Discussions of Mass Transfer Performance and Empirical Correlations

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Ind. Eng. Chem. Res. 2006, 45, 8689-8696

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SEPARATIONS Discussions of Mass Transfer Performance and Empirical Correlations for VOCs Absorbed by Triethylene Glycol Solution Honda Wu* Department of Information Management, Chungyu Institute of Technology, No.40, Yi-seVenth Road, Keelung, Taiwan, 201 R.O.C.

Theng-Ching Feng and Tsair-Wang Chung Department of Chemical Engineering, Chung Yuan Christian UniVersity, Chungli, Taiwan, 32023 R.O.C.

To improve indoor air quality, volatile organic compounds (VOCs) absorbed by absorbent solution were tested in a packed-bed absorber. On the basis of the principle that organics dissolve into organic solution easily, the triethylene glycol (TEG) solution was used as the working solution to discuss mass transfer performance of VOCs absorbed by the TEG solution in this study. Toluene, methanol, ethyl ether, and methyl ethyl ketone were absorbed by the TEG solution, and the mass transfer coefficients were calculated. The flow rates of liquid and gas were controlled in the ranges 1.24-1.43 kg/min and 2.9-2.97 kg/min. According to the volatile properties of VOCs and the environmental standard, the concentrations of toluene, methanol, ethyl ether, and methyl ethyl ketone were operated in the ranges 62-110, 210-369, 58-155, and 105-205 ppm, respectively. The concentrations of the TEG solutions were controlled to between 91.5 and 96.5 wt %. After experimental runs, effects of operating variables on mass transfer coefficients were discussed, and the influence of interfacial disturbance on mass transfer performance was also analyzed in this study. The interfacial disturbance occurred because the surface tension difference prompts the surface to renew and increases the contacting probability for liquid and gas phases, which leads to enhancement of the mass transfer performance. Finally, four correlations were regressed to predict the mass transfer coefficient, and the average error was (12%. Introduction Human life is always accompanied by economic development, and the improvement of quality of life gradually increases in importance. Since some VOCs come from the manufacture of plastic products, paints, perfume, varnish, etc., they are ubiquitous in indoor and outdoor environments. Therefore, reducing the level of VOCs is important in improving indoor air quality. The technologies for reducing the concentrations of VOCs include burning, burning with a catalyst, adsorption, absorption, condensation, and filtration with biomaterial. On the basis of the fact that organic matter dissolves into an organic solution easily and the advantage of reusing absorbent solution, the method of absorption was selected to treat VOCs in the gas phase. For a packed column, one of the applications is to remove VOCs from liquid solvent by air stripping. For example, different types of packing were used by Linek et al.1 to remove VOCs from wastewater, and the penetration theory was applied by Linek et al.2 to obtain the mass transfer coefficient for the liquid phase. The packed column has also been used in the absorption and distillation processes, too. Some studies3-6 had discussed mass transfer characteristics and performance for water vapor absorbed by desiccant solutions; however, the mass transfer phenomena for VOCs absorbed by glycol solution in the packed-bed absorber were discussed poorly, so the packedbed absorber was chosen to remove VOCs from the air. * To whom correspondence should be addressed. Tel.: 886-224237785-576. Fax: 886-2-24293375. E-mail: [email protected].

Discussions of mass transfer phenomena for the sorption processes could be divided into three fields, which included mass transfer correlations,7-11 the effect of operating variables on mass transfer performance,12-17 and indoor air quality.18,19 Although the indoor air quality has gained attention recently, studies of removing VOCs with a glycol solution are limited in the open literature. Therefore, the purpose of this study was to set the absorption system to absorb VOCs with the TEG solution and to discuss the effects of operating variables on mass transfer performance. Generally speaking, the absorbate absorbed by the absorber includes water vapor,7,11,20-22 inorganic pollutants,13-17,23,24 and volatile organic compounds.11,25 In the open literature, there are few studies discussing the mass transfer phenomena of VOCs absorbed by the absorber. The mass transfer phenomena for VOCs absorbed by the TEG solution are discussed, and more mass transfer data were also acquired from experimental operations so that it could be offered to researchers and absorber designers. The Marangoni effect is the phenomenon that interfacial fluid flow and interfacial mass transfer can be affected by the surface tension gradient. The interfacial area and mass transfer mechanism could be changed by the surface tension gradient during the mass transfer process. The Marangoni effect always occurs in the interface, and it is often studied with the continuous liquid flow. Furthermore, the terms Marangoni “positive” and “negative” systems are used in the fields of the interfacial phenomena. For example, the Marangoni positive and negative systems have

