Separation of Oxygen Isotopic Water by Using a Pressure-Driven Air

Ind. Eng. Chem. Res. 2009, 48, 5431–5438

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Separation of Oxygen Isotopic Water by Using a Pressure-Driven Air Gap Membrane Distillation Jaewoo Kim,*,† Dae-Shik Chang,‡ and Yun-Young Choi§ Nuclear Materials Research DiVision and Nuclear Fusion Research DiVision, Korea Atomic Energy Research Institute, 1045 Daedukdaero, Daejeon 305-353, Republic of Korea, and Korea Sunlight Energy Co. Ltd., 449 Wolyung-Dong, Masan, Kyungnam 631-701, Republic of Korea

Separation of oxygen isotopic water using a newly developed pressure-driven air gap membrane distillation (AGMD) process that is applicable to a multistage MD system was investigated. Pressure-driven (PD) AGMD is distinguished from a currently available AGMD by way of its permeation flux generation. It uses a peristaltic pump to generate the pressure gradient on the membrane interface. Permeation characteristics and isotopic selectivity of four different hydrophobic membranes were evaluated. Permeation fluxes under the various conditions were measured by weighing the collected membrane-permeated water. Permeation fluxes for PD AGMD were higher than those of AGMD as much as 4.2-fold whereas the degree of isotope selectivity was competitive. The oxygen isotopic concentrations of the samples were analyzed by measuring the isotopic ratio H218O/H216O using tunable diode laser absorption spectroscopy and isotopic ratio mass spectrometer. A multistage PD AGMD system was also successfully constructed and operated. 1. Introduction Stable isotope (O-18)-enriched (>90%) water is used as a cyclotron target for the production of the β-emitting radioisotope F-18 whose half-life is 109.7 min, which is essential for a synthesis of radiopharmaceutical [F-18]FDG (fluorodeoxyglucose) used for positron emission tomography diagnoses. For the separation of the oxygen isotopes whose natural abundances are 99.76% for O-16, 0.04% for O-17, and 0.2% for O-18, fractional distillation of water and cold distillation of nitric oxide (NO) have been developed, but only the former is available at the present time. Cold distillation uses different equilibrium vapor pressures between the isotopic species. Even though its stage separation factor (R ∼ 1.03 at T ) 77 K) for O-18 is relatively high, the separation system should be resistive to the corrosive and toxic natures of NO and thermal insulation is also required to maintain the process temperature as low as 77 K. More importantly, an additional material conversion system is necessary to transfer NO to water for cyclotron use. Fractional distillation of water, however, looks to be more efficient than cold distillation, even with its low separation factor (R ∼ 1.0037 at T ) 320 K) for O-18 because of its high process temperature. In addition, the product can be used directly as a cyclotron target without any material conversion steps. As the demand for O-18 stable isotope increases, the demand for a more efficient separation process also grows. Membrane distillation (MD)1-5 and laser processes6-8 have been investigated in recent decades, but MD appears more attractive for a large amount of isotope production. AGMD and VEMD (vacuum-enhanced membrane distillation) under various conditions have also been explored for application of the process to a production system.9,10 For the production of a feasible amount of isotopes, a multistage MD system is required to produce concentrated isotopic water. Permeation flux for AGMD is too * To whom correspondence should be addressed. E-mail: kimj@ kaeri.re.kr. † Nuclear Materials Research Division, Korea Atomic Energy Research Institute. ‡ Nuclear Fusion Research Division, Korea Atomic Energy Research Institute. § Korea Sunlight Energy Co. Ltd.

