Molecular modelling of the organic solvent nanofiltration process: a better understanding for a better use – MANIAC

Molecular simulations

Atomistic models of polymeric membranes (PIM-1, polyamide, polyimide) have been developed according to the following protocol. The polymer repetition units are randomly inserted into a simulation box. The system is then equilibrated in the NPzT ensemble at 300 K and 1 bar until a constant density is obtained. Polymerisation is then carried out using the Polymatic code. Chains smaller than 10-15 monomers are removed after polymerisation (washing of the polymer).
A compression-relaxation protocol is used in order to bring the density of the polymer back to a value close to the experimental value. This protocol consists of 21 steps and includes NVT and NPT simulations at high temperatures, as well as gradual compression up to a maximum pressure Pmax and gradual decompression down to 1 bar. We performed different simulations considering 5 different values of Pmax (10, 20, 30, 40, and 50 kbar). Our results showed that the final density of the polymer is constant from Pmax = 30 kbar. The structural analysis (density, accessible specific surface area and pore size distribution) of the membranes shows a good agreement with the few experimental results reported in the literature.
Solvent models relevant to OSN applications (methanol, ethanol, acetone, dichloromethane, toluene) have been developed and implemented in the LAMMPS code. In order to put the solvents in contact with the polymeric membrane, two solvent reservoirs are connected on both sides of the membrane in the simulation box. A heat-compression protocol is then applied in order to make the solvent molecules penetrate the polymer. The filling rate sequence of the different solvents is in accordance with the experiment, even if the values obtained by the simulation tend to be slightly lower than the experimental values.

A specific study has been launched on the PIM-1 membrane and methanol-toluene mixtures of different compositions. In this study, mixtures of methanol-toluene molecules with proportions ranging from 0 to 100% methanol and quantities similar to the filling rates reported in the literature were randomly introduced. This study will enable us to study the behaviour of solvent mixtures under extreme confinement, in particular the impact of the latter on the structure of the solvents, their translational and rotational dynamics, solvent-membrane interactions, etc.
Following this work, we will approach the study of the transport of different solvents through polymer membranes by means of non-equilibrium molecular dynamics simulations. For this purpose, rigid walls will be introduced into the simulation box in order to act as pistons and thus simulate the transport under pressure of the different solvents.

The first publication linked to the MANIAC project will be submitted in November 2020.

The fast depletion of energy resources together with environmental constraints force the various industries to strive for more sustainable separation and purification processes. In this respect, the recent organic solvent nanofiltration process has received increasing attention as an energy-efficient process offering an alternative to solvent-intensive purification techniques, and suited for the treatment of large volumes as well as thermo-sensitive products. However, its industrial development remains limited so far because of the lack of understanding of the molecular mechanisms involved in non-aqueous separations. MANIAC is a fundamental research program that aims at removing this bottleneck by rationalizing the molecular mechanisms governing membrane separations in non-aqueous media thanks to molecular simulations carried out with both polymer membranes and polymer/MOF mixed matrix membranes.