The major challenges of our time require innovative, robust processes to bring about rapid positive change. Membrane-assisted liquid-liquid extraction (MEMEX) is one of these disruptive technologies. Although extensively described since the mid-1980s (1), references for MEMEX are still rare. The reasons for this include membrane stability with the chemicals used, availability of corresponding membrane modules but also low(er) awareness of the technology - and its advantages and disadvantages. With more expertise in this area, the number of references could quickly increase.
MEMEX is not, contrary to what the name suggests, a conventional membrane process where pore size and operating pressure are key parameters. Instead, the (porous) membrane acts as a “phase immobilizer” to bring the two liquid phases into contact without mixing them.
MEMEX has decisive advantages:
- No mixing energy required
- No formation of stable emulsions
- No phase separation after extraction
- Position-independent operation (vertical and horizontal)
- No difference in density between the liquid phases necessary
In addition, an extreme solvent:feed phase ratio of up to 100:1 “built in” by suitable module geometry can be advantageous, enabling very high liquid-liquid extraction yields.
Nevertheless, there are also drawbacks:
- Availability of membranes with precisely known, different wettability
- Membrane resistance, especially to the usual solvents
- Availability of technical modules for direct use
While membranes for MEMEX in the form of tubes, capillaries or even flat-sheet membranes are relatively easy to investigate in an academic environment, further scale-up or piloting is always linked to availability of membrane modules. The FHNW School of Life Sciences in Muttenz (Switzerland) has been researching and developing membrane contactors for 15 years to prove the reliability of this technology up to pilot scale (2)
With a range of membrane contactor modules up to 5 m2, numerous successful tests have been carried out, with processes developed and realized up to production scale. The CUMERI Horizon Europe project (3) focuses on the use of ceramic membranes instead of PTFE to address the “PFAS problem”. In addition to the above-mentioned advantages, these state of the art membranes allow significantly higher operating temperatures, conducive to diffusion of the transferring components through the membrane pores. This project also demonstrated that MEMEX can be strongly predicted using rigorous mass transfer models, and that scale-up by a factor of 300 is possible without any issues.
MEMEX successfully extracted lactic acid directly from fermentation broths in continuous operation, without the need to terminate fermentation after a few days due to over-acidification. This means that simultaneous equimolar gypsum formation during fermentative lactic acid production is no longer necessary. Additionally, short contact times reduce potentially toxic microbiological contact with the solvent to such low levels that no influence on growth could be detected. Finally, the solvent could be replaced by a “green” alternative - a natural deep eutectic system (NADES). During conventional extraction this typically forms a stable emulsion, but MEMEX eliminates that problem (4).
The key to success and to the widespread adoption of MEMEX is to select, develop and pilot potential applications precisely in areas in which the benefits are a disruptive advance in process operation. Thus, the way is now wide open for renewed intense research and development to establish membrane-supported liquid-liquid extraction as a reliable extraction process.