Simultaneous Separation of H 2 S and CO 2 from Biogas by Gas–Liquid Membrane Contactor Using Single and Mixed Absorbents

The present work studied the simultaneous separation of H 2 S and CO 2 from biogas by gas–liquid membrane contactor (GLMC) using single and mixed absorbents. The synthetic biogas contained 300 to 900 ppm of H 2 S, 30% to 50% CO 2 and CH 4 . To better understand the effects of different absorbents on simultaneous separation of H 2 S and CO 2 from biogas, water, monoethanolamine (MEA, primary amine), potassium carbonate (K 2 CO 3, inorganic salt), potassium hydroxide (KOH, inorganic salt), and potassium sarcosine (PS, organic salt) were applied as absorbent solutions. Poly(vinylidene fluoride) (PVDF) hollow fiber membrane was used in the membrane contactor modules. The simultaneous absorption performance of CO 2 and H 2 S into single and mixed absorbents was investigated. In addition, the effects of liquid and gas velocities, absorbent concentration, acid gas content of the feed gas, and gas pressure on the absorption performance and the analysis of mass transfer coefficients were investigated. The results indicated that the highest H 2 S absorption flux was obtained when KOH and K 2 CO 3 were used as single absorbents, and the highest CO 2 flux was obtained using PS as the single absorbent. The use of promoted K 2 CO 3 with PS solutions could simultaneously improve the absorption flux of H 2 S and CO 2 . Increasing the liquid flow rate and absorbent concentration led to an increase in the CO 2 absorption flux, while increasing the gas flow rate led to a significant increase in H 2 S absorption; The change of liquid flow rate has little effect on H 2 S absorption flux. A long-term stability test revealed that partial wetting of membrane could reduce the CO 2 absorption flux but has little effect on H 2 S absorption flux. The detailed analysis of the mass transfer coefficients showed that liquid side resistance was negligible in comparison with membrane and gas side resistances for H 2 S absorption. On the contrary, the mass transfer process of CO 2 was controlled by liquid mass transfer resistance.