Environ. Sci. Technol. 2002, 36, 4170-4174
Electrostatic Separation of Volatile Organic Compounds by Ionization TAKAO ITO,† NORIKAZU NAMIKI,‡ MYONGHWA LEE,‡ HITOSHI EMI,‡ AND Y O S H I O O T A N I * ,‡ DAI-DAN Co., Ltd., 390 Kitanagai, Miyoshi-machi, Iruma-gun, Saitama 354-0044, Japan, and Department of Chemistry and Chemical Engineering, Kanazawa University, 2-40-20 Kodatsuno, Kanazawa 920-8667, Japan
Removal technique of trace volatile organic compounds (VOCs) from air or other gases is of great concern in order to obtain contamination-free indoor air and various process gases for semiconductor manufacturing process. We propose a new technique for separating trace gas components. This technique utilizes preferential ionization and electrical migration of ions. Nitrogen and oxygen gas flow containing toluene vapor is divided into two flows, while the flow is irradiated with alpha-ray from 241Am under a DC electric field. The ionized toluene vapor in one flow electrically migrates into the other flow causing toluene rich and free flows. The separation efficiency of toluene is 50% when 0.5 L/min of inlet nitrogen stream contains 0.15 ppm of toluene at the applied voltage of 250 V. The separation efficiency of toluene increases with the mole fraction of oxygen in the carrier gas. The cation concentration flowing out from the separator is lower than the number of separated toluene molecules by 6 orders of magnitude, but the dependency of toluene separation efficiency on the applied voltage is the same as that of cation separation efficiency. The dependency of separation efficiency on the applied voltage and the gas flow velocity is qualitatively explained by the separation model which accounts for the generation and neutralization of VOC ions in the separator.
Introduction Air contamination by volatile organic compounds (VOCs) released from various building materials has become a serious problem with increased airtightness of indoor space. Although VOCs can be removed by adsorption with activated carbons and other sorbents, this method has several drawbacks, e.g., the necessity of adsorbent replacement or regeneration after reaching adsorption equilibrium, as well as high-pressure drop across the adsorption bed. To avoid health risks due to VOCs, an effective technique is required to remove VOCs from indoor air with low energy consumption. In the semiconductor manufacturing process, various high purity gases are used. Since contamination of process gases by VOCs from pipe wall and gas transport system is inevitable, an effective direct purification technique at the point of use is required in order to ensure the high purity of process gas. Gaseous molecules in the atmosphere are ionized by irradiation with radioactive source, electrical discharge, * Corresponding author phone: +81 76 234 4813; fax: +81 76 234 4829; e-mail: otani@ t.kanazawa-u.ac.jp. † DAI-DAN Co., Ltd. ‡ Kanazawa University. 4170
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combustion, etc. (1). Irradiation of radioactive source or corona discharge first generates primary positive ions and free electrons. The free electrons become rapidly attached to electronegative air molecules forming negative ions. The primary positive and negative ions are not stable. Secondary ionization of ion-molecule reaction occurs by the collision between ions and neutral molecules. These phenomena are utilized for the analysis of trace gas component by atmospheric pressure ionization mass spectrometer (APIMS) (2, 3). The ion-molecule reactions finally produce cluster ions. Water vapor molecules and impurity molecules have a tendency of clustering on the ions (4-6). When ions are present in a saturated vapor, ion-induced nucleation takes place to form nanometer-sized particles by the cluster growth. The mechanisms of ion-induced nucleation from primary ion by alpha-ray radiolysis were investigated by Adachi et al. (7). Sato et al. (8) reported that VOC molecules such as toluene and ethanol eventually acquire positive charge during ionmolecule reaction as H+[VOC][H2O]n and VOC+[H2O]n (n ) 0-3). Iinuma (9) studied multiple-ion reactions which occur during ion-molecule reaction, ion drift, diffusion by using a drift-tube mass spectrometer. The apparent electrical mobility of ion migration in the atmosphere varies depending on ion-molecular reactions (10), because multiple ions and cluster species are the charge carriers during the ion drift. Trace impurities in negative corona discharge field trends to become negative ions by electron attachment (11). Negatively charged impurities such as SF6- (12) and chloronitrobenzene- (13) can be separated from carrier gas flow by an electrical field (14, 15). However, there is an obstruction to applied suitable electrical field for separation due to the common use of the discharge voltage and migration voltage. Electrostatic separation of ions generated by alpha-ray was carried out by Adachi et al. (16), Romay et al. (17), and Yun et al. (18), to obtain unipolar ions from bipolar ions. However a large fraction of ions loses their charge due to the collision onto the conduit wall by electrostatic dispersion, resulting in low ion concentration. Therefore, to separate VOC by ion drift, it is necessary to devise the flow field so that the separated VOC ions are not mixed with the main carrier gas flow after the VOC ions lose their charge. This paper proposes a new electrostatic separator of VOC molecules employing a splitting flow and low electrical field, which utilizes ion-molecule reaction in alpha-ray irradiation field. The separation performance of VOC molecules by ionization is measured in the nitrogen and oxygen flow containing VOC vapor. The ion concentration flowing out from separator is also measured in order to compare the ion separation efficiency with that of toluene vapor. Further, VOC separation mechanisms are discussed in terms of ionmolecules reaction kinetics.
