10
CONDUCTIVIT LECTRIQUE DES SOLUTIONS D’ASPHALTØNES Les interactions des asphaltnes avec leur environnement molculaire dans des solutions modles ont t tudies par la mesure de la conductivit lectrique de ces solutions. Les interactions avec le n-heptane, des rsines, des tensioactifs, l’eau, le phnol et le chlorure de sodium ont t explores. Les conclusions tires de cette tude ont confirm certaines hypothses faites sur les mcanismes d’agrgation des asphaltnes en solution, en particulier dans le cadre de la thorie des solutions collodales. STUDY OF ASPHALTENE SOLUTIONS BY ELECTRICAL CONDUCTIVITY MEASUREMENTS The asphaltene interactions in model solutions were studied using a technique based on the electrical conductivity measurement. Interactions with n-heptane, resins, surfactants, water, phenol and NaCl were investigated. The conclusions drawn from this study confirmed previous opinions on aggregation mechanism of asphaltenes in solutions. They confirmed also the interpretation of asphaltene behaviour in terms of colloidal solution theories. CONDUCTIBILIDAD ELCTRICA DE LAS SOLUCIONES DE ASFALTENOS Se ha procedido al estudio de las interacciones de los asfaltenos con su entorno molecular en soluciones modelo, por la medicin de la conductibilidad elctrica de stas. Tambin se han explorado las interacciones con el n-heptano, las resinas, los tensioactivos, el agua, el fenol y el cloruro de sodio. Las conclusiones que se derivan de este estudio han venido a confirmar ciertas hiptesis establecidas acerca de los mecanismos de agregacin de los asfaltenos en solucin, y bsicamente, situndose en el marco de la teora de las soluciones coloidales. REVUE DE L’INSTITUT FRANÇAIS DU PÉTROLE VOL. 53, N° 1, JANVIER-FÉVRIER 1998 41 STUDY OF ASPHALTENE SOLUTIONS BY ELECTRICAL CONDUCTIVITY MEASUREMENTS N. HASNAOUI, C. ACHARD and M. ROGALSKI* Laboratoire de thermodynamique et d’analyse chimique 1 E. BÉHAR Institut français du pétrole 2 (1) Universit de Metz, 57 045 Metz Cedex - France (2) 1 et 4, avenue de Bois-Prau, 92852 Rueil-Malmaison Cedex - France * Corresponding author

Study of Asphaltene Solutions by Electrical Conductivity

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Page 1: Study of Asphaltene Solutions by Electrical Conductivity

CONDUCTIVIT� �LECTRIQUE DES SOLUTIONSD'ASPHALTéNES

Les interactions des asphalt�nes avec leur environnementmol�culaire dans des solutions mod�les ont �t� �tudi�es par lamesure de la conductivit� �lectrique de ces solutions. Lesinteractions avec le n-heptane, des r�sines, des tensioactifs, l'eau,le ph�nol et le chlorure de sodium ont �t� explor�es. Lesconclusions tir�es de cette �tude ont confirm� certaineshypoth�ses faites sur les m�canismes d'agr�gation desasphalt�nes en solution, en particulier dans le cadre de la th�oriedes solutions collo�dales.

STUDY OF ASPHALTENE SOLUTIONS BY ELECTRICALCONDUCTIVITY MEASUREMENTS

The asphaltene interactions in model solutions were studied usinga technique based on the electrical conductivity measurement.Interactions with n-heptane, resins, surfactants, water, phenol andNaCl were investigated. The conclusions drawn from this studyconfirmed previous opinions on aggregation mechanism ofasphaltenes in solutions. They confirmed also the interpretation ofasphaltene behaviour in terms of colloidal solution theories.

CONDUCTIBILIDAD EL�CTRICA DE LAS SOLUCIONESDE ASFALTENOS

Se ha procedido al estudio de las interacciones de los asfaltenoscon su entorno molecular en soluciones modelo, por la medici�nde la conductibilidad el�ctrica de �stas. Tambi�n se han exploradolas interacciones con el n-heptano, las resinas, los tensioactivos, elagua, el fenol y el cloruro de sodio. Las conclusiones que sederivan de este estudio han venido a confirmar ciertas hip�tesisestablecidas acerca de los mecanismos de agregaci�n de losasfaltenos en soluci�n, y b�sicamente, situ�ndose en el marco dela teor�a de las soluciones coloidales.

