# Optimization of CO2 Absorption Characteristic under the Influence of SO2 in Flue Gas by Hollow Fiber Membrane Contactor

Authors: Ziyi Qu; Li Zhang; Yunfei Yan; Shunxiang Ju
Journal: Journal of Chemistry
 (2016)

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## Abstract

Hollow fiber membrane contactor is a new, highly efficient, and the most promising technology for CO2 absorption in flue gas. There is still SO2 that exists in the flue gas after desulfurization tower of power plant. This paper studied the influence of SO2 on CO2 absorption characteristic in flue gas by hollow fiber membrane contactor with absorbent of EDA, EDA + MEA (0.6 : 0.4), and EDA + MEA + PZ (0.4 : 0.4 : 0.2). The influences of SO2 concentration, cycle absorption and desorption characteristic of absorbent, absorbent concentration, and liquid-gas flow rate ratio are studied to analyze the influence of SO2 on CO2 absorption characteristic. The appropriate absorbent composition ratio and appropriate parameter range that can inhibit the influence of SO2 are proposed by studying the hybrid sorbent with activating agent, appropriate absorbent concentration, and ratio of liquid-gas flow rate. Among the three kinds of absorbents, EDA + MEA + PZ (0.4 : 0.4 : 0.2) had the best tolerance ability to SO2 and the highest efficiency. With comprehensive consideration of CO2 removal efficiency and operating cost, under the condition of 1000 ppm SO2, the appropriate concentration and liquid-gas flow rate ratio of EDA, EDA + MEA, and EDA + MEA + PZ are proposed.


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## Body

## 1. Introduction

In recent years, energy consumption has been increasing with the rapid growth of the world economy. The greenhouse effect of CO2 became increasingly serious, and hence energy conservation is urgent [1]. The hollow fiber membrane contactor is a new technology for the CO2 absorption process [2]. The membrane does not participate in the reaction, which isolates gas and liquid. In a hollow fiber membrane contactor, the absorbent flows in one side while the flue gas flows in the other side. The flue gas diffuses through the gas-liquid interface initially, and then CO2 reacts with the absorbent. Because of the CO2 concentration gradient in the gas and liquid phases, CO2 transfers from the gas phase to liquid phases through the membrane pores and continues to react with the absorbent. Because of the high reaction rate, simple operation, small volume membrane absorption technology, and low cost, the hollow fiber membrane contactor is one of the most promising decarburization technologies.The alcohol amine absorbent used in CO2 absorption has been used in SO2 removal by many researchers [3–8]. The hollow fiber membrane contactor is not only applied for CO2 removal, but also applied to remove SO2. Ogundiran et al. [9] studied SO2 capture in flue gas by porous hydrophobic hollow fibers and found that it was a more promising technology than conventional scrubbers used in desulfurization. Park et al. [10] studied the effects of operation parameters on SO2 removal by PVDF hollow fiber membranes and found that it is one of the most competitive alternatives in the future. There is still SO2 that exists in the flue gas after desulfurization tower. The influence of SO2 on CO2 capture gained more attention recently. Zhong [11] studied the effect of SO2 concentration on MEA, MEA/MDEA, MEDA/PZ, and DEA/AMP. The tolerance of SO2 was found to be as follows: MEA > DEA/AMP > MEDA/PZ > MEA/MDEA. Uyanga and Idem [12] studied the degradation of MEA caused by SO2 in a semibatch reactor. The results show that SO2 accelerates the rate of MEA degradation and established a dynamic model. Supap et al. [13] studied the kinetics of SO2− and O2− induced degradation of aqueous MEA during CO2 capture. The results show that an increase in temperature and concentration of MEA, O2, and SO2 causes a higher degradation rate of MEA. Gao et al. [14] studied the effect of SO2 on the CO2 capture process in a pilot plant. The results show that SO2 causes amine oxidative degradation, which is beneficial to remove SO2 induced heat stable salts using appropriate methods. Bonenfant et al. [15] studied the absorption of CO2 and SO2 mixtures with the absorbent of aqueous 2-(2-aminoethylamino)ethanol (AEE) solution and its blends with N-methyldiethanolamine (MDEA) and triethanolamine (TEA) to estimate the influence of SO2. The results show that SO2 decreases the CO2 absorption rate. The addition of 5 and 10 wt.% of MDEA and TEA does not influence the CO2 absorption rate in AEE. TEA decreases the absorption capacity of AEE. Yang et al. [16] studied the influence of SO2 on the CO2 capture in an absorption-desorption experimental setup using MEA as the absorbent. The results show that there were sharp decreases in CO2 removal efficiency and mass transfer rate of CO2 after the initial several days of operation; more progress is needed in high-efficiency and stable absorbents.According to the present research situation, SO2 causes the degradation of alcohol amine absorbent, resulting in the decrease of CO2 removal efficiency. The influence of SO2 on CO2 absorption by alcohol amine absorbent deserves attention. It is necessary to research the optimization of CO2 absorption characteristic under the influence of SO2. Research on appropriate absorbent and parameter range which can inhibit the influence of SO2 is needed. MEA, MDEA, DEA, and other amine solutions [11–16] are selected as absorbent of CO2 capture by many researchers. The disadvantage of MEA is the high energy consumption in the CO2 desorption. The tolerance ability to SO2 of MEA is better than that of MDEA [11]. Studies on EDA as an absorbent used in CO2 capture are relatively fewer. In the study of Shunxiang [18], the performance of the absorbents used in CO2 absorption is as follows: PZ > EDA > MEA > DEA. Considering that CO2 removal efficiency of EDA is higher than of MEA, the tolerance ability to SO2 of MEA is high and the activation ability of PZ is good, and EDA, EDA + MEA, and EDA + MEA + PZ are selected as absorbents of CO2 capture in this paper. This paper researches the performance of these absorbents under the influence of SO2 in order to study the appropriate absorbent composition ratio and specific operating parameters to optimize CO2 absorption under the influence of SO2. The results of this study are an important reference for the industrial application of CO2 absorption by hollow fiber membrane contactor.


