Characterization of Asphalt for Use as Binder to Make Coke Briquets from l o w Rank Coal Char Frank P. McCandless' and John M. Blake' Department of Chemical Engineering, University of Colorado, Boulder, Colo. 80302 Certain asphalts make excellent briquetting binders for use in the manufacture of metallurgical coke briquets from low rank coal char. To characterize asphalts for this use, a wide variety of asphalt products were used as binder in test briquets, and the principal dependent variables of briquet crushing strength and losses associated with curing and coking correlated with the composition (asphaltenes, resins, and oils) as determined by solvent analysis. Average briquet strength, coke residue, and net curing losses all greatly increased with increasing asphaltene content of the binder. The scatter of the data was greater when correlations were attempted with other properties of the binder such as resin plus asphaltene content, and C/H ratio. The effects of curing the green briquets in air compared with curing in an inert nitrogen atmosphere are also reported. The strength of briquets cured in air was more than double that of briquets cured in nitrogen alone.
IN
ONE successful approach to the manufacture of metallurgical coke from low rank ("noncoking") coals, briquets are made from coal char using a treated low temperature tar as binder (Work, 1966). The char is produced by pyrolysis of crushed coal in three successive steps, utilizing fluid bed techniques. Binder produced from tar recovered from the pyrolysis is mixed with the low volatile char and formed into green briquets which are then cured in air a t about 450" F and devolatilized to form low volatile coke, well suited for metallurgical use. A commercial plant utilizing this process is operated by the FMC Corp., Kemmerer, Wyoming. Some asphalts derived from petroleum also make excellent briquetting binders for this process, and they would be an important supplementary source of binder material if the process were applied to coals low in volatile matter which would not give off sufficient tar to briquet all of the solids produced. All asphalts do not behave alike, however; so to characterize asphalts for this use, a number of samples were evaluated by making test briquets from subbituminous coal char using the asphalts as binder. The principal dependent variables of briquet-crushing strength and losses associated with curing and coking were correlated with binder composition as determined by solvent analysis. I n addition, to show the need for curing in oxygen and to help determine the mechanism of the binder, briquets from two of the asphalts were cured in an inert atmosphere of nitrogen and their properties compared with those that had been cured in air.
Experimental
Fourteen samples of asphalt, with a wide range of physical properties and compositions, obtained from five manu-
' Present address, Department of Chemical Engineering, Montana State University, Bozeman, Mont. 59715. Present address, 871 Portola Road, Portola Valley, Calif. 94025.
facturers, were tested. Softening points, solvent analyses, and C / H ratios are shown in Table I . Blends of asphalts and blends of asphalts with an asphaltene fraction were used to obtain data at the compositions not covered by the samples as received. Data on four binders made from low temperature coal tar were also obtained for comparison. Calcined coal char (calcinate), made by devolatilizing a subbituminous coal from the Elkol mine in Wyoming, was used to make the briquets. This material was produced in the FMC coke plant in Wyoming. The char contained approximately 3% volatile matter and about 7% ash, and had a bulk density of about 38.4 pounds per cubic foot. Plant run calcinate was screened and the -10-mesh material used to make all the briquets in this study. Test briquets containing 15% by weight of binder were made by thoroughly mixing the solids and binder in an electrically heated kitchen-type mixer modified for this use. Mixing temperature was varied from about 195" to 250" F, depending upon the softening point of the binder. Briquets, each weighing about 13 grams before curing and coking, were made by compressing the warm mix in a l%-inch i.d. cylindrical die a t 15000 psi with a hydraulic hand press. The resulting green briquets were cylindrical, about 1% inches in diameter and % inch long. They were cured in air for 2 hours a t 230:C in a forced convection oven and then coked for 20 minutes a t 950°C in an electrically heated muffle furnace. Curing was carried out in a wire mesh container to ensure good contact of the briquets with the upflowing current of hot air. Samples were coked in a stainless steel beaker with a loose fitting lid, with a small quantity of loose calcinate placed on the lid to minimize oxidation of the briquets. The briquets were weighed before and after curing, and after coking. The strength was measured by crushing the briquets in a Riehle universal testing machine, with the bottom plate movement set at 0.35 inch per minute. The load Ind. Eng. Chem. Prod. Res. Develop., Vol. 9 , No. 2, 1970
183
Table I. Binder Behavior and Properties Briquet Asphalt Sample No. and Components of Blends
tosses, Wt. Yo of Green Briquet
SofteningC Pt., F
C/H Ratio, Atomic
0.676 0.731
PSI
Curing
Coking
Cokeb Residue
2. 3. 4. 5. 6. 16 & 3 7. 16 & 3 8. 9. 10. 11. 2 & 17 12. 2 & 17 13. 21 & 3 14. 16 & 3 15. 16. 17. 18. 16 & 21 19. 16 & 21 20. 16 & 21 21.
