以醇胺水溶液搭配旋轉填充床捕獲二氧化碳

CO2 Capture by Alkanolamine Aqueous Solutions in a Rotating Packed Bed

鄭旭翔(Hsu-Hsiang Cheng)

指導教授 ： 談駿嵩

國立清華大學/工學院/化學工程學系/博士(2012年)

https://doi.org/10.6843/NTHU.2012.00187

摘要

本研究是以化學吸收法利用醇胺水溶液捕獲CO2，並以超重力旋轉床取代傳統吸收塔及氣提塔以縮減傳統化學吸收製程所需之龐大設備體積並減少再生能耗。內容分成六章，包含緒論、以醇胺水溶液結合超重力法捕獲CO2、超重力操作對吸收液溶氧量之探討、超重力旋轉床中化學吸收製程之模擬、以超重力旋轉床再生化學吸收劑及總結。各章節內容如下：
第一章簡介化學吸收法及超重力技術，說明為何採用此兩種技術進行CO2捕獲。
第二章是利用超重力旋轉床搭配醇胺水溶液，自含有1000 ppm、10%及30% CO2等不同CO2濃度之氣體中吸收CO2。研究結果顯示，在處理不同CO2濃度之氣體時，增加吸收液流量及減少氣體流量對於提升CO2捕獲率及降低質傳單元高度是有利的，這是因為相對於氣體而言，有更多的吸收液存在於系統中；提高操作溫度可提高吸收劑之反應速率常數，且CO2吸收過程並未達到熱力學平衡，因此對於提升CO2捕獲率及降低質傳單元高度也是有利的；最適轉速之存在則是因為必須在氣液接觸面積與氣液接觸時間取得一妥協。在吸收液配方部分，反應速率常數愈高之吸收劑之CO2捕獲率也愈佳，這是因為吸收液於旋轉床中之停滯時間極短，因此必須採用反應速率常數較高之吸收劑較為有利。此外相較於傳統吸收塔之質傳單元高度30-100 cm，超重力旋轉床之質傳單元高度2 cm小許多，說明超重力旋轉床之設備體積可較傳統吸收塔為小之原因。
第三章利用實驗設計法探討不同操作變數對於吸收液溶氧量之影響，以推斷旋轉床製程對醇胺水溶液氧氣劣化之影響。研究成果顯示氣體流量是影響溶氧最大的操作變數，其次為氧氣去除劑Na2SO3含量、溫度及旋轉床轉速，且各因子間之交互作用並不明顯。藉由吸收液循環操作實驗可知，吸收液內之溶氧在實驗初期即快速地溶入吸收液中並導致溶氧快速提升，但有添加Na2SO3之吸收液的溶氧量提升較慢，但隨著Na2SO3逐漸耗盡，有添加Na2SO3之吸收液的溶氧值逐漸與未添加Na2SO3之吸收液的溶氧值相同。由於吸收劑氧氣劣化的速率與溶氧量成正比，因此在吸收液循環操作下，添加Na2SO3以避免吸收劑產生劣化是有必要的。
第四章則是建立以超重力旋轉床搭配醇胺水溶液吸收CO2製程之模型，以作為未來旋轉床放大設計之依據。研究成果顯示，無論攪拌槽串聯模型或微分方程模型，從低濃度CO2(1000 ppm)到高濃度CO2 (30%)、低CO2捕獲率(40%)至高CO2捕獲率(99%)、低溫(25 oC)至高溫(60 oC)、低氣體流量(6 L/min)至高氣體流量(70 L/min)及低吸收液流量(50 mL/min)至高吸收液流量(300 mL/min)，此兩種模型皆可正確預估CO2出口濃度，顯示此模型可靠且適用範圍極廣。
第五章則是進行MEA吸收液及DETA+PZ混合吸收液之再生，並分析旋轉床轉速、吸收液流量、再沸器溫度及壓力等操作因子對再生率及再生能耗之影響。由結果可知，相較於轉速及吸收液流量，再沸器溫度及壓力是影響再生率及再生能耗之主要操作變數。此外研究成果也顯示在相同再生率下，旋轉床之質傳單元高度較傳統填充床為短且其所需再生能耗也較傳統填充床為低，設備體積不到傳統氣提塔的十分之一，顯示以超重力旋轉床取代傳統氣提塔填充床之可行性。比較20wt% DETA+10wt% PZ混合吸收液與30wt% MEA吸收液之再生可知，可有效捕獲CO2之吸收液20wt% DETA+10wt% PZ也可於旋轉床操作中被有效再生，而且與30wt% MEA相較，其CO2捕獲效率高、再生率高且再生能耗較低，顯示選擇適當吸收劑配方之重要性。
第六章總結前五章之內容。

