Application of Proton-conducting by pengtt


									 Journal of NUCLEAR SCIENCE and TECHNOLOGY, Vol. 41, No. 8, p. 863–870 (August 2004)


             Application of Proton-conducting Ceramics and Polymer Permeable
                         Membranes for Gaseous Tritium Recovery
                    Yamato ASAKURA1; Ã, Takahiko SUGIYAMA1 , Takao KAWANO1 , Tatsuhiko UDA1 ,
                    Masahiro TANAKA2 , Naruhito TSUJI2 , Koji KATAHIRA3 and Hiroyasu IWAHARA4
                                  National Institute for Fusion Science, Oroshi-cho, Toki-shi, Gifu 509-5292
                                  Nippon Kucho Service Co. Ltd., Terugaoka, Meitou-ku, Nagoya 465-0042
                                             TYK Co. Ltd., Ohbata-cho, Tajimi-shi, Gifu 507-8607
                              Professor Emeritus, Nagoya University, Shikenya, Moriyama-ku, Nagoya 463-0034
                                (Received December 12, 2003 and accepted in revised form May 12, 2004)

                In order to carry out deuterium plasma experiments on the Large Helical Device (LHD), the National Institute for
            Fusion Science (NIFS) is planning to install a system for the recovery of tritium from exhaust gas and effluent liquid.
            As well as adopting proven conventional tritium recovery systems, NIFS is planning to apply the latest technologies
            such as proton-conducting ceramics and membrane-type dehumidifiers in an overall strategy to ensure minimal risk in
            the tritium recovery process. Application of these new technologies to the tritium recovery system for the LHD deu-
            terium plasma experiment is evaluated quantitatively using recent experimental data.
              KEYWORDS: tritium, tritium separation, tritium monitoring, proton-conducting ceramic, hydrogen pump,
              membrane filter, membrane dehumidifier, tritium recovery system, Large Helical Device

                                                                        sorption process have also been studied such as application
I. Introduction
                                                                        of gas separation membranes.10–14) In this study, a dehumidi-
   Deuterium plasma experiments are currently being plan-               fier using a permeable polymer membrane instead of molec-
ned for the Large Helical Device (LHD) of the National In-              ular-sieve adsorbers is evaluated in the treatment of exhaust
stitute for Fusion Science (NIFS), following the present hy-            gases emitted during inspection and maintenance work on
drogen plasma experiments. Under the deuterium experi-                  the LHD.
ment conditions, it is estimated that 430 MBq of tritium will              Application of these two technologies will be effective in
be generated in each discharge shot, involving the injection            reducing both the size of the treatment equipment and the
of a neutral deuterium (D) beam into D plasma, leading to a             quantity of radioactive waste after treatment in LHD deute-
maximum of 370 GBq (10 Ci) of tritium generated each                    rium experiments.
   The generated tritium must be recovered before emission,
                                                                        II. Specifications of Targeted Treatment System
with the annual environmental emission specified at less than
3.7 GBq (0.1 Ci). The Pd–Ag membrane permeator is one                      Configurations of the exhaust gas/liquid treatment sys-
candidate for tritium recovery in the LHD plasma exhaust                tems are shown in Figs. 1(a) and (b). Figure 1(a) shows
gases treatment system. Intensive study has focused on proc-            the system designed for recovering tritium during the plasma
esses using Pd–Ag membrane permeators for application in                experiment and Fig. 1(b) details the system during inspec-
plasma exhaust processing and recycling, especially for                 tion and maintenance of the LHD vacuum vessel.
use in the International Thermonuclear Experimental Reac-                  The design conditions of these two systems are summariz-
tor (ITER).2–6) However, application of these permeators to             ed in Table 1. Tritium concentrations at the different units
low tritium-concentrations and to water vapor processing                of these systems are shown in Table 2.
seems difficult. Instead of a Pd membrane, a blanket tritium                 In the deuterium experiment, exhaust gas from the vacu-
recovery system with an electrochemical hydrogen pump us-               um pump is stored in the temporary storage tank. Details
ing a proton-conducting membrane has been proposed.7) In                of the tritium chemical forms and composition of the stored
this study, an electrochemical hydrogen pump using a pro-               gas are shown in Sec. III-2(1)(a). The stored gases (He gas
ton-conducting membrane is evaluated in the treatment of                including H, D and T) are introduced into the vacuum pump-
plasma exhaust gases from the LHD experiments.                          ing-gas treatment unit and after tritium removal the gas is re-
   Among atmosphere detritiation systems, methods which                 leased from the stack. Treatment flow rate was assumed to
collect gaseous tritium as tritiated water using the combined           be 0.02 N m3 /h and the tritium concentration of the process-
processes of catalytic oxidation and adsorption have been               ed gas was calculated to be 5,000 Bq/cm3 , the mean value
widely applied in Japan and overseas.8,9) To develop more               for treating the annual tritium emission (10 Ci/yr) over
compact and cost-effective systems, alternatives to the ad-              4,000 h operation. Tritium-recovery efficiency was fixed at
                                                                        99%. At this efficiency and after dilution by ventilation with
                                                                        non-radioactive gas flow, the tritium concentration at the
     Corresponding author, Tel. +81-572-58-2321, Fax. +81-572-58-       stack outlet would be reduced to a value less than
     2610, E-mail:

