Comparative Study on the Regeneration of Flue-Gas Desulfurizing

by using ED. In comparison, ED has higher piperazine yield and current efficiency, and much lower voltage drop and energy consumption. However, its pr...
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Environ. Sci. Technol. 2006, 40, 5527-5531

Comparative Study on the Regeneration of Flue-Gas Desulfurizing Agents by Using Conventional Electrodialysis (ED) and Bipolar Membrane Electrodialysis (BMED) CHUANHUI HUANG AND TONGWEN XU* Laboratory of Functional Membranes, School of Chemistry and Material Science, University of Science and Technology of China, Hefei 230026, P. R. China

Piperazine is an ideal desulfurizing agent but the heatstable salts formed in desulfurization have caused secondary pollution and waste of resources. In the previous paper, a method was reported to regenerate piperazine by using BMED. To find the variety of that regeneration process, we performed experiments on the regeneration of piperazine by using ED. In comparison, ED has higher piperazine yield and current efficiency, and much lower voltage drop and energy consumption. However, its process cost is higher than that of BMED due to an extra expenditure for the base and its tank and pumps. The process cost is estimated to be 0.96 $/kg Pz for BMED and 1.14 $/kg Pz for ED. Notably, BMED has more environmental benefits and will be more economically attractive as the control on secondary pollution is strengthened and the bipolar membrane cost decreases.

Introduction Sulfur dioxide (SO2), one of the major air pollutants, has been under increasingly stringent control worldwide (1). As the dominant technology for SO2 pollution control, wet absorption has been applied to flue gas desulfurization (2). Although this technology has a verified efficiency, its competence with other desulfurization processes is threatened by its inherent shortcoming-secondary pollution and waste of resources. Namely, alkaline absorbents form heat stable salts in the process circulation due to the oxidation of SO2 and/or sulfites. Piperazine (Pz), one of the alkaline absorbents, has recently found more applications in flue gas desulfurization due to its advantage over other absorbents, i.e., Pz possesses a fast SO2 sorption kinetics and can be regenerated by heating the formed piperazine sulfites (PzH22+SO32- and [PzH+SO32-]-). However, unexceptionally, its application is limited by heat-stable salts-piperazine sulfates (PzH22+SO42- and [PzH+SO42-]-). In the previous paper (3), this shortcoming was overcome by coupling Pz absorption with bipolar membranes electrodialysis (BMED), which converted piperazine sulfate (Pz‚ H2SO4) into Pz and sulfuric acid. The fact that BMED is adequate to salt convention should be ascribed to its effective * Corresponding author phone: +86-0551-3601587; fax: +860551-3601592; e-mail: [email protected]. 10.1021/es060525c CCC: $33.50 Published on Web 08/08/2006

 2006 American Chemical Society

functional integration of the water splitting in bipolar membranes (4-5) and the salt dissociation in conventional electrodialysis (ED). Theoretically, ED can also be applied to treat spent desulfurizing agents if an additional base is employed. Accordingly, in this paper, in an attempt to find the better regeneration process, experiments were performed on the regeneration of piperazine by using ED. Subsequently, a comparative study between BMED and ED was conducted in terms of operation conditions and process economics. The comparison will also lead to a better understanding of these two processes and lay a foundation for treating the industrial waste solution discharged in the process of desulfurizing. See the Supporting Information for experimental apparatus and procedure.

