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Synthesis of polyaluminum chloride with a membrane reactor: parameters optimization for the in situ synthesis
Journal of Membrane Science 247 (2005) 221–226
Synthesis of polyaluminum chloride with a membrane reactor: parameters optimization for the in situ synthesis Fei He, Zhiqian Jia, Peijing Wang, Zhongzhou Liu∗ Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, P.O. Box 2871, Beijing, PR China Received 13 May 2004; received in revised form 15 May 2004; accepted 4 October 2004 Available online 21 November 2004
Abstract Polyaluminum chloride (PAC), containing about 70% Alb , with the Al concentration (0.35 mol/L) satisfying the in situ water treatment, was synthesized efficiently with a membrane reactor. The effects of the membrane molecular weight cut-off (MWCO), the membrane modules lengths and the reaction temperatures on the species distribution, pH values, pressure drops and flow rates of aluminum solutions were studied. It is found that lower membrane MWCO, shorter membrane module length and higher temperature are favorable for the synthesis of PAC with high Alb content. © 2004 Published by Elsevier B.V. Keywords: Synthesis; Polyaluminum chloride; Membrane reactor; In situ; Optimization
1. Introduction Polyaluminium chloride (PAC) is an important flocculant in water treatment. The species in PAC include three parts: “Ala ” (mononuclear Al, including Al3+ , Al(OH)2+ , Al(OH)2 + and Al(OH)4 − ), “Alb ” (medium polymer species), and “Alc ” (larger polynuclears and/or Al(OH)3 ) [1–3]. Among them, Alb species correspond to Al13 ([AlO4 Al12 (OH)(24+n) (H2 O)(12−n) ](7−n)+ ) in freshly prepared solutions [4], and are usually regarded as the performance flocculant due to their higher positive charge and stability. In order to improve the Alb content, a membrane reactor was employed for synthesizing PAC in our previous paper [5,6], in which NaOH solution permeated gradually through the micropores of hollow fiber ultrafiltration (UF) membranes into the fiber lumens under the transmembrane pressure and reacted with AlCl3 solution. Since the mi-
∗
Corresponding author. E-mail addresses: [email protected] (F. He), [email protected] (Z. Liu). 0376-7388/$ – see front matter © 2004 Published by Elsevier B.V. doi:10.1016/j.memsci.2004.10.009
cropores of UF membranes are on the nanometer scale, the droplets sizes of NaOH solutions are dramatically reduced; due to a larger contact interface between the strong base drops and Al solution, more Al(OH)4 − ions are presumed to be formed [7], leading to the increase in the Alb content. We have previously shown that the PAC products synthesized with the membrane reactor contain more Alb (Alb > 70%, at basicity of about 2.2) and show better flocculation effects than some commercial products [5]. However, it usually took about 3–4 h to prepare 220 mL PAC solutions, and the product concentration (0.14 mol/L) was also somewhat low for use in water treatment (0.25–0.5 mol/L). In order to industrialize the synthesis, there is a need to shorten the preparation time and to increase the product concentration. However, this is not an easy task. As described in our former paper [6], a lower injection rate of OH− and a lower reactant concentration favor the formation of Alb species. Fortunately, it was found that bi-directional flow significantly reduces the membrane fouling [6]. In this paper, the effects of the membrane molecular weight cut-off (MWCO), the membrane module length and the reaction temperature on the species distribution and process parameters are investigated.
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Table 1 Membrane module Membrane module
Material
MWCO
Internal fiber diameter (mm)
Length (m)
Number of fibers
Ia
PS/PDC PS/PDC PS PS
10,000 30,000 10,000 10,000
1.0 1.0 0.8 0.8
0.130 0.130 0.130 0.260
10 10 14 7
IIa IIIb IVb
a Made in Research Center for Eco-Environmental Sciences, Beijing, China. PS/PDC means a blend of polysulphone and PDC (polysulfone with phenolphthalein group). b Made in Motian Ltd., Tianjin, China.
We show that PAC products with high concentration and Alb content can be synthesized in a short time.
2. Materials and methods 2.1. Membrane modules The membrane modules used in this paper are listed in Table 1.
2.3. Analysis of Al species As described in the literature [1], the aluminum-Ferron complexation timed spectrophotometry can be used to determine the concentration of aluminum species. The reaction time for Ala -Ferron was 1 min, and the Al species that reacted with the Ferron reagent before 120 min represented [Ala + Alb ]. The total Al [AlT ] minus Ala and Alb was taken as Alc .
