ethane to propane B = (6 eq A-4
[c-c]
[
1 = - 3(1
- 12
12
+ 8)/(2 + 3) = 2.8. Now, applying
- 0.1312)
(i- + 2’s)
(0.1312) - l2 (3 (0.380) 224
[c-c]
K , = number of pellet diameter per radial stage K , = number of pellet diameter per axial stage M W = average molecular weight Pe = Peclet number = V l / D = (velocity)(characteristic length)/diffusivity
1
+ 1.107 - 1)
= 0.0378 mol/g of oil
Nomenclature a = stoichiometric coefficient A(i,j) = area of stage j in row i B = hydrogen to carbon ratio a t complete conversion C = total carbon atoms per molecule C(i,j) = concentration of reactants in stage j in row i CA = aromatic carbon atoms per molecule CO = olefinic carbon atoms per molecule C,,(i,j) = heat capacity a t constant pressure at the temperature in stage i,j [C-C] = concentration of c carbon-carbon bonds, mol/g [C=C] = concentration of aromatic x carbon-carbon bonds, mol/g [C=C]’ = concentration of olefinic x carbon-carbon bonds, mol/g d p = catalyst pellet diameter H = hydrogen atoms per molecule H D L = heat dissipation length AH = heat of reaction i = index for stirred tank in axial direction j = index for stirred tank in radial direction k = first-order rate constant, (lh)
R A = average number of aromatic rings per molecule RAS = average number of substantial rings per molecule RT = average total rings per molecule AT,,, = maximum temperature reached in the hot spot t = time, s T(i,j) = temperature of fluid in stage j or row i Ubulk = average fluid velocity in unobstructed region urnin = minimum velocity in obstructed region
u ( Q ) = fluid velocity in stage j or row i w ( r , j ) = mass flow rate of fluid in stage j of row i
Greek Letters t = void fraction p(i,j) = density of fluid in stage j or row i $(i - 1,j) = average concentration of fluid entering stage j in row i from row i - 1 $(i - 1j) = average temperature of fluid entering stage j in row i from row i - 1 Literature Cited Deans, H. A., Lapidus, L., AlChE J., 6 (4), 656 (1960). Jaffe, S. B., lnd. Eng. Chem., Process Des. Dev., 13, 34 (1974). Qader, S. A., Hill, G.R., lnd. Eng. Chem., Process Des. Dev., 8, 98 (1969). Stangeland, B. E., lnd. Eng. Chem., Process Des. Dev.. 13, 71 (1974). Wilhelm, R. H.. Pure Appl. Chem., 5 , 403 (1962). Zhorov, Yu. M., et al.. lnt. Chem. Eng., 11 (2), 256 (1971).
Received for review October 20, 1975 Accepted February 2,1976
The Oxidation of Bituminous Coal. 3. Effect on Caking Properties A. Y. Kam, A. N. Hixson, and D. D. Perlmutter’ Department of Chemical and Biochemical Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 79 7 74
A quantitative study is reported of the effect of oxidation on the caking propensity of bituminous coal. An empirical caking index is found to correlate well with a defined semiempirical oxidation parameter. The preferred oxidation temperature for decaking treatment was found to be about 200 OC,since the higher oxygen utilization at higher temperatures does not appreciably contribute to the desired effect.
In two previous papers of this series (Kam et al., 1976a,b), a study was reported on the modelling and experimental kinetics of the oxidation of bituminous coal. This paper presents a quantitative study of the effects of oxidation on the caking properties of the coal samples used in the kinetic evaluations of Part 2. The samples, whose oxidation history is given in Table I, were tested to determine how changes in their caking properties could be related to the oxidation rate and extent parameters. In order to gain insight into the phenomenon of caking and the effect of oxidation of bituminous coal on its caking properties, it is desirable to establish a quantitative correlation between the extent of oxidation and the corresponding reduction in caking tendency. Such a correlation will also facilitate the practical application of the caking test data obtained here to the design of pretreatment processes. 416
Ind. Eng. Chem., Process Des. Dev., Vol. 15, No. 3, 1976
The Plastic Properties of Coal When coal is heated, it undergoes two stages of change: the coal first softens and becomes “liquid-like,” and subsequently the particles cake and form a compact mass which swells and resolidifies into a coherent body with a porous structure, i.e., coke. At the same time the coal undergoes a chemical transformation, evolving gases and condensable vapors, and leaving a residue consisting almost exclusively of carbon and ash. The properties associated with these changes are generally referred to as the plastic properties of coal and have been studied extensively in researches on coke-making. Comprehensive reviews on the numerous papers published on this subject have been given by Howard (1963) and Loison et al. (1963). In general, the models proposed on the softening and caking phenomena of coal may be classified as physical or physico-
Table 1. Oxidation History of Caking Test Samples Distribution of 02 reacted, g/kg of coal Run no.
