Liquid Phase Adsorption Equilibria and Kinetics SAM EAGLE AND JOHN W. SCOTT, California Research Corporation, Richmond, Calif. Simple methods are presented for measuring and expressing the equilibria and rate y diagrams, processes involved in some liquid phase adsorption systems. Isotherms, x selective adsorptive capacities, diffusion coefficients, and fracture resistance of adsorbents are reported. Use is made of toluene selective capacity, diffusivity, and fracture resistance, in comparing a number of silica, silica-alumina, alumina, and charcoal adsorbents. Toluene-iso-octane is recommended as a useful binary system for evaluating and comparing adsorbent materials. The effect of temperature on preparation of adsorbents, on adsorption equilibria, and on kinetics is indicated briefly.
-
P
ROBLEMS in design and operation of liquid phase adsorption columns have two broad aspects, the adsorption equilibria and the rate processes involved in attaining these equilibria. This paper presents experimental data, laboratory procedures, and simple correlation methods that have proved useful for expressing these fundamental concepts in pilot plant studies, for designing processes, for evaluating commercial applications of refining by adsorption, and for selecting favorable adsorbents for specific separations. Adsorption fractionation over silica gel, alumina, and activated carbon has been used widely in recent years to separate mixtures of hydrocarbons into chemical types. Hundreds of new uses and modifications of the chromatographic technique have been described, and extensive literature in the field has been cited in recent publications (If , IS, 21, 22). Commercial applications of liquid phase adsorption techniques have been made in the separation of vitamins and antibiotics (21, 24), and a new commercial process for separation of aromatics, olefins, and.paraffins from petroleum has been recently announced (8). The primary attraction of adsorption in the petroleum industry has been associated with the high selectivity of silica gel or activated carbon for making practically quantitative separations of aromatics and olefins from paraffins in feed stocks of wide boiling range (8, 13, 15, 16). Process research and development work on such applications included the measurement of the properties of various adsorbent materials and a study of their behavior in typical liquid systems. ADSORPTION EQUILIBRIA Physical adsorption separations depend upon the fact that some compounds are more strongly attracted than others to a given adsorbent surface. Thus near an adsorbent surface a binary liquid phase undergoes a change in composition, one component, B, being more strongly attracted than the other, A. For example, the introduction of an adsorbent such as silica gel or activated alumina into a mixture of toluene and iso-octane results in a decrease in the concentration of toluene in the solution. The material adsorbed is a mixture of toluene and iso-octane in which the toluene concentration is higher than it was in the original solution. Toluene is, therefore, said to be preferentially adsorbed from the toluene-iso-octane solution. Similarly, it is found that in the treatment of a variety of liquid mixtures with solid adsorbents these porous materials show an adsorptive preference for one or more components of the mixture. It is well known that physical adsorption is a reversible equilibrium process. When a feed mixture is brought into contact with a solid adsorbent, this type of equilibrium is realized after the lapse of a definite period of time and, upon such realization,
no further change occurs in the relative concentrations of the components of the adsorbed and nonadsorbed fractions of the feed mixture. The adsorption equilibria may be simply expressed in terms of what has been designated as the selective adsorptive capacity of the adsorbent. For a binary system this is defined as the volume of B selectively adsorbed per unit weight of adsorbent. This property is determined experimentally as follows:
A measured volume of a binary mixture whose composition is known is added to a known weight of adsorbent in a flask. After equilibrium is established, a sample of the nonadsorbed liquid is analyzed. Selective adsorptive capacity is then calculated by the equation:
where a = selective adsorptive capacity v = volume of binary mixture m = weight of adsorbent zo = volume fraction of component B in the original mixture 5 = volume fraction of component B in the equilibrium nonadsorbed liquid The curve of a versus z a t a given temperature is the adsorption isotherm, This form of the isotherm is somewhat similar to other expressions that have been used in the literature to describe binary liquid phase adsorption systems (2, ld,lC, 17-19). Some isotherms are shown in Figure 1 for mixtures of toluene and iso-octane on carbon (coconut charcoal), on a commercial silica gel a t two temperatures, and on a commercial silica-alumina cracking catalyst. Similar isotherms are shown in Figure 2 for mixtures of toluene with various oxygenated compounds on silica gel. The more strongly adsorbed oxygenated compounds find use as desorbents for displacing aromatic and olefin adsorbates from silica-type adsorbents. The approach to a plateau in the selective adsorptive capacity for many binary, systems a t values of 5 greater than about 0.3 affords a convenient, quick method for estimating the comparative utility of various porous materials as adsorbents or of various liquids as desorbents. Thus a toluene-iso-octane mixture (20= 0.3) has been used in catalyst studies tg define an “aromatic adsorptive index” which is reported to vary directly with nitrogen surface areas and with measurements of catalyst activity ( d o ) . The simplicity of preparing equivolume binary solutions and the closer approach a t this composition to the maximum selective capacity in many systems resulted in tha choice of 20at 0.5 as a binary composition for quick comparative testing. The relative adsorptions of several liquids on silica gel are compared to toluene in Table I. 128’1
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
1288
Table 1. Selective Absorptive Capacities of Silica Gel for Different Liquids Selective adsorptive capacity, n
=
m l - r
Room temperature z = volume fraction 13 20 = 0.50 u = mi.of inixture m = g i a n ~ sof silica gel 1 C0m;)oneIlt A Iso-octane &'lonoch~oFobenzene To1.uene
Component 13 Toluene Toluene Anisole Benzyl Cellosolw Dichloroethyl e t h e r Diisopropyl sulfide Cyclohexanol Diisopropyl ketone Diethyl Cellosolve Dioxane Isobutyl acetate Isopropyl ether Ethyl acetate Methvl ethrl ketone Amyl-acetati Furfural Ethyl alcohol Mor holine Metfyl Cello~o11-e
a,
V/m
M,/k 1.25 1.25 1.25 1.25 1.5 1.25 1. 5 1.25 1.25 1.25 1.25 1.25 1.25 1.25 1.25 1.5
1.63 1.5 1.26
5
0 395
0.460 0.46 0.41 0.422 0.396 0.415 0.40 0.39 0.36 0.383 0.37 0.38 0.373 0.375 0.393 0.398 0.382 0.345
3II./G. 0.24 0.095 0.11 0.17 0.20 0.21
0.22 0.23 0.23 0.24
0.24 0.24 0.25 0.25 0.25 0.26 0.28 0.29 0.30
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9
d
a
m.