864                                                                                                                     Y. ASAKURA et al.

          During LHD Operation                                                                             Stack
      Large Helical Device(LHD) Building                                   Ventilation system
                                                                       T           5             Tritium gas monitor

           LHD plasma vacuum vessel                              (Inlet duct) (Outlet duct)

                                               Vacuum pumping                                   storage tank

                        Tritium safety
                         storage unit                                      Vacuum pumping–gas
                                                                              treatment unit
                                                                                                            Tritium gas
                 (Hydrogen absorbing alloy bed,etc)

                                                                 Flow of gas                           Development item


             During LHD Maintenance                                                                         Stack
         Large Helical Device(LHD)Building                                  Ventilation system
                                                                       T           5             Tritium gas monitor
                    (from inlet duct)

                                 LHD plasma
                  Air           vacuum vessel                    (Inlet duct) (Outlet duct)
               dehumidifier                           Ventilation equipment

                                                                  Vacuum-vessel purge–gas                  Tritium gas
                                                                       treatment unit                        monitor

                                        Recovery tritium
                                                                                              Deliver vessels to
                                        enrichment unit
                                                                                              collecting agency
              Recovery water tank                          Enriched-water storage vessel
                                                                                                         Tritium gas

              Flow of water
                                                                       Tritium monitor (Liquid scintillation counter)
              Flow of gas
               Development item                                                Drainage to sewerage
                                              Waste water tank


      Fig. 1 (a) Exhaust gas and effluent liquid treatment system for LHD during operation (b) Exhaust gas and effluent liquid
        treatment system for LHD during maintenance

                                                                            JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY
Application of Proton-conducting Ceramics and Polymer Permeable Membranes for Gaseous Tritium Recovery                                   865

                                        Table 1 Conditions for gaseous tritium recovery system of LHD

                                                                              Vacuum pumping-gas            Vacuum-vessel purge-gas
                                                                                 treatment unit                  treatment unit
           Maximum treatment flow rate (N m3 /h)                                        0.02                         1,000
           Operation time (h/yr)                                                   4,000                            4,000
           Tritium generation amount (GBq/yr)                                        370                              370
           Tritium concentration in treatment gas (Bq/cm3 )                        5,000                                0.1

                  Table 2 Tritium concentrations for different units for application of the gaseous tritium recovery system

                                                                         Vacuum pumping-gas                 Vacuum-vessel purge-gas
                                                                            treatment unit                       treatment unit
           Tritium-recovery efficiency (%)                                            99                              99
           Tritium conc. of the gas (Bq/cm )
             Inlet of treatment unit                                             5,000                              0.1
             Outlet of treatment unit                                               50                              0.001
             Outlet of stacka)                                                  8Â10À5                             8Â10À5
           Regulation limit concentration (Bq/cm3 )
             Chemical form of water vapor                                                          5Â10À3
             Chemical form of hydrogen gas                                                           70
                Total exhaust gas flow rate from the stack is 12,500 N m3 /h

