A parallel plate wet denuder for acidic gas measurement

A parallel plate wet denuder (PPWD) was designed and tested for acidic gas absorption in this study. The PPWD is composed of two frosted glass plates coated with nano-TiO₂ particles. The gap of the plates is 4 mm, the active surface is 7.5 cm in width and 15 cm in height. The DI water is flowing downward along the surfaces to form a uniform water film to absorb the gas flowing upward along the gap. The gas absorption efficiency and memory effect were tested using test gases of HF (3.57-3.80 ppb), HCl (0.48-15.63 ppb), and HNO₃ (8.43-20.92 ppb) generated from permeation tubes at the flow rate of 5-10 L/min, scrubbing DI water flow rate of 1.0 mL/min/ plate, and pH = 6.8 ±0.3. The experiment was also conducted for HF and HCl with scrubbing water pH value of 3.08 and 4.10 to examine the effect of pH on the absorption efficiency. The experimental data at different flow ratas match with the theoretical values very well assuming perfect absorption condition at the water film when the scrubbing water pH = 6.8 ±0.3. The absorption efficiency of the three gases at the sampling flow rate of 5 L/min was nearly 100% when the scrubbing water pH value was 6.8 ±0.3, but was found to decrease slightly when the pH value of scrubbing water was reduced from 6.8 to 3.08, which indicates slight modification of perfect absorption condition at the water film. The memory effect test showed that the rise time was 15 to 25 min and the fall time was 10 to 25 min for the three gases. Good uniformity of the water film was observed during the 9-day ambient air sampling for HF, HCl, and HNO ₃, which indicates the applicability of the present PPWD for ambient air sampling.