平板式薄膜蒸餾於溶液氨氮移除與回收之模擬與實驗探討

Simulation and experimental study on ammonia removal and recovery from aqueous solution by Flat sheet membrane distillation.

游采婕(TSAI-CHIEH YU)

指導教授 ： 莊清榮

中原大學/工學院/化學工程學系/碩士(2022年)

https://doi.org/10.6840/cycu202201492

摘要

為防止水體中氨氮濃度過高，避免造成河川湖泊優養化，現今已發展出許多技術用以處理廢水中的氨氮。其中，薄膜蒸餾法(MD)因操作上不受限於廢水中氨氮濃度高低，應用於氨氮處理上甚有發展潛力。目前業界普遍使用直接接觸式薄膜蒸餾(DCMD)操作，以硫酸為吸收液形成硫酸銨，若能於脫氨程序中產製氨水或其他更具回收利用之產物，其工業應用之經濟價值將提高。本研究進行空氣掃流薄膜蒸餾(SGMD)及真空式薄膜蒸餾(VMD)實驗，探討不同操作條件(進料溫度、pH值及氣體掃流流率等)下，對總質傳係數、通量及氨氣去除率之影響，並討論滲透端之冷凝裝置的大小對氨氮回收之影響。另外，亦建立直接接觸式薄膜蒸餾(DCMD)及真空式薄膜蒸餾(VMD)之氨氮移除模擬程式，探討進料溫度、pH值及氣體掃流流率等，對通量、總質傳係數(Overall mass transfer coefficient)及氨氣選擇率(Ammonia Selectivity, St)之影響，並與文獻實驗數據進行比較。
依據SGMD之實驗結果，顯示當進料氨氮濃度為1000 ppm、溫度為50 ℃，及有效膜面積與進料體積之比值 (A/V)為0.011 m-1，空氣掃流流率從2增加至20 L/min時，平板PTFE模組之總質傳係數由4.46×10^(-6)提高至9.94×10^(-6)m/s，經四小時操作後，氨氮去除率從45%提高至75%。而當以氨水形式回收，進料溫度設定為60℃、pH為12及滲透掃流流率為20 L/min並設置兩段式冷凝裝置進行回收，氨回收率僅20%，當增加冷凝瓶長度(磨砂口下500 mm)後，其氨回收率提升至39%；另外，以氯化銨產物回收，其進料溫度設定為60℃、pH 12及滲透掃流流率為10 L/min時，氨回收率為25%，當冷凝裝置加大並降低掃流流率至8 L/min，回收率提高至近40%。根據VMD實驗結果，滲透端壓力15 kPa時，氨移除率可達90%，總質傳係數為1.52×10^(-5)m/s，然而氨氣於低溫低壓狀態下之溶解度極低，氨回收率僅14%，因此增大冷凝系統並提高其滯留時間後，回收率提高至27%。
為驗證所建立之模擬程序之可行性，先以文獻之實驗條件進行模擬及實驗數據比較，於VMD模擬中，當進料pH值增加時，模擬之總質傳係數會隨之上升，但pH大於11後總質傳係數變化便趨於平緩，其模擬結果與文獻實驗數據之差距介於6~10 ％間；當進料流速增加時，氨氣通量及氨選擇率皆隨之增加，其氨氣通量之文獻數據與模擬結果相對差距約2~10%，與氨選擇率相對差距約0.7~2%間。另外，對於DCMD模擬中，當薄膜兩端液體溫度設定為等溫時，若操作溫度從20升至60℃，模擬之氨通量由5提高至近23 mol/m2‧hr，提高約4.6倍，其模擬結果與該文獻數據相對差距介於5~11%間，接著利用驗證後之DCMD程式碼進行批次進料模擬，將操作時間拉長至11.5小時候，透膜吸收液為硫酸及乙酸之氨氮去除率達92及86%。

關鍵字

薄膜蒸餾 ； 氨氮脫除及回收

並列摘要

In order to prevent the eutrophication of rivers and lakes, caused by the excess ammonia nitrogen concentration in the water, many technologies have been developed as the treatments of ammonia nitrogen in wastewater. Among them, membrane distillation (MD) has great potential for industrial applications since it is not only unlimited to the concentration of ammonia nitrogen in wastewater but also providing a compact operating space. At the present time, the direct contact membrane distillation (DCMD) is commonly applied in the industry while the sulfuric acid is used as the absorbent solution. The economic value of its industrial application will be enhanced on condition that ammonium or other more recyclable products can be produced during the deamination process. First part of this study discussed overall mass transfer coefficient, flux and ammonia removal rate for sweeping gas membrane distillation (SGMD) and vacuum membrane distillation (VMD) under different operating conditions, such as feed temperature, pH value and sweep gas flow rate for, and also discussed the influence of the size of the condensation device at the permeate side on ammonia recovery. In the second part of this study, simulation program for ammonia transport of direct contact membrane distillation (DCMD) and vacuum membrane distillation (VMD) was also established and compared with the experimental data of the literature in order to verify the feasibility of the simulation program.
According to the experimental condition of which the feed ammonia concentration was 1000 ppm, feed temperature was 50 ℃, and the ratio of effective membrane area to feed volume (A/V) was 0.011 m-1, SGMD results showed that the overall mass transfer coefficient of PTFE module was increased from 4.46to 9.94×10^(-6)m/s ａs sweep gas flow rate increased from 2 to 20 L/min. However, while recovered in the form of ammonium, the ammonia recovery rate was from 20% to 39 % when the length of the condenser was increased from 250mm to 500mm (under the frosted mouth) under the feed temperature of 60°C, the pH = 12 and sweep gas flow rate at 20 L/min. On the other hand, when the recovery product was ammonium chloride, the recovery rate of ammonia was only 25% under the feed temperature of 60°C, the pH = 12 and sweep gas flow rate at 10 L/min. By enlarging the condensing device and reducing sweep gas flow rate to 8 L/min, the recovery rate increased to nearly 40%. According to the results of VMD experiment where the permeate pressure was 15 kPa, the ammonia removal rate was 90%, as well as the overall mass transfer coefficient was 1.52×10^(-5)m/s. However, due to the solubility of ammonia under low temperature and pressure was extremely small, the ammonia recovery rate obtained was 14%. After enlarging the condensing system and increasing its residence time, the recovery rate was only increased to 27%.
In order to verify the feasibility of the simulation program established in this study, the experimental conditions in the literature are used to simulate and compare with the experimental data. In VMD simulation, when the pH value of the feed increased, the simulated overall mass transfer coefficient would increase. However, when the pH is greater than 11, the change of the overall mass transfer coefficient tends to be flat, and the deviation between simulation results and the literature data was between 6 ~ 10%. The deviations in ammonia flux and ammonia selectivity, respectively, between simulation results and the literature data was 2~10% and 0.7~2%, respectively. In addition, for the DCMD simulation, when the liquid temperature at both sides of the membrane is set to be isothermal and, if the operating temperature increases from 20 to 60 °C, the simulated ammonia flux increases from 5 to nearly 23 mol/m2·hr, an increase of about 4.6 times. The deviation between the simulation results and the literature data is between 5~11%. Then, the verified DCMD code is used for batch simulation. When the permeate absorbent is sulfuric acid and acetic acid, the removal rate are 92 and 86%, respectively, for 11.5 hours operation time.

並列關鍵字

Membrane distillation ； Ammonia removal and recovery

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