4.3. Effect of SO2 inlet concentration on SO2 absorption rate
Experiments were carried out at 20 _C, the volumetric gas flow rate of 2.083 _ 10_5 m3/s, and the (NH4)2SO3 concentration of 0.05 mol/L to test the effect of SO2 inlet concentration on SO2 absorption rate. When the SO2 inlet concentration was increased from 2.857 _ 10_3 kg/m3 to 11.429 _ 10_3 kg/m3, the SO2 absorption rate increased from 1.13 _ 10_4 mol/(m2 s) to 3.6 _ 10_4 mol/(m2 s). The experimental results shown in Fig. 3 indicate that the absorption rate of SO2 into (NH4)2SO3 solution increases with increasing SO2 concentration in the inlet gas. The relationship between the absorption rate of SO2 and its concentration in the inlet gas is nearly linear when the other parameters are held constant. This shows that the reaction is always limited by gas-film resistance, and the increasing SO2 concentration enhances the driving force for absorption in the gas side.
4.4. Effect of temperature on SO2 absorption rate
The experimental results detecting the effect of temperature on SO2 absorption rate were shown in Fig. 4. During the runs, the (NH4)2SO3 concentration was 0.1 mol/L, and the volumetric gas flow rate was 2.5 _ 10_5 m3/s. The absorption rate of SO2 increases with temperature. According to Eq. (8), the solubility of SO2 in the liquid solution becomes lower with the increase of temperature. On the other hand, as shown in Table 1, the diffusing capacity of SO2 molecules in the liquid solution enhances by the increase of temperature, and the mass transfer of SO2 in the solution increases with the increase of temperature. Moreover, the increasing of the temperature also accelerates the reaction due to the increasing quantity of activated molecular.
4.5. Analysis of experimental data
It is shown in Fig. 2 that the effect of the (NH4)2SO3 concentration above 0.05 mol/L on the SO2 absorption rate is not significant, and the reaction may be considered zero-order with respect to (NH4)2SO3 concentration. The value of n in Eq. (7) is zero in this case. Eq. (7) can be expressed as:
Combining with Eq. (2) and the phase equilibrium in the gas–liquid interface, we have
which can be written in logarithmic form as
Because the relations of Ns = kG(ps _ pi) and ci = pi/H, the value of ci can be calculated by:
Using data in Fig. 3 and Eq. (15), the value of ci shown in Table 2 can be obtained. Then, the plot of ln Ns versus ln ci, shown in Fig. 5, can be gotten. There is a linear relationship between ln Ns and ln ci (correlation coefficient 0.9973). The slope of the fitted line is 0.8, so the value of m is 0.6. This means that the reaction is of 0.6th-order with respect to SO2 concentration at interface. The absorption rate of SO2 into (NH4)2SO3 solution may be represented by the equation
The comparison between calculated values (by Eq. (16)) and experimental values (from Fig. 3) of the SO2 absorption rate can be found in Fig. 6. It is seen that the calculated values fits the experimental data well.