Heat Requirements for
Retorting Oil Shale 11. W. SOHNS, L. E. MITCHELL, R. J. COX, W. I. BARNET, . ~ N DW. I. R. MURPHY U . S . Bureau of Mines, Laramie, Wyo.
z
that the resulting data may be more truly applicable to comniercia1 size equipment. Certain factors, however, limited the maximum size that was feasible-namely, ease of preparation of uniform, hand-mixed shale samples for the necessary number of experiments, problems relating to the construction of the electrically heated adiabatic shield, and the preparation, metering, and heating of the inert gas a t a high enough rate to heat completely the shale charge in a reasonable time. On the basis of these considerations, the retort was designed to hold about 50 pounds of raw shale and was constructed of a 28-inch length of 10-inch steel pipe, with a flat plate welded on the bottom and another plate bolted on the top. A schematic diagram of the entire retort assembly is shown in Figure 1. An internal cone bottom permitted oil drainage, and a perforated plate above the cone supported the shale. The retorting gas entered the top of the retort and left the bottom through 2-inch pipe nipples. Orifices of 7/s-inch diameter were inserted in these nipples to increase the gas velocity past two thermocouples which measure the inlet- and outlet-gas temperature. A 14-inch-diameter sheet-steel cylinder surrounded the retort and was wound with three sections of resistance wire. These heaters, together with two ring heaters above and below the retort, served as an adiabatic shield. Five pairs of thermocouples, mounted opposite each other on the retort wall and the shield, were arranged with opposing e.m.f. so that, operating through a multiple-point temperature controller, the temperature of the shield could be maintained a t the retortwall temperature. Thus, heat exchange between the retort and its surroundings was reduced to a small value which could be calculated with reasonable accuracy.
T h i s study was made to determine the total heat necessary to retort Colorado oil shale to produce shale oil and accompanying by-products under conditions that would exist in commercial practice. The over-all heat requirements for retorting Colorado oil shale, when measured above 77" F., ranged from 105 B.t.u. per pound at 450' F. to 356 B.t.u. per pound at l l O O o F. for 28 gallon-per-ton shale, and from 261 R.t.u. per pound at 750' F. to 378 B.t.u. per pound at 1100" F. for si-gallon shale. Heat content of spent shale ranged from 91 B.t.11. per pound at 500' F. to 272 B.t.u. per pound at 1100' F. Design of efficient oilshale retorting plants and evaluation of their thermal efficiency require data on the heat needed to retort oil shale. Since the literature contains bat little such data and fails to provide the necessary correlation with retorting temperature and grade of shale, this study was made for the purpose of correcting this deficiency.
D
CRISG the past 100 years of oil-shale development and
utilization in various parts of the world, numerous retort designs have been proposed, and many of these have been demonstrated on a pilot plant or commercial plant scale. Some have been successful from a materials handling standpoint, but few have been designed or operated to utilize heat to the best advantage. The reason for this, of course, is that few data have been available on the quantity of heat required to retort an oil shale. In its study of oil-shah retorting problems, authorized by Public Law 290, the Bureau of Mines Petroleum and Oil-Shale Experiment Station, a t Laramie, Wyo., has obtained data on this property of oil shale, and the results are reported in this paper. These "heat-of-retorting" values for various grades of oil shale represent over-all heat requirements and include:
THERMOCOUPLE FOR INLET TEMPERATURE
RETORTING GAS INLET
The heat content of the mineral and other nonvolatile portions of the shale a t the final retort temperature The heat of reaction resulting from conversion of the organic matter in the shale to gas, oil, and coke The heat of decomposition of that portion of the mineral carbonates that decomposes under the experimental conditions and other heats of reaction due to changes in the mineral content of the shale The heat of vaporization of the oil and water The heat content of the gas and oil vapors a t the temperature of their exit from the retort
p-
__L
RING HEATERS
OUTER JACKET
REYOVtBLE
WINDINGS
These data have been correlated with assay values of Colorado oil shale and final retort temperatures to permit their use in retort design.
