COKING METHODS AND PRODUCTS
I
S. W. MARTIN and H. W. NELSON Great lakes Carbon Corp,, Morton Grove, 111.
Pitch Binder Coke Yields
Coking values for binder pitches used for carbon electrodes and anodes have usually been determined with little reference to commercial baking practices. A new method, described here, simulates conditions in largescale operations and can accurately predict behavior of these pitches
HIGH
molecular weight hydrocarbons, produced by distilling coal tar or petroleum residua to yield pitches, are used as binders for fine carbonaceous aggregate materials. Specifically, asphalt pitches are employed for briquetting coal and/or petroleum coke fines into products sold in domestic fuel markets (75). For producing shaped amorphous and graphite articles, coal-tar pitch binders ( 7 7) are preferred, primarily because high pitch-coke yields are obtainable from their high molecular weight aromatics. The stringent quality specifications imposed by so-called baked carbon products clearly differentiates high temperature tar pitches from straight petroleum or asphaltic binders. I t is not surprising to find in the literature various methods for determining coking propensity of pitch binders. These methods are essentially empirical and are subject to criticism; no attempt is made to simulate heating conditions usually employed to carbonize pitch binders when making shaped articles. Therefore, a definite need exists for a research tool, as well as a binder quality
specification, to determine the ultimate pitch bHder coke yield obtainable from commercial pitches.
Coking Value Test Methods Several empirical methods for determining coking values of pitch binders are currently used-e.g., Fixed carbon, Barrett Method B-8 (3), Conradson carbon residue (7), and the Norsk Method (6). Techniques in these tests vary, essentially in the rate and period of heating:
'
Method Fixed carbon Conradson carbon residue
Norsk
Typical Heating Coking Period, Values, Min. %
7 30
330
43 58 60
Each of these three methods is essentially an empirical adaptation of a test which was originally developed for other materials. The Norsk method comes the closest to simulating conditions which a coal tar pitch is subjected to in commercial baking practice. Typical values, given in the preceding
tabulation, apply to a straight run coaltar pitch with a melting point (cube in air) of 107" C. In a commercial baking practice, this pitch would give a coke yield of at least 65% which is computed from weights of the formed "green" carbon pieces before and after baking. The green pieces are placed in the cell of a gas- or oil-fired furnace and separated from one another by a packing material consisting of granular coke or a coke-sand mixture. During firing, pitch binder evolves hydrocarbon gases in the temperature range of 300' to 500' C., and becomes rigid at about 450" to GOO" C. Above this temperature, a fairly large shrinkage takes place as the pitch binder coke hardens (7). The maximum temperature reached depends upon the ultimate use of the baked carbon article. Baking cycles vary from plant to plant but usually require about a week for heating and a similar period for .subsequent cooling. A typical temperature cycle for baking in an electrode plant was published by Hader (77). I n some research laboratories, binders are checked for coking value by producing test carbons on a small scale. These VOL. 50, NO. 1
JANUARY 1958
33
are baked in the laboratory under conditions simulating commercial practice. Although the results obtained by this procedure are quite accurate, nevertheless, a considerable expenditure of time and effort is involved along with extensive facilities. Therefore, it appeared worth while to develop a laboratory coking technique which determines accurately and consistently the optimum coke yield obtainable from established or candidate pitch binder materials. Great lakes Coking Method A number of conventional furnaces were tried but, primarily because of unequal temperature distributions and variable upheat rates, the results were erratic. Therefore, it was first necessary to design a furnace with markedly improved operating characteristics. A symmetrical furnace with several unique features was constructed to provide identical heating characteristics for each of 12 specimen cavities. The two-piece core consists of a cylindrical graphite block machined in such a manner that the central portion is a hollow vertical axis with 12 specimen cavitirs in a single plane perpendicular to the axis, spaced equidistant from the axis at 30’ intervals. High thermal conductivity of the graphite block permits rapid and uniform heat transference from heat source to test specimen. Graphite was used for the furnace core because it was readily available and easy to machine.
