Solvent Resistant Nanofiltration Research Articles

Observations on solvent flux and solute rejection across solvent resistant nanofiltration membranes

Organic solvent nanofiltration is an emerging technology made possible by the recent development of solvent resistant nanofiltration (SRNF) membranes. These membranes have many potential applications from continuous operation over many months in refinery systems [1,2] to short term operation for a few hours in batch chemical processes [3]. In this paper, solvent flux decline and membrane separation properties are investigated (including their dependence on pressure), using methanol with quaternary alkyl ammonium bromide salts with molecular weights (MW) in the range 322 to 547 Daltons as solutes. The membranes are characterised in terms of an equivalent uniform pore size using three simple pore flow models: Ferry model, Steric Hindrance Pore (SHP) model and Verniory model.

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The valorization of deodorizer distillates from the oil refining industry requires a stable membrane suitable to separate steryl esters (bioactive compounds with high molecular weight (MW), 650<MW<800g/mol) from pesticides (150<MW<400g/mol), whose content is restricted by the actual legislation. This work aims at identifying a suitable solvent and a solvent resistant nanofiltration (SRNF) membrane to be used in a diananofiltration process for the removal of pesticides.Hexane, ethanol and oleic acid were investigated as potential solvents. The role of solvent–membrane interactions was found to be important in the permeability of the membrane, supported by a strong relationship with the swelling/solvent viscosity ratio. Selected membranes were compared in a dead-end filtration mode, which allowed for identifying the membrane(s) that showed the best compromise between permeability and discrimination between the target compounds. The membranes with the best performance were GMT-oNF2 from Borsig/GMT, PuraMemS600 from Evonik and 030306F from Solsep. The performance of the process was enhanced while operating in a cross-flow mode, being further improved after optimization of the concentration of deodorizer distillate and of the processing solutions and the transmembrane pressure.The membranes identified in this work proved to be suitable for valorization of deodorizer distillates, presenting high rejections of steryl esters (>96%) and significant low rejection of pesticides (<65%), under optimised conditions.

NEW CONTRACT AWARD FOR SYNGAS PLANT IN CHINA

The valorization of deodorizer distillates from the oil refining industry requires a stable membrane suitable to separate steryl esters (bioactive compounds with high molecular weight (MW), 650<MW<800g/mol) from pesticides (150<MW<400g/mol), whose content is restricted by the actual legislation. This work aims at identifying a suitable solvent and a solvent resistant nanofiltration (SRNF) membrane to be used in a diananofiltration process for the removal of pesticides.Hexane, ethanol and oleic acid were investigated as potential solvents. The role of solvent–membrane interactions was found to be important in the permeability of the membrane, supported by a strong relationship with the swelling/solvent viscosity ratio. Selected membranes were compared in a dead-end filtration mode, which allowed for identifying the membrane(s) that showed the best compromise between permeability and discrimination between the target compounds. The membranes with the best performance were GMT-oNF2 from Borsig/GMT, PuraMemS600 from Evonik and 030306F from Solsep. The performance of the process was enhanced while operating in a cross-flow mode, being further improved after optimization of the concentration of deodorizer distillate and of the processing solutions and the transmembrane pressure.The membranes identified in this work proved to be suitable for valorization of deodorizer distillates, presenting high rejections of steryl esters (>96%) and significant low rejection of pesticides (<65%), under optimised conditions.

Transport properties of a polyimide solvent resistant nanofiltration membrane

A series of solvent resistant polyimide membranes, with nominal molecular weight cut-offs (MWCO) between 230–700 Da, are evaluated through high pressure filtration of six solutes dissolved in toluene. These solutes range in MW from 142 to 311 Da, and include normal and branched alkanes, and aromatic compounds. The asymmetric membranes, made from a copolymer of benzophenone tetracarboxylic acid with toluene diisocyanate and methylene bis(phenyl isocyanate), shows preferential transport of straight chain over branched alkanes, and of aromatic over non-aromatic compounds. The normal alkanes allow for estimation of MWCO. Performance curves are generated for this family of polyimide membranes. Transport rates, in mm/h, are determined for each dissolved component based on a solution–diffusion equation. These rates are then used to predict membrane performance under alternative conditions of pressure and composition. The results are consistent over the whole range of MWCO’s with the membranes acting in a solution–diffusion mode. These membranes have industrial utility in the enrichment of aromatic streams and in nanofiltration applications.

Increased catalytic productivity for nanofiltration-coupled Heck reactions using highly stable catalyst systems

The recycle of homogeneous Heck catalysts from post-reaction mixtures using solvent resistant nanofiltration (SRNF) membranes is known to increase catalytic productivity. However, the technique is sensitive to catalyst stability when applied to conventional catalysts: catalyst deactivation will cause declining reaction rates and higher reactor occupancy with an increasing number of catalyst recycles. In this study, a conventional Heck catalyst {bis(acetato)bis(triphenylphosphine)palladium(II)}, was recycled six times, before the reaction rate dropped below 20% of the original value. A cumulative turnover number (TON) of 690 in 120 h was obtained, whilst providing a product substantially lowered in organometallic impurities. Significant improvements in system performance were realised by employing, at identical Pd loading, state-of-the-art catalysts with greater chemical stability. An imidazolylidene catalyst, bis(1,3-dibenzylimidazoline-2-ylidene)diiodopalladium(II), yielded an equal TON for six recycles in 40 h with substantially less reaction rate decline. Palladium(II) acetate stabilised by the quaternary phosphonium salt (quat) tetraphenylphosphonium bromide even gave six recycles in under 30 h after careful solvent selection. In all cases, the membrane showed good selectivity against the catalyst (up to 96% Pd rejection), while allowing the reaction product to permeate completely. Through the sustainable high reaction rates, reactor occupancy can be minimised, and waste generated by downstream processing reduced. Indeed, the imidazolylidene and the quat-stabilised catalyst systems surpassed 10 catalyst recycles with TONs >1000 within 60 and 45 h, respectively.

