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Effect of solvent on the viscosity changes of coal-oil slurry under high temperatureЦhigh pressure during heating.

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ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING
Asia-Pac. J. Chem. Eng. 2009; 4: 744–750
Published online 15 July 2009 in Wiley InterScience
(www.interscience.wiley.com) DOI:10.1002/apj.330
Research Article
Effect of solvent on the viscosity changes of coal-oil slurry
under high temperature–high pressure during heating
Yingjie Ren,1,2 * Lei Song,1 Dexiang Zhang,1 Jinsheng Gao,1 and Hong Xu2
1
2
Department of Chemical Engineering for Energy Resources, East China University of Science and Technology, Shanghai, China
School of Mechanical and Power Engineering, East China University of Science and Technology, Shanghai, China
Received 22 September 2008; Revised 12 March 2009; Accepted 12 March 2009
ABSTRACT: Heating coal-oil slurry is an important step in direct coal liquefaction. Some physical and chemical
properties and the viscosity of coal-oil slurry will be the changes in the heating process. A self-manufactured rotary
viscometer that can measure the viscosity of coal slurry at high-pressure temperature was designed. Two kinds of
solvents including anthracene oil, Shenhua recycled oil (the catalytic hydrogenation products of Shenhua direct coal
liquefaction) and Yanzhou coal were used. The results show that the viscosity of Yanzhou coal-anthracene oil and
Shenhua recycled oil slurry decrease firstly with the decreasing of solvent viscosity, then increase with measuring
temperature for the absorption and volatilization of the solvent in atmosphere. Both of them have a viscosity peak with
increasing measuring temperature in atmosphere. However, the Yanzhou coal-anthracene oil slurry has a viscosity peak
at about 583 K under high pressure during heating, while the Yanzhou coal-Shenhua recycled oil slurry does not have a
viscosity peak, but coke deposits appear at 623 K and above, under high pressure. In addition, the viscosity–temperature
characteristics of thermally treated coal-oil slurry at different temperatures are the same; both have a viscosity peak
with increasing heating temperature at the same measuring temperature.  2009 Curtin University of Technology and
John Wiley & Sons, Ltd.
KEYWORDS: coal-oil slurry; viscosity; high temperature and high pressure; heating; coal direct liquefaction
INTRODUCTION
Due to the present energy crisis, the way of extracting oil from coal through direct hydroliquefaction
will play an important role in maintaining the energy
safety of the world. The Chinese Government promoted advanced coal technologies through policies and
financial support for research, development, demonstration and early deployment during the past 10 years.
Since 2004, the Ministry of Science and Technology
has included coal direct liquefaction technology in
its National Basic Research Program of China (973
Program).
When coal-oil slurry is preheated up to 673 K from
ambient temperature, several physical and chemical
changes will occur. From a rheological point of view,
they lead, first, to a viscosity increase, and second, to a
rapid decrease. In a hydroliquefaction plant, these phenomena are mainly encountered in the preheating stage
of the hydroliquefaction system. The strong changes
*Correspondence to: Yingjie Ren, School of Mechanical and Power
Engineering, East China University of Science and Technology,
Shanghai, China. E-mail: jackey.ren@yahoo.com.cn
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
of heat and mass transfers have to be well understood to help in designing optimized hydroliquefaction
plants[1 – 8] . Some work has already been performed
in order to study the rheology of coal-oil slurry, but
not enough[7,9 – 13] , especially for Chinese coal. The
results have shown that apparent viscosity is a function of numerous parameters: solvent, gas, nature of
coal, ratio of coal to solvent mass, shear rate and so
on. Thermo-dissolved products are intermediate products such as asphaltene, which come from coal-oil
slurry during the heating stage. Those thermally dissolved products are one of the main reasons for the
viscosity change[9] . The abrupt changes of viscosity
of coal-oil slurry will lead to systematic resistance
increasing, deterioration of mass and heat transfer, and
even partial coking and solid depositing in running
piping of coal paste. Finally, the manipulation of the
whole liquefaction process will depend on the viscosity
problem.
