2290
Langmuir 1992,8, 2290-2294
Marked Increase of SO2 Removal Ability of Poly(acrylonitrile)-Based Active Carbon Fiber by Heat Treatment at Elevated Temperatures Xsao Mochida,' Teruo Hirayama, Seiki Kisamori, Shizuo Kawano, and Hiroshi Fujitau Institute of Advanced Material Study, Department of Molecular Science and Technology, Graduate School of Engineering Sciences, Kyushu University, Kasuga, Fukuoka 816, Japan Received January 7,1991. In Final Form: April 14, 1992
The adsorption capacity of poly(acry1onitrile)-baaed active carbon fibers(PAN-ACFs)for SO2 removal in flue gas was studied at a temperature range of 298-423 K, using a fixed bed flow reactor. Heat treatment at 1073 K tripled the capacity of ACFs until the breakthrough, the amount of adsorbed SO2 at 373 K being moat markedly increased from 1.1 mmol gl for as-received ACF-FE300 to 3.2 m o l gl for the heattreated one. Sucha marked"ni of adsorptioncapacityby heat treatmentm a y comefrom the appropriate numbersof oxygen functionalitieson the pore walls of PAN-ACF which are controlledthrough the evolution of CO and C02 with the least decrease on the surface area. Introduction Advanced technologyfor SO2 removal from flue gas has been aimed at protecting the atmoapheric environment from the pollutant with less coat. Although the firation of SO2 as CaSO4 has been commercialized for some time, more efficient dry proteases, in terms of smaller facility, more flexibility for the sulfur products, and less energy and water, have been the objective of continuous study to protect the global environment from acid rain, since SO2 leaked in the flue gas ia oxidized into SO3 to form H2S04 mist in the atmosphere. Active carbons and cokes have been recognized as excellent adsorbenta for SO2l3 and an industrial application has been successful, using active coke of high mechanical strength and moderate adsorption ability in the moving bed.- Such a process completely removes SO2 around 423 K, with recovery of concentrated SO2 around 673K, captures dusts, and has sufficientdurability in the moving bed. Simultaneous reduction of NO, with NHs on the same coke is another advantageof the process.6 Neverthelees carbon adsorbenta with a larger capacity for SO2 adsorption are strongly desired for smaller facilities. Active carbon fiber prepared from poly(acry1onitrile) (PAN-ACF)has been reported to exhibit a high activity for NO reduction with N H 3 , especially after the oxidation with Hfi04.7-g Ita adsorption ability for SO2 was found to be largest among the commercially available active carbons as reported in a previous paper.10 In the present study, the influences of heat treatment of PAN-ACF at a temperature range of 423-1273 K were (1)Yamamoto, K.;Seki,M.;Kawazoe, K.Nippon Kagoku Kaishi 1972, 1046. (2) Richter, E.; Knoblnuch, K.; Juntgen, H. Can Sep. Purij. 1987, 1,
35. (3) Knoblauch, K.; Richter,E.; Juutgen, H. Fuel 1981,60,832. (4) Juntgen, H.Erdol. Kohle 1986,39,546. (5) Komataubara, Y.; Yano, M.;Shiraiahi,I.; Ida, S.J. Fuel SOC.Jpn.
1985,64,255. (6) Komataubara, Y.; Teuji, K.; Shiraishi, I.; Ida, S.;Mochida, I. J. Fuel Soe. Jpn. 1986,64,840. (7) Mochida, I.; Ogaki, M.;Fujitau, H.; Komataubara,Y.; Ida, S. Fuel 1983,62,867. (8) Mochida.1.; Fujitau,H.; Shiraishi,I.; 1da.S. Nippon KagokuKaishi 1987,797. (9) Mochida.I.;Ogaki,M.;Fujitsu, H.; Komateubara,Y.; Ida,S.Nippon Kagaku Kaiehi 1986,680. (10) Mochida,I.; Maaumura, Y.; Hirayama, T.; Fujitau, H.; Kawano, 5.;Goto, K. Nippon Kagaku Kaishi 1991,269.
0743-746319212408229O$03.OO/0
studied to reveal the relation between adsorption capacity and oxygen functional groups on the PAN-ACF surface, with the aim of increasing ita adsorption capacity. Ishizuka et al.I1 reported the enhancing effecte of heat treatment on the specificoxidationrates of SO2for granular activecarbonsprepared from coal or coconut shell;however the capacity for SO2 removal was little affected by the treatment, being dependent principally on the sources of active carbons. Heat treatment must decompose the oxygen functionalgroups into COZ and CO at temperaturea which vary with their reactivity.12-16 Hence the number of the groups can be controlled by the treatment. The groups are believed to adsorb SO2 through oxidation. Hence their decrease may appear to lead to the reduction of adsorptioncapacity; however toomany sitesare expected to hinder the adsorptionthroughthe repulsiveinteractions of adsorbed SO2 (H2SO4)and H20 present in the adsorbed multilayers. Thus, proper heat treatment can control the number of oxygen functionalgroupswith the least decrease of surface area to maximize the effective active sites kinetically accessible for S02.
