BASIC PRINCIPLE OF ULTRAFILTRATION

Ultrafiltration is a pressure driven membrane separation process in which water and low molecular weight substances permeate through a membrane while particles, colloids, and macromolecules are retained. The primary removal mechanism is size exclusion, although the electrical charge and surface chemistry of the particles or membrane may affect the purification efficiency. Ultrafiltration pore ratings range from approximately 1000 to 500,000 Da, thereby making UF more permeable than Nano filtration (200 to 1000 Da). Since only high molecular weight species are removed in UF, the osmotic pressure differential across the membrane surface is negligible. Low applied pressures are, therefore, sufficient to achieve high flux rates from an ultrafiltration.

Membrane Modules

Generally ultrafiltration membranes are available in four basic modules. These are tubular, hollow fiber or capillary, flat sheet and ceramic monolith. Commercial diameters of the tubes are 5 to 25 mm. Tubular configurations are characterized by least susceptibility to plugging, small replacement area and ease of cleaning. Fibers range from a few mm down to 250 um. The advantage of hollow fiber configuration is a large membrane area per unit volume and the possibility of operating with back pressure for flushing the module. Fiat sheets membranes are most common, made into spiral wound modules, cassettes and plate and flame devices.

ULTRAFILTRATION MEMBRANES

The important characteristics for membrane materials are porosity, morphology, surface properties, mechanical strength and chemical resistance. Polymeric materials, viz. polysulfone, polypropylene, nylon 6, polytetrafluoroethylene (PTFE), PVC, acrylic copolymer, etc. have been used successfully as UF membranes. These membranes usually have two layers: a thin (0.1 to 0.5 um), semipermeable membrane made of cellulose ester and a substructure support material. During manufacturing, the membranes are cast onto the membrane support. Only the layer of semipermeable membrane is in contact with the sample during ultrafiltration. The support material below the membrane does not affect the filtration characteristics of the membrane. Species smaller than the rated molecular weight cut off (MWCO) of the membrane are capable of passing through the membrane. UF membranes are composed of a polymer such as polysulphone or polyamide, which is usually extruded into fíat sheets or hollow fibres or cut into disks as required by the specific application. A small disk of UF membrane may be subject to rapid fouling and produce a low flow rate for many processes. As a result, UF membranes are typically arranged in a configuration which maximizes surface área and reduces fouling by using

FLUX EQUATION FOR ULTRAFILTRATION

The flux equation for diffusion of solvent through the membrane in ultrafiltration is the same as that for reverse osmosis.

Jw = AW(∆P – π)

ln ultrafiltration, solutes are generally macromolecular in nature. So, the membrane does not allow the passage of solutes. The concentration in moles/1 of the large solutes is usually small. Henee the osmotic pressure is very low and can be assumed negligible. Equation becomes

Jw = AW(∆P)

Ultrafiltration units operate at about 5 to 100 psi pressure drop compared to 400 to 2000 psi for reverse osmosis. Since the high molecular weight solutes are retained on the membrane surface, a concentration gradient builds up