Ionization-Separator for VOCs The structure of ionization-separator for VOCs is shown in Figure 1. Flow containing VOC vapor is split into two flows, while the flow is being irradiated with alpha-ray from 241Am (2.6MBq) under an electric field. Primary ions are produced, and then ion-molecule reaction takes places. VOC molecules are selectively ionized and eventually acquire a positive charge. Although recombination of anion and cation occurs, ionized VOC vapor in one flow electrically migrates into the other flow. Consequently, we obtain VOCs-rich flow and VOCs-free flow. The virtue of this separator is the utilization of splitting flow in the ion separation region. The advantage of splitting flow is such that a small ion drift is sufficient to 10.1021/es025505z CCC: $22.00
2002 American Chemical Society Published on Web 08/29/2002
FIGURE 1. Schematic of ionization-separator for VOCs with wire mesh electrodes.
TABLE 1. Specifications of Carrier Gases N2
purity: 99.99995%
O2
purity: 99.9995%
O2 < 0.1, H2 < 0.3, CO < 0.1, CO2 < 0.1, THC < 0.1, H2O < 1 [ppm] N2 < 2, Ar < 2, THC < 0.2, H2O < 1 [ppm]
TABLE 2. Proton Affinity and Ionization Potential N2 O2 H2 Ar CO CO2 H2O toluene
proton affinity [kJ/mol]
ionization potential [eV]
494.5 422 424 371 594 548 697 794
15.58 12.071 15.43 15.76 14.0139 13.773 12.612 8.82
FIGURE 2. Change in toluene concentration with applied voltage. Carrier gas: dry air (1.0 L/min). Inlet toluene concentration: 0.26 ppm. (a) Ratio of the toluene outlet concentration to the inlet concentration. (b) Separation efficiency of toluene.
move ions from one flow to the other, and even if VOCs lose their charge after migration they follow the carrier gas stream.
Experimental Apparatus and Procedure Toluene vapor was used as the VOC, and nitrogen (N2) and oxygen (O2) were used as the carrier gas. The specification of carrier gas is listed in Table 1. N2 and O2 flow containing toluene vapor (diluted standard gas: toluene: 13 ppm in N2) was introduced into the ionization-separator shown in Figure 1. Inlet flow was divided equally into two outlet flows, the inlet and outlet concentrations of toluene vapor were measured by sampling the gas with a gasbag followed by the determination with a gas chromatograph (GC: GC-17A, Shimadzu). Ionization properties of gaseous components are shown in Table 2 (19, 20). Since toluene has a higher proton affinity and smaller ionization potential compared to the gases shown in Table 2, toluene tends to be in the form of cation in an irradiation region of alpha-ray.
Experimental Section Separation of Toluene. Figure 2(a) shows the ratios of toluene outlet concentration to the inlet concentration as a function of applied voltage. The concentration ratio of the flow from the cathode is larger than unity, while the ratio of the flow from the anode is less than unity, indicating that the separation of VOC does occur in the separator. The concentration ratio curves for cathode and anode outlets are in a mirror image of each other with respect to the inlet concentration line (outlet concentration/inlet concentration ) 1), suggesting that mass balance of toluene holds and therefore no decomposition and no dissociation of toluene takes place in the separator.
