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Susceptibility to regeneration of fly ash as an adsorbent material
Resources, Conservation and Recycling, 1 (1988) 137-143 Elsevier Science Publishers B.V./Pergamon Press pie - - Printed in The Netherlands
137
Short Communication
Susceptibility to Regeneration of Fly Ash as an Adsorbent Material* M. ROVATTI, A. PELOSO and G. FERRAIOLO
Istituto di Scienze e Tecnologie dell'lngegneria Chimica, 16145 Genova (Italy) (Received October 2, 1987~accepted in revised form February 6, 1988)
INTRODUCTION
In a previous work, mainly devoted to finding new areas for fly ash utilization [I-5], the possibilityof using fly ash to adsorb organic vapours was examined. Experiments carriedout through a laboratoryadsorption apparatus showed that a fly ash product, obtained afterparticleaggregation and thermal activation at a suitabletemperature, presented satisfactoryadsorption performar~ce for toluene vapours. Since the preliminary treatments of flyash to make the finalproduct showed the fly ash could adsorb a quite remarkable amount, research was devoted to verify whether the saturated sample could be regenerated and how many regeneration cyclesthe sample could sustainwithout detriment to itsadsorption capacity. Finally, some investigations were carried out to examine whether relation° ships exist between the susceptibility of the sample to be regenerated and the granular size of its particles. EXPERIMENTAL
Raw material and sample preparation The investigation was carried out on fly ash produced by combustion of coal from the U.S.A. in an Italian power station. The average chemical composition of untreated fly ash is reported in Table 1. The aggregation of raw material was obtained by chemical means, as referred to in ref. [6 ], by adding a solution of phosphoric acid at a ratio of 25 g of acid to 100 g of fly ash, a ratio which gives the final product the best fluid dynamic beha~iour and the highest abrasion resistence. *This work was suppo~d by a grant from the Italian Public Education Department in the rco search project, "Combustion Technology".
0921-3449/88/$03.50
© 1988 Elsevier Science Publishers B.V./Pergamon Press pie
138 TABLE 1 Averasechemicalcompositionof fly ash used in experiment Substance
% in fly ash
C SiO.~ AI~O~ Fe~O~ TiO~ CaO K.~O SO:~
5.31 45.60 23.50 6.00 1,54 1,65 2.47 0,85 Sect|on
A
Section B V~
FI
NI*C~Hs
F;~
I
m
Fig. 1, Apparatusfor evaluationthe susceptibilityto regeneration. After maturing and drying the viscous-pasty mixture over 24 h, the final p~du~t was milled and sieved; then from this ~natcrial two granular sizes (ranges 2.8m1,18 and 1,18ffi0,35 mm) were isolated and activated by heating for 2 h at 450 °C in a muffle furnace. The products s~ obtained were called sample A and sample B respectively.
Apparatus for regeneration tests The apparatus used to evaluate the samples' susceptibility to regeneration is shown in Fig, 1, It consisted of two sections, A and B. Section A served to produce a gaseous stream feeding the adsorption column and to ensure that the inlet parameters (gas flowrate, toluene concentration and flow temperature) were maintained constant. Schematically, section A consisted of:
139
rathe flowmeter F1C1 which controls nitrogen at flowrate F1 to the vaporizer E and the flowmeter F2C2 wich regulates the flowrate F2 of nitrogen used as diluting gas; - - a device to vaporize the toluene, equipped with a heating blanket to control the temperature of the liquid toluene and with a falling-film coolant to prevent any condensation of vapour into the downstream lines. Section B corresponded to the adsorption stage and consisted of: --two flowmeters FIC3 and FIC4 inserted in the feed and in the outlet stream from the adsorption column, respectively; --two three-way valves which allowed bypass of the adsorption column; --a glassadsorption column having the same characteristicsof that described in ref. [6 ],i.e.with an internaldiameter of 65 m m and a height of 300 ram; m t w o temperature indicatorslocated immediately upstream and downstream from the adsorption device. The toluene concentration in the inletand in the exitflow were analyzed by a gas chromatograph provided with a fame ionizationdetector. Experimental runs
