Ind. Eng. Chem. Res. 2003, 42, 2767-2772
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Low-Pressure Radio-Frequency Plasma for Surface Decontamination of Artillery Shell Casings. 1. Dinitrotoluene Ben W.-L. Jang† Department of Chemistry, Texas A&M UniversitysCommerce, P.O. Box 3011, Commerce, Texas 75429-3011
A novel low-pressure radio-frequency nonthermal plasma system has been successfully designed, installed, and tested for the decontamination of full-size shell surfaces. The destruction removal efficiency (DRE) of the new plasma system for dinitrotoluene (DNT), a simulant of high explosives, destruction depends on various plasma parameters including gases, power outputs, on/off cycles, and treatment times. Under the same decontamination conditions, H2 plasma is more uniform and efficient for all surface decontamination of DNT than O2 plasma. The best three decontamination conditions investigated so far are 100 W with a 6 ms/3 ms cycle, 75 W with continuous wave, and 200 W with a 3 ms/6 ms cycle of H2 plasma. The best DRE achieved is 99.86%. No unreacted DNT was detected bypassing the plasma treatment in the gas stream. N2, CO2, and H2O are the major products detected by the online mass spectrometer, from the process of plasmas reacting with DNT. There is some NOx, mainly NO, detected in the product stream, but it is a minor product of the process. It appears that the current nondestructive decontamination process is economical and environmentally friendly and has a great potential to replace the open burning/open detonation process currently practiced by the military for obsolete artillery shell casings. Introduction There is an urgent need for the Military to remove hazardous/explosive contaminants from structures, parts, and demiled artillery shells so they can be reused, sold, or disposed of. There are several hundreds of thousands of tons of artillery shells alone that need to be decontaminated in the United States, and the number increases by 50 thousand tons per year.1 Currently, open burning and open detonation (OB/OD) is a common disposal practice, which generates both particulate and gaseous emissions that are of environmental concern. Another more environmentally friendly practice to dispose of obsolete shells is to recover or remove most of the explosive materials from the artillery shells by steam melting. The shells, however, contain grams of explosives depending on their sizes after the steam melting steps. Those shells would then be required to go through another steam cleaning step to remove the contaminants to a nonvisual level. To transport the shells, they would still need to be decontaminated by a high-temperature oxidation process. However, most of them would be disposed of or sold as scrap after being exposed to high temperatures. The concept of the hot-gas decontamination (HGD) process, a high-temperature oxidation process, was originally investigated by the U.S. Army Toxic and Hazardous Materials Agency.2 Previous pilot studies have shown that decontamination of a structural component is possible using a heated gas to thermally decompose or volatilize explosives with subsequent incineration in an afterburner. A pilot study at Hawthorne Army Ammunition Plant in 1989 was followed to determine the feasibility of HGD of equipment containing explosives.2 Another similar, but more extensive, pilot study was demonstrated by the Tennessee † Phone: (903) 886-5383. Fax: (903) 468-6020. E-mail:
[email protected].
Valley Authority in 1994.3 The analytical results indicate that HGD is effective in decontaminating munitions to a quantity below the detection limits of the highperformance liquid chromatography. Although the HGD process has been proven to be safe and efficient in decontaminating explosives-residue-contaminated munitions, there are some disadvantages for the process including the following: (1) extended time to heat up and cool; (2) not adequate to treat nonsteel or aluminum parts such as clay; (3) not successful for intricate or mechanical components; (4) need for a large quantity of fuel. Above all, the process also generated a large volume of flue gas that needs to be monitored and may need additional treatments before being released into the air. It is obvious that a milder, more effective, and less costly process is needed for decontamination. Low-temperature radio-frequency (RF) plasma technology is a well-known tool to prepare a thin film coating on top of various materials to improve their physical and chemical properties to meet the needs of special applications.4-6 This technology can also clean the surfaces before the coating deposition.6-9 A special advantage of the process is that the system is operated under vacuum and requires only a few cubic centimeters per minute of flow. Therefore, the process emits only a minimal amount of exhaust. Although the process requires electricity for plasma generation, the energy efficiency is high because the electricity is used to generate high-energy electrons to sustain the plasma but not to heat the gas stream as other thermal plasmas do. Therefore, the RF nonthermal plasma technique is investigated in this study to economically remove the energetic materials in an environmentally friendly and to allow the shells to be reused while maintaining the integrity of the materials. Although there are many studies on nonthermal plasma for volatile organic compound and NOx/SOx control in the flue gas and chemical processing at atmospheric pressure,10-12 this
10.1021/ie020997+ CCC: $25.00 © 2003 American Chemical Society Published on Web 05/03/2003
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Figure 1. Schematic of the RF plasma system for full-size shell surface cleaning.
