Effects of Imidazolium-Based Ionic Liquids on the Rheological

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Effect of Imidazolium Based Ionic Liquids on the Rheological Behavior of Heavy Crude Oil under High Pressure and High Temperature Conditions Sugirtha Velusamy, Sivabalan Sakthivel, and Jitendra S. Sangwai Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b00521 • Publication Date (Web): 29 Jun 2017 Downloaded from http://pubs.acs.org on July 1, 2017

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Energy & Fuels

Research Article

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Effect of Imidazolium Based Ionic Liquids on the Rheological Behavior of

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Heavy Crude Oil under High Pressure and High Temperature Conditions

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Sugirtha Velusamy1, Sivabalan Sakthivel1,2, Jitendra S. Sangwai1,*

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1

Petroleum Engineering Program, Department of Ocean Engineering, Indian Institute of Technology Madras, Chennai – 600 036, India

8 9

2

Department of Chemistry, Madras Christian College, Chennai – 600 045, India

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Corresponding Author:

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Jitendra S. Sangwai: [email protected]; Phone: +91-44-2257-4825 (Office); Fax: +91-

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44-2257-4802.

1 ACS Paragon Plus Environment

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Abstract

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Production, processing and transportation of heavy crude oil (HCO) is difficult due to its

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high viscosity. For practical applications, the information on the rheological behaviour of HCO

4

plays an important role, especially in flow assurance investigations. In this work, six different

5

imidazolium ionic liquids (ILs) were tested for their effect on the rheological behaviour of HCO

6

under high pressure and high temperature conditions. The rheological studies were carried out at

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three different pressures (0.1, 5 and 10 MPa) and four various experimental temperatures

8

(298.15, 323.15, 348.15 and 373.15 K). The system of HCO+ILs showed a favourable viscosity

9

reduction of 26.5 and 31.5 %, respectively, for the system of HCO+1-butyl-3-

10

methylimidazolium chloride ([BMIM]+[Cl]-) and HCO+1-octyl-3-methylimidazolium chloride

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([OMIM]+[Cl]-) at 298.15 K and 0.1 MPa as compared to the pure HCO system. At 298.15 K

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and 0.1 MPa, the yield stress of the HCO+ILs was reduced by about 15-20 %,

13

increase in temperature to 373.15 K, it decreased in the range of 25-30 % as compared to the neat

14

HCO. The viscoelastic moduli of the HCO sample at 0.1 MPa, 298.15 K and at about 1.5% of

15

the strain are found to be G' (storage modulus) ~ 11 Pa and G" (loss modulus) ~ 7 Pa

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representing that the HCO sample is solid-like, whereas, for the system of HCO+ILs, the G' and

17

G" are reduced to ~ 7 and 3 Pa, respectively. The cross over frequency of the system of

18

HCO+ILs was reduced to the range of 25-30 % as compared to the pure HCO. From the various

19

measurements, it was observed that the addition of ILs to the HCO showed improved the

20

rheological properties over the pure HCO system. Further, results on the microscopic

21

investigation also supported the rheological studies indicating that addition of ILs helped to

22

break the large flocculated structures of HCO into smaller spheres. It has also been observed that


while with

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the IL with the longer alkyl chain length provided greater efficiency in the viscosity reduction

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with favorable viscoelastic behavior.

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Key words: Crude oil; Ionic liquids, Loss modulus, Microscopy, Storage modulus, Viscosity

5

reduction.

6 7

1. Introduction

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Reserves of heavy crude oil (HCO) in the world are twice as compared to those of

9

conventional light crude oil.1 However, heavy crude oil owing to its complex composition and

10

poor flow ability, poses innumerous problems and challenges in its production, transportation

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and refining.2 Resins and asphaltenes, being the major fraction of the unsaturates in the

12

composition of HCO, affect its viscosity. According to the American Petroleum Institute, HCO is

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the oil possessing an °API of ≤ 10 due to which it is extremely viscous and, therefore, cannot be

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mobilized freely. Viscosity of the crude oil plays a crucial role, both during upstream and

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downstream processes. The major difficulty in the production of HCO is that their mobility is

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very low and furthermore, their production from the reservoir and flow assurance issues either

17

during upstream or downstream processes is highly complicated.3-8 All the above technical

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hitches interrogated the economic feasibility of the production and processing of heavy and extra

19

heavy crude oils.9 In order to recover and process the heavy and extra heavy crude oils, the prime

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most task to be performed is to increase the mobility of the oil from the reservoir to the well bore

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and in the oil refineries.

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Rheology is an essential study for the basic understanding of fluid properties,

23

specifically considering its displacement from one place to another. Rheology also plays a major


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role accounting for the flow behaviour of the crude oil under various conditions. Viscoelastic

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materials exhibit both elastic and viscous properties when subjected to deforming stress. For

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practical applications, the measurement of the viscoelastic behaviour of fluids is more important

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especially for the oil and gas industries. The study of the viscoelastic properties hopes to provide

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useful information regarding the storage modulus (G′) and the loss modulus (G″) suitable for

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oilfield applications.

7

Mobility of crude oil can be improved by heating, diluting with light crude oil, blending

8

the crude oil with hydrocarbon gases or by means of emulsification using surfactants.9-11

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Production of heavy and extra heavy crude oil is mainly achieved using thermal and microbial

10

enhanced oil recovery techniques. In the case of microbial methods, microorganisms help to

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degrade the long chain hydrocarbons to short chain hydrocarbons, thereby, reducing the viscosity

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and increasing the mobility of the HCO.12 Steam flooding is the most frequently used thermal

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method for the reduction of viscosity of the heavy crude oil. The injection of heat breaks down

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the asphaltene molecules, thereby, causing reduction in the viscosity and further enables the

15

crude oil to flow towards the production well.9 The difficulties in the transportation of HCO can

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be overcome by heating which reduces its viscosity significantly and can be made easier to

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pump. The main shortcoming with respect to heating is that it requires huge amount of energy

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and involves operational challenges especially over long distances of pipelines. Heat losses are

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also more common within the reservoir area and the heat transportation within the porous media

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is not always uniform.5 The method of emulsification is the alternative to heating in which water

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forms the continuous phase and HCO forms the dispersed phase. The continuous phase is

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blended with surfactants so as to form an oil-in-water emulsion which reduces the apparent

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viscosity (the ratio of shear stress to shear rate for a non-Newtonian fluid). Al-Roomi et al.


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Energy & Fuels

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(2004) have employed new surfactant solutions to convert viscous crude oils into lighter oil-in-

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water emulsions.9 The foremost difficulty of this technique lies in the choice and cost of the

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surfactants and the technical trouble in the establishment of separation units at the receiving end

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of the transportation pipelines.4

5

Ionic liquids (ILs) are molten salts containing an organic cation and an anion fused

6

together by a poor coordination. They have tremendous physicochemical properties such as, low

7

melting point, negligible vapour pressure, non-flammability, etc., due to which they are widely

8

used in several fields of engineering, synthesis, biomass conversion, bitumen recovery,

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desulphurization, asphaltene degradation, enhanced oil recovery, etc.13-23 ILs can be tuned for

10

various preferred modulations in their behaviour by alteration of their anionic/cationic part.22-24

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Additionally, the used ILs can be recycled using a small amount of water and can be reused.24

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Nowadays, ILs are being explored for numerous oil and gas industrial applications. Imidazolium

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ILs

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methylimidazolium aluminium tetra chloride ([BMIM]+[AlCl4]-) had been observed to cause

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increased dissolution of asphaltene to a certain extent. [BMIM]+[Cl]- and [BMIM]+[FeCl4]- had

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been investigated by Shaban and co-researchers for upgrading heavy oil.5 It was found that

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[BMIM]+[FeCl4]- ILs had the best effect on upgrading HCO at the optimum temperature between

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70-90 °C. Their results showed reduction in both viscosity and sulfur content. C-S bonds have

19

been found to be broken by the formation of new complex between the IL and the sulfur

