Energy efficiency of a hybrid membrane/condensation process for VOC (Volatile Organic Compounds) recovery from air: A generic approach

• A systematic parametric analysis of hybrid membrane-condensation unit is presented.
• Hybrid process energy efficiency depends on the VOC boiling temperature and content.
• Standalone condensation remains more energy efficient for high boiling VOC.
• Hybrid process is more energy efficient for intermediate to low boiling VOC.
• A generic selection map of the most energy efficient process is proposed.

The recovery of VOC (Volatile Organic Compounds) from air is a major issue in terms of minimizing the environmental impact of numerous industrial processes (chemistry, food, pharmaceutical, metallurgy, refrigeration…). Non destructive VOC capture technologies are preferentially used in order to enable the recycling of a large ratio of the emitted compounds. To that respect, condensation is attractive because it offers the possibility to recover the VOC from the air stream under liquid state thanks to a physical, non destructive, separation process. Nevertheless, a very low (cryogenic) condensation temperature is often required in order to achieve that target. In that case, a membrane VOC pre-concentration step can be of major interest in order to increase the VOC content of the condensation unit and possibly improve the energy efficiency of the overall operation. In this study, a systematic analysis of the energy efficiency (overall electrical energy needed per kg of recovered VOC) of a standalone condensation process is compared to a hybrid process based on membrane concentration + condensation. It is shown that the standalone condensation remains more energy efficient for high boiling VOC (e.g. toluene, octane, acetone), while a significant improvement of the energy efficiency is obtained with the hybrid process for intermediate to low boiling temperature VOC (e.g. propane, ethane, ethylene…). A generic map of the most energy efficient VOC recovery process as a function of the VOC boiling temperature is finally proposed and potential improvement of the hybrid approach, based on a retentate recycling strategy is discussed.