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STUDY OF ASPHALTENESOLUTIONS BY ELECTRICALCONDUCTIVITYMEASUREMENTS

N. HASNAOUI, C. ACHARD and M. ROGALSKI*Laboratoire de thermodynamique et d’analyse chimique1

E. BÉHARInstitut français du pétrole2

(1) Universit� de Metz,57 045 Metz Cedex - France

(2) 1 et 4, avenue de Bois-Pr�au,92852 Rueil-Malmaison Cedex - France

* Corresponding author

bihorel
copyright C 1998, Institut Français du Pétrole
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INTRODUCTION

The asphaltenes are one of the most polar parts of thecrude oil. The polyaromatic core of asphaltenescontaining nitrogen, sulfur, oxygen, vanadium andnickel strongly interacts with the external electricalfield. The resulting migration of asphaltenes leads to theelectrical conductivity as was first reported by Lichaaand Herrera (1975).

The mechanism of this migration is not completelyexplained up to this day. Recently Fotland et al. (1993)proposed to determine asphaltene precipitation with atechnique based on measurement of electricalconductivity of the crude oil. It was demonstrated thatthe abrupt change of the electrical conductivityoccurring during precipitation phenomena made itpossible to determine not only the precipitation pointbut also the relative amounts precipitated.

In this study we used electrical conductivity ofasphaltenes to characterise their molecular interactionsin model solutions. Asphaltenes were obtained fromasphalts derived from various crude oils and furnishedby the Institut français du pétrole. Solutions wereprepared by dissolving the asphaltenes in polar solvents.The high solvent polarity increases the electricalconductivity of asphaltenes and facilitates measurements.

Measurements were carried on with tetrahydrofuranand nitrobenzene. Both compounds are excellentsolvent for asphaltenes.

1 EXPERIMENTAL SECTION

1.1 Experimental set-up

The apparatus is shown on Figure 1. Measurementswere performed in a glass sample cell (a) of the volumeV = 250 cm3 equipped with a magnetic stirrer (b). Thetemperature was maintained constant to within ± 0.1 °Cby a double jacket surrounding the cell and connectedto the Lauda RM6 thermostat (c). A platinum resistanceprobe (PT 100) with a Hewlett-Packard digitalmultimeter (34401 A) was used to measure the celltemperature (d). The conductivity measurements werecarried out using an electrode immersed in the solutionand connected to a Tacussel CDM 92 conductimeter(e). The working frequency of this apparatus rangedfrom 94 Hz to 50 kHz (conductivity from 10 nS ×cm-1 to

Figure 1

Experimental setup (a: sample cell; b: magnetic stirrer; c: thermostat; d: multimeter; e: conductimeter; f: piston buret;g: PC computer).

6 S × cm-1 when using a cell constant of 1 cm-1) andenabled the conductivity studies of both low and highconductivity solutions. An automatic piston burette(Schott T90/20) was used to modify the initial quantityof the solvent (f). The piston burette, the multimeter, theconductimeter and the magnetic stirrer were connectedwith the four serial ports of a PC computer via thestandard RS-232 interface (g). Measurement controland data acquisition was automated via the computer.

1.2 Experimental procedure

A known mass of asphaltenes was dissolved in 50 mlof solvent. The solution was stirred duringapproximately 3 hours up to the stability of themeasured conductivity was obtained. The measurementwas performed every 2 seconds during 1 minute afterthe stirring was stopped. The mean value was retainedand compared with the new conductivity measurementobtained after the next stirring-measuring cycle. Next,the solvent was introduced into the cell (0.1-2.5 ml) byan automatic piston buret and the measurement wasperformed as described above. Usually the experimentwas stopped when 50 ml of solvent was added. Allexperiment was controled by PC computer.

a

b

c

d

e f

g

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2 RESULTS

2.1 Conductivity of asphaltenes innitrotoluene and in tetrahydrofuran

Measurements were performed with solution ofasphaltenes in nitrotoluene (initial concentration wasCasph = 11.3 g/l) and in tetrohydrofuran (Casph = 7.27 g/l).Results obtained are presented in Figure 2. The specificconductivity presented in this figure is defined as theratio of the conductivity to the concentration ofasphaltenes in the solution. The specific conductivity innitrotoluene is less important than in THF in spite of thehigher polarity of the former. This can be attributed tothe higher viscosity of nitrotoluene. In both cases thespecific conductivity is sharply increasing in dilutedsolutions. This phenomenon corresponds probably tothe dissociation of asphaltene aggregates. It can besupposed that the limiting conductivity valuecorresponds to monomeric species. Active sites ofasphaltenes are the most accessible in the monomericform and become shielded when the aggregation rate

increases. They are supposed to be responsible for theelectrical conductivity. Therefore in more concentratedsolutions where asphaltenes are more aggregated asmaller specific conductivity is observed. Consequently,analysing the conductivity curve during dilution canafford arguments to modelling the aggregationmechanism of asphaltenes.

