Polydisperse Size Distribution of Monomers and Aggregates of Sulfur

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Polydisperse Size Distribution of Monomers and Aggregates of Sulfur-containing Compounds in Petroleum Residue Fractions Zhentao Chen, Junfeng Liu, Yun Wu, Zhiming Xu, Xuxia Liu, Suoqi Zhao, and Chunming Xu Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 29 Jun 2015 Downloaded from http://pubs.acs.org on June 30, 2015

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Polydisperse Size Distribution of Monomers and Aggregates of Sulfur-containing Compounds in Petroleum Residue Fractions Zhentao Chen*, Junfeng Liu, Yun Wu, Zhiming Xu, Xuxia Liu, Suoqi Zhao, Chunming, Xu* Abstract: Dimension of sulfur-containing compounds in residue is crucial to hydrodesulfurization catalyst design. Size distributions of sulfur compounds in fractions of Venezuela atmospheric residue were determined from the bulk-phase diffusion coefficients, which were measured at 298 K by a diaphragm cell by using 200 nm polycarbonate membranes. Sulfur compounds in all fractions show obvious size polydispersity. The size of four narrow SFEF (supercritical fluid extraction and fraction) fractions varies slightly with the concentrations of 1 g/L to 40 g/L. However, maltenes and asphaltenes from the end-cut showed significant variation in size over concentrations of 0.1 g/L to 40 g/L, which indicates a coexistence of various monomers and aggregates. The monomers are dominated in maltenens, whereas aggregates dominated in asphaltenes. The hydrodynamic diameter of sulfur-containing monomers of four SFEF fractions ranges from 0.74 nm to 1.45 nm at concentration of 1 g/L. The size of maltene monomers spans a range of 1.87 nm to 2.29 nm at 0.1 g/L and presents a more significant polydispersity than SFEF fractions. The size variation of the SFEF fractions and maltenes to yields demonstrates a continuous distribution in size for petroleum residue. However, asphaltene aggregates cover the span of diameters from about 4.29 nm to 5.54 nm at concentration of 0.1 g/L and the values reveal larger than that of 1-3 nm in most literatures for asphaltene molecules. The average diameters of asphaltene fractions decreased to 4.02 nm and 3.95 nm at concentrations of 0.05 g/L and 0.03 g/L,

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respectively. It reveals aggregation of asphaltene molecules can occur at concentration lower than 0.1 g/L and a state of coexistence of asphaltene monomers and aggregates at 0.05 to 0.1 g/L. 1. Introduction Growing worldwide energy demand and environmental concern promote production of clean transportation fuels from heavy crudes and residues. These feedstocks are structurally and compositionally complex mixtures and composed of polyaromatic rings, aliphatic chains, and heteroatoms such as sulfur, nitrogen, nickel and vanadium. The presence of numerous sulfur species reduces the quality of the products and poses serious environmental problem. The desulfurization is a major concern of the petroleum industry worldwide. In response to the heavier feedstocks, residue hydrotreating is expected to contribute in a decisive way to worldwide new demands for fuels with improved quality, with the emphasis on minimization of sulfur content. The heavy crudes are characteristically more difficult to process than petroleum distillates because of larger molecule size. Accurate design of the catalyst systems requires knowledge of both dimension of the residue species and their transport properties in the microporous solid. Although the chemical structures of residue and asphaltenes have been extensively studied, there is still some debate about the size of these feedstocks due to their complicated compositions and strong tendency of asphaltenes to aggregate. In spite of crucial importance of the information of residue dimension, there are three primary methods used to obtain average size of asphaltenes. They are direct molecular imaging and molecular diffusion and interfacial property. Direct molecular imaging by both scanning tunneling microscopy and high-resolution transmission electron microscopy shows the bulk of the polycyclic aromatic hydrocarbon ring systems have a length of scale slightly larger than 1.0 nm.1, 2 Unfortunately, the images represent the aromatic ring systems of asphaltenes due to the aromatic portion are readily imaged while the alkanes are not readily observed. Furthermore, it is difficult to distinguish aggregates versus individual structural units because of the heterogeneous nature of asphaltenes. Fluorescence depolarization (FD),3,