The pressure drops caused by the experimental empty apparatus and by fillingsA and B, were preliminarilyevaluated. In these tests,a current of nitrogen had to pass through the bed. The pressure change from upstream to do,vnst~eam of the adsorbing bed was gauged by a water filledpressure drop manometer; itstwo branches were connected to a pipe tap at the inletand at the outletof the column, respectively. The heightsof the beds were maintained at the same levelof 135 ram, which was estimated as the height of the beds in subsequent experiments. Thus, data could be compared to indicate actual pressure drops that would result from fillingthe beds in adsorption tests. The pressure drop caused by each singlefillingresultedfrom the difference between the contrasting levels in the manometer that were observed between the filled and empty columns. This difference was evaluated against equal riowrates of the gas being fed. A set of experiments was devised in order to investigate the susceptibility of the sample to be regenerated. The corresponding tests of adsorption-regeneration cycles were carried out using the laboratory apparatus shown in Fig. 1 and concerned two different systems: System A was represented by a bed of aggregated and activated fly ash of size 2.8-1.18 mm (sample A); System B was formed by a bed of fly ash prepared in the same way as sample A but with a size range of 1.!8-0.35 ram. Each cycle was composed of an adsorption and desorption phase, rumfing at the following operating conditions:
140
adsorption: inlet flowrate, F - 1 L/rain; height of the adsorption bed, h= 135 ram; toluene concentration, C - 17.6 mg/L (corresponding to 4700 ppm ); desorption: regeneration of the fly ash bed was realized by heating it for 12 hours at 200°C in a muffle furnace. The fly ash bed after regeneration was used for the following adsorption step for each cycle. During each adsorption phase the time needed to reach the breakpoint and the breakthrough curves was determined. RESULTS AND DISCUSSION
Pressure drop. The results of the tests for the evaluation of the pressure drops are summarized in Fig. 2, It shows, in logarithmic scale, the pressure drops expressed in rnm H~O, regarding the inlet flowrate (L/rain) of the gas passing through the bed. The pressure drops introduced from the lower grain size filling, as foreseen, are considerably higher than those due to sample A. Adsorption--desorption cycles. The results obtained during the dynamic abatement experiments, alternating with the desorption phases, are shown in Figs. 3 and 4, and look quite different for the two samples. As far as sample A is concerned, up to the third adsorption-desorption cycle the breakpoint position remains almost unchanged and the breakthrough curves look quite similar. On the contrary, in the two following cycles, the breakpoint appears earlier (about 25 minutes) and the breakthrough curves significantly differ from those corresponding to the first three tests. Therefore, the thermal .A~ (ram t4~O)
80 ~0 RO
~o
.................................. J ...........................................
l ...............
! .
I
............. ! ........... ! ........... ! . . . .
O
4 F ( I/ram.
Fig, 2, Pressure drop of granuhted samples: • sample A; A sample B.
)
141 vsvo 1
u
o
0 0.75
°o
OoO o
o
oO 0
o
0
Q
• Q
0
| 70
80
o
8
0
S0
o
BO
o
Oo 0.25
o
o
N
o
0.50
oe
u
100
120
140
160
180
200
~0
240
T Imin)
Fig. 3. Sample A: b~akthrough curves after: O no regeneration; D one regeneration; W two regenerations; • three regenerations; Q four regenerations. Y0yo 1
0
13
0
O
13
O.?S
Oral B
0.50 g
o m
o
o 0
G
0
0
,3 Q
B
t
o.e5
o
ul o
Ill o 0
'to/
~/
50 60
?0
80
90 t 0 0
o o
o 120
140
160
180
200
220
240
T (ram)
Fig. 4. Sample B: breakthrough curves after: O no regeneration; D one regeneration; I two regenerations.
