結合觸媒氧化及高級氧化以處理含氯揮發性有機物之研究

Abstract

本研究主要是以觸媒氧化探討含氯揮發性有機物(chlorinated volatile organic compounds, CVOCs)的處理成效。選擇的處理對象為二氯乙烷(1,2-dichloroethane, DCEA)及氯乙烯(vinyl chloride, VC)。前者是工業上最常使用的有機溶劑之一，而後者則是製造聚氯乙烯(polyvinyl chloride, PVC)類之塑膠產品的主要原料。CVOCs在環境中的轉化過程甚慢，易累積於環境中，且具有毒性，會對人類的健康造成危害，且大部分已被證實具有動物致癌性。 本研究使用的觸媒為Pt/γ-Al2O3。結果顯示觸媒氧化對於DCEA及VC的轉化效果甚佳。DCEA在638 K可達90%的轉化率，而VC之T90則為580 K。相較於傳統的熱氧化處理，其DCEA與VC之T90分別為925及997 K，兩者所需的T90分別下降287及417 K。研究中亦探討不同空間流速的影響，當空間流速為80,000 h-1時，轉化效率最高，在637 K時能使DCEA完全轉化，而VC則在611 K能完全轉化。並建立DCEA及VC在Pt/γ-Al2O3上觸媒氧化之反應動力模式。所使用之模式為Rideal-Eley model，並結合Arrhenius equation求得反應的活化能(Ea)及頻率因子(A)。其中DCEA反應之Ea與A分別為29.02 kJ mol-1及1.02 × 105 s-1，而VC則為43.48 kJ mol-1及4.56 × 106 s-1。 在產物部份，DCEA及VC其觸媒氧化的最終產物皆為CO2、Cl2及HCl。在尾氣中並未偵測到不完燃燒之產物CO。兩者之含氯產物皆以Cl2為主，其中VC又為DCEA經觸媒氧化的中間產物。此外，觸媒氧化後的產物之總碳原子及氯原子的量，可達進口之碳原子及氯原子的70-100%，較傳統熱氧化處理之30-50%為高，顯示觸媒氧化對於兩汙染物之轉化效率是有極大的提升。 此外，本研究亦添加臭氧於觸媒氧化系統中，探討臭氧的加入對於DCEA處理效果的影響。因為VC具有雙鍵，而臭氧之共振結構對於雙鍵會有選擇性氧化的情形，故VC會於短時間內與臭氧反應完全，因此此部分不予以討論。DCEA經過臭氧觸媒氧化系統反應後，達90%轉化率的溫度為610-620 K，隨著臭氧濃度的增加而所需的反應溫度隨之降低。此外，在同溫度下，臭氧觸媒氧化對於DCEA之轉化率亦較觸媒氧化為高。此結果顯示臭氧的存在對於DCEA之處理的確有實質之提升。 因為受臭氧加入的影響，各項DCEA臭氧觸媒氧化後的產物之產率有了明顯的變化。以CO2來說，最終產率或礦化效率皆可達100%。至於含氯產物部分，仍以VC為其中間產物，而Cl2及HCl為最終產物。但整體氯原子平衡情況不佳，在DCEA完全轉化時，氣體產物總氯原子的產率只有進口DCEA氯原子量的40%左右，代表觸媒表面有大量的含氯物質吸附於其上，也就是造成觸媒活性退化的原因之ㄧ。

This study investigated the catalytic oxidation for the decomposition of chlorinated volatile organic compounds (CVOCs). The target compounds are 1,2-dichloroethane (DCEA) and vinyl chloride (VC). The former is commonly used as solvents in the industry, and the latter is a main material for manufacturing polyvinyl chloride (PVC) products. CVOCs are difficult to be converted, so they accumulate easily in the environment. Further, CVOCs are hazardous to human health with most of them being proved to cause cancers.

Pt/γ-Al2O3 was used as catalysts in this study. The results indicated that it enchaced the conversions of DCEA and VC via oxidation. The reaction temperatures for 90% conversion (T90) of DCEA and VC are 638 and 580 K, respectively. On the other hand, the values of T90 of DCEA and VC for the traditional thermal oxidation are 925 and 997 K, respectively. Besides, a lower gas hourly space velocity (GHSV) gave a higher conversion. The data with various GHSV were further used to establish the reaction kinetic model of catalytic oxidation of DCEA and VC over Pt/γ-Al2O3. Rideal-Eley model was adopted to simulate the experimental results. The model combining with the Arrhenius equation then yielded the activation energy (Ea) and frequency factor (A). The values of Ea and A were 29.02 kJ mol-1 and 1.02 × 105 s-1 for DCEA, and 43.48 kJ mol-1 and 4.56 × 106 s-1 for VC.

As for the products, the ultimate products of decomposition of DCEA and VC were found to have CO2, Cl2 and HCl. No incomplete combustion product of CO was detected. The main chlorinated product of decomposition of DCEA and VC was Cl2, while VC was also an intermediate of catalytic oxidation of DCEA. The total carbon and chlorine atoms of products after catalytic oxidation reached 70-90% of inlet carbon and chlorine atoms, and these values were higher than those after traditional thermal oxidation. The results showed that catalytic oxidation obviously promoted the conversions of DCEA and VC.

Furthermre, the effect of adding ozone in catalytic oxidation of DCEA was examined. Because VC had a double bond, while ozone showed the selectivity to the double bond. Thus, VC was completely reacted in a short time. When DCEA was treated via the ozone-catalytic oxidation process, about 90% conversion was reached at 610-620 K. The needed reaction temperature to reach the same conversion decreased with the increase of ozone concentration. On the other hand, the conversion of DCEA via the ozone-catalytic oxidation at the same temperature was higher than that of catalytic oxidation. This indicated that the presence of ozone indeed assisted with the efficiency for treating DCEA.

The addition of ozone to the catalytic oxidation of DCEA had distinct effects on yields of products. For CO2, the ultimate yield reached 100%. As to the chlorinated products in ozone-catalytic oxidation of DCEA, VC was also an intermediate, while Cl2 and HCl were the ultimate products. For the chlorine balance, the total chlorine atoms of gaseous products were 40% of inlet chlorine atoms. Thus it indicated a large number of chlorinated products were adsorbed on the surface of the catalyst, standing for a reason for the deactivation of catalyst.