利用微水霧改善傳統填充塔對HCl氣體去除效率之研究

Abstract

現行高科技產業所排放的酸鹼廢氣皆使用填充式洗滌塔來做處理，但在先前的研究當中顯示，目前傳統的填充塔對於低濃度、高風量之酸鹼氣狀污染物的處理效果不佳，無法符合半導體法規對於酸性廢氣處理效率必需大於95 %的規定，因此處理效率必需大幅提昇。 為了達到提高酸鹼氣體污染物的去除效率，本研究設計並建造及測試一個高效率的洗滌系統模場，在原有的填充塔前加設一個噴霧塔，利用噴霧塔中產生之細微水霧的大比表面積、及極佳的吸收效果，來提昇洗滌系統的去除效率。本研究中針對高科技業常排放之HCl氣體做為模擬的污染氣體進行各種試驗，並將在各操作條件下的實驗結果與理論做一比較。 本研究所設計的高效率洗滌塔的測試風量為1.3~7.2 CMM，填充部分之滯留時間為0.7~2.5秒，其他之設計條件符合半導體製造業空氣污染管制及排放標準中所規定。噴霧塔之滯留時間為0.8~2.4秒；噴霧塔之氣液比為13000~60000 m3/m3，水滴之梭特直徑(Sauter diameter)為28.4 μm，水滴之pH值為5~7之間(去除鹼性氣體時)或7~10(去除酸性氣體時)。 本研究先針對填充塔部分進行研究測試。結果顯示填充塔對HCl的去除效率會和進口濃度有相當大的關係。當進口濃度低於1 ppm時去除效率會快速的下降，而當進口濃度逐漸上升後，去除效率會漸漸穩定而達到一平衡的效率。當改變洗滌水導電度時，發現導電度升高，去除效率會下降，且在愈低濃度的區間此現象會更加明顯。在洗滌水的pH方面，增加pH值由7到10，對於去除HCl上也有幫助，但並不是非常顯著。反而在降低風速以延長滯留時間由0.7秒到2.5秒時，在進口濃度為220 ppb的條件下可以得約11%的效率提升。 在噴霧塔的實驗結果上，最重要的操作條件為氣液比。當氣液比由19500下降至13000時，可以得到10%以上的效率提升，且在低濃度區去除效上有相當不錯的改善。其餘操作條件所得到的趨勢與填充塔的大致上相同，但得到的效率提升都要較填充塔來的優異。 本研究最終的目的為同時運作填充塔和噴霧塔，使其成為一新式的洗滌系統。此系統經在操作條件為噴霧塔氣液比13000、水霧pH=7、填充塔洗滌水pH=7、循環水量25 lpm的情況下測試後，改善結果顯示，在原本只有填充塔的時候，進口濃度需要到1350 ppb時才有辦法達到95%去除效率，在加入噴霧塔之後進口濃度只需要600 ppb便可以有95%的效率。300 ppb以上也可以達到90%的去除率，這表示此新式洗滌系統可以增加不少對於低濃度HCl污染氣體的去除能力。

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.