T h e activation o f trihydrate bauxites a t temperatures u p to 1600" F. has b e e n studied w i t h particular. reference to the production of a highly e f f i c i e n tdrying adsorbent. Data a r e presented to show the relationship bet w e e n activation temperature, residual volatile matter content, surface area, d r y gas capacity, and equilibrium capacity. The mechanism o f the thermal decomposition o f bauxite is discussed with reference to the results of differential thermal and x-ray diffraction analyses. The maximum d r y gas capacity is attained b y activation a t 700750" F. This treatment reduces the volatile matter content o f the mineral from about 28-30 to 6-8%',. U n d e r optimum activating and operating conditions bauxite will adsorb 1 1-I6% b y weight o f water b e f o r e a n y moisture is d e t e c t a b l e in the e f f l u e n t . A c t i v a t e d bauxite m a y be regenerated r e p e a t e d l y b y heating a t 300-500' F. The various other factors which influe n c e drying e f f i c i e n c y are discussed.
HE application of activated bauxite to the decolorization of lubricating stocks and waxes (27, 18) and to the refining of sugar sirups and liquors (25) has been described previously. The activation and capacity of bauxite for the adsorption of water from gases and liquids have also been investigated, and findings in this field are summarized here. Activated bauxite, especially the deironed Arkansas variety, is finding increasingly extensive application in all types of industrial drying. Specific uses include drying of the feeds t o catalytic isomerization and alkylation plants, the dehumidifying of natural gas to prevent hydrate formation, the protection of anhydrous organic chemicals from moisture, the drying of various gases, etc. Activated bauxite W. A. La Lande, Jr., W. S. W. McCarter, is durable and relatively inexpensive. It and 1. B. Sanborn can be regenerated repeatedly, and its high preferable to restrict the term efficiency and life make i t comparable, even "bauxite" to the ores containPOROCEL CORPORATION, PHILADELPHIA, P A . without consideration of the low cost, with ing bohmite or gibbsite. other industrial drying adsorbents. Laboratory experiments on The term "bauxite" is used universally to identify rocks conthe dehydration of liquids by adsorbents are tedious and time taining certain forms of hydrated alumina as their major comconsuming; after it was ascertained that bauxite is efficient in ponent. The alumina content of the rocks is always associated drying both liquids and gases, the complete characterization of with kaolinite and iron and titanium minerals. On the basis of the material as a drying adsorbent was carried out with gases. x-ray spectra, Bohm (4) identified three different natural hydrates of alumina-diaspore, bauxite, and hydrargillite (now MATERIALS INVESTIGATED called "gibbsite"). Bohm's second hydrate is now identified as Samples were prepared from raw lump ores by drying and bohmite, the name assigned to it by de Lapparent (96). Diaspore milling to the required mesh sizes. Unless otherwise noted, and bohmite are monohydpatts, AlO.OH; gibbsite is a tride-ironed, activated Arkansas bauxite manufactured by the hydrate, AI(0H)s. Bohmite and gibbsite are the characteristic Porocel Corporation was used. Analytical data on the bauxites minerals, respectively, of the bauxites containing (in dried are given in Table I. samples of average good quality) approximately 12 and 30% The mineral components of the bauxites were identified by of water. The French bauxites belong to the former group; x-ray analysis (Table 11) and differential thermal analysis the United States and Guirtna ores are representative of the (Figure 1). The differential thermal analyses of the three untrihydrate bauxites. Diaspore diiTers from bohmite and gibbsite activated trihydrate bauxites (curves A , B, and C) show, for i n being less soluble in sodium hydroxide solution. It appears each sample, an endothermic reaction beginning a t 480-500" F. * * * and reaching a maximum a t 645-680' F., another endothermic depression starting a t 880-900" F. (maximum a t 995-1040' F.), The above photograph show a bauxite dehydrator on an H F alkylation unit. and an exothermic peak a t 1770-1790O F. The first endothermic (Courtesy, Philiipc PeQoleum Company)
T
99
100
INDUSTRIAL AND ENGINEERING CHEMISTRY
