Carbon Adsorbents-Oxygen
Reaction
Chung Chi Chou Research and Development Division, Amstar Corp., 266 Kent Avenue, Brooklyn, N Y 11211
The kinetics of bone char oxidation were examined in detail for the first time. In the temperature range of 30O-45O0C, the surface oxidation was half order with activation energies of 20 kcaI/mol in the region affected by mass transport and 3 6 kcaI/mol in the chemical-control region. The study also suggested that the inorganic phase of the bone char functions as a catalyst in the bone char oxidation. Some characteristics of bone char carbon were elucidated from the oxidation data. It wascalculated that the active surface area of the bone char is about 7.3% of the total carbon surface area, and that the average diameter of a carbon crystallite is about 6 2 A.
T h e study of the kinetics of carbon oxidation has been the subject of many papers over the past 50 years. Most of these deal with materials ranging from coke to activated carbon and from carbon black to graphites and carbon filament (Walker et al., 1959; Ergun and ,Mentser, 1965; Thomas, 1970). The literature on the reaction of bone char with oxygen is limited. Despite considerable technical interest in the oxygenbone char reaction, a fundamental study on its reaction mechanism and kinetics has long been neglected. Bone char is the most widely used decolorizing and deashing adsorbent in the cane sugar refining process. It is a product of destructive distillation of animal bones. During this process collagen macromolecules undergo pyrolysis, and carbonaceous material gradually “accumulates” on the surface of hydroxyapatite crystals according to a mechanism which possibly involves various degradation, condensation, and hydrogen elimination reactions. After a period of use, bone char becomes exhausted owing to the accumulation of organic matter and ash on its surface and/or in its pores. It is then regenerated by washing and kilning. I n many technical applications both activated carbon and bone char are used to remove color and/or organic contamination from process streams. However, there is a striking difference in the method of the regeneration of exhausted adsorbents. Activated carbon is generally regenerated in a steam atmosphere a t a temperature of about 1800°F, whereas a bone char is regenerated in a low percent oxygen atmosphere at a temperature of about 1000’F. Questions as to the reasons for the difference in the regeneration method for the adsorbents have often been raised among sugar technologists. X fundamental understanding of the nature of bone char carbon and the reaction kinetics of bone char oxidation is of both technical importance and academic interest. I n a previous paper (Chou and Hanson, 1971) various techniques, such as differential thermal analysis, oxidation studies, infrared analysis, and electron microscopical examination were employed in a n attempt to define the nature of the carbon of bone chars. I n this u-ork the reaction of bone char with oxygen was studied, and some characteristics of bone char carbon were elucidated from oxidation data. Experimental Methods
Oxidation of Carbon Adsorbents. Two adsorbents were used in this study. A new bone char (A) made by a British 348
Ind. Eng. Chem. Prod. Res. Develop., Vol. 1 1 , No. 3, 1972
supplier with approximately 80% of calcium phosphate in t h e hydroxyapatite form, 107, carbon, other inorganic salts, and a Pittsburgh-type granular CAL carbon. The mean particle diameter of the carbon adsorbents used were about 1.0 mm. Before oxidation runs both adsorbents were heat-treated in a helium atmosphere a t a temperature of 550°C for 1 hr to remove organic-like materials. The detailed apparatus used and procedures for the oxidation study were described in a previous paper with slight modification (Chou and Hanson, 1971). The sample holder employed in this work was a stainless spring coil with a n inside diameter of about 4 mm and a length of about 6 em. The opening between the coil was about 0.4 mm which allows the oxidizing gas mixture t o flow uniformly through the samples and ensures uniform decarbonization of carbon adsorbents. About 