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Pretreatment of Isopropanol Solution from Pharmaceutical Industry and Pervaporation Dehydration by NaA Zeolite Membranes
SEPARATION SCIENCE AND ENGINEERING Chinese Journal of Chemical Engineering, 19(6) 904ü910 (2011)
Pretreatment of Isopropanol Solution from Pharmaceutical Industry and Pervaporation Dehydration by NaA Zeolite Membranes* YU Congli (ဥҶो), LIU Yanmei (ঞཧਜ), CHEN Gangling (чشক), GU Xuehong (ڄ༰**)܃ and XING Weihong (ฯ)܃ State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology, Nanjing 210009, China
Abstract NaA zeolite membranes with 80 cm in length and 12.8 mm in outer diameter were prepared by our research group cooperating with Nanjing Jiusi Hi-Tech Co., China. The influence of dissolved inorganic salts and pH value in the feed of isopropanol (IPA) solution on NaA zeolite membranes was investigated. It was found that both factors exhibited strong influence on the stability of NaA zeolite membranes. A set of pretreatment steps such as pH adjustment and distillation of the IPA solution were proposed to improve stability for pervaporation dehydration. An industrial-scale pervaporation facility with 52 m2 membrane area was built to dehydrate IPA solution from industrial cephalosporin production. The facility was continuously operated at 368378 K to dehydrate IPA solution from water mass content of 15%20% to less than 2% with a feed flow rate of 400500 L·h1 and an average water flux of 11.5 kg·m2·h1. The successful application of this facility suggested a promising application of NaA zeolite membrane for IPA recovery from pharmaceutical production. Keywords NaA zeolite membrane, isopropanol, dehydration, pervaporation
1
INTRODUCTION
Isopropanol (IPA) is widely used as solvent in many pharmaceutical syntheses such as cephalosporin production. However, the complexity of components in the solvent and the existence of azeotropic point for IPA and water at 12.6% water mass content under ambient pressure make the recovery of IPA very difficult. Conventional methods for separation of IPA/water mixtures include azeotropic distillation and extractive distillation, which are of high energy consumption and waste generation [1]. In past decades, pervaporation (PV) separation technology has gained great interest in many chemical industries [2]. Since only the permeant components have to be vaporized in this process, pervaporation is often considered to be an economic technology for dehydration of organics with azeotropic point [3]. Polymeric membranes are early used in organic solvent dehydration and hundreds of industrial facilities have been established. However, the applicability of such membranes is limited owing to their low thermal, mechanical and chemical stability [4]. Zeolite membranes are promising candidates to overcome these drawbacks, in which the zeolites present a class of highly ordered, porous and crystalline silica containing materials with multipurpose character and a far better stability [5]. In 1999, the first industrial facility for ethanol dehydration using NaA zeolite membranes was established by Mitsui Engineering and Shipbuilding Co., Japan [6]. Until now, there are over 100 industrial facilities based on NaA zeolite membranes
for organic solvents dehydration running in the world. For IPA dehydration, some literatures have been reported based on both polymeric and zeolite membranes [7, 8], in which NaA zeolite membranes presented excellent dehydration performance. van Hoof et al. [9] compared the IPA dehydration performance of commercial Mitsui NaA zeolite membranes and other two kinds of commercial polymeric pervaporation membranes, namely Celfa CMC–VS–11V and Sulzer Pervap 2510. The NaA zeolite membranes showed the best dehydration properties at water concentration in the azeotropic region. For recovery of IPA from pharmaceutical industry, Urtiaga et al. [10] studied the method of pervaporation using commercial polymeric membrane CMC-CF-23 from Celfa in laboratory. For a fixed water mass concentration of 10% in the feed, the water flux increased from 0.36 kg·m2·h1 at 323 K to 1.15 kg·m2·h1 at 353 K while the water mass content of the permeate increased from 86.1% to 93.4%. However, the long run stability of the membrane was not discussed. In this work, we investigated NaA zeolite membranes for dehydration of water-containing IPA from cephalosporin production. The influence of industrial feed solution properties and operating parameters on dehydration performance was evaluated. The method of pretreatment of the solution before pervaporation was developed. An industrial scale pervaporation facility based on NaA zeolite membranes was successfully built and applied in the recovery of IPA. The stability of NaA zeolite membranes in the industrial IPA solution was investigated.