study is the first nonthermal RF plasma study for decontamination of explosives. Experimental Section Design and Installation of a Full-Size Shell Plasma System. A novel RF (13.56 MHz) plasma system is designed and installed with the objective to decontaminate both the inside and outside surfaces of shell casings. A schematic of the plasma system is shown in Figure 1. The system consists of three major components: power supply, reaction chamber, and a gas delivery and pumping system. The power supply includes an EIN A-500 amplifier, Tektronix pulse generation (model 2101), and a Wavetech function generator (model 166). A custom-designed matching network is built to tune and minimize the reflectance power. The power wattage is measured by the combination of a wattmeter, Bird model 4412, and an oscilloscope, Leader model LS 1020. Both the chamber walls (6-in. diameter) and the stainless steel tube, for a gas delivery in the middle of the reaction chamber, are grounded. Shell casings (81 mm) are fitted on top of a three-leg Teflon stand and connected with the incoming power through the top. A quartz window on the side is for viewing and light emission detection purposes. The RF plasma is, therefore, generated at room temperature for the decontamination of both inside and outside surfaces of the shell. However, the chamber may be slowly heated to 70 °C depending on the time and power of the plasma used. The gas for plasma generation is controlled by a mass flow controller ranging from 0 to 10 cm3/min. H2O vapor for the plasma is controlled by a needle valve and the reservoir maintained at room temperature. The pumping system includes a Leybold 16 B pump and a MKS 252 pressure control system that can adjust the pressure ranging from 0 to 2 Torr. Most of the tests are carried at pressures from 300 to 1000 mTorr. Figure 2 shows the top view of the plasma chamber when plasma is on at a continuous wave (CW) mode. The plasma intensity looks relatively uniform throughout the chamber. Decontamination Analysis with a Gas Chromatography (GC)-Nitrogen Phosphorus Detector (NPD) and Mass Spectrometer (MS). Decontamination tests were carried out with 2,4-dinitrotoluene (DNT) as a simulant to high explosives such as trinitrotoluene, RDX, Comp B, etc. It is expected that different shell surface locations would have different cleaning efficien-
Figure 2. Top view of the plasma reactor during the CW mode.
