This work has demonstrated that sucrose inversion may be carried out successfully in fixed beds a t concentrations up to 45% sucrose. However, considerable caution must be exercised in this range because of the imoortance and effects of secondary degravdation reactions. External mass transfer effects are not significant, even for such highly concentrated feed solutions. Rather, measured reaction rates are undoubtedly characteristic of the basic reaction kinetics under strong intraparticle diffusion influence.
Literature Cited
Jones, c. M., Lewis, lv. ‘2. M.2 J.C‘hem. sod. 1149 1120 (1920). Lifshutz, Norman, M. S. thesis, Northwestern University, Evanston, Ill., 1966. Moelwvn-Hughes. E. .4.. Z. Phw. Chem. B26.281 (1934’3. Moran; T., Lewis,’W. C.’ M., f.Chem. SOC.iil, 1613 (1922). O’Connell, J. E., D. Sc. thesis, Massachusetts Institute of Technology, Cambridge, Mass., 1964. Reed, E. W., Dranoff, J. S., h d . Eng. Chem. Fundamentals 3, 304 (1964). RECEIVED for review April 14, 1967 ACCEPTED September 29, 1967
MIXED-GAS ADSORPTION AND VACUUM
Downloaded by UNIV OF WINNIPEG on September 9, 2015 | http://pubs.acs.org Publication Date: April 1, 1968 | doi: 10.1021/i260026a020
DESORPTION OF CARBON DIOXIDE
ON MOLECULAR SIEVE Thermo&amic and Rate Behauior PAUL FUKUNAGA AND K. C. HWANG AiResearch Manufacturing Co., Los Angeles, Calif. SAM H. DAVIS, JR., AND JACK WINNICK1 hTASAManned Spacecraft Center, Houston, Tex:
A mathematical model for adsorption and desorption processes is given with experimental data. The data verify the model proposed and are used to determine several important operating and potential design parameters for the adsorption of CO2 on 5 A molecular sieve. The fundamental data reported include isotherm data for COZadsorption on Linde 5A molecular sieve. Empirically determined coefficients for mass transfer during adsorption and desorption cycles are given together with pressure drop coefficients for low pressures (during vacuum desorption). These empirical coefficients are compared in a numerical simulation for the small experimental bed. ESIGN
procedures for adsorption beds used in gas purifica-
D tion are well developed (Chemical Engineer’s Handbook, 1963). Special problems or limitations such as those encountered in space technology may require modifications of these procedures or a re-examination of some of the basic assumptions used in the development of the procedures. I n COS adsorption beds (with molecular sieve the primary adsorbent) most of the current designs require that the bed remain free of interfering contaminants (in particular water vapor), and bed regeneration be carried out a t a temperature high enough to remove essentially all COZ before the bed is re-used. I n space applications both of these requirements may be impossible (or too costly in power requirements) to achieve. In addition, critical weight requirements in space flights make a complete analysis of such adsorption beds worthwhile in order to determine optimum design parameters such as bed size, gas flow rate through the bed, thermal swing size between adsorption and desorption cycles, and cycle time. T h e need to determine the influence of many operating parameters in a rather complex system dictated a model approach for analysis of the molecular sieve bed. This analysis would include the determination of pertinent thermodynamic data such as equilibrium isotherms and heats of 1
Present address, AiResearch Manufacturing Co., Los Angeles, Calif.
adsorption, the evaluation of the relative importance of different transport resistances, and the determination of the most important rate coefficients, including those associated with mass transfer between the gas and the adsorbent sites and those associated with mass flow through the bed. Theory
Mass transport during adsorption must proceed by the following steps: Adsorbate is carried by bulk transfer in the gas stream, mass transfer occurs between the bulk gas and the exterior portion of the solid surfaces, mass transfer also occurs within the pores or over the interior surfaces of the solid, and time variation in the amount of adsorbate on the solid surfaces accounts for the accumulation of adsorbate. A Wth step is sometimes included to account for a “reaction” step a t the solid-fluid interface but is probably fast enough to be neglected. A mathematical model which accounts for each of these processes in a relatively simple manner was utilized in this study and found to agree well with the small scale dynamic adsorption data reported here. T h e mathematical model used in this analysis was based on the following assumptions.