4

Fluorescence correlation spectroscopy (FCS)5 and nuclear

magnetic resonance (NMR)5-9 technique were performed on molecule diffusion measurements for 2 Environment ACS Paragon Plus

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asphaltenes and the hydrodynamic dimension of diffusing species was evaluated thereafter. Basic agreement among these methods indicates the average diameters of petroleum-derived asphaltenes range from 1.0 nm to 3.0 nm. Other molecular diffusion measurements including Taylor dispersion10, 11 and membrane diffusion12 have also provided consistent results. The average diameter for asphaltenes adsorbed at the interface has also been estimated from the areas.13, 14 The estimated average radii for asphaltenes at the interface range from 1.4 to 2.2 nm, which are also in agreement with the values mentioned above. Many recent advances are being made in asphaltene science and a consensus of strong tendency for asphaltenes self-association has been obtained. Aggregation of asphaltene molecules has been studied extensively by NMR diffusion,6 centrifugation,15 high-Q ultrasonic spectroscopy,16 and Direct-Current (DC) electrical conductivity.17 Most experimental results to date confirmed that asphaltenes in toluene form nanoaggregates with small aggregation numbers at concentration of roughly 150 mg/L, which is termed as the critical nanoaggregate concentration (CNAC) and used to define the transition from a true molecular solution to a nanoaggregate. It has also been shown that asphaltenes aggregation can start around 50 mg/L.18 More than ten years ago, distinct break points at several g/L in interfacial tension or calorimetric titration measurements have been observed and regarded as critical micelle concentration (CMC) or critical aggregation concentration (CAC) values.19-21 More recently, the formation of clusters of asphaltene nanoaggregates has been detected at concentrations of roughly 2 g/L in toluene by DC conductivity.22, 23 The average diameters of the asphaltene nanoaggregates obtained by forenamed methods mostly ranged from 3 nm to 10 nm.6, 24-30 The asphaltene clusters have larger size than the nanoaggregates.31, 32 Hydrodynamic radii of clusters of light asphaltene fractions are between 5 to 10 nm, whereas a small fraction of asphaltenes is made of larger clusters with radius of around 40 nm.33 Meanwhile, NMR9, 34 and SANS9 reveals the coexistence of big aggregates with smaller ones. Over the years, much attention has been given to asphaltenes and the size of asphaltenes was mostly described in terms of a mean value. As is well known, residue and its fractions are highly complicated 3 Environment ACS Paragon Plus

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systems which contain thousands of different molecules. The molecules behave differently during processes of reaction and transport. Furthermore, previous studies mostly provided static or gyration dimensions. Therefore, hydrodynamic size of the overall residue and its aggregates is crucial to design catalyst suitable for such heavy feeds, especially when deasphalted oil is used as feedstocks for catalytic upgrading processes. In our previous study, size and size distribution of petroleum residue fractions have been obtained by membrane diffusion.35 The results showed that there is an abrupt increase of the size for the end-cut other than continuous increase in the molecular size of residue SFEF fractions with respect to the accumulative yield. Strong tendency of asphaltenes to aggregate suggested that large size of the end-cut results from the aggregation of asphaltene enriched species therein. In the present study, the bulk-phase diffusion coefficients of sulfur-containing species in fractions of Venezuela atmospheric residue were determined by using a diaphragm diffusion cell. High dilute solutions were used in diffusion experiments to avoid interference from molecular aggregation. The aggregation behavior of residue fractions was investigated by increasing the concentrations. As a result, polydisperse size distributions of monomers and aggregates of sulfur-containing compounds were obtained from their diffusivities. 2. Material and methods 2.1. Feedstock preparation Atmospheric residue derived from Venezuela Orinoco heavy crude oil (VAR) was used as feedstocks. The residue was separated into 15 extractable fractions and an unextractable end-cut by the supercritical fluid extraction and fractionation (SFEF) technique. The separation process and operating procedure have been reported elsewhere.36, 37 Normal pentane was used as the supercritical solvent and operated in programming pressure with an increasing gradient of 1.0 MPa/h. A major advantage of SFEF technology is that it can be used to prepare sufficient quantities of narrow fractions of residue for further in-depth studies.