A silicon carbide resistance-heating element, sheathed by a fused silica tube, is placed a t the axis of the graphite block. This heating element provides practically a point source of radiant heat in line with the test specimens. Thus hot spots are eliminated. After the firing cycle is completed, the end plugs in the central ceramic cylinder can be removed and the chimney effect provides a means of rapid cooling. Auxiliary equipment required to ensure adequate control and operation of the furnace include a transformer, wattmeter, time-cycle program controller, and nitrogen purge system for introducing measured quantities of purified nitrogen. In general, this furnace with its compactness and simplicity of design is a most satisfactory laboratory tool. Procedure for Determining Great Lakes Coking Values. A 15-ml. cylindrical porcelain crucible (Coors No. 1) provided with size C lid is ignited and weighed. A 4.0-gram sample of dry pitch, crushed to pass a 20-mesh sieve, is weighed into the crucible which is then covered and placed in the center of the specimen cavity. Loading of the furnace is carried out at room temperature. To avoid graphite pickup on the unglazed bottom surfaces of the crucibles, a thin porcelain disk is inserted on the bottom of the specimen cavity. Duplicate samples are placed in diametrically opposite specimen cavities-e.g., in the 3 o’clock and 9 o’clock
positions. Duplicate samples of a pitch carefully prepared as a reference standard should also be included in each run. After the furnace block is loaded, the graphite lid is placed on the furnace block body, and powdered carbon (calcined petroleum coke pulverized to -50% 200-mesh) is distributed in both the annular space between the block and tile and on top of the graphite cove1 to the level indicated in illustration below (left). This prevents oxidation of samples. The refractory lid is then set on top of the powdered carbon and the insulating cover above the refracton lid. A wet paste of asbestos insulating cement is used to seal openings or cracks on top of the furnace, and finally thermocouple, electrical, and purge gas connections are made. The apparatus is placed on an automatic time-cycle control and heated to 1000° C. for 40 hours (solid line in Figure 1). Dry purified nitrogen is used to purge the furnace and maintain a nonoxidizing atmosphere. At the start, the volume of nitrogen is adjusted to a rate of 0.5 cubic foot per hour, but after 24 hours this is increased to 1.0 cubic foot per hour. When the maximum temperature of 1000O C is reached, the power is disconnected and the furnace allowed to cool while maintaining a nitrogen purge of I 5 cubic feet per hour. The crucible and coke residue are weighed together with cover, and the coke residue as weight per cent of the
Great Lakes laboratory furnace for measuring coking values Left.
Cross section Silicon carbide heating element 6. Ceramic spacer plugs C. Silica tube D. Graphite furnace block, body E. Specimen cavity F. Graphite furnace block, lid G, H. Powdered carbon 1. Insulating fire brick J. Ceramic tile K. Cast refractory lid I . Cast insulating lid M. Cast refractory support for tile N. Dicalite powder insulation P. Spacer ring support Q. Steel furnace shell R. Reinforcing ring S. Steel plate bottom U. Heating element support V . Control thermocouple port W. Indicator thermocouple port X . Purge gas port Y. Plastic seal 2. Asbestos insulating cement Below. Looking down into the furnace. Specimen cavities surround the axis
A.
34
INDUSTRIAL AND ENGINEERING CHEMISTRY
COKING METHODS A N D PRODUCTS using Fixed carbon, Norsk, and Conradson carbon methods, and may be regarded as close to the ultimate yield expected from a pitch binder coked under optimum conditions. The amount of coke obtainable from a pitch binder increases with the melting point which in turn is an indirect measure of distillate removed from the original tar feed stock. For a given melting point range, coal tar pitches vary as to specific gravity (4,benzene insolubles (Z), quinoline insolubles (5)and so-called beta resins which are defined as the difference between benzene insolubles and quinoline insolubles. These insolubilities, recognized by both producers and consumers of pitch binders, are commonly used as criteria of pitch binder quality. To this list of tests, the Great Lakes coking value obtained under carefully controlled conditions and with a high degree of reproducibility can now be added. Nevertheless, if this test is to qualify as an improved procedure for high temperature tar pitch binders, it should simulate coke yields usually realized when converting green to baked carbon products. To establish this, a number of different coal-tar pitch binders, used in making prebaked anodes for reducing alumina (73), were selected, and the Great Lakes coking value was determined. Laboratory-prebaked anode mixes were prepared and formed under conditions simulating commercial practice. Pitch concentration was fixed at 17.3 weight % ' and aggregate and pitch were thoroughly mixed at a temperature of 145' C. for 45 minutes. The anode was formed in a 5 X 9 inch cylindrical mold maintained at 120' C. and a pressure of 5000 pounds per square inch was applied for 3 minutes to form
The complete furnace with controls, ready for operation
data were analyzed to determine the variability or precision of the test. Variation between runs was not significantly different from that within runs; consequently, these variances were combined to estimate the standard deviation of the test. In 29 runs, the average coking value for 70 determinations was 71.5% and the standard deviation 0.52%. Great Lakes coking values, obtained for a variety of pitch products (Table I), are substantially greater than those
pitch charge is reported as the Great Lakes coking value. Variables in the Great Lakes Coking Test
To standardize the Great Lakes coking test, a carefully prepared standard pitch sample was selected (Table I). Numerous coking runs were made in which this standard sample was used and the
Table 1.