Results and Discussion Effect of Electrolyte Concentration on Piperazine Regeneration. As described in the experimental section of the Supporting Information, the Type I configuration (A-C-A for ED, and BP-C-A for BMED) was applied to all the experimental runs except the ones designed for the configuration effect hereinafter. Figure 1(a) demonstrates the change of piperazine yield as electrolyte concentration increases. Piperazine yield was not affected significantly by electrolyte concentration except for an increase with time elapse in the case of ED or BMED. However, ED has a slightly higher piperazine yield than BMED, and this advantage is more obvious at the beginning. This difference arises from the OH- source for Pz regeneration. As in ED, the OH- supply, through migration and diffusion, keeps almost at the same rate. As for BMED, H+ and OH- are provided by the water splitting in bipolar membranes, before which “transition time” (6) is needed to deplete the ions in the interface region. It also takes some time for OH- ions to take the dominance over other counter-ions in bipolar membranes and become the main current carriers. Figure 1(b) shows the effect of electrolyte concentration on the voltage drop across the stack. In the case of BMED, the higher the electrolyte concentration, the lower the voltage drop. The sharp voltage increase shortly after a current is applied, is due to a sharp increase in the bipolar membrane resistance (6) and a decrease in the apparent conductivity of the electrode rinsing solutions (3). As shown in the back part of the voltage drop-time curve, the electrical resistance of the stack increases because the piperazine sulfate was gradually replaced by piperazine (Pz), whose conductivity is much lower than PzH2SO4. In the case of ED, the voltage drop-time curves are very close, suggesting that increasing electrolyte concentration doesn’t lead to an apparent decrease in the electrical resistance of the corresponding compartments or membranes. Compared with BMED, ED has a much smaller voltage drop and less sharp resistance increase. This voltage difference (>20 V) shouldn’t be ascribed to the difference in the cathode compartment because its biggest difference caused by electrolytes is only 1.22 V (the voltage drop of a compartment with 0.5 mol/L NaOH or 0.2 mol/L Na2SO4 as the electrolyte is 0.52 and 1.74 V, respectively). All the above indicate that the bipolar membrane has a higher electrical resistance than monopolar membranes and causes a much higher voltage drop after the water splitting occurs. Figure 1(c) shows the effect of electrolyte concentration on energy consumption and current efficiency. In the case of BMED, as the electrolyte concentration increases, the energy consumption decreases since the electrolyte can VOL. 40, NO. 17, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Effect of electrolyte concentration on piperazine regeneration. (a) Effect of electrolyte concentration on piperazine yield; (b) effect of electrolyte concentration on the voltage drop across the stack; (c) effect of electrolyte concentration on energy consumption and current efficiency. The other operation conditions were current I ) 0.35 A (current density i ) 50 mA/cm2), the concentration of Pz‚H2SO4 c ) 0.13 mol/L, fluid flow speed v ) 27 L/h, and configuration Type I.

FIGURE 2. Effect of piperazine sulfate concentration on piperazine regeneration. (a) Effect of piperazine sulfate concentration on piperazine yield; (b) effect of piperazine sulfate concentration on the voltage drop across the stack; (c) effect of piperazine sulfate concentration on energy consumption and current efficiency. The other operation conditions were current I ) 0.35 A (current density i ) 50 mA/cm2), the concentration of Na2SO4 c ) 0.3 mol/L, fluid flow speed v ) 27 L/h, and configuration Type I.

reduce the electrical resistance of the stack, especially that of the bipolar membrane. In the case of ED, the electrolyte concentration has a negligible influence on the energy consumption. In regards to the current efficiency curves, high current efficiency is only achieved at middle concentrations of electrolyte solutions (c0(Na2SO4) ) 0.3∼0.4 mol/L) in both cases. Current efficiency increases with electrolyte concentration but decreases at high concentrations. Employing Na2SO4 at high concentrations leads to an excessive diffusion flux of competitive ions, and thus less migration flux of piperazine ions into the base compartment (3). In the case of BMED, the decrease in current efficiency is more obvious because high concentration of electrolyte solutions also leads to a decrease in the water supply into the bipolar membrane (7) and an increase in transition time (3).

Effect of Piperazine Sulfate Concentration on Piperazine Regeneration. Figure 2a-c show the effects of Pz‚2SO4 concentration on piperazine yield, the voltage drop across the stack, energy consumption, and current efficiency. Figures 1 and 2 have a lot of similarities because both Na2SO4 and Pz‚H2SO4 are electrolytes, but there are some differences. For one thing, increasing the concentration of Pz‚H2SO4 in the salt cycle results in a higher voltage decrease since Pz‚ H2SO4 has a smaller specific conductance than Na2SO4. For another, the main reason for the increase in current efficiency is that Na+/PzHnn+ (n ) 1,2) and H+/PzHnn+ ratios in the salt compartment decrease as the Pz‚H2SO4 concentration increases. However, in the case of BMED, after more Pz‚H2SO4 diffuses into the base compartment, current efficiency will finally decrease. It is because the organic diffusion increases

