DESALINATION

ELSEVIER Desalination 127 (2000) 155-164 ww w.elsevier.com/locate/desal

Alternative methods for dissolved oxygen removal from water: a comparative study

Vivek Sinha, K. Li* Department of Chemical and Environmental Engineering, National University of Singapore,

10 Kent Ridge Crescent, Singapore 119260 Tel. +65 874-6388; Fax +65 779-1936; email." chekangl@nus.edu.sg

Received 23 June 1999; accepted 8 August 1999

Abstract

Different methods for removing dissolved oxygen from water on the laboratory scale have been studied. They are membrane modules, membrane reactors and ion-exchange columns. The membrane modules and membrane reactors have basically a shell and tube type of construction with hollow-fibre polymeric (polypropylene) membranes acting as the tubes. The main difference in the membrane reactor compared to the membrane module is the presence of catalyst (palladium doped anion-exchange resin based on styrene and divinyl benzene) in the shell side. The ion-exchange column was fabricated from glass and packed with a type-II anionic resin (Purolite A-310) regenerated in the sulphite form. These methods use either physical stripping or chemical reduction or a combination of both for the removal of dissolved oxygen from water. The performance of each of these methods was analyzed, and it was found that the membrane reactor which reduced the dissolved oxygen in water by both physical stripping and chemical reduction gave the best results in terms of the outlet dissolved oxygen level.

Keywords: Ultrapure water; Dissolved oxygen removal; Membranes

1. Int roduct ion nology. Among these, the standards of high

The removal of dissolved oxygen (DO) from purity water used in the semiconductor industry are the most stringent. The presence of DO in

water is one of the primary concerns in various water causes a native oxide layer to be formed on industries such as semiconductor manufacture, the wafer surface when it is rinsed. It has been power plants, pharmaceuticals and biotech- found that a native oxide layer of 3 A may be

formed when the silicon wafer surface is washed *Corresponding author, by high purity water containing even a low level

0011-9164/00/$- See front matter © 2000 Elsevier Science B.V. All rights reserved PII: S O 0 1 1 - 9 1 6 4 ( 9 9 ) 0 0 2 0 0 - 3

156 V. Sinha, K_ Li / Desalination 127 (2000) 155-164

of 40-60ppb of DO [1]. The requirements for combination of ultraviolet radiation with the DO levels have become more stringent as the addition of reducing agents. integration levelofsemiconductorshasincreased Sodium sulphite reacts with DO to form over the years. It is estimated that less than sodium sulphate and hence increases the ionic 10ppb of DO is required for semiconductors of content of water. When hydrazine is used to 16 Mbits integration level [2]. avoid the extra ions, the reaction takes place only

DO can be removed by various physical above 90°C. Therefore, it is only suitable for use methods such as thermal degassing, vacuum in high-pressureboilers. Carbohydrazidedecom- degassing, nitrogen bubbling and degassing poses at 177°C inside the boilers of power plants through a membrane module. In thermal into hydrazine [2]. DEHA is also used as an degassing, water is in contact with high- oxygen scavenger in high-temperature, high- temperature steam. Large quantities of steam pressure boilers of power plants [2]. It is fed in make the process suitable only for treating water the ratio of 3:1 to the DO concentration. Thus, in power plants where steam is available [3]. carbohydrazide and DEHA are suitable for power Also, it cannot reduce the DO to a very low level, plant operations only. Vacuum degassing is mainly used for degassing In the above chemical methods, while low of cooling water. It consists of a tower packed levels of DO of less than 50 ppb can be achieved, with Raschig rings over which water is sprayed the disadvantages are addition of toxic substances from the top. A vacuum is created inside the such as hydrazine, which possesses occupational tower so that the dissolved gases are discharged, health hazards, and an increase in the solid In the nitrogen bubbling method, purified contents of the treated water if sodium sulphite is nitrogen gas is bubbled into the water through used. This increase in solid contents can further porous gas spargers. Large quantities of purified load the ion-exchangers used in the UPW system. nitrogen are required for this purpose and it also In this study three different methods of introduces an extra component, i.e., nitrogen, into removing DO from water are investigated. They the water [4]. The vacuum degassifiers and are microporous membrane modules, membrane nitrogen bubbling are also not flexible to changes reactors and ion-exchange columns. These in flow rate [4]. In membrane degassing, the methods use physical permeation, chemical water is passed through a non-porous hollow reaction oracombination ofphysicalpermeation fibre membrane module and vacuum is applied and catalytic chemical reduction to remove DO. on the shell side [3]. An outlet DO level of 200-500 ppb could be achieved by this method [5]. This level is quite high and vacuum pumps 2. Experimental are also required. A combination of nitrogen sweep gas and vacuum can also be used in the 2.1. Membrane module