o SILICAGEL I c SILICA GEL No. I 1 S I W - W I W No. 4
X , WWME FRACTION TOLUENE IN EQUILIBRIUM LIQUID
d
Figure 1. Adsorption Isotherm for Toluene-Iso-octane Mixtures on Typical Adsorbents
Vol. 42, No. 7
Similarly, an equivolume mixture of toluene and iso-octane has been used for comparing the selectivity of various porous materials in separating aromatics. For such comparative purposes, the "toluene selective capacity" is defined as the value of the selective adsorpt'ive capacity obtained with a toluene-isooctane mixture in which 20 is 0.5, and the voluine of mixturc: (ml.) per gram of adsorbent is about 1.25. Properties are conipared in Table I1 for several silica gels, silica-alumina catalysis, activated aluminas, and adsorbent charcoals. The parallel botween toluene selective capacity arid butane surface area (16) is evident from comparison or the fifth and sixth columns of Table 11. The grapliicd COI tion of tolucnc selective capacity \1-ith butane surface area is a h o m in Figure 3. The straight line through the origin v i t h a slop(>of 1/19jo indivntes the cori,elation between the toluene selective capncity (expressed as cubic ccntimeters of toluene per gram of idsorbent) and the butane surface area (in square meters per gram) that would be obtained if the surface of the adsorbent were completely covered with a motlolayer of toluene. Assuming tlic adsorbed toluene to havc the density of liquid t801ueneat room t,eniperatmure,a value of 34.4 square Angstrom units WLA estimated bv the method of Urunaucr ( 3 ) to be the area occupicci by an adsorbed toluene molecule. The experimental data in Figure :3 are within 55 t o 10070 of the values predicted by the corrc.l:ilioii line for ull of the adsorbcnts exaniined with the esceptioll of the cliarc.oals. The c1i:ircoals are presumed io deviilte Irom t,his correlation because of the knon-n extremely small pore size of these mat,erials, which restricts the area available to the toluene molcculc compared to that available t o butane. The toluene selective mpLcity is sensitive to changes in the idsorberit properties arising lrom exposure to high temperatures or ot,her process conctitiom that produce a decline in adsorbent actilrity. The effect of rsposing several silica-type adsorbents t o various temperaiures for 2-hour periods is shown in Figure 4. The highest area silica-type adsorbents are most susceptible to decline in adsorptive capacity when heated above 500" F. In order t o minimize change in adsorbent properties in testing, butane surface areas wore riormally measured ut 32" F. after oulgassing the adsorbent at mm. of mercury for about 2 hours a t 850" F., while toluene sclcctive capacities usudly were det,ermined on the adsorbent d t o r drying for 4 hours a t 400" F. Typi-
Table 11. Properties of Adsorbent Materials
Adsorbent Silica gel 1 IA 2 61 88
Silica-alununa
3
4 5 6 Silica-magnesia 7 Silica-alumina 8 Silica-alumina 9 Alumina 10
11
12 13 14 15 16
17
Carbon 20 21 22 23
24 25 28
29
Toluene Selective Capacity &*II./Gr&
Butane Surface Area, Sq. M./Gram
Sveeific
Pori
Avrraa~ ~~
Bulk
Volume, bIl./Gra~n
Density, Gram/Ul.
Pore-
Diameter, A.
T0111eno . - -. ..Diffusivitv Sq. Cm./Seh. ~
x
Form
Origin
Graniiles Granules Granules Beade Beads
Davison Dayison Davison Laboratory Laboratory
28-200 mesh 28-200 mesh 28-200 mesh 10-14 mesh 10-14 mesh
0.264 0.142 0.239 0.23(j 0.248
485 524 474 432
...
0.387 0.389 0.338 0.325 0.38
0.67 0.63 0.69 0.76 0.78
Beads Beads Beads Microspheres Miorospheres Microspheres Beads
Socony Socons Socony LOP
3.2% AlnOs 9.0% 4hOa 7.8% MzOa 1.5% A1203 10% icIg Aerocat 9.2% AI
0.211 0.12 0.006 0.198 0.173 0.154 0.086
417 298 178 415 400 418 162
0.323 0.41 0.36G 0.440
.. ...
