Microfiltration inversion

Membranes used for the pressure driven separation processes, microfiltration (MF), ultrafiltration (UF) and reverse osmosis (RO), as well as those used for dialysis, are most commonly made of polymeric materials. Initially most such membranes were cellulosic in nature. These ate now being replaced by polyamide, polysulphone, polycarbonate and several other advanced polymers. These synthetic polymers have improved chemical stability and better resistance to microbial degradation. Membranes have most commonly been produced by a form of phase inversion known as immersion precipitation.11 This process has four main steps ... 

Asymmetric Microporous Nonporous, skinned on microporous substrate Flat-sheet, tubular, hollow fiber Flat-sheet, tubular, hollow fiber Phase-inversion casting or spinning Phase-inversion casting or spinning Microfiltration, ultrafiltration, membrane reactors Reverse osmosis, gas separation, pervaporation, perstraction, membrane reactors... 

Inorganic Isotropic or Microporous Tubular, Sol-gel inversion, sintering, Microfiltration, ultrafiltration,... 

In microfiltration, the permeate flux increases inversely with the suspension viscosity and proportionally to the applied pressure, provided that there is no membrane fouling (Belford, 1988 Ho and Zydney, 2000). To accelerate the process, it is possible to decrease the solution viscosity by increasing the temperature, although not so much as to denature the protein. 

Today the majority of polymeric porous flat membranes used in microfiltration, ultrafiltration, and dialysis are prepared from a homogenous polymer solution by the wet-phase inversion method [59-66]. This method involves casting of a polymer solution onto an inert support followed by immersion of the support with the cast film into a bath filled with a non-solvent for the polymer. The contact between the solvent and the non-solvent causes the solution to be phase separated. This process involves the use of organic solvents that must be expensively removed from the membrane with posttreatments, since residual solvents can cause potential problems for use in biomedical apphcations (i.e., dialysis). Moreover, long formation times and a limited versatihty (reduced possibUity to modulate cell size and membrane stmcture) characterize this process. 

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). 

The literature describes numerous manufacturing methods for synthetic membranes. A recent review by Pusch and Walch (1) considers membranes from a number of techniques for manufacturing membranes and discusses applications ranging from microfiltration to desalination to gas separation. In this paper, a thermal phase-separation technique of preparing membranes Is presented. The method Is a development of an Invention described In US Patent 4,247,498 by Anthony J. Castro (,2). This technique Is similar In many respects to the classical phase-inversion methods however, the additional consideration of thermal solubility characteristics of the poly-mer/solvent pair offers new possibilities to membrane production. 

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 ... 

In on-line experiments with a plant effluent containing 2% detergents, 1% oil and 0.8% suspended solids (metal, dirt, etc.), tubular modules equipped with UF membranes of different pore-sizes and a microfiltration membrane have been tested. (The experiments were carried out in the gear and axle production of Daimler Benz AG.) As expected, the MF membrane produced the highest flux for detergents. Because of th relatively low retention capacity for oil, however, the use of MF membranes is limited to a maximum oil concentration of about 10% in the concentrate. In this case, the oil concentration of the permeate is about 50 mg/ , contrary to the UF membranes, where the oil concentration of the permeate is independent of the oil concentration of the concentrate up to the phase inversion concentration of about 41%. 

The limiting oil-concentration for the first stage, microfiltration, is determined by the tolerable concentration of oil in the recycled product. Since the oil retention capacity of the ultrafiltration stage is independent of feed concentration, the final concentration of the retentate is solely determined by the phase inversion "oil/water -> water/oil". At this concentration, the overall water recovery of the process is above 97.5%. However, the retentate of the process still contains too much water if incineration or refining is considered. Therefore, an evaporation step must be included in the process. Almost certainly evaporation will not be economical for the small capacities indicated in Figure 6.34 and a central evaporation station for the tentates from several production lines should be considered. 

Membranes used in microfiltration, reverse osmosis, dialysis, and gas separation are usually prepared by the wet-extrusion process, since it can be used to produce almost every membrane morphology. In the process, homogeneous solutions of the polymers are made in solvent and nonsolvent mixtures, while phase inversion is achieved by any of the several processes, such as solvent evaporation, exposure to excess nonsolvent, and thermal gelation. In most formulations, polymer solutions of 15-40 wt% concentration are cast or spun and subsequently coagulated in a bath containing a nonsolvent (usually water). 

The majority of polymer membranes used for microfiltration and ultrafiltration of liquids are prepared by the wet phase inversion process. Such membranes exhibit a typical asymmetric structure characterized by a thin dense surface layer and a thick microporous bulk. Poly(phthalazinone ether sulfone ketone) (PPESK) copolymers, c.f. Figure 7.10, show glass transition temperatures in the range of 263-305°C. The polymers show an outstanding chemical stability. They are soluble only in 98% H2SO4. Concentrated aqueous solutions of sodium chlorate, hydrogen peroxide, acetic acid, and nitric acid show no effect. ... 

The membrane can be a solid, a liquid, or a gel, and the bulk phases can be liquid, gas, or vapor. Membranes can be classified according to their structures. Homogeneous or symmetric membranes each have a structure that is the same across the thickness of the membrane. These membranes can be porous or have a rather dense uniform structure. Heterogeneous or asymmetric membranes can be categorized into three basic structures (1) integrally skinned asymmetric membrane with a porous skin layer, (2) integrally skinned asymmetric membrane with a dense skin layer, and (3) thin film composite membranes [13]. Porous asymmetric membranes are made by the phase inversion process [14,15] and are applied in dialysis, ultrafiltration, and microfiltration, whereas integrally skinned asymmetric membranes with a dense skin layer are applied in reverse osmosis and gas separation applications. 