Experimental Section Active Carbon Fibers. Active carbon fibers used in the present study are listed in Table I with some of their properties. ACF was heat treated in N2 atmosphere at 423-1273 K for 1 h. Some of the properties of the pre-heat-treated products are summarized in Table I. Their surface areas, external surface areas, and micropore volumes were measured according to the BET method and the t plot using N2.17-19 Procedures for SO2 Removal and Adsorption Meruurement. The SO2 removal was carried out in a fied bed flow reactor. The reactant gas contained lo00ppm S02,5% 02,10% H20, and the balance Nz. The weight of ACF and the total flow rate were 0.5 g and 100 mL min-l, respectively. The SO2 con(11)Ishimka, y.;Kuronuma, H.;Yama* H.;Imai, H.NipponKagoku Kaishi 1976,1046. (12) Puri, B. R.;Baneal, R. C. Carbon 1964, 1, 451. (13) Phillips, R.;Vaetola, F. J.; Walker, P. L., Jr. Carbon 1970,8,197. (14) Bansal, R.C.; Vastola,F. J.;Walker,P. L., Jr. Carbon l970,8,443. (15) Tremblay, G.; Vastola,F. J.; Walker, P. L., Jr. Carbon 1978,16, 35.
(16) Yamabe, K.; TaLahaehi, H . Tam0 1980,102, 106. (17) Gregg,S. J.;Sing,K.S.W. Adsorption, Surface Area andPomity, 2nd ed.; Academic Press: London, 1982; p 94. (18) Rodriguez-Rsioso,F.; Martin-Mattinez, J. M.;Prado-Bwguete, C.;Mchaney, B. J. Phys. Chem. 1987,91,515. (19) Kaneko, K.; Suzuki, T.; Kakei, K. Tam0 1989,140,288.
0 1992 American Chemical Society
Increased SO2 Removal Ability of PAN-ACF
Langmuir, Vol. 8, No. 9,1992 2291 Table I. Prowrties of ACFe
sample FE100 FE-200 Fa300 Fa400 ACNWS-10 KF-1600 F-15 0
elemental analysis (wt %) H N Odiff 1.8 9.7 11.0 1.7 5.8 16.7 1.4 4.5 16.0 1.5 2.3 19.4 0.9 0.1 7.0 1.5 1.5 10.5 1.2 0.4 8.8
C 77.5 75.8 78.1 76.8 92 86 89
ash 0.3 0.3 0.3 1.4 e 1.0 0.6
surface areaa (m2/g) 446 887
1141 1020 600 2000 1550
micropore volumeb (mL/g) 0.17 .~ 0.34 0.56
outer surface areac (m2/g) 10 ~. 7 9
0.57
17
raw material
PAN PAN PAN PAN Kynold Cellrose Pitch
FE-100,200,300 were measured by t plot. Others were measured by BET. Pore diameter less than 20 A, measured by t plot; error +0.02
to -0.05. Measured by t plot; experimental error +2 to -3. A kind of phenol resin. e Trace. 100-
-$
< V
0
1
2
3
Ti me ( h ) Figure 1. Breakthrough curvea of SO2 over PAN-ACFs: reaction temp., 373 K W/F, 5 x 10" g min mL-l; S02,lOOO ppm; 02 5%; H20,10%; NZbalance; (1)FE-100, (2) FE-200, (3) FE-300, (4) FE-400. centration in inlet and outlet gases waa analyzed by a flame photometric detector (FPD). The adsorption isotherms of SO2 and 02 on PAN-ACF were measured volumetrically at 373 K. Temperature-Programmed Decomposition of ACF. Temperature-programmed decomposition (TPDE) spectra of the ACFs were measured by using an apparatus made of quartz equipped with a maas spectrometer (TE-600, Nichiden-Anelva, Inc.). A sample of 0.3 g was heated to 1273 K by 10 K min-l and the evolved gases such as CO and CO2 were analyzed by the mass spectrometer.
