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Optimization of C[O.sub.2] Absorption Characteristic under the Influence of S[O.sub.2] in Flue Gas by Hollow Fiber Membrane Contactor.

1. Introduction

In recent years, energy consumption has been increasing with the rapid growth of the world economy. The greenhouse effect of C[O.sub.2] became increasingly serious, and hence energy conservation is urgent [1]. The hollow fiber membrane contactor is a new technology for the C[O.sub.2] 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 C[O.sub.2] reacts with the absorbent. Because of the C[O.sub.2] concentration gradient in the gas and liquid phases, C[O.sub.2] 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 C[O.sub.2] absorption has been used in S[O.sub.2] removal by many researchers [3-8]. The hollow fiber membrane contactor is not only applied for C[O.sub.2] removal, but also applied to remove S[O.sub.2]. Ogundiran et al. [9] studied S[O.sub.2] 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 S[O.sub.2] removal by PVDF hollow fiber membranes and found that it is one of the most competitive alternatives in the future. There is still S[O.sub.2] that exists in the flue gas after desulfurization tower. The influence of S[O.sub.2] on C[O.sub.2] capture gained more attention recently. Zhong [11] studied the effect of S[O.sub.2] concentration on MEA, MEA/MDEA, MEDA/PZ, and DEA/AMP. The tolerance of S[O.sub.2] was found to be as follows: MEA > DEA/AMP > MEDA/PZ > MEA/MDEA. Uyanga and Idem [12] studied the degradation of MEA caused by S[O.sub.2] in a semibatch reactor. The results show that S[O.sub.2] accelerates the rate of MEA degradation and established a dynamic model. Supap et al. [13] studied the kinetics of S[O.sup.2-] and [O.sup.2-] induced degradation of aqueous MEA during C[O.sub.2] capture. The results show that an increase in temperature and concentration of MEA, [O.sub.2], and S[O.sub.2] causes a higher degradation rate of MEA. Gao et al. [14] studied the effect of S[O.sub.2] on the C[O.sub.2] capture process in a pilot plant. The results show that S[O.sub.2] causes amine oxidative degradation, which is beneficial to remove S[O.sub.2] induced heat stable salts using appropriate methods. Bonenfant et al. [15] studied the absorption of C[O.sub.2] and S[O.sub.2] 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 S[O.sub.2]. The results show that S[O.sub.2] decreases the C[O.sub.2] absorption rate. The addition of 5 and 10wt.% of MDEA and TEA does not influence the C[O.sub.2] absorption rate in AEE. TEA decreases the absorption capacity of AEE. Yang et al. [16] studied the influence of S[O.sub.2] on the C[O.sub.2] capture in an absorption desorption experimental setup using MEA as the absorbent. The results show that there were sharp decreases in C[O.sub.2] removal efficiency and mass transfer rate of C[O.sub.2] after the initial several days of operation; more progress is needed in high-efficiency and stable absorbents.

According to the present research situation, S[O.sub.2] causes the degradation of alcohol amine absorbent, resulting in the decrease of C[O.sub.2] removal efficiency. The influence of S[O.sub.2] on C[O.sub.2] absorption by alcohol amine absorbent deserves attention. It is necessary to research the optimization of C[O.sub.2] absorption characteristic under the influence of S[O.sub.2]. Research on appropriate absorbent and parameter range which can inhibit the influence of S[O.sub.2] is needed. MEA, MDEA, DEA, and other amine solutions [11-16] are selected as absorbent of C[O.sub.2] capture by many researchers. The disadvantage of MEA is the high energy consumption in the C[O.sub.2] desorption. The tolerance ability to S[O.sub.2] of MEA is better than that of MDEA [11]. Studies on EDA as an absorbent used in C[O.sub.2] capture are relatively fewer. In the study of Shunxiang [18], the performance of the absorbents used in C[O.sub.2] absorption is as follows: PZ > EDA > MEA > DEA. Considering that C[O.sub.2] removal efficiency of EDA is higher than of MEA, the tolerance ability to S[O.sub.2] 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 C[O.sub.2] capture in this paper. This paper researches the performance of these absorbents under the influence of S[O.sub.2] in order to study the appropriate absorbent composition ratio and specific operating parameters to optimize C[O.sub.2] absorption under the influence of S[O.sub.2]. The results of this study are an important reference for the industrial application of C[O.sub.2] absorption by hollow fiber membrane contactor.