The aim of the present work is limited to an experimental study on the effect of two kinds of solvents on
the viscosity changes of Yanzhou coal-oil slurry during
the heating stage.
Asia-Pacific Journal of Chemical Engineering
TEMPERATURE AND HEAT CHANGES TO VISCOSITY OF COAL-OIL SLURRY
EXPERIMENTAL
Samples and solvents
Two kinds of solvents, including anthracene oil and
Shenhua recycled oil (which come from Shenhua coal
direct liquefaction pilot plant, the catalytic hydrogenation products) and Yanzhou coal (middle rank
and agglutinate) were used in these experiments. The
Yanzhou coal was pulverized to less than 200 mesh and
dried at 333 ± 2 K for 24 h in vacuum. The proximate
and ultimate analysis of these coal samples are shown
in Table 1.
Apparatus
A self-designed rotary viscometer that can measure the
viscosity of coal slurry online at high pressure and
high temperature was designed and manufactured for
this project. Fig. 1 illustrates a schematic diagram of a
high-temperature–high-pressure viscometer.
Another rotary viscometer (Model NDJ-31, Chengdu
Apparatus Factory, China) that can only measure the
viscosity of coal-oil slurry with increasing temperature
in atmosphere was used.
Figure 1.
Schematic diagram of a hightemperature–high-pressure viscometer. 1-Speed
motor; 2-Torque sensor; 3-Drive magnet; 4Induction magnet; 5-Support bearing of outer
cylinder; 6-Safety valve; 7-Bolt; 8-Gas import
valve; 9-Gas import; 10-Autoclave; 11-Furnace;
12-Outer cylinder; 13-Stirring wings; 14-Inner
cylinder; 15-Thermocouple; 16-Digital display;
17-Autoclave cover; 18-Support bearing of inner
cylinder; 19-Induction magnet of outer cylinder;
20-Synchronous motor; 21-Drive magnet of
outer cylinder; 22-Cooling water jacket; 23Pressure meter; 24-Gas export; 25-Gas export
valve; 26-Controller.
Procedure
The coal-oil slurry was prepared by mixing the
anthracene oil and Shenhua recycled oil with the pulverized coal according to a coal/solvent weight ratio equal
to 80 g/120 g (2/3) and 110 g/90 g (11/9), respectively.
The slurry was put into the high-temperature–highpressure viscometer. The viscometer was charged and
discharged by 2 MPa–3 MPa nitrogen and hydrogen
three times, respectively, before being pressurized to the
desired initial hydrogen pressure of 5 MPa. The slurry
was heated at the rate of 5 K/min in an electric furnace
up to the preset temperature. The coal-oil slurry was
stirred sufficiently by the outer cylinder and the viscosity value measured in the inner cylinder at intervals
of 10 K. The high-temperature–high-pressure viscometer was cooled to ambient temperature by cool water
immediately when it reached the preset temperature.
The thermally treated coal-oil slurry was then removed,
separated into oil (including a little amount of gas and
water), asphaltene, preasphaltene and residual coal by
extraction with n-hexane, toluene and tetrahydrofuran
(THF), respectively. The yield of oil (including less gas
and water), asphaltene and preasphaltene (coal dry ashfree basis) were calculated on the basis of ash balance
principle.
m
m
−
A
A1
Oil yield = m0
−m
A0
A0
1
1
=
−
1 − A0
A0 A1
× 100%
(1)
Table 1. Proximate and ultimate analysis of coal sample.
Sample
Yanzhou
Proximate analysis (%)
Ultimate analysis (daf) (%)
Mad
Aad
Vdaf
C
H
St
N
O
3.89
13.13
37.97
78.90
5.16
4.16
1.55
10.23
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pac. J. Chem. Eng. 2009; 4: 744–750
DOI: 10.1002/apj
745
746
R. YINGJIE, ET AL.
m
m
−
A
A2
Asphaltene yield = m1
−m
A0
A0
1
1
=
−
1 − A0
A1 A2
× 100%
(2)
m
m
−
A2 A3
Preasphaltene yield = m
−m
A0
A0
1
1
=
−
1 − A0
A2 A3
× 100%
(3)
where: the mass of ash is m, A0 is the ash content of raw
coal, A1 is the ash content of coal surplusage extracted
by n-hexane, A2 is the ash content of coal surplusage
extracted by toluene and A3 is the ash content of coal
surplusage extracted by THF.
