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The influence of salt in solutions on hydrochloric acid recovery by membrane distillation
Separation and Purification Technology 14 (1998) 183–188
The influence of salt in solutions on hydrochloric acid recovery by membrane distillation M. Tomaszewska *, M. Gryta, A.W. Morawski Technical University of Szczecin, Institute of Inorganic Chemical Technology, ul. Pułaskiego 10, 70-322 Szczecin, Poland Received 23 July 1997; received in revised form 27 February 1998; accepted 6 March 1998
Abstract The concentration and recovery of HCl by membrane distillation (MD) was performed in a capillary module with polypropylene membranes. The influence of acid concentration and salt presence in the feed on HCl molar flux through the membrane was systematically studied. The feed temperature was kept at 333 or 343 K, and the permeate temperature at 293 K. The studies show that for a low HCl content in the feed, the permeate was practically pure water. The increase of temperature and acid concentration in the feed caused a substantial increase in HCl molar flux through the membrane. The presence of FeCl in the feed and the increase of its concentration in the feed causes a 3 significant increase in HCl molar flux in comparison with the case of pure acid. Because the salt was retained in the feed, the permeate was pure hydrochloric acid with increasing concentration. The experiments were also performed with a real pickle solution. The results show that MD can be applied for the recovery of hydrochloric acid from the acidic industrial effluents. © 1998 Elsevier Science B.V. All rights reserved. Keywords: HCl separation; Membrane distillation; Vapour permeation
1. Introduction Membrane processes offer a number of advantages over conventional treatment of industrial effluents, among others that valuable products can be recovered and commercially utilised. Membrane distillation (MD) is one of the promising membrane processes, and is still under investigation. MD is the process whereby a hydrophobic, microporous membrane separates two aqueous solutions at different temperature and composition [1,2]. * Corresponding author. Fax: +48 91 4330 352; e-mail: [email protected] pl
The principle of direct contact membrane distillation is presented in Fig. 1. The liquid evaporates at a feed/air filling pores interface, vapour diffuses through the pores and condenses directly in a cold distillate. The driving force for MD is a partial pressure difference induced by the temperature gradient between the solutions on both sides of the membrane. The hydrophobicity of the membrane used, and maintaining a gaseous phase inside the membrane pores, are the necessary conditions of MD [1–3]. The separation mechanism is based on the vapour/liquid equilibrium of a liquid mixture [4]. MD is a highly selective operation for non-volatile solutes. For example, the retention coefficient reaches 100% for salts [5] and the MD
1383-5866/98/$ – see front matter © 1998 Elsevier Science B.V. All rights reserved. PII S1 3 8 3 -5 8 6 6 ( 9 8 ) 0 0 07 3 - 2
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M. Tomaszewska et al. / Separation and Purification Technology 14 (1998) 183–188
Fig. 1. The principle of direct contact membrane distillation.
permeate is a high purity water. Therefore, membrane distillation can be used for obtaining fresh water from the sea [6,7] and the production of high purity water [8,9]. On the other hand, MD can be applied to the concentration of solutions of non-volatile solutes, even to the supersaturated state [5,10]. The process can be performed at a feed temperature considerably lower than its boiling point. This permits the utilisation of waste heat existing at the various industrial sites. Until now MD has been investigated mostly in the aqueous solutions of non-volatile substances. The results of our earlier work have shown a good perspective for MD application to recovery of volatile hydrochloric acid from its solutions [11,12]. The aim of the present work was to study the influence of salt concentration in HCl solutions on the separation of hydrogen chloride during membrane distillation.