Received 2010-11-11, accepted 2011-07-31. * Supported by the National Basic Research Program of China (2009CB623403), the National High Technology Research and Development Program of China (2009AA034802), the National Natural Science Foundation of China (20706030, U0834004), the Science & Technology Support Program (Industry) of Jiangsu Province of China (BE2008141), the Natural Science Foundation of the Jiangsu Higher Education Institutions (09KJA530002) and 333 High-Level Personnel Training Project in Jiangsu Province. ** To whom correspondence should be addressed. E-mail: [email protected]
Chin. J. Chem. Eng., Vol. 19, No. 6, December 2011
2 2.1
EXPERIMENTAL Membrane preparation
Industrial scale NaA zeolite membranes were hydrothermally synthesized on the surface of tubular mullite substrates with 12.8 mm outside diameter, 7.8 mm inside diameter, ca. 1 ȝm pore diameter and ca. 40% porosity, which were made by our group in cooperation with Nanjing Jiusi Hi-Tech Co., China. The substrates were seeded with NaA zeolite particles by a dip-coating technique before hydrothermal crystallization. Aluminosilicate solution for membrane synthesis was prepared by dissolving commercial chemicals of sodium aluminate and water glass in deionized water at room temperature. The detailed synthesis procedure has been reported elsewhere [11]. The as-made membranes have the thickness of about 1015 ȝm and the permeation flux between 23 kg·m2·h1 for the separation of 90%Ή10% (mass ratio) ethanol/water mixtures at 343 K. The membranes with the permeate water mass content of >80% were used for construction of industrial facility. 2.2
Pervaporation
Dehydration through NaA zeolite membranes was performed by pervaporation. For laboratory test, the membranes used in the experiments were about 7 cm long with an effective outer surface area of 1.58×103 m2. The feed pressure was larger than 0.15 MPa (gauge pressure) to make sure the liquid does not vaporize. The permeate pressure was maintained at 50200 Pa (absolute pressure) in all experiments. Two cold traps cooled with liquid nitrogen were located between the membrane module and the oil vacuum pump for collecting permeates. The schematic diagram of experimental setup is shown in Fig. 1. For an industrial facility, eight membrane modules with membrane area of 5 and 7 m2 were connected in series. The used membranes were about 80 cm long with the same diameter as laboratory test. In the PV operation, the feed pressure was larger than
Figure 1
905
0.15 MPa (gauge pressure), and the permeate pressure was between 20003000 Pa (absolute pressure). The permeate was trapped by two condensers cooled down to 20 ºC by ternary ethylene glycol/ethanol/water mixtures. The low pressure was maintained by the vacuum pump. To evaluate the separation performance of membranes, permeate water content and total flux (J) were measured in these experiments since the industrial users are more concerned about these parameters. Moreover, the industrial pervaporation process generally dehydrates solvent from high water content to low water content, which is not convenient to calculate a precise separation factor at variable feed water content. The average flux (J) is described as m (1) J Am 't where m is the mass of the permeate, kg; Am the membrane area, m2; and ǻt the time interval, h. 2.3
Characterization
The membranes were characterized by scanning electron microscopy (FESEM, S-4800II, Hitachi) and energy dispersive spectroscopy (EDS, S-4800II, Hitachi). The compositions of feed and permeate were determined by gas chromatography (GC-8A, Shimadzu) with thermal conductivity detector (TCD) and a Porapak Q packed column. The pH value was determined by pH-meter (Model 6173pH, Jenco). The electric conductivity was measured by conductometer (DDS-307, Lei-Ci, China). 3
RESULTS AND DISCUSSION
3.1 Laboratory dehydration of industrial used water-containing solvent IPA 3.1.1 Influence of inorganic salts in IPA on membrane performance Industrial used water-containing solvent IPA from cephalosporin production contains very complex
Schematic diagram of experimental setup for pervaporation
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components such as IPA, water, EtOAc, organic acids & alkalis, salts, colored substances, and high boiling point organics. The PV performance of NaA zeolite membranes for dehydration of this IPA stream was first carried out in batch mode in laboratory. The results are shown in Fig. 2, in which the results on simulated IPA stream are also included for contrast. The water-containing solvent IPA was dehydrated from water mass content of about 17% to less than 3%. The total operation time was about 30 h. It can be seen that very high water flux up to 7.3 kg·m2·h1 was achieved at a feed water mass content of 17.3% at 373 K. As the feed water content decreases, the water flux shows a declining trend, which can be explained by the decrease of driving force, the water partial pressure drop across the membrane. The maintenance of high permeate water mass content (about 94%) indicates that the membrane remained high selectivity.
(a) New membrane
(b) Membrane tested in simulated system
Figure 2 Batch mode PV results of the industrial used IPA solution (IPA feed flow rate 80 ml·min1, total operation time 30 h, T 373 K, simulated system: IPA/water mixtures prepared with analytical pure grade IPA and deionized water in laboratory) üƶü J H O ; ü ü J H O , simulated system;üƸü permeate wa2 2
(c) Membrane tested in industrial IPA solution Figure 3 SEM image of the polluted NaA zeolite membrane for batch mode PV of the industrial used IPA solution Table 1
ter content
However, compared to the simulated IPA/water mixtures prepared with analytical pure grade IPA and deionized water in laboratory, the industrial IPA had lower water flux especially after long time running with low water content in the feed. This phenomenon implied that the membrane nanopores might be blocked. SEM (scanning electron microscope) analysis was carried out on the membrane surface after IPA separation. As shown in Fig. 3, compared to new membrane (a), the membrane surface faced to industrial IPA (c) was covered by contaminates, while no contaminate was found on the membrane tested in the simulated system (b). Table 1 shows the EDS analysis results on the surfaces of a new membrane and the polluted membrane tested in industrial IPA. Some new elements such as calcium and sulfur were detected on the surface of polluted membrane, which could be attributed to sulfuric salts from IPA solution. Besides, the content of sodium ion increased largely for the polluted membrane compared with new membrane, indicating IPA solution also contained more sodium compounds. With the reduction of water content during