The viscosity determination of themally treated coaloil slurry was carried out in a rotary viscometer (Model
NDJ-31) to compare the viscosity changes between raw
coal-oil slurry and thermally treated coal-oil slurry at
different thermal treatment temperatures. The viscosity
was measured at the interval temperature (e.g. 10 K).
RESULTS AND DISCUSSION
Variation of viscosity of raw coal-oil slurry in
atmosphere
Figure 2 shows the viscosity changes of the raw
Yanzhou coal mixes with anthracene oil and Shenhua recycled oil, respectively, with increasing measuring temperature under atmosphere. It can be seen
that the viscosities of Yanzhou coal–anthracene oil and
Shenhua recycled oil slurry decrease first, with the
decrease in solvent viscosity, then increases with measuring temperature for the absorption and volatilization
of the solvent in atmosphere. In addition, the viscosity
of Yanzhou coal–anthracene oil slurry is higher than
that of Yanzhou coal–Shenhua recycled oil at 303 K,
although the solvent content is more. Because the viscosity of anthracene oil is greater than that of Shenhua
recycled oil, anthracene oil cannot supply more free
moving space for coal particles. Above 313 K, the viscosity of Yanzhou coal–anthracene oil slurry is less than
that of Yanzhou coal–Shenhua recycled oil because the
viscosity of anthracene oil decreases and there is more
solvent content in the slurry.
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pacific Journal of Chemical Engineering
Figure 2. Viscosity changes of two kinds of coal-oil slurry
with increasing measuring temperature in atmosphere.
Variation of viscosity of coal-oil slurry under
high temperature–high pressure during
heating
The solvent, besides coal’s properties, has a great
effect on the viscosity changes of coal-oil slurry during
heating. Figures 3 and 5 show the viscosity changes of
coal-oil slurry with thermal treatment temperature under
high hydrogen pressure.
Variation of viscosity of Yanzhou
coal–anthracene oil slurry
The viscosity of Yanzhou coal–anthracene oil slurry
decreases from an initial viscosity of 2700 mPa at
300 K in Fig.3. The main effect of the solvent is supplying free moving space to coal particles and lubricating
them. When the coal-oil slurry is heated, the viscosity of anthracene oil is reduced for the acceleration of
Figure 3. Viscosity changes of Yanzhou coal–anthracene
oil slurry with increasing thermal treatment temperatures.
Asia-Pac. J. Chem. Eng. 2009; 4: 744–750
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
TEMPERATURE AND HEAT CHANGES TO VISCOSITY OF COAL-OIL SLURRY
molecular thermal motion. On the one hand, the solvent
viscosity decreases, which reduces the viscosity of the
coal-oil slurry. On the other hand, the rate pickup of solvent penetrating coal pores that decreases the free phase
in coal-oil slurry and leads to an increase in the viscosity of coal-oil slurry. The two factors mentioned before
compete each other, the viscosity will change towards
the preponderant factor. Obviously, during the initial
heated stage of Yanzhou coal–anthracene oil slurry, the
solvent viscosity decrease is the main factor, so the
slurry viscosity becomes debased.
The viscosity, however, begins to increase from
150 mPa around 433 K and reaches a peak viscosity
value about 1550 mPa at 583 K before falling again till
240 mPa as the temperature was raised further. It is
attributable to an increase in the solid volume fraction
of the slurry because of the solvent absorption and
the swelling of the coal particles. At this moment, the
soluble organics in coal will begin to dissolve and to
cover the surface of the particles and swell, and if
the thermal treatment temperature keeps on increasing,
that will augment the volume of the coal particles,
resulting in some viscous organics being transferred
from insides of the coal particles to cover the surface of
coal particles. With the thermal treatment temperature
increasing to 583 K the Yanzhou coal is pyrolysed and
forms the unstable free radical to be hydrogenated that
will generate some intermediate products. Also, some
high-viscosity products which cover the particle surface
have been melted and dissolved. The moving space of
coal particles increases again for the more fluid phase.
