MEMBRANE SEPARATION PROCESSES

Document Sample
MEMBRANE SEPARATION PROCESSES Powered By Docstoc
					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).