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It is known that properties, such as molecular weight, of SFEF fractions vary gradually with increased SFEF yield. Hence, four narrow SFEF fractions (SFEF-4, SFEF-8 and SFEF-12 and SFEF-15) of the residue were chosen as feedstocks for the diffusion experiments. Furthermore, our previous study show the end-cut has large polydispersity and asphaltenes are enriched in it.35 Therefore, maltenes and asphaltenes were further separated from the end-cut using n-heptane following the procedure of ASTM D6560 and were also performed to diffusion experiments. The SFEF fractions and the further separated maltenes and asphaltenes are all called VAR fractions herein. The residues and the corresponding fractions were subjected to various analyses. The average molecular weights were obtained using the Waters GPC515-2410 unit coupled with a refractive index detector. The elemental analysis was carried out on a Perkin-Elmer CHNS/O Analyzer 2400. Each fraction (∼20 g) was diluted with 0.5 L of toluene to make a stock solution (40 g/L) that was further diluted by using toluene to yield a series of solutions with required concentrations. The solutions were sonicated 1 hour prior to diffusion experiment to ensure complete dissolution and mixture homogeneity. HPLC-grade toluene was obtained from Sigma-Aldrich Chemical and was 99% pure. 2.2. Diaphragm diffusion cell The diffusion experiments were performed using a diaphragm diffusion cell described in detail elsewhere.38 A brief summary of the diffusion cell is given below. The apparatus illustrated in Figure 1 contains two glass chambers clamped together with a membrane between them. Teflon-coated magnetic stirring bars are mounted in the chambers and are driven by the rotation of an external magnet. To maintain a constant temperature, the cell was placed in a thermostatic bath during experiments. The lower and upper chamber hold a volume of 45.9 mL and 55.6 mL, respectively, which were measured by weighing the cell before and after filling with deionized water. Track-etched Nuclepore polycarbonate membranes with nominal pore diameters of 200 nm (Whatman plc.) were used in the diffusion experiments. This type of membranes is extensively used in the study of diffusional transport because of their ideal pore geometry.39-41 2.3. Diffusion coefficient measurements 5 Environment ACS Paragon Plus

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Diffusion coefficient measurements were performed using the diaphragm diffusion cell mentioned above. The method for measuring diffusion coefficient is described in detail elsewhere.38 A brief description is given below. The same procedure for measuring diffusivity was applied in each case. Initially, the lower chamber was filled with solution and the upper chamber with pure solvent. Discrete samples were withdrawn from the upper chamber at appropriate time intervals. Then, an equal volume of pure toluene was added to the upper chamber to keep the volume constant. The diffusion experiment ended when equilibrium was approached. The sulfur contents of the samples and the final solutions in the upper and lower chambers were determined by EA3100 analyser. The concentration in the lower chamber corresponding to the sampling point was determined from mass balance. For dilute solutions of 0.1 g/L, samples withdrawn were condensed to 10-20 times using nitrogen for fast evaporation in a concentration workstation. For solutions below 0.1 g/L, no sample was withdrawn during the diffusion experiment. The final solutions in two chambers were withdrawn and condensed to approximately 100 times using the method mentioned above. The sulfur contents of condensed solution were detected by EA3100 analyser and used to evaluate the average diffusion coefficient. In the pseudosteady state, the flux across the membrane in incremental time dt equals the change in the amount of solute in any chamber. So combining mass balance of two chambers and diffusion flux through membrane pores, one obtains

d S 1 1 (cL − cU ) = − ( + ) D(cL − cU ) , dt l VL VU

(1)

where D is the diffusion coefficient of solute, cL and cU refer to solute concentrations in the lower and the upper chamber of the diaphragm cell, VL and VU are the volumes of the lower and upper chamber, S is the effective diffusion area of the membrane pores and l is the effective diffusion path length. Integrating Eq. 1 between the jth and (j+1)th sampling, we have: ln

cL, j +1 − cU, j +1 cL, j − cU, j

= − β D j ∆t j , j +1 ( j = 0,1, 2) .


