Ind. Eng. Chem. Res. 2008, 47, 2203-2212
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Multistage Gas-Liquid Electrical Discharge Column Reactor For Advanced Oxidation Processes Frank Holzer† and Bruce R. Locke*,‡ Department of EnVironmental Technology, Helmholtz Centre for EnVironmental Research - UFZ, Leipzig, Germany, and Department of Chemical and Biomedical Engineering, Florida State UniVersity, FAMU-FSU College of Engineering, 2525 Pottsdamer Street, Tallahassee, Florida 32310
A three-stage gas-liquid electrical discharge reactor was developed based upon a tray column design. The reactor system was operated in both closed recirculation and multiple pass continuous flow modes. The energy yields of hydrogen peroxide and the amount of ozone formation in the multistage reactor were similar to those in the single staged reactor and were also not affected by running the multistage reactor in the recirculation (closed) or continuous (open) modes. The removal energy yields for operating the tray column reactor in the recirculation mode were 2.08, 0.88, 2.42, and 0.63 g/(kWhr) for reactive blue 137 dye, phenol, 2-chloroethyl phenyl sulfide (2-CEPS), and diphenyl chlorophosphate (DPCP), respectively. The phenol and 2-CEPS energy yields in the tray column reactor with recirculation were factors of 4 and 3, respectively, higher than those in the single stage, while the energy yield for DPCP did not change upon going from one to three stages. It is possible that the enhanced gas-liquid mixing and mass transfer of ozone from the gas in the tray column reactor design improved the energy yields for the primary species and byproducts susceptible to direct ozone attack. The removal energy yields for operating the reactor in continuous flow mode did not change for phenol in comparison to those in the recirculation mode. The removal energy yields increased to 3.02 and 2.15 g/(kWhr) for 2-CEPS and DPCP, respectively, after adding FeSO4 (initial concentration of 485 µM) to the reactor solution due to enhanced reactions with hydroxyl radicals from hydrogen peroxide. The variation of the aqueous flow mode from recirculation to continuous flow decreased the removal energy yields of CEPS and DPCP from those in the recirculation mode, but these values were still comparable to those of the one-stage reactor operating in the batch mode. 1. Introduction A large variety of high voltage electrical discharge reactors are under intensive study for applications to water treatment.1-3 High voltage discharge directly in the liquid phase has been shown to lead to the formation of many active radical and molecular species, including H2, O2, H2O2, O, and OH.1-3 As a generator of hydroxyl radicals, these systems can be classified as a type of advanced oxidation technology4 and can lead to effective oxidation of many organic compounds. Electrical discharges in hybrid systems consisting of both gas- and liquidphase discharges have also been demonstrated to degrade a number of organic compounds in the liquid phase.5-9 The gasphase discharge contributes partially through the formation of ozone in the gas phase as well as through the formation of other reactive species by the discharge at the interface of the gas and liquid phases. Many of these previous studies have been conducted in batch or semi-batch reactors, and in particular many of the hybrid gas-liquid discharges have utilized welldefined gas-liquid interfaces where mass transfer limitations may be important. In some cases, gases have been bubbled through the hollow needle electrodes in the liquid. A falling film reactor has an advantage of continuous operation and has been studied for both liquid-phase treatment10 and for gas-phase treatment.11 To overcome mass transfer limitations and to facilitate the development of continuous flow gas-liquid electrical discharge reactors, the present study focuses on the development and analysis of a staged gas-liquid tray column reactor. * To whom correspondence should be addressed. Tel.: (850) 4106165. Fax: (850) 410-6150. E-mail:
[email protected]. † Helmholtz Centre for Environmental Research. ‡ Florida State University.
