Physicochemical properties of bitumen-coal tar mixtures

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FuelVol. 75, No. 5, pp. 531-535, 1996 Copyright © 1996ElsevierScienceLtd Printed in Great Britain. All rights reserved 0016-2361/96 $15.00+ 0.00

Physicochemical properties of bitumen-coal tar mixtures Philippe Chambrion, Ronald Bertau* and Pierre Ehrburger Centre de Recherches sur /a Physico-Chimie des Surfaces So~ides, 24 avenue du prOs~dent Kennedy, 68200 Mu/house, France *H. G. O., Etab//ssement de Vendin-Loison, rue de/a Justice, BP 29, 62880 Vend/n-/e- Vie//, France (Received 26 October 1995)

The miscibility of bitumen-coal tar mixtures has been investigated. Bitumen-coal tar mixtures containing 10 to 50wt% of coal tar were examined by differential scanning calorimetry and optical microscopy to determine the threshold of rniscibility. A solid-like precipitate was observed after blending bitumen with coal tar. Up to 20wt% of added coal tar, only one liquid-like phase was detected by both techniques, while a second phase appeared at higher contents of coal tar. The chemical composition of demixed phases was investigated using an FT-i.r. microprobe. This indicated that a new distribution takes place between aromatics and aliphatic-like compounds. Copyright © 1996 Elsevier Science Ltd. (Keywords: bitumen-tar mixtures; miscibility; phase transitions)

Bitumen obtained by refining of petroleum is used as a binder for aggregates in road construction. To improve its adherence to stone, coal tar fractions may be added to the bitumen. Taking into consideration the complex chemical composition of bitumen and tar, it is important to determine their miscibility after blending. In fact, both components taken separately may already exhibit a heterogeneous character. It is well known that bitumen contains paraffinic compounds, e.g. waxes, which may crystallize at room temperature or below 1-3. In a recent paper 4, it was shown that during rapid cooling of bitumen to room temperature or below, segregation of paraffin wax leads to the formation of two interactive glass-type fractions. Coal tar usually contains solid particles, the so-called primary quinoline-insolubles or QI, which render them heterogeneous as well. Furthermore, an effect of polar compounds, phenols and pyridines in the tar on the extent of agglomeration of QI particles was recently reported 5. It was shown in particular that the presence of phenols tends to disperse the solid particles, whereas pyridines promote their agglomeration. The purpose of the present study was to investigate the homogeneity of bitumen-coal tar blends. The occurrence of various glass-forming liquids and solid phases was therefore investigated as a function of blend composition and temperature.

starting materials. The QI content of the CT was 1.5wt%. Bitumen-coal tar blends (B-CT) were prepared in the laboratory by heating bitumen to 430 K with mechanical stirring in air and adding CT preheated to 360 K to obtain 1 kg of the mixture. The mixture was then stirred for l h at 4 1 0 K before cooling to room temperature. Blends containing from 10 to 50wt% of coal tar were prepared.

Differential scanning calorimetry Differential scanning calorimetry (d.s.c.) measurements were performed using a Mettler TA 4000 thermoanalyser interfaced with a computer for data storage and processing. All d.s.c, experiments were carried out on -~20 mg of material under dry nitrogen flow. Samples kept at 278 K in the d.s.c, cell were cooled to 153 K at a linear cooling rate with absolute values covering 1 5 K m i n -1 The d.s.c, signal was thereafter recorded at a linear heating rate of 1 5 K m i n -1 in a manner described elsewhere 6. The glass transition temperature, Tg, was determined as the midpoint of the heat flow shift. The difference in specific heat capacity between the liquid and the glass, ACp, was taken from the thermoanalyser computer. The experimental uncertainty in the measurement of Tg and ACp was +1 K and 4-0.02 J g-1 K-1 respectively.

Image analysis EXPERIMENTAL

Sample preparation A bitumen sample, E L F Feyzin 40/50 (B), and a coal tar (CT) obtained in a modern coking plant were the

Samples were observed by optical microscopy using transmitted light. A small amount of material (0.5-1 mg) was set on a glass slide at room temperature and covered with another glass slide before heating to 470 K. The thin layer obtained was then cooled to room temperature

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Physicochemical properties of bitumen-coal tar mixtures: P. Chambrion et al.

a)

b) _ _ I

Figure I

50 ~tm.

I

Micrographs of B-CT 80-20 obtained under (a) natural light and (b) crossed polars

before observation by transmission light microscopy using natural and polarized light. Amounts of solid particles and liquid-like droplets were determined by image analysis (IA) at a magnification of x l00. Their total amount was determined as described elsewhere4.

Fourier transform infrared spectroscopy A small amount of material (1-5mg) was set on a potassium bromide slide at 370 K, covered by another slide and then cooled to room temperature before analysis. The thickness of the as-obtained sample was ~10 #m. Infrared spectra were recorded using a spectrophotometer interfaced with a computer and a microprobe to analyse areas from 20 to 100#m in diameter. FT-i.r. spectral treatment was carried out by using the software OPUS. Solid particles were filtered and cleaned up with trichloroethane before their spectra were recorded using diffuse reflectance. RESULTS AND DISCUSSION

the amount of solid particles originating from CT would be significantly smaller than that actually found, it is concluded that precipitation of heavy components takes place after blending. Considering the rather poorly defined shape of the precipitated particles, their size is defined simply as the square root of their cross-section. The cumulative size distribution of particles in the blend is shown in Figure 2 for 20 and 40 wt% CT respectively. It is seen that the particle size of the precipitate is also sensitive to the amount of CT present in the blend.

