3808
Ind. Eng. Chem. Res. 1996, 35, 3808-3811
Nonelectrolytic Production of Caustic Soda and Hydrochloric Acid from Sodium Chloride Vikram S. Nayak† Guelph Chemical Laboratories Ltd., 246 Silvercreek Parkway N., Guelph, Ontario, Canada N1H 1E7
A nonelectrolytic process for the production of sodium hydroxide and hydrochloric acid from sodium chloride is explored. This environmentally friendly process, in which production of chlorine and the use of hazardous mercury are eliminated, is based on the following principle. A thermally stable cation exchanger such as ZSM-5 zeolite in its H-form undergoes a cationexchange reaction with a sodium chloride solution to liberate hydrochloric acid, and the resulting NaZSM-5 zeolite undergoes a cation-exchange reaction with a ammonium hydroxide solution to liberate sodium hydroxide. The ammonium ion-exchanged zeolite is converted into its H-form by deammoniation in the temperature range of 400-500 °C. The liberated ammonia is trapped in water to form ammonium hydroxide for reuse. Introduction
Table 1. Ion-Exchange Capacities of Synthetic Zeolites
Caustic soda is mostly produced by the electrolysis of a sodium chloride solution using mercury, diaphragm, or ion-exchange cells. In all these electrolysis cells chlorine is produced at the anode. Although chlorine and its derivatives are at present useful and marketable commodities, due to harmful effects of some chloro and chlorofluoro compounds on the environment in general and human health in particular, the number of calls for a gradual phaseout to an outright ban on the production of chlorine and chlorocarbons is growing day by day (Hileman et al., 1994). Since caustic soda and chlorine are coproduced from the electrolysis of a sodium chloride solution, with the former being one of the most useful chemicals in chemical, pharmaceutical, paper, leather, and engineering industries, the production of caustic soda may receive a severe blow if production of chlorine and chlorocarbons is banned. Therefore, a nonelectrolytic process, which employs a thermally stable cation exchanger such as ZSM-5 zeolite, for the continuous production of caustic soda along with useful hydrochloric acid from sodium chloride is explored. The ion-exchange principle and cation-exchange capacity of a thermally stable zeolite such as ZSM-5 can be used to produce caustic soda and hydrochloric acid. Thus, when a protonated form of ZSM-5 (i.e., HZSM-5) zeolite is contacted with an aqueous sodium chloride solution, the cation exchange between H+ and Na+ ions takes place with the liberation of HCl, i.e.
HZSM-5 + NaCl / NaZSM-5 + HCl
(1)
The sodium-exchanged zeolite is then contacted with an ammonium hydroxide solution to undergo a cationexchange reaction to yield NaOH, i.e.
NaZSM-5 + NH4OH / NH4ZSM-5 + NaOH (2) The ammonium form of the zeolite is then deammoniated by heating at about 400-500 °C to get the zeolite back in its H-form for recycling, i.e.
NH4ZSM-5 f HZSM-5 + NH3
(3)
†
Present address: Brantford Chemicals Inc., 34 Spalding Dr., Brantford, Ontario, Canada N3T 6B8.