Element Na
Surface EDS results of a new membrane and the polluted membrane Atom/% New membrane
Polluted membrane
28.97
37.21
Al
34.55
29.75
Si
36.48
28.59
S
0
2.99
Ca
0
1.46
total
100.00
100.00
pervaporation, the solvable salts would precipitate from IPA solution and cover the entrance of membrane pores, resulting in a decreased water flux. Moreover, some cations show good ion-exchange behaviors with the sodium ions in the NaA zeolite, which would lead to change in size of pore apertures of zeolite A or transformation of zeolite structure, resulting in decreased separation performance for dehydration. For example, 3A and 5A molecular sieves could be obtained by the cation exchange of NaA with potassium and calcium, respectively [12]. A systematic
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study on divalent and trivalent ion exchange with zeolite A was reported by Wlers et al [13]. These results suggested that the salts in the solvent had a strong influence on the stability of the membranes. Pretreatment of industrial IPA is necessary to remove salts before PV dehydration. 3.1.2 Influence of pH value on membrane performance It is known that NaA zeolite structure is not stable in acid or alkaline systems due to the occurrence of de-aluminum [1416] or de-silicon [17, 18] from zeolite framework. For industrial application, it is very important to know the stability of NaA zeolite membrane in industrial used IPA solution. A distilled IPA solution with pH of ~9 was further adjusted to 69 by adding sulfuric acid in order to investigate the stability of NaA zeolite membranes. NaA zeolite membranes were immersed into the IPA solutions with different pH values at room temperature. After immersing for 100 d, the separation performance of membranes were evaluated by PV experiment with a feed of 90%Ή10% (mass ratio) ethanol/water mixtures. Time dependence of the membrane performances are shown in Fig. 4. It can be seen that the membrane performance dropped with time when immersed in the IPA solution with pH 6. The water mass content of permeate decreased from 97.8% to 82.3%. No significant change in water flux as well as permeate water content was observed for the membranes immersed in the solutions with pH 7, 8 and 9. The surface morphologies of the membranes after long time immersing are shown in Fig. 5. For pH value of 6, some defects and amorphous like materials were found on the membrane surface, indicating that the surface structure could be damaged to a certain extent. For acid feed system, the Al O bonds in NaA zeolite framework are apt to be dissociated since the acid could be acted as a catalyst for hydrolysis. As a result, the framework structure of the zeolite is destroyed, and the zeolite layer is converted to an amorphous silica-alumina layer. Sodium ions in the NaA-type zeolite may be also exchanged with hydrogen ions [16]. For membranes under pH 79 well-reserved zeolite crystals without any defects were found on the
(a) pH 6
(b) pH 7
(c) pH 8
(d) pH 9 Figure 5 SEM images of the NaA zeolite membranes after immersed in the IPA solvent with different pH values for 100 d (immersion solutions: 83% IPA, T 298 K)
membrane surfaces. These results suggest that NaA zeolite membranes are not stable for dehydration in acidic IPA systems. To improve the stability, the IPA solution should be controlled in basic environment, typically pH 79. However, too high pH (>10) is not beneficial to the stability of NaA zeolite membranes due to de-silicon of framework, which will be discussed in the industrial facility part. Figure 4 Time dependence of the PV performance of NaA zeolite membrane immersed in IPA solvent with different pH values (feed for PV: 90% ethanol, flow rate 80 ml·min1, T 343 K) pH: üƶü 6; üƻü7; üƸü8; üͪü9
3.1.3 Pretreatment method of the industrial used water-containing IPA stream Based on the above investigations, a pretreatment method was proposed to obtain appropriate IPA solution as a feed for PV dehydration. The pretreatment
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Figure 6
Pretreatment procedure of industrial used IPA stream for PV dehydration
procedure of industrial used IPA stream for PV dehydration is shown in Fig. 6. For IPA solvent S1 (IPA about 80%85%) from pharmaceutical production, a distillation step was carried out first to remove the salts and most colored substances. The distilled solution (S2) with pH 911 was then neutralized by sulfuric acid and further rectified by a plate distillation column to remove the newly formed sulfuric salts and residual impurity organics. The obtained IPA solvent (S3) had the pH value of about 78. Finally, the as-pretreated stream (S3) was preheated and introduced into the membrane modules for PV dehydration. The water mass content of S3 was about 15%20%, which is approximate to the azeotropic point. There were still some other organics such as ethanol, EtOAc, etc., which would not deteriorate membrane. The pH value and electrical conductivity of the streams from S1 to S3 are listed in Table 2. As can be seen, the IPA solution was successfully controlled at 78 by pH adjustment. The electric conductivity of the solution was reduced from >1000 to 210 for S1 to S3, which implied that the salt in the IPA solvent was effectively removed. The results suggested that appropriate IPA solution as a feed for PV dehydration can be obtained using the pretreatment method. Table 2
3.2
Properties of the IPA streams in different steps in the pretreatment procedure
Stream
pH
Electrical conductivity/ȝS·cm1
S1
68
>1000
S2
911
100200
S3
78
210
Industrial facility
3.2.1 Overview of the facility Cooperating with Nanjing Jiusi Hi-Tech Co., an industrial scale facility with a total membrane area of 52 m2 was built for IPA dehydration. The facility was equipped with 8 modules in series. It was designed for dehydration of IPA solvent from 15%20% H2O to less than 2%. The used IPA solvent from cephalosporin production was pretreated with the method mentioned above, which was then introduced into the facility with a feed flow rate of 400500 L·h1. The operating temperature was 368378 K. 3.2.2 Effect of operating parameters According to actual operation data, the average water flux was between 1.01.5 kg·m2·h1. The water fluxes at different operating parameters are shown in Fig. 7. For comparison, a 7 cm length membrane tested in laboratory was also included. It can be seen
