INDUSTRIAL AND ENGINEERING CHEMISTRY
242
oxygen per million parts by weight of water (1 part in about 156,000,000). Tests indicate that the limit of error of the titration is *0.001 ml. of oxygen a t 25” C. and 760 mm.
Advantages of the Method The author has used this equipment extensively in connection with power plant tests and has found the following advantstges in making dissolved oxygen determinations: The use of the simplified electrometric titration procedure eliminates the temperature, starch quality, and personal equation errors of the starch-iodide method. The analysis is based upon a difference determination whereby the influence of dissolved oxygen in the reagents, small ionic variations of foreign substances, secondary reactions, loss of dissolved oxygen by displacement, possible
VOL. 11, NO. 5
contamination of the sample by air when the reagents are added, and temperature variations are minimized. The complete equipment can be made in portable form.
Literature Cited (1) (2) (3) (4) (5) (6) (7) (8) (9) (10j
(11)
Foulk and Bowden, J . Am. Chem. SOC., 48, 2045 (1926). Hewson and Rees, J . SOC. Chem. Ind., 54,254t (1935). Jones and Kaplan, J. Am. Chem. SOC.,50, 2066 (1928). Kolthoff, Rec. trav. chim., 41, 172 (1922). Kolthoff and Furman, ”Potentiometric Titrations”, 2nd ed., p. 69, New York, John Wiley & Sons, 1931. Schwartz, M. C., Louisiana State Univ., Univ. Studies No. 21 (1935). Swartz and Gurney, Proc. Am. SOC.Testing Materials, 34, 796 (1934). Theriault, E. J., Pub. Health Bull. 151; Suppl. 90 (1925). Van Name and Fenwick. J . Am. Chem. SOC..47. 9.19 (1925). White, Leland, and Button, Proc. Am. So;. Tes>ing’Materials, 36, 697 (1936). Willard and Fenwick, J . Am. Chem. SOC., 44,2504,2516 (1922).
Measurement of Plastic Properties of Bituminous Coals Comparison of Gieseler and Davis Plastometer and Agde-Damm Dilatometer Methods R. E. BREWER
T
AND
J. E. TRIFF, Central Experiment Station, Bureau of Mines, Pittsburgh, Penna.
HE torsional principle-that is, the measurement of resistance to shear caused by movement of a stirring
device within the heated coal c h a r g e w a s first applied in 1931 by Davis (5) to the determination of the “plastic” properties of bituminous coking coals. Since then, this general principle has been employed in various forms of other instruments (9-13). Although the later designs of the Davis plastometer have incorporated a few minor changes to ensure smoothness in operation and improve general appearance, the original instrument, after more than seven years of continued use, has proved satisfactory. It has been found, however, that the procedure (5-8) gives more uniform test results when modified (S), especially by the use of a larger sample of representative coal and by operation of the retort a t a slower speed of rotation, and that “minor limitations lie in the difficulties of determining accurately small changes during the period of greatest fluidity and extremely high resistances, above 63.4 kg.-cm. (55 pound-inches) shown by certain coals.” These difficulties have been overcome by the use of tension springs with a sensitivity of less than 0.23 kg.-cm. (0.2 pound-inch), during the period of greatest fluidity, which permit accurate measurements of resistances up to 149.8 kg.-cm. (130 pound-inches). In the Davis plastometer method (6) the coal charge as a whole is rotated and stirred; the property measured is the resistance to shear of the partly fused coal adhering to the inner periphery of the retort. I n the Gieseler plastometer method (9) the coal is static a t the start, and later stirring is proportional to the fluidity of the coal. Accordingly, with increase in fluidity of the heated coal are noted (a) a decrease in resistance, or torque, measured in kilogram-centimeters, in the Davis rotary retort, and ( b ) an increase of the rate of rotation of the stirring shaft in the Gieseler stationary retort.
