Ultrafiltration solvent-resistent membranes

A variety of synthetic polymers, including polycarbonate resins, substituted olefins, and polyelectrolyte complexes, are employed as ultrafiltration membranes. Many of these membranes can be handled dry, have superior organic solvent resistance, and are less sensitive to temperature and pH than cellulose acetate, which is widely used in RO systems. 

The discussion so far implies that membrane materials are organic polymers, and in fact most membranes used commercially are polymer-based. However, in recent years, interest in membranes made of less conventional materials has increased. Ceramic membranes, a special class of microporous membranes, are being used in ultrafiltration and microfiltration applications for which solvent resistance and thermal stability are required. Dense, metal membranes, particularly palladium membranes, are being considered for the separation of hydrogen from gas mixtures, and supported liquid films are being developed for carrier-facilitated transport processes. 

For relatively porous nanofiltration membranes, simple pore flow models based on convective flow will be adapted to incorporate the influence of the parameters mentioned above. The Hagen-Poiseuille model and the Jonsson and Boesen model, which are commonly used for aqueous systems permeating through porous media, such as microfiltration and ultrafiltration membranes, take no interaction parameters into account, and the viscosity as the only solvent parameter. It is expected that these equations will be insufficient to describe the performance of solvent resistant nanofiltration membranes. Machado et al. [62] developed a resistance-in-series model based on convective transport of the solvent for the permeation of pure solvents and solvent mixtures ... 

In the petroleum industry, dewaxing solvents are separated by ultrafiltration from dewaxed oils by chemically resistant membranes made from polysulfone or polyimide. In a related process, pentane is separated from deasphalted heavy oil under conditions intermediate between reverse osmosis and ultrafilttation (ca. 15 bar applied pressure). High-molecular-weight hydrocarbons in the oil form a gel layer on the surface of a polysulfone support membrane. This gel restricts passage of heavier hydrocarbons but not pentane, which is recovered as permeate. To separate other hydrocarbon mixtures that do not contain gel-forming components, polymeric additives would be used as a rejecting barrier substitute. 

P. Zschocke and D. Quelhnalz. Integral asymmetric, solvent-resistant ultrafiltration membrane made ol partiahy sulphonated, aromatic polyether ether ketone. DE Patent 3 321860, assigned to Berghol Eorschungsinst (DE),... 

Ceramic membranes, which are tougher and longer lasting than polymeric membranes, offer many advantages in ultrafiltration applications but are more than 10 times more costly than equivalent polymer membranes. Thus their use has been limited to small-scale, high-value separations that can bear this cost. One area where ceramic membranes may find a future use is clarification of chemical or refinery process streams, where their solvent resistance is needed. However, it is difficult to see a major business developing from these applications unless costs are reduced significantly. 

Ultrafiltration is a membrane process whose nature lies between nanofiltration and microfiltration. The pore sizes of the membranes used range from 0.05 um (on the microfiltration side) to 1 am (on the nanofiltration side). Ultrafiltration is typically used to retain macromolecules and colloids from a solution, the lower limit being solutes with molecular weights of a few thousand Daltons. Ultrafiltration and microfiltration membranes can both be considered as porous membranes where rejection is determined mainly by the size and shape of the solutes relative to the pore size in the membrane and where the transport of solvent is directly proportional to the applied pressure. Such convective solvent flow through a porous membrane can be described by the Kozeny-Carman equation (see eq. VI - 27) for example. In fact both microfiltration and ultrafiltration involve similar membrane processes based on the same separation principle. However, an important difference is that ultrafiltration membranes have an asymmetric structure with a much denser toplayer (smaller pore size and lower surface porosity) and consequently a much higher hydrodynamic resistance. 

The ultrafiltration membrane with high permselectivity and solvent resistance has been prepared from the butadiene-styrene copolymer. The copolymer on hydroboration with 9-BBN, followed by NaOH-HjO oxidation affords ultrafiltration membrane [8] with good permeation property and separation behavior for several solutes. 

The information presented in Table 16.2 suggests PAN and PI are solvent-resistant polymers. Both polymers can be processed by the phase inversion technique. Polyimides are commonly used for integral asymmetric nanohltration (NF) membrane preparation and will be addressed in this section. PAN is mainly associated with the preparation of integral asymmetric ultrafiltration (UF) membranes and consequently serves as a support for composite NF membranes to be discussed in the next chapter. Pis are made by the reaction of diamines (DA) with dianhydrides to form the soluble polymer precursor known as poly(amic acid). This can then be processed into a useful shape by converting it into the final PI by cyclodehydration of the amic acid (imidization). Figure 16.1 shows the mechanism for the pyromeUitic dianhydride (PMDA) case. 

