Ind. Eng. Chem. Res. 1991,30, 2322-2327
2322
SEPARATIONS Capture of Ammonia by Active Carbon Fibers Further Activated with Sulfuric Acid Isao Mochida* and Shizuo Kawano Institute of Advanced Material Study, Kyushu University 86, Kasuga, Fukuoka, 816 Japan
Capture of ammonia (100 ppm) in the atmosphere was studied a t room temperature on active carbon fibers (PAN, pitch, and phenol resin based ACFs) and their further activated forms with sulfuric acid. T h e as-received ACFs absorbed only 0.2% of ammonia until its breakthrough. Activation with sulfuric acid by impregnation and heating a t 250 "C for 4-6 h increased remarkably adsorption up to 3% in repeated uses after regeneration a t 185 "C. Two kinds of acid sites were found after the activation; the remaining H2S04and acidic oxygen containing groups. The latter groups are produced by oxidation through the reduction of SO, to SO2to contribute the regenerable adsorption of ammonia. Hence the adsorption capacity of ACFs was linearly correlated with the amount of evolved C02 a t 250-400 "C in their temperature-programmed-decomposition profiles. The regeneration conditions were intended to remove ammonia from the surface of ACF without decomposing the acidic oxygen sites.
Introduction Ammonia is one of the unpleasant materials related to life. Its handy capture or removal is wanted for a pleasant environment (Kunibe, 1988). Its acidic capture with mineral acids such as sulfuric acid is simple but not handy. Solid or solidified acids may be more practical. Zeolites and silica-alumina can be excellent adsorbents for ammonia when they are dehydrated properly; however, humidity in the atmosphere hinders adsorption and the regeneration requires a rather high temperature around 450 "C (Hara and Takahashi, 1978). Although active carbons are not believed good adsorbents for ammonia (Matsumura, 1984), sulfuric acid impregnated on active carbon has been commercialized (Tsurumi Coal Co., 1989). However, its capacity for ammonia capture is rather limited when the amount is restricted to prohibit elution of the corrosive acid in humid atmosphere. It is difficult to regenerate the acid from the product ammonium sulfate. The present authors have proposed that the acidity introduced with sulfuric acid onto the active carbon surface (Komatsubara et al., 1984) captures ammonia at room temperature and releases it at an elevated temperature to regenerate the acidity (Mochida et al., 1990). Some active carbon fibers (ACF) exhibited a large capacity of ammonia capture until the breakthrough (Mochida et al., 1985a, 1987). In the present study, the optimum activation with sulfuric acid and regeneration conditions of an ACF were explored to find the largest capacity of regenerable ammonia capture. The best ACF was also explored among commercially available polyacrylonitrile (PAN), pitch, and phenol resin based ACFs. The oxygen functionalities introduced onto the ACF surface were analyzed by means of temperature-programmed decomposition (TPD) of oxygen-containing groups of the ACF surface. Influences of ammonia concentration and humidity in the atmosphere were also studied to reveal the applicability of the present ACF further activated with sulfuric acid under practical conditions.