(2)

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In this equation, the subscripts j and j+1 refer to the sampling numbers, t is the time of sampling, Dj is the average diffusion coefficient of the solute transporting through the pores between the jth and (j+1)th sampling, and β is the constant of the diaphragm cell and is given by Eq. 3:

β=

S 1 1 ( + ). l VL VU

(3)

An aqueous solution of potassium chloride at 298 K was used to determine β in this study. The method is described elsewhere42-44 and the diaphragm cell constant of 200 nm pore size membrane is evaluated to be 26.9. In order to compare dimensions of different fractions, the average diffusion coefficient was defined as follows: n

∑ (D

i, j

Di =

× mi , j )

j =1

(i=SFEF-4, SFEF-8…asphaltene; j= 1, 2…n),

n

(4)

∑m

i, j

j =1

where Di is the average diffusion coefficient of the ith fraction and mi,j is the amount of the ith fraction through the membrane in the jth interval of sampling.

2.4. Hydrodynamic diameter measurements The Stokes-Einstein equation is often used to extract the hydrodynamic diameter of a solute from its bulk-phase diffusion coefficient at infinite dilution: Db =

κT , 3πη d

(5)

where Db is the bulk-phase diffusion coefficient, κ is Boltzmann’s constant, T is the absolute temperature, η is the solvent viscosity, and d is the diameter for the spherical solute or the equivalent hydrodynamic diameter for nonspherical solute.

3. Results and Discussion 3.1. Properties of the residue and its fractions


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The main properties of VAR and their fractions employed in this work are summarized in Table 1. There is slight variation in the properties of the light fractions, whereas large variation in that of heavy fractions. Molecular weight, density and contaminant contents show steady growth as the SFEF fractions become heavier, whereas H/C atomic ratio has the opposite variation trend. However, the bulk properties of maltene and asphaltene fractions are remarkable different from the SFEF fractions. The molecular weight of maltenes and asphaltenes are significantly larger than the SFEF fractions, and the H/C atomic ratio of maltenes and asphaltenes are much less than that of SFEF fractions. The contaminants, especially nickel and vanadium, are mostly enriched in maltenes and asphaltenes.

3.2. Bulk-phase diffusion coefficient distribution of sulfur compounds in VAR fractions The bulk-phase diffusion coefficients of sulfur-containing compounds in VAR fractions are obtained by using 200 nm pore size membranes. As mentioned above, the diameters of petroleum asphaltene molecules are less than 3 nm. Therefore, the pore size of 200 nm is large enough for residue molecules to diffuse freely through the membrane. Figure 2 depicts the diffusion coefficient distribution of four SFEF fractions for 1 g/L solution and maltene and asphaltene for 0.1 g/L solution. As can be seen from Figure 2, the bulk-phase diffusion coefficients of sulfur compounds in each fraction decrease gradually as the experiment proceeds. The mass spectrum range and maximum peak of SFEF fractions has been detected to increase as they became heavier by electrospray ionization (ESI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS).45 It illustrates that the fractions are complex mixture of a large number of substances with different molecular weight. FD showed that the spherical diameters of UG8 asphaltenes are in the range of 1.2-2.4 nm and the values vary with the wavelength of fluorescence emission.3,

4, 46

These results strongly supported the

distribution of diffusion coefficients. The preferential diffusion of smaller species results in the decrease of the diffusion coefficients as the experiment progresses. As reviewed above, FCS and NMR techniques have been employed to study the asphaltene diffusion. Average diffusion coefficient of asphaltene molecules has been evaluated to be about 2.0-3.5×10-5 cm2/s at infinite dilution in toluene.5-8 Unfortunately, these advanced techniques only obtained an 8 Environment ACS Paragon Plus

