Rich Solution by Vacuum Desorption - American Chemical Society

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Study on the Regeneration of Basic Aluminum Sulfate SO2‑Rich Solution by Vacuum Desorption Min Chen,* Xianhe Deng, and Feiqiang He Department of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, P. R. China ABSTRACT: Basic aluminum sulfate (BAS) wet flue gas desulfurization (FGD) is a promising renewable process used to remove sulfur dioxide (SO2) from flue gas, in which the regeneration of BAS SO2-loaded solution is of great importance for the reuse of BAS solution. In this paper, a novel regeneration method by vacuum desorption was developed to achieve superior regeneration performance for a BAS-rich solution. The operating parameter effect on SO2 desorption performance was thoroughly investigated in a lab-scale reactor. The experimental results demonstrated that the great decrease of pressure could significantly improve the regeneration performance, and high desorption temperature was favorable for SO2 desorption. Furthermore, it is worth determining the optimum components of a BAS-rich solution and initial sulfite concentration, considering the contradiction between the SO2 absorption performance and the regeneration performance. The increase of stirring speed in the liquid had a considerable positive effect on SO2 desorption efficiency. In addition, through a simple comparison with direct heating regeneration, it indicates that the utilization of vacuum regeneration could have the potential to improve the regeneration rate and lower the total energy consumption. Finally, the recycling experiments of the absorption− desorption process show that the BAS solution could be reused successfully to capture SO2 from flue gas by vacuum regeneration, while SO2 absorption efficiency would decrease to below 90% after 11 cycles, attributed to the inevitable oxidation of byproduct in the desulfurization process. by researchers in recent years.1,10−12 In a comparison with the traditional regenerative processes mentioned above, these methods are more beneficial to the SO2 desorption due to weaker chemical and physical interactions with bonding SO2.11 However, most of them still stay at the laboratory level considering some disadvantages of high cost, lack of easy availability of the absorbents, poor stability, and potential environmental pollutions. As a promising regenerative process, BAS desulfurization has been used to control the SO2 emission from flue gas in practical applications. In 1943, Josef Barwasser et al.13 first presented a process for regeneration of BAS solution for the recovery of SO2. The result showed that it could successfully achieve the regeneration of absorbent and the recovery of SO2 by thermal desorption. Moreover, the desulfurization performance of BAS solution has been investigated in our recent work, indicating that the absorption solution can efficiently remove the SO2 from flue gas.14 For the BAS wet FGD regeneration process, however, little attention was received to research the SO2 desorption of the BAS-rich solution, which is a critical procedure for the process. Therefore, a systematic study on the development of regeneration techniques of BAS solution is urgently needed. Despite traditional thermal regeneration having a positive effect on the SO2 desorption from a BAS-rich solution, it suffers from several inherent drawbacks, such as poor regeneration performance and high energy consumption.15Accordingly, it is necessary to develop efficient and energy-saving regeneration methods. Along with the technical progress, novel technologies

1. INTRODUCTION SO2 emission mainly from industrial processes and from the use of hydrocarbon fuels containing substantial sulfur (S) is a significant source of air pollution, threatening the environment and human health owing to the formation of acid rain and smog.1−3 Until now, various desulfurization technologies, generally including precombustion, in-flame, and postcombustion methods, have been developed to control the SO2 emission in the atmosphere. On the basis of the postcombustion controls, limestone wet scrubbing is regarded as one of the most wide and effective technologies used for the flue gas treatment from thermal power plants.4 Unfortunately, this method suffers from some inherent drawbacks in practical applications, such as a large water requirement, numerous gypsum wastes, and even secondary pollution.5 In contrast, it is noteworthy that SO2 from flue gas would also become one of the important sulfur sources if it could be reversibly absorbed and converted into sulfuric acid or pure SO2. Thus, the development of renewable FGD processes has a profound significance from the sustainable principles and environmental protection. In past decades, many works were focused on the development of regenerative wet FGD technologies, some of which, such as the magnesia method, Wellman−Lord method, Cansolv method, and Elsorb method, have been commercially applied in industrial running.6−9 The categories of renewable desulfurization processes are mainly regenerated by direct heating, causing a large amount of high-temperature water vapor consumption. Through practice, these processes are greatly limited because of poor regeneration performance and high energy consumption in terms of the SO2 desorption process. In addition, the reversible absorption of SO2 using organic solvents or ionic liquids (ILs) has also been proposed © XXXX American Chemical Society

