Low-pressure ultrafiltration

Tanny (9 ) has prepared ultrafilters by depositing hydrous zirconium oxide on pliable porous materials in a fluted configuration, providing a large membrane area in a small volume suitable for low pressure ultrafiltration. 

Permeability measurements The hydraulic water permeability of the membranes under pressure was measured with a low-pressure ultrafiltration cell of Bio- ngineering Model MC-II. The exposed membrane was 12.57 cm. The measurements were carried out at temperatures between 25 and 60 C, and pressures between 1 and 4 atm by using compressed nitrogen gas. 

Low Pressure Ultrafiltration of Sucrose and Raffinose Solutions with Anisotropic Membranes," J. Pky. Ckm., 238 (1972) coauthors R. W. Baker and H. Strathmann. 

When fouling is present or possible, ultrafiltration is usually operated at high Hquid shear rates and low pressure to minimize the thickness of the gel polarization layer. 

The most common membrane systems are driven by pressure. The essence of a pressure-driven membrane process is to selectively permeate one or more species through the membrane. The stream retained at the high pressure side is called the retentate while that transported to the low pressure side is denoted by the permeate (Fig. 11.1). Pressure-driven membrane systems include microfiltration, ultrafiltration, reverse osmosis, pervaporation and gas/vapor permeation. Table ll.l summarizes the main features and applications of these systems. 

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. 

The types of hollow fiber membranes in production are illustrated in Figure 3.32. Fibers of 50- to 200-p.m diameter are usually called hollow fine fibers. Such fibers can withstand very high hydrostatic pressures applied from the outside, so they are used in reverse osmosis or high-pressure gas separation applications in which the applied pressure can be 1000 psig or more. The feed fluid is applied to the outside (shell side) of the fibers, and the permeate is removed down the fiber bore. When the fiber diameter is greater than 200-500 xm, the feed fluid is commonly applied to the inside bore of the fiber, and the permeate is removed from the outer shell. This technique is used for low-pressure gas separations and for applications such as hemodialysis or ultrafiltration. Fibers with a diameter greater than 500 xm are called capillary fibers. 

The goal of most of the early work on reverse osmosis was to produce desalination membranes with sodium chloride rejections greater than 98 %. More recently membranes with lower sodium chloride rejections but much higher water permeabilities have been produced. These membranes, which fall into a transition region between pure reverse osmosis membranes and pure ultrafiltration membranes, are called loose reverse osmosis, low-pressure reverse osmosis, or more commonly, nanofiltration membranes. Typically, nanofiltration membranes have sodium chloride rejections between 20 and 80 % and molecular weight cutoffs for dissolved organic solutes of 200-1000 dalton. These properties are intermediate between reverse osmosis membranes with a salt rejection of more than 90 % and molecular weight cut-off of less than 50 and ultrafiltration membranes with a salt rejection of less than 5 %. 

The effect of the gel layer on the flux through an ultrafiltration membrane at different feed pressures is illustrated in Figure 6.7. At a very low pressure p, the flux Jv is low, so the effect of concentration polarization is small, and a gel layer does not form on the membrane surface. The flux is close to the pure water flux of the membrane at the same pressure. As the applied pressure is increased to pressure p2, the higher flux causes increased concentration polarization, and the concentration of retained material at the membrane surface increases. If the pressure is increased further to p3, concentration polarization becomes enough for the retained solutes at the membrane surface to reach the gel concentration cgel and form the secondary barrier layer. This is the limiting flux for the membrane. Further increases in pressure only increase the thickness of the gel layer, not the flux. 

Ultrafiltration 1 Low pressure exclusion chromatography Cut-off membrane 10000 Da 1 Sephadex LH-20 RP-HPLC Ultrasphere ODS column Absorbance at 214 nm and fluorescence of the OPA derivatives Acedo et al. (1994)... 

The magnitude of the Peclet number indicates the importance of the convective relative to the diffuse process for solute transport. The solute concentration profiles for representative values of Pe are illustrated in Fig. 12.2 according to Bungay [7]. When diffusion is dominant (Pe 0) the concentration varies nearly linearly in z. For large absolute values of the Peclet number, diffusion is significant only in a thin zone adjacent to the low pressure face of the membrane in which the concentration profile is very steep. For micro- and ultrafiltration membranes, the solute concentration varies little from the value at high pressure face. For nanofiltration the Peclet number can vary considerably depending on membrane characteristic almost dense or porous membranes. 

Nanofiltration is one of promising technologies for the treatment of natural organic matters and inorganic pollutants. Low-pressure operation of nanofiltration is possible [115, 116]. The nanofiltration system, which has a pretreatment process of microfiltration or ultrafiltration, may be applicable to drinking water treatment. 

Replacing the ultrafiltration, nanofiltration pretreatment and reverse osmosis by BAHLM processes in desalination industry is the main idea of this proposal. Rejection characteristics of natural organic matters and inorganic salts in a low pressure nanofiltration (e.g., >99% at 1.5 MPa [115]) and capacity of polyanions to complex monovalent and especially bivalent cations [92, 95, 115-117] make this idea promising. 

The present contribution describes a novel low pressure, high flux system which utilizes an "in situ" dynamically formed silica membrane particularly suited for the ultrafiltration of emulsions. The support for this selective layer of silica was a pleated, thin channel crossflow module il (tradename "Acro-flux", Gelman Sciences, Inc.) containing 0.1 m of 0.2 urn pore size acrylonitrile copolymer membrane. 

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