以ZnMn2O4/Al2O3吸附劑去除氣流中硫化氫之研究

Removal of H2S from gas streams using Zn-Mn-based adsorbents

郭立峰(Li-Feng Guo)

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(2014年)

摘要

生質能是目前全球四大能源，僅次於石油、煤及天然氣，然而生質物經熱裂解產生之合成氣中含有數百ppm硫化氫，硫化氫會腐蝕後端管線及毒化合成二甲醚之觸媒，人體暴露在濃度200至1,500 ppm的環境有致死危險。目前以金屬轉換法處理高溫氣體中之硫化氫為技術主流，此類文獻較少探討低濃度硫化氫的處理，並且多數文獻僅以固定的硫化氫濃度為操作參數，忽略不同的硫化氫濃度與空間流速、操作溫度的交互影響，故本研究製備以鋅錳氧化物載擔於γ-氧化鋁之吸附劑，探討在空間流速4,000至30,000 h-1、反應溫度400至600oC和入口硫化氫濃度200至1,000 ppm對吸附效率的影響，並研究吸附劑再生反應與吸附效率衰退的關係；長效性反應測試；改質吸附劑之研究。
製備之吸附劑在鍛燒600oC時有高的比表面積150.6 m2/g，且其尖晶石結構之ZnMn2O4為混合型金屬吸附劑，具有高反應速率和減少鋅元素揮發的優點。研究中愈高的空間流速有愈低的吸附量；愈高的反應溫度有愈高的吸附量；愈高的入口硫化氫濃度有愈高的吸附量，然而當作用中的吸附劑與硫化氫的反應速率達平衡時，愈高的濃度反而會降低吸附劑的吸附量。整體而言，與硫化氫反應的最佳空間流速為不超出17,000 h-1，可適用於吸附溫度400至600oC的狀況，吸附量32.5至53.3 mg-S/g-adsorbent，較之理論吸附量42.6 mg-S/g-adsorbent高的原因，推測為Claus反應消耗掉貫穿吸附劑之硫化氫。當硫化氫濃度為200 ppm並提升空間流速從17,000至30,000 h-1時吸附效率剩60%，然而當濃度為1,000 ppm並隨空間流速提升，吸附效率維持在90%而不衰減，此因提升反應物濃度可增加碰撞機率而增加反應速率，進而提升吸附量。並以相同的操作條件僅提升反應溫度後，吸附量和吸附效率皆大幅提升，此結果符合熱力學和化學反應動力學原理。
吸附劑經長效性測試在100小時仍有87%極佳的吸附效率，XRD證實吸附劑內之鋅元素因再生之劇烈再生放熱反應而揮發，導致吸附效率下降，且未形成金屬硫酸鹽而影響吸附效率。本研究透過不同的再生實驗得知，再生溫度300oC即可將於600oC反應環境形成之鋅錳硫化物完全恢復成鋅錳氧化物，且能有效降低因再生放熱反應導致的吸附效率下降，操作以空氣為再生氣體以及較低的再生溫度具經濟和節省能源的優點。研究試圖添加鈰於鋅錳氧化物上作為改質之吸附劑，因鈰具有晶格氧的功能可有效去除硫化氫，然實驗結果顯示吸附量下降，原因推測為本研究吸附溫度為400至600oC，在此溫度區間內無法提供足夠的熱能活化鈰的特性，並且由XRD得知鈰氧化物覆蓋鋅錳氧化物，進而影響吸附效率。

關鍵字

生質能 ； 硫化氫 ； 金屬吸附劑 ； 鋅錳氧化物 ； 晶格氧

並列摘要

Biomass may be pyrolyzed to produce syngas, however, some impurity such as H2S may be generated. A few hundred ppm of H2S and medium reaction temperature in the biomass system need to be investigated in order to protect the pipelines and catalyst from damage. Currently, metal conversion is the major treatment method for H2S removal in high temperature. Many studies investigate how to remove high concentration of H2S (over thousands ppm) without caring the interaction of H2S concentration, space velocity and reaction temperature. In this study, Zn-Mn based adsorbents supported on γ-Al2O3 is used for H2S removal in a temperature range of 400 to 600oC, space velocity range of 4,000 to 30,000 h-1, and H2S concentration range of 200 to 1,000 ppm. These parameters will seriously affect the performance of adsorbent capacity, and even interaction. From the experimental results, adsorbent after calcination at 600oC has a high surface area of 150.6 m2/g with the structure of spinel. It is advantageous for removing H2S at high reaction rate and protecting Zn from vaporizing at high temperatures. Experimental results show high capacity of adsorbent with low space velocity, high reaction temperature and high H2S concentration. However, when the reaction rate reaches equilibrium, capacity of adsorbent will decrease with H2S concentration. With the space velocity of 17,000 h-1, temperature range of 400 to 600oC, the capacity of adsorbent will be in the range of 32.5 to 53.3 mg-S/g-adsorbent. Theoretical capacity is 42.6 mg-S/g-adsorbent. The over theoretical capacity is possibly due to physical adsorption of alumina. Increasing H2S concentration from 200 to 1,000 ppm will enhance adsorption capacity and maintain at 90 percent of efficiency even the space velocity is increasing from 17,000 to 30,000 h-1. This is because enhancing concentration of reactants can increase the probability of collision, and thus enhance the adsorption capacity. By the way, enhancing reaction temperature can also increase the activity of molecule to promote adsorption capacity.
Long-term tests have reached more than 100 h and been maintained at 87 percent of efficiency. The reason for decreasing efficiency is because ZnO of adsorbent will be vaporized by violent exothermic reaction. XRD analysis shows the result and has no Zn sulfate or Mn sulfate. By different parameter of regeneration, the results show that Zn and Mn sulfides after adsorption at 600oC can be completely recovered to Zn and Mn oxides at regeneration temperature 300oC, and regeneration at 300oC can mitigate the effect of violent exothermic reaction. It is economic and saving energy. In order to enhance the capacity of adsorbent, ZnMn2O4 doping with CeO2 as a new adsorbent, because Ce has a function of lattice oxygen to effectively enhance ability of H2S removal. However, adding Ce into Zn-Mn based adsorbent does not improve adsorption capacity. The possible reason is that operating at a temperature range of 400 to 600oC can not provide sufficient energy to activate Ce for H2S removal.

並列關鍵字

biomass ； hydrogen sulfide ； metal oxide adsorbents ； ZnMn2O4 ； lattice oxygen

參考文獻

[7] 李聖慧，以固體吸附法去除沼氣中之硫化氫，崑山科技大學環境工程研究所，碩士論文，2010。

[54] Huang, C.E., The study of the hydrogen damage mechanism of pressure vessel and its preventive strategy, Environment and Safety Engineering, YunTech 2010, National Yunlin University of Science and Technology, 2000.

[9] 兔束保之，生質能源利用科學，揚智文化事業股份有限公司，2011。

[6] 李宏台、吳耿東、萬皓鵬、徐瑞鐘，我國生質能源之應用，經濟前瞻，2005。

[20] 劉雅菁，以ZnMn2O4/SiO2吸收劑高溫去除硫化氫之研究，國立成功大學環境工程學系，碩士論文，2004。