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Optimal off-time in interrupted electroosmotic dewatering
ELSEVIER
Separations
Technology 6 (1996) 197-200
Optimal off-time in interrupted S. Gopalakrishnan,
electroosmotic
dewatering
A.S. Mujumdar, M.E. Weber*
Department of Chemical Engineering, McGill University, Montreal, Canada Received
28 December
1995; accepted
9 February
1996
Abstract Electroosmotic dewatering was enhanced by periodically interrupting the power (the off-time) and simultaneously short circuiting the electrodes. Noting that continuous application of power corresponds to an off-time of zero, the final amount of water removed increases with off-time, goes through a maximum and then decreases. For Hydrocol clay, the experimental optimum off-time was 0.1 s for an on-time of 30s. Keywords: Dewatering; Electroosmosis;
Interruption;
Optimal off-time
1. Introduction In chemical, electrochemical and metallurgical industries, dewatering is one of the most economically significant unit operations. Conventional dewatering techniques based on a single non-thermal driving force, such as vacuum, pressure or centrifugal force, are inefficient for dewatering of colloidal or gelatinous sludges which have low hydraulic permeabilities. Thermal drying is disadvantageous for wet sludges because it consumes uneconomical amounts of power. Electroosmotic dewatering (EOD), which utilizes an externally applied electric field to remove water without phase change, is a promising process for the dewatering of ultrafine particle suspensions. For example, EOD removed up to 70% of the initial water from a Bentonite clay suspension with an energy consumption much lower than that for thermal drying [l-4]. In EOD, water removal is based on the surface and colloid characteristics of the solid particles. Unlike conventional dewatering processes, EOD is essentially independent of hydraulic permeability. Rabie et al. [l] demonstrated that electroosmotic dewatering using interrupted power can remove more
xCorresponding
author.
0956-9618/96/$15.00 0 1996 Elsevier P/l S0956-9618(96)00153-l
Science
Ireland
water for the same energy consumption than EOD using continuous power. Their method of power interruption involved periodically disconnecting the direct current power supply and short circuiting the electrodes while the power was off. For a tied period of power application (on-time) of 3Os, more water was removed when the time of power interruption (offtime) was shorter. They used off-times as short as 0.5 S. AS the off-time approaches zero, we expect that the amount of water removed should decrease toward the value removed by continuous application of power. The objective of the present study was to determine whether there is an optimal off-time which yields maximum water removal.
2. Experimental
methods
Tests were performed on two clays: the Bentonite used by Rabie et al. [l] and Hydrocol (Allied Colloids, Yorkshire, UK). The median particle size of the Bentonite was 7 pm and of the Hydrocol, 16 pm. Each clay had a negative zeta potential at pH > 2 as shown in Fig. 1. The Bentonite tests were performed in the apparatus used by Rabie et al. [ll with modified electrical circuitry to permit off-times as short as 0.1 s. The
Ltd. All rights reserved.
198
S. Gopalatihnan
et al. /Sepamtions
Technology 6 (1996) 197-200 60
50 -10 B I F r
-20
a
I ;
1 :
j 3 ::
40
so
I -30
t 20
-40
“I
10
0
0 2
4
6
a
10
12
14
&
0
10
20
30
40
50
cm-timr
(min.)
60
70
60
P6
Fig. 1. Zeta potentials of Bentonite and Hydrocol.
Bentonite slurry was held in a vertical acrylic cylinder, 5 cm ID, between non-corroding electrodes. The anode was the upper electrode. The Hydrocol experiments were conducted in a similar apparatus which held the clay in a vertical acrylic cylinder, 9.4 cm ID. In the experiments, a constant direct current was applied to the electrodes. Two modes of operation were used: (1) continuous power, denoted DC; and (2) power interruption with a short circuit, denoted IS (fl/f2) where t, was the on-time and t, was the off-time, both in seconds. Gopalakrishnan [5] gives additional information.