The present reactor is a scaled-up version of the hybrid series pulsed electrical discharge reactor5,8,9 with three electrical discharge units incorporated into a tray column with countercurrent liquid-gas flows. The reactor is analyzed in both the batch (recirculation) and the continuous flow modes with respect to the formation of hydrogen peroxide and ozone and the decomposition of 2 chloroethyl phenyl sulfide (2-CEPS), diphenyl chlorophosphate (DPCP), phenol, and reactive blue dye as model pollutants. 2. Experimental Section 2.1. Reactor. All experiments were conducted in the multistage gas-liquid electrical discharge reactor (Figure 1 and photo 1). The discharge units were made of three glass tee joints (Ace Glass, Vineland, NJ) with an inner diameter and a vertical height of approximately 9 and 30 cm, respectively. The discharge units were labeled unit 1 to unit 3 from top to bottom of the reactor. The tee extension had the same inner diameter and a horizontal length of about 15 cm. The tee joints were connected with flanges, which pressed two adjacent glass bodies on a Teflon tray with a thickness of 1.5 cm (9). A normal straight glass tube (without a tee) with the same dimensions as the tee joints served as the bottom reservoir that did not have electrical discharge. The high voltage cable (5) was connected to the high voltage needle (3) through a lateral borehole in the Teflon tray. A nickel chromium wire with a diameter of 0.75 mm was applied as high voltage electrode (3). The grounding, a stainless steel tube (6), was introduced into the reactor through a Teflon plug, which also closed the horizontal tee joint opening. The inner end of the grounding tube was bent 90° and had an inner thread over
10.1021/ie071442n CCC: $40.75 © 2008 American Chemical Society Published on Web 03/06/2008
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Ind. Eng. Chem. Res., Vol. 47, No. 7, 2008
Figure 1. Schematic construction of the staged reactor (only two units) and photograph of the reactor in operation.
a length of about 2 cm. A 2 cm long threaded stainless steel rod connected the grounding tube with a threaded stainless steel tube soldered to a stainless steel mesh (diameter: 4 cm) to which the reticulated vitreous carbon (RVC) ground electrode (4) was attached. Both the grounded RVC electrode and the stainless steel mesh had two oppositely oriented cutouts in the form of half circles to provide clearance for the downcomers (7). The inner-outer-inner thread construction provided a maximum tolerance within the adjustable height of the grounded RVC electrode of about 2 cm. The distance between the water surface and the grounded RVC electrode depended on the water and gas flows. This distance was for most experiments between 0.5 and 1.5 cm. The overall distance between the high voltage needle and grounded RVC electrode was 9 cm. The placement of the grounded RVC electrode above the water surface resulted in a discharge in the reactor solution (1) as well as in the gas gap between the water surface and the grounded RVC electrode (2), which defined this kind of discharge as a hybrid series configuration (see photograph in Figure 1). The downcomers (7) made of Teflon had a length and an inner diameter of 42 and 1.5 cm, respectively. On the top of each downcomer, four oval openings (8) 0.5 cm in width and 1 cm in length were machined. The downcomers were threaded into the Teflon trays. Hence, the water level in each unit was determined by adjusting the height of the downcomer openings, resulting in a solution volume of 0.6 L in each unit. Two aluminum sparger disks (diameter: 2 cm) characterized by 10 boreholes of 0.75 mm diameter attached to each Teflon tray by two Teflon plugs and O-rings allowed the gas to pass from one unit to the next. The Teflon tray between the bottom reservoir and the bottom discharge unit was equipped with a pressure release valve, which connected the bottom reservoir with the ambient pressure environment of the lab. Opening of this valve enabled the operator to counteract possible flooding of discharge units. A horizontal borehole connected the water reservoir of each discharge unit with a sampling valve (10) on each Teflon tray.