2.2. Preparation of PAC To obtain steady permeation fluxes, a constant flow pump (Xingda LP Pulseless Pump, Satellite Manufactory of Beijing, China) was employed to inject NaOH solution into the lumens of a UF membrane at a constant rate (0.01–9.99 mL/min). At the same time, AlCl3 solution in the stirred tank, with an initial volume of 150 mL, was recycled through the lumens of the UF membrane by a BT00-300M peristaltic pump (Lan’ge Instrument Company of Baoding, China) and reacted with the NaOH to form PAC. Valves A–D were used to change the flow direction of the AlCl3 solution (see Fig. 1). The positive flow meant A → B → membrane module → C → D, and the negative flow meant A → E → C → membrane module → B → F → D. When the flow rate dropped, the negative flow was conducted for 1 min per 30–60 min according to the flow rate of solution.
3. Results and discussion 3.1. Effects of MWCO The membranes of MWCO 10,000 and 30,000 with equal membrane area and inner fiber diameter (modules I and II in Table 1) were employed to synthesize PAC. The other conditions were as follows: the permeation flux (J) was 0.3 mL/min; the initial flow rate of AlCl3 solution in the membrane lumens (u0 ) was 1.06 m/s; and the temperature (T) was 290 K. In the first group of experiments, the concentration of reactants AlCl3 (CA0 ) and NaOH (CB0 ) were 0.10 and 0.50 mol/L, respectively. The results of the species distribution (Fig. 2a) show that during the reaction, the proportion of Al present as the Ala species decreases almost in parallel. There exists
Fig. 1. Schematic of experimental apparatus.
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Fig. 2. Effects of MWCO (CA0 = 0.1, CB0 = 0.5 mol/L): (a) species distribution vs. B; (b) pH vs. B; and (c) flow rate vs. B. Here, u0 = 1.06 m s−1 , J = 0.3 mL/min, T = 290 K. () MWCO = 10,000 and () MWCO = 30,000.
a cross-point of the two Alb curves at a basicity (B)1 of about 2. The Alc content is observed to increase markedly in both cases after B = 1.5. For MWCO = 30,000, the maximum Alb content is 70% at B = 2. For MWCO = 10,000, more Ala species and less Alc species appear at the same B, and the maximum Alb content is attained 77% at B = 2.3. When the concentrations of AlCl3 and NaOH solution were increased to 0.4 and 1.0 mol/L, respectively, the changes in species distribution (Fig. 3) are similar to that shown in Fig. 2a, except that the maximum content of Alb species (60%) is relatively lower than that obtained at low concentration. The pH–B curve (Fig. 2b) shows that, for MWCO = 10,000, the polymerization process is maintained to a higher basicity than for MWCO = 30,000 [5], which is in accordance with Fig. 2a. Fig. 2c shows that the flow rate for MWCO = 10,000 never drops during the synthesis, and no flushing was needed in this experiment. Therefore, it can be concluded that the lower MWCO membrane is favorable for both increments in the maximum Alb content and inhibition of the membrane fouling. The differences in the species distribution for the two experiments are mainly attributed to the difference in the droplet size of added base. The ratio (c) of the surface area
1
Basicity (B) is defined as molar ratio of NaOH/AlT .
of base drops permeating the membrane of MWCO = 10,000 to MWCO = 30,000 can be described as follows: c=
n1 4πr12 n2 4πr22
(1)
where, n1 and n2 are the number of base drops passing through the membranes of MWCO = 10,000 and 30,000, respectively; r1 and r2 are the average radius of base drops,
Fig. 3. Effects of MWCO (CA0 = 0.4, CB0 = 1.0 mol/L): species distribution vs. B. Here, u0 = 1.06 m/s, J = 0.3 mL/min, T = 290 K. (䊉) MWCO = 10,000 and (–) MWCO = 30,000.