1. T h e bed and gas flow are homogeneous over every bed cross section. 2. Mixing or dispersion in the direction of flow is negligible. VOL. 7
NO. 2
APRIL 1968
269
3. T h e bed can be considered as a continuum except for the existence of two separate phases. 4. T h e gas phase a t a given distance, z, from the bed inlet has a n adsorbate concentration of c moles per unit volume, which depends only on z and time. 5 . T h e solid phase adsorbate concentration, w , pounds per pound of solid, may depend on a third variable (in addition to t and z ) , r , which measures the distance from the center of a n equivalent sphere for the solid. 6. At r = R the solid and gas are in contact and mass transfer occurs through a film with conductance proportional to a coefficient, k,. 7 . T h e driving potential between the gas and the solid surface is the difference between the partial pressure of the adsorbate in the gas and the equilibrium (isotherm) pressure of the adsorbate over a solid with w(R,z,t),P*. 8. Mass transfer of the adsorbate within the solid occurs by a diffusional process with flux proportional to the gradient in w . This assumption is open to question, since equilibrium partial pressure would appear to be a more logical driving force for diffusion than w but for low bed loadings should give reasonable results.
Downloaded by UNIV OF WINNIPEG on September 9, 2015 | http://pubs.acs.org Publication Date: April 1, 1968 | doi: 10.1021/i260026a020
T h e model equations describing gas concentration and bed loading changes are : E
ac + -vac - = ak, (P*- P) at A at
-
bw
D b
T h e coupling relation between gas and solid is the boundary condition.
(3) During a desorption cycle phenomena occur similar to those discussed for the adsorption cycle. I n addition to assumptions regarding homogeneity and linear transport laws, the special assumptions for a desorption model were: All inert gases are evacuated rapidly enough to make it possible to ignore their presence. Equations 2 and 3 describe phenomena in the solid and a t the gas-solid interfaces with possibly a different transfer coefficient kd.
Pressure drop between the evacuated and closed ends of the beds may be appreciable but can be calculated by an equation similar to the Blake-Kozeny equation for flow through packed columns. T h e particular relation used in this analysis for pressure drop calculations was m = -a(P)
aP -
ax
(4)
where m is the mass flow rate per bed cross section and a ( P ) is an experimentally determined coefficient that depends on bed pressure. T h e gas mass balance was then used in the modified form E
ap + am - = akd (P*- P) M A at ar
-
(5)
where p = cMA and k , is the transfer coefficient during desorption. A primary objective of the experimental program was to determine whether these model equations are sufficiently general to account for the most important factors governing the regenerative use of an adsorption bed. T h e most questionable assumptions made in the model are those associated with nonhomogeneous flow and the absence of mixing in the direc270
I & E C PROCESS D E S I G N A N D D E V E L O P M E N T
tion of flow. Although the experiments reported seem to verify these assumptions, there is still some question about extension to beds with a larger cross section or more complicated internal structure. Experimental
An experimental program was established in order to describe the equilibrium behavior of CO2 on 5A molecular sieve, discover which of the transport steps were most important, and determine values for the necessary transport coefficients. Gravimetric Apparatus. T h e equilibrium isotherms for COn on Linde 5A molecular sieve were determined using a McBain balance. T h e adsorbent was suspended on a quartz spring balance and its change in weight measured as the adsorbate gas was added incrementally. T h e weight change was recorded against system pressure. Constant temperature was provided through a glass jacket which controlled the temperature of the water circulated. Calibration of the spring revealed a linear relation between weight and spring extension with a constant of 3.9 mg. per mm. Spring extension was measured to +0.05 mm. with a cathetometer. T h e balance held about 1 gram of adsorbent. Pressure was measured with either a Televac thermocouple gage or a mercury manometer. T h e Televac gage was calibrated for low pressures (0 to 600 microns) with a McLeod gage. Vacuum was achieved with a Kinney forepump and mercury diffusion pump. Temperature was measured with a thermocouple located near the sample