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One
of the constituents in the crude oil other than wax is asphaltene which can
cause deposition and leads to blockage of pipelines in petroleum industries.
Asphaltenes are known as impure compounds as they consist thousands of
different species with different molecular weights but similar behaviours
chemically(Wei
et_al, 2016). Their nature is actually very complicated and complex
which makes the study of their characteristics becomes harder(Rocha
et_al,2006).

 

 

         Asphaltenes have higher solubility in the
non-polar solvent such as toluene and they are not soluble in the polar solvents
such as glycerine, water, n-heptane and n-pentane compounds(Adebiyi
& Thoss, 2014). Moreover, asphaltenes always exist
in suspended solid form or known as colloidal particles and the structures of
the asphalthenes being interconnected by bridges that are formed from some
aromatic compounds as sulphur or  alkyl(Zhang
et al., 2014). The existence of aliphatic or
aromatic chains caused them to be one of the heaviest elements in the crude
oil.

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Figure 2.14: The
illustration of asphaltenes structures by Speight

Source: Petroleum Science and
Technology 25(1-2):67-80(2007)

 

 

Table
2.1: The composition of asphaltenes that derived from four different countries

 

        

        

        

 

 

 

 

 

 

     Source: The Chemistry and Technology of
Petroleum (Speight, 1999).

 

 

 

 

 

2.9      Asphaltene Treatment

 

Before analysing the asphaltene treatments, the needs to
differentiate between three common terms that will be frequently used which are
precipitation, flocculation and finally deposition is very important.
Precipitation is a process that forms particles in semi-solid through
aggregation process and usually, they are in one-micrometre size particles.
After a big cluster of precipitate is formed, then they are known as flocculate
and this is so called as flocculation process. Finally, deposition is the last
stage process that forms the blockage in pipelines due to heavy or large
adsorbed particles on the surface of the pipelines(Shadman
et_al, 2017).

 

 

         Deposition
of asphaltene occurs due to its stability and the deposition always occurs at
many target places along the pipelines such as inside the pumps, valves, and
flow lines of the pipes(Adebiyi
& Thoss, 2014).Based on Kokal and Sayegh(1995),
they mentioned that the deposition or precipitation of the asphaltene occurred
due to many reasons such as temperature changes, the composition of the
chemicals themselves, chemical properties due to the mixture of oil and other
substances. These depositions caused a severe problem in terms of transport and
process facilities in petroleum industries almost all over the world.

 

 

         Thus,
there are many types of treatments that can be done to overcome the
precipitation of asphaltene in crude oil. The treatments or methods that had been
used by many types of previous researches are such as mechanical removal,
ultrasonic removal, removing the deposition by using any high temperature or
hot fluids such as water steam, and finally adding the inhibitors or other
types of dispersants that can breaks the structure of the asphaltene and
prevent them from growing bigger to form a precipitate (Rocha
et al., 2006). Based on Shadman (2017), he
introduced a treatment that can inhibit the deposition of asphaltene by using
the viscometer by using various types of amphiphiles. The result of the
treatment is that the stability of asphaltene is affected by the concentration
of amphiphiles and when the concentration increases, eventually the stability
increases too. Although there are many treatments or method applied to inhibit
the asphaltene deposition, the most efficient treatment is by using chemical
inhibitors.

 

2.10   Asphaltene Inhibitors

 

As
mentioned before, the best treatment is by using chemical inhibitors to reduce
the blockage in the pipelines due to the asphaltene deposition. Thus, solvent
treatment becomes the best treatment to treat this issue and the most common
solvents that can be used as inhibitors are such as toluene, benzene and xylene
which they are aromatic compounds. Treatment by using these chemical solvent
can saves the cost as well as acts a preventive method for this critical issue.
Usually, the inhibitors used to reduce the effect of this problem should be
used on a large quantity of crude oil as then the inhibitors can dissolve in a
solution that is highly concentrated. Based on Muhammad Ali Karambeigi (2016),
he mentioned that the most effective inhibitor that he had used for the Iranian
crude oil is  IR95 inhibitor with 34%
asphaltene reduction in precipitation compared to others tested inhibitors(Karambeigi
et_al,2016). This is because IR95 has higher polarity and they are
in aromatic form compounds which make the asphaltene to dissolve easily and improved
the flow of crude oil.

