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Results of mathematical modeling of oil shale retorting in an aboveground, internal combustion retort
Fuel Processing Technology, 9 (1984) 125--138 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
125
RESULTS OF MATHEMATICAL MODELING OF OIL SHALE RETORTING IN AN ABOVEGROUND, INTERNAL COMBUSTION RETORT
R.L. BRAUN, D.E. CHRISTIANSEN, J.C. DIAZ and A.E. LEWIS Lawrence Livermore National Laboratory, Livermore, CA 94550 (U.S.A.)
(Received November 29th 1983; accepted March 25th, 1984)
ABSTRACT The one-dimensional mathematical model developed at the Lawrence Livermore National Laboratory to simulate the detailed chemical reaction kinetics involved in modified in-situ retorting of oil shale has been applied to simulate aboveground retorting in a moving, packed-bed retort. Application of the model to simulate such a retort was made for the internal combustion mode of operation, using typical operating conditions of the Paraho semiworks retort. Comparison with the semiworks experimental data revealed the general accuracy of the model in calculating retort temperature profiles, oil yield, off-gas composition, and outlet shale composition. The model was also applied to a hypothetical set of commercial-scale conditions in order to identify key operating or design parameters. These parameter studies predicted that the oil yield and the overall process performance could be markedly improved by increasing the recycle gas flux, decreasing the inlet-air flux, and relocating the inlet-air distributors. A process flowsheet was developed for the complete retorting system.
INTRODUCTION A n oil shale r e t o r t is essentially a h e a t e x c h a n g e r f o r t r a n s f e r r i n g h e a t f r o m a h e a t i n g m e d i u m t o t h e shale. E i t h e r h o t gas o r h o t solid c a n b e used as t h e h e a t - c a r r y i n g m e d i u m . Processes using gas are usually l i m i t e d t o t h e use o f p a c k e d b e d s o f c o a r s e l y - c r u s h e d oil shale in o r d e r t o a l l o w a d e q u a t e h e a t a n d m a s s t r a n s f e r w i t h o u t excessive p r e s s u r e d r o p . T h e h o t gas can be g e n e r a t e d e i t h e r e x t e r n a l l y t o t h e oil shale r e t o r t o r i n t e r n a l l y f r o m c o m b u s t i o n r e a c t i o n s w i t h i n t h e r e t o r t itself. M u c h e x p e r i m e n t a l w o r k has b e e n d o n e o n t h e l a t t e r g a s - c o m b u s t i o n c o n c e p t o f oil shale r e t o r t i n g b y t h e U.S. B u r e a u o f Mines [ 1 ] . S u b s e q u e n t l y , f u r t h e r r e f i n e m e n t s o f t h e p r o c e s s w e r e m a d e t h r o u g h t h e P a r a h o p r o j e c t [2]. N o m a t h e m a t i c a l m o d e l i n g w o r k , h o w e v e r , was r e p o r t e d in c o n j u n c t i o n w i t h e i t h e r o f t h o s e e x p e r i m e n t a l p r o g r a m s . T h i s p a p e r , t h e r e f o r e , addresses t h e s i m u l a t i o n o f a b o v e g r o u n d r e t o r t i n g using i n t e r n a l c o m b u s t i o n in a m o v i n g , p a c k e d - b e d r e t o r t . T h e p r i n c i p a l o b j e c t i v e s are t o t e s t t h e v a l i d i t y o f t h e m a t h e m a t i c a l m o d e l a n d t o i d e n t i f y k e y o p e r a t i n g or design p a r a m e t e r s b y m e a n s o f p a r a m e t e r studies o n a h y p o t h e t i c a l set o f c o m m e r c i a l ~ c a l e c o n d i t i o n s . 0378-3820/84/$03.00
© 1984 Elsevier Science Publishers B.V.
126 MODEL DESCRIPTION A time-dependent, one-dimensional mathematical model has been developed for simulating the chemical and physical processes involved in the retorting of a packed bed of oil shale [3]. The physical processes included in the model are axial convective transport of heat and mass, axial thermal dispersion, axial pressure drop, heat transfer between gas stream and shale particles, thermal conduction within shale particles, wall heat loss, water evaporation and condensation, movement of shale countercurrent to gas flow, and additional cooling at air inlets. Chemical reactions within shale particles are release of b o u n d mineral water, pyrolysis of kerogen, coking of oil, pyrolysis of char, decomposition of t w o carbonate minerals, combustion of char and sulfur, and gasification of organic carbon residue with H20 and CO2. Chemical reactions in the bulk-gas stream are combustion and cracking of oil vapor, combustion of H2, CHx, CH4, and CO, and the w a t e r - g a s shift. Inlet-shale conditions (temperature, composition, particle size, and flux) may be specified as a function of time. Likewise, inlet-gas conditions (temperature, composition, and flux) may be specifed as a function of time. Inlet gas is permitted not only at the shale-outlet end o f the retort, b u t also at any other specified axial location. All or part of the wet or dry off-gas from the shale-inlet end of the retort may be recycled into any gas-inlet location. The governing equations for mass and energy balance are written in terms of the preceding processes and are solved numerically b y a semi-implicit, finite~lifference method. The bulk-gas flux as well as the composition and temperature of both the gas stream and the shale particles are calculated as a function of time and axial location in the retort. The time-dependent feature of the model is useful for examining transient responses to start-up conditions and to changes in operating conditions. In the present study we were interested only in the steady-state solution, so the model was run with constant conditions until a steady-state operation was reached. MODEL VALIDATION The internal combustion retorting process was experimentally investigated as part of the Paraho oil shale project [2, 4, 5]. The Paraho semiworks retort was operated by Development Engineering Inc. at the Anvil Points facilities under a lease agreement with the Department of Energy. This retort is shown schematically in Fig. 1. The retort dimensions and the locations for gas inlet and outlet reported by Laird [6] were used in our analyses. The distance between the b o t t o m cooling-gas distributor and the off-gas collector is approximately 8.2 m. This is the effective height for heat transfer and chemical reactions. The mid and t o p air distributors are located 3.81 and 5.64 m above the b o t t o m distributor, respectively. The retort has an internal crosssectional area of approximately 4.6 m 2. Samples and raw data from these ex-
127
perimental operations were provided to the Laramie Energy Technology Center (LETC). Test periods with complete data for material and energy balances were selected by LETC to represent steady-state operating conditions. Under a separate contract with LETC, JAYCOR Corporation developed a computer data base with material and energy balances for each of these test periods [6, 7]. This data base was utilized for our model validation. Raw shale Net product gas
Offgas I OiJ0asIoas Mist formation zone
i
~
T Retorting zone
Top
distributor ~ Combustion zone
separators
Productoil Topairl
Recycle gas blower
!Recycle gas
Top dilution gas Mid dilution gas
Mid
Retorted shale cooling zone
distributor
Mid air
Air Air blower
Bottom cooling gas
~
Retortedshale
Fig. 1. Schematic drawing of internal combustion retort.
