Improved Plastometer for Studying Agglutinating Behavior of Caking Coals G. L. BARTHAUER Research and Development Division, Pittsburgh Consolidation Coal Co., Library, Pa.
and is constructed entirely of Type 310 stainless steel. The ’/ss-inch wall thickness is adequate t o resist the rough handling occasionally necessary to remove the agglutinated bed. Two thermollells extend into the retort, one acting as a pin bearing to support the paddle, the other extendirg into the bed itself. R i t h the paddle design used, i t proved necessary t o meld a small steel pin to the crucible wall in order t o remove caked material which adhered to the paddle in the course of a run. T h e close-fitting stainless steel lid contairs two 3.75-inch lengths of black iron pipe opening directly into t h e crurible. One opening is used for sample entry, the other for the escape of gas and tar vapors. Paddle Design. The only design requirement for the paddle is t h a t no significant amount of material be removed from contact with the rotating arms during the test period. This condition is a prerequisite t o reproducibility. Furthermore, the carbonized bed should be completely granular at the end of a run. The torque level for a given coal 1%-illdiffer greatly with various paddle configurations; however, one is usually not interested In absolute torque values b u t onlv in the relative torques developed by various coals tested under identical conditions. I n order to achieve better mixing and better heat transfer from the hot retort w.alli to the bed proper, the top t n o paddles shown on Figure 1 are tilted In suc-h a m a m e r that the solids are driven toward the hottoll of the retort Conversely, the loner t’lree paddles drive the solids tonard the top.
In connection with a program for fluidized coal carbonization it became necessary to obtain both research and control data on the agglutinating behavior and plasticit) of various feed coals. For this purpose a rugged and dependable plastometer was constructed. This apparatus, which has been in use continuously for over 4 years, obviates many of the shortcomings of standard units. The unit continuously records both torque and temperature, 3 ielding permanent detailed records. -4s many as 1.5 samples can be procegsed per man-day.
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I G H L T caking coals, n-hen heated rapidly to ca. 750’ F., undergo a change that can best be described as a melting phenomenon. The cleavage edges of the individual particles become more rounded as the coal approaches this temperature and shortly thereafter the entire particle becomes fluid-i.e., behaves like a pitch particle heated t o its melting point. A group of these particles will flow into one another, forming a liquid mass which soon resolidifies to form the char or coke product. Coal t a r and gas are evolved rapidly throughout this stage. Several years ago, this laboratory became interested in carbonizing such coals in fluid beds. It soon became evident that this “melting” behavior controlled bed operability. As t,he particles became fluid, they also became sticky and tended to adhere to one another, finally forming agglomerates which in severe cases plugged the bed and resulted in forced shutdoivns. Various coals behaved differently. I n m a n - cases the fluidizing gas agitated the bed sufficiently t o break up the agglonicrates as rapidly as they n-rre formed The need for a research and control instrument to measure the agglomerating (or agglutinating) strengths of various coals was obvious. Several units, notably the Davis ( 2 ) and Gieseler (3) plmtometers, have been used for this purpose. Hon-ever, both are primarily research instruments, unsuitable for testing large numbers of saniples on a control hasis. The Davis unit provides data that require point-by-point plotting to be meaningful. The Gieseler plastomer is designed to measure fluidity alone : therefore the curves are discontinuous (in the sense that paddle rotation ceases shortly after the bed begins to solidify) and give no information on the force required to break up the agglomerate. Both units suffer from a multiplicity of guides, sleeve bearings, etc. Because of the limitations of the above units, the apparatus shown schematically in Figure 1 vias built.
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CONSTANT SPEED MOTOR
TOROUEMETER
TO RECORDING POTENTIOMETER EVE BEARING -UNIVERSAL
COUPLING
TO TEMPERATURE-
RECORDER CRUCIBLE TYPE THERMOCOUPL€WELL
-FURNACE (1200 WATTS)
DESCRIPTION OF APPARlTUS
Figure 1.
The unit is similar to the Gieseler and Davis units in t h a t the torque required to drive a stirrer through a carbonizing bed is measurrd. T h e unique features of the improved plastometer include provisions for easy dismantling and reassembly for control operation, continuous recording of both torque and temperature, and enough fleuibility to permit accurate measurements on both strongly and weakly caking coals with a single torque unit of limited range. This flexibility is accomplished b y diluting the coal n ith various ( b u t standard) percentages of inert material. The degree of dilution depends solely upon the interparticle bonding strength of the particular roal under study. Furnace and Retort Detail. The furnace is a Hoskins FD-104 crurible-type unit: poiver input is 1200 watts a t 110 volts. The crucible well ir refractory-lined, 5 inches in diameter, and approximately 6 inches deep. The retort itself is 4 inches in diameter and 4.5 inches deep,
Improved plastometer
The distance bet\%-eenthe motor (a I / ( hp., 7 2 r.p.m. Bost,on Gear unit) and the retort is kept a t a minimum, only one narrow sleeve bearing being necessary for perfect alignrent. As shoxn, connection with the paddle shaft is made through a universal coupling. Torque Recording Systsm. The heart of the continuous torque recording system is a 0- to 25-inch-pound torque. unit, originally designed for applying knovin torques to screw heads. This unit, manufactured by the Apco-Mossburg Co., Attleboro, Mass., is simple in design, inexpensive, and can be purchased in a variety of torque ranges. will be observed in Figure I, i t is incorporated in the drive shaft. Simply by substituting a slide-wire for the indicating scale on the spring-loaded inner 969
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ANALYTICAL CHEMISTRY
shaft of the torque unit and a voltage pickoff for the indicator, one has all of the elements necessary for continuous recording. Figure 2 shows the elements of the recording system in more det’ail. The indicating pointer, A, is the variable contact on slide-Tvire R3; the latter is integral rr-ith the spring-loaded inner shaft of the torque unit. .It any instant, the potent,ial removed from the slide-wire will be proport,ional to the torque applipd! tiius reflecting the resistance of t,he bed to paddle rotation. T h r necessary connections to a Brown Electronik Recording potentiometer, R, are made through slip rings. The sensitivity of the unit can be changed at will, simply by changing t h e voltage drop across the variable resistances R1aiid X?.