Typical Properties and Coking Values of Pitch Binders Insolubles. Wt. %
Sample x0.a
c.
67j 74b 76b 85 89 98 104 105 105 107 107 109 110 111 115 119 128 185
Sp. Gr. 25' C./25' C.
Benzene
Quinoline
1.274 1.292 1.247 1.293 1 303 1.308 1.317 1.303 1.300 1.303 1.320 1.270 1.318 1.341 1.289 1.317 1.329 1.353
17.7 24.6 19.4 20.6 21.1 32.3 26.5 31.9 37.0 30.2 31.5 24.4 30.1 39.4 20.3 30.2 40.1 44.8
6.7 7.9 6.8 7.6 8.5 13.3 14.5 15.3 10.8 10.8 12.4 19.2 13.5 18.5 3.0 9.1 12.5 14.0
Beta resins
11.0 16.7 12.6 13.0 12.6 19.0 12.0 16.6 26.2 19.4 19.1 5.2 16.6 20.9 17.3 21.1 27.6 30.8 B, I, R, G , and J are coal-tar pitches from different sources; 0 series are oil-gas tar pitches. C/W, but in all other cases, C/A. 1-1 B-1 0-1 1-2 1-3 I -4 B-2 G-1 G-2 J -1 Stand. 0 -2 1-5 I -6 F-1 , R-1 I -7 1-8
*
M.P.,
I
GL 63.2 64.5 61.1 68.0 69.7 70.5 69.8 68.3 67.4 65.5 71.5 68.2 72.5 73.0 71.4 72.4 73.5 83.9
Coking Value, Wt. % Conradson carbon Norsk 42.9 46.9 48.2 48.3 51.0 55.9 58.1 52.4 56.4 60.0 57.5
43.5 47.5 47.7 48.7 53.8
57.0
59.1 59.5 61.0 52.0 60.5 60.4 76.3-
57.3 56.0
53.0 56.8 56.2
59.9
57.9 61.0 52.5 60.5 59.9 75.0
Fixed carbon 30.5 36.2 35.3 37.8 38.6 42.2 40.9 40.1 44.8 43.2 43.0 44.7 43.6 46.8 37.4 46.3 49.2 60.4
,
~
VOL. 50, NO. 1-
~
JANUARY 1958
35
Table II. Sample KO."
1 2 3 4 5 6 7 8 9 10 11 12
M.P.,
c.
98 99 105 106 109 110 111 111 112 112 113 116
Great Lakes and Binder Coke Yields from Prebaked Experimental Anodes Insolubles, Wt. % G L Coking Anode Binder __
Sp. Gr. 250 CJ25 a
c.
1.308 1.298 1.300 1.311 1.305 1.310 1.330 1.341 1.292 1.373 1.319 1.285
Benzene
Quinoline
Beta resins
32.3 29.4 37.0 26.3 35.9 34.1 37.7 39.4 2!.4 37.8 32.4 29.6
13.3 6.6 10.8 9.7 15.0 12.9 14.5 18.5 7.2 18.7 10.1 10.7
19.0 22.8 26.2 16.6 20.9 21.2 23.2 20.9 14.2 19.1 22.3 18.9
GL Coking Value/
Value, Wt. %
Coking Valueb, Wt. yo
Anode Binder Value
70.5 69.2 67.4 70.6 68.7 71.2 69.0 73.0 72.3 72.2 72.1 70.4
65.4 63.8 66.0 68.0 69.2 67.3 69.3 70.0 67.0 71.2 68.5 68.0
1.08 1.08 1.02 1.04 0.99 1.06 1.00 1.04 1.08 1.01 1.05 1.04 1.04 f 0.03
Av. a
Sample 1, 3, 8, correspond t o 1-4, G-2, and 1-6, respectively, of Table I. Baked anode wt. - coke aggregate wt. x 100. Pitch binder wt.
