E;gE;ri
Pressure Feeder for Powdered Coal or Other Finely Divided
ng
rrocess development
K.
R. BARKER1, JOHN J . S. SEBASTIAN,
AND
L. D. SCHMIDT
U. S. BUREAU OF MINES, MORGANTOWN,W. VA.
H. P. SIMONS WEST VIRGINIA UNIVERSITY, MORGANTOWN, W. VA.
T h e high cost of compression of the synthesis gas prior to purification and synthesis to liquid fuels favors gasification a t elevated pressures of powdered coal with superheated steam and oxygen. Thus, it was necessary to develop a method of continuous charging of finely powdered coal into a pressurized gas generator. Experimental work under pressures up to 150 pounds per rquare inch proved that powdered coal can be conveyed \pneumatically, in a dense phase (26 pounds per cubic foot), qrom a fluidized bed and discharged a t constant rate. With other conditions equal, the discharge rate is solely a function of the pressure differential across a conveying line of given dimensions. Regardless of the operating pressure, the amount of coal conveyed per unit volume of gas, measured a t the existing pressure, is practically constant, but the weight of coal carried per unit weight of gas rapidly decreases with increasing operating pressures. The design information given on pressurized dense phase feeders has not been heretofore available.
EXCESS FLUIDIZING GAS
A
T ITS station in Morgantown, W. Va., the Federal Bureau of
Mines operates pilot plants t o produce synthesis gas, a niixture of carbon monoxide and hydrogen obtained by gasifying coal and used in manufacturing synthetic liquid fuels. Part of the operation, conducted by the Bureau's Synthesis Gas Production Branch, consists in charging finely powdered coal pneumatically from a feeder column, in which the coal is fluidized, into a synthesis-gas generator. Thus far, the generators have been operated a t pressures slightly above atmospheric but below the pressure maintained above the fluidized bed, so that a continuous stream of coal conveyed by a small quantit,y of air was caused t o flow into t'he generator chamber. The development and the mechanism of this method of feeding finely divided solids was covered in an earlier paper ( 9 ) . New plans call for the use of superatmospheric pressures b o t h in the feeder column and generat,or. To obt,ain design data for a high pressure, pneumatic feeding system on commercial scale, it
* Present address, Mine Safety Appliances Go , Pittsburgh,
Pa.
/HOPPER
COLUMN
FLU'DlzlNG
I
1
REGULATOR
&>.[
-----
\
i
METER
4
PURGE LINE NlTROGEN CYL"DERS
SIGHT GLASS
CONVEYING GAS
REGEWER
PNEUMATIC COAL FEEDER
Figure 1. Apparatus for Studying Continuous Feeding of Finely Powdered Coal at Superatmospheric Pressures
1204
May 1951
I N D U S T R I A L A N D E N G I: N E E R I N G C H E M I S T R Y
was necessary t o construct and operate a small scale pilot model a t various pressures and t o determine the effect of these pressures on operation. The objective of the investigation described in this paper was the development of a continuous method of conveying and charging finely powdered coal or other pulverized solids under superatmospheric pressures ahd a t a constantly uniform feed rate. No reference has been found in the technical literature t o the effect of pressure on the conveying of solids by gases, in the form of high density streams. This method of conveying is approached to some extent in certain parts of the fluid-catalytic cracking process, but the pressures used there are in the range of 8 t o 10 pounds per square inch gage ( 3 ) . Albright ( I ) , who carried out similar investigations a t atmospheric pressure, recommended that research be continued using higher pressures. He stated that an inverted, funnel-shaped inlet t o the feed line was used rather than a straight, funnelshaped mouth t o provide flexibility in the coal t o conveying-gas ratio. He also pointed out that a minimum velocity, ranging from 5 t o 10 feet per second, is required for steady flow through the feed line; otherwise the solids settle out and clog the conveying tube. Turneaure and Russell ( 4 ) studied the problem of conveying sand in water through horizontal pipes and found that the settling rate of the sand depended on the density ratio and the velocity of flow. The pilot model built in cooperation with the department of chemical engineering at West Virginia University, for the experimental work to be described, is capable of withstanding pressures up t o 750 pounds per square inch gage. Nitrogen instead of air was used as the fluidizing medium to prevent any possibility of explosive mixtures in the equipment. It was decided t o make a series of runs a t atmospheric pressure and a t multiples of 50 pounds per square inch gage until the effect of pressure could be established on the following variables: 1. Coal to conveying-gas ratio through the feed line 2. Rate of coal feed through the conveying feed line 3. Minimum linear velocity in the conveying line 4. Rate of fluidization in the feeder column (linear velocity of the fluidizing gas above the fluid bed) EXPERIMENTAL PROCEDURE
*
*
As seen from Figure 1,the apparatus consisted of equipment for providing the fluidizing nitrogen a t any required pressure; coal hopper and fluidizing column; receiver vessel; feed line connecting the fluidizing column with the receiver; cyclones and filters for dust removal; and metering and control devices. A photographic view of various parts of the equipment is shown in Figure 2.