RETORT SHELL THERMOCOUPLES
_MAGNESIA INSULATION
APPARATUS
The experimental retort was designed to utilize a heat-balance method wherein the difference in heat content of an inert gas as it enters and leaves the retort, corrected for such small heat losses to the surroundings as may have occurred, is equated to the heat required to retort the shale. Heat losses from the gas stream, other than to the shale, were reduced to small values that could be calculated accurately by surrounding the retort with an adiabatic shield maintained a t the same teniperature as the retort wall. For this type work, a reasonably large retort is desirable, so
ADIABATIC.
snsLo
GAS AND VAPOR OUTLET
THERMDCOUPLE FOR OUTLET GAS TEMPERATURE
Figure 1.
33
Schematic Diagram of Retort Assembly
-
34
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY b
r
THERMOSTATIC HEATER
voi. 43, No. 1
VENT
0-135v AC
-
~
7
A
PREHEATER BY-PASS
x)
.
RETORTING GAS
1 ,
RETORTING GAS
GAS AND OIL FOG I
FOG FILTERS
CHAMBER TRAP' r
VENT
BROKEN FIRE BRICK SPRAY OIL
LCOOLING TOWER
H A T U R A L GAS
OIL
OIL
I AIR BY-PASS
COLD GAS LINE
Figure 2.
Flow Diagram o€ Heat of Retorting Apparatus
A flow diagram for the heat-of-retorting unit is shown in Figure 2. Flue gas resulting from the combustion of natural gas in a deficiency of air was used to supply the heat to the shale charge in the retort, This gas consisted primarily of nitrogen, with lower percentages of carbon monoxide, carbon dioxide, hydrogen, and water vapor and less than 1% each of argon and methane; under the conditions existing in the retort the gas was assumed to be chemically inert. A preheater was installed on top of the inert gas production chamber. Hot flue gases flowed through the tubes of the preheater to enter the bottom of a water-spray cooling tower packed with l/y-iiich graphite Raschig rings. Leaving the spray tower a t about 60" F. the gas passed through a spray trap, after which a continuous sample was removed for mass spectrometer analysis. The temperature and pressure were then 1000 I
I
I
measured, and the gas was metered by a recording area-typc flowmeter. The retorting gas then was passed through th? shell of the preheater, countercurrent t o the hot flue gases flowiug through tBhetubes, and brought to the desired controlled teniperature in the thermostatic heater. This unit was equipped with three electric heater windings; power input was controlled by transformers and a temperature controller. After entering t,he top of the retort a t constant temperature, the gas lost part, of its heat to the retort and charge and left through the bott,orn of thc retort. The shale-oil vapors were condensed by passage thi~ougli a series of three water-cooled condensers, after which any oil fog remaining in suspension in the gas stream was removed in two fog filters packed with steel wool. After passing through this system, the mixture of retorting gas and shale gas was vented to the atmosphere. A gas bypass around the preheater permitted operation of the retort a t any controlled temperature above about 100" F., and a gas line carrying cold flue gas direct from the exit of the spray trap to the bottom of the retort permitted cooling the retort a t the end of a run with inert gas so that combustion of the shale did not take place. The inlet- and outlet-gas temperat'ures and the differential temperature bet,ween retort wall and shield were continuously recorded by means of a 12-point strip-chart recorder. Figure 3 is an example of the type temperature chart obtained for a run. Figure 4 pictures the heat-of-retorting apparatus. EXPERlMENTAL WORK
0
Figure 3.