regeneration does not seem to affect the adsorption bed formed with granules in the range size 2.8-1.18 ram, at least up to the third consecutive test of adsorption. After that, the system effectiveness progressively decreases and its performance is no longer reproducible. This fact also results from the flow measurements by means of flowmeter FIC3 and flowmeter F1C4; indeed, in the last two tests, the flowrate of the current leaving the filling fixed itself on a value, which was 10% less than the
142
value cf the three previous tests. There is a reduction of the bed's permeability to the toluene vapor current passing through it. With reference to sample B, the breakpoint appears in a more and more remarkable way at each cycle, while the form of breakthrough curves is not reproducible. The permeability of the bed to the currents passing through is clearly less than the one noticed with the larger granulometric size filling, and it is underlined through the higher pressure drops due to the smaller sizes of the particles. Moreover, the filling's particles stuck to each other to form aggregates. This tendency was more and more evident during the following cycles of adsorption-desorption. The desorption by heating clearly affects the performance of the adsorption bed beginning from the first regeneration treatment. Actually a progressive and marked decrease in the adsorptive efficiency of the bed occurs after each cycle while the reproducibility of the breakthrough curves vanishes. In a direct comparison between the performance of the two different kinds of fillings, the following was observed. Initially there wasn't any previous regeneration; the breakpoints for both kinds of filling appear nearly at the same time (104 and 108 rain, respectively) but, as far as the bed with a smaller particle size is concerned, the curve of the experimental breakthrough is much flatter (it takes much longer to reach saturation), so the total adsorbing power is higher than for sample A. However, the effect of the regenerations is so high as to make the performances worse; but sample A keeps its adsorbing power unchanged (at least until the third regeneration). CONCLUSIONS
The examination of experimental data show that fly-ash,after aggregating and activating,can produce a finalproduct not only suitable for adsorption of vapours but also susceptible to be regenerated. The resultspoint out, too,that the sizeof the particlesforming the solidbed affects,in a remarkable way, the adsorption performance of the system starting from the firstregeneration treatment. In particular, below a determined range of granulometric size,the fly ash can sustain ~o regeneration without a significantdecrease in the adsorption activity.
REFERENCES i Quarlcsvan Ufford, J,J,, 1981.Coal wastes and the environment. Resources and Conservation, 7:315~319, 2 Capp,J.P,, 1966. Fly ash utilization. Combustion, 37(8): 36-40. 3 Tehney,M.E. and Echelberger, W.F., 1970. Prec. 2nd Ash Utilization Syrup., Pittsburgh, PA, (March 1970): 237-268.
143 4 Chou, K.S., Klemm, W.A., Murtha, M.J. and Dunnet, G., 1976. TLe limv sinter process for production of alumina from fly ash. Proc. 4th Int. Ash Utilization Syrup., St. Louis, MO, pp. 433-449. 5 Peloso, A., Rovatti, M., Giordani, M. and Ferraiolo, G., 1984. Studio sperimentale sule recupero dell'allumina de ceneri volanti via solfato ammonico. Riv. Combustibili, 38 ( 1): 3-6 (in Italian ). 6 Peloso, A., Rovatti, M. and Ferraiolo, G., 1983. Fly ash as adsorbent material for toluene vapours. Resources and Conservation, 10: 211-220.
137
Short Communication
Susceptibility to Regeneration of Fly Ash as an Adsorbent Material* M. ROVATTI, A. PELOSO and G. FERRAIOLO
Istituto di Scienze e Tecnologie dell'lngegneria Chimica, 16145 Genova (Italy) (Received October 2, 1987~accepted in revised form February 6, 1988)
INTRODUCTION
In a previous work, mainly devoted to finding new areas for fly ash utilization [I-5], the possibilityof using fly ash to adsorb organic vapours was examined. Experiments carriedout through a laboratoryadsorption apparatus showed that a fly ash product, obtained afterparticleaggregation and thermal activation at a suitabletemperature, presented satisfactoryadsorption performar~ce for toluene vapours. Since the preliminary treatments of flyash to make the finalproduct showed the fly ash could adsorb a quite remarkable amount, research was devoted to verify whether the saturated sample could be regenerated and how many regeneration cyclesthe sample could sustainwithout detriment to itsadsorption capacity. Finally, some investigations were carried out to examine whether relation° ships exist between the susceptibility of the sample to be regenerated and the granular size of its particles. EXPERIMENTAL
Raw material and sample preparation The investigation was carried out on fly ash produced by combustion of coal from the U.S.A. in an Italian power station. The average chemical composition of untreated fly ash is reported in Table 1. The aggregation of raw material was obtained by chemical means, as referred to in ref. [6 ], by adding a solution of phosphoric acid at a ratio of 25 g of acid to 100 g of fly ash, a ratio which gives the final product the best fluid dynamic beha~iour and the highest abrasion resistence. *This work was suppo~d by a grant from the Italian Public Education Department in the rco search project, "Combustion Technology".