shell side [6]. Again large quantities of nitrogen A microporous membrane module reduces the and the maintenance of a vacuum are required. DO in the deionized water using the principle of Hence the conventional physical methods have physical permeation only. In this method the the disadvantages of being bulky, costly and microporous hydrophobic hollow-fibre mem- inflexible in operation, branes made from polypropylene are used. They

Chemical methods for removing DO from have an outer diameter of 4×10-am and a wall water include addition of reducing agents such as thickness of 3 x 10 -5 m and are manufactured by hydrazine or sodium sulphite, addition of carbo- Hoechst Celanese (US). The pores have a hydrazide or diethylhydroxylamine and a diameter of 30nm and cover about 33% of the

V. Sinha, K. Li / Desalination 127 (2000) 155-164 157

Table la Characteristics of membrane modules

Module number 1 2 3 4 5 6 7

Number of fibres 500 100 100 100 100 100 500 Length, m 0.10 0.20 0.50 0.10 0.25 0.75 0.75 Effective length, m 0.06 0.15 0.46 0.07 0.21 0.71 0.71 Potted length, m 0.04 0.05 0.04 0.03 0.04 0.04 0.04 Surface area, ×10-2m 2 3.48 1.74 5.34 0.81 2.44 8.18 40.9

Table lb Table lc Characteristics of membrane reactors Characteristics of ion-exchange columns

Reactor number Column number

1 2 3&4 1 2

Number of fibres 100 500 100 Diameter, m 0.031 0.031

Length, m 0.50 0.50 0.75 Resin height, m 0.36 0.30

Effective length, m 0.46 0.46 0.71 Resin volume, × 10 -4 m 3 2.72 2.27

Potted length, m 0.04 0.04 0.04

Surface area, m 2 0.0577 0.2889 0.0891

Packing fraction 0.04 0.20 0.04 2.2. Membrane reactor

This method uses the dual principles of physical permeation and catalytic chemical

fibre surface. The module shell is fabricated from reduction for the removal of DO from water. It poly(methylmethacrylate) with an inner diameter consists of a hollow-fibre membrane module of 1.9x10-2m. The required number of hollow packed with catalysts in the shell side. The fibres were assembled as a bundle and were hollow-fibre membranes used for construction of potted inside the shell using epoxy resin. Seven the membrane reactor are the same as for the different modules with varying length and membrane modules mentioned above. Four number of membranes were made and used in the different reactors were used in the experiment. experiments. The characteristics of the modules The characteristics of these are given in Table lb. are shown in Table la. The catalysts used in the void space of the shell

Deionized water saturated with purified air side were obtained from Bayer [7] and are (DO level of 8.3 ppm) was used as the feed on the palladium doped anion-exchange resin based on tube side, while purified nitrogen gas was used as styrene and divinyl benzene. They are trans- the sweep gas in the shell side. The flow patterns parent-gellular and opaque-microporous spherical were kept co-current or countercurrent. The beads. The doping process results in the experiment was conducted at various flow rates palladium being dispersed in the outer periphery ofwater and nitrogen. The experimental set-up is of the beads as extremely fine particles in shown in Fig. 1. metallic form. Therefore, the dissolved hydrogen

158 V. Sinha, K. Li / Desalination 127 (2000) 155-164

,,. Ve

1 ~ | ........ ,t

Deionizedwater --~ [

H IX)

Container Pump Membrane Hydrogen/Nitrogen meter Reactor/Module Gas

Fig. 1. Experimental set-up for membrane reactor/membrane module. F, flowmeter; P, pressure indicator.

