Znd. Eng. chem. Prod. Res. Dev. 1988,25,637-640
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Manufacture of Road-Paving Asphalt Using Coal Tar Tsoung-yuan Yan Central Research Laboratory, Mobil Research and Development Corporation, Princeton, New Jersey 08540
Road-paving asphalt meeting the required specifications can be prepared by mixing coal tar with fluid catalytic cracker (FCC) bottoms and whole used tire, excluding steel beits, at proper ratios. Suitable composition can be prepared by heating at 300-315 OC for 1 h, thus ensuring homogeneity of product and good quality. The group constituents of the final mixture are quite similar to those of conventional asphalts. The use of coal tar pitch and whole used tire saves petroleum residue, which can be upgraded to transportation fuels, and alleviates disposal problems of used tires.
Introduction Asphalt is defined by ASTM as a dark brown to black cementitious material in which the predominant constituents are bitumens that occur in nature or are obtained in petroleum processing. Since 1907, most asphalt has been produced from the refining of petroleum. According to production methods, asphalt can be classified into two types: 1. Straight-run asphalt is obtained by a straight reduction of crude and residue under vacuum. In this process, the viscosity of the asphalt is increased, but its chemical nature is not changed. The yield of asphalt depends on the asphaltene content initially present in the crude or residue. 2. Air-blown asphalt is produced by contacting asphalt stock with air at 200-275 "C. In this process, dehydrogenation and polymerization take place, leading to the formation of asphaltenes and the production of hard asphalt; the valuable hydrogen-rich oil (or resin) is downgraded to asphaltenes. Asphalt production peaked at 173 MM bbl/year in 1984 (Killilea, 1985). In recent years, the paving market consumed about 80% of the product, while the roofing industry accounted for about 15% of the asphalt market. Although the price of asphalt varies in different parts of the country, the approximate price level of asphalt cements in 1985 was $170/ton (Chemical Marketing Reporter, (1985) or $31/bbl. In comparison with crude oil at $27/bbl and gasoline at $36/bbl, asphalt is not a premium product. The petroleum residue used for asphalt production could be upgraded to a more valuable gasoline via hydrotreating and catalytic cracking. The processing cost for gasoline production from petroleum residue is high, but it is considerably lower than the cost from other synfuel processes, such as coal liquefaction or oil shale retorting (Parker and Williams, 1986). As the petroleum reserves are depleted, it becomes more and more desirable to produce more gasoline from each barrel of crude oil. One approach is to find other sources of bitumen to produce asphalt and to convert the residue to gasoline. Coal tar is a byproduct of coking and other synfuel processes, such as Lurgi's gasification. In the United States, production of coke oven coal tar in 1980was 13 MM bbl (Dickson, 1983). The current value of coal is $225/ton or $41/bbl according to the Chemical Marketing Reporter (1985). Even though this general value of coal tar is higher than that of asphalt, the relative value of coal tar and asphalt can vary significantly from place to place. In the future, when the synfuel industry is developed, the availability of coal tar will be greatly increased and its economics of utilization will be changed accordingly.
Particularly in some localities such as South Africa, new and unconventional applications should be developed to use the coal tar effectively. Coal tar is a ready source of asphaltenes needed in asphalt production. Coal tar pitch itself, however, is unsuitable for making road-paving asphalt since the resulting material has low ductility, high temperature sensitivity, and low resistance to wear. For this reason, in England, where replacing imported petroleum with local products was important 10-20 years ago, it was required that no more than 10-20% coal tar pitch be incorporated in road pavement. At higher concentrations, the pitch separates from the petroleum-derived asphalt, causing brittleness and cracking. To make a good asphalt from coal tar pitch, chemical modification of blending with additives appears necesstuy. In this study, the potentials for producing road-paving asphalt from coal tar and available inexpensive petroleum fractions were explored. The objective of the study was to develop new uses of coal tar for asphalt production and to free the petroleum residue for upgrading to gasoline and diesel fuels.
