Ind. Eng. Chem. Res. 2003, 42, 5617-5621
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SEPARATIONS Separation of Volatile Organic Compound Vapor by an Ionization and Electrical MigrationsSeparation Model Takao Ito,†,‡ Yoshio Otani,*,† and Norikazu Namiki† Departments of Chemistry and Chemical Engineering, Kanazawa University, 2-40-20 Kodatsuno, Kanazawa 920-8667, Japan, and DAI-DAN Company, Limited, 390 Kitanagai, Miyoshi-cho, Iruma-gun, Saitama 354-0044, Japan
We proposed a new separation technique for trace volatile organic compounds (VOCs) from gases (Environ. Sci. Technol. 2002, 36, 4170-4174) that utilizes the selective ionization of VOCs and electrical migration of ions in bifurcating flow. In the present work, we employed a soft X-ray source for the ionization of VOCs and investigated the effect of the X-ray intensity and VOC concentration on the separation efficiency. As a result, experimental results showed that a higher separation efficiency is attained with a higher intensity of the soft X-ray source and that the separation efficiency increases with decreasing the VOC concentration. The dependency of the VOC separation efficiency on these factors is well explained by the proposed separation model, which accounts for the reactions of VOC intermediates and reactive primary cations of carriergas molecules. Introduction Removal of gaseous contaminants or specific vapors from air and other gases has been of great concern for indoor air quality and semiconductor manufacturing processes, for example, volatile organic compounds (VOCs) released from various building materials into indoor air,1 water vapor or oxygen from ultrahigh purification gas used for semiconductor industries,2 and so forth. We proposed a new technique for gas purification (ionization separator), which utilizes the preferential and selective ionization of VOCs and electrical migration of ions in bifurcating flow and successfully separated toluene vapor from nitrogen and oxygen flow.3 The separation mechanisms, however, have not been well understood yet because even single species of a VOC goes through very complicated ion-molecular reactions in an R-ray irradiation zone. In this paper, we measure the separation performance of the ionization separator by introducing a soft X-ray generator and investigate the influence of the VOC concentration and the sort of intensity of the X-ray irradiation on the separation efficiency to seek the separation mechanisms of VOCs. Ionization Separator Figure 1 shows the structure of ionization separator that was used in the previous work.3 Flow containing VOC vapor is split into two flows, while the flow is being irradiated with R rays from 241Am (2.6 MBq) under an * To whom correspondence should be addressed. Tel.: +81 76 234 4813. Fax: +81 76 234 4829. E-mail: otani@ t.kanazawa-u.ac.jp. † Kanazawa University. ‡ DAI-DAN Company, Limited.
Figure 1. Schematic of the VOC separator.
electric field. In addition to the R ray that was used to generate bipolar ions in VOC-laden carrier gas in the previous work, soft X-rays from a photoionizer (model L6941, Hamamatsu Photonics, Japan; energy 3.0-9.5 keV) were also employed in the present work. The intensity of soft X-rays can be adjusted by passing the soft X-ray through various numbers of aluminum sheets (aluminum foil, thickness 12 µm), and the intensity of the X-ray was measured with a dose meter (ICS-311, Aloka). The soft X-ray generator has a dose rate of 4.7 × 10-6 Gy/s at a distance 1-m away from the source. According to the reference,4 the dose rate corresponds to the production rate of cations of 2.4 × 1016 m-3 s-1 at 4-cm away from the source when the
10.1021/ie0302286 CCC: $25.00 © 2003 American Chemical Society Published on Web 09/26/2003
5618 Ind. Eng. Chem. Res., Vol. 42, No. 22, 2003
Figure 2. Soft X-ray intensity versus ion current measured by ion counter. N2 gas flow rate ) 1.5 L/min, thickness of aluminum sheet ) 0, 12, 24, 36, 48, 60, and 72 µm.
Figure 4. Change in toluene concentration with applied voltage. Flow rate: 1.0 L/min. Inlet toluene concentration: 500 ppb. (a) Ratio of the toluene outlet concentration to the inlet concentration. (b) Separation efficiency of toluene.