2. Methods The experiments were performed using a system presented earlier [11]. The MD module (supplied
by Euro-Sep Ltd., Poland) was equipped with a capillary membrane Accurel, d /d =1.8/2.6 mm, in out made from polypropylene (PP), having an effective area of 120 cm2. The warm feed and cold distillate were circulated countercurrently in closed thermostated systems. The initial volume of HCl solution used as a feed was 2000 cm3. When FeCl in HCl 3 was a feed the initial volume was equal to 500 cm3. The cold system was initially supplied by distillate water (initial volume equal to 400 cm3). The experiments were carried out with inlet feed temperatures of 333 or 343 K. The inlet temperature of the cold distillate was kept at 293 K. When the inlet feed temperature was 333 or 343 K, the outlet temperatures were 4 or 6 K lower for the feed and 3.5 or 4 K higher for the distillate, respectively. The studies were performed using as a feed hydrochloric acid solutions or a model solution containing FeCl in hydrochloric acid. 3 The initial acid concentration in the feed was varied from 18 to 250 g/dm3 and the salt concentration was in the range 2–90 g Fe/dm3. The changes of cold distillate volume were measured every hour. The distillate is the solution in the cold system. The HCl flux (J ) was calculated from the material balance of HCl in the distillate every hour, taking into account the changes of the volume and the acid concentration in the distillate, according to the following equation: J=
24(c
V −c V ) t+1 t+1 t t AMt
C D mol
m2 d
where c and c are the distillate concentration t t+1 at time t and t+1, V and V the distillate t t+1 volume at time t and t+1, A the membrane area inside the capillaries, M the molar weight of HCl, t the time between the following measurements.
3. Results and discussion The results of the gradual concentration of hydrochloric acid solution, starting from 5% HCl, at a temperature of 333 or 343 K are presented in Fig. 2. As can be seen, during MD of hydrochloric acid solution initially, at low acid concentration only the water vapour passes through pores of a
M. Tomaszewska et al. / Separation and Purification Technology 14 (1998) 183–188
Fig. 2. Dependence of permeate flux and HCl molar flux through a hydrophobic membrane on the feed concentration during MD.
hydrophobic membrane, but at higher acid concentration in the feed both water vapour and gaseous HCl are transferred through the membrane from the feed to the distillate. The vapour is condensed directly in the cold distillate and gaseous HCl is dissolved. HCl is a volatile substance and its separation between the permeate and the concentrate depends upon the vapour/liquid equilibria on both sides of a hydrophobic membrane. The rise of the acid concentration in the feed over 180 g/dm3, at 333 K, caused a substantial increase in HCl molar flux through the membrane, up to 600 mol HCl/m2 d ( Fig. 2). The vapour composition is affected by a hydrochloric acid concentration in the feed and its temperature. The increase of feed temperature from 333 to 343 K causes an increase in HCl molar flux through the membrane. The higher value of HCl molar flux at low acid concentration in the feed, at 333 K, indicates a small leakage of the feed through the biggest pores of the hydrophobic membranes. But this was rather an exceptional case, and the effect was not observed in further experiments. As can be seen in Fig. 2, the volume permeate flux dropped initially with the acid concentration in the feed, and was next fixed on a practically constant level, higher for higher feed temperature. The observed effects are in agreement with the vapour and HCl
185
Fig. 3. Partial vapour pressure dependence on HCl content in the liquid phase [16 ].
