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Phase Behavior of Bituminous Materials Paul C. Painter, Boris A. Veytsman, and Jack Youtcheff Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b01728 • Publication Date (Web): 17 Sep 2015 Downloaded from http://pubs.acs.org on September 23, 2015
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Phase Behavior of Bituminous Materials Paul Painter, Boris Veytsman† and Jack Youtcheff†† The EMS Energy Institute, The Pennsylvania State University, University Park, PA 16802. †
School of Systems Biology & Computational Materials Science Center, George Mason University, MS 6A12, Fairfax, VA 22030 ††
Turner Fairbanks Highway Research Center, McLean, VA 22101.
CORRESPONDING AUTHOR: Paul Painter AUTHOR EMAIL ADDRESS
[email protected] ABSTRACT An attenuated association model previously developed to describe the aggregation of asphaltenes in solution is extended to a description of the phase behavior of bitumen and asphalts. Work reported in the literature using various experimental techniques shows that these materials have complex phase structures, but essentially consist of two amorphous phases, one predominantly asphaltenes and the other largely maltenes. In the work reported here, these materials were first modeled as pseudo two component mixtures, one consisting of a self-associating component that for the most part corresponds to asphaltenes, while the second component is a non-selfassociating component that essentially consists of maltenes. It is shown that the critical value of the Flory χ interaction parameter is significantly reduced in these mixtures relative to asphaltene
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solutions and calculated binodals (co-existence curves) are very broad and asymmetric, with a phase boundary that approaches the pure maltene composition limit. This indicates that above a critical value of χ, bitumen and asphalts phase separate into an almost pure maltene fraction. However, the asphaltene rich phase consists of an appreciable non-self associating fraction that varies with temperature and overall bitumen composition. Ternary phase diagrams were also calculated by assuming that bitumen is a pseudo three component system consisting of a selfassociating component identified as asphaltenes, saturates and aromatics plus resins (combined). These ternary mixtures are predicted to phase separate into two phases, one consisting of asphaltenes and aromatics plus resins, but with only small amounts of saturates, while the second phase consists of a mixture of saturates and aromatics plus resins with only trace amounts of asphaltenes. The calculations are consistent with experimental measurements of the glass transition temperatures of these materials and microscopic observations of phase behavior.
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INTRODUCTION In recent work we have applied an attenuated association model to the calculation of the phase behavior of asphaltene solutions.1,2 It has been shown that the association of asphaltenes in the form of nano-aggregates significantly reduces the critical value of the Flory χ interaction parameter relative to solutions with non-associating molecules of equivalent size and chemical structure. This strongly affects the calculated phase behavior of asphaltene solutions and should have an equivalently profound effect on the calculated phase behavior of bituminous materials. Heavy oil, bitumen and the residua of oil distillation (asphalt binders) are complex, multicomponent materials that display a rich microstructure. Here we will explore the extent to which the model can be used to interpret phase behavior in these materials. There is a very broad literature concerned with the phase behavior of bitumen and asphalt, but the experimental work most relevant to this study concerns thermal properties as measured by differential scanning calorimetry (DSC) and the observation of phase behavior using atomic forces microscopy (AFM). DSC thermograms are complex and show a range of transitions that vary with sample and thermal history.3-8 Most of the results have been interpreted in terms of the presence of two amorphous but complex phases, together with a fraction that crystallizes. For example, Masson and Polomark5 observed two glass transition temperatures (Tgs), a large Tg near –20˚C and a smaller, broad Tg near 70˚C, which they assigned to maltenes and asphaltenes, respectively. The detection of the 70˚C transition depended strongly on thermal history. Fulem et al.6 also observed a low temperature transition in maltene rich nano-filtered samples (210 K – 270 K) that was absent in asphaltene rich fractions. The latter showed a broad, complex solid to liquid transition that occurred between temperatures of 340 K and 520 K. Mouazen et al.8 also observed two Tgs, near –20˚C and 53˚C in the sample they studied, and assigned these transitions