A high volatile coal from Sewickley seam (West Virginia) was used in the experimental work described. The air-dried coal charged into the unit contained 1.1%moisture. The cumulative weight percentages retained on Tyler standard screens were: 1.7% on 100 mesh, 4.7% on 150 mesh 16.5% on 200 mesh, and 83.2% on 300-mesh screen. The true density of the coal was 84.0 pounds per cubic foot. Finely powdered coal, 83.5% of which passed a 200-mesh sieve, was charged from an overhead hopper into the 6-foot fluidizing column made from a 6-inch, Schedule 80, seamless steel pipe. A 4-inch-diameter porous micrometallic plate (powdered chromenickel alloy steel sintered and flattened to l / 4 inch thickness) a t the conical bottom of the fluidizing column served the purpose of distributing the metered fluidizing nitrogen evenly throughout the entire cross-sectional area of the column. Commercial compressed nitrogen, density 0.074 pound per standard cubic foot (60' F., 30 inches of mercury, dry) was used. The feeder column was approximately half filled each time so that the fluidized coal bed had a depth of about 4 feet. Thus, above the uniformly
Figure 2.
1205
Pressure Feeder for Finely Powdered Soli&
fluidized coal bed, there was a 2-foot disengaging space in the column. Above that, an additional 2-foot disengaging space was provided in a 2-inch diameter standpipe for gradual separation of the entrained coal dust from the excess fluidizing nitrogen. The surplus nitrogen was passed through a cyclone and glass-wool filter t o remove the last traces of dust before venting it through a throttling valve t o the atmosphere. By operating the l/(-inch needle valve a t the outlet for the excess nitrogen, it was possible to maintain any desired pressure in the fluidizing column. The flow of coal from the fluidizing column to the receiver, conveyed by nitrogen through copper tubing (12 feet by inch, 3 / ~inch inside diameter, called feed line) was started by opening a small plug valve a t the outlet from the column. The coalgas mixture passed through a sight glass, where the flow could be observed, and then into t h e receiver. Reaching the receiver vessel, the conveying nitrogen separating from the coal passed upward to the top and through a vertical 2-inch pipe, 2 feet long; this served as a disengaging column to the outlet. From there, the nitrogen was passed through a. throttling valve into a cyclone, a glass-wool filter, and finally t o a wet-test meter. The coal was removed from the bottom of t h e receiver through a 2-inch plug valve into a dustproof bag and returned to the system through a hopper above the feeder column. One man operated the equipment; all controls were manual and consisted of standard needle valves and pressure regulators. By maintaining constant pressures in the feeder column and receiver it was possible to obtain a uniform rate of flow of coal and gas through the feed line. Automatic pressure regulators and differential pressure controls probably would maintain constancy in the coal feed rate without manual regulation. The most disturbing difficulty in operation was caused by the harmful effect of powdered coal on the plug valves. Two nonlubricated 2-inch plug valves (made to withstand 500 pounds per square inch pressure) were used for filling and emptying the e q u i p ment, respectively. However, after short usage, these develop leaks from grooves formed on seating surfaces. The grooves were caused by pressure forcing abrasive coal particles a r t way around the plug. Similar effects were noted on small p g g valves used in the feed line. Difficulties also were experienced with manometer connectiona clogging with coal as the pressure in the fluidizing column was in-
INDUSTRIAL AND ENGINEERING CHEMISTRY
1206
TABLE I.
OPERATIKG
Pressure, LbJSq. Inch Gage Retion, F!uidMinutes u e r ceiver 0 a.00 8 3.00 9 0
Dura-
R u n 3.0. 2 3
.