0.5 gram of sample placed in the reaction tube was first heated in a helium to prevent oxidation. When the furnace reached the required temperature, and steady state was obtained, the oxidizing gas mixture was substituted for the helium for the desired time; the oxidizing gas mixture was then replaced by helium, and the reaction tube cooled t o room temperature. The weight loss was determined by weighing the sample before and after oxidation. Reproducibility of the data was poor a t a low percentage of decarbonization of carbon adsorbents. At a higher percent decarbonization, the reaction tended to be complicated by a n intraparticle diffusion process in the micropore region. For the CAL carbon oxidation, the situation may be further complicated by the changes in total surface area during the oxidation process. This is particularly true a t a high percent decarbonization. For these reasons the theoretical treatment of the experimental data was based on the results obtained between 2 0 4 0 % decarbonization for bone char (A) and 5-30% decarbonization for the CXL carbon. Ignition Temperature of Carbon Adsorbents. Inorganic salts presented in the carbonaceous materials play a n important role in the oxidation of carbon. Since bone chars and CAL carbons differ greatly in their composition, particularly in inorganic content, it was decided t o examine t h e ignition temperature of the adsorbents. This gave an insight into the catalytic effect of the inorganic materials of the adsorbents on the oxidation process. The ignition temperature of the adsorbents was determined by measuring the percent carbon burn-off in a pure oxygen atmosphere a t 3.3 psig for a series of temperatures starting
with a temperature of 275°C for the bone char and 375°C for the CAL carbon. As the temperature increased, the degree of decarbonization for a given period of time increased. T h e ignition temperature 'was taken as t h a t a t which a sudden increase in percent decarbonization occurred. T h e same apparatus used for the oxidation study was used for the ignition temperature determin a t'1011. Oxidation of Tagged Bone Char. I n t h e study of t h e reactivity of carbon, t h e carbon on t h e edges of crystallit,es was more react,ive t o oxygen t h a n t h e basal plane carbon (Laine et, al., 1963). An att'empt was made to develop a method t o obtain further evidence t o support t'his conclusion b y examining t'he effect of cryst'allite size on the rate of oxidat'ion. X tagged bone char was prepared by contacting i t with radioactive sucrose solution overnight a t i 5 " C . During this period sucrose molecules were sorbed b y the bone char onto its surface and into its pores. After washing off the excess sucrose, the char was dried and heated in a helium atmosphere for 30 min at a temperature of 350°C. Under such conditions the retained sucrose was pyrolyzed to give carbon residues on the char. The tagged bone char was then subjected to oxidation for 30 min in a nitrogen-oxygen mixture with 3.1% of oxygen a t 350°C. The percentage of total carbon and tagged carbon on bone char before and aft'er oxidation was determined by a combustion method and radioactivity measurement. The general procedure for isotopic carbon assay described b y Christman and his coworkers (19j5) was followed by use of a glass proportional counting tube and a Decade Scaler, 8703 series made by Nuclear Chicago, Des Plaines, IL. Results and Discussion
The overall reactioii of oxygen with a carbon surface may usually be separated into five consecutive steps, the slowest step determining the .rate of oxidation: (a) diffusion of oxygen molecules to the carbon surface; (b) adsorption of oxygen molecules on the surfa'ce; (c) chemical reaction a t the surface; (d) desorption of the oxidation products; and (e) diffusion of the desorbed products into the gas phase. Steps (a) and (e) are mass-transport processes and if the rate-determining step, the temperature dependence on the oxidation rate mill be relatively small and will result in a low value of activation energy. Steps (b), (c), and (d) are of a chemical nature and are affected by the nature of