GB2283015A

GB2283015A - Membrane reactor for the removal of dissolved oxygen from water

Info

Publication number: GB2283015A
Application number: GB9321867A
Authority: GB
Inventors: Wah Koon Teo; Wun Jern Ng; Kang Li; Chua Swee Hoe Ivy
Original assignee: National University of Singapore; Chemitreat Pte Ltd
Current assignee: National University of Singapore; Chemitreat Pte Ltd
Priority date: 1993-10-22
Filing date: 1993-10-22
Publication date: 1995-04-26
Anticipated expiration: 2013-10-22
Also published as: HK1010783A1; GB9321867D0; GB2283015B

Abstract

There is provided a process for the removal of dissolved oxygen from water, which process comprises providing a membrane reactor which is a hollow fibre module 52 packed with a catalyst 56 in the void space and a purified gas or a gas mixture acting as both a purge gas and a reducing agent. The invention finds particular utility in the semiconductor industry where ultra pure water with a very low level of dissolved oxygen is used as rinsing water during production of silicon wafers. <IMAGE>

Description

MEMBRANE REACTOR FOR THE REMOVAL OF DISSOLVED OXYGEN FROM WATER This invention pertains to a method i.e., use of a membrane reactor, for the removal of dissolved oxygen from water to a level less than 1.5 ppb.

This invention can be particularly utilized in the semiconductor industry where ultra pure water with very low level of dissolved oxygen content is often used as rinsing water during production of silicon wafers.

It is generally recognized that the removal of dissolved oxygen from water can be achieved by either physical or chemical methods. The former includes thermal degassing, vacuum degassing or purging with N2 gas, while the latter involves the addition of a reducing agent such as H2,N2H4, Na2SO23 or HCO2H.

The conventional physical methods such as the thermal or vacuum degassing systems have inherent drawbacks in terms of both operating costs and bulky constructions. Chemical reaction represents an attractive alternative for the removal of dissolved oxygen in the presence of catalytic resins (Bayer AG, Organic Chemical Division) or ultraviolet rays (European Patent No. EP 0 427 191 Al). The lowest DO level attainable by these methods is around 5 ppb.

In order to achieve an extremely low concentration of DO in water, hybrid systems are sometimes required, where a membrane module with oxygen selective membranes is utilised as degassing pretreatment prior to the main treatment such as nitrogen gas bubbling deaeration system (physical method) or catalytic reduction reaction (chemical method).

The experimental results for these two hybrid systems were obtained by Kasama et al. (appearing in Proceeding - Institute of Environmental Sciences, 1991 p.

344-349) who indicated that the best deoxygenation results achievable for these hybrid systems are around 3-5 ppb. However, Japan Patent No. 4-293503 claims that with the use of hybrid systems, the dissolved oxygen level in water can be reduced to less than 1 ppb.

An objective of the present invention is to seek to provide a process and use of a membrane reactor to remove dissolved oxygen from water.

Another objective is to provide a membrane reactor wherein the dissolved oxygen in water can be removed from 8800 ppb to less than 1.5 ppb. Other objectives and advantages shall become apparent in the following description of the invention.

According to the invention, there is provided a process for the removal of dissolved oxygen from water, which process comprises providing a membrane reactor which is a hollow fibre module packed with a catalyst in the void space and a purified gas or a gas mixture acting as both a purge gas and a reducing agent. The dissolved oxygen can be removed [to a level less than 1.5 ppb].

Hydrogen may act as both a reducing agent and as a purging gas and may be introduced into the hollow fibre lumen, while water containing saturated oxygen (8800 ppb) may be fed into the shell. Because of the highly hydrophobic character of the hollow fibre membranes used, it prevents water from penetrating through, but gives no resistance to gases. Thus, the dissolved oxygen diffuses through the hollow fibre from shell side to fibre lumen and is purged away by hydrogen flowing in the fibre lumen. Simultaneously, hydrogen gas will also diffuse through the membrane and dissolve into the water, and then react with the remaining dissolved oxygen in the presence of a catalyst to fonn water.

Thus, the dissolved oxygen level in water can be reduced to a minimum.

A process and membrane reactor embodying the inventor are hereinafter described, by way of example, with reference to the accompanying drawings.

Fig. 1 shows a conventional membrane module used for the dissolved oxygen removal; Fig. 2 illustrates a conventional way for the removal of dissolved oxygen from water using a catalytic method; Fig. 3 shows a longitudinal and a transverse cross-sectional view of a module constructed according to the invention, Fig. 4 shows a membrane degassing apparatus for the dissolved oxygen removal; Fig. 5 shows an apparatus for the catalytic removal of dissolved oxygen from water; Fig. 6 shows an experimental apparatus of the present invention; and Fig. 7 is a graph showing the relationships between the dissolved oxygen level in the treated water and the treating time using the apparatus shown in Figs. 4 to 6.