Experimental Section ACF. Table I summarizes the ACFs used in the present study with some of their properties. The original forms of ACFs were supplied by Toho Rayon (PAN base, FE-200 and FE-400), Osaka Gas (pitch base, OG-5A, OG-10A, OG-20A), and Nippon Kynol (phenol resin base, ACN210-20) and were used without further treatment. Activation. Chopped ACF was immersed into aqueous HzS04(12 N), containing the prescribed amount of H#04 for 24 h, dried in a rotary evaporator, and heat treated in a nitrogen flow. The amount of H2S04was varied from 1/1 (H2S04/ACFby weight) to 5/1. The heat-treatment temperatures and times were 200-500 "C and 1-4 h, respectively. Details of activation procedure were described previously (Mochida et al., 1985a,b, 1987). Capture of Ammonia. The capacity of ACF for capture of ammonia was measured at room temperature in a columnar reactor with packed ACF, the weight and length of the bed being 0.5-1 g and 77 mm, respectively. The flow rate and ammonia concentration in dry He were 100 mL/min and 100ppm, respectively, at atmospheric pressure. In some runs, ammonia concentration was varied to 10, 25, and 50 ppm and the carrier gas was humidified at 20 "C to carry 1.7 X lo9 g.min.mL-' (relative humidity 90%) for wet conditions. Ammonia at the outlet of the reactor was oxidized into NO to be quantified continuously with a NO, meter (Yanagimoto, ECL-77A). The adsorption amount of ammonia (wt %) until the breakthrough was calculated. Two kinds of breakthrough times were counted: Toand Tloo are the times until the first detection of ammonia elution and until 100% elution, respectively. Regeneration. ACF that had captured ammonia until its breakthrough was regenerated by heating under He flow (100 mL/min) until the ammonia concentration at the outlet of the reactor reached less than 1 ppm. The temperatures examined were 170, 185, 200, and 250 "C according to temperature-programmed desorption of am@ 1991 American Chemical Society
Ind. Eng. Chem. Res., Vol. 30, No. 10, 1991 2323 Table I. Some Properties of Activated Carbon Fibers sample FE-200 orig 3/250/4' 3/ 25015c 3/ 25O/Sc 3/250/7' FE-400 orig 3/250/5c OG-20A orig 3/250/4c 3/250/5c 3/ 25016' OG-1OA orig 3/ 25016' OG-5A orig 3/ 25016' ACN-210-20 orig 3/ 25014~ 3/ 25016' TSURUMI-AX'
C
ultimate analysis, wt % H N O S
ash
surf. area, m2.g-1
pore vol, mL.g-'
9.7 26.4 20.2 23.4
740 460 500 610 670
0.39 0.27 0.31 0.35 0.40
PAN base
remaining H2S04,wt %
oxygen! wt %
source
tr 6.1 5.3 1.0 0.7
2.1
0
4.4
9.7 38.4 30.6 25.4 21.4
2.3 2.9 3.3 2.0
19 16 3 2
1.1 1.6
2.3 1.6
9.0 36.0
tr 4.2
4.6 2.2
0 13
9.0 27.8
1190 630
0.64 0.44
PAN base
83.0 38.2 55.0 65.6
1.1 1.9 1.9 1.3
2.3 0.2 0.1 0.2
9.0 39.2 35.9 29.8
tr 20.3 6.8 2.7
4.6 0.2 0.3 0.4
0 61 21 8
9.0
980
0.51
pitch base
22.6 24.4
350 610
0.21 0.35
91.6 45.9
0.9
0.6 0.5
6.8 43.3
tr 8.5
0.1 0.7
0 26
6.8 26.7
710 230
0.37 0.14
pitch base
1.1
89.8 58.5
1.0 0.9
0.6 0.6
8.5 36.4
tr 3.4
0.1 0.2
0
8.5 29.7
480 420
0.25 0.22
pitch base
10
93.1 42.9 62.5 64.3
0.9 0.8 1.2 1.5
0.1 0.2 0.1 0.2
5.5 46.5 32.4 22.0
tr 8.9 3.1
0.4 0.7 0.7 12.0
0 27 9 13
5.5 28.9 26.3 9.3
1520 180 800 220
0.81 0.09 0.48 0.23
phenol resin
81.5 47.5 55.3 64.9 70.4
1.4 1.4 1.5 1.3
5.3 4.3 4.4
1.1
83.0 54.4
4.1
Determined by titration of H20 extract.
coal base
Oxygen other than that included in remaining H2SOI. 'Conditions for HzSOl activation:
H2S0,/ACF ratio, temperature ("(21, and time (h). Table 11. Activation Conditions and Amount amt of adsorbed NH3 by TO,wt % activation lsto 2nd" 3rda 4tha condition original 0.2 0.2 0.2 temp,* O C 200 11.7 250 4.3 2.6 2.7 2.6 275 3.0 2.2 1.9 1.9 1.7 1.7 300 2.3 1.8 350 1.5 1.3 1.2 400 1.3 1.1 1.1 500 0.6 0.5 0.5 H+3O,/FE-20Oc 1 1.6 1.2 0.9 2 2.9 1.4 4.3 1.1 3 4.3 2.6 2.7 2.6 4 21.8 ~~. 0.5 .. 5d 6.9 2.8 2.4 2.1
of Adsorbed Ammonia in ReDeated Runs for FE-200
remarks impregnation of H2S04,300 w t %; activation time, 4 h
adsorption: NH3 = 100 ppm (He balance); W / F = 5 X regeneration: 185 OC; 5 h; He flow; W / F = 5 X
gmin-ml-' at room temp
g.min-mL-'
activation temp and time, 250 OC for 4 h except for run 5," where heated for 8 h adsorption: NH3 = 100 ppm (He balance); W / F = 5 regeneration: 185 OC; 5 h; He flow; W / F = 5 X
X
10" g.min.mL-' at room temp
g.min.mL-'
'Run. bActivation temperature. cImpregnated by weight H2S04/FE-200ratio. dHeated for 8 h.