FIGURE 3. Influence of impurity removal from carrier gas by coldtrap on toluene separation efficiency. Carrier gas: N2 (1.0 L/min). Inlet toluene concentration: 0.26 ppm. Figure 2(b) shows the separation efficiency of VOC as a function of applied voltage. We defined the separation efficiency of VOC, Sef, by the following equation
Sef ) (Ncathode - Nanode)/Ninlet
(1)
where Ninlet is the inlet flux of VOC, and Ncathode and Nanode are the outlet VOC flux at the cathode and anode. The toluene separation efficiency has a peak, and the maximum separation efficiency is 1.25 times at applied voltage of about 500 V. Influence of Impurity. The influence of impurity in the carrier gas was studied by introducing liquid-N2 cold-trap for the carrier gas purification. Figure 3 shows the effect of impurity removal by the cold-trap on the toluene separation from nitrogen. The separation efficiency of toluene slightly increases by the use of cold-trap. Since impurities, such as water vapor, attract protons, they may suppress the proton attachment onto VOC molecules. Influence of Carrier Gas. The influence of carrier gas composition on toluene separation efficiency was studied by changing the mole fraction of nitrogen and oxygen in the carrier gas. Figure 4 shows the change of the separation efficiencies as a function of oxygen mole fraction in the carrier gas. The separation efficiency increases with oxygen mole VOL. 36, NO. 19, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 6. Modified ionization-separator with plate electrodes.
FIGURE 4. Change in separation efficiencies as a function of oxygen mole fraction. Inlet toluene concentration: 0.26 ppm. Flow rate: 1.0L/min.
FIGURE 5. Influence of volumetric flow rate on toluene separation efficiency. Carrier gas: N2. Inlet toluene concentration: 0.26 ppm. fraction, probably due to the enhanced ion-molecule reaction of toluene in a carrier gas at a higher oxygen mole fraction. Effect of Flow Rate. Influence of volumetric flow rate on toluene separation efficiency is shown in Figure 5. The separation efficiency curves have a peak at a given applied voltage in all cases. As the gas flow rate increases, the separation efficiency peak shifts to a higher voltage. Maximum separation efficiency increases with decrease in the flow rate. It is considered that effective separation of toluene requires a given residence time for toluene molecules to acquire electrical charge by ion-molecule reaction. Comparison of Toluene Separation with Ion Separation. The ionization-separator shown in Figure 1 has wire mesh electrodes and glass filters to form a uniform flow distribution near the electrodes. However, since the filters collect ions, no ions flew out from the separator. Consequently, we modified the structure of the separator as shown in Figure 6. The ion number concentration flowing out from the separator was measured by an ion-counter (18). Figure 7 shows the result of toluene separation with the modified ionization-separator. The maximum outlet concentration is 1.5 times the inlet toluene concentration at an applied voltage of 250 V and a volumetric flow of 0.5 L/min. The separation efficiency decreases with increase in the flow rate, and almost no separation of toluene occurs at 2.0 L/min in the modified separator. Figure 8a shows the change in cation concentration flowing out from the cathode outlet with the applied voltage. The cation concentration increases abruptly with applied voltage and reaches a constant value, which is a function of the volumetric flow rate because of increased ion loss at a lower flow rate. The presence of toluene raises the cation 4172
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FIGURE 7. Influence of flow rate on toluene separation as a function of applied voltage for modified ionization-separator. Inlet toluene concentration: 0.15 ppm. (a) Ratio of the toluene outlet concentration to the inlet concentration. (b) Separation efficiency of toluene. concentration at two flow rates. Furthermore no anions are detected at the cathode outlet, so that the separation of cations is complete by the separator. Figure 8b shows the change in anion concentration with applied voltage at the anode outlet. In the absence of toluene, the anion concentration increases with applied voltage, attains a peak at an applied voltage of 400 V, and then decreases with further increase in voltage. In the presence of toluene, the anion concentration is higher than that for pure nitrogen in low voltage region, and the peak voltage shifts to about 250 V. The increased ion concentration due to the presence of toluene may suggest that cations exist in more stable form in the presence of toluene. The concentration of cations flowing out from the anode is shown in Figure 8c. As shown in Figure 8(a),(b), both anions and cations flow out from the anode, indicating that the separation of anions is not complete by the separator. In the absence of toluene, the concentration decreases gradually with increase in voltage. In the presence of toluene, the concentration sharply decreases with voltage, becomes zero at 250 V, and again increases with further increase in voltage over 300 V. The complicated behavior of ions flowing out from the anode may appear because the separation of ions occurs in the bipolar ion generation zone where the carrier of electrical charge changes with applied voltage as well as by the presence of toluene. In Figure 8a, the cation concentration flowing out from the cathode is 2.3 × 1012 m-3, which is 6 orders of magnitude lower than the number of toluene molecules at the inlet (4.03
FIGURE 9. Comparison of separation efficiency of toluene vapor and that of cation.