Received: May 8, 2016 Revised: August 31, 2016

A

DOI: 10.1021/acs.energyfuels.6b01110 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

following reaction 2, forming the complex A12(SO4)3· Al2(SO3)3 which in aqueous solution is thermodynamically unstable.23,24

like microwave irradiation and ultrasound have attracted some attention for the recovery of SO2 in flue gas treatment. For instance, Xu et al.16 researched the influence of ultrasound on the SO2 desorption from sodium-alkali-desulfurization-rich solution. The results showed that ultrasound only promoted the decomposition of sulfurous acid, while it could not enhance the decomposition of sodium bisulfate. Xue et al.17 also used ultrasound to regenerate the citrate SO2-rich solution with heating, indicating that ultrasound can strengthen SO 2 desorption without changing the reaction mechanism. Meanwhile, some researchers proposed the microwave method for the regeneration of activated carbon in desulfurization.18,19 Although these methods mentioned above can strengthen the SO2 desorption, so far it has been difficult to commercially apply them in industrial running conditions considering some significant disadvantages, such as expensive equipment investment, instability, and immature techniques, etc. To date, vacuum technology has been widely used for the separation and purity of gas in liquid. For example, Teramoto et al.,20 Nii et al.,21 and Fang et al.22 have done deep research on the utilization of vacuum technology for the CO2 regeneration in rich solution, and their results demonstrated that it could greatly improve the CO2 desorption rate and reduce the energy cost by reducing pressure. Thus, vacuum technology has the potential to achieve the efficient regeneration of BAS with lower energy consumption for the BAS absorption−desorption process. With consideration of the unique property of BAS aqueous solution that free ionic Al3+ can prompt sulfite ionic hydrolysis to form SO2 in liquid, we infer that the utilization of vacuum pressure may accelerate the SO2 desorption rate from a rich solution, based on the two-film theory of mass transfer. To the best of our knowledge, there is no literature reported on the regeneration of BAS SO2-rich solution by vacuum desorption. In view of these considerations above, we propose a novel regenerative process using vacuum desorption, aiming to efficiently, reversibly, and economically achieve the reuse of BAS solution.

Al 2(SO4 )3 (aq) + 3xCaCO3(s) + 6x H 2O → (1 − x)Al 2(SO4 )3 ·x Al 2O3(aq) + 3xCaSO4 ·2H 2O(s) + 3xCO2 (g)

(1)

Al 2(SO4 )3 ·Al 2O3(aq) + 3SO2 (g) ↔ Al 2(SO4 )3 ·Al 2(SO3)3 (aq)

(2)

Δ

Further, SO2 in BAS SO2-rich solution will be desorbed with the regeneration of BAS by heating since reaction 2 is thermodynamically reversible. At the same time, the equilibria of ionization and hydrolysis between SO2 and H2O will also accompany the BAS-based absorption−desorption process by the following eqs 3−5:4,14 SO32 −(aq) + H 2O ↔ HSO−3 (aq) + OH−(aq)

(3)

HSO−3 (aq) + H 2O ↔ H 2SO3(aq) + OH−(aq)

(4)

H 2SO3(aq) ↔ SO2 (g) + H 2O

(5)

Moreover, on the basis of double hydrolysis, the presence of Al3+ in BAS solution will greatly promote the hydrolysis reactions above by reaction 6: Al3 +(aq) + 3H 2O ↔ Al(OH)3 (aq) + 3H+(aq)

(6)

However, a part of sulfite in rich solution may be inevitably oxidized by oxygen in the flue gas by reaction 7: Al 2(SO4 )3 ·Al 2(SO3)3 (aq) + 1/2O2 (g) → 2Al 2(SO4 )3 (aq) (7)

In summary, BAS in aqueous solution can be regenerated with the recovery of SO2 by heating and reducing pressure. It is noteworthy that Al3+ may play an important role on the regeneration of rich solution due to the double hydrolysis reactions with sulfurous(IV) ions. In terms of the unique properties of BAS-rich solution, reducing pressure will enhance the SO2 desorption rate based on the two-film theory.

2. REGENERATION MECHANISM For the BAS desulfurization process, BAS is the effective absorbent of SO2, which can be prepared by reaction 1.14,15 The model of BAS-based absorption−desorption process is clearly illustrated in Figure 1. When SO2 from flue gas enters into the BAS solution, BAS will react rapidly with SO2 by the

3. EXPERIMENTAL SECTION 3.1. Materials. Aluminum sulfate (purity ≥99%) and calcium carbonate (purity ≥99%) were purchased from Tianjin Kermel Chemicals Co., Ltd., China. Ethylene glycol (EG) (A.R. ≥99%) was purchased from Shanghai Sinopharm Chemicals Co., Ltd., China. All chemicals and reagents used in the experiments were of analytical reagent grade. The simulated flue gas with 21% (v/v) O2 was prepared by mixing compressed air and pure SO2 reagent. SO2 cylinder (purity ≥99% and the rest N2) was procured from Guangzhou Puyuan Gas Co., Ltd., China. Water used in all the experiments was deionized. 3.2. Preparation of BAS SO2-Rich Solution. First, BAS solution was prepared by the method reported in the literature.14,15 For instance, an aluminum sulfate solution was neutralized by addition of calcium carbonate powder, and after 24 h with stirring the slurry was filtered to obtain the clear solution. Subsequently, pure SO2 gas was directly introduced into the fresh BAS solution. The desired amount of SO2 absorbed in solution was adjusted by pH meter with an accuracy of ±0.02. Meanwhile, 2% (v/v) EG was added into BAS solution used in all the experiments for hindering the oxidation of sulfite, and sulfite oxidation in the SO2 desorption process could be neglected since the