pan. Samples of molecular sieve were baked out a t GOO0 F. before the determination of each isotherm. T h e water jacket was first removed and the sample tube wound ivith heating tape. Simultaneous with the heating, the system was evacuated to 1 X 10-5 mm. of Hg. A small increment of gas was then added and the pressure history observed. When no changes in system pressure and or balance spring position nere seen, equilibrium was assumed to exist and a point was recorded. This procedure was repeated to the highest pressure desired. Dynamic Bed. ADSORPTIOS.In order to determine dynamic adsorption parameters, a tubular bed was constructed. Figure 1 shows the 10-inch copper tube with an inside diameter of 5 / 8 inch. T h e interior has copper fins 0.01 inch thick offset each inch down the tube, which were designed to improve the heat transfer and discourage channeling. T h e apparatus consisted of a vacuum system, a gas-supply system, an analysis system, and a sample system. T h e tube was filled with about 35 grams of molecular sieve. During adsorption nitrogen-carbon dioxide or oxygen-carbon dioxide mixtures were passed through the tube with the inlet and outlet CO2 concentration continuously monitored. Fischer-Porter flowmeters were used to measure the flow rates of COZ and inert gas. Carbon dioxide concentrations were determined with Beckman 15A infrared analyzers. System pressure was monitored with a Wallace and Tiernan gage. Water vapor content was measured with an AiResearch dew point measuring instrument. T h e adsorbent was loaded into the tube bed while the bed was vibrated. Heating tape and glass insulation were wound around the bed and the system was brought to regeneration temperature. T h e bed was simultaneously evacuated to 10 microns or less. When the system pressure remained a t 10 microns, with no applied vacuum, the heat was removed and the bed cooled to ambient temperature. A constant-temperature bath was placed around the bed while the proper flow rate and CO2 concentration were set through use of a bypass. When proper conditions existed, the bypass was closed and the bed opened to the test gas. DESORPTION.For desorption studies, the same bed was used as for adsorption. However, the gas inlet was capped. Pure carbon dioxide was admitted to the regenerated bed. T h e pressure and temperature a t equilibrium were recorded. During vacuum desorption, pressure was continuously measured at four locations in the bed. I n addition, the mass of COZ remaining a t several intervals was estimated by isolating the system and allowing isothermal pressure equilibrium. Pressures were measured with a four-station Magnevac
GASIVAPOR INLET
Downloaded by UNIV OF WINNIPEG on September 9, 2015 | http://pubs.acs.org Publication Date: April 1, 1968 | doi: 10.1021/i260026a020
Q
GMA 140 thermal conductivity gage calibrated Tvith a Texas Instrument quartz Bourdon-tube gage. Temperatures were recorded continuously on an Offner recorder. T h e adsorbent was initially regenerated as for adsorption. T h e bed was brought to test temperature with the constanttemperature bath. Pure CO1 was admitted through one of the unused pressure probes through a valve and the bed was brought to the desired pressure and allowed to equilibrate overnight. T h e vacuum valve was opened and continuous pressure and temperature measurements were taken. T h e valve was closed when the system pressure reached about 10 microns. During some runs, the vacuum was shut off a t certain intervals and the bed permitted to equilibrate. T h e equilibrium pressure obtained was used to estimate average bed loading from the equilibrium isotherms. About 1 hour was necessary. T o determine the constant cy in Equation 4, pure nitrogen \vas admitted to a regenerated bed a t low pressure while 10 microns were maintained a t the outlet. Pressures were measured through the bed a t three different flow rates. Measurements were made a t four points in the bed. Nitrogen has a very low equilibrium adsorption loading, so that after the first few minutes, all the nitrogen admitted passed through the bed. Materials. T h e molecular sieve was 1/16-inch Linde 5A pellets. T h e carbon dioxide was supplied by the Liquid Carbonic Co. Analysis of the gas showed 99.9% COS, 32 p,p.m. of 0 9 , and 1.1 p.p.m. of H 2 0 . T h e in-line dew pointer indicated dew points below -80" F. a t all times. Results
Equilibrium Isotherms. Equilibrium isotherms were determined for Linde 5A molecular sieve pellets a t O o , 10.5', 25", and 50" C. All samples had been desorbed a t GOO0 F.
