Pore Structure Alteration of a Carbon Molecular Sieve for the Separation of Hydrogen Sulfide from Methane by Adsorption

• Main Authors: Z.J. Pan, S.G. Chen, J. Tang, R.T. Yang
• Format: Article
• Language: English
• Published: Hindawi - SAGE Publishing 1993-03-01
• Series: Adsorption Science & Technology
• Online Access: https://doi.org/10.1177/0263617499010001-418

The equilibrium adsorption of H 2 S is substantially stronger than that of CH 4 on carbons, including carbon molecular sieve (CMS). A carbon molecular sieve with a proper pore structure can provide a kinetic selectivity for H 2 S over CH 4 , thus further enhancing the overall selectivity (equilibrium plus kinetic) for H 2 S and providing the basis of natural gas desulfurization by adsorption. Kinetic selectivity requires a unique pore structure due to the small difference in the molecular dimensions of H 2 S and CH 4 (~0.2 Å). Equilibrium and diffusion rate data for CH 4 and H 2 S at 25°C have been measured in three commercial carbon molecular sieves: Bergbau Forschung CMS, Takeda CMS 3A and Takeda CMS 5A. The pores are either too small (in the two former carbons) or too large (in CMS 5A) for H 2 S/CH 4 separation. Alterations to the pore structure either by controlled oxidation or carbon deposition by pyrolysis have been studied. Optimal results were obtained by pyrolysis of propylene on CMS 5A under the following conditions: 0.05 atm, 700°C, 5 min, weight gain of 0.67%. The resulting carbon molecular sieve retained the high equilibrium adsorption capacities while yielding a diffusion time constant ratio for H 2 S/CH 4 of 8.2. This carbon is suitable for natural gas desulfurization by adsorption processes such as pressure swing adsorption. Temperature was the most important variable in pore structure alteration by carbon deposition. Under the optimal pyrolysis conditions, carbon was only deposited near the pore entrances.