0.81 0.70 0.69 0.74 0.76 0.72 0.70
, . .
11
Granuies Pellets
Alorco Alor~o
0.127
2 87
...
0.91
...
7
0.117 0.070 0.066 0.030 0.093 0.078 0.039
273 155 138 56 194
0.97 0.96 0.96 0.89
58
5 6
0.28 0.296 0.275 0.280 0.266 0.252 0.214 0.223
UOP
Ani. Cyanti n x d socon?
Identification
Pellets Pellets Pellets Granules Pellets Pellets
Alorco Alorco Harshaw Alorco Alorco I-Iarshair
R2200 4-8 rnesli R2200-3/15 X 3/16 inch A */6 X I/g inch -4 3/16 X 3/15 inch AI-12 H-40 H-40 AI-4
Granules Granules Granules Granules Granules Granules Granules Granules
Columbia Columbia Columbia Columbia Columbia Columbia Pitts. C . and C . Pitts. C. and C.
SW8 4SW 48CW 8-14G 4SXA 4SXW GW-30 X 100 R-30 X 100
184
51 .
I
.
774 1027 798
...
592 837
I
.
.
0.32 .
I
.
0.23 0.48
... ...
,..
29
... ... 27
... 29 53
... 37 44
...
... iko . . I
...
...
...
...
0.48 0.53 0.52 0.50
0.63 0.50 0.7G
0.50 0.55 0.45
0.66
0.64
...
0.65
...
0.50
... '
22 18
... ...
... . . I
100
..
.. o:b 0.8
0.9
5 10
..
.. ..
9 10
..
10 ' 4'
2 3 5 4 ,.
..
INDUSTRIAL AND ENGINEERING CHEMISTRY
July 1950
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1289
ing component partition between phases in distillation and absorption design. The value to be assigned to the specific pore volume, V p ,in Equation 3 may be higher than measured by butane adsorption as shown in Table 11, because strongly adsorbed liquids such as toluene or water usually give 5 t o 10% higher values for the apparent specific pore volume than are obtained with butane or iso-octane as the measuring liqu?d. Brunauer discusses these differences and several methods of measuring pore volumes ( 4 ) . For consistency the specific pore volumes used in conjunction with Equation 3 were determined with toluene or adsorbate as the me,asuring liquid. T i t h the same measuring liquid, the methods of vapor phase saturation, adsorbent densities, and liquid saturation followed by removal of void liquid gave concordant values. In any case, a small variation in the assigned value of V , in EquaFigure 2. Adsorption Isotherms of Desorbent-Toluene Equilibria tion has a effect On the shape O f the 2 - y diagram for process design purposes. Measured a t r o o m temperature on silica gel 1 Typical z - y diagrams for binary systems involving several hydrocarbon mixtures are shown in Figure. 6 for a commercial silica gel estimated t o have a specific pore cal butane isotherms for three ctdsorbents are shown in Figure 5 ; volume of 0.387 cc. per gram of dry adsorbent. the effect of preliminary treatment on the physical properties of The effects of different adsorbents and temperatures on the 6- t o 8-mesh adsorbent beads is evident from Table 111. shape of the x - y diagram are illustrated for the toluene-isooctane system in Figure 7 and for an assumed kerosene-aromatic paraffin binary in Figure 8. Table 111. Effect of Preliminary Treatment Experimental measurements of the equilibria betmeell aroHut.ane matic and nonaromatic components of,kerosene distillate were Adsorbent Silica gel 61 (6-8-mesh heads) Silica desiccant 3 (6-8-mesh beads) Silica-alumina 4 (6-8-mesh beads)
PretreatmentTemp., Time, Sq. ' F. hours h'l./G. 474 400 4 452 850 0.6 900 2 448 400 4 402 880 0.5 417 900 2 409 400 4 307 850 0.5 298 900 2 307
2:::::
"p",",",.
Vol., h'II./G. 0.328 0.311 0.299 0.325 0.309 0.300 0.416 0,398 0.397
Diam., Capacity, A. Ml./G. 27.4 0.236 27.5 26.8 0:200 32.3 0,210 29.6 29.4 o:iii 54.3 0.120 53.5 51.7 o:iio
The specific pore volume, V,, was Calculated from the volume The surface area, 8, was calculated by the Brunauer, Emmett, and Teller method (6). The average pore diameters were calculated from the measured pore volumes and surface areas, assuming uniform cylindrical pores (7,9). of gas adsorbed a t saturation pressure.
Pore diameter = 4V,/X
9
$.M
(2)
For some process design considerations the equilibrium curve is more useful in the form of an z - y diagram than as the simple isotherm of Equation 1. In these calculations z is again the volume fraction of B in the interstitial liquid, and y is the volume fraction of B in the adsorbent pore space in equilibrium with the interstitial liquid. The volume of B per unit weight of adsirbent in the pore space is expressed by
a
z,M
Figure 3.