I
2
3 TIME, HOURS
4
5
I
6
Typical Temperature Chart for Heating Oil Shale in Adiabatic Retort
This investigation included determination of the heat of retorting of a 28-gallon-per-ton ran- shale in the temperature range of 450' to l l O O o F. and a 57-gallon-per-ton raw shale in the temperature range of 750" to l l O O o F. as well as a determination of the heat content of spent shale in the temperature range of 500" to 1100" F. The raw shales were obtained from the Bureau of Mines Anvil Points mine near Rifle, Colo., and the spent shales were composited samples obtained from several raw shale retort-
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Janumy 1951
35
TABLE I. ANALYSES OF RAWAND SPENTSHALESUSEDIN HEAT OF RETORTING STUDIES Fischer Shale Raw Raw
(28) (54)
Spent Spent
1
Analysis, W t . Water Mineral (by assay) COz 0.5 20.0 1.7 11.4 ... 21.4 ... 16.0
Organic matter 14.2 30.7 2 ,R 7.8
Gt?%%n 27.9 57.0
Ash !,j,3 00.2
70.3 7R. 0
ing experiments in the heat-of-retorting apparatus; analyses of the shales are given in Table I. Before working with these shales, some preliminary work was done t o calculate the heat losses from the retort so that these corrections might be made. Although the tempei-atuw controller usually maintained the adiabatic shield at the same temperature as the retort wall, differences occurred at times, particularly during the first part of a run. T h e temperature differences were recorded on the chart, and the only additional information needed for calculating the quantity of heat interchange between the retort and shield was the heat transfer coefficient. This coefficient was determined by maintaining the shield temperature constant and passing cold air at a constant rate through the empty retort until equilibrium conditions were obtained. T h e heat transfer coefficient was calculated from the heat input t o the air and the temperature difference between the shield and the air. This coefficient includes the resistance to heat transfer from the retort wall t o the moving air stream, but the latter is small compared with the over-all resistance and therefore does not affect the calculations appreciably. The measured heat transfer rate was B.1 B.t.u./(hour) ( O F . ) and is equivalent t o a thermal conductivity for the magnesia insulation between the retort and shield of 0.050 B.t.u./(square foot) (hour) ( O F. foot). This value compares well with the published conductivity for magnesia ( 4 ) of 0.046 B.t.u./(square foot) (hour) ( O F. foot). The heat content of the empty retort was needed in calculating the net quantity of heat supplied t o the shale. Therefore, experiments were made with the empty retort at 100 O temperature intervals between 400" and llOOo F. Three runs were also made with a charge of quartzite to check the accuracy of the results obtained with the apparatus. Analyses of samples of the retorting gas taken a t intervals during the experiments and of a composite sample from each experiment showed t h a t the gas composition remained very close to the following average: 75.0% nitrogen, 6.2% carbon dioxide, 7.8% carbon monoxide, 7.8% hydrogen, 2.1% water vapor, 0.9% argon, and 0.270 methane. T h e heat content for this mixture
I
400
1
0 0
.
Figure 4.
was calculated a t intervals over the temperature range 50" t o 1200' F. from recently published data of the American Petroleum Institute, Research Project 44 ( 3 ) ,and was tabulated for convenience. Calculation of the heat-of-retorting values was simple, though somewhat tedious. The procedure is outlined by the equation (Heat of retorting) = (total heat transferred from gas t o retort and charge) - (heat loss by retort to the shield) - (heat content of empty retort) By means of a teniperature chart for each experiment, such as that shown in Figure 3, the total amount of heat translerred t o the retort and charge by the retorting gas was calculated according t o the following procedure:
I
1
I
I
I
I
I
800
900
1000
1100
I
The chart was divided into short time intervals so t h a t accurate averages could be determined for both the inlet- and the outlet-gas temperatures. The heat content of the gas a t these average temperatures was read from the previously prepared table. The difference in heat content of the gas at the inlet and outlet temperatures was then multiplied b y the flow rate of the gas and the time interval to obtain the amount of heat transferred from the gas to the retort and charge during this particular part of the run. The sum of these quantities for the entire experiment, corrected for the calculated quantity of heat transferred between retort wall and shield during the run, gave the net heat received by the retort and charge. The desired heat-of-retorting value was then obtained by subtracting from this sum the heat content of the empty retort at the final run temperature and dividing the difference by the weight of the shale charge.
