This is the equipment setup ,
..
LEO DIRICCO' and PAUL L. BARRICK Department of Chemical Engineering, University of Colorado, Boulder, Colo.
Pyrolysis of Oil
Shale
Retorting at reduced pressures in a fluidized solids system shows
b
Pyrolysis i s first-order reaction
b
Effect of pressure at 250-465" C. and 625-50 mm. is insignificant
S H A L E oil has been produced throughout the world for over 100 years by heating oil shale to a temperature such that the organic matter is converted to oil, gas, and coke. While considerable work has been done to establish the relationship among temperature, time, vapor removal, and quantity and quality of products, the essentially empirical manner in which the oil-shale pyrolysis has been studied is indicated by over 100 patented retorts in this country alone (4). 1 Present address, Engineering Service Department, E. I. du Pont de Nemours & Go., Inc., Wilmington, Del. 13 16
...
Data on the rate of pyrolysis a t various temperatures and pressures are essential for the economical recovery of shale oil in the future. A survey of the literature showed that three investigations of oil-shale pyrolysis rate and one investigation of oil-shale carbonate decomposition rate had been carried out (7-3, 6 ) . In general, the rate of decomposition of oil shale corresponds to that for a first-order reaction; it is proportional to the amount of kerogen (the insoluble organic matter found in oil shale) remaining undecomposed. Maier and Zimmerley (5) studied the
INDUSTRIAL AND ENGINEERING CHEMISTRY
rate of formation of bitumen (the benzene-soluble organic material formed during heating, which remained in the sample) in closed tubes over the range of 275' to 365" C. (527" to 688' F.). For tertiary shale from Soldier Summit, Utah, having a yield of 40 gallons per ton of shale, their data were adequately described by the following first-order reaction equation: log,, k
-9075
temp. (deg. Kelvin)
+ 13.46
Hubbard and Robinson (2) studied the kinetics of the thermal decomposition of
C H E M I C A L PROCESSES The equation is said to apply between 900' and 1200° F. Data from two different sources (7, 2 ) agree rather well when loglo k is plotted us. l / T (" Rankine), deviation being shown a t 500' C. (932' F.) and above. The work of Maier and Zimmerley, carried out a t a lower temperature level, gives values of rate constant smaller than would be expected by an extrapolation of the data a t higher temperature. These determinations were carried out in sealed tubes a t pressures considerably above atmospheric. As it is generally accepted that the sequence of material obtained upon retorting is:
(GLASS) SILICA GEL TUBE
THERMOCOUPL
Kerogen
FILLED)
(T C=THERMOCOUPLE) 200 MESH SCR SUPPORT
U L
NITROGEN GAS IN
t--
. . . and this is the flow d i a g r a m lated by two equations of the Arrhenius type, depending upon the temperature range (time in minutes).
+
-13,572 20.45 Iogl, k = temp. -( a K.) (437 a C. and below)
loglo )=K :- t
-5549
+ 9.14
(437' C. and above)
A British patent specification (7) indicates that the oil-shale pyrolysis follows a first-order reaction rate mechanisin which may be expressed, for purposes of calculation and design of fluidized solids type retorts a t a temperature of T degrees Rankine, as: -18'400
logLo = temp. (deg. Rankine)
bitumen
-f
oil and gas
and the experimental methods used were all directly comparable, the only significantly different variable in this work was the pressure, which had been estimated for some runs to be as high as 15 atm. From a study of the literature two statements of major interest may be made : The data of Hubbard and Robinson ( 2 ) and data calculated from the British patent specification ( 7 ) show disagreement a t 500' C. and above (932' F.). The effect of differing ambient pressures on the rate of kerogen decomposition is relatively unknown. The work of Maier and Zimmerley (5), done with sealed tubes having retorting pressures around 15 atm., would indicate a decrease of rate with increase in pressure. Thus one might surmise that subatmospheric pressures would give retorting rates greater than normally expected a t corresponding temperatures
(6).