Figure 1. Model of BAS-based absorption−desorption process. B

DOI: 10.1021/acs.energyfuels.6b01110 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels oxidation efficiency of sulfite was controlled within 2% during each desorption experiment.14 3.3. Experimental Apparatus and Procedure. 3.3.1. Vacuum Regeneration Experiments. Figure 2 shows the schematic diagram of

Figure 3. Experimental apparatus for the recycling of the absorption− desorption process based on BAS solution (1, air compressor; 2, needle valve; 3, rotameter; 4, SO2 cylinder; 5, glass tee; 6, gas tank; 7, thermostatic bath with magnetic stirring; 8, magnetic stirrer; 9, thermometer; 10, flask; 11, condenser tube; 12, vacuum pressure gauge; 13, vacuum pressure control; 14, flat bottom; 15, vacuum pump).

Figure 2. Experimental apparatus for the regeneration of BAS SO2-rich solution (1, thermostatic bath with magnetic stirring; 2, magnetic stirrer; 3, flask; 4, thermometer; 5, vacuum pressure gauge; 6, needle valve; 7, condenser tube; 8, trap; 9, vacuum pressure control; 10, flat bottom; 11, vacuum pump).

Table 1. Experiment Conditions for Recycling of the Absorption−Desorption Process

the vacuum desorption apparatus. Before each experiment is run, the leakage test of the apparatus system was conducted under vacuum produced by a vacuum pump (maximum absolute vacuum pressure, 10 kPa; pumping rate, 60 L/min). If the reading of the vacuum pressure gauge (−0.1 to 0 MPa, ±1 kPa) remains steady, a fixed vacuum pressure value in the experiments was then set up by adjusting the needle valve opening of the vacuum pressure control vessel. After the leakage check and pressure regulation, 250 mL of BAS SO2-loaded solution containing 2% (v/v) EG was poured into the three-necked flask with a capacity of 500 mL, which was placed in a constant temperature water bath and was magnetically stirred at fixed stirring speed. Once the solution reached the desired temperature measured by the thermometer with accuracy of ±1.0 K, vacuum desorption would run by opening the needle valve (6) in the confined condition. Then, the fixed vacuum pressure was acquired, and the time was recorded. The vapor steam from the three-necked flask was completely condensed through a straight condenser tube, and the condensed water was collected in the trap (8). Subsequently, the outlet gas steam was passed into a flat-bottomed flask containing a large of tap water to ensure that the SO2 in steam was thoroughly removed. Finally, the gas was emitted into the atmosphere. The procedure above was repeated to research the effects of various operating parameters (temperature, vacuum degree, initial SO2 concentration, and the component of BAS solution) on SO2 desorption. In addition, direct thermal regeneration was also carried out as shown in Figure 1, while heating was provided by an oil bath. For each 20 min interval, the vacuum desorption system was stopped, and a 1 mL sample was taken out from a three-necked flask for determining the sulfite concentration in solution. Meanwhile, the condensate in the trap was also weighed by electronic balance with accuracy of ±0.0001 g. After the procedure above was rapidly finished, the vacuum desorption was open again. In addition, the pH of solution before and after experiments was measured by a pH meter. 3.3.2. Repeated Experiments. The recycling experiments of the SO2 absorption−desorption process based on BAS solution were conducted in a batch lab-scale apparatus. As seen in Figure 3, it mainly consists of two parts: the SO2 absorption process and the SO2 desorption process. The operating parameters of repeated experiments were given in Table 1. The experimental procedure can be described by the following: First, the simulated flue gas with the fixed gas flow rate and initial SO2 concentration was introduced into a three-necked flask with a capacity of 1000 mL, which was filled with 500 mL of fresh BAS solution and placed in a constant temperature water bath with magnetic stirring. After the surplus SO2 in outlet gas was removed by a flask filled with a large amount of tap water, the outlet gas stream was discharged into the atmosphere. For each 20 min interval, the SO2

condition

SO2 absorption

regeneration

volume of solution (mL) temp (K) pressure (kPa) flow rate (L/min) flue gas SO2 conc (ppm) time (min)