Oxidation temp, OC
Total 02 reacted, g/kg of coala
CO”
41.5 6.3 2.1 200 200 66.8 13.0 4.8 21 200 53.4 9.6 3.1 22 200 75.8 12.6 3.9 42.7 8.2 2.4 23 200 63.2 11.6 3.3 28 200 37 250 110.6 28.2 10.3 39 225 72.0 17.0 8.9 40 277 163.2 49.1 16.5 43 225 59.2 14.8 7.2 44 225 87.5 20.1 9.3 45 225 66.6 14.8 7.8 46 225 55.2 11.7 6.5 47 225 103.6 22.9 10.5 48 225 55.9 13.4 7.2 67.5 16.0 8.2 49 225 72.4 17.5 8.9 50 225 Calculated by integration of rate data from part 2 (Kam e t al., 197613). Calculated by difference. 19 20
a
cop
chemical (Howard, 1963). In the physical models, the softening of coal is considered as a paste-like melting, similar to the change that occurs in thermoplastic materials, that results from the increase in thermal agitation due to higher temperatures. Subsequent flow of the softened material between adjacent particles leads to agglomeration, fusion, and caking. Parallel to this softening process, pyrolysis occurs and modifies the fusability of the particles by gas evolution and degradation of the chemical structure. The plastic behavior of coal is therefore a superposition of the physical phenomenon of “melting” and the chemical phenomenon of pyrolysis (Audibert, 1926; Berkowitz, 1949, 1950; Gillet, 1951; and Lahiri, 1951).In the physiochemical model, the softening and caking of coal are considered to be the consequences of pyrolysis. Van Krevelen (1954) and Hirsch (1954) regarded coal as a polymeric material made up of large structural units (lamelles) of various sizes connected by cross-linkages. A pyrolytic softening/caking model for coal has been suggested by Chermin and Van Krevelen (1957).In the terminology common in the technical literature, any treatment that reduces the caking propensity of coal is said to be a decaking, and the product is referred to as more or less decaked (Gasior et al., 1965). This phrase is retained here for its simplicity and in spite of its slightly awkward style, because it is well known in the field and avoids other cumbersome circumlocutions. Although oxidation is known to decake coal, the precise role of oxygen in the chemical and/or physical phenomena governing the caking process is uncertain. Fuch and Sandhoff (1942) represented the pyrolytic breakdown of bituminous coal as the rupture of the ether-type oxygen cross-linkages between the polynuclear aromatic micelles. Orchin et al. (1951) argued that the decaking effect of oxidation might be due to the formation of additional oxygen (unspecified) cross-linkages which then hinder the micellar structure from gaining mobility. They also suggested an alternative explanation in terms of “available oxygen” analogous to the situation in high explosives. They argued that the oxidized coal contained more oxygen available to oxidize or catalyze the rearrangement of the material in coal whose presence was essential for coke formation. Orechkin (1956) suggested that the softening of coal results from a depolymerization process which causes a decrease in the average molecular weight of the coal and its transformation into liquid or a low-melting material. This is followed by a condensation reaction between the
H20“
Fixedb
12.6 23.5 16.9 26.1 14.1 18.7 34.7 29.0 57.8 23.4 32.4 24.3 19.9 36.5 22.5 23.1 23.2
20.5 25.5 23.8 33.2 18.0 29.6 37.4 17.1 39.8 13.8 25.7 19.7 17.1 33.7 12.8 20.2 22.8
carbon complexes; this reaction produces water and leads to a growth of the carbon skeletons and a loss of fusibility. The effect of the oxidation of coal on its caking propensity is regarded as a shift of the reaction tendencies toward the condensation reaction. The more recent work of Ignasiak et al. (1974) and Wachowska e t al. (1974) further indicates that the loss of swelling and fluid properties of oxidized coal is due to the formation of ether type oxygen cross-linkage‘s formed by the condensation reaction of acidic hydroxyl groups. Measurement of the Degree of Caking A great many laboratory apparatus and test methods have been proposed and used to study the plastic properties of coals. Due to the imperfect knowledge of the softening and agglomerating mechanisms of coal, all the tests are empirical in nature and have been devised to characterize the plastic behavior of coal by means of numerical indices that are not generally physical constants. Rather, the indices serve as empirical reference points and bases for classifying coals or predicting the relative behavior of different coals. Comprehensive summaries and reviews of the many proposed test methods and subsequent modifications have been given by Brewer (1945) and Loison et