Use of Organic Solvent Nanofiltration (OSN) membranes for Counter-Current Chromatography (CCC) solvent recovery
Authors:   , Walters Matthew J. 
Published: April 2022

Solvent resistant membranes are a relatively new technology which has the potential to expand the possible utilities of membranes for process industries. Little is known in terms of generic characterisation and basic physical properties of the membranes, and the aim of this work is to explore and characterise such membranes in a systematic way and apply them in an industrial application. After characterisation of the Organic Solvent Nanofiltration (OSN) membranes are complete; solvent and solute properties, such as solubility, polarity and viscosity, are measured and compared to the rejection and permeate flux data of two charged dyes in alcohols of increasing molecular weight in an attempt to ascertain the dictating factors which govern separation in OSN membranes. Following on from this, Polyethersulfone (PES) membranes were fabricated and crosslinked using different materials and methods, with performance and FTIR data analysed to measure the effectiveness of the crosslinking for rejection of solutes in solvent systems. Finally, a process material from Counter-Current Chromatography (CCC) containing a mixture of solvents and a pesticide (MW = 350 g.mol-1) is screened using OSN and aqueous-system nanofiltration membranes using dead-end filtration, to determine the optimum membrane for solute concentration. Following this, two bovine gelatin (MW = 100,000 g.mol-1) in solvent mixtures are concentrated in a crossflow system for use before and after CCC to determine the viability of OSN as a counter-part to CCC. Initially, characterisation of solvent resistant membranes are completed by testing the performance of very tight (OSN) membranes with volatile and neutral solutes in aprotic solvents. The Duramem 150 and 200 membranes were tested using salt, volatiles and polyethylene glycols in both water and solvents using dead – end filtration cells. The rejection was >90% of solutes with MW 150 and 200 respectively in some solvents, but not in others, which indicate that the solubility of the solute in the solvent and the degree of swelling in the membrane play a role in the rejection performance of the membrane. A new application for the measurement of membrane surface zeta potential of positively charged membranes using the Laser Doppler Effect through the Malvern Zetasizer has been developed, by using positively charged particles in the place of negatively charged particles. This allows for the analysis of both negatively and positively charged membranes using the same system. This methodology was also tested using solvent systems, however, this proved to be challenging and little meaningful data was obtained. Following on from zeta potential measurements, other physical properties of OSN membranes, solute and solvent properties were analysed to determine which properties (such as system viscosity, solubility of solute in solvent, membrane swelling etc) govern permeate flux and rejection two charged dyes of identical MW (Orange II and Safranin O) using Duramem 500 membrane. By comparing the performance data of the membrane against each alcohol solution, the permeate flux shows the best correlation with the viscosity of the solution suggesting that this is the governing properties of the system with respect to permeate flux, while the rejection of the solutes in each solution show good correlation with the solubility of the dye in each alcohol, as well as the polarity of the dye and alcohol, suggesting that the degree of hydrogen bonding occurring in the system is the chief factor with regards to the rejection of the solute in OSN systems. PES membranes have been shown to robust physical properties and have been used to filtration of hazardous chemicals, and as such PES membranes were fabricated to examine the viability of this material for solvent filtration systems. Previous studies using volatiles for modification of polysulfone membranes have shown to modify the physical properties of the membranes, thus this was attempted with polyethersulfone to observe if similar results would occur. The rejection of the solute remained at a relatively constant level while the permeate flux significantly increased with the addition of volatile alcohols to the dope mixture. Following on from this, different materials and methods of crosslinking were used in an attempt to lower the MWCO of the PES membranes. The crosslinking agents used were PEG 200 and Hexane Diamine, these were applied onto the membrane through “post” casting modification (submersion in crosslinking bath), Simultaneous Phase Inversion and Crosslinking (SIM) modification and flux modification. The use of PEG 200 as a crosslinking agent, applied through the flux method appears to give the highest degree of crosslinking. Two pesticides from different cuts of Counter-Current Chromatography (CCC) were subjected to nanofiltration using different membranes, before and after chemical saturation to mimic aging. The GE DK membrane had the highest rejection of the range that was tested, while the HL membrane had the highest permeate flux rate. The solvents before and after filtration were measured to find that there was little change in solvent composition after filtration. In a cross – flow system, bovine gelatin (MW 100,000 g.mol-1) was dissolved in similar solvent compositions to that of the pesticides and concentrated using a 25 kDa MWCO membrane. There is total rejection of the solute, with a change in the solvent composition, namely ethyl acetate and hexane, depending on the composition of the initial feed solvent mixture.