low to be applied to a multistage system, even though AGMD is more suitable among other available processes. In this investigation, we increased the permeation flux of water by using a newly developed PD AGMD while the isotope selectivity was competitive. For several different hydrophobic membranes made of polyvinylidene fluoride (PVDF), polyether imide (PEI), polysulfone (Psf), and polytetrafluoroethylene (PTFE), the permeation fluxes and isotope selectivity for both AGMD and PD AGMD were evaluated and a multistage PD AGMD system with a PTFE membrane was constructed and evaluated. Figure 1 shows the currently available membrane distillation processes.11 The permeation cell for AGMD constitutes the upper part of the membrane for water flow in (feed) and flow out (product), the air gap for the collection of the membranepermeated water (tail), and the lower part for the cooling fluid as shown in Figure 1a. When hot feedwater flows on the membrane surface, water vapor is generated on the membrane surface based on the equilibrium vapor pressure of water causing different isotopic concentrations between water and water vapor at the same temperature; i.e., the concentration of the heavy molecule H218O in water is higher than that in the water vapor and vice versa for H216O. In addition to the equilibrium vapor pressure effect, the faster diffusion of the lighter gaseous molecules through the membrane pores causes a higher H216O concentration in the permeated water. As a result of the faster diffusion combined with the higher vapor pressure of the lighter species, the product contains more heavy water molecules whereas the tail contains more light water molecules. Because MD utilizes the differential diffusivity between the isotopic water molecules, the isotope selectivity for VEMD shown in Figure 1b can be greatly enhanced by the Knudsen diffusion in the absence of air in the membrane pores. Also, the permeation flux can be dramatically increased by the pressure gradient applied to the membrane interfaces. However, VEMD may not be efficient for the construction of a multistage system because an additional separation system for the tail delivered from the pumping system for each stage is required. For direct contact MD (DCMD) and sweep gas MD (SGMD) shown in parts (c) and (d), respectively, of Figure 1, the construction of a multistage system is also difficult and costly because the

10.1021/ie900277r CCC: $40.75  2009 American Chemical Society Published on Web 05/06/2009

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Ind. Eng. Chem. Res., Vol. 48, No. 11, 2009

Figure 1. Various membrane distillation processes.

Figure 2. Conceptual diagram of the PD AGMD process.

permeated water vapor in these processes is mixed with cooling fluids or sweep gas. Each stage must have a permeated water vapor separation system. 2. Experimental Section 2.1. Pressure-Driven AGMD. Whereas AGMD appears to be suitable for a multistage system, its permeation flux is too low to be refluxed efficiently through the stages. To promote water vapor permeation in AGMD, the feedwater temperature or temperature gradient on the membrane interface must be increased. However, maintaining these temperature conditions in the AGMD system is practically very difficult and the system operation is costly. But, PD AGMD as shown in Figure 2 uses the AGMD structure combined with the VEMD concept to increase the permeation flux. Peristaltic pump in this process (instead of a vacuum pump used in VEMD) applies the pressure gradient to the membrane interface. It also removes the air in the gap and the membrane pores while refluxing the permeated water to the previous stage as a feed at the same time in the case of a multistage system. Pressures in the air gap of the permeation cell could be reduced to about 20 Torr depending on the energy of the peristaltic pump (roller speed). 2.2. Hydrophobic Membranes. Four different types of hydrophobic membranes were used in this investigation, as shown in Figure 3, which were prepared by the Center for Chemical Process, Korea Research Institute of Chemical Technology, except for the PTFE membranes, which obtained from Millipore. Characteristics of the membranes are listed in Table 1. Pore diameters, porosities, and tortuosity factors were

estimated based on the scanning electron microscope (SEM) images of the membranes. 2.3. Experimental System and Procedure. Diagram of the experimental system of a used single permeation cell is shown in Figure 4. For AGMD, the permeation cell consists of a membrane permeation cell, a temperature-controlled feedwater circulation system, and a sample collection trap. For PD AGMD, a peristaltic pump was added to the AGMD setup. Four different types of hydrophobic sheet membranes with an effective area of 12.6 cm2 were used and each membrane was supported by a stainless steel grid. Feedwater was deionized to exclude the ion interactions with the membrane material. The temperature of the feedwater was controlled using a heat bath filled with ethylene glycol. Feedwater was recirculated by a peristaltic pump under the preset feed flow rate. Permeated water (tail) was collected in the trap through a stainless steel heat exchange funnel using the gravity for AGMD and using a peristaltic pump for PD AGMD. Each sample was weighed on a microscale within two decimal points. Temperatures of the feed and cooling fluid (water) were also measured. Samples were collected for more than 10 mL for most cases. The feed temperature, Tf, which was varied at Tf ) 30-60 °C, was defined as the temperature measured at the inlet feed port. The feed flow rate was set at Ff ) 10 mL/min or Ff ) 15 mL/min. The cooling fluid temperature was varied at Tc ) 10-25 °C by using a chiller or tap water. For multistage MD operation, a permeation cell with a larger membrane was used as shown in Figure 5. The ceramic heater inside the permeation cell was used to maintain the feed temperature during system operation because the feed temperature is decreased by the heat loss through the stages. Each