monia. Characterization of ACF. ACFs were characterized with microanalyses, BET surface area, pore volume, and TPD of CO and C02evolution. Temperature-programmed decomposition was carried out in a temperature range of room temperature to 1000 "C by 10 "C/min at a He flow rate of 100 mL/min. Desorbed gases of CO, COz, SO2,and H20 were identified and quantified by a quadrupole mass spectrometer (Anelva, TE 600) and a gas chromatograph, respectively. The amount of H2S04 remaining on ACF after the activation was quantified by extraction with boiling water and titrating with aqueous NaOH.
Results Survey of Optimum Activation Condition. Figure 1 illustrates some examples of breakthrough curves of ammonia through the packed beds of PAN-ACF FE-200 and its activated forms. The activation conditions were H2S04 impregnated, 300 wt %; and heat treatment, 200-500 OC for 2 h. The original ACF removed completely
loo1j
3 5-50 I
00
1
IO
20
30 Time(h)
50
so
lW
Figure 1. Breakthrough curves of NH, in the first run over PANFE-200 activated with 300 wt % H2SOI. Adsorption conditions: room temperature (20 " C ) ; NHs = 100 ppm; W / F = 5 X g minsml-'. Activation temperature: 1, original; 2,500 OC;3,400 "C; 4, 300 "C; 5, 250 "C (first run); 5', 250 O C (second run); 5", 250 "C (third run); 6,200 O C . Profiles of second and third runs (5' and 5") were observed after regeneration a t 185 "C.
ammonia of 100 ppm in the atomosphere only for 1 h, when the concentration in the eluent increased sharply to that at the inlet within 4 h. The adsorbed ammonia on the ACF until breakthrough (To) was 0.2 wt % as summarized in Table 11.
2324 Ind. Eng. Chem. Res., Vol. 30, No. 10, 1991 Table 111. Adsorbed Ammonia on Activated ACFs for First through Fourth Runs" amt of adsorbed NH,, w t % activation 1st 2nd 3rd ACF FE-200
time, h orig 3 4
5 6
I FE-400 OG-20A
OG-1OA OG-5A ACN-210-20
orig 4 5 orig 4 5 6 7 orig 6 orig 6 orig 4 5 6
TO 0.2 7.2 4.3 4.0 3.1 2.6 0.1 15.2 4.9 0.1 21.7 9.1 5.2 1.6 0.1 9.7 0.1 5.2 0.1 13.8 10.7 6.6
Tl, 0.24 9.1 4.7 4.4 3.5 2.9 0.11 19.4 5.5 0.12 23.7 9.7 5.8 1.8 0.12 11.0 0.13 6.5 0.11 14.6 11.9 7.7
TO 0.2 1.0 2.6 2.5 1.9 1.5 0.1 1.5 2.9 0.1 1.3 2.6 2.8 1.2 0.1 2.8 0.1 3.3 0.1 3.9 3.1 3.1
Tl, 0.28
4th
TO
TI,
0.2
0.25
2.7 2.2 1.8 0.9 0.1
2.8 2.3 2.0 1.1 0.10
2.5 0.1
TO
TIC@
2.6 2.3 1.8
2.7 2.4 2.0
2.7 0.11
2.4
2.6
2.5 2.6
3.1 2.7
1.1 0.1 3.1 0.1 3.0 0.1 3.1 3.1 3.1
2.4 2.5 1.0
2.9 2.7 1.2
0.12 3.8 0.12 3.6 0.11 3.4 3.8 3.3
2.8
3.4
2.7
3.1
3.2 3.3 3.1
3.5 3.7 3.3
1.4
3.1 2.8 2.1 1.6 0.12 1.7 3.0 0.11 1.5 3.1 2.9 1.2 0.11 3.5 0.12 3.9 0.11 4.2
3.8 3.4
1.2
'Impregnated H2S04/ACF= 3 by weight. Activation at 250 OC for 4, 5, or 6 h. Adsorption: HN, = 100 ppm (He balance); W / F = 5 x g.min.ml-'. lo-, gminsml-'. Regeneration: 185 "C;5 h; He flow; W / F = 5 X
Activation with 300% H2S04remarkably prolonged the time until the breakthrough (To).The heat treatment for 4 h at the lowest temperature of 200 "C allowed 100 h until breakthrough (To)while the highest one at 500 "C was only 5 h. The amount of adsorbed ammonia until the breakwas 12% by heat treatment at 200 "C, dethrough (To) creasing with higher temperatures to 0.6% at 500 "C as summarized in Table 11. However, the lowest temperature of 200 "C did not allow any adsorption of ammonia in the second run after the regeneration of ACF at 185 "C as shown in Table 11. Other ACFs exhibited reduced adsorption abilities in