Optimization of CO<sub>2</sub> Absorption Characteristic under the Influence of SO<sub>2</sub> in Flue Gas by Hollow Fiber Membrane Contactor

Qu, Ziyi; Zhang, Li; Yan, Yunfei; Ju, Shunxiang

doi:https://doi.org/10.1155/2016/5729503

Journal of Chemistry

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Abstract Introduction Materials and Methods Results and Discussion Conclusions Acknowledgments References Copyright Related Articles

Research Article | Open Access

Volume 2016 | Article ID 5729503 | https://doi.org/10.1155/2016/5729503

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

Ziyi Qu,^1,2,3Li Zhang ,^1,2Yunfei Yan,^1,2and Shunxiang Ju^1,2

Academic Editor: Naoki Haraguchi

Received09 Aug 2016

Accepted06 Nov 2016

Published27 Dec 2016

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.

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 firstly [11, 19]:

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:

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]:The reaction between SO₂ and MEA is as follows [19]:where 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 Table 1. 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.

2.3. Experimental Procedures

The system is shown in Figure 1. 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.

The experimental conditions and parameters are shown in Table 2. 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.%.

3. Results and Discussion

The CO₂ removal efficiency is the parameter that reflects the CO₂ removal performance; it can be calculated by [23]where η is the CO₂ removal efficiency, is the gas flow rate of inlet, is the gas flow rate of outlet, and and 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]where is the mass transfer rate, is gas temperature, and S 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.

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 Figures 4 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.

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.

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.

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.

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.

Nomenclature

C:	Concentration (mol·m⁻³)
d:	Average pore diameter (μm)
:	CO₂ mass transfer rate (mol·m⁻²·h⁻¹)
L:	Length (m)
n:	Number of fibers
S:	Contact area of fiber (m²)
t:	Time (s)
T:	Temperature (K)
U:	Velocity (m³·h⁻¹)
V:	Instantaneous velocity of gas at a point of module (m·s⁻¹).

Greek Letters

η:	CO₂ removal efficiency (%)
ε:	Porosity of fiber membrane (%)
φ:	Volume fraction of CO₂ in gas (vol.%)
τ:	Tortuosity factor of fiber membrane
():	Packing density of fiber membrane.

Subscripts

:	Gas
:	Liquid
in:	Inlet
out:	Outlet.

Abbreviations

EDA:	Ethylenediamine
PZ:	Piperazine
MEA:	Monoethanol
PP:	Polypropylene.

Competing Interests

The authors declare no competing interests regarding the publication of this paper.

Acknowledgments

This research is supported by the Science and Technology Project of Chongqing Municipal Education Commission (KJ1502503) and the Fundamental Research Funds for the Central Universities (no. CDJZR14145501).

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Copyright © 2016 Ziyi Qu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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