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Figure 3. SEM images of the various types of membranes (a-c, sectional images; d, plane image). Table 1. Characteristics of the Membranes characteristics

PVDF

PEI

Psf

PTFE

pore diameter (µm) porosity (%) thickness (µm) tortuosity factor

3.2 80 80 1

3.0 80 75 1

3.5 80 75 1

1.0 85 165 2

ceramic heater was driven by an ac power supply and the temperature of each cell was proportional integral derivative

Figure 4. Schematic diagram of the single permeation cell system.

(PID) controlled by monitored feed temperatures. To maintain the feed temperature at 50 °C, for example, about 30 W (20 V, ∼1.5 A) electric power per heater was used at each cell. PTFE hydrophobic membrane, for which the effective area was 85.5 cm2, was used for this system. The air gap between the membrane and the heat exchange plate was 1 mm for most cases, except for the air-gap-dependent experiments. In the six-stage PD AGMD system, the product produced from a stage was supplied as a feed to the next stage and the tail (permeated water) was combined with the previous stage feed as shown in Figure 6. Feed, product, and tail were circulated by the peristaltic pumps concurrently. The system was evaluated under the conditions obtained from the preliminary experiments. Oxygen isotopic concentrations were measured by an isotopic ratio mass spectrometer (IRMS; Finnigan MAT Delta Plus) for the samples from the AGMD permeation cell and by a tunable diode laser absorption spectroscopy (TDLAS) system (Figure 7) for the samples from the multistage PD AGMD system, respectively. The TDLAS system was used to measure the absorption peak ratios of the isotopic water H218O/H216O that can calculate the isotopic concentrations by comparing them. The TDLAS compares the rotational-vibrational absorption peak heights of the ν1 + ν3 (∼1.392 µm) combination band between the isotopic molecules in samples.12,13 The degree of the isotopic change R (separation coefficient) is defined as R ) (x/(1 - x))product/(x/(1 - x))tail, where x is a concentration of 18 O. The degree of the 18O isotope separation is also expressed by δ18O[‰], representing the change of 1/1000 in the concentrations. Isotope separation coefficient, therefore, is equal to R ) 1 - δ18O[‰]/1000. The precision of the analytical system was verified prior to measurement of the samples by comparing the isotopic concentrations of identical samples.

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Figure 5. Diagrams of the permeation cell used in a multistage PD AGMD system.

Figure 6. Configuration of the multistage PD AGMD system.

3. Results 3.1. AGMD and PD AGMD of the Various Hydrophobic Membranes. Permeation characteristics for AGMD and PD AGDM were evaluated under various feed temperatures and feed flow rates using four different types of hydrophobic membranes as shown in Figures 8 and 9. Permeation fluxes for the PEI membrane with AGMD were observed to be lower than those for the PTFE membrane. Also, a slight increase of the permeation fluxes for PEI, especially in a high feed temperature region, were observed for a faster feed flow (Ff ) 15 mL/min) than for a lower feed flow (Ff ) 10 mL/min), whereas those for PTFE were negligible. This might be due to the lower

porosity of the PEI membrane and the lower thermal conductivity of PEI (k ∼ 0.22 W/mK) than that of PTFE (k ∼ 0.25 W/mK) which causes a higher temperature polarization for a faster feed flow. Figure 9 shows the permeation fluxes dependent on the feed temperature for various membranes at Ff ) 10 mL/min and Tc ) 20 °C. PTFE membrane shows the highest permeation while the lowest is for the PEI membrane. Dependency of the permeation flux on the feed flow rate was much smaller than the dependency on the feed temperature as expected. Heat loss of the feed is due to its contact with the membrane whose lower surface is exposed to the cold air cooled by the cold heat exchange plate. Temperature difference between the inlet feed


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Figure 7. Layout of the TDLAS system for isotopic water analyses.