M
0,12
0.10 w
3
5"
0.08
w& 0 06 0
w 0
2
0.04
in
U
0.02
0 0.005 0 . 0 1
0.1
1 0 .o
1.0
50.0
SYSTEM PRESSURE, m n H g
Figure 2. Equilibrium isotherms of COz adsorption on Linde Type 5A molecular sieve '/ls-inch diameter pellets
arid mm. of Hg. The resultant isotherms are shown as Figure 2. T h e curves yield loadings about 0.02 gram per gram lower than those quoted by Linde (1959). Dynamic Studies. ADSORPTION.Sixteen breakthrough curves were determined for a variety of conditions (Table I). A sample curve is shown as Figure 3. The total adsorption capacity seemed about 10% below the equilibrium value. A series of tests was performed with other carrier gases to deterVOL. 7
NO. 2
APRIL
1968
271
Table 1.
Run
KO. 2 3 4 5 6 7 8 9
Carrier Gas N2 N2 N2 N2 Ng N2 N2 0 2
10
Downloaded by UNIV OF WINNIPEG on September 9, 2015 | http://pubs.acs.org Publication Date: April 1, 1968 | doi: 10.1021/i260026a020
16 17
Nz
Au. Inlet CO? Mass Velocity, Lb./Sq. Ft. In. 1.54 0.67 1.55 0.68 1.54 0.69 1.50 1.75 1.75
Au. Inlet
1.49 1.47
1 .04 1.01
COZ Partial Press., M m . Hg 7.15 7.13 7.12 7.03 7.18 7.14 7.13 7.19 7.25
Summary of Dynamic Carbon Dioxide Adsorption Time to Time to Initial 70070 Total Bed Bed BreakBreakPress., Tfmp., Length, Sample through, through, P.S.I.A. c. Inches Wt., G. Min. Mtn. 25 7.2 10.3 40 5.07 24.5 25 7.2 5.10 24.5 30.5 95 5 .OB 0 7.2 24.5 23.5 70 7.05 25 7.2 24.5 41.3 120 35.4 20 70 5 .OB 25 10.1 5 .OB 25 10.1 35.4 53 105 0 10.1 35.4 43.5 80 5 .OB 35.4 19 65 5.08 25 10.1 5 .OB 0 10.1 35.4 40.3 105
5.07 5.08
50 0
mine if this was due to coadsorption (Table I). The complete set of breakthrough curves is reported elsewhere (AiResearch Manufacturing Co., 1967). Carbon dioxide desorption runs were made to DESORPTION. determine the relative effects of applied vacuum, temperature, and intrapellet diffusion. The results are summarized in Table I1 and loadings are shown in Figure 4. Complete data are available from AiResearch. Runs 7, 8, 9, and 11 were halted a t times shown in Table I in order to estimate the loading. Two runs were made with the outlet pressure throttled to about 300 microns. The final loadings for these runs were approximately at equilibrium a t 300 microns, but showed desorption rates early in the run near those a t "full" vacuum (IO microns of Hg). One run made with pellets of '/g-inch diameter, twice the size of those used in all other runs, showed essentially the same desorption behavior. T h e bed temperature never varied more than about 1" F. during the run. Pressure histories a t three locations in the bed for one run are shown in Figures 5 and 6. Other runs were similar. The nitrogen flow data were found to fit Equation 4 with a = 0.0137 PO.*M
35.4 35.4
10.1 10.1
(6)
with P in mm. of Hg.