5Â10À4 Bq/cm3 , which is one-tenth of the regulation limit
                                                                              III. Evaluating Application of New Technology
for release as water vapor. Considering that the regulation
limit for release as hydrogen gas is 70 Bq/cm3 , recovery                     1. Application Plan of New Technology
of tritium in the form of hydrogen gas is preferable to that                     From the viewpoint of minimizing the risk of tritium han-
of water vapor.                                                               dling, the following basic development concepts are consid-
   During inspection and maintenance work of the inside of                    ered:
the LHD vacuum vessel, the vessel is ventilated with dry air,                 ‹ Treatment of tritium in the chemical form of hydrogen
and the remaining tritium is purged along with the air. The                       instead of water
tritium-purging gas (air including H, D and T) is flowed to                    › Reduction of radioactive waste contaminated with triti-
the vacuum-vessel purge-gas treatment unit and after tritium                      um.
removal the air is released from the stack. For this study, the               (1) Vacuum Pumping-Gas Treatment Unit
treatment flow rate was assumed to be a maximum of                                The conventional method of oxidation of tritium and re-
1,000 N m3 /h and tritium concentration in the processed                      moval of the tritiated moisture using adsorbents such as mo-
gas calculated to be 0.1 Bq/cm3 , the mean value for treating                 lecular sieves could be applied to the LHD system. However,
the generated tritium during continuous operation. Tritium                    for the present unit, it is expected that small amounts of tri-
recovery efficiency was again fixed at 99%. At this efficiency                     tiated water will be generated but with a comparatively high
and dilution by ventilation with non-radioactive gas flow, the                 concentration compared to the vacuum-vessel purge-gas
tritium concentration at the stack outlet would be less than                  treatment unit. Considering development concept ‹, the
5Â10À4 Bq/cm3 . The tritium is recovered in the form of wa-                   new system targets the recovery of tritium in the form of iso-
ter, and the tritiated water accumulated in a tank. It is expect-             topically enriched hydrogen gas.15) In order to pump out the
ed that by using dry air instead of natural, humid air the gen-               hydrogen gas selectively, proton-conducting ceramics7,16–20)
eration rate of tritiated water will be reduced to 30 t/yr. The               have been investigated. A proton-conducting ceramic is not
tritiated water accumulated in the tank will be reduced in                    suitable for generating perfectly dry hydrogen gas as in the
volume and thus enriched in tritium before handling by a                      case of a Pd-alloy diffusion membrane. However, it has
collecting agency.                                                            the ability to decompose hydrogen compounds such as water
   As well as considering these two tritium recovery units,                   vapor and methane into hydrogen molecules and can also ex-
the in-line monitor, which can detect tritium released from                   tract small amounts of hydrogen molecules selectively by
the stack with a sensitivity of less than 5Â10À4 Bq/cm3 , is                  application of a direct voltage between electrodes of the pro-
planned to be developed based on an electrochemical hydro-                    ton-conducting ceramic. That is, when a direct current is ap-
gen pump.                                                                     plied to an electrochemical cell with proton-conducting solid
                                                                              electrolyte and hydrogen gas is supplied to the anode, the hy-
                                                                              drogen ionizes to form protons. These protons are transport-
                                                                              ed through the electrolyte to the porous cathode, where they

VOL. 41, NO. 8, AUGUST 2004
866                                                                                                            Y. ASAKURA et al.

                                 LHD plasma
                                vacuum vessel

                                                                  Tritium gas monitor

                                                                                            Tritium gas monitor

                                           Hydrogen gas pumping
                         storage tank      (Proton conducting ceramic)
                                                                       Hydrogen isotope
                                                                     separation apparatus
                                                                    (Pressure Swing Adsorption)
                       Flow of gas                                                       Hydrogen absorbing bed

