Beer Filtration
Depth Filters
Pressure-leaf, candle, lenticular disc, and plate and frame filters, are the most common forms of depth filters used in filtering beer.
Pressure Leaf Filters
Pressure leaf filters, sometimes referred to as powder filters, consist of 10 to 60 leaves, stacked inside a vertical or horizontal vessel (Figure 15.1). The leaves are drainage systems that support a wire or textile cloth cover on both sides. The screen provides the support of the diatomaceous earth cake during the filtration. In a vertical leaf filter both sides of the support are coated with filter aid whereas in the horizontal leaf filter only the upper surface is used. The kieselguhr adheres to the stainless-steel septa because of the pressure at which the beer is forced into the filter.
Diatomaceous Earth
Pressure leaf filters use diatomaceous earth (DE, kieselguhr) as the filtration medium. There are various grades of DE depending on the fineness or particle size, which ranges from about 2.5 to 38 µm. The finer particle size produces a more polished filtration. Loose cellulose fibers, treated to have a positive charge, may be added to facilitate the adsorption of colloidal materials.
Safety and Disposal. Diatomaceous earth is very dangerous when inhaled; it can lead to irritation of the lungs and even long-term lung damage. It is considered carcinogenic to human beings and can also cause silicosis, a progressive and sometimes fatal lung disease, following long exposure to silica dust.
Perlite
An alternative to DE is perlite which consists of thermally expanded volcanic glass, crushed to form microscopic flat particles. Perlite is less efficient filter aids than DE but are perceived as being safer than DE. However, perlite lacks the remarkable skeletal structures of the diatoms that comprise DE and as a consequence, its filtration performance is not as good. In order to achieve the required filtration performance, secondary filtration (e.g., sheet or cartridge filters) is required.
Operation
Normally, use of DE in filtration is a two-step operation. First, a thin layer of clean filter aid is coated on the filter element (cloth or wire screen) by recirculating a dilute slurry of filter aid through the filter. This is referred to as precoating the filter. The precoat serves two purposes; it protects the filter element and also produces almost instant clarity when the filter goes on line. Following precoating operation, a small amount of filter aid (body feed) is continuously added to the liquid being filtered. The first 20 to 40 liters of filtered juice will have an earthy taste (from the DE) and is typically discarded.
Candle Filters
The candle filter is a type of precoat or pressure filter and is predominantly used for clarification of beer that has been fermented and lagered, although other applications, such as sterile filtration or wort filtration, are possible (Figure 15.2). The candles are mounted inside a cylindrical pressure vessel that receives unfiltered beer. The candle itself is the septum that holds the filter aid, which could be diatomaceous earth, perlite, cellulose, or other materials, during the solid–liquid separation.
Lenticular Disc Filters
A lenticular filter, also referred to as a disc filter, is simply a circular housing or canister that is secured to a base plate with a clamp closure (Figure 15.3). Built to housings hold circular stacks of filter discs, sometimes called frisbees, either 12 inches or 16 inches (30–41cm) in diameter. Several will be contained as a single package, hence the term stack. Discs are inserted, depending on the amount of beer to be filtered. Normally a 12-inch diameter stack contains approximately 18 square feet (1.67 sq. m) of surface area or roughly the equivalent of 12 pads in a 40 cm x 40 cm plate and frame.
Lenticular Module Sterilization
Lenticular modules are operated in much the same way as filter sheets. In preparation for filtration, the filters should be thoroughly rinsed with ambient water in a forward direction at a maximum of 1.5 times the operating flow rate for 5 to 10 minutes.
Lenticular Module Regeneration
Once the maximum differential pressure is reached, or the filtration batch is completed, the housing may be flushed with particle-free, ambient or warm water. If a back-flushable cartridge is used, reverse flow should be limited to half the operating flow rate and no more than 7 pounds per square inch (psi) (0.5 bar).
Plate and Frame Filters
Plate and frame filters, sometimes called pad or sheet filters, contain a series of plates that hold the pads between them (one plate, one pad) (Figure 15.4). Although plate and frame filters are more commonly used with filter pads (also commonly referred to as sheets) as the filtration medium, they can also can be setup using DE as the filtering medium. These plate and frame filters are equipped plastic or stainless-steel filtering plates for DE. The units are set up with multiple plates within the frame, and each plate is fed by a common manifold which provides access ports to all other plates.
Plates
The plates are generally square in shape and hang from two supporting bars at each side of the filter stack. The number of plates that are used determines the filter area and the number may be adjusted for the filtration run. Plate and frame filters are generally sold as holding 20, 40, 60, 80, or 100 plates. Units are usually sold in multiples of 20 plates.
Pads
Most pad filters are made of a blend of cellulose fibers, sometimes with the addition of granular components, such as DE, perlite, polyethylene fibers, and resins. The resins provide strength and also impart a zeta potential, a positive charge that increases the collection of small, negatively charged particles and microorganisms. The common dimensions of the commercial pads are squares of 20 x 20 or 40 x 40 centimeters.
Filter Operation
Before closing and compressing the plates for new filtering, wet the cloth pads with water. Check the edges of the pads before closing the plates, to assure that no bent pad is caught between the plates. If this happens, the filter will leak when the pressure rises. In preparation of filtration, the filter sheets must be thoroughly rinsed in a forward flow. Optimum rinsing temperature is 20 degrees C (68°F) at a capacity of 1.5 to 2.0 hL/m2/hour (7.1–9.5 gallons/square foot/minute (1.5-2.0 hL/m2/hour) with particle-free, soft brewing water.
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