Thermodynamics Insights into the Selective Hydrogenation of Alkynes in C₂ and C₃ Streams

Selective hydrogenation of alkynes in C₂ and C₃ streams from steam cracking of naphtha is significant for the production of polymer-grade ethylene and propylene. Herein, a comprehensive thermodynamics analysis for the selective hydrogenation of alkynes with focus on the formation of alkanes and green oil is carried out using the Gibbs free energy minimization method. For acetylene hydrogenation in C₂ stream, it is demonstrated that the formation of ethane is suppressed with decreasing ratio of H₂/C₂H₂ and temperature, but with increasing pressure. Besides, high temperature and low pressure as well as low H₂/C₂H₂ ratio are thermodynamically favorable for the formation of the green oil precursor, i.e., C₄ components. Similarly, high temperature and low pressure are also thermodynamically favorable for the formation of propane and hexadiene during the hydrogenation of methyl acetylene and propadiene (MAPD) in C₃ stream. In addition, the formation of propane is facilitated at high H₂/MPAD ratio, but that of hexadiene is suppressed. With more focus on the long-chain components, i.e., C₁₈ components, it is found that the green oil formed in the hydrogenations in C₂ and C₃ streams thermodynamically tends to be in the form of long-chain components, which prefer to decompose with increasing temperature and decreasing pressure. This thermodynamics analysis would bring more insights into the design of new hydrogenation catalysts and/or regeneration for the deactivated catalysts.