Miscibility of bitumen and coal tar As already mentioned, paraffin wax separates from bitumen after storage at 278 K for 12 h and the remaining bitumen matrix undergoes a glass-liquid transition between 239 and 273 K, as shown in Figure 3. Melting of paraffin wax gives a broad endothermic peak between 300 and 370 K. CT behaves in a similar way after storage, i.e. a glass transition temperature at 226K and an endothermic peak between 280 and 310K due to the melting of crystallized fractions are observed (Figure 3).

Formation of precipitate during blending The amount of solid present in the CT was ~2 vol.% and corresponded to the QI fraction (1.5 wt%). No solid particles were observed in bitumen except for the paraffinic domains, which disappeared on heating 4. Micrographs of bitumen blended with 20wt% CT (B-CT 80-20) are shown in Figure 1. Small black particles could be observed in natural light (Figure la). Their shape did not change during heating at 470 K and they corresponded to solid particles formed during blending. After storage at room temperature for 12h, anisotropic domains composed of crystallized paraffin were also detected under crossed polars (Figure lb). However, they vanished on heating, as observed with bitumen. The amount of precipitate determined by IA is indicated as a function of blend composition in Table 1. It varies from 5 to 7 vol.%. Since

lOO

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i °° '°

20

0

Table 1 Total amount of solid particles present in bitumen-coal tar mixtures CT added (wt%) Solid particles (vol.%)

532

0

lO

20

30

40

50

Size (lira) 0 0

20 5

30 7

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40 7

50 7

Figure 2 Cumulative size distribution of solid particles present in B-CT 80-20 (O) and B-CT 60-40 (0) blends

Physicochemical properties of bitumen-coal tar mixtures: P. Chambrion et al.

-0.6

/ - 423 K

"

v

"~'~.

278K-~ / d s c

~

~

/-- 423K

15tK~53K U ,5~min -0.6

~

~

153K ~-/

-r "~ -0.7 -0.7

~o -0.8 -0.8

-0.9

, 200

, 250

, 300 Temperature(K)

r 350

i 200

, 400

250

300

Temperature(K)

350

400

Figure 3 D.s.c. curvesof(l) bitumen, (2) B-CT 80-20 and (3) coal tar

Figure 4 D.s.c. curvesof(l) B-CT 60-40 and (2) B-CT 50-50 blends

Table 2 Comparisonof measured and calculated Tgvalues(K) for BCT 90-10 and 80-20

according to the equation 7

Calculated

Measured

1

252 248 -

256 252 249 226

Tgmix

M1 -

Bitumen B-CT 90-10 B-CT 80-20 CT

The d.s.c, curve of bitumen blended with 20 wt% CT ( B CT 80-20) stored at 278K for 12h also exhibits two phenomena, as shown in Figure 3. The first is a glass transition intermediate between those of CT and B, while the second is an endothermic peak attributed to the melting/dissolution of the paraffin wax and/or CT crystallized fractions. The occurrence of a single glass transition suggests nearly complete miscibility of the two constituents, except for fractions which crystallized during storage. The glass transition temperature of the mixture of two compatible non-crystalline polymers can be deduced from the Tg values of their components,

a)

Tgl

M2 -I

(1)

Tg2

where T ~ i x represents Tg of the mixture and M1 and M 2 are the mass fractions of the two components. Experimental and calculated values of Tg for blends containing 10 and 20wt% of CT are in good agreement (Table 2). Hence, neglecting the amount of precipitate and the crystallized fractions present in the mixture after storage, B - C T blends can be considered as being compatible up to 20 wt% added CT. D.s.c. curves for blends containing 40 and 50 wt% CT (samples B - C T 60-40 and B - C T 50-50) are shown in Figure 4. The melting peak of crystallized fractions is still present, but two glass transitions are now observed, suggesting the presence of two non-miscible phases, each undergoing a glass transition. Both Tg values are located between those of coal tar and bitumen. The examination of the B - C T 60-40 blend does indeed reveal the

b) I 50~m I

Figure 5 Micrographsof B-CT 60-40 obtained at room temperatureand two differentmagnifications.Dispersedphase: d; precipitate:p; paraffinic compounds: Pa

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Physicochemical properties of bitumen-coal tar mixtures." P. Chambrion et al. 50

] 2

"7

~- 25

/

t~ i. r,

/

/

:o

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f 10

20

30

40

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60

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CT Fraction(%)