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the hydrophobicity of the bipolar membrane to the extent that less water is supplied to generate OH- (3). As for energy consumption, it increases at the much lower current efficiency. In the case of ED, a hydrophobicity increase of the anion-selective membrane doesn’t decrease the OH- supply since it is provided by a NaOH solution. Furthermore, Pz‚ H2SO4 in the diffusion flux can react with the OH- migrated from the cathode compartment or the NaOH diffused from that compartment. Therefore, more piperazine can be regenerated and current efficiency increases. In general, as concerns ED, the current efficiency increases and the energy consumption decreases as Pz‚H2SO4 concentration increases. The results indicate that the low-energy consumption and high current efficiency are achieved when using a Pz‚H2SO4 solution of middle concentration (c0(Pz‚H2SO4) ) 0.08∼0.13 mol/L) for BMED and that of high concentration (c0(Pz‚H2SO4) ) 0.23 mol/L) for ED . Effect of Current Density on Piperazine Regeneration. Figure 3a-c indicate that, in the case of BMED or ED, as current density increases, the piperazine yield, the voltage drop across the stack, and energy consumption increase. However, current efficiency decreases and then increases. ED produces more piperazine than BMED. In all the experiments for both ED and BMED, a greater part of the total electrical energy is consumed to overcome the electrical resistance as the current density increases. This is because the irreversible contribution (the voltage drop or energy used to overcome the electrical resistance) to the energy consumption is much greater than the reversible one. One confirmation is that the voltage drops across the stack at the time when the current was switched off were all less than 4 V. When it comes to the current efficiency, there are many factors contributing to the experimental curve. As current density increases, the ion selectivity of all the membranes decreases, which results in a decrease in current efficiency at low current densities. However, a higher current density leads to (a) less relative salt ion transport (8) in both cases, and (b) a shorter transition time in the case of BMED, and less piperazine migrating out of the base compartment due to the concentration gradient. Accordingly, current efficiency increases steadily at high current densities. Effect of Configuration on Piperazine Regeneration. As shown in Figure S1a-e (see Supporting Information), there are four configurations applied to regenerate piperazine. Types I, II, and IV generate piperazine by basifying the piperazine ions (PzHnn+) with a difference in the number of anion or cation selective membranes. Type III produces piperazine by replacing the sulfate anions (SO42- and HSO4-) with the OH- generated in bipolar membranes (BMED) or supplied by a NaOH solution (ED). Figure 4a shows the effect of stack configuration on piperazine yield. The results indicate piperazine yield increases in the following order: Type III < Type I ≈ Type II ≈ Type IV for ED and Type III < Type I < Type II ≈ Type IV for BMED, and ED has a higher piperazine yield than BMED. The initial piperazine concentration, in the case of Type III, is much higher than that of any other configuration because there exists some piperazine in the salt compartment before a current is applied due to the hydrolysis of piperazine sulfate. Figure 4b shows the effect of configuration on the voltage drop across the stack. The results show that the voltage drop increases in the order: Type III < Type II < Type I ≈ Type IV for ED and Type III < Type IV < Type II and Type I for BMED. ED has a lower voltage drop than BMED for each configuration. For Type II (BP-C for BMED and A-C for ED), its voltage drop increases sharply and then decreases steadily. In one case, the electrical resistance of the anode compartment increases sharply when Pz‚H2SO4 is used as the anode rinsing electrolyte and the compartment is partly occupied by the gas bubbles. For another, the voltage drop decreases