2. Materials and Methods

2.1. Reaction Mechanism. S[O.sub.2] 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 S[O.sub.2] and amine solution can be assumed as a combination of the physical dissolution of S[O.sub.2] in water and the chemical absorption of amine solution. S[O.sub.2] in water generates SO32- firstly [11, 19]:

S[O.sub.2] + [H.sub.2]O [left and right arrow] [H.sup.+] + HSO[.sub.3.sup.-] (1)

HS[O.sub.3.sup.-] [left and right arrow] [H.sup.+] + S[O.sub.3.sup.2-] (2)

The solubility of S[O.sub.2] 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:

RR'NH + S[O.sub.2] + [H.sub.2]O [left and right arrow] RR'N[H.sub.2.sup.+] + HS[O.sub.3.sup.-] (3)

With oxygen: 2HS[O.sub.3.sup.-] + [O.sub.2] = 2HS[O.sub.4.sup.-] (4)

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 C[O.sub.2] removal efficiency in the system in the long run.

For MEA, the reaction between C[O.sub.2] and MEA is as follows [20-22]:

C[O.sub.2] + 2RN[H.sub.2][right arrow] RN[H.sup.+]C[O.sub.2.sup.-] + RN[H.sub.3.sup.+] (5)

The reaction between S[O.sub.2] and MEA is as follows [19]:

S[O.sub.2] + 2RN[H.sub.2] + [H.sub.2]O [right arrow] 2RN[H.sub.3.sup.+] + S[O.sub.3.sup.2-] (6)

where R is HOC[H.sub.2]C[H.sub.2].

2.2. Materials. The solutions of EDA (ethylenediamine), EDA + MEA (monoethanolamine), and EDA + MEA + PZ (piperazine) are selected as absorbents for C[O.sub.2] absorption in this experiment. The concentration of absorbent is 500 mol/[m.sup.3], 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[degrees]C.

2.3. Experimental Procedures. The system is shown in Figure 1. The flue gas is simulated by mixed gas of C[O.sub.2], S[O.sub.2], and [N.sub.2]. 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 C[O.sub.2] and becomes a rich liquid and is then introduced into the absorbent tank by a pump. The desorption tank desorbs C[O.sub.2] 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 [m.sup.3]/h, and absorbent flow rate is 0.7 [m.sup.3]/h. Volume fraction of C[O.sub.2] in gas is 14 vol.%.

3. Results and Discussion

The C[O.sub.2] removal efficiency is the parameter that reflects the C[O.sub.2] removal performance; it can be calculated by [23]

[eta] = [Q.sub.in] x [[phi].sub.in] - [Q.sub.out] x [[phi].sub.out]/[Q.sub.in] x [[phi].sub.in] x 100, (7)

where [eta] is the C[O.sub.2] removal efficiency, [Q.sub.in] is the gas flow rate of inlet, [Q.sub.out] is the gas flow rate of outlet, and [[phi].sub.in] and [[phi].sub.out] are the volume fraction of C[O.sub.2] at inlet and outlet, respectively.

The mass transfer rate reflects the performance of mass transfer. It can be calculated by [23]

[mathematical expression not reproducible], (8)

where [mathematical expression not reproducible] is the mass transfer rate, [T.sub.g] is gas temperature, and S is the total area of the membrane.

3.1. Influence of S[O.sub.2] Concentration on C[O.sub.2] Absorption. In order to emphasize the influence of S[O.sub.2] concentration, the concentration of S[O.sub.2] is amplified from 500 ppm to 2000 ppm in this research. The influences of S[O.sub.2] concentration on C[O.sub.2] removal efficiency and mass transfer rate are shown in Figures 2 and 3. The C[O.sub.2] removal efficiency and mass transfer rate of the three absorbents decrease with the increasing of S[O.sub.2] concentration. The solubility of S[O.sub.2] is much higher than of C[O.sub.2]. The pH value of S[O.sub.2] aqueous solution is smaller than the pH value of C[O.sub.2] equilibrium solution. The reaction between S[O.sub.2] and amine absorbent can be considered as instantaneous [11]. The reaction rates and opportunities for S[O.sub.2] and absorbent are much higher than those of C[O.sub.2] and absorbent, which leads to the decrease of C[O.sub.2] removal efficiency.