al. (1963). Of the many tests proposed to date, the only two which have been standardized by the ASTM (1972) are the Free Swelling Index (ASTM Test D720-67) and the Giesler Plastomer (ASTM Test D2639-71). Even these two tests are considered semi-quantitative in nature and are subjected to variation from operator to operator. The caking test chosen for this study was the Gas Flow test (Foxwell, 1924; Coffman and Layng, 1927,1928). This test war chosen for its simplicity in apparatus design and operation and for its close resemblance to the caking of coal in a packed bed reactor. In the Gas Flow test described by Coffman and Layng, a sample of the test coal is heated a t a constant rate of temperature rise (2 OC/min) inside a tube with a constant flow of nitrogen (40 cm3/min) through the coal sample. The differential pressure drop across the sample is monitored with a manometer. As the sample goes through its plastic range, the pressure drop increases, reaches a maximum, and then decreases. Pressure drop vs. temperature readings are taken as data for the test. The oxidized coal samples from Part 2 of this study were subjected to this test to investigate the effect of oxidation on their caking propensities. The extents of oxidation for the samples are shown in Table 11. Ind. Eng. Chem., Process Des. Dev., Vol. 15, No. 3, 1976 417
TAR TRAP
Table 11. Caking Test Results for the Oxidized Coal Samples
.TO VENT
Oxida- MPD,a Oxida- Coal size, tion mm Test Total02, tion U S . std. mesh runno. H2O columnb reactedC temp, O C 46 45 44 47 22
19 21 28
20 39 37 40 48 43 49 50 23
216 171
137 117 120 484 342 218 167 368 225 154 645 393 293 318 241
W W W F F C C C C C C C C C C C C
55.2 66.6 87.5 103.6 75.8 41.5 53.4 63.2 66.8 72.0 110.6 163.2 55.9 59.2 67.5 72.4 42.7
225 225 225 225 200 200 200 200 200 225 250 277 225 225 225 225 200
-18to+50 -18tof50 -18to+50 -18to+50 -18 to +50 -14to-l-18 -14to+18 -14to+18 -14to+18 -14to+18 -14to+18 -14to+18 -6tof14 -6t0+14 -6t0+14 -6t0+14 -6t0+14
MANOMETER
U
V-E->TO
BUBBLE FLOWMETER
@
THERMOMETER
@
P R E S S U R E GAGE
@
TOGGLE
VALVE
Figure 1. Schematic of flow system for caking test.
a MPD-maximum pressure drop across coal column. Physical condition of the coal column after the caking test; W = weakly caked, F = free-flowing, C = caked. Total oxygen reacted during the oxidation runs, g/kg of coal. b
Experimental Apparatus and Procedure The caking test apparatus consisted of a caking tube assembly, a furnace, a temperature control and monitoring system, a gas flow system, and a manometer for pressure drop measurements. Figure 1 is a schematic diagram of the apparatus. The caking tube was a 20-in. length of Type 304 stainless steel seamless tubing (0.50 in. 0.d. X 0.049 in. wall). As shown in Figure 2, a lower assembly provided gas access, thermocouple insertion, and a pressure tap. A tar trap was used a t the top to prevent plugging of the vent line. Nitrogen for the test was drawn from a cylinder of Ultrapure grade nitrogen (Airco) which contained less than 1 ppm of oxygen as impurity. A constant flow of 40 cm3/min of nitrogen to the caking column was maintained by imposing a relatively large pressure drop across a fixed orifice. A stable pressure of 50 psig was maintained on the up-stream side of the orifice by a standard two-stage gas cylinder pressure regulator. The down-stream side of the orifice was open to the atmosphere through the coal column. The Na flow rate did not change significantly even when the down-stream back pressure changed due to the caking of the coal column (maximum back pressure of about 1200 mm of H20). The caking tube was heated by a furnace with two heating zones. T o achieve a constant rate of temperature rise in the coal column and continuous monitoring of the column temperature, a temperature controller was'used in conjunction with a linear temperature programmer, a power controller, and a digital temperature display unit as shown in Figure 3. A current-adjusting-type temperature controller was used to eliminate any temperature cycling. The temperature programmer provided a consistently reproducible heating rate of 2.0 f 0.01 OC/min. The coal to be tested was mixed thoroughly and a 3.0 & 0.003 g sample was charged. The caking tube was tapped repeatedly until no further settling of the column occurred. This method of packing produced the consistent column heights shown in Table 111. The coal column was heated under manual control to 200 "C a t about 10 "C/min. When switched to automatic control about 30 min was required for the system to "settle down" to 418
