Page 10 - Filtration Resources

Have you noticed how your membrane performance is affected by the operating conditions? Depending on the type of membrane separation process, operating conditions may include hydraulic pressure, osmotic pressure, temperature and feed cross flow velocity. Operating conditions can affect both permeate flux and solute rejection. Among these parameters, permeate flux is very sensitive to the feed temperature. Permeate flux increases as the feed temperature increases. This is mainly due to the decrease of feed viscosity with an increase in the feed temperature.  More specifically permeate flux typically increases as temperature increases in a linear relationship with viscosity as described below (1):

In this equation, J is the permeate flux through the membrane,  m is the feed viscosity, J₀ is the permeate flux at a reference temperature, and μ₀ is the viscosity at the same reference temperature. J₀ and µ₀ are constants and are ordinarily defined by the membrane

Sterlitech offers bench-scale membrane test cells that can be used for a variety of filtration applications. The Developer series are the largest cells used with Sepa size flat sheet membrane coupons, i.e. membrane active area of 140 cm²/24 in². The Explorer series is used with CF042 size flat sheet membranes, i.e. membrane active area of 42 cm²/6.5 in². The Innovator series are the smallest cells used with CF016 size flat sheet membranes, i.e. membrane active area of 20.6 cm²/3.2 in². All three series of cells are available in a  variety of material of construction, including: Stainless steel, Hastelloy, Delrin, PTFE, Acrylic.

These are available in models suited for Cross/tangential Flow, Forward Osmosis, or both processes. For Cross Flow applications, the feed solution is circulated tangentially to the membrane filter. Molecules or materials which are smaller than the membrane's molecular weight cut-off (MWCO) or porosity pass through the membrane as permeate and the remainder

Because they have similar outward appearance, it is easy to presume that the various microporous membrane filters and flat sheet membrane filters offered by Sterlitech have similar pore structures.  Some consider symmetry to be the epitome of beauty, but with respect to filtration, it’s all relative.  In reality, there is a rich variety of pore structure morphologies resulting from the techniques and materials used to manufacture the membranes.

For the microporous membrane filters, the pore structures range from the essentially two-dimensional screen-like structures of the polycarbonate and polyester track-etch membranes, to the node and tendril structures of the expanded PTFE membranes, to the entangled nanofiber structure of the polyacrylonitrile (PAN) membrane, and to the convoluted lacey foam-like structures of the solvent cast membranes such as cellulose acetate (CA).

Some solvent cast microporous membranes, such as Nylon, have essentially symmetric pore structures where the

Sterlitech offers a broad variety of polyamide active layer thin film composite (TFC) reverse osmosis (RO) and nanofiltration (NF) flat sheet membranes.  It is quite common for these membranes to have visually apparent surface imperfections, most notably small dark spots, resulting from the manufacturing process.  Some inexperienced users may be concerned that these spots are mold or some other contamination, however, these are almost always the result of trace residual amounts of polyamide monomer that have reacted to ambient light.  While not visually appealing, you can rest assured that these spots are not a source of contamination and do not affect membrane performance.  Users should resist the urge to clean these spots as rubbing the membrane surface will cause damage.

Other types of common surface imperfections are light areas or spots that become visually apparent when observing membranes with strong transmitted light.  These imperfections are inherent to TFC membranes