A group of eight Paraho test periods, selected as having nearly the same operating conditions, was initially studied in order to distinguish statistical fluctuations from actual differences between the experimental data and the model calculations. The average operating conditions are given in Table 1. The recycle gas flux was approximately 2/3 of the total off-gas, and it contained 20 vol.% water vapor. Preliminary model calculations for these conditions indicated that there were some discrepancies that needed to be resolved before further application of the model. First, the experimental location of the main combustion zone and the retorting zone near the top air distributor was displaced from the calculated location by nearly one meter in the direction of gas flow. This is apparently due to a delay in the combustion heat reaching the thermocouple locations in the actual retort. This, in turn, is probably due to a combination of physical and chemical limitations: (a) mixing of the inlet air with the main gas stream, (b) induction-delay time for ignition, and, most importantly, (c) radial dispersion of the hot combustion gas toward the centerline thermocouples. Conversely, in the one-dimensional retort model complete mixing
128
is defined to occur at the point of air injection and, if the temperature is higher than a specified threshold value, combustion occurs immediately to completely consume either the oxygen or the fuel. Furthermore, the heat of combustion is immediately sensed at that location. Thus, more elaborate modeling of these processes is needed to arrive at the correct location of the combustion zone. It m a y not be possible to do this in a one-dimensional model. As a t e m p o r a r y measure, we introduced a one-meter offset of the combustion zone from each air-inlet location to approximately give the correct locations for the combustion and retorting zones for the base runs. TABLE 1 Average conditions for base group of eight Paraho runs Grade (gal/ton) Raw shale organic carbon (wt%) Raw shale mineral CO~ (wt%) Shale velocity (m/d) Bed void fraction Air flux (mol/m2.s) top middle bottom Recycle flux a (mol/m2.s) top middle bottom Cooling at air distributors (kJ/m2-s)
27.0 11.8 17.7 45.4 0.45 3.1 0.7 0.0 1.1 1.1 9.3 27.0
aIncluding approximately 20 vol.% water vapor.
Second, in the preliminary adiabatic model calculations, the calculated temperature exceeded the measured temperature b y a b o u t 100°C. This had the effect of causing more carbonate decomposition than was experimentally observed. In subsequent model calculations, loss of heat through the wall of the retort was permitted, assuming that the rate o f heat loss was proportional to the temperature gradient through the wall [3]. A wall-heat-loss coefficient of 12.5 J / m 2. K-s brought the calculated and measured axial temperature profiles into agreement over the entire height o f the retort. The a m o u n t o f mineral CO2 remaining in the outlet shale also came into agreement. Third, the average measured oil yield for the base group of eight runs was approximately 10% lower than the calculated yield. J A Y C O R estimated that approximately 10% of the oil generated in the retort was cracked to lighter h y d r o c a r b o n fractions. While the model does address cracking o f the initially generated oil vapor, it does n o t address any downward flow o f oil that could re-introduce the oil into a hot zone for revolatilization and further degradation. One hypothesis is that part of the oil mist is filtered o u t o f the bulk-gas stream b y the shale particles and is carried downward into the h o t zone
129
where the oil can undergo further cracking. More experimental work is needed to understand the details of this oil refluxing and degradation process before it can be adequately modeled. Therefore, as a temporary measure, we simply included an additional 10% of the Fischer assay oil yield in our CHx gas species. For the condensation conditions utilized in the semiworks operation (approximately 60°C), all of this hydrocarbon fraction remained in the gas phase and was largely recycled into the retort and burned. With these three modifications for the model calculations, final comparisons were made with the base group of Paraho runs. The calculated and measured temperatures are shown in Fig. 2 for one of these runs. The other seven runs gave similar agreement. Comparison of other results is summarized in Table 2. The product gas exhibits some discrepancies that may be due to model inaccuracies in the combustion reactions and to lack of knowledge of the stoichiometry for the kerogen pyrolysis at high heating rates. The principal nonstatistical discrepancy is that the calculated CO flux is approximately twice the measured flux. This may be so pronounced because of the assumption that oil combustion produced CO rather than CO2. Furthermore, CO (produced in the retort or introduced from the recycle gas) was the last fuel species to be oxidized in the sequential gas-combustion scheme used at each axial grid point (i.e., oil, H2, CHx, CH4, and finally CO). Table 2 also shows the close agreement between calculated and measured
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Measuredtemperature Calculated shaletemperature - - - Calculatedgastemperature j~
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6
Fig. 2. Comparison of measured and calculated temperature profiles for typical semiworks run.
130
composition of the outlet shale in terms of residual organic carbon and mineral COs. This indicates that the chemical reactions within the shale particles are adequately modeled. Interestingly, the residual organic carbon concentration corresponds to approximately only a 25% char utilization in the retort. Likewise, the residual mineral COs concentration also corresponds to only a 25% decomposition of the initial carbonate minerals. This is consistent with previous parameter studies [8] which showed t h a t the degree of carbonate decomposition closely followed the degree of char oxidation in an internal combustion retort. Moreover, the endothermic heat of carbonate decomposition (principally CaMg(CO3)2 = CaCO3 + MgO + COs) largely offsets the exothermic heat of char oxidation. Thus, the recycle gas and the shale oil itself furnish the principal fuel for operating an internal combustion retort. TABLE2 Comparison of measured and calculated results for a base group of eight similar Paraho runs.
Net product flux (mol/m2.s) N2 CO2 CO H2 CH, CHx Oil yield (% Fischer assay) Organic carbon (final wt%) Mineral CO 2 (final wt%) Wall heat loss (kJ/s)
Measured
Calculated
ave.
ave.
std. dev.
std. dev.