Ind. Eng. Chem., Process Des. Dev., Vol. 15, No. 3, 1976
1 THERMOCOUPLE Figure 2. Schematic of caking tube assembly.
0 lo20
115 V A C
POWER
SOURCE
CONTROLLER
0
t o I5ma
TEMPERATURE
0 l o I5 V D C
PROGRAMMER
(SET POINT)
'
AMPS.
TO 'HEATERS
SIGNAL
TEMPERATURE
TEMPERATURE
CONTROLLER
METER
the desired rate of temperature rise of 2 "C/min. Above 300 C pressure drop vs. temperature readings were recorded every 5 min up to 360 "C, and every minute (every 30 s or every 15 s in some cases where the changes of the pressure drop were rapid) up to 550 "C. After that power for the furnace was turned off and the coal column was allowed to cool under nitrogen flow, generally overnight. The caking tube assembly was removed from the furnace and disassembled. For decaked samples, the coal could be "poured" out of the caking tube and remained in particulate form. For caking samples, coke buttons were formed that had to be pushed with a rod to remove them from the caking tube.
500
Table 111. Caking Test Columns
U S . std. mesh coal size
Obsd range of column heights, in.
-6 to +14 -14 to +18 -18 to +50
2.07 f 0.04 2.01 f 0.03 1.97 f 0.03
r
i 3
2
300
c w
c v)
I
0
I
t
200
4
I
n R U N NO.
0 Q
2
100
Lo
a
L
x\\
I
0 IO
TEST
1
1 '\
r
T E Y P E R A T U R E , .C
LJL 41
OSBO
T E S T TEYPERATURE, *C
(AT 2'C/YIN.
RISE)
Figure 5. Caking test results for -14 to +18 mesh oxidized coal samples.
I
*fRCL-FLOWING
[ A T 2'C/MIN.
RISE)
Figure 4. Caking test results for -18 to +50 mesh oxidized coal samples.
RUN
OX10 T E Y P , ' C
20
200
39
225
37
250
40
277
300
Results and Discussion
The pressure drop vs. temperature data from the caking tests are shown in Figures 4 to 7. T o provide a basis for comparison, caking tests were also conducted with samples of the raw coal used as feed for the oxidation runs. These data are shown in Figure 8. All the results exhibit the typical bell shape reported for the Gas Flow test (Coffman and Layng, 1928). In addition to the pressure drop data, the physical condition of the coal column in the caking tube was also examined after each test. I t was found that all the samples for the -6 to +14 mesh (U.S. Standard) and -14 to +18 mesh sizes had caked and formed one-piece, coherent coke masses. The -18 to +50 mesh size samples showed significantly better reduction of caking propensity. In particular, the test samples for runs 47 and 22 were completely free-flowing, and all the other specimens of this mesh size were weakly caked, emerging as loosely formed clusters of coke particles intermixed with free-flowing particles. It is apparent that the smaller size particles are much more readily decaked by oxidative pretreatment than the larger size particles. For the raw coal, test samples of all three sizes caked and formed coherent coke masses. To correlate oxidation with the measured changes in caking propensity, a possible method is to relate a caking index to an oxidation parameter which characterizes the extent of oxidation. Caking Index. A comparison of Figures 4 to 7 with Figure 8 shows that the maximum pressure drops (MPD) for the oxidized coal samples were substantially lower than those of the raw, unoxidized samples. Furthermore, the free-flowing samples (runs 47 and 22) showed the lowest MPD. These observations indicate that the MPD is sensitive to oxidation and may be used as a caking index, with the degree of decaking varying inversely with the MPD; i.e., the smaller the MPD, the greater the degree of decaking. The MPD for the runs in Figures 4 to 7 are given in Table I1 together with the appropriate oxidation data. Oxidation Parameter. In order to correlate the reduction of caking propensity (characterized by the MPD) with the
01 360
1
1
400
1
I
440
TEST TEMPERATURE, 'C
1
1 480
( A T Z'CIHIN
I
I
520
RISE1
Figure 6. Caking test results for -14 to +18 mesh oxidized coal samples.
