Thermogravimetric Investigation of the Influence of Colloidal Phase on the Processing Properties of Crude Oil Residues
Oil & Gas Science and Technology - Rev. IFP, Vol.
Thermogravimetric Investigation of the Influence of Colloidal Phase on the Processing Properties of Crude Oil Residues
H. Laux 0
T. Butz 0
I. Rahimian 0
0 German Petroleum Institute , Walther-Nernst-Str. 7, D-38678 Clausthal-Zellerfeld - Germany
Crude oil residues are complex mixtures of different organic
compounds characterised by the existence of a colloidal
disperse phase. The formation of the colloidal disperse phase
is based on the low solubility of the asphaltenes compounds
which are insoluble in n-alkanes as n-pentane and n-heptane
but soluble in toluene. The asphaltenes associate to
crystallites as nucleus of colloid particles. The solvation of
these nuclei by resins and other compounds determines the
stability of the colloid particles and hinders the further
The processing characteristics of crude oils are adversely
influenced by the colloidal asphaltene phase. Depending on
colloidal stability the colloidal phase is among other things
the reason of fouling, coke formation and deactivation of
catalysts, especially at the conversion of residues (Zou and
Lin, 1994). Mixing of different products and the addition of dispersing agents are possibilities to stabilise the colloidal disperse phase and by that to improve the processing conditions.
But the effectiveness of such measures is difficult to assess, because the colloidal stability depends on different factors as the solubility parameters of asphaltenes and of dispersion medium (Laux et al., 1997a).
Kopsch (1995) has shown that information on the
processing behaviour of crude oils and crude oil residues at
higher temperatures can be obtained by thermogravimetry
analysis (TGA). Among other things it is possible to
calculate the activation energy of evaporation and cracking
examining both processes as first order reactions:
A → B
In this case the conversion is a function as described by Equation (2):
q = Aexp − (1 – x)
q rate of heating
x weight loss
A frequency factor
E activation energy
R universal gas constant
Using the thermogravimetric curves from at least three test
runs at different heating rates, A and E can be calculated on
the basis of the method according to
Flynn and Wall (1966)
In the temperature region below 400°C the activation energy corresponds to the evaporation enthalpy at the thermogravimetric conditions; above 400°C activation energy is determined by the cracking enthalpy.
It was found that the activation energies depend on the
interaction forces between the colloidal disperse phase and
components of the dispersion medium, too (Laux and
Kopsch, 1996; Laux et al., 1998). In this way the influence of colloidal disperse phase on the processing behaviour of residues can be evaluated.
In this article the results of thermogravimetric investigations of different residues in dependence on the composition and the colloidal stability are presented and discussed.
Three residues were chosen to investigate the influence of
colloidal disperse phase on the thermogravimetric behaviour:
an atmospheric residue, a vacuum residue and the
atmospheric residue of the visbreaking product of the
These residues were fractionated into asphaltenes, resins
and dispersion medium according to
Neumann and Wilkens
by precipitation of the complete colloids with ethyl
acetate. Afterwards, the resins were extracted by n-pentane
and the asphaltenes by toluene. The asphaltenes were
characterised by fractionation in three solubility classes
: asphaltenes were dissolved in
boiling cyclohexane and subfractions were precipitated by
stepwise addition of isooctane. At the cyclohexane/isooctane
ratio 1: 1 the low soluble fraction precipitated and at 1: 2 the
middle soluble fraction. The easily soluble fraction remains
dissolved at this ratio.
Supercritical fluid extraction (SFE) was applied to prepare
another subfraction of vacuum residue using CO2 at 40°C
and 14 MPa in the first step and CO + isooctane as entrainer
at 100°C and 30 MPa in the second step. The resulting
extraction residue was enriched with aromatic and polar
compounds in comparison to the vacuum residue.
The colloidal composition of the residues are given in Table 1.
According to Equation (4) the solubility parameter is a
function of the hydrogen deficit zRa in relation to alkanes and
of the carbon number nC. zRa is calculated by Equation (5):
zRa = 0.5 (2nC + 2 – nH)
The value ∆δ He considering the influence of the
heteroatom influence is obtained from Equation (6):
where ρ is the density of the sample, Fi is the increment value,
yi is the content of heteroatom i and Ai is the atomic weight.
At the flocculation point can be accepted:
δ∆ He =ρ ∑Fi i
δfp = δdm
χ Flory Huggins interaction parameter
υdm molar volume of the dispersion medium
R universal gas constant
δas solubility parameter of asphaltenes
δdm solubility parameter of the dispersion medium
δ∆ correction term considering the influence of asphaltene
χcrit critical Flory Huggins interaction parameter
The average solubility parameters of the solvent-precipitant
mixture at the flocculation point δfp was determined by
∆ fp = Φ δ + Φ δ
s s p p
φs and φp are volume fractions, δs and δp the solubility
parameters of solvent and precipitant.