10.1021/ie050951r CCC: $33.50 © 2006 American Chemical Society Published on Web 11/09/2006

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Figure 1. Absorption system of this study.

often been defined in liquid thin films accompanied by a surface tension gradient. When the ratio of partial surface tension to partial concentration of absorbent solution is larger than zero (Marangoni positive), the liquid will flow from the thicker region to the thinner region in the liquid surface. The liquid flow causes the liquid to spread on the supporting surface homogeneously, and the mass transfer performance will be higher than in a Marangoni negative system. On the contrary, the Marangoni negative system decreases the contact area for gas and liquid phases. As mentioned above, the meaning of the Marangoni positive system could be extended to describe the mass or heat transfer performance of a system that is increased by the Marangoni effect, and the term Marangoni negative system could be used to describe the mass or heat transfer performance of a system decreased by the Marangoni effect. In addition, Patberg et al.27 discussed the ratio of a packing area covered by liquid film to the intrinsic packing area of the packed-bed distillation column in relation to the surface tension gradient. Recently, the Marangoni index was defined by Wu et al.28 to elucidate the level of interfacial disturbance resulting from water vapor being absorbed by the TEG solution in the packed-bed absorber. Zhang et al.23 found that as a monoethanolamine solution absorbed carbon dioxide (CO2) in high partial pressure, the Marangoni effect was enhanced, which led the correlation to predict the absorption rate of carbon dioxide imprecisely in high partial pressure. On the basis of the discussion above, the mass transfer performance could be influenced by the surface tension difference between absorbent and absorbate in the absorption system. Since the surface tension difference also exists in the TEGVOC system, the interfacial instability will occur in the process of VOCs absorbed by the TEG solution. Therefore, not only the intrinsic dissolved capacity but also the interfacial behaviors should be considered to elucidate the mass transfer data in this study. Experiments Chemicals. Triethylene glycol was used as working solution to absorb VOCs in the packed-bed absorber. The VOCs included toluene, methanol, ethyl ether, and methyl ethyl ketone.

Absorption System. The absorption system consisted of an air compressor, a mass flow controller, an air impinger, and a packed-bed absorber. As shown in Figure 1, the packed-bed absorber was made of polypropylene, and the length, width, and height were 15, 15, and 55.5 cm, respectively. The ratio of absorber to packing diameter is about 9:1, and 1776 packing (34 cm) was packed in the packed bed of the absorber randomly. The vapor pressure of the pure TEG solution was less than 0.01 mm Hg at 25 °C under the open system (data source: A Guide To Glycol31). The TEG solution was barely volatile in the room temperature. Therefore, the carry over can be neglected for the system using TEG solution. On the basis of the properties of low volatility, low pollution, and noncrystallization, more and more scholars have selected the TEG solution as working solution in recent years. Generally speaking, the usual absorbers are designed to remove a few major gas mixtures. It is difficult to find an absorbent to absorb many kinds of gas pollutants well. Therefore, the absorption of individual VOCs by the TEG solution was tested in the packed-bed absorber. Experimental Procedures. TEG solution was sprayed into the packed bed by liquid distributors, and the liquid thin film contacted feeding air in the countercurrent. The liquid thin film would absorb VOC vapors. The solution was regenerated for reuse after the liquid thin film absorbed the vapors. The overall research procedures were divided into three steps. The first step included acquisition of a calibration curve and experimental operation. The known concentrations of VOCs were injected into the gas chromatograph (GC) to acquire the relationship (calibration curve) between concentrations of VOCs and the integral area. The absorption system was operated, and the equilibrium concentrations between VOCs and absorbent solution were measured. The second step was to calculate the mass transfer coefficient and to measure the surface tension of TEG and VOC solutions. The third step was to analyze the effects of operating variables on mass transfer performance. Finally, the mass transfer correlations for VOCs absorbed by the TEG solution were established in this study. Measurements of the Equilibrium Concentrations. Since the equilibrium concentrations were taken as the basis for calculating the mass transfer coefficient, the equilibrium con-

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Figure 2. Measuring system of the equilibrium concentration.