20

components of the crude oil.5

such

as

1-butyl-3-methylimidazolium

chloride

([BMIM]+[Cl]-)

and

1-butyl-3-

21

The effect of few imidazolium ILs was studied by Painter et al. (2010) for bitumen

22

processing. The researchers observed that an addition of minor quantity of ILs to the organic

23

solvents significantly improved the bitumen dissolution than with only neat organic solvents.16-18


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Few authors have also worked on the upgradation of heavy oil using ILs and have observed that

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the ILs improved the quality of heavy oil.19 The effect of a couple of ammonium ILs, namely,

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[Et3NH]+[AlCl4]-, [Et3NH]+[AlCl4]-Ni2+, [Et3NH]+[AlCl4]-Fe2+ and [Et3NH]+[AlCl4]-Cu+ were

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investigated by Fan et al. (2007) for the viscosity reduction of HCO at lower temperature.19 The

5

authors observed that the addition of minor quantity of ILs significantly reduced the viscosity

6

and asphaltene content of the heavy oil which could thereby help in minimizing the flow

7

assurance issues faced by the oil industry. Among the studied ILs, the IL, [Et3NH]+[AlCl4]-Ni2+

8

had been found to have worked better than the other ILs in efficiently reducing the viscosity and

9

asphaltene content.19 If the authors had extended their work for higher temperatures, then their

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study might have been very useful for consideration in practical applications.

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Functionalized molecules have been used by various researchers for enhancing the

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stability and lowering the viscosity of the crude oil as discussed below. In this regard, Hu et al.

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(2005) tested ILs for the first time for the reduction of asphaltene precipitation and to enhance

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the stability of crude oil.25 Various aspects featuring the ILs, such as, effect of alkyl chain length,

15

counter-ion charge density and charge density of cation head group were also investigated. It was

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observed that the ILs with greater anion charge density and lower cation charge density were

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better in inhibiting asphaltene precipitation effectively. The authors also observed that with

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increase in the concentration of the ILs from about 0.5 to 5 wt %, the stability of the asphaltene

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in the crude oil has been increased. The reason for this increased stability was attributed to the

20

electrostatic interactions between the IL and the asphaltene units which lowered the asphaltene

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aggregation size and reduced its precipitation.

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Recently, researches from our group have shown that the used ILs can be recycled and

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could be reused for further experiments.24 Processing techniques such as distillation, adsorption,


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extraction, induced phase separation, etc., could be performed to remove any dissolved

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contaminants like certain constituents of the crude oil along with ILs from the aqueous phase.26

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However, the oil phase can be separated out from the mixture using surface facilities and the

4

remaining aqueous phase could be subjected to further purification.

5

The main goal of the current study is to employ six various imidazolium ILs and to

6

observe their effect on the rheological behaviour of HCO under high temperature and high

7

pressure (HPHT) conditions. Based on the effect of various concentrations of one particular IL

8

on the rheology of the HCO, 5000 ppm was fixed as the concentration for all other subsequent

9

measurements with various other ILs. The present investigation focusses on examining the

10

viscoelastic behaviour as G′ and G″, using amplitude sweep and frequency sweep under varying

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pressures (0.1, 5 and 10 MPa) and temperatures (323.15, 348.15 and 373.15 K) for Indian crude

12

oil and ILs system. The results will then be compared and contrasted with the same crude oil

13

without ILs at 298.15 K and reported. Additionally, this study also explores the effect of alkyl

14

chain length and different cations and anions of the ILs on the crude oil rheology and the

15

microscopic characterization of the HCO with and without ILs at room temperature. Analysis of

16

the rheological properties of the HCO will help us in understanding its flow behaviour at HPHT

17

conditions of the reservoir and for processing heavy oil in oil refineries.

18 19

2. Experimental section

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2.1. Materials: Heavy crude oil and ionic liquids

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For the present investigation, heavy crude oil (HCO) was provided by Oil India Limited

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(OIL), Assam, India. The properties of the crude oil such as SARA (Saturates, Aromatics,

23

Resins, Asphaltenes), density, API gravity, surface tension and interfacial tension are


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summarized in Table 1 (SARA and density data was obtained from Oil India Limited, Assam,

2

India).

3

The current study utilizes six imidazolium ionic liquids for examining the effect on the

4

rheology of HCO. The six ionic liquids are: [BMIM]+[Cl]-, [BMIM]+[Br]-, [BMIM]+[HSO4]-,

5

[HMIM]+[Br]-, [HMIM]+[HSO4]- and [OMIM]+[Cl]- which were all synthesized according to

6

literature and characterized by 1H-NMR (Nuclear Magnetic Resonance: Brukar Avance 500

7

MHz spectrometer) using DMSO or CDCl3 as the solvents.27-30 Table 2 gives the list of ILs used

8

in the present work, their abbreviation and chemical structure. Prior to their usage, all the ILs

9

were dried under vacuum (0.1 Pa) at 353 K for 24 hrs. All the six ILs used in the current

10

investigation were tested for their water content using Karl Fischer reagent in Analab Karl

11

Fischer Titrator (Micro Aqua Cal 100, India). The water content was found to be below 2000

12

ppm for all the ILs. The titrator was calibrated using distilled water with a titre factor of 5.356

13

prior to the experiments. This instrument could detect the presence of water/moisture in the

14

range of 0.001 to 100 % through conductometric titration using dual platinum electrode. DSC

15

profile has been recorded for the IL, [BMIM]+[Cl]- and the melting point is observed to be

16

around 65.2 oC, which is very close to the reported value (65.5 oC) of the same.31 Similarly for

17

the IL, [BMIM]+[Br]-, the melting point is found to be 76.7 oC (Figure S1). For the other ILs, we

18

were unable to perform the melting point determination as they were almost liquids at room

19

temperature. Dynamic Contact Angle Tensiometer (Dataphysics DCAT 11EC, Germany) was

20

engaged to perform the surface tension and interfacial tension measurements. This Tensiometer

21

was controlled by the SCAT software and the measurements were done with the use of

22

Wilhelmy platinum-iridium plate. The chemicals used in the current research work are appended

23

in Table S1.


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2.3. Rheological studies

3

2.3.1. Viscosity measurements

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The viscosity and the dynamic rheological properties of the neat HCO and with various

5

ILs were measured with a stress controlled rheometer (Anton Paar Modular Compact Rheometer

6

MCR 52, Physica, Austria) using a double gap geometry (concentric cylinder) (model no.

7

DG35/26483) system. The dimensions of the system employed are as given in our recent

8

publications.32 The concentric cylinder system can withstand high pressures upto 40 MPa and is

9

also fitted with a jacket to circulate hot/cold fluid that could maintain the temperature of the

10

measuring cup. Rheological studies were carried out under varying conditions such as pressure,

11

temperature and varying shear rates. High pressure conditions within the measuring cell were

12

established using nitrogen gas during the measurements. Circulating water bath (Brookfield,

13

Model number: TC-650AP, temperature range: -20 °C to +200 °C) with insulated pipes was

14

connected to the jacket of the measuring system and was used to maintain the required

15

temperature. Before each measurement the pressure cell was cleaned thoroughly and completely

16

dried. Thereafter, for each experimental run, the samples of either HCO or HCO+IL were mildly

17

agitated to homogenize and then taken inside the measuring system. Prior to each trial of

18

measurements, the pressurized measuring system containing the experimental sample was

19

allowed to reach a thermally stable condition at the set experimental temperature for 15-20 min.

20

At the beginning of the rheological measurements, crude oil was taken and its viscosities

21

at varying shear rates from 0.1 to 1000 s-1 and at four different temperatures (298.15, 323.15,

22

348.15 and 373.15 K) and three different pressures (0.1, 5, 10 MPa) were measured and the

23

results were compared with various concentrations of IL, such as, 0, 100, 250, 500, 5000, 10000,


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1

25000 and based on the results a fixed concentration was maintained for further experiments.