the green specimen. Baking was accomplished in a 3.2 cubic foot Globar furnace employing a heating cycle of 10" C. per hour to the final baking temperature of 1050' C. The Great Lakes laboratory method averages 1.04 f 0.03 times the actual coke yield obtained in making prebaked anodes (Table 11); therefore, it is a desirable characterizing factor in evaluating pitch binder quality. Before accepting this conclusion, however, the effect of several well recognized variables on final results was evaluated. Specifically, the final temperature of 1000" C. specified in the Great Lakes procedure was arbitrarily chosen to conform with that commonly employed in commercial baking practice. Therefore, the final cycle temperature for two different pitches was varied (Table 111). Also two significant properties of pitch binder coke were determinedke., real density (9) and powder electrical resistivity (70). Final baking temperature did not significantly influence coking values. However, with increasing baking temperature, pitch binder coke real density increased and powder electrical resistivity decreased as expected. For this test procedure, a final temperature between 900' and 1000" C. is well established. Another significant factor is the selected up-heat rate as well as the reproducibility of this heating cycle from run to run. The effect of decreasing the cycle time from 40 to 13 and 10 hours is evident in Table IV. The heating rates used in these runs are given in Figure 1. With decreasing cycle time, the coking value decreases and tends to approach values for Conradson carbon and Norsk. Consequently. rapid up-heat rates may prove convenient for laboratory testing or commercially controlling pitch quality, but the results are of questionable significance unless a minimum cycle time of about 40 hours is chosen a t least t o approximate yields realized in commercial baking opera-
36
Table 111.
Effect of Final Temperature on Coking Value and Binder Coke Properties (Great Lakes method)
GL Cokine
I
Final Temp., a
c.
Value,
Wt. %
Real Density of Coke Residue, G./Co.
Electrical Resistivity Ohms/Cu. In.
Pitch Binder A
800 900 1000 1100
71.1 71.3 71.2 70.2
800 900 1000 1100
73.8 73.3 73.7 73.2
1.846 1.866 1.900 1.964
0.222 0.091 0.079 0.054
1.839 1.875 1.901 1.979
0.265 0.097 0.085 0.055
Pitch Binder B
tions. When special circumstances warrant lower coking yields than those of the Great Lakes method, it is necessary only to shorten the cycle time to the desired coking level and comparisons of pitch quality may then be made a t this level of coking propensity. The Great Lakes method still offers inherent
INDUSTRIAL AND ENGINEERING CHEMISTRY
advantages of coking reproducibility and control of heating cycle a t any predetermined cycle time. At best, any laboratory coking value test is the result of compromises among various opposing trends. T h a t the Great Lakes procedure represents an improvement over currently employed empirical
20 25 TIME I N HOURS Figure 1.
30
35
40
Heating rates used in standard and accelerated coking test runs
1
i
COKING METHODS AND PRODUCTS tests can be confirmed by demonstrating its utility in characterizing pitch binder quality from both conimercial and fundamental standpoints. Great lakes Coking Values of Various Materials
Coking propensity of carbonaceous material is directly related to the character of the material under examination (Table V). A paraffin wax, in the pitch binder melting point range, exhibits negligible coking propensity and low coking values obtained by various methods are comparable in characterizing ability. ' The wax has a low carbonhydrogen ratio which is in accordance with its molecular structure. A heavy catalytic cracking cycle oil with a carbon-hydrogen ratio of 8.21 exhibits higher coking propensity than a paraffin wax. At this low coking level, the Great Lakes value is higher than those for the other methods. A cracked petroleum residuum was vacuum-reduced to a straight petroleum pitch with a melting point of 110' C. and a carbon-hydrogen ratio of 12.4. The Great Lakes value is 55%, whereas
Norsk and Conradson's are about 50% and Fixed carbon only 31%. A coal tar-derived Resin C, similarly in the category of a one-phase pitch material, has a high carbon-hydrogen ratio of 18.8 and a corresponding high degree of aromaticity. A Great Lakes yield of 64.9y0 is in complete accord with this molecular structure. Therefore, as a first approximation, these coking tests definitely indicated aromaticity which in turn may be measured by the carbonhydrogen ratio. Two solid carbonaceous materials were also tested-Thermax carbon, a thermal black with a particle size of about 0.3 microns, a carbon-hydrogen ratio of about 250 and, as a result, a high Great Lakes value of 99.7%. The graphite flour had a value of 99.9% which is to be expected of pure carbon. The samples were selected for simplicity in structure as well as composition, on the one hand, representing a single-phase hydrocarbon system and, on the other, a single phase carbonaceous sytem. I n coal tar pitch binders, a well recognized two-phase system exists, composed of finely divided solid particles, measured by quinoline insolubles, and a
~
Table IV.
~~~
~
Effect of Heating Cycle Time on Coking Value (Great Lakes)
Sample No. M.P.,O C. C/W
B-1
Stand.
...
74
C/A
Sp. gr., 25" C./25O C.
1-5
...
...
107 1,320
1.292
0-1 76
...
