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IIUM Engineering Journal, Vol. 5, No. 1, 2004 S. Al-Zuhair et al.

THE EFFECT OF THE OPERATING CONDITIONS ON
THE APPARENT VISCOSITY OF CRUDE PALM OIL

DURING OIL CLARIFICATION

SULAIMAN AL-ZUHAIR*, MIRGHANI I. AHMED‡ AND YOUSIF A. ABAKR‡

* School of Engineering, Taylor’s College, Subang Jaya, Malaysia.

‡ Department of Mechanical Engineering, International Islamic University, Malaysia.

e-mail: mirghani@iiu.edu.my

Abstract: This paper discusses the apparent viscosity of crude palm oil, using rotary
viscometer, under different boundary conditions. It was experimentally shown that the
apparent viscosity of palm oil drops with increasing of the shear rate and the
temperature. However, the effect of temperature on the viscosity tends to fade at
temperatures beyond 80 oC. A correlation between the apparent viscosity of crude palm
oil and the operating conditions was developed. This correlation can be used in design of
crude palm oil settlers and in determining the optimum operating conditions.

Key Words: Crude palm oil, apparent viscosity, shear rate, modelling, separation

1. INTRODUCTION

The raw palm oil as pressed from the fruits is a product even cruder than the
commercial crude palm oil. It contains a lot of fibres, dirt, water, soluble impurities and
considerable amount of debris. The average composition (volume to volume) of the raw
palm oil as received from the screw press is 40-75% oil, 10-40% water and 6-25% non-
organic solids [1]. The crude palm oil is left in a clarifier to allow for the separation of oil
from water (together with most of the debris). Water and its content of debris form
immiscible liquids with the oil. The heavier water and debris settle to the bottom leaving
the relatively clean and light oil on the top to be skimmed off later.

Previous studies found that a lot of oil is lost in the settler and most of the waste water
is produced there. It is very important to determine the optimum design parameters of
crude palm oil settler. The separation of clear oil droplets from crude palm oil mixture is
treated as a coalescence process [2-4], with oil droplets rising upward. To determine the

 Corresponding author

IIUM Engineering Journal, Vol. 5, No. 1, 2004 S. Al-Zuhair et al.

optimum design parameters of crude palm oil settler, it is essential to determine the rising
velocity of oil droplets in the aqueous phase. There are two main factors affecting the
rising velocity, namely, density and viscosity [2]. The effect of the operating conditions,
such as temperature and shear rate on the density is relatively small compared to their
effect on viscosity [5-7]. Hence, the density may be treated as a constant and the viscosity
is considered the main operating condition influencing the separation process. Regardless
of the numerous research conducted previously, a reliable model to predict the effect of
the independent operating conditions on the viscosity of crude palm oil is still lacking.This
is due to the difficulty in studying crude palm oil, which behaves as a non-Newtonian fluid
[1], and also due to the composition complexity of the mixture itself (oil, water and
debris). The fact is that, in all previous works to determine the design equations of crude
palm oil settlers, the viscosity of crude palm oil mixture was always assumed, incorrectly,
to be an additional variable, independent of temperature and shear rate [1, 6].

In this study, a correlation between viscosity and the independent operating conditions
affecting it will be formulated (i.e., temperature and shear rate). This correlation will be
employed into the design Eqs. of crude palm oil settlers, and will be used to determine the
optimum operation conditions. The proposed correlation will reflect the combined effects
of both shear rate and temperature.

2. THE MATHEMATICAL MODEL

Previous experimental work proved that crude palm oil is a non-Newtonian fluid. The
apparent viscosity was shown to change with shear rate applied. The power law, Eq. (1),
is widely used in the literature to express the effect of shear rate on the viscosity of non-
Newtonian liquids at constant temperature [8-10],

n-1 = k  (1)

The apparent viscosity of crude palm oil was found to decrease with shear rate [1], this
behaviour is called shear thinning, which is characterised by the value of the power index
n being less than unity.