DATAAND RESCLTS FOR
Differential Pressure Across Feed Line, Inohas Water Gage 2 00
Fluidizing Gas
Cud
Ft.
6 8 9
0.119 0.133 0.126
130.83 152.96 141.90
0.627 0,729
11.00 11.50
0.758 0.866
16.76 18.36 17.56
220 230 225
368 346 357
27.30 25.66 26.48
7
0 0
170
3.8 3.8
0.108 0,108 0.108
113.07 113.07 113.07
0,603 0,616
10 20 10.40
0.724 0.740
16.51 16.89 16.70
204 208 206
340 339 340
25.19 25.12 25.16
3.00 3 .00
66
0 0
140 140
3.7 3.6
0.105 0.102 0.104
104.94 102.10 103.52
0,516 0,509
9.10 9 00
0,624 0.616
14.81 14.63 14.72
182 180 181
337 338 338
25.00 25.07 25.04
3.00 3.00
5 5
0
110 110
3.6 3.5
0.102 0,099 0.101
97.09 94.39 95.74
0.414 0,406
7.65 7.55
0.505 0,496
12.49 12.27 12.38
153
0
337 338 339
24.92 25.00 2 5.08
0.105 0.105 0.201 0.130 0.091 0 042 0.112
99.78 94.63 172.04 111.22 81.84 35.72 99.21
0.108 0.102 0.105
9 1 89 87.05 89.47
0,178 0.172
3.70 3.60
0.222 0.215
5.99 5.81 5.90
74 72 73
339 343 341
25.17 25.35 25.26
0.113
91.15 86.59 84.31 87.35
0.106 0.088 0,116
2.40 2.60 2.50
0.135 0.119 0 146
3.83 3 39 4 14
48 52 50 50
348 456 329 378
23.81 33.77 24.51 28 03
5
0 0 0
80 80 80
3.7 3.7 6.0 4.4 3.2 1.3
80 80
0
0 n
80
Arerage 3.00 3.00
3 3
0 0
50 50
3.8 3.6
3.00 3.00 3.00
2 2 2
0 0 0
38 38 38
4.0 3.8 3.7
-4verage 14
0.108 0.105 0.109
-4verage Measured under existing pressure in fluidizer at 60" F.
T4BLE
Rlln
so.
Conveying R a t i o Lb. Lb. coal/lb. ooal/cu. gas ft. gas"
7
10
15
Coal Feed Rate, Lb./ Hour
3.00 3.00
11 68 69 70 71
16
Coal Conveyed, Lb.
4.2 4.7
Average
12 13
Total Volume Coal and Linear Gas Con- Velocity veyed, in Feed Line Std. Cu. F t . Ft./Seh."
200
A yerage 7 .4verage
R U Y S C A R R I E D OCT AT A4TIv10SPHERIC P R E S S U R E
Superfioia! Velocity Conof Fluid- Fluidizing veying izing Gas Rate, Gas, Gas, 8td. Cu. Std. Ft./Sec." Ft./Hourb Cu. Ft.
-4verage 4 5
Vol. 43, No. 5
11.
OPER4TING
DATA.4SD RET-LTSFOR RC\S
Differential Pressure Across Pressure, Lb./Sq. r e e d Line, Inch Gage Inches Water ReDuration, FluidGage izer ceiver Minutes
Fluidizing Gas, C1lb Ft.
