Journal of Membrane Science 145 (1998) 111±117

Removal of dissolved oxygen using non-porous
hollow-®ber membranes
Akira Itoa,*, Kazuaki Yamagiwaa, Masato Tamuraa, Michio FurusawabaDepartmemt of Chemical and Material Engineering, Niigata University, Ikarashi 2-8050, Niigata 950-21, Japanb
Nagayanagi Kougyou Co., Kyoujima 1-1-1, Tokyo 131, Japan
Received 11 March 1997; received in revised form 19 February 1998; accepted 19 February 1998

Abstract
The removal of dissolved oxygen (DO) from water was studied using a silicone rubber hollow-®ber membrane module.
Dissolved oxygen in water was effectively reduced by the vacuum degassing or pervaporation process through a non-porous
membrane. The controlling transport property, gas permeability through the membrane, was independently measured by amethod using a differential transformer. A gas-phase driving force model was presented based on the permeabilities of thepermeates through the hollow-®ber. Experimental results of the oxygen removal from water were compared with the model
calculations. The present model proved to explain the effects of the pressure condition and the operation mode on the removalof oxygen from water. Although the liquid-phase mass transfer resistance was shown to have a large effect on the removal
performance of the membrane module, the membrane permeation step controlled the mass transfer during vacuum degassing.
# 1998 Elsevier Science B.V.
Keywords: Fiber membranes; Gas and vapor permeation; Pervaporation; Water treatment; Dissolved oxygen

1. Introduction
The removal of dissolved oxygen (DO) from wateris needed today in many production areas such as
semiconductors, pharmaceuticals and foods. In the
power industry, corrosion in a boiler or steel pipes
will be prevented if the DO level in the water is lower
than 0.5 ppm. There have been several methods available to remove DO from water, such as a thermal or
vacuum degassing process, and the chemical reduction
process using sodium bisul®te.
*Corresponding author. Tel.: +81 25 262 6789; fax: +81 25 262
6789; e-mail: aito@eng.niigata-u.ac.jp0376-7388/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved.
PII S0376-7388(98)00068-4

The membrane separation process has a possible
candidate as a removal process for DO. Several
vacuum degassing processes using a membrane arecommercially available today, e.g., a process by
Hoechest Celanese, USA. This kind of process useshydrophobic and microporous membranes or hollow®bers.
A vacuum is applied to one side of the microporous membrane, and DO in the feed water will
be removed through it. The mass transfer processof DO through a microporous membrane has
been well studied [1±3]. There have been related
studies [4,5] concerning the design of an arti®cial
gill.

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A. Ito et al. / Journal of Membrane Science 145 (1998) 111±117

As another membrane process for degassing, a nonporous membrane is also applicable. Such a processusing a non-porous membrane has several advantages.
For example, the permeate side pressure will be
arbitrarily chosen, since feed water would not penetrate to the other side of the membrane. There havebeen a few studies on the degassing process using a
non-porous membrane. Baker et al. [6] reported theseparation of oxygen from sea water using a composite membrane. Their objective was the use of oxygen
dissolved in sea water for a submarine life-support
system and for a fuel cell. They treated the permeation
process as a kind of pervaporation, but provided little
theoretical information about it.
Oxygen permeation through a non-porous membrane has been theoretically studied for the water/
membrane/water system [7±10]. Hwang et al. [10]reported the oxygen transfer rates between two water
streams through a silicone rubber ¯at sheet. They
apparently evaluated the permeability of oxygen
through the non-porous membrane from the view
point of the gas-phase driving force. Their conclusion
was that the apparent permeability agreed with thegas-phase permeability, which was measured during
the gas-phase permeation [11]. In addition, they
insisted that the liquid-phase mass transfer resistancewill play a large part in the mass transfer of oxygenfrom water through the membrane.
The objective of this study is to develop a model
based on the permeabilities of permeate gas and vapor