cies because plasma intensities are somewhat different as a result of the various distances between electrodes (shell body) and the grounds (chamber walls and the inside gas delivery tube) (see Figure 1). Silicon carbide samplers (Nicolet; self-stick soft disks with 1-cm diameter) were loaded with 10-40 mg of DNT and were stuck to four representative locations of shell wall surfaces, including the inside middle, inside bottom, outside top, and outside middle. The DNT sample on the disk will be openly accessed by the plasma treatments. CW or pulsed plasmas, including O2, H2, and H2O plasmas, were tested with various lengths of cycle time, pressures, duty cycles (on/off time), and power wattages. After the plasma treatment, samples were peeled off and kept in vials with 1.0 mL of toluene overnight. The resulting solutions were then analyzed by a HP 5890 GC with a NPD to determine the amount of DNT left after the plasma treatments. All solutions were analyzed. The residual DNT was obtained by averaging the duplicated analyses. About 0.2 µg/mL is the detection limit of our GC-NPD system for DNT with 1-µL injection. The destruction removal efficiency (DRE) is then calculated by the following equation:
DRE ) 1 - residue/loading × 100% Product stream is also monitored by an online mass selective detector HP model 5972, operating under the selective ion mode. Selected ions for monitoring are mass numbers 15 (CH4), 18 (H2O), 28 (N2 and CO), 30
Ind. Eng. Chem. Res., Vol. 42, No. 12, 2003 2769 Table 1. Cleaning Efficiency of Various Plasmas for DNT on Four Representative Locationsa plasma gas O2
H2
H2O
a
sample location
DNT loading (mg)
DNT residue (µg)
DRE (%)
inside middle inside bottom outside top outside middle inside middle inside bottom outside top outside middle inside middle inside bottom outside top outside middle
11.0 13.7 17.0 15.4 10.0 10.8 13.0 12.0 15.8 15.4 12.6 10.2
100.4 12.0 328.0 397.3 33.0 15.4 98.9 109.2 86.2 16.6 144.8 294.6
99.1 99.9 98.1 97.4 99.7 99.9 99.2 99.1 99.5 99.9 98.9 97.1
500 mTorr, 5 mL/min flow with 200 W for 25 min.
(NO), 44 (CO2 and N2O), 46 (NO2), 78 (benzene), 91 (toluene), and 165 (DNT). Results and Discussion Comparison among O2, H2, and H2O Plasmas. Table 1 summarizes the results of DNT decontamination tests with O2, H2, and H2O plasmas. The sample location, sample loading, DNT residue after the treatment, and calculated DRE are listed. As shown in Table 1, all tests were carried out at 500 mTorr with a 200 W CW for 25 min. DNT loading of samples ranges from 10 to 17 mg. The residue detected after the plasma treatment ranges from 12 to 400 µg. The calculated DRE ranges from 97.1 to 99.9%. It suggests that the three plasmas tested (O2, H2, and H2O) have similar decontamination effects for DNT, but H2 plasma appears to be more uniform throughout the plasma chamber, according to the more uniform DRE of four different sample locations under H2 plasma. In general, the inside bottom location shows the best decontamination efficiency and the outside middle locations show the worst. The result is somewhat surprising for the outside middle location because the distance between the outside middle of the shell and the chamber wall is shorter than the distance from the outside top surface to the chamber wall and the plasma intensity is expected to be higher for the outside middle location than the outside top location. However, the resulting DRE is always lower for the samples at the outside middle location. Systematic Study of H2 Plasma for Decontamination of DNT. Because the preliminary tests of DNT decontamination indicate that the H2 plasma is the more uniform and effective plasma to decontaminate DNT, our further investigations focus mainly on H2 plasma while changing other parameters such as treatment time, pressure, on/off duty cycle, and power. Also, only the surface at the outside middle location was further tested because it is the most difficult place to clean, according to the results reported in the previous section. The total flow of hydrogen is maintained at 5 mL/min. (a) Effect of the Treatment Time. As listed in Table 2, the length of the treatment time affects the DRE tremendously. For example, the DRE increases from 56.2% to 94.5% when the treatment time increases from 12 to 21 min. In addition, the DRE increases from 94.5% to 99.5% when the treatment time increases from 21 to 25 min. However, a further increase of the treatment time does not improve the DRE effectively. By visual inspection of the samples after the plasma
Table 2. Effect of the Treatment Time on DREa treatment time (min)
DNT loading (mg)
DNT residue (µg)
DRE (%)
12 21 25
41.6 72.0 39.1
18219 3959 209.2
56.2 94.5 99.5
a
500 mTorr; 200 W; 25 min.