110 1.318
1.247
wt. % Benzene insolubles Quinoline insolubles Beta resins GL coking value Cycle time, hr. 40 19 10
Conradson carbon Norsk coking value Fixed carbon
24.6 7.9 16.7
31.5 12.4 19.1
30.1 13.5 16.6
19.4 6.8 12.6
64.5 57.8 52.0 46.9 47.5 36.2
71.5 66.8 63.0 57.5 59.9 43.0
72.5 68.9 64.2 57.9 59.5 43.6
61.1 55.2 50.6 48.2 47.7 35.3
Figure 2. Quinoline insolubles under an electron microscope
hydrocarbon system specified by quinoline solubles, which in turn is further differentiated by the benzene insolubles. This is verified in Table I which points to the complexity of pitch binder properties. Selecting a desirable product for a carbon-bonding job is time consuming and expensive. Characterization
of Pitch Binders
I n pitch binder quality the quinolineinsoluble fraction, essentially a nonfusible powder, is important (Figure 2). T o further identify this material 150 grams of standard pitch (Table I) were dissolved in 4.5 liters of quinoline heated to 80' C. After 1 hour at this temperature, the solution was filtered through an electrically heated Buchner 18.5-cm. funnel using No. 50 Whatman filter paper. The residue was washed repeatedly with additional quinoline and a few benzene washes followed. To complete the purification of the quinoline insolubles, the residue was transferred to a large Soxhlet extractor containing 750 cc. of benzene and extracted continuously until the solvent drippings were completely clear. The thimble from the Soxhlet, containing the quinoline-insoluble material, was transferred to an evaporating dish and vacuum-dried a t 105' C. for 2 hours to yield 17.42 grams of product, which ~~
Table V. Property
Paraffin Wax
M.P." C. Sp. gr., 25O C./25O C.
Coking Values of Various Carbonaceous Materials Heavy Cycle
Oil
Petroleum Pitch
78 0.90
17.7a
11O(C/A) 1.138
5.88 0
8.21 0.19
0
0.0
...
Coal Tar Resin C
7
2 ~ 1.248
Thermax
~
)
Carbon
Graphite Powder
1.905
2.208
249
... ... ...
...
...
Wt. %
Carbon/hydrogen ratio Benzene insolubles Quinoline insolubles Beta resins
Ash Fixed carbon Conradson carbon Norsk GL Carbon Hydrogen
0 0 0.09 0.14 1.10 1.16 85.8 14.6
0.19 0.02 0.32 1.73 4.08 10.1 88.7 10.8
12.44 7.3 0.7 6.6 1.1 31.5 48.9 50.9 55.8 88.6 7.12
18.8 4.26 0.0
4.26 0.06 16.3 24.0 38.6 64.9 92.0 4.9
...
... ...
...
0.3 99.7
... ...
99.7 99.4 0.4
(I
0.5 99.7
... ...
99.95 99.0 0.35
* OAPI.
VOL. 50, NO. 1
JANUARY 1958
37
is 11.670 of the original pitch. This agreed satisfactorily with a quinolineinsoluble determination which gave 12.4%. Properties of the isolated quinoline insolubles are taken as representative of this component which is usually found in pitch binders derived from high temperature tars (Table VI). M'hen isolated before and after carbonization, these materials are a fine powder and may be regarded as a carbonaceous filler finely dispersed in aromatic hydrocarbons which is the true pitch binder component. Specific gravities for these insolubles from a number of pitches, averaged about 1.65; therefore, this value is considered well defined. Composition and coking values may prove more variable with increased sampling and testing. However, with this information, even accurate as a first approximation, it is possible to simplify much of the complexity associated with commercial pitch binders. Industrial background information on tar and pitch has been given by Rhodes (14). The specific gravity of a given pitch is assumed to be represented by the incremental contributions from quinoline insolubles and bindm hydrocarbons. Thus,
'"
=
100 - QI 100 - QI S, 1.65
(1)
Table VI. Properties of Carbonaceous Material Insoluble in Quinoline Property wt.% Sp. grav., ASTM D-167 Carbon-hydrogen ratio Carbon Hydrogen Sulfur Nitrogen Oxygen Ash GL coking value
1-1 B-1 0-1 1-2 1-3 1-4 B-2 G- 1 G-2 J -1 Stand. 0-2 1-5 I -6 F -1 R -1 1-7 1-8