500 30 101.3 1.4 2800−3200 180

500 340−343 19−21

120

concentration of inlet and outlet gas streams was sampled and determined by a method reported in our previous work.14 Followed by the procedure above, the vacuum desorption immediately starts on the conditions of the desired temperature and vacuum pressure as shown in section 3.3.1. It is noted that the condensate from the vapor stream was returned into the three-necked flask by a spherical condenser tube during the vacuum desorption process. Finally, the absorption− desorption procedure above was repeated until the SO2 absorption efficiency decreased below 90%. In addition, the pH and sulfite and sulfate concentration in liquid were measured before and after each experiment, respectively. The concentration of SO2 in both flue gas and solution was determined by a standard iodometric titration method, while the concentration of sulfate was measured by the means of ion chromatography (ICS-900, American DIONEX). 3.4. Data Analysis. In this study, SO2 desorption efficiency (ηDE) and SO2 absorption efficiency (ηAE) were determined to evaluate the absorption performance and regeneration performance, respectively. Furthermore, the average SO2 desorption rate (RSO2) and the average evaporation rate of water vapor (RH2O) were also considered. They can be defined as the following eqs 8−11:

C TS,0· V0 − C TS, i· Vi

ηDE =

ηAE =

C TS,0· V0

y0 − yi

R SO2 = R H2O = C

y0

× 100% (8)

× 100% (9)

C TS,0·V0 − C TS, i· Vi 60Δt

(10)

mH2O 60ΔtM H2O

(11) DOI: 10.1021/acs.energyfuels.6b01110 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Here, the following abbreviations apply: ηDE is the SO2 desorption efficiency, %; ηAE is the SO2 absorption efficiency, %; RSO2 is the average mass-transfer rate of SO2, mol s−1; RH2O is the average transfer rate of water vapor, mol s−1; CTS,0 and CTS,i are the concentration of total sulfite in BAS-rich solution before and after desorption, mol/L, respectively; V0 is the initial solution volume, mL; Vi is the solution volume after desorption (this can be obtained by the following: Vi = V0 − mH2O, mL; mH2O is the mass of water collected by the trap, g); MH2O is the molar mass of H2O, g/mol; y0 and yi are the inlet and outlet concentration of SO2 in the gas phase, ppm, respectively; Δt is the regeneration time, min. Every experiment was repeated at least three times, and for the calculations of values above, the oxidation of sulfite in rich solution was neglected with respect to the amount of SO2 desorbed in every desorption experiment because of the addition of 2% (v/v) EG in BAS-rich solution. In addition, the influences of some uncertainties caused by the readings and accuracies of the pressure gauge and temperature meter were also considered.

(about 27.3 kPa at 340 K25), a larger amount of water vapor will be generated, resulting in strong turbulence in the liquid phase and rapid update at the gas−liquid interface which can enhance the liquid−gas mass-transfer rate. In addition, the increase of water vapor will lower SO2 partial pressure in the gas side, which contributes to the distinct rise of SO2 masstransfer rate. To completely recognize the dependence of regeneration performance on regeneration time at various vacuum pressures, we also investigate the influence of regeneration time on SO2 desorption efficiency at different operating pressures (17, 20, 23 kPa). Figure 5 shows that the SO2 desorption efficiency has a

4. RESULTS AND DISCUSSION 4.1. Effect of Vacuum Pressure on the SO2 Desorption. Figure 4 shows the influence of vacuum pressure on both

Figure 5. Effect of regeneration time on SO2 desorption efficiency at different vacuum pressures (amount of aluminum, 15 g/L; basicity, 25%; initial sulfite concentration, 0.06 mol/L; stirring speed, 120 rpm; T = 340 K; P = 20 kPa).

remarkable increase from 69.4% to 99.1% at 60 min as the vacuum pressure decreases from 23 to 17 kPa. Furthermore, the SO2 desorption efficiency can reach 95% above at 80 min when the vacuum pressure is 20 kPa. However, it can be seen obviously that the regeneration time with 95% desorption efficiency needs at least 180 min as the vacuum pressure increases to 23 kPa. Thereby, the results indicate that lower pressure is greatly in favor of the SO2 regeneration from BASrich solution, and the pressure should be maintained below 20 kPa at 340 K for high regeneration performance in industrial applications. 4.2. Effect of Temperature on the SO2 Desorption. For the desorption process of the BAS-rich solution, temperature is closely related to the feasibility and energy consumption. Therefore, the influence of temperature on the SO2 desorption performance was experimentally investigated in Figures 6 and 7. As seen in Figure 6, it reveals that the desorption temperature has an important influence on RSO2 and RVP. The RVP value has a significant increase with an increase in the temperature. Meanwhile, the curve of RSO2 may be divided into three stages. In the first stage, the SO2 desorption efficiency is close to zero despite the temperature reaching 325 K. In the second stage, the RSO2 value has a sudden rise when the temperature increases to 328 K, and remains with a slight improvement with the rise of temperature from 328 to 334 K. This may be because the decomposition reaction 2 happens until the temperature is over 327 K. Finally, the RSO2 value increases rapidly from 0.23 × 10−5