for the removal process of DO using a silicone rubber
hollow-®ber membrane module. The controllingtransport property of the process, permeability
through the membrane, was independently measured
by a new method using a differential transformer. A
model based on the gas and water vapor permeabilities
for the simultaneous transport of oxygen, nitrogen andwater through the non-porous membrane was presented and experimentally tested.
2. Permeability through silicone rubber
microtube
The permeation model based on the gas-phase
driving force will require the permeability ofthe permeates in the gas or vapor state. In thisstudy, a membrane module made by silicone rubber
microtubes was used for the DO removal process.
The permeabilities of oxygen, nitrogen and water
vapor through the silicone rubber microtube
were independently measured. The microtube is asilicone rubber hollow-®ber with a 320 mm o.d.
and 200 mm i.d., which was purchased from Nagayanagi Kougyou, Japan [12]. A small membrane
module with a 66 cm2 permeation area was madeusing the microtubes and was used for the permeability measurement.
The precise measurement apparatus of permeability
is shown in Fig. 1. The apparatus consists of a glass

Fig. 1. Permeability measurement apparatus.

A. Ito et al. / Journal of Membrane Science 145 (1998) 111±117tube vessel for the liquid source, a gas reservoir, themini-module for the hollow-®ber membrane, a cold
trap and a vacuum pump. The permeate side pressure
was maintained below 0.3 kPa in all runs. For apermeation run with water, the vapor was fed fromthe liquid source. The permeation rate of water vapor
was measured based on the level change in the water
vessel [13]. For a permeation run with a gas, the feedline of the membrane module was connected to the gas
reservoir. The permeation rate of the gases through themembrane was measured based on the pressurechange in the feed side line of known volume.
Fig. 2 shows the vapor permeabilities of water
vapor, oxygen and nitrogen gas through the hollow®ber membrane. The permeability of water vapor has
been precisely measured at a low vapor pressure. The
water vapor permeability signi®cantly depends on thefeed vapor pressure, and the permeability in the shell
direction permeation is greater than that in the tube
direction. These are the characteristics of vapor permeability through rubbery polymers [12]. On the other
hand, the permeabilities of the gases, oxygen andnitrogen, have no dependency on the feed pressure
and the permeation direction. These permeabilities
well agree with that through a silicone rubber ¯atsheet membrane for water vapor [14] and for gases
[11].

113

3. Gas-phase driving force model for permeationof dissolved oxygen through a non-porous
membrane
According to Hwang's results [10], oxygen transfer
in a water/silicone rubber membrane/water system isabout the same as in the case of gas-phase permeation.
That is, the oxygen ¯ux in the water/non-porous
membrane/water system will be described by thepermeability and gas-phase equilibrium partial pressures, except for the effect of liquid ®lm resistance.
Although permeate side conditions are different
from that system, the gas-phase driving force model
is applied to a vacuum degassing process with a nonporous membrane. In the vacuum degassing process,permeate side pressure will be practically at a partial
vacuum higher than the saturation water vapor pressure at the operating temperature. Therefore, theoxygen transfer rate will be affected by the permeateside partial pressures of other permeate gases orvapors. A model calculation for the vacuum degassing
process will need to be treated as a simultaneoustransfer of dissolved gases and water vapor.
In our experimental setup, feed water ¯ows throughthe tube side of the membrane module, and permeate¯ows co-currently through shell side. The changes in
the molar ¯ow rates of each component along the ¯owdirection in the membrane module, Fi and Vi, aredescribed by the permeabilities, Qi, and the vapor
phase driving forces:ÿdFO2 =dA  dVO2 =dA  QO2 pO2 ÿ Pl yO2 =

(1)

ÿdFN2 =dA  dVN2 =dA  QN2 pN2 ÿ Pl yN2 =

(2)ÿdFH2 O =dA  dVH2 O =dA  QH2 O pH2 O ÿ Pl yH2 O =(3)where F is the molar ¯ow rate in the feed and V is thatin the permeate mixed gas. The permeabilities of eachcomponent have been independently measured, whichis shown in Fig. 2. Applying a plug-¯ow model, thepermeate side concentrations are the fraction of molar¯ow rates in the shell side of the membrane module:
yi  Vi =Vii  O2 ; N2 ; H2 O

