Ind. Eng. Chem. Res. 2010, 49, 3337–3341
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Integrated Membrane Process for the Treatment of Desulfurization Wastewater Na Yin,† Fei Liu,†,‡ Zhaoxiang Zhong,†,‡ and Weihong Xing*,† State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing UniVersity of Technology, Nanjing 210009, China, and Jiangsu Jiuwu Hitech Company, Ltd., Nanjing 210061, China
In the desulfurization process for purification of coking gas, a huge amount of wastewater is generated. In order to recover the usable substances such as suspended sulfur (SS) and ammonium salts, e.g., (NH4)2S2O3 and NH4SCN, in the wastewater and also to avoid severe environmental problems in the case of improper disposal, an integrated membrane process mainly consisting of ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO) was proposed to treat the wastewater. In the UF process, a ceramic membrane was employed to remove SS and an efficiency of 99.9% and a steady flux of about 500 L · m-2 · h-1 were achieved. In the NF step, (NH4)2S2O3 was separated from NH4SCN with a retention ratio of 95.0%, and finally 83.0% of (NH4)2S2O3 was recovered in the retentate, whereas 99.2% of NH4SCN was recovered in the permeate via dialysis with deionized water. In the RO process, NH4SCN can be recovered with an efficiency of 99.0% through a four-pass filtration process. Meanwhile, the RO permeate can be reused in the salt diafiltration process. The results show that the proposed integrated membrane process is technically feasible and economically efficient for the treatment of desulfurization wastewater generated in the coking industry. 1. Introduction In order to reduce environmental loads and improve economic benefits, more and more global coking industries are paying attention to the treatment of desulfurization wastewater and the recovery of usable substances. In China, a wet desulfurization process called HPF technology is widely used in the purification process of coking gas. The HPF represents a mixed catalyst system composed of hydroquinone, phthalocyanine cobalt dualcore sulfonate, and ferrous sulfate, which are used to convert H2S to elemental sulfur.1,2 However, in the HPF process, a huge amount of wastewater containing suspended sulfur (SS) and ammonium salts (NH4)2S2O3 and NH4SCN is produced.3 There is an urgent need for a further treatment to the desulfurization wastewater before it can be discharged; otherwise, serious environmental problems may arise. Because membrane operations for the treatment of different wastewaters is becoming of particular interest,4 extensive studies involving microfiltration, ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO) have been done in recent years.5-10 The cotreatment strategy was found to be both economically and technically advantageous to reduce the wastewater discharge and pollution load with a high efficiency. Concerning the wastewater from coal gasification, early research revealed that the low-pressure (NF) membranes generally showed superior salt rejections (higher than 94.4%),11 but no other report addressed the treatment of the desulfurization wastewater with an integrated membrane system. Its application in the wastewater treatment is challenged by the presence of the considerably small particle size of SS and mixed ammonium salts, which can cause fouling and subsequent system failure either temporarily or permanently. In addition, these technologies require the use of adequate membranes not only to separate the target substances with high efficiency but also to recover them economically. For the sake of solving the SS problem mentioned above, ceramic membranes are the best choice because they are * To whom correspondence should be addressed. Tel.: +86-2583172288. E-mail:
[email protected]. † Nanjing University of Technology. ‡ Jiangsu Jiuwu Hitech Company, Ltd.