reaction is characteristic of gibbsite (12, 22, 50, 81); the second depression, in conjunction with the exothermic peak, discloses the presence of kaolinite (18, SO, SI). The strongly endothermic reaction of French bauxite starting at 850" and reaching a maximum a t 1025" F. (Figure 1, curve P ) indicates bohmite. The first endothermic reaction for kaolinite occurs in the same temperature range; however the weak exothermic peak at Figure 1. Differential Thermal Analyses 1760' F. indiA Arkansas I1 D Arkansas I 600' F. cates the presence E.' Arkansas I,' 700" F. 8.' Brltbh Guiana of an amount of C. Arkansas I F. French kaolinite which would give only a relatively small depression at 995-1040" F. Conversely, large amounts of kaolinite would completely prevent the detection of comparatively small amounts of bohmite. The cause of the exothermic peak at 575" F. in curve F i s unknown. The slight reaction at about 450' F. (just preceding the gibbsite peak) in the curve for the Guiana bauxite probably indicates limonite. This reaction was not observed with the Arkansas bauxite, in which the iron is probably present as hematite. The dual nature of the kaolinite reaction at 970" F. and the exothermic reaction immediately following are attributed to the presence of organic matter. From these data and the chemical analyses, the rational analyses of the ores (Le., their quantitative mineralogical composition given in Table 111) were calculated. Since practically all of the Si08 in the bauxites was contained in the kaolinite, the percentage of this component was found by converting Si02 to A1203.2Si02.2H20. The calculated water content of the ores (Table 111) was determined from the water required by the calculated amounts of kaolinite and gibbsite. i 7 I I M E T H O D S AND A P P A R A T U S / I i i
L
I
Activation. The bauxite samples (1000 ml.) were activated in externally gas-fired rotary kilns (7 r.p.m., 6.5-inch diameter, 13.5-inch length), fitted with flights and with a port at each end to allow escape of water vapor. The samples were heated from room temperature to the required temperature in 30 minutes and maintained at the activating temperature (*2' F.) for 30 minutes unless otherwise noted. The products were removed from the kilns a t 400' F., quickly screened to remove fines, and charged to the apparatus under anhydrous conditions.
K
I
Vol. 36, No. 2
Residual Water Content. Five-gram samples were ignited at 1800" F. to constant weight. X-Ray and Differential Thermal Analysis. Measurements were made throu h the cooperation of R. E. Grim, of the Illinois State Geological %urvey, on samples prepared in our laboratory. The authors assume all responsibility for the interpretation of the data. Dry Gas Capacity (D.G.C.). The term "dry gas capacity'' is used as a measure of the amount of water vapor held by the adsorbent up to the point where it just permits water to appear in the gas passing from it. Following this break point, up to which the bauxite adsorbs all the water, moisture appears in the effluent in increasing amounts until the adsorbent is saturated. The results in Table IV show the course of drying up to the point where the bauxite is adsorbing only 50% of the water in the gas stream. The dry gas capacity is expressed as yoby weight of the adsorbent. Dry gas capacities were determined in the apparatus shown in Figure 2. The moisture content of the gas (air from the laboratory compressed air line, propane, nitrogen, etc.) was regulated by bubbling it through a sulfuric acid solution of known concentration while controlling the flow with needle valve A . A preliminary adjustment of the moisture content was effected in the two flasks, B , in which a large volume of the solution was used to avoid any appreciable change in concentration during a determination. The gas next passed through a preheating coil, C, to a series of six smallw humidifiers, D, containing sulfuric acid of the required concentration, and thence to large flask E which served as a sur e tank to eliminate pulsations in the gas Manifold F felivered the humidified gas to six parallel I n each train the gas received final treatment in the sintered glass disk gas-washing bottles, G, containing more of the same acid solution. The caps of the gas-washing bottles, as well as traps H , were packed with glass wool to remove any entrained liquid from the gas stream before it passed to the modified Turner bulb, I , which held the adsorbent. The final stages of humidification and sorption of water vapor were carried out in LL constant-temperature bath, J. Satisfactory constancy of the moisture content of the gas is indicated by the fact that the concentration of the sulfuric acid solution in the final washing
f%"n'
TsBLE
Bauxite Appearance
I.