Fig. 2. Percent water removed vs. cumulative on-time for Hydrocol (S, = 10 wt %, H, = 2 cm, E, = 6 V/cm).
is shown in Fig. 4 where the percent water removed is plotted versus the cumulative on-time. Two runs are shown for each field strength, the first for DC and the second for IS (30/0.1). The optimal off-time was 0.1 s for both initial field strengths. The IS (30/0.1) run removed 9% more water than DC at E, = 6 V/cm, but only 6% more water at E,, = 10 V/cm. The enhancement of EOD by power interruption for the optimal off-time decreased with increasing electric field strength. For Bentonite, the percent water removed increased with decreasing off-time, even down to t, =
3. Results 50
Fig. 2 shows the percent of initial water removed from a Hydrocol suspension as a function of the cumulative on-time for DC and IS runs with three off-times. The on-time was fixed at 30 s and the off-times were 0.05, 0.1 and 2 s. All data correspond to an initial solid content, S,, of 10 wt %, an initial bed height, H,, of 2 cm and an initial field strength, E,, of 6 V/cm. The most water was removed by IS (30/0.11, followed by IS (30/0.05) while IS (30/2) removed less water than DC. These results confirm the presence of an optimal off-time, here 0.1 s for a 30 s on-time. For the same DC and IS runs, the incremental energy of dewatering is plotted in Fig. 3 against the percent water removed. The energies of dewatering using electroosmotic dewatering were well below the latent heat of vaporization of water until near the end of dewatering. The IS (30/0.1) run removed the most water while consuming the least energy. The effect of the initial strength of the electric field
,
I
_.
20
-
15
10
-
0
10
20 Percent
30 “her
40 Remo*ed
50
60
70
(XJ
Fig. 3. Energy of dewatering vs. percent water removed for Hydrocol. (S, = 10 wt %, H, = 2 cm, E,, = 6 V/cm). Dashed \ine: latent heat of vaporization of water at 101 kPa.
S. Gopalakrishnm et al. /Separations
Technology 6 (1996) 197-200
199
water removal for IS (30/0.1) over that for continuous power was different for each clay. 4. Discussion 60
s
50
z 6 1
40
4 r’
30
;; f P
During the on-time, the following reactions occur at the electrodes [1,21: At the anode: 20H--+
i02 + H,O + 2e-
(1)
At the cathode: 20
2H,O + 2e-+
H, +20H-
(21
10
0
0
10
20
JO
40
50
on-time
60
70
60
90
100
(min.)
Fig. 4. Percent water removed vs. cumulative on-time for Hydrocol for two initial field strengths. (S,, = 10 wt%, H,, = 2 cm). Open symbols for DC; filled symbols for IS (30,‘O.l).
0.1 s. Since equipment limitations prevented the use of shorter off-times, an optimum was not obtained. The final water removals at the end of 70 min of dewatering for Bentonite and Hydrocol are shown in Fig. 5 as a function of the off-time for an on-time of 30 s. The percent removals for continuous application of power are plotted at zero off-time. An optimum was clearly established for Hydrocol. For Bentonite, the optimum is at t, < 0.1 s. The relative increase in
,,I
101 0.0
’ 0.2
1 0.4
I 0.6
0.6 Off-time
1.0
1.2
1.4
1.6
1.6
2.0
(*ec.)
Fig. 5. Relation between final percent water removed and off-time for Bentonite and Hydrocol for on-time of 30 s. Open triangle represents data of Rabie 121 for Bentonite (S, = 9.1 wt %, H,, = 1 Filled triangles represent present data for cm, E,, = 2.8 V/cm). Bentonite (S,, = 9.1 wt %, H,, = 1.5 cm, E,, = 6.7 V/cm). Filled circles represent present data for Hydrocol (S,, = 10 wt %, H,, = 2 cm, E,, = 6 V/cm).