                                     Fig. 2 Design of the vacuum pumping-gas treatment unit

are discharged to form hydrogen gas.18) The configuration           with sufficient accuracy to meet regulations using a liquid
and process flow of the treatment system for the proton-con-        scintillation method. However, a real-time monitor has not
ducting ceramic device are shown in Fig. 2. In this system,        been commercially available for concentrations under
vacuum pumping gas accumulated in the temporary storage            5Â10À4 Bq/cm3 , which is 1/10 of the regulation value for
tank is delivered to the proton-conducting ceramic device at       tritiated water vapor in the exhaust gas. Using the hydrogen
a constant flow rate of 0.02 N m3 /h, and the hydrogen gas          pump described in Chap. I, it is possible to lower the effec-
containing tritium is selectively extracted from the process-      tive detection limit by greater than an order of magnitude, by
ing gas. The tritium included in the hydrogen gas will then        concentrating the hydrogen-isotope gas (including tritium)
be enriched by Pressure Swing Adsorption (PSA) equipment           and by removing the radon gas which is mixed in at the mon-
operated at liquid nitrogen temperature. Tritium-enriched          itoring stage. The configuration and process flow of the mon-
hydrogen gas can then be stored in a stable form as a metal-       itoring system applying the proton-conducting ceramic are
lic hydride.                                                       shown in Fig. 4. The monitoring gas is introduced to the pro-
(2) Vacuum-Vessel Purge-Gas Treatment Unit                         ton-conducting ceramic device and only the hydrogen-iso-
   For this unit, it is difficult to apply the same system as for    tope gas is extracted to the circulating carrier gas. Extraction
the vacuum pumping-gas treatment unit because the gas              of hydrogen continues until the tritium concentration in the
treatment capacity is about 1,000 times larger. With respect       carrier gas is higher than the detection limit of the radiation
to development concept ›, waste generation could be re-            detector. A proportional counter is used as a radiation detec-
duced through the use of a polymer membrane dehumidifi-             tor for this high sensitivity monitoring system.
er15,21) instead of an absorbent column. When using molecu-
lar sieves, a dew point of less than À60 C is readily achieved    2. Evaluation of Potential Application
at room temperature if the amount of absorbed water is 10%         (1) Application of Proton-Conducting Ceramic to Vacuum
or less. However, if a dew point of less than À60 C could be           Pumping-Gas Treatment Unit
obtained using a polymer membrane dehumidifier, the                 (a) Gas Amount and Composition
equipment could be reduced in size and a more stable dehu-            The estimated amount and composition of the gas to be
midifying performance could be expected. Molecular-sieve           treated are shown in Table 3. The processed gas is a mixture
adsorption equipment requires regeneration by alternately          of the exhaust gas from helium glow discharge operations
switching two columns. The configuration and process flow            during cleaning of the vacuum vessel wall and the exhaust
of the system applying the polymer membrane dehumidifier            gas during plasma experiments, in which hydrogen is the
are shown in Fig. 3. The air used to purge the tritium from        main isotope. The maximum quantity of exhaust gas from
the vacuum vessel is delivered to the catalytic oxidation          the deuterium plasma experiments is estimated to be
equipment. The tritiated water vapor is recovered by the           80 N m3 /yr. Helium is the main component of the processed
polymer membrane dehumidifier. To reduce the volume of              gas stored in the temporary storage tank, and is present in the
tritiated water, dry air is used to purge the vacuum vessel.       exhaust gas from both deuterium plasma experiments and
The polymer membrane dehumidifier has additional applica-           helium discharge cleaning operations. The hydrogen compo-
tion in producing this dry air.                                    nent composition is estimated to be 8.3% hydrogen, 6.3%
(3) High Sensitivity In-line Tritium Monitor                       water vapor and 0.2% methane. That is, total hydrogen con-
   The tritium concentration in exhaust gas can be monitored       tent in the processed gas is 15% by molecular ratio or

                                                                         JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY
Application of Proton-conducting Ceramics and Polymer Permeable Membranes for Gaseous Tritium Recovery                            867

                                                                        (From inlet duct)


              LHD                                              Compressor       dehumidifier
              vacuum                                                                                       Tritium gas
              vessel                                                                                       monitor

                    Flow of water         Catalytic oxidation
                     Flow of gas             apparatus                                           Temporary storage tank

                                   Fig. 3 Design of the vacuum-vessel purge-gas treatment unit

                    Carrier gas                                     Circular pump
                 Monitoring gas                                                                              Exhaust