Road-Paving Asphalt Tests and Specifications. The important tests for road-paving asphalts are the following: (1) Viscosity test, measures fluidity at a given temperature. The viscosity at 140 OF (60 "C) is used to classify asphalt. (2) Penetration test, indicates asphalt consistency. (3) Flash point test, measures the temperature to which the asphalt may be safely heated without an instantaneous flash in the presence of open flame. (4) Solubility test, indicates the presence of nonorganic constituents. (5) Rolling thin-film oven test (TFOT), indicates the oxidation stability during the mixing with hot aggregate in the pug mill, simulates aging in the road. Loss in weight and changes in penetration, viscosity, and ductility due to the prescribed heating cycle are measured. (6) Ductility test, indicates the inverse brittleness of asphalt and seems to be related to the adhesiveness of the product. An asphalt of higher ductility exhibits greater cementing properties for the stone aggregate. It is also worth noting that ductility is an indirect indication of the wax content of the asphalt. (7) Ring and ball softening point test, measures the softening point. The effect of temperature on the test results for viscosity, penetration, or softening point indicates the temperature susceptibility of the asphalt. Typical asphalt specifications for the United States are shown in Table I. It is recognized that the specifications vary significantly among localities and countries. 0 1986 American
Chemlcal Society
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 4, 1986
Table I. Requirements for Asphalt Cement, Viscosity Graded at 140 OF (60 OC)O viscositv" grade test viscosity, 140 O F (60 "C), P viscosity, 275 O F (135 "C), min, cSt penetration, 77 O F (25 "C), 100 g, 5 s, min flash point, Cleveland open cup, min, O F ("C) solubility in trichloroethylene, min, % tests on residue from thin-film oven test viscosity, 140 O F (60 "C), max, P ductility, 77 O F (25 "C), 5 cm/min, min, cm
AC-2.5 250 f 50 80 200 325 (163) 99.0
AC-5 500 f 100 110 120 350 (177) 99.0
AC-10 1000 f 200 150 70 425 (219) 99.0
AC-20 2000 f 400 210 40 450 (232) 99.0
AC-40 4000 f 800 300 20 450 (232) 99.0
1250
2500 100
5000 50
10 000 20
20 000 10
loo*
"Grading based on original asphalt. bIf ductility is less than 100, material will be accepted if ductility a t 60 O F (15.5 "C) is 100 minimum a t a pull rate of 5 cm/min.
Table 11. Compositional Analysis asphaltene, wt composition asphalt range ideal coal tar pitch FCC bottoms Arab light residue
oil, wt %
resin, wt %
%
40-50 44 25 50 50
30-42 33 25 45 40
18-26 23 50 5 10
Chemical Compositions Requirements Asphalt is a colloidal dispersion of high molecular weight hydrocarbons (asphaltenes) in an oil medium (maltenes). The asphaltenes are insoluble in the oils, but may be dispersed through peptization with resins. The properties of the asphalt thus depend on the nature and concentration of its constituents, asphaltenes, oils (aromatics and saturates), and resins. Generally, the concentration of the asphaltenes determines the consistency of the asphalt: the higher the asphaltene content, the harder the asphalt. Similarly, a high aromatic oil content leads to high ductility and temperature susceptibility. Resins promote durability and ductility in the asphalt. To meet the specifications for asphalt, the composition of oil, resin, and asphaltene has to be balanced. Williford (1943) has proposed a group composition for an ideal asphalt (see Table 11). Survey indicates that the compositions of commercial asphalts, regardless of their crude source, fall within the proposed specification limits. The coal tar is too rich in asphaltenes and short on resins to meet the compositional requirement. Approach of This Study Coal tar pitch itself is not a good road-paving asphalt because its composition is so different from the ideal asphalt (Table 11). Coal tar pitch is characterized by high asphaltene and low resin and oil contents. As a result, coal tar pitch is hard, low in ductility, and high in temperature susceptibility. In the refinery, the fluid catalytic cracker (FCC) produces bottom products (FCC bottoms). In contrast to coal tar pitch, this stream is high in resin and oil but low in asphaltenes (Table 11). It is apparent that the mixtures of coal tar pitch and FCC bottoms can approximate the compositions of the ideal asphalt. Because of its high aromatic and resin contents, FCC bottoms are an excellent solvent for asphaltenes (Table 111). Thus, synthetic asphalt of the desired hardness can be prepared by mixing the coal tar pitch and FCC bottoms at proper ratios. The pitch provides asphaltenes for thickening, while the FCC bottoms provide resins and act as a dispersant to improve ductility. The oil in both FCC bottoms and coal tar pitch is highly aromatic, leading to products of high temperature sensitivity or low penetration index. To overcome this defi-
Table 111. Properties of FCC Bottoms and Coal Tar FCC bottoms coal tar elemental analysis 90.13 90.02 carbon, wt % 4.63 7.35 hydrogen, wt % 0.99 oxygen, wt % 2.53 0.44 1.42 nitrogen, wt % sulfur, wt % 1.09 0.40 hydrogen character," % aromatics 37 91 benzylic 30 phenolic 5 4 aliphatic 33 pour point, O F 50 distillation 5%, O F 800 95%, O F 905 By NMR analyses.
