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Ind. Eng. Chem. Res. 2008, 47, 5856–5860
KINETICS, CATALYSIS, AND REACTION ENGINEERING Industrial Scale Destruction of Environmental Pollutants using a Novel Plasma Reactor Alice M. Harling,† David J. Glover,‡ J. Christopher Whitehead,*,† and Kui Zhang‡ School of Chemistry, The UniVersity of Manchester, Oxford Road, Manchester, M13 9PL, U.K., and Plasma Clean Ltd., Broadstone Knowledge Mill, Broadstone Road, Stockport, Cheshire, SK5 7DL, U.K.
Results for the destruction of environmental pollutants, using a novel multistage dielectric packed bed discharge plasma reactor (DPBD), carried out at an industrial scale flow rate of 300 L min-1 are presented. Some initial results on the combination of a MnO2 catalyst with the system are also given. Complete destruction of toluene is seen for an initial concentration of 10 ppm at a deposited energy density of 23 J L-1 (0.006 kW h Nm3-). This is an order of magnitude better than previous values indicating high energy efficiency. No NOx, a previously common byproduct in plasma processing, can be detected 1. Introduction Plasma processing in air pollution control is now a proven technology that has been extensively tested by many groups on a laboratory scale1–4 but scale-up studies carried out at higher industrial flow rates are less common. Disadvantages of plasma technology limiting its acceptance for large scale gas cleanup include low energy efficiency and the formation of toxic byproduct such as NOx, CO, and nitric acid.1 Studies on the reduction of NOx formation in plasma systems have been carried out by several groups.5–8 A distinct advantage of plasma technology for removing waste gas streams is the ability to combine this system with catalysts in plasma-catalysis systems often with synergistic effects.9,10 Plasma-catalysis has been extensively evaluated in order to reduce the formation of unwanted byproduct, improve the energy efficiency of the system and extend catalyst lifetimes.1,11 Our previous work on NOx formation12 reveals that the combination of plasma and catalyst can restrict the formation of NOx.13 It reveals that catalysts and their properties play an important role not only in enhancing the conversion of volatile organic compounds (VOCs) but also in improving the product selectivity. It is important to gain a better understanding of plasma-promoted catalysis under nonthermal plasma conditions, in order to utilize the advantage of both plasma and catalysts, by optimizing the discharge properties and by selecting suitable catalysts under plasma conditions through the development of new technologies. Scale-up results for the destruction of environmental pollutants, using a novel, multistage, dielectric packed bed discharge plasma reactor (DPBD), are presented here. Proof of concept results for the plasma destruction of VOCs using this multicell plasma device in a laboratory scale reactor have been presented previously;14 this study introduces results at a much higher flow rate of 300 L min-1. This study also shows initial results for the destruction of toluene using a plasma-catalysis system. The previous study14 showed that the novel DPBD reactor is an effective technology for VOC remediation in real air and that the formation of byproduct such as NOx (NO, NO2) is * To whom correspondence should be addressed. E-mail:
[email protected]. † The University of Manchester. ‡ Plasma Clean Ltd.
essentially suppressed. The combination of plasma cells in series significantly improves the destruction of toluene and ethylene decomposition with increased energy efficiency, but it is not simply an additive effect. The system converts low concentrations of VOCs (such as toluene and ethene) in air into CO2 and water without producing other hazardous byproduct such as NOx. Ozone is a major byproduct that is formed at levels up to 120 ppm in this system although it can be effectively removed using suitable catalysts, such as the one used in this work. 2. Experimental Section The experimental setup is similar to that used previously14 and is shown schematically in Figure 1, where 1-3 correspond to the plasma cells in series (designated A, B, and C), 4 is the catalyst and 5 and 6 correspond to the two possible sampling points for product analysis. The multistage dielectric packed bed discharge plasma reactor (DPBD) device is a custom-made plastic device (dimensions 105 cm × 30 cm × 31 cm, Plasma Clean Ltd.) that consists of up to 3 plasma cells (dimensions 14 cm × 14 cm × 1.6 cm). Each plasma cell consists of 2 stainless steel mesh electrodes (12 cm × 12 cm) which are supported in a clear polycarbonate filter box. The distance between the two stainless electrodes in a plasma cell is 16 mm. The plasma cells are filled with glass beads (6 ( 0.3 mm in diameter), and each cell is individually powered by a high frequency, AC high voltage power supply (an electronic neon sign transformer). The input voltage to each of the plasma power sources is controlled by a variac connected to the mains. A plug-in power meter (Prodigit Electronics, 2000MU) is used to monitor the input (or wall-plug) power applied to each power supply; the power is varied by altering the input supply voltages. The output from the neon sign transformer is 10 kV, 30 mA at full supply voltage with a frequency of 21 kHz. There is no specific measurement of the plasma power in this study, only the input electrical power.14 In this study, a mixture of room air and toluene vapor passes through the reactor drawn by a fan at the outlet of the device. The speed of the fan controls the total air volume treated by the plasma device. The amount of toluene vapor introduced into the air flow was controlled by pushing a small amount of nitrogen (controlled by a mass flow controller (MFC)) through
10.1021/ie8001364 CCC: $40.75 2008 American Chemical Society Published on Web 07/16/2008
Ind. Eng. Chem. Res., Vol. 47, No. 16, 2008 5857
Figure 1. Schematic diagram of the scale-up experimental device.
