August 1952
IN D U S T R I A 2 A N D E N G I N E E R I N G C H E M I S T R Y
tions and on the changes in aromatics composition accompanying the partial destruction of polynuclear aromatics by the cracking process. Similar data are given for the thermal cracking and steam cracking of clarified oils. Data are also given t o show how the potency varics with blending ratio for blends of bunker fuel type containing high boiling cracked oils or thermal reformer tars and crude residua. Potencies of such blends are not simply additive; they increase in direct proportion t o the percentage content of the cracked component up t o about 6070, and then remain uncahanged a t higher perrentages, within the limits of error ACKNOWLEDGMENT
The authors w e pleased t o acknowledge the large amount of experimental work carried out by B. F. Dudenbostel, Jr., L. T. Eby, R. L. Mathiasen, G. G. Wanless, and other laboratory associates. Special thanks art) due to H. G. M. Fischer for his enthusiastic and able guidance and advice in t h e program. The extensive animal test data used in this work were obtained and supplied by W. E. Smith and D. Sunderland of the New York University Hellevue Medical Center, and by K. Sugiura of the Sloan-Kettering Institute. R. E. Eckardt of these laboratories, and W. C . Hueper of the United States Public Health Service, contributed much valuable discussion on the hygienic and medical aspects of the problem. c. L. Brown, R. M. Shepardson, and M. W. Swaney contributed valuable support and technical disrussion. Finally, thanks are due t o the managements of the Standard Oil Co. (New Jersey), the Esso Standard Oil Co., and
182?
the Standard Oil Development Co., for their generous financial support of t h e project and their encouragement t o publish t h e results, and t o the managements of affiliated refineries for their cooperation in the experimental plant rum. LII‘ERA’I‘URE CITED (1) Badger, G. M., Brit. J. Cancel., 2, 309 (1948). ( 2 ) Hlanding, F. H., King, W. H., Jr., Priestley, W., Jr., and Rehner, John, Jr., Arch. I d . Hug. Occup. Med., 4, 335 (1951). (3) Charlet, E. M., Lanneau, K. P., and Johnson, I?. D., preprint, p. 105, Petroleum Division Symposium, 119th Meeting AM. CHEM.SOC., Cleveland, April 9, 1952. (4) Eby, L. T., Wanless, G . G., and Rehner, John, Jr., IND.ENG. CHEN.,43, 954 (1951). ( 5 ) Fischer, H. G. M., Priestley, W., Jr., Eby, L. T., Wanless, G. G., and Rehner, John, .JI... Arch. Ind. Hyg. Occup. Med., 4, 315 (1951). (6) Hartwell, J. L., IT. 8 . Public Health Service, P u b . Health B u l l . 149 (1951).
(7) Holt, J. P., Hendricks, K. V., Eckardt, R. E., Stanton, C . L., and Page, R. C., Arch. I d . Hug. Occup. Med., 4, 325 (1951). (8) Hueper, W. C., “Occupational Tumors and Allied Diseases,” Chap. 11, Springfield, Ill., Charles C Thomas, 1942. (9) Ibid., pp. 1 4 6 5 3 . (10) Setlllll, K., and Ekwall, P., Nature, 166, 188 (1950). (11) Smith, W. E., Sunderland, D. A,, and Sugiura, K., Brdi. End,. Hug. Occup. Med., 4, 299 (1951). (12) Twort, C. C., and Twort, J. M., J. Ind. Hyg., 13, 204 (1931). (13) Wanless, G. G., Eby, L. T., and Rehner, John, Jr., Anal. Chem.,. 23, 563 (1951). RECEIVED for review Deoember 19. 1951.
ACCEPTEDApril 2 6 . 1952..
Granular Adsorbents for Sugar -
Refining SOME FACTORS AFFECTING POROSITY AND ACTIVITY IN SERVICE ELLIOTT P. BARRETT R a u g h and Sons Co., Philadelphia,Pa., and M e l l o n I n s t i t u t e , P i t t s b u r g h , P a .