(4)Equilibrium partial pressures of the dissolved gases
are calculated using Henry constants:
Fig. 2. Permeabilities of water vapor, oxygen and nitrogen throughthe hollow-fiber membrane.

pi  Hi xsi

i  O2 ; N2 

(5)

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A. Ito et al. / Journal of Membrane Science 145 (1998) 111±117(6)

the tube side of the membrane module. The shell sideof the module was evacuated by a vacuum pump. The
permeate side pressure was controlled by a vacuum
regulator (Kendall Model 16, Fairchild). The DOconcentrations of the inlet and outlet ¯ows were
measured by a DO meter (Horiba OM12). The operation parameters in the experiments were the permeateside pressure and the feed ¯ow rate.

(7)

5. Results and discussion

In a more precise model including the effect of theliquid side mass transfer resistance, the Hausen equation [15] for the mass transfer of laminar ¯ow in a tube
will give the concentration difference of the dissolved
gas between the bulk ¯ow and the membrane interface:
Sh  3:66 
xi ÿ xsi  ÿ

0:0668 D=lRe Sc
1  0:04 D=lRe Sc2=3
dFi =dAD
Sh  cDABi

i  O2 ; N2 

Under the assumption of plug-¯ow conditions on
both sides of the membrane module, simultaneous
integration of these differential equations gives the¯ow rate changes and concentration pro®les of eachcomponent along the module [16]. The model calculation provides the DO concentration of the outlet ¯owof the membrane module. Considering all permeates
including nitrogen, the present model could be applied
to other related membrane processes, such as oxygen
removal by a nitrogen purge or an oxygen supply from
air to water.
4. Experimental
Fig. 3 shows the apparatus for the removal of DO
from water.
A silicone rubber microtube membrane module(effective length: 14 cm; number of tubes: 6000;
effective membrane area: 4950 cm2) was made by
Nagayanagi, Japan, with the same microtubes usedin the permeability measurement. Ultra pure waterwith an adjusted oxygen concentration was fed into

Fig. 3. Experimental apparatus for removal of DO.

Fig. 4 shows the effect of the permeate side pressure
on the DO removal ef®ciency of the vacuum degassing
process. The feed ¯ow rate was kept constant and thepermeate side pressure was changed. As the permeateside pressure decreased, the DO level of the outlet ¯owapproached a minimum value. DO could be effectively
removed by the vacuum degassing process using a
non-porous membrane.
The model calculations have been compared withthe experimental results under the same operating
conditions. The solid curve in the ®gure is the calculation results including the liquid-phase mass transfer
resistance. The present model provides a good prediction of effect of the permeate side pressure over therange from vacuum to partial vacuum. The model will

Fig. 4. The effect of permeate side pressure to the DO removal
efficiency of the vacuum degassing process.

A. Ito et al. / Journal of Membrane Science 145 (1998) 111±117

115

Fig. 5. The effect of feed flow rate on the DO removal efficiency.

Fig. 6. DO removal process using a sweep gas (nitrogen).