The C[O.sub.2] removal efficiency of three absorbents decreased suddenly after 1000 ppm S[O.sub.2]. The effective components keep constant when the absorbent concentration is fixed. When S[O.sub.2] concentration increased to a certain value, the effective components of absorbent are consumed largely by S[O.sub.2],which leads to the sudden decrease of C[O.sub.2] removal efficiency. The C[O.sub.2] removal efficiency of EDA decreases by 4.05% with 500 ppm S[O.sub.2] and decreases by 37.3% with 2000 ppm S[O.sub.2]. It can be considered that the influence of S[O.sub.2] on C[O.sub.2] removal efficiency with EDA is not significant when the S[O.sub.2] concentration is under 500 ppm. Because of the low concentration of S[O.sub.2], even with the faster reaction rate of S[O.sub.2], there is still a chance for C[O.sub.2] to react with the absorbent. The concentration gradient of the gas and the liquid vapor interface increases with increasing S[O.sub.2] concentration, which improves the mass transfer dynamics. This is favorable for S[O.sub.2] molecules to diffuse to the surface and the interior of the absorption solution and speed up the reaction of S[O.sub.2] with EDA. Therefore, the absorption of C[O.sub.2] reduced greatly with the increasing of S[O.sub.2] concentration. The influence of S[O.sub.2] on C[O.sub.2] removal efficiency is more significant with the increase of S[O.sub.2] concentration. The S[O.sub.2] concentration in the outlet of flue gas is always zero, which indicates that S[O.sub.2] is absorbed by the absorbent completely, and the reaction rate of S[O.sub.2] and the absorbent is significantly higher than that of C[O.sub.2] and the absorbent.

The C[O.sub.2] removal efficiency of EDA + MEA decreases by 2.1% with 500 ppm S[O.sub.2], the efficiency is 53.1% with 1000 ppm S[O.sub.2], and the efficiency decreases to 29.3% with 2000 ppm S[O.sub.2]. Comparing the results of EDA + MEA (0.6: 0.4) and EDA, the C[O.sub.2] removal efficiency and mass transfer rate of EDA are higher than those of EDA + MEA without the addition of S[O.sub.2]. With the increasing of S[O.sub.2] concentration, the C[O.sub.2] removal efficiency and mass transfer rate of EDA + MEA (0.6: 0.4) are higher than those of EDA. The C[O.sub.2] absorption capacity of EDA is better than that of MEA [18]; therefore, the C[O.sub.2] removal efficiency and mass transfer rate of EDA are higher than those of EDA + MEA (0.6: 0.4) without the influence of S[O.sub.2]. The active ingredient increases after the addition of MEA, which promotes the tolerance ability of the absorbent to S[O.sub.2]. The C[O.sub.2] 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 S[O.sub.2] is better than that of EDA.

Zhong [11] researched the influence of S[O.sub.2] concentration on C[O.sub.2] absorption with the absorbent of 10% MEA and 10% MEA + 2% MDEA (liquid flow rate 18 L/h, temperature 40[degrees]C; flue gas flow rate 1800 L/h, temperature 15[degrees]C; S[O.sub.2] concentration is 500-1500 ppm). The results are shown in Figure 2. The results indicate that the C[O.sub.2] removal efficiency decreases with the increase of S[O.sub.2] concentration, and the influence of S[O.sub.2] is not significant till the S[O.sub.2] concentration becomes greater than 500 ppm, which agrees well with the results in this paper. The C[O.sub.2] removal efficiency of 10% MEA + 2% MDEA is higher than that of 10% MEA when S[O.sub.2] concentration is under 117 ppm, and the decrease extent of C[O.sub.2] removal efficiency in 10% MEA + 2% MDEA is more than that in 10% MEA with the increase of S[O.sub.2] concentration. The C[O.sub.2] removal efficiency of 10% MEA is higher than that of 10% MEA + 2% MDEA with 200 ppm S[O.sub.2]. This indicates that the reaction between 10% MEA + 2% MDEA and S[O.sub.2] is more rapid and intense, so the decrease extent of C[O.sub.2] 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 C[O.sub.2] absorption with the influence of S[O.sub.2]. The tolerance ability of 10% MEA + 2% MDEA to S[O.sub.2] is lower than that of 10% MEA.