57-G.P.T. RAW S H A L E 28-6 P T RAW S H A L E
A SPENT SHALE FROM 2 8 - G P T RAW SHALE x
SPENT SHbLE FROM 5 4 - G P T RAW SHALE
4 a
.'.
z
300-
5 7 - 6 P T SHALE
L.: L Y
P
eo0
-
IO0
-
Heat of Retorting Unit
28-6 P T S H A L E
$
DlSCUSSION OF RESULTS 0 400
I
I
500
600
I ?OO
TEMPERbTURE,'F
Figure 5.
Retorting Heat of Raw Shale and Heat Shale above 77' F.
~
1200
Content c,f spent
T h e heat content of the empty retort for temperatures in the range 400" t o 1100" F. is given in Table 11. Also listed are the respective retort weights calculated from the experimental heat
INDUSTRIAL AND ENGINEERING CHEMISTRY
36
Vol. 43, No. 1
TABLE IT. WEIGHTOF EMPTY RETORTC A L C T - L ~ TFROM E I ) ITS HC.iT CONTENT A B O \ I: 77' F. Experimental Heat Content of Retort, B.t.11. above 7 7 O r 3,888
Temperature,
F.
400 .>00
3,17!) 6,592 8,118
600
700 800 900 1000 1100
Calculated Weight of Retort. Lb. of I1mn 103 . 0 103.7 104 . I 1 104 . 9 108.2
9,514
11,101 12,800 14,046 Arerage
103.; 103.9 104.2 103.9
~
TABLE 111. HEATCOXTESTOF QCARTZITE F.
725 830
1000
Run
Temperatiire,
No.
65 OB 67 68 1 2
3 4 >
61 59
7
58
TABLE VI. Run SO.
I l e a t Content, B.t.ii. 'Lb above 77' F. Experimental Calculated from value litera t i r e 149 149 188 183 229 223
Temperature, 0
Run No. 54 53 32 r, 5
0
r.
Id0 430 800
700 7;,0 800 800 '300 900 1000
1100 1100
Heat of Retorting, n.t.11. 'Lh. 104 103 133 189 207 230 232 269 257 307 363 358
Calculated Heat Content, n.t .I1 .,'Lb. 102
102 152
188 207 220 226 267 207 310 356 355
content and published values ( 1 ) for the unit'heat content' of iron. Because some of the heat, is taken up b y t,he magnesia insulation l)etween the retort and shield, the calculat,ed retort weight, is not necessarily the actual i~eight, of thi? empty retort. It is, however, :t value that should be vonstant from one r u n t o another and a t different final temperatures. The agreeinent of the calculated weights, within *0.9%, indicates good reproducibility of resulk with this apparahs. The retort weight of 104 pounds, calculated from the heat content, agrees well with the weight of 105 pounds calculated from the dimensions of the unit. The weight of the retort was calculated as accurately as possible because construction of the apparatus precluded removal of the retort for weighing directly. D a t a for the t,hree check runs made with quartzite are given in Table 111. The experimental heat content of the quartzite a t 725", 850", and 1000" F. is compared wit,h published data (a) for quartz. A t 725" and 850" F. the experimental value agrees exactly with published data, and a t 1000" F. i t is high by 4 B.t.u. per pound or 2%. This range, 0 to 2%, is probably a good measure of the experimental error involved in detcrmining the heat of retorting of the raw shale and the heat content of the spent shale. The heats of retorting for 28- and 57-gallon-per-ton Colorado oil shales are given in Tables IV and V, for the temperature ranges 450" t o 1100" F. and 750" to 1100" F., respectively. The heat content of the spent shale is given in Table 1-1 €or the teinperat,ure range 500" to 1100" F'. These data are presented graphically in Figu1.e 5 . The experinlent,al values w e compared in the tables with heat quantities calculated 11sextrapolation of the following equations ( 5 ) in the temperature range 70" to 450' F. For raw shale,
S,,,= 0.172 -t (0.067
+ 0.00162 G)
T:or q x n t sli:~l(i,
s,, = 0.17-1 4-0,031 x
lo.-"'
32 33
'JOO
Heat of Retorting, R.t,.ii./I,b. 26 1 282 308
1000 1100
821 3 78
T c niperat.une, 0 I'.