                                                                              Tritium         Proportional
                                Hydrogen gas pumping                         enriched           counter
                                     apparatus                                  gas
                               (Proton conducting ceramic)
                            Flow of gas

                                   Fig. 4 Design of the in-line gas phase tritium monitoring unit

                     Table 3 Composition of the vacuum pumping gas stored in the temporary storage tank

                                            Q2          Q2 O          CQ4            He         CO2 etc.       Gas volume
                Gas component
                                           (%)          (%)           (%)            (%)          (%)            (N m3 )
             Deuterium discharge          59            15            1            0              25               10
             Cleaning discharge            1             5            0.1         89               4.9             70
             (He-glow discharge)
                    Total                   8.3          6.3           0.2        77.8             7.4             80

12 N m3 by gas volume.                                                  350 ml/min. Considering the hydrogen component composi-
(b) Required Performance for Proton-conducting Ceramic                  tion in Table 3, the required hydrogen-permeation capacities
   When evaluating performance of the proton-conducting                 are calculated as 29 ml/min for hydrogen gas and 22 ml/min
ceramic, both treatment capacity and stability of the device            for water vapor. The total annual volume of hydrogen gas in-
should be considered. In this treatment, operation was as-              cluded in the exhaust gas is thus estimated at about 12 N m3 .
sumed to be continuous for half a year (4,000 h/yr). Thus,              This volume corresponds to a total proton current value of
the required treatment flow rate to treat the annual quantity            about 7 A using the proton-conducting ceramic device for
of exhaust gas of 80 N m3 over these 4,000 h is about                   extracting the hydrogen.

VOL. 41, NO. 8, AUGUST 2004
868                                                                                                                      Y. ASAKURA et al.

                       Table 4 Performance of electrochemical hydrogen pumps using proton-conducting ceramic tubes

                                                              Vacuum exhaust-gas treatment                       Sensitive tritium monitor
 Experimental condition                             Hydrogen-pump                Water-vapor electrolysis         Water-vapor electrolysis
                                                     performance                      performance                      performance
 Electrolyte                                                       SrZr0:9 Yb0:1 O3À                                CaZr0:9 In0:1 O3À
 Electrode                                                           Plated platinum                                 Pasted platinum
 Heating temperature                                                      700 C                                          800 C
 Anode gas                                    1% H2 + 1.2% H2 O + Ar                  1.2% H2 O + Ar            1.2% H2 O + 20% O2 + Ar
 Cathode gas                                      1.2% H2 O + Ar                      1.2% H2 O + Ar                     Dry Ara)
                                                0.85 ml/min at 0.8 V
 Hydrogen extraction rate                                                            0.61 ml/min at 2 V            0.67 ml/min at 3.5 V
                                                (2.1 ml/min at 2 Vb) )
                                    c)                          2 b)
 Hydrogen extraction density                      45 ml/min/cm                         13 ml/min/cm2                  14 ml/min/cm2
 Reference                                              23)                                  23)                            27)
      H2 O<200 ppm, b) Estimated value, c) Effective electrode area: 47 cm2

