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Experimental Investigation of the Effects of DifferentParameters on the Rate of Asphaltene Deposition inLaminar Flow and Its Prediction Using Heat TransferApproachFarhad Salimi a , Mohsen Vafaie Seftie a & Shahab Ayatollahia ba Chemical Engineering Department , Tarbiat Modares University , Tehran , Iranb Enhanced Oil Recovery (EOR) Research Center and Shiraz University , Shiraz , IranAccepted author version posted online: 14 Jun 2013.

To cite this article: Journal of Dispersion Science and Technology (2013): Experimental Investigation of the Effects ofDifferent Parameters on the Rate of Asphaltene Deposition in Laminar Flow and Its Prediction Using Heat Transfer Approach,Journal of Dispersion Science and Technology, DOI: 10.1080/01932691.2013.763729

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Experimental Investigation of the Effects of Different Parameters on the Rate of Asphaltene Deposition in Laminar Flow and Its Prediction Using Heat Transfer

Approach

Farhad Salimi1, Mohsen Vafaie Seftie1,, Shahab Ayatollahia2

1Chemical Engineering Department, Tarbiat Modares University, Tehran, Iran,

2Enhanced Oil Recovery (EOR) Research Center and Shiraz University, Shiraz, Iran

Received 8 December 2012; Accepted 3 January 2013.

Address correspondence to Mohsen Vafaie Seftie, Chemical Engineering Department, Tarbiat Modares University, Tehran, Iran. E-mail: vafaiesm@modares.ac.ir

Abstract

In this study, asphaltene deposition from crude oil on the pipe surface has been studied

experimentally using a novel designed test loop. Washing technique is used to

quantitatively measure the rate of asphaltene deposition during laminar flow in the steel

pipe. The effects of oil velocity, asphaltene content and surface temperature on the

thickness of asphaltene deposition are investigated. The results show that the asphaltene

deposition rate increases with increasing surface temperature, results in asphaltene

content reduction of the flowing crude oil. As the oil velocity increases, less deposition

was noticed on the surface of the pipe. Besides, thermal approach was applied to the

experimental procedure which shows good agreements between the predicted thickness

and the measured value from the test loop.

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KEYWORDS: asphaltene, deposition, pipeline, Washing Method, laminar flow,

1. INTRODUCTION

Flow assurance is referred to a technique to assess the ability of production facilities to

transfer multi-phase fluids from the reservoir to the market. The fluid behavior is tested

for any possible interruption during this fluids flow process. Flow assurance includes

factors such as asphaltene and wax deposition, hydrate formation, scale, slugging and

corrosion1. Asphaltenes deposition insides the oil reservoirs and production facilities is

known as the main flow assurance problem in the oil industry.

Generally, asphaltenes are regarded as part of the crude oil that is insoluble in normal

alkanes such as pentane and heptane but soluble in aromatics such as toluene and

benzene. Also Asphaltenes are known as highest molecular weight and most polar oil

components2-4.Asphaltenes composition, structure and stability are dependent on its

source, and the type of solvent used for the extraction of oil5-9.Furthermore, the

absorption and deposition of asphaltenes on steel surface would restrict oil flow in the

transportation pipelines10, 11.The remediation of asphaltene is very costly which limits the

production design of many asphaltenic crude oil reserves9, 12.

In many cases, the potential of organic solids depositions force the field managers to rely

mostly on the chemical and mechanical remediation methods13-17. Therefore, better

understanding of the mechanism of solid deposition is required to better design

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treatments, including the effect of pressure, temperature, composition, additives and flow

conditions. These are the most important parameters during flow assurance process.

Literature study on asphaltene deposition reveals that few works on asphaltene deposition

at real pipe conditions have been reported18-21. The aim of this work is a mechanistic

study of asphaltene deposition in laminar flow to investigate the effects of oil velocity,

temperature and asphaltene content on the rate of asphaltene deposition.

2. EXPERIMENTAL PART

2.1. Experimental Procedure

A test loop is used to investigate asphaltene deposition inside the pipe using an Iranian

asphaltenic crude oil. The fractions of saturates, aromatics, resins and asphaltenes in the

oil were obtained from the so-called SARA test depicted in Table 1.

In this work, the deposited asphaltene inside of tube was measured by “washing method”.

