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Application of Low-Temperature, Activated-Carbon-Catalyzed SO2 Oxidation to Stack Gases.
K.J.Smith, E.C. Sanford (Editors),Progress in Catalysis 0 1992 Elscvier Scicnce Publishcrs B.V. All rights rcscrved.
203
Application of Low-Temperature, Activated-Carbon-Catalyzed SO, Oxidation to Stack Gases. W.Hasokowati. J. Metzinger, D. Stnldiotto, R.R. Hudgins and P.L. Silveston Department of Chemical Engineering, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, Canada N2L 3G I
Abstract Current technology in the area of a regenerative processes for the removal of SO, from stack gases is too expensive to pursue. As an alternative, during the 1970’s, work was done on the effectiveness of activated carbon operating in a trickle bed reactor to remove SO,. More recent experiments using pulsed liquid flow in a trickle bed reactor have demonstrated increased average reaction rutes and higher acid concentrations than can be achieved in steady state processes. 1. INTRODUCTION
Current methods of tlue gas clean-up can be classified into two categories: throwaway processes and regenerative processes. The most common throwaway process contains a limestone scrubber to remove SO,. Regenerative processes are designed to recover sulfur byproducts as concentrated SO,, elemental sulfur or sulfuric acid. However these processes are rarely used due to their high cost and complexity. It is not clear from the literature when the catalytic properties of activated carbon for the reactions
so,
+
1 -0, 2
+ so,
or SO, + -0, 1 + H,O
2
+ H2S04
were firht discovered. Several published studies [ 1-31 provide detailed descriptions of SO, oxidation over activated carbon. Catalytic oxidation involves the reaction between SO, chemisorbed on active centres with chemisorbed molecular oxygen to form SO, adsorbed on the surface [4]. The SO, does not desorb spontaneously at low temperatures. Two direct methods can be employed for desorption: thermal desorption and water flushing. Thermal
204
desorption (as practised in the Mitsui process) leads to carbon loss through CO, formation and loss of SO, conversion activity and adsorption capacity. The second method as mentioned by Davtyaii and Ovchinnikova [S] involves washing the catalyst with water to form sulfuric acid. The comprehensive review paper by Hartman et a].[ 1 J reports preliminary experiments on stripping SO, from a simulated flue gas in buth trickle beds and a slurry reactor. Hartman et al. achieved 70% SO, removal and generated 0.1M acid without optimizing either the trickle bed reactor nr the slurry reactor. Komiyama and Smith 161 studied the kinetics and mass transfer interference and concluded that catalytic nxidatinn in a three phase system is controlled by 0, mass transfer rate to the suiface. Our experimental reactor employs periodic water flushing to desorb the SO, from the surface of the catulyst.
2. EXPERIMENTAL In previous work by Haure 171 1.3% SO, in air flows continuously downward through the trickle bed. Water, saturated with O,, is periodically switched nn and flows cocurrently with the gas through the bed. In some experiments, flow cycling was symmetrical, e.g., the duration of liquid + gas flow equalled the duration of gas flow alone. In others, a pulse mode was used, i.e., a brief pulse of liquid was introduced into a continuuus gas flow. In these experiments, pressure drop was not a concern so 14x32 mesh type BPL activated carbon (Calgon Corp.) was employed. Current work is based on a simulated flue gas consisting of 2500 ppm SO,, 5% O,, 15% CO,, with the remainder N,. This gas is saturated with water and p through the reactor. A periodic pulse of water or dilute acid, saturated with 0,, is introduced through the reactor. I n these experiments we want to keep the pressure drop across the bed low so we are using 4 x 6 and 6 x 1 6 mesh activated carbon uf the same type as Haure. We are also going to optimize this process by doing a parametric study in which we vary cycle period, cycle split, liquid and gas flow rates.
3. RESULTS AND DISCUSSION It was found from Haure’s work that higher rates of SO, nxidation could be achieved through periodic water flushing over continuous water flushing. This is due to a lower resistance to oxygen transfer and a higher time average temperature in the trickle bed. Both arise because there is no liquid flow through the bed for most of the cycle. Figure 1 shows the increase in the average reactor rate compared to the comparable steady state. The concentration of the acid measured at different times during water flushing of the trickle bed is shown in Figure 2. The acid flushed initially from the bed is about ten times as concentrated as that leaving the bed when steady state is reached. These results are for symmetrical cycling (equal liquid flow to no flow durations). If a pulse operation is used il muck higher mean acid concentration can be achieved. This is illustrated in Figure 3. At a split of 0. I (split is the fraction of the flow-no flow cycle in which water flows through the bed) and a 40 min period, water and gas flow through the bed for 4 minutes in each cycle. For the remaining 36 minutes, only the SO,-air mixture passes
205
downward through the bed. Figure 3 shows that the mean concentration is more than four times the concentration produced with symmetrical cycling (Figure 3). In conclusion this process has great potential for the removal of SO, from stack gases and current research efforts proceeding in our laboratories are attempting to provide the necessary information for a potential pilot plant.
