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The process of activation of carbons by gasification with CO2-III. Uniformity of gasification
Cnrbon
1971, Vol. 9, pp. 79-85.
Pergamon
Press.
THE PROCESS GASIFICATION
Printed
in Great
Britain
OF ACTIVATION WITH CO, -111. GASIFICATION B. RAND*
Northern
OF CARBONS BY UNIFORMITY OF
and H. MARSH
Coke Research Laboratories, School of Chemistry, The University of Newcastle upon Tyne, Newcastle upon Tyne, NE1 7RU, England (Received 2 March 1970)
Abstract-An 850°C polyfurfuryl alcohol carbon has been gasified at 800°C in pure carbon dioxide and mixtures thereof with carbon monoxide to 25 per cent burn-off. The effect of flow rate and particle size of the carbon upon the development of the microporosity has been studied. At low flow rates (particle size -3O+ 44 B.S. mesh) the carbon monoxide retained at the carbon surface inhibits gasification of the particle exterior resulting in the development of a greater micropore volume than when high flow rates are used. At this high flow rate (200 cm3 (S.T.P.) min-r) only with small particels (cl20 B.S. mesh) is complete uniformity of gasification approached. Addition of carbon monoxide to the pure carbon dioxide results in an enhancement of the degree of uniformitv , of gasification and of micropore volume at this 25 per cent burnoffulevel. ”
1. INTRODUCTION
4. EXPERIMENTAL
In the two previous papers of this series [ 1,2], the effect of CO, gasification (800°C) on the microporosity of an 850°C polyfurfury1 alcohol (PFA) carbon, pure and containing known amounts of catalytic metallic impurities, has been studied. In those studies a COZ flow rate of 15cm3min-’ was arbitrarily chosen and it was thought that the temperature of 800°C with a resultant low gasification rate, might result in gasification being free from pore diffusion effects. However, evidence was produced which showed that uniformity of gasification did not occur. For example conical pores were developed at the particle exterior and the ratio of internal to total weight loss was only about 0.75. In these studies we have attempted to establish which physical factors controlled rates of gasification and porosity development of the PFA carbons studied earlier [ 11. *Present address: Sheffield, England.
University
of
The carbon was from the batch investigated earlier [ 1,2]. Gasification was carried out in a gravimetric flow apparatus[2] and all other procedures were as described previously. In addition to the use of pure carbon dioxide, mixtures were employed of carbon dioxide, carbon monoxide and nitrogen at 1 atm total pressure and a total flow rate of 200 cm3 (S.T.P.) min-‘. With mixtures it was necessary to raise the reaction temperature to 840°C since at 800°C the rates of the inhibited reaction were prohibitively low. 3. RESULTS AND DISCUSSION The rate of gasification (in pure carbon dioxide at 1 atm) of the pure 850°C PFA carbon gradually increased with burn-off to 3 per cent burn-off, and then remained constant at least up to 25 per cent burn-off, when the reaction was stopped. Figures 1 and 2 show the effect of CO, flow rate on the rate of gasification (particle size of carbon, -3O+
Sheffield, 79
80
‘I.
B. RAND and H. MARSH
Burn-off
Reaction *
Fig. 1. Effect
of flow rate
time 105 cm3
(hours1
+
[STPItnin-’
on the rate of gasification of pure 850°C polyfurfuryl (B.S. mesh -30 + 44) in CO, at 800°C.
250cm3(STPl alcohol
carbon
0 r-i
0.3 Rate ‘I.
burn- off hi’
I
50
8
I
100
150 Flow
Fig. 2. Effect
rate
I
200
250
cm31STPlnin-’
of flow rate on the rate of gasification of pure 850°C polyfurfuryl carbon (B.S. mesh -3O+ 44) in COz at 800%.
mif’
alcohol
THE
PROCESS
OF
ACTIVATION
81
OF CARBONS-III
concentration gradient of carbon dioxide throughout the pore structure as well as across the ‘stagnant’ layer at the carbon particle exterior. It could be that increasing the flow rate of the carbon dioxide may enhance its penetration into the inner porosity of the particle so enhancing the micropore volume over that obtained at low flow rates. This result was not obtained and it appears that the ratio of external to internal burn-off is changed, in in fact it is increased as a result of removal of the inhibiting carbon monoxide in the outermost sections of the particle. This results in a loss of porosity from the outermost sections of t.he particle greater than any increase in porosity developed in the particle interior by more uniform gasification. The ratios of external to total burn-off, calculated by the mercury density method [l] are in Table 1.