centrations had to be measured under the operating conditions. The equilibrium concentration is a function of the concentration of liquid absorbent, the temperature of the absorption process, and the concentrations of VOCs. On the basis of the required operating conditions, the equilibrium concentrations were measured in this study. As shown in Figure 2, the measured system for equilibrium concentrations consisted of an equilibrium region, an electromagnetic heater/agitator, a temperaturecontrolled bath, an air impinger, a mass flow controller, and an air compressor. The concentrations of VOCs were adjusted by the air compressor and the mass flow controller. The gas phase in the equilibrium region was sampled continuously until the concentration of the VOC was stable. After the concentration was stable, the TEG solution was injected into the equilibrium region rapidly, and then valves 1 and 2 were closed to form a closed system. The TEG solution was agitated by magnet to decrease the equilibrium time for gas and liquid phases. The equilibrium state was attained after ∼3-4 h. Then, the gas phase of the equilibrium region was sampled. After the calibration curve of the concentration of VOC was obtained by GC (gas chromatography), the sampled gas phase was subjected to GC to analyze the concentration of the VOC. Results and Discussion Overall Mass Transfer Coefficient for the Gas Phase. On the basis of the mass transfer balance for the gas and liquid contacting system, the mass transfer flux can be shown as follows (Hines and Maddox29).

NA )

Kya d(GyA) ) (y - yA*) adZ βV-V* A βV-V* )

(1 - yA*)M )

(1 - yA*)M 1 - yA

(1 - yA*) - (1 - yA) 1 - y A* ln 1 - yA

(1)

(2)

(3)

Equation 1 is replaced by eq 2 and is integrated with respect to the concentration of VOC in the gas phase. The final formula for mass transfer coefficient in the gas phase can be shown in eq 4.

(Kya)avg )

∫yyA

G Z

out

Ain

(1 - yA*)M dyA 1 - yA yA* - yA

(4)

As the concentration of VOC is dilute in the gas phase, (1 yA*)M and (1 - yA) approach unity. The overall mass transfer coefficient for the gas phase can be simplified into eq 5.

(Kya)avg )

∫yAyA

G Z

out

in

dyA G ) yA* - yA Z

∫yAyA

in

out

dyA G ) y A - y A* Z yAin - yA* ln (5) yAout - yA*

[

]

Therefore, selecting eq 4 or eq 5 to calculate the mass transfer coefficient is determined by the concentration of VOC in the gas phase. Effect of Operating Variables on Mass Transfer Coefficient. Figure 3 shows that the mass transfer coefficients were obtained from VOCs absorbed by 96.5 wt % TEG solution under 2.97 kg/min air flow rate. The mass transfer coefficients were increased by the increased flow rates of the TEG solutions. The results could be derived from the fact that the amount of treated VOCs in the gas phase would be increased with the increased flow rate of TEG solution. In addition, 2.9 and 2.97 kg/min air flow rates and 96.5 wt % TEG solution were set to absorb VOCs in different liquid flow rates, and the results are shown in Figure 4. Since the overall mass transfer coefficient for the gas phase was in proportion to the air flow rate theoretically (eq 5), the mass transfer coefficients for the high air flow rate were larger than those for the low air flow rate, as shown in Figure 4. From another point of view, the VOCs were carried into the packedbed absorber by air stream, and the total amount of VOCs would be increased with the increased air flow rate at the same inlet concentration of VOCs. Therefore, the amount of VOCs absorbed by the TEG solution would be increased with the increased air flow rate, and thus, the mass transfer coefficient should be increased. Both theory and experiment demonstrated that the mass transfer coefficients were increased as the air flow rate was increased. In addition to liquid and air flow rates, one of the operating variables affecting absorption performance is the concentration of VOC in the gas phase. The effects of the concentration of VOC on the mass transfer coefficients for high and low levels are shown in Figure 5. Since the concentration of VOC is dilute in the gas phase, the mass transfer coefficient is in proportion

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Figure 3. Effects of liquid flow rate on mass transfer coefficient.

Figure 4. Effects of high and low level of air flow rate on mass transfer coefficient.