Table IV. Mass Spectroscopic Analysis and Estimate of Labile Hydrogen of Torrance FCC Bottoms naphthenic aromataromatlabile H2, compd its, % ics, % % alkylbenzene 0.4 naphthene benzenes 1.0 0.03 dinaphthene benzenes 3.7 0.16 naphthalenes 0.1 acenaphthenes (biphenyls) 7.4 0.08 fluorenes 10.1 0.11 13.1 phenanthrenes naphthene phenanthrenes 11.0 0.18 pyrenes, fluoroanthenes 20.5 chr ysenes 10.4 benzofluoroanthenes 6.9 perylenes 5.2 benzothiophenes 2.4 dibenzothiophenes 5.4 naphthobenzothiophenes 2.4 0.04 total
64.4
35.6
0.61
ciency, inexpensive tires are used as polymer additives. With this approach, road-paving asphalt meeting the specificationswas produced by dissolving used tire and coal tar pitch in FCC bottoms (Yan, 1979, 1980). The FCC bottoms are a good solvent and powerful enough to dissolve wood, paper, and coal effectively (Yan, 1982, 1983). Experimental Section Materials. FCC Bottoms. The material was unconverted feed from FCC operation and was obtained as the bottom product from the FCC distillation tower in a refinery. It was used in this study as received. The elemental, NMR, and mass spectroscopic analyses are shown in Tables I11 and IV. The feed is high in aromatics and hydroaromatic content with a significant amount of labile or transferable hydrogen.
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 4, 1986 639
Coal Tar. The coal tar was a coke oven byproduct obtained from Bethlehem Steel. The elemental and NMR analyses are shown in Table 111. It was used directly for blending. In some runs, it was topped to a nominal 370+ OC, yielding approximately 75% bottoms or coal tar pitch. The pitch was rich in naphthalene-like material and solidified on the retort. Used Rubber Tire. The whole rubber tire (Dunlop brand), excluding steel belts, was chopped into approximately 1/4 in. x 1/4 in. X l/* in. particles. The particle size was chosen to facilitate the experiments in the laboratory. Because of its high dissolution rate, the particle size can be as large as several inches and is only limited by handling consideration. Thermal gravitational analysis shows that tires start to decompose at temperatures as low as 250 "C (perhaps as a result of devulcanization) and rapidly decompose at 440 OC. Whole used tires (not including steel belts) contain about 70% rubber. The remaining materials are approximately 25 % carbon black (nominally 30004000-A particle diameter), 2% sulfur, and a few percent tire cord. Procedure. Coal Tar-FCC Bottoms Mixture. The coal tar (or coal tar pitch) was added to FCC bottoms to a desired ratio, and the mixture was heated to about 400 O F (200 "C) with constant stirring until a homogeneous phase was formed. It was noted that the coal tar pitch separated out when the mixing temperature was too low and agitation not vigorous enough. Rubberization of Coal Tar Pitch-FCC Bottoms Mixture. About 5 wt % whole cut tire was added to a 50/50 mix of coal tar pitch and FCC bottoms. Experiments in a heated autoclave with agitation showed that 1h at 300-315 " C (575-600 O F ) or 2 h at 290 "C (550 OF) was required to dissolve the tire. During the reaction period, no degradation of the asphalt was noticed as indicated by blank runs. The homogeneity of the product was assured by visual inspection. The product was cooled to a solid and then split with impacting force for inspection of the new surface. A shining surface indicated it was homogeneous, while a dull surface indicated it was nonhomogeneous and dissolution was incomplete. Results and Discussion 1. Coal Tar-FCC Bottoms Mixture. Controlling the ratio of coal tar and FCC bottoms produces a homogeneous bitumen of desired hardness useful as road-paving asphalt. Coal tar pitch is not miscible with most petroleum feedstocks. When its concentration exceeds a certain level, generally about 20%, the coal tar separates from the oil and forms a new phase. This is because petroleum stocks, particularly