2154
Ind. Eng. C h e m . Res. 1990, 29, 2154-2159
for the strong base resin. Furthermore, the desorption rate for chelate resin is faster than for strong base resin. Though this report is a very preliminarynote, the results obtained here may suggest that normal chelate resins can be used economically for the recovery of Hg(I1) from wastewater by adopting the desorption method, which generates mercury metal on the cathode. Nomenclature CE = concentration of eluent, mol/dm3 CHg(II) = total concentration of Hg(I1) in the liquid phase, mol/dm3 CNaCl= concentration of NaCl in the liquid phase, mol/dm3 E = electrode potential, V E" = standard electrode potential, V i = current density, A/cm2 k , = equilibrium constant of eq 7, dm3/mol Q = exchange capacity as a concentration of Hg(II), mollkg q = resin-phase concentration of Hg(II), mol/kg WHg= weight of mercury metal deposition on the cathode, g/cm2
Registry No. Hg, 7439-97-6; NaC1, 7647-14-5; Dowex A l ,
9056-04-6.
Literature Cited Ciavatta, L.; Grimaldi, M. J . Inorg. Nucl. Chem. 1968, 30, 563. Freiser, H.; Fernando, E. In Ionic Equilibria in Analytical Chemistry, 10th ed.; Fujinaga, T., Sekido, E., Transl.; Kagaku Dojin: Tokyo, 1975. Kataoka, T.; Yoshida, H. Chem. Eng. J. 1988,37, 107. Toshima, S. Kiso Denki Kagaku (Basic Electrochemistry, in Japanese) 5th ed.; Asakura Shoten: Tokyo, 1969. Yoshida, H.; Kataoka, T. AIChE J. 1989, 35, 318.
* Correspondence concerning this paper should be addressed to H. Yoshida. Hiroyuki Yoshida,* Takeshi Kataoka Department of Chemical Engineering University of Osaka Prefecture 4-804, Mozu- Umemachi, Sakai 591, Japan Received for review June 29, 1989 Accepted June 18, 1990
Feedstock Effects in Coal Flash Pyrolysis The impact of lower rank coals on yields and product qualities is examined in a large-scale flash pyrolysis reactor capable of achieving heating rates of lo6 'C/s and reactor times of 10 ms. A Montana lignite and a Minnesota peat were pyrolyzed and the results compared to those for Illinois No. 6 bituminous coal. The peat was found to be more reactive than the Illinois No. 6 coal and the lignite less reactive. Relatively high liquid yields were also obtained from the peat. This result demonstrates that high oxygen content in a feedstock does not necessarily correlate with low production of liquids. At high reactor severity, the peat is an excellent source of aromatics (benzene, toluene, and xylenes) and medium Btu gas, which could be converted to other chemical feedstocks. Previously we reported on the flash pyrolysis of two bituminous coals at very short reaction times (Tatterson et al., 1988). As a continuation of that study, we have also examined the effect of feedstock type on coal conversion, yields, and product quality. The feedstocks studied were a Montana lignite and a Minnesota peat, both lower rank materials relative to our earlier studies with Illinois No. 6 bituminous coal. Our objective was to establish the impact of these alternative feedstocks on the economic feasibility of pyrolysis in a reaction system sufficiently large to provide results typical of a commercial reactor. Numerous studies have examined feedstock effects in pyrolysis. We compare our results to these. Edwards et al. (1985) pyrolyzed a number of various ranked Australian coals in a fluidized sand bed reactor. Liquid yield (tar yield) for the feedstocks studied peaked at approximately 600 "C and varied strongly with the hydrogen/carbon ratio of the feed coal. Furlmsky et al. (1984) pyrolyzed 12 Canadian coals of different rank in a Fischer assay retort. The feedstocks varied from lignite to semianthracite. Gas and tar yield were found to increase with the hydrogen/carbon ratio of the feedstock up to a point, and then tar yields began to gradually decrease. Furlmsky et al. also found that large amounts of oxygen in the coal did not necessarily lead to low liquid (tar) yields. Tyler (1980) also pyrolyzed numerous Australian coals in a fluidized sand bath. He also found a similar effect of feedstock hydrogen/carbon ratio on liquid (tar) yields. The purpose of this present study was to perform coal flash pyrolysis in a continuous reactor, such that the results 0888-5885/90/2629-2154$02.50/0
would be scalable to a commercial scenario. These results are additional data to test the devolatilization model of Solomon et al. (1988), which describes rank dependence of tar formation, extract yields, etc., on COz yields. Experimental Section Liquefaction Reactor. The liquefaction reactor used in these experiments is shown in Figure 1. This reactor was used in our previous studies, and complete details on the design and operation have been reported by Tatterson et al. (1988). Briefly, the reactor consists of a liquefaction zone into which an entrained stream of coal particles is injected through a converging nozzle. The liquefaction zone is filled with hot hydrogen genrated by burning excess hydrogen with oxygen in two side-arm combustors. The hydrogen helps to stabilize reactive coal fragments and decrease polymerization products. Once inside the liquefaction zone, the entrained coal stream forms a free jet that expands as it slows down, thereby entraining the hot gases surrounding the jet. Particle heating rates with this system are quite high, on the order of lo6 "C/s. The residence time of the coal particles could not be measured directly. It was calculated from the coal feed tank pressure, the reactor pressure, and the coal and carrier hydrogen flow rates. This calculation assumes that the coal particle velocity in the pyrolysis zone is the same as the nozzle discharge velocity. In all cases, the nozzle was operated under choked flow conditions (i.e., sonic velocity at the nozzle discharge point). Rudinger (1965) has developed expressions for the sonic velocity of gaslsolid 0 1990 American Chemical Society
Ind. Eng. Chem. Res., Vol. 29, No. 10, 1990 2155 C(IRL/HIOROGEN
I
1 NOZZLE
HYDROGEN
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Figure 1. Free-jet reactor.