17 116
40 220
co 2 % of Adsorbed, Equilibrium G. CO?/G. co 2 Sieve Adsorbed 0.0529 90.9 0.0492 84.7 0.0997 97.3 0,0504 87.7 0.0525 89.7 0,0524 90.0 0.0986 96.2 0.0493 84.3 0.0926 89.9
0.0065 0.0394
100 .o 91 .O
Treatment of Data
Differential Heat of Adsorption. The equilibrium isotherms were used to estimate the differential heat of adsorption of carbon dioxide o n the 5A molecular sieve. T h e ClausiusClapeyron equation, d l n P - AH d(l/T)
( 7)
R
can be integrated over conditions such that AH is constant to yield ;
For the same equilibrium loading, the equilibrium pressures were found by crossplotting the data at T I = 90" F. and T2 = 100" F. (AiResearch Manufacturing Co., 1967). Several loadings from 0.5 to 5.0 pounds of COa per 100-pound bed were used (Figure 7). Dynamic Adsorption. T h e best values for k, a and Da were found by fitting the isothermal breakthrough curves with Equations 1, 2, and 3 written in finite difference form. Ten lengthwise nodes were used in a simple forward difference
100 90
80
80 0 L
2
W 0
70
I
a m
E w
0 W
g 00
2
60
N
50
8
z
W
40
40 P W
0 R U N 8 THROTTLED
I-z
30
u W
5
20
20
a
10
0
0
10
20
40 50 TIME, M N U T E S
30
60
70
80
Figure 3. Typical breakthrough curve of COZfrom Linde Type A molecular sieve a t
2 5 " C. 272
l & E C PROCESS D E S I G N A N D D E V E L O P M E N T
0
10
20
30
40
50
TIME ,MIN.
Figure 4. Vacuum desorption of COZfrom Linde Type 5A molecular sieve at 25" C. Full vacuum and '/lC-inch pellets unless otherwise noted
Table II. Vacuum Desorption of COz from Linde Type 5 A Molecular Sieve Pellets
Pellet Diameter, Inch
Test
KO. 1 2 3 4
Initial COa Loading, Lb. COz/Lb. Sieve ( M m . H g ) 0.062 ( 7 . 9 8 ) 0.625 ( 8 . 1 ) ' 0.625 ( 8 . 1 ) 0.065 (8.74)
'/16
'116 1/16 '116
b C
d 8a
'/16
b 9a
'/16
C
Downloaded by UNIV OF WINNIPEG on September 9, 2015 | http://pubs.acs.org Publication Date: April 1, 1968 | doi: 10.1021/i260026a020
d '/16
lla
'/8
C
d
I
-BED
TEMP
-INITIAL
I
LOADING
- 0.061
10'.
4
L B C O z / L B M.S.
- 0,007 LB/COz/LB - 0,008L B / C O z / L B
~
M.S. (EXPERIMENTAL) M.S
30 30 10 30
0.001 0.007 0.038 0.027
30 30 2 2
(0.030) (0.258) (3.48) (1.97)
25 8 9 13 15 2 28 2 8 9 13
0.007 ( 0 . 2 5 1 ) 0.0175 11.04'1 0.010 ( 0 . 3 9 9 j 0.0055 (0.188) 0,0035 (0,094) 0.001 (0.025)
25 25 25 25 25 50 25 25 25 25
-ii°F
LOADING
-FINAL
I
I
I
0.007 10.241) o.001 io 027j 0 . 0 0 4 (0.120) 0.0035 (0.085)
0.019 (1 , l 5 ) 0.0155 ( 0 . 8 4 ) 0.025 ( 1 . 7 4 )
0.0625 ( 8 . 1 5 ) 0.035 ( 3 . 0 4 ) 0.013 (0.64) 0.007 (0.258)
b
10
25
0.025 ( 1 . 7 4 ) 0.0625 18.15'1 0.0175 (I . 0 4 j 0.010(0.399) 0.0055 (0.188) 0.062 ( 8 . 0 )
b 10
25 50
0.0135 ( 0 . 6 9 ) 0.0625 ( 8 . 1 5 ) 0.066 (8.9) (Accidentally bled air into system) 0.027 ( 1 . 9 7 ) 0.019 ( 1 . 1 5 ) 0.0625 ( 8 . 1 7 )
5 6 7a
Final CO1 Loading, Desorption Lb. CO2ILb.Siece ( M m . H g ) Time, Min.