Figure 3. Flowchart of the experimental setup.
generation of a pair of ions requires the energy of 34 eV. To find the relationship between the X-ray intensity and the ion-generation rate in the separator, we measured the ion concentration at the outlet of the separator with an ion counter5 by changing the relative intensity of the X-ray using aluminum sheets. The relationship is shown in Figure 2. As seen in Figure 2, the ion current is not proportional to the intensity of the X-ray, and 70% reduction in the X-ray intensity leads to a decrease in the ionization capability by about 46%. In the separator, VOC molecules are selectively ionized as a result of ion-molecular reactions and eventually acquire a positive charge. Although the recombination of the anion and cation occurs, ionized VOC vapor in one flow electrically migrates into the other flow, causing VOC-rich flow and VOC-free flow. The advantage of splitting flow is such that a small ion drift is sufficient to move ions from one flow to the other, and VOCs follow the carrier-gas stream even if they lose their charge after migration. Experimental Method Toluene vapor was used as a VOC. Because toluene (proton affinity 794 kJ/mol, ionization potential 8.82 eV) has a higher proton affinity and a smaller ionization potential compared to nitrogen (proton affinity 494.5 kJ/ mol, ionization potential 15.58 eV), toluene tends to be in the form of a cation in an ionization region. Figure 3 shows the experimental setup. The carrier gas of nitrogen (purity 99.999 95%, O2 < 0.1 ppm, H2 < 0.3 ppm, CO < 0.1 ppm, CO2 < 0.1 ppm, THC < 0.1 ppm,
H2O < 1 ppm) is further purified by passing it through a liquid-nitrogen cold trap. The purified nitrogen is mixed with nitrogen containing toluene vapor (13 ppm) to obtain a given concentration and then introduced into the ionization separator. Inlet flow is equally divided into two outlet flows, and the inlet and outlet concentrations of toluene vapor are measured by sampling the gas with a gasbag followed by determination with a gas chromatograph (Shimadzu, GC-17A). Experimental Results Figure 4a shows the ratios of toluene outlet concentration to the inlet concentration as a function of applied voltage. For both the irradiation of soft X-rays and that of R rays, 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 toluene occurs in the separator. The concentration ratio with the soft X-ray is higher than that with the R ray. The concentration ratio curves for the cathode outlet and anode are mirror images of each other with respect to the inlet concentration line (outlet concentration/inlet concentration ) 1), suggesting that the mass balance of toluene holds and, therefore, no decomposition of toluene takes place in the separator. Figure 4b shows the separation efficiency of toluene as a function of applied voltage. We defined the separation efficiency of the VOC, Sef, by the following equation.
Sef ) (Ncathode - Nanode)/N0
(1)
where N0 is the inlet flux of the VOC and Ncathode and Nanode are the outlet VOC fluxes at the cathode and anode. The toluene separation efficiency has a peak, and the maximum separation efficiency of the soft X-ray is 0.3 at the voltage of 600 V. Figure 5 shows the influence of the inlet toluene concentration on the toluene separation efficiency as a
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at the same time. Although many ion species are involved even in a binary gas mixture, here we consider only neutral VOC molecules and VOC ions. When the generation of VOC ions is proportional to the number of neutral VOC molecules and the depletion of VOC ions is proportional to the number of VOC ions, the mass balance of neutral VOC molecules and VOC ions yields the following one-dimensional convective diffusion equations.3
∂CV+ ∂2CV+ ∂CV+ ) (u - ZE) - RCV+ + βCV + DV+ ∂t ∂x ∂x2 ∂CV ∂2CV ∂CV )u + RCV+ - βCV + DV 2 ∂t ∂x ∂x Figure 5. Influence of inlet toluene concentration on toluene separation efficiency as a function of applied voltage for the R-ray separator. Flow rate: 1.0 L/min.
Figure 6. Influence of ion generation by R rays and soft X-rays and its intensity on the toluene separation efficiency as a function of inlet toluene concentration. Flow rate: 1.0 L/min. Voltage: 600 V.