Figure 7 Effect of water content in IPA on water flux at different operating temperatures (solid lines: industrial running, IPA feed flow rate 400500 L·h1; dashed lines: lab test, IPA feed rate 80 ml·min1) üƶü 378 K;üƻü368 K;üƸü358 K;ƶ378 K, lab test; ƻ 368 K, lab test;Ƹ358 K, lab test
that the operating temperature had a great effect on water flux. Rising temperature can significantly increase water flux, which can also be explained by the increase of water permeating driving force. However, compared to laboratory test results, the obtained industrial flux was lower, especially at high water concentration. It can be explained by the magnification effect of the membranes and the decrease of large module efficiency. The lab test could provide more uniform operating temperature and feed flow distribution. While for industrial operation, the vaporized permeates cause a large temperature drop in the module, which leads to lower temperature operation in some region of membrane modules. As a result, the modules for higher water content have a lower efficiency than the lab test results. Moreover, the temperature and concentration polarization of the large module are more severe than single channel module used in lab test. 3.2.3 The long run stability Figure 8 shows the long run performance of NaA zeolite membranes in PV using 52 m2 apparatus for industrial IPA dehydration. The raw IPA stream was first pretreated by a distillation and a rectification step without pH adjustment. The water mass content was 15%20% with a feed flow rate of 400500 L·h1 and the PV operating feed temperature was 368378 K. During the initial 600 h, the pH value of the feed was stable below 9. The water content of the permeate remained relatively stable around 80%. The pH value began to rise after about 600 h and remained above 9, meanwhile the permeate water content tended to decline. At the whole running time of about 1000 h, the permeate water mass content decreased to less than 40%, which suggested the NaA zeolite membranes in
Chin. J. Chem. Eng., Vol. 19, No. 6, December 2011
Figure 8 Effect of feed pH value on membrane performance in industrial process (inlet feed IPA mass content: 80%85%, feed rate: 400500 L·h1, operating temperature: 368378 K) ƶ pH value;ͩpermeate water content
the system were damaged by the IPA solvent. These observations clearly indicate that the higher pH value of IPA solvent has a significant effect on the life of membrane tubes. Figure 9 shows the SEM images of the used membrane. Many pinholes and zeolitic crystal boundary gaps can be seen on the surface, which implies that the structure of the membranes has been damaged at high pH value >9. ýižmek et al. pointed out that the rising temperature could accelerate the dissolve of amorphous gel embedded in NaA zeolite membrane in alkaline solvent [17], which would result in the loss of separation performance for NaA zeolite membranes. Moreover, Si element could be removed preferentially from the zeolite framework [18]. Thus, the structure of NaA zeolite membranes would be damaged in the highly basic system.
909
To improve the life of NaA zeolite membranes, the pH value of feed stream should be controlled less than 9, typically between 78. Consequently, a pH adjustment step was used to control the pH of feed stream as shown in Fig. 6. Under the pH 78 of the feed stream, the membranes showed stable permeation flux and stability with a permeate water mass content of about 80%. Fig. 10 shows the SEM images of NaA zeolite membranes (a) before and (b) after three months PV dehydration for industrial used water-containing IPA stream. New membrane has intergrowth zeolite crystals without defects. The membrane after 3 months running reserved good quality, where the intergrowth zeolitic crystals were clearly seen. The used membrane exhibited high water selectivity for the separation of 90%Ή10% (mass ratio) ethanol/water mixtures. At 343 K, the permeate water mass concentration was up to 97%. These results suggested that NaA zeolite membranes could be stable for dehydration of IPA solvent after de-salt and adjusted pH between 78. The facility has been running steadily since Sept. 2009.
(a) New membrane
(b) The used membrane (a) Surface
Figure 10 SEM images of the surface of NaA zeolite membrane
4
(b) Cross section Figure 9 SEM images of the damaged NaA zeolite membrane in industrial process
CONCLUSIONS
NaA zeolite membranes were applied for dehydration of used IPA solvent from industrial cephalosporin production. The salts dissolved in IPA and the pH value showed strong affect on stability of NaA zeolite membranes. The pretreatment of industrial used water-containing IPA stream were successfully established to improve stability of NaA zeolite membrane for PV dehydration. Dissolved salts were effectively removed by distillation of IPA. The pH adjusted between 78 could significantly improve the stability
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of PV process. Industrial PV apparatus was successfully set up for recovery of used IPA, suggesting a promising application of this technology in pharmaceutical production. REFERENCES 1
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5
6
7
Eliceche, A.M., Daviou, C.M., Hoch, P.M., Uribe, I.O., “Optimisation of azeotropic distillation columns combined with pervaporation membranes”, Comput. Chem. Eng., 26, 563573 (2002). Lipnizki, F., Field, R.W., Ten, P.K., “Pervaporation-based hybrid process: a review of process design, applications and economics”, J. Mem. Sci., 153, 183210 (1999). van Hoof, V., van den Abeele, L., Buekenhoudt, A., Dotremont, C., Leysen, R., “Economic comparison between azeotropic distillation and different hybrid systems combining distillation with pervaporation for the dehydration of isopropanol”, Sep. Purif. Technol., 37, 3349 (2004). Kondo, M., Komori, M., Kita, H., Okamoto, K., “Tubular-type pervaporation module with zeolite NaA membrane”, J. Membr. Sci., 133, 133141 (1997). Shah, D., Kissick, K., Ghorpade, A., Hannahb, R., Bhattacharyya, D., “Pervaporation of alcohol-water and dimethylformamide-water mixtures using hydrophilic zeolite NaA membranes: mechanisms and experimental results”, J. Membr. Sci., 179, 185205 (2000). Morigami, Y., Kondo, M., Abe, J., Kita, H., Okamoto, K., “The first large-scale pervaporation plant using tubular-type module with zeolite NaA membrane”, Sep. Purif. Technol., 25, 251260 (2001). Sommer, S., Melin, T., “Influence of operation parameters on the separation of mixtures by pervaporation and vapor permeation with inorganic membranes. Part 1: Dehydration of solvents”, Chem. Eng. Sci., 60, 45094523 (2005).
