Volumetric Mass Transfer Coefficient of SO 2 Gas Absorption into Aqueous Sodium Sulphite Solution in Plate Column

The volumetric mass transfer coefficient (KG.a) for SO2 removal from gas mixture into aqueous Na2SO3 solution was studied in a plate column at constant temperature ( 25 , and liquid holdup. The KG.a values were evaluated over ranges of operating independent variables: gas flow rate (QG), SO2 concentration in inlet gas (CSO2, in), and concentration of aqueous Na2SO3 solution (CNa2SO3). The experimental results showed that KG.a decreased with increasing of CSO2, in, increased with increase of QG, and CNa2SO3. The influence of gas flow rate on KG.a is more than the influence of SO2 concentration in inlet gas, and concentration of aqueous Na2SO3 solution respectively. Computer program Statgrhaphics/Experimental design was used to find the linear fitted models of the KG.a in terms of the dimensional and dimensionless of independent operating variables. The QG, CSO2, in, and CNa2SO3 have significant effects on KG.a, while the interactions of them have no significant effects on it, and could be neglected. The R-squared statistic indicates that the model as fitted explains 90.4949% of the variability in KG.a.


Introduction
Flue gas desulfurization (FGD) is presently receiving much attention in many countries.Removal of sulphur dioxide gas from flue gas is very important in the controlling of atmospheric pollution.The wide spread processes used for this purpose are the wet process absorption where the flue gas absorbed in lime slurries, the additives sometimes are used with slurries to increase the absorption rate, and dry process absorption when limestone and lime injected into the hot flue gas.These processes, besides being non regenerative, create the problem of disposal of large quantities of waste (Dutta, Basu, Pandit, & Ray, 1987;Duric, Omerovic, Brankov, Dzaferovic, & Stanojevic, 2011).Several regenerable processes of sulphur dioxide removal involving absorption into aqueous sodium citrate solution (Erga, 1980;Skribic, Cvejanov, & Paunovic, 1991) which have been received most attention in recent times due to regenerative viability of absorbent solution (Bravo, Camacho, Moya, & Aguado, 1993).Process developed by several investigators and has been used for various scales and even in commercial scale.The leading SO 2 recovery process is presently the Wellman-Lord (WL) process shall here be used in this study (Erga, 1988).In the WL process SO 2 is absorbed in an aqueous sodium sulphite solution producing sodium bisulphate: The SO 2 is a very toxic gas, has hard effects on the health and environment.The current OSHA standard for SO 2 is (5 ppm.) of air average over an eight-hour work shift.Several investigators studied absorption into different absorbents such as: aqueous reactive slurries of calcium and magnesium hydroxide (Sada, Kumazawa, Sawada, & Hashi-Zume, 1981;Dagaonkar, Beenackers, & Pangarkar, 2001), aqueous sodium hydroxide and sodium sulphite solution (Hikita, Asal, & Tsuji, 1977), aqueous solution of sodium carbonate (Witte & Kind, 1986), Urea solution (Barbooti, Ibraheem, & Ankosh, 2011), and aqueous sodium citrate solution (Skribic, Cvejaanov, & Paunovic, 1993).
The aim of present work to estimate the volumetric mass transfer coefficient (K G. a) of SO 2 gas into aqueous sodium sulphite solution using the following equation:

Experimental
Experiments were performed on absorption of SO 2 from SO 2 /N 2 gas mixture into aqueous sodium sulphite solution, at constant liquid holdup, and temperature ( 25 ) in plate column, according to experimental plan as shown in Table 1.

Results and Discussion
The estimated values of volumetric mass transfer coefficients (K G .a) and the experimental design plan are shown in Table 2.  (3) The fitted model with dimensionless independent variables is presented in the following form: The validity of the linear models is as follows: The independent variables: gas flow rate, SO 2 concentration in inlet gas mixture, and concentration of aqueous Na 2 SO 3 solution have significant effects on volumetric mass transfer coefficient (K G .a), while the interaction of independent variables have no significant effects on K G .a, and could be neglected.The influence of gas flow rate on K G .a is higher than that of SO 2 concentration in inlet gas mixture, and concentration of aqueous Na 2 SO 3 solution as shown in following Figure 2: The relationship between K G .a for SO 2 gas absorption into aqueous Na 2 SO 3 solution and independent variables: gas flow rate (Q G ), and SO 2 concentration in inlet gas mixture (C SO2, in ) could be seen at Figure 7.The K G. a increases with increasing in Q G , and decreases with increasing of C SO2, in .The K G .a for SO 2 gas removal from flue gas into aqueous Na 2 SO 3 solution was investigated in packed column with various operating conditions of independent variables (Wang, Yang, & Zhang, 2010).The K G .a values for SO 2 in flue gas of coal fired power plant were measured and calculated by aqueous ammonia in packed column as well (Qiu, Zhang, Guo, Li, Zheng, & Gong, 2010).The measured results of both investigators showed that the independent variables: gas flow rate, SO 2 concentration in flue gas, and absorbents concentrations have significant effects on K G .a of SO 2 absorption.These results are practically similar to the present results showing significant effects of independent variables on K G .a of SO 2 gas removal by different of aqueous solutions.

Conclusions
 Absorption of SO 2 gas from gas mixture (SO 2 /N 2 ) into aqueous Na 2 SO 3 solution were performed using different operating conditions of independent variables: gas flow rate, SO 2 concentration in inlet gas mixture, and concentration of aqueous Na 2 SO 3 solution. The K G .a of SO 2 gas absorption into aqueous Na 2 SO 3 solution was calculated.The results showed that K G .a increases with increasing gas flow rate and concentration of aqueous Na 2 SO 3 solution, and by decreases with increasing in the SO 2 concentration in inlet gas mixture. Using computer program (Statgraphics/Experimental design) to find the fitted linear models for dimensional and dimensionless of independent variables: Q G , C Na2SO3 , and C SO2 , in.The two fitted models for Ka.a reveal that Q G , C Na2SO3 , and C SO2 ,in have significant effects on KG.a, while the interaction of them have no significant effects on it, and could be ignored as shown in Figure 2.  The R-squared statistic indicates that the models as fitted explains 90.4949% of the variability in K G .a.The adjusted R-squared statistic, which is more suitable for comparing models with different numbers of independent variables, is 86.4212%.The standard error of the estimate shows the standard deviation of the residuals to be 0.19627. The K G .a of SO 2 absorption could be improved by increasing of gas flow rate, and concentration of aqueous Na 2 SO 3 solution, and by decreasing the SO 2 concentration in inlet gas mixture. Pilot plant designed based on the results of present work, and erected at sulphuric acid production factory in Baghdad to reduce the emission of SO 2 to atmosphere for preventing the air pollution.

(
Fig program (Statgraphics/experimental design) to find the linear model of K G .a in terms of independent variables: gas flow rate (Q G ), SO 2 concentration in inlet gas mixture (C SO2, in ), and concentration of aqueous Na 2 SO 3 solution (C Na2SO3 ).The fitted model with dimensional independent variables as follows: K G .a = 0.722 + 817.250Q G -1.509 C SO2, in + 0.501 C Na2SO3

Figure 7 .
Figure 7. Variation of K G .a with SO 2 concentration in inlet gas mixture (C SO2, in ) and gas flow rate (Q G )

Table 1 .
Experimental plan for SO 2 gas absorption into aqueous Na 2 SO 3 solution in plate column

Table 2 .
Data base for experimental design and results of K G .a for SO 2 gas absorption into aqueous Na 2 SO 3 solution in plate column