S0888-5885(96)00043-7 CCC: $12.00
zeolite
Si/Al ratio
ion-exchange capacity (mequiv/g)
zeolite A zeolite X zeolite Y mordenite ZSM-5 ZSM-5
1 1.25 2 5 20 10
7.0 6.4 5.0 2.6 0.8 1.5
comments Breck, 1974 Breck, 1974 Breck, 1974 Breck, 1974 Chu and Dwyer, 1983 calcd from unit cell composition
The liberated ammonia is then captured in water and recycled. Although cation exchange with both sodium chloride and ammonium hydroxide can be carried out with any of the organic and inorganic cation exchangers, the conversion of the ammonium form of the cation exchanger to its protonated form by deammoniation at higher temperature can only be carried out with thermally stable inorganic cation exchangers such as zeolites. It is important that the conversion of a cation exchanger from its ammonium form to its protonated form by a thermal method should not involve decomposition of ammonia into its constituent elements, which otherwise would result in net consumption of ammonia. Thus, the selection of a cation exchanger for the production of NaOH and HCl would be based mainly on its thermal stability and ion-exchange capacity. Although organic cation exchangers have good ion-exchange capacities, their limited thermal stability makes them unsuitable for the production of NaOH and HCl from sodium chloride. Synthetic zeolites such as A, X, Y, mordenite, ZSM-5, etc., have good thermal stabilities and ion-exchange capacities. The cation-exchange capacities of these synthetic zeolites, which are directly related to the framework aluminum contents, are in the order A > X > Y > mordenite > ZSM-5 (see Table 1). In the case of synthetic zeolites, the thermal stability generally increases with an increase in the Si/Al ratio. In other words, the thermal stabilities of synthetic zeolites are in the reverse order of their cation-exchange capacities (see Table 2). The thermal stabilities of some of the zeolites depend strongly upon the type of cations also. For example, zeolites A and X are very unstable even at 100 °C in their ammonium forms (see Table 2). Therefore, zeolites A and X are considered unsuitable © 1996 American Chemical Society
Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996 3809 Scheme 1. Ion-Exchange Reaction on the Surface of ZSM-5 Zeolite
Figure 1. Schematic illustration of the nonelectrolytic process for the continuous production of sodium hydroxide and hydrochloric acid from sodium chloride, wherein HCl is produced by the cation-exchange reaction of HZSM-5 zeolite with NaCl in Exchange Unit-I, NaOH is produced by the cation-exchange reaction of NaZSM-5 zeolite with NH4OH in Exchange Unit-II, and HZSM-5 zeolite is obtained by the deammoniation of NH4ZSM-5 zeolite in deammoniator. Table 2. Thermal Stabilities of Synthetic Zeolites
zeolite
onset temp for temp at which structural crystallinity degradation reduces by (°C) 50% (°C)
NaA NaX NaY NaZSM-5
660 660 710 >800
755 770 795 >1000
NH4A NH4X NH4Y NH4ZSM-5
120 80 400 >800
140 100 500 >1000
comments Breck, 1974 Breck, 1974 Breck, 1974 Lecluze and Sand, 1980; Nayak, 1983 Breck, 1974 Breck, 1974 Breck, 1974 Lecluze and Sand, 1980; Nayak and Choudhary, 1983
for the production of NaOH and HCl. Zeolites Y and mordenite although have relatively better thermal stability; repeated exposure of these zeolites to high temperatures is bound to reduce their lifetime. Among synthetic zeolites, only ZSM-5 zeolite (Argauer and Landoldt, 1972), which is known to be thermally stable up to about 1000 °C and quite stable in acid and alkali mediums, appears to be a potential cation exchanger for the process. As the cation-exchange capacity of a zeolite is proportional to the framework aluminum content (e.g., Breck, 1974; Jacobs, 1977), thermally stable zeolites with lower Si/Al ratios would be preferred for the process. The possible continuous production of sodium hydroxide and hydrochloric acid from sodium chloride using ZSM-5 zeolite as a cation exchanger is illustrated in Figure 1. The cation-exchange reaction between NaCl and HZSM-5 zeolite takes place in Exchange Unit-I in which HZSM-5 and NaCl solution (from Tank A) are introduced from the top and bottom, respectively. The liberated HCl is collected at the top of the Exchange Unit-I and concentrated to the required strength. The resulting NaZSM-5 zeolite is removed from the bottom of the Exchange Unit-I, filtered, and washed to free