Figure 4. Effect of vacuum pressure on RSO2 and RVP (amount of aluminum, 15 g/L; basicity, 25%; initial sulfite concentration, 0.06 mol/L; stirring speed, 120 rpm; T = 340 K; regeneration time, 20 min).

the SO2 desorption rate (RSO2) and the amount of water vapor (RVP). As can be seen from Figure 4, the vacuum pressure has significant influence on the SO2 desorption rate. With the decrease of vacuum pressure, the values of both RSO2 and RVP increase and have a consistent tendency toward increasing. Also, we can find that RSO2 has no linear correlation with the vacuum pressure. For instance, when the vacuum pressure is above 26 kPa, it has little positive effect on the RSO2 value. However, RSO2 is considerably sensitive to the change of pressure, and it increases significantly from 0.34 × 10−5 to 1.08 × 10−5 mol s−1 as the vacuum pressure ranges from 29 to 14 kPa. The reasons may be explained by the following aspects. On one hand, the decrease of pressure will reduce the partial pressure of SO2 in the gas phase, which can enhance the liquid−gas driving force to mass transfer and accelerates the desorption rate of BAS-rich solution. On the other hand, as vacuum pressure is reduced to reach the water vapor pressure D

DOI: 10.1021/acs.energyfuels.6b01110 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

When the temperature is 337, 340, and 343 K at 20 kPa, correspondingly, the regeneration time with above 99% desorption efficiency drastically reduces from 180, to 120, to 80 min, respectively. In view of the considerations above, the temperature should be adjusted to around 340 K, which is easy to satisfy through the reuse of low-grade energy or waste gas from industry, thereby reducing the cost of energy consumption. 4.3. Effect of Water Vapor on SO2 Desorption. From Figures 4 and 6, it is evident that the SO2 desorption performance is associated with the vaporization amount of water from solution, which has a favorable influence on SO2 desorption. One reason may be that the water vapor acts as the sweeping steam to lower the SO2 partial pressure in the gas phase, leading to stronger mass-transfer driving force from the liquid phase to the gas phase.20 Another reason for this is the greater gas−liquid contact area since a large amount of water vapor can sweep the gas−liquid interface. Therefore, water vapor is considerably necessary for the regeneration of BAS solution, and the increase of temperature or the decreased vacuum pressure in vacuum regeneration process can produce a greater amount of water vapor, which will be used to sweep the gas−liquid interface resulting in the improvement of regeneration performance.21 4.4. Effect of Component of BAS Solution on the SO2 Desorption. The components of the BAS solution are crucial for the desulfurization performance of the BAS solution. Also, it is determined by a combination of the amount of aluminum and basicity which are associated with the pH value of BAS solution.14 The influence of the amount of aluminum on the SO2 desorption rate was examined by adding various amounts of aluminum sulfate to the BAS solution (amount of aluminum 15 g/L and basicity 25%). As seen in Figure 8, this illustrates

Figure 6. Effect of temperature on RSO2 and RVP (amount of aluminum, 15 g/L; basicity, 25%; initial sulfite concentration, 0.06 mol/L; stirring speed, 120 rpm; P = 20 kPa; regeneration time, 20 min).

Figure 7. Effect of regeneration time on SO2 desorption efficiency at different temperatures (amount of aluminum, 15 g/L; basicity, 25%; initial sulfite concentration, 0.06 mol/L; stirring speed, 120 rpm; P = 20 kPa).

to 1.12 × 10−5 mol s−1 when the temperature ranges from 334 to 345 K. The main reason may be explained as not only is high temperature a benefit for accomplishing reaction 2, which is an endothermic reaction, but also it will speed up the thermal motion of solution and increase a greater amount of water vapor which in turn can reduce the SO2 ratio in the gas phase, resulting in a more rapid SO2 desorption rate. In addition, when the temperature approaches the boiling point of solution (about 333 K at 20 kPa25), the mass-transfer rate of water vapor has a considerable rise, which can enhance the SO2 desorption performance similar to the results discussed in section 4.1. Thus, increasing the temperature will also contribute a great improvement to the SO2 desorption rate if the operating pressure cannot be reduced to an optimum value. Furthermore, we also research the dependence of desorption efficiency on regeneration time at different temperatures. Figure 7 shows that there is a great difference for the regeneration time with 99% above desorption efficiency at various temperatures.

