一個蜂巢狀填充洗滌系統的開發

Abstract

現行的高科技產業(如半導體、光電廠)通常在製程設備出口端裝設小型、流量低的局部洗滌塔(local scrubbers)，為處理製程尾氣的重要設備。局部洗滌塔有電熱水洗、燃燒水洗、填充式水洗及乾式吸附等種類，其中以小型的濕式填充洗滌塔為主要的酸鹼廢氣處理設備，然其影響吸收效率的因素如氣流的滯留時間、氣體污染物的進口濃度、循環水的品質及其pH值及氣液比(G/L)等，均值得進一步探討，並且有提升氣體吸收效率的需要。

過去研究指出，濕式填充洗滌塔對於無機酸鹼廢氣HF、HCl、NH3之處理效率不佳。本研究為了提高濕式局部洗滌塔對酸鹼氣體污染物的去除效率，將設計並測試一個高效率的小型蜂巢式填充塔。此洗滌塔由前段的噴霧系統噴水將廢氣中的酸鹼氣體去除，再利用後端的蜂巢式填充塔的細小水霧將其表面潤濕，提供更大的吸收表面積，以進一步提昇洗滌系統的去除效率。本研究所設計的高效率洗滌塔的處理廢氣風量為209~2,694 L/min，前段噴霧塔及後段蜂巢式填充洗滌塔之滯留時間分別為0.5~10秒及0.5~2.5秒之間。本研究中將針對氣體滯留時間、氣體污染物的進口濃度及噴霧系統的氣液比之操作參數外，也將研究蜂巢式填充塔的比表面積、氣體流量、總去除效率及洗滌塔壓損的影響。

研究結果發現，利用蜂巢式洗滌塔(噴霧塔+蜂巢式填充塔)時，對HCl而言，在低風量(670 lpm)時，當進口濃度大於400 ppb時，總去除效率為90 %，且隨著進口濃度愈高，總去除效率也會愈趨於穩定為98~99 %。其中蜂巢式填充塔在HCl的進口濃度為500 ppb時可提升5 %的去除效率，總去除效率為85 %，而在進口濃度為50 ppb時可提升15~20 %的去除效率，總去除效率為95 %，也就是說愈低濃度之下，蜂巢式填充洗滌塔的提升效率愈高，而在高風量(1855 lpm)之下也有相似的結果；對NH3而言，當進口濃度大於2000 ppb時，總去除效率會漸漸趨於穩定為80 %。若進口濃度小於2000 ppb時，去除效率會隨著進口濃度降低而急遽下降，甚至經過蜂巢式填充洗滌塔後，反而未提升去除效率，進口濃度為330.2 ppb時，去除效率為26.2 %，此情形在愈低濃度時愈明顯。

若我們使用蜂巢式填充洗滌塔來去除HCl時，在進口濃度為400 ppb時仍然有90 %以上的去除效率，顯示蜂巢式填充洗滌塔對於低濃度的HCl仍有較高的去除效率；蜂巢式填充洗滌塔去除NH3時，其去除效果並不佳，實驗結果得知當NH3的進口濃度大於2000 ppb時，總去除效率會趨於穩定為60~65 %。

氣液比方面，我們進行了兩組實驗(Qg/Ql =19.3及53.4)，結果發現到增加氣液比後，對於HCl的去除效率與進口濃度的曲線趨勢並沒有明顯的改變，其最主要原因為氣體滯留在洗滌塔內的時間較長所致，氣液比(Qg/Ql)為19.3及53.4時，其氣體滯留時間皆大於2秒。故氣液比為53.4時的去除效率進與口濃度的曲線趨勢會與與氣液比為19.3的曲線相似。

In the high-tech industry, small local scrubbers with low flow rate are usually installed at the exhaust end of the process equipments to treat residual gases. They are important tail-pipe equipments for removing process exhaust gases. Local scrubbers have several types such as electric heating-wet scrubbing, combustion-wet scrubbing, packed tower scrubbing and dry adsorption. Among them, small packed towers are the primary equipments for controlling acidic and basic waste gases. However, factors such as residence time of gas flow, the inlet concentration of gas pollutants, quality of re-circulating water and its pH value, and gas-liquid ratio, all of them will affect the absorption efficiency and they are worth to be investigated. Also it is necessary to improve the gas absorption efficiency of the scrubbing tower. Previous researches indicate that the abatement efficiency of HF, HCl, and NH3 by the packed tower scrubber is not very well. This research intends to design and test a highly efficient small-scale honeycomb type packed tower to improve the removal efficiency of acidic and basic gases. In the front of the scrubber, the water droplets will be sprayed to absorb and remove the waste gas. The removal efficiency will be further increased by the larger surface area offered by a honeycomb type packed tower (surface is wetted by water spray) installed at the rear end of the scrubber. The exhaust gas flow rate is produced by fax that is designed to 209~2,694 L/min, and the residence time at the front-end spray tower and the rear-end honeycomb packed tower are 0.5~10 sec and 0.5~2.5 sec, respectively. The parameters to be investigated in my research are included the residence time of gas flow, inlet concentration of gas pollutants, and gas-liquid ratio. In addition, the relationship between the overall removal efficiency and the specific surface area of the honeycomb packed tower, the wetting flow rate, and the pressure drop of the gas flow will be studied. The experimental result showed that, for the HCl, when the inlet concentration is more than 400 ppb, the removal efficiency is 90%. As the inlet concentration increase, the removal efficiency will become into a stable value, 98~99% by using honeycomb packed tower scrubber. For honeycomb section at low flowrate(670 lpm), the total removal efficiency can be raised 5% to 85% for HCl in 500 ppb of the inlet concentration; then, 50 ppb of the inlet concentration, the total removal efficiency also can be raised 15~20% to 95%. In others words, the lower inlet concentrations, the higher improvement of the total removal efficiency, as the similar result as air flowrate is 1855 lpm; for NH3, when the inlet concentration is more 2000 ppb, the total removal efficiency will become 80%, and if it below 2000 ppb, it will be decrease rapidly by the inlet concentration. Even the NH3 pass through the honeycomb packed tower, the efficiency doesn’t rise anymore. This situation is obvious under very low concentration. For honeycomb packed tower treat HCl, the inlet concentration is 400 ppb, the total removal efficiency is 90%. There is no different between honeycomb packed tower and honeycomb packed tower scrubber. Undoubtedly, the honeycomb packed tower also has high removal efficiency for low concentration of the HCl. However, the total removal efficiency for NH3 is not very well. When the NH3 inlet concentration is more than 2000 ppb, the total removal efficiency is closed to 60~65%. We compare gas to liquid ratio between 19.3 and 53.4. We found that there is no different between them for removal efficiency curve, because both of them of the residence time are too long to distinguish. Their residual time are both more than 2.0 second.