Snperficial Velocity of Fluidizing Gas, Ft./Seca
152
8.98 10.20 9.49 7.19 9 86 8.42 9 19
6.20 6.30 5.10 4.20 6.10 4.70
' Standard cublo foot a t 60'
151
3.79
25.41 22.20 26.24 24.85 25.31 36.11 25.53
F., 3 0 lnrhes m e r c u r y
c iRRIED
OUT 4T
50 LB./SQ.IXCH G.4GE
Total T'olume Coal and Linear Coal Coal Gas Con- Velocity Feed Fluidizing reying Conreyed in Feed Rate, Gas Rate, Gap, reyed, Std. C.; Line, Std. Lb.1 Std Cu. Pt,b,c r t . / S e c . a Hour Ft./tIourb Cu. F t . b Lb. Con.~~ ~
Conveying R a t i o Lb. Lb.ooal/lb. coal 'cu. gas f t . $a*"
17 18 19
3.00 2.42 2.26
50 50 50
41 40 40
200 200 200
3.8 3.0 3.2
0.108 0,105 0,120
340 81 360.14 380.97 360.64
1.863 1.576 1.404
11.2 8.5 8.3
1.996 1.677 1.503
17.21 17.60 17.17 17.33
224 212 220 219
81.2 72.8 79.7 77.9
26.96 24.19 26.51 25.89
20 21
3.00 3.00
50 50
40 40
170 170
3.4 3.3
0,096 0.095
0,093
304.93 295.97 300.45
1.649 1.639
9.8 9.7
1.766 1.755
15.21 15.11 15.16
196 194 195
80.2 79.8
80.0
26.65 26.53 26.59
2.34 2.62 2.92
50 50 50
42 43 42
140 140 140
5.0 3.8 3.5
0.181 0,123 0,102 0.135
574.91 390.24 322.50 429.22
2.117 1.248 1.502
6.8 7.4 8.8
2,198 1.336 1,607
22.26 13.16 14.19 16.54
174 169 176 173
43.3 80.0 79.1 67.6
14.40 26.59 26.27 22.42
'2,5 26 -4 vei'ltge
3.00 3.00
50
42 42
110 110
3.4 3.4
0,096 0.096 0.096
804.93 304.93 304.93
1.279 1,290
2.5
50
1.368 1.381
11.75 11.89 11.82
150 152
151
79.1 79.5 79.3
26.30 26.41 26.36
27 28 '29
2.45 2.08 2.81
50 50 50
45 45 45
80 80 80
2.7 2.7 3.5
0.094 0,110 0.106 0.103
296.52 349.26 335.13 326.97
0 857 0 884 1,027
5.0 5.3 5 8
0.917 0.947 1.096
9.65 11.78 9.99 10.47
122 127 124 124
78.7 80.9 76.2 78.6
26.15 26 89 25.33 26.12
30
3.00 6.00
50 50
46 46
50 50
3.5 6.2
0,099 0,088 0.094
313.90 278.03 295.97
0 558 1,087
3.3 6.4
0.597 1,163
5.13 5.00 5.07
66 64 65
79.9 79.5 79.7
26.53 26.41 26.47
3.00 3.00
50 50
47 47
38 38
3.6 3.7
0.102 0,103
322.87 331.84
0.473 0,452
2.7 2.6
0,505 0.483
4.31 4.14 4.23
54 52 53
77.1 77.6 77.4
25.60 25.80 25.70
Average
A rerage 22 23 24
A rei'age
-4verage
'31 Average '32 33
0.111
0.104 327.36 Averaee a Measured under existing pressure in fluidizer a t 60' F. Standard cubic foot. at 60° F., 3 0 inches mercury. C Corrected for gas lost due t o valve leakage in some runs at higher pressures.
creased. This problem wafi solved by using porous nietallic snubbers screwed directly into the column of fluidized coal so t h a t they extended into the coal bed. Good results were obtained in this manner because of the sweeping effect of the fluidizing gas over the end of the snubbers, which vere kept clean thereby. AERODYNAMIC EFFECTS IN FLUIDIZING COLUMN
The first 30 preliminary runs were made by using an inverted funnel-shaped inlet t o the feed line inside the pneumatic feeder column. All these runs were made a t 50 pounds per square inch gage pressure, using various coal feed rates. HoFever, the coal t o conveying gas ratio varied widely (including results for runs
r.6
carried out under identical experimental conditions) owing to uneven entry of the conveying nitrogen into the feed line. It appeared that eddy currents were formed as the fluidizing gas n-as streaming upward around the rim of the inverted funnel; this caused the upsurge of bubbles of gas around the apex and thpir entrainment in the mouth of the feed line. I n an effort to eliminate flow variations at the point of the offtake, an upright funnelshaped mouth was substituted for the inverted funnel-shaped inlet to the feed line. Thereafter it was possible t o duplicate the coal to conveying gas ratio in runs carried out a t atmospheric pressure and under identical conditions. All further runs at
INDUSTRIAL AND ENGINEERING CHEMISTRY
May 1951
1207