Figure 8. Effect of amount of aluminum on SO2 desorption rate (basicity, 25%; initial sulfite concentration, 0.06 mol/L; stirring speed, 120 rpm; T = 340 K; P = 20 kPa; regeneration time, 20 min).

that the SO2 desorption rate slightly increases as the amount of aluminum increases from 15 to 45 g/L. However, it will have a little decrease in turn when the amount of aluminum rises to 55 g/L. The reason that aluminum sulfate promotes the SO2 desorption may be that the increasing concentration of ionic aluminum can strengthen the double hydrolysis reactions with free sulfite ionic and reduce the pH value of the solution, which E

DOI: 10.1021/acs.energyfuels.6b01110 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels is in favor of reactions 3−6. Nevertheless, excessive aluminum sulfate may hamper the SO2 desorption that contributed to the decrease of the liquid-phase mass-transfer coefficient with the changes of physical properties of the solution, such as density, viscosity, and diffusion coefficients, etc. 26 Hence, the experimental findings indicate that the amount of aluminum should be below 55 g/L. Furthermore, Figure 9 shows that the basicity as well as pH have a significant effect on the SO2 desorption rate of BAS-rich

Figure 10. Effect of concentration of sulfite on SO2 desorption rate (amount of aluminum, 15 g/L; basicity, 25%; stirring speed, 120 rpm; T = 340 K; P = 20 kPa; regeneration time, 20 min).

Figure 9. Effect of basicity on SO2 desorption rate and pH of the SO2loaded solution (amount of aluminum, 15 g/L; initial sulfite concentration, 0.06 mol/L; stirring speed, 120 rpm; T = 340 K; P = 20 kPa; regeneration time, 20 min).

solution. It is obvious that the SO2 desorption rate markedly reduces with an increase of the basicity. One reason may be due to the increase of pH, causing less SO2 in the liquid. Another reason is that higher basicity makes the formed complex more stable, hindering the decomposition of Al2(SO4)3·Al2(SO3)3.27 In view of these considerations above, high basicity is suitable for SO2 absorption of the BAS solution, while it should be maintained at below 30% for better regeneration performance. 4.5. Effect of Initial Concentration of Sulfite on the SO 2 Desorption. To examine the influence of the concentration of sulfite on the SO2 desorption rate, a number of experiments were carried out. Notably, the results summarized in Figure 10 indicate that the SO2 desorption rate has a remarkable rise with an increase of the concentration of sulfite. Furthermore, compared with that of 0.060 mol/L, the SO2 desorption rate can increase about 3 times when the concentration of sulfite is 0.140 mol/L. A reasonable explanation is that since reaction 2 is thermodynamically reversible by heating, high sulfite concentration in liquid will promote it to react rapidly toward the positive direction by heating. Besides, according to the stoichiometric ratio of the reaction, the amount of SO2 reacted in fresh BAS solution (amount of aluminum 15 g/L and basicity 25%) is 0.110 mol/ L, over which the liquid phase will form more free SO2 which can greatly strengthen the SO2 mass-transfer rate. Thus, a high concentration is suitable for the SO2 desorption of BAS-rich solution, but the SO2 absorption performance should be wellconsidered.14 4.6. Effect of Stirring Speed on SO2 Desorption. Figure 11 shows that the stirring speed has an important effect on the

Figure 11. Effect of stirring speed on SO2 desorption efficiency at different vacuum pressure (amount of aluminum, 15 g/L; basicity, 25%; initial sulfite concentration, 0.06 mol/L; T = 340 K; P = 20 kPa).

SO2 desorption. For instance, when the stirring speed increases from 120 to 360 rpm, the SO2 desorption efficiency will have a substantial increase in the range 74.6−90.5% at 40 min. Furthermore, in a comparison with results at 120 rpm, the regeneration time with high SO2 desorption efficiency 98% above can reduce from 100 to 60 min when the stirring speed is 360 rpm. This can be explained by the observation that increasing the stirring rate will expand the gas−liquid contact areas which are mainly contributions to the liquid−gas masstransfer rate, thereby causing more rapid SO2 desorption from the liquid phase to the gas phase. Furthermore, a larger stirring rate may lead to uniform concentration of sulfite in aqueous solution and a better heat-transfer effect which are favorable for SO2 desorption. Fang et al.28 proposed a novel membrane vacuum regeneration technology used in CO2 desorption from rich alkanolamine solution. They suggested that using polypropylene hollow fiber membrane contactors could acquire better regeneration performance and lower regeneration energy consumption compared with traditional thermal regeneration at the same temperature. In view of the considerations above, it is F

DOI: 10.1021/acs.energyfuels.6b01110 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

Figure 12. Effects of thermal regeneration and vacuum regeneration: (a) SO2 desorption efficiency with time; (b) amount of water vapor with time (amount of aluminum, 15 g/L; basicity, 25%; initial sulfite concentration, 0.06 mol/L; stirring speed, 120 rpm, P = 20 kPa, T = 340 K; direct heating desorption, 373−374 K).