do the calculating using various permeate side pressure, because of considering all permeation ¯uxes
including nitrogen and water vapor. The difference
between experimental data and the solid line is
explained by a deviation from plug-¯ow model forthe permeate side ¯ow. An increasing of permeate side
pressure will cause an uniform ¯ow through thehollow-®ber bundle. The dashed line in the ®gure
represents another calculation result for a case withoutliquid-phase resistance, where the mass transfer resistance in water was neglected (xixsi). During thevacuum degassing process, the liquid-phase masstransfer resistance is part of the total resistance.
Fig. 5 shows the effect of feed ¯ow rate on the DO
removal ef®ciency of the membrane module. The
permeate side pressure was kept at a moderate vacuumof 13 kPa and the feed ¯ow rate was changed in therange of laminar ¯ow. The DO of 6 mg/l in the feedwater was reduced to 2 mg/l in the outlet ¯ow. Whenthe feed ¯ow rate decreases, the OD of the outlet ¯owdecreases.
In the ®gure, the dashed line represents the calculated result of a case without liquid-phase resistance.
In this case, the model will predict a small effect of¯ow rate on the outlet DO. This is because that themembrane module was long enough for the feed ¯ow
to approach an equilibrium concentration with thepermeate side. So, if the liquid-phase resistance isignored, the outlet DO will be only slightly affected bythe ¯ow rate. The solid line is the model calculation
including the liquid-phase mass transfer resistance.
The prediction by the model is improved by considering the liquid-phase mass transfer. The difference
between experimental data and the solid line is
explained by the effect of ununiformity in permeateside ¯ow. Although the liquid-phase resistance is more
important with an increase in the feed ¯ow rate, themembrane permeation step is shown to control theentire mass transfer process of vacuum degassing.
Since the model is a simple and general descriptionof permeation through a non-swelling membrane, we
can apply it to other applications. Another DO
removal process uses a nitrogen ¯ow in the permeateside as a sweep gas. Fig. 6 shows the results of thistype of experiments. Feed DO is effectively reduced
during this mode of operation. A model calculation
was done using the same equations as the vacuum
degassing, where the partial pressure of nitrogen was
set to atmospheric pressure at the inlet of the module.
Present model also predicts entire tendency of DO
removal process. A failure of the model would be
attributed to the non-ideal ¯ow of the shell side ¯ow.
Fig. 7 shows the results of another operation involving oxygen feed from air to water. The low DO water
was fed to the membrane module and air ¯owed cocurrently through the shell side. Oxygen in the air on

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A. Ito et al. / Journal of Membrane Science 145 (1998) 111±117

processes of oxygen transfer into or from water
through a non-porous membrane.
The present gas-phase driving model with liquidphase resistance has been proven to be the basis for theoxygen transfer process through a non-porous membrane to water. Originally, the gas-phase driving force
model has been applied to a pervaporation process[14]. One could treat pervaporation, vacuum degassing and an arti®cial gill with a non-porous membranefrom the same view point of permeability. For a more
precise prediction, we must consider the effect of gasphase mixing on the shell side ¯ow of the hollow-®ber
membrane module.
7. List of symbols
Fig. 7. Oxygen transfer from air to water through the membrane.

the shell side of the module dissolves in the water
through the membrane. During this type of process,the direction of oxygen transfer is the reverse of the
DO removal process. This mode of mass transfer isreferred to as an arti®cial gill. The DO of the feedwater increases from 0.5 ppm at the inlet to nearly thesaturation DO level of air. The feed ¯ow rate affects
the DO feed ef®ciency of the module. In the ®gure, thesolid line is the calculation result of the present model.
In the calculation, model equations and transport
properties were the same as that for the DO reduction
process. Only the driving force of the oxygen isnegative compared to the DO removal process. The
present model also predicts the entire tendency of DO
supply process. A failure of the model would be
attributed to the non-ideal ¯ow of the shell side feed
gas.

A
c
D
DAB
Fi
H
l
Pl
p*
Qi
Re
Sc
Sh
Vi
xi
xs

6. Conclusionsyi

A model consideration for the removal of dissolved
oxygen (DO) from water using a silicone rubber
hollow-®ber membrane module was presented andcompared for the experimental results. The model
was able to explain the effects of pressure conditions
and operation mode on the removal of oxygen fromwater. The present model would be applicable to other

permeation area of the membrane module(m2)mole density of the feed (mol/m3)
inner diameter of the hollow-fiber membrane(m)diffusion coefficient in water (m2/s)
mole flow rate of a component in feed liquid(tube side) (kmol/s)
Henry constant (kPa)distance from the inlet along flow direction (m)permeate side pressure (kPa)equilibrium partial pressure (kPa)permeability (kmol m/(s m2 kPa))
Reynolds number
Schmidt number
Sherwood number
mole flow rate of a component in permeate
gas flow (shell side) (kmol/s)
DO or nitrogen concentration in bulk waterflow (mole fraction)
DO or nitrogen concentration in water on themembrane surface (mole fraction)
mole fraction of ith component in permeate
flow
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