L. G. JOYNER AND P. P. HALENDA Mellon I n s t i t u t e , P i t t s b u r g h , P a .
T
HE gradual decrease in color- and ash-removal power of bone char with increasing length of service in sugar refining is a familiar phenomenon, but the mechanisms t h a t operate t o produce the deterioration in activity have not been adequately studied. I n conjunction with a description of the development and refinery scale testing of a synthetic granular adsorbent for sugar refining, called Synthad (2-38, Barrett, Brown, and Oleck ( a ) studied the changes in distribution of pore volume and area of the synthetic and natural chars in service. T h e activity, indicated by either color- or ash-removal power, decreased less rapidly in proportion t o initial activity than area decreased in proportion t o initial area. In other words, activity per unit area increased in service. An exception t o this was displayed by Synthad in the early cycles, as it gained area instead of beginning to lose it immediately. On the baais of these results and those of t h e pore volume and area distribution studies, i t was concluded t h a t there was no evidence t o indicate t h a t any of the pores in either adsorbent were too small t o function in the removal of impurities from sugar liquors. The loss in small pore ( - 100 A. radius) area was shown
to occur by two mechanisms--filling of the pores with adsorbed impurities and the growth of hydroxyapatite crystallites. The latter phenomenon, apparently, would effectively reduce the area by a redistribution of the interstices between crystallites, these interstices constituting the so-ealled pores. Reduction in area b y this mechanism will occur whenever environmental conditions favor crystallite growth regardless of whether or not the char has been contacted with impure sugar liquors, Le., whether the char has done any “wo&” or not. This paper describes the results of laboratory scale experiments calculated t o assist in understanding some of the results reported in a previous paper (2)and t o determine the effect of various conditions on the rate of crystallite growth, and, consequently, on t h e rate of loss of area. The d a t a are presented in t h e form of pore volume and area distribution curves constructed from nitrogen desorption isotherms by the method of Barrett and Joyner (8). Prior t o determining the desorption isotherms, t h e samples were outgassed for 1 hour at 200 C. under a vacuum of approximately 2 microns. To test the reliability of the results calculated from the nitrogen desorption isotherms, an independent approach
1828
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 8
0.001
r r I
.
B W H Washed and Hsated a t 1000"E, 150 Hours
0.00:
0 00
-BWH
Washed and Heated o f IOOO"F, 150 Hours
z-
&
0.002
'a12
'ai ;1
-
B W T H Washed, NaC2H,0Z Treated and Heated at 1000-F, 150 Hours
0.00
0.00 I
0.ooc
0 001
PORE RADJUS, r ( A )
F i g u r e 1. Effect of Washing and Heating on Pore Volume D i s t r i b u t i o n of Bone Char
was made by means of a high pressure mercury porobimeter (6). The data obtained with this instrument agreed surprisingly v 011 with the results of the isotherm method. The precision of the nitrogen isotherm method of estimating the volume in pores of various sizes varies with pore radius. For examplr, inspection of Table I11 of reference (3)shows that for pores with an average radius of 27.5 =k 2.5 A, an erior of 0.1 cc. a t standard temperature and pressure in measuring the volume of nitrogen adsorbed results in an error of 0.00013 cc. per gram in ebtimating the pore volume. At an average pore radius of 265 i 5 -I.,however, the same error in volume of gas adborbed pioducrs an error in pore volume of 0.00018 cc. per gram. I n d r m i n g the pore volume distribution curves presented in this paper, variations as small as 0.0001 cc. per gram have been treated as possiblv significant. EFPE4XS OF WATER EXTRACTION AND ELEVATED TEMPERATURES
h sample of new bone char wm extracted with 10 successive portions of distilled water a t 75" C., each portion remaining in contact with the char for a t least 1 hour. The char was then dried and was found to have lost 1.1% in weight. A portion of thc washed and dried char waa heated in an alloy steel retort, in thc substantial absence of air, a t 1000" F. for 150 hours. This continuous high temperature treatment is roughly equivalent to the cumulative thermal treatment t o which the older char of a two-grade char house would have been exposed in the successive reactivations t o which it would have been subjected. This is, of course, only statistically true. Some of the char would be mu& newer and some would be much older. Figure 1 compares the pore volume distribution curve for new bone char, B, with that for the u-ater-washed sample, BW, and with that for t h e heated sample, BWH. It is evident that before washing the char contained considerable quantities of watersoluble solids separated by interstices (pores) varying betlyeen 20 and 70 A. in radius, since the char lost volume in pore6 within this range of radii. The solids were contained in pores varying in radius from about 70 t o about 140 A., since the char gained
Figure 2.