On the other hand, temperature changes also affect the apparent viscosity of crude palm
oil. Like any other liquid, the apparent viscosity of crude palm oil is expected to reduce as
temperature increases. This phenomenon is explained by the reduction of the inter-
molecular attractions with increasing temperature. In this work the effect of temperature
on the apparent viscosity of non-Newtonian liquids at constant shear rate is expressed
using Arrhenius expression [8],

 e avE RTa   (2)

Equations (1) and (2) describe the separate effects of either shear rate or temperature,
however, it is desired to have an Eq. that combines both effects (shear rate and
temperature) in one model Eq. that can be used in the design Eq.s of crude palm oil
settlers. McCabe et al [11] derived an expression that combines the effects of the shear
rate and the temperature by combining Eqs. 1 and 2,

IIUM Engineering Journal, Vol. 5, No. 1, 2004 S. Al-Zuhair et al.

  1
av

E
n R T

a ok e 
 
 

    (3)

The constants found in Eq (3) can now be determined experimentally.

3. MATERIALS AND METHOD

3.1 Materials

Screened crude palm oil samples were obtained from Dengkil mill, Malaysia, and
diluted with 40% (vol/vol) water to simulate the mixture that enters the crude palm oil
settler in the mill.

3.2 Apparatus

There are many methods to measure the apparent viscosity of non-Newtonian fluids
[11]. The apparatus used in this study is rotary viscometer (Cannon 2020 LV, viscosity
range 0.001 to 2000 Pa sec, with accuracy of 1% ) in which a motor with variable speed
drives a spindle immersing in the sample to be investigated with a spiral spring. The
viscosity of the sample generates a resisting torque at the spindle, which can be measured
with the aid of the torsion of the spiral spring and through a computer software. The shear
stress can be calculated for different shear rates. The computer software is also used to
calculate the apparent viscosity from the measured values of shear stress at various shear
rates. A water jacket, with circulating temperature-controlled water, surrounds the sample
pot to control the temperature. Different shear rates were obtained by changing the size
and the speed of the spindle. Figure 1 shows the apparatus used.

3.3 Methodology

A sample of 25 ml volume was contained in the sample pot each time. The temperature
of the sample was controlled by adjusting the temperature of the circulating water in the
jacket. Measurements were carried out for the changes of the apparent viscosity with shear
rate at five different temperatures 40 oC, 50 oC, 60 oC, 70 oC and 80oC.

4. RESULTS AND DISCUSSIONS

The measured apparent viscosity at different temperatures and different shear rates are
shown in Fig. 2. It can be clearly seen that the crude palm oil mixture is a shear thinning
fluid, as the viscosity decreases with the increase in shear rate. It can be also seen from
Fig. 2. that the apparent viscosity decreases with increasing the temperature, which is the
case for all liquids, since the intermolecular attraction decreases with increasing
temperature.

IIUM Engineering Journal, Vol. 5, No. 1, 2004 S. Al-Zuhair et al.

Fig. 1: The rotary viscometer (Cannon 2020 LV).

The natural logarithmic of a was drawn against the natural logarithmic of  at five
different temperatures and the results are shown in Fig. 3. It can be clearly seen that the
straight lines generated are almost parallel with identical slopes, which validates the
proposed model. The values of n-1 at each temperature were determined from the slopes
of the straight lines in Fig. 3. The average value of n was then calculated as 0.4 with
standard deviation of  0.036, which indicates a deviation of less that 10%.

To determine the activation energy,  1ln na   was plotted against (1/T), and
(Eav/R), and ln(k) were determined from the slope and the intercept, respectively of the
straight line generated (Fig. 4) as predicted from Eq (3). From the slope of the straight line
in Fig. 4., Eav/R = 1511.6

1k  , and from the intercept, k = 3.5x10-3. The standard
deviations of the results were low and in the range of 0.020 to 0.058, and presented in the
error bars found in Fig 4.

IIUM Engineering Journal, Vol. 5, No. 1, 2004 S. Al-Zuhair et al.

0.1

0.3

0.5

0.7

0.9

0 0.4 0.8 1.2 1.6 2

Shear rate,  (sec-1)

A
pa

re
nt

v
is

co
si

ty
, 

a (
P

a
se

40oC

50oC

60oC
70oC

80oC

Fig. 2: The effect of temperature and shear rate on the apparent viscosity of crude palm
oil.