as shown in Figure 4. The sudden change in direction of the smoke streams striking the underside of the spreading funnel base created a turbulence that resulted in the formation of eddies, which passed over the apex of the cone, first on one side and then on the other. On application of suction,the eddy currents passed into the mouth of the funnel, resulting in an uneven flow of smoke through the feed line. OPERATING RESULTS
Coal feed rates were varied, as planned for each pressure stage investigated. Tables I to I V give essential operating data and results of 75 runs carried out a t four different pressures with a wide range of feed rates. The results of the runs became closely reproducible when the causes of irregularities in operation were determined and eliminated. The over-all effect of pressure on
Figure 3. Aerodynamic Pattern Showing Streamlined Flow Around Funnel Inlet in Upright Position
various pressures were made using a n upright funnel (1.5-inch diameter rim and 60" spread on the inside), accurately machined of steel and highly polished. The considerable difference in the effects of the funnel inlets in "upright" against "inverted" position was proved also by an aerodynamic investigation carried out in a smoke tunnel. A model of the upright funnel placed in the tunnel gave the flow pattern shown in Figure 3, uniformly streamlined with few eddy currents discernible. Above the funnel, there was a quiet zone in which there was no movement until suction was applied to the feed line, resulting in a steady flow of smoke passing into the funnel. The inverted funnel gave an entirely different pattern,
Figure 4. Aerodynamic Pattern Showing Turbulent Flow and Eddies Entering Funnel Inlet in Inverted Position
FOR R U N S TABLE111. OPERATINGDATAAND RESULTS
Run S o .
Duration, Minutes
Differential Pressure Across Pressure, Lb./Sq. Feed Line, Inch Gage Inches FluidiReWater zer ceiver Gage
C A R R I E D O U T AT
100 LB./SQ. INCHG.4GE Total
Voiulne Superficial ConCoal and Linear Flpid- Velocity of Fluidizing veying Coal Gas Con- Velocity izing Fluidizing Gas Rate, Gas, Conveyed, in Feed Gas, Gas, Std. Cu. Std. veyed, Std CU Line Cu. Ft.& Ft./Seaa Ft./Hourb Cu. Ft.* Lh. Fi. b, ' Ft./Sed."
Coal Feed Conveying Ratio Lb. Lb. Rate, Lb./ coal/lb. coal/cu. gas ft. gasa Hour
3.00 3.00 3.00
100 100 100
96 96 96
200 200 200
3.2 4.0 3.8
0.091 0.113 0.108 0.104
509.99 637.49 605.62 584.37
2.908 2.992 3.385
10.6 11.0 11.2
3.034 3.123 3.518
15.41 15.90 17.51 16.27
212 220 224 219
49.2 49.6 44.6 47.8
29.0 29.3 26.4 28.2
37 38
3.00 3.00
100
97 97
170 170
39 3.8
0.110 0.108 0.109
621.55 605.62 613.59
3.088 3.042
10.0
100
10.0
3.207 3.161
15.90 15.72 15.81
200 200 200
43.7 44.4 44.1
25.8 26.2 26.0
39 40
2.00 3.00
100 100
98 98
140 140
3.0 4.4
0.127 0.125 0.126
717.18 701.24 709.21
1.811 2.775
5.8 8.7
1.880 2.879
13.95 14.19 14.07
174 174 174
43.2 42.3 42.8
25.5 25.0 25.3
41 42
3 .OO 3.00
100
100 100
110 110
4.2 4.0
0.119 0.113 0.116
669 37 637.49 653.43
2 418 2.337
77 7 6
2.510 2.427
12 41 12 03 12 22
1.54 152 153
43.0 43.9 43.5
25.4 25.9 25.7
645.46 485.05 1259.04 749.05 430.31 127.50 616.07
1,244 1.829 1.842 1.948 1.035 1.887
4.2 5.7 6.1 6.3 6.4 6.2
1.204 1.897 1.915 2.023 2.111 1.961
10.l o 9.56 10.03 10.40 9.76 9.93
5.72
126 124 122 126 128 124 125
45.6 42.1 44.7 43.7 42.4 44.3 43.8
26.9 24.8 26.4 25.8 25.1 26.2 25.5
494.06 621.55 557.81
1.067 1,090
34 3.5
1.107 1.132
5.46 5.61 5.54
68 70 69
43.0 43.3 43.2
25.4 25.6 25.5
637.49 3.00 100 104 38 40 0.113 653.43 3 00 100 104 38 4.1 0 116 645.46 0.115 Average Measured under existing pressure in fluidizer a t 60' F. Standard cubic foot a t 60° F., 30 inches mercury. Corrected for gas lost due to valve leakage in some runs a t higher pressure S.
