Macrostructure of asphaltene dispersions by small-angle X-ray scattering

Macrostructure of asphaltene small-angle X-ray scattering P. Herzog,

D. Tchoubar

dispersions

by

and D. Espinat*

Laboratoire de Cristallographie, U.A. 810, Universitd Francais du P&role, Rueil-Malmaison, France (Received 12 January 1987; revised 27 May 7987)

d’Orl&ans,

France

and * lnstitut

Three asphaltene dispersions have been studied by small-angle X-ray scattering. Pematang and Grenada are two benzene dispersions of asphaltenes from atmospheric residues (350°C). Safaniya is asphaltene dispersed in maltene from a vacuum residue. The scattering data present characteristic profiles which can be explained by comparing the experimental with simulated curves. The macrostructure model used for the curve simulations is based upon a distribution of thin discs which all have the same thickness (3.4 8) and a radius which follows a power law such as rB (13 A Q r d 800 A). The validity of such a model, discussed in the light of known properties of the asphaltenic particles, suggests that these membranes could result from the aggregation of small Yen’s elementary units. (Keywords: asphaltene; structural analysis; X-ray emission spectroscopy)

Many studies using different physical methods have described the size of asphaltenes which represent a large proportion of heavy crude oils and vacuum residues. Among these methods small-angle X-ray scattering (s.a.x.s.) appears to give the most realistic information about asphaltenic dispersions. As opposed to standard Xray diffraction operating on precipitated and dried asphaltenes, which may reveal artefacts due to autoassociation, s.a.x.s. can be used to study asphaltene dispersion in a crude or in a solvent. It has been applied to vacuum residues from crudes’ -4, coal-derived liquids5, high-ash coals6 and process effluents7-9. This paper describes s.a.x.s. experiments using a synchrotron X-ray source for some asphaltene dispersions. The main interest of such a source is to give spectra under good spot collimation conditions and with relatively good count statistics. The new approach consists of the comparison of experimental data with simulated curves such as a thin disc or a distribution model.

Data recording

Experiments were carried out using the synchrotron Xray radiation of the D.C.I. storage ring at L.U.R.E. (Universite d’orsay, Paris). The spectra were recorded every 100s during 1 h, to check the possibility of any variation in the sample during irradiation and to prevent any possible saturation of the detector. The wave length A selected by a variablecurvature monochromator was 1.608 A. The focal distance was 2 m, with 1 m between the sample and the linear sensitive detector. Taking into account the sizes of the beam stop and of the detector windows which were actually used, the accessible particle range of sizes was 4&800 A. As a result of spot collimation of the incident beam, the only correction necessary was the sample absorption correction. The petroleum solution was contained in a cell with parallel mica windows. The thickness was adjusted to 1 mm to obtain the optimum scattering power. Data analysis

EXPERIMENTAL Samples

Three samples were studied consisting of asphaltene dispersions contained in distillation residues. Samples 1 and 2, respectively Grenada and Pematang, were from atmospheric residues (35OC). After precipitation with n-heptane, asphaltene fractions were dispersed in benzene. Sample 3 was a vacuum residue from Safaniya oil. We asked for this sample directly without any separation with n-heptane. The s.a.x.s. diffusion was compared with the signal from maltene which is the soluble phase in the nheptane solvent. For a better comparison, the benzene dispersions were prepared with the same asphaltene weight concentration (- 10%) which approximately corresponds to the natural concentration of asphaltenes in Safaniya vacuum residue. 001f%2361/88/02024WI6%3.00 0 1988 Butterworth & Co. (Publishers) Ltd.

The experimental data analysis was performed first by direct observation of characteristic laws on the experimental intensity, and then by computing distance distribution as previously shown by Dwiggins”-“. The experimental curves were then compared with theoretical ones which were computed from different models. Direct

method

The general discussedi3*14.

theory of s.a.x.s. has been extensively The gist of this theory is summarized in

Appendix.