carbon and by impurities present in carbon. I n practice, it is oft.en not possible to distinguish between steps (c) and (d) because the rate of desorption is not known; consequently, in the treatment of t h e experimental data, t,he chemical reaction a t t'ne carbon surface (Step c) and desorption of the reaction products (Step d) are usually considered as a single step. T h e distinction of each step and the determination of the rate-determining step under a particular condition are not only of academic interest b u t also of practical importance to a n understanding of the carbon adsorbents revivification process. The effect of mass transport on the rate of carbon oxidation was studied by varying the flow rate of the oxidizing gas. If the rate of oxidation was limited by diffusion of oxygen molecules to the carbon surface, the rate should increase with increasing flow rate, and the diffusional limitation would be most severe for the runs a t a low percentage of oxygen in the osidizing gas mixture. Figure 1 shows the effects of gas flow rate and oxygen partial pressure on the rate of bone char oxidation a t 550°C by use of' a n oxygen-nitrogen mixture with 2.8%
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/ D
s
7 -
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6 -
c
6
5 -
4 -
3 -
I
50
100 Flow r o t e
150
200
ml/mln
Figure 1 . Effect of gas flow rate on rate of bone char oxidaticn
oxygen. T h e percent decarbonization is the % weight loss per 10 min. The rate of oxidation increases both with increasing gas flow rate and oxygen partial pressure a t 2.8% oxygen. A t a high percent of oxygen concentration (e.g., 44% oxygen), the gas flow rate had little effect on the rate of bone char oxidation. The results clearly indicate t h a t a t a low oxygen concentration (e.g., 2.8% oxygen), the rate of oxidation was affected by the diffusion of oxygen to the carbon surface. T h a t the slope mas not significantly affected by the oxygen partial pressures a t a constant oxygen percentage (2.8%) indicates the decreasing effect of the mass-transport step as the oxygen pressure increases. A similar conclusion was derived in the study of the oxidation of the CAL carbon. It must be noted t h a t the results provide no information as to t h e nature of the diffusional resistance, be it by mass transport in-pores and/or in the gaseous layer surrounding the particles. Further study is needed in this respect. I n the study of gas-solid reactions when the mass-transport resistance becomes important, the apparent activation energy tends to decrease. To further examine the effect of mass transport on the carbon oxidation, the activation energy of the reaction was determined a t various oxygen percentages with temperature ranges of 300-450°C for the bone char and 550460°C for the CXL carbon. Different temperature ranges were employed to obtain the degree of decarbonization within the range of 2 0 4 0 % for the bone char and 5-30% for the C.1L carbon for the reasons previously described. The results are shown in Figures 2 and 3. At a low percent oxygen (e.g., 3.1'%), t h e activation energies are about 20 kcal/mol for the bone char and about 40 kcal/mol for the CAL carbon oxidation. The values increase as the percent oxygen in the oxidizing gas mixture increases. A t 4491, oxygen it reaches values of 36 and 80 kcal/mol for the bone char and the CAL carbon, respectively. The low values of activation energy a t low oxygen percentages may be attributed to diffusional resistance of the mass transport of oxygen in the oxidizing gas mixture to the carbon surface. As the percent oxygen increases, the diffusional resistance becomes less important and finally a t high percent oxygen (e.g., 44%), the rate of oxidation depends mainly on the cheniical reactivity of the carbon as suggested by the leveling of the effect of percent oxygen on activation energy after about 44% oxygen. I n comparable experiments Hawtin and Gibson (1966) studied the effect of diffusion and bulk gas flow on the thermal Ind. Eng. Chem. Prod. Res. Develop., Vol. 1 1 , No. 3, 1972
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Table 1.
