Recovery of sulfur dioxide from gas mixture in packed bed column

Abstract

Recovery of SO^sub 2^ from SO^sub 2^/Air mixture into aqueous sodium carbonate solution was performed using packed bed column in pilot scale. The aim of the study was to improve the recovery efficiency of this process, to find the proper operation conditions in the packed bed column, and to contribute to the application of this process in the industry. The SO^sub 2^-recovery efficiency was measured while the gas mixture rate, the inlet SO^sub 2^ concentration, sodium carbonate solution concentration, liquid temperature, and the liquid hold-up were changed according to experimental design. Computer program (Statgraphics/Experimental Design) was used to estimate the fitted linear model of SO^sub 2^-recovery efficiency (η) in terms of (G, C^sub SO2^, C^sub Na2CO3^, T, and V^sub L^), and the economic aspects of the process. The accuracy of η model is ± 2.38 %. The linear model of η was adequate, and the operating parameters were significant, while the interactions were negligible.

Headnote

Abstract

Recovery of SO^sub 2^ from SO^sub 2^/Air mixture into aqueous sodium carbonate solution was performed using packed bed column in pilot scale. The aim of the study was to improve the recovery efficiency of this process, to find the proper operation conditions in the packed bed column, and to contribute to the application of this process in the industry. The SO^sub 2^-recovery efficiency was measured while the gas mixture rate, the inlet SO^sub 2^ concentration, sodium carbonate solution concentration, liquid temperature, and the liquid hold-up were changed according to experimental design. Computer program (Statgraphics/Experimental Design) was used to estimate the fitted linear model of SO^sub 2^-recovery efficiency (η) in terms of (G, C^sub SO2^, C^sub Na2CO3^, T, and V^sub L^), and the economic aspects of the process. The accuracy of η model is ± 2.38 %. The linear model of η was adequate, and the operating parameters were significant, while the interactions were negligible.

Copyright © 2017International Energy and Environment Foundation - All rights reserved.

Keywords: So2 absorption; Aqueous sodium carbonate solution; Backed bed column.

(ProQuest: ... denotes formulae omitted.)

1.Introduction

Thermal power plants are major sources of air pollutants, the large amount of SO2 emitted from the combustion of coal leads to severe air pollution and results in great harm to people's living, production and health, and has to be controlled. Several desulfurization schemes, such as fuel pretreatment, concurrent burning and adsorption, and flue gas post treatment, have been proposed [1]. The flue gas desulphurization (FGD) is the main technology used, and a great number of FGD methods have been developed, such as dry-, semidry- and wet-processes, among which the wet-process is more widely applied, due to lower operating cost and more stable operation, and limestone- or lime-based scrubbing is used. However, many of these FGD methods are not regenerative and pollute the environment. The traditional SO2 absorption process requires a large column packed with various packings [2] or spray tower [3], rotating-stream tray scrubber [4], etc. Since the mass transfer efficiency is poor, the size of columns is large, leading to high capital and operating costs. Therefore, researches have been focused on developing regenerative processes and equipment with process intensification for absorption of SO2 [57]. So2-removal by absorption into sodium citrate buffer solution is generally considered as a fast, safe, green, and economical method with the advantages of non-toxic reagent with negligible losses, simple process, no fouling problem, and low oxidation of SO2. In the process, SO2 in flue gas is absorbed by aqueous sodium citrate solution, and dissolved So2 is subsequently recovered from the solution by steam stripping or other regenerative method, or the resulting absorbent solution react with H2S to obtain sulphur, which can be separated by flotation [8-10]. The desulfurization of SO2 with absorption and desorption in sodium citrate buffer solution. SO2 is a major constituent in air pollution. SO2 which affects the environment by no. of ways like acid rain, corrosions and severe damage to the health. SO2 causes a wide variety of health and environmental impacts because of the way it reacts with other substances in the air. Particularly sensitive groups include people with asthma who are active outdoors and children, the elderly, and people with heart or lung disease. Removal of SO2 from gas mixtures by chemical absorption is one of the most important processes for environmental protection. A great number of gas desulphurization methods have been developed throughout the world [11,12]'The most widespread processes are limestone-, or lime-based scrubbing [11-14], beside these, magnesium hydroxide [13, 14], sodium hydroxide [15] and organic solvents are also used as absorbents [16, 17].

A lot of research has been focused on regenerative processes [1, 8, 16-26]. The SO2-removal by aqueous sodium carbonate solution received a considerable attention [18-28].