2

Thereafter, for each sample analysis, crude oil of 2.35 g of HCO and 2.35 g of HCO + 11.75 mg

3

of IL (0.5 wt % or 5000 ppm of ILs unless specified) was used subsequently in this study. The

4

recyclization procedure for the ionic liquid is given in the supporting information. The 1H-NMR

5

spectra of the fresh [BMIM]+[Cl-] and reused [BMIM]+[Cl-] are given as Figure S2, and there

6

was no considerable variation in the peaks observed on their comparison. However the efficiency

7

of the recycled IL, [BMIM]+[Cl-] is not insignificant (see supporting information). All the

8

measurements were repeated at least thrice to confirm reproducibility.

9 10

2.3.2. Viscoelastic measurements

11

In order to determine the linear viscoelastic region (LVE) for the HCO and HCO+IL,

12

strain-sweep measurements were carried out to find out the dynamic moduli in the range of 0.1

13

to 100 % strain amplitude during which the frequency was kept constant at 10 rad s-1.33 This

14

methodology is very much useful in the determination of the LVE region where G' remains

15

constant with change in strain amplitude and the crossover point (critical strain) above which G'

16

decreases in a nonlinear pattern with additional increase in the strain amplitude.33 Frequency

17

sweep measurements are typically performed at a fixed strain amplitude within the LVE region

18

so as to measure the dynamic moduli at variable frequencies. For the strain-sweep

19

measurements, considering the insensitivity of the rheometer (below 1.0 %), results obtained are

20

above 1.0 % of the strain amplitude. All the rheological measurements were repeated at least

21

thrice to confirm the reproducibility.

22 23

2.4. Morphological investigations


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Microscopic characterization of the HCO and HCO+IL systems were also performed

2

using a high magnification inverted microscope (Leica DMI3000B Inverted Microscope,

3

Germany) with magnifications 5X, 10X, 20X, 63X, 100X. This study was performed by taking

4

uniform ratio of 3:1 of HCO:ILs in small vials, followed by vigorous agitation using a magnetic

5

stirrer for 15 min. The systems of either pure HCO or HCO+ILs were then allowed to stabilize

6

for few minutes and then subjected for microscopic examination. The microscopic study is useful

7

to understand the morphological behaviour of the system formed using crude oil+ILs system.

8 9

3. Results and Discussion

10

The present study focusses on the shear rheological behaviour of HCO on the addition of

11

six ILs as a function of different pressures and temperatures. Consequently, the viscoelastic

12

properties (yield stress, G', and G") of HCO and HCO+IL systems were studied and compared.

13

The later part of this section deals about the morphological examination of the HCO and

14

HCO+IL systems and correlated with the rheological behavior and discussed.

15 16

3.1. Shear rheological characterization

17

3.1.1. Standard heavy crude oil

18

For the purpose of rheological measurements, weighed quantity of 2.35 g of the HCO had

19

been taken initially inside the double gap measuring system, sealed properly and then subjected

20

to further experimental measurement of viscosity at varying shear rates from 0.1 to 1000 s-1. This

21

experiment had been conducted for varying temperatures (298.15 to 373.15 K) and varying

22

pressures (0.1 to 10 MPa). Figure S3 depicts the shear rheological behavior of the HCO at

23

various experimental temperatures and pressures. A non-Newtonian shear thinning behavior was


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1

observed over the entire considered range of shear rate and a reduction in viscosity with

2

increasing shear rates was also detected.

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3

Increase in pressure from 0.1 to 5 and 10 MPa is observed to increase the viscosity and

4

non-Newtonian shear thinning behavior of the standard HCO as shown in Table S2 and Figure

5

S3. At 298.15 K, the viscosity increases from 17214 cP at 0.1 MPa to 27786 at 10 MPa,

6

providing about 61.42 % of increase in the viscosity of the crude oil with pressure. With increase

7

in the system pressure, the viscosity increase is detected due to the inhibiting of the Brownian

8

motion of the molecules as they come closer together. The effect of temperature on the rheology

9

of the standard HCO has also been observed. At all pressures (0.1 to 10 MPa), the viscosity of

10

the standard HCO is observed to decrease (see Table S2 and Figure S3) with increase in

11

temperatures above 298.15 K. When the temperature rises, the higher molecular weight fractions

12

of the crude oil, such as, asphaltenes, resins, etc., does not aggregate to form clusters and

13

eventually leading to the reduction in the crude oil viscosity.3 Over the tested experimental

14

temperatures, significant reduction in viscosity is observed (see Table S2). Figure 1 is a perfect

15

representation of the effect of temperature and pressure on the measurement of viscosity of the

16

standard HCO. Irrespective of the temperature, it is observed that there is an effect of pressure on

17

the viscosity of the HCO.

18 19

3.1.2. Effect of ILs on the viscosity of heavy crude oil

20

The effect of the addition of six ILs on the shear rheological behavior of the HCO was

21

also executed in a similar way as the previous section. Initially, the concentration (ppm) effect on

22

one particular IL ([BMIM]+[Cl]-) with the HCO was studied and is presented in Figure S4. From

23

this study, the optimum concentration of ILs for subsequent experiments was fixed as 5000 ppm.


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Figure S5 and Figure S6 depict the changes in the shear rheological behavior of the standard

2

HCO with the addition of 5000 ppm of ILs, [BMIM]+[Cl]- and [OMIM]+[Cl]-, respectively at

3

various temperatures and pressures. Irrespective of the added IL, the viscosity of the HCO was

4

reduced more significantly upon subsequent additions at low pressure (0.1 MPa) and low

5

temperature (298.15 K) more than at higher pressures. It can be clearly seen from Table S2, for

6

the system of HCO+[BMIM]+[Cl]- at 298.15 K and at 0.1 MPa, the viscosity reduced to 13599

7

cP and for the system of HCO+[OMIM]+[Cl]-, the viscosity was dropped to 13082 cP from the

8

standard value of 17214 cP giving 26.58 % and 31.58 % decrease in viscosity, respectively.

9

However, the IL with the lengthier alkyl chain, ([OMIM]+[Cl]-) worked more efficiently than the

10

one with lower chain containing IL ([BMIM]+[Cl]-).

11

Similarly, ILs containing [Br]- and [HSO4]- were also studied at 0.1 MPa and at four

12

various temperatures and their results are presented in Figure S7. At 298.15 K, the system of

13

HCO+[BMIM]+[HSO4]- showed a reduction in viscosity as 13771 from 17214 cP (24.94 %

14

decrease in viscosity) upon adding 5000 ppm of IL, wherein in the system of

15

HCO+[HMIM]+[HSO4]- reduced the viscosity to 13254 from 17214 cP ( 29.87 % decrease in

16

viscosity). Likewise, for HCO+[BMIM]+[Br]- system, the viscosity dropped to 13651 cP (26. 10

17

% decrease) and for HCO+[HMIM]+[Br]-, the viscosity was reduced to 13614 cP from the

18

standard value of 17214 cP (26.44 % decrease). The complete viscosity reduction profile of the

19

HCO with addition of six different ILs is presented in Figure 2.

20

In general, it has been observed that the ILs containing longer alkyl chain length

21

([OMIM]+[Cl]-, [HMIM]+[HSO4]- and [HMIM]+[Br]-) show better effectiveness towards the

22

viscosity reduction of HCO than the other studied ILs. The superior efficiency of the longer

23

chain containing ILs could be accredited due to the concept of ILs hydrophobicity that could


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Page 14 of 41

1

have played a major role.34 The reason for the better efficiency of longer alkyl chain containing

2

ILs could be due to their lesser polar nature (due to greater hydrophobicity) than the ILs with

3

shorter alkyl chain (more polar). In general, there will be polar-polar interactions between the

4

polar fractions (N, S, O moieties of the asphaltenes and resins units) of the crude oil and polar

5

moieties of the IL (charged species [BMIM]+ which is relatively more polar than [OMIM]+). It

6

means that the system containing more interaction ([BMIM]+-crude oil) shows lesser reduction

7

in viscosity than [OMIM]+, i.e., the shorter chain containing ILs shows lesser efficiency than the

8

longer chain containing ILs. However, once the interactions occur between asphaltene/resin and

9

IL, it helps to relax the flocculation forces of asphaltene/resin with other crude oil moieties

10

(saturates and aromatics), and eventually, the viscosity of the IL added system is found to be

11

lesser than the neat HCO. Another reason could be that the hydrogen bond (-S---H---N-) which

12

occurs in the crude oil due to the association of the two or more asphaltene/resin units could be

13

suppressed by the addition of the ILs, wherein the anion [Cl]- of the IL could try to create S Cl--- H

N

14

another set of interaction

with the crude oil by weakening the existing hydrogen

15

bond of the crude oil (-S---H---N-). i.e., the Cl---H bond will get stronger and the H---S or H---N

16

bond will get weaker. Also, by this action, the flocculation between the asphaltenes or resins

17

could be reduced, which leads to the reduction in the viscosity of the crude oil.

