Energy & Fuels 1992,6, 357-374
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Use of Continuous Flow Centrifugation Techniques for Coal Maceral Separation. 1. Fundamentals G.R.Dyrkacz* and C.A. A. Bloomquist Chemistry Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439 Received November 7, 1991. Revised Manuscript Received March 13, 1992
The utility of continuous flow centrifugation (CFC) for the separation of coal macerals was investigated. From a systematic study of the various factors affecting the efficiency of the separation, a methodology was developed for routine separations. Using CsCl/Brij-35 as the separation media, maceral enrichments of greater than 85% could be easily achieved for coals with a particle size of 92% purity could be obtained on a -100-pm particle size fraction. There are several notable advantages to using CFC for maceral separations. First, very dilute coal suspensions can be used. This reduces particleparticle interactions, which adversely affect separation. Second, large amounts of material can be separated; the major limitation is the capacity of the rotor (for the simplest rotors). Third, the separation efficiency is higher. Unlike batch sink/float centrifuge operations, there is less potential for crosscontamination problems between the light and heavy phases, since the light phase is continuously removed. Lastly, the separation is relatively fast; to produce an equivalent amount of purified coal material, much less manipulation is necessary, compared to batch sink/float separations. With these potential advantages in mind, we have again turned our investigations to CFC for maceral separations. However, this time a dedicated centrifuge, specifically designed for such work, was u5ed.I (5) Dyrkacz, G . R.; Bloomquist, C. A. A.; Horwitz, E. P. Sep. Sci. Technol. 1981,16,1571-1588. (6) Fremont, W.J. J.; Chermin, H. A. G.; Joziasse, J.; Nater, K. A. Roc., 1989 Int. Conf. Coal Sci., Tokyo, Japan, Oct. 23-27, 1989 1989, 113-116. (7)A preliminary report of the current work can be found in: Dyrkaa, G . R.; Bloomquist, C. A. A. Prepr. Pap.-Am. Chem. Soc., Diu.Fuel Chem. 1988,33(3),128-135.
0887-0624/92/2506-0357$03.00/00 1992 American Chemical Society
358 Energy & Fuels, Vol. 6, No. 4, 1992
Dyrkacz and Bloomquist
Experimental Section
of the rotor. The rotor effluent and deposit were filtered on 0.8rm polycarbonate membrane filters, waahed with water and ethanol, dried in a vacuum at 65 "C,and weighed. Flow Profile Runs. Safranine-0 (Aldrich) in water wm used to study the flow profiles in the New Brunswick centrifuge. Analogous to the usual operation of the centrifuge, the rotor was first filled with clear water, then brought up to the desired operating speed, and finally the proper flow rate set. Next, the inlet line was exchanged with a line containing the dye solution to minimize diffusion,and pumping wm continued. At periodic time intervals, aliquote of the effluent stream were taken at the outlet nozzle, which had been fitted with a funnel. The absorbance of each fraction was then measured at 520 nm. The raw data were converted to flow velocities, using a computer routine which interpolated a series of calculated constant volume increment radii against the absorbance data. The resulting data were then used to interpolate the specified radii relative to the flow rate data. (The flow rates were the reciprocal of the experimental time data multiplied by the size of the volume increment.) Dividing the results by the rotor height (19.0 cm) gave the flow velocity, in cm/sec. Density Gradient Monitoring of the Continuous Flow Separation. Aliquots of the phases derived from the continous flow centrifuge separation were analyzed by ADGC procedures. The overall procedure was similar to that previously described.' However, there have been some significant changes in the density gradient separation itself. Because of concern about very fine particles in the separated phases, we have increased the centrifugation time (at final speed) to at least 1h. Comparison with density gradient runs centrifuged for up to 8 h showed no definitive difference between the resultant density distributions. The major changes in procedure concerned primarily data acquisition and processing. An ISCO Model 640 fraction collector/syringe pump was used to pump out the density gradient through an ISCO UA-5 absorbance monitor flow cell. The data from the absorbance monitor (at 660 nm) was collected using a Data Translation DT2805 data acquisition board in a personal computer. The DT2805 provides 1/2048 resolution. However, since the absorbance monitor has provisions for changing the absorbance scale range, we often have an absorbance resolution that is about 1/40000. For most DGC work, this is far more precise than is necessary. The absorbance monitor output was also calibrated to the ADC board, since there was a linear voltage scaling problem between the two devices. Corresponding density data were obtained from a Paar DMA 45 density monitor, equipped with the optional 100-NL-capacity microcell. The microcell was used as an in-line flow cell and positioned after the absorbance monitor. Using the microcell as a flow cell does not allow us to take full advantage of its potential accuracy, but the density data can be obtained with at least *0.001 g cm-3 accuracy. For some early data,the temperature of the microcell was ambient. However, better reproducibility and fewer densitometer problems were encountered when the microcell was thermoetated at 25 OC. Density meter accuracy was checked periodically, and was maintained to within k0.003g cm-3 using CsCl solutions that had densities established by standard pycnometry. Density data was transferred to the computer via an RS232 connection. The computer queried the density meter and absorbance monitor for data under flow conditions (2 mL/min) which provided data at approximatelyfO.OO1 g resolution. Normally, 200-400points were collected for each gradient. The absorbance data were then properly scaled, densities were converted to 25 "C, and the density range of interest was defined. Further analysis consisted of smoothing the data, if necessary, and applying a linear baseline correction before integrating. Errors between duplicate analysis were on the order of i 5 % . Whole coals were subjected to preparative scale, 2-g separation and macerd analysis as described in ref 1. Particle Size Measurement. The volume particle size distribution was obtained using a Coulter LS laser scattering instrument.
