去除次微米微粒之文氏洗滌器理論分析

Abstract

一些工廠在洗滌器之後的排氣有高不透光率白煙的問題，研究發現，水蒸氣凝結於排煙中的細小微粒可能是造成白煙的主要原因。因此在廢氣排放前，如能降低次微米微粒的濃度，應可有效控制白煙的形成。 本研究擬探討利用蒸氣混合常溫廢氣使細微粒產生異相核凝增長，並配合文氏洗滌器去除增長後之微粒，藉以提高微粒去除效率。若排氣中含氯氣、氨氣、氯化氫及氟化氫等酸性氣體，本文氏洗滌器亦可在適當操作條件及適當吸收液濃度及流量下一併去除。文氏洗滌器設計時考慮實驗室可操作之流量，選擇喉部直徑10mm、流速60~100m/sec、流量283~471lpm之設計參數，另外喉部入口噴入液體流量與氣體流量比為2.5l/m3。 本理論研究結果顯示，在排氣溫度20℃、飽和比1.0時，對一正對數常態分佈（數目中間粒徑NMD為115.263nm、幾何標準偏差為1.92、總微粒數目濃度3.08 ×106/cm3）之細微粒而言，利用不同溫度之高溫蒸氣，皆有一達到最大蒸氣飽和比之蒸氣/廢氣質量混合比；而噴入100℃、飽和比1.0、質量混合比0.148的高溫蒸氣，混合氣體飽和比最大可達3.49，效果最佳。此最佳條件下粒徑50nm以上微粒在1.0 ms的核凝時間後均可成長至1.8 □m以上。另外高溫廢氣（150℃以上）中噴入常溫水的情況，當廢氣溼度在5%以下時，蒸氣飽和比皆有2.0以上，而在最佳混合條件下，NMD在1.0 ms的核凝時間後亦能成長至1.5 □m以上。 在文氏洗滌器效率及壓力降之計算結果可知，上述常溫廢氣中的細微粒未增長前總去除效率，在喉部速度為100、80及60m/sec時，Calvert計算效率分別為32.7、22.2、12.3%，Yung計算效率分別為0.1、0及0%，Slinn計算效率分別為17.7、10.8、5.2%；而增長後（以最佳混合條件）Yung計算效率分別為98、97及96%，Calvert及Slinn計算效率皆為100%；壓力降方面，在喉部速度為100、80及60m/s時，Leith et al.（1985）壓力降預估分別為76、49及27cm水柱。由此模擬可知，不論高溫或低溫廢氣，凝核程序在對於白煙控制應有其可行性。 關鍵詞：異相核凝增長、文氏洗滌器。

High flue gas opacity or white smoke problem exists in some factories. Water vapor condensed on small particles in the flue gas was found to be the main reason. If we could control the concentration of small particles in the flue gas before emission, the white smoke could be controlled. This study used high temperature steam（water vapor）to mix with normal-temperature flue gas so that small particles could grow by heterogeneous nucleation and condensation. Collecting the grown particles by a Venturi Scrubber results in a better collection efficiency. If the flue gas contains acid gases such as Cl2、NH-3、HCl、HF, the Venturi Scrubber can also collect them with proper operating conditions, proper absorbent concentration and flowrate. Considering the maximum operating flowrate in the laboratory when designing the Venturi scrubber, we designed the throat diameter to be 10mm, the throat velocity to be 60~100m/sec, the flowrate to be 283~471lpm, and the ratio of liquid-gas flowrate to be 2.5 l/m3. When the flue gas temperature is 20℃, the saturation ratio is 1.0, for a log-normally distributed aerosol with number median diameter（NMD）of 115.263nm, geometric standard deviation of 1.92, and total particle number concentration of 3.08×106/cm3, the theoretical results show that for different steam temperature, a specific steam/flue-gas mass mixing ratio produced a maximum saturation ratio. When the temperature is 100℃, saturation ratio is 1.0, and the mass mixing ratio is 0.148, the mixing high-temperature steam results in the highest maximum saturation ratio, 3.49. Under this condition, all particles with diameter greater than 50nm can grow to a diameter greater than 1.8□m after 1.0ms growth time. Mixing of normal-temperature water spray with high-temperature flue gas（150℃ and above） also produce the saturation ratio greater than 2.0 when humidity of flue gas is below 5%. In the best mixing condition, NMD can also grow to a diameter greater than 1.5 □m after 1.0ms growth time. When the throat velocities are 100, 80, and 60m/sec, the results show that collection efficiencies of the small particles in the normal-temperature flue gas are 32.7, 22.2, 12.3%, respectly, by Calvert’s theory, 0.1, 0, 0 % by Yung’s theory, and 17.7, 10.8, 5.2% by Slinn’s theory before particle growth. The corresponding collection efficiencies with the best mixing condition are 98, 97, and 96% by Yung’s theory, and 100% bt both Calvert’s and Slinn’s theories. The pressure drops are 76, 49, and 27cm-H2O, respectly, when throat velocities are 100, 80, and 60m/sec. In conclusion, this study shows that no matter it is high- or normal-temperature flue gas, the nucleation and condensation process is very useful for white smoke control. Key Words：heterogeneous nucleation and condensation growth、Venturi Scrubber