For this purpose the tube was rinsed with Heptane first to wash out non-asphaltene

hydrocarbons, where asphaltenic components remained there attached to the wall.In the

next step the deposited materials inside the tube were washed out using toluene and its

weight was measured after evaporating of the toluene.

The following procedure was used to measure the asphaltene fraction of deposited

materials inside the pipe. The collected asphaltenes from the previous step, “washing

method”, was first precipitated with n-heptane at 40:1 volume ratio of n-heptane to

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sample solution. Then the mixture was left to equilibrate for 20 to 24 hours, and finally

the sample was filtered by filter paper (Whatman Grade No. 42). The Filter with

asphaltene was crumpled and placed in a Soxhlet apparatus and refluxed with n-heptane

for 2 hours. The deposited material retained on the filter paper was considered as

asphaltene after drying process. Table 1

2.2. Experimental Apparatus

Figure 1 shows schematic view of the novel designed flow assurance test loop which has

been used to measure the thickness of asphaltene deposition as a function of time at

different condition. The apparatus is made of a well-controlled temperature bath

containing long stainless steel tube in coil shape. The temperature of the bath was

maintained constant using heat source, controlling unit and stirrer. The long test tube was

equipped with accurate pressure transducers and thermocouples at several intervals;

transferring all the information into a data acquisition system. The feed was prepared and

transferred into feed storage and its temperature was maintained at pre-set temperature

prior to flow through the pump into the flowing loop.

For this study, the pipe test section was made of1m length stainless steel tube (seamless,

Fitok Company)with 3.74mm inside diameter, which was coiled and placed inside the

bath. The bulk temperature of the oil is measured with K-type thermocouples which are

located in the tank and in mixing chambers before and after of the test section. The

temperature of the bath was controlled within ±0.1oC,holding the stainless steel pipe. The

absolute pressure at the outlet of the tube was controlled with back pressure control

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regulator (model of BP-66). The oil flow rate was controlled by the constant rate pump.

A data acquisition system is used to monitor the temperature at various point of both bath

and tube.Figure 1

3. RESULTS AND DISCUSSION

The results are tabulated in Table 2. These eight set of tests were designed to check the

effects of oil velocity and surface temperature on the rate of asphaltene deposition. This

procedure (Washing Method) could be used to find the deposition rate by carefully

monitoring the temperature difference between the outlet and inlet and also bath

temperature. The results for the asphaltene deposition rates are presented and discussed in

the following sections. Also the analytical tests of the deposited materials show that the

amount of asphaltene in the deposited layer is significant.

3.1.Concentration Measurement Of Flocculated Asphaltene

The first step to study the mechanisms of asphaltene deposition is to determine the

concentration of flocculated asphaltene particles in the oil at specified temperatures. One

of the methods used to measure the amount of precipitated asphaltenes due to the solvent

is scaling method. Mathematical correlations of this method is very simple and do not

need the oil specification. This method was first proposed by Rassamdana et al22. Their

results showed that all asphaltene titration curves of the dead oil by solvents of normal

alkane correlate into a single curve. This function is as follows:

2 30 1 2 3Y A A X A X A X= + + + (1)

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Parameters A0 to A3 are constant and indicate the scaling coefficients. The three main

variables of titration of dead oil curves are the weight percentage of precipitated

asphaltene W, the solvent to oil dilution ratio Rm and the molecular weight of the solvent.

Rassamdana et al23 lumped these three parameters into two variables X and Y of the

scaling equation:

mz z

m

R WX and YM R ′= = (2)

Adjustable parameters are z and z’ which must be carefully tuned to find the best fitting

of the experimental data. They suggested Z’ = -2 and Z = 0.25 in spite of oil and type of

precipitant material used. In recent years, several investigators have verified the scaling

model23-26.Ashoori et al (2010) modified the scaling equations and considered the scaling

variables X and Y as follows26:

( )n Z ZV W t VX R / T .M and Y W / R ′= = (3)

The exponent n is a constant and its value is between 0.10 and 0.25. Two other constants,

Z and Z', are the same as the first scaling equation, i.e. Z=0.25 and Z'=−2.

The measured mass of precipitated asphaltene as a function of the volumetric dilution

ratio (n-heptane/oil) for several temperatures in this work has been shown in figure 2.

