MEMBRANE SEPARATION PROCESSES
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MEMBRANE SEPARATION PROCESSES S. Bandini, G. Camera Roda, C. Gostoli, G.C. Sarti Foreward This research area includes the development of novel separation techniques (osmotic distillation, membrane distillation, affinity membrane separations) as well as the application of more conventional operations mainly in the field of biotechnology, food processing and water treatment. The research in this field at DICMA has been active since the seventies, starting from the development and modelling of hollow fibre dialysers and includes both theoretical and applied issues generally developed through collaborations with industry, public institutions and research centres. The current investigations can be grouped into the following areas: (i) Affinity membrane separations, (ii) Electrodialysis, (iii) Membrane contactors, (iv) Membrane distillation, (v) Membrane separation of hydrogen, (vi) Nanofiltration, (vii) Pervaporation. The main aims and results are presented separately below. Affinity membrane separations The selective separation of proteins from complex mixtures represents the most important cost determining step of the production process. An effective and widely used technique is represented by affinity chromatography based on the use of beads and gels as solid supports; they are characterised by elevated area per unit volume and allow to reach high purity; however some weak points are present as the long separation times, associated to the molecular diffusion process required for the biomolecules to reach the active ligand sites inside the dead-end pores of the solid support, the high pressure drop, required to have the solution flow through the columns, which may cause column clogging. The use of porous membranes as affinity supports, on the other hand, has the advantage of allowing the solution flow though the pores, thus greatly decreasing the mass transport resistance encountered by the biomolecules to reach the selective ligands attached onto the surface of the membrane pores. That makes more effective not only the adsorption stage of the chromatographic separation, but also the washing and the elution stages. the In addition, membranes are relatively thin and, by piling them in a column configuration allow to form a short and wide column requiring a pressure drop much smaller than the one existing in traditional chromatographic columns using beads. The research thus consists in selecting a porous membrane, and modifying it by attaching a ligand which is specific for the target protein to be purified from a complex solution. The affinity membrane thus obtained must be characterised by measuring the surface concentration of ligand and the sorption isotherm of the target protein, possibly at different ionic strength of the buffer. The membrane must then be inserted in a membrane unit properly designed. The membrane material used thus far is cellulose based, chosen in view of the limited unspecific binding observed versus protein, in comparison with other materials. As target protein domains the Maltose Binding Protein (MBP) has been considered first, present in different fusion proteins produced by genetically modified E. Coli strains; three different strains have been used, available from New England Biolabs, which over express MBP- galactoxidase, MBP-rubredoxin and MBP-intein-CBD respectively. Such proteins are produced endocellularly by the E. Coli strains used. The affinity membranes were obtained by using amylose as selective ligand for the MBP domain. The modification protocol set-up consists in two steps: first a spacer arm is bound to the membrane (a bis-epoxy compound has been used) and then binding the ligand to the active epoxy moiety of the spacer arm. The affinity membranes thus obtained have been finally used to purify the MBP fusion proteins directly from the cell lysate, after centrifugation for the separation of the membrane cell debris. In comparison with the chromatographic beads commercially available the affinity membranes obtained in DICMA labs produced purer solutions of the target proteins, in a single chromatographic step, in an overall time of about two hours instead of the 24 hours required by the commercially available beads. This has been patented (pat. N° BO2001A00058), including the membrane apparatus setup for the separations. The effects of flow rate on sorption and desorption rate are also inspected. Analogous procedure is currently under investigation for the separation of lectins obtained from Momordica Carantia, in use as carriers for cytotoxins, testing different polysaccharides as ligands. The separation and purification of immunoglobulins has been and is investigated, considering