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Chapmana, P.D., Oliveirab, T., Livingstona, A.G., Li, K., “Membranes for the dehydration of solvents by pervaporation”, J. Membr. Sci., 318, 537 (2008). van Hoof, V., Dotremont, C., Buekenhoudt, A., “Performance of Mitsui NaA type zeolite membranes for the dehydration of organic solvents in comparison with commercial polymeric pervaporation membranes”, Sep. Purif. Technol., 48, 304309 (2006). Urtiaga, A.M., Gorri, E.D., Ortiz, I., “Pervaporative recovery of isopropanol from industrial effluents”, Sep. Purif. Technol., 49, 245252 (2006). Liu, Y.M., Yang, Z.Z., Yu, C.L., Gu, X.H., Xu, N.P., “Effect of seeding methods on growth of NaA zeolite membranes”, Micropor. Mesopor. Mater., 143, 348356 (2011). Chudasama, D.C., Sebastian, J., Jasra, V.R., “Pore-size engineering of zeolite A for the size/shape selective molecular separation”, Ind. Eng. Chem. Res., 44, 17801786 (2005). Wlers, H.B., Grosse, J.R., Cllley, A.W., “Divalent and trivalent ion exchange with zeolite A”, Environ. Sci. Technol., 16, 617624 (1982). Cui, Y., Kita, H., Okamoto, K., “Zeolite T membrane: preparation, characterization, pervaporation of water/organic mixtures and acid stability”, J. Membr. Sci., 236, 1727 (2004). Hartman, R.L., Fogler, S.H., “Reaction kinetics and mechanisms of zeolite dissolution in hydrochloric acid”, Ind. Eng. Chem. Res., 44, 77387745 (2005). Hasegawa, Y., Nagase, T., Kiyozumi, Y., Hanaoka, T., Mizukami, F., “Influence of acid on the permeation properties of NaA-type zeolite membranes”, J. Membr. Sci., 349, 189194 (2010). ýižmek, A., Komunjer, L., Subotiü, B., Široki, M., Ronceviü, S., “Kinetics of zeolite dissolution. Part 1: Dissolution of zeolite A in hot sodium hydroxide”, Zeolites, 11, 258264 (1991). Groen, J.C., Peffer, L.A.A., Moulijn, J.A., Pérez-Ramírez, J., “On the introduction of intracrystalline mesoporosity in zeolites upon desilication in alkaline medium”, Micropor. Mesopor. Mater., 69, 2934 (2004).
Pretreatment of Isopropanol Solution from Pharmaceutical Industry and Pervaporation Dehydration by NaA Zeolite Membranes* YU Congli (ဥҶो), LIU Yanmei (ঞཧਜ), CHEN Gangling (чشক), GU Xuehong (ڄ༰**)܃ and XING Weihong (ฯ)܃ State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology, Nanjing 210009, China
Abstract NaA zeolite membranes with 80 cm in length and 12.8 mm in outer diameter were prepared by our research group cooperating with Nanjing Jiusi Hi-Tech Co., China. The influence of dissolved inorganic salts and pH value in the feed of isopropanol (IPA) solution on NaA zeolite membranes was investigated. It was found that both factors exhibited strong influence on the stability of NaA zeolite membranes. A set of pretreatment steps such as pH adjustment and distillation of the IPA solution were proposed to improve stability for pervaporation dehydration. An industrial-scale pervaporation facility with 52 m2 membrane area was built to dehydrate IPA solution from industrial cephalosporin production. The facility was continuously operated at 368378 K to dehydrate IPA solution from water mass content of 15%20% to less than 2% with a feed flow rate of 400500 L·h1 and an average water flux of 11.5 kg·m2·h1. The successful application of this facility suggested a promising application of NaA zeolite membrane for IPA recovery from pharmaceutical production. Keywords NaA zeolite membrane, isopropanol, dehydration, pervaporation
1
INTRODUCTION
Isopropanol (IPA) is widely used as solvent in many pharmaceutical syntheses such as cephalosporin production. However, the complexity of components in the solvent and the existence of azeotropic point for IPA and water at 12.6% water mass content under ambient pressure make the recovery of IPA very difficult. Conventional methods for separation of IPA/water mixtures include azeotropic distillation and extractive distillation, which are of high energy consumption and waste generation [1]. In past decades, pervaporation (PV) separation technology has gained great interest in many chemical industries [2]. Since only the permeant components have to be vaporized in this process, pervaporation is often considered to be an economic technology for dehydration of organics with azeotropic point [3]. Polymeric membranes are early used in organic solvent dehydration and hundreds of industrial facilities have been established. However, the applicability of such membranes is limited owing to their low thermal, mechanical and chemical stability [4]. Zeolite membranes are promising candidates to overcome these drawbacks, in which the zeolites present a class of highly ordered, porous and crystalline silica containing materials with multipurpose character and a far better stability [5]. In 1999, the first industrial facility for ethanol dehydration using NaA zeolite membranes was established by Mitsui Engineering and Shipbuilding Co., Japan [6]. Until now, there are over 100 industrial facilities based on NaA zeolite membranes
for organic solvents dehydration running in the world. For IPA dehydration, some literatures have been reported based on both polymeric and zeolite membranes [7, 8], in which NaA zeolite membranes presented excellent dehydration performance. van Hoof et al. [9] compared the IPA dehydration performance of commercial Mitsui NaA zeolite membranes and other two kinds of commercial polymeric pervaporation membranes, namely Celfa CMC–VS–11V and Sulzer Pervap 2510. The NaA zeolite membranes showed the best dehydration properties at water concentration in the azeotropic region. For recovery of IPA from pharmaceutical industry, Urtiaga et al. [10] studied the method of pervaporation using commercial polymeric membrane CMC-CF-23 from Celfa in laboratory. For a fixed water mass concentration of 10% in the feed, the water flux increased from 0.36 kg·m2·h1 at 323 K to 1.15 kg·m2·h1 at 353 K while the water mass content of the permeate increased from 86.1% to 93.4%. However, the long run stability of the membrane was not discussed. In this work, we investigated NaA zeolite membranes for dehydration of water-containing IPA from cephalosporin production. The influence of industrial feed solution properties and operating parameters on dehydration performance was evaluated. The method of pretreatment of the solution before pervaporation was developed. An industrial scale pervaporation facility based on NaA zeolite membranes was successfully built and applied in the recovery of IPA. The stability of NaA zeolite membranes in the industrial IPA solution was investigated.