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I 01 600
700
800
1000
Rx)
TEMPERATURE,*F
Figure 3.
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Figure 2.
Typical plastometer curve
Crush the sample to a top size of 3j-niesh (Tyler) and screen out the -200-mesh fraction. Particles smaller than 200-mesh normally are elutriated from the bed during the early stages of volatile evolution. Dilute a weighed portion of the sample with n standard amount of inert material, the ratio of samDle to inert heing chosen so that significant torque values will be re‘gistered duiicg the course of tbe iun. If the agglutinating properties of the salnple are complete11 unknortn, a trial run usually suffices to indicate a preferred COPcentration. The safety relay in any case i d 1 prevent damage to the unit. Diluents may be either sand or char; in any case, t h r qize limits should lie between 35 and 200 mesh. Calibrate the torque unit at two points, using two separate volumes of dense carborundum povder (Fisher Catalog KO C-IDO), the “standard” torques of nhich are knon-n Adjust RI and R2 until the torque recorder matches these knonn values. Preheat the empty furnace and retort to 550’ F. and, with the paddle rotating, introduce the 11-ell mixed charge through one of the openings in the retort lid. .Idjust the heating rate of the furnace (by means of a variable transformer) to the proper valLe and continue the run until the bed has passed well through the plastic zone-i.e , until the bed temperature reaches 1000° F. h t short intervals throughout the run, introduce a mark siniultaneously on both the “torque” and “temperature” charts; theqe marks facilitate later plotting of the data. Upon completion of the test, shut off the motor and furnace and remove the recorder charts. Using t h e v charts, plot tbe “torque us. temperature” curve as shonn on Figure 3.
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Schematic diagram of torque unit
T h e maximum torque which can be measured with this unit is 25 inch-pounds. -4s so?n as the torque exceeds this value, an electronic relay shuts off the motor through a manual reset power relav. This prevents da-nige in t h e event of excessive torques. OPER*TIYG PROCEDURE
As is usually the case with equipment which does not provide data in absolute units, the conditions of the test are dictated to a large extent by the specific problem at hand. For example, t h e heating rate used with the Gieseler and Davis units is about 3’ C. per minute, a rate closely corresponding t o that used in high temperature coking. This low rate is completely unrealistic in continuous fluid bed operations, where the feed coal is introduced directly into a large bed already a t a n elevated temperature. Consequently, the heating rate with this particular unit is ca. 30” C. per minute. This higher rate results in shorter test periods as well as more realistic information on coal behavior at these high rates. However, higher thermal gradients exist through the bed than is the case with either the Davis or Gieseler unit. The unit is so designed t h a t dilution of the bed with a n inert substance-usually char or sand (Fisher’s Berkshire grade)-is necessary, particularly in the case of strongly caking coals. This is a distinct advantage when one is working with small samples-e.g., individual petrographic components-as even a bed diluted to 2 % coal concentration will show easily measurable plasticity and agglutinating behavior. K i t h weakly caking coals, much higher concentrations are necessary. The operating procedure for a typical run is as follows:
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850
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an
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900
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92s
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TEMPERATURE, ‘F
Figure 4.
Effect of sodium carbonate on agglutinating strength
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V O L U M E 27, NO. 6, J U N E 1 9 5 5 ISTERPRETATION OF DATA
T.'ipure 3 is a t>-pical plot of the fluidity and agglutinating behavior of a \\-esrerll Pennsylvania high volatile A coal. I n obtaining the data for this particular curve, a mixture of 1 part of coal and nine parts of inert (loo- temperature char) was used. Size consistency of both coal and char was 35 X 200 mesh. All points of spccific interest are evident in Figure 3. These include: the iniri:il fusion tempei,:iture, -4; the point of maximum fluidity, B; ant1 the maximum r tance point, C. I n addition, the fluidity range ia readilj- a;iparent. Another example illustrating t h e applicability of the unit for research stlitlie.* is phoivn in Figure -1. .Is is well known, t h e addition of alkaline d t s t o a caking coal materially alters its behnvioi~during c:tihonization. This is well illustrated in Figure 4, n-hich slion-s the cffect of adding incrensing amounts of soclinm carl,oriate t o :t bed of caking coal and diluent char. Only the uice point is plotted, this point being of primarjinterwt in so far as fluid bed operability is concerned.
coking problems, using techniques similar t o those described by BreR-er and Xtkinson ( 1 ) . T h e dilution technlque presents no problem because a few standard coal-diluent ratios suffice to cover a broad range of caking strengths. T h e incl eased flexibility of the apparatus more than offsets the occacional need for 3, rerun on a borderline sample. Usually a general knowledge of the sample source permits one t o choo-e the proper dilution ratio immediately. ACK%OWLEDGRIEST
T h e author gratefully acknowledges the assistance of 8. .\. Jones for his many helpful suggestions in the design and construction of t h e described apparatus. LITERATURE CITED
(1) Brewer, R. E., and .Itkinson, R . G.. Tsn. Esc,. CHEY., AS.^,.
ED.,8, 443-9 (1936). ( 2 ) Davis, J. D., Ihid., 3, 43--5 (1931). (3) Gieseler, Ilamin? Diethvlamine
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