I-VDUSTRIAL A N D ENGINEERING CHEMISTRY
312
VOl. 22, No. 4
Figure 1-Panorama of Ford
Waste-Wood Utilization b y the Badger-Stafford Pr ocessl The Ford Wood-Distillation Plant at Iron Mountain W. G. Nelson FORDMOTORCOMPAVY, IRON MOUNTAIN. MICH.
PPROXIMATELY 400 toils of scrap wood are produced per day a t the sawmill and body plant of the Ford Motor Company a t Iron Mountain, U c h . Half of this comes from the sawmill in the form of slabs and edgings with an average moisture content of 36 per cent, the other half from the body plant in the form of trimmings from kiln-dried lumber containing approximately 10 per cent moisture. The scrap wood averages 70 per cent maple, 25 per cent birch, and 5 per cent ash, elm, and oak. Destructive distillation is the logical means of utilizing this waste material, but owing to the smallness and irregularity of the pieces it cannot be handled satisfactorily on cars in the customary horizontal retorts. A radical departure from established procedure appeared to be necessary when the problem of utilizing this material was attacked. After preliminary investigations which indicated the applicability of the Stafford vertical continuous retort to the problem a t Iron Mountain, a complete wood-distillation plant was put into operation in August, 1924. This plant differs in many respects from the standard units for wood distillation : it is the only plant using Stafford retorts a t the present time; it is designed for continuous operation with automatic control and mechanical handling of materials; since initial installation it has been modified to produce ethyl acetate by a direct process rather than through the customary intermediate stage of calcium acetate, and it is probably the cleanest wood-distillation plant in the world. The plant is housed in two buildings containing the departments related, respectively, to carbonization and distillation. (Figure 1) The carbonization building contains the wooddrying, retort, and charcoal departments, while the distillation building contains the primary separation, refining, direct ethyl acetate, and pitch departments. The main details of the operations in these departments are described in the following sections.
hog, which reduces it to pieces varying in size from that of a match up to 8 X 2 X "4 inches. From the hog it passes on another conveyor to storage bins in the carbonization building. The progress of the wood through the carbonization building may be followed on the flowsheet of Figure 2. As noted there, the sawdust and shavings from the sawmill and body plant are handled independently of the scrap wood in rotary retorts, the products from which are combined with the products from the Badger-Stafford retorts. The hogged scrap wood from storage goes t o six rotary driers 10 feet in diameter and 100 feet long, one of which is shown in Figure 3. These driers are heated by flue gas from the power house entering a t 315" C. (600" F.) and flowing countercurrent to the wood. Each drier has a capacity of 3 tons of wood per hour delivered, with an average water content of 0.5 per cent. The wood requires 3 hours for passage through a drier. Flue-gas pressure is controlled by automatic pressure regulators, which maintain a zero pressure differential a t the discharge end of the drier. The pressure drop through the drier causes a negative gas pressure of approximately 0.4 inch (10 mm.) of water a t the feed end of the drier, where the suction fan is located. The temperature distribution on each drier is checked by a six-point recorder. Wood is discharged from the driers a t a temperature of approximately 150" C. (302" F.), and is carried up on an inclined belt conveyor, evident a t the left of Figure 3, t o the top floor of the retort department. The conveyor discharges over a magnetic separator for the purpose of removing tramp iron from the wood, which is then picked up by a second conveyor equipped with a weightometer which records continuously the amount of wood transferred. The wood is discharged into a hopper, from which it is diverted by butterfly valves into chutes leading to the three Badger-Stafford retorts.
Handling and Drying of Scrap Wood
Carbonization i n Badger-Stafford Retorts
As the wood scrap is produced in the sawmill and body plant, it is picked up by belt conveyors and delivered to a
The retorts are 40 feet high and 10 feet in diameter with a heat-insulating wall 18 inches thick composed of firebrick, diatomaceous earth, and insulating brick. Barrel valves, prac-
A
1
Received January 22, 1930.