## 2. Materials and Methods

### 2.1. Reaction Mechanism

SO2 diffuses from gas phase to gas-liquid interface firstly and then diffuses from gas-liquid interface to liquid phase and dissolves in liquid. The reaction between SO2 and amine solution can be assumed as a combination of the physical dissolution of SO2 in water and the chemical absorption of amine solution. SO2 in water generates SO32- firstly [11, 19]:(1)SO2+H2O⟷H++HSO3-(2)HSO3-⟷H++SO32-The solubility of SO2 increased by the addition of amine solution, and the amine reacted with the hydrogen ion from the water and formed a compound of strong heat stability: (3)RR′NH+SO2+H2O⟷RR′NH2++HSO3-(4)With  oxygen:2HSO3-+O2=2HSO4-The compound generated in formula (3) is stable and cannot be regenerated by heating, which results in degradation and depletion of the amine solution. In addition to this, it will cause solution foaming and decrease CO2 removal efficiency in the system in the long run.For MEA, the reaction between CO2 and MEA is as follows [20–22]:(5)CO2+2RNH2⟶RNH+CO2-+RNH3+The reaction between SO2 and MEA is as follows [19]:(6)SO2+2RNH2+H2O⟶2RNH3++SO32-where R is HOCH2CH2.


### 2.2. Materials

The solutions of EDA (ethylenediamine), EDA + MEA (monoethanolamine), and EDA + MEA + PZ (piperazine) are selected as absorbents for CO2 absorption in this experiment. The concentration of absorbent is 500 mol/m3, and the mole ratio of the component in hybrid absorbent is EDA : MEA = 0.6 : 0.4 and EDA : MEA : PZ = 0.4 : 0.4 : 0.2.The pp (polypropylene) hollow fiber membrane contactor of KH-MF-4040N-PP is produced by Hangzhou Kaihong Membrane Technology Co., Ltd., and the specification and parameters are shown in Table1. The membrane is designed by internal pressure and stretch forming. The inlet and outlet of the gas and liquid are arranged on the side and the end, respectively. The maximum pressure designed is 0.3 MPa, applicable to the pH of 1~14, at 15~40°C.Table 1
Specifications and parameters of PP hollow fiber membrane contactor.
Physical Unit Value Parameter Liquid flux L/h 800–1000 U Outer diameter of module mm 94 D o u t Inner diameter of module mm 90 D i n Module length mm 1125 L m Fiber length mm 920 L Membrane pore diameter μm 0.1–0.2 d p Inner diameter of fiber mm 0.3 d i n Outer diameter of fiber mm 0.4 d o u t Module area m2 8~12 S Number of fibers  7000 n Tortuosity factor  2 τ Fiber porosity % 45 ε Packing density % 13.8 (1-Φ)


### 2.3. Experimental Procedures

The system is shown in Figure1. The flue gas is simulated by mixed gas of CO2, SO2, and N2. The flue gas is introduced into fiber membrane contactor from compressed gas cylinders. The absorbent is introduced into the contactor by a pump. The gas flows in the tube side and the absorbent flows in the shell side. There are regulating valves at the outlet of the simulative flue gas and absorbent pump which can control the flow rate of gas and absorbent. The absorbent reacts with CO2 and becomes a rich liquid and is then introduced into the absorbent tank by a pump. The desorption tank desorbs CO2 by heating the rich liquid. This is one cycle of absorption and desorption. The desorption tank is designed by electric heating. There are sample portions to analyze the gas component by a gas analyzer (ECOM-J2KN, German RBR Company) and gas chromatograph (GC7900, Shanghai Tianmei Scientific Instruments Co., Ltd.). Values are obtained when the reaction is stable for 5 min, and then an average value of three times is obtained, each time interval of 30 s.Figure 1
Flow chart of the experiment using the flue gas.The experimental conditions and parameters are shown in Table2. Gas flow rate is 4 m3/h, and absorbent flow rate is 0.7 m3/h. Volume fraction of CO2 in gas is 14 vol.%.Table 2
Parameters of operating conditions.
Parameter Unit Physical Value T g K Gas temperature 288 T l K Absorbent temperature 288 U g m3/h Gas flow rate 4 U l m3/h Absorbent flow rate 0.7 C mol/m3 Absorbent concentration 500 φ C O 2 vol.% Volume fraction of CO2 in gas 14 P MP Operating pressure 0.1


## 2.1. Reaction Mechanism

SO2 diffuses from gas phase to gas-liquid interface firstly and then diffuses from gas-liquid interface to liquid phase and dissolves in liquid. The reaction between SO2 and amine solution can be assumed as a combination of the physical dissolution of SO2 in water and the chemical absorption of amine solution. SO2 in water generates SO32- firstly [11, 19]:(1)SO2+H2O⟷H++HSO3-(2)HSO3-⟷H++SO32-The solubility of SO2 increased by the addition of amine solution, and the amine reacted with the hydrogen ion from the water and formed a compound of strong heat stability: (3)RR′NH+SO2+H2O⟷RR′NH2++HSO3-(4)With  oxygen:2HSO3-+O2=2HSO4-The compound generated in formula (3) is stable and cannot be regenerated by heating, which results in degradation and depletion of the amine solution. In addition to this, it will cause solution foaming and decrease CO2 removal efficiency in the system in the long run.For MEA, the reaction between CO2 and MEA is as follows [20–22]:(5)CO2+2RNH2⟶RNH+CO2-+RNH3+The reaction between SO2 and MEA is as follows [19]:(6)SO2+2RNH2+H2O⟶2RNH3++SO32-where R is HOCH2CH2.


