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Ind. Eng. Chem. Res. 1999, 38, 2156-2159
Volatile Organic Compound Emission from Diffused Aeration Systems: Experiment and Modeling Jia-Ming Chern* and Cheng-Fu Yu Department of Chemical Engineering, Tatung Institute of Technology, 40 Chungshan North Road, 3rd Section, Taipei, Taiwan 10451
A series of batch volatile organic compounds (VOC) emission tests were performed in a 500-L tank equipped with coarse-bubble diffusers at 0.82-3.29 m3/h diffused air flow rate and 289.2305.6 K water temperature. The unsteady-state dissolved concentrations of p-xylene and tetrachloroethylene were measured during the tests and compared with the results predicted by both the American Society of Civil Engineers (ASCE)-based model and the two-zone model. The VOC emission rate decreased with increasing air flow rate and water temperature. The results also confirmed that the two-zone model could give a better prediction of the VOC emission rates while the ASCE-based model underestimated the VOC emission rates.
(
Introduction
Sd ) 1 - exp
Air pollution problems caused by the emission of volatile organic compounds (VOCs) from wastewater treatment facilities have already received a great deal of attention. In wastewater treatment facilities, VOCs emit from wastewater collection systems as well as many treatment units.1-5 Among the many treatment units, the emission from aeration tanks is the major source of the VOC problem and thus becomes the focus of attention. The VOC mass-transfer model for diffused aeration systems, based on the American Society of Civil Engineers (ASCE) oxygen mass-transfer model,6 was first developed by Matter-Muller et al.7 and then modified by Roberts et al.8,9 and widely used to estimate the VOC emission rates from diffused aeration systems. A twozone VOC mass-transfer model was developed by Chern and Yu10 to estimate the VOC emission rates from diffused aeration systems. This paper presents the results of batch VOC emission tests and compares the results with those predicted by both the ASCE-based model and the new VOC mass-transfer model.
)
-KLaVOCVL QGHc
(2)
where KLaVOC is the volumetric mass-transfer coefficient of the VOC. Prediction of the Dissolved VOC Concentration by the Two-Zone Model According to the two-zone mass transfer model, the degree of saturation of a VOC in the gas bubble is calculated as follows:
Sd )
P0 - PW K2 RTHc 2
{ (
x
π K1b(ZS-a/2b)2 a e erf xK1b + K1b 2b a K b (3) erf ZS 2b x 1
[(
)
)
]}
where P0 is the atmospheric pressure, PW the water vapor pressure at temperature T, R the gas constant, and ZS the water depth and K1, K2, a, and b are parameters defined as
Prediction of Dissolved VOC Concentration by ASCE-Based Model
K1 )
KLBaBVOC(1 - )A RTHcG
(4)
The unsteady-state VOC concentration in a batch aeration tank is calculated by the following equation according to the ASCE-based model:
K2 )
KLBaBVOC(1 - )A G
(5)
(
CVOC ) CVOC,0 exp -
)
QGHc Sd × t VL
(1)
where CVOC,0 is the initial dissolved VOC concentration, QG the diffused airflow rate, Hc Henry’s law constant of the VOC, VL the liquid volume in the aeration tank, t the aeration time, and Sd the degree of saturation of the VOC calculated as follows: * To whom correspondence should be addressed. E-mail:
[email protected]. Tel: 011-886-2-25925252 ext. 3487. Fax: 011-886-2-25861939.
a ) P0 - PW + Fg(1 - )ZS b)
Fg(1 - ) 2
(6) (7)
where KLBaBVOC is the volumetric mass-transfer coefficient of the VOC in the gas bubble mass-transfer zone, the gas holdup, A the cross-sectional area of the aeration tank, G the nitrogen molar flow rate, F the water density, and g the gravity acceleration constant. The right-hand side of eq 3 is the ratio of the gas-phase VOC concentration to that in equilibrium with the liquid-phase VOC concentration. The unsteady-state VOC concentration in a batch aeration tank is calculated as follows:
10.1021/ie980565s CCC: $18.00 © 1999 American Chemical Society Published on Web 04/01/1999
Ind. Eng. Chem. Res., Vol. 38, No. 5, 1999 2157
{[
CVOC ) CVOC,0 exp -
{ (
a a erf xK1b + erf ZS 2b 2b
)
x
KLBaBVOC 2ZS
[(
Table 1. Correlation Parameters for the Volumetric Mass-Transfer Coefficients of Oxygen
π K1b(ZS-a/2b)2 e × K1b
)xK b]} + K 1
]}
LSaSVOC
t
(8)
where KLSaSVOC is the surface reaeration zone volumetric mass-transfer coefficient of the VOC. It is important to note that the atmospheric air above the aeration tank is assumed to be free of the VOC. Similarly to the ASCE-based model, the two-zone masstransfer model predicts that the dissolved VOC concentration decreases exponentially with the aeration time.
