(605d) Study of Factors Impacting Peptide Retention in Nanofiltration

Nanofiltration (NF) is a membrane separation unit operation which has applications across several industries such as water treatment, food and agriculture, fine chemicals, and pharmaceutics. It is used for concentration, purification by removal of low molecular weight species, and solvent exchange by applying high pressure across a nano-porous membrane orthogonal to the bulk flow. For the processing of larger molecules such as proteins, monoclonal antibodies, and peptides where intermediate or final isolation becomes challenging, membrane filtration can be a powerful tool. It also aligns well with other upstream and downstream continuous unit operations to make the overall synthesis telescoped and aids in improving process control and throughput. Membrane material and molecular weight cut off (MWCO) selection is done based on the physical and chemical properties of the solvents and solutes involved in the process.

The performance of an NF process is measured based on cycle time and yield. The parameters that impact these include feed flow and crossflow rate, transmembrane pressure (TMP), temperature, membrane pore size, and viscosity of solvent system. If chemistry modifications are not possible, the only controllable process parameters are flowrate and TMP. Being a high-pressure technology, one challenge in NF is the loss of solute molecules to the permeate even after selection of a tight membrane. This can be addressed by membrane conditioning using the process solution wherein a solute layer forms on the membrane which acts as a secondary sieve. Since this polarization layer adds resistance in the system, it causes reduction of permeate flux and increases cycle time. Operating conditions such as TMP and crossflow rate need to be adjusted to balance the flux reduction and yield loss.

In this work, we utilized ceramic NF membranes with an MWCO of 450 Da to study the effect of operating parameters on the retention of an ~2500 Da peptide molecule. TMP and crossflow rate were modified to span the flow regime spectrum from laminar to turbulent, and low to high pressure. Caustic regeneration was performed between each experiment to reset the membrane to the same baseline clean solvent flux. From experiments, we show that for this ratio of peptide size to membrane pore size, high crossflow velocity and low TMP aid in peptide retention.