Desorption Temp,, a C.
0.035 ( 3 . 0 4 ) 0.013 ( 0 . 6 4 ) 0.007 (0.258) 0.0037 (0.109)
I
I
I
I
I
I
1
,
I
I
-
BED T E M P 1 2 2 ' F I N I T I A L LOADING - 0 . 0 6 2 5 L B C O Z / L B M.S. F I N A L LOADING - 0,001L B C C > / L B M.S. (EXPERIMENTAL) 0,002 L B / C C z / L B M.S. (PREDICTED)
-
(PREDICTED)
1 .o
1.0
m
I E
I 0 E
A
N 0 a V
0 0. 0
2 1 / 2 INCHES ABOVE SCREEk I
0.1
y
1 / 2 INCH ABOVE SCREEN-
0.1
1
\ 1
I \
"
I
1
I
!
I
\ ..'
I
1
X
PREDICTED
-t
.01 0
10
TIME, MINUTES
20
30
Figure 5. Desorbing molecular sieve pressure profile Run 1
\ x I
I
\
' X
1
I
.01
0
Figure 6.
lo
,I I
\
TlhlE, MlluUTES
20
30
Desorbing molecular sieve pressure profile Run 2
VOL. 7
NO. 2 A P R I L 1 9 6 8
273
T I M E , MINUTES
Downloaded by UNIV OF WINNIPEG on September 9, 2015 | http://pubs.acs.org Publication Date: April 1, 1968 | doi: 10.1021/i260026a020
Figure 8. Test data and predicted breakthrough curves of COZon Linde Type 5 A molecular sieve
numerical solution. Four radial nodes inside the pellets were used to approximate Equation 2. T h e surface resistance, k,a, was found to control. Its best value was found by a few iterations to be 0.315 lb. mole/cu. ft. hr. mm. Hg. Any value of Da below 2.5 feet per hour caused the calculated curve to be below the experimental near the end of the run. The value of 2.5 was found to produce negligible internal concentration variations compared to the pressure variation across the surface resistance. Figure 8 shows two sample runs a t different temperatures and flow rates compared with their calculated curves using these values of k,a and Da. These runs are representative of all the fits obtained a t 77' and 122' F. Those a t 32' F. did not fit as well, probably because of errors in the analytical representation of the equilibrium data a t this low temperature. Desorption. The experimental desorption runs were fitted in essentially the same manner as the adsorption runs. For desorption, however, the empirical bed pressure relation (Equation 4), rvith the value of cy = 0.603 Po.2,was necessary. T h e experimentally observed vacuum duct pressure history was used as the downstream boundary condition. The best value for kda was found to be the same as k,a, 0.315 lb. mole/cu. ft. hr. mm. Hg. As for adsorption, no effect was found for the internal diffusion. Thc process was best described by the surface resistance condition alone, with equilibrium essentially assumed within the adsorbent. The pressure histories in the bed for two runs a t different temperature are shown in Figures 5 and 6. T h e predicted final loadings and pressure histories agree well. T h e discrepancy observed in the first few minutes of desorption may be due to the forward difference technique used in solving the equations, as conditions are changing rapidly at the start of desorption. Conclusions
The equilibrium isotherms for C O Zon Linde 5A molecular sieves were obtained at four temperatures in the low pressure region. Mixed-gas adsorption and vacuum desorption rates were measured and successfully correlated with a simple masstransfer coefficient. Adsorption and desorption mass-transfer coefficients were essentially the same and independent of temperature and flow rate in the range of the tests. The fact that no internal mass transfer, such as pore diffusion, was necessary to fit the experimental data was anticipated from the desorption results with the larger pellets. Since effectively no change in the rate was observed, pore diffusion 274
l&EC PROCESS D E S I G N A N D DEVELOPMENT