 

 

         The
purpose of chemical inhibitors which have different functional groups and
structures is to delay the formation of asphaltene deposition by increasing
adsorption to the surface of the asphaltene. Based on the Cenergy(2001), the
treatment that had frequently used is by direct injection process into the
asphaltene deposition and they are known as a physical-chemical process. The
relationship between Van der Waals and electrostatic force between inhibitor
increases the adsorption process to the surface of the crude oil.

 

 

         Based on
Chang and Fogler (1994), the concentration of amphiphiles as inhibitor will
affect the stabilisation of asphaltenes. When increasing the amphiphiles’
heads, then bonding between the asphaltene and amphiphiles becomes stronger due
to higher polarity. This proves that chemical inhibitor such as alkanes
improvised the flow of the crude oil in the pipelines by modifying the bonding
between the molecules in asphaltene so that they become more stable.
Rogel(2010) mentioned that the reduction in asphatene sizes does not change
when the inhibitor had reached the maximum or optimum concentration value and
this shows that the concentration of inhibitor plays a role in inhibiting the
deposition other than the temperature of the inhibitors with crude oil. The end
result of this inhibition process will be more efficient if using cationic
inhibitors. Cationic inhibitors can speed up neutralisation of asphaltenes’
polarity and caused them to be easily reacted with non-polar substances
eventually fasten up the reduction of the deposition process in the crude oil.
Most of the chemical inhibitors can react well at the temperature around 10°C
and if their concentration range is about from 500ppm to 5000 ppm. Thus, it is
very important in choosing the proper chemical inhibitors with accurate
structural functional groups to treat asphaltene issues in petroleum pipelines
so that this issue can be solved easily with less costing (Ridzuan
et_al, 2016).

 

 

Table
2.2: The degree of inhibition in percentage for various types of inhibitors at
5°C

Type
of inhibitor

Percentage
inhibition efficiency (%)

Blank
crude oil

NA

Acetone

23.2

Toluene

25.4

Cyclohexane

28.9

DEA

17.6

C-DEA

5.6

EVA

36.6

MA

32.4

 

Source:
Evaluation of the inhibitor selection on wax deposition for Malaysian crude oil
(2016)

 

 

 

2.11   Summary

 

In a summary for all types of researches that had been
studied, the most common used treatment is inhibitors method which is very
high-cost saving and the latest technology for this method is that by using the
different formulation of inhibitors where it can help to reduce the
precipitation of both major components in crude oil which is wax and
asphaltenes.

 

 

         Most of
the studies, they had been using the ethylene vinyl acetate (EVA) as the major
inhibitor with some aromatic compounds such as toluene and xylene which speed
up the reaction faster. These combinations of two different compounds such as
EVA and xylene really gives the best results and EVA 30 gives better end result
rather than other EVA. The presences of Methyl Ethyl Ketone (MEK) at 75% fasten
up the inhibition process if MEK is added together with another solvent as EVA
and any other aromatic compounds.

 

 

         Moreover,
from the previous studies, there are many parameters that had been analysed for
the solubility of wax and asphaltenes. The parameters are such as temperature,
carbon dioxide concentration, the concentration of the inhibitors and shear
rate of the concentration. Based on the previous researches, the optimum
temperature or the wax appearance temperature (WAT) is between 11 to 25°C and
to reduce the viscosity, the temperature of inhibitors should be increased. The
shear rate that been used for the most of the studies is around 800ppm to 1000
ppm but it is influenced by the concentration of the asphaltene in the crude
oil. The summary of the articles that been studies had been done in a form of a
table as below:

 

 

 

No.

