Wine Filtration
Membrane Filters for wineries
Traditional membrane filters, also referred to as cartridge filters, are the perpendicular flow membranes used for the sterile filtration of wines just prior to bottling. They are called membrane filters since they collect particles at the surface. Membrane filtration is different from depth filters. Membranes allow for a more precise pore size differentiation, so the different levels of product flow can provide the winemaker with choices, depending on the need for clarification. Depth filters rely on the fibrous nature of the path to trap these particles in a gradation of the weave fibers through the filter, starting from coarse to fine from entry to exit.
Membrane Materials
Membrane filter cartridges are constructed out of a wide range of synthetic materials, including cellulose acetate, cellulose nitrate (collodion), polyamide (nylon), polyvinylidene difluoride (PVDF), polytetrafluoroethylene (PTFE), polyethersulfone (PES), and polypropylene (PP).
Membrane Pore Sizes
Membranes are manufactured in a range of different pore sizes and an effective way of maximizing their life is to use them in sequence, the wine passing from larger pore size to smallest. The common sizes in membrane filters are 1.2, 1.0, 0.8, 0.65, and 0.45 ?m. Because of their small pore-diameter, membrane filters tend to have a slower flow rate than depth filters. They are also more likely to plug, because most of the filtering occurs at the surface.
Bubble Point Test
Although effective, a membrane filter has limitations based on length of use, the wine matrix, and volume of wine. Therefore, it is critical to perform an integrity test before and after each bottling run to ensure filtration performance. Filter failure due to a rip, tear, and defect, or plugging will be detected by these tests. Arguably, the most common integrity test being performed is known as the bubble point test.
Working Pressure
In a membrane filter the flowing particles are blocked on the membrane surface by the size of the holes which specify the filter mesh characteristic. As wine flows into the cartridge, it leaves microscopic particles on its outer surface. During operation, the accumulation of particles on the surface increases the resistance and causes pressure to build up.
Regeneration of Membrane Filter Cartridges
The membrane cartridge will last as long as it continues to let wine through, while also passing regular integrity testing. The point in which membranes will clog is dependent upon the rough and polish filtration of the wine, as well as the constituents of the wine (e.g., colloids and gums). Typically, membrane filters undergo cleaning (i.e., regeneration) after each daily filtration cycle to remove the soluble polysaccharides, sugars, and proteins trapped by the filter.
Water Regeneration
After filtering wine and while the membrane is still in the filter housing, the membrane is rinsed with cold water (to prevent denaturing and subsequent deposit of proteins), followed by a hot water flush at 40 degrees C (104°F) for 15 minutes, both in the direction of the original flow of wine.
Chemical Regeneration
When a hot water washing cycle does not properly restore flow rate, chemical cleaning may be recommended. Typical chemical cleaners include tri-sodium phosphate (TSP), sodium hydroxide, citric acid solution, peroxyacetic acid (peracetic acid), and many other commercially available clean-in-place (CIP, Section 14.2) chemicals.
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