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correspondingly. At the same base injection rate: 4 3 4 3 n1 πr1 = n2 πr2 3 3 c=
r2 >1 r1
the two curves of Alb content arises because of these effects. (2) 3.2. Effects of the length of the membrane module (3)
Thus, in the absence of any diffusion effects, the base droplets permeating through the 10,000 MWCO membrane have a larger “instantaneous” surface area than those of the 30,000 MWCO membrane. As we know, the formation of Al(OH)4 − , the precursor of Al13 , needs a suitable, but not too high, localized basicity [2,8]. The larger base drops lead to higher localized concentration of OH− , which favors the formation of Al(OH)4 − and Alb at the beginning of the reaction. However, with the permeation of OH− , the pH value in the bulk solution increases, and the localized concentration of OH− is high enough to form Al(OH)3 precipitates. Consequently, due to the gradual transformation of Ala to Alb , the solution viscosity increases and the diffusion rate of ions decreases. The larger base drops provide a smaller OH− –Al3+ interface than the smaller drops. The amount of the precursor and Al13 that is subsequently formed with the 30,000 MWCO membranes is smaller than that formed with the 10,000 MWCO membranes. The cross point in
In this group of experiments, membrane modules III and IV, as described in Table 1, were used respectively. The modules are different in length but the same in total membrane area. The other operating conditions were as follows: J = 0.3 mL/min; u0 = 1.18 m/s; CA0 = 0.20 mol/L; CB0 = 1.0 mol/L; and T = 303 K. Fig. 4a shows that there exists some differences in the species distribution when B > 1.5. For example, the content of Alc species is lower, and the maximum content of Alb species is larger for the short module than that for the long one. At the point of maximum Alb content, there are 19.7% Ala , 71.7% Alb and 8.6% Alc (B = 2.1) for the short module; while there are 14.4% Ala , 67.1% Alb and 18.5% Alc (B = 2) for the long one. This above result is in accordance with the pH–B plot (Fig. 4b). When B > 2, the pH values for the short module increase much more slowly, indicating that the polymerization reaction lasts longer [5]. Fig. 4c shows that the pressure drops of the long module are larger than that of the short module throughout the synthesis. The pressure drop reflects the resistance of inner
Fig. 4. Effects of the module lengths: (a) species distribution vs. B; (b) pH vs. B; (c) pressure drop vs. B; and (d) flow rate vs. B. Here, J = 0.3 mL/min, u0 = 1.18 m/s, CA0 = 0.20 mol/L, CB0 = 1.0 mol/L, T = 303 K. () The long module and () the short module.
F. He et al. / Journal of Membrane Science 247 (2005) 221–226
walls of the membrane lumens to the flowing solution and increases with increasing module length [9]: 2 l u p = −λ ρ (4) d 2 where λ is the friction coefficient, l is the length of the module, d is the diameter of the fibers, ρ is the solution density, and u is the flow rate of Al-solution. For the different length of modules, the average retention time (t) of the Al-solution in the lumens and the average molar ratio of OH− to aluminum (MOH /MAl ) in the out flow are different, i.e., t=
l u
JCB0 4JCB0 CB0 + CA0 B MOH = = MAl nπd 2 /4uCA πd 2 CA0 CB0 nu
225
tent of Alb species (at B = 2.25) reaches 60%, 63% and 80%, respectively (Fig. 5b). This result shows that the higher temperature favors the formation of Alb species. One of the reasons is that, when the temperature rises, the viscosity of the solution decreases and the Brownian motion induced diffusion is enhanced. Besides, the higher temperature favors the dissolution of precipitates, which is beneficial for the micromixing of base and Al-solution in the membrane lumens. Meanwhile, the hydrolysis process to produce H+ is enhanced by the higher temperature, which lowers the localized supersaturation and inhibits the formation of Al(OH)3 . Therefore, more Alb species are formed at higher temperatures.
