Full-scale Wetted-wall Column에서의 SO2 흡수에 관한 연구

김인원, 이재천, 이형근*

건국대학교 화학공학과, 한국에너지기술연구소 에너지환경연구부*

A Study on Absorption of SO2 in Full-scale Wetted-wall Columns

In-Won Kim, Jae-Cheon Lee, Hyung-Keun Lee*

Dept. of Chemical Engineering, Konkuk University

Energy and Environmental Research Department, Korea Institute of Energy Research*

I. Introduction

The wetted-wall column has been used for cleaning of flue gases. The mass transfer in most laboratory-scale wetted-wall columns is simple because of using a laminar liquid film without waves or ripples. In studies employing pilot- or full-scale columns are complex because of turbulent liquid films and unknown interfacial area.

The first objective of this work was to determine empirical correlations for the dimensionless mass transfer coefficients, ShG and ShL, in a full-scale wetted-wall column. The correlation can be used in analysing the SO2 absorption data in the column. The second objective was to measure the rate of absorption of SO2 into water and Mg(OH)2 solutions.

II. Theory

2.1 Mass transfer coefficient correlations

The rate of gas- and liquid-phase mass transport has investigated by a large number of researchers (Beenackers and Van Swaaij, 1993). Upon analysing dimensionless group and correlating the mass transport data, it was found that the well-known form describes the data well and given in Eq.(1) (Nielson et al., 1998)

[eqn: rm Sh ~=~ k_1 `Re_G^k2` Re_L^k3`S_c^k4]	(1)

2.2 Calculation of mass transfer coefficients

A differential mass balance over the gas phase for the absorbed component produces the following equation for calculation of gas-phase mass transfer coefficient, kg

[eqn: k_g CDOT pi CDOT d_i CDOT a_c (C_G_i ``-`` C_G )~=~ {dC_G} over dx CDOT Qg]	(2)

The following expression is obtained by the integration of Eq.(2).

[eqn: Q_G over {pi d_i} ln LEFT ( {C_G_i ``-``C_{G,i n}} over {C_G_i ``-``C_G,out} RIGHT )~=~k_g ~ a_C ~ L]	(3)

where CG,in and CG,out are inlet and outlet gas concentration respectively.

In the SO2/NaOH system, SO2 reacts instantaneously with the solution and CGi assumes to be zero.  The modified mass transfer coefficient, kg·ac, can be calculated as the slope of Eq.(4) by the method of least squares..

[eqn: Q_G over {pi d_i} ln {C_{G,i n}} over {C_G,out}~=~k_g CDOT a_C CDOT L]	(4)

An equation for physical liquid-phase mass transfer coefficient, klo, is obtained in a similar way

[eqn: Q_L over {pi d_i} ln LEFT ( {C_L,i - C_L,out} over{C_L,i - C_{L,i n}} RIGHT ) ~=~ k_L^o CDOT a_c CDOT L]	(5)

The concentration at the interface, CL,i, in Eq.(5) is calculated using Henry's law

[eqn: C_L,i ~= ~ p over H^o]	(6)

2.3 Rate of absorption and enhancement factors (Lancia et al., 1997)

The rate of absorption (the molar flux) of SO2 across the gas-liquid interface of a column can be written in terms of the gas or liquid side resistance as

[eqn: R_SO_2 = k_g (P_SO_2 - P_SO_{2 ````i}) = varphi k_l ^o ([SO_2 ]_i - [SO_2 ])]	(7)

The enhancement factor, □, takes into account the reaction of the diffusing SO2(aq) with components found in the liquid phase. The absorption of SO2 can be written as

[eqn: R_SO_2 = LEFT ( { 1} over {kg } + { 1} over { H varphi k_l ^o} RIGHT )^-1 (P_SO_2 - P_{SO_2}` ^* )]	(8)

where H is the Henry's law constant defined by [SO2] = H Pso2.

III. Experimental

The schematic diagram of wetted-wall column system is shown in Fig. 1. The wetted-wall column can simulate the practical absorber in an actual Flue Gas Desulfurization process. The glass column with an inner diameter of 15mm and a length of 5m was equipped with six intermediate measuring position for gas and liquid samples. Synthesis flue gas (SO2 in N2) was fed at the top of the wetted-column and came in contact with a co-current flow of liquid film falling down the column.