excellent in the separation of solids from wastewater.12,13 Because of inherently superior physical integrity, chemical resistance, and thermal stability, the membranes are suitable for extreme-condition applications, especially for the separation of micrometer and submicrometer particles.14 On the basis of the fundamental interplay between the membrane microstructure, the membrane performance, and the characteristics of the particle systems, the permeate quality can be ensured by choosing the right ceramic membrane. Moreover, NF membranes seem suitable for salt removal.15,16 However, most researches concern low salt concentrations because it has been realized that an NF membrane is unable to desalt seawater or other highly concentrated electrolyte solutions because of its characteristic features. It is demonstrated that the salt rejection is almost zero if the feed concentration is higher than 5%17 because the salt rejection is strongly pronounced as being dependent on the concentration of the feed electrolyte.18 Referring to the capacities of the NF and RO membranes to separate a high-concentration water solution, Karelin et al.19 found that NF membranes kept their ability of selectively retaining bivalent ions at electrolyte concentrations in solutions up to 200 g · L-1, but the ion rejection was very low ( 100% of NH4SCN [defined as P ) (Cp/Cf) × 100%]. The reason for the high-resolution separation of the two salts is that S2O32has higher charge density and larger steric hindrance than those of SCN-. Therefore, it has a stronger repulsion force toward the negatively charged NF membrane, and this leads to a high rejection of (NH4)2S2O3. In addition, according to Donnan equilibrium, to maintain electroneutrality on both sides of the membrane, a high permeation of NH4SCN was observed. A long-duration, continuous-salt-concentration experiment was conducted to examine the performances of the NF membrane (Figure 4). It can be seen that the flux decreased with time when an initial NF feed volume of 42 L was reduced to 7 L. Specifically, the steady-state permeate flux was only 6.1% of its initial value. This was caused by an increase in the salt concentration in the process, which led to a higher osmosis pressure and a decreased net pressure as the driving force. In our studies, the final Cp of NH4SCN decreased to 120 g · L-1 and that of (NH4)2S2O3 increased to about 100 g · L-1; meanwhile, the final Cf of NH4SCN decreased from 120 to 76 g · L-1 and that of (NH4)2S2O3 increased from 70 to 190 g · L-1. All of these changes were due to the maintenances of macroscopic electroneutrality in the feed and permeate sides. Generally, salt rejection decreases with an increase in the salt concentration because of concentration polarization.29 This means that the NF membrane permeability deteriorated when the salt was concentrated. However, the concentration of (NH4)2S2O3 in the NF feed can be increased to more than 3 times higher. These results indicate that the NF process works well for the separation of (NH4)2S2O3 from its mixture with NH4SCN, but the salt
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Figure 6. NF concentration process for (NH4)2S2O3 with TMP ) 2.0 MPa and T ) 30 °C.
Figure 5. NF diafiltration process with TMP ) 2.0 MPa and T ) 30-35 °C.
Figure 7. Dependencies of flux and R of NH4SCN versus RO 1-4 passes with TMP ) 3.0 MPa and T ) 30 °C.
Table 2. Mass Balance of the NF Diafiltration Process NF diafiltration
feed
volume (L) 7.0 (NH4)2S2O3 (g) 490.0 NH4SCN (g) 840.0
pure water total permeate 18.7 0.0 0.0
total retentate
18.7 7.0 83.0 (16.9%) 406.9 (83.0%) 833.5 (99.2%) 6.5 (0.8%)
balance (%) 100 99.9 100
concentration needs to be optimized to maintain a high rejection of (NH4)2S2O3 and complete permeation of NH4SCN. In order to avoid polarization and maintain the high rejection rate of (NH4)2S2O3, a diafiltration process was designed to increase the purity of (NH4)2S2O3 (adding water to decrease the concentration of NH4SCN in the retentate of NF).30 Deionized water was added to the NF feed tank at a flow rate of 5.1 L · h-1, equal to the permeate flow rate, which was done to maintain a stable feed volume. The permeate flux remained steady at 21 L · m-2 · h-1 like before, while R for (NH4)2S2O3 was in the range of 91.5-95.7%, as shown in Figure 5a. Most of (NH4)2S2O3 was in the final retentate, which contained NH4SCN with a concentration of no more than 1 g · L-1 (Figure 5b) after NF diafiltration treatment for 220 min. Therefore, the mass balance of the process was calculated in the former 220 min. The results shown in Table 2 confirm that 83.0% of (NH4)2S2O3 is recovered in the retentate, whereas 99.2% of NH4SCN is recovered in the permeate. The retentate from NF diafiltration was commonly subjected to a postconcentration step for a further increase in the salt concentration. Moreover, in order to investigate whether the presence of NH4SCN can affect the filtration of (NH4)2S2O3, a simulated solution was prepared to run the concentration process by dissolving (NH4)2S2O3 in 60 L of deionized water. The results are illustrated in Figure 6. The VRF, defined as the ratio between the initial feed volume and the volume of the resulting retentate,