partial pressure of the hydrochloric acid solutions shown in Fig. 3. At 293 K, the partial pressure of gaseous HCl rises sharply and, at the same time, the water partial pressure slowly decreases when HCl concentration in the solution increases above 30 wt%. An increase in the temperature of the liquid phase decreases the solubility of gaseous HCl, therefore the HCl partial pressure rises at lower content of this compound in the solution. The presence of FeCl in the feed containing 3 HCl varies the results of MD due to a change of the vapour compositions. The salt effect in vapour–liquid equilibrium was considered earlier [13]. It was ascertained that the salt effect is a complex function of all possible interactions between all components of the studied system. The vapour composition depends on the solubility of the salt in each of the solution components. Raoult’s law predicts that the salt would lower the vapour pressure of each volatile component in the absence of the other. However, when salt is added to a mixture of volatile compounds, a salt actually raises the partial pressure of one component while lowering that of the other. This is the salting out effect. For example, the salts ammonium, sodium, and potassium chloride that caused salting out of the methanol, ethanol and n-propanol from the investigated systems were much more soluble in water than in the alcohol [13]. The salts increased
186
M. Tomaszewska et al. / Separation and Purification Technology 14 (1998) 183–188
activity of the alcohol in the vapour phase (the ions exert a minor attraction for the alcohol molecules in the solution), while the partial pressure of water is reduced by the salt addition (the ions exert a preferential attraction for water molecules). Some agents can affect the activity coefficients in the H O/HCl system. These compounds added to 2 the aqueous solution of hydrochloric acid change the composition of gaseous phase (HCl/H O). It 2 was stated that H SO , LiCl, CaCl [14] and 2 4 2 MgCl [15] are effective in HCl separation. 2 The influence of FeCl concentration in the feed 3 on HCl molar flux through the membrane was systematically studied, starting from diluted and ending at concentrated solutions. A table of approximate initial compositions of the model solutions is presented below. HCl concentration (g/dm3)
FeCl concentration 3 (g Fe/dm3)
100 150 200 250
35 35 35 35
45 45 45 45
60 60 60 60
75 75 75 75
90 90 90 90
At these conditions, water vapour and gaseous HCl are transported through the pores of the hydrophobic membrane, whereas the salt was retained in the feed. The presence of salt in the feed decreases the solubility of the gas, causing vapour enrichment (the desalting out effect). Thus, a higher molar flux of HCl through the membrane can be achieved compared to the case without the salt in the feed. One should notice that molar HCl flux depends on a volume permeate flux and HCl current concentration achieved in the distillate during the process. Therefore, at high permeate flux and moderate concentration, the molar HCl flux can be the same as at a low volume permeate flux and its high concentration. Fig. 4 shows that the presence of FeCl in the feed causes a significant 3 increase in HCl molar flux. At acid concentration in the feed of 250 g/dm3 and feed temperature of 343 K, the increase of salt concentration from 2 to 19 g Fe/dm3 caused a rise in molar HCl flux from 300 to 500 mol HCl/m2 d, at similar volume permeate flux. The retention coefficient of FeCl 3 was >99.9%.
Fig. 4. The influence of acid and salt concentration in the feed on permeate flux: (1) constant FeCl concentration 2 g 3 Fe/dm3 and various HCl concentrations; (2) FeCl concen3 tration 17.5–19 g Fe/dm3 and various HCl concentrations.
The effect of the feed composition on MD performance was studied systematically. Fig. 5 is an example of changes of feed and permeate composition during 4 h of MD. Initially, at feed composition starting from 100 g HCl and 35 g Fe as ferric chloride per litre (Fig. 5) to 200 g HCl and 50 g Fe per litre, the acid concentration gradually increased in the feed during the MD process and the HCl molar flux was very small. From this point, when initial HCl concentration was equal to or higher than 20%, an increase of the salt concentration above 60 g Fe/dm3 caused so high
Fig. 5. The influence of the feed composition on the HCl molar flux through a hydrophobic membrane during MD.
M. Tomaszewska et al. / Separation and Purification Technology 14 (1998) 183–188
a flux of gaseous HCl through a hydrophobic membrane that the HCl concentration in the feed began gradually to decrease. At the same time, the volume permeate flux decreased from 68 dm3/m2 d (for the feed composition 35 g Fe and 100 g HCl per litre) to 20 dm3/m2 d at 250 g HCl and 90 g Fe per litre in the feed (T =333 K ), as F presented in Fig. 6. Higher salt and acid concentrations in the feed caused a decrease of the water vapour pressure, therefore a lower flux was achieved. On the other hand, under these conditions HCl partial pressure is higher, hence its separation is facilitated. In spite of such considerable decrease in volume permeate flux, the molar HCl flux achieved 908 mol/m2 d (Fig. 7). At higher feed temperature, 343 K, the volume permeate was higher and the molar flux increased to 1064 mol HCl/m2 d. Studying the salt concentration effect in the feed,
Fig. 6. The influence of the feed composition on the HCl molar flux through a hydrophobic membrane during MD.
Fig. 7. The influence of the feed composition on the volume permeate flux through a hydrophobic membrane during MD.
