Residual Hydrogen Sulfide in Recovered Liquid Sulfur: A Computational Investigation

Area of Science:

• Computational Chemistry
• Chemical Physics
• Materials Science

Background:

• The modified thermal Claus process is crucial for sulfur recovery.
• Understanding sulfur cluster (Sn) and hydrogen sulfide (H2S) reactions is key to process optimization.
• The behavior of liquid sulfur, including its paramagnetism, requires detailed explanation.

Purpose of the Study:

• To investigate the nature, structure, stability, and reaction of sulfur clusters (Sn) with H2S.
• To elucidate the formation of hydrogen polysulfanes (HSn+1H) in equilibrium.
• To explain the paramagnetic properties of liquid sulfur and residual H2S in the Claus process.

Main Methods:

• Density Functional Theory (DFT) calculations.
• Coupled Cluster (CC) theory computations.
• Analysis of spin state stability and reaction energetics.

Main Results:

• Sulfur clusters (Sn) exhibit a transition from open chain (n ≤ 4) to cyclic (S5, S8) ground state structures.
• A spin state stability switch occurs for open chain structures at S3 and S5.
• Triplet states of S5 and S8 are more stable than singlet states, explaining liquid sulfur's paramagnetism.
• Hydrogen polysulfanes (HSn+1H) form on the singlet surface of the Sn + H2S reaction.
• Unbranched HSn+1H form exergonically; branched forms can be exergonic or endergonic.
• Triplet surface reactions yield endergonic doublet species, potentially explaining residual H2S.

Conclusions:

• The study provides insights into sulfur chemistry relevant to the Claus process.
• Computational results explain the observed paramagnetic behavior of liquid sulfur.
• The findings suggest mechanisms for hydrogen polysulfane formation and residual H2S in sulfur recovery.