Figure 8. Feed flow rate dependent permeation fluxes of PTFE and PEI membranes for AGMD at Tc ) 20 °C.

and outlet product ∆Tf ) Tif - Top was 3 °C at Tf ∼ 35 and 15 °C at Tf ∼ 65 °C, respectively, for Tc ) 20 °C. Permeation flux is in general related to the membranes’ average pore diameters and other parameters such as the membrane porosity, the membrane thickness, and the membrane tortuosity, the membrane porosity being the most important factor affecting the permeation characteristics.14,15 For PD AGMD under most of the feed temperature ranges in these experiments, the pore sizes of the membranes are similar to the mean free path of the gaseous molecules producing the Knudsenviscous diffusion in the deaerated pores. At the feed temperature Tf ) 40, 50, and 60 °C, the mean free paths of the water molecules in the absence of air are ∼1.8, ∼1.1, and ∼0.7 µm, respectively. The driving force in PD AGMD can be increased greatly by the pressure difference, by the membrane interfacial temperature difference, and by the deaeration in the pores, whereas the driving force in AGMD is caused only by the

temperature difference. According to the SEM images in Figure 3, the pore sizes and porosity for the prepared membranes were about the same except for those of the PTFE membrane. As the temperature of the feed and the membrane interfacial temperature gradient increase for AGMD, the equilibrium vapor pressure of the feedwater and the membrane interfacial driving force increase simultaneously, causing an enhancement of the permeation of the water vapor. In addition to the equilibrium vapor pressure effect for AGMD, the membrane interfacial pressure gradient could be applied by a pressure reduction in the air gap for PD AGMD, causing an increase of the permeation flux. Permeation fluxes for PD AGMD could be increased by as much as 4.2-fold for the Psf membrane compared to those for AGMD whereas it is 2 times that for the other membranes. The degree of the isotopic separation of the permeated samples was measured by using IRMS as shown in Figure 10. The ratios of the oxygen isotopic concentrations H218O/H216O in the permeated water (tail) were compared with those in the feeds. The degree of the O-18 isotopic change in the tails compared to that in the feed was between -4‰ and -8‰ depending on the types of the membrane and the process. The PEI membrane produced the highest separation degree, whereas the lowest was produced by the Psf membrane. For most membranes, AGMD produced higher O-18 isotopic changes than PD AGMD. Presumably, isotopic changes for PD AGMD should be higher than those for AGMD according to the diffusion properties, especially when the mean free path of the water vapor is larger than the pore diameter. This must be explained by the increase of the permeation flux for PD AGMD, which offsets the increment of the isotopic concentrations according to the stage separation-cut relationship.16Even though AGMD produced a higher isotopic selectivity, PD AGMD has more advantages for application to the process from engineering standpoints because AGMD is not applicable to a multistage system as a result of its low permeation flux. From these permeation characteristics and the isotope selectivity of the membranes, the PTFE membrane

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Figure 9. Feed temperature dependent permeation fluxes for AGMD and PD AGMD (Tc ) 20 °C, As ) 12.6 cm2, and feed flow rate ) 10 mL/min).

Figure 10. Degree of 18O isotopic changes in the permeated samples for AGMD and PD AGMD at Tf ) 55 °C (35 °C for #3 and #4), Tc ) 25 °C, Ff ) 15 mL/min, and permeated sample volume ∼ 10 mL.