In (a) the resistance is created by the movement of the coal in the retort against the rabble arms on the inside shaft, which is prevented from free rotation by the tension springs; in (b) the rotation of the stirring shaft is caused by application of a constant force on the loading pan. Gieseler (9) criticized the method of Davis (5) because the coal is heated under conditions corresponding to those of a rotary retort, permitting volatile matter and tar to escape more freely than in a coke oven. Gieseler (9) made the broad assertion that all methods for the determination of plasticity in which the coal is not prevented from expanding are unsatisfactory, quoting a statement from Davies and Mott (4) that a “coal which is free to expand loses volatile matter readily and plasticity ends a t a comparatively low temperature.” Davies and Mott (4) showed, however, that the temperature of solidification, termed by them the “end of plasticity”, is higher than the temperature of final expansion as determined by the Sheffield laboratory coking test on a number of the better coking coals. At the beginning of the solidification of a melted coal mass into semicoke a coal rapidly loses its fluidity. At this stage the Gieseler instrument naturally shows with increasing temperature less and less movement of the stirring shaft per unit of time, and as semicoke formation proceeds this movement falls off rapidly to zero. The apparent discrepancy in the interpretation of the “end of plasticity” is, therefore, purely one of definition. The temperature limits of the plastic range for the Davis plastometer test are defined as the difference between the temperature a t which resistance develops and that a t which resistance ends. This latter temperature is some degrees higher, the magnitude varying with different coals, than that of maximum resistance or solidification, which, in turn, is higher than the temperature
ANALYTICAL EDITION
MAY 15, 1939
243
the binding between the stirrer shaft and the sieve plate caused by the accumulation of partly carbonized heavy tars is entirely eliminated. A radial type of ball spindle bearing assembly taking a horizontal thrust only was first used. Because of high friction in this bearing, a weight of 145 grams on the loading pan was re uired for satisfactory operation. Except for the7usion temperatures, which showed the same order of agreement, the data obtained with this bearing and 145 grams on the loading pan were less concordant than those obtained with the new ball-bearing assembly shown in Figure 1. This latter bearing takes both a vertical and horizontal thrust and shows but little friction in its operation. A 20-gram weight on the 18.7-gram loading pan is sufficient t o cause uniform rotation of the stirring shaft in the empty retort, no rotation in the coal charge until initial softening of the coal is reached, and a smooth movement during passage through the preplastic and plastic temperature ranges.
L
1
6
6 ‘mm.-----
FIGURE1. MODIFIED GIE~ELER APPARATUS
A comparative study of different mesh sizes of coal-passing through 20-, 35-, and 60-mesh (Tyler sieves)-showed that the 35mesh size was the most suitable. The length of the high fluidity temperature range was not affected .by changes in the particle size of the coal. However, the 20-mesh size showed a higher maximum fluidity a t a slightly higher temperature than the 35-mesh coal. This difference may be attributed to the fact that more time is required to heat through the larger size particles; in consequence, more fluid material would result a t the time of maximum fluidity. The 60-mesh size tended to become frothy and swell out of the retort, causing erratic indications. Since the capacity
a t the end of high fluidity or the start of solidification of the softened coal mass into semicoke. Furthermore, Gieseler has shown (9) that the end of the softening zone determined by his instrument agrees closely with that obtained by his penetrometer method on the same coals. This temperature, as shown in Table I1 of the present paper, agrees more nearly with the solidification temperature but always lies below the end of the plastic-range temperature, as determined by the Davis plastometer. The difference in the temperatures for the “end of the plastic range” in the Gieseler and Davis plastometer methods is to be expected, therefore, from the operating characteristics of the two instruments.
Development of the Modified Gieseler Plastometer Experience in this laboratory with retorts designed according to descriptions by Gieseler (9) and Jung (IO) showed difficulties in charging and uniform packing of the coal sample. The ‘[personal equation” involved in the tedious method of packing the coal charge employed by these authors was largely eliminated in the present work by the use of a specially designed loading device and a simplified retort threaded on the outside wall a t the top and without a removable cap bottom. Figure 1 is a sectional drawing of this retort, on a somewhat larger scale than that of the rest of the apparatus. The retort is 21 mm. in inside diameter and 16 mm. in depth. In testing strongly contracting coals a steel pin 1.6 mm. in diameter is inserted through a hold in the retort at a point 5 mm. from the top and t o a distance of 5 mm. radially toward the center from the inside wall. Without this pin arrangement the stirring shaft carries the contracted coal charge around with it and thereby falsely indicates a highly fluid mass. At the same time, the pin arrangement constitutes an improvement over the sieve-plate cover first used at the top of the retort, since
FIGURE 2. LOADING DEVICEFOR PACKING COALCHARGE
of the retort, 4.0 to 4.2 grams of coal, is believed to be too small to permit use of a representative 20-mesh coal sample, the 35-mesh size is taken as standard. This latter size,