44 B.S. mesh) in this constant rate region. The rate of gasification increases to a constant value at flow rates above 100cm3 (S.T.P.) min-‘. Figures 3 and Table 1 show the adsorption data for carbon dioxide at 195°K and nitrogen at 77°K on the carbons gasified to 25 per cent burn-off at flow rates of 15 and 105cm”(S.T.P.) min-i. It is seen that the higher micropore volume (adsorptive capacity) is developed by gasification at the lower flow rate, presumably because of jnhibition by the product, carbon monoxide, in the outermost sections of the particle. However, as there is considerable develstructure during of micropore opment gasification to 25 per cent burn-off, so there of the carbon is obviously penetration dioxide into the micropore structure of the carbon. Therefore, at low gas flow rates gasification is not in any of the three idealised zones of Wicke[3], but there is probably a
-
co2
-----
II2
at 195’K
at 77’K
lop;0 IPJPl 0
co2 fkw
rate I15 cm3~STP~lnin-’
A
CO2 flow
rate = 105 cm31SlPl
min.’
Fig. 3. Adsorption of N, and CO, on pure 850°C ~o~yfurfu~l alcohol carbon, 25 per cent burn-offin COz at 800°C (cf. Fig. I).
CAR. Vol. 9 No. 1 -F
200
200
200
200
200
-30 -t 44
-30+44
-30-k-44
-30+44
+-Fe -30 + 44
+Fe
105 200 200
-30 -t-44 -30 -t 44 -120
15
(cm3 (S.T.P.) min-*)
B.S. Mesh
-30+44
Total gas How rate
Particle size
840
840
840
840
840
Pcm = 45.6 PC0= 15‘2 PNz= 15.2 PC&= 45.6 ‘P,, = 5.6 PN,= 24.8 PC** = 45.6 I?~~=:0 P, = 30.4 P c0p= 45.6 PGo= 5.6 P,, = 24.8
0.17
0.6
0*16
0.094
0.36
0.23
0.38
0.41
0+33
O-330 0.340 o-445
800 800 800
Pcoz= 45.6 PC0= 0 PNZ= 30.4
0.352
800
volume cm3 g-’ (N, at 77°K)
Micro pore
Pure COz t arm. Zatm. I. atm. 1 atm.
(% hr-‘)
Rate of gasification
(“C.)
Gasification temperature
(P = cm Ng)
Reactant gas Composition
I *22
1.26
1.04
l-08
l-08 1.08 l-02
l-06
Particle (mercury) density (g . cmw3)
Table 1. Gasification data for 850°C PFA carbon at 25 per cent burn-off
044
0
0.89
O-76
O-76 0.76 O-97
a433
Ratio internal to total burn-off
‘ 9 rl ‘Triangle’ method
THE
PROCESS
OF ACTIVATION
In order to ascertain whether gasification at the high flow rates was affected by mass transport of reactant, two different particle size fractions of PFA carbon were gasified (Fig. 4, Table 1). Rates of gasification increased with decreasing particle size. It is possible to relate these rates using the effectiveness factor, ‘?I’, (a measure of chemical versus physical control of rate) using the triangle method of Weisz and Prater[4], assuming the reaction to be first order with respect to carbon dioxide (see Table 1). These values of ‘n’ suggest that, gasification of the smallest particle size fraction was chemically controlled, whereas gasification of the -30 + 44 mesh fraction was subject to control by ‘in-pore’ diffusion. -However, a caution must be mentioned. The determination of the effectiveness factor by this method cannot be unreservedly accepted because of an additional factor. Austin and Walker{51 have shown that carbon monoxide inhibition can
OF CARBONS-III
83
cause marked non-uniformity of gasification under conditions which by calculation of the dimensionless group @[6] appear to describe chemical reaction control. Therefore, a more realistic criterion of gasification under chemical control is the ratio of internal to total burn-off which should be equal to the ratio of surface area within pores to the total surface area. For chemical control this ratio should be equal to unity for carbons with a large micropore volume. Table 1 shows that for these samples of carbon the ratios of internal to total burn-off, as do the ‘n’ values, suggest that gasification of the smallest particle size fraction is practically uniform, whilst the -30 +- 44 mesh fraction is markedly non-uniform. These results show that the activated series of carbons described earlier 111were gasified under conditions where diffusion of CO, to the particle exterior and through the pore structure as well as inhibition by carbon mon-
LO Reaction
B. RAND and H. MARSH
84
oxide affect the rate microporosity.