(5)
3.4. Synthesis of PAC at high-concentration
(6)
Based on the above discussion, smaller MWCO membranes, shorter membrane module length and higher temperature are favorable for the synthesis of PAC exhibiting a larger proportion of Alb species. Therefore, to prepare PAC with higher Al-concentration and Alb content in a shorter reaction time, membrane module I (MWCO = 10,000, 0.13 m in length) was used, and the reaction temperature was controlled to be about 305–307 K. The other conditions were as follows: CA0 = 0.5 mol/L, CB0 = 2.5 mol/L, J = 1.0 mL/min and u0 = 1.09 m/s. Using bi-directional flow, a PAC product with B = 2.1 was synthesized within 75 min. The final product, with an AlT -
In this group of experiments, the flow rates of Al-solution are similar (Fig. 4d), so the retention time of Al-solution in the lumens and MOH /MAl in the out flow (at a certain B) for the long module are twice those for the short module. In the membrane module, the permeate base solution moves the hydrolysis to the right: Al3+ + OH− ↔ Al(OH)2+ Al(OH)2+ + OH− ↔ Al(OH)2 + Al(OH)2 + + OH− ↔ Al(OH)3 Al(OH)3 + OH− ↔ Al(OH)4 − ... After returns to the stirred tank, the out flow solution mixes with the acidic bulk Al solution. When B is low, the uncomplexed OH− ions in the out flow solution are neutralized quickly by the acidic bulk, so the differences in the species distribution and pH values for the long and short modules are not apparent. With increasing B value, the localized basicity is high enough to form Al(OH)3 precipitates, and the micromixing condition in the lumens deteriorates greatly, so the content of Alb begins to drop markedly. This is the reason for the significant difference in the species distribution that is apparent after B = 2. 3.3. Effects of temperature The reaction temperature was altered to be 291, 297 and 305 K, respectively, under the conditions of CA0 = 0.40 mol/L, CB0 = 2.0 mol/L, P0 = 0.0090 MPa, and u0 = 1.09 m/s. In this group of experiments, membrane module I was used. Because the hydrolysis of Al3+ is an endothermic reaction, the elevated temperature induces the hydrolysis to move to the right. Thus, at the same basicity, the pH decreases with increasing temperature (Fig. 5a). For temperatures of 291, 297 and 305 K, Alc species are apparent at B = 0.65, 1.0 and 1.4, and the maximum con-
Fig. 5. Effects of temperature: (a) pH vs. B and (b) species distribution vs. B. Here, MWCO = 10,000, CA0 = 0.40 mol/L, CB0 = 2.0 mol/L, P0 = 0.0090 MPa. () T = 291 K, () T = 297 K, () T = 305 K.
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concentration of 0.35 mol/L, is clear and transparent, with no precipitates apparent after 24 h aging. The species distribution of PAC is Ala = 22%, Alb = 70% and Alc = 8% at B = 2.1. This product concentration satisfies the requirement for in situ wastewater treatment.
n2
4. Conclusion
r2
It is found that smaller membrane MWCO, shorter module length and higher temperature are all favorable for the formation of Alb species. On the basis of these results, with bi-directional flow operation, PAC products (B = 2.1) containing Al-concentration of 0.35 mol/L and Alb content of 70%, have been successfully synthesized within 75 min at 305–307 K.
t
Acknowledgements
p r1
T u u0
number of base drops permeating the membrane of MWCO = 30,000 pressure drop (MPa) average radius of base drops permeating the membrane of MWCO = 10,000 average radius of base drops permeating the membrane of MWCO = 30,000 average retention time of Al-solution in lumens (s) temperature (K) flow rate of A1Cl3 solution in the membrane lumens (m/s) initial flow rate of A1Cl3 solution in the membrane lumens (m/s)
Greek letters λ friction coefficient ρ solution density (kg/m3 )
The authors are grateful for financial support from the national 863 High-tech Project 2002AA 601290, China. References Nomenclature B c
CA0 CB0 d J l MAl MOH n n1
basicity, molar ratio of OH/Al (i.e., NaOH/total aluminum) ratio of the surface area of base drops permeating the membrane of MWCO = 10,000 to MWCO = 30,000 initial concentration of A1Cl3 solution (mol/L) concentration of NaOH solution (mol/L) diameter of the hollow fiber membrane (m) permeation flux of NaOH of total membrane area (mL/min) length of a module (m) molar of aluminum at the outlet of lumens (mol) molar of OH− at the outlet of lumens (mol) number of fibers in a module number of base drops permeating the membrane of MWCO = 10,000
[1] J.Y. Bottero, J.M. Cases, et al., Studies of hydrolyzed aluminum chloride solutions. 1. Nature of aluminum species and composition of aqueous solutions, J. Phys. Chem. 84 (1980) 2933–2939. [2] J.W. Akitt, et al., New 27 Al NMR studies of the hydrolysis of the aluminium(III) cation, J. Magn. Reson. 32 (1978) 345–349. [3] P.M. Bertsch, W.J. Layton, R.I. Barnhisel, Speciation of hydroxyl aluminum solutions by wet chemical and Al-27 NMR methods, Soil Sci. Soc. Am. J. 50 (1986) 1449–1454. [4] J.W. Akitt, A. Farthing, Aluminum-27 NMR studies of the hydrolysis of aluminum(III). Part 4. Hydrolysis using sodium carbonate, J. C. S. Dalton Trans. (1981) 1617–1623. [5] F. He, Z. Jia, Y. Peng, P. Wang, Z. Liu, A novel method to synthesize polyaluminum chloride with a membrane reactor, J. Environ. Sci. 16 (2004) 482–486. [6] F. He, P. Wang, Z. Liu, Synthesis of polyaluminum chloride with a membrane reactor: effects of operation modes, J. Membr. Sci. 227 (2003) 15–21. [7] J.Y. Bottero, M. Axelos, D. Tchoubar, J.M. Cases, J.J. Fripia, F. Fiessingger, Mechanism of formation aluminum trihydroxide from keggin Al13 polymers, J. Colloid Interface Sci. 117 (1987) 47–57. [8] P.M. Bertsch, Conditions for Al13 polymer formation in partially neutralized aluminum solutions, Soil Sci. Soc. Am. J. 51 (1987) 825–828. [9] R.B. Bird, W.E. Stewart, E.N. Lightfoot, Transport Phenomena, Wiley John & Sons Inc, New York, 1960.