was increased to 10.0 through this process. Unfortunately, the exhibited flux decreased from about 5 to 1 L · m-2 · h-1, whereas the salt rejection decreased from 99.6% to 38.3% in about 7500 min. The permeate flux decrease with operating times can be divided into two periods: an initial period, up to VRF ) 1.2, in which a rapid decrease of flux occurred; a second period, up to VRF ) 10.0, in which a steady flux was maintained. This phenomenon can be attributed to the serious concentration polarization. The results showed that the NF membrane can merely recover (NH4)2S2O3 effectively only at a concentration of less than 70 g · L-1. Compared with the continuous concentration process of the UF permeate in Figure 4, either the concentrating folds or the permeate flux is lower in the retentate concentration process in Figure 6. It is concluded that the presence of an amount of NH4SCN is favorable for the concentration of (NH4)2S2O3. This was caused by the Donnan effect coupled with a particle scale and an electric charge force.31 When NH4SCN is present, the rejection of (NH4)2S2O3, as well as the permeate flux, is higher. 3.3. RO of the NF Permeate. The NF permeate after diafiltration mainly containing NH4SCN of about 45 g · L-1 was treated with the RO filtration process. That is, the permeate solution was further treated by repeated application of the RO process for the purpose of recovering more NH4SCN. This is a kind of typical operation called “passes” (the permeate of one is the RO feed to the next RO in series).32 The results are illustrated in Figure 7. As shown, both the permeate flux and salt rejection increase with the RO passes. The flux increases from 60 to 190 L · m-2 · h-1, while R of NH4SCN goes from 52.5% to 94.5%. Both of them are due to the decrease in the concentration polarization, as well as the increase in the effective
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pressure applied, i.e., the difference between the operating pressure and the osmotic pressure. In any case, by repeated operation of the process, water of any desired quality can be obtained as the product, along with concentrated solutions suitable for salt recovery. With RO passes of no more than four, 99.0% of NH4SCN can be recovered. The final permeate can be reused in the diafiltration process. 4. Conclusions An integrated membrane process (UF/NF/RO) used for the recovery of elemental sulfur and ammonium salts from the desulfurization wastewater of the HPF process was investigated in this work. The UF process provided 99.9% removal of suspended solids, and the NF process retained 95.0% of the divalent salt (NH4)2S2O3 while monovalent salt NH4SCN permeated completely. Finally diafiltration revealed that 83.0% of (NH4)2S2O3 was recovered in the retentate, whereas 99.2% of NH4SCN was recovered in the permeate. The four-pass RO process recovered 99.0% of NH4SCN, and its permeate can be reused in the diafiltration process. The proposed process provides a complete membrane treatment of the desulfurization wastewater, which not only reduces environmental loads but, more importantly, makes the recovery of three kinds of valuable substances [sulfur, (NH4)2S2O3, and NH4SCN] and the reuse of water possible. Acknowledgment This work is supported by the National Basic Research Program of China (Grant 2009CB623400), the National Natural Science Foundation of China (Grants 20636020 and 20806038), the Natural Science Foundation of Jiangsu (Grant BK2008504), the Joint Innovation Fund of Jiangsu Province (BY2009107), and the Program for New Century Excellent Talents in University (NCET-06-0506) of China. Literature Cited (1) Arthur, L. K.; Richard, B. N. Gas purification; ButterworthHeinemann Press: Oxford, U.K., 1997; pp 315, 762–764. (2) Mashapa, T. N.; Rademan, J. D.; van Vuuren, M. J. J. Catalytic performance and deactivation of precipitated iron catalyst for selective oxidation of hydrogen sulfide to elemental sulfur in the waste gas streams from coal gasification. Ind. Eng. Chem. Res. 2007, 46 (19), 6338–6344. (3) Yang, M.; Sun, Y.; Xu, A. H.; Lu, X. Y.; Du, H. Z.; Sun, C. L.; Li, C. Catalytic wet air oxidation of coke-plant wastewater on ruthenium-based eggshell catalysts in a bubbling bed reactor. Bull. EnViron. Contam. Toxicol. 2007, 79, 66–70. (4) Drioli, E.; Romano, M. Progress and new perspectives on integrated membrane operations for sustainable industrial growth. Ind. Eng. Chem. Res. 2001, 40 (5), 1277–1300. (5) Li, Z. Y.; H-Kittikun, A.; Youravong, W. Removal of suspended solids from tuna spleen extract by microfiltration: A batch process design and improvement. Biochem. Eng. J. 2008, 38, 226–233. (6) Capar, G.; Yilmaz, L.; Yetis, U. A membrane-based co-treatment strategy for the recovery of print- and beck-dyeing textile effluents. J. Hazard. Mater. 2008, 152, 316–323. (7) Russo, C. A new membrane process for the selective fractionation and total recovery of polyphenols, water and organic substances from vegetation waters (VW). J. Membr. Sci. 2007, 288 (1-2), 239–246. (8) Sudilovskiy, P. S.; Kagramanov, G. G.; Kolesnikov, V. A. Use of RO and NF for treatment of copper containing wastewaters in combination with flotation. Desalination 2008, 221, 192–201. (9) Cassano, A.; Della Pietra, L.; Drioli, E. Integrated membrane process for the recovery of chromium salts from tannery effluents. Ind. Eng. Chem. Res. 2007, 46, 6825–6830.
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ReceiVed for reView August 11, 2009 ReVised manuscript receiVed January 13, 2010 Accepted February 25, 2010 IE901267Q