40oC [y = -0.55x + 0.19]

-1.8

-1.4

-1

-0.6

-0.2

-1.2 -0.7 -0.2 0.3

ln ()

ln
(


50oC [y = -0.56x - 0.99]

60oC [y = -0.62x - 1.19]
70oC [y = -0.62x - 1.25]

80oC [y = -0.62x - 1.38]

Fig. 3: The variation of ln(a)with ln() at different temperatures.

IIUM Engineering Journal, Vol. 5, No. 1, 2004 S. Al-Zuhair et al.

y = 1511.6x - 5.6565

-1.5

-1.3

-1.1

-0.9

-0.7
0.0028 0.0029 0.003 0.0031 0.0032 0.0033

1/T (K-1)

ln
(

a/
n-

1
)

Fig. 4: Ln (an-1) versus 1/T

The correlation between the apparent viscosity of crude palm oil and the operating
conditions, including the shear rate and temperature is therefore,

3 0 61511 63 5 10 exp 0 02.
.

μ . γ .
T

     
 

(4)

Fig. 5 shows the comparison between the experimental results and the proposed model
curves that were based on Eq (4), for three different temperatures (40, 60 and 80 oC). It
can be noted that the model predicts fairly well the apparent viscosity at various shear
rates for all three temperatures.

The apparent viscosity correlation, Eq. (4), can be introduced directly into any design
Eq. of crude palm oil settler, instead of keeping the viscosity as an additional variable and
can be used to determine the optimum operating temperature. The rate of separation of
crude palm oil increases as the viscosity decreases, hence, the temperature and the shear
rate should be increased. The increase in temperature is always favourable for increasing
the rate of oil separation, and crude palm oil settlers should be designed to operate under
high temperatures, however, energy consumption and equipment materials costs should be
considered. Furthermore, it can be seen from Fig. 6 that the effect of temperature on the
viscosity tends to fade at temperatures beyond 80 oC, so increasing the temperature
beyond this point, in addition to the energy consumption and material cost stresses, will
not result in substantial decrease in the viscosity to justify this increase. On the other hand,

IIUM Engineering Journal, Vol. 5, No. 1, 2004 S. Al-Zuhair et al.

increasing the shear rate, although results in reducing the viscosity, enhances the
dispersion of oil, but it is not advisable to increase it by agitation or by any other means.

0.1

0.3

0.5

0.7

0.9

0.3138 0.6276 0.9414 1.8828

Shear rate,  (sec-1)

A
pa

re
nt

v
is

co
si

ty
, m

a
(P

a
se

c)
40˚C
50˚C
60˚C
70˚C
80˚C

Fig. 5: Comparison between experimental results and the proposed model curves
showing the effect of agitation speed on the apparent viscosity at three different

temperatures (40, 60 and 80 oC)

Figure 6 shows the combined effect of the shear rate and the temperature on the
apparent viscousity of crude palm oil. This figure is a graphical presentation of Eq. (4),
where the effects of two parameters are both presented on the same graph.

IIUM Engineering Journal, Vol. 5, No. 1, 2004 S. Al-Zuhair et al.

Fig. 6: The combined effects of temperature and shear rate on the apparent viscosity.

5. CONCLUSION

Analytical and experimental studies of the behaviour of the crude palm oil in the palm
oil settlers were conducted. A correlation between the apparent viscosity of crude palm oil
and the operating conditions, has been determined. The results showed a good agreement
between the experimental results and the estimated values of the obtained correlation. The
new correlation reflects accurately the combined effects of both shear rate and temperature
on the apparent viscosity of crude palm oil. This correlation can be employed into the
design of crude palm oil settlers, and can be used also to determine the optimum operation
conditions for palm oil settlers.

REFERENCES

[1] K.H Lim. and D.A.M Whiting, The Influence of Non-Newtonian Behaviour of Crude Palm
Oil on the Design of the Clarification Station Equipment. The Proceedings of The Malaysian
International Symposium on Palm Oil Proceedings and Marketing, International
Developments in Palm Oil. Malaysia, 1977.