2.99 1.05 0.11 0.07 0.10 0.12
0.05 0.04 0.02 0.01 0.02 0.01
3.02 1.09 0.24 0.10 0.11 0.10
0.01 0.06 0.01 0.01 0.01 0.01
89.2 1.83 13.0 57
3.0 0.10 0.5 3
89.0 1.95 13.6 350
0.8 0.06 0.8 2
The final comparison in Table 2 shows t h a t the wall heat loss required to give agreement between calculated and measured axial temperature profiles and residual carbonate concentrations was about six times the wall heat loss estimated by JAYCOR for the properties of a new refractory retort liner. This could simply be due to changes in the refractory liner after lengthy operation o f the retort. No source of error in the model calculations could be found to explain these discrepancies. Therefore, for all Paraho simulations the wall-heat-loss coefficient o f 12.5 J / m 2- Ko s was used. Measurements of heat loss on the semiworks plant could resolve this question. To further test the retort model, additional calculations were made for other Paraho test periods that had operating conditions different from the base group just discussed. In all these calculations we assumed the same one-
131 meter offset of the apparent combustion zone from each air-inlet location, a conversion of 10% o f the shale oil into a lighter hydrocarbon fraction t h a t was subsequently largely recycled into the retort and burned, and a wallheat-loss coefficient of 12.5 J/m2-K • s. The following ranges of conditions were investigated: shale velocity 40.9--50.3 m/d, air flux at top distributor 2.24--3.67 m o l / m 2. s, air flux at mid distributor 0.25--1.06 m o l / m 2- s, recycle gas flux at top distributor 0.80--0.98 mol/m2-s, recycle gas flux at mid distributor 0.34--1.78 mol/m2.s, recycle gas flux at b o t t o m distributor 6.27--8.06 m o l / m 2. s, and average particle size 1.6--2.9 cm. With one exception, the calculated and measured axial temperature profiles were in very good agreement. The exception was for the run with low air flux at the mid distributor. For this run, the calculated temperature was too low near the mid distributor because the cooling load (applied equally at b o t h air distributors for all runs) far exceeded the a m o u n t needed at the mid distributor. Even for this run, however, the temperature profiles were brought into good agreement when all of the cooling was applied at the top distributor where most of the combustion air was introduced. Comparisons o f other results are shown in Table 3 for all 27 Paraho test periods that were simulated. The average ratio o f calculated result to measured result for all 27 test periods is very similar to the ratio for the eight base test periods. Thus, the three principal assumptions t h a t were used for the base comparisons appear to apply equally to ranges of operating conditions tested in the Paraho retort. TABLE3 Ratio of calculated/measured results for Paraho retort simulations Base runs a
All runs b
Product N~ Product CO2 Product CO Product H~ Product CH, Product CHx
1.01 1.03 2.10 1.48 1.18 0.84
1.01 1.10 2.11 1.36 1.21 0.78
Oil yield Organic carbon remaining Mineral CO2 remaining Wall heat loss
1.00 1.07 1.05 6.14
0.99 1.10 0.98 6.08
a8 Test periods in the same category of operating conditions. b27 Test periods in 15 different categories of operating conditions. MODEL PREDICTIONS Having validated the retort model for the conditions of the Paraho semiworks retort, we applied the model to a hypothetical set of commercial-scale
132
conditions. Without knowledge of any proprietary plans for such a retort, we assumed that the effective height of the retort for heat transfer and chemical reactions could remain at 8.2 m, unchanged from the semiworks retort. The retort was further assumed to be 10 m wide and 40 m long, having a ratio of perimeter to cross-sectional area of 0.25 m -1, compared with a ratio of 1.7 m -1 for the semiworks retort. Thus, the wall area for heat loss is approximately 15% of that for the semiworks retort. Likewise, w i t h o u t knowledge of any proprietary operating conditions, we began with the operating conditions given in Table 1, with the following exceptions. A grade of 125 L/Mg (30 gal/ton) was taken as the standard grade for all commercial simulations. The total recycle gas flux was unchanged, b u t the recycle gas contained essentially no water vapor, in accordance with the assumed off-gas condensation temperature of 10°C for the commercial plant. Moreover, because of the low condensation temperature, we no longer assumed that part of the Fischer assay oil yield was included in the recycle gas. Thus, the loss in oil yield was only that calculated b y the model for oil cracking, coking, and combustion of the initially generated oil as it moved to the gas outlet. If it is n o t practical to cool the entire off-gas stream to 10°C, additional loss in oil yield would occur from combustion of the uncondensed oil in the gas that is recycled into the retort. The calculated shale and gas temperature profiles for the initial commercial conditions are shown in Fig. 3. These are, of course, very similar to
800
Shale temperature - - - - - Gas temperature
600 o
~ 400 e~
E
Gas
Shale
200 Mid distributor 0
0
I
2
3
4
Top distributor 5
6
7
8
Height (m) Fig. 3. Temperature profiles for the initially selected commercial-scale operating conditions.
133
those for the semiworks retort, except that the temperatures are slightly higher because of the lower wall heat loss. As for the semiworks retort, there are two undesirable features illustrated by these temperature profiles. First, the retorting zone at a height o f approximately 6.9 m is very close to the principal combustion zone at 6.6 m. It is not surprising that the oil yield was only 91.5% of Fischer assay, because oil was generated in close proximity to the available oxygen. Previous parameter studies [8] have illustrated that the separation between the retorting zone and the combustion zone in an internal combustion retort is largely a function of the total gas flux carrying heat from the combustion zone into the retorting zone. Thus, by increasing the recycle gas at the b o t t o m distributor, the separation should be improved. However, the location selected for the top air distributor is so close to the top of the retort that there is simply not enough r o o m in the lower part o f the retort for much additional separation between the combustion and retorting reactions. More r o o m can be allowed for this separation by moving both air distributors downward by 1 m. This can be done while still providing enough room in the lower part of the retort for heat transfer to occur between the cold recycle gas and the hot shale. With these changes in air distributor locations, the increase in oil yield as a function o f recycle gas flux at the b o t t o m distributor is shown in Fig. 4. A 50% increase in recycle gas (9.24 to 13.86 mol/m 2.s) increased the oil yield from 91.5% to nearly 98% of Fischer assay. This increase in yield can be attributed partly to the increased fuel in the additional recycle gas. It is also partly due to an increase by a factor o f three in the separation between the retorting zone (now at 6.5 m) and the principal combustion zone (now at 5.6 m). This will be illustrated later in the final temperature profiles. Returning to Fig. 3, the second undesirable feature is the elevated temperRecycle gas flux (SCFM/ft 2) 100
45
50
55
60
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I
65
98 u~
96 "0
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94
:~
92 ,
90
9
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10
l
11
,
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12
I
l
13
i
14
Recycle gas flux (mol/m 2 . s) Fig. 4. Effect of recycle gas introduced at b o t t o m distributor on oil yield.
134
ature of the outlet shale, nearly 250°C. If the sensible heat can be recovered from the hot outlet shale, the thermal efficiency of the process will be improved. One way of achieving that heat recovery is to increase the bottom recycle flux. The temperature of the outlet shale as a function of bottom recycle flux is shown in Fig. 5. Here, a 50% increase in recycle gas lowered the outlet shale from 240°C to a much more acceptable 62°C. In so doing, however, the temperature of the outlet gas at the top of the retort increased from 60°C to 125°C. Such a high off-gas temperature would place a heavy thermal load on the oil condenser, or else allow an unacceptable amount of light oil to be recycled into the retort and burned.
Recycle gas flux (SCFM/ft 2) 45
250
A
50
55
60
,
,
65 _
200
o
.= •~
150--
E ~-
100--
50
9
10
11
12
13
14
Recycle gas flux (mol/m 2. s) Fig. 5. Effect of recycle gas introduced at b o t t o m shale and o u t l e t gas.
distributor
on temperature of outlet
This problem can be alleviated in two ways. First, the retort design could be altered to give additional length at the top of the retort for ga~-solid heat transfer to more completely recover the sensible heat from the gas. Parameter studies showed that this change had the overall effect of raising the maximum temperature in the retort to nearly 800°C, since now sensible heat from both the outlet shale and the outlet gas has been efficiently recovered within the retort. This high retort temperature would aggravate any tendency for oil cracking or combustion, particularly for nonuniform flow conditions. A second, more acceptable solution to this problem is simply to generate less heat. This is readily accomplished by decreasing the air flux at the top air distributor. For example, decreasing the air flux by 35% (from 3.1 to 2.0 mol/m 2. s) brought the off-gas temperature down to 70°C. This change had the accompanying effect of raising the outlet shale temperature from 62°C to 80°C. The latter temperature was readily decreased to 70°C by a 5% further increase in the bottom recycle gas.