## 2.2. Materials

The solutions of EDA (ethylenediamine), EDA + MEA (monoethanolamine), and EDA + MEA + PZ (piperazine) are selected as absorbents for CO2 absorption in this experiment. The concentration of absorbent is 500 mol/m3, and the mole ratio of the component in hybrid absorbent is EDA : MEA = 0.6 : 0.4 and EDA : MEA : PZ = 0.4 : 0.4 : 0.2.The pp (polypropylene) hollow fiber membrane contactor of KH-MF-4040N-PP is produced by Hangzhou Kaihong Membrane Technology Co., Ltd., and the specification and parameters are shown in Table1. The membrane is designed by internal pressure and stretch forming. The inlet and outlet of the gas and liquid are arranged on the side and the end, respectively. The maximum pressure designed is 0.3 MPa, applicable to the pH of 1~14, at 15~40°C.Table 1
Specifications and parameters of PP hollow fiber membrane contactor.
Physical Unit Value Parameter Liquid flux L/h 800–1000 U Outer diameter of module mm 94 D o u t Inner diameter of module mm 90 D i n Module length mm 1125 L m Fiber length mm 920 L Membrane pore diameter μm 0.1–0.2 d p Inner diameter of fiber mm 0.3 d i n Outer diameter of fiber mm 0.4 d o u t Module area m2 8~12 S Number of fibers  7000 n Tortuosity factor  2 τ Fiber porosity % 45 ε Packing density % 13.8 (1-Φ)


## 2.3. Experimental Procedures

The system is shown in Figure1. The flue gas is simulated by mixed gas of CO2, SO2, and N2. The flue gas is introduced into fiber membrane contactor from compressed gas cylinders. The absorbent is introduced into the contactor by a pump. The gas flows in the tube side and the absorbent flows in the shell side. There are regulating valves at the outlet of the simulative flue gas and absorbent pump which can control the flow rate of gas and absorbent. The absorbent reacts with CO2 and becomes a rich liquid and is then introduced into the absorbent tank by a pump. The desorption tank desorbs CO2 by heating the rich liquid. This is one cycle of absorption and desorption. The desorption tank is designed by electric heating. There are sample portions to analyze the gas component by a gas analyzer (ECOM-J2KN, German RBR Company) and gas chromatograph (GC7900, Shanghai Tianmei Scientific Instruments Co., Ltd.). Values are obtained when the reaction is stable for 5 min, and then an average value of three times is obtained, each time interval of 30 s.Figure 1
Flow chart of the experiment using the flue gas.The experimental conditions and parameters are shown in Table2. Gas flow rate is 4 m3/h, and absorbent flow rate is 0.7 m3/h. Volume fraction of CO2 in gas is 14 vol.%.Table 2
Parameters of operating conditions.
Parameter Unit Physical Value T g K Gas temperature 288 T l K Absorbent temperature 288 U g m3/h Gas flow rate 4 U l m3/h Absorbent flow rate 0.7 C mol/m3 Absorbent concentration 500 φ C O 2 vol.% Volume fraction of CO2 in gas 14 P MP Operating pressure 0.1


## 3. Results and Discussion

The CO2 removal efficiency is the parameter that reflects the CO2 removal performance; it can be calculated by [23](7)η=Qin×φin-Qout×φoutQin×φin×100,where η is the CO2 removal efficiency, Qin is the gas flow rate of inlet, Qout is the gas flow rate of outlet, and φin and φout are the volume fraction of CO2 at inlet and outlet, respectively.The mass transfer rate reflects the performance of mass transfer. It can be calculated by [23](8)JCO2=Qin×φin-Qout×φout×273.15×100022.4×Tg×S,where JCO2 is the mass transfer rate, Tg is gas temperature, andS is the total area of the membrane.