Figure 6 Fraction of dark liquid-like phase estimated by image analysis (©) and differential scanning calorimetry (0)

presence of two liquid-like phases: a light and continuous one which contains droplets of a demixed darker one (Figure 5a). Upon heating to 450 K, the shape of the discontinuous darker phase changes and the droplets dissolve into the continuous liquid phase. At higher magnification, the discontinuous liquid-like phase is observed together with precipitated solid particles and paraffinic domains (Figure 5b). To a first approximation, the magnitude of each glass transition (ACp) may be considered as being proportional to the amount of matter undergoing the transition. Hence an estimate of their relative amounts can be determined either by d.s.c, or by IA. The demixed phase seems to correspond to the highest glass transition temperature• The fraction of demixed phase obtained by both methods as a function of the amount of CT in the blend is shown in Figure 6. Although both techniques lead to similar trends for the overall change in the fraction of demixed phase with the blend composition, it

3300 3200 3100 3000 2900 2800 2700 2600

Figure 8 FT-i.r. spectra o f precipitate from (1) B - C T 80-20 and (2) B - C T 50-50

seems that its amount determined by IA is significantly smaller than by d.s.c. This may result from an underestimation of the number of small droplets due to a lack of contrast and/or to the limit of resolution of the optical microscope. Nevertheless, the presence of two distinct liquids in the blend indicates that the components of bitumen and coal tar are redistributed into two nonmiscible phases according to mutual solubility and affinity when the content of CT is >20 wt%.

Chemical composition of blend constituents The two liquid-like phases of a B-CT 50-50 blend were analysed by FT-i.r. microprobe. Since the mean thickness of the sample was of the order of 10 #m, it was assumed that the microprobe analysis would correspond to a single phase, provided that the size of the examined phase was >30#m. FT-i.r. spectra of both liquid-like phases, bitumen and coal tar are shown in Figure 7. Three main absorption bands are present in the spectra• The band at 3047 cm- is attributed to the stretching of aromatic C - H bonds, while those at 2924 and 2854 cm -1 are attributed to the aliphatic C-H. Interestingly, the most important band in CT is the aromatic one, whereas this band is almost absent in bitumen. Hence the overall composition of the different phases present in blends can be estimated with respect to their petroleum or coal tar origin. The presence of the three absorption bands in the liquid-like phases of the blend confirms the partial miscibility of constituents originating from bitumen and coal tar, as suggested by thermal analysis. Furthermore, the comparison of the relative intensity of the bands at 3047 and 2854 cm-1 suggests that the aromatic character is more pronounced in the darker liquid-like phase than in the lighter one. FT-i.r. spectra of precipitates from blends of different composition are shown in Figure 8. All three absorption bands are again found, which indicates that the precipitate includes heavy components originating from petroleum and coal tar. Moreover, the aromatic character is more pronounced in the precipitate from B-CT 80-20 than in that from B-CT 50-50• Hence the

1

3.0

2.5 t~

ul

2.0

1) 1.5

2) 1.0

0.5

a)

~

4)

0.0

3300 3200 3100 3000 2900 2800 2700 2600 2500 Wave number (cm"l) Figure 7 FT-i.r. spectra of (1) bitumen, (2) light and (3) dark liquidlike phases in B - C T 50-50 mixtures, and (4) coal tar

534

Fuel 1996 Volume 75 Number 5

!500

Wave number (cm':)

Physicochemical properties of bitumen-coal tar mixtures: P. Chambrion et al. aromatic character of the precipitate is less marked when two liquid-like phases are present. This result is also consistent with the fact that the dark liquid-like fraction is highly aromatic.

CONCLUSIONS Blending of liquid fractions originating from petroleum and coal sources leads to the formation of a small but significant amount of solid precipitate and to one or two liquid phases at room temperature, depending on the relative amounts of the two components in the mixture. The chemical composition of the precipitate varies with the blend composition. The aromatic character of the solid formed decreases on increasing the proportion of coal tar in the blend, suggesting that heavy compounds from coal and petroleum are precipitating together in a manner depending on the blend composition. When a single liquid phase is observed at room temperature, its glass transition temperature can be determined from the Tg values of the bitumen and coal tar using the rule of

mixtures. Demixing into two non-miscible liquids at a temperature <415 K is observed at a higher fraction of coal tar in the blend without a pronounced change in the amount of precipitate. The phase which separates at room temperature has a more pronounced aromatic character than the continuous phase. These results indicate that a redistribution of compounds takes place in the blend, depending on their respective aromaticaliphatic character rather than on their petroleum or coal tar origin. REFERENCES 1 2 3 4 5 6 7

L&off~, J. M., Claudy, P., Kok, M. V., Garcin, M. and Voile, J. L. Fuel 1995, 74, 810 Noel,F. and Corbett, L. W. J. Inst. Pet. 1970,56, 261 Giavarini,G. and Pochetti, F. J. Therm. Anal. 1973,5, 83 Chambrion,Ph., Bertau, R. and Ehrburger,P. Fuel 199675, 144 Chambrion,Ph., Bertau, R. and Ehrburger, P. Fuel 1995, 74, 1284 Ehrburger,P., Martin, Ch. and Saint-Romain, J. L. Fuel 1991, 70, 783 Fox, J. G. Bull. Am. Phys. Soc. 1956, 1, 123

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