135 The temperature profiles for the final commercial retort are shown in Fig. 6. To summarize the changes that were made from the initial conditions: (a) both air distributors were lowered by 1 m, (b) the bottom recycle flux was increased by 55%, and (c) the top air flux was decreased by 35%. These changes had the desirable effects of (a) raising the oil yield from 91.5% to 98% of Fischer assay, (b) increasing the thermal efficiency of the retort by keeping both the outlet shale and the outlet gas at 70°C, and (c) keeping the maximum temperature of the principal combustion zone below 700°C, as well as separating it by nearly 1 m from the retorting zone at 450°C. The latter aspect is also important in minimizing loss in oil yield due to nonuniform-flow effects that cannot be addressed by the one
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800
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Shale temperature -
400
E=
p-
200
0
0
1
2
3
4
5
6
7
8
Height (m) Fig. 6. Temperature profilesfor finalcommercial-scale operating conditions. OVERALLPROCESSFLOWSHEET
A preliminary process flowsheet for a commercial-size, aboveground, countercurrent, gas-combustion retort system has been developed. Flowsheet data are based on an overall mass and energy balance code which uses the LLNL one-dimensional retort model in conjunction with calculations for the physical processes external to the retort (e.g., condenser and blowers). The principal purpose of the flowsheet is to predict the retorting results and the process interrelationships for conditions to be encountered in a typical commercial operation. After similar flowsheets are developed for a number of surface retort systems, they can be used to help assess the advantages and
136
disadvantages of the various generic processes and to help determine the most important areas for additional development. The mass- and energybalance code takes the retort data and results as predicted by the retort model and scales them up to a 50,000 bbl/day shale oil plant. The code then calculates temperatures, pressures, and mass flow rates as they occur in the overall process. Condenser cooling requirements, heater requirements, and compressor or blower power requirements are also calculated. The plant flowsheet, shown in Fig. 7, was designed by making a few modifications to the semiworks schematic drawing shown in Fig. 1. Following through the system, beginning with the shale input, 892 kg/s of raw shale must be crushed to produce the 758 kg/s needed for retorting. The difference represents the material that is t o o fine for processing (< 1.3 cm). Burned shale leaves the b o t t o m of the retort at 70°C, and must then be further cooled and wetted for disposal. The indicated retort cross-sectional area of 1 1 5 7 m 2 should not be interpreted as the cross-section o f a single retort unit; rather, it is the total area required for a 50,000 bbl/day plant and would probably consist of three or more retort units.
Rejected
Raw shale
134kg/s 892 kg/s 1O°C
Retort gas
Precipitator
& condenser ,~1
758 kg/s Gas & vapors
820 kg/s 70.4°C 0,85arm
~ £~
Load i "
Gas product
710 kg/s 1O°C 0.80 atm
117.5kg/s 10°C
Oil product
~
83.7 kg/s 10°C
Oil
Water Retort area
Gas & air
108kg/s20.2°C 0.889atm
1157m 2
Recycle blower power requirements
6.4 MW
Distributor cooler load
17.4MW
67.3 kg/sair 21.0°C0.889arm
Gas & air 61.0 kg/s 2O°C
0.8970.886atm
/.l
atm
22.0°C0.897atm
~
Air feed 10°C 90 kg/s0.80etm
~
I-
Air blower power requirements
1,0 MW
Burned shale
619 kg/s70°C
~
0.0 kg/sair
Recycle gas
514 kg/s 18.6°C
Recycle gas
Foul water 26.2 kg/s !0°C
592.5kg/s 0.80atm 10°C
l=
Fig. 7. Flowsheet for internal combustion returt system.
Pyrolysis and combustion products leave the retort and enter the cooling and separation train shown on the flowsheet as the precipitator and condenser. At this point the streams are cooled to 10°C, and the oil and foul
137 water are separated and leave the system. Approximately 29% of the dry gas stream is bled off as waste gas through a blower. The remainder of the stream is recycled. As air is fed to the system, its pressure is raised by blowers from the ambient 0.80 atm to a pressure equal to the sum of the pressure drops through the bed, the manifold, and the valve. The air and recycle gas temperatures are slightly raised as they pass through their individual blowers; the temperature rise of the recycle gas (though not shown) is between 6 and 8°C. The preliminary process flowsheet presented here is meant only to serve as an example of a 50,000 bbl/day shale oil plant that uses an internal combustion retort. These results will be useful for comparison with results of models which are being developed for other types of aboveground retort systems. CONCLUSIONS The LLNL mathematical model for simulating aboveground retorting of oil shale in a moving, packed-bed retort was tested by comparison with data from the Pamho semiworks retort. These comparisons illustrated the validity of the model in calculating retort temperature profiles, oil yields, off-gas composition, and outlet shale composition. The model calculations also pointed out three aspects that need further resolution; namely, (1) the apparent upward displacement of the combustion zone from the air inlet, (2) the loss of heat through the wall of the retort, and (3) the possible refluxing of oil mist between the gas outlet and the combustion zone. Even without further modification, however, the retort model can be used to rapidly investigate wide ranges of operating conditions that would be prohibitively expensive to do experimentally. Proposed improvements in the process can thus be readily identified and further studied in laboratory-scale or semiworks retorts. Application of the model to a hypothetical set of commercial-scale conditions helped to identify key operating or design parameters. The predicted oil yield was significantly improved by lengthening the spatial separation between the retorting zone and the principal combustion zone. This can be done by increasing the inlet recycle gas flux so that the convective heat transfer into the retorting zone is increased and by allowing sufficient distance in the reactor for the retorting zone to be thereby moved away from the combustion zone. Increasing the amount of recycle gas at the bottom of the retort has the additional benefit of increasing the thermal efficiency of the process by capturing the sensible heat from the outlet shale. These parameter studies also illustrated how the peak temperature in the combustion zone and the temperature of the off-gas at the top of the retort could be adequately controlled by adjusting the inlet air flux. We believe that the model can be a powerful tool in the development of oil shale retorts.
138 ACKNOWLEDGEMENTS
Helpful discussions with B.C. Sudduth are gratefully acknowledged. This work was performed under the auspices o f the U.S. Department of Energy by the Lawrence Livermore National Laboratory under contract number W-7405-ENG-48.
REFERENCES 1 Matzick, A., Dannenberg, R.O., Ruark, J.R., Phillips, J.E., Lankford, J.D. and Guthrie, B., 1966. Development of the Bureau of Mines gas-combustion oil-shale retorting process. U.S. Dep. Inter., Bur. Min. Bull. 635. 2 Jones, J.B., Jr. and Heistand, R.N., 1979. Recent Paraho operations. In: Gary, J.H. (Ed.), Proceedings of 12th Oil Shale Symposium, Colorado School of Mines, Golden, Colorado, pp. 184--194. 3 Braun, R.L., 1981. Mathematical modeling of modified in-situ and aboveground oil shale retorting. Lawrence Livermore National Laboratory Report UCRL-53119, Livermore, California. 4 Jones, J.B., 1976. Paraho oil shale retort. In: Gary, J.H. (Ed.), Proceedings of 9th Oil Shale Symposium, Colorado School of Mines, Golden, Colorado, pp. 39--48. 5 Jones, J.B., Jr. and Glassett, J.M., 1982. Paraho processes. In: Allred, V.D. (Ed.), Oil Shale Processing Technology, The Center for Professional Advancement, East Brunswick, New Jersey, pp. 107--120. 6 Laird, D.H., 1982. Validation and analysis of steady-state operating data from the Paraho semiworks retort. JAYCOR Final Report, Vol. 1, J510-80-005/2127. 7 Laird, D.H. and Scharff, M.F., 1983. Intercomparison and correlation of steady-state operating data from Paraho semiworks retort. JAYCOR Final Report, Vol. 2, J510-81029/2127. 8 Lewis, A.E. and Braun, R.L., 1981. Retorting and combustion processes in surface oilshale retorts. J. Energy, 5: 355--361.