Table 3. Effect of the On/Off Duty Cycle on DREa on/off cycle
DNT loading (mg)
DNT residue (µg)
DRE (%)
0.3 ms/0.6 ms 3 ms/6 ms 30 ms/60 ms
37.8 38.9 39.4
2343 111.2 299.0
93.8 99.7 99.2
a
500 mTorr; 200 W; 56 min.
treatment, we found a small dark orange residue still left on the top of the silicon carbide sampler. Because some portion of the residue does not dissolve in toluene even after an extended period, it is suspected that a polymer material derived from DNT may have formed and trapped some unreacted DNT. It is suspected that the heat generated during the reaction between plasma and DNT could induce direct decomposition of DNT, which can then release more heat and cause a chain reaction to form polymeric materials. The plasma may be unable to quickly penetrate the polymer and to further decontaminate the residual DNT trapped underneath. (b) Effect of the On/Off Duty Cycle. To increase the DRE, using milder conditions to avoid fast reaction and the formation of polymerization precursors seems to be a reasonable approach. Pulsed plasma provides short intense plasma pulses to react with surface DNT, while the heat generated can be dissipated during the plasma off cycle. Ideally, it should minimize the heat effect and the polymerization reaction and therefore increase the DRE. We, therefore, tested three different pulsed duty cycles of the H2 plasma with a longer treatment time of 56 min. The results are summarized in Table 3. As shown in Table 3, three duty cycles (on/ off cycles), including 0.3 ms/0.6 ms, 3 ms/6 ms, and 30 ms/60 ms, are tested with a power of 200 W and 500 mTorr total pressure. Although the total power outputs of the three tests are the same, the results are quite different. The 3 ms/6 ms on/off duty cycle is the most effective, with 99.7% DRE, followed by the 30 ms/60 ms cycle, with 99.2%. The 0.3 ms/0.6 ms cycle is the least efficient, with 93.8% DRE. The total power output under pulsed condition is about the same as the CW plasma (200 W) for 18.7 min. All three tests show higher or similar DREs compared with the result obtained under the CW plasma treatment for 21 min, 94.5%. Indeed, it demonstrated that a pulsed plasma could result in higher DRE than a CW plasma with similar power output. However, we do not understand at this point why these three pulsed cycles showed different DREs with the same power output. It is suspected that DREs depend on the lifetime and characteristics of intermediates generated during the pulsed plasma treatment process. Obviously, the amount of heat dissipated is more during the longer off time than the shorter off cycle time of the pulsed plasmas. On the other hand, more intermediates, which can polymerize and trap unreacted DNT, could be generated during the longer plasma pulses. There could be a maximum DRE for various pulsed plasmas because the competition between inter-
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Table 4. Effect of the Plasma Pressure on DREa
Table 6. Comparison between H2 Plasma and O2 Plasmaa
P (mTorr)
DNT loading (mg)
DNT residue (µg)
DRE (%)
plasma type
power/duty cycle
DNT loading (mg)
DNT residue (µg)
DRE (%)
200 300 500 700 900
39.7 38.7 38.9 39.7 40.5
85.0 94.6 111.2 1062 1237
99.8 99.8 99.7 97.3 96.9
H2 O2 H2 O2
200 W, 3 ms/6 ms 200 W, 3 ms/6 ms 100 W, CW 100 W, CW
38.7 39.2 38.1 39.2
94.6 617.0 118.4 615.5
99.8 98.4 99.7 98.4
a
Table 5. DREs under Various Power and Duty Cycle Combinationsa power (W)
duty cycle
DNT loading (mg)
DNT residue (µg)
DRE (%)
200 100 100 100 75 200 200
3 ms/6 ms 4.5 ms/4.5 ms CW 6 ms/3 ms CW 6 ms/3 ms 4.5 ms/4.5 ms
38.7 38.2 38.1 38.8 39.7 40.6 38.8
94.6 4402 118.4 55.7 95.9 170.0 350.5
99.8 88.5 99.7 99.9 99.8 99.6 99.1
a
a
300 mTorr; 56 min.
3 ms/6 ms; 200 W; 56 min.