Figure 5. Effects of high and low level of concentration of VOCs on mass transfer coefficient.

to ln[(yAin - yA*)/(yAout - yA*)]. The difference between yAout and yA* can be regarded as the basis on which to calculate the mass transfer coefficient. Since the higher outlet concentration of VOC was always accompanied with the higher inlet concentration of VOC, the basis, yAout - yA*, is larger for the higher inlet concentration than that for the lower inlet concen-

tration of VOC. The larger basis always leads to the lower mass transfer coefficient. Therefore, the mass transfer coefficients for the high inlet concentration of VOC were smaller than those for the low inlet concentration of VOC, as shown in Figure 5. The mass transfer performance for usual mass transfer equipment can be assessed with the mass transfer coefficient, the

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Figure 6. Effects of high and low level of concentration of TEG solutions on mass transfer coefficient.

Figure 7. Equilibrium diagram for methanol and TEG solution.

height of the transfer unit, the mass transfer flux, the total removal amount, etc. Assessing mass transfer performance by various definitions might lead to divergent results. For example, the mass transfer coefficient was increased as the inlet concentration of VOC was lowered. However, as the total removal amount, yAin - yAout, was used to assess the mass transfer performance, the higher removal amount would be accompanied with the higher inlet concentration of VOC. The mass transfer coefficients were larger for 96.5 wt % TEG solution than for 91.5 wt % TEG solution except for methanol, as shown in Figure 6. Because of the existence of hydrogen bonding, the methanol was almost completely dissolvable in the water. The equilibrium diagrams for methanol and TEG solution are presented in Figure 7. Figure 7 shows that the equilibrium concentration of methanol decreased with the decreased concentration of the TEG solution; namely, the lower concentration of the TEG solution would be advantageous to increasing the mass transfer performance for absorption of methanol. On the contrary, the equilibrium concentrations for other VOCs were increased with the increased concentrations of the TEG solutions, which would lead to the mass transfer coefficients being larger for the higher concentrations of the TEG solution. Therefore, the mass transfer coefficients for other VOCs were larger in the 96.5 wt % TEG solution.

As shown in Table 1, the equilibrium concentrations of VOCs for toluene are significantly smaller than those for other VOCs; that is, the vapor of toluene was more dissolvable in the TEG solution than the other VOCs under the experimental operating conditions. In addition, as the variables, including the concentration of absorbent solution, inlet concentration of VOC, and air flow rate, were adjusted to absorb VOCs well, the flow rate of the absorbent solution (liquid flow rate) is always the most direct factor to affect the mass transfer performance. Therefore, the changes of the mass transfer coefficients for toluene are larger than those for other VOCs (Figure 1). Influence of Surface Tension on the Absorption Mechanism. To compare the mass transfer performance for various kinds of VOCs absorbed by the TEG solution, the operating conditions were set at the inlet concentration of 210 ppm VOCs, 1.43 kg/min flow rate of absorbent solution, 2.97 kg/min air flow rate, and 91.5 wt % TEG solution. Table 2 shows that the mass transfer coefficients for methanol, ketone, ethyl ether, and toluene were 0.106, 0.058, 0.054, and 0.053, respectively. The equilibrium concentrations of the VOCs are also shown in Table 2. The equilibrium concentration refers to the minimum concentration of VOC existing in the gas phase under the experimental operating conditions. Generally speaking, the lower the equilibrium concentration is, the better the absorption performance is. Table 2 shows that the mass transfer performance for methanol absorbed by the TEG solution was larger than those for the other VOCs. The result could be deduced as follows. The equilibrium concentrations were measured in the static state, and the equilibrium concentrations were related to the affinity between TEG and VOC and the volatility of VOC; however, absorptions of VOCs in the absorber were in the dynamic state, and hydrogen bonding existed between methanol and the TEG solution. Therefore, the mass transfer performance was better for absorption of methanol. Except for the methanol, the sequence of the equilibrium concentrations for VOCs in the gas phase was ethyl ether, toluene, and methyl ethyl ketone (Table 2), so the mass transfer performance for VOCs absorbed by the TEG solution should be in the series of methyl ethyl ketone, toluene, and ethyl ether. However, the mass transfer coefficients shown in Table 2 are in the series of methyl ethyl ketone, ethyl ether, and toluene. The experimental data and deduced results were discrepant, and the reasons could be explained as follows. Brian30 mentioned that the mass transfer from one phase to another would cause the surface tension gradient, and the

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Table 1. Parts of Experimental Dataa VOC

air flow rate (kg/ min)

liquid flow rate (kg/ min)

VOCs inlet conc (ppm)

