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Ind. Eng. Chem. Process Des. Dev. 1986, 25, 168-171
chlorine flow rate, when such variations were introduced manually in the model. Implementation of the feedforward control would have been simpler if a digital control hardware was used. Acknowledgment This work was partly supported by a grant from the National Sciences and Engineering Research Council of Canada. Nomenclature A
B C E G K M N
8 R
T U V
X k 4 r S
t X
Y
Arrhenius frequency factor, dimensionless; area for heat transfer, m2 benzene mole fraction, NB/NB(O) heat capacity, kJ/(kgK) activation energy, kJ process transfer function constant monochlorobenzene mole fraction, NM/NB(O) number of kilomoles heat contribution due to chlorine feed, impeller action, condenser, and heat of solution of chlorine, kW universal gas constant, kJ/ (kmo1.K) temperature, sampling interval, s overall heat-transfer coefficient, kW/ (m2.K) initial volume of reacting mixture, m3 number of kilomoles of chlorine reacted at any instant reaction rate constant, m3/(kmol.s) volumetric flow rate, m3/s inlet flow rate of chlorine, kmol/s Laplace operator time, s input to noise filter output from noise filter
Symbols AH heat of reaction, kJ/kmol
P
benzene exponential decay index density, kg/m3 dimensionless time time constant, s
P
0 7
Subscripts
B C D H M P V
W Z i
benzene chlorine dichlorobenzene heating water monochlorobenzene reacting mixture valve cooling water trichlorobenzene reaction number
Registry No. Benzene, 71-43-2;monochlorobenzene,108-90-7.
Literature Cited Armstrong, W. S.; Coe, B. F. Chem. Eng. Prog. 1983, 7 9 , 56. Slakemote, N.; Aris, R. Chem. Eng. Sci. 1962, 17, 591. Carlson, A. Instrum. Control Syst. 1965, 38, 147. Himoe, A.; Stock, L. M. J . Am. Chem. SOC.1969, 9 1 , 1452. Macmullln, R. B. Chem. €ng. Prog. 1948, 4 4 , 183. Mosler, H. A.; Koppel, L. B.; Coughanowr, D.R. Ind. Eng. Chem. Process Des. Dev. 1967, 6 , 107. Mou, D. G.; Cooney, C. L. Biotechnol. Bioeng. 1983, 25, 225. Munick, H.AIChE J . 1965, 1 7 , 754. Ray, W. T.; Aris, R. Automatika 1987, 4 , 137. Siebenthal, C. D.; Aris, R. Chem. Eng. Sci. 1964, 19, 747. Soni, Y.; Albright, L. F. J . Appl. Polym. Sei., Appl. Polym. Symp. 1981, 3 6 , 113. Spellmann, R. A.; Quinn, J. B. ISA Trans. 1975, 74, 312. Stock, L. M.;Himoe, A. J . Am. Chem. Soc. 1961. 8 3 , 1973.
Received for review September 6 , 1984 Revised manuscript received February 25, 1985 Accepted July 10, 1985
Drying Characteristics of Morwell Brown Coal and Effects of Drying on Liquefaction Ryoro Toe1 and Hajlme Tamon' Department of Chemical Engineering, Kyoto Universiv, Kyoto 606, Japan
Katsuya Uehara and Saburo Matwmlya Toy0 Engineering Corporation, Kasumigaseki Building, 2-5, 3-Chome, Kasumigaseki, Chiyda-ku, Tokyo 100, Japan
Drying characteristics of Morwell brown coal were experimentally determined. About 80% of the waters contained in the coal was the free water and the capillary water, and the drying was easy. Though the coal was adsorptive and shrank during drying, the falling drying rate was proportional to water content. This result was useful to design a dryer. The effects of drying on coal liquefaction were investigated. During hot air drying below 164 O C , oxygen in the hot air did not affect the product yields and the hydrogen consumption during liquefactionand the composition of coal.