outlet pressure of vacuum pump, respectively, kPa. ηVP is the efficiency of the vacuum pump, which can be given as eq 16:28

extremely indispensable for efficient regeneration performance to enhance the gas−liquid interface area by fine contactors applied in the regeneration process. 4.7. Comparative Analysis between Vacuum Regeneration and Direct Thermal Regeneration. As shown in Figure 12a, it clearly shows that, in comparison with direct heating regeneration, SO2 desorption efficiency can be greatly improved and reach 95% above at 80 min by vacuum regeneration. Meanwhile, Figure 12b reveals that the evaporation rate of water with vacuum desorption is larger than the corresponding heating regeneration. Thus, the results above indicate that vacuum desorption is more conducive to regenerating BAS-rich solution with respect to thermal desorption. In addition, we have also made a simple analysis for energy consumption of two regeneration methods through the experimental data. For the calculations of total energy consumption, it is based on 95% above SO2 desorption efficiency. The total energy consumption for the regeneration process mainly consists of vacuum pump, cooling the pump, and water vapor without considering the energy consumption of SO2 decomposition reaction in solution, which can be estimated by the following eqs 12−15:22,29,30 ESO2 = E H2O + (WVP + WVP, COOL) × 60Δt /1000

(12)

E H2O = m H2OΔHH2O/1000

(13)

WVP

(k − 1)/ k ⎡ ⎤ GMW,VPRTκ ⎢⎛ PVP,out ⎞ ⎜⎜ ⎟⎟ = − 1⎥⎥ (κ − 1)ηVP ⎢⎝ PVP,in ⎠ ⎣ ⎦

WVP,COOL = 0.054ηVPWVP

⎛ PVP,out ⎞ ⎟⎟ + 0.8746 ηVP = −0.1058In⎜⎜ ⎝ PVP,in ⎠

(16)

Consequently, the calculations of energy consumption for vacuum regeneration and thermal regeneration were shown in Figure 13. With an increase in vacuum pressure, the total

Figure 13. Comparison of regeneration energy consumption between vacuum regeneration and direct thermal regeneration (amount of aluminum, 15 g/L; basicity, 25%; initial sulfite concentration, 0.06 mol/L; stirring speed, 120 rpm; T = 340 K; heating desorption, 373− 374 K; desorption efficiency, 95% above).

(14) (15)

energy consumption as well as the energy requirement of the vacuum pump has a significant increase, while the condensation energy always remains very low value with little change. The main reason may be that, as reported in Figure 3, increasing the pressure can decrease the SO2 desorption rate, resulting in more regeneration time for 95% above desorption efficiency; accordingly, a greater amount of sweeping steam which determines the total energy consumption directly will be consumed. Therefore, lower pressure is more conducive to the regeneration of BAS-rich solution compared with direct thermal regeneration. However, if the regeneration pressure is too low,

Here, the following abbreviations apply: ESO2 stands for the total energy consumption for 95% desorption efficiency of BAS-rich solution, kJ; EH2O is the energy consumption of water vapor, kJ; WVP and WVP,COOL are the work for the energy requirement of the vacuum pump and cooling pump, respectively, W; ΔHH2O is the heat of vaporization of water, kJ/kg; mH2O is the amount of water vapor, g; GMW,VP is the molar flow rate of the wet stripping gas, mol/s; R is the gas constant, J/(mol K); T is temperature, K; κ is the adiabatic constant of the gas mixture; PVP,in and PVP,out are the inlet and G

DOI: 10.1021/acs.energyfuels.6b01110 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels it will require a higher-performance vacuum pump to lower pressure and have a larger suction capacity, leading to greater equipment costs. Hence, the vacuum pressure should remain around 20 kPa, in which total energy consumption for vacuum regeneration could be lower than thermal regeneration and relatively high regeneration performance could be satisfied. In addition, if a low-temperature heat source and waste heat from power plants can be efficiently used in practical applications, vacuum regeneration will be a promising method for the SO2 desorption of BAS-rich solution with lower energy costs. 4.8. Recycling of BAS Absorption−Desorption Process by Vacuum Desorption. To investigate the feasibility of the BAS absorption−desorption process, repeated experiments were conducted in a batch lab-scale platform. As seen in Figure 14, the curve at the bottom shows that the SO2 desorption

Figure 15. FT-IR spectra of samples of (A) fresh BAS and (B) regenerated BAS.

agreement with the literature values reported by Sun et al.4 The absorption bands at 3460 and 1659 cm−1 suggest that there is crystalline water in two samples.31 Moreover, Figure 15 also indicates that there is no change in the structure of regenerated BAS compared with that of the fresh BAS. These obtained results indicate that the white solid samples could be BAS.32As a result, we conclude that BAS solution can be stably and repeatedly used to capture SO2, which further coincides with the mechanism shown in Figure 1.