VOCs outlet conc (ppm)

equilibrium conc of VOCs (ppm)

mass transfer coefficient (mol/ m3 s)

toluene (1) (2) (3) (4) (5) (6) methanol (7) (8) (9) (10) (11) (12) ethyl ether (13) (14) (15) (16) (17) (18) ketone (19) (20) (21) (22) (23) (24)

2.97 2.97 2.97 2.97 2.97 2.97 2.97 2.97 2.97 2.97 2.97 2.97 2.97 2.97 2.97 2.97 2.97 2.97 2.97 2.97 2.97 2.97 2.97 2.97

1.24 1.34 1.43 1.24 1.34 1.43 1.24 1.34 1.43 1.24 1.34 1.43 1.24 1.34 1.43 1.24 1.34 1.43 1.24 1.34 1.43 1.24 1.34 1.43

62 62 62 110 110 110 210 210 210 369 369 369 58 58 58 155 155 155 105 105 105 205 205 205

38.7 38.6 34.0 72 71.5 69.4 159.3 157.3 156.2 302.7 293.7 292.5 46.9 46.7 45.6 131.3 126.9 125 82.6 81.4 80.1 161.1 160.2 156.7

4.1 4.1 4.1 11.1 11.1 11.1 40.2 40.2 40.2 39.2 39.2 39.2 15.8 15.8 15.8 24.2 24.2 24.2 8.9 8.9 8.9 15.7 15.7 15.7

0.115 0.126 0.148 0.108 0.110 0.121 0.079 0.083 0.085 0.050 0.058 0.059 0.068 0.070 0.078 0.045 0.054 0.058 0.059 0.063 0.067 0.059 0.060 0.066

a

TEG concentration: 96.5 wt %, packing height: 0.34 m.

Table 2. Surface Tensions of Absorbent and Absorbatea

chemical

surface tension at 27 °C (mN/m)

equilibrium conc of VOCs at 91.5 wt % TEG soln (ppm)

mass transfer coefficient (mol/m3 s)

96.5 wt % TEG solution 91.5 wt % TEG solution toluene methanol ethyl ether methyl ethyl ketone

44.8 45.5 27.3 22.2 17.1 23.4

30.2 41.7 43.3 22.2

0.053 0.106 0.054 0.058

a Mass transfer coefficients were obtained under the conditions of 210 ppm inlet concentration of VOCs, 1.43 kg/min liquid flow rate, and 2.97 kg/min air flow rate.

induced Marangoni instability would enhance the mass transfer performance for the liquid-liquid and gas-liquid contact systems. In Table 2, the surface tension of ethyl ether is smaller than that of toluene; that is, the surface tension difference between ethyl ether and TEG solution is larger than that between toluene and TEG solution, and the interfacial disturbance will be stronger for the system of ethyl ether and TEG solution. The behavior of interfacial disturbance would prompt the interface to renew and increase the contacting probability for gas and liquid phases. Therefore, the mass transfer performance would be enhanced for ethyl ether more than for toluene in the same operating conditions, and the larger mass transfer coefficient was obtained for the ethyl ether. Mass Transfer Correlations. In addition to the surface tension effect, the mass transfer coefficient was also affected by other operating variables. Unless the values of mass transfer coefficients for four VOCs were in the immediate range, which would lead to the sequence of mass transfer coefficient being changed by the surface tension difference linearly, the term of surface tension was not suitable for inclusion into the mass transfer correlation. Furthermore, the inlet concentrations of the VOCs were difficult to control, and the intrinsic solubility of VOCs in the TEG solution were various, which led the values of mass transfer coefficients to spread widely, such that four mass transfer correlations were respectively regressed for four

VOCs. Parameters affecting the mass transfer coefficient include the physical properties of gas and liquid, the flow rates of gas and liquid, the packing material and diffusion coefficient of VOCs in the gas phase, etc. As the liquid flow rates were increased, the mass transfer coefficients increased (Figure 3). Therefore, the dimensionless term, L/V, was tested in the empirical regression. On the other hand, effect of the concentration of TEG solution on the mass transfer coefficient is significant; however, the general mass transfer correlation for the gas phase could not present the variables for the liquid phase. To enlarge the prediction range of the mass transfer coefficient and to raise the accuracy in predicting mass transfer coefficients, the dimensionless terms L/V and X were selected to establish mass transfer correlations. The mass transfer correlations were obtained by dimension analysis and nonlinear regression in this study. The parameters mentioned above were represented in the functional form as follows.

f(Kya, L, V, M, D, X, dp, dc, F, µ) ) 0

(6)

Applying the Buckinghan π method to reintegrate these parameters as a measurable dimensionless group, the mass transfer correlation was obtained as follows.