Relation of Toluene Selective Capacity to Butane Surface Area
+ (V, - a h
where a is the selective adsorptive capacity and V; is the specific pore volume available to the adsorbate. Hence, the volume fraction of 13 in the pore space is: 'y =
a
+ (V, - a b VP
(3)
The curve obtained by plotting y against z is analogous bo the z - y diagram used in represent-
PRETREAT E M P E R AT URE ,* f.
Figure 4.
Heat Treating of Adsorbents
2 hours a t pretreat temperature
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
1290
z
Vol. 42, No. 7
80
a e E
&!m
8 40 Y)
az X,VOLUME
Oo
I 05
06
IO
09
08
07
RELATIVE PRESSURE (P/Po)
04
OB
IO
00
FRACTION E IN EQUILIBRIUM LIQUID
Figure 6. Adsorption Equilibria on Silica Gel 1
*
Figure 5. Butane Isotherms a t 32" F. on Three SilicaType Adsorbents Outgassed
0.5 hour at 850' F. below 10-2
mrn. H g
OSILICA GEL e S I L I C A GEL OCUARCOAL A SILICA-ALUMINA A SILICA-ALUMINA
NO I NO I
0387 0 30
NO20
NO 4
066 040
NO 4
03s
LIQUID I N PORES
h-
75
'LOO 71 75 '210
CORRLCTLD
X,VOLUME FRACTION TOLUENE I N LIQUID
Figure 7. Toluene-Iso-octane Equilibria o n Four Adsorbents accomplished by using suitable blends of the aromatic and paraffin-naphthene fractions obtained from a silica gel fractionation of kerosene distillate. Volume fraction of aromatics in the kerosene boiling range liquids was determined by the acid absorption method (A.S.T.M. D 875-46T). Allowance for change in the density of the pore liquid with temperature was made in estimating x - y diagrams a t higher temperatures. The close approach of the kerosene system to the toluene z - y diagram on silica gel is evidence that such multicomponent saturated petroleum fractions may be treated as simple binary systems in the design of adsorption columns for separating aromatic from nonaromatic constituents. Linear correlations of the equilibrium data usually were obtained by plotting log y / ( l - y) versus log s/(l - z)as is shown in Figure 9. In the design and operation of adsorption units, graphs relating the feed-to-adsorbent ratio and feed composition are useful. The curves shown in Figure 10 have been calculated directly from the measured adsorption isotherm and are typical of a fresh commercial silica gel. Feed-to-gel ratios obtained from such graphs can be corrected by an adsoibent efficiency factor to compensate for any decline in adsorbent activity observed after prolonged use, ADSORPTION KINETICS At very low feed rates knowledge of the adsorption equilibrium may be sufficient t o predict operation of adsorption columns. At higher feed rates the rate of reaching adsorption equilibrium becomes important. Knowledge of the rate processes is required in predicting what separations can be effected when mass transfer between the liquid and adsorbed phase is limiting. T h e n a feed mixture is brought into contact with a solid adsorbent, a finite time must elapse before equilibrium is realized.
Figure 8. Kerosene Distillate Equilibria on Two Adsorbents
During this time interval the volunie fraction of component B in a binary liquid decreases from zo to se. If a sample of the interstitial liquid is removed before equilibrium is established, the volume fraction of B will be a t soine composition between zoand ze. Designating the time dependent composition as xt and the equilibrium value as z., the fractional approach to equilibrium, E 9is expressed by the equation
(4) The variation of E with time, t , is usually fitted closely by Wicke's ( 5 ) solution of the diffusion equation for radial diffusion in spheres:
L
1
-1
with 7 9
k = --D R2
where k is the ordinary rate constant, R is the radius of the sphere, and D is the diffusion coefficient which appears in the classical diffusion equation
(7)
A graphical evaluation of Equation 5 is shown in Figure 11, using the numerical values tabulated by Geddes (IO),who applied the same equation to radial diffusion in gas bubbles. Diffusional resistance in the external liquid phase is assumed to be
0 3ILICA GLL No
I
20
'0
lOn TOLULNL IN L I W O ION TOLULHt IN M L S 0 02
aoi
1291
INDUSTRIAL AND ENGINEERING CHEMISTRY
July 1950
1 , 1 / 1 1
aoz
0.05
0.10
0.20
a50
IO
I 2.0
I
, 1 , , , ' 1 M loo
zoo
X
40
€4
80
100
VOLUME PERCENT B IN FEED
Figure 10. Feed-to-Gel Ratios for Separations on Fresh Silica Gel
I
I-x
Figure 9. Toluene-Iso-octane Equilibria on Four Adsorbents at 75" F. negligible as compared to that encountered in the adsorbent pores when this equation is applied to adsorption. , The rate constant, k, in a liquid phase adsorption process may be expected t o be a function of fluid viscosity, temperature, adsorbent pore size, and particle size and shape. If the adsorbent particles are actually spheres of uniform radius, the particle size dependence of the rate constant can be removed by substituting the sphere radius for R in Equation 6 and calculating the diffusion coefficient, D. For a collection of irregularly shaped particles the value of D can be approached by replacing R by 3 V / A where V is the average volume per particle and A the average external surface area per particle. Comparison of k and D values from standardized experiments then makes it possible to evaluate differences in the diffusivity of various adsorbents for different liquids a t a given temperature. The diffusivities of a number of adsorbents have been determined using the following experimental procedure.
