International Conference on Global Trends in Engineering, Technology and Management (ICGTETM-2016)
Reducing Crude Oil Viscosity Using Diluents
Kulkarni A.D#1 and Wani K.S*2
#
Maharashtra Institute of Technology, Kothrud, Pune-411038, India
SSBT College of Engineering and Technology, Bambhori, Jalgaon-425001, India
*
Abstract- Viscosity reduction of crude oil for its
efficient transportation through cross country
pipelines is of prime importance in the petroleum
industry. One of the methods is to blend the crude oil
with light hydrocarbons and organic solvents. The
method is easy to implement and is widely used for
transporting heavy crude oils and bitumen. A number
of diluents like condensates, light oils, naphtha,
middle distillates and organic solvents are used. The
resultant viscosity depends on number of factors like
the composition of crude oil, properties of diluent,
oil/diluent ratio and the interaction between crude oil
and the diluent. This requires a good mathematical
model for accurate viscosity prediction. A number of
models available in the literature have been discussed
in this paper. Also an empirical model based on
experimentation for viscosity reduction of crude oil
diluted with n- hexane at different temperatures has
been developed. The model gives fair accuracy in a
limited operating range.
Key words: Flow Assurance, Hydrocarbons,
Viscosity Reduction, Crude Oil Dilution, Crude Oil
Transportation
I. Introduction
Transportation of crude oil through pipelines is a
major flow assurance challenge. This is attributed to
crude oil composition, density, viscosity and ambient
temperature conditions. Increased viscosities due to
the above factors lead to large pressure drops,
increased pumping costs, blocked pipelines and
production loss. This is true for heavy as well as
medium density crude oils. Heavy oil (API < 20) and
extra heavy oil (API < 10) have high proportion of
asphaltenes and paraffins compared to low molecular
weight hydrocarbons. During transportation these
asphaltenes and paraffins tend to get unstable and
precipitate causing multiphase flow and clogging of
pipelines. Estimated losses are in the range of millions
of dollars annually [1].Commonly used methods for
remediation or prevention include mechanical
methods, thermal treatment and chemical methods.
Along with these, dilution of crude oil is one of the
most widely used methods for overcoming the above
problem. This paper reviews the method of crude oil
dilution in detail.
II. Crude Oil Dilution
Dilution of crude oil is the oldest method (since
1930s) and can be accomplished by using condensates
ISSN: 2231-5381
obtained in natural gas production; light oils; light
hydrocarbons; organic solvents like methyl tert-butyl
ether (MTBE), tert-amyl methyl ether (TAME), etc;
alcohols like pentanol, hexanol, etc; gasoline and
middle distillates like kerosene. A dilution ratio of
0 –20 % for heavy crude oil and 25 to 50% for
bitumen is enough to carry out the method
successfully [2].
A. Advantages and Disadvantages
Dilution has number of advantages. It reduces the
viscosity of crude oil thereby facilitating its
transportation through pipelines. It avoids high
pressure drops and reduces the pumping cost.
Furthermore dilution helps the desalting and
dehydration operations downstream.
The method also suffers certain disadvantages.
Transportation of solvent requires an additional
pipeline along with the crude oil pipeline. This
demands considerable capital and operational
investment in pumping and pipeline maintenance.
Separation of solvent and returning it to oil production
site requires the creation of a separate facility. Choice
of the solvent is also affected by the oil composition.
This is due to the compatibility issues between the
asphaltenes and paraffins present in the oil with the
solvent. If due care is not taken, deposition of
asphaltenes and paraffins can cause further problems.
Availability of the diluents in the desired quantity is
also an important issue. Moreover, prediction of
solvent/oil ratio for achieving a required reduction in
viscosity is difficult as the number of governing
parameters is large and inconsistent such that simple
mixing rules are ineffective.
B. Diluents
(i) Natural gas condensates-Being the lightest
hydrocarbon component, dissolution of heavier
components and subsequent reduction in the viscosity
is facilitated by natural gas condensates. This was a
widely used method since 1930s. But the popularity
dropped after 80‟s due to the following reasons.
Firstly the availability of condensates depends on the
demand of natural gas. Secondly, with the increased
heavy oil recovery, the existing production of
condensates is insufficient. Thirdly, the distance
between the condensate production site and the oil
production site is a major issue. Lastly, asphaltenes
are not properly soluble in condensates. This may
results in flocculation which leads to partial plugging
of pipelines.
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International Conference on Global Trends in Engineering, Technology and Management (ICGTETM-2016)
(ii) Light oils-Crude oils having API gravity between
35 and 42 can be used for dilution. But these suffer
similar disadvantages as condensates like availability
and compatibility with asphaltenes.
(iii) Naphtha- Hydrocarbons ranging from C6 to C12
form the naphtha fraction of crude oil distillates. It has
low density which leads to efficient dilution of crude
oil. It has good compatibility with asphaltenes owing
to the presence of aromatic content in it. It is easily
recyclable and reusable.
(iv) Organic solvents/ distillates- Gasoline and
kerosene have been used as distillates owing to their
good solvent properties. Gasoline also helps to
improve the octane number in downstream processing.
Yaghi and Benami [3] have shown that 15% kerosene
mixed with heavy oil at 50°C achieves the same
viscosity reduction achieved by 20% kerosene at room
temperature. Use of Methyl tert-butyl ether (MTBE),
tert-amyl methyl ether (TAME) and dimethyl ether
(DME) have been considered [4]. Recovery of DME
is easier as compared to other solvents. Argillier et al
[5] have shown that alcohols are more effective in
reducing the viscosity. Hasan et al [6] have found that
10% ethyl alcohol reduced viscosity of crude oil by
almost 80% at 250C.This can be due to interaction
between the hydroxyl groups and asphaltenes.
(v) Carbon dioxide- In some recent developments,
R.Hu et al [7] have studied the effect of carbon
dioxide on the heavy crude oil. They have found that
crude oil saturated with carbon dioxide undergoes
significant reduction in viscosity at a given
temperature and pressure.
C. Prediction of Resultant Viscosity of the Crude oilDiluents Mixture
When oil and diluents are mixed together, the
resulting viscosity depends on the dilution rate,
viscosities and densities of oil and diluents. It is
observed in general that lower the viscosity of the
diluents, lower is the viscosity of the blended mixture.
A number of correlations have been developed till
date for prediction of resultant viscosity. But the
accuracy of these relations is limited owing to the
number of parameters involved in them. Gateau et al
[8] have discussed few such relationships.
The classical Arrhenius equation has been modified
by Lederer to represent the viscosity of a mixture.
 V



