微孔膜组件去除超纯水生产中的溶解氧

时间:2024-08-16 16:59:50 浏览量:0

Abstract 

 Dissolved oxygen is one of the major contaminants that have to be removed in the production of  ultrapure water. The removal of dissolved oxygen from ultrapure water employing microporous hydrophobic membranes has been studied. The membranes have a negligible resistance to the passage of oxygen and provide a larger surface area per unit volume for gas-liquid contact. A study of the mass transfer  in the membrane gas-liquid contactor showed that the resistance in the liquid film adjacent to the membrane phase controlled the rate of oxygen removal. The L&&que equation which describes adequately  the observed overall mass transfer coefficients, fails to predict satisfactorily the dissolved oxygen concentrations in the parts per billion range. The experimental results also indicate that the membrane  modules were capable of reducing the dissolved oxygen content in water to a level of around 8 ppb.


Introduction  

Ultrapure water, a quality chemical, is water  which is essentially ‘free’ of contaminants, e.g.  dissolved ions, silica, particles, organics, dissolved gases and bacteria. Generally, specifications for ultrapure water, call for only parts  per billion to trillion of impurities, depending  on the type of contamination and varying with  the process applications. Production of ultrapure water is one of the key support services for  manufacturing of semiconductors and pharmaceuticals, in biotechnology and power industries. In the semiconductor industry, one of  the major problems encountered is the presence of dissolved oxygen (DO) in the ultrapure  water which may result in the formation of a  native oxide layer during wafer cleaning. In order to suppress the occurrence of this native  oxide layer, the dissolved oxygen level in the  ultrapure water must be kept very low, less than  0.1 ppb is required in some specific applications.


Experimental  

The hollow fibre membranes used in the  present work were made of microporous polypropylene, of about 0.04 cm OD, and 0.003 cm  wall thickness and they were manufactured by  Hoechst Celanese. The pores, about 30 nm diameter, cover about 33% of the hollow fibre  surface. The hollow fibre modules designed for  the present experiments consisted of a 0.02 m  diameter polymethylmethacrylate tube acting  as a shell. The desired number of hollow fibres,  assembled as a bundle, were potted into the shell  and glued on both ends of the shell with a liquid  epoxy resin (70% epoxy resin and 30% hardener). A total of seven modules with different  length and packing fraction have been employed in this study. The characteristics of these  modules are shown in Table 1.


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Fig1


The apparatus and operating conditions for  the removal of dissolved oxygen from ultrapure  water using the modules are shown in Fig. 1.  The feed water at 22℃ containing about 8 ppm  dissolved oxygen was fed into the hollow fibre  lumen and purified nitrogen used as purge gas  was introduced at the shell side. The inlet flow  rates of water and nitrogen were controlled by  rotameters. The concentration of the dissolved  oxygen in the outlet water stream was measured by a DO meter (Martek) and was used  for calculation of the overall mass transfer  coefficient KL according to the following  equations.


Results and discussion  

The effect of operating conditions such as  water and sweep gas flow rates on the performance of hollow fibre modules has been studied.  The overall mass transfer coefficients were  found to be a function of the water flow rate. In  order to examine the dependence of the oxygen  transfer coefficient on the process parameters,  such as liquid and gas flows, the overall mass  transfer coefficient expressed in terms of the  Sherwood number (kLd/D) versus the water  velocity per module length, expressed in terms  of the Graetz number ( VLd2/ZD), has been  plotted as a log-log graph shown in Fig. 2. For  flow through small tubes, it has been suggested that the mass transfer in the fibre lumen  can be described by Leveque’s equation,  Sh=1.62Pe0.33 shown by the solid line in  Fig. 2. It can be seen from the figure that the  overall mass transfer coefficients KL obtained  from the modules used in this study fall closely  to the solid line indicating that the oxygen mass  transfer in the liquid phase is dominating.


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Fig2


The effect of sweep gas flow rates on the  overall mass transfer coefficient is illustrated  in Fig. 3 where the overall mass transfer coefficients KL are plotted against the gas flow rate  for three modules with fibre lengths of  and 10 cm, respectively. As expected the overall  mass transfer coefficients are independent of  the gas flow rate indicating a negligible gas film  resistance.


Firstly, the gas flow rates in Fig. 4 (O&S-  3.4 x 10m6 m”/sec) are considerably lower than  those in Fig. 3 (5-260 x 10m6 m3/sec). Based  on eqns. (1) and (2)) at a low gas flow rate, the  difference between the cocurrent and countercurrent flows is obvious. Of coume, when the  gas flow rate approaches infinity, i.e. QJHQo  becomes zero, their difference disappears and  eqns. (1) and (2) become identical. When the  gas flow rates are plotted against the dissolved  oxygen concentration in the product stream, the  effects of the flow pattern are significant as  shown in Fig. 4. If the data in Fig. 4 are transferred to Fig. 3, the effect of the flow pattern  becomes negligible as shown in the inset of Fig.  3. However, at the low gas flow rates operated,  the results indicate that the gas flow rate has  only a small effect on the overall mass transfer  coefficient. Thus, it may be concluded that the  flow pattern used with the hollow fibre modules  has a significant effect on the overall driving  force, but has little effect on the resistance to  mass transfer.


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Fig3


Conclusions  

Hollow fibre modules containing hydrophobic membranes have been assessed for their capability to reduce dissolved oxygen to the parts  per billion concentration range in ultrapure  water production. The overall mass transfer coefficients ofoxygen have been obtained in theliquid phase flowing through the fibre lumenand were observed to be dominated by the in-dividual mass transfer coefficient in the liquidfilm. The Lévèque equation, which describeswell the observed mass transfer coefficient dataobtained in the liquid phase, does not give asatisfactory prediction at very low gas flow ratesand dissolved oxygen levels in the parts per bil-lion range. Countercurrent flow operation wasfound to yield better results than cocurrent flowin terms of both the degree of dissolved oxygenremoval and the minimum sweep gas require-ments.Experimental results also reveal that thelargest reduction in dissolved oxygen obtainedwith the microporous hollow fibre modules usedin this study gives a value around 8 ppb.

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