中高壓雙反應器系統二氧化碳加氢生產甲醇

Abstract

自工業革命以來，污染呈指數級增長。二氧化碳是生態系統中最豐富的污染物之一，這是一種導致全球氣溫和海平面上升的溫室氣體。科學界投入了時間和資源來減少溫室氣體排放。隨著不可再生燃料資源的枯竭，回收溫室氣體作為再生燃料的前景日益迫切。二氧化碳可以使用氫氣合成再生燃料、產生甲醇，作為 更容易儲存的氫基燃料。在商業上，該過程是在高壓（50-100 bar）和低溫（200-250 oC）下使用銅基催化劑例如 Cu/ZnO/Al2O3進行的。然而，由於氧化鋁載體的親水性，會很容易發生催化劑失活並降低甲醇產率。 提高甲醇產量的一種方法是透過雙反應器系統除水。在雙反應器系統中，其中一個反應器用於透過反向水煤氣變換生產CO，然後流至乾燥器以除去水，第二個反應器用於減少水。為了提高還原能力，會在 Cu/ZnO/ZrO2 催化劑中加入額外促進劑（例如添加 La、Ce、Mo 和 W）。 加氫反應表明，添加六方氮化硼（hBN）的Cu/ZnO/ZrO2/CeO2催化劑顯示出最佳的逆水煤氣變換和CO2加氫性能。金屬和 hBN 的重量比例為 4:6，因此這裡將其稱為 40CZZC_hBN。使用 40CZZC_hBN 的逆水煤氣轉換的CO 選擇性為 99.64%，CO2 轉化率為 37.95%，CO產率 1026.01 mg/gcat h。 40CZZC_hBN的甲醇選擇性為36.42%，CO2轉化率為12.41%，甲醇產率為140.04 mg/gcat h。此外，40CZZC_hBN在CO加氫製甲醇的CO轉化率3.62%和甲醇產率為97.63 mg/gcat h方面比商業催化劑2.50%轉化率和甲醇產率91.57gcat h更好。 最後，將用於CO2 加氫製甲醇的雙反應器系統與一個反應器系統的性能進行比較，結果顯示甲醇產率幾乎相似，對於在第二個反應器中裝載0.8 g 40CZZC_hBN 的雙反應器系統，甲醇產率為127.69 mg/gcat h，與單一反應器系統的140.04 mg/gcat h 相似；兩者皆在10 bar 下運作。用於兩個反應器之間除水的乾燥劑對甲醇產率有顯著影響，使用 CaCl2 時甲醇產率為86.26 mg/gcath ，比使用分子篩 3A 降低了甲醇產率。增加雙反應器系統的壓力將顯著提高甲醇產率，因為將壓力增加至 30 bar 將使甲醇產率增加一倍以上，甲醇產率為266.11 mg/gcath。透過串聯兩次CO2加氫至甲醇反應，可以獲得最高的甲醇產率為337 mg/gcat h，30 bar。

Pollution has exponentially increased since the industrial revolution. One of the most abundant pollutants within our ecosystem is CO2, a greenhouse gas that caused rising global temperature and sea level. The scientific field has poured time and resource to reduce greenhouse gas. Along with the depleting source of non-renewable fuel, the prospect of recycling greenhouse gas as a renewable fuel has been on demand. CO2 pollution can be synthesized as a renewable fuel using H2 which produces methanol as an easier to store hydrogen-based fuel. Commercially, this process is conducted in at high pressure (50-100 bar) and low temperature (200-250 oC) with a copper-based catalyst, such as Cu/ZnO/Al2O3. However, due to the hydrophilic nature of alumina support, catalyst is easily deactivated and thus reduce methanol yield. One method to increase the production of methanol is water removal via the two-reactor system, where one reactor is used to produce CO by Reverse Water Gas Shift, RWGS, which is then flowed to a desiccator to remove water and the second reactor to reduce mixture of CO and CO2 to methanol by hydrogenation. To improve the reduction capability, an additional promoter to the Cu/ZnO/ZrO2 catalyst (such as addition of La, Ce, Mo, and W) was tested. Catalytic hydrogenation showed that Cu/ZnO/ZrO2/CeO2 (CZZC) catalyst being loaded with an addition of hexagonal Boron Nitride (hBN) showed the best performance for RWGS and CO2 hydrogenation. The ratio of the metal and hBN is 4:6, as such it is referred as 40CZZC_hBN. RWGS using 40CZZC_hBN had CO selectivity of 99.64%, CO2 conversion 37.95%, and CO STY 1026.01 mg/gcat h. 40CZZC_hBN performance of MeOH selectivity of 36.42%, CO2 conversion 12.41%, and methanol STY 140.04 mg/gcat h. In addition, the performance of 40CZZC_hBN in CO conversion of 3.62% and methanol STY of 97.63 mg/gcat h for CO hydrogenation to methanol had better performance than the commercial catalyst with a result of 2.50% conversion and 91.57 mg/gcat h methanol STY. Lastly, the two reactor system was for CO2 hydrogenation to methanol was compared to the performance of one reactor system and the result showed an almost similar methanol STY of 127.69 mg/gcat h for the two reactor system with 0.8 g of 40CZZC_hBN loaded in the second reactor compared to 140.04 mg/gcat h for the one reactor system; both operating at 10 bar. The desiccant that is used for water removal in between the two reactor had a significant effect on the methanol yield as using CaCl2 lowers the methanol STY of 86.26 mg/gcath than using molecular sieve 3A. Increasing the pressure of the two reactor system significantly improved the methanol space time yield as increasing the pressure to 30 bar more than double the methanol STY to 266.11 mg/gcath. By utilizing two CO2 hydrogenation to methanol reaction in series, the highest methanol STY can achieve 337 mg/gcat h at 30 bar.