has rapid access to the active sites and the as a reducing agent (chemical reaction). Due to catalyst can attain a very high degree of the simultaneous functions of both the physical efficiency. Purified hydrogen used as a reducing and chemical methods in this membrane reactor, agent was introduced into the fibre lumen, while the DO level in water at the outlet stream of the the deionized water saturated with air was fed reactor should be much reduced. into the shell side and flowed through the catalyst

packed around the hollow fibres. The experiment 2.3. Ion-exchange columns was carried out by varying the inlet flow rates of water and the volume of catalytic resins packed Ion-exchange columns packed with anionic in the shell side of the module. The experimental resins can be used for chemical removal of DO in set-up is also shown in Fig. 1. water. In this method, two columns made of glass

Because the hollow fibre membrane used is and containing reducing ion-exchange resins highly hydrophobic, it has a high resistance to were used. A combination ofamembrane module water but offers little resistance to gases, mentioned above and two columns of ion- Therefore, the DO in the water diffuses through exchange resins in series was also employed. The the hollow fibre into the gas stream and is purged experimental set-up is shown in Fig. 2. away by the purified hydrogen flowing in the The ion-exchange resins used were based on fibre lumen. Simultaneously, the hydrogen gas Purolite A-310 (commercial name) which is a also diffuses through the hollow fibre membrane type II gel anionic resin. It was generated in and dissolves into the water. Because of the sulphite form. The resins were in the form of presence of the catalyst, the dissolved hydrogen amber-coloured transparent beads. Packing of the reacts with the remaining DO to form water, solution. This was done to prevent them from which is not a subsequent contaminant. The scrubbing oxygen from the atmosphere, resulting purified hydrogen employed here not only acts as in a loss of capacity. The characteristics of the a purging gas (physical stripping), but also serves ion-exchange columns are given in Table 1 c.

V. Sinha, K. Li / Desalination 127 (2000) 155-164 159

i .J i

Deionized 1

Water ~ H

Container Pump DO Meter DO Meter

Fig. 2. Experimental set-up for ion-exchange columns. F, flowmeter; P, pressure indicator.

The deionized water obtained by treating tap sensor electrode is of the polarographic type. The water with a water purifier was passed through DO levels can be shown in ppm or ppb. the columns connected in series. The inlet water Calibration of the DO sensor was done in water- flow rate was kept at a fixed value and outlet DO saturated air at ambient conditions and confirmed levels was measured and recorded The experi- with water completely deoxygenated by addition ment was repeated with other inlet flow rates of of excess sodium sulphite. The sensor was water, serviced as per the instructions in the manual in

When using a combination of a membrane case of any problems associated with calibration module with the ion-exchange columns, the or measurement. Sample flow rate in the sensor membrane module was connected in series before sample chamber was kept at 100 ml/min. the columns and used as a pretreatment stage. The water was passed through the tube side of 3. Results and discussion the module that is inside the fibre lumens while

Experiments were conducted using all the purified nitrogen gas was used as a sweep gas in three methods for the removal of DO from water. the shell side to physically remove the DO. The The data obtained are presented in the following outlet water was then fed into the ion-exchange columns, sequence: studies of the DO removal using

microporous membrane modules, membrane reactors and ion-exchange columns; performance

2.4. Measurement of DO of each individual process and finally, compari- son of the processes.

To measure the inlet and outlet levels of DO, an Orbisphere Model 3600 DO analyzer (Orbisphere Laboratories, Switzerland) was used 3.1. Membrane modules

consisting of two parts: a series 3600 DO Membrane modules removed DO from water indicator and a model 31110 oxygen sensor. The by physical permeation only. The overall mass

160 V Sinha, K. Li / Desalination 127 (2000) 155-164

transfer coefficient was found to be a function of Table 2 that countercurrent flow patterns gave water flow rate but independent o f the sweep gas better results for DO removal as well as the flow rate. Under the operating conditions studied, sweep gas requirement. The outlet DO levels the overall mass transfer coefficient K L can be decreased with increasing membrane area and obtained using the following expression [8]' decreasing water flow rate. From Fig. 4 it can be

seen that when the membrane area was increased K L Q~ In C°ut t° 0"1 m2 f°r a water f l °w rate ° f ° 5× 10-6 m3/s' the

