Effects of pH on rheological characteristics and stability of petroleum coke water slurry
Effects of pH on rheological characteristics and stability of petroleum coke water slurry
Fu-Yan Gao 0 1 2
Eric-J. Hu 0 1 2
0 Edited by Xiu-Qin Zhu
1 School of Mechanical Engineering, University of Adelaide , Adelaide, SA 5005 , Australia
2 Ningbo Institute of Technology, Zhejiang University , Ningbo 315100 , China
In this study, the effects of pH on slurrying properties of petroleum coke water slurry (PCWS) were investigated. The slurrying concentration, rheological characteristics and stability of PCWS were studied with four different types of additives at pH varying from 5 to 11. The results showed that the slurrying concentration, rheological characteristics and stability of PCWS all increased at first and then decreased with increasing pH from 5 to 11, and a pH of around 9 was found to be the most favorable acid-alkali environment to all these three slurrying properties. It was also indicated that only in a moderate alkaline environment can the additives be active enough to react with particle surfaces sufficiently to obtain good slurrying concentration and form a stable three-dimensional network structure, which can support strong pseudoplastic characteristics and good stability. An acid environment was a very unfavorable factor to the slurrying properties of PCWS.
Petroleum coke; Petroleum coke water slurry pH; Slurrying concentration; Rheological characteristics; Stability
Petroleum oil and its products are important fuels and
chemical raw materials, which are widely used in almost
all aspects of production and life. Along with the rapid
development of the economy, the demand for petroleum oil
. More and more petroleum
coke, as an end product of the petroleum refining process,
(Ren et al. 2012; Zhang et al. 2012)
coke, with its characteristics of high carbon content, high
calorific value and low ash content, has become a popular
fuel for power generation
(Chen and Lu 2007; Milenkova
et al. 2005; Anthony et al. 2001; Wang et al. 2004; Sheng
et al. 2007)
and has started to become a potential
(Valero and Uso´n 2006; Fang et al. 2005)
With the development of coal water slurry (CWS)
technology, increasing attention has been paid to petroleum
coke water slurry (PCWS). CWS and PCWS are liquid
fuels of low pollution and high efficiency and can be
pumped like oil by pipeline and burned in power plants as
an oil substitute
(Zhan et al. 2010)
. They change the
traditional combustion of solid fuels and show huge
environmental protection and energy-saving advantages
et al. 1997; Wu et al. 2015)
. Because of the strong
hydrophobicity, PCWS generally possesses higher solid
concentration than conventional CWS. Moreover, PCWS
also can be a superior raw material for industrial
(Gao et al. 2012a, b; Zou et al. 2008)
. Hence, PCWS
has become an important way to utilize petroleum coke
efficiently and cleanly.
The slurrying properties are most important for
industrial application of the slurry fuels. The solid concentration
of the slurry fuels should be increased as much as possible
to reach a high level of heat value and thus ensure efficient
gasification and combustion, but the viscosity should be
low enough to facilitate preparation, pumping and
atomization of the slurry. Many studies are aimed at
influencing factors on PCWS’s slurrying properties
et al. 2012a, b; He et al. 2011; Xu et al. 2008; Vitolo et al.
1996; Wang et al. 2006)
. Yet, up to now, the effect of pH
on slurrying properties of PCWS is rarely reported. Acid–
alkali properties of the slurry can directly influence the
interactions between the additives and the surface of
petroleum coke particles and subsequently influence the
slurrying properties of PCWS. In this work, the effects of
pH on the slurrying properties of PCWS were investigated.
2 Materials and methods
A petroleum coke from America was used in the
experiments. The proximate and ultimate analysis results of the
petroleum coke used in this work are shown in Table 1.
The petroleum coke was ground in a ball mill to obtain the
pulverized sample, and particles below 149 lm were
selected by an electric sieve shaker to prepare PCWS. The
granularity distribution of the selected petroleum coke
particles was analyzed with a Mastersizer 2000 Granularity
Meter (Malvern, UK), as shown in Fig. 1. The average
particle diameter was approximately 27 lm.
Chemical additives are an important component of
slurry fuel, for they can help particles to disperse stably in
the slurry. Four kinds of anionic surfactants were used as
additives in PCWS preparation in this work. These were
sodium methylene naphthalene sulfonate-sodium styrene
sulfonate-sodium maleate (NDF), methylene naphthalene
sulfonate formaldehyde condensate (MF), lignin sulfonate
(LS) and petroleum sulfonate (PS). The additive dosage
was fixed at 0.8 wt% based on dry petroleum coke
et al. 2015)
The petroleum coke particles, deionized water, one
additive and moderate HCl or NaOH were mixed with an
electric mixer at 1000 r/min for 10 min to form a PCWS
sample. With each additive, PCWSs were prepared at four
different pH values (i.e., pH 5, 7, 9 and 11). The pH values
were measured by using an E200 Portable pH Meter (Mont,
The apparent viscosity and rheological properties of
PCWS were measured on a rotary viscometer (NXS-4C,
China). A PCWS sample was first loaded into the
viscometer, and then the shear rate was increased from 10 to
100 s-1. The relationship of the shear stress and the shear
rate can be revealed in this process. Keeping the shear rate
at 100 s-1 for 5 min, the apparent viscosity data were
recorded every 30 s during a 5-minute period. The average
apparent viscosity at 100 s-1 was calculated from the ten
apparent viscosity values recorded. During the entire
process, temperature was controlled at 20 ± 1 C.
The solid concentration of PCWS was determined by
drying the slurry in an oven at 105 C for 2 h and then
weighing the dried residue.