Results Adsorption Abilities of Active Carbon Fibers. Figure 1 illustrates breakthrough profiles of SO2 in the model flue gas (lo00 ppm S02, 5% 02,10% HzO in Nz) at W/F (the weight of ACF per flow rate of the model gas) of 6 X g min mL-l through as-received PAN-ACFs at 373 K. PAN-ACF-FE-100showed the largest adsorption capacity. It removed SO2 completely until 160 min (TO), when the leak of SO2 increased gradually to reach 50% escape at 180 min (Tw). A more gradual increase of SO2 escape was observable after Tw. Other PAN-ACFs exhibited less capacity for adsorption, which appears to decrease with the increased surface areas and micropore volumes of ACFs. Other ACFs prepared from Kynol, cellulose, and pitch exhibited very limited capacity, emphasizing the advantages of PAN-ACF. Influences of Heat Treatment. The breakthrough curves of SO2 at 373 K through PAN-ACF-FE-300 evacuated at a series of temperatures are illustrated in Figure 2. FE-300 heat treated at 423 K captured SO2 completely for 2 h, then SO2 started to leak, its concentration increasing gradually to reach 50 and 90% at 3.7 and 5 h, respectively. Heat treatment at higher temperatures markedly extended the period of completeremoval, 6 h being obtained by the heat treatment at 1073 K. The profiles of breakthrough curves were much the same regardless of the heat-treatment temperatures. Further higher temperature of 1273K decreased the time very drastically to 2 h.
50.
Mochida et al.
2292 Langmuir, Vol. 8, No. 9,1992 Table 11. Promties of Heat-Treated FE-300
~~
heat-treatment tamp(K) 423 673 873 973 1073 1173 1273 X
ultimata andmie H/C N/C O/C 2.0 1.9 2.0 1.6 1.4 1A 0.4
5.0 5.8 6.0 5.1 4.9 4.0 3.1
18.9 18.7 17.0 16.0 15.6 11.4 6.3
micropore outer surface areaa volumeb surface areac (mL/g) (m2/g) (m2/g) 1018 0.45 5 1057 0.46 8 0.47 9 1071 0.57 6 1144 1026 0.46 6 0.51 5 1010 0.34 5 787
Sa
amt of adeorbed before breakthrough b"mVg)
("OUg)
SOzd Old 0.24 0.09
50rH~Oa HzOr 0.24
1.3
TO
Tso 2.6 3.4
0.42
0.12
2.1
1.1 2.0 2.7
0.37
0.12
3.0
3.2
3.9
0.36
0.12
2.0
1.1
1.8
0 M easured by t plot. b,c See Table I. At 1.33 X 10-1 kPa, at 373 K. e SO2 = 1.33 X 10-1 kPa, H2O = 2.09 X 1W2 P a , at 373 K. At 2.09 10-2 P a , at 373 K. By TO. By Tw.
Table 111. Amounts of Adsorbed 802 before Breakthrough on FE-300/1073at Variable Temwraturer ~
amt of adsorbed SO2
temp(K)
To00
TsoM
298 343 373 413 473
9.5 7.9 5.9 5.0 4.3
10.5 9.5 7.3 6.2 5.4
mmoUg 5.1 4.2 3.2 2.7 2.3
wt%
33 27 20 17 16
Table IV. Ef€mt of Ha0 and 0 2 on Desulfurisotion Activities of PAN-ACP
0 0
2 C O 4 M 6 C O . s x r o m
Treatment temperature (T)
Figure 3. Influences of the heat-treatment temperature on the deaulfurhtionactivitiesof PAN-ACFa: reaction temp., 373 K; W/F, 5 X 1o-S g min mL-l; SO2,lOOO ppm; O2,5%; HzO, 10%; Nz balance; (0)F E W , (A)FE-200, ( 0 )FE-300, ( 0 )FE-400.
sample/tempb/gas compc FE-300/160/S02 FE300/800/SO2 FE300/150/S02 + 0 2 FE300/800/so~+ 0 2 FE300/150/SO2 + 0 2 + H2O ~ 3 0 0 / 8 0 0 / s o 2+ 02 + Ha0
To (h) 0.2 0.3 0.4 0.8 2.0 5.9
TW TdSOz TdSOn (h) (mmoVg) (mmoVg) 0.2 0.3 0.4 0.9 2.6 7.2
0.1 0.2 0.2 0.4 1.1 3.2
0.1 0.1 0.2 0.5 1.2 3.5
0 Adsorption temp, 373 K W/F, 5 X 1o-S g min mL-'; SO~,lOOO ppm; 0 2 , 5 % ; HzO, 10%;N2 balance. Evacution. Composition of
let gas.