5. CONCLUSIONS According to the regeneration model proposed, this paper first develops the novel regeneration method by vacuum desorption for a BAS wet FGD regeneration process. The experimental results show that the SO2 desorption efficiency could reach a high level above 95% (in 80 min) at 340 K when the pressure was controlled below 20 kPa, and the SO2 desorption rate as well as the mass-transfer rate of water vapor would have a considerable increase with the rise of pressure or temperature. The optimal operating conditions of vacuum regeneration of BAS solution under the laboratory scale were fixed as described in the following: amount of aluminum, below 45 g/L; basicity, 25−30%; absolute vacuum pressure, around 20 kPa; temperature, 338−345 K; initial sulfite concentration, 0.06−0.1 mol/L. Considering the significance of stirring rate on regeneration performance, further study of efficient gas−liquid contactors is needed in future works. In a comparison with traditional thermal regeneration, vacuum regeneration not only performed better on the SO2 desorption, but also had the potential to economically lower the total energy consumption and achieve the efficient utilization of a low-grade heat source like industrial waste gas and solar energy. In addition, regenerated BAS solution could be efficiently recycled 11 times, while after which the desulfurization efficiency would decrease to below 90% due to the inevitable oxidation of byproduct in running processes. The present study would be useful for the pilot-scale design and process optimization relevant to the BAS-based wet FGD regenerative system, and provides a benign alternative for the regeneration of other renewable FGD processes.

Figure 14. Regeneration performance for recycling of the BAS absorption−desorption process.

efficiency can be always well-maintained within 98% after every absorption process during all of the cycles. In addition, the curves on the top show the changes of both the SO2 absorption efficiency and the pH value with cycles. Meanwhile, from Figure 14, it can be clearly found that the SO2 absorption efficiency slowly decreases with an increase of cycles, while it can keep a high value with 90% above. Meanwhile, the pH value has a slight decline in range 3.43−3.17 after 11 cycles. The main reason may be due to the tardy oxidation of byproduct during the absorption−desorption process, causing the decrease of basicity in BAS-lean solution. Although 2% (v/ v) EG as an oxidation inhibitor of sulfite was contained into the BAS solution, it is inevitable that a small part of the byproduct was oxidized to form aluminum sulfate. Thus, it is necessary for the BAS-based wet FGD regeneration process to regularly improve the basicity of BAS-lean solution by the method reported in the literature.14,15 In addition, to develop more efficient oxidation, inhibitors used in BAS SO2-loaded solution may be a promising attempt for high recycling performance, resulting in lower operating costs. Additionally, after removal of the water of BAS solution by sequential evaporation and drying steps, white solid samples before and after a cycle of the BAS absorption−desorption process without adding EG were obtained and characterized by FT-IR spectroscopy (VERTEX 70, Germany BRUKER), respectively. As shown in Figure 15, the absorption brands at 1100 and 612 cm−1 could be assigned to SO42−, which is in H

DOI: 10.1021/acs.energyfuels.6b01110 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

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Corresponding Author

*Phone/fax: +86-020-87111814. E-mail: [email protected]. cn. Notes

The authors declare no competing financial interest.

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NOMENCLATURE CTS,0 = concentration of total sulfite in BAS-rich solution before desorption (mol/L) CTS,i = concentration of total sulfite in BAS-rich solution after desorption (mol/L) ESO2 = total energy consumption for 95% desorption efficiency (kJ) EH2O = energy consumption of water vapor (kJ) GMW,VP = molar flow rate of wet stripping gas (mol/s) ΔHH2O = heat of vaporization of water (kJ/kg) mH2O = amount of water collected by trap (g) MH2O = molar mass of H2O (g/mol) PVP,in = inlet pressure of vacuum pump (kPa) PVP,out = outlet pressure of vacuum pump (kPa) R = gas constant (J mol−1 K−1) RSO2 = average mass-transfer rate of SO2 (10−5 mol s−1) RH2O = average transfer rate of water vapor 10−4 mol s−1) Δt = regeneration time (min) T = temperature (K) V0 = initial solution volume (mL) Vi = solution volume after desorption WVP = energy requirement of vacuum pump cooling pump (W) WVP,COOL = energy requirement of vacuum pump cooling pump (W) y0 = inlet concentration of SO2 in gas phase (ppm) yi = outlet concentration of SO2 in gas phase (ppm)

Greek Symbols

ηAE = SO2 absorption efficiency (%) ηDE = SO2 desorption efficiency (%) ηVP = efficiency κ = adiabatic constant

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DOI: 10.1021/acs.energyfuels.6b01110 Energy Fuels XXXX, XXX, XXX−XXX