Contamination of Ultra-High-Purity (UHP) Gas Distribution Systems by Back Diffusion of Impurities

Abstract

A combination of experimental investigation and process simulation was used to analyze the effect of various operational parameters on impurity back diffusion into ultra-high-purity (UHP) gas distribution systems. The process model developed in this work was validated by comparing its predictions with data from the experimental test bed. Surface diffusion as well as convection and dispersion in the bulk fluid played a strong role in the transport of moisture from vents and lateral branches into the main line. Parametric studies on the effect of key operational and design parameters were performed experimentally and with the application of a process simulator. In this analysis, a dimensionless number (Peclet Number) was derived and applied as the key indicator of the relative significance of various transport mechanisms in moisture back diffusion. Guidelines and critical values of Peclet Number were identified for assuring the operating conditions meet the purity requirements at the point of use while minimizing UHP gas usage. These guidelines allowed the determination of lateral lengths, lateral diameters, flow rates, and restrictive flow device configurations to minimize contamination.