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Determining the Particle Size ofAsphaltenesS. Ashoori a , M. Jamialahmadi a , S. H. Müller b , M. Fathi a , Kh.Qorbani Neshaghi a , A. Abedini a & H. Malakkolahi aa Petroleum University of Technology, Faculty of Petroleum, Ahwaz,Iranb University of Stuttgart, Stuttgart, Germany

To cite this article: S. Ashoori , M. Jamialahmadi , S. H. Müller , M. Fathi , Kh. Qorbani Neshaghi , A.Abedini & H. Malakkolahi (2012) Determining the Particle Size of Asphaltenes, Petroleum Science andTechnology, 30:16, 1639-1646, DOI: 10.1080/10916460903394193

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Petroleum Science and Technology, 30:1639–1646, 2012

Copyright © Taylor & Francis Group, LLC

ISSN: 1091-6466 print/1532-2459 online

DOI: 10.1080/10916460903394193

Determining the Particle Size of Asphaltenes

S. ASHOORI,1 M. JAMIALAHMADI,1 S. H. MÜLLER,2

M. FATHI,1 KH. QORBANI NESHAGHI,1 A. ABEDINI,1

AND H. MALAKKOLAHI1

1Petroleum University of Technology, Faculty of Petroleum, Ahwaz, Iran2University of Stuttgart, Stuttgart, Germany

Abstract Study of heavy organics and their behavior is of great importance whileoccurrence of their deposition in production and processing of hydrocarbon fluids.

The physical properties of heavy organics, especially asphaltenes, have been a subjectof controversy for several years. The aim of the present work is to determine and

measure particle size of asphaltenes. Several mixtures of crude oil and n-heptane wereprepared with various dilution ratios (Rv). Two different techniques were employed

to determine and measure the particle size of asphaltenes. The first was utilizing anOLYMPUS BX60, a polarizing microscope with an appropriate magnification, and

the second was using high-resolution scanning electron microscopy (SEM) and X-raydiffraction techniques. The results revealed that the size of asphaltene particles is in

the range of 1–4 �m for each mixture, regardless of its n-heptane content. Althoughthe quantity of deposited asphaltene was increased by increasing the concentration of

n-heptane in mixture, the particle size of asphaltene was independent of the n-heptane

concentration.

Keywords asphaltene, dilution ratio, particle size, polarizing microscope, scanningelectronic microscope, X-ray diffraction

1. Introduction

Crude petroleum can be represented by three major fractions: oils, asphaltenes, andresins. Oils are mixtures of saturated hydrocarbons and aromatics of moderate molecularweight. Asphaltenes are operationally defined as the nonvolatile and polar fraction ofpetroleum that is soluble in benzene and toluene but insoluble in alkanes (i.e., pentaneand heptane) at room temperature. This property is used for extraction of asphaltenes. Thechemical structure and physicochemical properties of asphaltenes are not well understood.Several theories, models, and studies have been reported for this problem but because ofthe fuzzy nature of asphaltene and the large number of parameters affecting asphalteneprecipitation, none of them can express the properties of asphaltene and/or mechanisms ofits precipitation adequately (Speight and Moschopedis, 1981; Mansoori, 1997; Vazquezand Mansoori, 2000).

Particle size of asphaltene has been another area of uncertainty, with early estimatesranging from 30 to 100 Å. Ray et al. (1957) reported a particle size of 30–40 Å using anultra-centrifuge. It seems that strong centrifugal force may cause asphaltene precipitation

Address correspondence to Siavash Ashoori, Department of Petroleum Engineering, PetroleumUniversity of Technology, Kut Abdolla, Ahwaz, Iran. E-mail: ashoori39@yahoo.com