the second run and gave stationary values after the third run. Although the heat treatment at higher temperatures allowed smaller reduction of adsorption in the successive runs, the heat treatment at 250 "C provided the largest adsorption amount of 2.6-2.790 in the repeated runs. Table I11 summarizes the influences of heat-treatment time at 250 "C on the amounts of adsorbed ammonia by Toand TIm Heat-treatment time of 3 h allowed 7.2% in the first run but only 1.0% in the second run until To. Longer times reduced the amount in the first run but thereafter tended to maintain the amount, giving by 4 h the largest stationary adsorption of 2.6-2.7% in the repeated runs. The amounts of adsorbed ammonia by ?',, and TI*were very different in the first two runs; the difference was smaller in the successive runs. H2S04in the deep pore may contribute the slow adsoprtion in the first two runs. Table I1 also summarizes influences of the amount of impregnated H2S04on the adsorption of ammonia when the heat-treatment temperature and time were fixed at 250 OC and 4 h, respectively. A greater amount of H2S04 increased markedly the amount of adsorbed ammonia in the first run; however, the largest amount of adsorption provided the smallest amount in the second run by the present regeneration conditions. The longer time of 5 h, which reduced the amount of adsorption in the first run, provided a more reasonable but still decreasing amount of 2.8-2.4% by Toin the successive runs. So far the optimum activation conditions were 300% of H2S04and 250 "C and 4 h of temperature and time, allowing stationary
I85 115
o\
,
1
2
Run
3 No.
4
Figure 2. Adsorption of ammonia on regenerated ACFs. Regeng.min.mL-' for fourth eration conditions: He flow; W / F = 5 X run. Adsorption conditions: NH, = 100 ppm (balance He); W / F = 5 X lo-, gmin-ml-'; at room temperature. Regeneration temperature ("C) 185,200,220; PAN-FE-200 (3/250/4), circles; ACN-210-20 (3/250/6), squares. Table IV. Amount of Adsorbed Ammonia until the Breakthrough (To) on ACF Activated with H2S04under Variable NHS Concentration amt of adsorbed NH,, mL/e 100 25" 50" loo0 sample run PAN-ACF-200 (3/250/4)b 1st 9.6 21.6 36.5 55.7 8.1 16.6 25.4 34.4 2nd OG-5A (3/250/6)b 1st 30.2 52.6 68.3 81.5 2nd 19.7 32.7 41.8 48.6 NH, concentration, ppm. Conditions for H2S04 activation: H2S0,/ACF ratio, temperature ("0, and time (h).
adsorption of ammonia of 2.62.7% and breakthrough time (To) of 25-27 h in the repeated uses after the regeneration. Regeneration Conditions. Figure 2 illustrates influences of the regeneration temperature on the amount of adsorbed ammonia in the repeated runs. PAN-ACF FE200 activated with 300% H2S04at 250 "C for 4 h (3/250/4)
Ind. Eng. Chem. Res., Vol. 30, No. 10, 1991 2325 Table V. Influences of Water VaDor w o n the Ammonia Adsorption Abilities for First thioughThird Runsa amt of adsorbed NH3 by To,wt % carrier gas 1st 2nd 3rd sample dry 4.3 2.6 2.7 FE-200 (3/250/4)b wet 5.0 2.9 2.6 ACN-210-20 (3/250/6)b dry 6.6 3.1 3.1 wet 7.4 3.7 3.5 OG-5A (3/250/6)b dry 5.2 3.3 3.0 wet 5.7 3.7 3.4 TSURUMI-AX dry 1.4 0.7 0.3 wet 5.3 1.5 1.0 ,
I
OHumidifying method: bubbling (He flow) in water at 20 "C. Moisture amount: 1.7 X g.min.mL-' (RH = 90%). Adsorpg.min.mL-l at tion: NH3 = 100 ppm (He balance); WIF = 5 X room temperature. Regeneration: 185 "C; 5 h; He flow; W/F= 5 X gminaml-'. *Activation conditions, see Table IV.