The experimental intensity scattered by the sample at any angle is termed Z(s) where s = 2 sine/A, and 8 is the half scattering angle. For homogeneous particles which are widely dispersed in a solvent, essentially two characteristic laws are generally predicted to be observed: the first, which

FUEL, 1988, Vol 67, February 245

Macrostructure

of asphaltene

dispersions

by small

angle

X-ray

scattering:

P. Herzog

et al.

of the shape of particles: for example the intensity scattered by very thin discs varies as sm2. The experimental spectral analysis was performed first by direct observation of these characteristic laws and then by computing the distance distributions F(r) and P(r) (cf. Appendix). These are variably sensitive to the geometry and the density of particles as previously discussed by Glatter ’ 4. The F(r) distribution is more sensitive to the size of smaller particles or their internal structure. The P(r) distribution is more sensitive to the particle volume; the latter is proportional to the scattering matter volume within the particle. The P(r) distance distribution is also characteristic of the long distance interactions in the case of particles which are not well dispersed in the solvent. Another way to perform the analysis is to compare the experimental and simulated curves using particle models. RESULTS

-7 .

-6 .

-5 .

-4 . Log(s)

Figure 1 Plot of normalized scattering intensity in Napierian log-log plots. 0, Grenada; a, modelling by R” size distribution of thin disc: a = 1 S9. 0, Modelling by a thin disc 3.4 A thick and radius 400 A

15.

Characteristic

laws in the experimental

curves

For the three samples, a linear variation of log[Z,(s)] log(s) is observed for a large portion of the experimental scattering range. IN(s) is the normalized intensity defined in Appendix. This indicates that Z(s) can be fitted by a power law: smn with tl equal to 1.69 for Pematang and Safaniya; 1.59 for Grenada (see Figures 1 and 2). At high values of s, for all samples the scattering curves vary with sC2. As is shown in Figures 3 and 4, the s2Z,(s) products reach a plateau fairly sharply. This behaviour could be produced by a membrane shape of the particle. Thus shape could be given by the equation: versus

1 IN(S) # -ps s-rm k2r2

(1)

-2

which could correspond

to an infinitesimally

thin disc.

13 . 11. a

50

-7 .

-6 .

-5 .

-4.

30

Log!s J Figure 2 Plots of normalized scattering intensity inNapierian log-log plots. n , Pematang; 0, Safaniya; a, modelling by R” size distribution of thin disc: a = 1.69

concerns the very low angles of scattering, depends on the size of particles through their gyration radius; the second, which concerns the largest scattering angle, depends on the interface particlesolvent and is predicted to vary as s-4. The middle part of the scattering curve is characteristic

246

FUEL, 1988, Vol 67, February

10 &is

0.30

oA5

IO3 S2

Figure

3

Asymptotic

behaviour

of 4ns21&)

(symbols

as Figure 1)

Macrostructure

of asphaltene

dispersions

by small

angle

X-ray

P. Herzog et al.

scattering:

The physical meaning of all these features are discussed below with the help of theoretical curves. Comparison of experimental and theoretical curves Choice of the different models. It can be seen from the spectroscopy results that asphaltene structures are described as polycyclic systems15 which have some features in common with carbon mesophase units”. Therefore the 47rs2Z,(s) plots (Figure 3 and 4) which reach a plateau at the highest s values, can be related to scattering particles such as thin two-dimensional species. A disc-like shape of 3.4 A thick seems to be the first possible approximation to the asphaltenic units. A first theoretical curve Z,(s) is computed by using a disc with radius 400 A and thickness 3.4 A which could represent

I

0.15 0.30 0.45 1o-3 S2 Figure 4

Asymptotic

behaviour

of 47rs21~(s) (symbols

as Figure

2)

260

-

500

800 -I

Figure 6

F(r) distribution

(symbols

as Figure

2)

P(r)

200 Figure 5

F(r) distribution

500 (symbols

as Figure

800

1800

/’

r

I)

F(r) and P(r) distance distributions Figures 5, 6, 7 and 8 show the F(r) and P(r) distributions which were obtained for the three samples. We find unusual profiles for the F(r) curves with a narrow peak at a very small value of r and a slowly decreasing tail. The P(r) distributions are characterized by the small amplitude of their maximum which is about ten times lower than the curve that would be computed for particles such as spheres or ellipsoids or thick cylinders14.

r Figure 7

P(r) distribution

(symbols

FUEL,

as Figure

1988,

I)

Vol 67,

February

247

Macrostructure

of asphaltene

disparsions

by smail angle X-ray scattering:

P. Herzog et al.

The normalization ‘mar

P(r)

is then:

r@dr=l

s *min

Average

disc. The thin-disc

simulation

shows two

principal failures: 1. The

n’ l

47cs21N(s) product explains the asymptotic behaviour of the experimental intensity but it does not respect the general shape; 2. The corresponding log-log plot of the model which is shown in Figure 5 has a normal slope IX= 2. Therefore the actual experimental values which are all between 1 and 2 are not explained by this model.