3545 3370 3516 3710 4046 4062 4097 4614 4332 4012 5240 4960 5310 5300 5572 5541 5647 5692 5543 5332 4888
0.9 0.6 0.2 1.1 0.7 0.6 0.6 0.7 0.7 0.7 1.0 0.9 0.7 1.4 1.1 1.9 1.4 1.0 1.4 1.2 1.1
11.9 14.2 12.9 11.5 12.0 11.6 11.8 10.2 11.9 11.7 10.1 9.8 9.4 9.3 8.2 9.2 8.0 8.2 7.7 8.0 7.6
25.3 13.0 23.2 26.4 25.6 29.3 27.8 38.4 27.0 27.8 35.8 39.7 43.4 39.9 49.1 36.5 47.8 49.1 50.0 52.5 53.0
141 156 140 127 144
22. Oxidized 23. asphalts 24.
2920 3460 2600
1.3 1.1
12.2 12.4
20.4 28.0
228 151 160
0.676
25. Low temperature 26. coal tars 27. 28.
3375 3914 4006 4240
3.6 3.4 3.2 2.3
8.8 9.3 8.2 9.1
28.6 26.2 34.5 34.8
...
0.84 0195
I.
...
...
...
Solvent Analysis, Wt.
... 0.662 0.660
...
...
... ...
... ...
170
0.716 0.728
...
... ...
... ...
...
...
...
...
139 156 151
1.08 1.22 1.28
...
... ... ... ...
... ... ... ...
...
144
... ...
... ...
Yo
Asphaltenes
Resins
Oils
15.9 24.7 27.3 28.3 34.8 34.8 35.0 39.9 41.5 42.5 47.4 54.7 62.4 65.2 70.1 77.5 79.8 86.1 88.5 93.0 100.0
17.0 18.6 23.6 17.4 12.1 21.2 20.5 14.5 12.9 9.5 12.4 10.5 12.2 8.0 4.5 2.9 3.8 1.9 1.5 1.0
67.1 56.7 49.1 54.3 53.1 44.0 44.5 45.6 45.6 48.0 40.2 34.8 25.4 26.8 25.4 19.6 16.4 12.0 10.0 6.0
58.9 40.5 43.8
13.3 15.8 46.3
27.8 43.7 9.9
37.4 64.0 93.2 100.0
...
...
...
...
3.5 2.0
32.5 4.8
...
...
“Average of 5 briquets. ‘Assumes 2% of calcinate to volatilize on curing and coking and rest of weight loss to come from binder. Ring and ball method. Pentane-insolubles from 16. Pentane-insolubles from 27.
causing the first downward deflection of the indicator was taken as the crushing strength. The strength reported is the average strength of five briquets in a given batch. For several of the whole asphalts, the data reported in Table I are averages of two or three separate determinations. Agreement among these separate determinations was satisfactory (McCandless, 1963). A method similar to those described in the literature was used to analyze the asphalts (Furby, 1950; Hubbard and Stanfield, 1948; O’Donnell, 1951). Asphaltenes were precipitated from the asphalt by dissolving the oils and resins in n-pentane. The pentane-soluble fraction was then separated into oils and resins by adsorbing the resins on activated alumina. The oils were eluted from the alumina first using n-pentane and then the adsorbed resins were removed by leaching with a mixture of benzene containing 10% ethanol in a Soxhlet extraction apparatus. Solvent was distilled from the asphalt fractions under vacuum.
c
4l BO
20
Results and Discussion 60001
Crushing strength and the losses associated with the process correlate well with the asphaltene content of the binder. This is shown in Figure 1, where these variables, together with the calculated coke residue (per cent of the binder remaining in the coked briquet), are plotted as a function of the asphaltene content of the binder. As can be seen, total coking losses decreased from about 14 down to 7% and the net curing losses increased from about 0.5 to 1.5% as the asphaltene content of the binder was increased from about 20 to 100%. Briquet strength went through a maximum. It increased from about 3500 184
Ind. Eng. Chem. Prod. Res. Develop., Vol. 9,No. 2, 1970
4000
a 30000
PO
40
I
1
80
80
ASPHALTENE CONTENT OF BINDER, W T .h
Figure 1. Binder behavior as a function of asphaltene content
psi a t 20% asphaltenes to about 5600 psi a t 80% asphaltenes and then decreased to about 4800 psi as the asphaltene content was increased from 80 to 100%. I n the range of 80 to 100% asphaltenes, the asphaltene fraction of sample 16 was used to increase the asphaltene content of the binder. The carbon residue or coking value of the asphalts increased greatly from about 15 to 53% as the binder composition was increased to 100% asphaltenes. There was also a trend of increasing strength and decreasing losses as the amount of the pentane-insoluble fraction in the coal tar binders increased, although the strength of this coke was somewhat lower than that made with the asphalt binders. Three asphalts which had been air-blown to increase their softening points made poorer binders than unmodified asphalts-that is, they gave lower strengths and higher coking losses than nonoxidized asphalts of comparable asphaltene contents (Table I). Basically, the purpose of a binder is to provide a strong bond of carbon in the final coke which will hold the original calcinate particles together in the form of a briquet. Actually, in this coke, carbon from the binder and that from the original particles are not easily distinguished, and the particles appear to have been fused together. Fracture occurs through, rather than around, the original particles. T o accomplish this result, a large fraction of the binder must remain as coke in the final product, and this coke must be firmly and intimately bonded to the original carbon particles. The amount of carbon residue or coke deposited by the binder appears to be controlled by its asphaltene fraction. This would be expected when the chemical nature of asphaltenes is considered. They are the most reactive portion of an asphalt, have a high carbon content (or C / H ratio), and are capable of forming a high percentage of coke upon pyrolysis (Barth, 1962). Another factor which may contribute to greater strength with increased asphaltene content is that asphaltenes are preferentially adsorbed from the asphalt by solids such as the highly adsorbent calcinate particles (Barth, 1962). Hence, the higher asphaltene asphalts probably adhere to the calcinate particles better and so form stronger bonds between them.
