半自動化酸性氣體監測系統的研發

Abstract

傳統的酸性氣體採樣及分析過程繁複，且無法即時地監測空氣中酸性氣體的濃度，易增加人為的誤差，因此自動化酸性氣體採樣分析儀器的發展十分重要。

本研究建立一套能即時偵測酸性氣體濃度之半自動化採樣和分析系統，此系統的主體為一台研究室自行開發的高效率平行平板濕式分離器，其功用為進行氣體的自動化採集，此採樣器和一台IC離子層析儀連接後，可自動監測空氣中的酸性氣體。本研究使用的測試氣體為有機醋酸以及無機硝酸氣體，測試氣體源來自本研究室自行研發之氣體擴散管。本研究評估此套新系統適用性的方法，係將此系統的氣體採集濃度分析結果，和傳統的氣體採樣法：吸收瓶採樣法（醋酸）或多孔金屬片固氣分離器採樣法（硝酸）的氣體採集及濃度分析結果進行比較。平行平板濕式分離器之吸收液為超純水，其流量為2 mL/min，採樣氣體流量為5 L/min，採樣時間為持續1-3小時，離子層析儀設定的每次樣本分析間隔時間為每19 min。吸收瓶採樣法之吸收液為超純水，容量為150 mL，採樣氣體流量為3 L/min，採樣時間為持續1-3小時，採樣時間結束後進行離子層析儀注入分析。多孔金屬片固氣分離器採樣法使用之多孔金屬片塗敷劑為1 % （w/v）碳酸鈉，採樣氣體流量為2 L/min以及採樣時間為1小時，採樣時間結束後進行樣品萃取和離子層析儀注入分析。

實驗結果顯示，本系統的方法偵測極限，對醋酸氣體為1.24 ppbv、硝酸氣體為0.17 ppbv。在上述操作參數下，傳統吸收瓶採樣法對於醋酸氣體的採集效率高達99%。在測試的醋酸氣體濃度於10-190 ppbv時，經由吸收瓶採樣法測得的氣體濃度值和本系統偵測的氣體濃度值相近，兩者的線性判定係數R2為0.993。實驗結果亦顯示，多孔金屬片固氣分離器採樣法對於硝酸氣體的採集效率可達99%。在測試的硝酸氣體濃度在17-170 ppbv時，經由多孔金屬片固氣分離器採樣法測得的氣體濃度值和本系統偵測的氣體濃度值相近，兩者的線性判定係數R2為0.995。另外，透過吸收效率實驗，顯示本系統對於醋酸和硝酸氣體的吸收效率分別為96%和＞99%，和理論值近似。反應時間結果亦顯示，本系統在監測醋酸及硝酸氣體濃度變化時，其上升時間為20±1 min，下降時間為30±2 min。因此本半自動化氣體採樣及分析系統，能即時監測空氣裡特定酸性氣體濃度，並能記錄其短時間內的變化。

關鍵詞：平行平板濕式分離器、醋酸氣體、硝酸氣體、半自動化採樣分析系統

The sampling and analysis procedure is complicated in the traditional method for acidic gases. The traditional method is unable to monitor acidic gases in real time, and prone to making errors due to manual handling of the sampling. Therefore, it is important to develop a semi-automatic instrument to sample and analyze acidic gases.

This study has developed a semi-automatic sampling/analysis system for acidic gas. The system consists of a high efficiency parallel-plate wet denuder（PPWD）for gas sampling and an ion chromatography（IC）for real-time analysis. The PPWD is to sample the acidic gases, which is connected to IC for automatic monitoring. A home-made gas diffuser was developed to generate the test gases: acetic acid and nitric acid gas. The automatic system was validated by comparing the concentration measured by the system with that measured by the impinger(acetic acid gas) or the porous-metal denuder sampler(nitric acid gas). Operation parameters of the three methods were: (1) parallel-plate wet denuder－ultra pure water for gas absorption at the flow rate of 2 mL/min, gas flow rate of 5 L/min, sampling time of 19 min/sample; (2) impinger－ultrapure water for gas absorption at a fixed volume of 150 mL, gas flow rate of 3 L/min, sampling time of 1-3 hour/sample, and (3) porous-metal denuder－sodium carbonate was used as coating material (1 %, w/v), gas flow rate of 2 L/min, sampling time of 1 hour/sample.

Experimental results show that the method detection limit is 1.24 and 0.17 ppbv for acetic acid and nitric acid gas, respectively. For acetic acid, with the concentration range from 10 to 190 ppbv, good agreement between the concentration of the parallel-plate wet denuder and the impinger that has a good collection efficiency of >99% can be found with coefficient of determination（R2）＞0.993. For nitric acid, with concentration range from 17 to 170 ppbv, again good agreement between the concentration of the parallel-plate wet denuder and the porous-metal denuder that has a good collection efficiency of ＞99% can be found with R2＞0.995. Furthermore, the absorption efficiency test of the current system showed that the system has high efficiency of 96% and ＞99%, respectively, for gaseous acetic and nitric acids. Experimental results of the response time indicate that the rise time is 20±1 min and fall time is 30±2 min, respectively. Accordingly, the present parallel-plate wet denuder coupled with ion chromatography can detect gaseous acidic gases in real-time and can record short-term fluctuation of the concentrations.

Keywords: parallel-plate wet denuder, acetic acid gas, nitric acid gas, semi-automatic sampling/analysis system.