2.2 Conductivity of asphaltenes + resins in nitrotoluene

The initial solution was prepared with 0.5654 g ofasphaltenes and 0.1547 g of resins in 50 ml ofnitrotoluene. Therefore, the concentration in asphaltenesand resins were respectively 10.13 g/l and 3.24 g/l.Results obtained are given in Figure 3 together withresults obtained previously without resins. The patternof both curves is very similar but the conductivity inpresence of resins is significantly lowered. Probablyresins are bound to certain active sites. It can beobserved that the limiting conductivity remainsunchanged. Therefore asphaltene-resin complex

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1.10

1.20

1.30

1.40

1.50

1.60

1.70

1.80

1.90

2.00

0.00 2.00 4.00 6.00 8.00 10.00 12.00

Casphaltene (g/l)

Spe

cific

con

duct

ivity

(S

.cm

2 /g a

spha

ltene

) .1

04

Figure 2

Specific conductivity of asphaltenes versus asphaltene concentration in: (D) THF solution, (❏) nitrotoluene solution.

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dissociates at high dilution. The influence of resins onthe conductivity confirms the previous conclusionsconcerning the relationship conductivity/aggregation.The comparison of both curves lead to the quantitativeinterpretation to these results. In fact, supposing that theloss of the conductivity can be attributed to theasphaltene-resin binding the following relationships canbe written:

mads = Casp – C(asp+res) (1)

The mass of asphaltenes bound with resins mads iscalculated as a difference of asphaltenes concentrationsCasp – C(asp+res) (in g per liter of solvent) determined atthe same solution conductivity.

Xads = mads/Cres (2)

Xads is a mass ratio of bound asphaltenes and of thetotal quantity of resins present in the solution. The plotof Xads versus asphaltene concentration is given inFigure 4. After an initial abrupt rise in the range of

diluted solutions Xads remains almost constant forasphaltene concentrations higher than 4 g/l. This valueis close to the CMC of asphaltene in toluene (3.24 g/lat 298.2 K) as reported by Andersen and Speight(1993). It is clear from Figure 4 that approximately 1.9 g of asphaltenes is bound with 1 g of resins atconcentration higher than 4 g/l.

2.3 Flocculation of asphaltenes fromtetrahydrofuran solutions with n-heptane.

Initial solutions of asphaltenes in tetrahydrofurancontaining respectively 2.58, 5.19 and 13.6 g/l ofasphaltenes were diluted with n-heptane. Thecorresponding curves are presented in Figure 5. Theirshape is very similar to this obtained by Fotland et al.(1993) with crude oil conductivity. The characteristicslope changes of the conductivity curve which wasattributed by Fotland et al. to the beginning of theflocculation process occurs at the volumetric ratio of n-heptane/THF being 0.30, 0.29 and 0.27 respectivelyfor three solutions studied. Therefore, the beginning of

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0.80

0.90

1.00

1.10

1.20

1.30

1.40

1.50

0.00 2.00 4.00 6.00 8.00 10.00 12.00Casphaltene (g/l)

Spe

cific

con

duct

ivity

(S

.cm

2 /g a

spha

ltene

) .1

04

Figure 3

Specific conductivity of asphaltenes versus asphaltene concentration in (D) nitrotoluene, (❏) nitrotoluene + resins.

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STUDY OF ASPHALTENE SOLUTIONS BY ELECTRICAL CONDUCTIVITY MEASUREMENTS

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1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2

2 3 4 5 6 7 8 9 10 11

Casphaltene (g/l)

Xad

s (g

asph

alte

ne/g

resi

ne)

Figure 4

Asphaltene-resin binding in function of asphaltene in THF concentration.

0

0.5

1.0

1.5

2.0

2.5

0 1 2 3 4 5 6Casphaltene (g/l)

Spe

cific

con

duct

ivity

(S

.cm

2 /g a

spha

ltene

) .1

04

Figure 5

Flocculation of asphaltenes from THF solution with n-heptane.

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flocculation is nearly insensitive on the initialasphaltene concentration and occurs at almost the samen-heptane/solvent ratio. This observation is importantfor the comprehension of the flocculation mechanism.

2.4 Flocculation of asphaltenes from tetrahydrofuran/water solutionswith n-heptane

The knowledge of interactions of water withasphaltenes is an important topic. In fact, the importantpolarity and the hydrophobicity of asphaltenes letexpect the existence of complex entropy drivenstructures. THF is an ideal solvent for this studydissolving both water and asphaltenes.

