| Summary: | 碩士 === 國立交通大學 === 環境工程系所 === 92 === The packed scrubbers are used to control the acid and basic exhaust gas in high-tech industries. However the earlier researches indicate that the efficiency of packed tower is limited when the inlet concentration is low and flow rate of the acid and basic gas pollutant is high and thus fails to meet the emission standard of semiconductor manufacture. Thus there is urgent need to improve the removal efficiency. This study aims to design and build a new scrubbing system to increase efficiency of removing acid and basic gas pollutant. A spray tower was designed to produce the high surface ratio of the fine mist before the packed tower to absorb the pollutant with better efficiency. An experiment was conducted using HCl gas as pollutant in the new scrubbing system to study its performance under different conditions & the results were compared with the theory.
The test flow rate of the new scrubbing system was kept between 1.3 and 7.2 CMM; the retention time of the packed tower was set 0.7~2.5 sec, and other design parameters was met the semiconductor manufacturing emission standard. The retention time in the spray was adjusted between 0.8 to 2.4 sec; the gas to liquid ratio of the spray tower was varied from 13000 to 60000 m3/m3; the sauter diameter of the mist was 28.4μm; The pH of the mist was used between 5 and 7 when removing the basic gas & it was set 7 and 10 when removing acid gas.
Firstly the efficiency of packed tower of our new scrubbing system was tested & the results show the removal efficiency is highly related to the HCl inlet concentration. When the HCl inlet concentration is lower than 1 ppm, the removal efficiency was 35~90% & when inlet concentration become higher than 1 ppm, the removal efficiency increased to about 95% and reach a equilibrium removal efficiency. The removal efficiency show decreases with increase in the conductivity of the scrubbing water. The removal efficiency of HCl gas show when pH of scrubbing was increased from 7 to 10. However the increase of the retention time from 0.7 for 2.5 sec under 220 ppb inlet concentration show 11% improvement of HCl removal efficiency.
The most important parameter in spray tower experiment is Qg/Ql. When the ratio decreases from 19500 to 13000, spray tower can increase the HCl removal efficiency more than 10%. Besides this, it can work very well in low inlet concentration region. The trends under other operation parameters are the same with the packed tower, but the performances of the spray tower are better than the packed tower.
The new scrubbing system containing the spray tower and packed tower was tested with Qg/Ql=13000, mist pH=7, scrubbing water pH=7 and scrubbing water flow rate is 25 lpm & the results show that the removal efficiency reach to 95% only with the inlet concentration of 600 ppb by adding the spray tower instead of inlet concentration of 1350 ppb while using packed only. The removal efficiency of 90% was achieved with inlet concentration of 300 ppb can also reach to 90%. These results show that the new scrubbing system dose strengthen the ability of removing HCl gas pollutant in low inlet concentration.
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