As a result, the pH near the cathode increases while the pH near the anode decreases. This lower pH decreases the magnitude of the zeta potential (see Fig. 1) near the anode, thus reducing the electroosmotic velocity. When the DC power is interrupted and the electrodes are short-circuited during the off time, current flows through the sludge bed in the direction opposite to that during the on-time [l]. This short circuit current reverses the electrode reactions and thus the pH near the anode increases during the off-time and the zeta potential near the anode retains its magnitude longer than it would with continuous power. If the off-time is too short, the maximum pH recovery at the anode will not be obtained. The short circuit reactions also decrease the pH at the cathode. Hence, if the reactions occur for a sufficient time, the pH at the lower electrode can decrease sufficiently to reduce the magnitude of the zeta potential. In addition, if the off-time is too long, water from the lower part of the bed will be drawn upward by capillary forces toward the drier part of the bed, i.e. water moved downward during the on-time will move in the opposite direction during the off-time. Thus, if the electrodes are short-circuited for a long time, water removal by IS will be lower than DC. At the optimal off-time, the relative enhancement of water removal decreases with increasing initial electric field strength. Application of higher electric fields increases the electroosmotic driving force as well as the electrode reaction rates. While the former effect increases the dewatering rate, the latter decreases it by decreasing sludge pH and hence the magnitude of the zeta potential near the anode. This suggests that for high fields, the rapid decrease in sludge pH near the anode might be compensated by short circuiting the electrodes for a longer time. Acknowledgement The experiments with Hydrocol were performed at the Combustion and Thermal Engineering Labora-
S. Gopalatihnan
200
et al. /Sepamtiom
tory, Technical Research Centre of Finland in Jyvakyhi, Finland by S. Gopalakrishnan as part of his M. Eng. thesis. We thank Messrs. Pentti Pirkonen, Timo Tuori and Ms. Eliisa Jarvela of the Technical Research Centre of Finland for their suggestions and cooperation which contributed to the completion of this work. Messrs. Hannu Sekki, Hannu Mursunen and Jorma Ihalainen of the Centre are acknowledged for their help in conducting the experiments. References [l]
Rabie, H.R., Mujumdar, A.S. and Weber, M.E. (1994) Inter-
Technology 6 (1996) 197-200
rupted electroosmotic dewatering of clay suspensions. Sep. Technol. 4, 38-46. 121 Rabie, H.R. (1992) Continuous and interrupted electroosmotic dewatering of clay suspension. M. Eng. diss., Montreal, Quebec: McGill University. [31 Ju, S., Weber, M.E. and Mujumdar, A.S. (1991) Electroosmotic dewatering of Bentonite suspensions. Sep. Technol. 1.214-221. Ju, S. (1990) Electroosmotic dewatering of Bentonite suspen[41 sions. M. Eng. diss., Montreal, Quebec: McGill University. [51 Gopalakrishnan, S. (1995) Electroosmotic and combined field dewatering of sludges. M. Eng. diss., Montreal, Quebec: McGill University.
Separations
Technology 6 (1996) 197-200
Optimal off-time in interrupted S. Gopalakrishnan,
electroosmotic
dewatering
A.S. Mujumdar, M.E. Weber*
Department of Chemical Engineering, McGill University, Montreal, Canada Received
28 December
1995; accepted
9 February
1996
Abstract Electroosmotic dewatering was enhanced by periodically interrupting the power (the off-time) and simultaneously short circuiting the electrodes. Noting that continuous application of power corresponds to an off-time of zero, the final amount of water removed increases with off-time, goes through a maximum and then decreases. For Hydrocol clay, the experimental optimum off-time was 0.1 s for an on-time of 30s. Keywords: Dewatering; Electroosmosis;
Interruption;
Optimal off-time
1. Introduction In chemical, electrochemical and metallurgical industries, dewatering is one of the most economically significant unit operations. Conventional dewatering techniques based on a single non-thermal driving force, such as vacuum, pressure or centrifugal force, are inefficient for dewatering of colloidal or gelatinous sludges which have low hydraulic permeabilities. Thermal drying is disadvantageous for wet sludges because it consumes uneconomical amounts of power. Electroosmotic dewatering (EOD), which utilizes an externally applied electric field to remove water without phase change, is a promising process for the dewatering of ultrafine particle suspensions. For example, EOD removed up to 70% of the initial water from a Bentonite clay suspension with an energy consumption much lower than that for thermal drying [l-4]. In EOD, water removal is based on the surface and colloid characteristics of the solid particles. Unlike conventional dewatering processes, EOD is essentially independent of hydraulic permeability. Rabie et al. [l] demonstrated that electroosmotic dewatering using interrupted power can remove more
xCorresponding
author.
0956-9618/96/$15.00 0 1996 Elsevier P/l S0956-9618(96)00153-l
Science
Ireland
water for the same energy consumption than EOD using continuous power. Their method of power interruption involved periodically disconnecting the direct current power supply and short circuiting the electrodes while the power was off. For a tied period of power application (on-time) of 3Os, more water was removed when the time of power interruption (offtime) was shorter. They used off-times as short as 0.5 S. AS the off-time approaches zero, we expect that the amount of water removed should decrease toward the value removed by continuous application of power. The objective of the present study was to determine whether there is an optimal off-time which yields maximum water removal.