4 I

=\ l

200 Figure 8

500

\

800 r

P(r) distribution (symbols as Figure 2)

an average scattering particle. In this case, I,(s) is given by equation (2): n/2

Z,(s) =

s

r2*

sin2(2rcsH cos 0) 4n2s2H2cos28

x

45:(2~ sr sin@ . 4n2sZr2sin28

s1ned0’2’

0

where 8 is the half scattering angle Yis the radius of the disc H is the half thickness of the disc s=2sin6/L J, is the Bessel function J,(x) d is the number of dimension of the scattering particle; e.g. for flat particle, d = 2 Previous work on the structural analysis of asphaltenes and specially gel permeation chromatography”, which shows that asphaltenes can be separated into several fractions, guided our second approach. In considering a size distribution of the asphaltene particles, we assume in a first approximation that all the particles are thin discs with the same thickness of 3.4 A and with a distributed radius. The average scattered intensity f(s) obtained from a system of independently scattering randomly orientated particles” is given by:

(3)

where p(r) is the numerical distribution of particles giving the number of particles having a radius r; Z,(s) is the intensity scattered by one particle. In our case we admit that the characteristic power-law variation of the log-log plot of scattered intensity is due to a power-law size distribution of the scattering particles as discussed in a previous reportf8. In this content, we assumed that in Equation (3): p(r)r2d = rB

248

FUEL, 1988, Vol 67, February

(4)

The difference observed between the average disc model and the actual samples clearly appears with P(r) and F(r) functions (see Figures 5 and 7). Figure 5 shows a very marked difference between the experimental and theoretical F(r). Figure 7 shows that the amplitude of the theoretical P(r) is much too high. As already discussed in the section dealing with analysis, the amplitude of P(r) is directly related to the actual volume of matter within the particles. We can adjust the difference between experimental and theoretical P(r) by reducing the thickness of the model disc; however, in that case it would be necessary to use a thickness value of about 1 A which is not chemically valid. Such a paradox can be eliminated by postulating that the difference in amplitude between the experimental and theoretical P(r) is attributable to an internal porosity of the scattering particles; this porosity is equal to 79 Y0 for Grenada, 77 % for Pematang and 81% for Safaniya. Thin-disc distribution. The computed curves are very similar to the 47rs21,(s) experimental products (see Figures 3 and 4) in both shape and intensity. These results were obtained by choosing the fi coefficient in (4) equal to the c1coefficient of the experimental power laws (i.e. 1.59 for Grenada and 1.69 for Pematang and Safaniya) (Figure 9 shows the two corresponding weight distributions). The shape and the amplitude of the F(r) and P(r) distance distribution confirm this good agreement. In Figure 8, the difference of amplitude between the theoretical and the experimental curves shows that the particles present an internal porosity of about 25% for Pematang and 50% for Safaniya.

DISCUSSION The results described previously lead to two macrostructural models: the first describes asphaltenes as large porous membranes; the second is a size dist~bution of thin discs which follows a power law, and could be represented by Figure 10. The fitting by this second model, which gives the better simulation, is not the only way to justify the experimental behaviour. A structure like a fractional aggregation of the particles could equally fit the experimental curves19. Such a structure could be represented by a Witten and Sanders model (see Figure ZZ)20. Thus, there is an ambiguity between a radius distribution of thin dense and independent discs and particles with an internal structure of a fractional aggregation type. The elementary twodimensional units

Macrostructure

of asphaltene dispersions by small angle X-ray scattering: P. Herzog et al.

Particles nb + in a/. 100

P

50

P

I3

10

0’

*

of

100

0

500

1000 Radius in A

Figure 9

average

Weight distributions used for simulations: (0) p= 1.59 radius (I?) = 178 li; (A) b = 1.69 average radius (R) = 200 A

Il.ke.*.*. . . . . . l-. .;al

l.*** l..:

.*

0.

.

l

‘***.

.

l .:.:f:’

l

.

.

0”

.

. .*0’

.

.

l. l l.*..* l.*...”

l

l

. a

l.

l

a”

l

l.

‘0

0. .*t.

.

.

.A.

l. l.***. ’ l l .,p* l l.

. . . ..* . . .

.