18

The asphaltene structures of the HCO could have been broken down to lower molecular

19

fractions, mainly by hydrogen bonds upon treatment with IL.25 The formation of these smaller

20

fractions could have radically reduced the viscosity and the density of the HCO.35 The charges

21

present in this HCO could get attracted to other surrounding charges, here it could be the IL

22

which also contain charges and, therefore, has an affinity towards the larger asphaltene fractions


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forming bonds with the polar moieties through an acid-base interaction or by the concept of

2

electron donor-acceptor.36 The above result has provided the evidence that the designed IL could

3

be further employed for easy transportation of the HCO. The IL blended HCO could also enable

4

the oil and gas industries to mitigate various issues related to the HCO, such as transportation,

5

pipeline deposition, enhanced oil recovery, heavy crude oil processing, etc.

6

Basic understanding of flow assurance requires the temperature and pressure dependency

7

of the material since viscosity determines the flow behavior involving transportation processes

8

particularly for heavy crude oil. For this purpose, here we have investigated the temperature and

9

pressure dependency of the HCO and HCO+ILs. From literatures, it has been found that the

10

dynamic viscosity can be fitted using Arrhenius-type equation for temperature dependence.37-38

11

In this present study, it has been observed that both the HCO and HCO+ILs show Arrhenius

12

dependence. Figure S8 and S9 shows the plot of temperature dependence of viscosity (η) for the

13

systems of HCO and HCO+ILs, respectively. These figures (ln (η) versus (1/T)) shows clearly

14

the Arrhenius behavior of viscosity.37-38 Similarly, the system of HCO were also observed to

15

exhibit Arrhenius dependence of viscosity on pressure. These have been plotted in Figure S10 of

16

ln (η) against pressure P for four temperatures of 298.15, 323.15, 348.15 and 373.15 K.

17 18

3.2. Yield stress measurements

19

The yield stress is defined as the limiting stress below which a sample behaves like a

20

solid. At lower stress, elastic deformation occurs which disappears when the applied stress is

21

released. The relationship between the elastic deformation and the applied stress is linear.10

22

However, above the yield stress point, any further increase of the applied stress leads to

23

indefinite deformation which causes the sample to flow.39 This section discusses the results of


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1

the yield stress measurements of the heavy crude oil with and without ILs at different

2

temperatures employing the Anton Paar Rheometer.

Page 16 of 41

3

The stress-sweep curves (Figure 3) for HCO and HCO+IL systems were developed from

4

the viscosity profiles obtained from the earlier measurements as shown in Figures S3, S4 and S5.

5

No shear flow is observed upon progressively increasing the shear stress until the yield stress is

6

reached. Figure 3 (a-c) shows the given shear stress and the response shear rate relationship of

7

the standard HCO, HCO+[BMIM]+[Cl]-, HCO+[OMIM]+[Cl]-, respectively for different

8

temperatures and pressures. Table S3 shows the reduction in the yield stress of the various

9

systems: HCO, HCO+[BMIM]+[Cl]- and HCO+[OMIM]+[Cl]- at four different temperatures in

10

0.1 MPa pressure. Increase in temperature caused the flow of the sample to begin a little early by

11

lowering the yield point, whereas the reverse is true for increase in pressure of the system. The

12

yield point of the HCO reaches a value of 0.7 Pa at 298.15 K and further decreases to 0.4 Pa

13

upon reaching 373.15 K. On adding 5000 ppm of ILs, [BMIM]+[Cl]- and [OMIM]+[Cl]- with

14

HCO, it is observed that the apparent yield stress reduces considerably as shown in Figure 3 (a-

15

c). This implies that the addition of the ILs to HCO reduced the need for any further stress to be

16

applied so as to initiate the steady flow. This indicates that the system comprising both HCO+IL

17

can easily flow under the effect of shear alone as compared to the system of neat HCO. The

18

above results are also found to be in consistent with the results of the earlier sections based on

19

the effect of pressure (0.1 to 10 MPa) and temperature (273.15-373.15 K) on the rheological

20

behavior of HCO.

21

Figure 3 also indicates that the samples of HCO and HCO+ILs experienced a marginal

22

change in the yield stress values on increasing the shear rate from 0 to around 10-13 s-1. This

23

may also be credited to the low magnitude of applied stress, during which the resistance to


Page 17 of 41

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1

deformation was not enough to affect the internal structure of HCO responsible for yielding

2

behavior.40-41 When the shear rate is increased beyond 13-14 s-1, the samples showed a

3

significant flow-ability with nonlinear increase in yield stress with increasing shear rate. Though,

4

yield stress is difficult to measure accurately and may show uncertainties of the order of 6-7 %,

5

the interpretations made on yield stress in the current study are more empirical and useful for

6

practical applications.

7 8

3.3. Dynamic viscoelastic properties of HCO and HCO+IL solutions

9

Investigation of the viscoelastic properties is a dynamic testing tool to study the

10

rheological behavior of the HCO sample. The effect of oscillating stresses or strains are

11

investigated using the above test. These dynamic oscillatory measurements compute the

12

elasticity and the maximum strain that could be tolerated by the sample under study before shear

13

thinning. The storage or the elastic modulus, G', designates the contribution of the stress energy

14

that is stored temporarily during the analysis and is reversible/recoverable. While, the term

15

viscous or loss modulus, G", refers to the energy required to initiate the flow of the sample which

16

is dissipated into shear heat and is irreversible. The prevalence of the viscoelastic behavior of

17

HCO indicates that the interaction of the moieties and structural components of the sample gets

18

affected by temperature and by the addition of additives (ILs) as discussed further.

19 20

3.3.1. Standard heavy crude oil

21

In order to understand the effect of temperature and pressure on the viscoelastic

22

properties of the pure HCO, the strain-sweep measurements were carried out. The dynamic

23

rheological properties (G' and G") of the pure HCO is shown in Figure 4. Figure 4a shows the


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Page 18 of 41

1

viscoelastic moduli of the HCO sample at 0.1 MPa and at 298.15 K which are found to be G' ≈

2

11.02 and G" ≈ 7.05 Pa at about 1.5% of the strain representing that the HCO sample is solid-

3

like. At 0.1 MPa, the strain dependence of G' of HCO sample specifies a linear viscoelastic

4

region with a plateau over a narrow region of small strain (1.5 to 20.0 %), above which G'

5

exhibits a very slight declination, which could be the result of relaxation and extrication of the

6

HCO molecules. The loss modulus G", of the pure HCO begins to drop down to 2.5 % of the

7

strain from the initial value and then exhibits a plateau in G" upto 10 % of the strain and,

8

thereafter, begins to moderately increase (see Figure 4a). This trend indicates that, at 0.1 MPa,

9

the HCO sample exhibits viscoelastic behavior, with elasticity being the major component for

10

half the range of the strain and then for the rest of the strain amplitude it behaves like a liquid.

11

From the Figure 4a, the crossover/gel-point (where G' ≈ G") is identified to be at about 44.55 %

12

of the strain amplitude.