Total pressure, 10.32 inches Hg Pco2inlet, mm. Hg Bed temp., ' F. Gas flow, Ib./sq. ft./min. Prediction
- - -Run 2 7.05
77 1.54
-Run 1 3 7.21 122 0.67
0
should not have been a controlling factor. Thus, Assumption 8 becomes a moot point, neither internal diffusional process being observed. O n the other hand, the fact that the surface resistance coefficients were the same for both adsorption and desorption was a surprise. These two processes occur under completely different circumstances. During adsorption, the carbon dioxide must diffuse through a film resistance to the adsorbent surface, then either adsorb and migrate along the pellet to an area of low COZ concentration or else diffuse in the gas phase through the pellet interior until the concentration driving force is great enough to cause adsorption. During desorption, however, no carrier gas is present. Therefore, there can be no film diffusional resistance. A possible explanation would stem from the porous nature of the pellets, which are composed of small granules of the silicate held together on an inert matrix. The controlling mechanism could be the transfer onto or off the internal surfaces, the passages within the matrix being wide enough to permit rapid diffusion to and from the bulk stream. The '/*-inch and '/le-inch pellets have essentially the same surface area per pound, if it is the surface area of the silicate that must be considered. This observation agrees with the results reported by Geser and Canjar (1962) for adsorption of hydrocarbons on activated carbon rather than the results reported by Masamune and Smith (1964) for adsorption of IL'2 on Vycor glass particles. T h e flow rates investigated all were in the low Reynolds number (Re < 10) range. Good fits with all the data were obtained using a single value for the coefficient k,a or kda of 0.31 5 lb. mole/cu. ft. hr. mm. Hg. Nomenclature
solid-gas specific contact area, sq. ft./cu. ft. bed cross-sectional area, sq. ft. adsorbate gas phase concentration, lb. moles/cu. ft. adsorbate solid phase diffusivity, sq. ft./hr. heat of adsorption of adsorbate, B.t.u./lb. mole adsorption gas-phase mass-transfer coefficient, lb. moleihr. sq. ft. mm. Hg kd = desorption gas-phase mass-transfer coefficient, Ib. mole/ hr. sq. ft. mm. Hg m = gas mass velocity, lb./sq. ft. hr. M = molecular Fveight, 1b.ilb. mole M A = adsorbate molecular weight, lb./lb. mole P = pressure, Inm. Hg
a
= = c = D = AH = k, =
A
P* Y
R R t T V w
equilibrium pressure of adsorbate in adsorbed phase radial distance in an equivalent sphere, ft. gas constant: cu. it. mm. Hg/lb. mole " R. radius of an equivalent sphere of adsorbent, ft. time, hr. = temperature, " R. = volume rate of gas flow, cu. ft./hr. = adsorbate loading on adsorbent, lb./lb.
= = = = =
GREEKLETTERS a: E
= =
p ps
= =
gas density, lb./cu. ft. bulk density of adsorbent bed, lb./cu. ft.
Literature Cited
AiResearch Manufacturing Co., Rept. 67-1751 (March 1967). "Chemical Engineer's Handbook," J. H. Perry, Ed., 4th ed., Section 16, McGraw-Hill, New York, 1963. Geser, J. J., Canjar, L. N., A.I.CI1.E. J . 8, 494 (1962). Linde Co., Linde Rept. 9691-E (1959). Masamune, S., Smith, J. M., A.I.Ch.E. J . 10, 247 (1964).
empirical constant in flow equation, lb. ft./hr. mm. H g void fraction on adsorbent bed, cu. ft./cu. ft.
RECEIVED for review July 3 , 1967 ACCEPTED November 14, 1967
HYDROTORTING OIL SHALE WARREN G. SCHLINGER AND DALE R . JESSE Downloaded by UNIV OF WINNIPEG on September 9, 2015 | http://pubs.acs.org Publication Date: April 1, 1968 | doi: 10.1021/i260026a020
Montebello Research Laboratory, Texaco. Inc., Montebello, Calif.