187
Fig. 8. The influence of the feed composition on the HCl molar flux through a hydrophobic membrane during MD; T =333 K. F in
Fig. 9. Membrane distillation of a real metal pickling solution: (1) HCl concentration in the feed; (2) molar permeate flux (mol HCl/dm3); (3) volume permeate flux (dm3/m2 d); (4) distillate concentration (g HCl/dm3).
we have found that a high salt concentration causes HCl separation at acid concentration in the feed significantly lower than without the salt. For example, for a solution without salt, a significant molar HCl flux was observed from acid concentration in the feed to about 180 g/dm3, whereas when salt was present in the feed as well, the HCl separation was observed at feed containing 160 g HCl and 65 g Fe per litre ( Fig. 8) or 106 g HCl and 100 g Fe per litre (T =333 K ). When the feed F temperature was higher, 343 K, the same effect was observed at a lower salt concentration. The retention coefficient of salt for all model solutions
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M. Tomaszewska et al. / Separation and Purification Technology 14 (1998) 183–188
was higher than 99.9%. Figs. 5–8 represent values obtained during 4 h of MD performance. Experiments were also performed with a real pickle liquor, containing 8.9 g Fe3+ and 101.3 g HCl/dm3 ( Fig. 9). The HCl concentration in the feed increased gradually. The distillate obtained was pure water up to 15% HCl and about 28 g Fe/dm3 in the feed. From this point the molar HCl flux grows rapidly. Due to the constant volume permeate flux, equal to 120 dm3/m2 d, the concentration of the acid in the permeate has increased. The results of the experiments were satisfactory. The salt retention in the MD process was 100%, therefore the distillate was pure hydrochloric acid. The MD concentration was performed periodically. Thus periodic rinsing of the warm side of the membrane with water was performed (after shut-down of MD installation) for the protection of the membrane against the formation of the salt crystals in the membrane pores. The salt crystals could be formed from the supersaturated feed after cooling the installation. The crystals could cause the diffusion of the feed through the membrane, thus causing the pollution of the permeate [5]. During the experiments (above one year) the hydrophobicity of the polypropylene membrane was maintained and no changes in membrane properties were observed. Damage of the module with polypropylene membranes was caused rather by damage of a membrane gasket.
4. Conclusions The presence of salt in the feed causes a significant increase in molar HCl flux through a hydrophobic membrane during MD. The separation of HCl takes place at acid concentration in a feed significantly lower than without a salt. The results show that MD can be applied for the recovery of hydrochloric acid from industrial effluents. The useful products will be pure water, pure hydrochloric acid and metal salts after crystallisation from the saturated feed.
Acknowledgment This work was supported by a grant from The Polish State Committee for Scientific Research, No. 3T 09B00310.
References [1] E. Drioli, Y. Wu, V. Calabro, Membrane distillation in the treatment of aqueous solutions, J. Membrane Sci. 33 (1987) 277. [2] G.C. Sarti, C. Gostoli, S. Matuli, Low energy cost desalination process using hydrophobic membranes, Desalination 56 (1985) 277. [3] E. Drioli, V. Calabro, Y. Wu, Microporous membranes in membrane distillation, Pure Appl. Chem. 58 (1986) 1657. [4] M. Mulder, Basic Principles of Membrane Technology, Kluwer Academic, 1991. [5] M. Tomaszewska, Concentration of the extraction fluid from sulfuric acid treatment of phosphogypsum by membrane distillation, J. Membrane Sci. 78 (1993) 277. [6 ] L. Carlsson, The new generation in sea water desalination SU membrane distillation system, Desalination 45 (1983) 221. [7] S.I. Andersson, N. Kjellander, B. Rodesjo, Design and field tests of a new membrane distillation desalination process, Desalination 56 (1985) 345. [8] K. Schneider, W. Holz, R. Wolbeck, S. Ripperger, Membranes and modules for transmembrane distillation, J. Membrane Sci. 39 (1988) 25. [9] M. Tamura, New water distillation