Crop Process Engineering

Lesson 27. Types of Filters

27.4 Types of Filters

 27.4.1 Plate and frame filter press: In the plate and frame filter press, a cloth or mesh is spread out over plates which support the cloth along ridges but at the same time leave a free area, as large as possible, below the cloth for flow of the filtrate. This is illustrated in Fig. 7.2 (a). The plates with their filter cloths may be horizontal, but they are more usually hung vertically with a number of plates operated in parallel to give sufficient area.

 Filter cake builds up on the upstream side of the cloth, which is the side away from the plate. In the early stages of the filtration cycle, the pressure drop across the cloth is small and filtration proceeds at more or less a constant rate. As the cake increases, the process becomes more and more a constant-pressure one and this is the case throughout most of the cycle. When the available space between successive frames is filled with cake, the press has to be dismantled and the cake scraped off and cleaned, after which a further cycle can be initiated.

 The plate and frame filter press is cheap but it is difficult to mechanize to any great extent. Variants of the plate and frame press have been developed which allow easier discharging of the filter cake. For example, the plates, which may be rectangular or circular, are supported on a central hollow shaft for the filtrate and the whole assembly enclosed in a pressure tank containing the slurry. Filtration can be done under pressure or vacuum. The advantage of vacuum filtration is that the pressure drop can be maintained whilst the cake is still under atmospheric pressure and so can be removed easily. The disadvantages are the greater costs of maintaining a given pressure drop by applying a vacuum and the limitation on the vacuum to about 80 kPa maximum. In pressure filtration, the pressure driving force is limited only by the economics of attaining the pressure and by the mechanical strength of the equipment.

 27.4.2 Rotary filters: In rotary filters, the flow passes through a rotating cylindrical cloth from which the filter cake can be continuously scraped. Either pressure or vacuum can provide the driving force, but a particularly useful form is the rotary vacuum filter. In this, the cloth is supported on the periphery of a horizontal cylindrical drum that dips into a bath of the slurry. Vacuum is drawn in those segments of the drum surface on which the cake is building up. A suitable bearing applies the vacuum at the stage where the actual filtration commences and breaks the vacuum at the stage where the cake is being scraped off after filtration. Filtrate is removed through bearings. Rotary vacuum filters are expensive, but they do provide a considerable degree of mechanization and convenience. A rotary vacuum filter is illustrated diagrammatically in Fig. 7.2 (b).

 27.4.3 Centrifugal filters: Centrifugal force is used to provide the driving force in some filters. These machines are really centrifuges fitted with a perforated bowl that may also have filter cloth on it. Liquid is fed into the interior of the bowl and under the centrifugal forces; it passes out through the filter material. This is illustrated in Fig. 7.2 (c).

 27.4.4 Air filters: Filters are used quite extensively to remove suspended dust or particles from air streams. The air or gas moves through a fabric and the dust is left behind. These filters are particularly useful for the removal of fine particles. One type of bag filter consists of a number of vertical cylindrical cloth bags 15-30 cm in diameter, the air passing through the bags in parallel. Air bearing the dust enters the bags, usually at the bottom and the air passes out through the cloth. A familiar example of a bag filter for dust is to be found in the domestic vacuum cleaner. Some designs of bag filters provide for the mechanical removal of the accumulated dust. For removal of particles less than 5 μm diameter in modern air sterilization units, paper filters and packed tubular filters are used. These cover the range of sizes of bacterial cells and spores.

Fig. 7.2 Filtration equipment

(a)    Plate and frame press (b) Rotary vacuum filter (c) Centrifugal filter

27.5 Application of filtration

 Filtration efficiency is defined by how well a filter cleans indoor air by removing airborne particles. Low efficiency filters – those that are 25% efficient in removing particles 3 to 10 μm in size – typically are used to keep lint and dust from clogging the heating and cooling coils of HVAC systems. Medium and high efficiency filters – those that are up to 95% efficient in removing particles measuring 3-10 μm in size – typically are used to remove mold, pollen, soot, and other small particles.

 Advances of HVAC:

Using PLCs (programmable logic controllers) in HVAC is the trend nowadays. Companies are adopting wireless technology after they found out that networking HVAC controllers, which often use sensors, can eventually cut installation and labor costs. A lot of engineers are also focused on further improving this technology through the use of mesh wireless setup, which will work for both the wireless sensor and wireless controller networks.

 Advances in filtration technology are making new products possible in food & beverage: Micro filtration has served the food industry in a variety of areas for years, but refinements in membrane technology and a better understanding of the impact membranes have on the molecules that pass through are opening up a new world of possibilities. For e.g., bacteria and spoilage organisms in milk are easily removed by micro filters with pore sizes ranging from 0.1 to 20 microns. Canada's dairy land dairy, now a unit of Saputo, promotes this benefit with pure and fresh micro filtered milk. Ultra filtration units with pores ranging from 0.01-0.2 microns have been shown to affect the appearance and sensory properties of fluid milk because of the protein molecules that can be retained and then added back. Molecules that manage to work their way through the membrane exhibit different organoleptic properties, with a richer mouth feel attributed to the squeezed proteins. A significant body of research on the sensory, nutritionaland bacteria-removal effects of membrane filtration has been compiled in the last decade by David M. Barbano and other food scientists at the northeast dairy foods research centre at Cornell University.

 Application of filtration in cane & beet sugar industries;

• The sugar industry in developed countries has been under pressure for some time due to high-energy and labor costs, and environmental challenges. Many technologies are being constantly explored to improve sugar yields and quality with reduced energy consumption.

• Membrane filtration technology offers economic and technical advantages, when used either as a standalone process or in combination with other more established technologies such as ion exchange and chromatographic separators.

• Ultra filtration/ Micro filtration process in cane sugar production acts as a pretreatment prior to other separation technologies by removing impurities from the raw juice, including starch, gums, waxes, proteins and polysaccharides.

 Application of filtration in starch & sugar industry:

In a very short duration, cross flow membrane filtration has become a mainstream unit operation in the starch and sweetener industry. Membrane filtration processes, namely reverse osmosis, nanofiltration, and microfiltration by their versatility have gained acceptance. Microfiltration of saccharification tank liquor removes unliquified starch, polysaccharides, proteinaceous matter and other impurities. The process has been successfully applied to sweeteners derived from various starch sources–corns, wheat, tapioca, potatoes or cassava. The process eliminates use of diatomaceous earth (kieselgur) in rotary vacuum filters, while at the same time producing a superior quality product. Microfiltration is used for clarification of maltodextrins, depyrogenation of dextrose, final filtration of dextrose and fructose syrups. Reverse osmosis is used for concentration of dilute sugar streams and in some cases as a pre-concentration step prior to an evaporator.