Membrane Preconcentrator for Portable Trace VOC Detectors

A volatile organic compound (VOC) detection system that is portable, sensitive to the low-ppb level, and able to provide reliable measurements in real time is needed for environmental compliance monitoring and process control in chemical plants. Recently, hand-held and battery-powered ppb-level VOC photoionization detectors (PID) have become commercially available. However, the presence of water in the sample air interferes with the detection accuracy in PIDs. This project describes the development of a membrane device to remove water vapor from the gas being sent to portable PID-based VOC detectors. The membrane uses a perfluoro polymer that is water-permeable and VOC-rejecting. The membrane device is compact, lightweight and energy efficient, which makes it convenient to integrate into a portable detection system. To reduce water relative humidity in the air from 100 percent to 10 percent while sending 200 mL/min gas to the detector, a membrane with an area of 0.1 m2 is required. In the Phase I project, membranes with the required permeance and selectivity were prepared and made into bench-scale modules with various configurations and the required membrane area of 0.1 m2. These modules were evaluated for nitrogen/toluene separation, and the configuration was optimized to achieve better separation performance. In addition to removing water from the air, these modules have been demonstrated to enrich VOC content by 10- to 100-fold in the gas sent to the detector. In Phase II, we will optimize module configurations using more innovative designs for improved water removal performance, and construct a prototype membrane device to deliver to our business partner, RAE Systems, Inc., for testing and evaluation. Integrating such a membrane device with a hand-held ppb-level, VOC detector, such as the ppbRAE Plus detector made by RAE Systems, only adds 30-40 percent by volume and 100 percent by weight, which would provide an easily portable system with much improved performance.