IO0 200 PORE RADIUS, r ( A )
3 0
Effect of Various Treatnienm on Pore Volume D i s t r i b u t i o n of Bone Char
volume in pores of this radius range. On heating, the washed char lost volume in pores between 10 and 30 A. radius and gained it in pores larger than 75 A . radius. The loss in small pore volume cannot be ascribed t o grain shrinkage. The char actually gained about 0.02 cc. per grnni in pore volume during the heat,ing. This gain is due to a loss of about, 1% of volatile matter and to the oxidation of about 1% of carbon, and accounts for ii part of t h r increme in volume of the larger pores. The balance of tht, i n creme in large pore volume is contributed by the loss of small pore voluine resulting from the deposition of ions from small crystallites upon the surface of larger ones, the small pore volume, which prior t o heating represented interstices between small cry-stallite~, becoming automatically part of the volume of larger pores as the crystallites disappeared. The fact that this redist,ribution occurs is of the greatjest prcctical importance because bone char is deficient in large pore volume, and, if it did not possess this redistribution mechanism with which to augment its initial deficiency, it would lose pore volume much more rapidly than it does. Comparison of the pore volume distribution curve for the washed and heated char, HWH, with that for 32-cycIe char (Figure 2 ) shows that although the washing and heating produced an increase in large pore volume which was similar t o that produced in new bone char by refinery service, quantitatively the treatments produced a much smaller reduction in the volume in pores below 125 A. radius, and a smaller increase in the volume of pores above 125 A . radius than was produced by the first, 32 cycles of refinery service. The failure of the treatments t o produce a reduction in the volume of smaller pores comparable t o that of even so few as 32 cycles of service is not surprising, since the treatments did not require the char t o do any work, and consequently it contained no foreign material such as a char in service would contain when it entered the reburning kiln. To discover the effect of the presence of a small amount of foreign material on the behavior of the char during reburning, a 175-gram portion of the washed and dried char was contacted with 350 cc. of 1 N sodium acetate solution a t 75" C. for 6 hours, the mixture being shaken every half hour. The mixture WRS then
August 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
transferred t o a 4-inch Buchner funnel, sucked dry, and rinsed with a single 350-ml. portion of distilled water at room temperature. The rinsed char was dried in a n oven at 105" t o 110' C., and was found t o have gained 0.4201, in weight. A portion of the dried char was then heated t o 1000" F. for 150 hours t o duplicate the procedure used t o make sample BWH. A nitrogen desorption isotherm was then determined on the previously heated product and a pore volume distribution computed from the isotherm. Figure 2 compares the pore volume distribution for the waterwashed, sodium acetate-treated, heated char (BWTH) with that for the water-washed, heated char (BWH) and with that for 32cycle char (B-32). Inspection of the figure shows t h a t the presence of the sodium acetate during heating modified the pore size distribution resulting from heating in its absence as follows: 1. The volume in pores between 20 and 40 and between 70 and 100 A. was diminished. 2. The volume in pores between 10 and 20, between 40 and 70, and above 100 A. radius was increased. The relatively large loss in volume in pores between 70 and 100 .L radius shows t h a t pores in this size range retained a considerable portion of the sodium acetate residue; t h e gain in pore volume between 40 and 70 A. radius shows t h a t one effect of retaining t h e residue was t o produce a considerable volume of pores of this size range, Le., t h e larger pores were not, of course, completely filled but they were reduced in size. However, i t must not be assumed t h a t only pores in t h e radius range of 70 t o 100 A. were functioning, because the h a 1 result is only a statistical one. II'hether the volume in pores of a particular size increases or decreases depends on the relationship between t h e partial filling of pards of that size with residue from the contacting solution, and upon the production of pores