Hydrophilic MF membranes can be made by the dry-wet phase inversion technique, which can also be used to make PVDF membranes. On the other hand, other hydrophobic microflltration membranes are made by the thermally induced phase separation technique. In particular, semicrystalline PE, PP, and PTFE are stretched parallel to the direction of film extrusion so that the crystalline regions are aligned in the direction of stretch, while the noncrystalline region is ruptured, forming long and narrow pores. Hydrophobic membranes do not allow penetration of water into the pore until the transmembrane pressure drop reaches a threshold called the liquid entry pressure of water (LEPw). These membranes can therefore be used for membrane distillation. The track-etching method is applied to make microfiltration membranes from PC. 

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. 

Data in this report are generated from both commercial and developmental flat-sheet CA membranes. CA manbranes are prepared by dissolving commercial grades of CA polymers into a solvenl/non-solvent mixture to give a highly viscous dope solution. After microfiltration a knife blade is used to spread the dope onto a woven nylon substrate. The commercial equipment utilized allows for a 1-m width to be cast. The thin dope film is quenched into a water bath to form the microporous structure by the phase inversion process. Membrane is washed with water and post-treated to give finished product in dry state as roll stock. 

Separation processes such as ultrafiltration and microfiltration use porous membranes which allow the passage of molecules smaller than the membrane pore size. Ultrafiltration membranes have pore sizes from 0.001 to 0.1 pm while microfiltration membranes have pore sizes in the range of 0.02 to 10 pm. The production of these membranes is almost exclusively based on non-solvent inversion method which has two essential steps the polymer is dissolved in a solvent, cast to form a fUm then the film is exposed to a non-solvent. Two factors determine the quality of the membrane pore size and selectivity. Selectivity is determined by how narrow the distribution of pore size is. In order to obtain membranes with good selectivity, one must control the non-solvent inversion process so that it inverts slowly. If it occurs too fast, it causes the formation of pores of different sizes which will be non-uniformly distributed. This can be prevented either by an introduction of a large mun-ber of nuclei, which are uniformly distributed in the polymer membrane or by the use of a solvent combination which regulates the rate of solvent replacement. 

When solvents are removed solely by evaporation, the membrane formation is known as a dry phase inversion process (Resting 1985). When the phase separation and structure formation are achieved by immersion of a cast membrane in a quench medium, the process is known as a wet phase inversion process (Heffelfinger 1978). The latter process is used to prepare asymmetric membranes for either microfiltration (Roesink 1989), ultrafiltration (Michaels 1971), reverse... 

As will be described in chapter III, quite a number of techniques exist for preparing microfiltration membranes, i.e. sintering, stretching, track-etching and phase inversion. These techniques are not generally used to prepare ultrafiltrarion membranes, because the pore sizes obtained are only in... 

Let us take polysulfone as an example. This is a polymer which is frequently used as a membrane material, both for microfiltration/ultrafiltration as well as a sublayer in composite membranes. These applications require an open porous structure, but in addition also asymmetric membranes with a dense nonporous top layer can also be obtained which are useful for pervaporation or gas separation applications. Some examples are given in table ni.S which clearly demonstrate the influence of various parameters on the membrane structure when the same system, DMAc/polysulfone(PSf), is employed in each case. How is it possible to obtain such different structures with one and the same system To understand this it is necessary to consider how each of the variables affects the phase inversion process. The ultimate structure arises through two mechanisms i) diffusion... 

In the previous section the thermodynamic and kinetic relationships have been given to describe membrane formation by phase inversion processes. These relationships contain various parameters which have a large impact on the diffusion and demixing processes and hence on the ultimate membrane morphology. It has been shown that two different types of membranes may be obtained, the porous membrane (microfiltration and ultrafiltration) and the nonporous membrane (pervaporation and gas separation), depending on the type of formation mechanism, i.e. instantaneous demixing or delayed onset of demixing, involved. 

This simple relationship between the osmotic pressure k and the solute concentration Cj. is called the van t Hoff equation. It can be seen that the osmotic pressure is proportional to the concentration and inversely proportional to the molecular weight. If the solute dissociates (as for instance in salts) or associates, eq. VI - 21 must be modified. When dissociation occurs the number of moles increases and hence the osmotic pressure increases proportionally, whereas in the case of association the number of moles decreases as does the osmotic pressure. The osmotic pressure difference in microfiltration and ultrafllu ation applications are quite low, whereas it has to be taken into account in reverse osmosis. 

Figure VI - 4. Polymeric microfiltration membranes (a) phase inversion (b) stretching and (c) track etching...

The flux is proportional to the driving force where the proportionality constant can be considered a.s the inverse sum of all the resistances (see figure VII - 1). In those cases where concentration p>olarisation is very severe (microfiltration/ultrafiltration), flux decline can be quite considerably (it should be mentioned that fouling is the dominating factor in flux decline as will be discussed later) whereas in other processes, such as gas separation where concentration piolarisation hardly occurs, the flux remains reasonablyjconstant with... 

Cellulose acetate (CA) can also be used as a hydrophilic additive to enhance the performance of PVDF microfiltration membranes cast by the phase inversion process... 

We observed earlier in dead-end cake filtration (equation (6.3.135k)) that Rcs varies inversely with the square of the particle radius therefore, in effect, the filtration flux varies with the square of the particle radius for cake dominated filtration. The larger the particle radius, the higher the filtration flux. As shown in equation (7.2.145), in cross-flow microfiltration also the averaged filtration flux increases with particle radius, here as Romero and... 

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