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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.

The purging of stagnant or dead volumes in gas distribution systems is an important method for removing impurities and maintaining cleanliness. A combination of experimental investigation and computational process modeling is used to study the dynamics of impurity removal under variety of purge conditions. The controlled cycling of pressure during purge is found to enhance the cleaning process significantly, particularly in dead spaces. The process simulator was used to develop and analyze a pressure‐cyclic purge (PCP) method and understand the conditions that would make PCP advantageous over steady‐state purge (SSP). In particular, the effect of geometric factors, impurity surface interactions, flow rate, and cycle characteristics on PCP and its comparison with SSP was studied. The advantage of the PCP method, in terms of both purge time and gas usage, becomes more pronounced in systems with larger number and size of dead spaces and impurities that interact strongly with the surfaces. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3973–3980, 2015