700
600 0
I
2
500 2 z 0
400 c Y 0
a 300
0 Y
200
100
0 3
0 T E S T T E M P E R A T U R E . 'C
( A T 2'CIMIN.
RISE1
Figure 7. Caking test results for -6 t o +14 mesh oxidized coal samples. Ind. Eng. Chem., Process Des. Dev., Vol. 15, No. 3, 1976
419
OXID. TEMP.,'C
0
200
0
225 250
it 1 k0
\A37
E 200
3
i 5 =
10030
U
= 10030
A P MEASURED BY CALIBRATED PRESSURE 0IO€
\
\
40
I
I
I
I
I
I
I
I l
l 140 OMlK0M
I I I 60 80 100 120 I20 AMOUNT O F OXYOEN R E A C T E D ,
I
40 TOTAL
I
I I
I
40
\
\
~
,
I
1
D
I60 COAL
Figure 10. Relationship between decaking and amount of 0 2 reacted (-14 to t18 mesh).
L \ \
,
440
-0
T E S T TEMPERATURE, *C
I
I
I 620 6 RISE)
480 (ATZ*C/MIN.
feoo
I
OXID. TEMP., * C
-
a
E Y
200 0
$240
r
I PARTICLE
-
3
200
I
-
0 2 2
I I 50
TOTAL
I
BO AMOUNT
I
70
OF
O2
1
80 REACTED,
I
90 GMlKGM
I
100 COAL
I
I10
Figure 9, Relationship between decaking and amount of oxygen reacted.
oxidation of the coal sample, it is desirable to establish an oxidation parameter (OP) which will reflect the chemical changes in the coal due to oxidation, rather than the specific operating conditions of the oxidation. Toward this end, a preliminary examination of the data in Table I1 shows that at a given temperature, the MPD can be correlated with the total amount of oxygen reacted during the oxidation run. More quantitative tests are shown as Figures 9 to 11 for the three different sizes of coal particles. It is evident from these figures that, for a given amount of oxygen reacted, its effectiveness in decaking the coal is dependent on the temperature a t which the oxidation is done. Specifically, the effectiveness decreased when the oxidation temperature was raised from 200 to 225 OC. This loss of effectiveness was not offset by the higher oxygen reaction rate a t 225 "C since the oxidation time of the various runs were all relatively close to each other (5.5 to 6.5 h). In order to achieve the same degree of decaking as at 200 "C, a run a t 225 OC must be carried out over a longer time interval despite the higher oxygen reaction rate a t the higher temperature. This unexpected phenomenon was exhibited by all three sizes of coal particles. Referring to Figure 10, for example, the samples for runs 40 (277 "C) and 20 (200 ") showed comparable degrees of decaking despite the large disparity in the amounts of oxygen reacted during their 6.5 h oxidations. By increasing the oxidation temperature to 250 and 277 OC, the loss in oxygen effectiveness is increasingly compensated for by the higher oxygen reaction rates. This apparent 420
I
I 80
O F OXYDEN R E A C T E D ,
1 b 70 G Y l K O Y COAL
(-6 to +14 mesh).
Y
80 40
SO AMOUNT
Figure 11. Relationship between decaking and amount of 02 reacted
ln
120
I
I TOTAL
2 100 -
100
0 23
SOLID P O I N T S s FREE- FLOWING
180-
$ I 5
-
200 40
a
140-
300
x
9
a
E
-
E
0 x 1 0 . T E M P .C
0
46
400
200
225
a
S I Z E : -18 T O + S O M E S H
g,",
g 220 a-200 0 Y
5
0 0
\
g BOO -
Figure 8. Caking test results for feed coal samples.