system incorporating a membrane mist separator, Desalination 80 (1991) 105. [10] Y. Wu, Y. Kong, J. Liu, J. Zhang, J. Xu, An experimental study on membrane distillation — crystallization for treating waste water in taurine production, Desalination 80 (1991) 235. [11] M. Tomaszewska, M. Gryta, A.W. Morawski, Study on the concentration of acids by membrane distillation, J. Membrane Sci. 102 (1995) 277. [12] M. Tomaszewska, M. Gryta, A.W. Morawski, The recovery of hydrochloric acid from industrial effluents by membrane distillation, in: W.R. Bowen, R.W. Field, J.A. Howell (Eds.), Proc. Euromembrane ’95, Bath, 18–20 September 1995, vol. 1, A. Rowe Ltd., Chippenham, UK, p. 540. [13] A.J. Johnson, W.F. Furter, Salt effect in vapor–liquid equilibrium, part II, Can. J. Chem. Eng. (1960) 78. [14] J.J. McKetta, W.A. Cunningham (Eds.), Encyclopedia of Chemical Processing and Design, vol. 26, Marcel Dekker, New York, 1987. [15] P. Loetzsch, G. Scherz, Beitrag zur Auslegung von Rieselkolonnen fu¨r die Destillation des Dreistoffgemisches HCl–H O–MgCl , Chem. Tech. 25 (1973) 339. 2 2 [16 ] Handbook of Chemistry and Physics, WNT, Warsaw, 1974 (in Polish).
The influence of salt in solutions on hydrochloric acid recovery by membrane distillation M. Tomaszewska *, M. Gryta, A.W. Morawski Technical University of Szczecin, Institute of Inorganic Chemical Technology, ul. Pułaskiego 10, 70-322 Szczecin, Poland Received 23 July 1997; received in revised form 27 February 1998; accepted 6 March 1998
Abstract The concentration and recovery of HCl by membrane distillation (MD) was performed in a capillary module with polypropylene membranes. The influence of acid concentration and salt presence in the feed on HCl molar flux through the membrane was systematically studied. The feed temperature was kept at 333 or 343 K, and the permeate temperature at 293 K. The studies show that for a low HCl content in the feed, the permeate was practically pure water. The increase of temperature and acid concentration in the feed caused a substantial increase in HCl molar flux through the membrane. The presence of FeCl in the feed and the increase of its concentration in the feed causes a 3 significant increase in HCl molar flux in comparison with the case of pure acid. Because the salt was retained in the feed, the permeate was pure hydrochloric acid with increasing concentration. The experiments were also performed with a real pickle solution. The results show that MD can be applied for the recovery of hydrochloric acid from the acidic industrial effluents. © 1998 Elsevier Science B.V. All rights reserved. Keywords: HCl separation; Membrane distillation; Vapour permeation
1. Introduction Membrane processes offer a number of advantages over conventional treatment of industrial effluents, among others that valuable products can be recovered and commercially utilised. Membrane distillation (MD) is one of the promising membrane processes, and is still under investigation. MD is the process whereby a hydrophobic, microporous membrane separates two aqueous solutions at different temperature and composition [1,2]. * Corresponding author. Fax: +48 91 4330 352; e-mail: [email protected] pl
The principle of direct contact membrane distillation is presented in Fig. 1. The liquid evaporates at a feed/air filling pores interface, vapour diffuses through the pores and condenses directly in a cold distillate. The driving force for MD is a partial pressure difference induced by the temperature gradient between the solutions on both sides of the membrane. The hydrophobicity of the membrane used, and maintaining a gaseous phase inside the membrane pores, are the necessary conditions of MD [1–3]. The separation mechanism is based on the vapour/liquid equilibrium of a liquid mixture [4]. MD is a highly selective operation for non-volatile solutes. For example, the retention coefficient reaches 100% for salts [5] and the MD
1383-5866/98/$ – see front matter © 1998 Elsevier Science B.V. All rights reserved. PII S1 3 8 3 -5 8 6 6 ( 9 8 ) 0 0 07 3 - 2
184
M. Tomaszewska et al. / Separation and Purification Technology 14 (1998) 183–188
Fig. 1. The principle of direct contact membrane distillation.