INDUSTRIAL AND ENGINEERING CHEMISTRY
April, 1930
313
P l a n t a t Iron Mountain
tically air-tight, a t the top and bottom allow the iiitroduction of wood and removal of charcoal, respectively, without escape of gas. The pas passes off a t the top of each retort to four condensers mounted around it. Figure 4 shows the operating floor of the retort department, the wood chutes leading to the motor-operated barrel valves on top of the retorts, and the heads of the condensers projecting above the floor. The heads of the retorts are flush with the floor. The Badger-Stafford retorts operate continuously by utilizing the heat developed by the exothermic carbonization of the mood near the center of the retort to heat the descending column of wood up to the temperature necessary to start the reaction. When operated a t capacity each retort runs on a 2-week cycle. At the end of 2 weeks of continuous operation a retort must be taken off the line in order to burn out the tar which has accumulated in it. During regular operation two of the three retorts are always on the line, while the third retort is being burned out. Some of the noncondensable gas from the retorts in operation is burned in the cleaned retort t o bring it up to the required temperature of 540" C. (1004" F.), when it is put back on the line. The heating process requires about 24 hours. Teniperatures a t six points in each retort are recorded contiiiuously by instruments during operation. The average temperature in the central zone is 515' C. (950" F.), while near the bottom it is 255" C. (490' F.1. d constant positive
4 R SCCTR E E EX N
BALL
Table I-Average
Composition of Noncondensable Gas f r o m Badger-Stafford Retorts PERCEVTB Y VOLUME,D R Y GAS CONSTITUEVT H. 2 2 ~~. 16.8 1.2 23.4 37.9
CHI
CnHm
co
CO? 0 2
"4
N?
16.0
Charcoal Conditioning and Processing
When a retort is started up, wood is fed in until it reaches to within 7 feet of the top of the retort. The operator then
1 WATERSPRAY
2
pressure of 0 1 inch (2 5 mm.) of water is maintained within each retort by means of a pressure regulator controlling the rate of removal of noncondensable gaq from the condensers. The direct products from the retorts are charcoal, which is removed through the barrel valve a t the bottom of each unit and is sent to coolers and conditioners; pyroligneous acid or "green liquor," which is pumped from the condensers to the primary separation department; and noncondensable gaq, which is passed through scrubbers and is blown over to the power house and burned under the boilers, as well as being used for heating u p retorts. The composition of this gas varies somewhat; Table I gives average values on the basis of five analyses made a t considerable intervals. The average heating value is 290 B. t. u. per cubic foot.
MILLS
HARCOAL
HARCOA
W A ~ E R6 STARCH
DCIDOMETER
ilOTARYI SCREEN
FROM
OUTSIDE
BAR SCREEN
..
P H
SCALE
FLLLER-CINYON
I
CHARCOAL r n
PULVERIZED I V
Figure 2-Flow
S h e e t for Carbonization Building, Including Wood-Drying, Retort, a n d Charcoal D e p a r t m e n t s
314
s l m w thc bottom of
V d . 22, s o . 4 recirculated to the original screen when briquets only are tieing produced; is diverted to a separate series of screens when graded charcoal is required; or is sent to hall mills alii1 blown to the power plant by pumps when it is to be i i s r d as fuel. The fines dclivered to storage are made into charcoal Lriqiiets with the aid of a starch hiiider. A poidonieter propnrtions both charcod and starch solution a t a eonstalit rate to a paddle mixer. where it is worked up into a paste. Further incorporation takes place in a fluxcr atid an edge roller. The tliick paste from the latter goes to a rotary press, which delivers formed briquets to the apron of a coiitiiruous drier 100 Seet loirg, heated by flue gas Srom the power plant. During the 3-hour trio throuch the drier the moisture content of t,hk liriqucts is reduced from approxinratelg 40 per cent to less tlinis 1 per cent. The dry briquets are discharged from tke drier i l l t i , a squirre-cage screen, mhicls removes broken or (lcfect,ivcbriquets. Tlicse are recirculated to the condit,ioners Iig a pileumatic conveying system, wlrile the whole briquets arr discharged to storage in large concrete silos. Milch charcoal dust is inevitably produced during tlie various operations OS handling and processing the charcoal. 111ordcr to maintain good working conditions and eliminate irdi, a largr dust-recovery installabion of twveiity-three air-filter uiiits is uscd. Air from the c o n d i timers, screens, a i d mills is ~iolledtlirougli these filters atid llic recovered ckarcoal dust is returned to the storage bin for fine charcoal. All the eyuipniint throughout the clieniical plnnt is iiidividually driven by direct-current motors, which, wiih the exception of those iii tlie wood-drier department, are of t,lie totally enclosed explosion-proof type.
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