Energy & Fuels 1994,8, 567-569
567
A Comparison of the Macrostructure of Ratawi Asphaltenes in Toluene and Vacuum Residue D. A. Storm,* E. Y. Sheu, M. M. DeTar, and R. J. Barresi Texaco R&D, P.O. Box 509,Beacon, New York 12508 Received October 4,1993. Revised Manuscript Received January 10,1994"
Combining results from small-angle X-ray scattering, small-angle neutron scattering, and shear rheology studies we find that the average unsolvated asphaltenic colloidal particle in Ratawi vacuum residue has an apparent molecular weight of -100 000 amu and volume of -150 000 A3. The asphaltenic colloidal particles are solvated by the maltenes in vacuum residue. The average solvated particles has an apparent molecular weight of approximately -300 000 amu and volume of -450 000 A3.
Introduction Asphaltenes have received considerable study over the years since they have been implicated in many refining problems.lJ Although it is easy to accept that asphaltenes exist after they have been precipitated from the vacuum residue, some question their existence as a distinguishable species in the vacuum residue. In other words, one could question whether properties measured for asphaltenes in solvents are really relevant to the chemical and physical properties of the vacuum residue. This is a very important question in petroleum chemistry. One way to provide at least some information about this point is to see whether there are properties of vacuum residue that depend an asphaltene concentration in a systematic manner. Previous work with vacuum residua has shown both that the viscosity of the residua depends on asphaltene concentration in a systematic manner3t4 and that the presence of scattering centers in small-angle X-ray scattering experiments(SAXS) depend on the presence of the asphaltenes.5 In fact, these scattering centers are the same size as those observed for the corresponding asphaltenes in deuterated toluene according to small-angle neutron scattering (SANS) studiesm6 One interpretation of these results is that asphaltenes are colloidal particles in the vacuum residue. Colloidal particles, although small, are macroscopic in comparison to the molecules which form the surrounding medium, and so they behave as though they were a distinguishable species. A caveat should be made, however. The referenced experiments are not able to distinguish between the cases where the whole asphaltenic mixture caused the exhibited behavior (all the asphaltenic molecules were colloidal particles), or where only an unidentified subfraction of the heptane insoluble asphaltenes caused the exhibited behavior. Abstract published in Advance ACS Abstracts, March 1, 1994. (1) Speight, J. G. The Chemistry and Technology of Petroleum, 2nd ed.; Marcel Dekker, Inc.: New York, 1991. (2) Storm,D.A.;Edwards,J.C.;DeCanio,S.J.;Sheu,E.Y.Symposium on Resid Upgrading. Prepr.-Am. Chem. SOC.,Diu.Pet. Chen. 1993, 0
457-459. . -. . ...
(3) Storm, D. A.; Barresi, R. J.; Decanio, S. J. Fuel 1991, 70,779-782. (4) Storm, D. A.; Sheu, E. Y. Fuel 1993, 72, 233-237. (5) Storm, D. A.; Sheu, E. Y.; DeTar, M. M. Fuel 1993, 72, 977-981. (6) Sheu, E. Y.; Storm,D. A.; DeTar, M. M. J. Non-Cryst.Solids 1991, 131-133, 341-347.
0887-0624/94/2508-0567$04.50/0
It is one of the purposes of this paper to show that the physical nature, or macrostructure, of Ratawi asphaltenes is essentially the same in toluene as it is in vacuum residue. This result implies that toluene can be a model solvent in which to study asphaltenes. There is an important reservation, however. Asphaltenes are colloidal particles in both environments, and these colloidal particles are solvated by their respective mediums. The amount of solvation depends on the temperature. The size of the solvation shell is smaller in toluene than in vacuum residue at the same temperature, a fact that suggests that the forces holding the solvating molecules to the asphaltenic colloidal particle are less strong in toluene. Finally, we combine results from the rheological and scattering studies and estimate the "molecular weight" and size of the solvated asphaltenic colloidal particle in Ratawi vacuum residue; we estimate that the unsolvated particle has an apparent molecular weight of approximately 100 000 m u , and a solvated volume a t 93 "C of approximately 400 000 A3.
Experimental Section Asphaltenes were precipitated from Ratawi vacuum residue by mixing one part of vacuum residue with 40 parts of heptane and stirring overnight at room temperature. Deasphalted oil (DAO)was prepared from the nonasphaltenicliquid by removing the heptane. Samplesof syntheticvacuum residue were prepared by dispersing appropriate amounts of vacuum residue into the DAO, as described previously? These samples were used for the rheological studies3v4and for the SAXS experiments6described previously. The heptane-insoluble asphaltenes were dissolved in toluene for the rheological measurements,' and in deuterated toluene for the small-angle neutron scattering measurements: also described previously. Briefly, viscosities for the asphaltene/toluene and vacuum residue samples (natural and synthetic) were measured with Couttee-type viscometers. All samples behaved as Newtonian fluids. The SANS experiments were conducted at room temperature with the asphaltene/deuteratedtoluene mixtures on the time of flight small-anglediffractometerat Argonne National Laboratory. The scattering wave vectors were in the range of 0.007-0.35 A-l. The small-angle X-ray scattering experiments (SAXS) were conducted with the Ratawi vacuum residue and (7) Sheu, E. Y.; DeTar, M. M.; Storm, D. A. Fuel 1991,70,1151-1156.