from NaCl and HCl. It is then charged into Exchange Unit-II from the top where it undergoes a cationexchange reaction with an upcoming NH4OH solution. The sodium hydroxide liberated is discharged from the top of the Exchange Unit-II along with unconverted ammonium hydroxide in the form of a dilute solution. The sodium hydroxide is then separated from the ammonium hydroxide by evaporation at about 100 °C. The ammonium form of the zeolite, i.e., NH4ZSM-5, is then sent to the deammoniator to convert into the protonated form, i.e., HZSM-5, which is sent back to Exchange Unit-I. The ammonia released during deammoniation is trapped in the water in Tank B and recycled. Experimental Section All three steps, viz., production of HCl by the cationexchange reaction between HZSM-5 and NaCl, production of NaOH by the cation-exchange reaction between NaZSM-5 and NH4OH, and deammoniation of NH4ZSM-5 to HZSM-5, were tested separately to see the feasibility of the process. The production of HCl and NaOH from NaCl was tested using a 500 mL glass reaction unit with a jacket to circulate water. The reaction temperature could be varied using a thermostat. The contents of the reactor unit could be stirred by using a magnetic stirrer. Typically, about 4 g of the H+-form or Na+-form of ZSM-5 (Si/Al ) 36) zeolite was added with stirring to 100 mL of NaCl (2%) or a NH4OH solution. At the end of the run the content was filtered and rinsed with deionized distilled water. The amount of HCl liberated by the reaction of HZSM-5 zeolite with NaCl was determined by titrating the filtrate against a standard alkali. Similarly, the amount of NaOH liberated by the reaction of NaZSM-5 zeolite with NH4OH was determined by analyzing the corresponding filtrate for sodium by ICP-AES. The results obtained are described below. Results and Discussion Production of HCl. The cation exchange between NaCl and HZSM-5 zeolite, as shown in Scheme 1, appears to proceed very fast at 20 °C. It was found that each gram of ZSM-5 zeolite (Si/Al ) 36) liberated HCl equivalent to about 2.2 mg of sodium ion. It is estimated that, using HZSM-5 zeolite with a Si/Al ratio of 10, HCl equivalent to about 8 mg of sodium ion/g of zeolite can be generated in one exchange. In a batch system, as the production of HCl is limited by the ion-exchange equilibrium, more acid can be produced if it can be removed from the system. This can be accomplished by increasing the number of exchanges at a particular temperature. In the integrated process, as shown in Figure 1, as the zeolite
3810 Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996
Figure 2. Dependence of sodium hydroxide production or ammonium exchange selectivity (% Na released) on contact time at 20 and 50 °C [volume of NH4OH ) 25 mL/g of zeolite].
Figure 3. Dependence of sodium hydroxide production or % ammonium exchange selectivity (% Na released) on the strength and volume of ammonium hydroxide at 20 °C.
moves in the opposite direction of acid flow, a maximum amount of HCl per unit mass of zeolite is expected to be formed due to unattainment of ion-exchange equilibrium. Production of NaOH. The production of a stronger sodium hydroxide by an ion-exchange reaction of NaZSM-5 with weaker NH4OH is the key step in the overall process. It depends upon many factors such as the ion-exchange capacity, ion-exchange equilibrium, and ion-exchange kinetics. The results show that the rate of attainment of cation-exchange equilibrium between NaZSM-5 and NH4OH depends very much on the temperature. For example, with a 1 N NH4OH solution (25 mL/g of zeolite) the establishment of ion-exchange equilibrium took more than 30 min at 20 °C, whereas the equilibrium was established quickly at 50 °C (see Figure 2). As the percentage ion-exchange selectivity, which is defined as the equivalent fraction of ingoing cation in the zeolite multiplied by 100, is expected to be maximum, with the equivalent fraction of ingoing cation in the solution being unity, the ammonium exchange selectivity data were collected with only ammonium cation in the solution. The results obtained on the ion-exchange reaction between NaZSM-5 and NH4OH show that about 75% of Na+ ions from the zeolite can be replaced with NH4+ ions. The ammonium ion-exchange selectivity (% Na released) does not seem to change with variation in temperature. Since 100% ammonium ion-exchange