Arkansas I1
British Guiana
French
Light brown- Light creamgray, p&r: gray, Pretially oolitic dominantly oolitic
Pinkishcresm, massive
Dark reddish brown, massive
Arkansas I
1
CHEMICAL ANALYSES
Bulk densitya, lb./cu. it. Analysis: Ignition lossb, $& A1203. % SiOs, 3' 5 FetOs, % TiOz, %
70.3
70.3
73.0
89.3
28.76 55.81 10.08 2.72 2.63
31.12 59.69
30.18 59,45
12.46 57.42
2.24 2.05
1.42 2.16
23.41 1.49
4.90
6.79
5.22
a After activation the bulk density of the Arkansas and Guiana ores wae 65-57 lb./cu. ft t h a t of t h e French bauxite, S1 lb. b After heat& t o constant weight a t 220' F.
.
bottles remained unchanged throughout a determination. Detector tube K which followed the adsorption bulb was sled with anhydrous magnesium perchlorate (Anhydrone). Moisture was detected by an increase in weight of this desiccant before a color change could be observed in a cobaltous bromide iedicator. Bower (6) found that anhydrous magnesium perchlorate can dry air until the residual water vapor content per liter is 0.0020.003 mg. According to Smith (W), "the determination of the vapor pressure of the anhydrous product (magnesium perchlorate) and its lowest hydrate has demonstrated perfect drying efficiency. Gases dried by its use showed zero vapor pressure. We obtained the same dry gas capacities by substituting phosphorus pentoxide for magnesium perchlorate. Each train terminated in a flowmeter, L. For each determination, unless otherwise noted, approximately 65 grams of activated bauxite were charged to the bulb. This produced a cylindrical bed having a height to diameter ratio of 2.3-2.4. I n all experiments the moisture-containing gas was caused t o flow downward through the adsorbent bed in order to avoid "lifting" at the gas velocities used. Flow rates of 15
cubic feet per hour per pound of adsorbent (61 liters per hour for the 65-gram samples) were used unless otherwise noted. I n operation, needle valve A was opened to furnish a slightly greater gas sup ly than was required by the system. The gas velocity througf each train was adjusted by means of the manifold stopcocks to the required value, with the adsorption bulb and detector tube shunted out. Excess gas was vented through valve M . Minor readjustments of the manifold stopcocks could thus be made rapidly when the bulbs and tubes were connected. Once the manifold stopcocks had been set for a given series of adsorbents, the slight variations in gas supply could be taken care of by adjusting the single vent valve, M . The maximum variation in the weight of water vapor delivered to the different adsorption trains in a given time was 2.5%. This was shown t o consist of a 1% error due to uncertaint in reading the flowmeter and a 1.5% error attributable to the cazbration of the flowmeters and the use of the calibration curves. During the early sta es of a run the adsorbent holder and detector tube were weig%ed a t hourly intervals. As the anticipated break point (gain in weight of the detector tube due to the
TABLE 11. X-RAY DIFFRACTION ANALYSISOF d, in A. 7.2 6.2 6.3 4.85 4.39
4.34 4.30 4.20 4.18 4.04 3.56 3.54 3.62 3.34 3.28 3.24 3.16 3.09 2.70 2.64
2.66 2.52 2.50 2.46 2.38
,
2.37 2.34 2.33 2.29 2.27 2.25 2.20 2.16 2.10 2.04
1
w
G G
s
G+K .
-
E
.
I . .
K
w
K
........ ........ K ........
m m
K
m
K
8
An K G
m w
An K
8
.......
........
......G' G ....... w G
........
ww w m
B
ww
H
K ....... K
w ww w
K
w-m
K
m
....... m K
ww m
An K+B
w
........
w
w m-s B
G G
H
m
G
........ ........ w K
m
G
m-s
G
........ ........
.......
.......
.......
....... s G
h'
w
....... G
. . . . .An ..
w-m ww
G G
An
w-m
....... m K+An G G G
m
G
.......