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average value for certain asphaltenes. The application of membrane can detect diffusion coefficient distribution of residue fractions. The diffusion coefficients of asphaltene fractions provide consistent results with the literatures mentioned above. The larger diffusivities of SFEF fractions and maltenes seem reasonable since they are lighter than asphaltenes. Comparison of the curves shows that the diffusion coefficients of sulfur-containing compounds decrease as fractions become heavier. It is in agreement with the variation trend of their average molecular weight as listed in Table 1. Furthermore, Figure 2 shows that the experimental time approaching to the diffusion equilibrium increases as fractions become heavier. As known, the Brownian motion of the smaller solute is more active, which results in longer time to attain the diffusion equilibrium for heavier fractions.

3.3. Size distributions of sulfur compounds in VAR fractions Sphere equivalent hydrodynamic diameter of a solute can be deduced from its experimental Db value by using Eq. 5. Hydrodynamic diameter distributions of sulfur-containing compounds in VAR fractions are determined from their Db values (see Figure 2) and plotted as a function of the accumulative yield in Figure 3. The accumulative yield for each fraction is based on the SFEF results and the diffusion experiments (Figure 1), which was described in detail elsewhere.35 There is a continuous increase in the dimensions of residue fractions with respect to the accumulative yield. Comparison of the results clearly showed the diameters of sulfur compound in SFEF fractions increase gradually and presented a continuous size distribution. The diameters of SFEF-4, 8 ,12 and 15 range from 0.74 to 0.79 nm, 0.80 to 0.83 nm, 1.06 to 1.16 nm and 1.36 to 1.50 nm, respectively. However, the size of maltene and asphaltenes increase abruptly. And they present larger polydispersity with size distribution of 1.87-2.29 nm and 4.29-5.54 nm, respectively. Boduszynski et al.47 conducted a comprehensive study of heavy oil composition and they concluded that crude oil composition increased gradually and continuously with regard to molecular weight and heteroatom content. ESI FT-ICR MS provided detailed compositional evidence in support of the Boduszynski model that describes the progression of petroleum composition and structure.48 The size distribution of SFEF fractions 9 Environment ACS Paragon Plus

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demonstrates that petroleum residue is continuous in composition. To be noted, experimental results to date confirmed that asphaltenes in toluene form nanoaggregates at low concentration of roughly 0.1 g/L6, 15-18, 23

and the average diameters of petroleum-derived asphaltenes range from 1 nm to 3 nm.3-6, 9 The

diameters of VAR asphaltenes in Figure 2 are larger than that in most literatures. It indicates the value might represent the size of asphaltene aggregates. In order to investigate size of monomers and aggregates of sulfur compounds in residue, diffusion experiments are carried out over a series of solution concentrations. The size distributions of VAR fractions are obtained and introduced in the following two sections.

3.4. Size distributions of monomers of sulfur compounds in SFEF fractions The dimensions of four chosen SFEF fractions over concentrations of 1-40 g/L are determined by the same method presented in Figure 4. The detailed dimension values and average diameters of sulfur compounds in SFEF fractions are provided in Table A1 of the Appendix. Figure 4 shows that the size of all SFEF fractions follows the similar variation trends under three concentrations. The diameters gradually increase with the accumulative yield. Comparison the results show that there are slightly increase of dimensions for AVR SFEF fractions as the concentrations increase from 1 g/L to 40 g/L. While asphaltenes are generally acknowledged to form nanoaggregates at very low concentrations, no aggregation has been detected for maltene by high-Q ultrasonic spectroscopy49 and resin by DC conductivity17 and nanofiltration50 under the same conditions. Interactions between residue molecules in SFEF fractions are expected to be rather small because of their lower polarity than asphaltenes. The slight variation of dimension deduced no aggregation for sulfur compounds in SFEF fractions over the wide concentration range. On the other hand, enhancement of collision probability between diffusing molecules occurs as the concentrations increase, which leads to an increase in the obstruction effects and thus a decrease of diffusion coefficients. As a result, it seems reasonable that there is a slight increase of dimensions for SFEF fractions with the increase of concentrations. Size of sulfur compounds in SFEF-15 fraction varies more significantly than other three light fractions, which might due to the stronger intermolecular forces of the former. 10 Environment ACS Paragon Plus