2. Experimental
methods
Tests were performed on two clays: the Bentonite used by Rabie et al. [l] and Hydrocol (Allied Colloids, Yorkshire, UK). The median particle size of the Bentonite was 7 pm and of the Hydrocol, 16 pm. Each clay had a negative zeta potential at pH > 2 as shown in Fig. 1. The Bentonite tests were performed in the apparatus used by Rabie et al. [ll with modified electrical circuitry to permit off-times as short as 0.1 s. The
Ltd. All rights reserved.
198
S. Gopalatihnan
et al. /Sepamtions
Technology 6 (1996) 197-200 60
50 -10 B I F r
-20
a
I ;
1 :
j 3 ::
40
so
I -30
t 20
-40
“I
10
0
0 2
4
6
a
10
12
14
&
0
10
20
30
40
50
cm-timr
(min.)
60
70
60
P6
Fig. 1. Zeta potentials of Bentonite and Hydrocol.
Bentonite slurry was held in a vertical acrylic cylinder, 5 cm ID, between non-corroding electrodes. The anode was the upper electrode. The Hydrocol experiments were conducted in a similar apparatus which held the clay in a vertical acrylic cylinder, 9.4 cm ID. In the experiments, a constant direct current was applied to the electrodes. Two modes of operation were used: (1) continuous power, denoted DC; and (2) power interruption with a short circuit, denoted IS (fl/f2) where t, was the on-time and t, was the off-time, both in seconds. Gopalakrishnan [5] gives additional information.
Fig. 2. Percent water removed vs. cumulative on-time for Hydrocol (S, = 10 wt %, H, = 2 cm, E, = 6 V/cm).
is shown in Fig. 4 where the percent water removed is plotted versus the cumulative on-time. Two runs are shown for each field strength, the first for DC and the second for IS (30/0.1). The optimal off-time was 0.1 s for both initial field strengths. The IS (30/0.1) run removed 9% more water than DC at E, = 6 V/cm, but only 6% more water at E,, = 10 V/cm. The enhancement of EOD by power interruption for the optimal off-time decreased with increasing electric field strength. For Bentonite, the percent water removed increased with decreasing off-time, even down to t, =
3. Results 50
Fig. 2 shows the percent of initial water removed from a Hydrocol suspension as a function of the cumulative on-time for DC and IS runs with three off-times. The on-time was fixed at 30 s and the off-times were 0.05, 0.1 and 2 s. All data correspond to an initial solid content, S,, of 10 wt %, an initial bed height, H,, of 2 cm and an initial field strength, E,, of 6 V/cm. The most water was removed by IS (30/0.11, followed by IS (30/0.05) while IS (30/2) removed less water than DC. These results confirm the presence of an optimal off-time, here 0.1 s for a 30 s on-time. For the same DC and IS runs, the incremental energy of dewatering is plotted in Fig. 3 against the percent water removed. The energies of dewatering using electroosmotic dewatering were well below the latent heat of vaporization of water until near the end of dewatering. The IS (30/0.1) run removed the most water while consuming the least energy. The effect of the initial strength of the electric field
,
I
_.
20
-
15
10
-
0
10
20 Percent
30 “her
40 Remo*ed
50
60
70
(XJ
Fig. 3. Energy of dewatering vs. percent water removed for Hydrocol. (S, = 10 wt %, H, = 2 cm, E,, = 6 V/cm). Dashed \ine: latent heat of vaporization of water at 101 kPa.
S. Gopalakrishnm et al. /Separations
Technology 6 (1996) 197-200
199
water removal for IS (30/0.1) over that for continuous power was different for each clay. 4. Discussion 60
s
50
z 6 1
40
4 r’
30
;; f P
During the on-time, the following reactions occur at the electrodes [1,21: At the anode: 20H--+
i02 + H,O + 2e-
(1)
At the cathode: 20
2H,O + 2e-+
H, +20H-
(21
10
0
0
10
20
JO
40
50
on-time
60
70
60
90
100
(min.)