The asphaltene fraction alone and blends of the asphaltene fraction with the parent asphalt made somewhat weaker briquets, however, than the whole asphalt, even though the residues of binder coke were higher when the asphaltenes were used. This strongly suggests that the asphaltenes were modified in some way during separation, so that their adhesive qualities were poorer. Less complete coating of the calcinate particles because of increasing binder softening point or insufficient plasticizing oils and resins also could cause the strength to decrease somewhat a t asphaltene contents close to 100%. The green briquets made from straight asphaltenes were friable and appeared poorly bonded, so that the decrease in strength above 80% asphaltenes is probably caused by poorer adhesion between binder and solids. Curing in an atmosphere containing oxygen is necessary to obtain the full benefit of these binders. This is shown in Table 11, which compares losses and strength when briquets are cured in air and in an inert nitrogen atmosphere. For both binders tested in this way, strength is more than doubled and coking losses are reduced by over 30% by curing in air as compared to curing without oxygen. The net result of oxidative curing is that the binder is converted to an infusible and insoluble polymer and COn and H20 are given off from the briquets along with tars and traces of CO and light hydrocarbons. This is also shown in Table 11. A large percentage of the weight loss when curing in air is from the formation of CO, and H20, and this carbon and hydrogen come from the binder material. The briquets adsorb some oxygen in the curing process, so that the weight of products distilled should exceed 100% of the weight lost by the briquet, even with the product weights on an oxygenfree basis. The reactions taking place during curing are probably similar to those that occur when asphalts are blown with air to increase their softening points. These air-blowing reactions have been shown to form high molecular weight asphaltenes from lower molecular weight asphaltenes, resins, and oils. Unsaturated bonds are formed by a dehydrogenation reaction with oxygen and these unsaturated compounds condense to form the heavy materials. Aromatization and decarboxylation reactions also occur (Barth, 1962). The curing conditions are much more severe than air blowing, however, since there is a very
Table II. Effect of Oxidative Curing on Binder Behavior for Two Asphalts Binder
21
17 Cured in air
Briquet strength, psi" Net losses on curing, wt. "c of green briquet Losses on coking, wt. 5% of green briquet Coke residue, 5% from binderb Products of curing,
a b c
Tar HjO COL Total
Yo
Cured in 100% Nz
5425. 1.4 3.0 47.6
2490. 1.6 12.3 18.3
Cured in air
0-Free' Basis
of net curing loss
Cured in 100% N2
4935. 1.1 7.6 53.0
2310. 1.3 11.2 27.1 0-Free' Basis
68.5 176.0 93.5
64.0 16.0 21.4
52.2 31.5 5.1
81.9 209.0 98.1
75.0 18.8 25.0
59.0 28.6 3.5 -
338.0
101.4
88.8
389.0
118.8
91.1
'Average of 3 briquets. of binder left in coked briquet. 'Assumes 5% 02 in tars and difference between Hz0 and CO, when cured in air and in N2 from reaction with 02.
Ind.
Eng. Chem. Prod. Res. Develop., Vol.