the toluene bubbler at room temperature (293 K) to maintain a toluene inlet concentration of 25 ppm, except when a catalyst was used and it was set to 71 ppm. To investigate the effect of the initial concentration of toluene, it was varied from 10-110 ppm. The airflow velocity at the surface of each plasma cell was set to 0.4 m s-1, corresponding to a volume flow rate of 300 L min-1; this was measured by a hot wire anemometer (AM4204). When the effect of differing initial concentrations of toluene was measured the air velocity was reduced to 0.25 m s-1, which equates to 187.5 L min-1. The relative humidity of the air is measured (N09AQ, Environment Meter) and found to be between 35 and 50% throughout the experiments. An industrial VOC analyzer (Signal Instruments 3030 p.m.) equipped with a flame ionization detector (FID) was used to measure the concentration of toluene before and after remediation. The FID signal is recorded by a computer through a Simple Logger (AEMC Instruments). Samples are pumped out of the system, as shown in Figure 1, either before the catalyst (position 5) or after the catalyst (position 6) and then into the FID. The FID provides a measure of total organic concentration including toluene and any organic endproducts. Experience from previous work14 shows that the endproducts are predominantly CO and CO2 which are not detected by the FID. Gas-sampling tubes (Gastec 11L) were used to measure the concentration of NOx (NO + NO2 + HNO3) in the air stream after plasma treatment. The detection limit for NOx was 0.01 ppm. In this work, separate experiments were conducted with power applied to the first cell (A), the first and second cells (A + B), and all three cells (A + B + C), to study their effect on the destruction of toluene with no catalyst. The initial concentration of toluene was varied from 10-110 ppm to look at its effect on the final toluene concentration, at a range of deposited energy density (DED) values of 4.5-29 J L-1. (DED is defined as being the input electrical power divided by the gas flow rate; 1 J L-1 ) 0.000 278 kW h Nm3-) The system was also run alone with just the plasma cells operating and no pollutant gas in order to study the byproducts that are produced from just running the plasma cells in air, regardless of the pollutant being destroyed. A catalyst (Honeycle ZA ozone destruction catalyst, Nichias Corporation) was then placed downstream after the plasma cells, as shown by position 4 in Figure 1. This is a copper oxide/ manganese dioxide oxidation catalyst, capable of decomposing ozone and oxidizing carbon monoxide. It is used in this work to look at its effect on the decomposition of toluene. 3. Results and Discussion 3.1. Toluene Destruction. The results for the destruction of toluene with one cell (A), two cells (A + B), and three cells (A + B + C) without a catalyst are given in Table 1, along with the deposited energy density (DED), which is a measure of the
Table 1. Toluene Destruction with Different Plasma Cell Configurations plasma cell configuration
toluene conversion (%)
input supply voltage (V)
input power (W)
DED (J L-1)
A A+B A+B+C
13 45 100
55 55 55
30 60 92
6 12 18.4
energy at a given flow rate. These experiments were carried out at 300 L min-1, with an initial toluene concentration of 25 ppm and input supply voltage of 55 V. Toluene conversion is seen to increase as the number of plasma cells used increases, from 13% with one cell to 100% with three cells. The DED can be also seen to increase as more plasma cells are used. This nonadditive increase in conversion with the number of plasma cells has also been shown in our previous study;14 it has significant implications in plasma gas processing as very low power operation of multiple cells can be used to achieve process efficiencies that have previously only been observed with high input energy plasma devices using a single cell configuration. This synergistic effect is attributed to the generation of activated species in the plasma, which impart increased efficiency to processing by the upstream cells.14 3.2. Effect of Initial Toluene Concentration. The destruction of toluene using 3 plasma cells without a catalyst, with input levels of 10, 25, 50, 70, and 110 ppm, was measured at a range of different DED values (4.5-29 J L-1), at a flow rate of 187.5 L min-1, and is shown in Figure 2. These results show that the lower initial toluene concentration give greater destruction of toluene, and that increasing the DED increases the destruction. At the highest DED (29 J L-1; 0.008 kW h Nm3-), 100% destruction of toluene is seen at both 10 and 25 ppm. The results demonstrate that this three plasma cell system design enables high levels of VOCs to be removed at commercially viable flows and powers. The experimental energy efficiencies are found to lie between 3 and 23 g kW h-1, which is up to an order of magnitude better than previous laboratory scale work,15 illustrating the high energy efficiencies seen in this work. At higher initial input concentrations, the energy efficiency scales with increasing DED. This is shown further in Figure 3, which shows that the total amount of toluene removed increases as the DED is increased independent of the input concentration of toluene. An alternative way of showing the energy efficiency of the system is illustrated in Figure 4, where we plot the deposited energy density divided by the concentration of toluene removed as a function of deposited energy density. The deposited energy per unit concentration of removed toluene decreases with increasing DED, indicating higher energy efficiency at higher DED reaching about 0.5 J L-1 ppm-1 for a DED of ∼30 J L-1.
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Figure 2. Destruction of toluene at varying initial concentrations and deposited energy values (where A ) 4.5, B ) 16, C ) 19.5, D ) 23, and E ) 29 J L-1) for a total flow of 187.5 L min-1.
Figure 3. Toluene removal (ppm) as a function of DED for 3 cells (with errors shown). The initial toluene concentrations (ppm) are 24.5, 50, 51, 55, and 112 at deposited energies of 4.5, 16, 19.6, 23, and 29 J L-1. Table 2. Ozone and NOx (NO + NO2) Concentrations with Three Plasma Cells Operating in Air Only plasma cell configuration
ozone (ppm)
NOx (ppm)
input voltage (V)
input power (W)
A+B+C A+B+C
80 120