k
IO
Time (h)
t 12
Figure 4. Desulfurization activitiea of PAN-ACF-FE-300/1073 at variable reaction temperatures: reaction temp, (1)298 K,(2) 343K,(3)373 K,(4) 413 K,(5) 473 K;W/F = 5 X 10-8g min mL-1; S02,lOOO ppm; 0 ~ ~ 5 H20,10%; % ; N2 balance.
capacity was very low. Oxygen in the flue gas increased significantlythe adsorption of SO2 to about double. Heat treatment at higher temperature provided twice as much adsorption. The presence of both oxygen and water enhanced the adsorption very much, emphasizing the favorable influences of heat treatment. Adsorption under Static Conditions. Adsorptionof SO2 and 0 2 on Fa300 heat treated at some temperatures was observed at 373 K under static conditions to find adsorption isotherm. SO2without 0 2 indicated Freudlichtype isotherms on FE-300. The equilirium amounts of adsorption at 1.33 X 10-1 kPa of SO2 were summarized in Table 11. The higher temperature of heat treatment increased the amount of adsorption significantly; however small differences were
observable above 873 K. It is estimated that the equilibrium adsorption of SO2 on FE-300 heat treated at 1073 K was 1.2 X 1W2 mmol gl at SO2 concentration of 10oO ppm. Water in the gas showed no influence on the adsorption of so2 without 0 2 . Amounts of adsorbed 0 2 at 1.33 X 10-l kPa are also summarized in Table 11. The amount was much smaller than that of 502 on the same ACF. The heat treatment increased slightlythe amount; however the differencewas negligible when the heatrtreatment temperature was above 873 K. Properties of PAN-ACF Modified by Evacuation. Figure 5 illwtratss tam erature-programmed decomposition profiles of PAN-A Fs. ACFs liberated CO and COz according to the temperature rise, their amounts and profdes being dependent upon the extent of activation. Evolution of CO started at 573 K and increased its amount sharply above 673-823 K to reach the maximum at 1023K and then decreased sharply. Different features of profdes were observable below 1173.K, where the evolution increased sharply with FE-100 and FE-300, whereas no hcreaee was observed up to 1273 K with FE400. The amount of evolved CO below 973 K was most from FE-400 and least with FE-100, appearing to reflect their extent of activation. However, the oxygen evolved as CO from ACF was incomparablyless than that carried by ACF. Evolution of C02 startad at much lower temperature of 523K, reachingthe maximum at 773K, and then decreased gradually to be null at 1073-1123 K. Again FE-300 and
8
Increased SO2 Removal Ability of PAN-ACF
Figure 5. TPDE profiles of PAN-ACFs:
-
Langmuir, Vol. 8, No. 9, 1992 2293
(- -) FE-100, (-
FE-300, (-) FE-400; He carrier; W/F, 3.7 X TR = 10 "C min-l.
- -1
g min mL-l;
FElOO gavethe largest and smallest amounta,respectively, which were much less than those of CO from the same ACFs. Some properties of FE-300 heat treated at a series of temperatures are summarized in Table 11. Heat treatment above 673 K reduced the BET surface area but changed very slightly micropore volume and external surface area. Heat temperature decreased atomicratios of H/C and O/C and the surface area. Heat treatment at 673 K slightly increased N/C but decreased the surfacearea significantly with unchanged H/C and O/C ratios. Treatment at 873 K decreased only the O/C ratio, other factors being unchanged. Further higher temperatures of 973 and 1073 K provided similar values, which were significantlysmaller than those obtained at 673 K except for the surface area. Above 1173 K, the values decreased very markedly, the most marked decreases of N/C and O/C values being obtained at 1273 K, although the ACF still exhibited the ratios of 3.1 and 6.3, respectively. Thus, it should be noted that major amounta of oxygen and nitrogen atoms stayed in ACF even after the treatment at 1273K. Their complete liberation may be achieved above 1973 K.20i21 Discussion The present study revealed that the heat treatment of PAN-ACF at a proper temperature markedly extended the period for the complete removal of SO2 in the flue gas. Since PAN-ACF showed the largest adsorption capacity among the carbons examined, such improvement may contribute to significant reduction of the size of facility. Such marked increases of capacity for SO2 removal, particularlyfor PAN-ACF,are notable because the specific oxidation rates of SO2 over granular active carbons was reported to be increased with only a slight increase of capacity.l1 It is of value to discuss why the heat treatment extended the breakthrough time. SO2 has been proved to be adsorbed through the oxidation into so3 and successive hydration on the active carbon surface. The present study revealed that the copresenceof oxygen and water enhanced the adsorption, suggesting the same mechanism on PANACF regardless the heat-treatment temperature. Hence (20) Fujimoto, K.; Yamada, M.; Iwase,M.; Nagino, H.High Temp.High Pressures 1984,16, 669. (21) Fujimoto,K.; Ohtauka,K.; Iwasa,M.;Yamashita,R.HighTemp.High Pressures 1984, 16,617.