13

The viscoelastic property of the pure HCO exhibit significant features at high pressure

14

conditions (> 0.1 MPa). A pronounced enhancement in the magnitude of G' indicating solid

15

behavior (see Figure 4a) is observed with increase in pressure. The storage modulus of the neat

16

HCO increases to a larger magnitude of about 138 Pa at 5 MPa (see Figure 4a), whereas, at 10

17

MPa, it further increases to 155 Pa (at 1.5 % strain) which is much greater than the value of G' at

18

0.1 MPa. A characteristic behavior of gel is observed at high pressure conditions where G' is

19

always higher in value with plateau behavior as compared with G" (see Figure 4a).42 At high

20

pressures of 5 and 10 MPa, no considerable change is observed in G" with increase in strain

21

amplitude and also there is no occurrence of crossover point indicating that amplitude strain does

22

not affect the structural property of the sample under study as compared to 0.1 MPa, as the

23

sample (HCO) behaves more like a solid (see Figure 4a).


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1

Effect of increasing temperatures on the HCO sample was also performed using the

2

strain-sweep analysis and their results are presented in Figures 4b, 4c and 4d at 323.15, 348.15

3

and 373.15 K, respectively. It is observed that both G' and G" are reduced in magnitude

4

significantly after increasing the temperature of the system irrespective of the pressure. At

5

323.15 K and 0.1 MPa, the crossover point (G' ≈ G") was observed at about 9.1 % of strain

6

amplitude (see Figure 4b). However, at higher temperatures of 348.15 and 373.15 K (Figure 4c

7

and 4d, respectively), no crossover point was observed at low pressure of 0.1 MPa owing to the

8

liquid-like property of the system rather than solid (G" is much higher than G' values). In

9

particular, at 348.15 K (Figure 4c) a crossover point of 9.3 and 11 % of the strain amplitude has

10

been observed at higher pressures of 5 and 10 MPa, respectively. Ultimately, the system of HCO

11

under investigation has been observed to behave completely as liquid-like at high temperature

12

(373.15 K) irrespective of the pressure of the system.

13

Subsequently, frequency-sweep measurements were performed to support the results

14

obtained from the strain-sweep analysis. Figure 5 (a-d) shows the frequency sweep

15

measurements carried out at a fixed strain amplitude of 1.5 % in the LVE region at three

16

different pressures (0.1, 5 and 10 MPa) and four various temperatures (273.15-373.15 with 25 K

17

interval). The crossover point is observed at 11.65 rad s-1 for the HCO system at low temperature

18

(298.15 K) and low pressure (0.1 MPa). As realized from strain-sweep experiments, it has been

19

observed here that the storage modulus G' of the HCO shows higher values at 5 and 10 MPa

20

which is basically independent of the frequency as compared to loss modulus values confirming

21

the characteristic behavior of the gel. The viscoelastic moduli of the HCO as shown in Figure 5a

22

indicate that at high pressures of 5 and 10 MPa, the crossover point in moduli does not appear

23

over the whole range of applied frequency as compared to 0.1 MPa. In other words, the HCO


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1

sample shows viscoelastic behaviour at atmospheric pressure (0.1 MPa) and behaves like a gel at

2

high pressures (5 and 10 MPa).

3

In addition, experiments at higher temperatures (323.15, 348.15 and 373.15 K) and three

4

pressures (0.1, 5 and 10 MPa) were also performed. At 323.15 K and 0.1 MPa, until a certain

5

region of frequency (4.5 rad s-1) both the moduli remain very close and then start deviating,

6

however, the crossover point occurred around 3.55 rad s-1. At the same temperature and at 5 and

7

10 MPa, G' > G", signifying a solid-like behavior of the HCO. Whereas, with increase in

8

pressure from 0.1 to 10 MPa, G' increases from 8.1 Pa (0.1 MPa) to 135 Pa (10 MPa, see Figure

9

5b) indicating that the HCO has become liquid-like and is transforming into gel-like. At 348.15

10

K, for the lower pressures of 0.1 and 5 MPa the G" > G', displaying that HCO behaves as a clear

11

viscous fluid than being elastic, whereas, at higher pressure of 10 MPa, viscoelastic behavior is

12

observed at the crossover frequency of 7.95 rad s-1. Similarly, at 373.15 K and 10 MPa, reduction

13

in the values of G' and G" is observed at crossover frequency at about 3.55 rad s-1.

Page 20 of 41

14

At higher temperatures of 348.15 and 373.15 K and at low frequencies, the response of

15

the HCO sample seemed to be rather viscoelastic because of the presence of both G' and G". As

16

the frequency increases above 5-8 rad s-1, the G" increases and dominates over G' indicating

17

viscous behavior at each experimental temperature. Figure 6 shows the effect of temperature

18

(298.15 to 373.15 K) on the viscoelastic properties of the HCO at three different pressures of 0.1,

19

5 and 10 MPa. It is observed that at low temperature of 298.15 K, G' is slightly higher than G",

20

exhibiting an elastic behavior. It has also been seen that the values of G' decrease with an

21

increase in temperature irrespective of the experimental pressures. The reason may be attributed

22

to the temperature induced marginal decrease in the viscosity of the HCO sample. In addition,


Page 21 of 41

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the increase in temperature also transforms the solid-like HCO into liquid-like at about 332, 358

2

and 367 K corresponding to the pressures of 0.1, 5 and 10 MPa (Figure 6), where G' ≈ G".

3 4

3.3.2. Effect of ILs on the dynamic viscoelasticity of heavy crude oil

5

Dynamic viscoelastic measurements were conducted to recognize the variation of elastic

6

(G') and viscous responses (G"), of HCO+IL systems. Two types of viscoelastic measurements,

7

viz., strain-sweep and frequency-sweep measurements were conducted. Figure 7 shows the strain

8

dependence profiles of G' and G" moduli for the various HCO+IL samples at 298.15 K as a

9

function of different pressures. At 0.1 MPa, it is observed that all the six HCO+IL systems (see

10

Figure 7 (a-f)) show a viscoelastic nature with visible crossover points at 2.95, 3.92, 19.16,

11

11.92, 21.23 and 21.51 % of the strain amplitude for the corresponding systems of

12

HCO+[BMIM]+[Cl]-, HCO+[OMIM]+[Cl]-, HCO+[BMIM]+[HSO4]-, HCO+[HMIM]+[HSO4]-,

13

HCO+[BMIM]+[Br]- and HCO+[HMIM]+[Br]-. However, the observed crossover point of

14

HCO+IL systems are much lesser than the pure HCO system (44.55 % of the strain amplitude

15

(see Figure 4a)). As the pressure increases from 0.1 to 5 and 10 MPa, both G' and G" increase

16

and no crossover point has been noticed, since G' dominates at higher pressures over the

17

complete range of strain amplitude indicating solid behavior of the systems. Furthermore, each

18

and every G' and G" of the various HCO+IL systems have been observed to be reduced more

19

significantly than the standard HCO system, that could be attributed to the reduction in the

20

viscosity of the HCO+IL systems compared to the neat HCO system. The measurement of the G'

21

and G" of the various HCO and HCO+IL systems has been summarized in Table S4. It is being

22

understood from the Table S4 that the addition of the ILs which contains longer alkyl chain

23

shows more significant effect on the improvement of the flow-ability of the system.