4. EFFECT
and
development
OF CARBON MONOXIDE THE GAS STREAM
of
IN
If the above considerations of the gasification of 850°C PFA carbon in pure carbon dioxide are correct, the addition of carbon monoxide into the gas-stream (at the higher flow rate) should result in a greater micropore volume developed at 25 per cent burn-off, as well as decreasing the rate of gasification. That this is the case is shown in Fig. 5 and Table 1. Although the uniformity of gasification and the development of microporosity, of the -30+44 mesh particles, is improved (e.g. ratio of internal to total burn-off increased from 0.76 to O-89) complete uniformity was not obtained with the partial
Reaction
pressures of carbon monoxide employed. These results do, however, support the suggestion of Austin and WaIker[5] that complete uniformity of gasification of microporous carbons can only be attained using CO/CO, reactant mixtures or very small carbon particles. The effect of carbon monoxide on the gasification of the iron contaminated carbon [Z] was also investigated (Fig. 6, Table 1). Again, the development of microporosity was improved. However, a surprising feature was that the reaction rate was only inhibited after the iron catalyst was rendered inactive. Thus, it would seem that carbon monoxide has no inhibiting effect upon the catalysed CO,-carbon reaction. However, this result opens up a new area of study, that of gasification of impure carbons using CO/CO, or H,/CO, mix&es.
time
(hours)
Fig. 5. Effect of addition of CO on the rate of gasification of pure 850°C polyfurfuryl alcohol carbon (B.S. mesh-30 -t-44) in CO, at 840°C (total flow rate = 200 cm3 (S.T.P.) min-‘1.
THE
PROCESS
OF ACTIVATION
OF CARBONS-III
30
20 Reactcontrmt A
PcO,=C5.6; Pn2r 30Icm
0
i hoursi
PC0 ~b56.P~ :216.PCOz56em 2
2
Fig. 6. Effect of addition of CO on the rate of gasification of 850°C polyfurfury~ alcohol carbon (B.S. mesh -30+44), containing iron Fe: C = 1: 1000, in COZ at 840°C (total flow rate = 200 cm3 (S.T.P.) min-I). Acknowledgements-This study forms part of the fundamental research programme of the British Coke Research Association and we are grateful to the Council for permission to publish. One of us (B.R.) acknowledges financial support from the B.C.R.A.
1. Marsh
REFERENCES H. and Rand B., Carbon 9,47 (1971).
2. Marsh H. and Rand B., Carbon 9,63 (1971). 3. Wicke E., Fifth Symp. on Combustion, p. 245. Reinhold, New York (1955). 4. Weisz P. B. and Prater C. D., Adu. Catalysis 6, 143 (1954). 5. Austin .J. G. and Walker P. L.. .Jr.. .I. Alit. Inst. Chsm. Eng. 9,303 (1963). 6. Walker P. L., Jr., Rusinko F., Jr. and Austin L. G.,A&. Catal@ 1x, 133 (1959).