Coal. The primary coal used in this study was obtained from the Argonne Premium Sample Program: APCS 7 is from the Lewiston-StocktonSeam, WV. Some early work was done using PSOC-732, obtained from Pennsylvania State University. Coal Pretreatment. Coal samples, initially -100 mesh, were ground to 1-2 pm average particle size using a fluid energy mill (FEM) (Sturtevant). The coal was then demineralized using HCl and HF, as has previously been reported.' Solution Preparation. AU solutionswere prepared using CsCl from a variety of suppliers, and Brij-35 surfactant (Aldrich). The CsCl was recycled in the same manner as previousy published.'v8 Solution densities were based on 25 OC values, but all runs were conducted under ambient conditions. Room temperature was generally no more than a few degrees different than 25 OC; roughly 1 OC difference corresponds to about 0.001 g cm" density change. Solution densities were usually checked with a Paar DMA 45 density meter. Unless otherwise noted, all quoted densities have an accuracy of i0.003 g ~ m - For ~ . all work, the Brij-35 concentration was 8 g/L. In some cases, large amounts of CsCl/Brij-35 stock solutions were made up for a series of runs. These solutions were checked often, since bacterial growth develops fairly rapidly in the solutions. Coal slurries, in 250-mL portions, were ultrasonically dispersed using an ultrasonic cell disrupter with a 1-in. horn (Heat Systems-Ultrasonics Inc, Model W-375) at 70% maximum power (maximum -350 W). The power was applied intermittently, using a 10% duty cycle. Each sample was treated for four 15-min cycles. Polystyrene latex particles were obtained from Duke Scientific (0.652 pm average diameter, standard deviation 0.0048 pm; lot no. 1A75). Continuous Flow Centrifugation Procedure. An unmodifed, continuous flow centrifuge (CEPA-LE laboratory centrifuge, Carl Padberg Zentrifugenbau GmbH; U.S. supplier: New Brunswick Scientific Co., Inc.) with a Model 'K" clarifier rotor was used for most of the studies. Initially, cooling coils were installed, but since there was little temperature change under our operating conditions, the coils were subsequently removed. A Sharples Super Centrifuge, Model T-1 (PenWalt Corp.), with a "1-H" clarifier rotor was also used for some early work. Rotor speed was monitored with a strobe lamp, and maintained at *3OOO rpm. To avoid grease contamination, and still provide a seal, the threads of the lower rotor cap were wrapped with Teflon tape rather than using the grease supplied with the centrifuge. (Contamination of sink material resulted if grease was used on the end cap threads, and no analytical density gradient centrifugation (ADGC) data could be obtained, due to severe particle agglomeration. Hexane washing of the sink fraction seemed to remove most of the grease contamination.) Several injection nozzles are available for the centrifuge; we found that there was no separation difference between a 1mm or 2 mm i.d. injector. A 1-mm injector was used for almoet all the work described here. A variable-speed peristaltic pump (Masterflex, Model 7520-00, Cole-Parmer) was used to pump the coal slurry into the spinning rotor. Silicone rubber tubing was used throughout the system because of ita compatibility with the peristaltic pump. With the rotor spinning at 12000-15000 rpm, a clear solution of the same density as the coal slurry was used to initially fii the rotor (-250 mL). (This rotor speed range was just below the first harmonic vibration of the centrifuge and mounting table system.) Once filled, the rotor was accelerated to the proper speed and the desired solution flow rate established. The coal slurry was then pumped into the rotor. Typically, 500 mL of slurry was used. The slurry was then followed by clear solution (200-300 mL), which served as a chase solution to ensure that the floating coal had been removed from the rotor. Generally, the effluent was clear of coal at the end of the flushing cycle. The rotor was then slowly decelerated to minimize disturbing the sink fraction on the wall of the rotor. Near 2000 rpm the fluid in the rotor rapidly drained out the bottom of the rotor. This drained material was collected and analyzed separately. Plastic film inserts cut from colorless Xerox transparency sheeta were used to facilitate removal of the sink deposit from the wall (8) Dyrkacz, G. R.; Ruscic, L.; Fredericks, J. To be published.
Coal, Analytical and Theoretical Considerations Coal and Coal Pretreatment. T h e coal sample which was chosen for t h e bulk of our investigations was t h e
Energy & Fuels, Vol. 6, No. 4, 1992 359
Coal Maceral Separation. 1 Table I. Elemental Analyms (wt % daf) of Coals Used in This Study coal C H N 00 Sm ashb PSOC-732 PSOC-732 demin APCS 7 APCS 7 demin a
80.8 83.4 78.6 80.1
4.79 4.7 4.9 5.0
1.4 1.4 1.5 1.5
12.16 9.67 14.16 12.56
0.85 0.74 0.81 0.82
16.5 0.33 18.8 0.35
"""1
(CP).
h
6
system, the Reynolds number. For particles of less than about 100 pm (Reynolds number