With this equation. specific gravities of the binder hvdrocarbons were computed (Table VII) for those products in Table I. Specific gravity of binder hydrocarbons is of particular importance because it mav be related to pitch binder aromaticity as measured bv the carbonhvdrogen ratio This is based on the fact that the higher the specific gravity of a hydrocarbon, within a given boiling point range, the higher is the aromaticitv of the compound. This concept was introduced bv Martin (12) in the characterization of hydrocarbons found in creosotes derived from high temperature tars. T o extend this approach to pitch
=
59.7
- 55.5
Sp. Gr.a
Wt. %
Binder Hydrocarbons Sp. gr.* C / H ratio
1.274 1.292 1.247 1.293 1.303 1.308 1.317 1.303 1.300 1.303 1.320 1 270 1.318 1.341 1.289 1.317 1.329 1.353
6.7 7.9 6.8 7.6 8.5 13.3 14.5 15.3 10.8 10.8 12.4 19.2 13.5 18.5 3.0 9.1 12.5 14.0
1.255 1.268 1.225 1.269 1.277 1.267 1.272 1.255 1.267 1.269 1.282 1.203 1.277 1.286 1.281 1.289 1.293 1.314
Further confirmation of this correlation is obtainable by correcting observed carbon-hydrogen ratios of whole pitches for the average ratio of the quinolineinsoluble material. This gives the computed carbon-hydrogen ratio of the binder hydrocarbon phase. Plotted specific gravities of pitch binder hydrocarbons computed in this way are also represented by the straight line relationship shown in Figure 3. The specific gravity of a pitch binder can be used for computing the specific gravity of its hydrocarbons and their equivalent carbonhydrogen ratio (Figure 4). Specific gravities of pitch binder hydrocarbons and corresponding carbon-hydrogen ratios for the commercial pitches found in Table I are presented in Table VII. Coking yields in Table V rise with increasing carbon-hydrogen ratios of the feedstock. This generalization can be used in characterizing pitch binders by taking advantage of the fact that the Great Lakes coking value of the
~~
Incremental Binder Binder hydrocarbons, QI hydrocarbons calcd.
~
Whole pitch
19.4 20.1 17.6 20.2 20.7 20.1 20.4 19.4 20.1 20.2 21.0 16.3 20.7 21.3 20.9 21.4 21.7 23.0
63.2 64.5 61.1 68.0 69.7 70.5 69.8 68.3 67.4 65.5 71.5 68.2 72.5 73.0 71.4 72.4 73.5 83.9
6.0 7.1 6.1 6.8 7.7 12.0 13.1 13.8 9.7 9.7 11.2 17.3 12.2 16.7 2.7
57.2 57.4 55.0 61.2 62.0 58.5 56.7 54.5 57.7 55.8 60.3 50.9 60.3 56.3 68.7 64.2 62.2 71.3
8.2
11.3 12.6
61.3 62.3 59.0 66.2 67.8 67.5 66.3 64.3 64.7 62.6 68.8 63.0 69.7 69.1 70.8 70.6 71.1 82.9
25" C./25" C.
Table VIII. Material M.P.,C/A, O C. Sp. gr., 25' C.!25' C. Carbon/hydrogen ratio
Properties of Pitches Completely Soluble in Quinoline
Petroleum Pitch 110 1.138 12.4
0.7
QI Carbon Hydrogen GL coking value a
38
88.6 7.12 55.8
c/w. INDUSTRIAL AND ENGINEERING CHEMISTRY
(2)
Characterizafion of Commercial Pitch Binders
QT,
I
C/H ratio
where S, and S, = specific gravities of pitch and binder hydrocarbons, respectively (25' C / 2 5 O C . ) ; and QI = quinoline insolubles in weight per cent.
Table VII.
Sample No.
1.65 51.6 92.9 1.8 0.43 0.80 0.80 1.35 90.0
binders, a number of pitches free of quinoline insolubles, were selected, and carbon-hydrogen ratios along with specific gravities were determined (Table VIII). When these values are plotted, a straight line relationship is established (Figure 3). Thus,
Topped Tar
...
1.207 16.9
...
92.4 5.46
...
Resin
Residue from T a c . Dist. of Resin ___. C 114 150 1.278 1.286 20.6 22.7
C 72" 1.248 18.8
93 1.269 19.8 m7t.
0 92.0 4.9 64.9
97,
0.1 91.6 4.64 66.7
0 91.7 4.46 68.6
0.33 91.9 4.04 73.3
COKING METHODS A N D PRODUCTS binder hydrocarbons have been further corrected to the Great Lakes coking values of pitch hydrocarbons on a 100% quinoline soluble basis; thus, GLCV of binder = hydrocarbons
GLCV 100
- 0.9QI x - QI
100
(3)
where in weight per cent, GLCV = Great Lakes coking value and QI = quinoline insolubles.
