去除奈米微粒的低壓旋風分離器

Abstract

本研究設計並測試一個低壓旋風分離器來去除奈米微粒，此設備包含一個臨界流孔板和連接於其下游的一個軸向旋風器，臨界流孔板用於降低旋風器的壓力，使微粒滑動校正係數提高、阻力降低，進而提升旋風器對奈米微粒的收集效率。 由於微粒通過流孔板時可能會損失在其中，所以本研究探討微粒流經流孔板時的損失情形，並研究減少損失的方法。本研究探討的流孔板為O’Keefe公司的 E-9流孔板(O’Keefe Control Co.)，其孔徑為0.231 mm，上下游各接有入口管(內徑10.4 mm,長度90 mm)和出口管(內徑6.2 mm,長度60 mm)，臨界流量為0.455 slpm。當上游壓力為760 Torr，下游為260 Torr時，奈米微粒(氣動直徑小於100奈米)於流孔板內的擴散損失很小，粒徑15奈米時僅為3.4%，慣性衝擊損失也接近於零。當下游壓力降低至5.4 Torr時，奈米微粒擴散損失仍很小，但慣性衝擊損失會高達50%，主要發生在流孔板下游管壁上，解決方法為加大下游管徑，如將下游管徑從6.2 mm增加至25 mm，則奈米微粒慣性衝擊損失會降為零。 本研究的軸向旋風器有一個旋轉三圈的導翼片，其內徑為15 mm，中心軸半徑為10 mm。實驗時的旋風器進口壓力為4.3~7 Torr, 流量為0.351~0.566 slpm，測試微粒為固體氯化鈉和液體油酸微粒，直徑12~100奈米。結果顯示當流量固定，旋風器的效率會隨進口壓力降低而增加，如流量為0.455slpm時，當旋風器進口壓力從6.0 Torr降為5.4 Torr，旋風器對油酸和氯化鈉微粒的截取氣動直徑分別會從49.8和47.1奈米減少為23.1和21.2奈米。此外本研究發現氯化鈉和油酸微粒有相近的收集效率，所以固體微粒在此旋風器內的彈跳問題幾乎不存在。 以三維數值模擬方法計算旋風器的流場，發現在導翼片內的切線速度分布近似拋物面，根據這個發現，本研究推導出旋風器的收集效率的理論值，其結果與實驗數據符合，最大誤差在15%以內。本研究也推出一個可預測不同旋風器壓力及操作流量下旋風器截取氣動直徑的半經驗公式，此公式可準確預測截取氣動直徑，誤差在9%以內，根據此半經驗公式，我們算得半經驗的截取史托克數平方根 為0.241的常數值。 上述的理論收集效率僅考慮導翼片中微粒受離心力之去除作用，未考慮到導翼片下游腔體中的微粒去除，且沒有考慮到細微粒的擴散作用，因此誤差較大。為進一步準確計算旋風器的微粒收集效率，本研究先以三維數值模擬求得旋風器內全部的流場，再運用布朗尼動力模擬方法進行微粒收集效率的計算，同時考慮微粒受離心力及擴散作用的影響。結果顯示在不同操作條件下，旋風器的收集效率和截取粒徑都和實驗數據相當接近，最大誤差在3.5%以內。此外發現微粒的擴散損失主要發生於導翼片之後的微粒收集腔體中，因為氣流出了導翼片之後速度大幅降低，微粒因而有較長的停留時間由於擴散作用而被收集。 本研究的低壓旋風分離器可有效去除奈米微粒，推導出的旋風器截取氣動直徑半經驗公式和布朗尼動力模擬的結果，可設計低壓旋風分離器用於篩選某粒徑以下的奈米粉體、去除高科技製程反應腔真空排氣中的有毒微粒、及作為奈米微粒的採樣之用。