A sample of adsorbent was saturated with iso-octane and centrifuged to remove the excess liquid. About equal volumes of the iso-octane-wetted el and of toluene were then mixed in glassstoppered flasks and t f e time of mixing was taken as zero. The toluene displaced iso-octane from the adsorbent pores, resultin in a gradual decrease in the toluene concentration in the extern3 liquid. Small samples of the li uid were removed at regular time intervals and the decrease in t8uene concentration was followed by measuring the refractive index. Both composition and time were recorded until a constant value of the refractive index indicated equilibrium between the adsorbed and external liquid. The fractional approach to equilibrium was then calculated t o give a value of E for each t following initial mixing of the toluene and wet adsorbent. Figure 12 shows the typical rate curves obtained by plotting E against t for silica gels and a silica-alumina cracking catalyst. Values of kt were obtained from Figure 11 corresponding t o the measured values of E. The values of kt were then plotted against time as shown in Figure 13. The slope
Table IV.
Rates of Adsorption of Toluene in Iao-octane Filled Adsorbent (Average liquid temperature 7 7 O to SOa F.)
Adsorbent
(S heroidal
scads)
Silica gel 6 1
Mesh Range (Tyler)
14-20 10-14 8-10 6-8
Silica-alumina 4
8-10 6-8
R2/T2,
Sq. Cm. X 104 2.53 6.47 12.3 18.3 12.3 21.2
IC, (MinJ-1
0.11 0,053 0.029 0.022 0.27 0.117
D
Sq. Cd./Sec.
x
108 0.47 0.57 0.57 0.67 5.5 4.2
Figure 11.
Graphical Evaluation of Equation 5
of the straight line was then taken to be the rate constant k (espressed in reciprocal minutes). If the particles are not uniform spheres, the plot of kt against t may be expected t o have a slight curvature. I n cases where curvature did occur, the initial ,310 es were used in estimating the rate constants. Diffusion coegcients, D, were calculated from the adsorbent bead radii using Equation 6. Fair agreement for values of D calculated from k values with four mesh ranges of fairly uniform silica gel beads from the same gel preparation is shown in Table IV. Likewise, comparison is shown for a silica alumina bead catalyst in which the toluene-iso-octane diffusion proceeds a t a rate about ten times that of the silica gel. The larger value of D for the 6- to %mesh silica gel beads may b e attributed to the presence or formation of small cracks in the larger beads resulting from stresses set up during the original drying or during the adsorption process itself. In this connection it was established that wetting of silica gel beads actually results in fracture of some of the beads. Likewise, repeated change of environment from iso-octane to toluene results in progressive cracking and fracture of the adsorbent. This adsorptive fracture effect increases a t larger bead sizes but becomes negligible for very small particles. Fracture of the beads is greater in more strongly adsorbed liquids. Thus cracking of adsorbent increases greatly in going from iso-octane to toluene to oxygenated compounds and water. Some rate measurements were made with iso-octane displacing toluene from silica gel beads initially wet with toluene. The rate constants were about 50% greater in these experiments with initial wetting of the adsorbent by toluene than with those with initial wetting by iso-octane. This higher observed diffusiv-
1292
Vol. 42, No. 7
INDUSTRIAL AND ENGINEERING CHEMISTRY
-
A
SILICA-ALUMINA
NO4 6 - 8 BLAOS
0 SILICA GEL W.61 0
01 0
I
10
10-14 W A D S 6-8 W A D I
SILICA CCL N O S 1
50
30
40
50
60
70
I
80
t, MINUTES Figure 12. Rates of Adsorption Toluene displacing iso-octane at SOo F.
t MINUTES Figure 13.
Rates of Adsorption
Toluene displacing iso-octane at 80. P.