Referring to the drawings, there is shown schematically a membrane for carrying out a process for the removal of dissolved oxygen from water, which membrane comprises a hollow fibre membrane reactor and utilises purified hydrogen or a hydrogen gas mixture not only as a reducing agent but also as a purge gas.

In order to render a better understanding of the invention, two typical methods are illustrated in Figs. 1 and 2 and its principle for removal of dissolved oxygen (DO) are described below.

As previously stated, there are many types of methods for removing DO from water. One typical example is shown in Fig. 1, a physical method where module 20 comprises a bundle of fibres 22 encased in a shell 24 which is also cylindrical and is slightly larger than bundle 22 so that when the bundle is inserted and axially secured within the shell, a cylindrical space 26 is left between the bundle 22 and the inner side wall of shell 24. Water containing 8800 ppb dissolved oxygen is introduced into the inlet (of the hollow fibre lumen) 28 and flows axially along the bundle of the fibres to fibre outlet 30, while purified nitrogen is fed into shell 32 and flows along the cylindrical space to shell outlet 34. As previously mentioned, the hollow fibre is made of hydrophobic materials. It has no resistance to gases, but has high resistance to water. Therefore, as water flows through the fibres, the dissolved oxygen diffuses through the hollow fibres and is purged away by purified nitrogen flowing along the cylindrical space 26.

Thus, the mass transfer of oxygen is achieved based on the concentration gradients and the DO level in fibre outlet 30 is reduced.

Another typical method for removal of DO from water is shown in Fig. 2, a chemical method which consists of a reactor 44 and a mixer 42. Hydrogen gas is first evenly dispersed in the water stream from inlet 36 as small bubbles by a distributor 38 and then completely dissolved through packed bed 40. The water stream, thus containing both dissolved oxygen and dissolved hydrogen is then fed into the reactor 42 packed with a catalyst 46. The following reaction takes place in the presence of the catalyst 46: 2H2 + O2 catalvst 2 H2O Therefore, the removal of the dissolved oxygen from water is achieved based on the above chemical reaction and oxygen content in the water at the outlet stream 48 is much reduced.

An embodiment of a membrane reaction which has been constructed in accordance with the present invention is shown in Fig. 3. It comprises a module 52 having hollow fibres 64, a catalyst 56 and a shell 58. The hollow fibres are well placed in the shell shown in the transversal cross-sectional view A-A of Fig. 3. The catalyst is packed in the void space and a well surrounds each of the hollow fibres. Purified hydrogen or a hydrogen gas mixture is introduced into the fibre inlet 50 and flows along each of the hollow fibres to the fibre outlet 60, while water containing saturated oxygen (at 8800 ppb) is fed into the shell inlet 62 and flows through the catalyst packed around each hollow fibre. As mentioned previously, the hollow fibre membrane used is highly hydrophobic.

It has high resistance to water, but has little resistance to gases. Therefore, based on Fick's Law, the dissolved oxygen in water diffuses through the hollow fiber from the water side to the gas side and is purged away by purified hydrogen or a hydrogen gas mixture flowing in the fibre lumen. Simultaneously, hydrogen gas is also diffused through the hollow fibre membrane and dissolves into the water. Because of the presence of the catalyst, the dissolved hydrogen reacts with the remaining dissolved oxygen to form water which is not subsequently contaminated. The purified hydrogen or hydrogen gas mixture employed here not only acts as a purge gas (physical method), but also serves as a reducing agent (chemical method). Due to the simultaneous functions of both physical and chemical methods in this invented membrane reactor, the dissolved oxygen level in water at the outlet 60 can be reduced to a level less than 1.5 ppb (parts per billion) .

EXAMPLE 1 The objectives of this example is to remove the dissolved oxygen from water using a membrane degassing apparatus i.e., a physical method, shown in Fig.4.

Tap water was treated with three filters having pore size of 10, 0.45, 0.1 microns connected in series prior to a water tank 1. The purified water in tank 1 is pumped to the hollow fibre lumen of a membrane module 4 using a pump 2, while the purified nitrogen in gas cylinder 3 is introduced into the shell of the membrane module 4. The water flow rate is operated at 400 ml/min, whereas the nitrogen purge gas flow rate is controlled in a range between 1000 and 1500 ml/min.

In Fig. 4, 5 refers to a DO meter. Figure 7 shows the relationships between the amount of dissolved oxygen present in the product stream and the operating time. The result of Example 1 is represented by a curve A, wherein the dissolved oxygen level is reduced to 200 ppb when the operating time is more than 50 minutes.

EXAMPLE 2 The objective of this example is to remove the dissolved oxygen from water using a catalytic reactor i.e., a chemical method, shown in Fig. 5.