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3.5. Size distributions of monomers and aggregates of sulfur compounds in maltenes and asphaltenes While it is well-established that asphaltenes tend to self-associate, the size distributions of asphaltene aggregates remain uncertain. To explore the size distributions of aggregates, diffusion measurements for maltenes and asphaltenes are performed over a wide range of concentrations. The size distributions of these two fractions in varying concentrations were obtained and also presented in Figure 4. The detailed dimension values and average diameters of sulfur compounds in maltenes and asphaltenes are provided in Table A2 of the Appendix. The size distribution of sulfur compounds in maltenes gradually increases from 1.87-2.29 nm to 2.362.89 nm as the concentration increases from 0.1 g/L to 40 g/L. While no aggregation has been found for maltene or resin,17, 49, 50 it should be mentioned that the feedstocks in the literatures were separated from crude oil or extracted by n-pentane. However, the maltenes of this study are obtained by nheptane extraction of ultraheavy oil. Parts of maltene molecules might have slight tendency to aggregate due to their large polarity. Further diffusion experiment shows that the average diameter of maltene fraction is 2.04 nm at 0.03 g/L and 2.06 nm at 0.05 g/L, which approaches to the average value of 2.10 nm at 0.1 g/L. While there might be aggregates in the maltene solution above 0.1 g/L, the monomers dominating over the whole range of concentrations can be deduced by the limited increase of the diameters. In contrast to SFEF fractions and maltene, dimensions of sulfur-containing compounds in asphaltenes increase abruptly and show significant variation over the wide concentration range. The size distribution of asphaltenes ranges from 4.29 to 5.54 nm at concentration of 0.1 g/L, whereas ranges from 6.74 to 10.20 nm at 40 g/L. On the one hand, basic agreement among diverse methods indicates the average diameters of petroleum-derived asphaltene molecules range from 1 to 3 nm.3-6,

9

FD

technique has also shown the molecular sizes of resins and asphaltenes form a continuous distribution with major axis diameters of 1.3 to 2.7 nm.3 On the other hand, recent studies have reported that asphaltenes aggregation can start at around 50 mg/L.16-18 Therefore, it is deduced from Figure 4 that aggregation of sulfur compounds in VAR asphaltenes can occur at concentration lower than 0.1 g/L. It 11 Environment ACS Paragon Plus

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is confirmed by the further diffusion experiments for 0.03 g/L and 0.05 g/L solutions in which the obtained average diameters of asphaltene fraction are 3.95 nm and 4.02 nm, respectively. It indicates that the asphaltene fractions are dominated by monomers at concentration lower than 0.05 g/L and aggregation occurs between 0.05 and 0.1 g/L. These results are in close agreement with reported CNACs in a large number of literatures. In comparison, the average diameter of VAR asphaltene molecules is larger than that of petroleumderived asphaltene determined by advanced instrument techniques.3-6, 9 Asphaltene molecules have been found to expand by approximately factor of 4 when in contact with liquid toluene.51 The solvation effects of toluene might contribute to the larger size value of VAR asphaltenes in this study. Numerous studies have been performed to investigate asphaltene aggregation and have provided consistent results of small asphaltene aggregate number of 2-8.17,