Fig. 4. Percent water removed vs. cumulative on-time for Hydrocol for two initial field strengths. (S,, = 10 wt%, H,, = 2 cm). Open symbols for DC; filled symbols for IS (30,‘O.l).
0.1 s. Since equipment limitations prevented the use of shorter off-times, an optimum was not obtained. The final water removals at the end of 70 min of dewatering for Bentonite and Hydrocol are shown in Fig. 5 as a function of the off-time for an on-time of 30 s. The percent removals for continuous application of power are plotted at zero off-time. An optimum was clearly established for Hydrocol. For Bentonite, the optimum is at t, < 0.1 s. The relative increase in
,,I
101 0.0
’ 0.2
1 0.4
I 0.6
0.6 Off-time
1.0
1.2
1.4
1.6
1.6
2.0
(*ec.)
Fig. 5. Relation between final percent water removed and off-time for Bentonite and Hydrocol for on-time of 30 s. Open triangle represents data of Rabie 121 for Bentonite (S, = 9.1 wt %, H,, = 1 Filled triangles represent present data for cm, E,, = 2.8 V/cm). Bentonite (S,, = 9.1 wt %, H,, = 1.5 cm, E,, = 6.7 V/cm). Filled circles represent present data for Hydrocol (S,, = 10 wt %, H,, = 2 cm, E,, = 6 V/cm).
As a result, the pH near the cathode increases while the pH near the anode decreases. This lower pH decreases the magnitude of the zeta potential (see Fig. 1) near the anode, thus reducing the electroosmotic velocity. When the DC power is interrupted and the electrodes are short-circuited during the off time, current flows through the sludge bed in the direction opposite to that during the on-time [l]. This short circuit current reverses the electrode reactions and thus the pH near the anode increases during the off-time and the zeta potential near the anode retains its magnitude longer than it would with continuous power. If the off-time is too short, the maximum pH recovery at the anode will not be obtained. The short circuit reactions also decrease the pH at the cathode. Hence, if the reactions occur for a sufficient time, the pH at the lower electrode can decrease sufficiently to reduce the magnitude of the zeta potential. In addition, if the off-time is too long, water from the lower part of the bed will be drawn upward by capillary forces toward the drier part of the bed, i.e. water moved downward during the on-time will move in the opposite direction during the off-time. Thus, if the electrodes are short-circuited for a long time, water removal by IS will be lower than DC. At the optimal off-time, the relative enhancement of water removal decreases with increasing initial electric field strength. Application of higher electric fields increases the electroosmotic driving force as well as the electrode reaction rates. While the former effect increases the dewatering rate, the latter decreases it by decreasing sludge pH and hence the magnitude of the zeta potential near the anode. This suggests that for high fields, the rapid decrease in sludge pH near the anode might be compensated by short circuiting the electrodes for a longer time. Acknowledgement The experiments with Hydrocol were performed at the Combustion and Thermal Engineering Labora-
S. Gopalatihnan
200
et al. /Sepamtiom
tory, Technical Research Centre of Finland in Jyvakyhi, Finland by S. Gopalakrishnan as part of his M. Eng. thesis. We thank Messrs. Pentti Pirkonen, Timo Tuori and Ms. Eliisa Jarvela of the Technical Research Centre of Finland for their suggestions and cooperation which contributed to the completion of this work. Messrs. Hannu Sekki, Hannu Mursunen and Jorma Ihalainen of the Centre are acknowledged for their help in conducting the experiments. References [l]
Rabie, H.R., Mujumdar, A.S. and Weber, M.E. (1994) Inter-
Technology 6 (1996) 197-200
rupted electroosmotic dewatering of clay suspensions. Sep. Technol. 4, 38-46. 121 Rabie, H.R. (1992) Continuous and interrupted electroosmotic dewatering of clay suspension. M. Eng. diss., Montreal, Quebec: McGill University. [31 Ju, S., Weber, M.E. and Mujumdar, A.S. (1991) Electroosmotic dewatering of Bentonite suspensions. Sep. Technol. 1.214-221. Ju, S. (1990) Electroosmotic dewatering of Bentonite suspen[41 sions. M. Eng. diss., Montreal, Quebec: McGill University. [51 Gopalakrishnan, S. (1995) Electroosmotic and combined field dewatering of sludges. M. Eng. diss., Montreal, Quebec: McGill University.