selectivity is reported for the cation-exchange reaction between NaZSM-5 and an aqueous ammonium salt solution (Chu and Dwyer, 1983), the results from the present study indicate that the ammonium exchange selectivity is dependent upon the type of anion in the solution. The dependence of rate of liberation of NaOH on the strength and amount of the NH4OH solution is shown in Figure 3. It appears that the liberation of NaOH from NaZSM-5 zeolite with a 1 N NH4OH solution is slightly higher than that with a 2 N NH4OH solution. This is an indication that ammonium ion-exchange selectivity is dependent upon the pH of the solution at least to some extent. The rate of cation exchange appears to increase with an increase in the volume of the NH4OH solution. For example, when the volume of 1 N ammonium hydroxide was increased from 25 to 50 mL for every gram of zeolite, the ammonium exchange in 15 min increased from 58 to 74% (see Figure 3). The increase in contact time from 15 to 30 min for the ammonium exchange carried out
with 50 mL of 1 N ammonium hydroxide/g of zeolite did not result in any increase in ammonium exchange selectivity. This is an indication that the ratio of the amount of ammonium hydroxide to the amount of zeolite affects only the ion-exchange rate and not the ion-exchange equilibrium. The results from the present study show that about 2.8 mg of NaOH can be produced from 1 g of ZSM-5 zeolite with a Si/Al ratio of 36. It is estimated that by using ZSM-5 zeolite with a Si/Al ratio of 10 about 10 mg of NaOH/g of zeolite in one exchange can be produced. Deammoniation of NH4ZSM-5 to HZSM-5. The deammoniation conditions and their effects on the activity and acidity of ZSM-5 zeolite have been reported elsewhere (Nayak, 1982). It is reported that deammoniation of NH4ZSM-5 zeolite can be easily accomplished by heating the zeolite in the temperature range of 400500 °C (Nayak, 1982; Nayak and Choudhary, 1983). The ammonium ion-exchanged zeolite obtained by contacting NaZSM-5 with ammonium hydroxide was therefore heated in the temperature range of 400-500 °C, and the resulting HZSM-5 zeolite was again tested for its cation-exchange capacity by contacting with a sodium chloride solution. It was found that the results on the sodium ion-exchange capacity were almost 100% reproducible. It can be concluded from the results that sodium hydroxide and hydrochloric acid can be produced from sodium chloride using a thermally stable cation exchanger. The process is environmentally friendly as sodium hydroxide is produced without the production of chlorine. The absence of hazardous materials such as mercury and chlorine also makes the process safer. In the absence of restriction on the production of chlorine, the new process at present does not look to be economically competitive with the existing electrolytic processes. With the development of a new thermally stable cation exchanger with a ion-exchange capacity at least 10 times higher than that of ZSM-5 zeolite with a Si/Al ratio of 10, the process can gain attention from the industrial sector. More research in the development of new thermally stable cation exchangers is therefore recommended. Literature Cited Argauer, R. J.; Landolt, G. R. U.S. Patent 3,702,886, 1972. Breck, D. W. Zeolite Molecular Sieves; John Wiley & Sons: New York, 1974.
Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996 3811 Chu, P.; Dwyer, F. G. Inorganic Cation Exchange Properties of Zeolite ZSM-5; ACS Symposium Series 218; American Chemical Society: Washington, DC, 1983; p 59. Hileman, B.; Long, J. R.; Kirschner, E. M. News FocussChlorine Industry Running Flat Out Despite Persistent Health Fears. C&E News 1994, Nov 21, 12. Jacobs, P. A. Carboniogenic Activity of Zeolites; Elsevier Scientific Publishing Co.: Amsterdam, The Netherlands, 1977. Lecluze, V.; Sand, L. B. Recent Progress Report presented at the 5th International Conference on Zeolites, Naples, Italy, 1984. Nayak, V. S. Studies In Synthetic Zeolites. Ph.D. Thesis, University of Poona, Pune, India, 1982.
Nayak, V. S.; Choudhary, V. R. Acid Strength Distribution and Catalytic Properties of H-ZSM-5: Effect of Deammoniation Conditions of NH4-ZSM-5. J. Catal. 1983, 81, 26.
Received for review January 23, 1996 Revised manuscript received June 10, 1996 Accepted June 12, 1996X IE960043H
X Abstract published in Advance ACS Abstracts, August 15, 1996.
