Polymeric membranes microfiltration

The solid-liquid separation of shinies containing particles below 10 pm is difficult by conventional filtration techniques. A conventional approach would be to use a slurry thickener in which the formation of a filter cake is restricted and the product is discharged continuously as concentrated slurry. Such filters use filter cloths as the filtration medium and are limited to concentrating particles above 5 xm in size. Dead end membrane microfiltration, in which the particle-containing fluid is pumped directly through a polymeric membrane, is used for the industrial clarification and sterilisation of liquids. Such process allows the removal of particles down to 0.1 xm or less, but is only suitable for feeds containing very low concentrations of particles as otherwise the membrane becomes too rapidly clogged.2,4,8... 

In bioprocesses, a variety of apparatus that incorporate artificial (usually polymeric) membranes are often used for both separations and bioreactions. In this chapter, we shall briefly review the general principles of several membrane processes, namely, dialysis, ultrafiltration (UF), microfiltration (MF), and reverse osmosis (RO). 

We can use the same filtration principle for the separation of small particles down to small size of the molecular level by using polymeric membranes. Depending upon the size range of the particles separated, membrane separation processes can be classified into three categories microfiltration, ultrafiltration, and reverse osmosis, the major differences of which are summarized in Table 10.2. 

The u% of synthetic polymeric membranes for water purification is now an established technoli. Historically, this developn nt dates to the beginning of this century, when Zsigmondy and Bachmann prepared the first microporous membrane from cellulose esters. SimOar microfiltration membranes are now widely used in applications ranging fiom sterile filtration to fine particle removal. 

Microfiltration of whey prior to ultrafiltration in the production of whey protein concentrates (WPC) was reported among others by Maubois et al. [75], van der Horst [76], and Wnuk et al. [77]. The microfiltration step cdso prevents fouling of the UF-membranes (either polymeric membranes or ceramic membrane) e.g. Daufin et al. [78] by phosphates and calcium. 

Three different techniques are used for the preparation of state of the art synthetic polymeric membranes by phase inversion 1. thermogelation of, a two or more component mixture, 2. evaporation of a volatile solvent from a two or more component mixture and 3. addition of a nonsolvent to a homogeneous polymer solution. All three procedures may result in symmetric microporous structures or in asymmetric structures with a more or less dense skin at one or both surfaces suitable for reverse osmosis, ultrafiltration or microfiltration. The only thermodynamic presumption for all three preparation procedures is that the free energy of mixing of the polymer system under certain conditions of temperature and composition is negative that is, the system must have a miscibility gap over a defined concentration and temperature range (4). 

Polymeric membranes are prepared from a variety of materials using several different production techniques. Table 5 summarizes a partial list of the various polymer materials used in the manufacture of cross-flow filters for both MF and UF applications. For microfiltration applications, typically symmetric membranes are used. Examples include polyethylene, polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE) membrane. These can be produced by stretching, molding and sintering finegrained and partially crystalline polymers. Polyester and polycarbonate membranes are made using irradiation and etching processes and polymers such as polypropylene, polyamide, cellulose acetate and polysulfone membranes are produced by the phase inversion process.f Jf f ... 

During sintering, a powder of particles of a given size is pressurized at elevated temperatures in a preformed shape so that the interface between the particles disappears. Microfiltration membranes can thus be obtained from PTFE (polytetra-fluoroethylene), PE (polyethylene), PP (polypropylene), metals, ceramics, graphite and glass, with pore sizes depending on the particle size and the particle-size distribution. Porosities up to 80% for metals and 10-20% for polymeric membranes can be reached with pore sizes varying between 0.1 and 10 pm. Most of these materials have excellent solvent and thermal stability. 

Thin-layer MIP composite membranes (cf. Section III.E, see Table 2) had been prepared by surface functionalization of various commercial porous membranes, which had already been optimized towards high performance in microfiltration. These membranes have also a moderate specific surface area (e.g., 0.22 pm PVDF 4m /g, 0.2 pm PP 20m /g). By photo-initiated graft copolymerization [81] or cross-linking polymerization [76], the entire internal pore structure could be coated evenly with thin MIP layers without formation of agglomerates. Most important, at suited degrees of functionalization, no pore blocking occurred as indicated by the preserved high membrane permeability and specific surface area [81]. 

For many years, polymeric membranes have been widely utilized in practical appHca-tions without having precise information on their pore size and pore size distribution, despite the fact that most commercial membranes are prepared by the phase inversion technique, and the performance of those membranes is known to be governed by their pore characteristics in a complicated manner [1]. These pore characteristics are influenced both by the molecular characteristics of the polymer and by the preparative method [2]. Crudely, membranes applied for pressure-driven separation processes can be distinguished on the basis of pore diameter as reverse osmosis (RO, < 1 nm), dialysis (2-5 nm), ultrafiltration (UF, 2-100 nm), and microfiltration (MF, 100 nm to 2 J,m). Nanofiltration (NF) membranes are a relatively new class and have applications in a wide range of fields [3]. The pore sizes of NF lie between those of RO and UF membranes. 

Regarding the microfiltration process. Gomes, Curvelo, and Davantel de Barros (2010) reported that the quantity of molecular glycerol and free glycerol dissolved in biodiesel is a vital factor in the quality control of biodiesel (Murphy, Kanani, Zydney, 2010). They observed that the recovery of biodiesel in permeate solution could possibly be due to interaction between the hydrophobic membranes and nonpolar biodiesel. The authors stated that transmembrane pressure was instrumental for the separation of water from biodiesel, and the preliminary results showed that biodiesel can be efficiently purified via polymeric membranes. Choi, Kai, Dionysios, Daniel, and George (2005) reported that the microfiltration membrane process is more easily fouled than the ultrafiltration membrane process. 

Only microfiltration membranes can be prepared via sintering, however. Tne porosity of porous polymeric membranes is generally low, normally in the range of 10 to 20% or sometimes a little higher. 

These arious techniques allow to prepare microfiltration membranes from Trtually all kinds of materials of which polymers and ceramics are the most important. Synthetic polymeric membranes can be divided in two classes, i.e. hydrophobic and hydrophilic. Various polymers which yield hydrophobic and hydrophilic membranes are listed below. Ceramic membranes are based mainly on two materials, alumina (A1203) and ziiconia (Zr02). However, other materials such as titania (TiOj) can also be used in principle. A number of organic and inorganic materials are listed below ... 

Pretreatment. Prefiltration with polymeric membranes (microfiltration), Treatment with active carbon. Deionization (ion exchange membranes)... 

Polymers commonly used to make such microfiltration membranes are polyvinylidene fluoride (PVDF), Nylon 66, polytetrafluoroethylene (PTFE), polysuUbne, cellulose, cellulose acetate/cellulose nitrate, polypropylene (PP), polyester, polycarbonate, etc. Ceramic microfiltration membranes are not uncommon. Polymeric membranes may have the following stmctures. 

The potential applications of such a polymerization technique for preparing novel polymeric materials include microfiltration, separation membranes, polymer blends with a unique microstructural morphology, and porous microcarriers for cultures of living cells and enzymes [7]. Some other interesting ideas about the preparation of novel materials include the conductive composite film [95] and microporous silica gel [96]. 

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