125
RESULTS OF MATHEMATICAL MODELING OF OIL SHALE RETORTING IN AN ABOVEGROUND, INTERNAL COMBUSTION RETORT
R.L. BRAUN, D.E. CHRISTIANSEN, J.C. DIAZ and A.E. LEWIS Lawrence Livermore National Laboratory, Livermore, CA 94550 (U.S.A.)
(Received November 29th 1983; accepted March 25th, 1984)
ABSTRACT The one-dimensional mathematical model developed at the Lawrence Livermore National Laboratory to simulate the detailed chemical reaction kinetics involved in modified in-situ retorting of oil shale has been applied to simulate aboveground retorting in a moving, packed-bed retort. Application of the model to simulate such a retort was made for the internal combustion mode of operation, using typical operating conditions of the Paraho semiworks retort. Comparison with the semiworks experimental data revealed the general accuracy of the model in calculating retort temperature profiles, oil yield, off-gas composition, and outlet shale composition. The model was also applied to a hypothetical set of commercial-scale conditions in order to identify key operating or design parameters. These parameter studies predicted that the oil yield and the overall process performance could be markedly improved by increasing the recycle gas flux, decreasing the inlet-air flux, and relocating the inlet-air distributors. A process flowsheet was developed for the complete retorting system.
INTRODUCTION A n oil shale r e t o r t is essentially a h e a t e x c h a n g e r f o r t r a n s f e r r i n g h e a t f r o m a h e a t i n g m e d i u m t o t h e shale. E i t h e r h o t gas o r h o t solid c a n b e used as t h e h e a t - c a r r y i n g m e d i u m . Processes using gas are usually l i m i t e d t o t h e use o f p a c k e d b e d s o f c o a r s e l y - c r u s h e d oil shale in o r d e r t o a l l o w a d e q u a t e h e a t a n d m a s s t r a n s f e r w i t h o u t excessive p r e s s u r e d r o p . T h e h o t gas can be g e n e r a t e d e i t h e r e x t e r n a l l y t o t h e oil shale r e t o r t o r i n t e r n a l l y f r o m c o m b u s t i o n r e a c t i o n s w i t h i n t h e r e t o r t itself. M u c h e x p e r i m e n t a l w o r k has b e e n d o n e o n t h e l a t t e r g a s - c o m b u s t i o n c o n c e p t o f oil shale r e t o r t i n g b y t h e U.S. B u r e a u o f Mines [ 1 ] . S u b s e q u e n t l y , f u r t h e r r e f i n e m e n t s o f t h e p r o c e s s w e r e m a d e t h r o u g h t h e P a r a h o p r o j e c t [2]. N o m a t h e m a t i c a l m o d e l i n g w o r k , h o w e v e r , was r e p o r t e d in c o n j u n c t i o n w i t h e i t h e r o f t h o s e e x p e r i m e n t a l p r o g r a m s . T h i s p a p e r , t h e r e f o r e , addresses t h e s i m u l a t i o n o f a b o v e g r o u n d r e t o r t i n g using i n t e r n a l c o m b u s t i o n in a m o v i n g , p a c k e d - b e d r e t o r t . T h e p r i n c i p a l o b j e c t i v e s are t o t e s t t h e v a l i d i t y o f t h e m a t h e m a t i c a l m o d e l a n d t o i d e n t i f y k e y o p e r a t i n g or design p a r a m e t e r s b y m e a n s o f p a r a m e t e r studies o n a h y p o t h e t i c a l set o f c o m m e r c i a l ~ c a l e c o n d i t i o n s . 0378-3820/84/$03.00
© 1984 Elsevier Science Publishers B.V.
126 MODEL DESCRIPTION A time-dependent, one-dimensional mathematical model has been developed for simulating the chemical and physical processes involved in the retorting of a packed bed of oil shale [3]. The physical processes included in the model are axial convective transport of heat and mass, axial thermal dispersion, axial pressure drop, heat transfer between gas stream and shale particles, thermal conduction within shale particles, wall heat loss, water evaporation and condensation, movement of shale countercurrent to gas flow, and additional cooling at air inlets. Chemical reactions within shale particles are release of b o u n d mineral water, pyrolysis of kerogen, coking of oil, pyrolysis of char, decomposition of t w o carbonate minerals, combustion of char and sulfur, and gasification of organic carbon residue with H20 and CO2. Chemical reactions in the bulk-gas stream are combustion and cracking of oil vapor, combustion of H2, CHx, CH4, and CO, and the w a t e r - g a s shift. Inlet-shale conditions (temperature, composition, particle size, and flux) may be specified as a function of time. Likewise, inlet-gas conditions (temperature, composition, and flux) may be specifed as a function of time. Inlet gas is permitted not only at the shale-outlet end o f the retort, b u t also at any other specified axial location. All or part of the wet or dry off-gas from the shale-inlet end of the retort may be recycled into any gas-inlet location. The governing equations for mass and energy balance are written in terms of the preceding processes and are solved numerically b y a semi-implicit, finite~lifference method. The bulk-gas flux as well as the composition and temperature of both the gas stream and the shale particles are calculated as a function of time and axial location in the retort. The time-dependent feature of the model is useful for examining transient responses to start-up conditions and to changes in operating conditions. In the present study we were interested only in the steady-state solution, so the model was run with constant conditions until a steady-state operation was reached. MODEL VALIDATION The internal combustion retorting process was experimentally investigated as part of the Paraho oil shale project [2, 4, 5]. The Paraho semiworks retort was operated by Development Engineering Inc. at the Anvil Points facilities under a lease agreement with the Department of Energy. This retort is shown schematically in Fig. 1. The retort dimensions and the locations for gas inlet and outlet reported by Laird [6] were used in our analyses. The distance between the b o t t o m cooling-gas distributor and the off-gas collector is approximately 8.2 m. This is the effective height for heat transfer and chemical reactions. The mid and t o p air distributors are located 3.81 and 5.64 m above the b o t t o m distributor, respectively. The retort has an internal crosssectional area of approximately 4.6 m 2. Samples and raw data from these ex-
127
perimental operations were provided to the Laramie Energy Technology Center (LETC). Test periods with complete data for material and energy balances were selected by LETC to represent steady-state operating conditions. Under a separate contract with LETC, JAYCOR Corporation developed a computer data base with material and energy balances for each of these test periods [6, 7]. This data base was utilized for our model validation. Raw shale Net product gas
Offgas I OiJ0asIoas Mist formation zone
i
~
T Retorting zone
Top
distributor ~ Combustion zone
separators
Productoil Topairl
Recycle gas blower
!Recycle gas
Top dilution gas Mid dilution gas
Mid
Retorted shale cooling zone
distributor
Mid air
Air Air blower
Bottom cooling gas
~
Retortedshale
Fig. 1. Schematic drawing of internal combustion retort.