7 50 800
HE.\rr C o r m s i .
ABOVE
77" F.
Calculated IIeat Content, B.t.11. /Lh. 24:
20'3
319
373 430 FOR
SPENTSHALI:
lleat, ~~~~ _Content, _ ~ ~ _ _R.t.:i./Lb. ______ Temperat,ure, Experimental Calculated 0 1:. value value Si)ent Shale froin 28-GaLITon Ran, Shale
91)ent Sliale froin ~j4-Cal.iTonRaw Shale 600 112 1000 220
119 22'3
The terms So,and S,, are the mean speclific heats oi raw shaic antl spent shale, respect,ively, between 7 7 " .:I and 7'" Iianliine. 0 is the modified Fischer assay in gallons of oil per ton of shale. The esperiniental heat cont'ent of the spent shale devia,tes from the calculated values by an average of 2 7 , . This close correlation is an int,eresting and valuable check of the results obta,ined by the two entirely different experimental methods, which were npplictl over two separate teniperature ranges. The experimental ht::itof-retorting data for the t'wo raw shales agree well with the c d culated heat content values a t the lower tcniperat,ures from 450 t o 800 F. N o n r v c r , a t higher tompcrat,urcs, thc cxperinient ;il rcsults are considernbly lon-er than the valucs c.:tlculated from thci ~ vapo heat content eyuat ion. This is probably b i ~ a u s coil gas, produccd during t h e course of the espctrimitiit, left the rr>toi,t somtJ\vhnt c~oolvr1h:tri thc final experinicrit XI tein11~raturc.. CONCLUSIOSS
The heat of retorting of 28- and 57-gallon-per-ton Colorado oil shale \\-as det'ermined a t 450" to 1100" F. and 750" t801100" k'., wspectively. The heat of retort'ing, measured a h o w 77" I?., ranged from 104 to 356 I3.t.u. per pound for the 2S-ga~llon-per-tori raxy shale and from 261 to 378 B.t.u. ppr pound for the 57-galionper-ton shaie. Heat content ahovc 77" F. for the spent shahs was determined at, t,emperature intervals from 500" to 1100" F. and ranged from 94 to 272 B.tm.u. per pound. The heat contcnt, determinations made on quartzite, the empty retort, and t,hc spent shale indicate that the data probably are reproducible antl accurst,? wit,hin * 2 % . ACKPQW LEDGBI ENT
The authors ?\-ish t o acknoiTledgr the general guidaricc ol R.:I. Cattell, chief of the Oil-Shale Research and Demonstratioil Plant Branch, and IT. 1'. Rue, supervising engincri, of the Petroleum and Oil-Shale Experiment Station. The vmrk \\-as cai,rictl out under the direction of H. 31. Thorne, engineer in chai~geof oil-shale research and developmrnt. LlTERATURE CITED
(1) Kelley, K. R., C . AS'. B u y . M i n e s Bull. 476, 85 (1019). ( 2 ) I b i d . , p. 153. (3) Sational Bureau of Statidads, \Vashington, n. C . , "Selected T'alues of Properties of Hydrocarkons," Vol. 2 , h.P.1. Research Project 44,1944. (4) Periy, J. H., editor-in-chief, "Chemical Engineers' Handbook," 2nd ed.. p. 055, K e n - York. McGraw-Hill Book Co., 1941. ( 5 ) Shax, 11. J . , C:. S . B u r . MinesRe7A.Pnoest. 4151 (Sovember 1947;). W E D .Ipril 17, 1950. Presented before the Division of Gas a n d Fuel Houston, Tex. Chemistry, 117th hIeeting . ~ . N B R I C A X CtImsrIcaL SOCIETY, ' P h i s moyl; as authorized by I'iiblic Lair 290, 78th Congress, and was performed under the general iupervision of JV. C . Schroeder, chief of the Office of Synthetic Liquid Fiirla, throiigll a cooperative apreeincnt between t h e B u r e m of >line?, 1-nited State.: 13cpartnient of tlie Interior, a n d the L-niversits- of J\ yonliilg.