permeate is a high purity water. Therefore, membrane distillation can be used for obtaining fresh water from the sea [6,7] and the production of high purity water [8,9]. On the other hand, MD can be applied to the concentration of solutions of non-volatile solutes, even to the supersaturated state [5,10]. The process can be performed at a feed temperature considerably lower than its boiling point. This permits the utilisation of waste heat existing at the various industrial sites. Until now MD has been investigated mostly in the aqueous solutions of non-volatile substances. The results of our earlier work have shown a good perspective for MD application to recovery of volatile hydrochloric acid from its solutions [11,12]. The aim of the present work was to study the influence of salt concentration in HCl solutions on the separation of hydrogen chloride during membrane distillation.
2. Methods The experiments were performed using a system presented earlier [11]. The MD module (supplied
by Euro-Sep Ltd., Poland) was equipped with a capillary membrane Accurel, d /d =1.8/2.6 mm, in out made from polypropylene (PP), having an effective area of 120 cm2. The warm feed and cold distillate were circulated countercurrently in closed thermostated systems. The initial volume of HCl solution used as a feed was 2000 cm3. When FeCl in HCl 3 was a feed the initial volume was equal to 500 cm3. The cold system was initially supplied by distillate water (initial volume equal to 400 cm3). The experiments were carried out with inlet feed temperatures of 333 or 343 K. The inlet temperature of the cold distillate was kept at 293 K. When the inlet feed temperature was 333 or 343 K, the outlet temperatures were 4 or 6 K lower for the feed and 3.5 or 4 K higher for the distillate, respectively. The studies were performed using as a feed hydrochloric acid solutions or a model solution containing FeCl in hydrochloric acid. 3 The initial acid concentration in the feed was varied from 18 to 250 g/dm3 and the salt concentration was in the range 2–90 g Fe/dm3. The changes of cold distillate volume were measured every hour. The distillate is the solution in the cold system. The HCl flux (J ) was calculated from the material balance of HCl in the distillate every hour, taking into account the changes of the volume and the acid concentration in the distillate, according to the following equation: J=
24(c
V −c V ) t+1 t+1 t t AMt
C D mol
m2 d
where c and c are the distillate concentration t t+1 at time t and t+1, V and V the distillate t t+1 volume at time t and t+1, A the membrane area inside the capillaries, M the molar weight of HCl, t the time between the following measurements.
3. Results and discussion The results of the gradual concentration of hydrochloric acid solution, starting from 5% HCl, at a temperature of 333 or 343 K are presented in Fig. 2. As can be seen, during MD of hydrochloric acid solution initially, at low acid concentration only the water vapour passes through pores of a
M. Tomaszewska et al. / Separation and Purification Technology 14 (1998) 183–188
Fig. 2. Dependence of permeate flux and HCl molar flux through a hydrophobic membrane on the feed concentration during MD.
hydrophobic membrane, but at higher acid concentration in the feed both water vapour and gaseous HCl are transferred through the membrane from the feed to the distillate. The vapour is condensed directly in the cold distillate and gaseous HCl is dissolved. HCl is a volatile substance and its separation between the permeate and the concentrate depends upon the vapour/liquid equilibria on both sides of a hydrophobic membrane. The rise of the acid concentration in the feed over 180 g/dm3, at 333 K, caused a substantial increase in HCl molar flux through the membrane, up to 600 mol HCl/m2 d ( Fig. 2). The vapour composition is affected by a hydrochloric acid concentration in the feed and its temperature. The increase of feed temperature from 333 to 343 K causes an increase in HCl molar flux through the membrane. The higher value of HCl molar flux at low acid concentration in the feed, at 333 K, indicates a small leakage of the feed through the biggest pores of the hydrophobic membranes. But this was rather an exceptional case, and the effect was not observed in further experiments. As can be seen in Fig. 2, the volume permeate flux dropped initially with the acid concentration in the feed, and was next fixed on a practically constant level, higher for higher feed temperature. The observed effects are in agreement with the vapour and HCl
185
Fig. 3. Partial vapour pressure dependence on HCl content in the liquid phase [16 ].
