Feasibility study of polydimethylsiloxane (PDMS) membranes for siloxane removal from biogas

Membrane separation, a green and efficient gas purification method, has been underexplored for removing volatile methyl siloxanes (VMS) from biogas. This study investigates the feasibility of removing VMS from biogas through polydimethylsiloxane (PDMS) membranes. Permeation tests revealed that PDMS membranes exhibit exceptional permeability to D4 (octamethylcyclotetrasiloxane, a representative VMS compound), with a permeance of 1492.37 GPU, significantly higher than for CH₄ and CO₂. The selectivity values for D4/CH₄ and CO₂/CH₄ were 75.26 and 9.80, respectively, demonstrating the membrane’s dual potential for D4 removal and CH₄ recovery. The permeation mechanism of D4 in PDMS was elucidated using density functional theory (DFT) calculations and molecular dynamics (MD) simulations. The results revealed that D4 permeation in PDMS is driven by favorable solubility compatibility, with dissolution being the rate-limiting step. Furthermore, the 40-hour continuous permeation test demonstrated the long-term stability of the PDMS membrane. Process simulation using PRO II software evaluated the engineering application of PDMS membranes for biogas purification, identifying a trade-off between maximizing D4 removal and minimizing CH₄ loss. To address this challenge, an optimized strategy was proposed through PDMS layer thickening. By increasing the thickness of the PDMS layer from 3.32 μm to 15.27 μm, the selectivity of the PDMS membrane for D4/CH₄ was enhanced. PRO II simulation results demonstrated that the thickened membrane could reduce the D4 concentration in simulated biogas from 50 mg/m³ to 0.79 mg/m³, with a CH₄ loss rate of only 3.39 %, meeting the requirements for engineering applications. This study provides a novel and sustainable approach for removing siloxane from biogas, highlighting its potential for practical applications and future research.