Tap water is first treated in the same manner as in example 1. The treated water containing about 8800 ppb of dissolved oxygen is then stored in the water tank 1 and is used as raw water. The raw water is pumped to the bottom of a packed bed mixer 8 using a pump 2. The hydrogen gas is introduced at the bottom of the mixer via a gas distributor 7 which disperses the hydrogen gas in the water stream as small bubbles and subsequently dissolving it completely in water. In order to ensure that the catalyst functions optimally, the water stream at the outlet of the mixer should not contain any bubbles. The water containing both saturated hydrogen and oxygen is then fed into the reactor 9 wherein the dissolved oxygen is removed by a reaction between the dissolved oxygen and hydrogen in the presence of the catalyst. The treated water has a low DO content and is measured by a DO meter 5. Figs. 5 and 6 refer to a hydrogen gas cylinder containing purified hydrogen gas. Water pressure is operated at 2 bars and other operating conditions such as water and gas flow rates are the same as those in Example 1. The relationships between the amount of dissolved oxygen present in the product stream and the operating time are also shown in Fig. 7 where the results of Example 2 are represented by curve B, indicating that the DO level is reduced to 40 ppb when the operating time is more than 30 minutes.

EXAMPLE 3 The objective of this example is to illustrate the present invention where the dissolved oxygen is removed from water by a membrane reactor. The process, shown in Fig. 6, combines both physical and chemical abilities and is capable of removing the dissolved oxygen to a level below 1.5 ppb.

The membrane reactor is constructed using a module with the same membrane area as the module used in Example 1. The same quantity of catalyst as in Example 2 is then packed into the void space of the module i.e., on the shellside. The raw water in tank 1 contains 8800 ppb dissolved oxygen and is fed into the shell side of the membrane reactor 10 by a pump 2. The purified gas in cylinder 6 is introduced into the hollow fibre lumen of the membrane reactor 10.

The unreacted or excess hydrogen in the product stream will affect the measurement of DO, therefore, a membrane module 11 is employed for the purpose of hydrogen removal. The relationships between the amount of dissolved oxygen present in the product stream and the operating time are also shown in Fig. 7, where the result of Example 3 is represented by curve C, indicating that the dissolved oxygen level is reduced to 1.3 ppb at the operating time of more than 60 minutes. When a hydrogen gas mixture i.e., 30% of hydrogen in nitrogen, replaces the purified hydrogen, identical DO results are obtained as shown in Table 1, however the operating time is slightly longer.

TABLE 1

WATER GAS OPERATING PRESSURE bars OPERATING TIME DO LEVEL FLOWRATE PRESSURE mins ppb ml/min bars 100%hydrogen 30%hydrogen 400 2.5 2.0 - 60 4.0 400 2.5 - 2.0 80 4.2 400 3.0 2.0 - 60 1.8 400 3.0 - 2.0 90 1.8 The operating pressure of the membrane reactor gives an effect on the dissolved oxygen level in the product stream. This is shown in Table 2 where it indicates that the increase of the operating pressure will result in lower DO levels in the product stream.

The catalyst may be a resin doped with palladium, or any other suitable catalyst.

TABLE 2 ~

WATER FLOWRATE OPERATING PRESSURE, bars DO LEVEL ml/min ppb WATER SIDE HYDROGEN SIDE 400 2.5 2.0 4.0 400 3.0 2.0 1.8 400 4.0 2.0 1.3

Claims

CLAIMS 1. A process for the removal of dissolved oxygen from water, which process comprises providing a membrane reactor which is a hollow fibre module packed with a catalyst in the void space and a purified gas or a gas mixture acting as both a purge gas and a reducing agent.

2. A process according to Claim 1, the purified gas comprising hydrogen as both purge gas and a reducing agent, which dissolves into water containing oxygen via a hydrophobic membrane.

3. A process according to Claim 1, in which the gas mixture comprises purified hydrogen and nitrogen and is used as both purge gas and as a reducing agent which dissolves into water containing oxygen via a hydrophobic membrane.

4. A process according to Claim 3, in which the hydrogen content of the gas mixture used as both purge gas and the reducing agent mixture is as low as 30%.

5. A process according to any preceding claim, in which after the catalytic reaction betwen the dissolved oxygen and the added hydrogen, the unreacted or excess dissolved hydrogen is removed from the water using a hydrophobic membrane module.

6. A process according to Claim 1, substantially as hereinbefore described.

7. A membrane reactor for the removal of dissolved oxygen from water, comprising a hollow fibre module packed with a catalyst in a void space surrounding the fibre.

8. A membrane reactor according to Claim 7, the hollow fibres being well placed in a shell.

9. A membrane reactor according to Claim 7 or Claim 8, the catalyst being packed in the void space of the shell side of a membrane module and a well around each of the hollow fibres.

10. A membrane according to any of Claims 7 to 9, the catalyst comprising palladium doped resin.

111. A membrane rector substantially as hereinbefore described, with reference to Figs. 3 to 7 of the accompanying drawings.