The differences between the solubility parameter of low soluble asphaltenes and of solvent-precipitant mixture at the flocculation points are given in Table 2.
The small difference of solubility parameters of the visbreaking residue indicates the low colloidal stability of this product. The colloidal stability of the atmospheric and vacuum residues are nearly identical.
Solubility parameters of the low soluble asphaltenes δlsas
and of the solvent-precipitant mixtures at the flocculation point δfp
and their differences in (MJm–3)0,5
Further information about the composition can be taken
Laux et al., 1999
Besides the original residues the following fractions and
mixtures were subjected to thermogravimetry:
– the maltene fractions of residues;
– mixtures of the atmospheric and vacuum residues;
– mixtures of the atmospheric residue with supercritical
extraction residue from the vacuum residue;
– mixtures of the atmospheric resp. the visbreaking residues
with different amounts of two dispersing agents (one of
phenolic basis, one of amine amide basis).
Thermogravimetry was carried out using the TA
Instruments thermal analyzer 951. The sample size for the
analysis was 8 to 10 mg. The heating rates were 5, 10 and
20 K/min at a stream of argon of 50 cm3/min. The kinetic
parameters of evaporation and cracking were calculated by
the TA Instruments software according to Flynn and Wall
The results of the thermogravimetric analysis of the
n-alkanes with carbon numbers 18, 32, 44 and 54 were used
to ascertain the correlation between TGA-temperature and
normal boiling point and for the calculation of enthalpy of
evaporation in dependence on the carbon number
et al., 1998)
. It was found that the correlation coefficients are
analogous to those of
The flocculation points of the products as the measure of
colloidal stability were determined by a titration method as
described in Laux et al. (1997a). A precipitant is added at
constant rate to the solution of the petroleum product under
intensive stirring. The titration is monitored by a light
intensity meter. After flocculation onset the light intensity
decreases and the maximum of the curve is defined as the
flocculation point. In this case chlorobenzene was used as the
solvent and isooctane as the precipitant. The concentration of
the solution was 5 wt%.
On the basis of the flocculation point determination it was
found that the Flory Huggins interaction parameter χ can be
used as a criterion of stability
(Laux et al., 1997a, 1997b)
χ = υdm (δas – ∆ δ – δdm )2 ≤ χcrit
Difference 19.98 15.21 4.77
Vacuum 19.92 15.23 4.69
Visbreaking 20.45 16.16 4.29
The comparison of the activation energies of the original
products and maltenes (Figs. 1 to 3) shows that:
– at temperatures up to 375°C the original products have
higher evaporation enthalpies than their maltenes. The
difference increases with the asphaltene content;
– the influence of asphaltenes on the cracking energy is not
The influence of colloidal stability is represented by the
mixtures of the atmospheric and vacuum residues. In
Figure 4 the activation energies (evaporation enthalpies) at
different temperatures in the region from 300 to 375°C are
displayed as a function of vacuum residue percentage. It is
evident that the evaporation enthalpies show extreme
dependencies. A similar behaviour was found for the
mixtures of atmospheric residue and the residue of
supercritical extraction (Fig. 5).
The flocculation points show a minimum only in
dependence on the vacuum residue percentage (Fig. 6).
The thermogravimetric residues at 500°C as measure of
the coke formation are shown in Figure 7.
The influence on the weight loss by evaporation at
different temperatures is demonstrated by Figure 8. One can
conclude that the addition of the components of vacuum
residue to the atmospheric residue may lead altogether to
increasing distillate yield.
The addition of dispersing agents to the atmospheric and
visbreaking residues produces the following results:
– the effects of the dispersing agents are more pronounced
in the relatively unstable visbreaking residue than in the
– in the case of the visbreaking residue an extreme
dependence of activation energies on the concentration of
dispersing agent is found (Fig. 9). The corresponding
flocculation points are shown in Figure 10;
– the TGA residues at 500°C are decreased by addition of
dispersing agents (Table 3), but at higher concentrations
the residue begins to increase again;
– the distillation yield was not significantly influenced by
the dispersing agents.
Activation energies of the components of the visbreaking residue.
20 40 60 80
Percentage of vacuum residue (wt%)
Percentage of SFE residue (wt%)
Activation energies of the components of the visbreaking
residue in dependence on the dispersing agent concentration.
Dispersing agent concentration of solution (ppm)
Flocculation points of the visbreaking residue in dependence on the dispersing agent concentration.
Influence of the dispersing agents on the TGA residues at 500°C (wt%)
agents need an experimental verification, which can be
realised by thermogravimetry.
Since the difference of the activation energies of the crude
oil residues and their dispersion media (maltenes) is a
measure of interaction forces between the disperse phase and
the components of the dispersion medium we may conclude.