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Page 22 of 41

1

Similarly, frequency sweep measurements have also been performed for the six different

2

HCO+IL systems at 298.15 K and three different experimental pressures (0.1, 5 and 10 MPa) and

3

are presented in Figure 8 (a-f). Analogous to the strain sweep studies, in the frequency sweep

4

experiments, the crossover frequency is being observed only at lower experimental pressure of

5

0.1 MPa and, they are 10.19, 8.5, 10.51, 10.55, 10.96 and 11.02 rad s-1 corresponding to the

6

systems

7

HCO+[HMIM]+[HSO4]-, HCO+[BMIM]+[Br]- and HCO+[HMIM]+[Br]-, respectively, which is

8

slightly lesser than the crossover frequency of the standard HCO system (11.65 rad s-1 of the

9

frequency). However, at high pressure conditions (5 and 10 MPa), no crossover frequency has

10

been observed and the complete region of frequency has been dominated by storage modulus

11

than the loss modulus. In fact, the systems of HCO+ILs (Figure 8 (a-f)), show a strong difference

12

by the significant reduction of the both the G' and G" moduli from the standard HCO system

13

(Figure 5 (a)).

of

HCO+[BMIM]+[Cl]-,

HCO+[OMIM]+[Cl]-,

HCO+[BMIM]+[HSO4]-,

14 15

3.4. Microscopic characterization

16

Micrographs of HCO and 5000 ppm of ILs+HCO are obtained using the Leica inverted

17

microscope using uniform magnification of 100X and the same is presented in Figure 9. It has

18

been observed under the microscope that the standard HCO particles are irregularly shaped

19

spherulites (see Figure 9 (a)). It is clear from the Figure 9 (a) that the HCO contains relatively

20

large flocculated structures, mainly the polar fractions of the HCO, such as asphaltenes and

21

resins along with the aromatics and paraffins.20 Upon addition of the different ILs, the large

22

flocculated structures have been observed to be broken into smaller spheres (Figure 9 (b-g)),

23

thus, allowing easier flow of the HCO in the presence of ILs. The effect of different ILs and its


Page 23 of 41

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Energy & Fuels

1

alkyl chain length (Figure of 9 (b-g)) has also been observed to comprehend the structural

2

(chemical) influence of the ILs on the microscopic study of the HCO. The IL with longer alkyl

3

chain (Figure 9 (c,e,g)) has been observed to break the aggregated HCO more vigorously than

4

the ILs of shorter alkyl chain (Figure 9 (b,d,f)).

5

Finally, it has been noticed during the investigation that the IL with the longer alkyl chain

6

length provides greater efficiency in viscosity reduction and is further supported by the results of

7

the microscopic investigation. The greater efficiency of the ILs with lengthier alkyl chain could

8

be due to the concept of hydrophobicity that could have played a major role.34 The reason for the

9

better efficiency of longer alkyl chain containing ILs could be due to their lesser polar nature

10

than the ILs with shorter alkyl chain. Additionally, another reason for the better efficiency of the

11

ILs could be due to that the hydrogen bond which occurs in the crude oil could have be

12

suppressed by the addition of the ILs, this could have resulted in the superior reduction in the

13

viscosity of the HCO, followed by greater efficiency in flow assurance.22-24

14

The above discussion has now been extended for the prediction of a possible mechanism

15

for the reduction of the crude oil viscosity. The efficiency of the additives largely depend upon

16

the composition and rheological behavior of the crude oil. We propose that the

17

asphaltenes/resins/aromatics of the crude oil which are large molecular structures (heavier

18

fractions) could aggregate to form lumps of hydrocarbons which can cause increase in the

19

viscosity of the crude oil. It becomes necessary to involve additives that could prevent or

20

mitigate the aggregation/coagulation of the heavier components, eventually, reducing the

21

viscosity of the crude oil. The ILs could intervene the aggregation of the larger hydrocarbons by

22

forming weak interactions.


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Page 24 of 41

1

We have chosen functionalized molecules of ILs which could interact with the

2

asphaltenes/resins/aromatics via charge-transfer interactions. As a result, the aromatic macro

3

sized cores could be broken down into smaller fragments, which would in turn lower the

4

viscosity. Figure 10 represents the possible mechanism that could have been responsible for the

5

reduction in the viscosity of the HCO via weak interactions between the positively charged

6

imidazolium ring of the IL and the lone pair of electrons of the heteroatoms of the macro core.

7

From the current investigation, it was also observed that the IL with a longer alkyl chain

8

greatly reduced the viscosity of the HCO. These findings were further supported by the recent

9

findings of Jian et al. (2013) who have studied the effect of alkyl chain length on the core of the

10

larger hydrocarbons via molecular simulations.43 Moreover, there exists the possibility of steric

11

hindrance to the asphaltenes/resins/aromatics due to the presence of aliphatic side chains of the

12

functionalized molecules (here, the ILs), thereby, inhibiting their accumulation.8 This steric

13

hindrance could eventually lead to smaller aggregate of the asphaltenes/resins/aromatics and thus

14

higher viscosity reduction.

15 16

4. Conclusions

17

This work utilized six different imidazolium ILs to study their effect on the rheology of

18

HCO. The system of HCO+ILs showed a favourable viscosity reduction maximum upto 31.5 %

19

at 298.15 K and 0.1 MPa as compared to the pure HCO system. The addition of ILs to the HCO

20

reduced the yield stress to about 15-20 % at 298.15 K and 0.1 MPa and to about 25-30 % at

21

373.15 K and 0.1 MPa as compared to neat HCO. The cross over frequency of the system of

22

HCO+ILs was reduced to 8.5 rad s-1 from the initial value of 11.65 rad s-1 (standard HCO). From

23

the various measurements, it was observed that the addition of ILs to the HCO showed improved


Page 25 of 41

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1

the rheological properties over the pure HCO system. Further experiments on microscopic

2

examination also supported the rheological studies indicating that the addition of ILs inhibited

3

the flocculation of the heavier fractions of the HCO. It was also observed that the ILs with longer

4

alkyl chain provided greater efficiency in the viscosity reduction and favourable rheological

5

behaviour over the ILs of shorter alkyl chain.

6 7

Acknowledgments

8

The authors would like to acknowledge IIT Madras for extending its financial support

9

throughout this research work. The authors would like to thank Oil India Limited for providing

10

crude oil sample and its SARA property. The authors would also like to extend their gratitude to

11

Ionic Liquid Laboratory (Dr. Ramesh Gardas) and Polymer Engineering and Colloid Sciences

12

Laboratory (Dr. Ethayaraja Mani) for permitting us to use their lab facilities for ionic liquid

13

synthesis and microscopic examination, respectively, during this investigation.

14


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Page 26 of 41

References 1. Shah, A.; Fishwick. R.; Wood, J.; Leeke, G.; Rigby, S.; Greaves, M. Energy. Environ. Sci. 2010, 3, 700-14. 2. Saniere, A.; Henaut, I.; Argillier, J. F. Oil Gas Sci Technol Rev IFP. 2004, 59, 455-466. 3. Ghannam, M. T.; Hasan, S. W.; Abu-Jdayil, B.; Esmail, N. J. Pet. Sci. Eng. 2012, 81, 122128. 4. Alomair, O. A.; Almusallam, A. S. Energy Fuels. 2013, 27, 7267-7276. 5. Shaban, S.; Dessouky, S.; Badawi, A. E. F.; Sabagh, A. E.; Zahran, A.; Mousa, M. Energy Fuels, 2014, 28, 6545-6553. 6. Yaghi, B. M.; Al-Bemani, A. Energy Sources. 2002, 24, 93-102. 7. Meriem-Benziane, M.; Abdul-Wahab, S. A.; Benaicha, M.; Belhadri, M. Fuel. 2012, 95, 97107. 8. Subramanian, D.; Wu, K.; Firoozabadi, A. Fuel. 2015, 143, 519-526. 9. Al-Roomi, Y.; George, R.; Elgibaly, A.; Elkamel, A. J. Pet. Sci. Eng. 2004, 42, 235-243. 10. Hasan, S. W.; Ghannam, M. T.; Esmail, N. Fuel. 2010, 89, 1095-1100. 11. Soni, H. P.; Kiranbala.; Bharambe, D. P. Energy Fuels, 2008, 22, 3930-3938. 12. Sakthipriya, N.; Doble, M.; Sangwai, J. S. J. Ind. Eng. Chem. 2015, 31, 100-111. 13. Welton, T. Chem. Rev. 1999, 99, 2071–2083. 14. Plechkova, N. V.; Seddon, K. R. Chem. Soc. Rev. 2008, 37, 123-150. 15. Li, X.; Sun, W.; Wu, G. Energy Fuels. 2011, 25, 5224-5231. 16. Painter, P.; Williams, P.; Lupinsky, A. Energy Fuels. 2010, 24, 5081-5088. 17. Hogshead, C. G.; Manias, E.; Williams, P.; Lupinsky, A.; Painter, P. Energy Fuels. 2011, 25, 293-299. 26 ACS Paragon Plus Environment