Received 2010-11-11, accepted 2011-07-31. * Supported by the National Basic Research Program of China (2009CB623403), the National High Technology Research and Development Program of China (2009AA034802), the National Natural Science Foundation of China (20706030, U0834004), the Science & Technology Support Program (Industry) of Jiangsu Province of China (BE2008141), the Natural Science Foundation of the Jiangsu Higher Education Institutions (09KJA530002) and 333 High-Level Personnel Training Project in Jiangsu Province. ** To whom correspondence should be addressed. E-mail: [email protected]
Chin. J. Chem. Eng., Vol. 19, No. 6, December 2011
2 2.1
EXPERIMENTAL Membrane preparation
Industrial scale NaA zeolite membranes were hydrothermally synthesized on the surface of tubular mullite substrates with 12.8 mm outside diameter, 7.8 mm inside diameter, ca. 1 ȝm pore diameter and ca. 40% porosity, which were made by our group in cooperation with Nanjing Jiusi Hi-Tech Co., China. The substrates were seeded with NaA zeolite particles by a dip-coating technique before hydrothermal crystallization. Aluminosilicate solution for membrane synthesis was prepared by dissolving commercial chemicals of sodium aluminate and water glass in deionized water at room temperature. The detailed synthesis procedure has been reported elsewhere [11]. The as-made membranes have the thickness of about 1015 ȝm and the permeation flux between 23 kg·m2·h1 for the separation of 90%Ή10% (mass ratio) ethanol/water mixtures at 343 K. The membranes with the permeate water mass content of >80% were used for construction of industrial facility. 2.2
Pervaporation
Dehydration through NaA zeolite membranes was performed by pervaporation. For laboratory test, the membranes used in the experiments were about 7 cm long with an effective outer surface area of 1.58×103 m2. The feed pressure was larger than 0.15 MPa (gauge pressure) to make sure the liquid does not vaporize. The permeate pressure was maintained at 50200 Pa (absolute pressure) in all experiments. Two cold traps cooled with liquid nitrogen were located between the membrane module and the oil vacuum pump for collecting permeates. The schematic diagram of experimental setup is shown in Fig. 1. For an industrial facility, eight membrane modules with membrane area of 5 and 7 m2 were connected in series. The used membranes were about 80 cm long with the same diameter as laboratory test. In the PV operation, the feed pressure was larger than
Figure 1
905
0.15 MPa (gauge pressure), and the permeate pressure was between 20003000 Pa (absolute pressure). The permeate was trapped by two condensers cooled down to 20 ºC by ternary ethylene glycol/ethanol/water mixtures. The low pressure was maintained by the vacuum pump. To evaluate the separation performance of membranes, permeate water content and total flux (J) were measured in these experiments since the industrial users are more concerned about these parameters. Moreover, the industrial pervaporation process generally dehydrates solvent from high water content to low water content, which is not convenient to calculate a precise separation factor at variable feed water content. The average flux (J) is described as m (1) J Am 't where m is the mass of the permeate, kg; Am the membrane area, m2; and ǻt the time interval, h. 2.3
Characterization
The membranes were characterized by scanning electron microscopy (FESEM, S-4800II, Hitachi) and energy dispersive spectroscopy (EDS, S-4800II, Hitachi). The compositions of feed and permeate were determined by gas chromatography (GC-8A, Shimadzu) with thermal conductivity detector (TCD) and a Porapak Q packed column. The pH value was determined by pH-meter (Model 6173pH, Jenco). The electric conductivity was measured by conductometer (DDS-307, Lei-Ci, China). 3
RESULTS AND DISCUSSION
3.1 Laboratory dehydration of industrial used water-containing solvent IPA 3.1.1 Influence of inorganic salts in IPA on membrane performance Industrial used water-containing solvent IPA from cephalosporin production contains very complex
Schematic diagram of experimental setup for pervaporation
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components such as IPA, water, EtOAc, organic acids & alkalis, salts, colored substances, and high boiling point organics. The PV performance of NaA zeolite membranes for dehydration of this IPA stream was first carried out in batch mode in laboratory. The results are shown in Fig. 2, in which the results on simulated IPA stream are also included for contrast. The water-containing solvent IPA was dehydrated from water mass content of about 17% to less than 3%. The total operation time was about 30 h. It can be seen that very high water flux up to 7.3 kg·m2·h1 was achieved at a feed water mass content of 17.3% at 373 K. As the feed water content decreases, the water flux shows a declining trend, which can be explained by the decrease of driving force, the water partial pressure drop across the membrane. The maintenance of high permeate water mass content (about 94%) indicates that the membrane remained high selectivity.