0,796 0,772
2.6 2.5
0.827 0.802
4.11 3.98 4.05
52 50 51
44.1 43.7 43.9
26.0 25.8 25.9
34 35 36 Average
Average Average
.4verage
45 46 Average
47 48
3 00 3.00
100
100
100
102 103
50 50
3.1 39
0.088 0.110 0,099
1208
Iv.
TABLE
Run Xo. 49 50 51 Average
Vol. 43, No. 5
INDUSTRIAL AND ENGINEERING CHEMISTRY
OPERATING D.kT.4 AKD
RESULTS FOR RUNSCARRIED O U T 4T 150 LB./SQ.I X C H G.4GE Total Volume Coal and Linear Coal Gas Con- Velocity in Feed Rate) veyed, ConLine Lb., veyed, Std. Cu. Ft. Lb. Ft./Seb.4 Hour 10.4 5.151 17.67 208 16.53 222 10.3 4.305 226 17.79 10.8 4.860 17.33 219
Differential Pressure Across Superficial conFeed Line, Fluid- Velocity of Fluidizing veying Pressure, Lb,/Sq. Inches Gas, izing Fluidizing Gas Rate, Std. Duration, InohGage Water Gas, Gas, Std. Cu. Gage Cu. Ft.a Ft./Seo.a Ft./Hourb Cu. Ft.’ Minutes Fluidizer Receiver 732.99 5.027 146 200 3.2 0,091 3.00 150 0.098 791.00 4.182 2.78 I50 146 200 3.2 0.101 814.08 4.731 146 200 3.4 2.87 150 779.36 0.097
’*
Conveying Ratio Lb. Lb. coal/lb. coal/cu. ft. gaa” gas 23.7 27.9 33.2 28.2 26.1 30.8 30.6 26.0
4.0 3.5
0.113 0,099 0.106
916.24 801.71 858.98
4.554 4.667
9.8 10.2
4.671 4,788
16.15 16.59 16.37
196 204 200
29.0 29.5 29.3
24.6 25.0 24.8
4.2 3.1 3.2
0.135 0.088 0.091 0.105
1089.11 710.08 740,39 846.53
3,428 3.828 3,756
6.6 8.8 8.6
3.507 3.933 3.856
13.44 13.79 13.60 13.61
149 176 172 166
26.0 31.0 30.6 29.2
22.1 26.3 25.9 24.8
110 110
3.1 3.4
0.088
710.08 778.80 744.44
3.508 3,267
7.7 7.4
3.600 3.355
12.50 11.72 12.11
154 148 151
29.6 30.6 30.1
25.1 25.9 25.5
149 149 149 149
80 80 80 80
3.4 3.4 3.0 3.4
0.096 0.115 0.088
0.096 0,098
778.80 934,56 687.18 778.80 794.84
2.731 2.324 2.939 2,783
5.7 5.3 6.6 6.1
2 799 2.387 3.018 2.856
9.62 10.02 10.53 9.92 10.02
114 127 132 122 124
28.2 30.8 30.3 29.6 29.7
23.9 26.1 25.7 25.1 25.2
150 151
50
3.2 3.4
0.091 0,096 0.094
732,99 778.80 755.90
1.393 1.532
3.2 3.4
1.431
1.572
5.01 5.45 5.23
64 68 66
31.0 30.0 30.5
26.3 25.4 25.9
897.54 755.89 824.61 826.01
0.828 1,198 1.091
1.9 2.7 2.4
0.851 1.230 1.120
3.66 4.29 3.90 3.95
47 54 48 50
30.9 30.4 29.7 30.3
26.3 25.8 26.2 25.8
146 146
52 53 Average
3.00 3.00
150 150
54 55 56 Average
2.65 3.00 2.97
150 150 150
57 58 Bverage
3.00 3.00
150 150
148 148
59 60 61 62 Average
3.00 2.50 3.00 3.00
150 150
150 150
63 64 Average
3.00 3.00
150 150
170 170
50
0.096 0,092
0.111 2.45 150 152 36 3.2 65 0.093 151 36 3.3 3 00 150 66 0.102 153 36 3.6 3.00 150 67 0.102 Average a Measured under existing pressure in fluidizer a t 60’ F. Standard cubic foot a t 60’ F., 30 inches mercury. C Corrected for gas lost due t o valve leakage in some runs a t higher pressures.