mixtures. The presence of solids decreases the sonic velocity of the gas. The relationship is
(5)'
r
= y(1
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The test conditions in all experiments were such that t (the volume fraction of solids) was less than 0.15. On the basis of this relation, the exit velocity of the coal from the nozzle was estimated to be greater than 100 m/s. The residence time was calculated by dividing the length of the liquefaction zone by the particle velocity. In reality, the coal particles slow down in the liquefaction zone; thus, the residence times calculated are minimum possible values. We found it extremely difficult to measure the temperature in the jet directly. As a result, the reactor temperature was taken as the wall temperature (measured by an R-type thermocouple), 7.6 cm down from the top of the liquefaction zone. This distance is sufficient to avoid reactor end effects. In addition, the highly turbulent flow in the reactor helps to provide a fairly accurate reflection of gas temperature in the reactor. A conduction/radiation heat balance on the thermocouple revealed that the measured temperature could be in error by as much as 30 "C. However, this is relatively small compared to the temperature range of the reactor, 550-1100 O C . Using a one-dimensional model (I-DICOG) for heat transfer to a reacting coal particle, developed by Smith and Smooth (1980), we estimated that a 115-pm particle would heat from room temperature to 925 OC in less than 2 cm of reactor length. As a result, we feel that the wall thermocouples also give accurate estimates of the temperature that the coal particles are experiencing in the liquefaction zone. Coal Feed. Three coals are compared in this study: (1) Illinois No. 6 bitumenous; a (2) Montana lignite; and (3) a Minnesota peat. The Illinois No. 6 was obtained from the Burning Star Mine No. 2 (Consolidation Coal Company) in southern Illinois; the Montana lignite was from the Knife River Coal Company Mine in Sidney, MT; and the Minnesota peat was obtained from the Northern Peat Bog (Northern Peat Company near Hill City, MN. The peat was dewatered and dried to approximately 10% moisture to facilitate handling and grinding. (In a commercial scenario, this would be undesirable to do, but larger nozzle diameters would eliminate the requirement for the extremely fine grind of the coal.) The Illinois No. 6 and Montana lignite were fed to the liquefaction reactor on an as-received basis. All three feedstocks were ground to -37
Table I. Feed Properties
prox anal., wt % volatile matter fixed carbon ash moisture ult anal., wt % (maf) carbon hydrogen oxygen (by diff) nitrogen sulfur H/C ratio heating value, cal/g (maf) petrographic anal. vitrinoids type exinnoids resinoids semifusinoids inerts av vitrinoid reflectance, % in oil coal particle size distribution mesh size, pm +220 150 105 74 55 45 37 -37
Illinois No. 6
Montana lignite
Minnesota Deat
32.3 44.5 17.4 5.8
32.9 30.1 9.9 27.1
58.2 20.7 13.8 7.3
80.3 6.2 9.3 1.6 2.6 0.92 6215
65.8 3.7 28.9 0.4 0.67 5438
57.0 6.1 33.7 2.8 0.4 1.28 4400
83.6 4.2 1.4 1.0 15.8
27.7
0.57
0.41
1.2
%
4.3 1.7 (6.0) 0.9 (6.9) 1.9 (8.8) 3.5 (12.3) 4.0 (16.3) 5.7 (22.0) 78.0 (100.0)
Im. All feeds were passed through a 150-pm screen before entering the feed tank to remove oversized particles that might plug the reactor injector nozzle. A typical particle size distribution for Illinois No. 6 coal is given in Table I. Table I indicates that the properties of these feedstocks vary greatly. The Illinois No. 6 is a high volatile bituminous coal with relatively high H/C ratio and high sulfur content. The lignite and peat are low rank feedstocks with high oxygen content (29% and 3470, respectively). The lignite and peat also have a very low sulfur content. The lignite also has a very low H/C atomic ratio of 0.67, while the peat is hydrogen rich with a ratio of 1.28. The peat is also distinguished by its relatively high nitrogen content (2.8 wt 5%). Test Conditions. The experimental program was designed to examine the impact of feedstock on yield structure and product quality. As a result, reactor temperature was the only variable that changed from test to test. Obviously, the heating rate of the coal particles was also influenced by the reactor temperature with the lower temperatures also corresponding to lower heating rates. Varying the temperature was found to be an excellent method of changing coal conversion. The experimental conditions for each test are given in Table 11. The ranges of operating conditions for the reactor are as follows: coal flow rate (kg/h), 2-25; reador temperature ( " 0 , 550-1400; reactor pressure (bar), 1.7-8.6; nozzle upstream pressure (maximum bar), 20.7; carrier hydrogen flow rate (m3/h), 1.3-2.7; water quench rate (kg/h), 23-34. Sample Analysis. Each liquefaction test generated three samples for analysis: (1) a gas sample; (2) a liquid product receiver containing quench water and converted products; and (3) a charcoal sample containing entrained tar and benzene-toluene-xylene (BTX). We experimented with a number of alternate product collection schemes to the water quench but were unable
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