In this study, a low pressure cyclonic separator for nanoparticle removal was designed and tested. The device included a critical orifice and an axial flow cyclone connected downstream of the orifice. The orifice was used to reduce the pressure of the cyclone. At reduced pressure, particle slip correction factor is increased and particle drag force decreased by a significant amount resulting in an increasing collection efficiency of nanoparticles. Particle loss may occur as particles pass through the orifice. Therefore, this study investigated particle loss in the orifice and the method to reduce the loss at first. The investigated orifice was the O’Keefe E-9 (O’Keefe Control Co.) orifice whose inner diameter was 0.231 mm and critical flow rate was 0.455 slpm. At the upstream and downstream of the orifice, there is an inlet tube (inner diameter=10.4 mm, length=90 mm) and outlet tube (inner diameter=6.2 mm, length=60 mm), respectively. As the upstream pressure (Pou) and downstream pressure (Pod) of the orifice was 760 Torr and 260 Torr, respectively, nanoparticle (smaller than 100 nm in aerodynamic diameter) diffusion loss in the orifice was found to be very low and impaction loss was nearly zero. Diffusion deposition loss was only 3.5% for 15 nm particles. When Pod was reduced to 5.4 Torr, nanoparticle diffusion loss was still low however inertial impaction loss was increased to 50%, which mainly occurs at the tube wall downstream of the orifice. Increasing the inner diameter of the outlet tube was found to reduce particle loss due to inertial impaction. For example, increasing inner diameter from 6.2 mm to 25 mm, particle loss was reduced to zero. The axial flow cyclone tested in the present study has one vane which makes three complete turns. The inner radius of the cyclone was 15 mm and the radius of the spindle was 10 mm. In the experiment, the operated pressures at cyclone inlet (Pin or Pod) and the flow rates were ranged from 4.3 to 7 Torr and 0.351 to 0.566 slpm, respectively. Liquid OA (oleic acid) and solid NaCl particles in size between 12 and 100 nm were used to examine the collection efficiency of the cyclone. Results showed that at a fixed flow rate, particle collection efficiency of the cyclone was increased with decreasing Pin. For example, when the flow rate was fixed at 0.455 slpm, the cutoff aerodynamic diameters of OA and NaCl were reduced from 49.8 and 47.1 to 23.1 and 21.2 nm, respectively as Pin was reduced from 6 to 5.4 Torr. In addition, it was found the collection efficiencies of NaCl and OA particles were close to each other in the size range from 25 to 180 nm in aerodynamic diameter. This is to say the effect of solid particle bounce on collection efficiency does not exist in the cyclone. Using 3-D numerical simulation to calculate the flow field of the axial flow cyclone, it was found the tangential flow velocity distribution in the vane section was paraboloid. Based on this finding, theoretical equation for particle collection efficiency of the cyclone was derived and showed good agreement with the experimental data with the maximum error of 15%. A semi-empirical equation for predicting the cutoff aerodynamic diameter at different inlet pressures and flow rates was also obtained. The semi-empirical equation is able to predict the cutoff aerodynamic diameter accurately within 9 % of error. From the empirical cutoff aerodynamic diameter, a semi-empirical square root of the cutoff Stokes number, , was calculated and found to be a constant value of 0.241. The above theoretical collection efficiency only considered particle centrifugal force in the vane section, without considering particle removal in the chamber downstream of the vane. Also the diffusional effect of fine particles was not included. These all led to errors in theoretical collection efficiency. In order to improve the accuracy of particle collection efficiency, 3-D numerical simulation was conducted to obtain the total flow field first. Then Brownian Dynamic (BD) simulation was applied to calculate particle collection efficiency considering both particle centrifugal and diffusional effects. The simulated results of both particle collection efficiency and cutoff aerodynamic diameter are in good agreement with the experimental data with the maximum derivation of less than 3.5% at different operating conditions. The increase in the diffusional deposition was found to occur mainly in the chamber after the vane section when the gas expands and slows down. Therefore, particles have longer residence time to be collected in the chamber by diffusion. The low pressure cyclonic separator developed in this study can remove nanoparticles efficiently. The derived semi-empirical equation of cutoff aerodynamic diameter and the results of BD simulation can facilitate the design of the low pressure cyclonic separator to classify nanopowders below a certain diameter, to remove toxic nanoparticles from the vacuum exhaust of process chambers commonly used in high-tech industries, and can be used for nanoparticle sampling.