Comparing the results of EDA and 10% MEA, the C[O.sub.2] removal efficiency of 10% MEA is higher than of EDA without S[O.sub.2], and the decrease extent is smaller than EDA with the increase of S[O.sub.2] concentration. So, the tolerance ability to S[O.sub.2] of 10% MEA is greater than 500 mol/[m.sup.3] EDA. After adding of MEA, the C[O.sub.2] removal efficiency of EDA + MEA (0.6: 0.4) is higher than of EDA with the increase of S[O.sub.2] concentration. So, the hybrid absorbent of EDA + MEA (0.6: 0.4) is appropriate for removal of C[O.sub.2] in the flue gas containing S[O.sub.2].

The absorbent of EDA + MEA + PZ is the most efficient in the three absorbents. The C[O.sub.2] removal efficiency of EDA + MEA + PZ decreases by 1.2% with 500 ppm S[O.sub.2], decreases by 3.4% with 800 ppm S[O.sub.2], and decreases by 27.1% with 2000 ppm S[O.sub.2]. The C[O.sub.2] removal efficiency reduction of EDA + MEA + PZ is not significant until 1000 ppm S[O.sub.2]. Because of the activity of PZ, the influence of S[O.sub.2] on C[O.sub.2] absorption with EDA + MEA + PZ is not significant under the condition of low S[O.sub.2] concentration in a short time. And the tolerance ability of EDA + MEA + PZ to S[O.sub.2] is greater than that of EDA and EDA + MEA. The influence of S[O.sub.2] is getting more significant when the PZ active effect is gradually consumed. Therefore, the C[O.sub.2] removal efficiency decreases significantly with 2000 ppm S[O.sub.2].

The results of the three absorbents indicate that the influence of S[O.sub.2] on C[O.sub.2] is not significant with low S[O.sub.2] concentration in the short run performance; the hybrid absorbent with high absorptive capacity component and high tolerance ability to S[O.sub.2] can inhibit the influence of S[O.sub.2] on C[O.sub.2] absorption effectively.

3.2. Cycle Absorption and Desorption Characteristic of Absorbent. According to the previous research, the influence of S[O.sub.2] on C[O.sub.2] absorption is not significant in the low concentration of S[O.sub.2]. 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 S[O.sub.2] concentration on C[O.sub.2] removal efficiency, the influence is not significant when the S[O.sub.2] concentration is below 500 ppm of the three absorbents. In order to study the influence of S[O.sub.2] on C[O.sub.2] absorption in low S[O.sub.2] concentration, the S[O.sub.2] 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 C[O.sub.2] removal efficiency and mass transfer rate decrease with the increase of cycle number. The C[O.sub.2] removal efficiency of EDA decreases by 26.85% with S[O.sub.2] and decreases by 21.8% without S[O.sub.2]; the C[O.sub.2] removal efficiency of EDA + MEA decreases by 24.7% with S[O.sub.2] and decreases by 22.1% without S[O.sub.2]; the C[O.sub.2] removal efficiency of EDA + MEA + PZ decreases by 14.6% with the influence of S[O.sub.2].

Desorption of the absorbent by heating the rich liquid results in absorbent degradation. S[O.sub.2] reacts with the absorbent and generates stable salts, which cannot be regenerated by heating. The existence of S[O.sub.2] accelerates the degradation of most amine solutions. Strazisar et al. [24] studied the effect of S[O.sub.2] on the degradation of MEA. The results show that S[O.sub.2] accelerates the degradation rate of MEA, which is significant in higher concentration of S[O.sub.2]. Therefore, the C[O.sub.2] removal efficiency and mass transfer rate decrease significantly with the increasing cycle of absorption and desorption.