Synthesis of polyaluminum chloride with a membrane reactor: parameters optimization for the in situ synthesis Fei He, Zhiqian Jia, Peijing Wang, Zhongzhou Liu∗ Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, P.O. Box 2871, Beijing, PR China Received 13 May 2004; received in revised form 15 May 2004; accepted 4 October 2004 Available online 21 November 2004
Abstract Polyaluminum chloride (PAC), containing about 70% Alb , with the Al concentration (0.35 mol/L) satisfying the in situ water treatment, was synthesized efficiently with a membrane reactor. The effects of the membrane molecular weight cut-off (MWCO), the membrane modules lengths and the reaction temperatures on the species distribution, pH values, pressure drops and flow rates of aluminum solutions were studied. It is found that lower membrane MWCO, shorter membrane module length and higher temperature are favorable for the synthesis of PAC with high Alb content. © 2004 Published by Elsevier B.V. Keywords: Synthesis; Polyaluminum chloride; Membrane reactor; In situ; Optimization
1. Introduction Polyaluminium chloride (PAC) is an important flocculant in water treatment. The species in PAC include three parts: “Ala ” (mononuclear Al, including Al3+ , Al(OH)2+ , Al(OH)2 + and Al(OH)4 − ), “Alb ” (medium polymer species), and “Alc ” (larger polynuclears and/or Al(OH)3 ) [1–3]. Among them, Alb species correspond to Al13 ([AlO4 Al12 (OH)(24+n) (H2 O)(12−n) ](7−n)+ ) in freshly prepared solutions [4], and are usually regarded as the performance flocculant due to their higher positive charge and stability. In order to improve the Alb content, a membrane reactor was employed for synthesizing PAC in our previous paper [5,6], in which NaOH solution permeated gradually through the micropores of hollow fiber ultrafiltration (UF) membranes into the fiber lumens under the transmembrane pressure and reacted with AlCl3 solution. Since the mi-
∗
Corresponding author. E-mail addresses: [email protected] (F. He), [email protected] (Z. Liu). 0376-7388/$ – see front matter © 2004 Published by Elsevier B.V. doi:10.1016/j.memsci.2004.10.009
cropores of UF membranes are on the nanometer scale, the droplets sizes of NaOH solutions are dramatically reduced; due to a larger contact interface between the strong base drops and Al solution, more Al(OH)4 − ions are presumed to be formed [7], leading to the increase in the Alb content. We have previously shown that the PAC products synthesized with the membrane reactor contain more Alb (Alb > 70%, at basicity of about 2.2) and show better flocculation effects than some commercial products [5]. However, it usually took about 3–4 h to prepare 220 mL PAC solutions, and the product concentration (0.14 mol/L) was also somewhat low for use in water treatment (0.25–0.5 mol/L). In order to industrialize the synthesis, there is a need to shorten the preparation time and to increase the product concentration. However, this is not an easy task. As described in our former paper [6], a lower injection rate of OH− and a lower reactant concentration favor the formation of Alb species. Fortunately, it was found that bi-directional flow significantly reduces the membrane fouling [6]. In this paper, the effects of the membrane molecular weight cut-off (MWCO), the membrane module length and the reaction temperature on the species distribution and process parameters are investigated.