Table VI. Adsorbed Ammonia of Activated ACFs for First through Fourth Runsa amt of adsorbed NH, by To, wt 9 i sample 1st 2nd 3rd 4th FE-200 0.19 0.18 0.18 orig 4.3 2.6 2.7 2.6 250/4b FE-400 0.12 0.13 0.12 orig 4.9 2.9 2.5 2.4 250/5* OG-20A orig 0.02 0.02 0.02 9.1 2.6 2.5 2.4 250/5* OG-1OA 0.04 0.03 0.04 orig 9.7 2.8 3.1 2.8 250/6b OG-5A 0.07 0.05 0.06 orig 5.2 3.3 3.0 2.7 250/6b ACN-210-20 orig 0.06 0.05 0.05 10.7 3.1 3.1 3.3 250/5b "Adsorption: NH, = 100 ppm (He balance); W/F = 5 X lo-* gmin.mL-' at room temperature. Regeneration: 185 OC; 5 h; He flow; W/F = 5 X lo-, g.min.mL-'. *Impregnated ACFs with 300 w t % H2S04. Heat treated at 250 "C for 4, 5, or 6 h.
and 69 h, respectively, in the first run. The second and successive runs after the regeneration a t 185 "C allowed Figure 3. Breakthrough profiles of ammonia on pitch and phenol both ACFs to exhibit stable capture of ammonia by their resin based ACFs and their activated forms. Activation conditions: of as long as 34-36 h, which was breakthrough time (To) impregnated H2S0,/ACFs = 3 by weight, activation at 250 "C for certainly superior to that of activated PAN-ACF FE-200. 6 h. Adsorption conditions: room temperature NH3 = 100 ppm; Table I11 included the amounts of adsorbed ammonia W/F = 5 X 10-3/g.min.mL-1. Regeneration conditions: 185 "C; carrier gas He = 100 mL.min-l; (- - -) OG-5A; (-*-) ACN-210-20. on ACFs activated by variable heat-treatment times at 250 "C. Heat-treatment time of 5-6 h appeared optimum to a series of OG-ACFs and ACN-210-20, giving the largest adsorbed 4.3% of ammonia in the first run. The regeneration at 185 "C for 7.5 h allowed 2.6-2.7'70 by Toin the stationary adsorption of 3.0-3.270 on OG-5A and ACNsecond and following runs as shown in Table IV. A large in the repeated uses 210-20 until the breakthrough (To) decrease after the first regeneration was noted; however, as shown in Table 111. the adsorption became very stable after the second and Adsorption of ammonia in a humid atmosphere inespecially the fourth regeneration. Adsorption by Tloo creased slightly but distinctly on ACN-210-20 and OG-5A showed a similar trend, although the amount by TI, was as shown in Table V, being favorable for their use in the significantly larger in the first two runs. atmosphere. The regeneration at a lower temperature of 170 "C took Amounts of adsorbed ammonia until the breakthrough 5 h for the ammonia concentration of eluted gas to reach on activated OG-5A in the first and second runs are sum1ppm, allowing a stable but smaller adsorption of 1.5%. marized in Table IV where the concentration of ammonia Higher regeneration temperatures of 200 and 220 "C for was varied from 10 to 100 ppm. The amounts decreased 4 h reduced the amounts to 2.4 and 1.4%, respectively, in with the decreasing partial pressure of ammonia more the second runs, which further decreased in the following moderately than those on PAN-ACF. runs. A greater amount of H 8 0 4 , 500%, failed to increase the Ammonia Concentration and Humidity of Eluent stationary adsorption of