0921-3449/88/$03.50
© 1988 Elsevier Science Publishers B.V./Pergamon Press pie
138 TABLE 1 Averasechemicalcompositionof fly ash used in experiment Substance
% in fly ash
C SiO.~ AI~O~ Fe~O~ TiO~ CaO K.~O SO:~
5.31 45.60 23.50 6.00 1,54 1,65 2.47 0,85 Sect|on
A
Section B V~
FI
NI*C~Hs
F;~
I
m
Fig. 1, Apparatusfor evaluationthe susceptibilityto regeneration. After maturing and drying the viscous-pasty mixture over 24 h, the final p~du~t was milled and sieved; then from this ~natcrial two granular sizes (ranges 2.8m1,18 and 1,18ffi0,35 mm) were isolated and activated by heating for 2 h at 450 °C in a muffle furnace. The products s~ obtained were called sample A and sample B respectively.
Apparatus for regeneration tests The apparatus used to evaluate the samples' susceptibility to regeneration is shown in Fig, 1, It consisted of two sections, A and B. Section A served to produce a gaseous stream feeding the adsorption column and to ensure that the inlet parameters (gas flowrate, toluene concentration and flow temperature) were maintained constant. Schematically, section A consisted of:
139
rathe flowmeter F1C1 which controls nitrogen at flowrate F1 to the vaporizer E and the flowmeter F2C2 wich regulates the flowrate F2 of nitrogen used as diluting gas; - - a device to vaporize the toluene, equipped with a heating blanket to control the temperature of the liquid toluene and with a falling-film coolant to prevent any condensation of vapour into the downstream lines. Section B corresponded to the adsorption stage and consisted of: --two flowmeters FIC3 and FIC4 inserted in the feed and in the outlet stream from the adsorption column, respectively; --two three-way valves which allowed bypass of the adsorption column; --a glassadsorption column having the same characteristicsof that described in ref. [6 ],i.e.with an internaldiameter of 65 m m and a height of 300 ram; m t w o temperature indicatorslocated immediately upstream and downstream from the adsorption device. The toluene concentration in the inletand in the exitflow were analyzed by a gas chromatograph provided with a fame ionizationdetector. Experimental runs
The pressure drops caused by the experimental empty apparatus and by fillingsA and B, were preliminarilyevaluated. In these tests,a current of nitrogen had to pass through the bed. The pressure change from upstream to do,vnst~eam of the adsorbing bed was gauged by a water filledpressure drop manometer; itstwo branches were connected to a pipe tap at the inletand at the outletof the column, respectively. The heightsof the beds were maintained at the same levelof 135 ram, which was estimated as the height of the beds in subsequent experiments. Thus, data could be compared to indicate actual pressure drops that would result from fillingthe beds in adsorption tests. The pressure drop caused by each singlefillingresultedfrom the difference between the contrasting levels in the manometer that were observed between the filled and empty columns. This difference was evaluated against equal riowrates of the gas being fed. A set of experiments was devised in order to investigate the susceptibility of the sample to be regenerated. The corresponding tests of adsorption-regeneration cycles were carried out using the laboratory apparatus shown in Fig. 1 and concerned two different systems: System A was represented by a bed of aggregated and activated fly ash of size 2.8-1.18 mm (sample A); System B was formed by a bed of fly ash prepared in the same way as sample A but with a size range of 1.!8-0.35 ram. Each cycle was composed of an adsorption and desorption phase, rumfing at the following operating conditions:
140
adsorption: inlet flowrate, F - 1 L/rain; height of the adsorption bed, h= 135 ram; toluene concentration, C - 17.6 mg/L (corresponding to 4700 ppm ); desorption: regeneration of the fly ash bed was realized by heating it for 12 hours at 200°C in a muffle furnace. The fly ash bed after regeneration was used for the following adsorption step for each cycle. During each adsorption phase the time needed to reach the breakpoint and the breakthrough curves was determined. RESULTS AND DISCUSSION
Pressure drop. The results of the tests for the evaluation of the pressure drops are summarized in Fig. 2, It shows, in logarithmic scale, the pressure drops expressed in rnm H~O, regarding the inlet flowrate (L/rain) of the gas passing through the bed. The pressure drops introduced from the lower grain size filling, as foreseen, are considerably higher than those due to sample A. Adsorption--desorption cycles. The results obtained during the dynamic abatement experiments, alternating with the desorption phases, are shown in Figs. 3 and 4, and look quite different for the two samples. As far as sample A is concerned, up to the third adsorption-desorption cycle the breakpoint position remains almost unchanged and the breakthrough curves look quite similar. On the contrary, in the two following cycles, the breakpoint appears earlier (about 25 minutes) and the breakthrough curves significantly differ from those corresponding to the first three tests. Therefore, the thermal .A~ (ram t4~O)
80 ~0 RO
~o
.................................. J ...........................................
l ...............