(c) Estimation of Practical Application                                      which is 1/10 of the regulation value for tritiated water in
   Kawamura et al. reported7) hydrogen extraction perform-                   exhaust gas. The minimum detectable amount of tritium,
ances for a H2 –H2 O gas mixture using a test proton-conduct-                Amin (Bq), for the present proportional counter can be esti-
ing ceramic tube constructed of SrCe0:95 Yb0:05 O3À (O.D.:                  mated (Eq. (1)):
12 mm, Length: 100 mm). SrZr0:9 Yb0:1 O3À 22) seems to be
                                                                                  Amin ¼ Cmin  V  k;                                       ð1Þ
a suitable ceramic for the present application because it is
chemically stable against carbon dioxide, which is present                   where Cmin (Bq/cm3 ) is the tritium concentration detection
in the processing gas. We have reported23) the hydrogen ex-                  limit for the proportional counter, V (cm3 ) is the gas volume
traction performance for a commercially available                            of the counter and k is the volume ratio of the monitoring gas
SrZr0:9 Yb0:1 O3À ceramic tube with one end closed and with                 to the total gas volume in the counter.
plated platinum electrodes (O.D., 15 mm, I.D.: 12 mm,                           Assuming that tritium can be extracted perfectly from the
Length: 200 mm, Effective electrode area: 47 cm2 ). Experi-                   monitoring gas stream using the proton-conducting ceramic
mental conditions and hydrogen extraction rates are summa-                   device, the feed-gas flow rate F (ml/min) required for the
rized in Table 4.                                                            proton-conducting ceramic can be calculated (Eq. (2)):
   As mentioned in the estimations in Sec. III-2(1)(b), the re-
                                                                                  F ¼ Amin =ðC Â TÞ;                                         ð2Þ
quired hydrogen-permeation capacities are 30 ml/min for
hydrogen gas and 22 ml/min for water vapor. Using the ex-                    where C (Bq/cm3 ) is the tritium concentration to be meas-
perimentally obtained performances shown in Table 4, the                     ured, and T (min) is the time for tritium accumulation in
required electrode surface area of the proton-conducting ce-                 the carrier gas.
ramic for actual tritium removal is estimated at about                          Substituting the data, Cmin ¼1:3Â10À3 Bq/cm3 , V¼1:3Â
2,500 cm2 . In other words, more than 50 of the present pro-                 103 cm3 , k¼0:2, C¼5Â10À4 Bq/cm3 and T¼10 min into
ton-conducting ceramic tubes would be required. With pres-                   Eqs. (1) and (2), F was calculated to be 68 ml/min. The
ent technology, it is possible to make the ceramic tube 2 to 3               practicality of this value for the proton-conducting ceramic
times longer and reduce the requirement to less than 20                      device will be estimated in the following section.
tubes. This number is judged to be tolerable in terms of ac-                 (c) Estimation of Practical Application
tual equipment construction. However, further improve-                          In terms of stability during long-term operation,
ments in the hydrogen-permeation rate and water-vapor elec-                  CaZr0:9 In0:1 O3À 25) is desirable as the proton-conducting ce-
trolysis rate are desired in order to ensure the performance                 ramic for the tritium monitor. Considering that the primary
margin during long-term operation.                                           hydrogen component in exhaust from the stack is water va-
(2) Application of the Proton-Conducting Ceramic to High                     por in air, we have performed water-vapor electrolysis ex-
     Sensitivity In-line T Monitor                                           periments in an atmosphere containing 20% oxygen.25–27)
(a) Monitoring Gas Composition                                               We have used a commercially available CaZr0:9 In0:1 O3À
   It is assumed that the released gas from the stack is diluted             tube with one end closed and with platinum electrodes
with ventilated air from the laboratory building. Therefore,                 (O.D.: 15 mm, I.D.: 12 mm, Length: 200 mm, effective elec-
composition of the gas to be monitored at the stack is mainly                trode area: 47 cm2 ). Typical results of the experiment are
air including water vapor of about 1%.                                       summarized in Table 4. When considering a treatment gas
(b) Required Performance for Proton-Conducting Ceramic                       flow rate of 68 ml/min and a water-vapor content of 1.0%,
   The commercially available proportional counter (EG&G                     the required water-vapor treatment capacity for the cell is
Berthold Co.: LB 110 equipment capacity: 1.3 L)24) detects                   0.7 ml/min. In order to electrolyze all the water vapor using
tritium to a level of 1:3Â10À3 Bq/cm3 over 10 min measure-                   the proton-conducting ceramic and recover it as hydrogen
ment. The targeted detection limit value is 5Â10À4 Bq/cm3 ,                  gas, a total electrode area of 54 cm2 was required. This area

                                                                                  JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY
Application of Proton-conducting Ceramics and Polymer Permeable Membranes for Gaseous Tritium Recovery                                            869

                                                                                                 Purge gas


          Feed gas
                                                                                                                         Residual gas

                 Compressor    Cold
                 (Feed pump)   trap                       Polyimide membranes
                                                           (Hollow-fiber type)
                                          Permeated gas