[2] E. Barnea, and J. Mizrahi, Separation Mechanism of Liquid/liquid Dispersion in Deep layer
Gravity Settler, Part I, PartII, Part III, and Part IV. Trans. Inst. Chem. Eng., Vol. 53, pp. 61-
92, 1975.

IIUM Engineering Journal, Vol. 5, No. 1, 2004 S. Al-Zuhair et al.

[3] S. Hartland, and S.A.K. Jeelani, Choice of Model for Predicting the Dispersion Height in
Liquid-liquid Gravity Settlers from Batch Settling Data. Chem. Eng. Science, Vol. 42, No. 8,
pp. 1927-1938, 1978.

[4] A. Fasano, The Dynamics of Two-Phase Liquid Dispersions: Necessity of a New Approach,
Milan Journal of Mathematics, Vol. 70, No. 1, pp. 245-264, 2002.

[5] B. Marcia, S. Günter, J.S. Milan, O.A. Elseoud, Vegetable Oils-Based Microemulsions:
Formation, Properties, and Application for "ex-situ" Soil Decontamination, Colloid Polym
Sci, Vol. 280, pp. 973-983, 2002.

[6] K.H. Lim, Theory and Application of Clarification in Palm Oil Mills. Harrisons and Crosfield
(M) Sdn. Bhd. Malaysia 1982.

[7] PORIM. Data Sheet for Engineers. E. Engineering News 1987.

[8] N. Aksel, A Model for the Bulk Viscosity of a Non-Newtonian Fluid, Continuum Mechanics
and Thermodynamics, Vol. 7, No 3, pp. 333-339, 2002.

[9] B.R. Munson, D.F. Young and T.H. Okishi, Fundamentals of Fluid Mechanics, third ed.,
John Wiley & Sons Inc., New York. 1998.

[10] G.B. Wallis, One-Dimensional Two-Phase Flow, McGraw-Hill, New York. 1969.

[11] W.L. McCabe, J.C. Smith and P. Harrott, Unit Operations of Chemical Engineering. 5th
edition. McGraw Hill Inc., New York. 1993.

[12] N.A. Park, T.F. Irvine, An alternative method of simultaneously measuring viscosity and
density of Newtonian and power-law fluids using the falling needle viscometer. Proc. Of the
XIIIth international congress of rheology, Cambridge, UK, Vol. 3, pp. 140-142, 2002.

NOMENCLATURE

aα Constant defined by Eq. (2) (Pa sec)

Eav Activation energy (J mol-1)

k Constant defined by Eq (1)

ko Constant defined by Eq (3)

n Constant defined by Eq (1)

n The average value of the measured n

R Universal gas constant (J mol-1 K-1)

T Temperature (K)

Greek letters

 Shear rate (sec-1)

 Apparent viscosity (Pa sec)

IIUM Engineering Journal, Vol. 5, No. 1, 2004 S. Al-Zuhair et al.

BIOGRAPHIES

Sulaiman Al-Zuhair received his B.Sc. in Chemical Engineering from Jordan University
of Science and Technology in 1996, M.Sc. in Chemical and Environmental Engineering
from University Putra Malaysia in 1998, and Ph.D. in Biochemical Engineering from
University of Malaya in 2003. At present, Dr. Al-Zuhair works as a Chemical Engineering
lecturer at Taylor’s College, Subang Jaya, Malaysia.

Mirghani I. Ahmed was born in Singa, Sudan in 1956. He received his M.Sc. and Ph.D.
in Mechanical Engineering from Budapest Technical University, Hungary in 1987. He
worked with the Sudan University of Science and Technology as Asst. Prof. from 1987-
1989. He worked with IBM – Canada as Research Associate from 1989 - 1996. He is
currently an Associate Professor at the department of Mechanical Engineering,
International Islamic University Malaysia (IIUM). Dr. Ahmed’s primary research interests
are renewable energy, thermal comfort problems, electronic component cooling, modeling
and simulation, computational fluid dynamics applications and heat transfer.

Yousif A. Abakr received his B.Sc. and M.Sc. in Mechanical Engineering from
University of Khartoum. He worked as a Mechanical Engineering lecturer at Sudan
University of Science and Technology, 1995 – 1997. Currently, he is a Ph.D. student at
the International Islamic University, Malaysia.