0: l .*. **.* . . . :. l 0.‘.

..‘.’

‘***.

. l ** ... . l.

0.’

p’..

l *. l .* 0’.

l. .

.

Although we have not fitted the experimental curves with theoretical computed curves from the aggregate model, we consider that it is in good agreement with various data from the literature. the existence of large Dwiggins l1 has demonstrated particles in a Red-Wash crude through research with Bonse Hart small-angle diffusion equipment. It also can be seen that many studies22-24 have revealed large particles, albeit smaller than those we found. By ultracentrifugation, Reerink22 found ellipsoids varying between 50 A and 100 A in size. In a preliminary study by low-angle neutron diffusion24 thin discs were determined with a size between 80 A and 180 A. Our experiments have been made with a higher concentration (10%) than these, using neutron diffusion (~4.5 %). The variation of contrast between particles and deuterated solvent which is used in neutron scattering could show some scattering domain different from those observed by s.a.x.s. Particles in different states of aggregation can explain these different results. It seems possible to either by solvent or by aggregates dissociate concentration. Furthermore, an aggregated structure can explain how low and high molecular mass can be observed25*26. The models resulting from s.a.x.s. studies also explain the catalyst macroporosity required in hydrotreatment depositing as the result of hydrometallation on two types of microporous and macroporous catalysts2’. For a catalyst with only medium (80 A) micropores, the metal is distributed at the outside of the catalyst pellets. However, if the catalyst support has a monomodal distribution centred around 13OOA, the metal is then uniformly distributed into the catalyst pellets. This confirms that the asphaltenes are very large particles with a small mass:volume ratio. This could be in good agreement with an aggregational structure. By making use of n.m.r. techniques2a and infrared absorption29, various studies of different crude oils have generally shown the existence of elemental nuclei that are more or less condensed and linked together by aliphatic chains3’. In this way a natural porosity appears, which can be explained by an aggregational structure. The particle described by Dickie and Yen’ 5 in the solid state could be satisfactorily explained from the coiling-up

m

b

Figure 10

(a) Size distribution of thin discs with an internal porosity of about 50%; (b) elementary units described by Yen*]

of this aggregate could correspond to the primary molecules defined by Yen *I . This last solution includes the possibility of any particle size distribution. This aggregation could be favoured by the relatively high concentration of our dispersion (10%).

Figure

11

Figure

IO)

Model

of Witten

and Sanders”

(primary

units are as in

FUEL, 1988, Vol 67, February

249

Macrostructure

of asphaltene

dispersions

by small

of very ramified fractional flocculation process.

aggregates

during

angle

the

X-ray

scattering:

30

P. Herzog

et al

Speight, G. S. ‘The Chemistry and Technology Marcel Dekker, New York, p, 181

of Petroleum’,

APPENDIX CONCLUSION Small-angle X-ray scattering is a powerful tool for studying heavy petroleum fractions both in organic solvents and a natural solvent (maltenes). The study presented here raises new points: first, the particle dispersions have been described in their natural solvent; second, we observed two characteristic behaviours-a power law at the lowest angle and an asymptotic law in ksm2 at the highest angles which can be interpreted in terms of scattering by two-dimensional species. This work is a first tentative approach, which gives the macrostructural principle of the structure. Asphaltene species appear as thin large and porous particles with varying radius and a lateral extension possibly greater than 800 A. Further work is necessary for more precise elucidation of the nature of the aggregation structure given different sizes of particles, different solvents and different treatments.