4. REFERENCES 1 M. Hartman, J . Polek and R. Coughlin, Chem. Eng. Progr. Symp. Ser., 115, 67, 7 (1971). 2 M. Hartman and R. Coughlin, Chem. Eng. Sci. 27, 867-880 (1972). 3 Y.U. Siedlewski, Int. Chem. Eng. 5, 608 (1965). 4 T. Otake, S. Tone, Y. Yokota and K. Yoshimura, J. Chem. Eng. Japan 4,155 (1971). 5 O.K. Davtyan and E.N. Ovchinnikova, Doklady Mad. Nauk. SSSR, 104, 857 (1955). 6 H. Komiyama and J.M. Smith, AIChE J. 21,664-676 (1975) 7 P. Haure, Ph.D Thesis. Department of Chemical Engineering. University of Waterloo (1989).
Average Reactlon Rate ( mol/kg 8 ) 100
80
0 08
60
0
0
V A
40
-
20
-
0
;
0
I
I
I
I
I
10
20
30
40
50
60
PERIOD (min) I
I
Figure 1: Time-average rate of SO, oxidation vs. period for symmetric cycling. Data
206
H,S04 Concentratlon (mol/L) 0.06
Qae wloclty
BPL Carbon
0.05
2 cm/r 1.3% SO,, 98.7% aIr
0.04
Cycle Perlod 20 mln
0.03
A 0
0.02
30 mtn 60 mln
0.01
0 0
5
10
15
20
25
30
Time (rnln) I
Figure 2: Instantaneous concentration of H,SO, leaving the reactor during the liqui flow half cycle
H,S04 Concentratlon (mol/L) 0.03
Qao mloclty Pam/r 1.3% SO,, 98% alr On-off cycllng BPL Carbon Cycle Perlod
0.02
10 mlnuhr
0.01
0 0
0.1
0.2
0.3
Spllt
0.4
0.5
0.6
'igure 3: Time-average concentration of H,SO, vs. cycle-split at different periods
203
Application of Low-Temperature, Activated-Carbon-Catalyzed SO, Oxidation to Stack Gases. W.Hasokowati. J. Metzinger, D. Stnldiotto, R.R. Hudgins and P.L. Silveston Department of Chemical Engineering, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, Canada N2L 3G I
Abstract Current technology in the area of a regenerative processes for the removal of SO, from stack gases is too expensive to pursue. As an alternative, during the 1970’s, work was done on the effectiveness of activated carbon operating in a trickle bed reactor to remove SO,. More recent experiments using pulsed liquid flow in a trickle bed reactor have demonstrated increased average reaction rutes and higher acid concentrations than can be achieved in steady state processes. 1. INTRODUCTION
Current methods of tlue gas clean-up can be classified into two categories: throwaway processes and regenerative processes. The most common throwaway process contains a limestone scrubber to remove SO,. Regenerative processes are designed to recover sulfur byproducts as concentrated SO,, elemental sulfur or sulfuric acid. However these processes are rarely used due to their high cost and complexity. It is not clear from the literature when the catalytic properties of activated carbon for the reactions
so,
+
1 -0, 2
+ so,
or SO, + -0, 1 + H,O
2
+ H2S04
were firht discovered. Several published studies [ 1-31 provide detailed descriptions of SO, oxidation over activated carbon. Catalytic oxidation involves the reaction between SO, chemisorbed on active centres with chemisorbed molecular oxygen to form SO, adsorbed on the surface [4]. The SO, does not desorb spontaneously at low temperatures. Two direct methods can be employed for desorption: thermal desorption and water flushing. Thermal
204
desorption (as practised in the Mitsui process) leads to carbon loss through CO, formation and loss of SO, conversion activity and adsorption capacity. The second method as mentioned by Davtyaii and Ovchinnikova [S] involves washing the catalyst with water to form sulfuric acid. The comprehensive review paper by Hartman et a].[ 1 J reports preliminary experiments on stripping SO, from a simulated flue gas in buth trickle beds and a slurry reactor. Hartman et al. achieved 70% SO, removal and generated 0.1M acid without optimizing either the trickle bed reactor nr the slurry reactor. Komiyama and Smith 161 studied the kinetics and mass transfer interference and concluded that catalytic nxidatinn in a three phase system is controlled by 0, mass transfer rate to the suiface. Our experimental reactor employs periodic water flushing to desorb the SO, from the surface of the catulyst.