various parts of the equipment, such as valves, was closely observed. The effect of pressure on the coal to conveying gas ratio is illustrated in Figure 5 . The upper curve shows that on increasing the pressure from atmospheric to 150 pounds per square inch gage the coal to conveying gas ratio, expressed as pounds of coal per pound of gas, decreases from 339 to 30. The lower curve, on the other hand, shows that the coal t o conveying gas ratio expressed as pounds of coal per actual cubic foot of gas (measured under pressure), is constant a t all pressures investigated; its average value WAS 2 5 . i pounds per cubic foot. From these facts it is obvious that each actual cubic foot of conveying gas, whatever its pressure, is saturated by a given amount of coal. For the upright funnel inlet and size of feed line used, the amount of coal carried by the conveying gas in “dense phase’’ depends either on the size and shape of the funnel or on the type coal conveyed-particularly its specific gravity and particle size-or both. The important effect of the differential pressure, AP-that is, pressure in the fluidizing vessel minus pressure in the receiver chamber-was investigated in a series of runs made under various operating pressures. I n each instance the differential pressure was decreased in increments of 30 inches of water from a maximum of 200 inches to the minimum that still allowed a flow in the feed line. Figure 6 shows that the coal feed rate increases with the differential pressure. However, the slope of the curve shows a gradual decrease with a noticeable tendency t o flatten out a t higher differential pressures. This would be expected, since the flow rates of fluids in pipes approach asymptotically a maximum value as the driving force (pressure) is increased but remain constant after a critical velocity has been reached. Thus, it is advantageous t o operate with considerably higher differential pressures than those used in these experiments, which were limited by the capacity of the differential pressure gage employed. Any unavoidable fluctuation in A P would not then cause variations in the coal feed rate. Attention is called t o the significance of the fact that the feed rate remained practically constant (within experimental error)
for the same differential pressure, irrespective of the magnitude of the operating pressure (Figure 6 ) . However, for any given operating pressure, the coal feed rate is not only the function of the differential pressure, A P , by which it is controlled, but also of the diameter and length of the conveying tube (feed line) and of the coal t o conveying-gas ratio, which appears to be determined by the funnel-mouth characteristics and the type coal handled. The higher the coal t o conveying gas ratio, the higher the density of the coal-gas mixture conveyed, on which the coal feed rate to a large extent depends. Xevertheless, for a given type of coal fluidized and discharged into a feed
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IW 160 PRESSURE IPSlG)
Figure 5. Effect of Operating Pressure o n Coal to Conveying G a s Ratio
INDUSTRIAL AND ENGINEERING CHEMISTRY
May 1951
line of a given diameter and length through a n upright funnel of a given shape and size, the coal feed rate is solely a function of the differential pressure and appears t o be unaffected by any other variable. Among the objectives of the experimental work was determination of the minimum coal feed rate for a feed line of a given diameter and length. With a 12-foot by S/le-inch inside diameter copper tube t o convey the coal into the receiver vessel, the minimum feed rate occurred a t 36 t o 38 inches of differential water pressure. Reducing the A P below this value resulted in an un-
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DIFFERENTIAL PRESSURE ACROSS FEED LINE (INCHES OF WATER )
Figure 6. Effect of Differential Pressure Across Feed Line on Coal Feed Rate
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even slug-type discharge of the coal from the feed line. This uneven flow was undoubtedly caused by the low velocity of the conveying gas which allowed the settling of coal particles and their accumulation in the feed line. The minjmum linear velocity necessary for steady flow in the feed line a t all operating pressures used was about 4 feet per second; this is approximately the same as t h a t required for water-sand mixtures t o flow through horizontal pipes, as reported by Turneaure and Russell ( 4 ) . The effect of the fluidization rate (superficial upward velocity of fluidizing gas in the pneumatic feeder column) on the coal t o conveying gas ratio was investigated in a series of runs made a t atmospheric pressure and at 100 pounds per square inch gage. The rate was decreased from about 0.20 foot per second t o the minimum a t which the coal still fluidized, as indicated by a continuous, uniform discharge through the feed line. The feed rate was kept constant at 124 pounds of coal per hour, corresponding t o a A P of 80 inches of water. The minimum fluidization rates were 0.042 and 0.023 foot per second at atmospheric pressure and 100 pounds per square inch gage, respectively. In spite of this wide range in the fluidization rates (0.20 t o 0.02 foot per second), the coal t o conveying gas ratio remained constant a t 25.7 pounds of coal per actual cubic foot of gas. This probably was due t o the use of the upright funnel inlet t o the feed line inside the fluidizing vessel. Any change in the velocity of the fluidizing gas left the quiet zone above the upright funnel unaffected. Since eddy currents did not penetrate the funnel zone, the coal t o conveying gas ratio did not change, and the feed rate remained constant. This furnished additional proof that the feed rate is not controlled by any other variable except the differential pressure across the feed line and is not affected by variations in the fluidized coal bed.