The reactor solution was fed into the top discharge unit by a pump (Gear Pump Drive, Cole Parmer, Vernon Hills, IL) and flowed from top to bottom by passing through the downcomers. The gas, which was for most studies an argon oxygen mixture (150 mL/min oxygen and 200 mL/min argon), was introduced from the bottom directly through the aluminum sparger disks of the bottom Teflon tray into the bottom discharge unit. The gas flow bypassed the bottom reservoir to avoid the passage of gas through the water drain. The gas left the reactor through the top of the unit. Therefore, the gas and liquid ran through the reactor in a countercurrent mode. However, the actual hydrodynamics in each unit can be more accurately described as in the crossflow regime. The reactor was operated either in a recirculation or in a continuous flow mode. The total water volume treated in the recirculation mode was 5 L. The solution flow was adjusted to 0.5 L/min. In the recirculation mode, 36, 24, and 40 vol % of the reactor solution (5 L) was distributed in the discharge units, in the periphery reactor parts (bottom reservoir and pipes), and in a solution storage tank, respectively. The water volume for the continuous flow mode was for all one-pass experiments (3 discharge units) at least 8 L. The solution flow was adjusted to 0.1 L/min, resulting in a residence time of 6 min in each discharge unit. In the case of the phenol removal experiment in the continuous flow mode, realizing 4 passes (12 discharge units) the initial water volume was 22 L. After each pass, the reactor was switched off for approximately 45 min to readjust the needle electrode protrusions due to electrode erosion. 2.2. Power Supply. The high voltage power supply and pulse forming network are the same as those used in previous work.5,7,8,12 To ensure a continuous discharge in all three units, the operating voltage was set to 55 kV, which corresponded to an average total power for the reactor of about 80 W. 2.3. Materials and Methods. A KCl (Fisher Scientific) solution was used to adjust the initial conductivity of the deionized water to 150 µS/cm. Decomposition experiments were conducted using phenol (A.C.S., Fisher Scientific), a reactive
Ind. Eng. Chem. Res., Vol. 47, No. 7, 2008 2205 Table 1. Solvent Program for 2-CEPS Analysis on HPLC minutes
water %
acetonitrile %
rate %/min
0-3 3-4 4-9 9-13
90 70 20 20
10 30 80 80
0, isocratic 20 10 0, isocratic
Table 2. Solvent Program for DPCP Analysis on HPLC minutes
water %
acetonitrile %
rate %/min
0-5 5-6 7-9 9-13
90 70 40 30
10 30 60 70
0, isocratic 20 10 10
Table 3. Solvent Program for Phenol Analysis on HPLC minutes
water %
acetonitrile %
rate %/min
0-12
80
20
0, isocratic
blue 137 dye, 2-chloroethyl phenyl sulfide (2-CEPS) (purity 98%, Sigma Aldrich), and diphenyl chlorophosphate (DPCP) (purity 99%, Sigma Aldrich) as test compounds. It was found that both compounds dissolved slowly. Both substances, 2-CEPS and DPCP, hydrolyzed. The solution was stirred for 48 h to ensure complete dissolution and hydrolysis. As a result of hydrolyzation, the conductivity of the reactor solution increased from 1 to about 150 µS/cm, so that no KCl was added. The pH dropped after adding the two compounds to deionized water from 5.4 to about 3.3. To avoid post-corona decomposition of the compounds in the sample vials after sample taking, 20 vol % methanol was added to each sample vial. All sample vials were stored in the refrigerator and analyzed on the same day of the experiment. A Perkin-Elmer HPLC (Perkin-Elmer Inc., Wellesley, MA) equipped with a UV detector was then used to measure the concentration of all of the organic test compounds. A Supelco Supercosil C18 column (25 cm × 4.6 mm, Supelco, Bellefonte, PA) and the Perkin-Elmer software Turbochrom were used for separation and quantification, respectively. The concentration of the reactive blue 137 dye was measured spectrophotometrically by measuring the absorbance at 610 nm. For analysis of phenol, 2-CEPS, and DPCP, a mobile phase consisting of deionized water containing 0.05% acetic acid and aceonitrile (HPLC grade, Fisher Scientific) at a flow rate of 1 mL/min was used. The compositions of the mobile phases are given in Tables 1-3. When run separately, the wavelengths used to analyze 2-CEPS and DPCP were 250 and 260, respectively. The analog output factor (signal intensity amplification) on the UV detector was set to 0.2 and 1.0 for 2-CEPS and DPCP, respectively. When 2-CEPS and DPCP were run as a mixture, the solvent program for 2-CEPS (Table 1) was used, the analog output factor was set to 1.0, and the UV detector wavelength was set to 260 nm. To analyze phenol, the wavelength and the analog output factor were set to 274 nm and 0.2, respectively. The initial concentrations of the reactive blue 137 dye and phenol were 60 and 87 ppm, respectively. In separate decomposition experiments, the concentrations of CEPS and DPCP varied from 49 to 53 and 42 to 46 ppm, respectively. In the case of studies with a mixture of 2-CEPS and DPCP, the initial concentrations were adjusted to 29 and 40 ppm for 2-CEPS and DPCP, respectively. In the case of experiments using Fenton chemistry, the initial FeSO4 (Fisher Scientific) concentration in the reactor solution was adjusted to 485 µM. Because of hydrolization of 2-CEPS and DPCP, Fenton’s reaction occurred in a pH environment of
Figure 2. H2O2 formation in the reactor for the recirculation and continuous flow regime.