### 3.1. Influence of SO2 Concentration on CO2 Absorption

In order to emphasize the influence of SO2 concentration, the concentration of SO2 is amplified from 500 ppm to 2000 ppm in this research. The influences of SO2 concentration on CO2 removal efficiency and mass transfer rate are shown in Figures 2 and 3. The CO2 removal efficiency and mass transfer rate of the three absorbents decrease with the increasing of SO2 concentration. The solubility of SO2 is much higher than of CO2. The pH value of SO2 aqueous solution is smaller than the pH value of CO2 equilibrium solution. The reaction between SO2 and amine absorbent can be considered as instantaneous [11]. The reaction rates and opportunities for SO2 and absorbent are much higher than those of CO2 and absorbent, which leads to the decrease of CO2 removal efficiency.Figure 2
Influence of SO2 concentration on the CO2 removal efficiency (Ug = 4 m3/h,Ul = 0.7 m3/h, andT = 288 K).Figure 3
Influence of SO2concentration on the CO2mass transfer rate (Ug = 4 m3/h,Ul = 0.7 m3/h, andT = 288 K).The CO2 removal efficiency of three absorbents decreased suddenly after 1000 ppm SO2. The effective components keep constant when the absorbent concentration is fixed. When SO2 concentration increased to a certain value, the effective components of absorbent are consumed largely by SO2, which leads to the sudden decrease of CO2 removal efficiency. The CO2 removal efficiency of EDA decreases by 4.05% with 500 ppm SO2 and decreases by 37.3% with 2000 ppm SO2. It can be considered that the influence of SO2 on CO2 removal efficiency with EDA is not significant when the SO2 concentration is under 500 ppm. Because of the low concentration of SO2, even with the faster reaction rate of SO2, there is still a chance for CO2 to react with the absorbent. The concentration gradient of the gas and the liquid vapor interface increases with increasing SO2 concentration, which improves the mass transfer dynamics. This is favorable for SO2 molecules to diffuse to the surface and the interior of the absorption solution and speed up the reaction of SO2 with EDA. Therefore, the absorption of CO2 reduced greatly with the increasing of SO2 concentration. The influence of SO2 on CO2 removal efficiency is more significant with the increase of SO2 concentration. The SO2 concentration in the outlet of flue gas is always zero, which indicates that SO2 is absorbed by the absorbent completely, and the reaction rate of SO2 and the absorbent is significantly higher than that of CO2 and the absorbent.The CO2 removal efficiency of EDA + MEA decreases by 2.1% with 500 ppm SO2, the efficiency is 53.1% with 1000 ppm SO2, and the efficiency decreases to 29.3% with 2000 ppm SO2. Comparing the results of EDA + MEA (0.6 : 0.4) and EDA, the CO2 removal efficiency and mass transfer rate of EDA are higher than those of EDA + MEA without the addition of SO2. With the increasing of SO2 concentration, the CO2 removal efficiency and mass transfer rate of EDA + MEA (0.6 : 0.4) are higher than those of EDA. The CO2 absorption capacity of EDA is better than that of MEA [18]; therefore, the CO2 removal efficiency and mass transfer rate of EDA are higher than those of EDA + MEA (0.6 : 0.4) without the influence of SO2. The active ingredient increases after the addition of MEA, which promotes the tolerance ability of the absorbent to SO2. The CO2 removal efficiency and mass transfer rate reduction of EDA + MEA (0.6 : 0.4) are smaller than those of EDA; therefore, the tolerance ability of EDA + MEA (0.6 : 0.4) to SO2 is better than that of EDA.Zhong [11] researched the influence of SO2 concentration on CO2 absorption with the absorbent of 10% MEA and 10% MEA + 2% MDEA (liquid flow rate 18 L/h, temperature 40°C; flue gas flow rate 1800 L/h, temperature 15°C; SO2 concentration is 500–1500 ppm). The results are shown in Figure 2. The results indicate that the CO2 removal efficiency decreases with the increase of SO2 concentration, and the influence of SO2 is not significant till the SO2 concentration becomes greater than 500 ppm, which agrees well with the results in this paper. The CO2 removal efficiency of 10% MEA + 2% MDEA is higher than that of 10% MEA when SO2 concentration is under 117 ppm, and the decrease extent of CO2 removal efficiency in 10% MEA + 2% MDEA is more than that in 10% MEA with the increase of SO2 concentration. The CO2 removal efficiency of 10% MEA is higher than that of 10% MEA + 2% MDEA with 200 ppm SO2. This indicates that the reaction between 10% MEA + 2% MDEA and SO2 is more rapid and intense, so the decrease extent of CO2 removal efficiency with the absorbent of 10% MEA + 2% MDEA is more significant than of 10% MEA. Because of the poor absorptive capacity and slow absorption rate of MDEA, MDEA is not suitable for CO2 absorption with the influence of SO2. The tolerance ability of 10% MEA + 2% MDEA to SO2 is lower than that of 10% MEA.Comparing the results of EDA and 10% MEA, the CO2 removal efficiency of 10% MEA is higher than of EDA without SO2, and the decrease extent is smaller than EDA with the increase of SO2 concentration. So, the tolerance ability to SO2 of 10% MEA is greater than 500 mol/m3 EDA. After adding of MEA, the CO2 removal efficiency of EDA + MEA (0.6 : 0.4) is higher than of EDA with the increase of SO2 concentration. So, the hybrid absorbent of EDA + MEA (0.6 : 0.4) is appropriate for removal of CO2 in the flue gas containing SO2.The absorbent of EDA + MEA + PZ is the most efficient in the three absorbents. The CO2 removal efficiency of EDA + MEA + PZ decreases by 1.2% with 500 ppm SO2, decreases by 3.4% with 800 ppm SO2, and decreases by 27.1% with 2000 ppm SO2. The CO2 removal efficiency reduction of EDA + MEA + PZ is not significant until 1000 ppm SO2. Because of the activity of PZ, the influence of SO2 on CO2 absorption with EDA + MEA + PZ is not significant under the condition of low SO2 concentration in a short time. And the tolerance ability of EDA + MEA + PZ to SO2 is greater than that of EDA and EDA + MEA. The influence of SO2 is getting more significant when the PZ active effect is gradually consumed. Therefore, the CO2 removal efficiency decreases significantly with 2000 ppm SO2.The results of the three absorbents indicate that the influence of SO2 on CO2 is not significant with low SO2 concentration in the short run performance; the hybrid absorbent with high absorptive capacity component and high tolerance ability to SO2 can inhibit the influence of SO2 on CO2 absorption effectively.


### 3.2. Cycle Absorption and Desorption Characteristic of Absorbent

According to the previous research, the influence of SO2 on CO2 absorption is not significant in the low concentration of SO2. The absorption experiment is conducted in a short time, and the cycle absorption and desorption of the absorbent are not considered. It is necessary to study the cycle absorption and desorption of the absorbent. Based on the influence of SO2 concentration on CO2 removal efficiency, the influence is not significant when the SO2 concentration is below 500 ppm of the three absorbents. In order to study the influence of SO2 on CO2 absorption in low SO2 concentration, the SO2 concentration of 500 ppm is selected in this experiment.The results are shown in Figures4 and 5. The absorbent from absorption to desorption is one cycle. The CO2 removal efficiency and mass transfer rate decrease with the increase of cycle number. The CO2 removal efficiency of EDA decreases by 26.85% with SO2 and decreases by 21.8% without SO2; the CO2 removal efficiency of EDA + MEA decreases by 24.7% with SO2 and decreases by 22.1% without SO2; the CO2 removal efficiency of EDA + MEA + PZ decreases by 14.6% with the influence of SO2.Figure 4
Influence of cycle absorption and desorption on the CO2 removal efficiency (Ug = 4 m3/h,Ul = 0.7 m3/h, andT = 288 K).Figure 5
Influence of cycle absorption and desorption on the CO2 mass transfer rate (Ug = 4 m3/h,Ul = 0.7 m3/h, andT = 288 K).Desorption of the absorbent by heating the rich liquid results in absorbent degradation. SO2 reacts with the absorbent and generates stable salts, which cannot be regenerated by heating. The existence of SO2 accelerates the degradation of most amine solutions. Strazisar et al. [24] studied the effect of SO2 on the degradation of MEA. The results show that SO2 accelerates the degradation rate of MEA, which is significant in higher concentration of SO2. Therefore, the CO2 removal efficiency and mass transfer rate decrease significantly with the increasing cycle of absorption and desorption.Gao et al. [17] studied the influence of SO2 on the absorption character of absorbent in the following campaign: no SO2, 214 ppm SO2, and 317 ppm SO2, respectively. The result is shown in Figure 6. The CO2 removal efficiency decreases gradually with increasing circulating time. And the decrease extent of CO2 removal efficiency increases with SO2 concentration. The absorbent degradation and heat stable salts formation are the main reasons for the significant influence of SO2. The trend of Figure 4 in this experiment agreed with the trend of Figure 6.Figure 6
The influence of circulating time on the CO2 removal efficiency [17] (Ug = 86 Nm3/h,Ul = 0.6 m3/h, andT = 319 K).The study in this section indicated that there is a significant influence of SO2 on CO2 absorption even in low SO2 concentration in the long run performance. The order of tolerance ability to SO2 is EDA + MEA + PZ (0.4 : 0.4 : 0.2) > EDA + MEA (0.6 : 0.4) > EDA. The CO2 removal efficiency of EDA + MEA + PZ (0.4 : 0.4 : 0.2) decreased to 60% after ten absorption and desorption cycles. Therefore, it is necessary to study the appropriate parameter range in the operation to inhibit the influence of SO2.