Gao et al. [17] studied the influence of S[O.sub.2] on the absorption character of absorbent in the following campaign: no S[O.sub.2], 214 ppm S[O.sub.2], and 317 ppm S[O.sub.2], respectively. The result is shown in Figure 6. The C[O.sub.2] removal efficiency decreases gradually with increasing circulating time. And the decrease extent of C[O.sub.2] removal efficiency increases with S[O.sub.2] concentration. The absorbent degradation and heat stable salts formation are the main reasons for the significant influence of S[O.sub.2]. 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 S[O.sub.2] on C[O.sub.2] absorption even in low S[O.sub.2] concentration in the long run performance. The order of tolerance ability to S[O.sub.2] is EDA + MEA + PZ (0.4: 0.4: 0.2) > EDA + MEA (0.6: 0.4) > EDA. The C[O.sub.2] 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 S[O.sub.2].

3.3. The Influence of Absorbent Concentration on C[O.sub.2] Absorption. The concentration of absorbent is one of the most important parameters of C[O.sub.2] absorption in operation. The influence of absorbent concentration on C[O.sub.2] removal efficiency is studied in this section for confirming the appropriate concentration of the absorbent which can inhibit the influence of S[O.sub.2].

The C[O.sub.2] removal efficiency of the three absorbents decreased suddenly after 1000 ppm S[O.sub.2] in the study of influence of S[O.sub.2] concentration. Therefore, the experiment is conducted under the condition of 1000 ppm S[O.sub.2]. The concentration of the absorbent is from 400 mol/[m.sup.3] to 800mol/[m.sup.3]. The C[O.sub.2] removal efficiency and mass transfer rate increase with the in[c.sub.reasing] absorbent concentration which is shown in Figures 7 and 8. Compar[i.sub.ng] the result of 1000 ppm S[O.sub.2] with that of no S[O.sub.2], the C[O.sub.2] removal efficiency of 400 mol/[m.sup.3] EDA, 600 mol/[m.sup.3] EDA, and 650 mol/[m.sup.3] EDA decreases by 13.27%, 6.5%, and 5.3%, respectively. The decreasing extent of C[O.sub.2] 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 S[O.sub.2] keeps constant in flue gas. There are more active ingredients in the absorbent to improve the C[O.sub.2] absorption with the increase of absorbent concentration. Therefore, increasing absorbent concentration can inhibit the influence of S[O.sub.2] on C[O.sub.2] 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 S[O.sub.2] on C[O.sub.2] absorption with low cost and low energy consumption. The C[O.sub.2] removal efficiency of 800 mol/[m.sup.3] EDA, 750 mol/[m.sup.3] EDA + MEA, and 650 mol/[m.sup.3] EDA + MEA + PZ is 75%, 74%, and 83%, respectively, with 1000 ppm S[O.sub.2]. Continuing to increase the absorbent concentration, the increment of C[O.sub.2] 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 S[O.sub.2], the appropriate concentrations of EDA, EDA + MEA, and EDA + MEA + PZ are 800 mol/[m.sup.3], 750 mol/[m.sup.3], and 650 mol/[m.sup.3], respectively.

3.4. Influence of the Liquid-Gas Flow Rate Ratio on C[O.sub.2] Absorption. The flow rates of absorbent and gas are important parameters which can affect the C[O.sub.2] absorption significantly. It is necessary to study the appropriate ratio of liquidgas flow rate for inhibiting the influence of S[O.sub.2].

