, 0.015,
0.024, and 0.019 mg/mg.min to 0.019, 0.018, 0.037, and 0.026 mg/mg.min, respectively, in the presence of K. In
addition, chemical and thermal pre-treatment enhances the reactivity even with no catalyst. These results suggest
that the effectiveness of pre-treatment methods is related to their effects on physical, chemical and crystal
characteristics of petcoke. Further analysis and reactivity tests in different atmospheres are underway.
100
chemically
pretreated
petcoke+K
90
raw
petcoke+K
80
char+K
Carbon conversion, %
70
thermally pretreated
petcoke+K
raw
petcoke
60
50
40
30
20
10
0
5
15
25
35
45
55
Time, min
Fig. 2. Conversion of the petcoke samples modified with potassium as a function of time at 900 ºC.
4.
CONCLUSIONS
The reactivity tests indicated, as expected, that the pre-treatment process used to modify the petcoke enhanced the
reactivity. The increase in reactivity due to the addition of a potassium catalyst is higher for chemically and
thermally pretreated samples than for raw petcoke and char. The characterization of the petcoke samples suggested
that these methods are effective because they enhance the concentration of anchoring sites (such as carboxylic acid
groups), increase the surface area, and increase the interlayer spacings within petcoke.
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