### 3.3. The Influence of Absorbent Concentration on CO2 Absorption

The concentration of absorbent is one of the most important parameters of CO2 absorption in operation. The influence of absorbent concentration on CO2 removal efficiency is studied in this section for confirming the appropriate concentration of the absorbent which can inhibit the influence of SO2.The CO2 removal efficiency of the three absorbents decreased suddenly after 1000 ppm SO2 in the study of influence of SO2 concentration. Therefore, the experiment is conducted under the condition of 1000 ppm SO2. The concentration of the absorbent is from 400 mol/m3 to 800 mol/m3. The CO2 removal efficiency and mass transfer rate increase with the increasing absorbent concentration which is shown in Figures 7 and 8. Comparing the result of 1000 ppm SO2 with that of no SO2, the CO2 removal efficiency of 400 mol/m3 EDA, 600 mol/m3 EDA, and 650 mol/m3 EDA decreases by 13.27%, 6.5%, and 5.3%, respectively. The decreasing extent of CO2 removal efficiency and mass transfer rate of absorbent reduce with the increasing absorbent concentration.Figure 7
Influence of SO2 on the CO2 removal efficiency in different concentrations of absorbent (Ug = 4 m3/h,Ul = 0.7 m3/h, andT = 288 K).Figure 8
Influence of SO2 on the CO2mass transfer rate in different concentrations of absorbent (Ug = 4 m3/h,Ul = 0.7 m3/h, andT = 288 K).A certain amount of absorbent is needed when the concentration of SO2 keeps constant in flue gas. There are more active ingredients in the absorbent to improve the CO2 absorption with the increase of absorbent concentration. Therefore, increasing absorbent concentration can inhibit the influence of SO2 on CO2 absorption. The cost and the energy consumption increase with the increasing absorbent concentration. Hence, there is an appropriate concentration of absorbent which can inhibit the influence of SO2 on CO2 absorption with low cost and low energy consumption. The CO2 removal efficiency of 800 mol/m3 EDA, 750 mol/m3 EDA + MEA, and 650 mol/m3 EDA + MEA + PZ is 75%, 74%, and 83%, respectively, with 1000 ppm SO2. Continuing to increase the absorbent concentration, the increment of CO2 removal efficiency will reduce because the absorbent viscosity and the mass transfer resistance increase with the increasing absorbent concentration. Furthermore, continuing to increase the absorbent concentration will increase investment and operating costs. Considering the above factors, under the condition of 1000 ppm SO2, the appropriate concentrations of EDA, EDA + MEA, and EDA + MEA + PZ are 800 mol/m3, 750 mol/m3, and 650 mol/m3, respectively.


### 3.4. Influence of the Liquid-Gas Flow Rate Ratio on CO2 Absorption

The flow rates of absorbent and gas are important parameters which can affect the CO2 absorption significantly. It is necessary to study the appropriate ratio of liquid-gas flow rate for inhibiting the influence of SO2.With the increasing ratio of liquid-gas flow rate under the condition of 1000 ppm SO2, the CO2 absorption characteristic of EDA, EDA + MEA, and EDA + MEA + PZ is shown in Figures 9 and 10. The CO2 removal efficiency and mass transfer rate rise with the increasing ratio of liquid-gas flow rate. With the addition of SO2, the CO2 removal efficiency of EDA decreases by 7.9%, 6.28%, and 5.26%, respectively, when the ratio of liquid-gas flow rate is 0.1, 0.15, and 0.25 individually. The CO2 removal efficiency difference between the case with SO2 and that without SO2 decreases with the increasing ratio of liquid-gas flow rate. The experimental results of EDA + MEA and EDA + MEA + PZ are similar to that of EDA. Therefore, the influence of SO2 on CO2 absorption decreases with the increasing ratio of liquid-gas flow rate. Lv et al. [25] studied the simultaneous removal of CO2 and SO2 in a polypropylene hollow fiber membrane contactor using MEA. The CO2 removal efficiency of MEA with 1600 ppm increased with the liquid flow rate. The experimental result in this section of this paper agrees with the result of Lv et al.Figure 9
Influence of SO2 on the CO2 mass transfer rate in different liquid-gas flow rate ratios (T = 288 K).Figure 10
Influence of SO2 on the CO2 mass transfer rate in different liquid-gas flow rate ratios (T = 288K).With a certain concentration of SO2, the reaction of absorbent with CO2 increases gradually with the increasing ratio of liquid-gas flow rate. The mass transfer rate increases with the ratio of liquid-gas flow rate under a certain gas condition. Because of the increment of concentration gradients, the mass transfer and the reaction of absorbent and CO2 are improved. The increase of absorbent flow rate accelerates the membrane wetting and wear, which result in a mass transfer resistance increase. Therefore, the increments of CO2 removal efficiency and mass transfer rate reduce with the increasing ratio of liquid-gas flow rate. Meanwhile, the consumption of absorbent and pump increases with the increasing liquid-gas flow rate ratio, which raises the operation cost. In the liquid-gas flow rate ratio of 0.2–0.25, the lowest CO2 removal efficiencies of EDA, EDA + MEA, and EDA + MEA + PZ are 60%, 63%, and 77%, respectively; the highest CO2 removal efficiencies of EDA, EDA + MEA, and EDA + MEA + PZ are 67%, 70%, and 92%, respectively. In order to inhibit the influence of SO2 on CO2 absorption and maintain high CO2 removal efficiency and low operating cost, under the condition of 1000 ppm SO2, the appropriate liquid-gas flow rate ratio of EDA, EDA + MEA, and EDA + MEA + PZ is from 0.2 to 0.25.