approximately 3.3. The conductivity increased after addition of 485 µM FeSO4 from 150 µS/cm (due to hydrolization) to approximately 300 µS/cm. For removal experiment with phenol, the addition of FeSO4 adjusted the water conductivity to 150 µS/cm. In the case of all corona-Fenton studies, no KCl was added. Total organic carbon (TOC) measurements were carried out with a total organic carbon analyzer (TOC-V WS, Shimadzu, Japan). The carbon analyzer was operated in the non-purgable organic carbon (NPOC) mode, where all inorganic carbon compounds react with phosphoric acid to CO2. The sample solution was purged with nitrogen to eliminate carbon dioxide and volatile organic compounds before all dissolved organic carbons were oxidized to CO2. The dissolved organic carbon concentration was determined by measuring and integrating the CO2 concentration. The gas effluent flow on top of the multi-stage gas-liquid electrical discharge column reactor passed through a condensation stage with a temperature of 0 °C to minimize the water vapor content before measuring the ozone concentration. The gas-phase ozone concentration was determined with a commercial ozone monitor HC-12 (PCI Ozone Corp., W. Caldwell, NJ). Dissolved ozone concentrations were determined as described in Standard Methods13 and by using the equation:
cO3 )
(A600nm‚100) - (AS‚VT) f‚VS‚b
(1)
where cO3 is the dissolved ozone concentration (mg/L), A600nm is the sample absorbance at 600 nm, VT is the total volume (mL), VS is the sample volume (mL), b is the optical path length (cm), and f equals 0.42. The quartz cuvettes used to measure the absorbance had a path length of 10 cm. Hydrogen peroxide was determined colorimetrically using the reaction of H2O2 with titanyl ions. The absorbance of the yellow peroxotitanium(IV) complex was measured at the wavelength of 410 nm. 3. Results and Discussion 3.1. H2O2 Formation in Reactor Operating in the Recirculation and Continuous Flow Modes. Differences within the H2O2 concentration of samples taken from all discharge units and the solution storage tank were very small, so that a mean H2O2 concentration was calculated for experiments in the recirculation mode. Operating the reactor in recirculation mode resulted in an almost linear increase in the H2O2 concentration (Figure 2) as commonly observed in single stage batch reac-
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Table 4. H2O2 Formation Rates and Efficiencies for the Reactor in Recirculation and Continuous Flow Mode operation mode
corona power /W
H2O2 formation rates /µmol/s
H2O2 formation energy yield /nmol/J
recirculation continuous flow
80 80
0.725 0.72
9.3 9.22
tors.8,14 In the case of operating the reactor in a continuous flow mode, the steady-state concentrations increased stepwise by equal amounts from stage to stage (Figure 2). This equal stepwise increase implies equal power dissipation in the liquid phase of each stage. The steady-state concentration in the last stage of the continuous mode operation was reached after about 30 min of operation, and this lag time corresponds very well with an overall residence time of about 29 min. In the preceding two units, the H2O2 concentrations leveled off after 15 and 20 min, respectively. The H2O2 formation energy yield for the continuous flow mode is defined as
G)
c*H2O2‚Q P
(2)
where c*H2O2, Q, and P stand for the steady-state H2O2 concentration, the liquid mass flow, and the electrical power that is dissipated in the reactor. Table 4 summarizes the H2O2 formation rates and energy yields for both operational modes. Despite significantly different operating conditions (e.g., residence times, H2O2 concentrations), both H2O2 formation rates and energy yields were very similar for the batch and recirculation modes. These results are consistent with the low pH in the reactor preventing ozone reactions with hydrogen peroxide in the liquid. Previous work has also demonstrated similar levels of hydrogen peroxide generation in liquid-phase electrical discharges with various gasphase discharges over the surface.8 It can be concluded from both studies that the gas-phase discharge and mass transfer of various species from the gas to the liquid have little effect on the hydrogen peroxide concentration in the liquid under the given conditions in both the single stage and the multistage reactors. This result