Figure 11. Permeation fluxes for AGMD and PD AGMD for a permeation cell used in the multistage system: Ff ) 20 mL/min without and with cooling fluid (Tc ) 10 °C).

is the best for application to a multistage system with PD AGMD because it produces the highest permeation flux whereas its isotope selectivity is competitive. 3.2. Permeation Characteristics of the Large PTFE Membrane. Permeation cell applicable to a multistage system using a large PTFE sheet membrane (Aeff ) 85.5 cm2) was designed and fabricated as shown in Figure 5a. Feed temperature could be maintained constant by using a submerged ceramic heater installed in the upper part of the permeation cell (Figure 6b) while the feed flows on the membrane. Permeation fluxes were measured under the same conditions as the previous experiments performed using smaller membranes without a heater. In these experiments, the feed temperature Tf was maintained constant during the cell operation and the cooling

fluid temperatures were controlled by using a chiller from 10 to 20 °C. The feed was not recirculated but was collected in a separate reservoir as a product. Figure 11 shows the feed temperature dependent permeation fluxes of the PTFE sheet membrane for both AGMD and PD AGMD. Permeation fluxes produced from these experiments were similar to the previous results, showing about twofold higher permeation flux for PD AGMD over AGMD. Graph also shows the permeation fluxes with and without the cooling fluid so that we can compare the effects of the temperature gradient applied to the membrane interfaces. Permeation fluxes without the cooling fluid were very small, even for PD AGMD. Without cooling the heat exchange plate, the permeation fluxes at a temperature lower than Tf ) 30 °C


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Figure 13. H218O/H216O precision measurement for oxygen isotope production (18O(δ‰) ) ∼1.67‰) by means of TDLAS for a PD AGMD system. Figure 12. Permeation fluxes for AGMD and PD AGMD for a permeation cell for a multistage system temperature gradient dependent: Tf ) 40 °C, Ff ) 5 mL/min.

for PD AGMD and Tf ) 40 °C for AGMD were hardly measurable. Effect of the temperature gradient on the permeation flux is shown in Figure 12. When ∆T ) Tf - Tc is small, the slope of the graph was gradual while it starts to increase beyond ∆T ∼ 15 °C. Also, the air gap dependent permeation fluxes were measured by using different air gaps, 1 and 9 mm, with Ff ) 10 mL/min, Tf ) 45 °C, and ∆T ) 35 °C. For AGMD, the permeation flux for the 9 mm air gap was ∼0.85 L/m2 · h and the 1 mm air gap was ∼1.7 L/m2 · h, respectively. For PD AGMD, the permeation flux was ∼3.8 L/m2 · h with the 1 mm air gap. It is clear that a narrower air gap with PD AGMD could produce a higher permeation flux, which accords with other results.17,18 Furthermore, it was also reported that the permeation flux in similar cases could be increased by as much as 10 times by using DCMD over AGMD.5 On the basis of these results, it can be concluded that the permeation flux is strongly dependent on the feedwater temperature and the membrane interfacial pressure difference. Hence, efficient cooling to generate a membrane temperature gradient with PD AGMD to produce a pressure difference on the membrane interfaces is important for increasing the permeation flux and for use of a higher feed temperature.19 In this regard, the PD AGMD process, as a result, could be applied to efficient operation of a multistage membrane system. 3.3. Multistage PD AGMD System. Because the degree of the oxygen isotopic changes from a single permeation cell is too small, construction of a multistage MD system is required to enrich the O-18 isotopic water to a usable concentration. PD AGMD might be a solution as shown in Figure 6, which shows a multistage PD AGMD system configuration. A constructed multistage system consists of six PD AGMD cells connected in series. The tap water was used as a cooling fluid at Tc ) 20 °C. Feed temperature of each cell was controlled by using a PID (proportional integral derivative) controlled ceramic heater. The feed for the ith stage consists of the product from the (i - 1)th stage combined with the permeated water(tail) from the (i + 1)th stage. The product from the (i - 1)th stage is hence slightly O-18 enriched and is supplied to the ith stage as a feed for further enrichment. A peristaltic pump was connected to the tail outlet of the (i + 1)th stage to reflux the tail and to flow the feed simultaneously. Permeation flux could determine the cut

Figure 14. Degree of isotope enrichment for the six-stage PD AGMD system.