,COOLING WATER IN
Colorado oil shale having a Fischer assay of 26.7, 52.6, and 75.0 gallons per ton over the temperature range of 350' to 525' C. (662" to 977' F.). They heated samples of ground shale in a tubular electric furnace for varying lengths of time in an atmosphere of helium, and extracted the samples with dry benzene. The difference between the loss of weight of the sample and weight of the condensate was considered to be the gas liberated. The oil formed was taken to be the total condensate less the water content of the shale as determined by Fischer assay. The authors conclude that a firstorder reaction mechanism adequately describes the data. Results were corre-
+
+ 13.45
Hubbard and Robinson ( 2 ) recommend two equations of the Arrhenius type for correlating their work, which would indicate a change in energy of activation in the range of 425' to 450' C. This indicated change may be due to preheating time a t the higher temperatures. Rate constants calculated ( 7 ) over the temperature range of 483' to 539' (900' to 1000° F.) follow essentially the continuation of the line representing the data of Hubbard and Robinson over the range of 350' to 425' C. (622' to 797' F,). This indicates that the energy of activation is essentially the same for these two sets of data over the temperature ranges noted. O n the basis of this information, and because considerable interest had been shown in retorting a t reduced pressures in a fluidized solids system because of the character of the liquid products produced (6),it was decided to study oilshale retorting rates a t pressures of 625
c.
VOL. 48, NO. 8
AUGUST 1956
13 17
I
Table
1.
Modified Fischer Assay Data
Table II.
Material Balance Data for Retorting Runs
(7) (Charge, 700.0 grams)
% bv Wezght Oil
Water Spent shale Gas plus loss
9.2 1.5 86.5 2.8
Gallo~i.~ per Ton of Shale 24.2 3.6
to 50 mm. of mercury (absolute) over the temperature range of 250" to 465" C. (482" to 8 6 7 " F.) using the fluidized solids technique.
Run No. 1 2 3 4 5 6 7 8 9 10 11 12 13
14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49
Experimental Work Apparatus. Ideally, the equipment to be used would satisfy the following conditions:
Provision for extremely rapid heating and cooling of the shale sample, in order that exposure time to retorting temperature may be measured accurately. Temperature control over the range of interest. Pressure control over the range of interest. Provision for recovery of retorted material (oil and spent charge), so that a material balance could be carried out. Use of an inert gas to sweep the oil vapors from the retorting zone in order to minimize oxidation and cracking. Analyses based on weights, for the sake of accuracy, speed, and simplicity. Consideration of the above factors led to the design and construction of the apparatus shown in Figure 1. Essentially it consisted of a stainless steel retort approximately 22 inches in length and 0.9 inch in inside diameter, wound externally with two sections of Nichrome heating wire; the lower section supplied heat for retorting the fluidized bed, while the upper section prevented condensation of oil in the retort. The retort was fitted with a sintered stainless steel plate for use as support for the bed. This plate was mounted on a shaft which could be turned to dump the retorted charge into the cooling system. The cooling system, which also served as a support for the retort, was essentially a duplicate of the retort, except that it was fitted, for the most part, with a water jacket. The oil-collecting system consisted of four U-tubes connected in series and filled with silica gel. The first tube was air-cooled, the remaining tubes were cooled with ice water. A glass wool filter was used to prevent any solids from being carried over into the gel tubes. Oil-pumped nitrogen was used as fluidizing gas and was brought in a t the bottom of the cooling section. Oil-Shale Samples. The oil shale was
13 1 8
Temperature c. F. 46 5 465 465 465 415 415 415 365 365 365 315 315 315 250 250 250 250 445 445 365 365 250 250 250 250 445 445 40 5 405 365 365 250 250 250 250 445 445 40 5 40 5 365 365 250 250 250 250 250 250 250 250
Table 111.
869 869 869 869 779 779 779 689 689 689 599 599 599 482 482 48 2 482 833 833 689 689 482 482 482 482 833 833 761 761 689 689 482 482 482 482 833 833 761 761 689 689 482 482 482 482 482 482 482 482
Heating Period, Min.