## 3.1. Influence of SO2 Concentration on CO2 Absorption

In order to emphasize the influence of SO2 concentration, the concentration of SO2 is amplified from 500 ppm to 2000 ppm in this research. The influences of SO2 concentration on CO2 removal efficiency and mass transfer rate are shown in Figures 2 and 3. The CO2 removal efficiency and mass transfer rate of the three absorbents decrease with the increasing of SO2 concentration. The solubility of SO2 is much higher than of CO2. The pH value of SO2 aqueous solution is smaller than the pH value of CO2 equilibrium solution. The reaction between SO2 and amine absorbent can be considered as instantaneous [11]. The reaction rates and opportunities for SO2 and absorbent are much higher than those of CO2 and absorbent, which leads to the decrease of CO2 removal efficiency.Figure 2
Influence of SO2 concentration on the CO2 removal efficiency (Ug = 4 m3/h,Ul = 0.7 m3/h, andT = 288 K).Figure 3
Influence of SO2concentration on the CO2mass transfer rate (Ug = 4 m3/h,Ul = 0.7 m3/h, andT = 288 K).The CO2 removal efficiency of three absorbents decreased suddenly after 1000 ppm SO2. The effective components keep constant when the absorbent concentration is fixed. When SO2 concentration increased to a certain value, the effective components of absorbent are consumed largely by SO2, which leads to the sudden decrease of CO2 removal efficiency. The CO2 removal efficiency of EDA decreases by 4.05% with 500 ppm SO2 and decreases by 37.3% with 2000 ppm SO2. It can be considered that the influence of SO2 on CO2 removal efficiency with EDA is not significant when the SO2 concentration is under 500 ppm. Because of the low concentration of SO2, even with the faster reaction rate of SO2, there is still a chance for CO2 to react with the absorbent. The concentration gradient of the gas and the liquid vapor interface increases with increasing SO2 concentration, which improves the mass transfer dynamics. This is favorable for SO2 molecules to diffuse to the surface and the interior of the absorption solution and speed up the reaction of SO2 with EDA. Therefore, the absorption of CO2 reduced greatly with the increasing of SO2 concentration. The influence of SO2 on CO2 removal efficiency is more significant with the increase of SO2 concentration. The SO2 concentration in the outlet of flue gas is always zero, which indicates that SO2 is absorbed by the absorbent completely, and the reaction rate of SO2 and the absorbent is significantly higher than that of CO2 and the absorbent.The CO2 removal efficiency of EDA + MEA decreases by 2.1% with 500 ppm SO2, the efficiency is 53.1% with 1000 ppm SO2, and the efficiency decreases to 29.3% with 2000 ppm SO2. Comparing the results of EDA + MEA (0.6 : 0.4) and EDA, the CO2 removal efficiency and mass transfer rate of EDA are higher than those of EDA + MEA without the addition of SO2. With the increasing of SO2 concentration, the CO2 removal efficiency and mass transfer rate of EDA + MEA (0.6 : 0.4) are higher than those of EDA. The CO2 absorption capacity of EDA is better than that of MEA [18]; therefore, the CO2 removal efficiency and mass transfer rate of EDA are higher than those of EDA + MEA (0.6 : 0.4) without the influence of SO2. The active ingredient increases after the addition of MEA, which promotes the tolerance ability of the absorbent to SO2. The CO2 removal efficiency and mass transfer rate reduction of EDA + MEA (0.6 : 0.4) are smaller than those of EDA; therefore, the tolerance ability of EDA + MEA (0.6 : 0.4) to SO2 is better than that of EDA.Zhong [11] researched the influence of SO2 concentration on CO2 absorption with the absorbent of 10% MEA and 10% MEA + 2% MDEA (liquid flow rate 18 L/h, temperature 40°C; flue gas flow rate 1800 L/h, temperature 15°C; SO2 concentration is 500–1500 ppm). The results are shown in Figure 2. The results indicate that the CO2 removal efficiency decreases with the increase of SO2 concentration, and the influence of SO2 is not significant till the SO2 concentration becomes greater than 500 ppm, which agrees well with the results in this paper. The CO2 removal efficiency of 10% MEA + 2% MDEA is higher than that of 10% MEA when SO2 concentration is under 117 ppm, and the decrease extent of CO2 removal efficiency in 10% MEA + 2% MDEA is more than that in 10% MEA with the increase of SO2 concentration. The CO2 removal efficiency of 10% MEA is higher than that of 10% MEA + 2% MDEA with 200 ppm SO2. This indicates that the reaction between 10% MEA + 2% MDEA and SO2 is more rapid and intense, so the decrease extent of CO2 removal efficiency with the absorbent of 10% MEA + 2% MDEA is more significant than of 10% MEA. Because of the poor absorptive capacity and slow absorption rate of MDEA, MDEA is not suitable for CO2 absorption with the influence of SO2. The tolerance ability of 10% MEA + 2% MDEA to SO2 is lower than that of 10% MEA.Comparing the results of EDA and 10% MEA, the CO2 removal efficiency of 10% MEA is higher than of EDA without SO2, and the decrease extent is smaller than EDA with the increase of SO2 concentration. So, the tolerance ability to SO2 of 10% MEA is greater than 500 mol/m3 EDA. After adding of MEA, the CO2 removal efficiency of EDA + MEA (0.6 : 0.4) is higher than of EDA with the increase of SO2 concentration. So, the hybrid absorbent of EDA + MEA (0.6 : 0.4) is appropriate for removal of CO2 in the flue gas containing SO2.The absorbent of EDA + MEA + PZ is the most efficient in the three absorbents. The CO2 removal efficiency of EDA + MEA + PZ decreases by 1.2% with 500 ppm SO2, decreases by 3.4% with 800 ppm SO2, and decreases by 27.1% with 2000 ppm SO2. The CO2 removal efficiency reduction of EDA + MEA + PZ is not significant until 1000 ppm SO2. Because of the activity of PZ, the influence of SO2 on CO2 absorption with EDA + MEA + PZ is not significant under the condition of low SO2 concentration in a short time. And the tolerance ability of EDA + MEA + PZ to SO2 is greater than that of EDA and EDA + MEA. The influence of SO2 is getting more significant when the PZ active effect is gradually consumed. Therefore, the CO2 removal efficiency decreases significantly with 2000 ppm SO2.The results of the three absorbents indicate that the influence of SO2 on CO2 is not significant with low SO2 concentration in the short run performance; the hybrid absorbent with high absorptive capacity component and high tolerance ability to SO2 can inhibit the influence of SO2 on CO2 absorption effectively.