FIGURE 3. Effect of current density on piperazine regeneration. (a) Effect of current density on piperazine yield; (b) effect of current density on the voltage drop across the stack; (c) effect of current density on energy consumption and current efficiency. The other operation conditions were the concentration of Na2SO4 c ) 0.3 mol/L, the concentration of Pz‚H2SO4 c ) 0.18 mol/L, fluid flow speed v ) 27 L/h, and configuration Type I. steadily when there is more H+ generated by electrolysis. For Type III, the stack of that configuration has fewer membranes and compartments than that of the Type I configuration, and its voltage drop is the lowest. For Type IV, the stack of that configuration has the same number of membranes and compartments as that of the Type I configuration, but its voltage drop finally becomes lower since the H+ migration leads to a decrease in the electrical resistance of the salt cycle (3). In both cases, the stack of the Type II configuration has the same number of membranes and compartments as that of the Type III configuration, but the former has much higher energy consumption, and the value is almost near that of the Type I or Type IV configuration. This gives a clue to the specialty of the cation-selective membrane adjacent to the Pz‚H2SO4 solution, i.e., it has a much higher electrical resistance than the other cation-selective membrane. This VOL. 40, NO. 17, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. An Estimation of the Process Cost. operation conditions current density, mA/cm2 effective membrane area, cm2 the concentration of Na2SO4, mol/L the concentration of Pz‚H2SO4, mol/L fluid flow speed, L/h stack configuration the energy consumption, kW‚h/kg Pz the process capacity, kg Pz/year energy Cost electricity charge, $/kW‚h the energy cost for the regeneration, $/kg Pz the Energy cost for the peripheral equipment, $/kg Pz the total Energy cost, $/kg Pz investment Cost membrane life-time and the amortisation of the peripheral equipment, years membrane prices (monopolar membrane), $/m2 membrane prices (bipolar membrane), $/m2 membrane cost, $ stack cost, $ peripheral equipment cost, $ the cost of the base tank and its pump, $ total Investment cost, $ amortisation, $/year interest, $/year maintenance, $/year the total fixed cost, $/year the total fixed cost, $/kg Pz running material cost (2 mol NaOH/mol Pz), $/kg Pz treatment of spent base solutions, $/kg Pz the total process cost, $/kg Pz

FIGURE 4. Effect of configuration on piperazine regeneration. (a) Effect of configuration on piperazine yield; (b) effect of configuration on the voltage drop across the stack; (c) effect of configuration on energy consumption and current efficiency. The other operation conditions were current I ) 0.42 A (current density i ) 60 mA/cm2), the concentration of Na2SO4 c ) 0.3 mol/L, the concentration of PzH2SO4 c ) 0.18 mol/L, and fluid flow speed v ) 27 L/h. is mainly because piperazine ions and molecules make a hydrophobic section of this membrane and cause a much higher electrical resistance. Figure 4c shows the effect of configuration on energy consumption and current efficiency. The energy consumption increases in the following order: Type IV < Type II < Type I < Type III for BMED and Type II < Type IV < Type I < Type III for ED, and current efficiency in the order: Type III < Type I < Type II ≈ Type IV for both cases. Generally, ED has a higher current efficiency and lower energy consumption than BMED. Among the four configurations, Type III has the lowest current efficiency and highest energy consumption. This is because the H+, migrating from the anode compartment, is easily accessible to the piperazine or OH- in the salt/base compartment, and much less piperazine is formed. 5530

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BMED

ED

60 7.07 0.30 0.18 27 BP-C-C 5.4 5.55

60 7.07 0.30 0.18 27 A-C 2.7 5.82

0.1 0.54 0.03

0.1 0.27 0.01

0.57

0.28

3

3

135 1350 1.15 1.72 2.58 0.00 4.30 1.43 0.34 0.43 2.20 0.40 0.00

0.19 0.62 1.93 0.97 3.52 1.17 0.28 0.35 1.81 0.31 0.45

0.00 0.96

0.10 1.14

Type IV and Type II have the higher current efficiency and lower energy consumption because in the buffering range of Pz‚H2SO4, a certain amount of H+ leads to an increase in PzHnn+ ions in the salt compartment without an increase in H+/PzHnn+ ratio. According to the experimental data, the most favorable configuration is Type IV (BP-C-C) for BMED and Type II (A-C) for ED. Process Economics. The cost estimation is made on the basis of the laboratory-scale experimental equipment (Table 1), and the corresponding calculation was conducted by the method reported in the literature (9). BMED has the higher energy cost and fixed cost; ED has the lower energy cost and fixed cost. However, ED has an extra cost for the running material (NaOH) as well as auxiliary equipment such as tanks and pumps, etc. The process cost for piperazine regeneration is 0.96 $/kg Pz for BMED and 1.14 $/kg Pz for ED, both are much less than the current market price of piperazine (ca. 31 $/kg Pz‚6H2O available in China). Many parameters for the cost estimation depend on process scales and market prices. Table S1 (see Supporting Information) presents a sensitivity analysis on these parameters on the basis of the laboratory-scale experimental equipment. The results suggest the sensitivity order as follows: running material (NaOH) cost > electricity charge > treatment of spent solutions > bipolar membrane cost > monopolar membrane cost > base tank and pump cost > peripheral equipment cost. The economic competence of BMED with ED is in positive correlation with the increment of running mater cost, treatment of spent solutions and base tank and pump cost, and the decrement of bipolar membrane cost, monopolar membrane cost, peripheral equipment cost and the rest parameters listed in Table S1 (see Supporting Information). Notably, the constant NaOH supply involves some operational problems, such as the material shipment, storage of

