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Ind. Eng. Chem. Res. 2011, 50, 378–381
Method for the Production of Deuterium-Depleted Potable Water Feng Huang and Changgong Meng* Department of Chemistry, Dalian UniVersity of Technology, Dalian 116024, China
A study of the utilization of dual-temperature catalytic exchange between water and hydrogen for the production of deuterium-depleted water is presented. We use a novel catalyst with excellent physical properties for the hot tower of the isotopic exchange. The deuterium-depleted water obtained from the experiment is in agreement with the theoretical consideration on deuterium content at 80 °C when λ is about 1.5. The deuterium-depleted water with 126.3 ppm D2O is gained when λ is about 2 under 80 °C. This kind of water can be used as ordinary drinking water and in cosmetic and hygiene products. 1. Introduction The exceptional properties of heavy water as a neutron moderator make it useful in nuclear reactors. The significance of deuterium in the nuclear industry is also well-known. Various methods have been developed for the separation and purification of deuterium,1-5 especially for the production of heavy water,6 such as, chemical exchange, liquid hydrogen distillation, cryogenic adsorption, and thermal diffusion. Nevertheless, accumulating evidence indicates that deuterium in drinking water can be detrimental to health. For instance, there is evidence to show that high concentration of deuterium in water (heavy water) will cause the loss of activity and various diseases in higher animals such as quail,9 trout,11 and others,7and in addition, some aquatic plants stop growing and developing in heavy water.8,9 Contrarily, decreasing of the deuterium concentration will improve the biological activity of the water. Natural water is a mixture of H2O and D2O in which the concentration of deuterium is approximately 150 ppm. Deuterium-depleted water in which the concentration of deuterium is less than 80 ppm is usually used in medical treatment. Deuterium-depleted water with about 125 ppm deuterium is generally used in ordinary drinking water and in the cosmetic industry. The function of deuterium-depleted water can be divided approximately into two aspects. First, deuterium-depleted water could promote animal and plant growth and play a significant role in health care. For instance, there is an increase in the rate of photosynthesis of plants and growth promotion effects for agricultural products and aquatic animals with use of low deuterium water as compared to control groups using standard water.10-13 Second, deuterium-depleted water can be applied to cancer therapy. The daily drinking water of the patients is replaced by deuterium-depleted water, which is administered as an anticancer agent besides conventional therapy, and it remarkably prolonged the survival time of the patients.14-17 Therefore, the subject of the production of deuterium-depleted water has attracted much interest of scientists. Deuterium enrichment and depletion are simultaneous processes. Various technologies have been developed for the production of deuterium poor water, such as electrolysis,18 distillation,19,20 desalination from seawater,7 and Girdler-sulfide (G-S) process.21 Separation by electrolysis is based on the difference between the mobility coefficients of the ions. This method is very expensive as electrolysis is a high-power consumption process. * To whom correspondence should be addressed. Tel.: +86-41184708545. Fax: +86-411-84708545. E-mail:
[email protected].
Separation by distillation is based on the difference between the boiling points of the molecules. This method uses simple equipment and is a simple operation technology, but has highenergy consumption. Desalination from seawater by using solar energy is an energy-saving and environment friendly method, but it is inefficient. The G-S process is a dual-temperature exchange method between hydrogen sulfide and water. This process uses the very hazardous material hydrogen sulfide. Besides these four methods, there are two processes that could be applied for producing deuterium-depleted water. One process is combined electrolysis and catalytic exchange (CECE). An alternative process is bithermal hydrogen water (BHW). In each stage there is an upper cold tower where the deuterium transfers from the hydrogen to the liquid water, and a lower hot tower where the deuterium transfers from the water to the hydrogen gas. The CECE and BHW processes rely upon hydrophobic catalysts to catalyze the exchange reaction between hydrogen and water. After several decades of research and development, there are mainly three types of hydrophobic catalyst used in the liquid-phase catalytic exchange process now, including a Pt/C/inert carrier (Pt/C/IC),22,23 a Pt/C/polytetrafluoroethylene (Pt/C/PTFE),24,25 and a Pt/styrene divinylbenzene copolymer (Pt/SDB).26,27 The Pt/C/IC has high strength, good chemical stability, and strong activity, but at the same time it has a complex forming technique. The size and shape of the Pt/C/ PTFE are easily controlled, whereas the utilization ratio of platinum is low. The Pt/SDB has good activity, yet it shows low strength and small particle size. In comparison with these three types of catalyst, our new catalyst used in this research has higher strength, lower pressure drop, and simpler forming technique besides basic catalytic performance. The aim of our work is to provide an economical, efficient, and environmental protection technique for producing deuteriumdepleted water. In this paper, we use the dual-temperature liquidphase catalytic exchange to producing deuterium-depleted water. During the hydrogen-water isotopic exchange process, a novel composite hydrophobic catalyst consisting of platinum directly supported on Teflon (Pt/PTFE) is used.28 The present work mainly focuses on the comparison between the experimental results and the theoretical results on the deuterium concentration of the deuterium-depleted water. 2. Methods and Materials 2.1. Experimental Flow Scheme. Figure 1 shows a schematic flow diagram of the experimental apparatus, which mainly
10.1021/ie101820f 2011 American Chemical Society Published on Web 11/29/2010
Ind. Eng. Chem. Res., Vol. 50, No. 1, 2011
CaH2 + 2D2O ) Ca(OD)2 + 2HD
379
(1)
Figure 2 shows a simple device of the HD preparation. 3. Theoretical Background The overall catalytic exchange of hydroen and deuterium between liquid water and hydrogen gas consists of the following two-step reactions:29 HDO(vapor) + H2O(liquid) S HDO(liquid) + H2O(vapor)
(2)
HD(gas) + H2O(vapor) S HDO(liquid) + H2(gas)
(3)
which totalized becomes the isotopic exchange HD(gas) + H2O(liquid) S HDO(liquid) + H2(gas)
Figure 1. Experimental apparatus.