In order to measure the gas- and liquid-phase mass transfer coefficients, SO2/NaOH and O2/H2O systems were used respectively. The absorption of SO2 into water and Mg(OH)2 solutions were measured with several operating conditions.

IV. Results and Discussion

4.1 Correlation for gas-phase mass transfer coefficient

To obtain data for kg, a system in which lean SO2(∼1000ppm) was absorbed into a strong NaOH solution(pH>11.9) was used. In this system, the liquid-phase mass transfer resistance can be considered negligible by an instantaneous irreversible reaction (Levenspiel, 1973).

The modified mass transfer coefficient, kg·ac, was measured at different gas and liquid flow rates and calculated by Eq.(4). The correlation by fitting the experimental data is given by Eq.(9).

ShG=0.0023ReG0.7463ReL05866ScG0.5	(9)

Experimental data and the fitted correlation are compared in Fig. 2. The correlation is valid for 3,400 < ReL < 7,700 and 380 < ReG < 870. It can be seen that ShG is a function of both ReG and ReL. The dependency on ReL is probably due to changes in the interfacial area and increased turbulence of the fluid phase (Nielson et al., 1998).

4.2 Correlation for physical liquid-phase mass transfer coefficient, klo

Data for klo was obtained by absorbing O2 from air into distilled water with a low concentration of O2. The O2 in the feed water was removed by blowing N2 through the air spiral in the bottom of the holding tank. The gas-phase mass transfer resistance was assumed to be negligible because O2 has a low solubility in water and the O2 concentration in atmospheric air is as high as 20 vol%.

The modified physical mass transfer coefficient, klo·ac, was investigated and the correlation of Eq.(10) was found.

ShL=0.111ReL0.801ScL0.5	(10)

The correlation is valid for 750 < ReL < 2,000 and 4,200 < ReG <10,000.

4.3 SO2/Water system

Experimental results on the absorption of SO2 into water are shown in Figure 3, where the experimental mass transfer enhancement factors, □, are plotted against the SO2 concentrations. The solid curve in this figure represents the theoretical enhancement factor predicted by surface renewal model. The theoretical enhancement factors were calculated by the following Eq.(11)

[eqn: □`````=~~`1~~&+ ~ LEFT [ sqrt{D_{{HSO_3}^-}} left([{HSO_3}]_i -[{HSO_3}]_o right) + sqrt{D_{{SO_3}^2-}} left([{{SO_3}^2-}]_i -[{{SO_3}^2-}]_o right) RIGHT ] ##&~/~ {sqrt{D_{SO_2}}`left( [{SO_2} ]_i `-`[{SO_2} ]_o right)}]  (11)

where []o and []i are the concentration in bulk liquid and gas-liquid interface respectively.

4.4 SO2/Mg(OH)2 system

Experimental results for the absorption of dilute sulfur dioxide into aqueous magnesium hydroxide solutions are shown in Figure 4 as a plot of rate of absorption versus SO2 concentration. Upper solid line represents absorption rate with gas film control (no liquid-side resistance) and lower line with physical absorption rate (no liquid-phase reaction).

V. Conclusions

The gas- and liquid-phase mass transfer coefficients were measured in a full-scale wetted wall column at atmosphere pressure and ambient temperature. Two empirical dimensionless correlation were found. The experimental data on the absorption of dilute sulfur dioxide into water and magnesium hydroxide were measured.

Acknowledgement

The authors gratefully acknowledge the partial support from the Automation Research Center at POSTECH.

References

Beenackers, A.A.C.M., and W.P.M. Van Swaaij, "Mass Transfer in Gas-Liquid Slurry

Reactors," CES, 48, 3109-3139 (1993).

Lencia, A., D. Musmarra and F. Pepe, "Modeling of SO2 absorption into Limestone

Suspensions," I&EC Res., 36, 197-203 (1997).

Levenspiel. O., The Chemical Reactor Omnibook, Wiley. 1993.

Nielson, C.H.E., S. Kill, H.W. Thomsen and K. Dam-Johansen, "Mass Transfer in

Wetted-wall columns: Correlations at high Reynolds Numbers," CES, 53, 495-503

(1998).

Figure 1. Schematic diagram of wetted-wall column system

Figure . Experimental gas phase mass transfer coefficient data compared with fitted correlation

Figure . Comparison of theoretical enhancement factor with experimental data for SO2/H2O system

Figure 4. SO2 absorption vs. absorption driving force