( )( )( )

FVdc KyaMdp )b µ µ

c

µ DF

d

L e f X V

(7)

The mass transfer coefficients obtained from the experiments and the related factors were brought into eq 7, and the nonlinear regression was conducted to obtain the values of b, c, d, e, and f. Finally, mass transfer correlations were obtained in the following. For toluene

Sh ) 30 × {Re}0.8 × Sc1/3 ×

(VL)

R-square ) 0.992

1.68

× X-0.13

(8)

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For methanol

Sh ) 13.37 × {Re}1.30 × Sc1/3 ×

(VL)

0.71

× X5.61

(9)

R-square ) 0.981 For ethyl ether

Sh ) 30 × {Re}0.75 × Sc1/3 ×

(VL)

1.40

× X-0.26

(10)

× X-0.2

(11)

R-square ) 0.971 For methyl ethyl ketone

Sh ) 30 × {Re}0.71 × Sc1/3 ×

(VL)

1.2

R-square ) 0.984 These four mass transfer correlations could be categorized as semiempirical correlations, and the values of the powers did not always have a physical meaning. We just could make a simple description for Figure 4 to analyze the effects of factors on the mass transfer performance. Since the trend of effect of the TEG concentration on the mass transfer coefficient for methanol was different from that of the other VOCs, the subsequent analysis for methanol was neglected. The air flow rate could be regarded as the most important factor in the Reynolds number to affect the mass transfer coefficient. Viewing the correlations for toluene, ethyl ether, and methyl ethyl ketone, the powers of Re were 0.8, 0.75, and 0.71, respectively. Against the gap between 2.97 and 2.9 kg/ min air flow rate in Figure 4, the sequence was toluene, ethyl ether, and ketone. The results revealed that the mass transfer correlations agreed with the experimental data. Similarly, the other dimensionless groups also could describe effects of the liquid flow rate and the concentration of TEG solution on the mass transfer coefficient, and the results also agreed with Figures 3 and 6. The average error for the regressed result was defined as the ratio of the difference between the predicted and experimental mass transfer coefficients to the experimental mass transfer coefficient, and the average error was 12%. Figure 8 showed that most of the data points were within the range of (15% error. The experimental mass transfer coefficients were taken as the denominator in the formula of the average error; that is, the experimental values were regarded as the referring points. As the mass transfer coefficient is located in the lower region, the smaller difference between predicted and experimental values might cause the larger error than that in the higher mass transfer coefficient. Therefore, the data points beyond the error of (15% mostly occurred in the region of the lower mass transfer coefficients. In addition, the values of R-squares for the regressed correlations ranged from 0.0971 to 0.992. Not only Figure 8 but also the R-squares demonstrated that the mass transfer coefficients predicted by these correlations were precise under the operating conditions of this study, and the results could be provided as a reference for absorber designers. Conclusion The absorption system for absorbing VOCs with a TEG solution was established successfully. The operating conditions of 2.9-2.97 kg/min air flow rates, 1.24-1.43 kg/min liquid flow rates, and 91.5-96.5 wt % TEG solution were controlled to absorb VOCs in this study. Since the contacting probability for gas and liquid phases was increased by the increased gas and

Figure 8. Relationship between predicted and experimental mass transfer coefficient.

liquid flow rates, the mass transfer coefficients were increased by the increased gas and liquid flow rates. Generally speaking, the higher concentration of the TEG solution would obtain the lower equilibrium concentration of VOCs in the gas phase, so that the VOCs would be absorbed easily under the higher concentration of the TEG solution. Therefore, the mass transfer performance was better for the absorbent with the higher concentration. Combining the principle of mass transfer and the Marangoni effect, the sequence of absorption performance for absorbing VOCs by the TEG solution is methanol, methyl ethyl ketone, ethyl ether, and toluene, as proven in this study. In addition, both Figure 8 and the R-squares demonstrated that the mass transfer coefficients could be predicted by these mass transfer correlations precisely. Not only the predicted results but also the experimental data can be provided to absorber designers in the future. Appendix 1. The absorption system was designed in a batch system. During our experimental operation, four experimental runs proceeded continuously. Since the concentration of the TEG solution lowered insignificantly, the TEG solution was regenerated after four experimental runs. The TEG solution was heated and put in contact with inlet air to strip the VOCs in the regenerating process. Taking data point 9 in Table 1 as an example, the mass transfer coefficient is 0.085 mol/m3 s. The value 0.085 mol/m3 s can be translated into 2.72 g/m3 s by the dimension translation. The column was designed as 0.15 × 0.15 × 0.34 m3, and the concentrations of VOCs for inlet and outlet of the air tunnel would attain the steady state after 15 min. Therefore, the removal amount could be sketchily estimated as 74.91 g.