consists of a nitrogen gas source, a hydrogen gas source, a cold tower, a hot tower, and some water tanks. 2.2. Preparation and Process of the Experiment. For the isotopic exchange reaction between the hydrogen gas and the liquid water, two types of hydrophobic catalysts have been used, i.e., platinum on Teflon (Pt/PTFE, 3 mm long, 3 mm wide, and 1.5 mm thick) applied in the hot tower and platinum on styrene-divinylbenzene copolymer (Pt/SDB, 2 mm sphere) applied in the cold tower. The Pt/SDB catalyst support is fashioned by the droplet agglomeration method, using a benzene solution of divinylbenzene to form small spheres. The Pt/PTFE catalyst support is formed by compression molding. Both types of catalysts consist of 0.8 wt % Pt deposited on the catalyst supports. The packing density of the Pt/SDB is 0.35 g mL-1, and the packing density of the Pt/PTFE is 1.2 g mL-1. The reactor tower is made of a Pyrex glass tube (Φ ) 2 cm, l ) 60 cm). The tower is equipped with a water jacket, through which the thermostatted water flows to maintain a constant temperature along the tower, and a hydraulic guard is installed at the bottom. The upper hot tower is filled with Pt/PTFE catalyst (150 mL, 180 g), and the lower cold tower is filled with Pt/SDB catalyst (150 mL, 53 g). The bottom and top of both catalyst beds are packed with hydrophilic packing. The system is initially purged with nitrogen gas to prevent hydrogen explosion. The hydrogen gas is electrolytic hydrogen with 142 ppm D2, which is preheated in the heater and fed into the bottom of the cold tower. The deuterated water is distilled water with 145 ppm D2O, which is fed in at the top of the cold tower and the hot tower. The hydrogen, flowing countercurrent to the water, passes through the catalyst bed where the isotopic exchange takes place. The hydrogen stream is released from the top of the hot tower and the condensed water vapor returns to the tower. 2.3. Analysis of Deuterium Content. The deuterium content is measured by gas chromatography (GC). For the gas phase, the deuterium content is measured in-line by GC (STCD g 4000 mV mL mg-1). For the liquid phase, the deuterium-depleted water with different deuterium content is converted into HD with corresponding deuterium content by chemical reaction. The reaction equation is as follows:
(4)
The first equation expresses the vapor-liquid equilibrium, that is, the transfer of deuterium from water vapor to liquid water accompanied by evaporation and condensation. The second equation is the transfer of deuterium between hydrogen and water vapor. The second reaction occurs only on the surface of the catalyst, whereas the first reaction takes place at any gas-liquid interface. The determination of the separation factor between two chemical species such as hydrogen and water generally requires simultaneous analysis of both components under conditions that do not disturb the isotopic equilibrium. R ) x(1 - ye)/ye(1 - x)
(5)
where x is the molar fraction of deuterium in liquid phase, ye is a molar fraction of deuterium in gas phase in equilibrium, and R is the separation coefficient. This procedure can be simplified when there is a large molar excess of one component relative to the other. When the molar fraction of deuterium in liquid phase x is very low, the ye is expressed as ye ) x/R
(6)
The temperature dependence is observed for all the binary mixtures of hydrogen isotopes on the separation and purification. The separation coefficient between hydrogen and deuterium RH-D could obtain from an empirical formula as follows:30,31 ln RHD ) -0.1636 + 333.7/T + 33840/T2
(7)
For the two-step reaction of isotope exchange, it has been well-known that the equilibrium constant of the transfer of deuterium between hydrogen and water vapor increases with decreasing temperature.30 Nevertheless, the equilibrium constant of the vapor-liquid equilibrium decrease with decreasing temperature. Thus, there is optimal temperature in this isotope
Figure 2. HD preparation installation.