Pressure, M m Hg 625 625 625 625 625 625 625 625 625 625 625 625 625 625 625 625 625 500 500 500 500
.
0.33 0.83 1.50 2.30 8.0 10.0
12.0 30 60 90 30 60 180 60 120 180 600 1.5 2.0 60 90 60 120 180 600 1.5 2.0 10 30 60 90 60 120 180 600 1.5 2.0 10 30 60 90 60 120 180 600 60
500
500 500 500 250 250 250 250 250 250 250 250 250 250 100 100 100 100 100 100 100 100 100 100 50 50 50 50
Average Values of
Yield Based on 72.9% Kerogen, %-Oil Gas Bitumen Total 18 7 0.3 25 36 54 63 32 37 43
15 21 26 8 10 18
0.3
4
Trace Trace Trace Trace Trace Trace Trace
3.0
Trace Trace Trace Trace
9 14 4 9
1.5 1.4 4 4
Trace Trace Trace Trace 9
Trace Trace Trace Trace
3 9
Trace Trace Trace 5 39 48 19 23
Trace Trace Trace 5 39 50
Trace
Trace 5 38 50 14 40 19 24
Trace Trace Trace 6
Trace
120
Trace
180 600
Trace
k
5
0.5
Trace Trace Trace Trace
Trace Trace Trace Trace
9 14 8 13 7 9
Trace Trace Trace 5 49.5 63.4 27 36
Trace Trace Trace 5 48.4 63.5 21.0 53.0 29.0 35.0
Trace Trace Trace 5 52 .o 68.0 25.0 57.0 31 .O 38 .O
5 4 3 4 5 5
Trace Trace Trace Trace Trace Trace Trace Trace
(% per Minute) for
7 3 9.5
..
13 8 12 7 9
37 18 22
Trace
0.3 4.0 4.0 3.1
0.5 0.4 3.0 4.0 4.0 4.0
10
51 75 89 44 51 64
0.3
Trace Trace Trace Trace Trace
Trace Trace Trace
Trace
Trace
Trace Trace
Trace
6
Trace 5
Current Work
(Based on kerogen content of 12.9%)
Pressure, Mm. Hc
Temfierature
c.
' F.
620-630
500
250
465 445 415 405 365 315 250
867 832 777 762 687 598 482
2.6 0.61 0.05 0.009
2.9 0.54 0.06 0.009
2.5 0.59 0.07 0.009
a
supplied by the U. S. Bureau of Mines Demonstration Unit at Rifle, Colo., from the Mahogany Ledge of the Green River Formation. The Fischer assay data as supplied by the U. S. Bureau of Mines Petroleum and Oil-Shale Experi-
INDUSTRIAL AND ENGINEERING CHEMISTRY
100
2.9 0.61 0.06 0.009
50
,. .. ..