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568 Energy & Fuels, Vol. 8,No. 3, 1994
samples of synthetic vacuum residue at 93 O C on the 10-m spectrometerat Oak Ridge National Laboratory. The scattering wave vectors in this case were 0.1425A-1. The scatteringdata was analyzed in the two cases by applying the constraintproposed by Sheu.a 0
e" .6
The shear rheology of suspensions of solid particles interacting by short-range repulsive interactions is quite well understood. Basically, the relative viscosity can be written as a power series in the fraction of the volume (4) occupied by the solid particles as follows:
+
+ k,f#? + k243 + ...
[?&I
(1)
where [73 is the intrinsic viscosity and kl and kz are constants. It is well-known that 171 is 2.5 for spheres. If there is a distribution of spheres with different sizes, eq 1can in effect be summed to yield? qr =
(1 - 91-2.6
(2)
In the case of asphaltenes, however, we do not know the volume they occupy in the vacuum residue, and so if we write
4=KWA
(3)
where K is a solvation constant and W Ais the weight fraction of asphaltenes; we have after substituting eq 3 into eq 2: 7,
= (1 - KWA)-'.'
(4)
We note that in the dilute limit eq 4 reduces to the Einstein equation where the apparent "intrinsic Viscosity" is 2.5 K. An important point to note is that eq 4 allows one to test if the particles are spherical, solvated, and polydispersed. The exponent 2.5 arises in the derivation given by Roscoes because the particles are spherical, and the functional form arises because these is a polydispersity of sizes. A parameter K different from p l p ~indicates solvation. Although eq 4 has theoretical justification, one might be uncomfortable that so much information will be obtained by the fit to a single equation. It was recently shown, however, that the rheological description of vacuum residue provided by eq 4 is consistent with that provided by several independent rheologicaltheories that have been proposed to describe the viscosity of suspensions of spherical particles.4 The SAXS or SANS scattering intensities are given for a dilute sample of identical particles in wave vector space as
1(Q)= N ( A P ) ~ V ~ ( F ~ )
(5)
where Q is the magnitude of the scattering wave vector, N i s the number of identical particles per unit volume, Ap is the contrast between the particles and the surroundings, V , is the volume of the particle, and ( F $ ) is the form factor for that particular shaped particle averaged over all orientations. (8)Sheu, E. Y . Phys. Reu. A 1992,45, 2428-2438, (9) Roscoe,R.Br. J. Appl. Phys. 1952,3, 267-269.
.El "I
Theory
7, = 1
1
1
.4 0
.2
I
I
.05
.1
I
.15
I
I
.2
.25
n .3
WA Figure 1. Relative viscosity versus asphaltene concentration according to eq 4: (B) in vacuum residue at 93 "C,( 0 )in toluene at room temperature. Table 1. Rheological and Size Parameters for Ratawi Asphaltenes*
[rllK2 K1 K2 R. % PD toluene 6.8 2.7 2.7 32.4 17.6 VR 7.3 2.9 2.9 33.8 15.4 a K1 obtained from eq 4. Kz obtained from eq 1 in dilute limit assuming spheres. asphaltenein
In the simplest case, where all the particles have the same shape and size, one can deduce the shape and size of the particles by fitting the observed intensity I ( Q ) with form factors and particle volumes corresponding to welldefined shapes. As discussed previously, however, eq 5 is only approximate, when the sample is polydispersed618 (contains particles with different sizes, and perhaps different shapes). The fit provided by eq 5 is not unique in this case: one can fit the observed intensity using average form factors and average particle volumes corresponding to very differently shaped particles, cylinders, and disks, for example. In order to remove some ambiguity, we applied a constraint suggested by Sheu.8 One requires that the shape of the particle and the type of distribution describing the sizes not change for a series of samples with different concentrations. Basically,this is the requirement that the contrast between scattering centers and surroundings not be a function of shape, size, or distribution parameters.
Results Equation 4 describes the observed rheology for both the asphaltenes in toluene a t 25 "C and for the samples of vacuum residue (natural or synthetic), as shown in Figure 1. Solvation constants are given in Table 1. The excellent agreement of the data with eq 4 implies that the asphaltenic particles are spherical, they are solvated by the surrounding medium, there is a polydispersity in sizes, and the direct interparticle interactions are not large.4 Table 1 also shows that if we divide the apparent viscosity obtained in the dilute limit by 2.5, the intrinsic viscosity for spheres, we obtain the same value for the solvation constant as obtained from eq 4. As mentioned above, the self-consistencyillustrated here extends to other rheological theories for suspensions of sphere^.^ The data in Table 1indicate that the Ratawi asphaltenic particle is solvated to the same extent in toluene at 25 "C as in vacuum residue at 93 "C.