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Energy & Fuels

18. Williams, P.; Lupinsky, A.; Painter, P. Energy Fuels. 2010, 24, 2172-2173. 19. Fan, H.-f.; Li, Z.-b.; Liang, T. J. Fuel Chem. Technol. 2007, 35, 32-35. 20. Sakthivel, S.; Velusamy, S.; Gardas, R. L.; Sangwai, J. S. RSC Adv. 2014, 4, 31007-31018. 21. Sakthivel, S.; Velusamy, S.; Gardas, R. L.; Sangwai, J. S. Energy Fuels. 2014, 28, 61516162. 22. Sakthivel, S.; Velusamy, S.; Gardas, R. L.; Sangwai, J. S. Colloids Surf. A. 2015, 468, 62-75. 23. Sakthivel, S.; Velusamy, S.; Gardas, R. L.; Sangwai, J. S. Ind. Eng. Chem. Res. 2015, 54, 968-978. 24. Velusamy, S.; Sakthivel, S.; Gardas, R. L.; Sangwai, J. S. Ind. Eng. Chem. Res. 2015, 54, 7999-8009. 25. Hu, Y. F.; Guo, T. M. Langmuir. 2005, 21, 8168. 26. Mai, N. L.; Ahn, K.; Koo, Y. M. Process Biochem. 2014, 49, 872-881. 27. Nejad, N. F.; Shams, E.; Adibi, M.; Miran Beigi, A. A.; Torkestani, S. K. Pet. Sci. Technol. 2012, 30, 1619-1628. 28. Dubreuil, J. F.; Bourahla, K.; Rahmouni, M.; Bazureau, J. P.; Hamelin, J. Catal. Commun. 2002, 3, 185-190. 29. Tshibangu, P. N.; Ndwandwe, S. N.; Dikio, E. D. Density, viscosity and conductivity study of 1-Butyl-3-Methylimidazolium Bromide. Int. J. Electrochem. Sci. 2011, 6, 2201-2213. 30. Nassor, E. C. O.; Tristao, J. C.; dos Santos, E. N.; Moura, F. C. C.; Lago, R. M.; Araujo, M. H. J. Mol. Catal. A: Chem, 2012, 363-364, 74-80. 31. Dharaskar, S. A.; Wasewar, K. L.; Varma, M. N.; Shende, D. Z.; Yoo, C. K. Procedia Engineering, 2013, 51, 416-422.


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32. William, J. K. M.; Ponmani, S.; Samuel, R.; Nagarajan, R.; Sangwai, J. S. J. Pet. Sci. Eng. 2014, 117, 15-27. 33. Aben, S.; Holtze, C.; Tadros, T.; Schurtenberger, P. Langmuir. 2012, 28, 7967-7975. 34. Gardas, R. L.; Ge, R.; Manan, N. A.; Rooney, D. W.; Hardacre, C. Fluid Phase Equilib. 2010, 294, 139-147. 35. Aske, N.; Kallevik, H.; Sjoblom, J. Energy Fuels. 2001, 15, 1304. 36. Nezhad, E. R.; Heidarizadeh, F.; Sajjadifar, S.; Abbasi, Z. J. Pet. Eng. 2013, DOI: 10.1155/2013/203036. 37. Haj-Kacem, R. B.; Ouerfelli, N.; Herráez, J. V.; Guettari, M.; Hamda, H.; Dallel, M. Fluid Phase Equilib. 2014, 383, 11-20. 38. Gangopadhyay, D.; Ganguly, B. N.; Mukherjee, T.; Dutta-Roy, B. Chem. Phys. Lett. 2000, 318, 161-167. 39. Ghannam, M. T.; Esmail, N. J. Pet. Sci. Eng. 2005, 47, 105-115. 40. Torres, L. G.; Iturbe, R.; Snowden, M. J.; Chowdhry, B. Z.; Leharne, S. A. Colloids Surf. A. 2007, 302, 439-448. 41. Huang, X.; Garcia, M. H. A. J. Fluid Mech. 1998, 374, 305−333. 42. Barnes, H. A. Appl. Rheol. 2000, 10, 248−253. 43. Jian,

C.;

Tang,

T.;

Bhattacharjee,

S.

Energy


Fuels,

2013,

27,

2057-67.

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Energy & Fuels

Table 1: Physicochemical properties of the heavy crude oil (HCO)+. No. Composition Density API Water Surface of at 298.15 gravity content tension at S Ar R As trials (%) (%) (%) (%) K (%) 353.15 K (kg/m3) (mN/m) 1 25.3 63.4 5.1 6.2 0.9974 2 25.2 67.6 3.5 3.7 0.9970 3 28.6 63.5 3.8 4.2 0.9972 Avg. 26.4 64.8 4.1 4.7 0.9972 + S: Saturates; Ar: Aromatics; R: Resins; A: from Oil India Limited, Assam, India.

Interfacial tension with pure water at 353.15 K (mN/m) 10.6 3.853 56.23 21.32 10.2 3.797 56.29 21.35 10.4 3.822 56.27 21.36 10.4 3.824 56.26 21.34 Asphaltenes. SARA and density data was obtained

The standard uncertainties are u(density) = 0.001 kg/m3, u(surface/interfacial tension) = 0.03 mN/m, u(water content) = 0.001 %. Table 2: List of synthesized ionic liquids for this study. Cation

Anion

N

N

+ C6H13

N

N

Abbreviation

[Cl]-

1-butyl-3-methylimidazolium chloride

[BMIM]+[Cl]-

[Br]-

1-butyl-3-methyl imidazolium bromide

[BMIM]+[Br]-

[HSO4]-

l-butyl-3-methylimidazolium hydrogensulfate l-hexyl-3-methylimidazolium bromide

[BMIM]+[HSO4]-

[HSO4]-

l-hexyl-3-methylimidazolium hydrogensulfate

[HMIM]+[HSO4]-

[Cl]-

1- octyl-3-methylimidazolium chloride

[OMIM]+[Cl]-

+ C4H9

Name

[Br]-

[HMIM]+[Br]-

+ C8H17

N

N


Energy & Fuels

10000 0.1 MPa 5 MPa 10 MPa Viscosity (cP)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 41

1000

100

10 300

320

340

360

380

Temperature (K) Figure 1: Effect of temperature and pressure on the viscosity (at shear rate of 1.0 s-1) of the standard heavy crude oil.

30

ACS Paragon Plus Environment

Page 31 of 41

Crude Oil + Crude Oil+[BMIM] [Br] + Crude Oil+[HMIM] [Br] + Crude Oil+[BMIM] [HSO4]

1000 Viscosity (cP)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

+

-

Crude Oil+[HMIM] [HSO4] +

100

-

Crude Oil+[BMIM] [Cl] + Crude Oil+[OMIM] [Cl]

10 300

320

340

360

380

Temperature (K) Figure 2: Effect of ionic liquids on the viscosity of the heavy crude oil as a function of temperature at 0.1 MPa.

31


Energy & Fuels

100

100 Applied stress is more than the yield stress

Shear stress (Pa)

Applied stress is less than the yield stress 298.15 K 323.15 K 348.15 K 373.15 K

10

298.15 K 323.15 K 348.15 K 373.15 K

10

1

1 1

10

100

1

1000

10

100

1000

-1

-1

Shear rate (s )

Shear rate (s )

(b)

(a) 100

Shear stress (Pa)

Shear stress (Pa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 41

298.15 K 323.15 K 348.15 K 373.15 K

10

1 1

10

100

1000

-1

Shear rate (s )

(c) Figure 3: Plots of shear stress vs shear rate for standard (a) HCO, (b) HCO+[BMIM]+[Cl]and (c) HCO+[OMIM]+[Cl]- sample as a function of different temperature at the pressure of 0.1 MPa.