Measurement of stability of PCWS was taken after the
slurry was sealed in a container for 7 days. In order to
ensure the reliability of the experimental results, the
stability of PCWS was measured by both the rod-insertion
and a visual method
(Li et al. 2008)
In the rod-insertion method, a steel rod was inserted
vertically and freely from the slurry surface, and the first
traveling length through the slurry was recorded. Then the
steel rod was strongly pressed down to the bottom of the
container, and the second traveling length through the
slurry was recorded as well. The relative height of the hard
sediment layer can be obtained by calculation, which is an
index to evaluate slurry stability. A large relative height of
hard sediment layer indicates poor stability of the PCWS.
In the visual method, the changes in slurry properties, such
as separated water, could be found through observation.
The mass ratio of separated water to total slurry is used to
evaluate the stability of slurry. A higher water-to-slurry
ratio indicates a worse stability.
3 Results and discussion
3.1 Effects of pH on slurrying concentration of PCWS
Solid concentration at a specific viscosity of 1000 mPa s
with the shear rate of 100 s-1 is used to evaluate the
slurrying concentration of petroleum coke. The higher the
solid concentration, the better the slurrying concentration
of petroleum coke
(Hu et al. 2009)
. Figure 2 shows the
relationship of slurrying concentration of PCWSs (with
different additives) with pH.
Figure 2 shows that the slurrying concentration
increased first and then decreased with increasing pH and
that an acid environment of pH 5 resulted in the worst
slurrying concentration and an alkaline environment of pH
9 resulted in the best slurrying concentration. The reason is
that the additives themselves possess moderate alkalinity,
and the activity of the additives can be restrained in acid or
strong alkali conditions, leading to the dispersion of
particles worsened and slurrying concentration decreased.
3.2 Effects of pH on rheological characteristics of PCWS
Rheological characteristics are very important to the
industrial application of slurry fuels. These are not only
related to the slurrying properties, but also directly affect
the pumping, atomizing and combustion performances of
the slurry fuels
(Ma et al. 2012, 2013a, b; Li et al. 2010;
Meikap et al. 2005; Chen et al. 2009)
. Usually PCWS is
expected to be of high viscosity to promote stability during
storage and low viscosity to ensure fluidity during
transport; hence, ‘‘shear-thinning’’ pseudoplastic characteristics
are generally required in industry.
Figure 3 shows the relationship of rheological
characteristics of PCWS (with different additives) with pH at
solid concentration of 69 wt%. It can be seen that the
PCWSs with NDF and MF additives were dilatant fluids
and had shear-thickening properties at pH from 5 to 11 and
exhibited the feeblest shear-thickening at pH of 9, while
the PCWSs with LS and PS additives possessed
shearthinning pseudoplastic characteristics at pH of around 9.
A three-parameter Herschel–Bulkley model
(Ma et al.
expressed by Eq. (1) was used to fit the shear
stress–shear rate data.
c_ ¼ 0
s ¼ sy þ kc_n
s [ sy
where c_ is the shear rate, s-1; s is the shear stress, Pa; sy is
the yield stress, Pa; k is the consistency coefficient, Pa sn; n
is the dimensionless flow characteristic exponent.
The parameter ‘‘n’’ can correctly reflect rheological
characteristics of PCWS
(Ma et al. 2013a, b)
. When n [ 1,
the PCWS is a dilatant fluid; otherwise, it is a pseudoplastic
fluid. Furthermore, the smaller the value of n, the greater
the pseudoplastic characteristics.
Figure 4 shows the relationship of flow characteristic
exponents of the PCWSs with pH when the PCWSs were
prepared with different additives at solid concentration of
69 wt%. It can be seen that the flow characteristic
exponents all decreased first and then increased with increasing
pH, and the smallest flow characteristic exponent with each
additive appeared at pH of 9, indicating that pH of around 9
was the most favorable acid–alkali environment to
strengthen pseudoplastic characteristics of PCWS.
This is because pH can affect interactions between the
additive and particles and then affect the rheological
characteristics of slurry. The alkaline additives can
maintain their own activity and react sufficiently with surface of
particles only in moderate alkaline environment, and make
3.3 Effects of pH on stability of PCWS
There is a positive correlation between stability and
pseudoplastic characteristics of PCWS. The more stable the
three-dimensional network structure of slurry is, the better
the stability turns out to be. Therefore, a pH of around 9
can also be the best acid–alkali environment to obtain good
stability of PCWS.
Through the study, the following conclusions can be
The slurrying concentration of the PCWS increased
first and then decreased with increasing pH from 5 to
11. An acid environment was an unfavorable factor to
the slurrying concentration. The optimal pH to obtain
best slurrying concentration was 9. The additives used
in this work themselves possess moderate alkalinity,
and their activity can be restrained in acid or strong
alkali conditions, leading to worse dispersion of
particles and decreased slurrying concentration.
The pseudoplastic characteristics of the PCWS
increased first and then decreased with increasing pH
from 5 to 11, and a pH of around 9 was the most
favorable acid–alkali environment to strengthen the
pseudoplastic characteristics. The alkaline additives
can maintain their own activity and react sufficiently
with surface of particles only in a moderate alkaline
environment, and make it possible to form a relatively
stable three-dimensional network structure in slurry
and present relatively strong pseudoplastic
The stability of the PCWS increased first and then
decreased with increasing pH from 5 to 11, and the
best stability occurred when the pH was around 9. A
pH of around 9 was the best acid–alkali environment to
obtain good stability of PCWS. It shows a positive
correlation between stability and pseudoplastic
characteristics of PCWS.
Acknowledgments The authors would like to acknowledge the
financial support from the National Natural Science Foundation of
China (No. 51506185) and the Zhejiang Provincial Natural Science
Foundation of China (No. LQ15E060002).
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