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to maltenes and asphaltenes, respectively. The presence of a second Tg was confirmed by an observed change in modulus near 55˚C. An additional layer of complexity results from the recent observations that asphaltene fractions can also form a liquid crystalline phase,9-11 although the thermal transition characteristic of the melting of this phase was not observed in asphaltene/maltenes mixtures.9 Finally, a crystallizable wax fraction is present in some materials and this can also result in the observation of a melting transition (see below). Recent AFM studies have provided additional insight into the phase behavior of these materials12-24 and this work has been recently reviewed by Yu et al.24 Essentially, three surface structures have been observed. One of these structures consists of elliptical domains called the peri-phase embedded in a second, continuous so-called para or perpetua phase. However, the peri-phase shows an additional structure (catana phase) wonderfully labeled “bees”, because of the pattern of pale and dark lines they display.12 This structure is now considered to be part of the peri-phase. The appearance of bee structures is associated with the presence of a crystallizable wax fraction together with a wrinkling or buckling of the surface.17-19,24 In the samples studied by Lyne et al.21 the bees disappeared above a temperature of 57˚C. Nevertheless, even in a sample with no crystallizable wax, a microstructure could still be observed by AFM,7 although one that is less rich, consisting of small clusters of phase separated material that were attributed to flocculated asphaltenes. It is interesting that bitumen samples that showed a well-defined, extensive peri-phase and bee structures contained only a small amount of wax (1-2%).7 These distinguishable phases disappear above a temperature that depends upon the sample. In the bitumen studied by Nahar et al.,20 this temperature was 90˚C and these authors considered their material to be a single phase at that temperature. De Moraes et al.15 observed a largely
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homogeneous mixture at temperatures above 70˚C in the material they studied, but also noted the presence of a barely observable, residual fine phase structure. There are two additional points from the AFM work that need to be considered in the context of calculations of phase behavior. First, the continuous or para-phase is softer than the peri-phase.18 Second, the size of the elliptical domains increases with the asphaltene content of the samples.20 Both these observations suggest that the peri-phase has a higher concentration of asphaltenes. If the peri-phase and its “bees” are associated with wax content, they also have a higher concentration of crystallizable alkanes. Given the thermodynamic incompatibility of alkanes and asphaltenes, this observation is at first sight counter-intuitive. However, Alcazar-Vara et al.25,26 point to various publications that suggest asphaltene aggregates can act as nucleation sites for wax precipitation. Be that as it may, asphaltenes and waxes are not the principal components in many bituminous materials, while the observed peri-phrase elliptical domains appear to be the major component. In this regard, the recent work of Fischer and Dillingh is important.22 These authors showed that the surface region probed by AFM is significantly enriched in the peri-phase and that in the bulk there is a larger fraction of the softer paraphrase, which is presumably mostly maltenes. Previously, Lynne et al.21 had also concluded that the peri/catana phase, which they called the bee laminate phase, separates from the bulk of the bitumen and blooms to the surface. To summarize, there appears to be two amorphous phases with distinct glass transitions temperatures and in many samples there is also a crystallizable wax fraction. The phase behavior of the amorphous components of bituminous materials has most often been modeled using regular solution or the Flory Huggins theory. As Wang and Buckley pointed out,27 in order to make the problem of calculating the phase behavior of these complex mixtures more tractable, various simplifying assumptions have been made. These include assuming that one phase is pure
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asphaltene,28 or on the other hand that it has no asphaltenes at all.29 Wang and Buckley27 took a different approach and assumed that a crude oil could be modeled as a two-component system (asphaltenes and everything else) and calculated tangents to a Flory Huggins free energy curve to define the equality of chemical potentials in co-existing phases. The solubility parameter and molar volume of the asphaltene were used as adjustable parameters and the precipitation of asphaltenes from crude oils was modeled. More recently, Dechaine et al.30 and Nikooyeh and Shaw31,32 have cast serious doubts on the applicability of regular solution theory to asphaltene mixtures. This work included measurements of the solubility of model compounds30 and the determination of the enthalpy of mixing asphaltenes with a range of solvents.31,32 They concluded that even when the Flory Huggins theory is used to account for size disparities between asphaltenes and solvents, solubility parameters do not provide a good description of interactions and solubility. It is not surprising that a direct application of the Flory-Huggins theory (with solubility parameters used to calculate the χ interaction parameter) does not do a good job of modeling asphaltene phase behavior. Solubility parameters were originally developed to describe interactions involving dispersion and weak polar forces. They do not apply to interactions that lead to association, such as hydrogen bonds and the more complicated collection of interactions that are probably central to asphaltene association.33 In addition, for mixtures where there are large disparities in thermal expansion coefficients, there are free volume differences that contribute to the free energy of mixing. Finally, the association of asphaltenes to form nanoparticles affects both the entropy and enthalpy of mixing in a way that falls outside the usual assumption of a random mixing of components.