R U N NO
Ind. Eng. Chem., Process Des. Dev., Vol. 15, No. 3, 1976
anomaly with respect to temperature must be considered in establishing an oxidation parameter to characterize the extent of oxidation. As reported in part 2 of this series (Kam, e t al., 1975b), a change in the oxidation temperature of coal causes changes in the oxygen reaction rate and changes in the relative production rates of the oxidation products ((202, CO, H20, and "fixed" oxygen). The change in oxygen reaction rate, discussed above in terms of total oxygen reacted, fails to account for the temperature anomaly. One must therefore conclude that the distribution of the oxidation products affects decaking. This distribution is shown in Table I. An extensive search of the published literature shows that the specific effect of the oxidation products distribution on decaking has not been investigated previously. I t is nevertheless instructive in developing the OP to examine the available information on the caking phenomenon to seek some general directions and probable decaking mechanisms. It has been shown (Ihnatowicz, 1952; Blom et al., 1957) that the oxidation of coal causes an increase in the -COOH and -OH oxygen functional groups in the coal structure. Assuming that the formation of water during oxidation proceeds via condensation-type reactions such as: 0
0
II -C-OH
+
-C-OH
II
--t
+ H1O
-C-O-C-
and
-C-
I I
OH
+
I I
-C-OH
I I
-C-O-C-
I I
+
HzO
it can be seen that the formation of water would simultaneously result in the formation of cross-linkages in the coal structure.
O X I D . T E M P . , ‘C
0 0 a zoo a
RUN NO.
200
OXID. T E M P . , .C
0.
225
SOLID
POINTS
\
=
FREE-FLOWINB
0
200
0
225
A
250 277
q ‘“a-n
2
300
eo0
5
~
1 a0
40
“30
O X IOATION
1
,
,
Bo
70
80
PARAYETER,
+
a y H E 0 ( - ) yH 0
YCO* +
1
1
40
100,
90
,
YFIX
0
I
I
I
a0
60
70
I 80
I
SO
0
OM/KOM
yco
Figure 12. Correlation of degree of decaking and extent of oxidation (-18 to +50 mesh).
OXID TEMP,’C
0
0
RUN NO. 700-
o*eoo
200 2 2 5
-
I
I I
am0
*
p 400 v)
I
z 300-
100
30
I 39
OXlOATlON PARANETER,
I
40
1 42
I
41
(QyH d(yHZo+ y F , , ) / ( t 0 + I
nature and location of the carbon-carbon bonds being ruptured are unknown, such a bond breakage may be regarded in a general sense as a depolymerization step. (Probable routes include ester pyrolysis followed by decarboxylation, or ester rearrangement followed by decarboxylation.) In this context, the formation of COz and CO during oxidation may be expected to enhance the caking tendency. The development of a universal, analytical correlation between the MPD and an OP is beyond the scope of this study since it would require intimate knowledge of the chemistry and rate of the softening and pyrolysis of coal. The arguments presented above do, however, provide the general directions for the development of an empirical OP based on the distribution data of Table I. Furthermore, a satisfactory empirical correlation can be established between the degree of decaking, characterized by the MPD’s from Table I1 and the extent of oxidation, characterized by an OP of the form:
y 1
co
), 0 M I K I
Figure 13. Correlation of degree of decaking and extent of oxidation (-14 to +18 mesh).
