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標題: 低溫二氧化碳吸附劑篩選之研究
Screening Test of Low Temperature CO2 Sorbents
作者: 蘇峰生
Su, Feng-sheng
關鍵字: CO2 adsorption/desorption;CO2吸脫附;screening of sorbents;concentration of CO2;dual bed adsorption;CO2吸附劑篩選;CO2濃縮;雙塔循環吸附
出版社: 環境工程學系所
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本研究之目的主要針對五種具潛力之固體吸附劑,包含3-aminopropyl-triethoxysilane改質之CNT (CNT(APTS))、GAC、NaY、13X、N-[3-(trimethoxysilyl)propyl]ethylenediamine改質之Y60 (Y60(EDA))等,篩選出具高CO2吸附量、快速吸脫附動力、低脫附熱、高CO2選擇性吸附、高CO2濃縮率之材料,並求取其CO2吸脫附與濃縮上之較佳操作參數,以利未來應用於產業CO2之捕獲及濃縮之參考依據。第一部份研究:吸附劑篩選之結果顯示,13X在乾燥氣流條件下表現較佳,工作吸附量(qw)比其他吸附劑高,在常見的煙道氣溫度50oC、CO2濃度15%下,13X的qw為55.6 mg/g;在CO2高濃度50%、溫度25˚C下,13X的qw為136.2 mg/g。在含有水氣的氣流中(含水量10 vol%),則是CNT(APTS)的qw最高,在CO2濃度50%時qw可達89.5 mg/g。CNT(APTS)與Y60(EDA)於濕度環境下可增加吸附量,原因在於表面含胺官能基與水氣和CO2作用進而增加qw。在再生熱分析方面,未改質之材料(GAC, NaY, 13X)之吸附熱(∆Ha)皆屬於物理吸附,而改質過之材料(CNT(APTS), Y60(EDA)),其∆Ha則介於物理與化學吸附之間,並且脫附熱(∆Hd)也高於未改質之材料。吸附劑再生所需總熱量(Qregen)依序為Y60(EDA)>GAC >NaY>CNT(APTS)>13X。這五種乾式固體吸附劑之Qregen皆遠小於濕式吸收劑MEA,顯示乾式固體吸附劑在操作較節能,能夠較MEA節省許多脫附成本。利用質量傳輸方程式模擬各吸附劑吸附速率常數(ka)與脫附速率常數(kd),結果發現ka與kd皆隨著溫度上升而增加,在低溫25˚C下,5種吸附劑的ka依序為CNT(APTS) > Y60(EDA) > GAC > NaY > 13X;kd依序為CNT(APTS) > GAC > Y60(EDA) > 13X >NaY。吸附劑篩選以(1)工作吸附量(qw);(2)吸附指標(Adsorption index, AI);(3)水氣影響;(4)再生熱;(5)吸脫附速率;(6)CO2選擇性等指標,綜合評估5種吸附劑,在乾燥氣流25°C條件下,13X具較高吸附量、高吸附指標(AI)、相對N2競爭吸附較小,可選擇作為後續CO2濃縮測試所使用的吸附劑。在含水氣之吸附部分,篩選在50°C具最高的吸附量、高AI、N2競爭較小的CNT(APTS)作為含水氣之CO2濃縮測試所使用的吸附劑,進行後續CO2脫附/濃縮測試。
第二部份研究則探討吸附劑再生與CO2脫附/濃縮,利用變壓/變溫脫附程序(VSA/TSA)水蒸汽脫附法等兩種脫附方法,分別以13X為吸附劑進行乾燥氣流環境吸脫附,以及CNT(APTS)進行含水氣之CO2吸脫附。研究結果發現,13X較佳VSA/TSA脫附溫度與壓力分別為140°C及0.01 bar,可以得到CO2純度93.3%的脫附氣體,並且連續吸脫附十次後,qw能維持平衡在76.9 mg/g、AI為91.6%。CNT(APTS)在含水氣之環境下吸附後,以較佳脫附溫度與壓力為140°C及0.1 bar可得到CO2純度82.7%的脫附氣體。在十次循環吸脫附後,qw能維持平衡至88.3 mg/g、AI為90.7%。水蒸汽脫附程序顯示,提高蒸汽壓力至4 kg/cm2可將13X脫附出高純度CO2(99.9%); CNT(APTS) 脫附蒸汽壓力提高至2 kg/cm2可將脫附出高濃度CO2 96.7%。雖水蒸汽脫附法比VSA/TSA有較高的CO2脫附濃度,但水蒸汽對13X與CNT(APTS)的結構或表面官能基影響甚大,其造成13X結構改變,以及將CNT(APTS)上含胺官能基釋出,使得無法再生此兩種吸附劑。故本研究最後以雙塔選擇VSA/TSA法進行多次循環吸附。在乾燥氣流條件下(25°C、15% CO2) 13X進行100次吸脫附,濃縮的CO2濃度最高值為96.9%、最低值為87.1%,平均值為92.7%,AI則維持穩定於93.0~87.21%之間。在含水氣之條件下(50°C、含水量2%、15% CO2)以CNT(APTS)進行10次吸脫附,CO2濃縮最高達85.5%、最低值為64.1%,平均值為74.4%。十次吸脫附後AI及CO2濃度皆降低,原因為脫附時間不足,無法完全脫附CO2,使再生後吸附量降低,並降低了CO2濃縮率。綜合上述結果,以13X利用VSA/TSA吸附法可以有效濃縮CO2並且可重複吸脫附及再生,可應用在實廠上CO2捕獲及濃縮。由成本評估分析發現,當循環吸附次數達1900次時,CO2累積吸附量達到119567.79 mg/g,CO2吸附材料成本可降至歐盟碳排放交易價格$50/ton-CO2以下,在產業界應用上具潛力。

The aim of the research was to screen the low-temperature solid CO2 sorbents such as 3-aminopropyl-triethoxysilane modified CNT (CNT(APTS)), GAC, NaY, N-[3-(trimethoxysilyl)propyl]ethylenediamine modified Y60 (Y60(EDA)), 13X for better performances on CO2 capacity, fast sorption rate and high CO2 concentration level. The first part of this research was screening test of low temperarue CO2 sorbents. The results show that the 13X has greater adsrotpion working capacity (qw) than other sorbents. The qw of 13X under 15% of CO2 concentration at 50oC were 55.6 mg/g. The greater qw of 13X under 50% of CO2 concentration at 25oC were 136.2 mg/g. Under the same condition with 10 vol% of water content, CNT(APTS) has greater qw than other sorbents. The qw of CNT(APTS) under 15 and 50% of CO2 concentration with 10 vol% of water content at 50oC were 68.4 and 89.5 mg/g, respectively. The qw of CNT(APTS) and Y60(EDA) could be enhanced by the present of water vapor, which is due to the amino groups