1971, Vol. 9, pp. 79-85.
Pergamon
Press.
THE PROCESS GASIFICATION
Printed
in Great
Britain
OF ACTIVATION WITH CO, -111. GASIFICATION B. RAND*
Northern
OF CARBONS BY UNIFORMITY OF
and H. MARSH
Coke Research Laboratories, School of Chemistry, The University of Newcastle upon Tyne, Newcastle upon Tyne, NE1 7RU, England (Received 2 March 1970)
Abstract-An 850°C polyfurfuryl alcohol carbon has been gasified at 800°C in pure carbon dioxide and mixtures thereof with carbon monoxide to 25 per cent burn-off. The effect of flow rate and particle size of the carbon upon the development of the microporosity has been studied. At low flow rates (particle size -3O+ 44 B.S. mesh) the carbon monoxide retained at the carbon surface inhibits gasification of the particle exterior resulting in the development of a greater micropore volume than when high flow rates are used. At this high flow rate (200 cm3 (S.T.P.) min-r) only with small particels (cl20 B.S. mesh) is complete uniformity of gasification approached. Addition of carbon monoxide to the pure carbon dioxide results in an enhancement of the degree of uniformitv , of gasification and of micropore volume at this 25 per cent burnoffulevel. ”
1. INTRODUCTION
4. EXPERIMENTAL
In the two previous papers of this series [ 1,2], the effect of CO, gasification (800°C) on the microporosity of an 850°C polyfurfury1 alcohol (PFA) carbon, pure and containing known amounts of catalytic metallic impurities, has been studied. In those studies a COZ flow rate of 15cm3min-’ was arbitrarily chosen and it was thought that the temperature of 800°C with a resultant low gasification rate, might result in gasification being free from pore diffusion effects. However, evidence was produced which showed that uniformity of gasification did not occur. For example conical pores were developed at the particle exterior and the ratio of internal to total weight loss was only about 0.75. In these studies we have attempted to establish which physical factors controlled rates of gasification and porosity development of the PFA carbons studied earlier [ 11. *Present address: Sheffield, England.
University
of
The carbon was from the batch investigated earlier [ 1,2]. Gasification was carried out in a gravimetric flow apparatus[2] and all other procedures were as described previously. In addition to the use of pure carbon dioxide, mixtures were employed of carbon dioxide, carbon monoxide and nitrogen at 1 atm total pressure and a total flow rate of 200 cm3 (S.T.P.) min-‘. With mixtures it was necessary to raise the reaction temperature to 840°C since at 800°C the rates of the inhibited reaction were prohibitively low. 3. RESULTS AND DISCUSSION The rate of gasification (in pure carbon dioxide at 1 atm) of the pure 850°C PFA carbon gradually increased with burn-off to 3 per cent burn-off, and then remained constant at least up to 25 per cent burn-off, when the reaction was stopped. Figures 1 and 2 show the effect of CO, flow rate on the rate of gasification (particle size of carbon, -3O+
Sheffield, 79
80
‘I.
B. RAND and H. MARSH
Burn-off
Reaction *
Fig. 1. Effect
of flow rate
time 105 cm3
(hours1
+
[STPItnin-’
on the rate of gasification of pure 850°C polyfurfuryl (B.S. mesh -30 + 44) in CO, at 800°C.
250cm3(STPl alcohol
carbon
0 r-i
0.3 Rate ‘I.
burn- off hi’
I
50
8
I
100
150 Flow
Fig. 2. Effect
rate
I
200
250
cm31STPlnin-’
of flow rate on the rate of gasification of pure 850°C polyfurfuryl carbon (B.S. mesh -3O+ 44) in COz at 800%.
mif’
alcohol
THE
PROCESS
OF
ACTIVATION
81
OF CARBONS-III
concentration gradient of carbon dioxide throughout the pore structure as well as across the ‘stagnant’ layer at the carbon particle exterior. It could be that increasing the flow rate of the carbon dioxide may enhance its penetration into the inner porosity of the particle so enhancing the micropore volume over that obtained at low flow rates. This result was not obtained and it appears that the ratio of external to internal burn-off is changed, in in fact it is increased as a result of removal of the inhibiting carbon monoxide in the outermost sections of the particle. This results in a loss of porosity from the outermost sections of t.he particle greater than any increase in porosity developed in the particle interior by more uniform gasification. The ratios of external to total burn-off, calculated by the mercury density method [l] are in Table 1.