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Table 1 Membrane module Membrane module
Material
MWCO
Internal fiber diameter (mm)
Length (m)
Number of fibers
Ia
PS/PDC PS/PDC PS PS
10,000 30,000 10,000 10,000
1.0 1.0 0.8 0.8
0.130 0.130 0.130 0.260
10 10 14 7
IIa IIIb IVb
a Made in Research Center for Eco-Environmental Sciences, Beijing, China. PS/PDC means a blend of polysulphone and PDC (polysulfone with phenolphthalein group). b Made in Motian Ltd., Tianjin, China.
We show that PAC products with high concentration and Alb content can be synthesized in a short time.
2. Materials and methods 2.1. Membrane modules The membrane modules used in this paper are listed in Table 1.
2.3. Analysis of Al species As described in the literature [1], the aluminum-Ferron complexation timed spectrophotometry can be used to determine the concentration of aluminum species. The reaction time for Ala -Ferron was 1 min, and the Al species that reacted with the Ferron reagent before 120 min represented [Ala + Alb ]. The total Al [AlT ] minus Ala and Alb was taken as Alc .
2.2. Preparation of PAC To obtain steady permeation fluxes, a constant flow pump (Xingda LP Pulseless Pump, Satellite Manufactory of Beijing, China) was employed to inject NaOH solution into the lumens of a UF membrane at a constant rate (0.01–9.99 mL/min). At the same time, AlCl3 solution in the stirred tank, with an initial volume of 150 mL, was recycled through the lumens of the UF membrane by a BT00-300M peristaltic pump (Lan’ge Instrument Company of Baoding, China) and reacted with the NaOH to form PAC. Valves A–D were used to change the flow direction of the AlCl3 solution (see Fig. 1). The positive flow meant A → B → membrane module → C → D, and the negative flow meant A → E → C → membrane module → B → F → D. When the flow rate dropped, the negative flow was conducted for 1 min per 30–60 min according to the flow rate of solution.
3. Results and discussion 3.1. Effects of MWCO The membranes of MWCO 10,000 and 30,000 with equal membrane area and inner fiber diameter (modules I and II in Table 1) were employed to synthesize PAC. The other conditions were as follows: the permeation flux (J) was 0.3 mL/min; the initial flow rate of AlCl3 solution in the membrane lumens (u0 ) was 1.06 m/s; and the temperature (T) was 290 K. In the first group of experiments, the concentration of reactants AlCl3 (CA0 ) and NaOH (CB0 ) were 0.10 and 0.50 mol/L, respectively. The results of the species distribution (Fig. 2a) show that during the reaction, the proportion of Al present as the Ala species decreases almost in parallel. There exists
Fig. 1. Schematic of experimental apparatus.
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223
Fig. 2. Effects of MWCO (CA0 = 0.1, CB0 = 0.5 mol/L): (a) species distribution vs. B; (b) pH vs. B; and (c) flow rate vs. B. Here, u0 = 1.06 m s−1 , J = 0.3 mL/min, T = 290 K. () MWCO = 10,000 and () MWCO = 30,000.
a cross-point of the two Alb curves at a basicity (B)1 of about 2. The Alc content is observed to increase markedly in both cases after B = 1.5. For MWCO = 30,000, the maximum Alb content is 70% at B = 2. For MWCO = 10,000, more Ala species and less Alc species appear at the same B, and the maximum Alb content is attained 77% at B = 2.3. When the concentrations of AlCl3 and NaOH solution were increased to 0.4 and 1.0 mol/L, respectively, the changes in species distribution (Fig. 3) are similar to that shown in Fig. 2a, except that the maximum content of Alb species (60%) is relatively lower than that obtained at low concentration. The pH–B curve (Fig. 2b) shows that, for MWCO = 10,000, the polymerization process is maintained to a higher basicity than for MWCO = 30,000 [5], which is in accordance with Fig. 2a. Fig. 2c shows that the flow rate for MWCO = 10,000 never drops during the synthesis, and no flushing was needed in this experiment. Therefore, it can be concluded that the lower MWCO membrane is favorable for both increments in the maximum Alb content and inhibition of the membrane fouling. The differences in the species distribution for the two experiments are mainly attributed to the difference in the droplet size of added base. The ratio (c) of the surface area
1
Basicity (B) is defined as molar ratio of NaOH/AlT .
of base drops permeating the membrane of MWCO = 10,000 to MWCO = 30,000 can be described as follows: c=
n1 4πr12 n2 4πr22
(1)
where, n1 and n2 are the number of base drops passing through the membranes of MWCO = 10,000 and 30,000, respectively; r1 and r2 are the average radius of base drops,
Fig. 3. Effects of MWCO (CA0 = 0.4, CB0 = 1.0 mol/L): species distribution vs. B. Here, u0 = 1.06 m/s, J = 0.3 mL/min, T = 290 K. (䊉) MWCO = 10,000 and (–) MWCO = 30,000.