關鍵字

二氧化碳捕獲 ； 醇胺 ； 旋轉床 ； 化學吸收法 ； 溫室氣體

並列摘要

Capture of CO2 by aqueous alkanolamine solution was carried out in a rotating packed bed. The conventional packed bed absorber and stripper were replaced by a rotating packed bed in this study for reducing the volumes of conventional absorber and stripper and the regeneration energy of chemical absorbent in the CO2 capture process. There are six chapters in this dissertation, including introduction, capture of CO2 by alkanolamine solution in a rotating packed bed, the effect of operating variables on the amount of dissolved oxygen in the alkanolamine solution in the rotating packed bed operation, the simulation of CO2 capture process in a rotating packed bed, thermal regeneration of alkanolamine solutions in a rotating packed bed, and summary.
In chapter 1, chemical absorption and higee technique were described, and the reason why choosing these techniques for CO2 capture was illustrated.
In chapter 2, capture of CO2 from flue gases with 1000 ppm, 10%, or 30% CO2, respectively, by alkanolamine solution in a rotating packed bed system was studied. The results showed that increasing the liquid flow rate and decreasing the gas flow rate were favorable for enhancing the CO2 capture efficiency and reducing the height of transfer unit because the molar ratio of liquid to gas was increased. Because the reaction rate constant was increased with the increase of temperature, and the absorbent did not achieve the maximum CO2 loading in the CO2 absorption, the increase of temperature was also favorable for enhancing the CO2 capture efficiency and reducing the height of transfer unit. Besides, a proper rotational speed existed because of the trade off between gas-liquid contact area and contact time. The results also showed the absorbent with higher CO2 reaction rate performed a better CO2 capture efficiency because of the short residence time of gas and liquid in the rotating packed bed. Compared with the conventional packed bed, the height of transfer unit of rotating packed bed is much smaller than the former, explained why the volume of rotating packed bed could be much smaller than that of conventional packed bed.
In chapter 3, the effect of operating variables on the amount of dissolved oxygen in the alkanolamine solution was examined by experimental design method for inferring the effect of rotating packed bed on oxygen degradation of absorbent. The results showed that the gas flow rate was the most dominating factor affecting dissolved oxygen, followed by the content of oxygen scavenger Na2SO3 in the solution, temperature, and rotational speed. Besides, the interaction between factors was not obvious. In a circulation operation, dissolved oxygen was increased in the beginning of experiment through physical absorption. However, dissolved oxygen in the solution with the presence of Na2SO3 was smaller than that in the solution with the absence of Na2SO3 in the starting period, due to the reaction between dissolved oxygen and Na2SO3. After a certain period of time, Na2SO3 was mostly consumed by O2, and Na2SO3 in the solution was eventually exhausted. Because the degradation rate of alkanolamine is proportional to dissolved oxygen, the addition of the scavenger Na2SO3 definitely beneficial for CO2 removal. With this concern, addition of more Na2SO3 is needed if the absorbent solution is to be re-circulated for subsequent use.
In chapter 4, two models were built up for simulating the CO2 capture process in a rotating packed bed for the scale up of rotating packed bed in the future. The ranges of operating variables used in the simulation included CO2 concentrations: 1000 ppm to 30%, CO2 capture efficiencies: 40 to 99%, temperatures: 25 to 60 oC, gas flow rates: 6 to 70 L/min, liquid flow rates: 50 to 300 mL/min, respectively. The results showed that both the stirred tanks connected in series model and differential equation model can simulate the outlet CO2 concentration in the rotating packed bed operation accurately, showing that these two models are reliable, and a wide of applicable ranges of operating variables of these models.
In chapter 5, CO2-loaded 30wt% monoethanolamine aqueous solution and CO2-loaded 20wt% diethylenetriamine +10wt% piperazine blended aqueous solution were thermally regenerated in a rotating packed bed. The effects of rotational speed, liquid flow rate, reboiler temperature, and pressure on regeneration efficiency and regeneration energy were investigated. The results showed that the reboiler temperature and pressure were the dominant operating factors affecting regeneration efficiency and regeneration energy, and the volume of the rotating packed bed apparatus could be reduced to no more than one-tenth that of a conventional packed bed. The CO2-loaded 20wt% diethylenetriamine +10wt% piperazine blended aqueous solution demonstrated a more effective CO2 capturing ability, higher regeneration efficiency and the lower regeneration energy consumed as compared with the performance of a 30wt% MEA aqueous solution, suggesting that the proper choice of absorbent for CO2 capture was of paramount importance.
In chapter 6, a summary based on previous chapters was provided.

並列關鍵字

CO2 capture ； Alkanolamine ； Rotating Packed Bed ； Chemical Absorption ； Greenhouse Gas

參考文獻

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