function of applied voltage. The separation efficiency of toluene increases with the applied voltage and reaches a maximum at a voltage between 600 and 1000 V in all cases. Furthermore, Figure 5 shows that the separation efficiency is higher for a low concentration of VOC, implying that the present separator is more effective for removing a low concentration of VOC. The maximum separation efficiency is 0.43 at the inlet concentration of 190 ppb at an applied voltage of 600 V. Figure 6 shows the change in separation efficiency with the inlet toluene concentration for both R-ray and soft X-ray sources. The separation efficiency of toluene increases with decreasing the inlet toluene concentration. An increase in the soft X-ray intensity brings the increased separation efficiency, and the rate of separation efficiency reduction (37% at 0.5 ppm) is similar to that of the ion current (46%) shown in Figure 2 because the increase in the soft X-ray intensity leads to a higher generation rate of primary bipolar ions which in turn charge toluene molecules. Discussion In an ionization region of applied electrical field, the generation of primary bipolar carrier-gas ions, formation of VOC ions by ion-molecule reactions, neutralization of ions by recombination, and electrical migration occur
(2)
(3)
where C is the concentration, u is the flow velocity, D is the diffusion coefficient, and R and β are the net depletion rate constant and the net generation rate constant of the VOC ions, respectively. Z is the electrical mobility of the VOC ions, and E is the electrical field strength. ZE is equal to the ion drift velocity. We observed in Figures 5 and 6 that both the decrease in inlet toluene concentration and the increase in irradiation intensity bring a higher separation efficiency. The increase in separation efficiency at a lower concentration may be attributed to the change in the net generation rate constant of VOC ions rather than the net depletion rate constant because the depletion of VOC ions by ion recombination is determined by the collision of VOC ions with other anion or free electrons,6 which seems less influenced by the VOC concentration. In the present work, we hypothesize that VOC ions are generated via the reaction intermediates of the VOC, which are formed by the collisions of neutral VOC molecules with the reactive primary cations of carriergas molecules. Multiple absorptions of photons into the carrier-gas molecules, X, by continuous irradiation of soft X-rays or R rays generates the reactive primary cation, X+*, by photoauger ionization.7
X f X+ *
(4)
The neutral VOC molecules, V, collide with the reactive primary cations and change their state into the excited intermediates, V*, competing the reaction of reactive primary cations into the carrier-gas cations, X+. Then, the reaction intermediates of the VOC, V*, collide with the carrier-gas cations, X+, to form VOC cations, V+.8 The generation of VOC cations via the intermediates, V*, is given by k1
k2
V + X+* 98 V* + X+ 98 V+ + X k3
X+* 98 X+
(5) (6)
where k1, k2, and k3 are the reaction rate constants. By introducing the steady-state approximation for the reactive primary cations and the intermediates
dCX+*/dt ) S - k1CVCX+* - k3CX+* ) 0
(7)
dCV*/dt ) k1CVCX+* - k2CV*CX+ ) 0
(8)
where S is the production rate of reactive primary cations.
5620 Ind. Eng. Chem. Res., Vol. 42, No. 22, 2003 Table 1. Transport Coefficients of VOC Molecules and Ions and the Net Depletion Rate Constant of VOC Ions electrical mobility of VOC ion, Z [m2 V-1 s-1] diffusion coefficient of VOC ion, DV+ [m2/s] diffusion coefficient of VOC molecule, DV [m2/s] net depletion rate constant, R [s-1]
1.4 × 10-4 0.3 ×10-5 1.0 × 10-5 1600
From eqs 7 and 8, the generation rate of VOC ions, G ()βCV in eqs 2 and 3) is given by
G ) dCV+/dt ) k2CV*CX+ )
S C (1/K) + CV V
(9)
with
K ) k1/k3
(10)
where K is the selectivity of the VOC ionization over the self-ionization of the carrier gas by the reactive primary cations. VOC ion generation is given by a function of the production rate of the reactive primary cation and inlet VOC concentration. Equations 2 and 3 are numerically solved by using eq 9. The boundary conditions for the bifurcating flow separator imposed for solving eqs 2 and 3 are
Figure 7. Concentration distributions of the neutral VOC molecule, VOC ion, and total VOC in the separator calculated by VOC ion transport equation at the voltage of 600 V.