Reaction Order for Bone Char Oxidation a t Various Experimental Conditions
1
Oxygen concn, % oxygen in 02-Nz mixture
1.1 3.1 5.8 11.8 Air 44
I
I
I
I
I
5
10
IS
PO a i
I
25 OIyqe”
30
I
I
I
35
40
45
Figure 2. Effect of percent oxygen in oxidizing gas mixtures on activation energy of bone char oxidation
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I
5
IO
15
20
21,
30
35
40
45
X OXYGEN
Figure 3. Effect of percent oxygen in oxidizing gas mixtures on activation energies of CAL carbon oxidation
ovidation of porous carbon. For ungraphitized carbon, the diffuqion effect n as important at temperatures in the range 300-400°C. For graphite the evperiments were carried out a t temperatures ranging from 550-675’C and o y g e n concentratioiis betn een 2.5 and 20.8Yc’,.Under these conditions the carbon o\idatioii is controlled by the rate of in-pore mass transport The experimental results give a n apparent activation energy of 25 kcallg atom which is comparable t o the activation energy obtained for the bone char oxidation aiid considerably below a value of 40 kcal/mol for the C h L carbon oxidation in the region affected by the diffusion. Comparison of the results of the CAL carbon and the bone chai oxidation shows a striking feature. The activation energy of the C.1L carbon is much greater than that of the bone char, i e., 80 and 36 kcal mol in the chemical-control region and 40 and 20 kea1 mol in the diffusional-control region for the CAL carbon and the bone char oxidation, respectively. Ailthough the activation energies for carbon oxidations vary [for example, Kicke (1955) reported a value of 58 =t4 for crushed electrode carbon, and Blyholder and E y i n g (1957) obtained a value of 80 kcal/mol a t pressures below 100 Hg and temperatures around 800°C for thin coatings of spectroscopic graphite], it is generally accepted that the actiaation energy for the oxidation of graphite ranges from 55-65 heal mol These values are considerably higher than 36 kcal/ mol for the bone chai oxidation. ,inalysis of the oxidation data obtained b3 Loebenstein and his cok\oikers (1949) shons t h a t the rate constant for bone 350
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Table II.
Reaction temp,
OC
375 350 350 325 32 5 330
Reaction order
0.53 0.52 0.67 0.51 0.50 0.55
Reaction Order for CAL Carbon Oxidation at Various Experimental Conditions
Oxygen concn,
70oxygen in 02-Nz mixture
Reaction temp, ‘C
Reaction ordei
3.1 5.8 Air 44
475 477 452 450
1 0.95 0.92 1.1
char oxidation is about twice that of activated carbon. KO explanation is offered by these workers, b u t their finding is consistent with the present work. The implication that the bone char oxidation is catalyzed by the inorganic phase of bone char is in agreement with our previous observation by a n independent technique (Chou and Hanson, 1971). The conclusion is further substantiated by the measurement of the ignition temperatures of the bone char and the CAL carbon. They are 325 =t2 O C and 450 =t 4°C for the bone char and the CXL carbon, respectively. Gallagher and Harker (1964), in a study of the effect of impurities on the carbon-oxygen reaction, found that the addition of small concentrations of impurity produced a comparatively large drop in ignition temperature. They suggested that the transition metal compounds function as true oxidation catalysts. The present work shows that the ignition temperature of the bone char is about 125OC lower than t h a t of the CALLcarbon, implying the catalytic effect of the inorganic phase of bone char on carbon oxidation. T h e low value of activation energy of the bone char mag partially account for the fact that although the temperature of CAL carbon regeneration can be as high as 1900°F, the temperature of bone char regeneration should not evceed 1200°F. The order of carbon oxidation was also examined. The results are shown in Tables I and 11. The order of the CAL carbon oxidation is approximately first order in both chemical control and diffusion-affected regions. These values are in agreement with results obtained by other workers (Walker et al., 1959). The order of the CAL carbon oxidation will not be affected by diffusional limitation, since a n .Yth order becomes l / 2 ( i S 1) under diffusional-control region. The order of the bone char oxidation is close to half-order reaction. This value may be expected for the carbon-oxygen reaction in a n intermediate pressure range, since in zone 1 (Walker et al., 1959), Le., the chemical-control region, the reaction order can vary from zero to one depending on degree of surface coverage. Another explanation is possible; for example, a n immobile adsorption with the dissociation of oxygen molecules as the rate-determining step would also give the observed reaction order (Laidler et al., 1940; Glasstone et a]., 1941).
+
T h e reaction order of bone char obtained in the diffusionaffected region (percent oxygen