k1 k2 k3 θ
ψ)
(
)
kL KLaVOC 14.86n ) 0.6288n 1 + KLaO2 k V GHc c
-1
(9)
where the exponent n is between 0.5 and 1, depending upon the hydrodynamic condition in the tank, Vc is the critical volume of the VOC, and kL and kG are the individual liquid-phase and gas-phase mass-transfer coefficients, respectively. The oxygen volumetric masstransfer coefficient is a function of the water temperature and diffused air flow rate:11
KLaO2 ) (k1 + k2Qk3)θT-293.15
KLSaSO2, s-1
0 2.753 × 10-2 0.673 1.059
10-4
1.200 × 5.975 1.385 1.019
KLaO2, s-1 6.91 × 10-13 0.1076 0.717 1.036
Table 2. Correlation Parameters for the Fugacity of p-Xylene and Tetrachloroethylene A B D E m
Determination of VOC Volumetric Mass-Transfer Coefficients The volumetric mass-transfer coefficient for the VOC, KLaVOC, can be calculated from that for oxygen using the following equation:10
KLBaBO2, s-1
p-xylene
tetrachloroethylene
85.475 -7595.8 -9.378 5.6875 × 10-6 2
58.764 -6191.2 -5.3312 2.1269 × 10-6 2
Table 3. Summary of the Experimental Conditions and Parameters for the VOC Tests and Calculations atmospheric pressure water temperature diffused air flow rate tank cross-sectional area water depth gas holdup VOC charge Vc for p-xylenea Vc for tetrachloroethylenea Hc for p-xylene at 25 °Cb Hc for tetrachloroethylene at 25 °Cb a
Data from Hwang et al.16
b
758.3-762.1 mmHg 289.2-305.6 K 0.82-3.29 Nm3/h 0.541 m2 0.84-0.87 m 0.003 10-25 g 379.0 cm3/mol 289.6 cm3/mol 0.273 1.099
Data from Reid et al.21
(10)
where k1, k2, k3, and θ are correlation parameters shown in Table 1. Determination of the VOC Henry’s Law Constant Many experimental data and methods to estimate Henry’s law constants of VOCs are available.12-15 Because most experimental data were given at 25 °C, the effect of the water temperature on Henry’s law constant can be estimated by the following equation:16
HT ≈ H298.15(f°T/f°298.15)
(11)
where HT and H298.15 are the VOC Henry’s law constants at T and 298.15 K, respectively, and f°T and f°298.15 are the VOC fugacities at T and 298.15 K, respectively. The VOC fugacity f° can be estimated from the following correlation:
ln f° ) A +
B + D ln T + ETm T
(12)
where A, B, D, E, and m are correlation parameters that can be found in the AIChE DIPPR (Design Institute for Physical Property Data) databank.17 The correlation parameters for p-xylene and tetrachloroethylene are shown in Table 2. Experiment Extra pure reagent-grade p-xylene (Osaka Hayashi, Japan) and tetrachloroethylene (Alps Chem. Co.) were used as the model VOCs for the tests. The experimental apparatus was basically the same as that used for the
Figure 1. Experimental and predicted dimensionless tetrachloroethylene and p-xylene concentrations.
oxygen-transfer study11 except that an electrical fan was added to blow away the stripped VOC to keep the atmospheric air above the aeration tank free of VOC. The experimental procedure was also similar to that employed in the oxygen-transfer study except that the predissolved VOC (in methanol) solution was pumped into the tank at a position close to the bottom of the tank. Liquid samples were taken at certain time interval, and the VOC concentrations were analyzed by an UV spectrophotometer (Hitachi model U-3200, Tokyo, Japan). The experimental conditions and parameters for the 17 tests are summarized in Table 3. Results and Discussion As soon as the predissolved VOC solution was added, the tank water was oversaturated with respect to the VOC. The VOC concentration in the water then decreased with the aeration time, as shown in Figure 1. The dimensionless VOC concentrations are calculated as the actual VOC concentrations divided by the initial
2158 Ind. Eng. Chem. Res., Vol. 38, No. 5, 1999
concentration. Also shown in Figure 1 are the dimensionless VOC concentrations predicted by the ASCEbased and the two-zone models, respectively. In the calculations, the oxygen volumetric mass-transfer coefficients were calculated by eq 10, Henry’s law constants were calculated by eqs 11and 12, and the exponent in eq 9 was taken as 0.6.18 The gas-phase to liquid-phase individual mass-transfer coefficient ratio, kG/kL, was taken as 150 for the surface reaeration zone19 while the ratio was taken as 2.5 for the gas bubble zone and the ASCE-based model.20 The air above the aerated water surface was blown turbulently by an electrical fan; the gas-phase mass-transfer resistance was therefore negligible compared