微粒負載對熱脫附管效能影響以及前置濾材開發與評估

Abstract

近年來，熱脫附管被廣泛地應用於作業環境空氣中揮發性有機化合物的採樣。雖然市售熱脫附管在吸附劑前端通常以填充玻璃綿作為保護，但是，根據過去的實驗經驗得知，該段玻璃綿對微粒的收集效率並不高，這表示微粒仍會穿透玻璃綿進而沉積在吸附劑上。有鑑於此，本研究的目的在於，瞭解微粒負載對吸附劑吸附效能的影響，以及設計可替換式前置濾材以增加熱脫附管的使用壽命。 本研究自行填充熱脫附管進行測試，選用吸附劑為Carbopack X，填充重量為100 mg，測試流率為0.2 L/min。在吸附能力測試方面，使用丙酮進行評估，當達飽和時，隨即以氮氣進行脫附。在吸、脫附的過程中，以火焰離子偵測器進行即時濃度監測。研究中，分別以定量微粒輸出產生器 (TSI 3076) 與超音波霧化器 (Sono Tek) 分別產生次微米以下以及微米粒徑範圍的測試氣膠微粒，而在評估各式濾材或熱脫附管的微粒穿透率時，則分別搭配微粒電移動度掃描分徑器 (Scanning Mobility Particle Sizer, SMPS) 與氣動微粒分徑器 (Aerodynamic Particle Sizer, APS) 進行上、下游微粒粒徑濃度分布的量測；在微粒負載實驗時，熱脫附管則改用定量抽氣幫浦連續操作。微粒的材質有氯化鈉 (NaCl) 固體微粒與癸二酸二異辛酯 (DEHS) 液體微粒兩種。測試過程中，利用壓力轉換器來監測濾材壓降變化。所測試的前置濾材包括：原廠填充之玻璃綿、不鏽鋼篩網 (400、1500 Mesh)、海綿 (110 ppi) 以及N95等級之纖維性濾材。驗證前置濾材效能時，同時將兩支熱脫附管進行液態微粒負載，只是其中一支有加裝前置濾材；微粒負載後，再量測吸附效率曲線，並且分別與各自初始的曲線進行比較。 結果顯示，固態次微米與微米微粒負載重量為0.6 mg時，對吸附劑吸附能力的影響 (減少) 分別為5% 與13%。液態微粒同樣會降低吸附能力，且由於其會在吸附劑表面形成薄膜，大幅減少比表面積，所以影響較固態微粒明顯，當負載重量為0.8 mg時，差異為45%。原廠填充之玻璃綿的壓降為21.2 ± 8.1 mmH2O，最易穿透粒徑介於0.3 ~ 0.5 μm，其穿透率為60 ~ 75%。而400 Mesh的不鏽鋼篩網在表面風速為17 cm/s時，需要40層才能達到與玻璃綿有相當的過濾效率，但是所造成的空氣阻抗則約為32 mmH2O。增加篩網網目數至1500，同時配合降低表面風速至0.31 cm/s時 (篩網直徑約37 mm)，則可以在較低的空氣阻抗下達到預期的過濾效率，但1500 Mesh篩網不容易取得，其成本也較高。另外，110 ppi海綿 (厚度30 mm) 在表面風速0.68 cm/s時，最易穿透粒徑 (約0.6 μm) 的效率約30% (空氣阻抗約0.25 mmH2O)，因此如果要讓該海綿具有90% 的收集效率，則厚度需增加至約300 mm，雖然可透過壓縮來提高收集效率，但品質不易控管。N95等級纖維性濾材在表面風速為10.37 cm/s (直徑6.4 mm，厚度0.8 mm) 時，最易穿透粒徑 (約0.04 μm) 穿透率約5%，壓降為9.8 mmH2O，是目前唯一可以同時達到前置濾材所需具備之高效率、低阻抗、體積小、取得方便、價格便宜等特性的材料。最後，在液態微粒負載的情況下，加裝前置濾材 (N95等級之纖維性濾材) 可以大幅降低微粒負載對吸附劑吸附能力的影響程度。

Thermal desorption tubes are commonly used to quantify trace amount of VOCs in the workplace. Despite a small piece of glass wool normally placed in front of the sorbent, it is unlikely an absolute filter, and aerosol penetration and deposition on the sorbent are inevitable. Therefore, this study aimed to characterize the effect of aerosol loading on the performance of thermal desorption tubes. The ultimate goal was to design a pre-filter for a better performance of thermal desorption tubes in practical dusty working environments. Homemade thermal desorption tubes loaded with 100 mg Carbopack X sorbents were used in the present study. The sampling flow rate was 0.2 L/min. Acetone vapor was generated to perform the adsorption tests and the nitrogen gas was used for desorption. A flame ionization detector (FID) was employed to measure the acetone concentrations upstream and downstream of the thermal desorption tubes. A constant output aerosol generator (TSI 3076) and an ultrasonic atomizing nozzle (Sono Tek) were used to generate sub-micrometer-sized and micrometer-sized aerosol particles, respectively. For aerosol penetration test, a scanning mobility particle sizer (TSI SMPS) and an aerodynamic particle sizer (TSI APS) were employed to measure the aerosol concentrations and size distributions upstream and downstream of the test filters. Both solid (NaCl) and liquid (DEHS) particles were generated and the pressure drop across the filter was simultaneously monitored. Glass wool plug, stainless steel mesh (#400, #1500), polyurethane foam (110 ppi) and fibrous filter disc cut from N95 filtering facepieces were tested in this work. Only the glass wool installed upstream of sorbent was tested for aerosol penetration. To verify the performance of the pre-filter, two thermal desorption tubes (one with the pre-filter) were simultaneously challenged with DEHS particles and then compared with the initial breakthrough curves. The experimental results showed that the sub-micrometer-sized and micrometer-sized solid particle loading (loaded mass 0.6 mg) decreased the adsorption capacity, 5% and 13% less, respectively. The liquid particles could significantly deteriorate the sorption performance, because the deposited liquid particles might form the film covering the activate sites. The most penetrating particle size (MPPS) of the glass wool was 0.3 ~ 0.5 μm and the aerosol penetration of MPPS was about 60 ~ 75% with the pressure drop of 21.2 ± 8.1 mmH2O under the sampling flow of 0.2 L/min. As for the 400 mesh stainless steel, the aerosol penetration of 40 pieces was comparable to that of the glass wool with the face velocity of 17 cm/s and pressure drop of 32 mmH2O. High aerosol collection efficiency (for example, 90%) can be achieved by increasing the mesh number and decreasing the face velocity. However, the use of stainless steel with high mesh number (1500 mesh in this case) was not cost effective. With the use of 110-ppi foam, the total length of the foam was estimated to be as long as 300 mm to attain the required collection efficiency (90%) at a face velocity of 0.68 cm/s. The aerosol collection efficiency can be enhanced by increasing the foam packing density. However, it was difficult to guarantee the foam packing quality to gain reliable performance. Moreover, the N95 filter disc (D = 6.4 mm, H = 0.8 mm) showed an excellent performance on aerosol collection with a fairly low pressure drop of 9.8 mmH2O. Among the filter materials tested, the N95 filter disc worked best, for low cost, low pressure drop and stable quality. Finally, the devastating effect of aerosol loading on the adsorption performance of thermal desorption tubes can be significantly leveraged with the use of a N95 pre-filter.