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

Selective hydrogenation of alkynes in C-2 and C-3 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 C-2 stream, it is demonstrated that the formation of ethane is suppressed with decreasing ratio of H-2/C2H2 and temperature, but with increasing pressure. Besides, high temperature and low pressure as well as low H-2/C2H2 ratio are thermodynamically favorable for the formation of the green oil precursor, i.e., C-4 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 C-3 stream. In addition, the formation of propane is facilitated at high H-2/MPAD ratio, but that of hexadiene is suppressed. With more focus on the long-chain components, i.e., C-18 components, it is found that the green oil formed in the hydrogenations in C-2 and C-3 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.

Automated summary

The selective hydrogenation of alkynes in C-2 and C-3 streams from naphtha steam cracking is crucial for producing polymer-grade ethylene and propylene. A thermodynamic analysis using Gibbs free energy minimization method revealed optimal conditions for alkane and green oil formation under different ratios of hydrogen to alkyne, temperatures, and pressures.