with the liquid-phase resistance. However, the gas-phase mass-transfer resistance in the gas bubble zone was significant, and a value of kG/kL between 2.2 and 3.6 was reported by Hsieh et al.20 for a diffused aeration system. As is clearly shown in Figure 1, the two-zone model gives a better prediction of the dimensionless VOC concentrations while the ASCEbased model overestimates the VOC concentrations. The ASCE-based model, using one single volumetric masstransfer coefficient, totally neglects the VOC transfer from the aerated water surface and therefore underestimates the VOC emission rate. For the VOCs with 1 order of magnitude difference in Henry’s law constant (Hc ) 0.267 for p-xylene and Hc ) 1.079 for tetrachloroethylene), the ASCE-based model predicts significantly different VOC concentrations while the two-zone model gives no significant difference. In addition to the difference in Henry’s law constant, the two VOCs possess different critical volumes (Vc ) 379 cm3/mol for p-xylene and Vc ) 289.6 cm3/mol for tetrachloroethylene); this leads to different volumetric mass-transfer coefficient ratios (Ψ ) 0.60 for p-xylene and Ψ ) 0.65 for tetrachloroethylene) for the same water temperature and diffused air flow rate, according to eqs 9 and 10. The degrees of saturation of p-xylene and tetrachloroethylene in the gas bubble calculated by eq 3 are 0.019 and 0.018, respectively. This suggests that the amounts of VOC stripped by the rising bubbles are almost the same for the two compounds. In the surface reaeration zone, the volumetric masstransfer coefficients for the two compounds are independent of Henry’s law constant and they are almost the same because of close Ψ values. Therefore, the total amounts of VOC emitted from the tank are almost the same for the two compounds although Henry’s law constants of the two compounds are significantly different. Figure 2 shows the experimental and predicted tetrachloroethylene concentrations at 296.5 K water temperature and three different air flow rates. As is clearly shown in Figure 2, the dimensionless tetrachloroethylene concentration decreases with increasing air flow rate, and this phenomenon can be predicted satisfactorily by the two-zone model. Similar results for p-xylene at 298.4 K water temperature are shown in Figure 3. In this case, the two-zone model underestimates the p-xylene concentration. Because p-xylene is less volatile than tetrachloroethylene, probably a different kG/kL ratio should be used to best fit the experimental data. The effects of diffused air flow rate and water temperature on the kG/kL ratio remain unknown and need further research. Figure 4 shows the experimental and predicted tetrachloroethylene concentrations at 2.47 m3/h air flow
Figure 2. Experimental and predicted dimensionless tetrachloroethylene concentrations at 296.5 K water temperature and three different air flow rates.
Figure 3. Experimental and predicted dimensionless p-xylene concentrations at 298.4 K water temperature and three different air flow rates.
Figure 4. Experimental and predicted dimensionless tetrachloroethylene concentrations at 2.47 m3/h air flow rate and three different water temperatures.
rate and three different water temperatures. As is clearly shown in Figure 4, the dimensionless tetrachloroethylene concentration decreases with increasing water temperature, and this phenomenon can also be predicted satisfactorily by the two-zone model. Conclusions The two-zone mass-transfer model for diffused aeration systems has been validated by a series of batch aeration tests. The model takes into consideration that
Ind. Eng. Chem. Res., Vol. 38, No. 5, 1999 2159
the VOC mass transfer occurs in two separate zones instead of lumping the overall VOCs transfer in the whole aeration tank as is done in the ASCE-based model. Only the VOC Henry’s law constant and the performance data of the aeration system, i.e., the volumetric mass-transfer coefficients of oxygen in the two mass-transfer zones, are required to predict the dissolved VOC concentrations under a given set of operating conditions. The experimental results show that the two-zone model satisfactorily predicts the VOC emission rates during batch aeration tests while the ASCE-based model underestimates the VOC emission rates. For any diffused aeration systems, with the correlation of the oxygen mass-transfer coefficients obtained, the effects of diffused