- - - (1) outlet DO level decreased sharply to less than n d i Ln C m I ppm. The DO level remained at almost the same

level o f 8 ppb on increasing the membrane area where K L is the overall mass transfer coefficient, from 0.4 to 0.58 m 2 by connecting modules 3,5,6 QL is the liquid (water) flow rate, d r is the internal and 7 in series. diameter o f the hollow-fibre membrane, L is the length o f each hollow fibre, n is the number o f hollow fibres, Cou t is the outlet concentration of DO and Cin is the inlet concentration o f DO. 6

Fig. 3 shows the variation o f overall mass ~ 5 **o transfer coefficient with the water flow rate. It -" "~ ~ 4 * * • • can be seen that the mass transfer coefficient _ ~ % a * • • • increases initially with the increase in water flow ~'~ ~ "~ * rate and then decreases. This may be attributed to 6 ~ the wetting o f the hydrophobic membranes at ~ 1

0 ,.~ i high water flow rates, which reduced the mass 0 1 2 3 4 5 6

transfer o f the DO and hence the lower mass Water flow rate (Ql.jxlO6m~/s

transfer coefficient obtained. Similar observation

was also made by Malek et al. [9]. Table 2 shows Fig. 3. Effect of water flow rate (QL) on overall mass the outlet DO levels when the two different flow transfer coefficient (KL) for membrane module (module patterns were used for module 7. Water was fed 1, fibre-water, shell-gas). Gas flow rate = 2.8 x 10 -6 m3/s. into the fibre lumen at a flow rate o f 1.7× 10 -6 m3/s while nitrogen was passed in the shell side at different flow rates. It can be seen from

I(X)O0 ~1 OO •

I(R)O • • • •

Table 2 -~ O 1(~)

DO outlet levels, ppm e~ "~ I0 • Q l . = 0 . 5 x 10 .6 ln~'/s • • •

Nitrogen/ Countercurrent Cocurrent ~ • QL=I.7 x 10 -6 m3ts water ratio flow flow I

. . . . . . . . 0. I (1.2 0.3 (1.4 0.5 0.6

0 . 5 0 . 6 0 . 8 M e m b r a n e area , m 2

1.1 0.45 0.56 Fig. 4. Effect of membrane area on outlet DO level,

2.1 0.30 0.40 Qc, = 3.4× 10 -6 m3/s, countercurrent flow.

V. Sinha, K. Li / Desalination 127 (2000) 155-164 161

3.2. Membrane reactor water flow rate but increased with the height of

The membrane reactor reduced the DO by resin bed. This can be explained by the fact that with increase in the water flow rate there was less

both physical permeation and chemical reduction, residence time for the reduction reaction to occur The palladium-doped catalysts in the anion- and for the DO to permeate to the tube side. The exchange resin have the advantages of a high increase in height of the resin bed increases the affinity for dissolved hydrogen and do not require catalyst loading and hence the area available for regeneration or removal. The microporous reaction to occur. The results shown in Figs. 6 hollow-fibre membranes functioned essentially as and 7 further indicate that the resistance across an efficient gas distributor for bubbleless the liquid film adjacent to the hollow fibre and dissolution of hydrogen in the water and also as the catalyst surface control the rate of DO a collector for the oxygen that permeated

removal. The lowest DO level that could be physically into the fibre lumen. At low catalyst achieved was less than 1.5 ppb. loading the physical stripping and chemical reduction occurred simultaneously while at high 50 catalyst loading the chemical reduction was more .~ influential. As can be seen in Fig. 5, at an area ~. 40 • ratio of catalyst to membrane surface >1.0 more ~ 30 than 90% of the DO is removed by chemical O • reduction. The membrane resistance was ~ 20 negligible because of the porous and hydrophobic nature of the membrane. As the fibre was not O • wetted by water, its pores were filled with gas 0 , A ~ , , , through which diffusion was faster. Fig. 6 shows 4 8 12 16 20 the variation of outlet DO level with the water Water flow r a t e x 106 , ma/s flow rate while Fig. 7 shows the variation of DO removal efficiency with the height of the catalytic resin bed at d i f ferent water f low rates. The D O Fig. 6. Effect of water flow rate on outlet DO level

r emova l e f f i c i ency decreased with increasing (module 3 and 4 connected in parallel).