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As mentioned above, in recent papers we have used an association model to account for the effect on asphaltene nano-aggregation on the free energy of mixing1,2 and approximated free volume contributions to the Flory χ parameter using an approach described by Graessely34 and Milner et al.35 This eliminates some of the problems in applying the Flory-Huggins theory to asphaltene mixtures referenced above and results in the definition of a bare χ parameter that is more appropriately related to solubility parameters. Here we will apply this approach to the phase behavior of the amorphous components of bitumen. The crystallization of the wax component will be neglected. We will first review the model for the convenience of the reader then consider its extension to ternary mixtures. ATTENUATED ASSOCIATION MODEL Following Wang and Buckley,27 we will initially assume that bituminous materials can be modeled as a pseudo two component system. One component is identified as being capable of self-association to form nano-aggregates, while the second does not self-associate. The former is largely asphaltenes while the latter is largely maltenes. Rather than applying the Flory-Huggins theory in its original form, however, we will apply an association model to account for the formation of nano-aggregates and the variation in the degree of aggregation with composition. This model accounts for the entropic and enthalpic contributions to the free energy of mixing that is a result of association using a single parameter, an equilibrium constant. The association model we use also has its roots in Flory’s lattice model. In an appendix to his paper on heterogeneous polymer solutions, Flory36 described a theory for treating mixtures where there is a chemical equilibrium between (covalent) polymeric species, such as those formed in condensation polymerizations. In a subsequent note, Flory37 pointed out the generality of his
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treatment and its applicability to reversible association. He also identified the state where the individual associated i-mers are separate and oriented as the proper standard reference state. A general description of association models can be found in the book by Acree38 and is the basis for the model used here and described in more detail in our preceding publications.1,2 The equations become tractable through the use of Prigogine’s observation that the chemical potential of a stoichiometric component in a mixture containing as associating component in equilibrium is equal to the chemical potential of the monomer.39 It is assumed that asphaltene association can be described by equations of the form: + ⇌ (1) where is the volume fraction of associated i-mers. To account for the steric limitations on nano-aggregate size, an equilibrium constant (KA) describing an association that is continuously attenuated as monomers are added to the cluster is defined as: = + 1 + 1 (2) This is a dimensionless equilibrium constant that is equal to an equilibrium constant defined in terms of molar concentrations divided by a molar volume per structural unit. As Flory pointed out,36 because this latter quantity is a constant, either equilibrium constant definition is acceptable. Essentially, the enthalpic and entropic changes that result from association are
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captured by this single parameter. Using this approach, the free energy per mole of lattice sites for a binary mixture (A, B) is given by:1,2 Δ ΔG = ( ($%& '& − 1) ($%& '& − 1) = ln ⁄ + ! + " − + + , (3) V is the total volume of the mixtures and an arbitrary reference volume, Vref, equal to the molar volume of a lattice site, has been defined; also rA = VA/Vref, rB = VB/Vref. The quantities and
are the volume fractions of (unassociated) asphaltene “monomer” molecules present in the mixture and pure state, respectively. The overall volume fraction of asphaltene in the mixture is , while is the volume fraction of the second (non-associating) component. For comparative purposes, the terms have been collected into two sets of parentheses. The first set contains terms that have the appearance of the Flory-Huggins combinatorial entropy of mixing, but with the asphaltene log term expressed in terms of the monomer concentration. It is therefore not just an entropy term, but also depends on the enthalpy of association. The first two terms in the second set of parentheses also contain terms associated with free energy changes due to association in the mixture relative to the pure state. These arise from the change in the degree of association on going from the pure state to the mixture and do not appear in the Flory-Huggins equation for polymer solutions, because in the latter the (covalent) polymer chain length does not change. Finally, + is an “effective” interaction term accounts for non-specific interactions between the asphaltene i-mers and the second component of the mixture, together with excess free energy terms due to factors such as free volume differences.1,2
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The equation for the spinodal is: - . (Δ ⁄) 1 - 1 = + − 2+ =0 . - - (4) + is the + term determined from the second derivative of the free energy and
- 1 = 1 2 - (1 + ) (5) In order to calculate as a function of composition we use the stoichiometric relationship: = $ %& '& (6) We are using a model where the interaction parameter is independent of concentration, so + = + = + 3 + +
(7) Graessley34 and Milner et al.35 have discussed and applied a simplified form of the contribution to + from free volume effects. In polymer solutions this is expressed in terms of a mismatch of
polymer and solvent thermal expansion coefficients, 4, as:
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+
3
.
5 45 65∗ 9∗ − 5∗ = 8 : 2 5∗ (8)
where 65∗ is related to the cohesive energy density of the solvent (in turn, related to
