以加氫脫氯方式降解液相OCDD/F之效率及控制參數探討

Removal of liquid-phase OCDD/F by catalytic hydrodechlorination

劉美辰(Mei-chen Liu)

指導教授 ： 張木彬

國立中央大學/工學院/環境工程研究所/碩士(2015年)

摘要

本研究探討加氫脫氯技術於溫和環境下對於OCDD/F之去除及對廢水中PCDD/Fs之去除與降解特性。PCDD/Fs係持久性有機污染物(Persistent Organic Pollutants, POPs)，國內廢水PCDD/Fs排放標準依既(新)設業者規範不同限值，新設業者之限值為5 pg I-TEQ/L，既設業者之限值為10 pg I-TEQ/L。傳統的廢水處理法以活性污泥或碳吸附系統來降解及去除水中汚染物，但這些程序除了可能效果不彰外，也可能僅是對污染物進行分佈相的轉移，未能完全的去除或破壞污染物結構。本研究以具化學還原特性的鈀觸媒加氫脫氯程序進行效率測試。文獻已證實鈀觸媒的產氫效能，證明鈀觸媒可有效轉化有機溶劑產生氫原子，因此加氫脫氯乃利用鈀觸媒的產氫及捕捉氫來還原PCDD/Fs，脫氯成無毒性之產物。然而此技術對於PCDD/Fs破壞的影響因子尚未釐清，因此本研究探討各操作參數的影響及PCDD/Fs的降解途徑。將特定OCDD/F濃度，攪拌速度及溫度條件下，探討溶劑、鹼劑、觸媒劑量、載體效應對OCDD/F催化脫氯降解的影響。實驗結果顯示不同溶劑下，鈀觸媒加氫脫氯OCDD/F以甲醇為溶劑之去除效率及破壞效率最佳，在反應60分鐘內PCDD/Fs總質量去除效率達96%，因此選擇甲醇作為後續試驗溶劑。添加不同劑量鹼劑NaOH於反應系統中，在反應前30分鐘，以1 mM NaOH條件下之PCDD/Fs質量去除效率較高，且去除效率未隨鹼劑添加量增加而上升；過量的NaOH會阻塞觸媒孔洞導致表面積下降，於50 mM NaOH條件下之總PCDD/Fs去除效率最低；反應時間進行至60分鐘，以未添加NaOH條件下之質量PCDD/Fs去除效率較高，故後續實驗不再添加NaOH。此外，觸媒載體對催化結果也有影響，本實驗選擇氧化鋁及活性碳作為載體進行比較，結果顯示Pd/Al2O3的PCDD/Fs去除效率比Pd/C高，特別是破壞效率的差別較大。本研究團隊以自主研發之連續熱裂解模組處理國內中石化安順場受高濃度汞、PCDD/Fs污染土壤，此系統之驟冷塔可捕捉熱處理污染土壤產生之高濃度有機化合物廢氣及高濃度PCDD/Fs冷凝廢水，廢水之PCDD/Fs濃度高達16~44 ng I-TEQ/L，因此針對冷凝廢水PCDD/Fs5之去除進行試驗。首先，使用球狀活性碳進行吸附試驗，結果顯示在液固比為3:1條件下，放流水PCDD/Fs及總汞可符合法規管制標準，高活性碳使用量導致成本過高。進一步，利用加氫脫氯技術處理冷凝廢水。在未添加還原劑條件下，反應180分鐘後PCDD/Fs質量去除效率為53.21%；在添加5%還原劑條件下，反應180分鐘後PCDD/Fs質量去除效率提升為71.86%。然而其毒性濃度仍未達法規管制標準(5 pg I-TEQ/L)，因此先將冷凝廢水曝氫氣，使冷凝廢水之氫氣達飽和，再將此廢水進行鈀觸媒催化試驗。結果顯示反應180分鐘後PCDD/Fs質量去除效率提升至97.34%。

關鍵字

戴奧辛 ； 鈀觸媒 ； 加氫脫氯

並列摘要

Adsorption with activated carbon is the most commonly applied technology for removing polychlorinated dibenzop-dioxins/ polychlorinated dibenzofurans (PCDD/Fs) from wastewater, but it just transfers PCDD/Fs from liquid phase to solid phase and may lead to secondary pollutions. On the other hand, it is proved that palladium speeds up hydrogenation and dehydrogenation reactions. In this study, hydrodechlorination of PCDD/Fs was investigated over palladium catalyst with select solvents, alkali agents, catalyst weights and catalyst supports. Experimental results indicate that with the initial OCDD/F concentration of 20 μg/L, stirred speed of 250 rpm and operating temperature of 25 oC, the removal efficiency of PCDD/Fs achieved with 5% (wt.) Pd/Al2O3 varies in different solvents. 5% (wt.) Pd/Al2O3 catalyst exhibited higher activity in methanol than in iso-propanol, n-hexane or toluene. With the treatment time of 60 minutes and methanol as solvent, the mass removal efficiency of PCDD/Fs reaches 96%. Different alkali concentrations were added in the reaction system to evaluate the effect and the results indicate that the mass removal efficiency of PCDD/Fs with 1 mM NaOH is better than that without NaOH at a treatment time 30 minutes. However, after 60 minutes treatment the mass removal efficiency of PCDD/Fs in 0 mM NaOH is better than that of 1 mM NaOH as a result of catalyst surface fouling with sodium. In addition, mass removal efficiency of PCDD/Fs decrease as the concentration of NaOH is increased. Moreover, catalyst support is also an important parameter affecting dehydrogenation reactions. Comparison of the destruction efficiency over 5% (wt.) Pd/Al2O3 and 5% (wt.) Pd/C shows that 5% (wt.) Pd/Al2O3 is more active than 5% (wt.) Pd/C for PCDD/Fs removol. Furthermore, this study investigates the effectiveness of AC adsorption and hydrodechlorination in removing PCDD/Fs from wastewater. First, the effectiveness of activated carbon in adsorbing PCDD/Fs from wastewater is evaluated. The result shows that the toxicity concentration of PCDD/Fs is lower than the standard as the ratio of liquid and activated carbon weight is controled at 3:1, however, it is not cost-effective. Hence, hydrodechlorination process is applied to degrade PCDD/Fs. The PCDD/Fs mass removal efficiency without adding reducing agent is 53.21% after operating 180 minutes; with adding 5% reducing agent (methanol), the removal efficiency increases to 71.86%. In addition, to better understand the differences between hydrogen donor and hydrogen molecules, this test had preaerated hydrogen gas into waste condensed water and then palladium catalyst was added for catalytic hydrodechlorination test. The test results show the PCDD/Fs mass removal efficiency increases to 97.34% with the treatment time of 180 minutes.

並列關鍵字

PCDD/Fs ； palladium catalyst ； hydrodechlorination

參考文獻

49. 洪保鎮，「熱裂解系統對戴奧辛之去除特性研究」，博士論文，國立中央大學，2014.

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