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The pressure loss across the fluidized coal bed, measured as differential pressure in inches of water, was found t o be proportional, regardless of the operating pressure used: (1) t o the quantity of coal fluidized and (2), t o a lesser extent, t o the fluidization rate (feet per second). Since the latter was maintained sensibly constant for anygiven series of runs, the value of pressure differential across the fluidized bed indicated the bed level and the time for charging the column with coal. SUMMARY AND CONCLUSIONS
Regardless of the magnitude of the operating pressure, fluidized powdered coal or other finely divided solid flows freely and uniformly from the fluidizing chamber into a receiving vessel a s long as a constant pressure differential ( A P ) is maintained across the small diameter pipeline through which the coal is carried in dense phase by a conveying gas. The quantity of material thus carried per unit volume of the conveying gas, through a feed line of a given diameter, length, and shape (straight pipe or coiled), depends on the size and shape of the upright funnel inlet to the feed line and on the type of solid thus conveyed, such as its specific gravity and particle size. The fluidization rate (superficial velocity of the fluidizing gas in the column) was found t o have no effect on the coal t o conveying gas ratio if the coal is passed through a n upright funnel inlet to the feed line. The operating pressure had no appreciable effect on the coal to conveying gas ratio, expressed as pounds of coal per actual cubic foot-that is, volume measured under the given operating pressure-of gas. For the coal investigated (high volatile bituminous from West Virginia) and the type upright funnel inlet used, this ratio was 25.7 pounds per cubic foot at all operating pressures from atmospheric to 150 pounds per square inch gage. The coal feed rate was found to be the sole function of the differential pressure across the feed line, irrespective of the magnitude of the operating pressure. As the differential pressure is increased, the coal feed rate increases toward a maximum constant rate. Thus, at sufficiently high differential pressures, the feed rate would not be sensitive to slight fluctuations in the pressure differential (AI‘). The obvious advantages of high differential pressures for any desired coal feed rate are thereby indicated. The differential pressure across the fluidized coal bed is a direct function of the amount of coal fluidized and, to a lesser extent, of the fluidization rate, b u t it is not influenced b y the operating pressure. For a given rate of fluidization, the amount of coal in the fluidized bed is conveniently gaged by the pressure drop between the bottom and the top of the column. Further work should be done to determine the effects of using various fluidizing gases, such as synthesis gas (carbon monoxide plus hydrogen), air, carbon dioxide, and steam a t various pressures and temperatures up to 500’ F. There is need also for the experimental determination of the flow-rate equation, expressing the coal feed rate as the function of the diameter, length, and shape of the feed line, fluid density, viscosity, and friction factor a t various operating pressures and temperatures. The effects of the funnel offtake characteristics and of the type, specific gravity, and size of solids fluidized on the solid t o conveying gas ratio also should be investigated further. LITERATURE CITED
(i)Albright, C. W., Master of Science thesis in chemical engineering, West Virginia University (1949). ( 2 ) Albright, C. W., Holden, J. H., Simons, H. P., and Schmidt, L. D., Chem. Eng.; 56, 108 (1949). (3) Kellogg Co., M. W., Bull., “Fluid Progress,” 1946. (4) Turneaure, F. E., and Russell, B. E., “Public Water Supply,” 4th ed., p. 442, New York, John Wiley & Sons, 1948. RECEIVED August 3, 1950. The work reported here was performed in cooperation with the West Virginia University.