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and aggregation. Katz and Beu (1945) postulated that the particles must be less than 65Å, and Espinat and Ravey (1993) utilized a small-angle neutron and X-ray scatteringmethod to analyze different asphaltenic suspensions. They found that asphaltenes areflat aggregates whose size and molecular weight mainly depend on the solvent and thetemperature. As temperature increases, one can notice a decrease of the particle size ofasphaltene. It is important to note that at �27ıC, the asphaltene particle diameter was 200Å, whereas at 77ıC it 100 Å. Addition of n-heptane induces an increase in the particlesize of asphaltene. At 0% concentration of n-heptane the particle size of asphaltene was130 Å, whereas at 40 wt% concentration of n-heptane the reported value was 200 Å forparticle size of asphaltene precipitated. Leontaritis et al. (1994) explained that the processof asphaltene flocculation appears violent and random. During flocculation, the asphaltenemicelles rearrange by losing (or gaining) resin molecules to form larger entities, some ofwhich are larger than 1,000 Å in diameter as determined from filtration measurements.Asphaltene micelle size distributions before asphaltene flocculation are expected to beabout 20 to 350 Å. After flocculation, asphaltene particles larger than 1,000 Å are formed,as deduced from filtration measurements. The amount of suspended asphaltene particleswith a diameter greater than 1,000 Å increases with the severity of asphaltene flocculation.They expressed that such a dramatic size increase in the asphaltene particles explains therock permeability impairment. Rassamdanna and Sahimi (1996) concluded that the sizeof the asphaltene particles is very large, ranging from about 1,600 Å to about 7,200 Å.They believed that particle size is, in general, a time-dependent quantity that evolves asaggregation of the asphaltene particles takes place.

2. Experimental

2.1. Selection and Preparation of Crude Oil

The fluid of one oilfield in the southwest of Iran with an asphaltene content of approxi-mately 8.3% was selected. The crude oil under study was relatively heavy tank oil with20 ıAPI. The crude was stored under laboratory conditions for up to 3 months, to allowany volatile components to evaporate, in order to provide a stable condition and to obtaina crude oil with relatively fixed composition. The oil had to be filtered to remove itssolid content. For filtering the crude, a piece of standard filter paper (Whatman 42) wasplaced on a Buchner funnel and the crude was poured on it. Because the crude was veryviscous, the filtration process was very slow. To solve this problem, a vacuum (about0.45 bar suction) was applied.

2.2. Particle Size Determination

Particle sizes of asphaltenes were determined in the following two ways.Different mixtures of oil and n-heptane were prepared, with 10, 20, 30, 40, 50, 60,

and 80% of n-heptane. Measurement of the particle sizes of precipitated asphaltenes sus-pended in the solutions was accomplished with an Olympus BX60 polarizing microscopewith 1,000-fold magnification. Figure 1 shows a photograph of this microscope.

Deposits of asphaltenes were collected from different mixtures of oil and n-heptanewith various dilution volume ratios Rv . To characterize and measure the particle sizes ofthe precipitated asphaltenes, these solid samples were analyzed by using a high-resolutionscanning electron microscope (SEM) and X-ray diffraction techniques.

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Determining the Particle Size of Asphaltenes 1641

Figure 1. Photograph of the Olympus BX60 polarizing microscope.

The former set of measurements with SEM was produced using samples preparedby centrifugation of the oil mixtures at 10,000 rpm. The effect of centrifugation resultedin large crystalline formations of asphaltene that were in the size range 3–500 �mand therefore much larger than those that would be expected in a homogeneous finesuspension. The particle size distributions obtained for optical microscope observationswere much finer (1–4 �m) and plate-like and are believed to be much smaller aggregatesof the primary asphaltene particles. Clearly, the effect of the agitation of the samplesis very profound and the particle sizes produced by streams of n-heptane and crude oilare believed to give rise to particles much closer in size to those observed in opticalmicroscope samples. The results of the two sets of particle size measurements arepresented later.