of that size as a consequence of the partial filling of still larger pores. When an adsorbent has a definite pore volume maximum, such as sample B W H exhibits at 80 A. radius, it is to be expected that the partial filling of all the pores JTith a foreign substance will produce another maximum a t a lower pore radius, as is illustrated by the maximum exhibited by sample B W T H at 50 A. radius. However, the fact t h a t there is some reduction in volume of pores between 20 and 40 A. radius shows that pores in this size range also contained decomposition products of sodium acetate. Superimposed upon the effects produced by the presence of the sodium acetate are the effects of crystallite growth. There is nothing unexpected about the nature of the distribution curve for sample B W T H below 90 A. radius, but the fact that, at radii above 90 A., the effect of crystallite growth overcomes the effect of pore filling so t h a t above about 105 A. radius the acetatetreated char exceeds the untreated char is remarkable. Consequently, it must be concluded that the presence of the sodium acetate accelerates the rate of recrystallization of the hydroxyapatite crystallites of the bone char. Sample B W T H has a pore volume distribution much more like that of 32-cycle char (B-32) than has sample BWH. It seems that several cycles of acetate treatment and heating would convert the distribution resulting from a single cycle into fair approximation t o t h a t produced by the 32 cycles of refinery service. Figure 3 compares the cumulative area distribution for the three treated samples with those for new bone char (B) and for 32 cycle char (B-32). Total areas are listed for all of the samples and the results of laboratory color-removal tests for the three treated samples are compared. It is apparent that there are no significant differences among the three treated samples from the standpoint of distribution of area in pores of various radii. However, although both the heated samples (BWH and B W H ) lost considerable area as a consequence of the thermal treatment, they experienced no deterioration in color-removal power. I n short, heating the char decreased its total area without decreasing its activity, with the result t h a t the increase in its activity per unit area was similar t o t h e increase in color removal per unit area for bone char in service ( 2 ) .
1829
0
X
B
BW
112.5
120.2 93.7
b
A
BWH BWTH 104.5' 103.4 94.7 94.5
m
6-32 66.C
f
- \k
PORE
Figure 3.
RADIUS, r(A.1
Effect of Various Treatments on Cumulative Pore Area of Bone Char
Since the augmented activity per unit area cannot be explained on the basis of availability of surface, it is necessary t o recognize that some change has been produced by the thermal treatment which has increased the activity of the available surface. No suggestion as t o t h e source of the augmented activity can be made, but there is no reason t o suppose t h a t i t differs in origin from the lower activity of unheated char and this, in turn, is definitely dependent on the chemical nature of the surface. Figures 4, 5, and 6 provide the same information for a sample of new Synthad C-38 aa do Figures 1, 2, and 3 for t h e new bone char. Water extraction of t h e new Synthad changed the monomodal character of its pore volume distribution curve (S) t o a bimodal one (SW), much as was t h e case with bone char, although in contrast t o bone char, which lost 1.1%of its weight as a result of the aqueous extraction, the Synthad lost less than 0.1%. To check the systematic character of the changes occurring during thermal treatment, two samples of washed Synthad were heated at 1000" F., one, as in the case of bone char, for 150 hours (SWH-l), the other for only 75 hours (SWH-2). Comparison of the distribution curves for these heated samples with t h e washed but unheated sample (SW) in Figure 4 shows that the change from the condition of sample SW to that of SWH-1 is one which proceeds systematically with time. The height of the pore volume maximum for sample SWH-2 is only 0.0037 (cc. per A.) per gram, while t h a t for SWH-1 is 0.0053 (cc. per A.) per gram. The maximum for SWH-2 occurs at 100 A. radius, while t h a t for SWH-1 occurs at 110 A. radius. The results can be explained only as a consequence of crystallite growth since the total pore volume of the Synthad did not change as a result of the heating and, consequently, the aug-
voi. 44, N ~ a.