! .
I
............. ! ........... ! ........... ! . . . .
O
4 F ( I/ram.
Fig, 2, Pressure drop of granuhted samples: • sample A; A sample B.
)
141 vsvo 1
u
o
0 0.75
°o
OoO o
o
oO 0
o
0
Q
• Q
0
| 70
80
o
8
0
S0
o
BO
o
Oo 0.25
o
o
N
o
0.50
oe
u
100
120
140
160
180
200
~0
240
T Imin)
Fig. 3. Sample A: b~akthrough curves after: O no regeneration; D one regeneration; W two regenerations; • three regenerations; Q four regenerations. Y0yo 1
0
13
0
O
13
O.?S
Oral B
0.50 g
o m
o
o 0
G
0
0
,3 Q
B
t
o.e5
o
ul o
Ill o 0
'to/
~/
50 60
?0
80
90 t 0 0
o o
o 120
140
160
180
200
220
240
T (ram)
Fig. 4. Sample B: breakthrough curves after: O no regeneration; D one regeneration; I two regenerations.
regeneration does not seem to affect the adsorption bed formed with granules in the range size 2.8-1.18 ram, at least up to the third consecutive test of adsorption. After that, the system effectiveness progressively decreases and its performance is no longer reproducible. This fact also results from the flow measurements by means of flowmeter FIC3 and flowmeter F1C4; indeed, in the last two tests, the flowrate of the current leaving the filling fixed itself on a value, which was 10% less than the
142
value cf the three previous tests. There is a reduction of the bed's permeability to the toluene vapor current passing through it. With reference to sample B, the breakpoint appears in a more and more remarkable way at each cycle, while the form of breakthrough curves is not reproducible. The permeability of the bed to the currents passing through is clearly less than the one noticed with the larger granulometric size filling, and it is underlined through the higher pressure drops due to the smaller sizes of the particles. Moreover, the filling's particles stuck to each other to form aggregates. This tendency was more and more evident during the following cycles of adsorption-desorption. The desorption by heating clearly affects the performance of the adsorption bed beginning from the first regeneration treatment. Actually a progressive and marked decrease in the adsorptive efficiency of the bed occurs after each cycle while the reproducibility of the breakthrough curves vanishes. In a direct comparison between the performance of the two different kinds of fillings, the following was observed. Initially there wasn't any previous regeneration; the breakpoints for both kinds of filling appear nearly at the same time (104 and 108 rain, respectively) but, as far as the bed with a smaller particle size is concerned, the curve of the experimental breakthrough is much flatter (it takes much longer to reach saturation), so the total adsorbing power is higher than for sample A. However, the effect of the regenerations is so high as to make the performances worse; but sample A keeps its adsorbing power unchanged (at least until the third regeneration). CONCLUSIONS
The examination of experimental data show that fly-ash,after aggregating and activating,can produce a finalproduct not only suitable for adsorption of vapours but also susceptible to be regenerated. The resultspoint out, too,that the sizeof the particlesforming the solidbed affects,in a remarkable way, the adsorption performance of the system starting from the firstregeneration treatment. In particular, below a determined range of granulometric size,the fly ash can sustain ~o regeneration without a significantdecrease in the adsorption activity.
REFERENCES i Quarlcsvan Ufford, J,J,, 1981.Coal wastes and the environment. Resources and Conservation, 7:315~319, 2 Capp,J.P,, 1966. Fly ash utilization. Combustion, 37(8): 36-40. 3 Tehney,M.E. and Echelberger, W.F., 1970. Prec. 2nd Ash Utilization Syrup., Pittsburgh, PA, (March 1970): 237-268.
143 4 Chou, K.S., Klemm, W.A., Murtha, M.J. and Dunnet, G., 1976. TLe limv sinter process for production of alumina from fly ash. Proc. 4th Int. Ash Utilization Syrup., St. Louis, MO, pp. 433-449. 5 Peloso, A., Rovatti, M., Giordani, M. and Ferraiolo, G., 1984. Studio sperimentale sule recupero dell'allumina de ceneri volanti via solfato ammonico. Riv. Combustibili, 38 ( 1): 3-6 (in Italian ). 6 Peloso, A., Rovatti, M. and Ferraiolo, G., 1983. Fly ash as adsorbent material for toluene vapours. Resources and Conservation, 10: 211-220.