Microfiltration phase-inversion process

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 ... 

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. 

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. 

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... 

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. 

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... 

The study of dual-layer asyimnetric hollow-fiber membranes formed by the phase-inversion process started in the late 1980s. In 1987, Yanagimoto invented dual-layer asymmetric flat-sheet and hollow-fiber membranes to improve the antifouling properties of membranes for ultrafiltration and microfiltration (Yanagimoto, 1987, 1988). Since then, Kuzumoto and Nitta (1989) simultaneously extruded inner and outer dopes containing the same polymer but different solvents and additives to improve water permeability. Ekiner et al. (1992) disclosed the procedures for the fabrication of dual-layer hollow fibers for gas separation. Li et al. (2002) developed a delamination-fiee dual-layer asymmetric... 

The thermal phase inversion process is also employed for ultra- and microfiltration membranes. Crystalline polymers such as polyethylene and polypropylene are generally preferred as solutions these can be prepared at temperatures above the melting point, but cooling below the melting point will yield rapid crystallization and phase separation. These membranes are often employed in microfiltration and dialysis applications. 

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 ... 

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. 

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). 

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. 

One specific advantage of this process is that the dense polymer layer firmly adheres to a porous support offering the ability to withstand high pressure. Hollow fiber membranes utilized in gas separation need to withstand continuous pressures up to and exceeding 1000 psi. For gas separation, it is desirable to have a continuous porous stmrture as the support without macrovoids, which can often be produced if the phase inversion variables (such as polymer solution concentration) are not optimized. For ultra- and microfiltration membranes, macrovoids often exist but do not critically restrain the... 

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