INDUSTRIAL AND ENGINEERING CHEMISTRY
1830
had been placed in the thermostat. Following this treatment thr inlet and outlet tubes to the flasks were closed. Two days after the start of the experiment it was noted that hubbles of gas were forming in the liquor in contact with bone char. During the third day the ~ : L Ppressure became high enough to blow the stopper out, of t h e flask. Water seals to permit escape of gas without allowing access of ail, were thereafter attached t o the flasks. After about 750 hours, slight evolut,ion of gas was noted in the Synthad flask hut the ratr of evolution was small in contrast to that from t,he bone char. When the flasks were opened a t the end of the contact period, the liquor in contact with bone char had a strong acidic odor ivliich was not noted in tho liquor in contact with Synthad. The liquors were decanted froin the adsorbents. They were highly colored, that from the bone char being much darker in color than that from the Synthad. The adsorbents were rinsed several times with hot distilled water and an attempt was made to extract all the colored material from them by soaking them with distilled n-ater a t 75' C., the water bring renewed once every hour during the working day. At the end of 1 week the wash water 'ifas d l l picking up color from the adeorbents, but much more from the bone char than from the Synthad, and the attempt to leach out all the color was disconbinued. The adsorbents were dried a t 105' to 110" C. and a portion of each was heated to 1000" F. for 2 hours. Kitrogen desorption isotherms mere determined for the four samples so obtained arid pore yolume distributions were computed for them. Figure 7 compares the distributions for sugar-contacted, waterc!xt>racted,dried bone char (RWLW) arid sugar-contacted, watcr-
0.00
SWH-I Washed and Heated a t 1000°F,, I50 Hours
0.00
-$1
S As Made
0.00
a
v 0
'a1G 0.00
0.00
000
Figitre I .
Effect of WaEihing and Heating F-oluineDistrihution of Synthad C-38
0x1
Pore 0.00
mente(1volume in pores i n the 90 to 120 A. radius iaiige could only have come from thc rediptribution of volume present prior t o the heatiiig. Consequently, it is seen that although in the refinery trial ( 2 ) the Synthad appeared t o g a h no new volume in pores larger than 90 -1.radius during refinery service, the apparent failure to do b o was merely due t o a counterbalancing of the effect of c~ystallitegrowth by a filling of pores with residue from adsorbed impurities. It would indeed be surprifiing if no crystallite gron t h occurred in the Synthad because its major coniponciut, like that of bone char, is hydroxyapatite Figurr 5 compares the washed, sodium acetate-treatrd, h c ~ ~ t e d (150 hours) oample of Synthad (SWTH) with the \ ~ a s h e dand heated (150 hours) sample SWH-1 and with 32-cycle Synthad (S-32). The effect of the sodium acetate treatment is similar to that on Imne char and requires no discussion. Likenise, Figuie 6 gives the analogous information for Synthad that Figure 3 giwh for hone char, and pointq to t h r iarnr g c m t d c*onclusions COV'VAC TI%G U3SORHhh
r\
WITH SUG4H LIQLOH
0 00
.---. m 201 a v
'a1;1 0 00 WTH-Washed, Na&H,O,-Treated and Heated at 1000°F, 150 Hr$
0 00
It lias txen shown that (51jhtallite giowth occurs in bone char b l e recrystallizaand Synthad at 1000 F I t is also c o n r c ~ i ~ ~ ithat tion niight oc(wr during rontact of the rtdsorbents with sugai liquoi 5 To investigate this possibilitj , portions 01 the watei-% ashed adsorbents were contacted u l t h 47.5" Brix granulated sugai liquor for 1000 hours a t 75" C Air n a s displaced from the flasks containing the adsorbents and liquor by bubbling oxygenfree nitrogen through the liquor for ahont 6 houls after the flash.
100
200
PORE RADIUS, r (A4.)