0 .bo9
ment Station at Laramie, Wyo., are given in Table I. I t was found, when the oil shale was ground for fluidized-solids retorting using a laboratory type tube mill, that the optimum grinding rate using rods amounted to 73 pounds per
CHEMICAL PROCESSES
I
This is the equation of a straight line when loglo (1 - R) is plotted against time, with slope equal to - k / 2 . 3 0 3 at one temperature. A simpler way to obtain k for a firstorder reaction is to use Equation 1 directly. If the reaction conforms to a first-order rate equation, then a t any one temperature the value of k obtained should be the same, regardless of the degree of retorting carried out. This is the method used here. Data on the decomposition of kerogen in oil shale are given in Table I11 and are plotted in Figure 3 against the reciprocal of the absolute temperature. The equation of the line shown is :
I
3 -2
\1 PREVIOUS WORK+
\ \
0.2 LOO~,,'K'=
^ ,
I"'
\4
-18,480
t 15.5
TEUP.I*R)
---C
L
L
loglo
=
-17,800 temp. (deg. Rankine)
+
15.4
Figure 3 also gives data from the literature for kerogen decomposition. Figure 3. Plot of literature and current work for kerogen decomposition
0 (2) X 14)
hour compared to 26 pounds per hour using balls. In addition, over 90% of the material ground using rods and approximately 5601, of the material ground using balls was in a size range suitable for fluidized-solids retorting. Method. At the start of a run, a charge of 85 grams of decarbonized spent shale was added to the retort, fluidized, and brought u p to retorting temperature. Fifteen grams of raw shale were placed in the feed device, after which the weighed gel tubes were connected to the retort. The raw shale was then added to the hot fluidized spent shale over a period of 3 to 5 seconds. After retorting the required length of time, the retorted charge was dumped into the cooling section, where retorting stopped, for practical purposes, instantaneously. The same procedure was followed for runs at reduced pressure, except that the system was evacuated to the desired pressure after the gel tubes were connected. The oil shale was ground so that 98% passed through a 35-mesh screen and contained less than 270 of -200-mesh material. Kerogen decomposition rates were determined over the temperature range of 250" to 465" C. and pressures of 625 to 50 mm. of mercury. Data and Results
The data taken were primarily material balances for each run based on raw shale, retorted shale, and oil picked up by the adsorption train (Table 11). Each retorted charge was extracted with benzene to determine the amount of
bitumen present which had not been removed from the shale as oil. These extracts were dried at room temperature. The data reported were based on a kerogen content of the raw shale of 12.9y0and were determined by material balance as follows: Weight gain of the gel was treated as being due to oil plus all the Fischer assay water of the raw shale. Subtracting the water content of the raw shale from the weight of the gel gave the oil removed. Bitumen was determined by extracting the retorted charge for 8 hours after it was weighed. Net weight loss was then considered as gas plus oil vapor which passed out the system without being adsorbed. Loss of weight of shale due to decomposition of inorganic carbonates was not considered in determining per cent organic material removed, because there is insignificant carbonate decomposition at the temperatures employed ( 3 ) . All weights were determined using an analytical balance. In order to obtain a valid comparison with previous work on rate constants, the concentration of organic material in the shalf: is represented in the same way (2, 5): If the concentration of the kerogen a t the beginning of the heating period is taken as 1, and the fraction that has been decomposed over any time t is taken as R, then the equation for the rate of a first-order reaction may be written : -k2.303
=
logm(1
- R)
(1 1
Summary
Kerogen in oil shale decomposes in a manner that may be represented by the rate equation for a first-order reaction. The value of the rate constant, k , in per cent per minute may be represented as a function of temperature by an Arrhenius-type equation as: loglo
-17,800 temp. (deg. Rankine)
k
+ 15.4
over the temperature range of 250" to 465" C. (482" to 867"F.), for retorting in a fluidized-solids system. The effect of pressures between 625 and 50 mm. of mercury on the rate of retorting of oil shale was insignificant over the temperature range of 250' to 465" C. Literature Cited
(1) Brit. Patent Spec. 677,140 (to Standard Oil Development Co.), Aug. 13, 1952. ( 2 ) Hubbard, A. B., Robinson, W. E., U. S. Bur. Mines Rept. Invest. 4744 (1950). ( 3 ) Jukkola, E. E., Denilailer, A. J., Jensen, H. B., Barnet, W. I., Murphy, W. I. R., IND.ENG.CHEM.45, 2711 (1953). (4) Klosky, Simon, U. S. Bur. Mines Bull. 467, 468 (1949). ( 5 ) Maier, C. G., Zimmerley, S. R., Univ. Utah Bull. 14, 62-81 (1924). (6) Pringle, J. W., Barrick, P. L., Wigton, H. F., IND. ENG. CHEM.44, 1489 (1952). (7) Stanfield, K. E., Frost, I. C., U. S. Bur. Mines Rept. Invest. 3977 (1946); 4477 (1949). RECEIVED for review August 29, 1955 ACCEFTED May 16, 1956
END OF SYMPOSIUM VOL. 48,
NO. 8
AUGUST 1956
1319