A group of eight Paraho test periods, selected as having nearly the same operating conditions, was initially studied in order to distinguish statistical fluctuations from actual differences between the experimental data and the model calculations. The average operating conditions are given in Table 1. The recycle gas flux was approximately 2/3 of the total off-gas, and it contained 20 vol.% water vapor. Preliminary model calculations for these conditions indicated that there were some discrepancies that needed to be resolved before further application of the model. First, the experimental location of the main combustion zone and the retorting zone near the top air distributor was displaced from the calculated location by nearly one meter in the direction of gas flow. This is apparently due to a delay in the combustion heat reaching the thermocouple locations in the actual retort. This, in turn, is probably due to a combination of physical and chemical limitations: (a) mixing of the inlet air with the main gas stream, (b) induction-delay time for ignition, and, most importantly, (c) radial dispersion of the hot combustion gas toward the centerline thermocouples. Conversely, in the one-dimensional retort model complete mixing
128
is defined to occur at the point of air injection and, if the temperature is higher than a specified threshold value, combustion occurs immediately to completely consume either the oxygen or the fuel. Furthermore, the heat of combustion is immediately sensed at that location. Thus, more elaborate modeling of these processes is needed to arrive at the correct location of the combustion zone. It m a y not be possible to do this in a one-dimensional model. As a t e m p o r a r y measure, we introduced a one-meter offset of the combustion zone from each air-inlet location to approximately give the correct locations for the combustion and retorting zones for the base runs. TABLE 1 Average conditions for base group of eight Paraho runs Grade (gal/ton) Raw shale organic carbon (wt%) Raw shale mineral CO~ (wt%) Shale velocity (m/d) Bed void fraction Air flux (mol/m2.s) top middle bottom Recycle flux a (mol/m2.s) top middle bottom Cooling at air distributors (kJ/m2-s)
27.0 11.8 17.7 45.4 0.45 3.1 0.7 0.0 1.1 1.1 9.3 27.0
aIncluding approximately 20 vol.% water vapor.
Second, in the preliminary adiabatic model calculations, the calculated temperature exceeded the measured temperature b y a b o u t 100°C. This had the effect of causing more carbonate decomposition than was experimentally observed. In subsequent model calculations, loss of heat through the wall of the retort was permitted, assuming that the rate o f heat loss was proportional to the temperature gradient through the wall [3]. A wall-heat-loss coefficient of 12.5 J / m 2. K-s brought the calculated and measured axial temperature profiles into agreement over the entire height o f the retort. The a m o u n t o f mineral CO2 remaining in the outlet shale also came into agreement. Third, the average measured oil yield for the base group of eight runs was approximately 10% lower than the calculated yield. J A Y C O R estimated that approximately 10% of the oil generated in the retort was cracked to lighter h y d r o c a r b o n fractions. While the model does address cracking o f the initially generated oil vapor, it does n o t address any downward flow o f oil that could re-introduce the oil into a hot zone for revolatilization and further degradation. One hypothesis is that part of the oil mist is filtered o u t o f the bulk-gas stream b y the shale particles and is carried downward into the h o t zone
129
where the oil can undergo further cracking. More experimental work is needed to understand the details of this oil refluxing and degradation process before it can be adequately modeled. Therefore, as a temporary measure, we simply included an additional 10% of the Fischer assay oil yield in our CHx gas species. For the condensation conditions utilized in the semiworks operation (approximately 60°C), all of this hydrocarbon fraction remained in the gas phase and was largely recycled into the retort and burned. With these three modifications for the model calculations, final comparisons were made with the base group of Paraho runs. The calculated and measured temperatures are shown in Fig. 2 for one of these runs. The other seven runs gave similar agreement. Comparison of other results is summarized in Table 2. The product gas exhibits some discrepancies that may be due to model inaccuracies in the combustion reactions and to lack of knowledge of the stoichiometry for the kerogen pyrolysis at high heating rates. The principal nonstatistical discrepancy is that the calculated CO flux is approximately twice the measured flux. This may be so pronounced because of the assumption that oil combustion produced CO rather than CO2. Furthermore, CO (produced in the retort or introduced from the recycle gas) was the last fuel species to be oxidized in the sequential gas-combustion scheme used at each axial grid point (i.e., oil, H2, CHx, CH4, and finally CO). Table 2 also shows the close agreement between calculated and measured
L,
i,
i,
i
'
i
'
i'
I
'1
i
800 ~-- •
Measuredtemperature Calculated shaletemperature - - - Calculatedgastemperature j~
E
l
0
Gas
t
,
,
0
1
2
w~
-t _~
~
~
7
8
' tri; t°r d,i trii t°r, ~
3 4 5 Height (m)
6
Fig. 2. Comparison of measured and calculated temperature profiles for typical semiworks run.
130
composition of the outlet shale in terms of residual organic carbon and mineral COs. This indicates that the chemical reactions within the shale particles are adequately modeled. Interestingly, the residual organic carbon concentration corresponds to approximately only a 25% char utilization in the retort. Likewise, the residual mineral COs concentration also corresponds to only a 25% decomposition of the initial carbonate minerals. This is consistent with previous parameter studies [8] which showed t h a t the degree of carbonate decomposition closely followed the degree of char oxidation in an internal combustion retort. Moreover, the endothermic heat of carbonate decomposition (principally CaMg(CO3)2 = CaCO3 + MgO + COs) largely offsets the exothermic heat of char oxidation. Thus, the recycle gas and the shale oil itself furnish the principal fuel for operating an internal combustion retort. TABLE2 Comparison of measured and calculated results for a base group of eight similar Paraho runs.
Net product flux (mol/m2.s) N2 CO2 CO H2 CH, CHx Oil yield (% Fischer assay) Organic carbon (final wt%) Mineral CO 2 (final wt%) Wall heat loss (kJ/s)
Measured
Calculated
ave.
ave.
std. dev.
std. dev.
