Recovering Hydrogen From Ethylene Cracked Gas By Membrane-Cryogenic Combined/Hybrid Processes

The cryogenic process of traditional ethylene process has several issues including the low condensation temperature needs extensive energy consumption, which is against the development requirement of energy conservation. Therefore, it is imperative to design an energy-efficient, facial process to recover hydrogen from ethylene cracked gas. To this end, the hydrogen separation membrane-cryogenic combined/hybrid processes designed by UniSim Design software are proposed to remove hydrogen from ethylene cracked gas. The refrigeration compressor power and the ethylene loss from the top of demethanizer significantly decrease due to the partial recovery of the hydrogen by the membrane system, which meets the requirements of recovering hydrogen from the ethylene cracked gas efficiently and at low cost.To recover hydrogen from ethylene plant of 800 kt·a-1, this thesis analyzes and studies the ethylene cracked gases from ethane and naphtha, which are the main sources in the ethylene cracking feedstocks, and in which the hydrogen concentrations are typical. According to the different combinations of hydrogen separation membrane modules and cryogenic separation process, the membrane-cryogenic hybrid process and the hydrogen separation membrane-cryogenic combined process are proposed. In the hydrogen separation membrane-cryogenic combined process, the hydrogen separation membrane modules and the cryogenic separation unit are simply stacked. In the hydrogen separation membrane-cryogenic hybrid process, the hydrogen-rich gas from chilling process back to the second membrane module. Based on the different ethylene cracking feedstocks, the optimal process and operating conditions of the process are identified. The membrane-cryogenic combined process is appropriate for the cracked gas from naphtha, and the membrane areas of the first and second stages are 40000 and 6667m2, respectively. In this case, the compressor power in the chilling process of this work is 38947kW, which is 8169kW lower than that in the traditional process. The hydrogen recovery rate is 97.51% with the purity of 95.53%, both exceeding those for the cryogenic process. Furthermore, the loss percentage of ethylene is 0.11%, lower than 0.22% in cryogenic process. The annual gross cost is 253.3 million yuan, and the overall recovery effectiveness outperforms that in the initial process. The membrane-cryogenic hybrid process is appropriate for the cracked gas from ethane, and the membrane areas of the first and second stages are 28000 and 10110m2, respectively. In this case, the compressor power in the chilling process of this work is 39496kW, which is 8996kW lower than that in the traditional process, The hydrogen recovery rate is 98.52% with the purity of 99%, both exceeding those for the cryogenic process. Furthermore, the loss percentage of ethylene is 0.46%, lower than 1.29% in cryogenic process. The annual gross cost is 279.1 million yuan, and the overall recovery effectiveness outperforms that in the initial process.Subsequently, this thesis compares the ternary refrigeration with the ethylene-propene cascade refrigeration and researches the effect of tertiary refrigerant composition on the refrigeration compressor power. The evaporation curve of the ternary refrigeration is close to the cooling curve of the cracked gas, which can reduce the mean heat transfer temperature difference and the refrigeration compressor power. For the cracked gas from naphtha, the minimum refrigeration compressor power is 37055kW as the ratio of methane, ethylene and propene in the ternary refrigeration is 7:9:84. For the cracked gas from ethane, the minimum refrigeration compressor power is 37664kW as the ratio of methane, ethylene and propene in the ternary refrigeration is 8:9:83. The refrigeration compressor power of tertiary refrigeration is less than that of ethylene-propene cascade refrigeration, which could provide specific guidance for actual production.