the capacity of SO2 removal before the breakthrough is suggestedto be defined kinetically by the rate of oxidation and the saturation of H2S04 to cover the active sites on the ACF. Within the range of our present study, physical properties of the surface of active carbon fibers such as surface area, micropore volume, and external surface area appear less influential on their performances. The removal capacity of PAN-ACF increased significantly at lower temperatures, suggesting that the rate of oxidation of SO2 can be ruled out as the major factor to define the capacity although the oxidhion and hydration reactions are indespensable for the removal of S02. The SO2 adsorption was completely reversible and ita amount without 0 2 and Hz0 was much less in comparison with removal capacity in their copresence. Adsorption amount of SO2 at lo00 ppm estimated from ita isotherm by the static method was much smaller than that obtained by a flow method probably due to a small leak of oxygen into the flowing gas in the flow method. Adsorption amount of 0 2 was also much smaller than that of SO2 under the presence of 02. hence the adsorption of SO2 and 02is a rather physical one;however their simultaneous adsorption may rapidly form SO3 on the active sites, which is much larger than that for their single adsorption. so3 is hydrated to be transferred from the active site to open it, filling up the polar surface of the pores to be saturated until ita breakthrough. Thus, the removal capacity of PAN-ACF for H2S04 under the present conditions may be defined by the extent of saturation of surface with H2sod. The number of oxidationsites may be influential on the removal capacity of SO2 only when the SO2 concentration in the flue gas is very high. The heat treatment at between 673 and 1173 K very slightly changed the physical properties of the surface but certainly decreased the oxygen content of ACF, although ACF still carries ita major amount even after the evacuation at 1273 K. A particular range of temperatures for the evacuation appears to provide, with the least reduction of surface area, a particular number of the groups which can provide sites for adsorption and oxidation of SO2 and for holding H2S04 on the ACF surface. At the moment, the location and structure of the particular active site are not identified. Neverthelessthe small externalsurfaceof ACFs suggests the importance of pore walls. The size of the pore mouth may also be very important to exclude the influences of diffusion control. The adsorption of S02,02, and H2O under the static conditions exhibited increases similar to that of SO2 removal before the breakthrough. Thus, very probably a particular number of the oxygen groups on ACF appears to allow the maximum adsorption of SOZ, hydrated so3 produced from SO2 through ita oxidation, H20, and 0 2 , although their amounta were very different, suggesting the different sites. The most adequate temperature of the heat treatment is expected reasonably to vary with ACFs because the number and natures of oxygen groups on the surface are strongly subjective to the extent of the activation. The reason why an appropriate number of oxygen groups allows the maximum adsorption is not fully solved. There may be two kinds of sites on the ACF surface: One is for the adsorption and probably oxidation site for SO2 and the other is for holding H2S04 on the surface. The former one is increased by the heat treatment, probably because very active forms of particular oxygen functionalities are produced through their partial decomposition, although any detail on the type of the groups is
2294 Langmuir, Vol. 8, No. 9, 1992
unknown except that the group providea COZ when heated to higher temperature. The whole polar sitea on the d a c e can hold HzS04; however too many sites in the unit area may cause the repulsion of polar adsorbates, or form the liquid phase of HB04 at the pore mouth, inhibiting the diffusion of HzSO4 as well as SO2 into pores. Hence an adequate number of oxygens on the surface of highly activated ACF may "izethe holding activity of HzSod. Excess removal of oxygen groups accompanied with a drastic decrease of surface area naturdy leads surely to the less extent of adbrption. Thus, the surface of PANACF with a balanced number of oxygen groups prepared by the heat treatment at a proper temperature dows the maximum capacity for complete removal of SO2. The
Mochida et al. location of functional groups should be also defined either on the external surfaceor pore wall to discuss more details of adsorption scheme because the groups on the external surface may be more influential on the blockage of pore mouth. Extraordinary high activity of PAN-ACF among the active carbons may reflect ita surfacenitrogen groups. The groups are found to be intimately related to the marked activity of NO reduction with a"onia7f8 Oxygengroups on the pyridine ring may be activated for the oxidation activity. Further detailed study is necessary to revealtheir roles in the adsorption and catalysis. Registry NO. SOz, 7446-09-5.