those rich in saturates, are not good solvents for asphaltenes. Aromatics, on the other hand, particularly naphthenic aromatics in FCC bottoms, have resin-like qualities, and the observed miscibility of the pitch in the FCC bottoms may be explained by peptization of naphthenic aromatics with the asphaltenes available in coal tar. This high solvent power of the FCC bottoms for the asphaltenes permits good binding material (Le., roadpaving and roofing asphalt) to be made from a mixture of coal tar pitch and FCC bottoms. The pitch provides asphaltenes for thickening, while the main-column bottoms act as a dispersant and lead to ductility. The hardness of the mixture increases as the concentration of coal tar pitch increases. The effect of concentration of coal tar pitch, C, on the penetration number is shown in Figure 1. Any penetration number can be obtained by varying C; for an AC-10 grade (penetration of 70-80), the necessary value of C appears to be 50-60 wt
\
0
\ 8\
10 40
45 50 55 60 Concentration o f Coal Tar Pitch, C , w t %
65
Figure 1. Penetration number vs. concentration percent coal tar pitch in FCC bottoms. Table V. Properties of Coal Tar-FCC Bottoms Mix (50/50) a n d Buffalo AC-10 Asphalt coal tar-FCC Bottoms AC-10 penetration at 77 OF, 100 g/5 s 73 66 ductility a t 25 OC, 5 cm/min before TFOT >140 100.20* after TFOT 120 7b viscosity at 140 O F , P before TFOT 750 1109 after TFOT 12 092 16454 weight loss in TFOT, w t 70 2.2 +0.4 "TFOT condition: 325 OF/75 min. bDuctility measured at 15 5 cm/min.
OC,
%, depending on the natures of the initial coal tar pitch and the FCC bottoms. The properties of a 50/50 mixture of coal tar (not topped) and FCC bottoms are shown in Table V. In comparison with AC-10 air-blown asphalt (also shown in Table V), this mixture is superior in ductility and oxidation or durability. However, the weight loss in the TFOT is 2.2%, exceeding the 1%maximum specifications. The TFOT specification could be met if the coal tar were topped, but the optimum cut-point has not yet been determined. I t might be desirable to retain as much as possible of the light fraction, since it is rich in phenolic antioxidants. 2. Rubberization of Coal Tar Pitch-FCC Bottoms Mixture. There is one important property for road-paving asphalt in which the mixture of FCC bottoms-coal tar pitch is deficient, namely, high temperature susceptibility or low penetration index. This deficiency is inherent in the aromatic nature of the mixture. Polymer additives are known to be useful in obtaining decreased temperature susceptibility, which usually covers the whole range of temperatures from below the brittle point to well above the softening point (Barth, 1968). In
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 4, 1986
ber tire are shown in Figure 2. In this figure, the viscosities of the products at 140 OF (in poise) and 275 OF (in centistokes) are cross-plotted. For comparison, those values for AC grades 2.5,5,10,20,and 40 are also plotted. They form a reasonably straight line which can be used to distinguish and judge the temperature susceptibility of the products in question. Those falling on the left of the line are not acceptable, while those on the right are acceptable. Without rubber addition, two preparations of 50/50 pitch-FCC bottoms failed to be acceptable (points 1 and 2). The addition of 5% whole tire to a 50150 mix (point 3) decreased its temperature susceptibility and made the material acceptable. The whole tires also contain about 25% carbon black, 2% sulfur, and a few percent tire cord. Because of the high dissolution power of the FCC bottoms, the whole tires can be used directly without removing these extraneous materials, leading to an inexpensive process. The steel belt can be removed readily after dissolution of the tire. In addition, whole tires used for asphalt production will alleviate problems associated with waste disposal.