Figure 5. Effect of Various Treatments on Pore Volume Distribution of Synthad C-38
0
INDUSTRIAL AND ENGINEERING CHEMISTRY
Auqust 1952
extracted, dried, reactivated ( l O O O o F., 2 hours) bone char (BWLWH) with water-extracted, dried bone char (BW) and water-extracted, dried bone char heated t o 1000" F. for 150 hours (BWH). Comparison of the distribution for BWLW with t h a t for BW shows no indication of crystallite growth during contact with the sugar solution. I n spite of the prolonged water extraction, it is evident t h a t a considerable amount of solid matter was left in pores of all sizes, since the curve for BWLW is displaced toward smaller pore radius values relative t o the curve for BW. The total loss in pore volume of t h e sample was about 0.02 cc. per gram. The curve for BWLWH shows t h a t a 2-hour reactivation at 1000" F. not only accomplished a fairly thorough cleaning out of the pores but also allowed some crystallite growth t o occur, since the curve is displaced toward larger pore radius values not only relative to sample BWLW but also with respect to BW. The reactivation caused an increase of about 0.03 cc. per gram in pore volume so that not only was the bulk of the organic matter expelled but, in addition, some volatile material was expelled from the char itself. Comparison of BWLWH with BWH shows that crystallite growth in 2 hours was, of course, much less than in 150 hours; it also shows that the presence of the residue from the sugar liquor influenced crystallite growth in the range from 20 t o 65 A. pore radius, since, within these limits, the curve for BWLWH is displaced toward larger pore radius values relative t o that for BWH. Figure 8 compares the cumulative area distributions for the four samples and shows total areas and the results of laboratory decolorization tests. If availability of area were the factor-determining activity, sample BWH ehould be the best decolorizer, but BWLWH is better. If all the area is available, and total area is the determining factor in decolorization, then sample BW, the water-washed new char, should be the best. It is inferior t o both the heated samples. Sample BWLW is next t o the highest in area and the area is only slightly less available to molecules of any chosen size than is the area of sample BW, but it is grossly inferior to BW in decolorizing power.
An inspection of Figures 3,6, and 8 might suggest that pores of the order of 180 t o 200 A. radius are those which function in decolorization, since the cumulative area plots for adsorbents which differ in decolorizing power are slightly dissimilar in that region. However, the total area of pores in this radius range is so small as t o make such an explanation seem improbable. Moreover, the type of carbonaceous adsorbent described by Juhola, Matz, and Zabor (6) has even less area in pores of this size than bone char or Synthad, but is a more active decolorizer than either of them (1). 0.004
0.003
c:
Q E
Y
L
X
S 82.5
SW 92.7 89.8
e
A
SWH-I SWTH 79.3 80. I 90.1 89.4
0
S-32 72.6
PORE RADIUS, r (A)
Figure 6.
Effect of Various Treatments on Cumulative Pore Area of Synthad C-38
1
B W H Washed and Heated at 1000eE, I50 Hours
'
BW Washed and Dried
WLWH Washed, Contocted With Liquor, Washed and Heated at 1000°F., 2 Hrs. LW Washed, Contacted With Liquor, Washed and Dried
0.00 I
0.OOC
Code Sornple Area (rn*/g.) Decolorization (%I
1831
IO0
200
0
PORE RADIUS, r ( A )
Figure 7 . Effect of Contact with Granulated Sugar Liquor' and of Reactivation on Pore Volume Distribution of Washed New Bone Char
The data lead inescapably to the conclusion that it is not the extent of available surface which governs the activity of the adsorbent. The chemical and/or physical condition of t h a t surface is the all-important factor. Figure 9 gives for Synthad C-38 information analogous t o that given for bone char in Figure 7. Sample SW is water-leached, dried Synthad and sample SWH-1 is the same heated at 1000 O F. for 150 hours. Sample SWLW is sample SW after contacting with granulated sugar liquor for 1000 hours, washing and drying, and sample SWLWH is sample SWLW after reactivating at 1000" F. for 2 hours. Qualitatively, the discussion of the analogous curves for bone char applies without modification to the Synthad samples. This is also true of the data presented in Figure 10. I n contrast t o that of bone char, the decolorizing power of Synthad was not diminished as a result of contacting it with sugar liquor, washing it, and drying it. The improvement in Synthad performance as a result of reactivating it after contacting it with sugar liquor is about 50% greater, relative t o the activity of washed new adsorbent, than is t h a t of bone char (see Figures 8 and 10). Furthermore, like the corresponding bone char samples, the Synthad that had been contacted with sugar liquor and water washed, and Synthad after reactivation, possessed more area than did the new, water-washed adsorbent. This is similar to the gain in area exhibited by Synthad in the early cycles of the full-scale test ( 8 ) .