synthetic ligands as Protein A Mimetic and IgM as the target protein. Monoclonal antibodies are considered as target within the AIMs Integrated Project ongoing within the 6th European Frame Programme. Electrodialysis for phenols production A new method is developed for the conversion of aqueous solutions containing sodium salts of phenols into the corresponding undissociated phenols. The method is alternative to conventional processes which are characterized by a relevant environmental impact caused by a high level of pollutants existing in wastewaters. Use of Nafion® electromembranes (cationic membranes), appropriately arranged, allows to perform conversion as well as products separation in a single step. Innovations of the research are development, design and experimentation of two membrane arrangements suitable to the purification purpose. The research is currently devoted to test bipolar membranes applications. Membrane contactors for food and wine processing Membrane contactors are an emerging technology in which the membrane is used as a tool for interphase mass transfer operation. The main applications considered is Osmotic distillation, a concentration technique for aqueous mixtures in which water is removed from the feed by a hypertonic solution flowing downstream a porous hydrophobic membrane. Concentration of citrus, tomato and grape juices were considered in detail. The main contributions given by the research at DICMA were: (i) the use of glycols or glycerol as extractants to avoid the corrosion and scaling related to the use of concentrated brines, (ii) the design of plate and frame modules, (iii) the role of heat transfer (and not only of mass transfer) in the process performance. Other operations under investigations are the ethanol removal from wine (alcohol adjustment) and the removal of volatile acidity from wine. The first goal is a sort of dialysis and can be achieved by a process scheme similar to osmotic distillation, but using pure water as exctractant. The former by a combined dialysis - ion exchange process. Two patents were developed: Ital. Pat. N. RM97A000678; Ital. Pat. N. RE2000A000120. Membrane distillation Membrane distillation is a separation process for aqueous solutions, based on the use of hydrophobic microporous membranes. The membranes are not wetted by the aqueous phase, until the operating pressure remains lower than the minimum penetration pressure of the membrane, so that the entrance of the pores acts as the physical support for a liquid vapour interface (meniscus) which can originate the separation of components of different volatility. The driving force for mass transfer across the membrane is a difference in the partial pressure between the two ends of the membrane pores. That can be maintained by acting on the temperature difference across the membrane, as in direct contact MD, by using a sweeping gas on the permeate side, by introducing an air gap or by applying vacuum in the permeate side. The MD separation process has been long studied at DICMA laboratories in all the different configurations: direct contact MD, air gap MD, sweeping gas MD and vacuum MD. The analysis has been performed experimentally and specific simulation models have been developed and compared with experimental data. Vacuum MD has been applied in recent periods as a method to extract VOC dissolved in water and the effects of both heat and mass transfer resistances have been studied. Economic evaluation of the process have been assessed. The process has also been applied as a method to concentrate grape must, in which case it has been observed that the aroma components present remain essentially in the must with an important beneficial effect for the quality of the product. Nanofiltration modelling The recent extensive development of Nanofiltration (NF) technology has put in evidence the real application potentialities of this innovative pressure-driven membrane process for the separation or purification of liquid mixtures. Researchers attention has been also focussed to the development and optimization of mathematical models able to predict membrane performances in the case of multicomponent systems (aqueous solutions containing electrolytes, as well as neutral solutes). The key points of the question about the mathematical modelling of electrolytes transport through charged membranes can be basically identified both in the characterization of the membrane, which can be seen as a homogeneous or as a porous medium, and in the understanding of the electrostatic phenomena giving rise to ion partitioning. Some important innovative results have been already obtained. A general model has been developed called Donnan Steric Pore Model & Dielectric Exclusion, DSPM&DE, to