Therefore, the viscosity of Yanzhou coal–anthracene oil
falls again.
Figure 4 shows the intermediate products yield of
Yanzhou coal–anthracene oil slurry with the thermal
treatment temperature. The oil yield rises slowly till
it reaches 573 K, however, it quickly increases after
it reaches 573 K. The result can explain the viscosity
decrease at 583 K during heating. The asphaltene and
preasphaltene yields have similar tendencies that always
increase till attaining 673 K, then begin to fall. Thus,
the viscosity of Yanzhou coal–anthracene oil decreases
sharply again for the content of high-viscosity substances decreasing and oil increasing.
Variation of viscosity of Yanzhou
coal–Shenhua recycled oil slurry
The viscosity changes of Yanzhou coal–Shenhua recycled oil slurry is different. The viscosity decreases
from 1620 mPa at the initial temperature of 303 K to
510 mPa at 723 K. Thus, there are no viscosity peaks
during the whole thermal treatment.
From Figs3 and 5, it can be seen that the viscosity of Yanzhou coal–Shenhua recycled oil slurry is
less than that of Yanzhou coal–anthracene oil slurry
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
Figure 4. The intermediate products yield changes of
Yanzhou coal–anthracene oil with the thermal treatment
temperatures.
Figure 5. Viscosity changes of Yanzhou coal–Shenhua
recycled oil slurry with increasing thermal treatment
temperatures.
at an initial temperature of 303 K because the viscosity of anthracene oil is higher than that of Shenhua recycled oil. This means that Shenhua recycled oil
can supply more free moving space for coal particles,
but the viscosity of Yanzhou coal–Shenhua recycled
oil slurry decreases more slowly than that of Yanzhou
coal–anthracene oil slurry during heating. Because the
molecular size of Shenhua recycled oil is smaller, it
is absorbed more easily by the coal particles that will
reduce the moving space among these particles and
enhance the viscosity of the slurry. In addition, Shenhua recycled oil is a kind of light oil and is induced
by Shenhua coal, therefore, it has a good solubility
for Shenhua coal. It cannot dissolve the asphaltene and
preasphaltene produced by Yanzhou coal (the heavier
part of intermediate products) well enough to lead to
Asia-Pac. J. Chem. Eng. 2009; 4: 744–750
DOI: 10.1002/apj
747
748
R. YINGJIE, ET AL.
the agglomeration of asphaltene and preasphaltene. The
asphaltene and preasphaltene are unstable and can easily be converted into coke when the heating wall is
overheated and the hydrogen is less. It was found that
the tendency of coke to get deposited appears when
the thermal treatment temperature is above 623 K. And
there are more coke products at 723 K than at 623 K
and 673 K.
Figure 6 shows that the intermediate products yield
changes of Yanzhou coal–Shenhua recycled oil with
the thermal treatment temperature. The content of
oil increases all the time, as also the content of
asphaltene and preasphaltene which increase first but
fall at 623 K for the coke deposit. In addition, the
content of intermediate products is less than that of
Yanzhou coal–anthracene oil slurry. And the asphaltene
and preasphaltene adhere to the heating wall instead of
being dissolved in the solvent. Meanwhile, the oil keeps
getting produced and the free moving space is more.
Consequently, there is no viscosity peak appearing
during the whole thermal treatment.
Asia-Pacific Journal of Chemical Engineering
Figure 7.
Viscosity changes of preheated Yanzhou
coal–anthracene oil slurry through different thermal
treatment temperatures with measuring temperature.
Comparison of viscosity changes between raw
and preheated coal-oil slurry
Figures 7 and 9 illustrate the viscosity changes of two
kinds of preheated coal-oil slurry through different thermal treatment temperatures with measuring temperature. Figures 8 and 10 illustrate the viscosity changes
of two kinds of preheated coal-oil slurry under different measuring temperatures with thermal treatment
temperatures.
The different solvents have the different viscosity
changes with the measuring temperature through different thermal treatment temperatures.