52, 53

Furthermore, a continuous

decrease of diffusion coefficients for asphltenes has been observed at concentrations of 0.3-2.1 g/L,6 which deduced an increase of dimensions. At certain concentrations in toluene, asphaltenes are believed to exist either as molecularly dispersed entities or as oligomers of asphaltene molecules.54 The aggregation propensity increases with an increase in the asphaltene concentration. Addition of more asphaltene significantly causes an increased number of aggregates. Therefore, asphaltenes of VAR go from the monomeric entity to aggregates and the multimers increase in abundance relative to monomers at increasing concentrations. To be noted, each diameter of asphaltenes presented in Figure 4 represents an average value of diffusing species over the monomers and aggregates in solution. Moreover, we caution that, as with EA3100 analyser, it is not the size distribution of residue fractions but rather that of sulfur-containing compounds and aggregates are obtained. Due to the detectability of EA3100 analyser, the minimum concentration of sulfur compounds in residue fractions for size distribution measurement is chosen to be 0.1 g/L. The results of this study show sulfur-containing compounds in asphaltenes can aggregate into small oligomers under the solution concentrations. Further work on diffusivities and size distribution of molecules in asphaltenes at higher dilute concentrations is in progress and will be presented in a subsequent paper. 12 Environment ACS Paragon Plus

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At last, comparison of these results shows that dynamic size and aggregation states are different among the VAR fractions. The size of asphaltenes is more than two times as large as that of SFEF fractions. Furthermore, the former has a larger tendency to aggregate compared to the latter. Dimension and aggregation state of maltenes fall between that of SFEF fractions and asphaltenes. It can be deduced that hindered diffusion of the end-cut (include maltenes and asphaltenens) through small pores is more severe than that of the four SFEF fractions, especially under high concentrations. With respect to residue hydrodesulfurization, a number of large pores are required to provide sufficient channels for easy access of heavy components into the catalyst. Alternatively, selectively remove of maltenes and asphaltenes from residue can benefit the catalytic conversion of petroleum residue.

3.6. Average diameters of VAR fractions The size distribution data has improved the understanding of the polydispersity of residue fractions. The average diameters are crucial to develop quantitative structure/diffusivity correlations for hindered diffusion of residue molecule in porous materials. As a result, the average size of sulfur compounds in each fraction under the lowest concentration was obtained on the base of diffusion experiment, which is described in detail elsewhere.35 Figure 5 shows the variations of average radius of residue fractions as a function of their average molecular weight. To be noted, 1500 Da has been considered as the upper limit to the molecular mass for asphaltenes.55 Asphaltene molecules might aggregate in GPC measurement due to its large value of molecular weight. Therefore, regression analyses of the results for SFEF and maltene fractions in figure 5 are performed and yield the following relationship: rF = 0.039 × M F 0.75 ,

(6)

where rF and MF refer to the average radius and the average molecular weight of residue fractions, respectively. Previously, a power law relation between average size and molecular weight for asphaltenes has been developed.11, 56 Comparison of the results shows that the expression obtained in this study is different from that of the literatures. The size and molecular weight measurements in previous studies are all


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carried out under higher concentrations. Therefore, their expressions represent the relation of asphaltene aggregates instead of molecules.

4. Conclusions In the current study, membrane diffusion measurements were conducted on VAR fractions over a wide range of concentrations. Size distributions of monomers and aggregates of the sulfur-containing compounds in the fractions were evaluated from the diffusion coefficients. The results show that six fractions of VAR are all polydisperse mixture. The size of sulfur compounds in SFEF fractions gradually increases from 0.74 nm to 1.50 nm with respect to the accumulative yield and shows slight variation over a wide range of concentrations. It indicates that the monomers of the sulfur compounds in the narrow SFEF fractions form a continuous distribution in dimensions. The diameters of sulfur compounds in maltenes gradually increase with an increasing concentration, whereas the asphaltenes increase significantly. Combining size variation of sulfur compounds in maltenes and asphaltenes with the literatures, different aggregate degree was deduced for Venezuela maltenes and asphaltenes at concentrations of 0.1-40 g/L. In spite of coexistence of molecules and aggregates, maltenes are dominated by monomers and range from 1.87 nm to 2.89 nm at concentrations larger than 0.1 g/L. However, asphaltenes are dominated by aggregates with the size distribution of 4.29-10.20 nm over concentrations of 0.05-40 g/L and monomers with the average diameter around 4 nm at concentration lower than 0.05 g/L.

