Detection of fugitive emissions from valve stems : DC resistance response and gas adsorption over tin dioxide mixed with alumina
Creators: name: MacKay, James Elliott
Publisher: name: University of British Columbia
Year: 2009

description: Stringent fugitive emission limits for process equipment including valves, as proposed by European regulatory bodies for example, require significant improvements in valve sealing technology and maintenance techniques and will create a need for monitoring and containment of very small leakage rates to reduce ambient air concentrations (to 1 ppm). Conceptually, the potential of a combined adsorbent / metal oxide sensor bed provides a novel solution to this problem and is studied to determine the feasibility of achieving the dual purposes of containment and monitoring at levels typical of default valve leakage rates, E = 6.56 x 10⁻⁷ kg/hr/source. Simultaneous adsorption breakthrough and electrical resistance measurements (dc) were obtained in a quartz reactor utilizing 2 co-centric tantalum electrodes. A loosely packed sensor bed consisting of 22.5 cc of a mechanical mixture of up to 40 %vol. Al₂O₃ (adsorbent) in SnO₂ (metal oxide sensor) was studied. 1 to 10 %vol. propylene in He was passed through the bed alternately with He and air gas cycles, and the adsorption breakthrough and electrical resistance monitored at temperatures of 50 - 150 °C. Results showed that the adsorption uptake was linearly proportional to the %vol. mixture of Al₂O₃ in the sensor bed, indicating that Al₂O₃ was responsible for the adsorption in the bed and that SnO₂ was essentially acting as a non-porous media limited to relatively low levels of surface adsorption responsible for very large changes in the bed electrical resistance. The change in electrical resistance between the oxidized state of the bed and the reduced state of the bed was used as the sensor bed's measured variable for the present system. Equilibrium adsorption uptake of 10 %vol. C₃H₆ in He over 100% Al₂O₃ at 50 °C and 12 kPa was 0.12 mmol of C₃H₆ per g of Al₂O₃. The uptake dropped significantly to 0.03 mmol/g at 100 °C with –ΔH[sub ads] = 29.2 kJ/mol (calculated from the vant Hoff equation). Maximum sensor bed sensitivity of 5.29 (the ratio of the bed resistance in air to the bed resistance after reduction by C₃H₆) was recorded for sensor bed composition consisting of 40 %vol. Al₂O₃ in SnO₂ at 150 °C but sensitivity dropped to 1.67 at 50 °C. The contrast between the temperature relationship of adsorption and that of electrical sensitivity indicates a significant challenge for the dual purpose sensor bed. That is, elevated operating temperatures favour fast sensor response and sensitivity but are unfavourable for adsorption uptake and the time between adsorbent regeneration (sensor life). Practical perspectives dictate that strong adsorbents should be utilized for light hydrocarbon or VOC recovery at elevated temperatures, in the 150 - 200 °C range, and metal oxide sensing materials prepared to improve sensitivity and selectivity for target gases in this same temperature range or lower, should be explored in combination to optimize sensor performance. Literature suggests that this is potentially viable, using additives such as Pt and Pd in sensor bed preparation and potentially using newly developed bed geometry such as wire mesh honeycomb (WMH) designs to further enhance this novel sensor concept. The electrical resistance in the sensor bed is modeled as a function of adsorption breakthrough using the axially dispersed plug flow model of Levenspiel and Bischoff (1963). The model was a good fit at 150 °C, but the modeled electrical resistance response was much faster than the observed response at lower temperatures indicating that kinetic effects were dominant at lower temperatures. A simple first order reaction mechanism in [O₂] was proposed in which an empirical reaction rate fitting parameter, a, was used to obtain a good fit between the model and the experimental data for all conditions between 50 to 150 °C. An activation energy, E[sub a] = 42.4 kJ/mol, was determined. A modified plug flow model (inclusive of dispersive and mass transfer effects) was compared to the axially dispersed plug flow model for adsorption and was not found to be of any additional benefit. It is concluded that the sensor bed concept is viable for containment and sensing of the default valve emission rate over a period of at least 1 year, but that further research and development is required to optimize materials, preparation and operating temperature.