(θ), which is the ratio of the product flow rate and the feed flow rate determining the capacity of the system. Feed flow rate and feed temperature were set at 5 mL/min and 40 °C, respectively. Under these conditions, the cut for the system was θ ) 85%. To determine the oxygen isotopic change in the samples, the TDLAS system shown in Figure 7 was used. Prior to measurement of the isotopic ratios of the samples, two identical samples were injected into the multipath Herriot reference and sample cells, respectively. Precision of the system was evaluated. As shown in Figure 13, the deviation from the center line, which indicates the degree of a measurement error, was ∼1.67‰ for H218O/H216O. Each data point in the graphs was obtained by averaging more than 40 scans of the H218O/ H216O ratios of the samples. Figure 14 shows the isotopic ratios of the final product (6th stage product) and the feed, which produce the enrichment factor β ) 1.023 (isotopic ratio of the product/isotopic ratio of the feed). This means that the product contains 0.205% of O-18 if the O-18 abundance is 0.2% in the feed. Permeation flux was not diminished for a 5 days continuous operation unlike the other MD systems such as desalination since pure water was used as a feed in our case. Even though the system was simply multistaged, this investigation shows the possibility of membrane distillation for oxygen isotope production by using PD AGMD.

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4. Conclusions

Literature Cited

Separation of O-18 isotopic water using a newly designed PD AGMD that was developed from AGMD was carried out for the first time. Because AGMD has a very low permeation flux, it is almost impossible to build a multistage MD system using AGMD for isotope production. Here, we increased the water permeation flux by as much as 4.2-fold with a Psf membrane and by 2.5-fold with a PTFE membrane using PD AGMD compared to AGMD, but the isotopic selectivity was competitive. Under various experimental conditions including the feed temperature, cooling fluid temperature, feed flow rate, and air gap, the permeation fluxes were measured for both AGMD and PD AGMD. Four different types of hydrophobic membranes made of PTFE, Psf, PEI, and PVDF were evaluated while PTFE appears to be the most reliable for the MD production system. As the temperature of the processing water and the membrane interfacial temperature difference increase, the permeation flux of the water increases as a result of higher equilibrium vapor pressure and a greater membrane interfacial driving force. Permeation flux was also increased significantly when the membrane interfacial pressure difference was raised by a peristaltic pump in the PD AGMD process. Oxygen isotope separation using MD was enhanced by a faster diffusion of the lighter water molecules in addition to the differential equilibrium vapor pressures between the isotopic water. Two different analytical methods, IRMS and TDLAS, for isotopic concentration measurement were used, but the latter appears more convenient since there is no complicated conversion of water to CO2. Isotope separation of a single stage was between -4‰ and -8‰ depending on the types of the membrane and the process. Even though a slightly higher degree of separation was observed for AGMD than for PD AGMD, the latter appears to be more efficient from an operational system point of view. About a 3% O-18 enrichment was achieved by operating the simply constructed six-stage PD AGMD system. Consequently, for future application of the MD process to isotope production, it is important to find the optimum conditions between the experimental parameters such as the feed flow rate, the permeation rate, and the isotope selectivity, which determine the system efficiency.

