Abstract
We are currently witnessing an increase in biogas production in the context of fossil fuel decline and the future circular economy. However, raw biogas contains undesirable compounds such as hydrogen sulfide, volatile organic compounds (VOCs) and carbon dioxide that should be removed to upgrade biogas to biomethane. Absorption is an effective method to capture undesirable compounds from biogas. In this work, we evaluated and compared the efficiency of hydrophobic deep eutectic solvents (DESs) based on fatty acids and conventional absorbents for the absorption of three major VOCs found as impurities in biogas (toluene, limonene and octamethylcyclotetrasiloxane). The vapor–liquid partition coefficient (K) of the VOCs in the studied solvents were determined using static headspace-gas chromatography. The effect of temperature and water content was assessed. Results show a decrease of K value in all solvents compared to water, hydrophobic DESs being the most effective solvents with an up to 20000-fold lower K value in the case of limonene. Interestingly, lowering temperature allows the conventional solvents to reach absorption efficiency close to DESs. Negative values of ΔG (from −6.95 to −22.89 kJ⋅mol−1) were obtained showing that all VOCs are favorably solubilized in absorbents. Obtained negative ΔH values (from −34.86 to −51.17 kJ⋅mol−1) indicate that the dissolution of VOCs is an exothermic process. For all studied absorbent the |ΔH| of VOC absorption is always higher than |TΔS|, showing an enthalpically driven process. The obtained results indicate that efficient VOC absorption can be achieved by the proposed hydrophobic DESs and conventional solvent also providing easy recycling, and recovering process.
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