ammonia on OG-5A, OG-20A, and Gas. Table IV summarizes the adsorption amounts of ACN-210-20 by the heat treatment at 250 "C for 8 h as ammonia until the breakthrough (To) against the partial shown in Table VI. pressure of ammonia. The adsorption amounts of amTSURUMI-AX (good on the market, commerically monia in the first and second runs decreased markedly available active carbon impregnated with HzSO4 apparwith decreasing concentration although the breakthrough ently without any heat treatment; Tsurumi Coal Co.) adtime was prolonged with decreasing concentration. Since in the first sorbed only 1.4% until the breakthrough (To) the adsorbent under the present situations adsorbed all run under dry conditions and adsorbed a severely deammonia in the atmosphere until the breakthrough (To), creasing amount in the successive runs as shown in Table the amount of adsorption may be governed by not equiV. Humidity increased the adsorption in the first and librium but kinetic factors. successive runs; however, the amounts decreased in every Influences of humidity on the adsorption of ammonia run to 1.5 and 1.0% in the second and third runs, reare summarized in Table V. Humidity increased slightly spectively. The superiority of the present ACFs activated the amounts in the first and repeated runs. further with HzS04is definite. Adsorption Abilities of Other ACFs. Figure 3 ilCharacterization of ACFs and Their Activated lustrates breakthrough profiles of ammonia on pitch based Forms. Table I summarizes elemental analyses and sur(OG-5A) and phenol resin based (ACN-210-20)ACFs and face areas of ACFs. The original ACFs carried fairly large their activated forms. The original forms of these ACFs amounts of oxygen (5.5-9.7'70) and BET surface areas were very poor for the capture of ammonia as PAN-ACF. (480-1520 m2/g). Activation with H 8 0 4tended to reduce Activation with 300% H2S04at 250 "C for 6 (3/250/6) the surface area considerably but increased markedly oxremarkably prolonged the breakthrough times (To)to 58 ygen and sulfur contents, which decreased rapidly with Break-through time ( h )
2326 Ind. Eng. Chem. Res., Vol. 30, No. 10, 1991
Hedtreotment time ( h )
Figure 4. Amount of remaining H2S04on PAN-FE-200 after activation at 250 OC with 300 w t % H2S0,. N2 flow; flow rate = 50 mlsmin-'; heating rate 10 OC/min to 250 OC.
g
o",t(wt%)
Figure 6. Relation between amount of adsorption ammonia and net oxygen content on ACF. Averaged amount of adsorbed ammonia until breakthrough (except for first run). ACFs and their activated forms: 1, FE-200 (original); 2, FE-200 (3/250/4); 3, FE-200 (3/ 250/5); 4, FE-200 (3/250/6); 5, FE-400 (original); 6, FE-400 (3/ 250/5); 7,OG-20A (original);8, OG-20A (3/250/5); 9, OG-20A (3/ 250/6); 10, OG-1OA (original); 11, OG-1OA (3/250/6); 12, OG-5A (original); 12, OG-5A (original); 13, OG-5A (3/250/6); 14, ACN210-20 (original); 15, ACN-210-20 (3/250/4); 16, ACN-210-20 (3/ 250/6); 17, TSURUMI-AX(goods on the market).
1.25-
Y
e
a 1.00. 5
temperature (200-400 "C) should be noted. ~
W),.."
.,.