Macrostructure of Ratawi Asphaltenes
Also shown in Table 1 are the average radii of the asphaltenic particles in toluene at room temperature according to SANS, and for scattering centers in Ratawi vacuum residue at 93 "C according to SAXS. In both cases, it was found that the particles are spherical, and the sizes are distributed according to the Schultz distribution. The percent polydispersity is a measure of the breadth of the distribution. These results show the average size, and distribution of sizes of the asphaltenic particles is also the same in toluene as in vacuum residue.
Discussion One could conclude from Table 1that toluene is a good model solvent for asphaltenes. The asphaltenic colloidal particles are spherical in both cases, there is an equivalent amount of polydispersity, and the amount of solvation by the surrounding medium is the same. However, the amount of solvation is temperature dependent. The solvation constant for the Ratawi asphaltenes in vacuum residue is 3.7 at room temperature and 1.0 at 300 "C. By analyzing the temperature dependency of the solvation constant, we find that the forces responsible for solvation in vacuum residue are relatively weak; the barrier for them to escape the solvation shell is approximately 1-2 kcallmol. The fact that the solvation constant is the same in toluene at 25 "C as in vacuum residue at 93 "C implies that the forces between the toluene moleculesand the asphaltenic colloidalparticle are weaker than those between the molecules in the nonasphaltenic fraction and the particle. Thus toluene is a good model solvent for asphaltenes as long as one takes into account that the interactions between the surface of the asphaltenic particles and the surrounding molecules are different at the same temperature. Next we consider the implications of SAXS and SANS indicating that the scattering centers in vacuum residue and in asphalteneldeuterated toluene are the same size. First of all, these results imply that it is the asphaltenes in vacuum residue that are scattering the X-rays, since there is no scattering if the asphaltenes are not present, and the asphaltenes when in toluene have the same average size and distribution of sizes as observed by SAXS for the vacuum residue. One could argue that only a fraction of the heptane asphaltenes cause the scattering, but then one would have to argue that the part of the asphaltenes that do not scatter X-rays in SAXS also do not scatter neutrons in deuterated toluene; Le., the noncolloidal part of heptane-insoluble asphaltenes is dispersed on a molecular length scale in both vacuum residue and toluene. This of course could be true but appears rather remarkable. Another striking implication that follows from the equivalence of sizes in the SAXS and SANS experiments is that the scattering centers in these experiments are the unsolvated asphaltenic particles.
Energy & Fuels, Vol. 8, No. 3, 1994 569
The scattering centers in SAXS are regions of a different electron density (X-rays scatter from electrons). Since we expect the asphaltenes to be polynuclear aromatics,2 it is easy to surmise that the X-rays are scattering from centers of high electron density (regions of concentrated %-electronic density caused by associated PNAs) in a continuum of lower electron density (sea of aliphatic and simple aromatic molecules). On the other hand, the scattering centers in SANS are regions of high proton density (neutrons scatter from protons). The contrast in proton density between the asphaltenes and their surroundings is provided in the experiment by using deuterated toluene. The fact that we observe a good signal to noise ratio in the SANS experiments implies that the deuterated toluene does not penetrate the asphaltenic core, which is consistent with the rheological measurements. Second, because the size of the centers of high electron density is the same as the size of the centers of high proton density, we conclude that the asphaltenic colloidal particle is rather impervious to the nature of its surrounding. This is supported by SANS studies that show the size of the asphaltenic colloidal particles does not change as toluene is systematically replaced by pyridine, although the dielectric constant of the surrounding medium changes from 2.6 to 12.5.7 Finally, on consideration of these observations we surmise that we are measuring the size of the unsolvated asphaltenic colloidal particle in these experiments. The average volume is approximately 1.5 X W9cm3 (150 000 A3), and taking the density of the asphaltenes to be 1.1 g/cm3, we estimate the apparent molecular weight of the average colloidalparticle to be approximately 100 000 amu. The solvated particle would be larger. The solvation constant in Table 1is the ratio of the volume fraction of solvated particles to the weight fraction of dry asphaltenes, as shown in eq 3. If N is the number of particles
K = (NVA/v) ( W/NWA) where V and W are the volume and weight of the vacuum residue and V Aand W Aare the volume and weight of the asphaltenic colloidal particle. If we divide the solvation constant by the density of the vacuum residue, and multiply by the density of the asphaltenes, we obtain the ratio of the volume of the solvated particles to the unsolvated particles: v A ( S O l ) / VA(dry)= PA/PK
This calculation indicates that the solvated volume (-450 000 A3) is approximately 3 times the unsolvated volume and that the diameter of the solvated particle (-90 A at 93 "C) is approximately 1.4 times the diameter of the unsolvated particle.