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Table 1. Chemical Parameters and Calculated Valuesa
I II a
pressure (kPa)
temperature of column (°C)
R
hydrogen feed (ppm D2)
hydrogen extracted (ppm D2)
water feed (ppm D2O)
water extracted (ppm D2O)
101 101 101 233
30 80 30 125
3.689 2.863 3.689 2.429
142 39.3 142 39.3
39.3 50.7 39.3 59.7
145 145 145 145
247.7 133.7 247.7 124.6
Molar flow ratio of gas to water (λ): 1.
exchange. To enhance the separation efficiency, it is necessary to decrease temperature without reducing the molar fraction of water vapor. In the course of the hydrogen-water exchange, in low temperature, the deuterium transfers from hydrogen to water, and in high temperature, the deuterium transfers from water to hydrogen, and thus, to applied dual-temperature exchange to get deuterium-depleted water. 4. Results and Discussion 4.1. Theoretical Consideration. Chemical parameters during the liquid-phase catalytic exchange are considered to be stable within the ranges determined by the technological process, such as temperature, pressure, and molar flow ratio of hydrogen gas to feedwater. The concrete data is given in Table 1. In the course of the isotope exchange, in the cold tower, the deuterium transfers from the hydrogen to liquid water, at the same time, the deuterium-depleted hydrogen gas is obtained, whereas in the hot tower, the deuterium transfers from the water to the hydrogen gas and the deuterium-depleted water is produced. The deuterium content of the deuterium-depleted water depends on the deuterium content of exchange hydrogen. Thus, the deuterium-depleted hydrogen gas from the cold tower is fed into the bottom of the hot tower, from which the deuteriumdepleted water could be obtained. In the process of the hydrogen-water isotopic exchange in countercurrent, the feed gas is electrolytic hydrogen with 142 ppm D2 and the feedwater is distilled water with 145 ppm D2O. On the basis of the mass balance, yt - y b ) x t - x b
(8)
where y is a molar fraction of deuterium in gas phase and x is a molar fraction of deuterium in liquid phase. The subscripts “b” and “t” mean the bottom of the catalyst column and the top of the catalyst column. As the exchange reaction reaches equilibrium, yt ) ye ) xt /R
(9)
xb ) (1 - 1/R)xt + yb
(10)
Table 2. Influence of the Molar Flow Ratio of Hydrogen Gas to Feedwater on the Deuterium Content of the Watera molar flow ratio of gas to water (λ)
water feed (ppm D2O)
water extracted (ppm D2O)
1 1.5 2
145 145 145
139 132.7 126.3
a b c
a Water flow rates: 24 mL h-1 for a; 18 mL h-1 for b; 12 mL h-1 for c. The following conditions were common for all tests: Temperature of cold tower: 30 °C; temperature of hot tower: 80 °C; pressure: 101 kPa; gas flow rate: 8.6 mL s-1.
temperature of the cold tower is 30 °C and the testing temperature of the hot tower is 80 °C. The deuterium concentrations of the water are shown in Table 2. The relative standard deviation RSD % of the experimental data is less than 3%. Data in Table 2 show that the experimental data are very consistent with the theoretical prediction when the molar flow ratio (λ) is about 1.5, and the deuterium-depleted water with 126.3 ppm D2O is obtained when λ is about 2. Generally, in the experimental process, the hydrogen-water exchange is not complete. Consequently, the hydrogen-water exchange rate increases when the hydrogen flow rate increases to a certain value. In summary, through optimization of the experimental conditions, the deuterium-depleted water with requisite deuterium content can be gained in the hydrogen-water isotopic exchange process. 5. Conclusions In this paper, a new method for the production of deuteriumdepleted potable water is successfully developed. This method is simple and convenient, highly efficient, and an energy-saving source. When the test conditions are reformed and adjusted, the experimental results are in agreement with the theoretical results. We have obtained the deuterium-depleted water with 132.7 ppm D2O when λ is about 1.5, and with 126.3 ppm D2O when λ is about 2 at 80 °C. Therefore, because of its advantages, such as being economical, environmentally friendly, and pollution-free, this method could be applied to the industrial-scale introduction of deuterium-depleted water.
So
In the theoretical consideration, the molar flow ratio of hydrogen gas to feedwater (λ) is considered as 1. In Table 1, the chemical parameters and the calculated results are summarized. Data in Table 1 illustrate that the deuterium content of the water all decreases to some extent. During 30-80 °C dualtemperature catalytic exchange, the deuterium content of the water decreases from 145 to 133.7 ppm. During 30-125 °C dual-temperature catalytic exchange, the deuterium content of the water decreases from 145 to 124.6 ppm. 4.2. Experimental Results. Considering the effect of the temperature and the pressure on glass apparatus, the testing
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ReceiVed for reView August 31, 2010 ReVised manuscript receiVed October 31, 2010 Accepted November 16, 2010 IE101820F
