Technology

Cross-flow membrane filtration technology is quickly gaining global acceptance as an important manufacturing step in many of the process lines in the food, dairy, pharmaceutical/biotechnology and starch and sweetener industries world wide. The ability to produce very specific separations at low or ambient temperatures with no phase change can, in many applications, make membrane filtration a much more cost-effective solution than more conventional methods such as rotary vacuum filtration or filter presses.

Please navigate to the following pages to learn more:

• Cross Flow Filtration
• Membrane Filtration Process
• Membrane Types
• Plant configurations

Membrane filtration is a pressure driven technology with pore sizes ranging from 100 molecular weight to 5 microns. The technologies included in membrane filtration are:

• Reverse osmosis - Reverse osmosis (RO), sometimes called hyperfiltration, describes the tightest of these molecular-level separations. In reverse osmosis, hydraulic force is applied in excess of the natural osmotic pressure of a solution to provide the driving energy for water molecules to diffuse into and through the membrane. Typical operating pressure can be in the hundreds to even a thousand pounds per square inch (25 to 68 bar). RO membranes are generally characterized by their ability to reject sodium chloride (NaCl) at given pressure, temperature, and concentration conditions. Typical rejection values can be on the order of 98 to 99.5%.
• Nanofiltration - Nanofiltration (NF) is the next, more open cross-flow membrane filtration type. In solutions of mixed ionic species, monovalent ions will tend to permeate (pass through) the membrane whereas divalent or multivalent species will tend to be highly rejected at the membrane interface. Since some ionic species, the monovalent ions, are transmitted through the membrane, the difference in chemical potential between the two solutions is less and therefore lower driving forces are required. Hence, typical NF operating pressures may be only one to a few hundred pounds per square inch (7 to 40 bar). NF membranes are generally characterized by their ability to retain a divalent ionic species, often magnesium sulphate (MgSO4) or calcium chloride (CaCl2). Since more variability in applications exist with NF, retention of MgSO4 might range from around 80% to 98%.
• Ultrafiltration - With ultrafiltration (UF), the membranes comprise a discrete porous network. As a mixed solute solution is pumped across the membrane, smaller molecules pass through the pores while larger molecules are retained. We end up with one solution depleted of larger molecules, the permeate stream, and another enriched of larger molecules, the retentate. The open membrane structure means that mass transfer is now more flow dependent than pressure dependent, so operating pressure is further reduced. Typical operating pressures for UF are tens of pounds per square inch to a hundred or so (1 to 10 bar). Membrane classification convention shifts to atomic mass for UF membranes. Molecular weight cut-off (MWCO) is generally expressed in standard dalton (Da) or kilodalton (kDa) units. For example, a 10 kDa membrane would highly retain molecules of that molecular mass or greater while highly permeating smaller molecules. While this system is inherently flawed since the atomic mass does nothing to describe the actual size or geometry of a molecule, it remains the standard convention for UF membrane classification.
• Microfiltration - Finally, microfiltration (MF) describes the coarsest of filtration in the cross-flow membrane filtration range. Membrane porosity is, at last, conventionally depicted as a distance measure generally from a fraction of a micron up to a micron (10-6 m) or so. Large and small molecules then can be separated from very large or complex molecular structures. Typical operating pressures for MF are a few pounds per square inch to perhaps one hundred (0.5 to 6 bar).