When sulfur dioxide in gas phase is absorbed into aqueous sodium carbonate solution, the following overall reaction between the dissolved sulfur dioxide and aqueous sodium carbonate solution may take place in the liquid phase [29], and can be expressed as:

... (1)

Some FGD systems go a step further and oxidize the Na2SO3 to produce marketable Na2SO4 (Sodium Sulphate) [30, 31]:

... (2)

The removal of sulfur dioxide from gas mixture by aqueous carbonate solutions is an important industrial absorption process for control of air pollution. Further, this chemical absorption process is of theoretical interest, since it is one in which the absorption is accompanied by a chemical reaction and the subsequent desorption of the volatile reaction product. However, there have been a few studies on the mechanism of chemical absorption of sulfur dioxide into aqueous carbonate solution [32].

2.Experimental work

2.1 Experimental apparatuses

The main equipment of the experimental apparatus as shown in Figure 1 is the packed bed column (1), and its heat exchanger (2), the size to gather of 1.5 m height and 0.10 m in diameter. The main complementary apparatus and pipe lines are as follows: Temperature gage (3), discharge point (4), digital pH- meter (5), compressed air in (6) , sulfur dioxide gas in (7), air rotameter (8), sulfur dioxide rotameter (9), mixing chamber (10), gas mixture in (11), aqueous Na2CO3 solution in (12), condenser (13), liquid recycle to top of the column (14), cold water out from condenser (16), cold water in to condenser (17), gas mixture (18), droplet collector (19), SO2 gas analyzer (20), gas mixture out to atmosphere (21), water to heat exchanger from thermostat (22), water from heat exchanger to thermostat (23), solid Na2CO3 (24), process water (25), mixing tank to prepare aqueous Na2CO3 solution (26), feeding pump of aqueous Na2CO3 solution to packed column (27), aqueous Na2CO3 solution feeding tank (28), thermostat water bath (29), (Na2SO4+Na2SO3, and CO2) in solution from bottom of column (30), (Na2SO4+Na2SO3, and CO2) solution tank (31) CO2 gas out (32), discharge pump of (Na2SO4 + Na2SO3) solution (33) solution for further processing plant to oxidized Na2SO3 to Na2SO4, and producing dry powder Na2SO4 (34).

2.2 Operating parameters

The preliminary experiments were carried out to absorb of SO2 gas from gas mixture into aqueous Na2CO3 solution in pilot scale packed column by using experimental apparatus as shown in Figure 1 to find the proper operating parameters could be used in this work. Operating variable parameters were as follows:

* Gas mixture flow rate (G): 25-35 m3/h.

* SO2 gas inlet concentration (CSO2): 500-3500 ppm.

* Aqueous Na2CO3 solution concentration (CNa2CO3): 5-35 wt %.

* Liquid temperature (T): 20-50 °C.

* Liquid holdup in the column (VL): 0.02-0.04 m3.

2.3Recovery of SO2 from SO2/Air gas mixture

Experiments of absorption of SO2 gas from SO2/Air gas mixture into aqueous Na2CO3 solution have been carried out using the mentioned various operating parameters by using experimental apparatus as shown in Figure 1, according to experimental design plan seen in Table 1. The gas mixture (11) enters the packed column (1) from lower part, while Na2CO3 solution from solution tank (28) by feeding pump (27) enters the upper part of the column. The heat exchanger (2) is maintain the desired liquid temperature constant in packed column (1) during the all experiments runs by circulation water [22, 23] from and to thermostat water bath (29) through the heat exchanger (2). Gas mixture from the top of column enters the condenser (13) to condense any liquid drops with it by cold water (17). The liquid (14) returns back to upper part of the column as recycle liquid or drain out from drain line (15). The gas from condenser pass through droplet collector (19), to separate any liquid drops in it. The SO2 gas concentration in dry outlet gas mixture is measured by SO2-gas analyzer (20), then the gas mixture to atmosphere air with few traces of SO2 gas (21). In the column, the liquid temperature measured by temperature gage (3), while the pH of the liquid is measured by digital pH-meter (5). The resultant reaction product (30) contains (Na2SO4 + little amount of Na2SO3 + CO2) in solution from downer part of column sent to solution tank (31). The CO2 gas (32) evolves to atmosphere air. In stirred tank (31) there is (Na2SO4 and little amount of Na2SO3) solution, it is difficult to separate them from each other, for that reason transferred by the discharge pump (33) to further processing to oxidize Na2SO3, and to producing dry powder of Na2SO4.

3.Results and discussion

The absorption of SO2 gas from SO2/Air gas mixture into aqueous sodium carbonate (Na2CO3) solution were carried out according to experimental design plan in Table 1 with the variation of gas mixture flow rate (G), SO2 gas inlet concentration (CSO2), , aqueous Na2CO3 solution concentration (CNa2CO3), experimental liquid temperature (T) and liquid holdup in the column (VL).

3.1 Definition of recovery efficiency

The SO2 Recovery efficiency (η) was defined as [8, 33, 34]:

... (3)

where, η = Recovery efficiency in (%), CSO2, in = SO2 gas inlet concentration in gas mixture in (ppm), CSO2, out = SO2 gas outlet concentration in gas mixture in (ppm).