With the increasing ratio of liquid-gas flow rate under the condition of 1000 ppm S[O.sub.2], the C[O.sub.2] absorption characteristic of EDA, EDA + MEA, and EDA + MEA + PZ is shown in Figures 9 and 10. The C[O.sub.2] removal efficiency and mass transfer rate rise with the increasing ratio of liquid-gas flow rate. With the addition of S[O.sub.2], the C[O.sub.2] 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 C[O.sub.2] removal efficiency difference between the case with S[O.sub.2] and that without S[O.sub.2] 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 S[O.sub.2] on C[O.sub.2] absorption decreases with the increasing ratio of liquid-gas flow rate. Lv et al. [25] studied the simultaneous removal of C[O.sub.2] and S[O.sub.2] in a polypropylene hollow fiber membrane contactor using MEA. The C[O.sub.2] 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 S[O.sub.2], the reaction of absorbent with C[O.sub.2] 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 C[O.sub.2] 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 C[O.sub.2] 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 C[O.sub.2] removal efficiencies of EDA, EDA + MEA, and EDA + MEA + PZ are 60%, 63%, and 77%, respectively; the highest C[O.sub.2] removal efficiencies of EDA, EDA + MEA, and EDA + MEA + PZ are 67%, 70%, and 92%, respectively. In order to inhibit the influence of S[O.sub.2] on C[O.sub.2] absorption and maintain high C[O.sub.2] removal efficiency and low operating cost, under the condition of 1000 ppm S[O.sub.2], 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 S[O.sub.2] on C[O.sub.2] absorption in the long run performance, which affects the industrial application prospects of this technology. It is necessary to study the S[O.sub.2] influence characteristic on C[O.sub.2] absorption and the measure to optimize the C[O.sub.2] absorption under the influence of S[O.sub.2].

This paper studied the optimization of C[O.sub.2] absorption characteristic under the influence of S[O.sub.2] with the absorbent of EDA, EDA + MEA, and EDA + MEA + PZ by hollow fiber membrane contactor. The S[O.sub.2] concentration, cycle absorption and desorption characteristic of absorbent, absorbent concentration, and ratio of liquid-gas flow rate are analyzed to evaluate the influence of S[O.sub.2] on C[O.sub.2] absorption characteristic. The reaction rate and absorption performance of S[O.sub.2] with amine solution are much greater than those of C[O.sub.2] with amine solution, resulting in decreases of C[O.sub.2] removal efficiency and mass transfer rate in different extent with the absorbent of EDA, EDA + MEA, and EDA + MEA + PZ. The C[O.sub.2] removal efficiency and mass transfer rate decrease with the increasing S[O.sub.2] 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 S[O.sub.2] on C[O.sub.2] absorption and optimize the C[O.sub.2] absorption under the influence of S[O.sub.2]. 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 S[O.sub.2] on C[O.sub.2] absorption effectively. EDA + MEA + PZ (0.4: 0.4: 0.2) has the best tolerance ability to S[O.sub.2] among the three absorbents. Under the condition of 1000 ppm S[O.sub.2] in flue gas, the appropriate absorbent concentrations of EDA, EDA + MEA, and EDA + MEA + PZ are 800mol/[m.sup.3], 750mol/[m.sup.3], and 650mol/[m.sup.3], respectively, and the appropriate ratio of liquid-gas flow rate is in the range from 0.2 to 0.25.

Nomenclature
C:               Concentration (mol x [m.sup.-3])
d:               Average pore diameter ([micro]m)
[mathematical    C[O.sub.2] mass transfer rate (mol x [m.sup.-2]
expression not    x [h.sup.-1])
reproducible]:
L:               Length (m)
n:               Number of fibers
S:               Contact area of fiber ([m.sup.2])
t:               Time (s)
T:               Temperature (K)
U:               Velocity ([m.sup.3] x [h.sup.-1])
V:               Instantaneous velocity of gas at a point of
                 module (m x [s.sup.-1]).

Greek Letters

[eta]:          C[O.sub.2] removal efficiency (%)
[epsilon]:      Porosity of fiber membrane (%)
[phi]:          Volume fraction of C[O.sub.2] in gas (vol.%)
[tau]:          Tortuosity factor of fiber membrane
(1-[PHI]):      Packing density of fiber membrane.

Subscripts

g:              Gas
l:              Liquid
in:             Inlet
out:            Outlet.

Abbreviations

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


http://dx.doi.org/ 10.1155/2016/5729503

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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Ziyi Qu, (1, 2, 3) Li Zhang, (1, 2) Yunfei Yan, (1, 2) and Shunxiang Ju1, (2)

(1) Key Laboratory of Low-Grade Energy Utilization Technologies and Systems, Ministry of Education, Chongqing University, Chongqing 400030, China

(2) College of Power Engineering, Chongqing University, Chongqing 400030, China

(3) Chongqing Electric Power College, Jiulongpo District, Chongqing 400053, China

Correspondence should be addressed to Li Zhang; lizhang@cqu.edu.cn

Received 9 August 2016; Accepted 6 November 2016

Academic Editor: Naoki Haraguchi

Caption: FIGURE 1: Flow chart of the experiment using the flue gas.