An
........ ........ ww K
...... m-8 G' .......
m-8 m m
K
B ........ ........ ........
......
w
'A1
........ ........ ........ ........
....... .......
m-s m-s
''
........
G+K
m
' *
An
........ m K+An
ww
B
........ ...... ........ m B+A1
- w
B+K
w
K
K
s
K
An K
8
........
ww
m w
K
........
R? B
........ K
An K+B
s
........ ........ w K
....... An
ww
w
K
m
i
......
.......
.......
w 8
....... ....... w
An B
m
........
m
An
w
? K+An B
........
8
w
........ ........ ........ m B+A1
- w
weak- m = medium- 8 strong. h haze. 7-aldmina; An ahatase; B -'btihmite; 0
B+K
'A1
m
R?
.......
s
w
$+An B
....... .......
....... m B+AI w
gibbsite; H
-
B+K
An K G
....... G G ....... w G
......
w
......
......
w
G'
....... w K+B ....... m G
......
m
G
m
An+Al
w
H
h'
' '
'ki
...... ...... ...... ...... ...... ...... An ...... H ...... ......
......
R? ...... An ...... ...... m H+An ...... w H
m w
...... ...... m A1 w
hematite; K
K
m w m
.......
....... .......
ww
w
...... ......
m
-
H
kaolinite; L
s
G
......K + B
m m
ww
K
s
G
....... ....... s G
.......
G ....... ....... .......
....... ....... m-s G
e
m-8
G ....... ? ....... m a
s
ww
ww
ww ww
An
ww
m m
G G
B .......
....... .......
m ww ww
G+K
An G
.......
G
G ....... t ....... m G An ....... ....... m-s G
m-s
....... An
w
G
ww m w
m
G
m
....... .......
G
....... ....... 6 ; 2" w G
zw 2
.......
........ ........
........ ........
G+K ....... ....... ww L?
........ ...... ....... s G
m
French
........ m-s B ........
8
...... H ...... ......
H ......
w
An
. . . . .G. . G ....... w G
8%
Guiana K B G G
m m
m
....... K
G
British w ww w
...... ......
An B
ww m
....... K+An ....... w K
.. m
....... ww K ....... .......
L?
ww
w
An
w
K
m w
G+K .......
.......
......
m
.......
........
........ ........ m K
"
8
.......
....... ....... w K
m
8
....... .....
An K+B
h' *
K
R? B
....... ....... .......
h" ' ' "A1
W
K
ww w
........ ........
........ K ........ ......
......
.......
........ ........
........ K ........ ........
...... ......
....... .......
m
K
...... ......
....... .......
........ ........
m
w
e w b A1
I
RAW AND THERMALLY ACTIVATEDBAUXITES
(Cu radiation, filtered: 4 hours, 40 kv., 20 ma.) Estimated Intensitye of Observed Lies, and Mineralsa Indicated Arkansas I 700' F. 800° F. 1200O F. Arkansas I1 8 K m * K w K m B w B w B w G 8s G m K w K
w w m
1.88 1.85 1.84 1.80 1.74
1.40 1 .as 1.30
........ K ........ ........
w
.......
8
1.64 1.48 1.45 1.44 1.41
........ ........
........
.......
W
600' F, K B
m w
.......
8s
1.99 1.97 1.96 1.93 1.92
1.70 1.68 1.67 1.66 1.68
Raw K
m
101
INDUSTRIAL AND ENGINEERING CHEMISTRY
February, 1944
G G
.......
G ....... .......
limonite: R = rutile.
........ ........ ........
........
w
An ........ ........
8
........ B ........
H ......... ........ ........ ........ ........ ........ ........ ........ 88 B ........ ........
m-s
........
m-w
H ........ ........ ........ ........ w B ........ ........ ........
........