partial pressure of the hydrochloric acid solutions shown in Fig. 3. At 293 K, the partial pressure of gaseous HCl rises sharply and, at the same time, the water partial pressure slowly decreases when HCl concentration in the solution increases above 30 wt%. An increase in the temperature of the liquid phase decreases the solubility of gaseous HCl, therefore the HCl partial pressure rises at lower content of this compound in the solution. The presence of FeCl in the feed containing 3 HCl varies the results of MD due to a change of the vapour compositions. The salt effect in vapour–liquid equilibrium was considered earlier [13]. It was ascertained that the salt effect is a complex function of all possible interactions between all components of the studied system. The vapour composition depends on the solubility of the salt in each of the solution components. Raoult’s law predicts that the salt would lower the vapour pressure of each volatile component in the absence of the other. However, when salt is added to a mixture of volatile compounds, a salt actually raises the partial pressure of one component while lowering that of the other. This is the salting out effect. For example, the salts ammonium, sodium, and potassium chloride that caused salting out of the methanol, ethanol and n-propanol from the investigated systems were much more soluble in water than in the alcohol [13]. The salts increased
186
M. Tomaszewska et al. / Separation and Purification Technology 14 (1998) 183–188
activity of the alcohol in the vapour phase (the ions exert a minor attraction for the alcohol molecules in the solution), while the partial pressure of water is reduced by the salt addition (the ions exert a preferential attraction for water molecules). Some agents can affect the activity coefficients in the H O/HCl system. These compounds added to 2 the aqueous solution of hydrochloric acid change the composition of gaseous phase (HCl/H O). It 2 was stated that H SO , LiCl, CaCl [14] and 2 4 2 MgCl [15] are effective in HCl separation. 2 The influence of FeCl concentration in the feed 3 on HCl molar flux through the membrane was systematically studied, starting from diluted and ending at concentrated solutions. A table of approximate initial compositions of the model solutions is presented below. HCl concentration (g/dm3)
FeCl concentration 3 (g Fe/dm3)
100 150 200 250
35 35 35 35
45 45 45 45
60 60 60 60
75 75 75 75
90 90 90 90
At these conditions, water vapour and gaseous HCl are transported through the pores of the hydrophobic membrane, whereas the salt was retained in the feed. The presence of salt in the feed decreases the solubility of the gas, causing vapour enrichment (the desalting out effect). Thus, a higher molar flux of HCl through the membrane can be achieved compared to the case without the salt in the feed. One should notice that molar HCl flux depends on a volume permeate flux and HCl current concentration achieved in the distillate during the process. Therefore, at high permeate flux and moderate concentration, the molar HCl flux can be the same as at a low volume permeate flux and its high concentration. Fig. 4 shows that the presence of FeCl in the feed causes a significant 3 increase in HCl molar flux. At acid concentration in the feed of 250 g/dm3 and feed temperature of 343 K, the increase of salt concentration from 2 to 19 g Fe/dm3 caused a rise in molar HCl flux from 300 to 500 mol HCl/m2 d, at similar volume permeate flux. The retention coefficient of FeCl 3 was >99.9%.
Fig. 4. The influence of acid and salt concentration in the feed on permeate flux: (1) constant FeCl concentration 2 g 3 Fe/dm3 and various HCl concentrations; (2) FeCl concen3 tration 17.5–19 g Fe/dm3 and various HCl concentrations.
The effect of the feed composition on MD performance was studied systematically. Fig. 5 is an example of changes of feed and permeate composition during 4 h of MD. Initially, at feed composition starting from 100 g HCl and 35 g Fe as ferric chloride per litre (Fig. 5) to 200 g HCl and 50 g Fe per litre, the acid concentration gradually increased in the feed during the MD process and the HCl molar flux was very small. From this point, when initial HCl concentration was equal to or higher than 20%, an increase of the salt concentration above 60 g Fe/dm3 caused so high
Fig. 5. The influence of the feed composition on the HCl molar flux through a hydrophobic membrane during MD.