Caption: FIGURE 2: Influence of S[O.sub.2] concentration on the C[O.sub.2] removal efficiency ([U.sub.g] = 4 [m.sup.3]/h, [U.sub.l] = 0.7 [m.sup.3]/h, and T = 288 K).

Caption: FIGURE 3: Influence of S[O.sub.2]concentration on the C[O.sub.2] mass transfer rate ([U.sub.g] = 4 [m.sup.3]/h, [U.sub.l] = 0.7 [m.sup.3]/h, and T = 288 K) .

Caption: FIGURE 4: Influence of cycle absorption and desorption on the C[O.sub.2] removal efficiency ([U.sub.g] = 4 [m.sup.3]/h, [U.sub.l] = 0.7 [m.sup.3]/h, and T = 288 K).

Caption: FIGURE 5: Influence of cycle absorption and desorption on the C[O.sub.2] mass transfer rate ([U.sub.g] = 4[m.sup.3]/h, [U.sub.l] = 0.7 [m.sup.3]/h, and T = 288 K).

Caption: FIGURE 6: The influence of circulating time on the C[O.sub.2] removal efficiency [17] ([U.sub.g] = 86 N[m.sup.3]/h, [U.sub.l] = 0.6 [m.sup.3]/h, and T = 319 K).

Caption: FIGURE 7: Influence of S[O.sub.2] on the C[O.sub.2] removal efficiency in different concentrations of absorbent (([U.sub.g] = 4 [m.sup.3]/h, [U.sub.l] = 0.7 [m.sup.3]/h, and T = 288 K)

Caption: FIGURE 8: Influence of S[O.sub.2] on the C[O.sub.2] mass transfer rate in different concentrations of absorbent ([U.sub.g] = 4 [m.sup.3]/h, [U.sub.l] = 0.7 [m.sup.3]/h, and T = 288 K).

Caption: FIGURE 9: Influence of S[O.sub.2] on the C[O.sub.2] mass transfer rate in different liquid-gas flow rate ratios (T = 288 K).

Caption: FIGURE 10: Influence of S[O.sub.2] on the C[O.sub.2] mass transfer rate in different liquid-gas flow rate ratios (T = 288 K).
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.sub.out]
Inner diameter of module      mm          90      [D.sub.in]
Module length                 mm         1125      [L.sub.m]
Fiber length                  mm         920           L
Membrane pore diameter     [micro]m    0.1-0.2     [d.sub.p]
Inner diameter of fiber       mm         0.3      [d.sub.in]
Outer diameter of fiber       mm         0.4      [d.sub.out]
Module area                [m.sup.2]     8~12          s
Number offibers                          7000          n
Tortuosity factor                         2          [tau]
Fiber porosity                %           45       [epsilon]
Packing density               %          13.8      (1-[PHI])

TABLE 2: Parameters of operating conditions.

Parameter                 Unit

[T.sub.g]                   K
[T.sub.l]                   K
[U.sub.g]              [m.sup.3]/h
[U.sub.l]              [m.sup.3]/h
C                     mol/[m.sup.3]
[mathematical             vol.%
  expression
  not reproducible]
P                          MP

Parameter                           Physical                 Value

[T.sub.g]                        Gas temperature              288
[T.sub.l]                     Absorbent temperature           288
[U.sub.g]                         Gas flow rate                4
[U.sub.l]                      Absorbent flow rate            0.7
C                            Absorbent concentration          500
[mathematical         Volume fraction of C[O.sub.2] in gas    14
  expression
  not reproducible]
P                              Operating pressure             0.1
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Title Annotation:Research Article; carbon dioxide, sulfur dioxide
Author:Qu, Ziyi; Zhang, Li; Yan, Yunfei; Ju, Shunxiang
Publication:Journal of Chemistry
Article Type:Technical report
Date:Jan 1, 2017
Words:6959
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