44 B.S. mesh) in this constant rate region. The rate of gasification increases to a constant value at flow rates above 100cm3 (S.T.P.) min-‘. Figures 3 and Table 1 show the adsorption data for carbon dioxide at 195°K and nitrogen at 77°K on the carbons gasified to 25 per cent burn-off at flow rates of 15 and 105cm”(S.T.P.) min-i. It is seen that the higher micropore volume (adsorptive capacity) is developed by gasification at the lower flow rate, presumably because of jnhibition by the product, carbon monoxide, in the outermost sections of the particle. However, as there is considerable develstructure during of micropore opment gasification to 25 per cent burn-off, so there of the carbon is obviously penetration dioxide into the micropore structure of the carbon. Therefore, at low gas flow rates gasification is not in any of the three idealised zones of Wicke[3], but there is probably a
-
co2
-----
II2
at 195’K
at 77’K
lop;0 IPJPl 0
co2 fkw
rate I15 cm3~STP~lnin-’
A
CO2 flow
rate = 105 cm31SlPl
min.’
Fig. 3. Adsorption of N, and CO, on pure 850°C ~o~yfurfu~l alcohol carbon, 25 per cent burn-offin COz at 800°C (cf. Fig. I).
CAR. Vol. 9 No. 1 -F
200
200
200
200
200
-30 -t 44
-30+44
-30-k-44
-30+44
+-Fe -30 + 44
+Fe
105 200 200
-30 -t-44 -30 -t 44 -120
15
(cm3 (S.T.P.) min-*)
B.S. Mesh
-30+44
Total gas How rate
Particle size
840
840
840
840
840
Pcm = 45.6 PC0= 15‘2 PNz= 15.2 PC&= 45.6 ‘P,, = 5.6 PN,= 24.8 PC** = 45.6 I?~~=:0 P, = 30.4 P c0p= 45.6 PGo= 5.6 P,, = 24.8
0.17
0.6
0*16
0.094
0.36
0.23
0.38
0.41
0+33
O-330 0.340 o-445
800 800 800
Pcoz= 45.6 PC0= 0 PNZ= 30.4
0.352
800
volume cm3 g-’ (N, at 77°K)
Micro pore
Pure COz t arm. Zatm. I. atm. 1 atm.
(% hr-‘)
Rate of gasification
(“C.)
Gasification temperature
(P = cm Ng)
Reactant gas Composition
I *22
1.26
1.04
l-08
l-08 1.08 l-02
l-06
Particle (mercury) density (g . cmw3)
Table 1. Gasification data for 850°C PFA carbon at 25 per cent burn-off
044
0
0.89
O-76
O-76 0.76 O-97
a433
Ratio internal to total burn-off
‘ 9 rl ‘Triangle’ method
THE
PROCESS
OF ACTIVATION
In order to ascertain whether gasification at the high flow rates was affected by mass transport of reactant, two different particle size fractions of PFA carbon were gasified (Fig. 4, Table 1). Rates of gasification increased with decreasing particle size. It is possible to relate these rates using the effectiveness factor, ‘?I’, (a measure of chemical versus physical control of rate) using the triangle method of Weisz and Prater[4], assuming the reaction to be first order with respect to carbon dioxide (see Table 1). These values of ‘n’ suggest that, gasification of the smallest particle size fraction was chemically controlled, whereas gasification of the -30 + 44 mesh fraction was subject to control by ‘in-pore’ diffusion. -However, a caution must be mentioned. The determination of the effectiveness factor by this method cannot be unreservedly accepted because of an additional factor. Austin and Walker{51 have shown that carbon monoxide inhibition can
OF CARBONS-III
83
cause marked non-uniformity of gasification under conditions which by calculation of the dimensionless group @[6] appear to describe chemical reaction control. Therefore, a more realistic criterion of gasification under chemical control is the ratio of internal to total burn-off which should be equal to the ratio of surface area within pores to the total surface area. For chemical control this ratio should be equal to unity for carbons with a large micropore volume. Table 1 shows that for these samples of carbon the ratios of internal to total burn-off, as do the ‘n’ values, suggest that gasification of the smallest particle size fraction is practically uniform, whilst the -30 +- 44 mesh fraction is markedly non-uniform. These results show that the activated series of carbons described earlier 111were gasified under conditions where diffusion of CO, to the particle exterior and through the pore structure as well as inhibition by carbon mon-
LO Reaction
B. RAND and H. MARSH
84
oxide affect the rate microporosity.