Aromatics Enrichment in Refinery Streams using Solvent Resistant Membranes

ABSTRACTPermselective separation of aromatic from non-aromatic hydrocarbons in feed streams can be accomplished by using membrane systems in hyperfiltration mode. In petrochemical or refinery operations, feed streams containing aromatics such as benzene, toluene, or xylenes can be selectively enriched or depleted in aromatics content. The development program for the STARMEM™ series of membranes led to a commercial process for their large-scale manufacture. A range of membranes in terms of flux and selectivity has been made, and both 2-inch and commercial scale 8-inch diameter spiral wound elements manufactured from these membranes. Tests at 800 psi (55 bar) and 50 °C with a toluene recycle stream have shown stable operation of these modules in hot hydrocarbons. The high efficiencies of these modules allow for high stage cuts (50% and greater) from process streams of thousands of barrels per day. By combining existing refinery operations such as distillation with these new membrane units, cost savings through better purity, recovery, or through-put can be achieved. With these membranes acting as tight hyperfiltration devices, the permeation properties can be modeled through a solution-diffusion equation for molecular transport. Since these membranes also act as solvent resistant nanofiltration devices, unusual benefits can be found while practicing aromatics enrichment of refinery streams, including fractionation of different molecular weight species.

The separation of homogeneous organometallic catalysts using solvent resistant nanofiltration

The non-destructive separation of homogeneous catalysts from organic solutions, allowing catalyst recycle, would be of considerable interest in commercial organic synthesis. Many of the catalysts are of high value, and the destructive separation methods currently employed often generate substantial amounts of waste. A study designed to test the potential of solvent resistant nanofiltration to achieve such separations is reported. An investigation was conducted into the nanofiltration of three organometallic catalysts commonly used in commercial organic synthesis (the Jacobsen catalyst, the Wilkinson catalyst and Pd-BINAP) from a selection of organic solvents (ethyl acetate, tetrahydrofuran and dichloromethane) using a range of polymeric solvent resistant membranes (W.R. Grace, STARMEM™ series; Osmonics, Desal-5; Koch Membrane Systems, MPF-50). The compatibility of the solvent–membrane combinations (membrane stability in solvent plus non-zero solvent flux at 2.0 MPa) was assessed. The solvent flux and membrane rejection of catalyst was then determined for each compatible catalyst–solvent–membrane combination in a dead-end pressure cell. Good catalyst rejection (>0.95) coupled with good solvent fluxes (>50 l m −2 h −1 at 2.0 MPa) were obtained in the majority of systems tested, indicating that the technology has considerable potential in this area. An extension of the investigation to assess the effect of pressure, temperature, and catalyst concentration was conducted on a selection of the most successful catalyst–solvent–membrane systems. Trends in the variation of solvent flux and catalyst rejection with variation of these parameters were identified. Increasing pressure substantially improved both solvent flux and catalyst rejection, whilst increasing catalyst concentration was found to be beneficial in terms of substantial increases in catalyst rejection without significantly affecting solvent flux. Increasing temperature generally resulted in improved solvent fluxes but lower catalyst rejection.

Phase-transfer catalyst separation and re-use by solvent resistant nanofiltration membranes

This communication describes the use of nanofiltration (NF) membranes for efficient separation and recycling of phase-transfer catalysts, using the conversion of bromoheptane into iodoheptane with tetraoctylammonium bromide as the PT catalyst, as an example; a solvent flux of >10 L m−2 h−1 was achieved with >99% catalyst recycle and no loss in PT catalyst activity over a cycle of three consecutive reactions.

Modeling of nanofiltration - assisted organic synthesis

The molecular weight of target compounds in many organic syntheses in the pharmaceutical industry is in the range of 300–1000 Da. Separation processes based on organic solvent-resistant nanofiltration membranes are being explored to separate these valuable products from the smaller molecular weight products and residual reactants present in the reaction mixture. Through numerical modeling and use of realistic values for the parameters in organic syntheses involving compounds of diverse molecular weights and hence, diverse sizes, this study shows that the production of the target compound can be enhanced significantly using nanofiltration. Enhancement is examined from the perspectives of conversion (yield), reduction of reaction time, selectivity and potential elimination of subsequent downstream separation steps.

Effect of solvent properties on permeate flow through nanofiltration membranes. Part I: investigation of parameters affecting solvent flux

The objective of the present study was to characterize transport properties of solvents permeating through solvent resistant nanofiltration membranes that have only recently become available. Permeation flows of a number of solvents of different chemical families were measured in a batch cell. The solvents studied were alcohols, paraffins, ketones, acetates, and water, as well as their binary mixtures. The experimental data revealed a marked variation in the level of permeate flux among the various solvents. For instance, permeation flow of pentane was about 60-times faster compared to that of water while permeation flow of ethanol was about 10-times faster than that of water. The dependence of solution flux on the fractional composition of the solvents in the mixture was found to be highly non-linear. The flux of either pure or mixed solvents was mainly affected by surface tension and viscosity of the solvents. The flux of paraffins mixtures in particular was affected by the dielectric constant also.