Figure 3. Correlation of specific gravity and carbon-hydrogen ratio of pitches completely soluble in quinoline
isolated quinoline insolubles (Table VI) is 90%. Consequently, coking values of pitch binders, containing quinoline insolubles, consist of increments contributed by the quinolineinsoluble phase (90% of quinoline insolubles) and the pitch binder hydrocarbons or quinoline solubles. The calculated incremental values for the pitches found in Table I have also been assembled in Table VII. For characterization and possible specification, the incremental contributions of the
-
1.00
Pitch binder comparisons or characterizations should start with products within a specified melting point range. The melting point is usually an index of the amount of distillate material removed from a tar to produce the binder pitch. To illustrate, prebaked anode binder pitch for the aluminum industry is usually specified a t 110' rt 5' C. Within this range, the standard pitch (Table VII) is characterized by benzene insolubles of 31.5%, quinoline insolubles of 12.4% and a beta resin difference of 19.1%. The carbon-hydrogen ratio for the binder hydrocarbons is computed as 21.0. The Great Lakes value of 71.5% is attributable to a 11.201, increment from the quinoline insolubles and 60.3% from the pitch binder hydrocarbons. O n a 100% basis, the pitch binder hydrocarbons are characterized by a Great Lakes value of 68.870. Pitches of comparable quality in all respects are samples 1-5, 1-6, and F-I. Within this melting point range, an oil-gas tar pitch, 0-2, is characterized as a low specific gravity binder pitch. Quinoline insolubles are high a t 19.2% with beta resins amounting to only 5%. The carbon-hydrogen ratio of 16.3 for these binder hydrocarbons is definitely
Figure 4. A nomograph for calculating carbon-hydrogen ratio of binder hydrocarbons from specific gravity of whole pitch 1.10 I
'
0
.
k
Q
w
A straight edge across the chart connecting the specific gravity of a pitch and its corresponding quinoline insoluble indicates the specific gravity and carbon-hydrogen ratio of the pitch binder hydrocarbons on the center scale
-
1
-
0
-
s :-
&I 20 d .
m
1.30-
1
30
low. Relatively lower aromaticity is further corroborated by a poor pitchbinder hydrocarbon coking value of 63.0 as compared to 68,870 for the standard which is a high quality, coal-tar pitch binder, well established in the aluminum industry. That the over-all propertiesofsampleo-2 are not markedly different from those of the standard pitch is attributable to the high quinoline insolubles of 0-2. By separating quinoline insolubles from quinoline solubles and characterizing each fraction, pitch-binder quality differences can be established. A European pitch, G-2, is outstanding for a high beta resin difference of 26%. However, its Great Lakes value is 'only 67.4 as compared to 71.5% for the standard. Similarly, its pitch binder hydrocarbons, have a coking value of 64.7 as compared to 68.8% for those of the standard. Here, high beta resins are not correspondingly superior in terms of pitch-binder hydrocarbon coking propensity. Production of experimental electrodes also failed to confirm the claimed binder superiority for the G-2 product. Another foreign pitch, J-1, had acceptable properties, such as specific gravities and insolubilities, but both the pitch binder and its hydrocarbons showed poor coking ability. This product would normally meet purchasing specifications for the whole pitch but could exhibit relatively poor performance in commercial operations. F-1 is another unique product which contains 20.3% benzene insolubles and only 3.00/, quinoline insolubles, giving a beta resin difference of 17.3Y0. Great Lakes values of the entire pitch and binder hydrocarbons, however, indicate good binder quality. With increasing melting point and removal of distillate material to obtain such products, benzene insolubles and beta resin increase significantly. Pitch binder coking values also increase with rising carbon-hydrogen ratios. Finally, Great Lakes coking values of the binder hydrocarbons rise with higher carbonhydrogen ratios, as expected. Carbon-hydrogen ratios of pitch binder hydrocarbons in Table VI1 start a t 20 for a Soderberg liquid type pitch (B-1) and at a melting point of 185' C., increase to 23.0. Low melting-point pitches differ from a high melting product in that more low coking fractions have been removed during distillation of the original tar. Thus, high temperature pitch binders consist of a broad range of highly aromatic, high molecular weight compounds. Removing the lighter compounds obviously concentrates high molecular weight compounds of high coking propensity. Therefore, structural interpretations of VOL. 50, NO. 1
JANUARY 1958
39
Table IX.
a
Carbon-Hydrogen Ratio and Molecular Weights of Pitch Fractions (8)
Description
Wt. %
Original pitch Decalin solubles Xylol solubles Pyridine solubles Pyridine insolubles
100 46 20 23 11
c,
Av. Mol.
M.P.,
O
C.=
Wt.
Wt.
...
68.5 15 70 240
92.1 92.5 91.9 90.4 92.3
240 280 360
...
E
H, Wt. %
70
4.5 5.1 4.7 4.1 3.0
S,
N, Wt.