Method/
Technology

Results/Description

Table
2.3: Summary of Literature Reviews

References
 

1

Thin layer chromatography
flame-ionization(TLC-FID) which is high-liquid performance chromotography

If pour point is below the
environment temperature, there will be higher formation of gel in the form of
matrix. The WAT is 41?C based on the TLC-FID. The viscosity and pour points
differs for each crude oils and wax represents the n paraffin content. The
viscous of the crude oil depends on the n paraffin. The fluid will achieve
equilibrium with proper temperature whenever there are long shutdowns.
 

“Flow Assurance Study for Waxy
Crude Oil”, Marcia et al. (2011)

2

Chemical addition by adding
chemical polymers

The WAT temperature is based on
27.6?C to 11?C and WAT can be reduced together with pour-point as there are
growing of polyacrylate and acrylated monomer at the edge of crystals due to
the addition of phenol aromatic naphtha. The inhibitors will affect the
viscosity of the crude oil by increasing monomers
by preventing wax crystals to be
formed.
 

“Experimental Study on the Effect
of Inhibitors on Wax Deposition”, Muhamad Ali Theyab and Pedro Diaz (2016)

3

Solvent extraction

The line of isotherm reaches the
asymptote at the time of 20 minutes which gives the maximum yield of
wax.  The temperature increases with
wax yield because there is a decrease in viscosity due to the heating process
and caused the rate of distribution of solvent in the mixture. As crystal
growth being prevented by presence of solvent so the more solvent used to oil
ratio can reduce the growth. Indirectly the viscosity of the mixture will
reduce but still there are no changes if the ratio of solvent to oil is is
20:1.Moreover, in this literature done by the researcher, the pure Methyl
Ethyl Ketone (MEK) can cause the formation of the third phase where there is
a new phase to be formed and there are some molecules formed to clogs the
filter and reduce the rate of filtration which directs to blockage of waxy
crude oils in pipelines.

“Wax Separation using MEK Toluene
Mixtures”, Amel A Nimer et al. (2009)

4

Differential Scanning Calorimetry
Technique (DSC) which is thermoanalytical
Technique based on temperature and
heat

The rate of wax formation is
affected by the one of the parameters which is temperature. They have
tendency to crystallise the wax if the temperature is below the solid liquid
equilibrium temperature. They have stronger effect to the Wax Appearance
Temperature (WAT) where they have the shorter hydrocarbon chains. If the
temperature is lower than WAT, then they are able to form waxy crude oils.
This is because nucleation process occurs here to make nuclei to be more
stable. Moreover the increasement in carbon dioxide causes the WAT to be
decreased. In this research they had used the light ends of crude oil where
they have huge capabilities to act as a flow improver for the waxy crude oils
transportation. WAT in crude oils really helps us to ensure awareness on safe
inhibition for the formation of wax and enable smoother flow in the
pipelines.

“The Effect of CO2 on
WAT Crude Oils”, Arya et al. (2016)

5

SPSS technology software for
analytical purposes

The wax deposition is being
affected by the crude oil temperature and they increases until the maximum
range of temperature which is 38°C. Insulation layer thickness also affects
the deposition of wax and it decreases with increasement of temperature. The suitable
temperature during 3°C in winter and 15°C in summer and if there is no
insulation layer then the thickness of wax is large. The deposition of
wax   outside the insulated is thicker
and obvious rather than wax deposition inside the insulated pipeline.

“Prediction of Wax Deposition in An
Insulation Pipeline”, Z. Hu et al. (2015)

6

Acid-catalysed esterification where
reaction of alcohol and acid

In this research, the best flow
improver occurs at higher concentrations where they act as the most effective
pour point depressant. The viscosity and the yield value of virgin crude oil
will be reduce by flow improver from oleic acid. Wax growth inhibited by
oleic acid to improve the flow due to increase in alkyl chain length. This is
because of polar part helps to block wax growth.