The mass of 0.260 g of asphaltenes was dissolved in60 ml of solvent containing 50 ml of THF and 10 ml ofwater (Casph = 4.33 g/l). This solution was diluted withn-heptane. Results are presented on Figure. 6. Thepresence of water increases the dielectric constant ofthe solutions and makes rise its conductivity. The curvebreak corresponding to the beginning of flocculationoccurs at the n-heptane/solvent ratio of 0.32.Surprisingly the presence of water in the solvent makes

the solution more stable in regards of flocculation(previously for the same amount of asphaltenes the ratio0.29 was obtained). Moreover, the comparison of theshape of both curves indicates that the amount ofasphaltenes flocculated is less important in this case(for details of argumentation see Fotland et al., 1993).Consequently, it can be thought that water-asphaltenesinteractions lead to structures involving bothcomponents and increasing their stability in thesolution. However, the solution seems to be metastableand flocculates with time.

2.5 Flocculation of asphaltenes fromtetrahydrofuran/water/surfactantsolutions with n-heptane

This experiment was carried out in the same way asthe former one. A small quantity of an anionicsurfactant (SDS, 2.8 10-5 mole/l) was added to themixed solvent (50 volume parts of THF and 10 volumeparts of water). Resulting conductivity curve is given inFigure 6. In this case the conductivity changes withconcentration nearly linearly. The small curve break isobserved at concentration corresponding to n-heptane/

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0

0.5

1.0

1.5

2.0

2.5

1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

Casphaltene (g/l)

Spe

cific

con

duct

ivity

(S

.cm

2 /g a

spha

ltene

) .1

04

Figure 6

Flocculation of asphaltenes with n-heptane from: (D) THF-water solution, (❏) THF-water-SDS solution.

Page 7: Study of Asphaltene Solutions by Electrical Conductivity

solvent ratio of 0.52. Therefore, about 40% more of n-heptane is needed to initiate the asphalteneprecipitation. Moreover, the analysis of this curve shapeindicates that the quantity of asphaltenes deposited issignificantly smaller than in the former case.

2.6 Conductivity of the system(asphaltenes + THF) + (H2O + Phenol)

The influence of water on the asphaltene stability insolution encouraged us to continue studies on thisproblem. In the present experiment a solution ofasphaltenes dissolved in THF was diluted either withwater either with the solution of phenol in water (9.66 gof phenol per liter of water).

In the later case phenol was used to follow theavailability of binding sites of asphaltenes. This methodwas proposed by Barbour and Petersen (1974) tocharacterise the basicity of asphalt and can be applied

to asphaltenes also. It can be considered that theamount of bound phenol expresses the basicity ofasphaltenes and the number of available binding sites.

Following systems were studied:A: asphaltenes + THF + water + phenolB: asphaltenes + THF + waterC: THF + water + phenolD: THF + waterConductivity of systems C and D were determined as

reference for A and B.In the case of systems A and B measurements were

performed for three initial concentrations of asphaltenesbeing respectively 3.23, 4.50 and 8.76 g/l. In Figure 7conductivity curves obtained in function of water orwater/phenol solution added are presented. Thecontribution of phenol and THF to the totalconductivity is small as is evident from C and D curvesbehaviour. The main contribution to the conductivity isdue to the presence of asphaltenes. The increase of the

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0

1

2

3

4

5

6

7

8

9

0 10 20 30 40 50 60

Volume of water (ml)

Con

duct

ivity

S/c

m)

,106

Figure 7

Conductivity of the system (asphaltene + THF) + (water + phenol)

◆ THF + water + phenol + asphaltenes (3.23 g/l) ● THF + water + phenol + asphaltenes (8.76 g/l)

✧ THF + water + asphaltenes (3.23 g/l) ❍ THF + water + asphaltenes (8.76 g/l)

▲ THF + water + phenol + asphaltenes (4.50 g/l) ■ THF + water + phenol

D THF + water + asphaltenes (4.50 g/l) ❏ THF + water

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dielectric constant of the solvent gives rise to the abruptincrease of the conductivity. When the initial volume ofsolvent is doubled the conductivity becomes nearlyconstant. In spite of the important water content noasphaltene deposition was observed duringexperiments. The phenol presence diminishes, asexpected, asphaltene conductivity in the case of lowerconcentration of asphaltenes (3.23 g/l). This is probablydue to the asphaltene-phenol binding. At higherconcentration this phenomenon is no more observed. Itcan be explained by increasing auto-aggregation ofasphaltenes but also by a probable formation ofasphaltenes/water/THF structures involving active sitesof asphaltenes.