70
C/IX
Wt. %
Ratiob
0.8 0.8 0.7 0.6 1.0
20.5 18.1 19.5 22.0 30.8
1.3 1.1 1.2 1.5 0.8
Kraemer-Sarnow method. Calculated for whole pitch by Equation 4,20.2. Nonfusible powder.
carbon-hydrogen ratios of pitch binder hydrocarbons are unfortunately limited -they are only the average for a n unknown composition of high and low molecular weight compounds contained, in the final pitch. These considerations have been demonstrated by Franck (8). A briquetting coal-tar pitch was separated by solvent extraction into fractions soluble in xylol, and pyridine and also insoluble in pyridine. The carbon-hydrogen ratios, computed from the published analytical data (Table IX) vary for pitch binder hydrocarbons from 18 to 22, whereas the pyridine insolubles (approximately equivalent quinoline insolubles) have a ratio of 30.8. Molecular weights determined for the binder hydrocarbons vary from 240 to 360. Furthermore, if the fractional percentages and the values for the carbon and hydrogen of each fraction are substituted, C XlCl li(wholepitch) =
+ xzcz + ..
xiHi f ~ z H z + . .
whole pitch, as compared to the determined value of 20.5. Finally, carbon-hydrogen ratios even determined on a n over-all average basis may prove a useful criterion of pitch binder quality, especially because they relate to the coking propensity of pitch binder hydrocarbons. Even with approximate values known for such ratios and molecular weights for the binder hydrocarbons, there is little doubt that paraffins, naphthenes, olefins, and straight chain aromatics can be eliminated from the composition of pitch binder hydrocarbons (Figure 5). Polynuclear aromatics definitely fall within the domain of molecular compounds which satisfy the carbonhydrogen and molecular weight relationships previously established for the binder hydrocarbons found in commercial pitch binders.
.+xncn
. + x,Hn (4)
where x1 and CI are per cent of fraction and per cent carbon of fraction, respectively. Thus, a computed carbonhydrogen ratio of 20.2 is obtained for the
Conclusion No attempt is made to recommend exact pitch binder quality specifications. Instead, a new coking method is introduced to simulate the results usually obtained when commercially baking pitch binders for carbon aggregate systems. This research tool, coupled with carbon-hydrogen ratios, is recom-
30
mended as a quantitative approach to define pitch binder qualities which may be used to correlate properties and qualities with actual performance in experimental or commercial applications.
Acknowledgment The authors wish to express their indebtedness to Glen Stecker, Joseph J. Patelczyk, and David C. Vartanian, Great Lakes Carbon Corp. for their contributions to this investigation. Literature Cited Am. SOC.Testing Materials, Philadelphia, Pa., Standards, Pt. V, ASTM D-189-52, pp. 114-20 1955. Barrett Division, Allied Chemical and Dye Corp. New York, N. Y., “Methods of Testing Coal Tar Products,” p. 24, 1950. Ibid., p. 28. Ibid., p. 52. Barrett Division, Allied Chemical and Dye Corp., New York, Tu’. Y., “Manual of Barrett Test Methods,” B-21, Ser. 760, January 1953. Charette, L. P., Bischofberger, G. T., IND. ENC. CHEM. 47, 1412-15 (July 1955). Currie, L. M., Hamister, V. C., MacPherson, H. G., Proc. Intern. Conf. on Peaceful Uses of Atomic Energy, Geneva, 1955, 8, 451-73 (1956). Franck, H. G., Brennstoff-Chem.36, 12-20 (Jan. 12, 1955). Great Lakes Carbon Corp., “Test Procedures for .4nalysis of Petroleum Cokes,” C-13, September 1957
Ibz,-G12A.
25
y20 G’
I
6 15 k Q
a
;.c*
? l O L
c,
5
0
1 5 I
0
100
I
-G-C-C-CI I
200 MOLECULAR
I
I
300
400
500
WEIGHT
Figure 5. Molecular structure of hydrocarbons related to carbon-hydrogen ratios
40
INDUSTRIAL AND ENGINEERING CHEMISTRY
Hader, R. N., Gamson, B. W., Bailev. B. L.. IND. ENC. CHEM. 46, 2 L l l (January 1954). Martin, S. W., Proc. Am. WoodPreservers‘ Assoc. 45, 100 (1949). Martin, S. W., Nelson, H. W., J . Metals 7, 540-3 (April 1955). Rhodes, E. O., “Encyclopedia of Chemical Technology,” vol. 13, pp. 614-32, Interscience, New York, 1954. Swartzman, E., Proc. Coal Briquetting Conf., Univ. of Wyoming, Inform. Circ. 3, pp. 70-92, October 1949. RECEIVED for review March 5, 1957 ACCEPTED November 13, 1957 Division of Gas and Fuel Chemistry, Symposium on Coking Methods and Products, 131st Meeting, ACS, Miami, Fla., April 1957.
