| Ethylene glycol is an important organic compound. The hydrogenation of dialkyl oxalate from syngas to ethylene glycol is considered as a green chemical process with potential benefits in both environmental and economic consideration. The key step is the synthesis of dialkyl oxalate via CO gaseous phase coupling reaction, which involves two reactions: CO coupling reaction to produce dialkyl oxalate and the regeneration of alkyl nitrite. Either methanol or ethanol can be used in this process to produce dimethyl oxalate and diethyl oxalate respectively but the comparison of dimethyl oxalate and diethyl oxalate synthesis on same catalyst was not reported.In this paper, the synthesis of dimethyl oxalate and diethyl oxalate by CO coupling reaction in gaseous phase was investigated in a fixed bed reactor over a Pd/Fe-Al2O3catalyst. The results showed that the space time yield of dimethyl oxalate was higher than that of diethyl oxalate under same reaction conditions. The different performance of dimethyl oxalate and diethyl oxalate formation was attributed to the different decomposition performance of methyl nitrite and ethyl nitrite. Moreover, Density Functional Theory (DFT) was used to simulate the decomposition process of methyl nitrite and ethyl nitrite and the results indicated that ethyl nitrite can be decomposed more easily than methyl nitrite. Regeneration reaction of methyl nitrite and ethyl nitrite was further compared. The results showed that the yield of methyl nitrite was higher than that of ethyl nitrite under same reaction conditions. So the production of ethylene glycol through dimethyl oxalate was more competitive than the route through diethyl oxalate. The regeneration of methyl nitrite was studied in detail.A model-reactor system suitable for kinetics study of gas-liquid reaction was set up and the kinetics of methyl nitrite synthesis from NO, NO2and methanol was obtained in the consideration of NO2as the main by-product accompany with methyl nitrite regeneration. The methyl nitrite regeneration reaction was considered as a two-step irreversible series reaction: the first reaction was NO oxidation to form NO2; the second reaction was the reaction of NO, NO2with methanol to form methyl nitrite, whose kinetics was obtained at atmospheric pressure by using the model reactor in this work. Based on the kinetics equation and simplified plug flow model, a mathematical model depicting methyl nitrite regeneration reaction in a packed bubble column reactor was proposed to predict methyl nitrite yield at varied conditions. As a result, the calculated results by the model coincided well with the experimental data, which proved that the model was effective for guiding the scaling-up of methyl nitrite regeneration process. Using this model, the effect of reaction temperature, N2volume fraction, NO/O2molar ratio and superficial gas velocity on methyl nitrite yield was predicted and compared with the experimental data. The suitable operation conditions were determined to be as follows: methanol concentration of40~99.9wt.%, gas flow rate of400~500mL/min, NO/O2molar ratio of5:1~6:1and N2content of60~80%volume fraction, reaction temperature of30~40°C,. the ratio of height to diameter of8~10. The gaseous composition and gas flow rate are considered the most sensitive factors affecting on the yield.The effect of reaction pressure and other gases on the methyl nitrite regeneration was further discussed. It was found that the suitable pressure is0.2~0.3MPa. The effect of CO on methyl nitrite is not obvious, while the existence of CO2in the system will decrease the O2conversion and methyl nitrite yield. |
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