Ultrafiltration, applications

Fig. 23. Two types of hollow-fiber modules used for gas separation, reverse osmosis, and ultrafiltration applications, (a) Shell-side feed modules are generally used for high pressure appHcations up to - 7 MPa (1000 psig). Fouling on the feed side of the membrane can be a problem with this design, and pretreatment of the feed stream to remove particulates is required, (b) Bore-side feed modules are generally used for medium pressure feed streams up to - 1 MPa (150 psig), where good flow control to minimise fouling and concentration polarization on the feed side of the membrane is desired.

Two other major factors determining module selection are concentration polarisation control and resistance to fouling. Concentration polarisation control is a particularly important issue in liquid separations such as reverse osmosis and ultrafiltration. Hollow-fine-fibre modules are notoriously prone to fouling and concentration polarisation and can be used in reverse osmosis applications only when extensive, costly feed solution pretreatment removes all particulates. These fibres cannot be used in ultrafiltration applications at all. 

Plate-and-frame units have been developed for some small-scale applications, but these units are expensive compared to the alternatives, and leaks through the gaskets required for each plate are a serious problem. Plate-and-frame modules are now only used in electrodialysis and pervaporation systems and in a limited number of reverse osmosis and ultrafiltration applications with highly fouling feeds. An example of one of these reverse osmosis units is shown in Figure 3.39 [111],... 

Tubular modules are now generally limited to ultrafiltration applications, for which the benefit of resistance to membrane fouling due to good fluid hydrodynamics outweighs their high cost. Typically, the tubes consist of a porous paper or fiberglass support with the membrane formed on the inside of the tubes, as shown in Figure 3.40. 

For ultrafiltration applications, hollow fine fibers have never been seriously considered because of their susceptibility to fouling. If the feed solution is extremely fouling, tubular systems are still used. Recently, however, spiral-wound modules with improved resistance to fouling have been developed these modules are increasingly displacing the more expensive tubular systems. This is particularly the case with clean feed solutions, for example, in the ultrafiltration of boiler feed water or municipal water to make ultrapure water for the electronics industry. Capillary systems are also used in some ultrafiltration applications. 

Because of the effect of the secondary layer on selectivity, ultrafiltration membranes are not commonly used to fractionate macromolecular mixtures. Most commercial ultrafiltration applications involve processes in which the membrane completely rejects all the dissolved macromolecular and colloidal material in the feed solution while completely passing water and dissolved microsolutes. Efficient fractionation by ultrafiltration is only possible if the species differ in molecular weight by a factor of 10 or more. 

Fouling is one of the most prevalent operational problems associated with microfiluation and ultrafiltration applications. The high mechanical strength and chemical as well as structural stabilities of many inorganic membranes (especially the ceramic types) and... 

Membrane materials for reverse osmosis and ultrafiltration applications range from polysulfone and polyethersulfone, to cellulose acetate and cellulose diacetate [12,18-23]. Commercially available polyamide composite membranes for desalination of seawater, for example, are available from a variety of companies in the United States, Europe, and Japan [24]. The specific choice of membrane material to use depends on the process (e.g., type of liquid to be treated and operating conditions) and economic factors (e.g., cost of replacement membranes and cost of cleaning chemicals). The exact chemical composition and physical morphology of the membranes may vary from manufacturer to manufaemrer. Since the liquids to be treated and... 

In 1983 Celanese began the production of polybenzimidazole (PBI) by using diphenyl isophthalate and 3,3 -diaminobenzidine, DAB. PBI is a high-temperature and flame-resistant fiber, used in the production of safety gloves and various items of protective clothing as well as for the production of PBI-based membranes for reverse osmosis and ultrafiltration applications. 

Dedest Corporation (USA) has announced the production of porous inox steel membrane systems (ultrafiltration applications). 

Major developments of membrane processes using ceramic membranes have been aimed at microfiltration or ultrafiltration applications. Up to now the most important applications for these membranes are found in aqueous media for the separation of particles, bacteria, colloids, macromolecules. Recently, ceramic nanofilters based on sol-gel derived microporous materials have been described [20]. They extend separation capability of ceramic membranes to ions and organics. 

R.O. systems utilizing externally wound tubular membrane element in modular assemblies have been used in the desalination of brackish and sea waters, the treatment and/or concentration of industrial waste waters, the separation/concentration of fluid food, pharmaceuticals and chemical solutions, and the manufacture of water purifiers for domestic use. Generally, externally wound tubular membrane systems have been found to be highly suitable for ultrafiltration applications in the processing Industry and in water pollution control applications. 

The tubular element Is the least susceptible to fouling and the easiest to clean. This element is widely used in ultrafiltration applications where the process streams contain suspended solids. The tubular element can be cleaned not only by chemical action but also by mechanical means. A sponge rubber ball is pumped through the tubular element with the chemical cleaning solution to scour the membrane surface. 

Sivakumar, M., Malaisamy, R., Sajitha, C. J., Mohan, D., Mohan, V., Rangarajan, R. (1999). Ultrafiltration application of cellulose acetate-polyurethane blend membranes, EurPoJjimJf, 35,1647-1651. 

Arthanareeswaran, G. and Thanikaivelan, P. 2010. Fabrication of cellulose acetate-zirconia hybrid membranes for ultrafiltration applications Performance, structure and fouling analysis. Separation and Purification Technology lA 230-235. 

Hamza A, Pham VA, Matsuura T, Santerre JP. Development of membranes with low surface energy to reduce the fouling in ultrafiltration applications. J. Membr. Sci. 1997 131 217. 

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. 

The parylene-coated membrane prepared shows higher diffusion values than conventionally prepared materials. In addition, the platelet activation and the adhesion are suppressed after coating with parylene. This indicates an improved biocompatibility. Potential proposed uses are ultrafiltration applications [121]. 

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