Recovery efficiency (η) was calculated by using equation (3). The Recovery efficiency of SO2 gas from gas mixture SO2/Air was in the range of η = 70.10 - 95.60 %. The results of Recovery efficiency are summarized in Table 2.

The effects of operating parameters on Recovery efficiency of SO2 gas from SO2/Air gas mixture could be seen as in the following Figures:

From Figures 2-8, the Recovery efficiency of SO2 gas from SO2/Air gas mixture (η) is increased with the increase in inlet SO2 gas concentration (CS02,in), aqueous Na2CO3 solution (CNaC03), and liquid holdup (VL), and decreased with an increase in gas mixture flow rate (G), and liquid temperature (T). The biggest effect of operating parameters on Recovery efficiency was G, and smallest one was ( VL).

3.2 Correlation model of Recovery efficiency

Computer program (Statgraphics/Experimental Design) was used to estimate the fitted linear model of Recovery efficiency (η) for SO2 gas recovery from SO2/air gas mixture into aqueous Na2CO3 solution in packed bed column in terms of operating parameters: G, CS02, CNaC03, T, and VL. The output shows the results of fitting a multiple linear regression model to describe the relationship between Recovery efficiency and 5 independent variables.

The equation of the fitted model is:

... (4)

Since the P-value in the ANOVA Table 3 is less than 0.05, there is a statistically significant relationship between the variables at the 95.0% confidence level.

The R-Squared statistic indicates that the model as fitted explains 91.4654% of the variability in Recovery efficiency. The adjusted R-squared statistic, which is more suitable for comparing models with different numbers of independent variables, is 89.994%. The standard error of the estimate shows the standard deviation of the residuals to be 2.38442. This value can be used to construct prediction limits for new observations by selecting the Reports option from the text menu. The mean absolute error (MAE) of 1.72457 is the average value of the residuals. The Durbin-Watson (DW) statistic tests the residuals to determine if there is any significant correlation based on the order in which they occur in your data file. Since the P-value is greater than 0.05, there is no indication of serial autocorrelation in the residuals at the 95.0% confidence level.

In determining whether the model can be simplified, notice that the highest P-value on the independent variables is 0.0581, belonging to Liquid holdup. Since the P-value is greater or equal to 0.05, that term is not statistically significant at the 95.0% or higher confidence level. Consequently, you should consider removing Liquid holdup from the model.

The validity range for the model in equation (4) is:

20 < T< 50 °C; 25 < G < 35 m3/h; 0.02 < VL < 0.04 m3; 5 < CNa2ca3 < 35 wt %; 500 < Cso2 < 3500 ppm.

The accuracy of the η model is ± 2.38 %.

The linear model in equation (4) is adequate, and the operating parameters were significant and were in ordered of CSO2 > CNa2CO3 > G > T> VL. The interactions of operating parameters were negligible.

The Pareto Chart of Recovery efficiency (η) of SO2 gas from SO2/Air gas mixture could be seen in Figure 9. The main effects of operating parameters on Recovery efficiency of SO2 gas from gas mixer SO2/Air is seen in Figure 10. The observed and predicted Recovery efficiency of SO2 gas from SO2/Air gas mixture represents in Figure 11.

3.3Optimal response

The optimum operating parameters for present work were obtained using the computer program to analyze the experimental results. The goal of optimizing was to maximize the Recovery efficiency (η) of SO2 gas from SO2/Air gas mixture. The results of optimizing were summarized in Table 4.

4.Conclusions

* Recovery of SO2 gas from SO2/Air gas mixture into Na2CO3 solution was carried out in pilot scale packed bed column. The Recovery efficiency (η) of SO2 gas was measured at various operating conditions (G, CS02, CNa2C03, T and VL) according to experimental design. The measured Recovery efficiency was in the range of η = 70.10 - 95.60 %. The η could be improved and increases by increasing in the CS02, CNa2C03, and VL and with decreasing of G, and T.

* A computer program (Statgraphics/Experimental Design) was used to estimate the linear fitted model of Rcovery efficiency (η) in terms of operating conditions (G, CSO2, CNa2CO3, T, and VL. The linear fitted model of η was adequate, and operating parameters were significant, while the interactions were negligible. The accuracy of Recovery efficiency model is ± 2.38 %.

* Using the same computer program the optimum operating conditions were obtained with values of G = 25 m3/h, CSO2 = 3500 ppm, CNa2CO3 = 35 wt %, T = 20 °C, and VL = 0.04 m3. The optimum Recovery efficiency (η) was in value of 96.97 %.

* On the base of results of measured Recovery efficiency, we conclude to scaling up the size of the pilot plant used in present work by 3-4 times to commercial size plant and using the optimum operating parameters obtained in future work.

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