REFERENCES

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

250

Dwiggins, C. W., Jr J. Phys. Chem. 1965, 69, 3500 Dwiggins, C. W., Jr J. .AppL Cryst. 1978, 11, 615 Kim, H. and Long, R. B. ACS, New Orleans Meeting, 1977,53 Biktimirova, T. G., Novoselov, V. F. and Timerbulatova, A. T. SC. Nauk. tr. Bashk. NII po Pererab. Nefi. 1982. 21, 99 (in Russian) Ho, B. and Briggs, D. I. Colloids and Surfaces 1982, 4, 275 Gonzales, J., Torriani, I. I. and Carlos, A. L. J. Appl. Cryst. 1982, 15, 251 Bosch, G. S., Marquez, R. M. and de Teresa, Y. C. Rev&a des Institute Mexicano del Petroleo 1980, XII, 77 (in Spanish) Espinat, D., Tchoubar, D., Boulet, R. and Freund, E. ‘Proc. Intern. Symp. on Heavy Crude Oil and Petroleum Residues’, 1984, Edit. Technip, Lyon, France, p. 147 Favre, A. and Boulet, R. ‘Proc. Intern. Symp. on Heavy Crude Oil and Petroleum Residues’, 1984, Edit. Technip., Lyon, France, p. 520 Dwiggins, C. W., Jr J. Appl. Cryst. 1979, 10,401 Dwiggins, C. W., Jr J. AppL Cryst. 1980, 13, 572 Dwiggins, C. W., Jr J. Appl. Crysr. 1982, 15, 564 Guinier, A., Fournet, G., Walker, C. B. and Yudowitch, K. L. ‘Small Angle Scattering of X-rays’, John Wiley, New York, 1955 Glatter, 0. and Kratky, 0. ‘Small Angle Scattering’, Academic Press, New York, 1955 Dickie, J. P. and Yen, T. F. Appl. Cryst. 1980, 11, 39 Guet, J. M. and Tchoubar, D. Fuel 1986,65, 197 Gourlaouen, C. Thesis, ENSPM, 1984, Rueil-Malmaison, France Schmidt, P. W. J. Appl. Cryst. 1982, 15, 567 Axelos, M., Tchoubar, D., Bottero, J. Y. and Fiessinger. J. Phys. 1985,46, 1587 Witten Jr., T. A. and Sanders, L. M. Phys. Rev. Letters 1981.51, 1123 Yen, T. F., Erdman, J. G. and Pollak, S. S. Anal. Chem. 1961,33, 1587 Reerink, H. Ind. Eng. Chem. Prod. Res. Develop. 1973, 12, 82 Dickie, J. P., Hailer, M. N. and Yen, T. F. J. Colloid Interj&c. Sci. 1969, 29, 475 Ravey, J. C., Ducouret, G. and Espinat, D., in press Tyrion, F. C. Bull. Sot. Chim. France 1981,9, 333 Moschopedis, S. E., Fryer, J. F. and Speight, J. G. Fuel 1976,55, 227 Plumail, J. C. Thesis, ENSPM, 1983, Rueil-Mahnaison, France Speight, J. C. Fuel 1970, 49, 81 Diamond, R. Acta Cryst. 1957, 10, 359

FUEL, 1988, Vol 67, February

The experimental data analysis was performed first by direct observation of characteristic laws on the experimental intensity, and then by computing distance distribution as previously shown by Dwiggins’O-12. The experimental curves were then compared with theoretical curves which were computed from different models. The general theory of scattering has been extensively discussed13q’4. We merely recall that it is related to the electron density fluctuations of the scattering medium. Let p(z) be the electron density at any point of the medium and p be the average density of this medium: The fluctuation T@) is such that p(I’x)=p+T@) for each point. The scattering intensity depends on the correlation function g(r) which is defined by the integral: g(r) = ( J

T(3z)T@+T)crX) I^

= F2 Vy(r)

(1)

where: 7 is the vector which relates two points of the medium; ( ) means the average for all the orientations of 2, in the case of a statistically isotropic medium such as a dispersion; r2 is the mean value of the density fluctuation; I/ is the volume of the irradiated sample; y(v) is the geometrical characteristic correlation function which depends on size, shape of particles and possible interactions between them. For a sample consisting of particles which are dispersed in a solvent, the r2 parameter is equal to (pp - p,)‘II/,$,. In this product pp and ps are the electron density of the particle and solvent; +p and $, are their respective volume concentration. The intensity Z(s) is then expressed by the Fourier transform for the correlation function:

47cr2Y(r)sin(2nsr)/(2nsr) dr

Z(s) = ZoZ,~2 V

(2)

s where

I, is the intensity of the incident beam; I, is the scattering by one electron.

The experimental and simulated characteristic laws are compared through I,(s) given by the normalization equation:

I(s) Z,(s)= I,I,Tz=

r

Z(s)

Inax

Jsm

47cs2Z(s) ds

where s,,,~”and s,,, limits are kept all the same for the samples and the models. Using Equation (2), y(r) can be deduced directly from IN(s): 4ns21,(s)eds s Two distance distributions v(r)=

F(r) = ry(r)

and

(4) can be considered

from y(r):

P(r) = r2y(r)

the shape and amplitude of which differ in sensitivity the geometry of the particles as previously discussed Glatter14.

to by