describe mass transfer of electrolytes and neutral solutes through Nanofiltration membranes. The transport equations of ions through the membrane are based on the extended Nernst-Planck equation, accounting for ionic diffusion, electromigration and convection in the membrane pores; the dielectric exclusion phenomenon has been recognized as a determining partitioning mechanism at the interfaces between the membrane and the external solutions, in addition to steric hindrance and Donnan equilibrium. Simplified versions of the model were also developed and a general assessment was presented for membrane characterization. Present activities are focussed to understand the correct mechanism of the membrane charge formation, with the aim to develop a charge model able to describe ionic adsorption mechanisms as well as amphoteric behaviour typically observed for NF membranes. Membrane processes in the dairy industry The application of Nanofiltration (NF) for the concentration and partial demineralisation of whey is well established, based on the fact that NF membranes exhibit low retention towards monovalent ions, whereas retention of higher ions and sugars is comparable to that obtained by reverse osmosis. The research investigated the milk pre-concentration by NF for fresh Quarg-type cheese manufacturing. Quarg obtained using milk NF retentates was naturally sweeter (61.5 g·L–1 lactose) than traditional fresh cheese and had a high calcium content (2.0 g·L–1) and no bitter taste. Whey had a higher total solids and lactose content and, therefore, required an easier treatment than normal Quarg acid whey. The process has been patented (It. Pat. N. B099A000269). Pervaporation The study is on the potentiality of pervaporation in different separation fields and processes and on the understanding of the diffusive phenomena which take place in the polymeric material in order to enhance the membrane performances. Pervaporation has been tested in the removal of persistent organic pollutants from aqueous streams alone or coupled with a photocatalytic reaction. The results are very interesting since a significant synergy has been demonstrated between the two processes when they are integrated in the same apparatus. The analysis of the transport phenomena inside the membrane has allowed to establish the effect of the concentration of a zeolite filler inside the polymeric matrix for preparing composite pervaporation materials with better performances. Membrane separation of hydrogen The research is focused on the experimental analysis and model simulation of membranes and membrane units to be used for the high temperature separation of hydrogen from the stream resulting from a novel methane reforming reactor. The membranes will be obtained in collaboration with the University of Messina, formed by coatings of palladium copper alloys over suitable ceramic supports. Selected Publications G. Mucchetti, G. Zardi, F. Orlandini, C. Gostoli, “The pre-concentration of milk by nanofiltration in the production of fresh cheese Quarg type”, Le Lait, 80, 43-50 (2000). M. Celere, C. Gostoli, “The heat and mass transfer phenomena in osmotic membrane distillation”, Desalination, 147, 133-138 (2002). M. Celere, C. Gostoli, “Osmotic distillation with propylene glycol, glycerol and glycerol-salt mixtures”, J. Membrane Sci., 229, 159-170(2004). F. Cattoli, G.C. Sarti, “Purification of MBP-βgalactosidase and MBP-rubredoxin through affinity membrane separations”, Separation Sci. & Technol., 37, 1699-1724 (2002). F. Cattoli, G.C. Sarti, “Separation of MBP fusion proteins through affinity membranes”, Biotechnology Progress, 18, 94-100 (2002). S. Bandini, G.C. Sarti, “Concentration of must through vacuum membrane distillation”, Desalination, 149, 253- 259 (2002). F. Cattoli, G.C. Sarti, “Characterization of amylose affinity membranes through adsorption of pure recombinant MBP-intein-CBD: Equilibrium and kinetic data”, Desalination, 149, 465-470 (2002). F. Cattoli, G.C. Sarti, “Use of micro-porous affinity membranes for protein purification: a case study”, in New Insights into Membrane Science and Technology: Polymeric and Biofunctional Membranes, (D. Bhattacharyya, D.A. Butterfield, Eds.), Chap. 13, Elsevier, 2002 S. Bandini, “Cationic Membranes for Phenols Production”, Ann. Chim. (Rome), 91, 134-144 (2001). D. Vezzani, S. Bandini, “Donnan equilibrium and dielectric exclusion for characterization of nanofiltration membranes”, Desalination, 149, 477-483 (2002). S. Bandini, “Nafion Membranes for conversion of sodium phenoxides into undissociated phenols”, J. Membrane Sci., 207, 209-225 (2002). S. Bandini, D. Vezzani, “Nanofiltration modeling: the role of dielectric exclusion in membrane characterization”, Chem. Eng. Sci., 58, 3303-3326 (2003).