                               Fig. 5 Schematic of the hollow-fiber membrane dehumidifier module

can be realized using two of the ceramic tubes tested in this                           100
experiment. These estimated results indicate that the hydro-
gen extraction performances of the present proton-conduct-
ing ceramic are adequate for application in the high sensitiv-                           95
ity in-line tritium monitor. However, further improvements
in the water-vapor electrolysis and hydrogen-permeation
                                                                    Recovery rate (%)

rates are desired to reduce the monitoring interval to less                              90
than 10 min.                                                                                                                 0.8 MPa (γ = 0.13)
(3) Application of Polymer Membrane Dehumidifier to                                                                           0.6 MPa (γ = 0.17)
     Vacuum-Vessel Purge-Gas Treatment Unit                                                                                  0.4 MPa (γ = 0.26)
                                                                                         85                                  0.3 MPa (γ = 0.35)
(a) Treatment-Gas Amount and Composition
   The treatment gas is moist air that includes tritiated water                                                    Experimental conditions
vapor. The maximum treatment flow rate was assumed to be                                                            Feed flow rate   : 50 NL/min
                                                                                         80                        Feed water-vapor : 1000 ppm
1,000 N m3 /h. The water-vapor concentration at the inlet of                                                       concentration
the polymer membrane dehumidifier was estimated to be                                                               Module temperature: 30 °C
1,000 to 2,000 ppm. Such low levels are achieved because                                 75
the gas supplied to the polymer membrane dehumidifier is                                    0.1               0.2               0.3       0.4
compressed up to 0.80–0.95 MPa by a compressor and dried                                                     Cut ratio (-)
at the dew point of about 10 C with a drain cooler.
(b) Required Performance of the Dehumidifier                       Fig. 6 Necessary operating conditions for achieving a water-
   A flow diagram of the membrane dehumidifier is shown in            vapor recovery rate greater than 99%
Fig. 5. The permeate side is at atmospheric pressure. Thus, it
is possible to control the drying conditions by returning a       The shaded area indicates the region in which the water-va-
portion of the dry outlet gas to the permeate side of the poly-   por recovery rate is greater than 99%. The necessary cut ra-
mer membrane module. Assuming a feed-gas flow rate of              tio, or effectively the purge flow rate, increases as the supply
50 N l/min and a water-vapor concentration of 1,000 ppm,          pressure decreases. Thus, it is possible to achieve a water re-
the dew point of the dry gas must be less than À60 C in or-      covery rate of 99% at a supply pressure of 0.3 MPa if the cut
der to satisfy the water recovery rate of 99% at the cut ratio    ratio is over 0.32.
(ratio of permeated to feed flow rates) of 0.4.                    (d) Estimated Results for Application
(c) Performance of current Polymer Membrane Dehumidifier              Using the largest treatment flow rate of 50 N l/min, the
   Water-vapor separation performances of commercially            number of filter modules necessary for realizing a treatment
available polyimide hollow fiber films have been report-            capacity of 1,000 N m3 /h was estimated to be about 300. The
ed10–14) and analyzed28) under decompression conditions at        total volume of the membrane dehumidifier was evaluated to
the permeate side. In order to elucidate the performances un-     be about 2.5 m3 . On the other hand, using molecular sieve
der atmospheric pressure at the permeate side, we carried out     adsorbers would require two columns of 1-m O.D. and 3-
experiments using a commercially available polyimide hol-         m length to realize the same performance under the condi-
low-fiber filter module (Ube Industries, UM-C10HF, O.D.:            tion of daily regeneration, necessitating a total volume dou-
90 mm, Length: 1,160 mm). It has been demonstrated that           ble that of the membrane dehumidifier. Results to date sug-
a dew point of less than À60 C can be sustained by optimiz-      gest that the polyimide membrane dehumidifier is applicable
ing the dry state of the polymer membrane.21) Experimental        as an alternative method to molecular-sieve adsorption. In
results for a treatment flow rate of 50 N l/min are summariz-      the future, the number of filter modules will be expected
ed in Fig. 6, which correlates water-vapor recovery rates and     to decrease once dehumidifying characteristics have been
cut ratios for different pressure ratios of permeate to feed.      evaluated for a treatment flow rate of over 50 N l/min.