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correspondingly. At the same base injection rate: 4 3 4 3 n1 πr1 = n2 πr2 3 3 c=
r2 >1 r1
the two curves of Alb content arises because of these effects. (2) 3.2. Effects of the length of the membrane module (3)
Thus, in the absence of any diffusion effects, the base droplets permeating through the 10,000 MWCO membrane have a larger “instantaneous” surface area than those of the 30,000 MWCO membrane. As we know, the formation of Al(OH)4 − , the precursor of Al13 , needs a suitable, but not too high, localized basicity [2,8]. The larger base drops lead to higher localized concentration of OH− , which favors the formation of Al(OH)4 − and Alb at the beginning of the reaction. However, with the permeation of OH− , the pH value in the bulk solution increases, and the localized concentration of OH− is high enough to form Al(OH)3 precipitates. Consequently, due to the gradual transformation of Ala to Alb , the solution viscosity increases and the diffusion rate of ions decreases. The larger base drops provide a smaller OH− –Al3+ interface than the smaller drops. The amount of the precursor and Al13 that is subsequently formed with the 30,000 MWCO membranes is smaller than that formed with the 10,000 MWCO membranes. The cross point in
In this group of experiments, membrane modules III and IV, as described in Table 1, were used respectively. The modules are different in length but the same in total membrane area. The other operating conditions were as follows: J = 0.3 mL/min; u0 = 1.18 m/s; CA0 = 0.20 mol/L; CB0 = 1.0 mol/L; and T = 303 K. Fig. 4a shows that there exists some differences in the species distribution when B > 1.5. For example, the content of Alc species is lower, and the maximum content of Alb species is larger for the short module than that for the long one. At the point of maximum Alb content, there are 19.7% Ala , 71.7% Alb and 8.6% Alc (B = 2.1) for the short module; while there are 14.4% Ala , 67.1% Alb and 18.5% Alc (B = 2) for the long one. This above result is in accordance with the pH–B plot (Fig. 4b). When B > 2, the pH values for the short module increase much more slowly, indicating that the polymerization reaction lasts longer [5]. Fig. 4c shows that the pressure drops of the long module are larger than that of the short module throughout the synthesis. The pressure drop reflects the resistance of inner
Fig. 4. Effects of the module lengths: (a) species distribution vs. B; (b) pH vs. B; (c) pressure drop vs. B; and (d) flow rate vs. B. Here, J = 0.3 mL/min, u0 = 1.18 m/s, CA0 = 0.20 mol/L, CB0 = 1.0 mol/L, T = 303 K. () The long module and () the short module.
F. He et al. / Journal of Membrane Science 247 (2005) 221–226
walls of the membrane lumens to the flowing solution and increases with increasing module length [9]: 2 l u p = −λ ρ (4) d 2 where λ is the friction coefficient, l is the length of the module, d is the diameter of the fibers, ρ is the solution density, and u is the flow rate of Al-solution. For the different length of modules, the average retention time (t) of the Al-solution in the lumens and the average molar ratio of OH− to aluminum (MOH /MAl ) in the out flow are different, i.e., t=
l u
JCB0 4JCB0 CB0 + CA0 B MOH = = MAl nπd 2 /4uCA πd 2 CA0 CB0 nu
225
tent of Alb species (at B = 2.25) reaches 60%, 63% and 80%, respectively (Fig. 5b). This result shows that the higher temperature favors the formation of Alb species. One of the reasons is that, when the temperature rises, the viscosity of the solution decreases and the Brownian motion induced diffusion is enhanced. Besides, the higher temperature favors the dissolution of precipitates, which is beneficial for the micromixing of base and Al-solution in the membrane lumens. Meanwhile, the hydrolysis process to produce H+ is enhanced by the higher temperature, which lowers the localized supersaturation and inhibits the formation of Al(OH)3 . Therefore, more Alb species are formed at higher temperatures.