## 3.2. Cycle Absorption and Desorption Characteristic of Absorbent

According to the previous research, the influence of SO2 on CO2 absorption is not significant in the low concentration of SO2. The absorption experiment is conducted in a short time, and the cycle absorption and desorption of the absorbent are not considered. It is necessary to study the cycle absorption and desorption of the absorbent. Based on the influence of SO2 concentration on CO2 removal efficiency, the influence is not significant when the SO2 concentration is below 500 ppm of the three absorbents. In order to study the influence of SO2 on CO2 absorption in low SO2 concentration, the SO2 concentration of 500 ppm is selected in this experiment.The results are shown in Figures4 and 5. The absorbent from absorption to desorption is one cycle. The CO2 removal efficiency and mass transfer rate decrease with the increase of cycle number. The CO2 removal efficiency of EDA decreases by 26.85% with SO2 and decreases by 21.8% without SO2; the CO2 removal efficiency of EDA + MEA decreases by 24.7% with SO2 and decreases by 22.1% without SO2; the CO2 removal efficiency of EDA + MEA + PZ decreases by 14.6% with the influence of SO2.Figure 4
Influence of cycle absorption and desorption on the CO2 removal efficiency (Ug = 4 m3/h,Ul = 0.7 m3/h, andT = 288 K).Figure 5
Influence of cycle absorption and desorption on the CO2 mass transfer rate (Ug = 4 m3/h,Ul = 0.7 m3/h, andT = 288 K).Desorption of the absorbent by heating the rich liquid results in absorbent degradation. SO2 reacts with the absorbent and generates stable salts, which cannot be regenerated by heating. The existence of SO2 accelerates the degradation of most amine solutions. Strazisar et al. [24] studied the effect of SO2 on the degradation of MEA. The results show that SO2 accelerates the degradation rate of MEA, which is significant in higher concentration of SO2. Therefore, the CO2 removal efficiency and mass transfer rate decrease significantly with the increasing cycle of absorption and desorption.Gao et al. [17] studied the influence of SO2 on the absorption character of absorbent in the following campaign: no SO2, 214 ppm SO2, and 317 ppm SO2, respectively. The result is shown in Figure 6. The CO2 removal efficiency decreases gradually with increasing circulating time. And the decrease extent of CO2 removal efficiency increases with SO2 concentration. The absorbent degradation and heat stable salts formation are the main reasons for the significant influence of SO2. The trend of Figure 4 in this experiment agreed with the trend of Figure 6.Figure 6
The influence of circulating time on the CO2 removal efficiency [17] (Ug = 86 Nm3/h,Ul = 0.6 m3/h, andT = 319 K).The study in this section indicated that there is a significant influence of SO2 on CO2 absorption even in low SO2 concentration in the long run performance. The order of tolerance ability to SO2 is EDA + MEA + PZ (0.4 : 0.4 : 0.2) > EDA + MEA (0.6 : 0.4) > EDA. The CO2 removal efficiency of EDA + MEA + PZ (0.4 : 0.4 : 0.2) decreased to 60% after ten absorption and desorption cycles. Therefore, it is necessary to study the appropriate parameter range in the operation to inhibit the influence of SO2.


## 3.3. The Influence of Absorbent Concentration on CO2 Absorption

The concentration of absorbent is one of the most important parameters of CO2 absorption in operation. The influence of absorbent concentration on CO2 removal efficiency is studied in this section for confirming the appropriate concentration of the absorbent which can inhibit the influence of SO2.The CO2 removal efficiency of the three absorbents decreased suddenly after 1000 ppm SO2 in the study of influence of SO2 concentration. Therefore, the experiment is conducted under the condition of 1000 ppm SO2. The concentration of the absorbent is from 400 mol/m3 to 800 mol/m3. The CO2 removal efficiency and mass transfer rate increase with the increasing absorbent concentration which is shown in Figures 7 and 8. Comparing the result of 1000 ppm SO2 with that of no SO2, the CO2 removal efficiency of 400 mol/m3 EDA, 600 mol/m3 EDA, and 650 mol/m3 EDA decreases by 13.27%, 6.5%, and 5.3%, respectively. The decreasing extent of CO2 removal efficiency and mass transfer rate of absorbent reduce with the increasing absorbent concentration.Figure 7
Influence of SO2 on the CO2 removal efficiency in different concentrations of absorbent (Ug = 4 m3/h,Ul = 0.7 m3/h, andT = 288 K).Figure 8
Influence of SO2 on the CO2mass transfer rate in different concentrations of absorbent (Ug = 4 m3/h,Ul = 0.7 m3/h, andT = 288 K).A certain amount of absorbent is needed when the concentration of SO2 keeps constant in flue gas. There are more active ingredients in the absorbent to improve the CO2 absorption with the increase of absorbent concentration. Therefore, increasing absorbent concentration can inhibit the influence of SO2 on CO2 absorption. The cost and the energy consumption increase with the increasing absorbent concentration. Hence, there is an appropriate concentration of absorbent which can inhibit the influence of SO2 on CO2 absorption with low cost and low energy consumption. The CO2 removal efficiency of 800 mol/m3 EDA, 750 mol/m3 EDA + MEA, and 650 mol/m3 EDA + MEA + PZ is 75%, 74%, and 83%, respectively, with 1000 ppm SO2. Continuing to increase the absorbent concentration, the increment of CO2 removal efficiency will reduce because the absorbent viscosity and the mass transfer resistance increase with the increasing absorbent concentration. Furthermore, continuing to increase the absorbent concentration will increase investment and operating costs. Considering the above factors, under the condition of 1000 ppm SO2, the appropriate concentrations of EDA, EDA + MEA, and EDA + MEA + PZ are 800 mol/m3, 750 mol/m3, and 650 mol/m3, respectively.