300
400
500
600
Temperature ("C) Figure 5. TPD profile of original and activated OG-5As with 300 w t % HaO, for 6 h. He carrier gas; W/F= 3.7 X g-mimml-'; ramp 10 OC-min-'; (-) CO; (- - -) CO,; -) SO2. 1, 2, 5, OG-5A (3/250/6); 3,4, OG-5A. (-a
higher temperature and longer time of activation. The amounts of remaining HzS04 after the activation are also summarized in Table I. A large amount of H2S04stayed on ACF when heat treatment was mild, but decreased steadily with longer heat treatment. Figure 4 illustrates the amount of remaining H2S04 on PAN-ACF-FE-200 by the activation at 250 "C. The amount decreased very slowly at temperatures below 250 "C and monotonously at 250 "C with time and became almost nil by 7 h at 250 "C. Taking into account remaining H2S04, net increases of oxygen content in ACF by activation were calculated according to the following equation and are summarized in Table I: Onet
= Odif - (OS04-k
Oorig)
(1)
where 0, = net increase of oxygen content, Om = oxygen content of activated ACF, 090,= oxygen in the remaining HzS04, and Oorig= oxygen content of original ACF. The net increase increased first with longer heat treatment to exhibit the maximum by the times of 4-6 h according to ACFs and then decreased rather rapidly. Figure 5 illustrates TPD profiles of the original form of OG-5A and its activated form with 300% H#04 at the 250 "C. The original ACF released CO and COz only above 400 "C. The activated ACF released SO2by two peaks at 70-120 "C and 200-400 "C, the peak at higher temperatures being dominant. The peaks may originate from adsorbed SO2 and remaining HzSO4 after the activation, respectively. Releases of CO and C02 also started at 200 "C and increased with rising temperature, giving a large peak of C02at 300 "C and monotonous increase of CO with a small peak at 280 OC. Marked increase of C02 evolution by the activation and its rather narrow range of evolution
Discussion Further activation of ACFs with H2S04was found to enhance very much their capture of ammonia for their practical application as deodorants. Another characteristic of the present activated ACFs is that the activity is regenerated by heating at 185 "C to release ammonia. Such a regeneration can be never expected with HzS04 impregnated on the ACFs. The averaged amount of adsorbed ammonia on ACFs in the repeated runs except for the first run is plotted against the net oxygen content in Figure 6 since HzSO4 remaining on ACFs is not expected to contribute to such an adsorption of ammonia. A fair correlation is obtained with some scattering in Figure 6. The oxygen functionalities are not always acidic: some of them are basic or neutral according to their chemical structures. The acidic sites among the oxygen functionalities are principally carboxylic groups (Donnet, 1968) which are expected to give carbon doxide when they decompose by heating. Phenolic groups are also acidic, but their acidity may not be strong enough to capture ammonia irreversibly. The amount of carbon doxide released from ACF in the temperature range of 250-400 "C is correlated with the averaged amount of adsorbed ammonia until as shown in Figure 7, where an excellent breakthrough (To) linear correlation was obtained. The acid strength of surface carboxylic groups is expected to be strong enough to capture ammonia at room temperature and release it at 185 "C. Carbon dioxide evolved above 400 "C may originate from esters and lactones which are not acidic. Such acidic sites are introduced by oxidation with H#04 but also tend to decompose thermally. Hence, the temperature and time of activation should be carefully selected to maximize the number of sites by more oxidation and to minimize their decomposition. Experimentally, 250 "C appears to be the optimum temperature with all ACFs of the present study and the time is in the range of 4-6 h according to kinds of ACF. Slightly different sizes of conjugated carbon planes of ACF surface may influence reactivity for oxidation and stabilities of surface carboxylic groups.
Ind. Eng. Chem. Res., Vol. 30,No. 10,1991 2327
,
I
Evolved CO2 (m mol g-'
Ob 10
io h i o $0
I
100
I
'JP
150
Figure 7. Relation between absorbed ammonia on ACFs and C02 amounts evolved at 260-400 OC by TPD analyses. 1, FE-200 (original); 2, FE-200 (3/250/4); 3, FE-200 (3/250/5); 4, FE-200 (3/ 250/6); 5, FE-400 (original); 6, FE-400 (3/250/5); 7, OG-20A (original); 8, OG-20A (3/250/4); 9, OG-5A (original); 10, OG-5A (3/ 250/6); 11, ACN-210-20 (original); 12, ACN-210-20 (3/250/5); 13, ACN-210-20 (3/250/6).
Figure 8. Langmuir plots of ammonia adsorption on activated ACFs until Toand TIw (m) FE-200 (3/250/4), first run until To; (0) FE-200 (3/250/4), second run until To;( 0 )FE-200 (3/250/4), first FE-200 (3/250/4), second run until Tlw; (A) run until Tlw;(0) OG-5A (3/250/6), first run until Tlw; (A)OG-5A (3/250/6), second run until Tim
The roles of remaining H#04 may be of value to discuss. It is always present to a certain extent when the surface
Nomenclature ACF = active carbon fiber PAN = polyacrylonitrile TPD = temperature-programmed decomposition F = flow rate of gas, mL/min Onet = net increase of oxygen content, wt % Odir = oxygen content of activated ACF by difference in microanalyses, wt % Oso, = oxygen in the remaining H2S04, w t % Oodg= oxygen content of original ACF, wt % P = equilibrium pressure of adsorption, mmHg To= time until the first detection of ammonia elution TlW= time until 100% elution of ammonia V = volume of adsorbed gas, mL/g W = weight of adsorbent, g Registry No. NH3, 7664-41-7; H2S04,7664-93-9.