The influences on these interactions are not additive when
different crude oil residue products are mixed. Obviously the
simultaneous variation of degree of aggregation and degree
of solvation of asphaltenes leads to extreme changes. Hereby,
a maximum of interaction is found if the flocculation point is
minimal. This condition causes also decreased distillate
yields and increased coke residues. Such behaviour was also
found by Syunayev et al. (1990, 1997) while investigating
disperse crude oil systems.
The stabilisation by dispersing agents displays other
results. In spite of continuously increasing stability the
activation energy develops extreme. This behaviour indicates
that the mechanisms of stabilisation by dispersing agents on
the one hand and by solvation by petroleum components on
the other hand have to be distinguished.
It has been shown that the thermogravimetry can be used to
investigate the influence of the colloidal phase on the
distillation and cracking behaviour of crude oil residues.
This influence is exhibited for the activation energies, for the
distillation yield and for the cracking residue at 500°C. The
complexity of interactions is illustrated especially by the
extreme dependence of evaporation enthalpies on the mixing
of residues and on the additivation. Therefore, calculation
methods for evaporation enthalpies as have been developed
for hydrocarbon mixtures cannot be used in the case of
crude oil residues
(Laux et al., 1998)
. The mixing of
different crude oil residues and the addition of dispersing
This work was supported through a grant from Deutsche
Flynn , J.H. and Wall , A.L. ( 1966 ) A Quick Direct Method for Determination of Activation Energy from Thermogravimetric Data . Polymer Letters , 4 , 323 - 328 .
Kopsch , H. ( 1995 ) Thermal Methods in Petroleum Analysis , VCH Verlagsgesellschaft , Weinheim.
Laux , H. , Butz , T. and Rahimian , I. ( 1998 ) Einfluß der kolloiddispersen Phase auf das thermodynamische Verhalten von Erdölrückständen (Influence of Colloid Disperse Phase on the Thermodynamic Behaviour of Crude Oil Residues) . Proceedings of DGMK Conference , 30 September, Hamburg-Reinbeck (DGMK-Tagungsbericht 9804) , 41 - 51 .
Laux , H. ( 1992 ) Löslichkeitsparameter und Verteilung von Erdölrückstanskomponenten (Solubity Parameters and Distribution of Crude Oil Residue Components) . Erdöl, Erdgas, Kohle, 108 , 227 - 232 .
Laux , H. and Kopsch , H. ( 1996 ) Verdampfungsenthalpien höher siedender Erdölkomponenten (Evaporation Enthalpies of Higher Boiling Crude Oil Components) . Chem . Techn., 48 , 267 - 270 .
Laux , H. , Rahimian , I. and Butz , T. ( 1997a) Thermodynamics and Mechanism of Stabilization and Precipitation of Petroleum Colloids . Fuel Proc. Techn ., 53 , 69 - 79 .
Laux , H. , Rahimian , I. and Butz , T. ( 1997b) Phase Behaviour of Colloidal Crude Oil System . Revue de l' Institut français du pétrole , 52 , 2 , 226 - 227 .
Laux , H. , Rahimian , I. and Schorling , P. ( 1999 ) The Colloidal State of Residues in Dependence of the Crude Oil Processing . Petrol. Sci. and Techn ., 17 , 3 - 4 , 349 - 368 .
Neumann , H.J. and Wilkens , J. ( 1974 ) Methode zur Bestimmung der Asphaltene und Erdölharze in Bitumen und Rückstandsölen (Method for the Determination of Asphaltenes and Petroleum Resins in Bitumen and Distillation Residues) . Bitumen, Teer, Asphalte und Pech , 25 , 246 - 247 .
Syunyaev , R. , Safieva , R. and Syunyaev , Z. ( 1997 ) The NonLinear Behaviour of Oil Disperse Systems at the Technological Processes of Oil Industry . Revue de l' Institut français du pétrole , 52 , 2 , 240 - 241 .
Syunyaev , Z.I. , Syunyaev , R.Z. and Safieva , R.Z. ( 1990 ) Neftjanye dispersnye sistemy (Disperse Petroleum Systems) , Chimija, Moscow (in Russian).
Zenke , G. ( 1989 ) Zum Löseverhalten von Asphaltenen. Anwendung von Löslichkeitsparameter-Konzepten auf Kolloidfraktionen schwerer Erdölprodukte (About the Solubility Behaviour of Asphaltenes. Application of Solubility Parameter Concepts on Colloidal Fractions of Heavy Petroleum Products) . PhD Thesis , Technical University of Clausthal.
Zou , R. and Lin , L. ( 1994 ) Role of Asphaltenes in Petroleum Cracking and Refining , in Asphaltenes and Asphalts , 1 , Yen , T.F. and Chiligarian , G.V. (eds.), Elsevier, Amsterdam, 339 - 363 .