air flow rate and water temperature on the VOC emission rates can be predicted by the two-zone model. Acknowledgment This work was supported by the National Science Council of Taiwan, Republic of China (Grant NSC 872214-E-036-002). Nomenclature a ) parameter defined in eq 6 A ) cross-sectional area of the aeration tank, m2 b ) parameter defined in eq 7 CVOC,0 ) initial dissolved VOC concentration at t ) 0, kmol m-3 CVOC ) liquid-phase VOC concentration at time t, kmol m-3 C*VOCS ) equilibrium dissolved VOC concentration at the water surface, kmol m-3 g ) gravity acceleration constant, m s-2 G ) nitrogen molar flow rate, kmol h-1 Hc ) Henry’s law constant of VOC K1 ) parameter defined in eq 4 K2 ) parameter defined in eq 5 KLBaBVOC ) bubble zone volumetric mass-transfer coefficient of VOC, h-1 KLSaS,VOC ) surface reaeration zone volumetric masstransfer coefficient of VOC, h-1 P0 ) atmospheric pressure, atm PW ) water vapor pressure, atm QG ) air flow rate, m3 h-1 R ) gas constant, atm m3 kmol-1 K-1 Sd ) degree of saturation of VOC in the gas bubble t ) aeration time, h T ) temperature, K Vc ) critical volume of VOC, m3 kmol-1 ZS ) water depth, m ) gas holdup F ) water density, kg m-3 ψ ) ratio of VOC and oxygen mass-transfer coefficients
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(2) Blackburn, J. W. Prediction of Organic Chemical Fates in Biological Treatment Systems. Environ. Prog. 1987, 6, 217. (3) Pincince, A. B. Transfer of Oxygen and Emissions of volatile Organic Compounds at Clarifier Weirs. Res. J. Water Pollut. Control Fed. 1991, 63, 114. (4) Corsi, L. R.; Chang, D. P. Y.; Schroeder, E. D. A Modeling Approach for VOC Emissions from Sewers. Water Environ. Res. 1992, 64, 734. (5) Rahme, Z. G.; Zytner, R. G.; Corsi, R. L.; Madaniisfahani, M. Predicting Oxygen-Uptake and VOC Emissions at Enclosed Drop Structures. J. Environ. Eng. Div. (Am. Soc. Civ. Eng.) 1997, 123, 47. (6) ASCE. ASCE Standard Measurement of Oxygen Transfer in Water; ASCE: New York, 1984. (7) Matter-Muller, C.; Gujer, W.; Giger, W. Transfer of Volatile Substances from Water to the Atmosphere. Water Res. 1981, 15, 1271. (8) Roberts, P. V.; Munz, C.; Dandiker, P.; Matter-Muller, C. Volatilization of Organic Pollutants in Wastewater Treatments Model studies; EPA-600/S2-84-047; U.S. Environmental Protection Agency, U.S. Government Printing Office: Washington, DC, 1984. (9) Roberts, P. V.; Munz, C.; Dandiker, P. Modeling Volatile Organic Solute Removal by Surface and Bubble Aeration. Res. J. Water Pollut. Control Fed. 1984, 56, 157. (10) Chern, J.-M.; Yu, C.-F. Volatile Organic Compound Emission Rate from Diffused Aeration Systems. 1. Mass Transfer Modeling. Ind. Eng. Chem. Res. 1995, 34, 2634. (11) Chern, J.-M.; Yu, C.-F. Oxygen Transfer Modeling of Diffused Aeration Systems. Ind. Eng. Chem. Res. 1997, 36, 5447. (12) Mackay, D.; Shiu, W. Y. A Critical Review of Henry’s Law Constants for Chemicals of Environmental Interest. J. Phys. Chem. Ref. Data 1981, 10, 1175. (13) Munz, C.; Roberts, P. V. Air-Water phase Equilibria of Volatile Organic Solutes. J. Am. Water Works Assoc. 1987, 79, 62. (14) Ashworth, R. A.; Howe, G. B.; Mullins, M. E.; Rogers, T. N. Air-Water Partitioning Coefficients of Organics in Dilute Aqueous Solutions. J. Hazard. Mater. 1988, 18, 25. (15) Yaws, C. L.; Yang, H.-C.; Hopper, J. R.; Hansen, K. C. Organic Chemicals: Water Solubility Data. Chem. Eng. 1990, July, 115. (16) Hwang, Y.-L.; Olson, J. D.; Keller, G. E., II. Steam Stripping for Removal of Organic Pollutants from Water. 2. Vapor-Liquid Equilibrium Data. Ind. Eng. Chem. Res. 1992, 31, 1759. (17) Daubert, T. E., Ed. The DIPPR Project 801 Data Compilation; Design Institute for Physical Property Data; AIChE: New York, 1990. (18) Corsi, L. R.; Card, T. R. Estimation of VOC Emissions Using the BASTE Model. Environ. Prog. 1991, 10, 290. (19) Munz, C.; Roberts, P. V. Gas- and Liquid-Phase Mass Transfer Resistances of Organic Compounds during Mechanical Surface Aeration. Water Res. 1989, 23, 589. (20) Hsieh, C.-C.; Babcock, R. W., Jr.; Stenstrom, M. K. Estimating Emissions of 20 VOCs. II: Diffused Aeration. J. Environ. Eng. Div. (Am. Soc. Civ. Eng.) 1993, 119, 1099. (21) Reid, R. C.; Prausnitz, J. M.; Poling, B. E. The Properties of Gases and Liquids, 4th ed.; McGraw-Hill: New York, 1987.
Received for review September 2, 1998 Revised manuscript received February 16, 1999 Accepted March 2, 1999 IE980565S