Figure 6. The intermediate products yield changes of
Yanzhou coal–Shenhua recycled oil with the thermal
treatment temperatures.
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
Figure 8.
Viscosity changes of preheated Yanzhou
coal–anthracene oil slurry under different measuring
temperatures with thermal treatment temperatures.
The viscosity of Yanzhou coal–anthracene oil slurry
first decreases and then increases with increasing measuring temperature when the thermal treatment temperature is 373 K, 423 K and 473 K. And the viscosity
under the same measuring temperature increases with
the thermal treatment temperature. At measuring temperature 343 K, the viscosity is 170 mPa, 460 mPa and
3350 mPa, respectively, after being thermally treated
through 373 K, 423 K and 473 K. All of them are
greater than the viscosity 120 mPa of raw coal-oil
slurry. The viscosity through preheated slurry increases,
and cannot be measured by the viscometer when the
thermal treatment temperature is 523 K, 573 K, 623 K
and 673 K, because of the absorption of the solvent
and the swelling of coal particles. However, its viscosity falls again as more intermediate products are
Asia-Pac. J. Chem. Eng. 2009; 4: 744–750
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
TEMPERATURE AND HEAT CHANGES TO VISCOSITY OF COAL-OIL SLURRY
viscosity under the same measuring temperature has
a peak appearance with the thermal treatment temperature (Fig.10). The viscosity is 790 mPa, 1090 mPa,
2250 mPa, 2440 mPa and 1930 mPa, respectively, at
the measuring temperature of 333 K after thermal treatment through 373 K, 473 K, 573 K, 623 K and 723 K.
The viscosity increases all the time between 303 K and
573 K (thermal treatment temperature) because there
is no appearance of coke deposit and the content of
asphaltene and preasphaltene increases (Fig.6). However, the viscosity falls above 623 K as coke deposit is
formed and the contents of asphaltene and preasphaltene
decrease while the content of oil increases.
Consequently, the viscosity changes of coal-oil slurry
are affected by the characteristics of the solvent.
Figure 9.
Viscosity changes of preheated Yanzhou
coal–Shenhua recycled oil slurry through different thermal
treatment temperatures with measuring temperatures.
CONCLUSIONS
(1) Both the slurries mixed by Yanzhou coal with
anthracene oil and Shenhua recycled oil display
that the viscosity decreases first, then increases with
increasing measuring temperature in atmosphere.
(2) The Yanzhou coal–anthracene oil slurry has a
viscosity peak at 583 K under high pressure during
heating, the Yanzhou coal–Shenhua recycled oil
slurry has no viscosity peak, but coke deposits
appear above 673 K under high pressure in the
whole studied temperature range.
(3) The viscosity–temperature characteristics of preheated coal-oil slurry with both solvents at different
thermal treatment temperatures are same; both of
them have a peak appearance with increasing heating temperature at the same measuring temperature
except for the viscosity values.
Figure 10.
Viscosity changes of preheated Yanzhou
coal–Shenhua recycled oil slurry under different measuring
temperatures with thermal treatment temperatures.
Acknowledgement
produced and the size of the coal particles decrease after
673 K thermal treatment. During the whole process of
measuring viscosity in atmosphere, its viscosity hardly
increases through 723 K thermal treatment because the
solvent is not absorbed and the coal particles do not
swell any more, the viscosity of intermediate products
decreases all the time.
The viscosity changes of Yanzhou coal–Shenhua
recycled oil slurry have similar tendencies to that of
anthracene oil. All the viscosity of preheated coaloil slurry decreases first and increases again for the
volatilization of solvents with increasing measuring
temperatures. And all of them are greater than that of
raw coal-oil slurry as the free moving space among
the coal particles decreases with increasing thermal
treatment temperature for the solvent absorption. The
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
This study was supported by the National Development Programming of Key Fundamental Researches
of China (2004CB217601), Chinese National Programs
for High Technology Research and Development (863
Plan) (2006AA05Z314) and Shanghai Leading Academic Discipline Project (B506).
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DOI: 10.1002/apj
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