Conversion of the CO and CO2 mixture to alcohols and hydrocarbons by hydrogenation under the influence of the water-gas shift reaction, a thermodynamic consideration
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摘要:
受水煤气变换反应(或其逆反应)的干预,CO(或CO2)加氢反应制烃类或醇类化合物经常会遭遇较高的CO2(或CO)选择性,而目标产物烃和醇的选择性往往较低,这使得对相关反应过程的评估显得非常混乱。为此,本工作对水煤气变换反应作用下的CO、CO2及其混合物的加氢转化制烃(以乙烯为例)和醇(以甲醇为例)反应进行了详细的热力学研究。结果表明,对于CO(或CO2)加氢反应,水煤气变换(或逆水煤气变换)反应作为连接CO和CO2的连通器,虽然会给CO(或CO2)的平衡转化率带来很大的改变并生成大量的CO2(或CO),但其对目标醇和烃产物的碳基平衡收率影响相对较小。CO加氢反应的烃醇产物的碳基平衡收率比CO2加氢反应的高,而CO和CO2混合物加氢的烃醇产物的总碳基平衡收率位于两者之间。对于CO和CO2混合物加氢,尽管CO或CO2的平衡转化率随原料组成的不同有较大幅度的变化,但烃醇产物的总碳基平衡收率变化较为简单,即随着CO2/(CO + CO2)摩尔比的增大而线形降低。鉴于CO或CO2加氢过程的尾气均为CO和CO2混合物,用CO和CO2混合物加氢制烃醇或许更为有利;无论CO、CO2还是其混合物的加氢过程,都应以目标产物的碳基总收率作为评估指标。
Abstract:Due to the intervention from the water-gas shift (WGS) reaction (or the reverse one (RWGS)), the hydrogenation of CO (or CO2) into alcohols and hydrocarbons often displays rather high selectivity to CO2 (or CO), which makes it rather puzzling to evaluate such conversion processes by using the relatively low selectivity to the target products. Herein, a thermodynamic consideration is made to elaborately evaluate the effect of the WGS/RWGS reaction on the hydrogenation of CO, CO2, and their mixture to typical alcohols (e.g. methanol) and hydrocarbons (e.g. ethene). The results indicate that for the hydrogenation of CO (or CO2), although the WGS (or RWGS) reaction, acting as a communicating vessel connecting CO and CO2, may have a severe influence on the equilibrium conversion of CO (or CO2), forming a large amount of CO2 (or CO), it only has a relatively minor impact on the C-based equilibrium yield of the target alcohol/hydrocarbon product. The hydrogenation of CO shows a higher C-based equilibrium yield for the target product than the hydrogenation of CO2, while the overall C-based equilibrium yield of target product for the hydrogenation of the CO and CO2 mixture just lies in between. For the hydrogenation of the CO and CO2 mixture, although the equilibrium conversion of CO and CO2 may vary greatly with the change in the feed composition, the relation between the overall C-based equilibrium yield of the target product and the feed composition is rather simple; that is, the overall C-based equilibrium yield of alcohol/hydrocarbon product decreases almost lineally with the increase of the CO2/(CO + CO2) molar ratio in the feed. These results strongly suggest that the mixture of CO and CO2 is credible in practice for the production of alcohols and hydrocarbons through hydrogenation, where the overall C-based yield should be used as the major index for the hydrogenation of CO, CO2, and their mixture.
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Key words:
- thermodynamic consideration /
- CO2 hydrogenation /
- CO hydrogenation /
- water-gas shift reaction /
- overall C-based yield /
- methanol /
- ethene /
- CO and CO2 mixture
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Figure 1 (a) Equilibrium yield of ethene and selectivity to CO2 for the hydrogenation of CO to ethene via the reaction of 2CO + 4H2 = C2H4 + 2H2O; (b) equilibrium yield of methanol and selectivity to CO for the hydrogenation of CO2 to methanol via the reaction of CO2 + 3H2 = CH3OH + H2O; and (c) equilibrium yield of ethene and selectivity to CO for the hydrogenation of CO2 to ethene via the reaction of 2CO2 + 6H2 = C2H4 + 4H2O. The first reaction has a H2/CO molar ratio of 2 in the initial reaction mixture of H2 and CO, whereas the later two reactions have a H2/CO2 molar ratio of 3 in the initial reaction mixture of H2 and CO2. The solid lines are for the individual CO or CO2 hydrogenation reactions alone, whereas the dashed lines are for those having the intervention from the WGS/RWGS reaction of CO + H2O = CO2 + H2
Figure 2 Overall C-based equilibrium methanol yield, CO conversion, and CO2 conversion for the hydrogenation of the CO and CO2 mixture to methanol via the reactions of CO + 2H2 = CH3OH and CO2 + 3H2 = CH3OH + H2O, where the water-gas shift (WGS) reaction of CO + H2O = CO2 + H2 as a nonindependent reaction occurs inevitably; depending on the CO2/(CO + CO2) molar ratio (z), the initial reaction mixture has a H2/CO/CO2 molar ratio of (2 + z)/(1−z)/z. For the pure CO2 hydrogenation (z = 1), the equilibrium selectivity to CO was displayed instead of the equilibrium CO conversion
Figure 3 Overall C-based equilibrium methanol yield, CO conversion, and CO2 conversion varied with the feed CO2/(CO + CO2) molar ratio (z) for the hydrogenation of the CO and CO2 mixture into methanol at 225 °C and different pressures (left) and at 5 MPa and different temperatures (right), via the reactions of CO + 2H2 = CH3OH and CO2 + 3H2 = CH3OH + H2O, where the water-gas shift (WGS) reaction of CO + H2O = CO2 + H2 as a nonindependent reaction occurs inevitably; depending on the CO2/(CO + CO2) molar ratio (z = 0–1), the initial reaction mixture has a H2/CO/CO2 molar ratio of (2 + z)/(1−z)/z
Figure 4 Overall C-based equilibrium ethene yield, CO conversion, and CO2 conversion for the hydrogenation of the CO and CO2 mixture to ethene via the reactions of 2CO + 4H2 = C2H4 + 2H2O and 2CO2 + 6H2 = C2H4 + 4H2O, where the water-gas shift (WGS) reaction of CO + H2O = CO2 + H2 as a nonindependent reaction occurs inevitably; depending on the CO2/(CO + CO2) molar ratio (z), the initial reaction mixture has a H2/CO/CO2 molar ratio of (2 + z)/(1−z)/z. For the pure CO hydrogenation (z = 0), the equilibrium selectivity to CO2 was displayed instead of the equilibrium CO2 conversion, whereas for the pure CO2 hydrogenation (z = 1), the equilibrium selectivity to CO was displayed instead of the equilibrium CO conversion
Figure 5 Overall C-based equilibrium ethene yield, CO conversion, and CO2 conversion varied with the feed CO2/(CO + CO2) molar ratio (z) for the hydrogenation of CO and CO2 mixture into ethene at 350 °C and different pressures (left) and at 3 MPa and different temperatures (right), via the reactions of 2CO + 4H2 = C2H4 + 2H2O and 2CO2 + 6H2 = C2H4 + 4H2O, where the water-gas shift (WGS) reaction of CO + H2O = CO2 + H2 as a nonindependent reaction occurs inevitably; depending on the CO2/(CO + CO2) molar ratio (z = 0–1), the initial reaction mixture has a H2/CO/CO2 molar ratio of (2 + z)/(1−z)/z
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