carboxyl groups are maximized since its oxidative release takes place simultaneously with the evolution of carbon dioxide. It certainly adsorbs ammonia, forming ammonium sulfate in the first run to increase markedly the amount of captured ammonia. However, it is never regenerated again at 185 "C where the decomposition of surface carboxyl groups is minimized. Ammonium sulfate produced in the first run may cover the surface or block the pore mouth to hinder the adsorption of ammonia in the successive runs. Furthermore, remaining HzS04has a chance to elute out in a humid atmosphere where water may be condensed on the ACF surface. Thus, remaining H2S04 should be minimized by maximizing the surface carboxylic groups. Figure 8 illustrates Langmuir plots of ammonia adsorption on activated PAN-ACF-FE-200 (3/250/4) and OG-5A (3/250/6) under its variable concentrations until and Tloo)in the first and second runs. the breakthough (To Excellent linear correlations are obtained, with both amounts of adsorption regardless of ACFs, suggesting validity of the Langmuir type of adsorption. There may be kinetic and equilibrum views for this adsorption. The amount of adsorption by Toon PAN-ACF-FE-200 is obtained under no ammonia left in the gas phase, with no eqilibrium being achieved before the breakthough (To). Thus, the Langmuir type kinetics may govern the adsorption amount. Two parameters, the equilibrium constant ( K ) , including the number of active sites, and the rate constant of adsorption (k),are calculated from the linear correlations. The adsorption amounts by TI,in the first and second runs on PAN-ACF-FE-200 provide lines parallel to those by To.The difference between pairs is too small for further discussion. It should be mentioned that Toand TI, became similar in repeated use. The sites for slow adsorption influencing TI, may not be regenerated. The difference of two ACFs in the amounts by TIoois found to be definite in their equilibrium constants, which may include the number of active sites, being consistent with a linear correlation in Figure 7. Repeated use tends to decrease significantly the rate constants of both ACFs, suggesting the physical blockade of ammonium sulfate. The accuracy of the equilibrium constant may not be enough to discuss their change by repeated use.
Literature Cited Donnet, J. B. The Chemical Reactivity of Carbons. Carbon 1968,6, 161-176. Hara, N.; Takahashi, H. Zeolite, Fundamental and Application; Kodansha: Tokyo, 1978; pp 89-110. Komatsubara, Y.; Ida, S.; Fujitsu, H.; Mochida, I. Catalytic activity of PAN-based active carbon fibre (PAN-ACF) activated with sulphuric acid for reduction of nitric oxide with ammonia. Fuel 1984,63, 1738-1742. Kunibe, S. New Technique for Removal of Bad Smells; Kogyo Chosakai: Tokyo, 1988; p 175. Matsumura, Y. Actiuated Carbon, Fundamental and Application; Tanso, Zairyou, Gakkai, Eds.; Kodansha: Tokyo, 1984; p 133. Mochida, I.; Ogaki, M.; Fujitsu, H.; Komatsubara, Y.; Ida, S. Mechanism in the Reduction of Nitrogen Monoxide with Ammonia on the Coke Activated with Sulfuric Acid. Nippon Kagaku Kaishi 1985a, No. 4,680-684. Mochida, I.; Mizoziri, T.; Fujitsu, H.; Komatsubara, Y.; Ida, S. Reaction of Nitrogen Monoxide with Ammonia over a Series of Pan-Based Active Carbon Fibers (PAN-ACF) Treated with Sulfuric Acid. Nippon Kagaku Kaishi 198513, No. 9, 1676-1684. Mochida, I.; Fujitsu, H.; Shiraishi, I.; Ida, S. Catalytic Activities of an Active Cabon and a PAN-ACF for the Reduction of Nitrogen Monoxide with Ammonia. Nippon Kagaku Kaishi 1987, No. 5 , 797-801. Mochida, 1.; Kawano, S.; Fujitsu, H. Regenerative Removal of Ammonia in the Atmosphere with Active Carbon Fibers. Chem. Lett. 1990, NO.9, 1627-1630. Tsurumi Coal Co. Actiuated Carbon for Remouing Alkaline Gases; Product information; Tsurumi Coal Co., Ltd.: Tokyo, 1989; pp 1-14. Receiued for review June 5, 1991 Accepted June 25, 1991