The mass of asphaltene deposition versus dilution volume ratio found here is based on

work of Ashoori et al26. Figure 2

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3.2. Mechanism Of Asphaltene Deposition

The published works show that no comprehensive model to describe the effect of

operating conditions on the mechanism of asphaltene deposition in the pipes is

available27. Broseta et al28 (2000) and Wang et al (2004)19 investigated asphaltene

deposition in a capillary tube. Jamialahmadi et al (2009) investigated the mechanisms of

deposition of flocculated asphaltene under forced convective conditions and turbulence

condition21. Results showed that the rate of asphaltene deposition increases with

increasing flocculated asphaltene concentration and temperature while it decreases with

increasing oil velocity.

There are likely several steps in deposition process for asphaltenes: precipitation,

flocculation, surface contact and adhesion18. Asphaltenes could be directly absorbed on

the solid surfaces if no precipitation has been occurred however, it is expected that for

this case the amount of absorption is negligible29. To predict asphaltene deposition

accurately one needs to understand the fundamental mechanism for each step. Fouling in

the heat transfer cases can be used to make an analogy for the asphaltene

deposition/release mechanism that was presented by Kern and Seaton30 (1959). The net

rate of growth of asphaltene deposit is the difference between the rate of deposited of

material md and the rate of its removal mr expressed as follows:

d rdm m mdt

= − (4)

3.3. Effect Of Asphaltene Concentration

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One of the main reasons of asphaltene deposition is the concentration of flocculated

asphaltene in the flowing oil. So long as the removal rate can be ignored and all particles

arriving at the heat transfer surface are deposited, the rate of deposition may be generally

expressed as:

( )nd t Asm k C= (5)

where CAS is the flocculated asphaltene concentration at the surface conditions. For mass

transfer and surface deposition controlled processes n generally varies between 1 and 2.

Equation 5 shows that the concentration has a strong effect on deposition, regardless of

the mechanism of deposition. The effect of asphaltene concentration on rate of deposition

at a Reynolds numberabout1000 and constant bulk and bath temperature is shown in

Fig.3. The results indicate that the increase of asphaltene concentration lead to enhance

the rate of asphaltene deposition. The results indicate that there are a linear relation

between rate of asphaltene deposition and asphaltene concentration and the best n for

fitting �� versus concentration of asphaltene is approximately equal to 1.2.

3.4. Effect Of Wall Temperature

To study the effect of wall temperature on asphaltene deposition, three different tests at

different wall temperatures were performed at constant oil velocity, bulk temperature and

asphaltene concentration. The rate of asphaltene deposition is plotted at different wall

temperature in Figures 4 where shows that the deposition thickness increases at higher

wall temperature.

3.5. Effect Of Oil Velocity

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In this section, the effect of velocity on the deposition thickness in the range of 0.23 to

0.67 m/s is discussed. Figure 5 shows the rate of asphaltene deposition for different oil

velocities which were measured using the “Washing Method”. This clearly indicates that

the deposition rate is decreased significantly when the oil velocity is increased. Figures 3,

4 and 5

4. THE MODEL

Fouling method was used to predict the rate of asphaltene deposition on pipe surface, and

an equation was developed. The models describing fouling usually are based on the well-

known concept of Kern and Seaton (1959) approach where the net fouling rate is the

difference between the rates of deposition and removal: Fouling Rate=Rate of deposition-

Rate of removed

According toEquation17, it can be put the fouling model in Kt coefficient(overall transfer

coefficient) and used it for predicting of deposition. Soitcan bedoneas follows:

Kt =Rate of deposition-Rate of removed

The type of the description of the deposition and removal terms is the basic differences

between various models reported in literature. The rate of deposition is described by

either a transport-reaction model or reaction alone model while the rate of removal is

described either by shear-related or mass-transfer related expressions. The first term of

the right-hand side of this expression is dependent on both the surface temperature and

also rate of transport the particles from the fluid bulk toward the wall.

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The rate of fouling increases exponentially with increasing surface temperature for

almost all fouling mechanisms31-34. which is generally expressed by an Arrhenius-type

equation.

However, the rate of particle transport from balk toward wall depends on the type of flow

regime. In this work, regime of flow is laminar and the Sieder–Tate correlation35 is used.