INDUSTRIAL AND ENGINEERING CHEMISTRY
1832
DISCUSSION
The results appear t o confirm the conclusions tentatively arrived at in (2). The rapid loss in area which occurs from cycle to cycle when new bone char is put in service does not occur as a primary consequence of filling of small pores by adsorbed impurities, but, on the contrary, results from the relatively rapid gron-th of hydroxyapatite crystallites which is probably accclerated by
i2 o h
Code SampJe Oecolorizotion (%) Areo (mP4J
0
X
0
BW BWH BWLW BWLWH 93.7 94.7 83.5 95.8 120.2 104.5 114.5 I 11.3
E
Vol. 44, No. 8
stances a large increase in volume in pores of intermediate radius occurs while t,he volume in pores of still larger radius is sonicwhat decreased. Compare, for example, samples SWH-1 and SWH-2 with sample SW in Figure 4. This observation is ctntirely (sonsistent with the view t]hat changes in pore size distribution produced by thermal treatment occur through the mrchanism of crystallite growth. A inass of small crystallites will, in general, be separated by rather narrow interstices. However, there may be regions where irregularly shaped clumps of crystals, contacting each other a t only a few points, are separated by much largw interstices than those separating the individual crystallites. Coilsequently, when the crystallites grow these larger pores disappear along with the small ones. This situation is very likely to occur with materials which, likc bone char and Synthad, exhihit initially a broad distribution of crystallite sizes. However, for materials like the hydroxyapatite used in Synthad, which prior to heating display a very narrow cryst'allite size distribution, hcating reduces only small pore volume and enhances large port' volume [see Figure 3 in rcference ( 2 )1.
Washed, Contacted With Liquor Washed and
w-SWH-I
Washed and Heated a t 100O0E, 150 Hours
0.003
W
'at ;1 0.002
PORE RADIUS, r ( A )
Figure 8. Effect of Contact with Granulated Sugar Liquor and of Reactivation on Cumulative Pore Area of Washed New Bone Char
prwence of iiiorgaiiic residues from the sugar liquors. In any ~ystem where crystallite growth is occurring the rate of growth diniinishes exponentially with time under constant conditions. This accounts for the exponential character of the curve for area versm number of cycles ( 2 ) . Reduction in area by the mechaniam of pore filling proceeds much more slowly than the reduction observed in the early cycles of use. In washed, new chars thepresence of organic matter which can be carbonized during r e x tivation actually increases the area (compare sample BWH with BWLWH and sample SmTH-lwith SWLWH). It is obvious that if the loss of area in early cycles were due t o filling of pores, then the rate of loss should have increased in the later cycles of the refinery trial when the loading was of the order of 10 times as great as in the early ones ( 2 ) . Also, if the rate of loss of area had continued as high as it was in the early cycles of the refinery trial, the char would have no appreciable area after 40 or 50 cycles and would be completely worthless. Although the principal effect of crystallite growth is t o decrease small pore volume and increase large pore volume, in some in-
Washed and Dried
I
0000~"
100
PORE RADIUS,
200
0
r (A)
Figure 9. Effect of Contact with Granulated Sugar Liquor and of Reactivation on Pore Volume Distribution of Washed New Synthad C-38
The foregoing observations explain the curve of area vcraus number of cycles previously reported for Synthad (2). Crystallite growth proceeds more slowly in Synthad than in bone char in the early cycles because the most rapid growth of hydroxyapatite crystallites in Synthad occurred during the carbonization process when it was being manufactured. Because of this slower rntc of
INDUSTRIAL AND ENGINEERING CHEMISTRY
August 1952 I 2(
Code X
0
Somple Decolorization (%) Areo (m*/g3
0
SW S W H - I 90.1 89.8 92.7 79.3
X
a t the same rate, due t o equal rates of crystallite growth, then if one has more area than the other and t h a t area is contained in larger pores, it will, under the same conditions of use and reactivation, continue t o maintain its excess of area. Since, with proper reactivation the activity per unit area increases with cycles of use, the adsorbent with the larger area will eventually do more work than the one with smaller area. From the foregoing it is evident t h a t an adsorbent consisting principally of hydroxyapatite, whether it be t h e natural apatite of bone char or the synthetic apatite of Synthad, should not deteriorate in color- and ash-removal power once its crystallites have ceased t o grow rapidly. Thereafter, loss of area is proceeding very slowly in so far as the effect of crystallite growth is concerned. Consequently, if all the mineral matter and all the organic matter picked up by the adsorbent in a cycle of use could be eliminated before it was sent to the reburning kiln, itti area would diminish at a negligible rate from cycle to cycle. Since there is no evidence that with proper reactivation activity per unit area decreases with length of service, it follows t h a t except for attrition losses the adsorbent would last forever. The complete elimination of mineral matter still presents a problem, a solution t o which has been suggested by Deitz (4),but there is no reason why any organic residue should be allowed t o remain in the adsorbent. A properly designed reburning kiln, which allowed adequate control of :bar temperature and of the oxidizing potential of t h e gaseous environment, would make possible the maintenance of service chars at a markedly higher level of efficiency than is possible with the equipment in current use.