9,No. 2, 1970 185
~~
Table Ill. Infrared Spectra for Asphalts 17 and 2 and Their Constituents
Beckman IR-5 spectrophotometer. All samples 0.5 gram/100 grams CC1, in 1-mm NaCl cell Per Cent Transmittance
17 Wave No., C m - l
1300 1380 1460 1620 1925 2870 2960 3030
Characteristic Group
Methylene Methyl Methyl Aromatic Methylene Methyl Methyl Aromatic
2
Asphalt
[ ?:!E1-]
Resins
Oils
Asphalt
90 82 66 85 96 79 61 75
91 83 68 85 98 80 65 75
83 76 63 80 95 73 52 74
90 80 61 85 95 66 40 71
88 74 54 95
large surface area exposed which enables the condensation reactions to occur to a much greater extent and results in solidification of the asphalt. Curing temperature is comparable to the temperature of air blowing. Apparently curing in an atmosphere containing oxygen produces asphaltenes and other highly condensed compounds by polymerization of the more reactive portion of the asphalt. These highly crosslinked polymers then form a high percentage of coke in the subsequent pyrolysis. The three oxidized asphalts gave lower strengths and higher losses than unmodified asphalts of comparable asphaltene content. Since it is known that the asphaltene content is increased by air blowing, the asphaltenes or pentane-insolubles formed in this way are apparently less effective binders than those occurring in the unoxidized asphalts. Infrared spectra of two asphalts (samples 2 and 17) showed all fractions of 17, which was high in asphaltenes, to be much more aromatic in character than 2, which was highly paraffinic, and low in asphaltenes (Table 111). I n this work the asphaltene content of unoxidized asphalts appears closely related to the crushing strength, but these two infrared spectra show that the chemical nature of the asphaltenes varies widely. Among these samples, the carbon-hydrogen ratio and thus the aromatic character of the asphalts increase along with their asphaltene contents. This parallel may not always exist-in fact, the oxidized asphalts are comparatively high in pentaneinsolubles, yet the single sample analyzed has a low C / H ratio. Thus, from this fragment of data, the C / H ratio might appear to be the best criterion on which to predict the effectiveness of a binder. However, when the strength is plotted against C / H ratio, for these samples the points scatter much worse than those in Figure 1. Furthermore, the ratio of resins to oils in the petrolene fraction and their respective chemical composition would be expected to influence the behavior of the binder. The effects would, of course, be more pronounced in the low asphaltene asphalts. The asphaltenes are the dispersed phase of a colloidal suspension in the outer petrolene fraction. This fraction, characterized only as resins and oils in this work, is much more variable in character than the asphaltenes (Corbett, 1964) and has recently been further characterized by a breakdown into three generic groups: saturates, naphthene-aromatics, and polar-aromatics (Corbett, 1969). The proportion of these components varies widely with asphalt source. They also have very different chemical properties, as shown by their behavior to air oxidation (King and Corbett, 1969). As a result, a breakdown only 186
Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 2, 1970
[ %kl-] 80 71 59 85
Resins
Oils
80 67 52 90
88 70 46
98
...
...
...
...
26 7 93
42 16 87
32 9 86
16 2 98
into resins and oils probably is not sufficient to characterize asphalts fully for the coking binder use, and this is probably reflected in the scatter of data, which appears to be more pronounced in the low asphaltene content range (Figure 1). However, the effects are small and appear to be obscured by other factors in this work. The above considerations point out the fact that with materials as complex as asphalts, no single, easily measured characteristic can possibly correlate the behavior of all samples, so that the degree to which the asphaltene content has succeeded in classifying the behavior of these samples is striking. I n addition, the character of the calcinate itself also probably influences the behavior of a binder in the process. However, because of the limited scope of the investigation, only one calcinate was investigated. Conclusions
The property of an asphalt which controls the amount of coke residue, and hence briquet strength, appears to be primarily the asphaltene content. Losses decrease and briquet strength increases with increasing asphaltene content of the binder. However, strength decreases when the binder is blended with the separated asphaltene fraction to increase the asphaltene content further, although coke residue continues to increase, so that some other factor, probably adhesion between binder and solids, also affects the strength. Oxidized (air-blown) asphalts make poorer binders than unmodified asphalts of comparable asphaltene content. Curing in an atmosphere containing oxygen is necessary to obtain the full benefit of the binder. This treatment probably forms highly condensed compounds which are capable of depositing a high percentage of coke in the subsequent baking step. Literature Cited
Barth, E. J., “Asphalt,” 1st ed., pp. 40, 123, 140, 391, Gordon and Breach Science Publishers, New York, 1962. Corbett, L. W., Anal. Chern. 36, 1967-71 (1964). Corbett, L. W., Anal. Chern. 41, 576-9, (1969). Furby, N . W., Anal. Chern. 22, 876-81 (1950). Hubbard, R. L., Stanfield, K. E., Anal. Chern. 20, 4605 (1948). King, W . H., Corbett, L. W., Anal. Chern. 41, 580-3 (1969). McCandless, F. P., M.S. thesis, University of Colorado, Boulder, Colo., 1963. O’Donnell, G., Anal. Chern. 23, 894 (1951). Work, Josiah, J . Metals 18, 635-42 (1966). RECEIVED for review October 27, 1969 ACCEPTED January 12, 1970