ity n-as attributed to initial cracking of the adsorbent beads which decreases the effective particle radius, rather than to a faster rate process for isouctane displacement of toluene in t,he adsorbent pores. The edect of changing the voluinc i,atio of toluene to iso-octancwet gel from 0.5 to 2.0 did not produ L significant change in the measured diffusivity. Likewise, stirring of the reaction mixtur during the process of an experiment did not appreciably affec the rate constants. In vien- of, these results, the simple esper mental procedure was adopted of using about equal volumes of wet adsorbent and displacing liquid with agitation only for initial iiiising arid immediately bcfore ~rithdriirvalof a liquid sampltx ior analysis. The temperaturc rise in the liquid from heat of adsorption \vas small, not esceeding 5' to G o F. during the fast rate experiments. I t is apparent that t h c local variations in temperature within the actual adsorbent pui'tts may be still highci,, so that, the nicasured rate proecss in tlic ntlsorbent may be chnracteristic of a higher temperature t h n n t h e apparent SO" F. 1 3 ~ onuse of the unrwtaiiiiy in thiy 1101'(' tc~nipelature,the test pro-
cedure w t i s standardized for measurcnients a t room tcmperaturcl wit,hout. attempting t o attain precision temperature control 11)thermostat,ing. ' l\S?rh recognit,ion of these limitations on the esperirnerit:d procedure, the measured diffusivities have been used to cvaluai.ct the difierence in the rate for different adsorbents (last. column of Table 11),for di cosity liquids, and t o a limited degree for a higher temperature. Thus Table V shoxs diffusivitics varying over ten thousandfoltl for different liquid systems with difierent, adsorbenis. The diffusion rate for kerosene wasv increased 17-fold in silica gel m d 10-fold in silica-alumina 4 ;it 210" F. over that obs il at room temperature. The very loiv diffusivit,ies observed luhricuting oil stoclts show bhe irnporlance of viscosity- and iiiolecular size 011 the rate process. The, diffusion coefficients for the liquid-adsorbent systems of Table, 1-m:ij. l ~ rc ~ ) n i p a r t ~n iIl l i t lit: c:ocfficicntsestimated for ordinal.!liquid di'ri'usion ,by the metho d s of W i l k e ( 2 3 ) anti hrnoltl ( 1 ) . Thus for countcrTable V. Rates of Adsorption in Adsorbent Beads diffusion or toluene and isoKerosene Kerosene Lubricating Oil octane in the bulk liquid :if I,i ~ ' Adsorbent Liquid Liquid r. second were estimated for thct 0J Iso-octane Silica gel cil Toluene 80 liquid diffusivity in keroscno. Kerosene Ipo-octane 0.2 80 Raffinate The sma,ller coefficients CIIIio-octanc 0 , 00:3 Lub. oil 80 Raffinate c o u n t e r e d f o r diffusion ill Kerosene Kerosene 0.02 80 porous niaterials may arisc: Aromatics Raffinate Iso-octane Iierovenc 80 0.02 from the following factors: Aromatics ~
~
O
43 184
Bilira gel 88
G6
20 27
Silica-aliiiiiina 4
Lilt). oil Raffinate
Lub. oil Aromatics
203
Iso-oct.ane Kerosene Raffinate IGroPene Raffinate
Toluene Kerosene Aromatics Kerosene Aromatics
80 80
Iso-octane Iso-octane
Toluene Kerosene Raffinate Kerosene Aromatics Iso-octane
80
1.8
80
0.5
Kerosene Aromatics Kerosene Aromatics Lub. oil Aromatics
80
0.9
205
9.0
205
0.02
28
lo-octane
26
Iierosene kroniatic's Kerosene Raffinate Kerosene Raffinate Lub. oil Raffinate
170
67 65
210 80 80
0.0003 0.8 0.03
0.5 5 2
1. Diffusion in an adsoibent particle must be less than in the bulk liquid because thr solid structure surrounding the pores is not available to th(, diffusing liquid. For euamplr in silica-alumina 4 the ratio of pore to particle volume was e+ timated a t 0.48 using a poi( volume of 0.41 ml. per grani and a particle density of 1.18 gram per ml. Similarly, 0.4 was estimated for the pore to particle volume ratio in d i r x gels 61 and 88.
INDUSTRIAL AND ENGINEERING CHEMISTRY
July 1950
Table VI.
Effect of Heat Pretreating of Silica Gel
(Silica gel 88, 10-14-meshbeads, dried a t 350’ F.1 Pretreatment -
Hours 0 20 4 7 12
24 1 2 3
7.5 12 19
Temp.,
F.
...
400 625 625 625 625 740 740 740 740 740 740 865 865 865 910 910 915 915 915 915 1000 1000 1000 1000 1100 1100 1100 1100 1200 1200 1200 1200
Bulk Density, G./MI. 0.76
0.77 0.79 0.81 0.82 0.80 0.80 0.80 0.79 0.82 0.81 0.82 0.81 0.80 0.81 0.82 0.83 0.83 0.82 0.81 0.82 0.82 0.83 0.83 0.83 0.83 0.83 0.84 0.85 0.85 0.85 0.87 0.86
* Fracture resistance = toluene and iso-octane.
Fracturea Resistance 0 2 3 12 14 6 13 16 27 36 46 82 9 26 38 50 74 72 77 77 76 50 70 79 85 48 84 87 88 87 g5
99 99
Toluene Selectivity, Ml./G. 0.24 0.24 0.22 0.22 0.22 0.21 0.23 0.22 0.22 0.21 0.21 0.21 0.22 0.21 0.21 0.21 0.20 0.20 0.20 0.19 0.19 0.21 0.20 0.19 0.19 0.20 0.19 0.18 0.17 0.18 0.17 0.16 0.16
Toluene Diffusivity
k, (Min.)-i 0.068 0 074 0.053 0.056 0.05 0.053 0.054 0,083 0,053 0.053 0.053 0.051 0.052 0.041 0.038 0.043 0,037 0.038 0.040 0.040 0.036 0.042 0.043 0,041 0.037 0.038 0.037 0,039 0.043 0.036 0.042 0.037 0.035
perfect beads after 38 alternate iinmersions in
2. Tortuosity of the pores through the adsorbent particle may result in the average path for the diffusing liquid being greater within an adsorbent bead than in a liquid sphere of the same radius. 3. Diffusion in very small pores may be a slow surface migration process as compared to free diffusion in the ordinary external liquid, When all the pores :Ire small, surface migration may reuresent the rate-determining diffusion process. The effect of sirface diffusion, likewise, may be expectgd to be more important with larger molecules, because the adsorbed monolayer in such cases occupies a greater proportion of the pore space.