2.99 1.05 0.11 0.07 0.10 0.12
0.05 0.04 0.02 0.01 0.02 0.01
3.02 1.09 0.24 0.10 0.11 0.10
0.01 0.06 0.01 0.01 0.01 0.01
89.2 1.83 13.0 57
3.0 0.10 0.5 3
89.0 1.95 13.6 350
0.8 0.06 0.8 2
The final comparison in Table 2 shows t h a t the wall heat loss required to give agreement between calculated and measured axial temperature profiles and residual carbonate concentrations was about six times the wall heat loss estimated by JAYCOR for the properties of a new refractory retort liner. This could simply be due to changes in the refractory liner after lengthy operation o f the retort. No source of error in the model calculations could be found to explain these discrepancies. Therefore, for all Paraho simulations the wall-heat-loss coefficient o f 12.5 J / m 2- Ko s was used. Measurements of heat loss on the semiworks plant could resolve this question. To further test the retort model, additional calculations were made for other Paraho test periods that had operating conditions different from the base group just discussed. In all these calculations we assumed the same one-
131 meter offset of the apparent combustion zone from each air-inlet location, a conversion of 10% o f the shale oil into a lighter hydrocarbon fraction t h a t was subsequently largely recycled into the retort and burned, and a wallheat-loss coefficient of 12.5 J/m2-K • s. The following ranges of conditions were investigated: shale velocity 40.9--50.3 m/d, air flux at top distributor 2.24--3.67 m o l / m 2. s, air flux at mid distributor 0.25--1.06 m o l / m 2- s, recycle gas flux at top distributor 0.80--0.98 mol/m2-s, recycle gas flux at mid distributor 0.34--1.78 mol/m2.s, recycle gas flux at b o t t o m distributor 6.27--8.06 m o l / m 2. s, and average particle size 1.6--2.9 cm. With one exception, the calculated and measured axial temperature profiles were in very good agreement. The exception was for the run with low air flux at the mid distributor. For this run, the calculated temperature was too low near the mid distributor because the cooling load (applied equally at b o t h air distributors for all runs) far exceeded the a m o u n t needed at the mid distributor. Even for this run, however, the temperature profiles were brought into good agreement when all of the cooling was applied at the top distributor where most of the combustion air was introduced. Comparisons o f other results are shown in Table 3 for all 27 Paraho test periods that were simulated. The average ratio o f calculated result to measured result for all 27 test periods is very similar to the ratio for the eight base test periods. Thus, the three principal assumptions t h a t were used for the base comparisons appear to apply equally to ranges of operating conditions tested in the Paraho retort. TABLE3 Ratio of calculated/measured results for Paraho retort simulations Base runs a
All runs b
Product N~ Product CO2 Product CO Product H~ Product CH, Product CHx
1.01 1.03 2.10 1.48 1.18 0.84
1.01 1.10 2.11 1.36 1.21 0.78
Oil yield Organic carbon remaining Mineral CO2 remaining Wall heat loss
1.00 1.07 1.05 6.14
0.99 1.10 0.98 6.08
a8 Test periods in the same category of operating conditions. b27 Test periods in 15 different categories of operating conditions. MODEL PREDICTIONS Having validated the retort model for the conditions of the Paraho semiworks retort, we applied the model to a hypothetical set of commercial-scale
132
conditions. Without knowledge of any proprietary plans for such a retort, we assumed that the effective height of the retort for heat transfer and chemical reactions could remain at 8.2 m, unchanged from the semiworks retort. The retort was further assumed to be 10 m wide and 40 m long, having a ratio of perimeter to cross-sectional area of 0.25 m -1, compared with a ratio of 1.7 m -1 for the semiworks retort. Thus, the wall area for heat loss is approximately 15% of that for the semiworks retort. Likewise, w i t h o u t knowledge of any proprietary operating conditions, we began with the operating conditions given in Table 1, with the following exceptions. A grade of 125 L/Mg (30 gal/ton) was taken as the standard grade for all commercial simulations. The total recycle gas flux was unchanged, b u t the recycle gas contained essentially no water vapor, in accordance with the assumed off-gas condensation temperature of 10°C for the commercial plant. Moreover, because of the low condensation temperature, we no longer assumed that part of the Fischer assay oil yield was included in the recycle gas. Thus, the loss in oil yield was only that calculated b y the model for oil cracking, coking, and combustion of the initially generated oil as it moved to the gas outlet. If it is n o t practical to cool the entire off-gas stream to 10°C, additional loss in oil yield would occur from combustion of the uncondensed oil in the gas that is recycled into the retort. The calculated shale and gas temperature profiles for the initial commercial conditions are shown in Fig. 3. These are, of course, very similar to
800
Shale temperature - - - - - Gas temperature
600 o
~ 400 e~
E
Gas
Shale
200 Mid distributor 0
0
I
2
3
4
Top distributor 5
6
7
8
Height (m) Fig. 3. Temperature profiles for the initially selected commercial-scale operating conditions.
133
those for the semiworks retort, except that the temperatures are slightly higher because of the lower wall heat loss. As for the semiworks retort, there are two undesirable features illustrated by these temperature profiles. First, the retorting zone at a height o f approximately 6.9 m is very close to the principal combustion zone at 6.6 m. It is not surprising that the oil yield was only 91.5% of Fischer assay, because oil was generated in close proximity to the available oxygen. Previous parameter studies [8] have illustrated that the separation between the retorting zone and the combustion zone in an internal combustion retort is largely a function of the total gas flux carrying heat from the combustion zone into the retorting zone. Thus, by increasing the recycle gas at the b o t t o m distributor, the separation should be improved. However, the location selected for the top air distributor is so close to the top of the retort that there is simply not enough r o o m in the lower part o f the retort for much additional separation between the combustion and retorting reactions. More r o o m can be allowed for this separation by moving both air distributors downward by 1 m. This can be done while still providing enough room in the lower part of the retort for heat transfer to occur between the cold recycle gas and the hot shale. With these changes in air distributor locations, the increase in oil yield as a function o f recycle gas flux at the b o t t o m distributor is shown in Fig. 4. A 50% increase in recycle gas (9.24 to 13.86 mol/m 2.s) increased the oil yield from 91.5% to nearly 98% of Fischer assay. This increase in yield can be attributed partly to the increased fuel in the additional recycle gas. It is also partly due to an increase by a factor o f three in the separation between the retorting zone (now at 6.5 m) and the principal combustion zone (now at 5.6 m). This will be illustrated later in the final temperature profiles. Returning to Fig. 3, the second undesirable feature is the elevated temperRecycle gas flux (SCFM/ft 2) 100
45
50
55
60
1
I
I
I
65
98 u~
96 "0
"~ "~
94
:~
92 ,
90
9
]
i
10
l
11
,
I
12
I
l
13
i
14
Recycle gas flux (mol/m 2 . s) Fig. 4. Effect of recycle gas introduced at b o t t o m distributor on oil yield.
134
ature of the outlet shale, nearly 250°C. If the sensible heat can be recovered from the hot outlet shale, the thermal efficiency of the process will be improved. One way of achieving that heat recovery is to increase the bottom recycle flux. The temperature of the outlet shale as a function of bottom recycle flux is shown in Fig. 5. Here, a 50% increase in recycle gas lowered the outlet shale from 240°C to a much more acceptable 62°C. In so doing, however, the temperature of the outlet gas at the top of the retort increased from 60°C to 125°C. Such a high off-gas temperature would place a heavy thermal load on the oil condenser, or else allow an unacceptable amount of light oil to be recycled into the retort and burned.