“Oleic Acid Based Polymeric Flow
Improves in Laghnaj”, Mayur et al. (2015)

7

Chemical Addition and Artificial
Neural Network (ANN) where analysed based
computational model

Shear rate decreases the wax
formation by affecting the viscosity of the wax crude oil and it had been
reduced to 780 cp from 2250 cp. If it is below the pour point, solid crude
oil will be formed where the viscosity will increase. Viscosity is being affected
by temperature and shear rate where if temperature and shear rate increases
can cause the viscosity to decrease. At 30°C there is no effect from the
improvers to reduce the wax deposition. The factors that affect the flow
improvers are the concentrations and the types of
of flow improvers themselves. If
the temperature is below the pour point depressant, the viscosity reduces
because there is formation of paraffin crystals. Moreover, in this study they
found that higher molecular weight of flow improvers has higher efficiency
for wax inhibition by affecting pour point and the rheological behaviour. The
Artificial Neural Network (ANN) model gives better results than Solid
Solution (SS) model as they do not involve wax critical properties.
Artificial Neural Network (ANN) method gives better results than Solid
Solution (SS) model.

“Investigation on Wax Precipitation
in Crude Oil”, Tarareh et al. (2015)

8

Commercially available wax
inhibitor chemical addition

The precipitation of wax will occur
at 25 °C and the purpose of wax inhibitor is to change the rheological
behaviour of the wax itself. When the experiments done at 23°C, at 125 ppm
the thickness of the wax had been observed to have a very obvious reduction
until 70% for uninsulated layer while to 90% for insulated layer. At 250 ppm,
there is no significant decrease in wax deposition at this temperature. It
just had been decrease about 10% from the deposition of wax 125ppm. At 20°C
the wax deposition is pretty higher than the wax deposition at
23°C. If the temperature is below
than 23°C, 250 ppm of inhibitor does not affect the wax inhibitor as there
are no changes with the wax deposition but only 500 ppm is eligible at this
lower temperature. If the temperature below than 23?C for 250 ppm of an
inhibitor has no effect on wax depositor but for 125 ppm, it reduces to 70%.

“Influence of Wax Inhibitor on
Fluid and Deposit Properties”, Hoffman et al. (2013)

9

Magnetic treatment by using
sintered magnets such as MQII and LDC magnet

Sedimentation and wax formation
lower at temperature 2?C to 3?C compared to other temperature at 0.1 kg/m2
per hour.

“Influence of Processing Conditions
on Sedimentation Kinetics of Highly Waxy Crude Oil”, Y.V. Loskutova et al.
(2015)

10

Polarized Light Microscopy (PLM) by
using
optical microscopy tecnhiques

A higher driving force or light
intensity required to build higher asphaltene molecules.

“Effect of Asphaltenes on Crude Oil
Wax”, Pavel Kriz and Simon I. Andesen (2004)

11

Asphaltene Stabiliser and Solid
Detection System(SDS) where it is a PVT cell with a piston floats and
attached to an impeller works magnetically

The samples characterised by their
stability through SARA test and the results shows that the higher aromatic
compounds has the best inhibition efficiency to asphaltenes.
 

‘Effect on Inhibitors on asphaltene
precipitation for Marrat Kuwaiti Reservoirs’
(
Ghloum et.al, 2010)

12

Physical MCR 301,apparatus attached
with rheometer that rotate together with attached cone and plate

 
The results is if the solvent used
has parameter of solubility less than 16.5 Mpa, then the solvent considered
as the best inhibitor for the asphaltene precipitation in the crude oil.

‘Asphaltene in Heavy Crude Oil’
(
Sergey et al., 2017)

13

SARA test to separate the crude oil
based on its solubility

From this research, they obtained
that the salicylic acids is the best inhibitors and they also come to a
conclusion that the higher polar and aromatic compounds reduce the
precipitation of asphaltene better than other compounds.