(a) New membrane
(b) Membrane tested in simulated system
Figure 2 Batch mode PV results of the industrial used IPA solution (IPA feed flow rate 80 ml·min1, total operation time 30 h, T 373 K, simulated system: IPA/water mixtures prepared with analytical pure grade IPA and deionized water in laboratory) üƶü J H O ; ü ü J H O , simulated system;üƸü permeate wa2 2
(c) Membrane tested in industrial IPA solution Figure 3 SEM image of the polluted NaA zeolite membrane for batch mode PV of the industrial used IPA solution Table 1
ter content
However, compared to the simulated IPA/water mixtures prepared with analytical pure grade IPA and deionized water in laboratory, the industrial IPA had lower water flux especially after long time running with low water content in the feed. This phenomenon implied that the membrane nanopores might be blocked. SEM (scanning electron microscope) analysis was carried out on the membrane surface after IPA separation. As shown in Fig. 3, compared to new membrane (a), the membrane surface faced to industrial IPA (c) was covered by contaminates, while no contaminate was found on the membrane tested in the simulated system (b). Table 1 shows the EDS analysis results on the surfaces of a new membrane and the polluted membrane tested in industrial IPA. Some new elements such as calcium and sulfur were detected on the surface of polluted membrane, which could be attributed to sulfuric salts from IPA solution. Besides, the content of sodium ion increased largely for the polluted membrane compared with new membrane, indicating IPA solution also contained more sodium compounds. With the reduction of water content during
Element Na
Surface EDS results of a new membrane and the polluted membrane Atom/% New membrane
Polluted membrane
28.97
37.21
Al
34.55
29.75
Si
36.48
28.59
S
0
2.99
Ca
0
1.46
total
100.00
100.00
pervaporation, the solvable salts would precipitate from IPA solution and cover the entrance of membrane pores, resulting in a decreased water flux. Moreover, some cations show good ion-exchange behaviors with the sodium ions in the NaA zeolite, which would lead to change in size of pore apertures of zeolite A or transformation of zeolite structure, resulting in decreased separation performance for dehydration. For example, 3A and 5A molecular sieves could be obtained by the cation exchange of NaA with potassium and calcium, respectively [12]. A systematic
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study on divalent and trivalent ion exchange with zeolite A was reported by Wlers et al [13]. These results suggested that the salts in the solvent had a strong influence on the stability of the membranes. Pretreatment of industrial IPA is necessary to remove salts before PV dehydration. 3.1.2 Influence of pH value on membrane performance It is known that NaA zeolite structure is not stable in acid or alkaline systems due to the occurrence of de-aluminum [1416] or de-silicon [17, 18] from zeolite framework. For industrial application, it is very important to know the stability of NaA zeolite membrane in industrial used IPA solution. A distilled IPA solution with pH of ~9 was further adjusted to 69 by adding sulfuric acid in order to investigate the stability of NaA zeolite membranes. NaA zeolite membranes were immersed into the IPA solutions with different pH values at room temperature. After immersing for 100 d, the separation performance of membranes were evaluated by PV experiment with a feed of 90%Ή10% (mass ratio) ethanol/water mixtures. Time dependence of the membrane performances are shown in Fig. 4. It can be seen that the membrane performance dropped with time when immersed in the IPA solution with pH 6. The water mass content of permeate decreased from 97.8% to 82.3%. No significant change in water flux as well as permeate water content was observed for the membranes immersed in the solutions with pH 7, 8 and 9. The surface morphologies of the membranes after long time immersing are shown in Fig. 5. For pH value of 6, some defects and amorphous like materials were found on the membrane surface, indicating that the surface structure could be damaged to a certain extent. For acid feed system, the Al O bonds in NaA zeolite framework are apt to be dissociated since the acid could be acted as a catalyst for hydrolysis. As a result, the framework structure of the zeolite is destroyed, and the zeolite layer is converted to an amorphous silica-alumina layer. Sodium ions in the NaA-type zeolite may be also exchanged with hydrogen ions [16]. For membranes under pH 79 well-reserved zeolite crystals without any defects were found on the
(a) pH 6
(b) pH 7
(c) pH 8
(d) pH 9 Figure 5 SEM images of the NaA zeolite membranes after immersed in the IPA solvent with different pH values for 100 d (immersion solutions: 83% IPA, T 298 K)
membrane surfaces. These results suggest that NaA zeolite membranes are not stable for dehydration in acidic IPA systems. To improve the stability, the IPA solution should be controlled in basic environment, typically pH 79. However, too high pH (>10) is not beneficial to the stability of NaA zeolite membranes due to de-silicon of framework, which will be discussed in the industrial facility part. Figure 4 Time dependence of the PV performance of NaA zeolite membrane immersed in IPA solvent with different pH values (feed for PV: 90% ethanol, flow rate 80 ml·min1, T 343 K) pH: üƶü 6; üƻü7; üƸü8; üͪü9
3.1.3 Pretreatment method of the industrial used water-containing IPA stream Based on the above investigations, a pretreatment method was proposed to obtain appropriate IPA solution as a feed for PV dehydration. The pretreatment
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Figure 6
Pretreatment procedure of industrial used IPA stream for PV dehydration
procedure of industrial used IPA stream for PV dehydration is shown in Fig. 6. For IPA solvent S1 (IPA about 80%85%) from pharmaceutical production, a distillation step was carried out first to remove the salts and most colored substances. The distilled solution (S2) with pH 911 was then neutralized by sulfuric acid and further rectified by a plate distillation column to remove the newly formed sulfuric salts and residual impurity organics. The obtained IPA solvent (S3) had the pH value of about 78. Finally, the as-pretreated stream (S3) was preheated and introduced into the membrane modules for PV dehydration. The water mass content of S3 was about 15%20%, which is approximate to the azeotropic point. There were still some other organics such as ethanol, EtOAc, etc., which would not deteriorate membrane. The pH value and electrical conductivity of the streams from S1 to S3 are listed in Table 2. As can be seen, the IPA solution was successfully controlled at 78 by pH adjustment. The electric conductivity of the solution was reduced from >1000 to 210 for S1 to S3, which implied that the salt in the IPA solvent was effectively removed. The results suggested that appropriate IPA solution as a feed for PV dehydration can be obtained using the pretreatment method. Table 2
