PF - Plate and Frame SW - Spiral Wound

PS - Pleated Sheet T - Tubular (including wide channel)

FS - Flat Sheet HFF - Hollow Fine Fiber

PF - Plate and Frame SW - Spiral Wound

PS - Pleated Sheet T - Tubular (including wide channel)

FS - Flat Sheet HFF - Hollow Fine Fiber

Since the majority of UF membranes have dense surface layers, it is difficult to characterize them with a true pore size distribution. Therefore, polymeric UF membranes are described by their ability to retain or allow passage of certain solutes. The MWCO values for UF membranes can range from as low as 1000 dalton (tight UF) to as high as 200,000 Dalton (loose UF). This roughly corresponds to an "equivalent" pore diameter range from about 1 nanometer (nm) to 100 nm (0.1 fim) as described in Ref. 10.

Different membrane materials with similar or identical MWCO value may show different solute retention properties under otherwise similar operating conditions. If adsorption effects are negligible, such a result can be attributed primarily to the differences in their pore size distributions. This is illustrated in Fig. 3. It can be seen that, although the two membranes are rated by the same MWCO value, their retention characteristics are distinctly different (sharp versus diffuse).

Polymeric cross-flow filters are available in many geometries. These are listed in Table 6. It is obvious that no single geometry can provide the versatility to meet the broad range of operating conditions and wide variations in properties. Some cross-flow filters such as cartridge filters have low initial capital cost but high replacement costs and tubular filters may show longer service life but higher operating costs. The optimization of CFF for a specific application may depend on economic and/or environmental factors and is almost impossible to generalize.

Mwco Curve Membrane Inorganic

Molecular Weight

Figure 3. Rejection coefficient as a function of molecular weight cutoffof an ultrafiltration membrane.

Molecular Weight

Figure 3. Rejection coefficient as a function of molecular weight cutoffof an ultrafiltration membrane.

Table 6. Polymeric Cross-flow Filters: Module Geometries Module Geometry_Special Features/Remarks

Flat Sheet Hollow Fine Fiber

Plate and Frame Pleated Sheet

Spiral Wound Tubular

Tubular (Wide Channel)

Typical spacing between sheets Is 0.25 to 2.5 mm and are used for laboratory evaluations (small surface area modules).

The Internal diameter generally ranges form 0.25 to 1 mm. This type of module geometry cannot handle large amounts of suspended solids or fibrous materials.

Flat sheet membrane elements are assembled in plate and frame devices to handle larger processing volumes.

Typical spacing between sheets is 0.25 to 2.5 mm. The sheets are enclosed in cylindrical cartridge. Not suitable to handle high solids.

Typical spacing between the membrane sheets Is 0.25 mm. Not suitable to handle high solids.

The internal diameter can range from 2 to 6 mm. Suitable for handling higher solids loading.

The Internal diameter Is typically greater than 6 mm and can be as high as 25 mm. The advantage is lower pressure drop and ability to handle high solids/fibrous materials at the expense of higher energy cost.

4.2 Inorganic Microfilters and Ultrafilters

Cross-flow membrane filters made from inorganic materials, primarily ceramics and metals, utilize entirely different manufacturing processes compared with their polymeric counterparts.[3] Although carbon membranes do not qualify under the inorganic definition, they will be included here due to the similarities with inorganic membranes with regard to their material properties such as thermal, mechanical and chemical resistance as well as similarity in production techniques. Table 7 lists the various commonly used materials and membrane geometries in MF and UF modules.

Commercial ceramic membranes are made by the slip-casting process. This consists of two steps and begins with the preparation of a dispersion of fine particles (referred to as slip) followed by the deposition of the particles on a porous support.'111

A majority of commonly used inorganic membranes are composites consisting of a thin separation barrier on porous support (e.g., Membralox® zirconia and alumina membrane products). Inorganic MF and UF membranes are characterized by their narrow pore size distributions. This allows the description of their separative performance in terms of their true pore diameter rather than MWCO value which can vary with operating conditions. This can be advantageous in comparing the relative separation performance of two different membranes independent of the operating conditions. MF membranes, in addition, can be characterized by their bubble point pressures. Due to their superior mechanical resistance bubble point measurements can be extended to smaller diameter MF membranes (0.1 or 0.2 jim) which may have bubble point pressure in excess of 10 bar with water.191

Typical pore size distributions of inorganic MF and UF membranes are shown in Fig. 4. The narrow pore size distribution of these membrane layers is evident and is primarily responsible for their superior separation capabilities. The manufacturing processes for inorganic membranes have advanced to the point of delivering consistent high quality filters which are essentially defect free. Inorganic MF and UF membranes also display high flux values (see Table 8) which they owe to their composite/asymmetric nature combined with the ability to operate at high temperatures, pressures and shear rates.

Two kinds of membrane geometries are predominantly used, the tubular multi-lumen and the multichannel monoliths with circular, hexagonal or honeycomb structures. The number of channels can vary from 1 to 60.

Table 7. Inorganic Cross-flow Filters: Membrane Materials and Module Geometries Membrane Material

Manufacturer/ Trade Name

Module Geometry a-A lumina


Titania Zirconia

Ceramic oxides/ Cordierite

Zirconium hydroxide (dynamic)


Stainless Steel

USF/Membralox® Ceraflo®


USF/Membralox® TechSep/Kerasep



Asahi Glass Schott Glass/Bioran

Mott. Pall

Tubular/multichannel monolith

Multichannel Multichannel

Honeycomb monolith





UF with pore diameter 20 nm to 100 nm

MWCO 1000 and 5000 Dalton MWCO 10,000

MF/UF with internal channel diameter of 1.5 to 2 mm

Dynamically formed zirconium hydroxide or Zr(OH)4 - polyacrylic acid membranes for MF/UF

Mostly for UF, although some MF membranes are available

Mostly for MF pore diameters 0.5 Hm and higher

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