dCV/dx ) 0, dCV+/dx ) 0 at x ) L/2, -L/2 (separator outlets) (11) CV ) C0 at x ) 0 (separator inlet)
(12)
where L is the distance between the anode and the cathode. Unfortunately, no complete set of reaction rates and kinetic constants for this system is available at present. The values of electrical mobility and diffusion coefficient of VOC ions and molecules are assumed to be in the range of those for the atmospheric ions and molecules reported by previous work.9,10 We also assigned the rate constant of R to be on the order of 103 referring to the previous work.3 The transport coefficients of the VOC molecules and the net depletion rate constant of the VOC ions used in the present calculation are given in Table 1. The unknown values of S and K are varied parametrically for fitting the experimental data. Figure 7 shows the concentration distributions of neutral VOC molecules, VOC ions, and total VOC in the separator for various K and S values, which are calculated by the present model. We see in this figure that (i) the concentrations of neutral VOC molecules, VOC ions, and total VOC on the cathode side are higher than those on the anode side and are uniform on both sides and (ii) the decreases in both K and S lower the concentration of VOC ions on the cathode side, that is, reduce the separation efficiency. Figure 8 compares the separation efficiencies calculated for various values of S and K with the experimental data. As shown in the figure, S determines the VOC concentration at which the separation of the VOC takes place, whereas K affects the increasing rate of separation efficiency with the decrease in VOC concentration. The values of S that give the best fit for the experimental data are 2.0 × 1022 and 7.5 × 1021 m-3 s-1 respectively for 100 and 70% X-ray intensity. The reduction in S due to the decrease in X-ray intensity nearly corresponds to the ion concentration reduction due to the X-ray intensity reduction shown in Figure 2. The values of S that give the best fit for the experimen-
Figure 8. Change in the calculated separation efficiency as a function of the inlet VOC concentration. Applied voltage ) 600 V.
tal data are on the order of 1021-1022 m-3 s-1, which is 5-6 orders of magnitude higher than that predicted by the X-ray intensity (2.4 × 1016 m-3 s-1 at 4-cm away from the source). The big discrepancy might be attributed to the involvement of reactive species other than reactive cations of carrier-gas molecules in the ionization of VOC molecules. For instance, it is conceivable that the X-ray directly ionizes or excites VOC molecules by electron emission and photon absorption. The value of K that gives the best fit for the experimental data is 1.0 × 10-19 m3 at different X-ray intensities. Because K is the selectivity of VOC ionization over the self-ionization of reactive primary cations, it is conceivable that the value of K is independent of the X-ray intensity. Incidentally, it is worth noting that the collision of a very small fraction of reactive primary cations and neutral VOC molecules is sufficient to
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Notation
Figure 9. Comparison of experimental separation efficiencies with those predicted by the present model. K ) 8.0 × 10-19 m3, S ) 7.5 × 1021 m-3 s-1.
separate the VOC molecules because the value of K is on the order of 10-19 m3. Figure 9 compares the experimental separation efficiencies of the R-ray separator with those calculated by using the best fitting values of S ) 7.5 × 1021 m-3 s-1 and K ) 8.0 × 10-19 m3. In Figure 9, at a VOC concentration of 190 ppb, the separation efficiency increases with applied voltage, attains its maximum at 400 V, and then decreases with a further increase in voltage. An increase in the inlet VOC concentration brings the reduction in the separation efficiency with the peak voltage shift toward a higher voltage. The predicted separation efficiency well expresses the dependencies of the experimental separation efficiency on both the VOC concentration and the applied voltage, which in turn prove the validity of the present model accounting for VOC intermediates and reactive primary cations. Conclusion We proposed an ionization separator for VOCs by selective ionization and electrical migration of ions. The maximum separation efficiency was 43% at the inlet concentration of 190 ppb by R-ray ionization. The separation efficiency by soft X-ray is higher than that in the R-ray charger of 241Am (2.6 MBq), and it increases with decreasing the inlet concentration. The dependencies of the separation efficiency on the applied voltage and inlet VOC concentration are successfully explained by the present separation model. Acknowledgment This work was supported in part by grants from the Smoking Research Foundation and Grant-in-Aid for Developmental Scientific Research (No. 14350407) from the Ministry of Education, Culture and Science of Japan.
C ) number concentration [m-3] C0 ) inlet concentration [m-3] D ) diffusion coefficient [m2/s] E ) electrical field strength [V/m] G ) generation rate of VOC cation [m-3 s-1] k ) reaction rate constant [m3 s-1, s-1] K ) selectivity [m3] L ) distance between anode and cathode [m] N ) VOC flux [s-1] N0 ) inlet VOC flux [s-1] R ) depletion rate of VOC cation [m-3 s-1] S ) production rate of reactive primary cation [m-3 s-1] Sef ) separation efficiency 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 depletion rate constant [s-1] β ) net generation rate constant [s-1] Subscripts V ) VOC molecule V* ) intermediate of VOC V+ ) VOC ion X ) carrier gas X+* ) reactive primary cation X+ ) carrier-gas cation
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Received for review March 12, 2003 Revised manuscript received July 24, 2003 Accepted August 4, 2003 IE0302286