4. EFFECT
and
development
OF CARBON MONOXIDE THE GAS STREAM
of
IN
If the above considerations of the gasification of 850°C PFA carbon in pure carbon dioxide are correct, the addition of carbon monoxide into the gas-stream (at the higher flow rate) should result in a greater micropore volume developed at 25 per cent burn-off, as well as decreasing the rate of gasification. That this is the case is shown in Fig. 5 and Table 1. Although the uniformity of gasification and the development of microporosity, of the -30+44 mesh particles, is improved (e.g. ratio of internal to total burn-off increased from 0.76 to O-89) complete uniformity was not obtained with the partial
Reaction
pressures of carbon monoxide employed. These results do, however, support the suggestion of Austin and WaIker[5] that complete uniformity of gasification of microporous carbons can only be attained using CO/CO, reactant mixtures or very small carbon particles. The effect of carbon monoxide on the gasification of the iron contaminated carbon [Z] was also investigated (Fig. 6, Table 1). Again, the development of microporosity was improved. However, a surprising feature was that the reaction rate was only inhibited after the iron catalyst was rendered inactive. Thus, it would seem that carbon monoxide has no inhibiting effect upon the catalysed CO,-carbon reaction. However, this result opens up a new area of study, that of gasification of impure carbons using CO/CO, or H,/CO, mix&es.
time
(hours)
Fig. 5. Effect of addition of CO on the rate of gasification of pure 850°C polyfurfuryl alcohol carbon (B.S. mesh-30 -t-44) in CO, at 840°C (total flow rate = 200 cm3 (S.T.P.) min-‘1.
THE
PROCESS
OF ACTIVATION
OF CARBONS-III
30
20 Reactcontrmt A
PcO,=C5.6; Pn2r 30Icm
0
i hoursi
PC0 ~b56.P~ :216.PCOz56em 2
2
Fig. 6. Effect of addition of CO on the rate of gasification of 850°C polyfurfury~ alcohol carbon (B.S. mesh -30+44), containing iron Fe: C = 1: 1000, in COZ at 840°C (total flow rate = 200 cm3 (S.T.P.) min-I). Acknowledgements-This study forms part of the fundamental research programme of the British Coke Research Association and we are grateful to the Council for permission to publish. One of us (B.R.) acknowledges financial support from the B.C.R.A.
1. Marsh
REFERENCES H. and Rand B., Carbon 9,47 (1971).
2. Marsh H. and Rand B., Carbon 9,63 (1971). 3. Wicke E., Fifth Symp. on Combustion, p. 245. Reinhold, New York (1955). 4. Weisz P. B. and Prater C. D., Adu. Catalysis 6, 143 (1954). 5. Austin .J. G. and Walker P. L.. .Jr.. .I. Alit. Inst. Chsm. Eng. 9,303 (1963). 6. Walker P. L., Jr., Rusinko F., Jr. and Austin L. G.,A&. Catal@ 1x, 133 (1959).