NaOH from CO2 depletion, and the treatment of spent NaOH solutions. However, BMED is free from those problems since the OH- is produced in situ by the water splitting in bipolar membranes. Furthermore, as the manufacture cost of bipolar membranes is reduced (the cost of a bipolar membrane is assumed, in this paper, to be ten times that of a monopolar membrane), BMED will obtain a more distinctive economical competence. In general, BMED is not only scientifically novel but also economically feasible and attractive, in addition to its environmental benefits. When BMED or ED is applied to treat the actual discharge from desulfurization, extra cost has to cover the prevention of scale formation in order to achieve process efficiency and stability. First, suspending particles, colloids, and multivalent ions (especially, Ca2+, Mg2+, Ba2+, and Sr2+) should be removed from the feed solution in case they would precipitate on the surface of membranes or block membrane channels. Second, hydrodynamic conditions should be improved in order to suppress concentration polarization, which may result in local supersaturation of solutes. Last but not least, the product piperazine should be extracted before its concentration exceeds its solubility. Naturally, more research needs to be conducted before these processes are put to industrial use.

Acknowledgments This research was supported in part by the National Science Foundation of China (no. 20376079), the Specialized Research Fund for the Doctoral Program of Higher Education (no. 20030358061), and National Basic Research Program of China (973 Program, no. 2003CB615700). We thank Michelle Corbin (School of Education, University of Maryland, U.S.) for proofreading the manuscript.

Supporting Information Available Experimental apparatus and procedure, the figure for stack configurations, and the table for sensitivity analysis. This material is available free of charge via the Internet at http:// pubs.acs.org.

Literature Cited (1) Taylor, M. R.; Rubin, E. S.; Hounshell, D. A. Effect of government actions on technological innovation for SO2 control. Envrion. Sci. Technol. 2003, 37, 4527-4534. (2) Lancia, A.; Musmarra, D. Calcium bisulfite oxidation rate in the wet limestone-gypsum flue gas desulfurization process. Environ. Sci. Technol. 1999, 33, 1931-1935. (3) Huang, C. H.; Xu, T. W.; Jacobs, M. L. Regenerating flue-gas desulfurizing agents by bipolar membrane electrodialysis. AIChE J. 2006, 52, 393-401. (4) Xu, T. W. Ion exchange membranes: state of their development and perspective. J. Membr. Sci. 2005, 263, 1-29. (5) Mafe´, S.; Ramı´rez, P.; Alcaraz, A. Electric field-assisted proton transfer and water dissociation at the junction of a fixed-charge bipolar membrane. Chem. Phys. Lett. 1998, 294, 406-412. (6) Wilhelm, F. G.; van der Vegt, N. F. A.; Strathmann, H.; Wessling, M. Comparison of bipolar membrane by means of chronopotentiometry. J. Electroanal. Chem. 2002, 199, 177-190. (7) Aritomi, T.; van den Boomgaard, Th.; Strathmann, H. Currentvoltage curve of a bipolar membrane at high current density. Desalination 1996, 104, 13-18. (8) Wilhelm, F. G.; Pu ¨nt, I.; van der Vegt, N. F. A. Asymmetric bipolar membranes in acid-base electrodialysis. Ind. Eng. Chem. Res. 2002, 41, 579-586. (9) Strathmann, H.; Koops, G. H. In Handbook on bipolar membrane technology; Kemperman, A. J. B., Ed.; Twente University Press: Enschede, 2000; pp 191-220.

Received for review March 7, 2006. Revised manuscript received June 10, 2006. Accepted July 17, 2006. ES060525C

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