DEOXYGENATION OF ULTRAPURE WATER BY CHEMICAL REDUCTION AND PHYSICAL PERMEATION PROCESSES

Abstract

The importance of development of deoxygenation system has taken an unprecedented significance in ultrapure water (UPW) production in recent years. Advances in high-level integration for the semiconductor industry, pharmaceutical industry and power plants require very stringent control of dissolved oxygen (DO) level in water. For instance, the formation of native oxide film on the silicon wafer surface due to the presence of trace amounts of DO in UPW used in the wet-cleaning process has caused great concern in the manufacturing of semiconductors. This project aims to develop a process for the efficient removal of DO in water from parts-per-million (ppm) range down to low parts-per-billion (ppb) range. The performance of DO removal by a catalytic reduction process using hydrogen, and physical permeation process using a hydrophobic microporous hollow fibre membrane have been examined. The former process was observed to be highly effective and DO removal efficiency of up to 99. 9% could easily be achieved using a packed bed palladium-doped catalytic reactor provided that an efficient hydrogen dissolution and distribution system could be designed. The latter process of DO removal in water by physical permeation using polypropylene microporous hollow fibres alone was less promising when the DO saturated water was introduced through the shell-side. Under the experimental conditions employed, the permeator achieved only a maximum efficiency of 51.1 %, due mainly to liquid channelling in the shell-side. A membrane reactor which incorporated both physical permeation and chemical reduction in one unit has been designed and evaluated in its effectiveness in deoxygenation under various conditions. The membrane reactor was fabricated from a polypropylene hollow fibre membrane module packed with palladium-doped catalytic resins in the void space of the shell-side. Water saturated with DO was pumped through the shell-side of the reactor whereas hydrogen gas was introduced counter-currently through the fibre lumen. In the membrane reactor, the hydrogen gas served both as a reducing agent for the DO as well as a sweep gas for the DO that permeated through the membrane into the tube-side. The performance of the membrane reactor was investigated under various conditions. The effects of liquid flowrates and the area-ratio of the catalyst to the membrane on the overall mass transfer coefficient of DO, and the performance of the membrane reactor have been examined. Studies on mass transfer of DO in the membrane reactor reveal that in general, the resistance to gas transport through the gas phase and the microporous membrane phase is negligible, therefore the overall mass transfer coefficients for physical permeation (KL) and catalytic reduction (KS) are independent of the gas flowrate, but depend significantly on the water flowrate. The results suggest that the resistances across the liquid films adjacent to the hollow fibre membrane and the solid catalyst surface control the rate of DO removal. The hollow fibre membranes essentially served as an efficient gas distributor for the supply of bubble-less dissolved hydrogen as well as a collector for dissolved oxygen that permeated from the shell-side to the tube-side. The removal of DO was achieved by both the physical stripping and the chemical reduction at low catalyst loadings whereas the chemical reduction became the dominant step at high catalyst loadings. For instance, at an area ratio of the catalyst to the membrane above about 1.13, the results indicate that the chemical reduction accounted for higher than 90% of the DO removal. A design equation has been developed for the prediction of DO content at the outlet of such a membrane reactor based on a mass transfer correlation obtained in this work and other available correlations.