(5)
3.4. Synthesis of PAC at high-concentration
(6)
Based on the above discussion, smaller MWCO membranes, shorter membrane module length and higher temperature are favorable for the synthesis of PAC exhibiting a larger proportion of Alb species. Therefore, to prepare PAC with higher Al-concentration and Alb content in a shorter reaction time, membrane module I (MWCO = 10,000, 0.13 m in length) was used, and the reaction temperature was controlled to be about 305–307 K. The other conditions were as follows: CA0 = 0.5 mol/L, CB0 = 2.5 mol/L, J = 1.0 mL/min and u0 = 1.09 m/s. Using bi-directional flow, a PAC product with B = 2.1 was synthesized within 75 min. The final product, with an AlT -
In this group of experiments, the flow rates of Al-solution are similar (Fig. 4d), so the retention time of Al-solution in the lumens and MOH /MAl in the out flow (at a certain B) for the long module are twice those for the short module. In the membrane module, the permeate base solution moves the hydrolysis to the right: Al3+ + OH− ↔ Al(OH)2+ Al(OH)2+ + OH− ↔ Al(OH)2 + Al(OH)2 + + OH− ↔ Al(OH)3 Al(OH)3 + OH− ↔ Al(OH)4 − ... After returns to the stirred tank, the out flow solution mixes with the acidic bulk Al solution. When B is low, the uncomplexed OH− ions in the out flow solution are neutralized quickly by the acidic bulk, so the differences in the species distribution and pH values for the long and short modules are not apparent. With increasing B value, the localized basicity is high enough to form Al(OH)3 precipitates, and the micromixing condition in the lumens deteriorates greatly, so the content of Alb begins to drop markedly. This is the reason for the significant difference in the species distribution that is apparent after B = 2. 3.3. Effects of temperature The reaction temperature was altered to be 291, 297 and 305 K, respectively, under the conditions of CA0 = 0.40 mol/L, CB0 = 2.0 mol/L, P0 = 0.0090 MPa, and u0 = 1.09 m/s. In this group of experiments, membrane module I was used. Because the hydrolysis of Al3+ is an endothermic reaction, the elevated temperature induces the hydrolysis to move to the right. Thus, at the same basicity, the pH decreases with increasing temperature (Fig. 5a). For temperatures of 291, 297 and 305 K, Alc species are apparent at B = 0.65, 1.0 and 1.4, and the maximum con-
Fig. 5. Effects of temperature: (a) pH vs. B and (b) species distribution vs. B. Here, MWCO = 10,000, CA0 = 0.40 mol/L, CB0 = 2.0 mol/L, P0 = 0.0090 MPa. () T = 291 K, () T = 297 K, () T = 305 K.
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concentration of 0.35 mol/L, is clear and transparent, with no precipitates apparent after 24 h aging. The species distribution of PAC is Ala = 22%, Alb = 70% and Alc = 8% at B = 2.1. This product concentration satisfies the requirement for in situ wastewater treatment.
n2
4. Conclusion
r2
It is found that smaller membrane MWCO, shorter module length and higher temperature are all favorable for the formation of Alb species. On the basis of these results, with bi-directional flow operation, PAC products (B = 2.1) containing Al-concentration of 0.35 mol/L and Alb content of 70%, have been successfully synthesized within 75 min at 305–307 K.
t
Acknowledgements
p r1
T u u0
number of base drops permeating the membrane of MWCO = 30,000 pressure drop (MPa) average radius of base drops permeating the membrane of MWCO = 10,000 average radius of base drops permeating the membrane of MWCO = 30,000 average retention time of Al-solution in lumens (s) temperature (K) flow rate of A1Cl3 solution in the membrane lumens (m/s) initial flow rate of A1Cl3 solution in the membrane lumens (m/s)
Greek letters λ friction coefficient ρ solution density (kg/m3 )
The authors are grateful for financial support from the national 863 High-tech Project 2002AA 601290, China. References Nomenclature B c
CA0 CB0 d J l MAl MOH n n1
basicity, molar ratio of OH/Al (i.e., NaOH/total aluminum) ratio of the surface area of base drops permeating the membrane of MWCO = 10,000 to MWCO = 30,000 initial concentration of A1Cl3 solution (mol/L) concentration of NaOH solution (mol/L) diameter of the hollow fiber membrane (m) permeation flux of NaOH of total membrane area (mL/min) length of a module (m) molar of aluminum at the outlet of lumens (mol) molar of OH− at the outlet of lumens (mol) number of fibers in a module number of base drops permeating the membrane of MWCO = 10,000
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