‘ Experimental Evaluation of
Asphaltene Inhibitors Selection for the Standard and Reservoirs’
(Karambeigi et al.,2016)

14

IR spectrometer to analyse the
compounds structure in crude oil

The result showed that the
asphaltene formation in crude oil is due to the higher content of porphyrins
and heterogeneous compounds.

‘Organic Elemental Elucidation of
Asphaltene Fraction Nigerial’
(Adebiyi
& Thoss, 2014)

15

Varian 400 NMR spectrum to
determine structure of molecule

The research proved that the
phenolic aromatic compounds seems removed higher amount of asphaltene
compared to other compounds which they only contain 1.22 intensity of
asphaltene in the final product.

‘Characterisation of Asphaltene
Extracted from the Indonesian Oil Sand Using NMR,DEPT and MACDI’
(Zheng
et al., 2015)

16

UV Visible Spectrometry Shidmanzu
Class Vp Version 6.13 SP2 software to analyse the solubility

Dodecyl benzene sulfonic acid
(DBSA) and BisACII   act as asphaltene
inhibitor. The final result is determined by the wavelength of the inhibitors
and the software which shows that BisACII in mixture of xylene or toluene can
reduce the asphaltene inhibition better than DBSA.

An ITC Study of Interaction and
Complexation of Asphaltene Model Compounds in a polar solvent II’
(Wei
et al., 2016)

17

Chemical Inhibitors such as
toluene,n-heptane and n-hexane

The efficiency of asphaltene
increases with chemical inhibitors with addition of some organic acids.
Moreover, it is proven that the aromatic compound toluene has better
inhibitor characteristics rather than aliphatic compounds.

‘Inhibition of Asphlatene Precipitation
in Brazilian Crude Oil using New Oil Soluble Amphiphiles'(Junior et al.,2005)

18

Transmission of Electron Microscope
to view the morphology structure of asphaltene to determine the end deposit.

The result concluded that the
larger polarity of C7 in toluene and n-hexane solution decreases the
solubility of apshaltene and found that the deposition of asphatene is less
than other carbon through microscope.

‘Study on the Polarity, Solubility
and Stacking Characteristics of Asphaltene
(Zhang et al.,2014)
 
 

19

Brookfield DV-II and programmable
viscometer, FTIR and H-NMR spectrocopy

The result is the copolymer which
has average molecular weight, higher index of polydispersity and higher
amount of nitrogen affect the pour point efficiently which eventually
decreases the concentration of inhibitors. The viscosity decreases as
temperature increases as 7.29 mPa at 39°C, 190.95 mPa at 27°C and 358mPa at
15°C.

‘Synthesis of Phthalimide and
Succimide copolymers and their Evaluation as Flow Improvers for an Egyptian
Waxy Crude Oil’
(Al-Sabagh et al.,2013)

20

SPSS software to determine the
equation for wax deposition rate

The wax deposition rate increases
when the insulated layer of the pipelines becomes thicker and temperature
decreases.

‘Prediction of wax Deposition in an
Insulation Crude Oil Pipelines’
Hu et al.(2015)

 

 

 

2.12   Limitations of Literature Review

 

After
many researches that had been studied, the limitations that had been found from
the review of all previous articles that there had no justifications done
on  viscosity of crude oil based on effect
of aromatic and non-aromatic compounds on various temperature and form different
inhibitors which are EVA,MEK and toluene or butanol. They only managed to come
with a result for the aromatic compound formulations but not yet with the non-aromatic
compounds. So eventually,this proves that there had been a small space between
all these previous researches where the researchers had been neglected the
effect of combination of inhibitors with aromatic compounds and non-aromatic
compounds in different temperature ranges. Thus, using the different
combination of inhibitors can help to do better comparison on solubility of the
wax and asphaltene and together with the viscosity of the crude oil.

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