1.0 11o ~ o~ ~ 0 0 . 1 5 m

==0.30 m 0.8 ~' 100 i l • A0.43 r~

• = & =~ 90 I I

0 6 ~ ao "

"~ 0 .4 "~ " - ~ 70 •

0 2 -

• 0 • .o 0.0 . . . . . . . . . • ' , ' ' , 50

0 0.5 1 1 5 100E-05 2.00E-05 3.00E-05 4.00E.-05 5.00E-05 6.00E-05

Surface area ratio, A,/A ,. W ater flow r a t e (m3/s)

Fig. 5. Contribution of the DO removal, data obtained Fig. 7. Oxygen removal efficiency versus water flow rate from the hollow-fibre membrane reactor at L = for different heights of catalytic resin (module 2). Gas 300 ml/min, with different catalyst loadings, flow rate -- 1.11 xl0 -4 m3/s.

162 V. Sinha, K. Li / Desalination 127 (2000) 155-164

3.3. 1on-exchange columns 3.4. Comparison o f the methods

In this method two columns of reducing ion- The use of any one of the above methods exchange resins were used which were anionic depends on the area of application. It can be seen resins regenerated in sulphite form. Water was from Table 3 that the lowest level of DO in the passed through the columns and an outlet DO outlet water could be achieved when the level of 13.1 ppb could be achieved. When using membrane reactor was used. While a membrane a combination ofmembranemodule with the ion- reactor may be a necessity in semiconductor exchange columns an outlet DO level of 9.60 ppb industries where the lowest levels of DO are could be achieved. Fig. 8 shows the outlet DO required, the other methods can be used in levels when membrane module, ion-exchange applications where the DO levels are not so columns or a combination of both are used. It can stringent. be seen that the lowest DO levels were obtained In terms of operating conditions, all the using a combination of the membrane module modules operated at the same ambient conditions with the ion-exchange columns. The reduction in the laboratory. The advantage of all these reactions occur on specific active sites on the systems was that the only power requirement was sulphite resins and the reaction product, i.e., for the pump needed for the delivery of water sodium sulphate, is bound by the functional into the module. Hence these systems have lower group of the resins. Thus there is negligible operating costs compared to conventional change in the ionic content of water in contrast systems where steam or vacuum is required. with what occurs if sodium sulphite is added Though the initial costs may be higher due to the directly. The disadvantage of this method was expenditure involved in membranes and catalysts, that the catalysts required packing in the columns in long-term operation they are definitely in an inert atmosphere by using a nitrogen economical and have ease of operation. The blanket and saturated sodium bisulphite solution, membrane systems are also relatively mainten- Otherwise, they scrub oxygen from the atmos- ance free in comparison to conventional systems. phere on exposure resulting in a loss of capacity. A distinct advantage in operation of the Also they had to be regenerated periodically, membrane module or the membrane reactor over

2500

.= 2000 •Membrane module alone

- : • Two ion-exchange columns i n

1500 • series • Membrane moduleand two i o n -

¢= 1 0 0 0 exchange columns in series

0 500 • • •

0 , & . . n . . . . . . . . . . . =,

0 0.01 0.02 0.03

T h r o u g h p u t , m j

Fig. 8. Outlet DO levels using membrane module, ion-exchange columns or a combination of both.

V. Sinha, K Li / Desalination 127 (2000) 155-164 163

Table 3 [title of table??]