300 mTorr; 56 min.
mediate generation by plasma and heat dissipation slows down the secondary intermediate generation. The result of pulsed plasmas suggests that minisecond range pulses could achieve the highest DREs. (c) Effect of the Plasma Pressure. As shown in the previous section, the on/off cycle in the minisecond range seems to be more efficient than other on/off cycles. Therefore, the 3 ms/6ms duty cycle is the focus for the investigation of pressure effect. As listed in Table 4, the effect of pressure ranging from 200 to 900 mTorr on DRE is investigated with a 200 W, 3 ms/6 ms on/off duty cycle and a total treatment time of 56 min. The sample loadings of DNT are similar (38-40 mg). The DRE order is 200 mTorr ≈ 300 mTorr ≈ 500 mTorr > 700 mTorr > 900 mTorr. The results suggest that lower pressure favors higher DRE under our testing conditions. It is considered that the efficiencies under 200, 300, and 500 mTorr are the same within experimental error. When the pressure is above 500 mTorr, the DRE decreased considerably. As reported in the literature, the pressure of the plasma system affects the plasma generation and ion and radical distributions tremendously.13 There are more gas molecules available to be excited or ionized by high-energy electrons under high-pressure conditions. However, ions and radicals are short-lived because they have more chances to collide with other molecules and lose their energy at high pressures. Apparently, the lifetime of species affects the plasma more than the quantity of molecules does based on the lower intensity of plasma emission under highpressure conditions. It reasonably explains the trend of lower DRE at higher system pressure. (d) Effect of the Power and Duty Cycle Combination. We further lowered the power of the plasma system to attempt to achieve higher DREs. Based on the results obtained in the previous section, we chose to focus our investigation at 300 mTorr system pressure. Table 5 lists the decontamination results of the combination of power from 75 to 200 W and the duty cycle from 3 ms/6 ms on/off cycle to CW. As shown in Table 5, except the result of 100 W with a 4.5 ms/4.5 ms on/ off duty cycle (total output: 50 W) showing relatively low DRE (88.48%), all other DREs were higher than 99% after 56 min of treatment. This further confirms that plasmas with lower output wattage can have a
better surface decontamination efficiency toward explosives because of the reduced heat effect. However, a minimum total energy output is required to achieve >99% DRE. The best three conditions were 100 W with 6 ms/3 ms cycle, 75 W with CW, and 200 W with 3 ms/6 ms cycle, with DREs of 99.9%, 99.8%, and 99.8%, respectively. Among all, the pulsed plasmas with a 6 ms/3 ms cycle and 100 W and a 3 ms/6 ms cycle and 200 W are most economical with a low power output, 66.7 W, and show the best DREs. To further test that H2 plasma is more efficient than O2 plasma for DNT decontamination, we again compared H2 plasma and O2 plasma under 200 W (3 ms/6 ms) and 100 W (CW) conditions. Higher DREs (99.8% and 99.7% vs 98.4%) obtained at both conditions with H2 plasma, as listed in Table 6, confirm that H2 plasma is more efficient for DNT destruction than O2 plasma under our testing conditions. Online MS Analysis. As shown in the previous sections, both pulsed and CW plasmas are effective in decontaminating the explosive simulant, DNT, on shell surfaces. To understand the product formation resulting from the plasma decontamination and the reaction mechanism, a MS was installed down stream to analyze the products during the decontamination process. The tests monitored are H2 plasmas with 0.3 ms/0.6 ms, 3 ms/6 ms, and 30 ms/6 ms duty cycles at 200 W and a total pressure of 500 mTorr. As shown in eqs 1 and 2,
C7H6N2O4 f CxHy + CO + CO2 + H2O + N2 + N2O + NOx (1) C7H6N2O4 + H f CxHy + CO + CO2 + H2O + N2 + N2O + NOx (2) possible products from DNT decomposition and DNT reaction with hydrogen plasma include hydrocarbons, carbon monoxide, carbon dioxide, water, nitrogen, nitrous oxide, and nitrogen oxides. DNT is also monitored to determine if any unreacted gaseous DNT slips through without reacting with the plasma. The only hydrocarbon products monitored are methane, ethane, benzene, and toluene. There is some small amount of methane, estimated to be