Orechkin (1956),Ignasiak et al. (1974),and Wachowska et al. (1974) have proposed that the formation of cross-linkages by condensation reactions contributes to a loss of plasticity and fusibility of the coal, and therefore lessens its caking propensity. This argument suggests that the formation of water during oxidation would contribute to decaking. Similarly, the fixation of oxygen onto the coal structure may be regarded as the direct formation of additional oxygen crosslinkages (Orchin et al., 1951) and would therefore also be expected to contribute to this change. The effect of CO2 and CO formation on decaking has not been discussed directly in the literature. A possible explanation may be derived by using the model of Chermin and Van Krevelen (1957), in which the softening and resolidification of coal is postulated to occur via depolymerization, causing partial liquefaction of the coal, pyrolytic cracking, and then resolidification to form coherent coke masses by means of repolymerization. The fluidity and fusability of the coal is dependent on the relative rates of depolymerization and repolymerization. A higher depolymerization rate yields a higher concentration of the “metaplast” (liquid coal), making the coal more fluid and fusable. For the present case, the formation of cross-linkages due to water formation and oxygen fixation may be regarded as a polymerization step in a broad sense. In order to form COa and CO, it is obviously necessary to cleave some carbon-carbon bonds in the coal structure. Although the
Figures 12 to 14 show such correlations for the three sizes of coal particles. They represent the replotting of Figures 9 to 11, respectively, using the OP as the revised correlating variable. On this new scale a single curve can be used for each particle size for the entire temperature range (200 to 277 OC) for the Pittsburgh Seam coal studied here. Figure 12 shows for example that -18 to +50 mesh Pittsburgh Seam bituminous coal can be rendered free-flowing by carrying out the oxidation to the extent that the OP equals or exceeds 76 g/kg of coal. Implications of the Oxidation Parameter. The empirically derived oxidation parameter defined by eq 1, which includes the combination ( Q Y H ~ O )to represent the amount of water formed during the course of the oxidation, has been shown to correlate satisfactorily with the degree of decaking. Figures 12 to 14 show that the degree of decaking improves with higher values to the OP, and hence higher values of ( Q Y H ~ ~ This ). implies, in direct agreement with the mechanisms proposed in the references cited earlier, that the greater the quantity of water formed during oxidation, the greater the degree of decaking. The OP given by eq 1also contains the factor ( Y H ~ O+ yfi,)/ (yco2 yco). From the correlations shown in Figures 12 to 14, it is apparent that decaking is favored when the formation of water and “fixed” oxygen is maximized and the formation of the carbonic gases is minimized, thus confirming the negative effect of COz and CO formation on decaking as discussed earlier. The factor ( y ~ +~yfi,)/(yco, 0 + yco) may be regarded as the ratio of polymerization (water formation and oxygen
+
Ind. Eng. Chem., Process Des. Dev., Vol. 15, No. 3, 1976
421
fixation) to depolymerization (carbonic gases formation), the former contributing to greater decaking and the latter having the opposite effect. The results of this caking study permit the following conclusions to be drawn. (1) Pittsburgh Seam bituminous coal can be rendered free-flowing by air oxidation in a packed bed reactor a t 200 or 225 "C. (2) Under a given set of oxidation conditions, the extent of change is strongly dependent on the coal particle size; a reduction in the particle size leads to significant improvement in the degree of decaking. (3) A quantitative correlation exists between the degree of decaking and the extent of oxidation of the coal when the latter is characterized by an oxidation pafameter defined by eq 1. (4) In the oxidation temperature range of 200 to 277 "C, the use of the oxidation parameter as a correlating variable leads to a unique relationship between the degree of decaking and the extent of oxidation of the coal, which is independent of the conditions a t which the oxidation of the coal is carried out. (5) The formation of water and "fixed" oxygen during the oxidation of coal contribute to a reduced caking tendency (probably through the formation of oxygen cross-linkages in the coal structure), whereas the formation of the carbonic gases (CO:! and CO) has an adverse effect on decaking (probably due to the "depolymerizing" effect of the breakage of carbon-carbon bonds). (6) Under similar oxidation conditions, better decaking is attained for the oxidation temperature of 200 "Cthan for temperatures of 225 or 250 "C; Le., raising the oxidation temperature to above 200 "C has a detrimental effect when other operating conditions are kept constant. For an oxidation temperature of 277 "C, decaking is comparable to that found a t 200 "C. However, a t 277 "C,a significantly larger amount of oxygen is needed for reaction and a greater loss of the fuel value o f t h e coal (as CO2 and co) Occurs without improved decaking. Within the oxidation temperature range studied, 200 "C appears to be the preferred treatment temperature.
422
Ind. Eng. Chem., Process Des. Dev., Vol. 15, No. 3, 1976
Nomenclature
MPD = maximum pressure drop from caking test O P = oxidation parameter Q = total amount of oxygen reacted, g/kg of coal y c o = fraction of reacted oxygen distributed as CO yco2 = fraction of reacted oxygen distributed as COz yfix = fraction of reacted oxygen distributed as "fixed" oxygen in the coal Y H ~ O= fraction of reacted oxygen distributed as water Literature Cited ASTM, "Annual Book of ASTM Standards," Part 19.ASTM, Philadelphia, Pa.,
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(1974).
Received for review October 2 2 , 1975 Accepted M a r c h 22,1976 T h e f i n a n c i a l s u p p o r t generously p r o v i d e d t o A.Y.K. b y t h e M o b i l Research a n d D e v e l o p m e n t C o r p o r a t i o n m a d e possible t h e u n d e r t a k i n g of t h i s research work.