32


Energy & Fuels

1000

100

100 G' and G" (Pa)

1000

10 G' (10 MPa) G' (05 MPa) G' (0.1 MPa)

1

G" (10 MPa) G" (05 MPa) G" (0.1 MPa)

0.1

10

G' (10 MPa) G' (05 MPa) G' (0.1 MPa)

1

G" (10 MPa) G" (05 MPa) G" (0.1 MPa)

0.1 1

10

100

1

10

Strain (%)

100

Strain (%)

(a) 298.15 K

(b) 323.15 K

1000

1000 G' (10 MPa) G' (05 MPa) G' (0.1 MPa)

G" (10 MPa) G" (05 MPa) G" (0.1 MPa)

G' (10 MPa) G' (05 MPa) G' (0.1 MPa)

100 G' and G" (Pa)

100 G' and G" (Pa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

G' and G" (Pa)

Page 33 of 41

10

1

G" (10 MPa) G" (05 MPa) G" (0.1 MPa)

10

1

0.1

0.1 1

10

1

100

10

100

Strain (%)

Strain (%)

(c) 348.15 K (d) 373.15 K Figure 4: Strain-sweep measurements (G' and G") of heavy crude oil samples at various temperatures and pressure of 0.1, 5, 10 MPa. G' (solid symbols) and G" (open symbols).

33


1000

100

100 G' and G" (Pa)

1000

10

1 G' (10 MPa) G' (05 MPa) G' (0.1 MPa)

0.1 1

G" (10 MPa) G" (05 MPa) G" (0.1 MPa)

10

Page 34 of 41

10

1

G' (10 MPa) G' (05 MPa) G' (0.1 MPa)

0.1

100

1

G" (10 MPa) G" (05 MPa) G" (0.1 MPa)

10

-1

100 -1

Frequency (rad s )

Frequency (rad s )

(a) 298.15 K

(b) 323.15 K

1000

1000 G' (10 MPa) G' (05 MPa) G' (0.1 MPa)

G" (10 MPa) G" (05 MPa) G" (0.1 MPa)

10

1

0.1 1

10

G' (10 MPa) G' (05 MPa) G' (0.1 MPa)

100 G' and G" (Pa)

100 G' and G" (Pa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

G' and G" (Pa)

Energy & Fuels

G" (10 MPa) G" (05 MPa) G" (0.1 MPa)

10

1

0.1

100

1

-1

Frequency (rad s )

10

100 -1

Frequency (rad s )

(c) 348.15 K (d) 373.15 K Figure 5: Frequency-sweep measurements (G' and G") of heavy crude oil samples at various temperatures and pressure of 0.1, 5, 10 MPa. G' (solid symbols) and G" (open symbols).

34


Page 35 of 41

G' and G" (Pa)

1000 G' (10 MPa) G" (10 MPa) G' (05 MPa) G" (05 MPa) G' (0.1 MPa) G" (0.1 MPa)

100

10

1 300

320

340

360

380

Temperature (K) Figure 6: Effect of temperature (298.15 to 373.15 K) on G′ (solid symbols) and G″ (open symbols) for HCO systems at 0.1, 5 and 10 MPa.

1000

1000

100

100 G' and G" (Pa)

G' and G" (Pa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

10 G' (10 MPa) G' (05 MPa) G' (0.1 MPa)

1

G" (10 MPa) G" (05 MPa) G" (0.1 MPa)

10

1

G' (10 MPa) G' (05 MPa) G' (0.1 MPa)

G" (10 MPa) G" (05 MPa) G" (0.1 MPa)

0.1

0.1 1

10

1

100

10 Strain (%)

Strain (%) +

(a) [BMIM] [Cl]

-

(b) [OMIM]+[Cl]-

35


100

1000

100

100 G' and G" (Pa)

1000

10 G' (10 MPa) G' (05 MPa) G' (0.1 MPa)

1

G" (10 MPa) G" (05 MPa) G" (0.1 MPa)

Page 36 of 41

10 G' (10 MPa) G' (05 MPa) G' (0.1 MPa)

1

G" (10 MPa) G" (05 MPa) G" (0.1 MPa)

0.1

0.1 1

10

1

100

10

+

(c) [BMIM] [HSO4]

(d) [HMIM]+[HSO4]-

-

1000

100

100 G' and G" (Pa)

1000

10 G' (10 MPa) G' (05 MPa) G' (0.1 MPa)

1

100

Strain (%)

Strain (%)

G' and G" (Pa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

G' and G" (Pa)

Energy & Fuels

G" (10 MPa) G" (05 MPa) G" (0.1 MPa)

10 G' (10 MPa) G' (05 MPa) G' (0.1 MPa)

1

G" (10 MPa) G" (05 MPa) G" (0.1 MPa)

0.1

0.1 1

10

1

100

10 Strain (%)

Strain (%)

(f) [HMIM]+[Br](e) [BMIM] [Br] Figure 7: Strain-sweep measurements (G' and G") of heavy crude oil samples at 298.15 K and pressure of 0.1, 5, 10 MPa. G' (solid symbols) and G" (open symbols). +

-

36


100

Energy & Fuels

1000

100

100 G' and G" (Pa)

1000

10

1

G' (10 MPa) G' (05 MPa) G' (0.1 MPa)

0.1 1

G" (10 MPa) G" (05 MPa) G" (0.1 MPa)

10

10

1 G' (10 MPa) G' (05 MPa) G' (0.1 MPa)

0.1 100

1

(a) [BMIM] [Cl]

Frequency (rad s )

-

(b) [OMIM]+[Cl]1000

100

100 G' and G" (Pa)

1000

10

1 G' (10 MPa) G' (05 MPa) G' (0.1 MPa)

0.1 1

100 -1

Frequency (rad s ) +

G" (10 MPa) G" (05 MPa) G" (0.1 MPa)

10

-1

G' and G" (Pa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

G' and G" (Pa)

Page 37 of 41

G" (10 MPa) G" (05 MPa) G" (0.1 MPa)

10

10

1 G' (10 MPa) G' (05 MPa) G' (0.1 MPa)

0.1 1

100

G" (10 MPa) G" (05 MPa) G" (0.1 MPa)

10

100 -1

Frequency (rad s )

-1

Frequency (rad s )

(c) [BMIM]+[HSO4]-

(d) [HMIM]+[HSO4]-

37


1000

1000

100

100 G' and G" (Pa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

G' and G" (Pa)

Energy & Fuels

10

1 G' (10 MPa) G' (05 MPa) G' (0.1 MPa)

0.1 1

G" (10 MPa) G" (05 MPa) G" (0.1 MPa)

10

10

1 G' (10 MPa) G' (05 MPa) G' (0.1 MPa)

0.1

100

Page 38 of 41

1

-1

10

100 -1

Frequency (rad s ) +

G" (10 MPa) G" (05 MPa) G" (0.1 MPa)

Frequency (rad s )

-

(e) [BMIM] [Br] (f) [HMIM]+[Br]Figure 8: Frequency-sweep measurements (G' and G") of heavy crude oil samples at 298.15 K and pressure of 0.1, 5, 10 MPa. G' (solid symbols) and G" (open symbols).

38


Page 39 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

(a) HCO

(b) HCO+[BMIM]+[Cl]-

(c) HCO+[OMIM]+[Cl]-

(d) HCO+[BMIM]+[HSO4]-

(e) HCO+[HMIM]+[HSO4]-

(f) HCO+[BMIM]+[Br]-

(g) HCO+[HMIM]+[Br]Figure 9: Surface morphology (Microscopic images) of the standard HCO and HCO+IL systems at 298.15K (each image has been captured at uniform magnification of 100X ,subsequently after stirring for 15 min, followed by few minutes of stabilization).

39


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 10: Proposed mechanism showing the interaction of the ionic liquids with the asphaltene/resin/ aromatic components of the heavy crude oil. Black colored structure represents the asphaltene/resins/aromatics and green colored structure denotes the ILs.

40


Page 40 of 41

Page 41 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Graphical Abstract:

Proposed mechanism showing the interaction of the ionic liquids with the asphaltene/resin/ aromatic components of the heavy crude oil. Black colored structure represents the asphaltene/resins/aromatics and green colored structure denotes the ILs.

41


Effects of Imidazolium-Based Ionic Liquids on the Rheological

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