(1) Hook, W. A. van.; Chmielewski, A. G.; Zakrzewska-Trznadel, G.; Miljevic, N. R. Method of Enrichment of Oxygen-18 in Natural Water. U.S. Patent 5057225, 1991. (2) Chmielewski, A. G.; Zakrzewska-Trznadel, G.; Miljevic, N. R.; Hook, W. A. van. Investigation of the Separation Factor between Light and Heavy Water in the Liquid/Vapor Membrane Permeation Process. J. Membr. Sci. 1991, 55, 257. (3) Chmielewski, A. G.; Zakrzewska-Trznadel, G.; Miljevic, N. R.; Hook, W. A. van. 16O/18O and H/D Separation of Water through an Hydrophobic Membrane. J. Membr. Sci. 1991, 60, 319. (4) Zakrzewska-Trznadel, G.; Chmielewski, A. G.; Miljevic, N. R. Separation of Protium/Deuterium and Oxygen-16/Oxygen-18 by Membrane Distillation. J. Membr. Sci. 1996, 113, 337. (5) Chmielewski, A. G.; Zakrzewska-Trznadel, G.; Miljevic, N. R.; Hook, W. A. van. Multistage Process of Deuterium and Heavy Oxygen Enrichment by Membrane Distillation. Sep. Sci. Technol. 1997, 32 (1-4), 527. (6) Sander, R. K.; Loree, T. R.; Rockwoon, S. D.; Freund, S. M. ArF Laser Enrichment of Oxygen Isotopes. Appl. Phys. Lett. 1977, 30, 150. (7) Bergman, R. C.; Homicz, G. F.; Rich, J. W.; Wolk, G. L. C-13 and O-18 Isotope Enrichment by Vibraional Energy Exchange Pumping of CO. J. Chem. Phys. 1983, 78 (3), 1281. (8) Sugita, K.; Majima, T.; Arai, S. 18O-Selective Infrared Multiple Photon Decomposition of Natural and 18O-Enriched Diisopropyl Ethers. J. Phys. Chem. A 1999, 103, 4144. (9) Kim, J.; Park, S. E.; Kim, T. S.; Jeong, D. Y.; Ko, K. H.; Park, K. B. Separation Characteristics of Oxygen Isotopes with Hydrophobic PTFE Membranes. Membr. J. (Korean written) 2003, 13 (3), 154. (10) Kim, J.; Park, S. E.; Kim, T. S.; Jeong, D. Y.; Ko, K. H. Isotopic Water Separation using AGMD and VEMD. Nukleonika 2004, 49 (4), 137. (11) Krell, E. Handbook of Laboratory Distillation; Elsevier Publishing Company: London, 1963. (12) Park, S. E.; Jeong, D. Y.; Kim, J.; Ko, K. H.; Jung, E. C.; Kim, C. J. Measurement of Oxygen Isotope Ratios using Tunable Diode Laser Absorption Spectroscopy. Proc. CLEO/Pacific Rim. Taipei, Taiwan. 2003, 1, 241. (13) Kerstel, E. R. Th.; Gagliardi, G.; Gianfrani, L.; Meijer, H. A. J.; Trigt, R. van.; Ramaker, R. Determination of the 2H/1H, 17O/16O, and 18O/ 16 O Isotope Ratios in Water by Means of Tunable Diode Laser Spectroscopy at 1.39 µm. Spectrochim. Acta, Part A 2002, 58, 2389. (14) Alklaibi, A. M.; Nior, N. Membrane-distillation Desalination: Status and Potential. Desalination 2004, 171, 111. (15) Schneider, K.; Holz, W.; Woolbeck, R. Membranes and Modules for Transmembrane Distillation. J. Membr. Sci. 1988, 39, 25. (16) Benedict, M.; Pigford, T. H.; Levi, H. W. Nuclear Chemical Engineering; McGraw-Hill Book Company: New York, 1981. (17) Guijit, C. M.; Racz, I. G.; Heuven, J. W. van.; Reith, T.; Haan, A. B. de. Modeling of a Transmembrane Evaporation Module for Desalination of Seawater. Desalination 1999, 126, 119. (18) Liu, G. L.; Zhu, C.; Cheung, C. S.; Leung, C. W. Theoretical and Experimental Studies on Air Gap Membrane Distillation. Heat Mass Transfer. 1998, 34, 329. (19) Schofield, R. W.; Fane, A. G.; Fell, C. J. D. Heat and Mass Transfer in Membrane Distillation. J. Membr. Sci. 1987, 33, 299.

Acknowledgment This work was supported by the Mid-Long Term Nuclear Energy Research Program operated by the Ministry of Education, Science and Technology in the Republic of Korea. We also greatly appreciate the help of Dr. Grazyna ZakrzewskaTrznadel from INCT (Institute of Nuclear Chemistry and Technology) in Poland for analysis of isotopic concentration of the samples in this investigation.

ReceiVed for reView February 18, 2009 ReVised manuscript receiVed April 13, 2009 Accepted April 20, 2009 IE900277R

Separation of Oxygen Isotopic Water by Using a Pressure-Driven Air

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