o  log   1  Vo  log 
log   
s
o
 V  V 
 V  V 
s
o
s
 o

Where Vo, o, Vs and s are the volume fractions and
viscosities of oil and solvent respectively and  is an
empirical constant varying between 0 and 1.
ISSN: 2231-5381
A generalized expression for  has been developed
by Shu [9] to predict the viscosity crude oil diluted
with light hydrocarbons.

17.04(  o   s ) 0.5237  o 3.2745  s 1.6316
 
ln  o 
 s 
Where, ρo and ρs are densities of oil and solvent
respectively.
Viscosity reduction is affected by the concentration of
asphaltenes in maltenes. There exists a critical
concentration above which there is entanglement of
the colloidal particles which results in increasing the
resultant viscosity of crude oil. Reduction in viscosity
can be achieved by limiting the entanglement. This
can be achieved by increasing the interaction between
solvent and the polar part of the crude oil viz.
asphaltenes which reduces the interactions amongst
the asphaltenes themselves.
These molecular interactions can be measured with
the help of the solubility parameter t as given by the
Hildebrand and Scott theory. The solubility parameter
is a representation of the combined effect of the
dispersion forces (comprising of London forces and
van der Waals forces), polar interactions and hydrogen
bonding. Hansen parameter  takes into account the
above factors.