0
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89.7 114.0
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93.1 95.5
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2
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ACKNOWLEDGMENT
3
The authors wish t o thank Baugh and Sons Co. m d Mellon Institute for permission t o present this paper. Frank Wood assisted in determining the nitrogen desorption isotherms.
PORE RADIUS, r (A)
Figure 10. Effect of Contact with Granulated Sugar Liquor and of Reactivation on Cumulative Pore Area of Washed New Synthad C-38
hydroxyapatite crystallite growth in Synthad during the early cycles, the rate of production of new area by the carbonization of organic matter from the sugar liquors a t first exceeded, and then balanced, the loss of area due t o crystallite growth, but by cycle 13, the rate of production of new area no longer exceeded that of the loss of area from crystallite growth and the area began t o fall systematically. Also, by cycle 13, the rate of crystallite growth in the bone char had decreased t o about that in t,he Synthad and the two curves become nearly parallel. When two adsorbents have reached the condition of losing area
LITERATURE CITED
(I) Barrett, E. P., “Advances in Carbohydrate Chemistry,” Hudson, C. S., and Cantor, S. M., eds., Vol. 6,pp. 220, 227, New York, Academic Press, Inc., 1951. ( 2 ) Barrett, E. P., Brown, J. M., and Oleck, S. M., IND. ENG.CHEM., 43,639 (1951). (3) Barrett, E. P., and Joyner, L. G., J . Am. Chefti. SOC.,73, 373 (1951). (4) Deitz, V. R. (to United States of America), U. S. Patent 2,557,948 (June 26,1951). (5) Joyner, L.G.,Barrett, E. P., and Skold, R. E., J . Am. Chein. SOC., 73,3155 (1951). (0)Juhola, A. J., Matz, W.H., and Zabor, J. W., paper presented at the Symposium on Adsorbent Refining Aids in the Sugar Industry, 119th Meeting AM.CHEM.SOC., Boston. Mass., 1951. RECEIVED for review December 1 1 , 1951.
ACCEPTEDApril 21, 1952.
Correction In the paper “Partial Combustion of Gas with a Deficiency of Air” [F. E. Vandaveer and C . George Segeler, IND. ENG.CHEM., 37, 816-20 (1945)], the following corrections should be made. On page 817 in the second paragraph of the second column, t h e sentence starting in line 12 should read “Under the same conditions the carbon monoxide content increased from 0 t o 22%, the hydrogen content increased gradually from 0 t o 41.2%, and methane appeared in traces at 90% air and gradually increased to 4% a t 20% aeration. In Figure 4 on page 819 the hydrogen curve is incorrect in that it should be a straight line from 0% hydrogen and 100% aeration to 22% hydrogen and 20% aeration. Also, under the words
“Gas Constituent-Per Cent” at the left side of the curve the phrase “For hydrogen multiply by two“ should be added. On page 819 in the last paragraph the third sentence should be changed to read “On natural gas the ratio of carbon monoxide t o hydrogen varies from 1 to 0.53 as air is decreased from 100 t o 25%.” On page 820, in Figure 8 the curve for natural gas is incorrect, It should be a straight line extending from the point located at 1 0 0 ~ aeration o and a ratio of 1.0 downward t o a ratio of 0.53‘at 10% aeration.
F. E. VANDAVEER