Method Condition Outlet DO level (ppb)

Membrane module N 2 flow rate: 1.53× 10 -6 m3/s 8 Water flow rate: 5× 10 -7 m3/s Countercurrent flow pattern

Membrane reactor H 2 flow rate: 1.11 x 10 -4 m3/s 1.5 Water flow rate: 1.67× 10 3 m3/s Countercurrent flow pattern

Ion-exchange columns Water flow rate: 1.67× 10 3 m3/s 13.1

Combination of ion-exchange columns and N 2 flow rate:>l.39× 10 -5 m3/s 9.6 membrane module in series Water flow rate: 5× 10 -7 mB/s

Countercurrent flow pattern

the ion-exchange column was that the membrane module using physical permeation only to interface between the gas and the liquid allowed remove the dissolved oxygen is economical to independent control over their flow parameters, use in areas where the DO level required is not

In comparison to membrane reactors, the very low. For applications in semiconductor membrane module and ion-exchange columns are industries, a membrane reactor has to be used. less hazardous as they use nitrogen as the sweep gas. When hydrogen gas is used, due precautions have to be taken so that it does not leak into the

4. Conclusions atmosphere and form an explosive mixture with the air. Different methods such as membrane

A membrane reactor involves the additional modules, membrane reactors and ion-exchange expenditure on the catalysts but its performance columns which employ the principles of physical with respect to the outlet DO levels achieved is stripping or chemical reduction have been studied the best. Similarly, the ion-exchange columns for their ability to remove DO from water and also have the additional cost of resins associated reduce it to very low levels in the ppb range. with them. On the industrial scale, especially Based on a comparative study, it was found that where low DO levels are required, this using a membrane reactor having a palladium- investment has to be borne if further losses in doped catalyst in the void space of the shell side production due to high DO levels in the water are is a very suitable method for deoxygenation of to be avoided. Nitrogen gas which is prepared by water to low levels. At high catalyst loading the air separation is cheaper than hydrogen and the removal of DO from water was mainly by cost o f operating the membrane module will be chemical reduction, and the hollow-fibre cheaper compared to the membrane reactor using membranes functioned as an efficient gas distri- hydrogen for the catalytic chemical reduction of butor. The membrane interface between gas and DO. liquid allowed independent operating of their

Thus the use of a particular system is defined flow rates, and constraints like flooding and more by the area of its application. A membrane loading associated with the conventional packed

164 V. Sinha, K. Li / Desalination 12 7 (2000) 155-164

towers are prevented. The catalytic reduction of References DO using hydrogen gas does not increase the total dissolved solids in the water unlike other [1] M. Morita, T.Ohmi, E. Hasegawa, M.Kawakamiand

M. Ohwada, J. Appl. Physics, 68(3) (1990) 1272. chemical methods involving addition of [2] T.H. Meltzer, High Purity Water Preparation for chemicals to reduce the DO. Outlet DO levels of Semiconductor, Pharmaceutical and Power Industries, less than 1.5 ppb could be achieved. 2nd ed., Tall Oaks, USA, 1997, p. 24, 298.

Hence, it may be concluded that methods [3] T. Saito, Processforremovingdissolvedoxygenfrom studied here for removing DO from water water and system therefor, European Patent definitely have more advantages in comparison to Application EP0427191A1, 1991. the conventional methods. The membrane reactor [4] D. Mark, Semiconductor Internat., (1996) 36. was found to be best suited at present for the [5] Y.Kasama, Y.Yagi, T. Imaoka, M. KawakamiandT. deoxygenation of water to achieve the lowest Ohmi, Proc., Institute of Environmental Science, levels of DO. 1990, pp.344-349.

[6] J. Bujedo and P.A. Peterson, Semiconductor Intemat., 12 (1997) 137.

Acknowledgements [7] A.G. Bayer, Organic Chemical Division, Forschung/ Anwendungstechnik, Levwatit, Ionenaustauscher/

The authors gratefully acknowledge the Katalystaoren, Geb, B106,Leverkusen, 1990,pp.l-45. research funding (RP3972690) provided by the [8] M.S.L. Tai, K. Li, I. Chua, W.J. Ng and W.K. Teo, J. National University of Singapore. A research Membr. Sci., 87 (1994)99. scholarship provided by the National University [9] A. Malek, K. Li and W.K. Teo, Ind. Eng. Chem. Res., of Singapore to Vivek Sinha is also gratefully 36 (1997)784. acknowledged.