It is evident from the last, columns of Tables I1 and V that the diffusion coefficients for toluene and iso-octane in the aluminas and silica aluminas are one third to one half the values estimated for liquid phase diffusion. I t is reasonable to assume that the combined effect of solid structure and pore tortuosity is sufficient to account for the observed coefficients in the large pore adsorbents and that essentially the same diffusion process is prevailing in the pore and external liquid. On the other hand, with the snlall pore silica gels the fraction of pore liquid free from surface forces must be small, inasmuch as the diffusion coefficients are an order of magnitude lower than would be predicted for ordinary liquid diffusion. The high diffusivities observed lor high-area, activated carbons indicate that they must be permeated with sufficient macropores to provide channels for free diffusion, since the observed coefficients approach the values estimated for the external liquid although the calculated average pore size is less than that estimited for the silica gels. Review of the assumptions involved in Equation 5 will reveal that the method is best applied in cases where resistance to diffusion is primarily within the ;&orbent. Thus, unless there is turbulence in the interstitial fluid, D values found to approach those existing in the bulk liquid are subject to question. SELECTION OF ADSORBENTS The toluene-iso-octane diffusion coefficients have been found extremely useful for selecting suitable adsorbents for liquid phase separations when used in combination with selective adsorptive capacities and other physical properties. Table VI illustrates
1293
the use of such a combination of properties in selecting optimum time-temperature treatment for a pilot plant batch of silica gel beads t o realize maximum durability and bulk density without excessive sacrifice in selective adsorptive capacity and diffusivity . “Fracture resistance” as indicated in the table was evaluated in the following manner: About 100 perfcct adsorbent beads were selected by visual inspection and placed in small screcn baskets, which TT-ere then alternately immersed in toluene and iso-octane with 2 hours allowed between changes. After 35 such cycles the beads were examined visually, and the percentage of beads remaining uncracked was talien as the fracture resistance of the sample. Table VI1 further indicates the influence of mesh size on the adsorptive fracture effect observed for 35 alternate immersions of silica-alumina and silica gel beads in iso-octane and toluene. It is apparent that susceptibility to adsorption fracture could impose a serious limitation on the use of an otherwise favorable adsorbent for a liquid phase adsorption application. Review of the properties of commercially available adsorbents indicates that none combines all the features t h a t could be desired for large scale commercial applications-namely, high selectivity, high diffusivity, high durability-resistance to adsorptive frarture-high bulk density, low pressure drop, amenability t o low cost regeneration process, low cost, and long life. Obviously, some compromise must be made in selecting a suitable adsorbent for a specific application. I n any case, simple evaluation methods are desirable. The toluene-iso-octane system has proved valuable as a basis for comparison of selectivity, diffusivity, and fracture resistance of adsorbents considered for petroleum refining applications. It is recommended as a standard for evaluating potential new adsorbents. The measurements of selective adsorptive capacity, diffusivity, and fracture iesistance may be readily extended to other systems using the general methods outlined in this paper. It is mticipateJ that wider application of these simple measurements may be of value in the development of better adsorbents for liquid phase adsorption separations and in predicting the effect of process variables in new applications.
Table VII. Fracture Resistance of Adsorbents hd3oibent Beads Silica gel 61
Mesh Size 14-20 10-14 8-10 6-8
TI actlire Resistance,
D6 Perfect Beads 39 7 1 0
1c0 98 95 83
Toluene Selectivity, hll./Grain 0.236 0,236 0.23G 0,236
0.08ii 0.096 0.12 0.12
ACKNOWLEDGMENT The cont,ributions of c:o-~rorkerswho assisted in developing laboratory and correlation met,hods and in obtaining experimental data are gratefully acknowledged. The authors also wish t o express appreciat,ion t o the management of California Research Corporation for suggestions and encouragement during the progress OF the work and for permission t o publish this paper. NOMENCLATURE a c
A A I3
D E
selective adsorptive capacity, defined by 15quation 1 concentration external surface area, of an adsorbent particle, sq. cm. less strongly adsorbed component in binary mixture more strongly adsorbed component in binary mixturc = diffusion coefficient, sq. cm. per second = fractional approach to equilibrium
= = = = =
1294
INDUSTRIAL A N D ENGINEERING CHEMISTRY
= diffusion rate constant, (min.)-I, defined by Equation 5 = weight of adsorbe:t, grams n = an integer c, = radius in spherical coordinates R = equivalent spherical radius of an adsorbent particle total surface area of an adsorbent particle, sq. meters per gram t = time after initial contact of feed and adsorbent, minutes 2 : = volume of a binary feed mixture, ml. average volume of an adsorbent particle, ml. = specific pore volume of an adsorbent, ml. per gram x = volume fraction of component B in liquid phase a t equilibrium xc = volume fraction of component B in liquid phase at equilibrium zo = volume fraction of component B in a feed mixture .Tt = volume fraction of component B in liquid phase after t . minutes ? / =volume fract>ion of component, B in pore liquid at equilibrium k
(6)
v = v,
(1) (2) 13) j
I
(4)
LITERATURE CITED Arnold, J. E., J . Am. Chem. SOC.,52, 3943 (1930). Bartell, P. E., and Sloan, C. K., Ihid., 51, 1637, 1643 (1929). Brunauer. 8.. “Adsorotion of Gases and VaDors.” I). 287. Princeton, Princeton-University Press, 1943. Ihid., Chap, XI.