is also consistent with the interpretation that the high-intensity discharge channel where hydrogen peroxide is formed is not affected by external conditions such as gas bubbling,15 as long as no reactions occur in the bulk phase to affect the hydrogen peroxide after it diffuses away from the discharge channel. Therefore, the hydrogen peroxide concentration should be the same for various reactor operational modes and configurations. The H2O2 formation rate (0.725 µmol/s) was higher than the value measured in the single stage batch reactor [0.49 µmol/ s8], which might be due to the higher voltage and power needed to initiate discharge in all three units of the present reactor (80 W as compared to 66 W in the single stage reactor).14 Lukes et al.8 found an energy yield of 0.5 g/kWhr at 66 W for the single stage semi-batch reactor (with gas flow over the liquid surface), which is close to the energy yield found in the present reactor (0.6 g/kWhr) at the higher power. These values are also similar to those of the reference reactor where both electrodes are submerged.8,16 3.2. Ozone Generation - Gas-Phase Measurements. In addition to the formation of OH radicals and H2O2 as reactive species, the hybrid serious configuration and the countercurrent, liquid-gas flow regime should also produce significant amounts of ozone in the gas passing through the reactor and in the reactor
Figure 3. Ozone concentration in the effluent of each discharge unit of the reactor.
solution. To measure the ozone concentration in the gas atmosphere of each discharge unit, 4 tiny holes (diameter: 0.75 mm) were drilled into the vertical part of each stainless steel grounding tube. By connecting the gas effluent tubing of the reactor from each single grounding tube to the ozone monitor, the ozone formation in each discharge unit was studied (Figure 3). To avoid dripping of water through the holes of the metal disks from the units above, the pressure in each discharge unit had to be controlled by using a valve regulating the flow from each discharge unit to the ozone monitor. Additionally, the flow was measured by a rotameter. After passing through the first unit (unit 3, bottom), the gas contained between 200 and 300 ppm ozone. However, after passing through the second unit (unit 2), ozone concentrations of 400-500 ppm were measured, which are similar to the final ozone concentration at the reactor outlet (unit 1). Clearly, humidity and the nature of the gas and liquid-phase discharges lead to a balance between ozone production in the gas phase and destruction (absorption/reaction) in the liquid phase after passing through the second stage. The ozone concentration in the gas reached its maximum after bubbling through the reactor solution in unit 2. Thereafter, the ozone concentration does not increase with further increasing unit number. It is interesting to note that the final ozone concentration (ca. 500 ppm in units 1 and 2) was approximately the same as that found by Lukes et al. for a single stage reactor.8 Similar gas flow rates and compositions were used in both studies; yet in the single stage work gas was not sparged through the liquid as in the column reactor. In addition, no differences in the ozone concentrations were observed between the recirculation and the continuous flow modes of the reactor. 3.3. Dissolved Ozone Concentrations. For a better understanding of the phenomena of dissolved ozone, ozone was produced in an external ozone generator, and the resulting ozone containing Ar/O2 mixture of 350 mL/min was passed through the reactor operating in recirculation mode. Both gas-phase and dissolved ozone concentrations were measured. Figure 4 illustrates the gas-phase ozone concentration during the three phases of the experiment. During the first phase, the ozone concentration produced by the external ozone generator (reactor inlet) was measured directly by bypassing the column reactor. After a constant ozone concentration of about 550 ppm was reached in the gas stream, the ozone containing Ar/O2 mixture was passed through the reactor and the corresponding ozone concentrations in each discharge unit were measured (phase 2). The high voltage remained switched off during phase 2. After about 300 min, the corona discharge was switched on (phase 3) and the ozone concentration was measured in the outlet of the reactor.