2. EXPERIMENTAL In previous work by Haure 171 1.3% SO, in air flows continuously downward through the trickle bed. Water, saturated with O,, is periodically switched nn and flows cocurrently with the gas through the bed. In some experiments, flow cycling was symmetrical, e.g., the duration of liquid + gas flow equalled the duration of gas flow alone. In others, a pulse mode was used, i.e., a brief pulse of liquid was introduced into a continuuus gas flow. In these experiments, pressure drop was not a concern so 14x32 mesh type BPL activated carbon (Calgon Corp.) was employed. Current work is based on a simulated flue gas consisting of 2500 ppm SO,, 5% O,, 15% CO,, with the remainder N,. This gas is saturated with water and p through the reactor. A periodic pulse of water or dilute acid, saturated with 0,, is introduced through the reactor. I n these experiments we want to keep the pressure drop across the bed low so we are using 4 x 6 and 6 x 1 6 mesh activated carbon uf the same type as Haure. We are also going to optimize this process by doing a parametric study in which we vary cycle period, cycle split, liquid and gas flow rates.
3. RESULTS AND DISCUSSION It was found from Haure’s work that higher rates of SO, nxidation could be achieved through periodic water flushing over continuous water flushing. This is due to a lower resistance to oxygen transfer and a higher time average temperature in the trickle bed. Both arise because there is no liquid flow through the bed for most of the cycle. Figure 1 shows the increase in the average reactor rate compared to the comparable steady state. The concentration of the acid measured at different times during water flushing of the trickle bed is shown in Figure 2. The acid flushed initially from the bed is about ten times as concentrated as that leaving the bed when steady state is reached. These results are for symmetrical cycling (equal liquid flow to no flow durations). If a pulse operation is used il muck higher mean acid concentration can be achieved. This is illustrated in Figure 3. At a split of 0. I (split is the fraction of the flow-no flow cycle in which water flows through the bed) and a 40 min period, water and gas flow through the bed for 4 minutes in each cycle. For the remaining 36 minutes, only the SO,-air mixture passes
205
downward through the bed. Figure 3 shows that the mean concentration is more than four times the concentration produced with symmetrical cycling (Figure 3). In conclusion this process has great potential for the removal of SO, from stack gases and current research efforts proceeding in our laboratories are attempting to provide the necessary information for a potential pilot plant.
4. REFERENCES 1 M. Hartman, J . Polek and R. Coughlin, Chem. Eng. Progr. Symp. Ser., 115, 67, 7 (1971). 2 M. Hartman and R. Coughlin, Chem. Eng. Sci. 27, 867-880 (1972). 3 Y.U. Siedlewski, Int. Chem. Eng. 5, 608 (1965). 4 T. Otake, S. Tone, Y. Yokota and K. Yoshimura, J. Chem. Eng. Japan 4,155 (1971). 5 O.K. Davtyan and E.N. Ovchinnikova, Doklady Mad. Nauk. SSSR, 104, 857 (1955). 6 H. Komiyama and J.M. Smith, AIChE J. 21,664-676 (1975) 7 P. Haure, Ph.D Thesis. Department of Chemical Engineering. University of Waterloo (1989).
Average Reactlon Rate ( mol/kg 8 ) 100
80
0 08
60
0
0
V A
40
-
20
-
0
;
0
I
I
I
I
I
10
20
30
40
50
60
PERIOD (min) I
I
Figure 1: Time-average rate of SO, oxidation vs. period for symmetric cycling. Data
206
H,S04 Concentratlon (mol/L) 0.06
Qae wloclty
BPL Carbon
0.05
2 cm/r 1.3% SO,, 98.7% aIr
0.04
Cycle Perlod 20 mln
0.03
A 0
0.02
30 mtn 60 mln
0.01
0 0
5
10
15
20
25
30
Time (rnln) I
Figure 2: Instantaneous concentration of H,SO, leaving the reactor during the liqui flow half cycle
H,S04 Concentratlon (mol/L) 0.03
Qao mloclty Pam/r 1.3% SO,, 98% alr On-off cycllng BPL Carbon Cycle Perlod
0.02
10 mlnuhr
0.01
0 0
0.1
0.2
0.3
Spllt
0.4
0.5
0.6
'igure 3: Time-average concentration of H,SO, vs. cycle-split at different periods