Ultrafiltration polymeric membranes

It is possible to separate a soap-LSDA dispersion by ultrafiltration through a polymeric membrane [33]. The filtrate contained sodium and some magnesium ions but no calcium soaps or LSDA. The separated substances on the membrane could be readily dispersed in water in which they retained a high degree of surface activity. 

Since membranes no longer had important nuclear applications in future, SPEC was sold in 1987 by the CEA to the French company Rhone-Poulenc which merged them with their polymeric membrane division to form the new subsidiary, currently known as Tech Sep. Zr02 based ultrafiltration membranes on 6 mm inner-diameter carbon tubes continues to be the main product line of Tech Sep in terms of inorganic membranes. 

Membrane reactors have been investigated since the 1970s 11). Although membranes can have several functions in a reactor, the most obvious is the separation of reaction components. Initially, the focus has been mainly on polymeric membranes applied in enzymatic reactions, and ultrafiltration of enzymes is commercially applied on a large scale for the synthesis of fine chemicals (e.g., L-methionine) 12). Membrane materials have been improved significantly over those applied initially, and nanofiltration membranes suitable to retain relatively small compounds are now available commercially (e.g., mass cut-off of 400—750 Da). 

Membrane reactors can be considered passive or active according to whether the membrane plays the role of a simple physical barrier that retains the free enzyme molecules solubilized in the aqueous phase, or it acts as an immobilization matrix binding physically or chemically the enzyme molecules. Polymer- and ceramic-based micro- and ultrafiltration membranes are used, and particular attention has to be paid to the chemical compatibility between the solvent and the polymeric membranes. Careful, fine control of the transmembrane pressure during operation is also required in order to avoid phase breakthrough, a task that may sometimes prove difficult to perform, particularly when surface active materials are present or formed during biotransformahon. Sihcone-based dense-phase membranes have also been evaluated in whole-cell processes [55, 56], but... 

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. 

A straightforward way to collect solutes from the interstitial fluid (ISF) space would be to have a semipermeable, hollow fiber, membrane-based device as originally described by Bito et al.1 Two semipermeable membrane-based devices that have been used to collect different types of analytes from various mammalian tissues include microdialysis sampling probes (catheters) and ultrafiltration probes. The heart of each of these devices is the semipermeable polymeric membrane shown in Figure 6.1. The membranes allow for collection of analytes from the ISF that are below the membrane molecular weight cutoff (MWCO). Each of these devices provides a sample that has a significantly reduced amount of protein when compared to either blood or tissue... 

The first widespread use of polymeric membranes for separation applications dates back to the 1960-70S when cellulose acetate was cast for desalination of sea and brackish waters. Since then many new polymeric membranes came to the market for applications extended to ultrafiltration, miciofiltration, dialysis, electrodialysis and gas separations. So far ultrafiltration has been used in more diverse applications than any other membrane processes. The choice of membrane materials is dictated by the application environments, the separation mechanisms by which they operate and economic considerations. Table 1.4 lists some of the common organic polymeric materials for various membrane processes. They include, in addition to cellulose acetate, polyamides. 

The ultrafiltration process is operated in a batch mode at a temperature of about 50 C. Ceramic membranes with 0.1 or 0.2 pm pore diameter enable processing of this highly viscous and concentrated raw or pasteurized whole milk due to their inherent mechanical strength. The viscosity of the concentrate has been found to increase exponentially with the rise of protein content in the precheese. Polymeric membranes have also been considered not suitable for this process in view of their structural compaction under pressure and their difficulty of cleaning. 

In recent years, two other important applications of synthetic polymeric membranes in water purification have become establMied. Chronologically the first of these, reverse osmosis, is rapidly becoming the principal method of water desalination worldwide. Another membrane process, ultrafiltration, is even newer and is finding important use in the renurval of h molecular wei t, colloidal, and emulsified materials from aqueous streams. 

T. S. Chung, J. J. Shieh, J. Qin, W. H. Lin, and R. Wang, Polymeric membranes for reverse osmosis, ultrafiltration, microfdtration, gas separation, pervaporation, and reactor applications. In Advanced Functional Molecules and Polymers, H. S. Nalwa (ed.). Chapter 7, Gordon Breach, pp. 219-264 (2001). 

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. 

The separation of proteins and enzymes is performed with ultrafiltration membranes. Branger et al. [93] use Carbosep Ml and M4 (40,000 and 20,000 Dalton respectively) for the separation of enzyme hydrolysates. The fluxes with these membranes compare favourably with polymeric membranes 37-102 l/m hvs. 7-41 l/m h. 

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). 

Typical UF performance for pyrogen removal with a polymeric and ceramic membrane is shown in Table 13. It can be seen that both types of UF membranes can adequately remove pyrogens. The choice of UF membrane (ceramic or polymeric) will depend on operating conditions or other special process requirements. Ceramic membrane ultrafiltration can achieve a 5 log reduction in pyrogen level. These UF membranes have been validated for the production of water meeting the requirements of pyrogen-free water for injection (WFI) standards.f ... 

Membrane Separations. Separation processes using polymeric membranes (30) have become important techniques because of their simplicity and low consumption of energy in comparison to alternatives such as distillation. Membranes for ultrafiltration are porous, and no diffusive transport actually occurs through the polymer itself. However, for separation at the molecular level, diffusion through the polymer provides a possible mechanism for selective passage of the desired small molecule. Reverse osmosis for desalination of water can occur by this mechanism, and large commercial processes using this technique are now in operation. 

---