FIGURE 8. Ion number concentrations at the outlets of separator as a function of applied voltage. (a) Cation number concentration at the cathode. (b) Anion number concentration at the anode. (c) Cation number concentration at the anode. × 1018 m-3 at 0.15 ppm of toluene concentration). This implies that only a small fraction of cations flowing out from cathode are in the form of ion downstream of the cathode. Nevertheless, the voltage that gives the zero cation concentration at the anode (Figure 8c) coincides with the voltage that yields the maximum toluene separation (Figure 7). Consequently, it seems that the cation separation determines the separation of toluene although the concentration levels are different in large orders of magnitude. To see the correlation between the cation separation efficiency and the toluene separation efficiency, they are plotted against applied voltage in Figure 9. We defined the cation separation efficiency as the ratio of the difference in cation concentration at the electrodes to the sum of the cation concentrations, by assuming that the number of cations generated in the separator is equal to the sum of cation numbers flowing out from the two electrodes. The dependency of toluene vapor separation efficiency on the applied voltage is very similar to that of the cation separation efficiency, indicating that separation efficiency of toluene is determined by the cation separation efficiency.
Discussion In an ionization region with applied electrical field, the generation of VOC ions by ion-molecular reaction, the neutralization by recombination with anion or free electron (21), and the electrical migration with applied electric field occur at the same time. Figure 10 illustrates the population balance of VOC ions and molecules in the ionization-separator. Although many ion species are involved even in a binary gas mixture (9), the separation efficiency of VOC is discussed by a simple transport model for neutral VOC molecules (V), VOC cations (Vi). In the model, we assumed that the net generation rate of VOC ions is proportional to the number of neutral VOC molecules and that the dissipation rate of VOC ions is proportioned to the VOC ion concentration. For simplicity, we also assumed that the flow after splitting is uniform and that there is no ion loss onto the separator wall. The mass balance of VOC molecules and VOC ions for a thin region of δx shown in Figure 10 yields the following
FIGURE 10. Population balance of VOC ions and molecules in the separator. one-dimensional convective diffusion equations
{
∂CVi ∂2CVi ∂CVi ) (u - ZE) - RCVi + βCV + DVi 2 ∂t ∂x ∂x
(2)
∂CV ∂2CV ∂CV )u + RCVi - βCV + DV 2 ∂t ∂x ∂x
(3)
where C is the concentration, u is the gas flow velocity, D is the diffusion coefficient, R is the net dissipation rate constant of VOC ions, β is the net generation rate constant of VOC ions, and Z and E are the electrical mobility of VOC ions and the electrical field strength, respectively. ZE is equal to the ion drift velocity, v. The boundary conditions are shown in Figure 10. Unfortunately, no complete set of reaction rates and kinetic constants for this system is available at the present. Therefore we assigned the rate constant of R and β in the order of 103 s-1 referring to the previous work (9). Equations 2 and 3 are numerically solved with the restriction of the following mass balance for VOC.
Qin Qin [CV + CVi]x)-L/2 + [C + CVi]x)L/2 ) C0Qin 2 2 V
(4)
The values of electrical mobility and the diffusion coefficient of VOC ions and molecules are assumed to be in the range of those for the atmospheric ions and molecules reported by the previous works (16, 22-24). Figure 11 shows the separation efficiencies of VOC calculated by the present model as a function of electric field strength. The separation efficiency curves have a peak at a certain applied voltage in all cases. When the gas flow velocity VOL. 36, NO. 19, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 11. Change in the separation efficiency calculated by VOCion transport equation as a function of electric field strength at various dissipation rate constant, r, the generation rate constant, β, electrical mobility, and the gas flow velocity. T ) 293K. DV ) 1.0 × 10-5m2/s. DVi ) 0.3 × 10-5 m2/s. increases with leaving the other parameters unchanged (compare curve a and b), the separation efficiency peak shifts to a higher voltage without changing the value of maximum separation efficiency. The decrease in electrical mobility also brings the same shift in the peak voltage (compare curve b and c). However, we observed in Figure 5 that the increase in flow velocity brings the peak shift to a higher voltage as well as the reduction in the maximum separation efficiency. The reduction in maximum separation efficiency may be attributed to the change in flow field due to the flow rate increase, i.e., we assumed a uniform flow field in the model, whereas the actual flow pattern in the separator changes significantly with the increase in flow rate. Therefore, the gas flow velocity and the electrical mobility of VOC ion seem to influence the peak voltage which gives the maximum separation efficiency. We also know from Figure 11 that a decrease in the net generation rate constant, β, brings the reduction in the separation efficiency with the peak voltage shift toward a higher voltage (compare curve b and d) and that a decrease in the dissipation rate constant, R, leads to an increase in the separation efficiency with the peak voltage shift toward a lower voltage (compare curve d and e). The enhanced separation efficiency at a higher oxygen mole fraction in a carrier gas, which was shown in Figure 4, may be explained by the changes in generation and dissipation rate constants, R and β, due to the presence of oxygen because oxygen would be the main carrier of negative charge. What follows from the discussion is that the separation efficiency of VOC is a complicated function of the electrical mobility of ions, the gas flow velocity, the dissipation, and generation rate constants of VOC ions.