n
 V
i i
i 1
Where,  is the Hansen parameter of the mixture
i is the Hansen parameter of the pure solvent
„i‟
Vi is the volume fraction of the solvent „i' in
the
mixture
Using the above theory, Gateau et al [8] have proved
experimentally that dispersion forces have negligible
effect on asphaltene aggregation whereas polar
interactions and hydrogen bonding have appreciable
effect. With mixtures of solvents like 2-butanone,
butyronitrile, butryraldehyde and ethyl acetate with
naphtha and nonane, they have proved that the solvent
with more polarity gives better viscosity reduction.
This was concluded from the observation that the
polar parameter p has less value in naphtha based
solvent mixtures as compared to nonane based solvent
mixtures. This means that the polar sites of
asphaltenes in naphtha are more accessible to the polar
solvents leading to higher efficiency and better
viscosity reduction. Also using small angle X-ray
scattering (SAXS) they have shown that polar solvents
like hexyl alcohol can reduce the gyration radius of
the asphaltene particles and the relative viscosity of
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International Conference on Global Trends in Engineering, Technology and Management (ICGTETM-2016)
Motahhari et al [10] have used Expanded Fluid (EF)
viscosity model for predicting the viscosities of 3 to
30 % by weight condensate diluted heavy oils and
bitumen at temperatures up to 1750C and pressures up
to 10 MPa.
% change for S1
% change for S2
100
% Change in Viscosity
the crude oil mixture. This is attributed to the ability
of the polar solvents to establish hydrogen bonding
with asphaltenes and thus reducing the interactions
amongst the asphaltenes themselves. They also found
that there was no specific relationship between the
hydrogen bonding parameter and viscosity reduction
efficiency. Thus, the candidate solvents for a
particular application can be selected using the
Hansen‟s theory.
90
80
70
60
50
40
30
35
45
55
65
75
Temperature (deg C)
III. Experimental
Using a diluent of single type in two different
proportions, it is proposed to predict the reduction in
viscosity of a crude oil at different temperatures.
A. Materials and Methods
Mixtures of crude oil (35 0API) and commercially
available n- hexane (MERCK) as a diluent were taken
in proportions of 10 % (Sample S1) and 20 % Sample
S2) by weight for experimentation. Viscosities for the
mixtures were measured at different temperatures
ranging from 400C to 650C with the help of a U-tube
viscometer kept in a temperature controlled bath.
Viscosities for pure crude oil (Sample S0) were also
measured and the percentage changes in viscosity for
the mixtures were calculated.
Fig.2. Changes in Viscosities of the Mixtures
A non-linear regression model is proposed to relate
the reduction in viscosity (V) with respect to
temperature (T) and concentration (C) of the diluent.
V =  TC
The parameters ,  and  are found using non-linear
regression routine in MS-EXCEL. The values found
were as follows:
 =5024.1
= -0.9
= 0.314
Hence the model becomes
V = 5024.1(T-0.9C0.314)
B. Results and Discussion
As seen from Fig. 1, the reduction in viscosity is a
function of temperature and the concentration of the
diluents.
The experimental results and the model predicted
results are given in Table 1 to evaluate the
performance of the developed model.
TABLE 1
REDUCTION IN VISCOSITY: ACTUAL VS. MODEL
PREDICTED
V = f (T, C)
S0
S1
S2
T
(0C)
C
(Weight
Fraction)
Viscosity
Reduction:
Actual (%)
Viscosity
Reduction:
Model
Predicted (%)
%
Deviation
65
60
55
50
45
65
60
55
50
45
0.1
0.1
0.1
0.1
0.1
0.2
0.2
0.2
0.2
0.2
52.586
56.445
71.482
76.294
76.622
71.346
74.910
86.395
88.873
89.848
56.941
61.195
66.179
72.107
79.279
70.787
76.074
82.271
89.640
98.556
-8.282
-8.415
7.417
5.488
-3.468
0.784
-1.555
4.774
-0.863
-9.692
Viscosity (cP)
80
60
40
20
0
35
45
55
65
75
Temperature (deg C)
Fig.1. Viscosity vs. Temperature
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International Conference on Global Trends in Engineering, Technology and Management (ICGTETM-2016)
Reduction in Viscosity (%)
110
100
90
80
Actual
70
Model
60
50
35
45
55
65
75
Temperature (deg C)
can be easily transported at lower temperatures. This
will reduce the requirement of heating and thermal
insulation for the transportation pipelines necessary to
keep the oil below WAT. With 20% diluent
concentration, a reduction of around 90% is observed,
indicating a lesser requirement of diluent for medium
density crude oil. The observation is supported by the
results given by Hasan et al [6]. This will result in an
overall reduction in the operating cost.
The model parameters  and  indicate that viscosity
reduction is inversely proportional to temperature and
directly proportional to concentration. The lesser
value of  underlines the importance of diluent
concentration in the overall viscosity reduction. The
maximum deviation for the actual and the model
predicted values for viscosity reduction is around 10%
which gives a fair accuracy of the predicted results.
Fig. 3. Reduction in Viscosity vs Temperature
Reduction in Viscosity (%)
[IV] Conclusion
The method of dilution of crude oil for its efficient
transportation has been discussed along with its
110
advantages and disadvantages. Amongst the various
diluents used commercially, mixtures of polar solvents
100
with naphtha gives better efficiency in viscosity
reduction of the crude oil. Resultant viscosity of the
90
crude oil-diluent mixture depends on the dilution ratio,
asphaltene concentration, solvent polarity and
80
Actual hydrogen bonding capability. This is demonstrated by
70
number of models as seen from the literature. From
Model
the experimentation performed using a single diluent
60
for given crude oil, it is observed that an optimum
concentration of the diluent is required to achieve the
50
desired viscosity reduction. An empirical model
0
0.1
0.2
0.3
relating viscosity reduction with temperature and
Concentration of diluent (Weight Fraction) diluent concentration has been developed. The model
gives good accuracy for prediction of viscosity
reduction. The model can work in a limited range
owing to the lesser number of data points used for
Fig. 4. Reduction in Viscosity vs Diluent Concentration
generation. It can be tuned to greater accuracy with
rigorous
experimentation
involving
varying
A steep change in the slope of the graph for viscosity concentration ranges with different diluents. Also the
versus temperature gives the value of the wax method of dilution if used independently can prove to
appearance temperature (WAT) for a crude oil. As be expensive due to the large volumes of diluents
seen from Fig. 1, the WAT for sample S0 (crude oil) is required. The quantity of diluents can be reduced if
between 550C-600C, for sample S1 (Crude + 10% n- the method is used in conjunction with other viscosity
hexane) is between 500C-550C and for sample S2 reduction methods like heating or magnetic field
(Crude + 20% n-hexane) it is between 450C- conditioning. These possibilities need to be explored
500C.Thus the WAT reduces with increasing diluent further.
concentration. However as seen from Fig.1, the
variation is slopes for S1 and S2 at the point of
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International Conference on Global Trends in Engineering, Technology and Management (ICGTETM-2016)
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