A key factor determining the performance of ultrafiltration membranes is concentration polarization due to macromolecules retained at the membrane surface. In ultrafiltration, both solvent and macromolecules are carried to the membrane surface by the solution permeating the membrane. Because only the solvent and small solutes permeate the membrane, macromolecular solutes accumulate at the membrane surface. The rate at which the rejected macromolecules can diffuse away from the membrane surface into the bulk solution is relatively low. This means that the concentration of macromolecules at the surface can increase to the point that a gel layer of rejected macromolecules forms on the membrane surface, becoming a secondary barrier to flow through the membrane. In most ultrafiltration appHcations this secondary barrier is the principal resistance to flow through the membrane and dominates the membrane performance. 

Since none of the commercially available nano- or ultrafiltration membranes so far shows real long-term resistance against organic solvents under the reaction conditions needed for a commercially interesting hydroformylation process and since no prices are available for bulk quantities of membranes for larger scale applications, considerations about the feasibility of such processes are difficult and would be highly speculative. 

Membranes used for the pressure-driven separation processes, microfiltration, ultrafiltration and reverse osmosis, as well as those used for dialysis, are most commonly made of polymeric materials 11. Initially most such membranes were cellulosic in nature. These are now being replaced by polyamide, polysulphone, polycarbonate and a number of 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. This process has four main steps (a) the polymer is dissolved in a solvent to 10-30 per cent by mass, (b) the resulting solution is cast on a suitable support as a film of thickness, approximately 100 11 m, (c) the film is quenched by immersion in a non-solvent bath, typically... 

Membrane Properties. The performance range of ammonia-modified membranes in low pressure operation is indicated in Figure 6 along with the performance of the reference membrane (I, reference membrane IV, ammonia-modified membrane). The lower boundary of the performance range refers to a solvent-to-polymer ratio of 3, the upper boundary to a ratio of 4. While the salt rejection towards univalent ions of the ammonia-modified membrane is limited to below 80 %, the maximum low pressure flux is over 15 m /m d (approaching 400 gfd) at a sodium chloride rejection of the order of 10 %. This membrane thus exhibits the flux capability of an ultrafiltration membrane while retaining the features of reverse osmosis membranes, viz. asymmetry and pressure resistance. 

In case of ultrafiltration membranes, resistance of membrane is owing to membrane material. Therefore, selection of membrane material is the most important. Figure 9 shows comparison of engineering plastics which have ever investigated for ultrafiltration membranes with solvent and high temperature resistance. It is the most difficult to find materials which satisfy both excellent resistivity and excellent processibility. Polyimide is an only material commercialized for a rather resistant ultrafiltration membrane. 

In ultrafiltration membranes, solvent and heat resistance is one of the most expected demands to expand their application. Owing to excellent processibility of PPSS polymer and... 

The pressures used in reverse osmosis range from 20 to 100 bar and in nanofiltration from about 10 to 20 bar which are much higher than those used in ultrafiltration. In contrast to ultrafQtration and microfiltration the choice of material directly influences the separation efficiency through the constants A and B (see eq. VI -34). In simple terms, this means that the constant A must be as high as possible whereas the constant B must be as low as possible to obtain an efficient separation. In other words, the membrane (material) must have a high affinity for the solvent (mostly water) and a low affinity for the solute. Hiis implies that the choice of material is very important because it determines the intrinsic membrane properties. The difference to ultrafiltration/ microfiltration, whi m the dimensions of the pores in the material determine the separation properties and the choice is mainly based upon chemical resistance, is obvious. 

Although ultrafiltration can easily control resist-bath conductivity, any small water-soluble molecule is allowed to pass through the membrane, including solvent, which constitutes an environmental/waste treatment problem. Many ED resists have been formulated to include water-soluble solvents, and maintaining bath solvent level therefore requires periodic analysis and addition of fresh solvents. (This is not the case with resists that have been ultrafiltered during manufacture to remove water-soluble solvent (section 2.4.10).)... 

At this stage some resists can be ultrafiltered to remove the water-soluble polymerization solvent. Ultrafiltration removes small molecules, but large aggregations of molecules (i.e. micelles) cannot pass through the membrane. Small amounts of other, water-soluble resist components may also be removed, but this can be compensated for by adding slightly more of them at the beginning. 

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