Recycle gas flux (SCFM/ft 2) 45
250
A
50
55
60
,
,
65 _
200
o
.= •~
150--
E ~-
100--
50
9
10
11
12
13
14
Recycle gas flux (mol/m 2. s) Fig. 5. Effect of recycle gas introduced at b o t t o m shale and o u t l e t gas.
distributor
on temperature of outlet
This problem can be alleviated in two ways. First, the retort design could be altered to give additional length at the top of the retort for ga~-solid heat transfer to more completely recover the sensible heat from the gas. Parameter studies showed that this change had the overall effect of raising the maximum temperature in the retort to nearly 800°C, since now sensible heat from both the outlet shale and the outlet gas has been efficiently recovered within the retort. This high retort temperature would aggravate any tendency for oil cracking or combustion, particularly for nonuniform flow conditions. A second, more acceptable solution to this problem is simply to generate less heat. This is readily accomplished by decreasing the air flux at the top air distributor. For example, decreasing the air flux by 35% (from 3.1 to 2.0 mol/m 2. s) brought the off-gas temperature down to 70°C. This change had the accompanying effect of raising the outlet shale temperature from 62°C to 80°C. The latter temperature was readily decreased to 70°C by a 5% further increase in the bottom recycle gas.
135 The temperature profiles for the final commercial retort are shown in Fig. 6. To summarize the changes that were made from the initial conditions: (a) both air distributors were lowered by 1 m, (b) the bottom recycle flux was increased by 55%, and (c) the top air flux was decreased by 35%. These changes had the desirable effects of (a) raising the oil yield from 91.5% to 98% of Fischer assay, (b) increasing the thermal efficiency of the retort by keeping both the outlet shale and the outlet gas at 70°C, and (c) keeping the maximum temperature of the principal combustion zone below 700°C, as well as separating it by nearly 1 m from the retorting zone at 450°C. The latter aspect is also important in minimizing loss in oil yield due to nonuniform-flow effects that cannot be addressed by the one
I '
800
I
'
['
1 ~ I
' '
'
I '~
Shale temperature -
400
E=
p-
200
0
0
1
2
3
4
5
6
7
8
Height (m) Fig. 6. Temperature profilesfor finalcommercial-scale operating conditions. OVERALLPROCESSFLOWSHEET
A preliminary process flowsheet for a commercial-size, aboveground, countercurrent, gas-combustion retort system has been developed. Flowsheet data are based on an overall mass and energy balance code which uses the LLNL one-dimensional retort model in conjunction with calculations for the physical processes external to the retort (e.g., condenser and blowers). The principal purpose of the flowsheet is to predict the retorting results and the process interrelationships for conditions to be encountered in a typical commercial operation. After similar flowsheets are developed for a number of surface retort systems, they can be used to help assess the advantages and
136
disadvantages of the various generic processes and to help determine the most important areas for additional development. The mass- and energybalance code takes the retort data and results as predicted by the retort model and scales them up to a 50,000 bbl/day shale oil plant. The code then calculates temperatures, pressures, and mass flow rates as they occur in the overall process. Condenser cooling requirements, heater requirements, and compressor or blower power requirements are also calculated. The plant flowsheet, shown in Fig. 7, was designed by making a few modifications to the semiworks schematic drawing shown in Fig. 1. Following through the system, beginning with the shale input, 892 kg/s of raw shale must be crushed to produce the 758 kg/s needed for retorting. The difference represents the material that is t o o fine for processing (< 1.3 cm). Burned shale leaves the b o t t o m of the retort at 70°C, and must then be further cooled and wetted for disposal. The indicated retort cross-sectional area of 1 1 5 7 m 2 should not be interpreted as the cross-section o f a single retort unit; rather, it is the total area required for a 50,000 bbl/day plant and would probably consist of three or more retort units.
Rejected
Raw shale
134kg/s 892 kg/s 1O°C
Retort gas
Precipitator
& condenser ,~1
758 kg/s Gas & vapors
820 kg/s 70.4°C 0,85arm
~ £~
Load i "
Gas product
710 kg/s 1O°C 0.80 atm
117.5kg/s 10°C
Oil product
~
83.7 kg/s 10°C
Oil
Water Retort area
Gas & air
108kg/s20.2°C 0.889atm
1157m 2
Recycle blower power requirements
6.4 MW
Distributor cooler load
17.4MW
67.3 kg/sair 21.0°C0.889arm
Gas & air 61.0 kg/s 2O°C
0.8970.886atm
/.l
atm
22.0°C0.897atm
~
Air feed 10°C 90 kg/s0.80etm
~
I-
Air blower power requirements
1,0 MW
Burned shale
619 kg/s70°C
~
0.0 kg/sair
Recycle gas
514 kg/s 18.6°C
Recycle gas
Foul water 26.2 kg/s !0°C
592.5kg/s 0.80atm 10°C
l=
Fig. 7. Flowsheet for internal combustion returt system.
Pyrolysis and combustion products leave the retort and enter the cooling and separation train shown on the flowsheet as the precipitator and condenser. At this point the streams are cooled to 10°C, and the oil and foul
137 water are separated and leave the system. Approximately 29% of the dry gas stream is bled off as waste gas through a blower. The remainder of the stream is recycled. As air is fed to the system, its pressure is raised by blowers from the ambient 0.80 atm to a pressure equal to the sum of the pressure drops through the bed, the manifold, and the valve. The air and recycle gas temperatures are slightly raised as they pass through their individual blowers; the temperature rise of the recycle gas (though not shown) is between 6 and 8°C. The preliminary process flowsheet presented here is meant only to serve as an example of a 50,000 bbl/day shale oil plant that uses an internal combustion retort. These results will be useful for comparison with results of models which are being developed for other types of aboveground retort systems. CONCLUSIONS The LLNL mathematical model for simulating aboveground retorting of oil shale in a moving, packed-bed retort was tested by comparison with data from the Pamho semiworks retort. These comparisons illustrated the validity of the model in calculating retort temperature profiles, oil yields, off-gas composition, and outlet shale composition. The model calculations also pointed out three aspects that need further resolution; namely, (1) the apparent upward displacement of the combustion zone from the air inlet, (2) the loss of heat through the wall of the retort, and (3) the possible refluxing of oil mist between the gas outlet and the combustion zone. Even without further modification, however, the retort model can be used to rapidly investigate wide ranges of operating conditions that would be prohibitively expensive to do experimentally. Proposed improvements in the process can thus be readily identified and further studied in laboratory-scale or semiworks retorts. Application of the model to a hypothetical set of commercial-scale conditions helped to identify key operating or design parameters. The predicted oil yield was significantly improved by lengthening the spatial separation between the retorting zone and the principal combustion zone. This can be done by increasing the inlet recycle gas flux so that the convective heat transfer into the retorting zone is increased and by allowing sufficient distance in the reactor for the retorting zone to be thereby moved away from the combustion zone. Increasing the amount of recycle gas at the bottom of the retort has the additional benefit of increasing the thermal efficiency of the process by capturing the sensible heat from the outlet shale. These parameter studies also illustrated how the peak temperature in the combustion zone and the temperature of the off-gas at the top of the retort could be adequately controlled by adjusting the inlet air flux. We believe that the model can be a powerful tool in the development of oil shale retorts.
138 ACKNOWLEDGEMENTS
Helpful discussions with B.C. Sudduth are gratefully acknowledged. This work was performed under the auspices o f the U.S. Department of Energy by the Lawrence Livermore National Laboratory under contract number W-7405-ENG-48.
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