The total reserves of brown coal (lignite) around the world are nearly 2.5 X 10l2tons. Brown coal has a high water content and therefore a low energy density, so that it is correspondingly expensive in terms of transport. In order to take full advantage of the energy potential of the 0196-4305/86/1125-0168$01.50/0
coal, it is utilized as briquette and coal liquid. The dewatering is very important in these technologies. The dewatering such as the heating under pressure in saturated steam (the Fleissner process (Fleissner, 1927)) or oil a t around 200 OC (von Staden, 1927) has been con0 1985 American Chemical Society
Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 1, 1986 Table I. Proximate and Ultimate Analysis of Morwell Brown Coal proximate anal., YO 60.0 (raw coal base) total water volatile matter 49.9 (dry coal base) fixed carbon 47.7 (dry coal base) 2.4 (dry coal base) ash ultimate anal., YO 69.84 (daf base) C H 4.86 (daf base) N 0.55 (daf base) S 0.31 (daf base) 24.44 (daf base) 0
sidered to be available. However, this dewatering method needs much energy and troublesome wastewater treatments. As for drying, the indirect-heat rotary dryer which is the multitubular type is often used. However, the capital and operation costs are high. Also much brown coal cannot be treated by this type of dryer. The drying cost using hot air around 150 OC is very low. Dried coals are commonly oxidized under relatively mild conditions. It may be considered that the yield of coal liquid becomes low and the spontaneous combustion occurs during drying. Therefore, it is important to develop the hot air drying process of brown coal, in which the lowering of the yield of coal liquid and the spontaneous combustion are prevented. Allardice and Evans (1971a, 1971b), Murray and Evans (1971), and Evans (1973) have published a series of investigations on Yallourn brown coal. They reported the evolution of water from the coal, the water sorption isotherm, the thermal dewatering, and the shrinkage on drying. The effects of drying and oxidation on coal pore structure have been reported by Karsner and Perlmutter (1982). However, the effects of drying on brown coal liquefaction have not been examined. The purpose of the present work is to obtain experimental data on the drying characteristics of brown coal and the effects of drying on its liquefaction. Water Sorption Isotherms on Brown Coal Experiment. The water sorption equilibrium on brown coal was determined as follows. Air containing arbitrary humidity was introduced to the carefully weighed coal sample. The amount desorbed was obtained from the decrease in weight of the sample, and the amount adsorbed was obtained from the increase in weight of it. The humidity of air was measured by a dew point meter using a quartz oscillator (Yokogawa Electric Works, Model 2586). The desorption isotherm was measured from the initial water content of coal (around 1.6 kg/kg) to the dried state. Then the measurements of readsorption and redesorption were repeated on the same sample. The coal tested in this work is Morwell brown coal. It was mined in Australia and immediately stored in plastic bags encased in metal barrels. All samples were kept in a refrigerator and protected against moisture loss and oxidation during storage prior to testing. The results of the proximate and ultimate analysis for this coal are shown in Table I . Experimental Results. The desorption and readsorption isotherms of water vapor on the coal sample were determined at 5, 20, and 35 "C. Figure 1 shows these isotherms. It can be seen that the initial desorption isotherm is of the standard sigmoid shape, typical of porous adsorbents, and usually classified as type I1 of the five isotherm shapes recognized by Brunauer et al. (1940). This sigmoid shape
169
Desorption
20'C 354c Desorption v
35'C
P 1% ( - 1 Figure 1. Sorption isotherm of water vapor on Morwell brown coal.
Table 11. Water Contained in Morwell Brown Coal water content, kg/kg 0.9 free water
is generally attributed to the following sorption processes: (1)capillary condensation, which is the dominant factor in the upper concave portion of the isotherm, (2) multilayer adsorption, which occurs in the middle portion of the isotherm, and (3) monolayer adsorption, which occurs in the lower concave portion of the isotherm. There is also present free water. This water would be evaporated before removal of the capillary water at a vapor pressure close to saturation. It is responsible for the large, near-vertical part of the isotherm shown in Figure 1. Consequently, the water contained in the coal is estimated by applying the BET equation (Brunauer et al., 1938) to the desorption isotherm. Table I1 indicates