VOL. 41, NO. 8, AUGUST 2004
870                                                                                                                    Y. ASAKURA et al.

                                                                       10) T. Hayashi, M. Yamada, T. Suzuki, et al., ‘‘Gas separation
IV. Conclusion                                                             performance of a hollow-filament type polyimide membrane
   In order to realize a safer waste reduction recovery system             module for a compact tritium removal system,’’ Fusion Tech-
for tritium released from the Large Helical Device (LHD)                   nol., 28, 1503 (1995).
deuterium experiment, a hydrogen pump using a proton-con-              11) S. Hirata, T. Kakuta, H. Ito, et al., ‘‘Experimental and analyt-
                                                                           ical study on membrane detritiation process,’’ Fusion Tech-
ducting ceramic and a dehumidifier using a polymer mem-
                                                                           nol., 28, 1521 (1995).
brane were quantitatively evaluated and the following con-
                                                                       12) H. Ito, T. Suzuki, T. Matsuda, et al., ‘‘Separation of tritium us-
clusions were obtained.                                                    ing polyimide membrane,’’ Fusion Technol., 28, 1376 (1995).
(1) Applying a commercially available proton-conducting                13) T. Hayashi, K. Okuno, T. Ishida, et al., ‘‘Effective tritium
     ceramic tube (O.D.: 15 mm, Length: 200 mm, one end                    processing using polyimide films,’’ Fusion Eng. Des., 39–40,
     closed) would require more than 50 ceramic tubes for                  901 (1998).
     tritium recovery from the LHD vacuum pumping gas.                 14) M. Le Digabel, P. A. Truan, D. Ducret, et al., ‘‘Glovebox at-
(2) A high-sensitivity gaseous tritium monitoring system                   mosphere detritiation process using gas separation mem-
     was proposed using a proton-conducting ceramic as                     branes,’’ Fusion Eng. Des., 69, 61 (2003).
     the hydrogen pump from the monitoring gas. By com-                15) Y. Asakura, ‘‘Development of exhaust gas and effluent liquid
     bining two of the commercially available proton-con-                  treatment system for LHD,’’ J. Plasma Fusion Res., 78[12],
                                                                           1319 (2002), [in Japanese].
     ducting ceramic tubes (O.D.: 15 mm, Length: 200 mm,
                                                                       16) H. Iwahara, T. Esaka, H. Uchida, et al., ‘‘Proton conduction in
     Shape: one end closed) and using a commercially avail-
                                                                           sintered oxides and its application to steam electrolysis for hy-
     able proportional counter would realize the targeted tri-             drogen production,’’ Solid State Ionics, 3/4, 359 (1981).
     tium detection limit value of 5Â10À4 Bq/cm3 with a 10-            17) H. Iwahara, ‘‘Technological challenges in the application of
     min monitoring interval.                                              proton-conducting ceramics,’’ Solid State Ionics, 77, 289
(3) Using a commercially available polyimide hollow-fiber                   (1995).
     filter module (O.D.: 90 mm, Length: 1,160 mm), the tar-            18) H. Iwahara, ‘‘Hydrogen pumps using proton-conducting ce-
     geted dew point (less than À60 C) could be sustained at              ramics and their applications,’’ Solid State Ionics, 125, 271
     a treatment gas flow rate of 50 N l/min. The volume                    (1999).
     necessary for the dehumidifier used for atmospheric de-            19) H. Iwahara, Y. Asakura, K. Katahira, et al., ‘‘Prospect of hy-
     tritiation in the LHD was estimated to be less than half              drogen technology using proton-conducting ceramics,’’ To
                                                                           be published in Solid State Ionics.
     of that using molecular-sieve adsorbers under the condi-
                                                                       20) H. Matsumoto, S. Hamajima, H. Iwahara, ‘‘Electrochemical
     tion of daily regeneration.                                           hydrogen pump using a high-temperature-type proton conduc-
                                                                           tor: improvement of pumping capacity,’’ Solid State Ionics,
                                                                           145, 25, (2001).
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