B ........ ........
as
w
?
m
A
........ ........ ........ ........
m
H
B+H .... .... ........ w B B
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
102
Vol. 36, No. 2
Figure 3. Estimation of Break Point in Determination of Dry Gas Capacity
passage of water through the adsorbent) was approached, weighings were made every 15 minutes. After the break point a final weighing mas made a t the end of 15 minutes. Figure Experiments under pressure were made by placing two of the modified Turner adsorbers in a steel txessure vessel. The outlets of the adsorbers were attached to separate connections passing through the wall of the vessel and terminating in pressure relief valves. Flowmeters were attached to the valves. Gas admitted t o the vessel passed into the inlets of the adsorbers, through the adsorbents, and from thcnce through pressure reliei valves to the detector tubes and flowmeters. The prrssure vessel was maintained at constant temperature by immersion in a thermostatically controlled water bath.
MIXERALOGICA L COMPOSITION TABLE 111. CALCULATED (IN
Bauxite Kaolinite AlzOs. dSiOz. 2Hz0 Gibbsite Al(0H)a BBhrnite: AlO. OH Anatase, Ti02 Hematite, Fet0s
PERCENT)
Aikansas I Arkansas I1 21 7 72.3
10.6 84 9
2.6 2.7
2.0 2 2
99.3
99.6
28.75 28.00
31.11 30,90
...
...
~
Total
Hz0 (ignition loss) Hz0 (calculated) a Present as limonite.
British Guiana 14.6 82.2
....
2.2 1.4n
French 12.8
....
ACTIVATION TEMPERATURE - OF. 4. Relation of Residual Water Content, Dry Gas Capacity, and Surface Area to Activation Temperature
"Dry gas" may be further explained by consideration of the sensitivity of the method. An undetected increase of 0.5 mg. in the weight of the detector tube during the shortest interval employed (15 minutes) would mean that the air delivered during this period had a moisture content of 0.033 mg. per liter which corresponds to a dew point of -58" F. An undetected increase of 0.5 mg. over a 6-hour active life would represent, for the total volume of air produced, a moisture content of 0.0015 mg. per liter which corresponds to a dew point of -98' F. Since our weighing technique permitted the detection of weight increases of 0.2-0.3 mg., i t may be concluded that extremely dry air (Le., with a dew point considerably below -98" F.) is produced during the active life of the adsorbent. The reproducibility of the method for determining dry gas capacity is shown in Table V. Triplicate determinations were made on three different samples of 10-20 mesh bauxite a t atmospheric pressure and 79.5" F. with air of 75% relative humidity.
63.2 1.7 26.7
--
__
100.4
104.4
30.17 30.50
500 700 900 1100 1300 1500
12.46 11.30
Typical data for the estimation of a dry gas capacity are plotted in Figure 3. During the first part of the determination the water vapor adsorbed is linear with respect to time (curve -4). Deviation from linearity occurs when the bauxite is no longer able to take up all the water vapor passing to it. The point of deviation, however, is not sufficiently well defined to serve as a graphical estimate of the break point. The break may be determined by plotting the percentage moisture content of the air adsorbed by the bauxite during regular time intervals; curve C shows the sharp break at 6.27 hours. The simplest device, however, is to plot the total increases in the weight of the detector tube against time (curve B ) . Extrapolation of the line through the first and second (total) weight increases indicates a break a t 6.30 hours. This break corresponds to a dry air capacity of 11.46 %.
TABLE IV. Gas DRYING CHARACTERISTICS OF ACTIVATED BAUXITE (Gas velocity, 15 cu. ft. per hr. per Ib.; 75% relative humidity a t 79.5' F.)
Time, Hours0
Water Adsorbed, %b
2.5-3,0 3 .O-3.5 3.6-4.0 4.0-4.5 4.5-5.0
0.94 1.86 2.77 3.68 4.58 5.49 6.40 7.30 8.19 9.20
5.0-6.6
10,OI
0-0.5 0.5-1.0 1.0-1.5
1.5-2.0 2.0-2,5
Time, Hoursa 5.5- 6 . 0 6.0- 6 . 5 6.6- 7 . 0 7.0- 7 . 5 7.5- 8 . 0 8.0- 8 . 5 8.5- 9 . 0 9.0- 9 . 5 9.5-10.0
Water Adsorbed, %b
Moisture .% Retained Dew Point by of Dried Adsorbent Air, O F. 100