that most of the waters contained in the coal sample are the free water and the capillary water, and the dewatering is easy. The drying characteristic curves mentioned later also show that the critical water content is 0.9 kg/kg and there is present the free water above this water content. The readsorption branch of the isotherm indicates that the coal does not adsorb moisture to the initial water content (1.6 kg/kg) but to around 0.3 kg/kg once it is dried up. The second desorption branch is in fact a scanning curve which crosses the hysteresis region and closes the loop by joining the initial desorption curve at a relative pressure of approximately 0.4. Below this point, the initial and second desorption curves are coincident. The water sorption equilibria are expressed by these isotherms independent of the temperatures. Figure 1shows that there is a distinct difference between the paths of the initial desorption and readsorption branches of the isotherm. The hysteresis has been widely observed in sorption systems but is usually confined to a loop in the upper portion of isotherms and is attributed to capillary condensation. In the present work, the hysteresis extends over most of the isotherm, including the lower portion of the isotherm. This hysteresis at the low-pressure region is considered to be attributed to shrinking and swelling during dewatering and rewetting of the brown coal (Allardice and Evans, 1971b). Drying Characteristics of Brown Coal Experiment. Figure 2 shows a schematic drawing of the hot air drying apparatus. Air was heated by electric heaters to get a constant air temperature. The turbulent
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Table 111. Experimental Conditions and Results of Through-Flow Drying of Morwell Brown Coal run hot air hot air initial initial initial water drying no. temp, "C wt, kg thickness, cm content, kg/kg humidity, kg/kg time, h 1 87 0.0077 0.57 6.8 1.60 2.0 2 87 0.0074 0.58 6.4 1.60 1.3 3 127 0.0072 0.58 6.8 1.50 1.7 4 127 0.0067 0.58 7.0 1.60 0.9 5 127 0.0076 0.58 7.0 1.60 0.7 6 153 0.0082 0.57 7.0 1.61 1.0 7 164 0.0069 0.57 7.1 1.67 0.8
final water content, kg/kg 0.020 0.035 0.005 0.079 0.120 0.002 0.001
Table IV. Effects of Drying Conditions on Liquefaction of Morwell Brown Coal" run no. 1 2
3 4 5 6 7 8 9
drying method hot air hot air hot air hot air hot air hot air hot air vacuum vacuum
drying temp, "C 87 87 127 127 127 153 164 58 125
coal conversion, wt % 92.3 93.6 95.5 94.6 94.5 91.5 95.2 94.0 92.3
oil yield,
SRC yield,
gas yield,
H consumption,
wt%
wt%
wt%
wt%
16.6 14.2 18.0 14.9 15.6 12.1 15.1 18.2 13.8
55.9 59.2 57.4 58.7 55.6 58.1 56.8 55.2 57.9
24.8 24.7 25.1 25.8 28.0 26.4 27.6 25.4 25.4
5.0 4.5 5.0 4.8 4.7 5.1 4.3 4.8 4.8
"SRC yield = SRC amount/input coal amount (daf base). Hydrogen consumption = hydrogen amount/input coal amount (daf base). Ultimate analysis of SRC: C, 89.5; H, 5.4; N, 1.1; S, 0.1; 0, 3.9. Boiling range of oil: light oil, 230 " C , 50 wt %. BALANCE
w ( k g l kg)
l d l
. - 0.5=01 GI
THERMOCOUPLE REGULATED HEATER
3
CONTROLLER
e t ? PRE-HEATER
BLOWER
0-
VOLTAGE S L I D E R
Figure 2. Hot air drying apparatus.
velocity component was reduced through wire meshes in the calming section. The guard heaters were wound on the outside surface of the drying chamber to protect against heat loss. The spherical sample of Morwell brown coal whose diameter was around 3 cm was suspended in the drying chamber. The time changes of temperature and water content were measured by the thermocouples and the balance, respectively. The hot air temperatures were 49, 60, 70 and 80 "C. Experimental Results. One of the drying characteristic curves at the hot air temperature of 70 "C is shown in Figure 3. This figure shows the time change of the center temperature of the spherical sample t,, the surface temperature t,, and the water content w. The distinct constant drying rate and falling drying rate periods are found from the drying rate curve. The critical water content is 0.9 kg/kg. The falling drying rate is proportional to the water content. Though Morwell brown coal is adsorptive and shrinks during drying, the very interesting falling drying rate is observed. This result makes the design of the hot air dryer very easy. Under other experimental conditions, the almost same critical water content is obtained and the falling drying rate is proportional to the water content. Consequently, it is considered that the water transfer through the coal sample is fast.
Figure 3. Drying characteristics of Morwell brown coal (hot air velocity, 0.8 m/s; diameter of sample, 2.89 cm; humidity of air, 0.0195 kg/kg).
0.5
10
1.5
2.0
8 (h)
Figure 4. Through-flow drying characteristics of Morwell brown coal.