3.2
Properties of the IPA streams in different steps in the pretreatment procedure
Stream
pH
Electrical conductivity/ȝS·cm1
S1
68
>1000
S2
911
100200
S3
78
210
Industrial facility
3.2.1 Overview of the facility Cooperating with Nanjing Jiusi Hi-Tech Co., an industrial scale facility with a total membrane area of 52 m2 was built for IPA dehydration. The facility was equipped with 8 modules in series. It was designed for dehydration of IPA solvent from 15%20% H2O to less than 2%. The used IPA solvent from cephalosporin production was pretreated with the method mentioned above, which was then introduced into the facility with a feed flow rate of 400500 L·h1. The operating temperature was 368378 K. 3.2.2 Effect of operating parameters According to actual operation data, the average water flux was between 1.01.5 kg·m2·h1. The water fluxes at different operating parameters are shown in Fig. 7. For comparison, a 7 cm length membrane tested in laboratory was also included. It can be seen
Figure 7 Effect of water content in IPA on water flux at different operating temperatures (solid lines: industrial running, IPA feed flow rate 400500 L·h1; dashed lines: lab test, IPA feed rate 80 ml·min1) üƶü 378 K;üƻü368 K;üƸü358 K;ƶ378 K, lab test; ƻ 368 K, lab test;Ƹ358 K, lab test
that the operating temperature had a great effect on water flux. Rising temperature can significantly increase water flux, which can also be explained by the increase of water permeating driving force. However, compared to laboratory test results, the obtained industrial flux was lower, especially at high water concentration. It can be explained by the magnification effect of the membranes and the decrease of large module efficiency. The lab test could provide more uniform operating temperature and feed flow distribution. While for industrial operation, the vaporized permeates cause a large temperature drop in the module, which leads to lower temperature operation in some region of membrane modules. As a result, the modules for higher water content have a lower efficiency than the lab test results. Moreover, the temperature and concentration polarization of the large module are more severe than single channel module used in lab test. 3.2.3 The long run stability Figure 8 shows the long run performance of NaA zeolite membranes in PV using 52 m2 apparatus for industrial IPA dehydration. The raw IPA stream was first pretreated by a distillation and a rectification step without pH adjustment. The water mass content was 15%20% with a feed flow rate of 400500 L·h1 and the PV operating feed temperature was 368378 K. During the initial 600 h, the pH value of the feed was stable below 9. The water content of the permeate remained relatively stable around 80%. The pH value began to rise after about 600 h and remained above 9, meanwhile the permeate water content tended to decline. At the whole running time of about 1000 h, the permeate water mass content decreased to less than 40%, which suggested the NaA zeolite membranes in
Chin. J. Chem. Eng., Vol. 19, No. 6, December 2011
Figure 8 Effect of feed pH value on membrane performance in industrial process (inlet feed IPA mass content: 80%85%, feed rate: 400500 L·h1, operating temperature: 368378 K) ƶ pH value;ͩpermeate water content
the system were damaged by the IPA solvent. These observations clearly indicate that the higher pH value of IPA solvent has a significant effect on the life of membrane tubes. Figure 9 shows the SEM images of the used membrane. Many pinholes and zeolitic crystal boundary gaps can be seen on the surface, which implies that the structure of the membranes has been damaged at high pH value >9. ýižmek et al. pointed out that the rising temperature could accelerate the dissolve of amorphous gel embedded in NaA zeolite membrane in alkaline solvent [17], which would result in the loss of separation performance for NaA zeolite membranes. Moreover, Si element could be removed preferentially from the zeolite framework [18]. Thus, the structure of NaA zeolite membranes would be damaged in the highly basic system.
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To improve the life of NaA zeolite membranes, the pH value of feed stream should be controlled less than 9, typically between 78. Consequently, a pH adjustment step was used to control the pH of feed stream as shown in Fig. 6. Under the pH 78 of the feed stream, the membranes showed stable permeation flux and stability with a permeate water mass content of about 80%. Fig. 10 shows the SEM images of NaA zeolite membranes (a) before and (b) after three months PV dehydration for industrial used water-containing IPA stream. New membrane has intergrowth zeolite crystals without defects. The membrane after 3 months running reserved good quality, where the intergrowth zeolitic crystals were clearly seen. The used membrane exhibited high water selectivity for the separation of 90%Ή10% (mass ratio) ethanol/water mixtures. At 343 K, the permeate water mass concentration was up to 97%. These results suggested that NaA zeolite membranes could be stable for dehydration of IPA solvent after de-salt and adjusted pH between 78. The facility has been running steadily since Sept. 2009.
(a) New membrane
(b) The used membrane (a) Surface
Figure 10 SEM images of the surface of NaA zeolite membrane
4
(b) Cross section Figure 9 SEM images of the damaged NaA zeolite membrane in industrial process
CONCLUSIONS
NaA zeolite membranes were applied for dehydration of used IPA solvent from industrial cephalosporin production. The salts dissolved in IPA and the pH value showed strong affect on stability of NaA zeolite membranes. The pretreatment of industrial used water-containing IPA stream were successfully established to improve stability of NaA zeolite membrane for PV dehydration. Dissolved salts were effectively removed by distillation of IPA. The pH adjusted between 78 could significantly improve the stability
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of PV process. Industrial PV apparatus was successfully set up for recovery of used IPA, suggesting a promising application of this technology in pharmaceutical production. REFERENCES 1
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