3. Determination of Particle Size of Precipitated Asphaltene

3.1. Optical Microscope Studies

Different mixtures of oil and n-heptane were prepared in order to determine the particlesize of asphaltene precipitated in a particular mixture. In these mixtures the concentrationof n-heptane varied as 10, 15, 20, 25, 30, 40, 50, 60, 80, and 95% by volume. Themixtures were left in a dark place for about 6 hr and measurement was accomplishedwith a microscope with a magnification of 1,000�.

Visual observations showed no deposits of asphaltenes for mixtures of 10, 15, and20% n-heptane concentration. The minimum concentration of n-heptane at which solidparticles of asphaltene were observed was 25 vol% equivalents to a dilution volumeratio Rv of 0.3. In other words, the dilution volume ratio at the onset of asphalteneprecipitation was obtained as 0.3. The deposits of asphaltenes were observed clearly forother mixtures (40, 50, 60, 80, and 95%) above the precipitation onset.

The results showed that the size of particles was in the range 1–4 �m for eachmixture, regardless of its n-heptane content. The quantity of deposited asphaltenes in-

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Figure 2. Microscopic image of asphaltene particles obtained from mixture at 40% concentration

of n-C7.

creased with the increase in concentration of n-heptane, whereas the particle size wasindependent of the n-heptane concentration in each mixture. Figures 2 through 5 depictthe results for mixtures of 40, 50, 60, and 80% n-c7 concentration, respectively.

3.2. SEM Measurements

To characterize and measure the size of particles of precipitated asphaltene, anothertechnique was also employed. The deposits of asphaltenes were collected from different

Figure 3. Microscopic image of asphaltene particles obtained from mixture at 50% concentration

of n-C7.

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Determining the Particle Size of Asphaltenes 1643

Figure 4. Microscopic image of asphaltene particles obtained from mixture at 60% concentration

of n-C7.

mixtures of the oil and n-heptane with various dilution volume ratios, Rv . These solidsamples were analyzed using high-resolution SEM.

The crystalline particles of asphaltene produced by centrifugation are much larger insize due to the strong agitation. It is believed that the particles formed by contacting feedstreams of n-heptane and crude oil are much smaller and not in this crystalline form.They are much more likely to be in the form of loose aggregates, as seen in Figures 6and 7. These figures show the SEM photographs for asphaltene solid particles obtainedfrom the mixtures with dilution volume ratios of 1 and 5, respectively.

Figure 5. Microscopic image of asphaltene particles obtained from mixture at 80% concentration

of n-C7.

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Figure 6. SEM photograph of asphaltene particles obtained from mixture with dilution volume

ratio of Rv D 1.

Figure 7. SEM photograph of asphaltene particles obtained from mixture with dilution volume

ratio of Rv D 5.

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Determining the Particle Size of Asphaltenes 1645

Figure 8. Typical X-ray analysis for asphaltene particles obtained from mixture with Rv D 1.

Figure 9. Typical X-ray analysis for asphaltene particles obtained from mixture with Rv D 5.

3.3. X-Ray Diffraction Studies of Elemental Analysis

Elemental analysis of deposits was made by X-ray diffraction with the aim of distin-guishing asphaltene particles from possible sand particles or clay minerals containedin oil during production. Figures 8 and 9 illustrate the X-ray analysis results for theparticulate matter obtained from the mixtures of n-heptane-to-oil dilution volume ratiosof 1 and 5, respectively. According to these results, all sediments consist of asphaltene.

4. Conclusions

This study was conducted to determine the particle size of asphaltenes. Two distinct setsof experiments were carried out to investigate and determine particle size of asphaltenes.Based on the obtained results, the following conclusions were made:

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1. The solid particles of precipitated asphaltene fall in the size range of 1–4 �m for eachmixture, regardless of n-heptane (precipitant) concentration.

2. Asphaltene solid samples were analyzed using high-resolution SEM and a polarizingoptical microscope. The size distribution of loose aggregates produced by gentleswirling action in the preparation of the optical microscope samples was observedto be in the size range 1–4 �m. The conditions for precipitation in the floodingpractices in porous media appear to give rise to similar or smaller size aggregates.

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