has reaction with CO2 and H2O. The adsorption heats (∆Ha) of GAC, NaY and 13X were less than 20 kJ/mol can be assigned to physisorption. The ∆Ha of CNT(APTS) and Y60(EDA) were between 20-50 kJ/mol, which is thus between a physical interaction and a weak chemical interaction. The regeneration heats (Qregen) were in the order: Y60(EDA) >GAC>NaY>CNT(APTS)>13X. The Qregen for these 5 sorbents were less than that for the wet solvent MEA. It reveals that the regeneration heat panelty of dry sorbents and be lower than wet solvent. A numerical model that considers mass transfer of CO2 in the gas phase and simultaneous adsorption and desorption of CO2 in the solid phase was developed to predict system performance of a fixed-bed CO2 adsorber. The adsorption rate (ka) and desorption rate (kd) were fitted with adsorption breakthrough curves of sorbents, which are increased with rising of temperature. Under the condition as 15% of CO2, the ka and kd are following the order as CNT(APTS)> Y60(EDA)>GAC>NaY>13X. The screening of sorbents are based on the 6 conceptual points as (1)qw (2)adsorption index (AI) (3)moisture effect (4)regeneration heat (5)adsorption/desorption rates (6)selectivity of CO2. 13X and CNT(APTS) were choosen as potential sorbents into second part of this research because of their abilities as higher qw, higher AI and relatively lower N2 competetive adsorption.
The second part of this research was desorption and concentration of CO2 by using VSA/TSA and steam processes. The results show that the 93.3% of CO2 was found from 13X under the VSA/TSA conditions as 140°C and 0.01 bar. During 10 cyclic adsortptions in dry condition, the qw of 13X was stable which has decreased a little from 83 to 76.9 mg/g (AI=91.6%). The 82.7% of CO2 was found under the VSA/TSA conditions for CNT(APTS) are 140°C and 0.1 bar. During 10 times cyclic adsortption, the qw of CNT(APTS) shows stable adsorption/desorption in humidity condition has decreased form 97.3 to 88.3 mg/g (AI=90.7%). In steam regeneration process, the optimal conditions for 13X and CNT(APTS) were found which were 4 kg/cm2 and 2 kg/cm2 of steam pressure, respectively. The concentrations of CO2 from 13X and CNT(APTS) were obtained 99.9% and 96.7% with their steam optimal conditions. However, the steam regeneration has caused decomposition of 13X and CNT(APTS) as well. It can be attributed the low hydrothermal stability of zeolite and APTS, the water molecules at high temperature can act with Si-O-Si and resultant structure changes. This show that the steam regeneration is not capable with 13X and CNT(APTS) in spit of they have very high CO2 concentration level. Using dual beds to operate 100 cycles of adsorption/desorption with VSA/TSA process, 13X shows the good performance and obtained 96.9% of CO2 in average and stablely AI (93%~87.21%). This result suggests that VSA/TSA process with 13X is possibly a promising CO2 capture technology. The results of VSA/TSA of 13X via 100 times cyclic adsorption were cost analyzied. It shows that when adsorption cycles was greater than 1900 times, the accumulated CO2 adsorbed was 119567.79 mg/g and cost of adsorption was reduced to below $50/ton-CO2(European carbon tax). This reveals that the 13X has feasibility on CO2 capture in practical industry.
其他識別: U0005-2607201106595800
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