M. Tomaszewska et al. / Separation and Purification Technology 14 (1998) 183–188
a flux of gaseous HCl through a hydrophobic membrane that the HCl concentration in the feed began gradually to decrease. At the same time, the volume permeate flux decreased from 68 dm3/m2 d (for the feed composition 35 g Fe and 100 g HCl per litre) to 20 dm3/m2 d at 250 g HCl and 90 g Fe per litre in the feed (T =333 K ), as F presented in Fig. 6. Higher salt and acid concentrations in the feed caused a decrease of the water vapour pressure, therefore a lower flux was achieved. On the other hand, under these conditions HCl partial pressure is higher, hence its separation is facilitated. In spite of such considerable decrease in volume permeate flux, the molar HCl flux achieved 908 mol/m2 d (Fig. 7). At higher feed temperature, 343 K, the volume permeate was higher and the molar flux increased to 1064 mol HCl/m2 d. Studying the salt concentration effect in the feed,
Fig. 6. The influence of the feed composition on the HCl molar flux through a hydrophobic membrane during MD.
Fig. 7. The influence of the feed composition on the volume permeate flux through a hydrophobic membrane during MD.
187
Fig. 8. The influence of the feed composition on the HCl molar flux through a hydrophobic membrane during MD; T =333 K. F in
Fig. 9. Membrane distillation of a real metal pickling solution: (1) HCl concentration in the feed; (2) molar permeate flux (mol HCl/dm3); (3) volume permeate flux (dm3/m2 d); (4) distillate concentration (g HCl/dm3).
we have found that a high salt concentration causes HCl separation at acid concentration in the feed significantly lower than without the salt. For example, for a solution without salt, a significant molar HCl flux was observed from acid concentration in the feed to about 180 g/dm3, whereas when salt was present in the feed as well, the HCl separation was observed at feed containing 160 g HCl and 65 g Fe per litre ( Fig. 8) or 106 g HCl and 100 g Fe per litre (T =333 K ). When the feed F temperature was higher, 343 K, the same effect was observed at a lower salt concentration. The retention coefficient of salt for all model solutions
188
M. Tomaszewska et al. / Separation and Purification Technology 14 (1998) 183–188
was higher than 99.9%. Figs. 5–8 represent values obtained during 4 h of MD performance. Experiments were also performed with a real pickle liquor, containing 8.9 g Fe3+ and 101.3 g HCl/dm3 ( Fig. 9). The HCl concentration in the feed increased gradually. The distillate obtained was pure water up to 15% HCl and about 28 g Fe/dm3 in the feed. From this point the molar HCl flux grows rapidly. Due to the constant volume permeate flux, equal to 120 dm3/m2 d, the concentration of the acid in the permeate has increased. The results of the experiments were satisfactory. The salt retention in the MD process was 100%, therefore the distillate was pure hydrochloric acid. The MD concentration was performed periodically. Thus periodic rinsing of the warm side of the membrane with water was performed (after shut-down of MD installation) for the protection of the membrane against the formation of the salt crystals in the membrane pores. The salt crystals could be formed from the supersaturated feed after cooling the installation. The crystals could cause the diffusion of the feed through the membrane, thus causing the pollution of the permeate [5]. During the experiments (above one year) the hydrophobicity of the polypropylene membrane was maintained and no changes in membrane properties were observed. Damage of the module with polypropylene membranes was caused rather by damage of a membrane gasket.
4. Conclusions The presence of salt in the feed causes a significant increase in molar HCl flux through a hydrophobic membrane during MD. The separation of HCl takes place at acid concentration in a feed significantly lower than without a salt. The results show that MD can be applied for the recovery of hydrochloric acid from industrial effluents. The useful products will be pure water, pure hydrochloric acid and metal salts after crystallisation from the saturated feed.
Acknowledgment This work was supported by a grant from The Polish State Committee for Scientific Research, No. 3T 09B00310.
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