Acknowledgements The authors acknowledge the supports by the National Natural Science Foundation of China (NSFC) (No. 21106183, 21476257 and U1463207) and Science Foundation of China University of Petroleum (No. 01JB0195).

Supporting Information Tables showing size distribution and average diameters of sulfur compounds in four SFEF fractions (Table A1) and maltenes and asphaltenes (Table A2), respectively. This material is available free of charge via the Internet at http://pubs.acs.org. 14 Environment ACS Paragon Plus

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Notation c

=

concentration of solution

d

=

diameter of solute

D

=

diffusion coefficient of solute

D

=

average diffusion coefficient of solute

l

=

effective diffusion path length

m

=

amount of the solute

M

=

molecular weight

r

=

radius of solute

S

=

effective diffusion area of the membrane pores

t

=

time of sampling

T

=

absolute temperature

V

=

volume of chamber of diaphragm cell

Greek letters β

=

constant of the diaphragm cell

η

=

solvent viscosity

κ

=

Boltzmann’s constant

b

=

bulk diffusion

i

=

the ith fraction of VAR

F

=

fraction of VAR

j

=

sampling number

L

=

lower chamber

subscript


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=

U

Page 16 of 28

upper chamber

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13

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Table Caption Table 1. Properties of VAR and its fractions. Table A1. Size distribution and average diameters of sulfur compounds in four SFEF fractions. Table A2. Size distribution and average diameters of sulfur compounds in maltenes and asphaltenes.


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Page 22 of 28

Table 1. Properties of the VAR and its fractions. Feeds

Molecular Weight

Density

H/C

Sulfur

Nitrogen

Vanadium

Nickel

(g/cm )

-

(wt%)

(wt%)

ppm

ppm

VAR

764

0.9857

1.42

3.9

0.74

533.59

154.95

SFEF-4

471

0.963

1.55

3.2

0.38

35.22

12.81

SFEF-8

491

0.976

1.55

3.5

0.44

39.93

13.70

SFEF-12

699

1.013

1.46

4.1

0.80

115.05

47.41

SFEF-15

1041

1.068

1.36

4.8

1.31

608.2

82.55

Maltenes

1751

-

1.18

4.9

1.62

713.7

171.6

Asphaltenes

2565

-

1.17

5.1

1.95

1649.6

361.7

3


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Figure caption Figure 1. Schematic diagram of diaphragm diffusion cell apparatus. Figure 2. Diffusion coefficients of sulfur-containing compounds in VAR fractions through 200-nm pore size membranes. Figure 3. Size distribution of sulfur-containing compounds in VAR fractions as a function of the accumulative yield. Figure 4. Size distribution of monomers and aggregates of sulfur-containing compounds in VAR fractions. Figure 5. Average radius of sulfur compounds in VAR fractions as a function of average molecular weight.


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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

Fig 1. Schematic diagram of diaphragm diffusion cell apparatus. 304x282mm (96 x 96 DPI)

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Page 24 of 28

Page 25 of 28

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Fig 2. Diffusion coefficients of sulfur-containing compounds in VAR fractions through 200-nm pore size membranes. 157x83mm (300 x 300 DPI)


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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

Fig 3. Size distribution of sulfur-containing compounds in VAR fractions as a function of the accumulative yield. 157x83mm (300 x 300 DPI)


Page 26 of 28

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Fig 4. Size distribution of monomers and aggregates of sulfur-containing compounds in VAR fractions. 157x83mm (300 x 300 DPI)


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Fig 5. Average radius of sulfur compounds in VAR fractions as a function of average molecular weight. 207x144mm (300 x 300 DPI)


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Polydisperse Size Distribution of Monomers and Aggregates of Sulfur

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