2.72 g/m3s × 0.15 × 0.15 × 0.34 × 60 s/min × 15 min × 4 runs ) 74.91 g The total amount of the TEG solution is about 40 L, and the concentration of TEG solution was 96.5 wt %. Since the density of the TEG solution is 1.11 g/L, 44400 g TEG solution was cycled in the absorption system.

40 L × 1.11 kg/L × 1000 g/kg ) 44400 g As the 74.91 g of methanol was absorbed into the TEG solution, the concentration of the TEG solution would be

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decreased from 96.5 to 96.3 wt %. Since the concentration of TEG solution was decreased insignificantly, the effect of reusing the cycled solution on absorption performance was limited during the experimental runs. 2. Could the empirical correlations regressed by this study be applied to the system of multicomponent VOCs? Once the following assumptions are held, the authors trust that these empirical correlations could be used to predict the mass transfer coefficients for a multicomponent system: (1) The packed-bed absorption system scales up and down reasonably. (2) The system provides enough contacting area and exposure time to absorb gas pollutant. (3) The chemistry between VOCs is inert; that is, the gas phase is almost approaching the state of ideal gas. 3. The directions for the mass transfer correlations can be described simply as follows. First, required operating conditions and variables were placed into the relevant dimensionless group, and then the mass transfer coefficient could be predicted before construction of the absorption system. Second, the size of the absorber was designed according to the predicted mass transfer coefficient. Finally, the designed absorber would be connected to the flooding line and the minimum wetting rate to assess the flow rates of gas and liquid. Acknowledgment This study is supported by National Science Council of the Republic of China under Grant NSC 92-2626-E-148-001. Nomenclature D ) diffusion coefficient, m2/s Dc ) column diameter, m dp ) packing diameter, m G ) molar flux of gas phase, mol/m2 s Kya ) overall mass transfer coefficient for the gas phase, mol/ m3 s L ) liquid flow rate, kg/min M ) molecular weight of the transferred molecule, g/g mol Re ) Reynolds number, FVdc/µ Sc ) Schmidt number, µ/FD Sh ) Sherwood number, kyaMdp/µ V ) air flow rate, kg/min X ) mole fraction of TEG to water of the TEG solution yA* ) mole fraction for VOCs equilibrating in the air yAin ) mole fraction of component A in the inlet gas yAout ) mole fraction of component A in the outlet gas Z ) height of absorber, m Literature Cited (1) Linek, V.; Sinkule, J.; Janda, V. Design of Packed Aeration Tower to Strip Volatile Organic Contaminants from Water. Water Res. 1998, 32 (4), 1264. (2) Linek, V.; Petricek, P.; Benes, P.; Braun, R. Effect of Interfacial Area and Liquid Side Mass Transfer Coefficients in Absorption Columns Packed with Hydrophilized and Untreated Plastic Packings. Chem. Eng. Res. Des. 1984, 62, 13. (3) Gandhidasan, P. Quick Performance Prediction of Liquid Desiccant Regeneration in a Packed Bed. Sol. Energy 2005, 79 (1), 47. (4) Gandhidasan, P. A Simplified Model for Air Dehumidification with Liquid Desiccant. Sol. Energy 2004, 76 (4), 409. (5) Alizadeh, S.; Saman, W. Y. An Experimental Study of a Forced Flow Solar Collector/Regenerator Using Liquid Desiccant. Sol. Energy 2002, 73 (5), 345. (6) Chung, T. W.; Wu, H. Comparison between Spray Tower with and without Fin-coils forAir Dehumidification Using Triethylene Glycol Solutions and Development of the Mass Transfer Correlations. Ind. Eng. Chem. Res. 2000, 39 (6), 2076.

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ReceiVed for reView August 19, 2005 ReVised manuscript receiVed July 14, 2006 Accepted September 25, 2006 IE050951R