Ultrafiltration Processes

Three different membrane processes, ultrafiltration, reverse osmosis, and electrodialysis are receiving increased interest in pollution-control applications as end-of-pipe treatment and for inplant recovery systems. There is no sharp distinction between ultrafiltration and reverse osmosis. In the former, the separation is based primarily on the size of the solute molecule which, depending upon the particular membrane porosity, can range from about 2 to 10,000 millimicrons. In the reverse-osmosis process, the size of the solute molecule is not the sole basis for the degree of removal, since other characteristics of the... 

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. 

Membrane degumming. Membrane separation has also been evaluated as an alternative process to conventional oil refining processing. Ultrafiltration (UF) and nanofiltration (NF) membranes separate phospholipids almost completely, and FFAs, pigments, and other components can also be removed with the phospholipids to a certain extent. Less effort is required in the later processing steps. 

A simple centrifugation or MF step in the primary clarification may directly be succeeded by a secondary clarification or an enrichment step, such as extraction, precipitation, or adsorption. The concentrated product may then be subjected to membrane filtration processes. Ultrafiltration might be done earlier and then followed by extraction or precipitation with salts. Diafiltration units can subsequently be used to remove the... 

Khayet, M. and Matsuura, T. 2003a. Progress in membrane surface modification by surface modifying macromolecules using polyethersulfone, polyetherimide and polyvinylidene fluoride base polymers Applications in the separation processes ultrafiltration and per-vaporation. Fluid Particle Sep. J. 15(1) 9-21. 

Particularly by preparation of SLN from warm microemulsions, emulsifiers are used in high excess and are normally removed by an adequate purification process. Heydenreich et al. investigated different purification processes (ultrafiltration, ultracentrifugation, and dialysis) and studied the cytotoxicity of the purified cationic SLN dispersions beside particle size and zeta potential. The removal of excess polysorbate decreased the toxicity about 10-fold and dialysis was found to be the preferred method to remove the excess of polysorbate whereas particularly ultrafiltration was not as efficient. 

Membranes with diverse structures (porous, dense and composites) and from different materials (regenerated cellulose, polysulfone, polyamide/polysulfone) currently used in filtration processes (Ultrafiltration, Nanofiltration and Reverse Osmosis) and other separation applications were studied. 

Membranes from different materials and with diverse structures (porous, dense, asymmetric, composites) currently used in different filtrations processes (ultrafiltration, nanofiltration, reverse osmosis) and other separation applications as well as the effect of fouling on porous membranes were considered by studying the following samples ... 

In this section, we will briefly describe two membrane processes, ultrafiltration and membrane gas permeation, using configurations where we can assume that the feed mixture region is well-mixed its composition equals that of the concentrate stream. 

Cross-Flow Filtration in Porous Pipes. Another way of limiting cake growth is to pump the slurry through porous pipes at high velocities of the order of thousands of times the filtration velocity through the walls of the pipes. This is ia direct analogy with the now weU-estabHshed process of ultrafiltration which itself borders on reverse osmosis at the molecular level. The three processes are closely related yet different ia many respects. 

The idea of ultrafiltration has been extended ia recent years to the filtration of particles ia the micrometer and submicrometer range ia porous pipes, usiag the same cross-flow principle. In order to prevent blocking, thicker flow channels are necessary, almost exclusively ia the form of tubes. The process is often called cross-flow microfiltration but the term cross-flow filtration is used here. 

K. marxianus var. fragilis which utilizes lactose, produces a food-giade yeast product from cheese whey or cheese whey permeates collected from ultrafiltration processes at cheese plants. Again, the process is similar to that used with C. utilis (2,63). The Provesteen process can produce fragiUs yeast from cheese whey or cheese whey permeate at cell concentrations ia the range of 110—120 g/L, dry wt basis (70,73). 

Pish protein concentrate and soy protein concentrate have been used to prepare a low phenylalanine, high tyrosine peptide for use with phenylketonuria patients (150). The process includes pepsin hydrolysis at pH 1.5 ptonase hydrolysis at pH 6.5 to Hberate aromatic amino acids gel filtration on Sephadex G-15 to remove aromatic amino acids incubation with papain and ethyl esters of L-tyrosine and L-tryptophan, ie, plastein synthesis and ultrafiltration (qv). The plastein has a bland taste and odor and does not contain free amino acids. Yields of 69.3 and 60.9% from PPG and soy protein concentrate, respectively, have been attained. 

A newer juice concentration process, requiring minimal heat treatment, has been appHed commercially in Japan to citms juice concentration. The pulp is separated from the juice by ultrafiltration and pasteurized. The clarified juice containing the volatile flavorings is concentrated at 10°C by reverse osmosis (qv) and the concentrate and pulp are recombined to produce a 42—51 °Brix citms juice concentrate. The flavor of this concentrate has been judged superior to that of commercially available concentrate, and close to that of fresh juice (11). 

The seminal discovery that transformed membrane separation from a laboratory to an industrial process was the development, in the early 1960s, of the Loeb-Sourirajan process for making defect-free, high flux, asymmetric reverse osmosis membranes (5). These membranes consist of an ultrathin, selective surface film on a microporous support, which provides the mechanical strength. The flux of the first Loeb-Sourirajan reverse osmosis membrane was 10 times higher than that of any membrane then avaUable and made reverse osmosis practical. The work of Loeb and Sourirajan, and the timely infusion of large sums of research doUars from the U.S. Department of Interior, Office of Saline Water (OSW), resulted in the commercialization of reverse osmosis (qv) and was a primary factor in the development of ultrafiltration (qv) and microfiltration. The development of electro dialysis was also aided by OSW funding. 

Fig. 25. Reverse osmosis, ultrafiltration, microfiltration, and conventional filtration are related processes differing principally in the average pore diameter of the membrane filter. Reverse osmosis membranes are so dense that discrete pores do not exist transport occurs via statistically distributed free volume areas. The relative size of different solutes removed by each class of membrane is illustrated in this schematic.

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