Ind. Eng. Chem. Res., Vol. 47, No. 7, 2008 2207
less Henry constant for ozone HO3 of 4.17 for a temperature of 20 °C and ambient pressure17 and the equation
H O3 )
Figure 4. Gas-phase ozone concentration in the inlet and in the outlet of the discharge units 1, 2, and 3 of the reactor (recirculation mode).
Figure 5. Dissolved ozone concentration in the units of the reactor during feeding ozone containing Ar/O2 mixture (phase 2) and electrical discharge operation (phase 3).
Despite the rapid increase of the ozone concentrations in each discharge unit after switching the ozone containing Ar/O2 mixture to run through the reactor (phase 2), the steady-state concentration decreased from the inlet concentration of 550 ppm to 400, 350, and 250 ppm after units 3, 2, and 1, respectively. These results can be explained by dissolution and/or decomposition of ozone in the liquid phase. Switching on the corona discharge after 300 min of no-corona operation resulted in a rapid increase of ozone concentration in the reactor gas phase outlet by about 500 ppm, which was similar to the ozone concentration measured in the outlet of the reactor operated at 55 kV (Figure 3) in a conventional recirculation mode. Significant dissolved ozone concentrations were found in all units in the reactor in the absence of electrical discharge (Figure 5). Because measuring dissolved ozone was conducted during the same experiment as the gaseous ozone measurement (Figure 4), the ozone containing Ar/O2 mixture bubbled for the first 35 min only through unit 3 (bottom discharge unit) and from 35 to 75 min through units 3 and 2 (bottom and middle units). After 75 min, the gas flow was fed through all units of the reactor. The increase in dissolved ozone concentration was largest for unit 3, purged with an Ar/O2 mixture containing the highest ozone concentrations (about 550 ppm), and lowest for unit 1 where the feed ozone concentration was approximately 350 ppm. As is obvious from the still increasing concentrations of dissolved ozone, it can be concluded that the dissolved ozone had not yet reached its equilibrium concentration after 250 min. Moreover, kinetic limitations like too little contact time of ozone containing gas bubbles with the reactor solution might prevent an ozone equilibrium between gas and liquid phase. To estimate the liquid equilibrium concentration in each stage, a dimension-
cO3,gas c*O3,liquid
(3)
with c*O3,liquid as the liquid ozone concentration in equilibrium with the bulk gas-phase concentration cO3,gas were used. Applying as discharge unit inlet ozone concentrations 550, 400, and 350 ppm, equilibrium liquid-phase concentrations of 0.26, 0.19, and 0.17 mg/L were calculated for units 3, 2, and 1, respectively. Hence, after approximately 250 min of bubbling of ozone containing Ar/O2 mixture through all units, the dissolved ozone concentration was 46%, 32%, and 18% saturated for units 3, 2, and 1, respectively. These results indicate a reasonably sufficient rate of mass transfer between the gas and the liquid phase. After the corona discharge was switched on at about 300 min, the dissolved ozone concentration decreased almost to zero. This can only be explained by ozone destruction mechanisms caused by the corona operation such as thermal effects, UV-light initiated ozone decomposition, or chemical reactions. The bulk temperature of the liquid increased only slowly with time (