## 3.4. Influence of the Liquid-Gas Flow Rate Ratio on CO2 Absorption

The flow rates of absorbent and gas are important parameters which can affect the CO2 absorption significantly. It is necessary to study the appropriate ratio of liquid-gas flow rate for inhibiting the influence of SO2.With the increasing ratio of liquid-gas flow rate under the condition of 1000 ppm SO2, the CO2 absorption characteristic of EDA, EDA + MEA, and EDA + MEA + PZ is shown in Figures 9 and 10. The CO2 removal efficiency and mass transfer rate rise with the increasing ratio of liquid-gas flow rate. With the addition of SO2, the CO2 removal efficiency of EDA decreases by 7.9%, 6.28%, and 5.26%, respectively, when the ratio of liquid-gas flow rate is 0.1, 0.15, and 0.25 individually. The CO2 removal efficiency difference between the case with SO2 and that without SO2 decreases with the increasing ratio of liquid-gas flow rate. The experimental results of EDA + MEA and EDA + MEA + PZ are similar to that of EDA. Therefore, the influence of SO2 on CO2 absorption decreases with the increasing ratio of liquid-gas flow rate. Lv et al. [25] studied the simultaneous removal of CO2 and SO2 in a polypropylene hollow fiber membrane contactor using MEA. The CO2 removal efficiency of MEA with 1600 ppm increased with the liquid flow rate. The experimental result in this section of this paper agrees with the result of Lv et al.Figure 9
Influence of SO2 on the CO2 mass transfer rate in different liquid-gas flow rate ratios (T = 288 K).Figure 10
Influence of SO2 on the CO2 mass transfer rate in different liquid-gas flow rate ratios (T = 288K).With a certain concentration of SO2, the reaction of absorbent with CO2 increases gradually with the increasing ratio of liquid-gas flow rate. The mass transfer rate increases with the ratio of liquid-gas flow rate under a certain gas condition. Because of the increment of concentration gradients, the mass transfer and the reaction of absorbent and CO2 are improved. The increase of absorbent flow rate accelerates the membrane wetting and wear, which result in a mass transfer resistance increase. Therefore, the increments of CO2 removal efficiency and mass transfer rate reduce with the increasing ratio of liquid-gas flow rate. Meanwhile, the consumption of absorbent and pump increases with the increasing liquid-gas flow rate ratio, which raises the operation cost. In the liquid-gas flow rate ratio of 0.2–0.25, the lowest CO2 removal efficiencies of EDA, EDA + MEA, and EDA + MEA + PZ are 60%, 63%, and 77%, respectively; the highest CO2 removal efficiencies of EDA, EDA + MEA, and EDA + MEA + PZ are 67%, 70%, and 92%, respectively. In order to inhibit the influence of SO2 on CO2 absorption and maintain high CO2 removal efficiency and low operating cost, under the condition of 1000 ppm SO2, the appropriate liquid-gas flow rate ratio of EDA, EDA + MEA, and EDA + MEA + PZ is from 0.2 to 0.25.


## 4. Conclusions

There is a significant influence of SO2 on CO2 absorption in the long run performance, which affects the industrial application prospects of this technology. It is necessary to study the SO2 influence characteristic on CO2 absorption and the measure to optimize the CO2 absorption under the influence of SO2.This paper studied the optimization of CO2 absorption characteristic under the influence of SO2 with the absorbent of EDA, EDA + MEA, and EDA + MEA + PZ by hollow fiber membrane contactor. The SO2 concentration, cycle absorption and desorption characteristic of absorbent, absorbent concentration, and ratio of liquid-gas flow rate are analyzed to evaluate the influence of SO2 on CO2 absorption characteristic. The reaction rate and absorption performance of SO2 with amine solution are much greater than those of CO2 with amine solution, resulting in decreases of CO2 removal efficiency and mass transfer rate in different extent with the absorbent of EDA, EDA + MEA, and EDA + MEA + PZ. The CO2 removal efficiency and mass transfer rate decrease with the increasing SO2 concentration and absorption and desorption cycle of absorbent.This paper proposes appropriate absorbent composition ratio and operation parameters range which can inhibit the influence of SO2 on CO2 absorption and optimize the CO2 absorption under the influence of SO2. Depending on the results in this research, hybrid absorbent with activator agent, appropriate absorbent concentration, and ratio of liquid-gas flow rate can inhibit the influence of SO2 on CO2 absorption effectively. EDA + MEA + PZ (0.4 : 0.4 : 0.2) has the best tolerance ability to SO2 among the three absorbents. Under the condition of 1000 ppm SO2 in flue gas, the appropriate absorbent concentrations of EDA, EDA + MEA, and EDA + MEA + PZ are 800 mol/m3, 750 mol/m3, and 650 mol/m3, respectively, and the appropriate ratio of liquid-gas flow rate is in the range from 0.2 to 0.25.



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*Source: 5729503-2016-12-27.xml*