Acknowledgments This work was supported in part by grants from the Smoking Research Foundation and Grant-in-Aid for Developmental Scientific Research (No. 13450315 and 13555218) from the Ministry of Education, Culture and Science of Japan.
Nomenclature
L
distance between anode and cathode [m]
N
VOC flux [s-1]
N0
inlet VOC flux [s-1]
Qin
inlet flow rate [L/min]
R
dissipation rate [m-3 s-1]
Sef
separation efficiency [-]
T
temperature [K]
t
time [s]
u
gas flow velocity [m/s]
v
drift velocity of ion ()ZE) [m/s]
V
voltage [V]
x
coordinate system [m]
Z
electrical mobility of ion [m2 V-1 S-1]
R
net dissipation rate constant [s-1]
β
net generation rate constant [s-1]
Subscript V
VOC molecule
Vi
VOC ion
Literature Cited (1) Ungethu ¨ m, E. J. Aerosol Sci. 1974, 5, 25-37. (2) Mitsui, Y.; Kambara, H.; Kojima, M.; Tomita, H.; Katoh, K.; Satoh, K. Anal. Chem. 1983, 55, 477-481. (3) Mitchum, R. K.; Korfmacher, W. A. Anal. Chem. 1983, 55, 14851499. (4) Mohnen, V. A. Pageoph 1971, 84, 141-153. (5) Tokunaga, O.; Suzuki, N. Radiat. Phys. Chem. 1984, 24(1), 145165. (6) Sakata, S.; Okada, T. J. Aerosol Sci. 1994, 25, 879-893. (7) Adachi, M.; Okuyama, K.; Seinfeld, J. H. J. Aerosol Sci. 1992, 23, 327-337. (8) Sato, K.; Takahashi, H.; Sakata, S.; Okada, T. Proceeding of the 12th International Symposium on Contamination Control; Yokohama, 1994; p 7. (9) Iinuma, K. Can. J. Chem. 1991, 69, 1090-1099. (10) Nagato, K.; Ogawa, T. J. Geophys. Res. 1998, 103, 13917-13925. (11) Caledonia, G. E. Chem. Rev. 1975, 75, 333-351. (12) McKeown, M.; Siegel, M. W. Am. Lab. November, 1975; pp 8999. (13) Mitchum, R. K.; Korfmacher, W. A. Spectra 1982, 8, 12-18. (14) Tamon, H.; Imanaka, H.; Sano, N.; Okazaki, M.; Tanthapanichakoon, W. Ind. Eng. Chem. Res. 1998, 37, 2770-2774. (15) Babko-Malyi, S. Fuel Processing Technol. 2000, 65-66, 231246. (16) Adachi, M.; Kousaka, Y.; Okuyama, K. J. Aerosol Sci. 1985, 16, 109-123. (17) Romay, F. J.; Pui, D. Y. H.; Adachi, M. Aerosol Sci. Technol. 1991, 15, 60-68. (18) Yun, C. M.; Otani, Y.; Emi, H. Aerosol Sci. Technol. 1997, 26, 389-397. (19) Lias, S. G.; Liebman, J. F.; Levin, R. D. J. Phys. Chem. Ref. Data 1984, 13, 695. (20) Lias, S. G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R. D.; Mallard, W. G. J. Phys. Chem. Ref. Data 1988, 17, suppl. No.1. (21) Graham, W. G.; Fritsch, W.; Hahn, Y.; Tanis, J. A. Recombination of Atomic Ions; NATO ASI Series B, Physics, Plenum: New York, 1992; Vol. 296. (22) Bricard, J. Problems of atmospheric and space electricity; Coroniti, S., Ed.; Elsevier: Amsterdam, 1965; p 82. (23) Ma¨kela¨, J. M.; Jokinen, V.; Mattila, T.; Ukkonen, A.; Keskinen, J. J. Aerosol Sci. 1996, 27, 175-190. (24) Ghosh, S. J. Atmos. Chem. 1993, 17, 391-397.
C
number concentration [m-3]
C0
inlet concentration [m-3]
D
diffusion coefficient [m2/s]
E
electrical field strength [V/m]
G
generation rate [m-3 s-1]
Received for review January 4, 2002. Revised manuscript received June 29, 2002. Accepted July 3, 2002.
J
diffusion flux [m-2 s-1]
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