Removal of low concentrations of acid gases: issues and solutions

Part per billion concentrations of acid gases such as SOx and NOx have been detected in both high purity gases and CDA lines. These contaminants can have deleterious effects on a number of high purity applications such as the optics found in lithography equipment steppers, scanners, and inspection tools. In addition, acidic gases have also been shown to reduce the life of masks and reticles, decrease fuel cell output due to catalyst poisoning, and cause hard disk drive crashes due to surface contamination and corrosion. Consequently, acid gas control in these applications has become a critical part of the required filtration system. SOx concentrations are typically used as the baseline for acid gas filter exposure guidelines and performance testing. However, this approach has been shown to provide poor filter life predictions, which has been attributed to the presence of other acidic and organic contaminants that compete with SOx for the available adsorption sites. Equally important, the type of sorbents and methods used to control acid gases can significantly affect the ability to remove SOx. In this work we will compare the performance of various sorbents, structures, and methods for the removal of SOx and NOx.