Experimental results using hydrogen as a retorting fluid for recovery of shale oil from Rocky Mountain Using hydrogen at pressures between 1000 and 2000 p.s.i.g. and a t temperatures between 900" and 1000" F., shale oil yields ranging up to 1 1 5 vol. % o f Fischer assay are obtained. Properties of both the shale oil and spent shale resulting from the hydrogen treatment or hydrotorting are discussed.
oil shales are presented.
Y THE
past year, a number of U. S. corporations have in-
I.creased the research effort devoted to the development of
economic methods for recovering and refining oil produced from oil shales. The potential oil reserves contained in domestic oil shale deposits are known to be very substantial and form a large fraction of the known energy reserves of the world. Estimates as to the potential shale oil content of the formations in the Colorado, TVyoming, and Utah areas vary. However, Childs (1965) stated that 600 billion barrels of oil are available from deposits containing more than 25 gallons of oil per ton of shale and that an additional 1.4 trillion barrels are contained in shales assaying from 10 to 25 gallons per ton. TVhen these figures are compared with the annual 'L'nited States consumption of liquid hydrocarbons (4.5 billion barrels in 1965) (Oil and Gas Journal, 19651, it is apparent that oil shale a t the present consumption rate could supply the domestic liquid hydrocarbon demand for 440 years. The proved United States oil reserves are nearly 34.4 billion barrels, whereas the Free \Vorld reserves amount to 319 billion barrels (Enright, 1965). I n the past 100 years, several commercial ventures involving the processing of oil shale have been undertaken. These operations in all cases were attractive only under very special economic environments and invariably consisted of small-scale operations compared to today's refining operations. T h e oil is recovered from the shale in three major phases: mining and transporting the crude oil shale to the primary processing site; extracting the shale oil from the large fraction of ash and foreign material in the raw shale; and refining the crude shale oil to marketable products. Each step is a major operation and contributes significantly to the cost of obtaining finished products from oil shale. The following discussions are devoted to the second of these stages, extraction or retorting. I n nearly all methods for extracting the shale oil, heat is supplied by combustion of a
portion of the residual organic matter to break doxvn thermally and to distill the organic matter contained in the natural shale. A number of retorts have been proposed and demonstrated employing a wide range of flow conditions. However, almost without exception, heated air has been employed to burn the residual organic material left on the shale after a major portion of the original organic material called kerogen has been decomposed and removed by the hot combustion gases. The quantity of recoverable oil contained in oil shales is generally measured by the Fischer assay (Stanfield and Frost, 1949). I n this laboratory test, a 100-gram sample of shale is heated in a closed aluminum retort a t a controlled rate until a final temperature of 932" F. is reached. The quantity of Lvater and hydrocarbon distilling off is measured and is reported in gallons per ton. Large quantities of shale deposits containing more than 25 gallons of oil per ton of shale are known to exist. Certain sedimentary layers in these deposits may contain up to 50 or 60 gallons per ton. The Fischer assay does not convert the entire content of organic material to recoverable oil in the laboratory evaluation procedure. The spent shale from the assay typically contains from 3 to 1lY0 organic and free carbon, depending upon the oil shale assay (Stanfield et al., 1951). This carbon represents 6 to 25 gallons per ton of material that could be recovered as liquid hydrocarbon under more suitable retorting conditions. Tests are described in which the oil shale was almost entirely stripped of its organic material by use of hot hydrogen. In these tests hydrogen a t pressures ranging from 1000 to 2000 p.s.i.g. was employed in a recycle system. By controlling the temperature of the recycle gas. complete and efficient retorting was realized. All but a few tenths of a per cent of the organic carbon in the shale can be converted to distillable hydrocarbons. Yields of oil ranging from 105 to 115% of Fischer assay can be repeatedly obtained. A typical yield VOL. 7
NO. 2
APRIL 1 9 6 8
275
