選定非還原性二氧化碳高值化反應途徑之熱力學分析

Abstract

將二氧化碳(CO2)轉化為其他高附加價值的化學品被視為是一種潛在的減少碳排放策略。然而，由於二氧化碳是一種具有低生成自由能的穩定化合物，其轉化在熱力學上於常溫常壓條件下是不利的，或者需要與高生成自由能的反應性化學品(如氫氣)進行反應。若該過程需要極端的反應條件、涉及高能耗的分離程序，或使用具高碳足跡的反應物，那麼實際的減碳效益可能大幅降低。因此，釐清不同二氧化碳轉化途徑的減碳潛能是相當重要的。於本研究中，我們使用固碳量作為評估各反應減碳能力的指標。理論固碳量的計算方式是將反應消耗的二氧化碳量減去反應進行所需的能量消耗造成的碳排放量，這個碳排放量與反應自由能呈正比關係。因此，若能得到不同二氧化碳轉化途徑的反應自由能，不僅能用來比較反應的自發性，也能藉由固碳量來評估各個反應的減碳能力。

在本研究我們探討非還原性中三種反應途徑，分別是將二氧化碳轉化為碳酸鹽類(Carbonate)、氨基甲酸酯類(Carbamate)以及尿素類(Urea)。我們使用Aspen Plus中的RGIBBS反應器，得到不同溫度、壓力與進料比下的平衡轉化率，進而得出熱力學上推薦的操作條件範圍，也藉此得到不同反應的固碳量上下限。此外，我們也使用分離器(Separator)來模擬物理除水對於平衡轉化率的提升效果。當有參數缺少時，我們使用G3(Gaussian-3)方法來計算理想氣體下的比熱、生成熱與生成自由能，並使用COSMO-SAC模型和Peng-Robinson 狀態方程式來進行相態修正。從我們的研究結果來看，轉化為尿素類的反應自發性最高，其次是氨基甲酸酯類，轉化為碳酸鹽類的自發性則最差，而在固碳量上，我們發現同樣是尿素類最高，氨基甲酸酯類次之，轉化為碳酸鹽類的固碳量則最低。因此，從熱力學的分析角度而言，尿素類的合成相較其他兩種反應途徑更容易達成較高的二氧化碳轉化率，而其反應固碳量與減碳潛能也較高。

Transformation of CO2 to other value-added chemicals is considered a potential measure of carbon reduction. However, as a stable (low formation free energy) compound, the conversion of CO2 to other chemicals is either thermodynamically unfavorable (under ambient conditions) or requires reactions with other reactive chemicals (high formation free energy, such as H2). The actual amount of carbon reduction may be much less if the process requires extreme reaction conditions, involves energy-intensive separation, or requires a reactant with a high carbon footprint. It is thus desirable to understand the potential of carbon reduction for various CO2 conversion pathways. In this study, we used theoretical carbon fixation as a measure to assess the carbon reduction potential of various reactions. The calculation of carbon fixation is executed by deducting the carbon emissions produced by the energy required to drive the reaction from the amount of CO2 consumed in the reaction. This carbon emission is proportional to the reaction free energy. Therefore, obtaining the reaction free energy of different CO2 conversion pathways allows for a comparison of the spontaneity of reactions and enables the assessment of carbon reduction potential through carbon fixation.

In this study, we explore three non-reductive pathways: the conversion of CO2 into carbonates, carbamates, and ureas. We used the RGIBBS reactor in Aspen Plus to obtain equilibrium conversion under varying temperatures, pressures, and feed ratios, which in turn provides a thermodynamically recommended range of operating conditions. This also enables us to determine the upper and lower limits of carbon fixation for different reactions. Moreover, we used a separator to simulate the enhancement effect of physical water removal on equilibrium conversion. In cases where certain parameters were missing, we used the Gaussian-3 (G3) method to calculate the heat capacity, heat of formation, and free energy of formation under ideal gas conditions. Phase corrections were performed using the COSMO-SAC model and Peng-Robinson equation of state. Our results suggest that among the pathways investigated, the reaction spontaneity is highest for the conversion into ureas, followed by carbamates, with the least spontaneity observed for the conversion into carbonates. In terms of carbon fixation, ureas also exhibited the highest, followed by carbamates, with the lowest observed for the conversion into carbonates. Thus, from a thermodynamic analysis perspective, the synthesis of ureas is more likely to achieve higher CO2 conversion rates compared to the other two pathways, and its carbon fixation and carbon reduction potential are also higher.