1
/
4000 -
P / 0
2000-
.-W
VI
3Q
n 0 .
LL
2
1000 -
4
/
800.-3
8
"
600
-
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v1
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>
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0 AC S p e c i f i c a t i o n t
Coal t a r p i t c h
t
Used t i r e
-
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Coal t a r p i t c h FCC bottoms
1
40
, 60
I
1
80 100 V i s c o s i t y a t 275'F,
I
I
200
400
600
C.S.
Figure. 2. Temperature susceptibility of coal tar pitch-FCC bottoms vs. U.S. AC specification.
addition, polymers are also effective in improving qualities of asphalt, such as ductility, adhesiveness, toughness, elasticity, impact resistance, etc. (Barth, 1968). Government rubber-styrene (GR-S), nitrile, and other neoprene rubbers have been used for this purpose. Beneficial effects have been obtained when these "rubbers" are dispersed uniformly at the molecular level in the asphalt. Consequently, they are added in the form of latex, which is costly. Although reclaimed rubber from used tires has been employed to rubberize road-paving and sealing asphalts (Lewicke, 1973), such an approach is rather expensive. Whole used tires (except the steel belts) were completely dissolved in FCC bottoms and mixtures of FCC bottoms-coal tar pitch when heated for 1 h at 300-315 OC for 2 h at 290 "C. Such dissolution ensures uniform dispersion of the rubber in the asphalt mixture and leads to maximum effectiveness of the rubber for improving the asphalt quality. It is noted that during the high-temperature dissolution of whole used tires, no degradation of the asphalt was observed. However, about 5% of the rubber tire was degraded to light fragments. Stripping was required in order to meet the weight-loss specification in the TFOT test. The temperature susceptibilities of the coal tar pitchFCC bottoms mixtures with and without addition of rub-
Summary In our exploratory study, it was shown that road-paving asphalt meeting general specifications can be produced by mixing whole tire, coal tar pitch, and FCC bottoms at proper ratios. AC-10 type asphalt was formulated with 5% whole used tire and coal tar pitch and asphalt bottoms each at 47.5%. The chemical composition of this mixture is quite similar to that of conventional asphalt. The mixture can be prepared by heating at 300-315 "C for 1 h. FCC bottoms are a powerful solvent for dissolving whole used tires and peptizing the asphaltenes in the coal tar pitch to form homogeneous synthetic asphalts. Whole used tire is an inexpensive rubberizing compound that improves asphalt properties by decreasing its temperature sensitivity and increasing its ductility. The use of coal tar pitch for asphalt production results in the saving of petroleum residues which, in turn, can be upgraded to gasolines and diesel fuels. The use of whole tire in asphalt production leads to an inexpensive rubberization process and a solution for waste disposal problems. Literature Cited Barth, E. J. Asphalt; Gordon and Breach: New York, 1968; pp 624, 640. Cbem. Mark. Rep. 1985, 228(2), 31. Dickson, S. E. Chemical Economics Handbook, SRI International: Menlo Park, CA, 1983; p 212.1001W. Killilea, T. F. ChemicalEconomics Handbook; SRI International: Menlo Park, CA, 1985; p 229.3001A. Lewicke, C. K. Environ. Sci. Technol. 1973, 7, 188-190. Parker, M. A,; Williams, B. Oil. Gas J. 1986, 84 (July 20),50-51. Williford, C. Univ. Tex. Publ. 1943, No. 73, 14. Yan, T. Y. I n Anern. Energy Sources: [Roc. Miami Int. Conf.] 1977 1978, 6, 79-102. Yan, T. Y. US. Patent 4139397, 1979. Yan, T. Y. US. Patent 4211 576, 1980. Yan, T. Y. Fuel Process. Techno/. 1983, 7, 121-133
Received for review F e b r u a r y 24, 1986 Accepted J u n e 24, 1986