(5) Ibid., p. 465.
Brunauer, S., Emmett, P. H., and Teller, E., J . AWLChem. SOC., 6 0 , 3 0 9 (1938).
??Z
s =
Vol. 42, No. 7
( i )Drake, L. C., and Ritter, H. L., IKD. ENG.CHEM.,ANAL.ED., 17, 787 (1946).
(8) Eagle, S., and Scott, J.
W., Petroleum Processing, 4, No. 8 , 8S1
(1949). (9)
Emmett, P. H., and Demitt, T., J . Am. Chem. Soc., 65, 1257 (1943).
(10) Geddes, R. L., Trans. Am. Inst. Chenz. Engrs., 42, 88 (1946). (11) Harris, B. L., IND. ENG.CHEM.,41, 15 (1949). (12) Hartman, R. J., Kern, R. A., and Bobalek, E. G., J . Collozd Sci., 1, 271 (1946).
Hibbard, R. R., IND. ENG.CHEX..41, 197 (1949). Jones, D. C., J . Phys. Chem., 29,326 (1925). (15) LIair, B. J., J . Research Natl. Bur. Standards, 34,435 (1945). (16) Mair, B. J., and Forziati, A. F., Ibid., 32, 151 (1944). (13) (14)
(17) Ibid., p. 165. (18) Patrick, W.A., and Jones, D. C., J . Phys. Chem., 29, 1 (1926). (19) Rao, B. J., Ihid., 36,616 (1932). (20) Rescorla, A. A., Ottenmeller, J. H., and Freeman, R. S., A m ! . Chem., 20, 196 (1948). (21) Shearon, W.H., Jr., arid Gee, 0. F., IND. EXG.CHEM.,41, 218 (1949).
Strain, H. H., Anal. Chem., 21, 7 5 (1949). Wilke, C. R., Chem. Eng. Progress, 45, 218 (1949). (24)Williams, R., Jr., and Hightower, J. l‘., Chem. Eng., 55, S o . 11, (22) (23)
133 (1948).
RECEIVED October 21, 1949.
esulf urization aphtha
tion and
of Cracke
Application of Cyclic Adsorption P r o c e s s SAM EAGLE
AND
CHARLES E. RUDY, JR.,
California Research Corporation, Richmond, Calif.
A new processing scheme for hydrodesulfurizing thermally cracked naphtha without appreciable octane number loss is presented. Preliminary separation of the cracked naphtha into a paraffin-naphthene-olefinraffinate and a high sulfur aromatic extract is accomplished in a cyclic adsorption plant using pentane as a stripping liquid. The adsorption extract is hydrodesulfurised and reblended with adsorption raffinate to yield a premium low sulfur gasoline blending component. Operating features of a n eightcolumn cyclic adsorption pilot plant are discussed. Experimental data are also presented for several runs in t h e pilot adsorption unit and for hydrodesulfurization of the high sulfur extract. Octane number studies were made using t h e desulfurized-cracked naphtha as a gasoline blending stock.
A
CYCLIC liquid-phase adsorption process for refining various
petroleum fractions has been described recently by Eagle and Scott ( 4 ) . Briefly, the process permits the separation of aromatics and olefins from paraffins and naphthenes in wide boiling range distillates by adsorption on silica gel or other adsorbents in a multiple fixed-bed plant. D a t a were presented in this paper on typical separations achieved in a pilot unit. These included the removal of aromatics from a hgdroformed naphtha, a kerosene distillate, and a gas-oil, and the separation of both aromatics and olefins from a thermally cracked naphtha. The present paper describes in a more detailed manner the adsorption separation of the aromatics and sulfur compounds from a high sulfur-thermally cracked naphtha and the hydrodesulfurization of the aromatic extract obtained therefrom for the purpose of preparing low sulfur gasoline blending components. It has been recognized for some time that hydrogenation over sulfur-active catalysts is an effective way of eliminating sulfur from petroleum stocks. Numerous recent publications attest t o the efficacy of these methods (1-3, 5-8) for a variety of petroleum stocks. However, such a process has not been applied
commercially to the desulfurization of thermally cracked naphthas, primarily because the olefins present in these naphthas are more or less completely hydrogenated while reducing the sulfur content to a sufficiently low value to be attractive. Xot only does this result in an excessive consumption of hydrogen but also in a decrease in the unleaded octane number of the desulfurized product. The octane number loss has been observed t o be as much as ten to twelve units. A decrease in octane number. however, can largely be avoided by making a preliminary separation of the naphtha by silica gel adsorption into a paraffinnaphthene-olefin raffinate and an aromatic-sulfur compound extract. This high-sulfur aromatic extract can then be hydrodesulfurized over a suitable sulfur-active catalyst. The purpose of this paper is to describe the operation of a cyclic adsorption pilot plant in making such a separation of thermally cracked naphtha, to present data on the hydrodesulfurization of the aromatic extract, and to discuss octane number results on blends of the desulfurized product x ith the low-sulfur adsorption raffinate. Preliminary adsorption studies made in batch columns demon-