Effects of Drying on Brown Coal Liquefaction Through-Flow Drying of Brown Coal. Morwell brown coal was crushed to 8-20 mesh to avoid moisture loss and oxidation and was fed to the iron cylinder whose diameter and height were 14 and 13 cm. The cylinder was inserted into the drying chamber shown in Figure 2. The hot air was introduced to it, and the through-flow drying was conducted under the experimental conditions listed
Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 1, 1986 171 Table V. Effects of Drying Conditions on Element Composition of Morwell Brown Coal ultimate anal., w t '70 run no. C H N ash (S) 0 0.5 4.7 1.3 27.5 1 66.0 0.6 1.7 27.9 2 65.1 4.7 4.6 0.7 1.3 27.9 65.5 0.6 2.2 28.1 64.5 4.6 64.2 4.8 0.6 1.4 29.0 4.4 0.7 66.6 1.5 26.8 4.4 0.6 65.7 1.8 27.5 4.7 0.6 0.4 28.8 65.5 0.6 1.8 27.4 4.5 65.7
in Table 111. In this experiment, the hot air rates were 0.31 m/s in the center and 0.36 m/s near the wall. A typical example of the experimental results is shown in Figure 4. This figure shows the temperatures in the various portions and the thickness of the packed bed. The thickness of the packed bed gradually decreases because of the shrinkage of each coal particle. Coal Liquefaction. Liquefaction of Morwell brown coal was conducted in a thick-walled stirred autoclave. Normally, 0.0375 kg of the coal dewatered by the through-flow drying was charged to the 500-mL autoclave with 5 X lo4 kg of iron oxide catalyst and 0.113 kg of solvent, which was the dissolved oil obtained from brown coal. Air was removed from the reactor before sealing. Hydrogen was inserted into this autoclave at the initial pressure of 1.18 X lo7 Pa. The sample was heated to 420 "C by the rate of 3 "C/min with agitating 900 times a min. After it was held at 420 "C for 60 min, it was quenched to room temperature. The liquid product was separated from hydrogen and product gases and also was divided into the filtrate and the filter cake. The filtrate was distilled at the pressure of 1333 P a and the bottom temperature of 350 "C, and the coal distillate and tFe still residue (solvent-refined coal; SRC) were obtained. The filter cake was washed by benzene and acetone and was vacuumdried. I t was regarded as the reaction residue. Samples of Morwell brown coal under study were vacuum-dried (pressure < 133 Pa) at 58 and 125 "C to allow water removal without accompanying chemical changes such as surface oxide, COz,CO, and H20 formation. The dried samples were then submitted to the above liquefaction experiment and provided reference cases. The experimental results are shown in Table IV. In this table, the run number corresponds to one of the drying experiments listed in Table 111. Table IV means that the drying of the brown coal does not affect the conversion, the yield of SRC, and the hydrogen consumption. The ultimate analysis for the coal sample which was dried under various conditions was conducted by using CHN Corder (Yanagimoto, Co., Ltd.: MT-3). The ex-
perimental results are given in Table V. The drying conditions do not affect the element composition of brown coal, especially the oxygen content. In the hot air drying before coal liquefaction, it has been considered that brown coal is oxidized and the yields of coal liquid become low. These experimental results, however, show that oxygen in hot air below 164 "C does not affect the composition of the coal and its liquefaction. Morwell brown coal was tested in the present work. Since it is not clear that the results are applicable to other coals, more investigations are needed.
Conclusion (1) About 80% of the waters contained in Morwell brown coal are the free water and the capillary water, and the drying of it is easy. (2) Though Morwell brown coal is adsorptive and shrinks during drying, the falling drying rate is proportional to the water content. This result is useful to design a dryer. (3) Hot air drying below 164 OC does not affect liquefaction of Morwell brown coal. Acknowledgment We are grateful to H. Hikosaka for his assistance in the experimental work and to Mitsui SRC Development Co., Ltd., for conducting coal liquefaction experiments. This work waa supported in part by the Ministry of Education, Science, and Culture of Japan for a Grant-in-Aid for Energy, No. 58045084 (1983). I
Nomenclature L = height of packed bed, cm p = pressure, Pa p s = saturated vapor pressure, Pa t , = hot air temperature, "C t , = temperature in center of brown coal sample, "C t, = temperature on surface of brown coal sample, "C t,, = temperature in lower portion of packed bed, O C t,, = temperature in middle portion of packed bed, O C t,, = temperature in upper portion of packed bed, "C w = water content based on dry material, kg/kg 6 = time, h Literature Cited Allardlce, D. J.; Evans, D. G. Fuel 1971a, 50, 201. Allardlce, D. J.; Evans, D. G. Fuel 1971b, 50, 236. Brunauer. S.; Deming, W. E.; Teller, E. J. J . Am. Chem. SOC. 1940, 62, 1723. Brunauer, S.; Emmett, P. H.;Teller, E. J. J . Am. Chem. Soc. 1938, 60, 309. Evans, D. G. Fuel 1973, 52, 186. Flelssner, H. Sonderb. Sparwltfsh. 1927, 10, 11. Karsner, G. G.; Pertmutter, D. D. Ind. Eng. Chem. Process Des. Dev. 1982, 21, 348. Murray, J. B.; Evans, D. G. Fuel 1971. 51, 290. von Staden, H. A. US Patent 783 757, 1927.
Received for review August 28, 1984 Accepted June 27, 1985