2.7 Conductivity of the systemasphaltenes + H2O + NaCl

The strong interactions of ions with colloidal speciesand their influence on aggregation and flocculationprocesses justify a study of asphaltene-ion systemconductivity.

In the present experiment two asphaltene solutions inTHF with concentration 3.8 g/l and 9.4 g/l respectivelywere diluted by the water or aqueous solution of NaCl(3.44 g/l). For each asphaltene concentration two seriesof measurements were performed:

A: asphaltenes + THF + NaCl/H2OB: asphaltenes + THF + H2OThe total conductivity of A is composed of the

contributions of NaCl and of asphaltenes. We define thedifference:

c(NaCl/Asph) = cA – cB (3)

as the conductivity due essentially to the salt. Thecorresponding specific conductivity is given by:

L(NaCl/Asph) = c(NaCl/Asph)/c(NaCl/Asph) (4)

with c(NaCl/Asph) being the concentration of the salt in thesolution.

The third series of measurements

C: THF + NaCl/H2O

was performed to determine L(NaCl), the conductivitypattern of NaCl without asphaltenes. The correspondingspecific conductivity was defined as in Equation 4. InFigure 8 L(NaCl/Asph) and L(NaCl) are plotted in functionof salt concentration. The observed decrease of the

NaCl conductivity can be attributed to the NaCl-asphaltene binding. Therefore, it can be assumed thatthe mass of the bound salt is given by expressiondefined at the constant specific conductivity:

mads(NaCl) = c(NaCl) – c(NaCl/Asph) (5)

Amount of bound salt in function of saltconcentration is reported in Figure 9. In the range ofdiluted salt solutions this amount is small and nearlyconstant when salt composition increases. Moreovernearly the same value is observed with both asphalteneconcentrations. This behaviour changes for saltconcentrations higher than 0.8 g/l. The amount of thesalt retained increases exponentially with the saltconcentration. This rise is smaller in the case of moreconcentrated asphaltene solution. This is probably dueto a higher auto-aggregation.

CONCLUSION

Measurements of the electrical conductivity wereused to study molecular interactions of asphaltenes insolutions. This technique can be applied to quantitativecharacterising of binding phenomena also.

The most important conclusions concerningasphaltenes behaviour can be summarised as follows:– asphaltenes aggregate in diluted solutions and

this tendency is only slightly dependant on theirenvironment;

– at low concentration (below 3-4 g/l) behaviour ofasphaltenes is significantly different than at higherconcentration; the limit between the two domainscorresponds probably to the CMC;

– at higher concentrations asphaltenes seem to be moresensible to their chemical environment and they areable to form organised structures including polarmolecules;

– water can be involved in the structures stabilisingasphaltenes in solution.

All above formulated opinions should be confirmed byfurther investigations.

ACKNOWLEDGEMENT

The authors thank Institut français du pétrole forfinancial help and for permission to publish the results.

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0

1

2

3

4

5

6

7

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

Spe

cific

con

duct

ivity

(S

.cm

2 /g N

aCl)

.104

CNaCl (g/l)

Figure 8

Specific conductivity of: (❏)THF + NaCl/H2O solution and (D)THF + NaCl/H2O + asphaltene.

0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

CNaCl (g/l)

(gN

aCl/g

asph

alte

ne)

.102

Figure 9

Asphaltene-NaCl binding in function of the salt concentration. The initial concentration of asphaltenes in THF (D) 3.8 g/l, (❏) 9.4 g/l.

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REFERENCES

Achard C, Jaoui C., Schwing M. and M. Rogalski (1996)Aqueous Solubilities of Phenol Derivatives by ConductivityMeasurements, J. Chem. Eng. Data, 41, 504-507.

Andersen S.I. and Speight J.G. (1993) Observation of the CriticalMicelle Concentration of Asphaltenes. Fuel, 72 1343-1344.

Barbour, A. and Petersen J.C. (1974) Molecular Interactions ofAsphalts: An Infrared Study of the Hydrogen-Bonding Basicityof Asphalt. Anal. Chem., 46, 273.

Fotland P., Anfindsen H. and Fadnes F.H. (1993) Detection ofAsphaltene Precipitation and Amounts precipitated byMeasurement of Electrical Conductivity. Fluid Phase Equilibria,82, 157-164.

Lichaa, P.M. and Herrera L. (1975) Electrical and other EffectsRelated to the Formation and Prevention of AsphalteneDeposition in Problem in Venezuelan Crudes. SPE 5304.

Final manuscript received in December 1997

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