Rate of deposition is obtained by substituting of terms includes wall temperature and rate

of particle transport as follows:

Rate of deposition (Re. Pr (D/L))1/3 (µ/µw)0.14 * exp(Ea/RTW)

Both length and diameter are constant and do not change during experiments, so the

equation can be simplified as follows:

Rate of deposition = Kd*(Re. Pr)1/3* exp(Ea/RTW)

where Kd is constant and can be determined from the experimental data. As it was already

described, there are two proposed mechanisms for the removal rate which are expressed

as follows:

1. The rate of removal depends on shear-related or velocity flow which is expressed

as follows: Rate of removed=Kremoval*τw

2. The rate of removal instead of being dependent on the shear stress is affected

bytherate of mass-transfer which was already proposed by Polley et al36. Rate of

removed=Kremoval*Ren

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Which Kremoval and n constants have been obtained by curve-fitting experimental data in

final model. After substituting equations in equations, following equations are obtained:

( )1

1.23 exp ad e r removal W Ab

W

Em k R P K CRT

τ

= − (6)

( )1/3 1.2exp nad e r removal e Ab

W

Em k R P K R CRT

= −

(7)

Results of curve-fitting will tabulate in table 3. Results in table 3 indicate that the

equation 7 is better than the equation 6 for fitting of data. It seems that curve-fitting is

better when the rate of removal is dependent on rate of the rate of mass-transfer. Table 3

5. CONCLUSIONS

In the present study, after carefully verifying of the use of “Washing Method” for the

measurements of asphaltene deposition thickness, the effects of operating parameters

such as oil velocity and pipe surface temperature on the deposition process in asphaltenic

crude oil in a tube was investigated. The results showed that the deposition rate was

increased as the surface temperature and asphaltene concentration were increased. The

experimental results also indicate that the deposition rate is inversely proportional to the

oil velocity and the thickness of deposited components was decreased as the oil velocity

was increased. Besides, fouling model was employed to predict value of asphaltene

deposition from the test loop and good agreement between the predicted thickness and

the measured value was noticed.

ACKNOWLEDGMENTS

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The authors would like to express their sincere gratitude to NPF Co (Mr. Tohidi and Mr.

Moazed), for their kind collaboration and help during the installation and setup of the

apparatus used in this study.

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31. Eaton, P. and Lux, R. (1984) ASME HTD., 35: 33-42

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TABLE 1. Analysis of SARA

Fractions %wt

Asphaltenes 13.84

Resins 13.46

Saturates 30.31

Aromatics 42.4

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Table 2. A summary of the experimental results

Test

Number

Casph Toil1 Tbath Velocity Time Mass of

deposit(kg/m2)

%

Asphaltene

(gr/cm3) (oC) (oC) (m/s) (hr) (Washing

method)

in

the deposit

Test 1 1.5 51.5 70 0.23 70 0.4 63.3

Test 2 1.5 52.5 70 0.51 70 0.22 82

Test 3 1.5 50 70 0.67 70 0.118 87

Test4 3 50 70 0.27 52 0.144 83.3

Test5 5 50 70 0.17 47 0.208 85

Test6 5 57 70 0.333 40 0.094 85

Test 7 1.5 57 80 0.333 24 0.103 84

Test 8 1.5 57 90 0.333 23 0.21 87

1Average oil temperature of the input and output in the pipe

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TABLE 3. Results of curve-fitting

Constants %Absolute Average Error

Equation 6

Kd=148 m/s

Kremoval=4.23*10-8 .

mPa s

EaR

= 7788.6 K

38

Equation 7 Kd=-139266m/s

Kremoval=-101.59 m/s

EaR

= 10291.9 K

n=-3.267

13.9

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FIG.1. Schematic of the experimental apparatus.

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FIG.2. Variation of mass of asphaltenes precipitation with dilution ratio at various

temperatures.

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FIG.3. Variation of asphaltene deposition rate with flocculated asphaltene concentration.

(Re=1000, Tbath=70C, Tbalk=50C)

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FIG.4. Variation of asphaltene deposition rate with wall temperature(v=0.33m/s,

Tbalk=57C).

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akot

a St

ate

Uni

vers

ity]

at 0

1:55

08

Sept

embe

r 20

13

ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT 22

FIG.5. Variation of asphaltene deposition rate with velocity(Tbath=70C,Tbalk=50C).

Dow

nloa

ded

by [

Nor

th D

akot

a St

ate

Uni

vers

ity]

at 0

1:55

08

Sept

embe

r 20

13