Mass filter

- Is no longer used
- The filter mass consists of cotton linter to which 1% asbestos was added to obtain a sharper filtration
- The costs are very high;
- After every filtration the filter must be unpacked
- The mass cake must be taken apart, washed & sterilized
- The filter mass must be pressed into a filter mass cake
- The filter must be reassembled again

Types of filter

The following types of filter are used for filtration Industry;

- Powder filters – filter aid powder is used in sheet filters (plate & flame filter), candle filters, screen filters (horizontal or vertical), leaf filter
- Sheet filters without filter aid (plate filters)
- Chamber filters (e.g. mash filter)
- Sectional filter presses (e.g. mass filters)
- Membrane filters

Filter aid

- Filter aid are powdery substances, such as kieselguhr or perlite, which are deposited on a filter (tissue or layer) & as a result of their shape & arrangement make filtration possible
- Filter aids can not be used without a supporting filter
- Filter aids used in the present are Kieselgurh and Perlite

Kieselguhr
- Is the term used for fossils of diatoms (single cell algae containing silicon dioxide, SiO2) of which there are more than 15,000 types in the sea
- They must have existed for millions of year in isolated seas or regions of the sea in such quantities that their fossils in the course of time covered the bottom of the sea in a very thick layer
- Depend on their processing there are three groups of guhrs;
- Dried kieselguhrs
- The material is dried at about 300 oC & then milled
- So called fine guhrs are produced in this way
- Ignited & calcine guhrs
- The raw material is heated in large rotating ovens at 600 to 800 oC & then screened
- This produces depending on the combustion temperature & sieving, fine & medium guhrs
- Guhrs ignited with a flux additive
- These are produced by recombustion at 1000 oC of the already ignited guhrs with the addition of CaCO3 or other flux additive & the diatom skeletons are thereby cemented together into larger structures which are require for precoating
- The white guhrs are formed in this way
- The finer the guhr, the better it clarifies but at the same time the lower the filtration speed
- An important feature for the economic use of kieselguhr is the wet density
- By this meant the volume which the guhr occupies under pressure
- Kieselguhrs with a wet density less than 300 g/l are most suitable for filtration
- With higher wet densities must expect to use more guhr & to obtain a higher pressure increase
- Kieselguhr usage can vary between 80 to 200 g/hl
- Kieselguhr is not only very expensive, it also result in costs for disposal of the kieselguhr slurry

Perlite
- Is a material of volcanic origin & consists principally of aluminium silicate
- Raw perlite is heated to 800 oC
- The water contained in it expands & leads to swelling & bursting
- The glassy structures produced are milled
- This give rise to a very light, loose powder which weighs 20 to 40% less than kieselguhr
- At low pH values perlite release chalk & iron

Filters

Filter include...

- Sieves of all kinds, e.g. metal sieves, slotted sieves or parallel arrangement of wedge wire in candle filters

Metal or cloth tissue
- The metallic tissues are easier to clean & sterilize
- But modern textile cloths e.g. polypropylene are not use for beer filtration because they can not be sterilized so well

Filter sheets
- Made of cellulose, cotton, kieselguhr, perlite, glass fibres or other materials (asbestos, is no longer used on health safety grounds)
- Filter sheets are now very widely used & are available for very different filtration cut off limits down to sterile filtration
- Particles smaller than the cut off limit may pass through the filter
- The lower the cut off limit the sharper the filtration

Loosely packed beds
- For example small pebble filters for water filtration & layers of powdery filter aids used as “precoats” on filter sheets

Porous bodies
- Such as sintered metal or metal frits as used for atomizing air & to an increasing

Membrane
- These are made of polyurethane, polyacrylate, polyamide, polyethylene, polycarbonate, cellulose acetate & other substances
- The membranes are very thin (0.02 to 1 mm) & therefore applied on large – pored support layers, because they would otherwise tear
- They are manufactured by impregnation, spraying or deposition
- The pores themselves are made;
- By incorporating salts which are later dissolved out again to form pores
- By etching

- Because of the different manufacturing materials used, nowadays membranes with any desired pore size can be made & they can correspondingly filter out materials of any chosen molecular size
- Because these filters contain very fine pores, one speaks of;
- Microfilters when the range is concerned 10-1 to 10-2 mm
- Ultrafilters or nanofilters when the range is concerned 10-3 to 10-4 mm
- Ceramic material with very fine channels are often used for microfiltration

Separation mechanisms

Sieving or surface filtration
- The particles cannot pass through the pores in the filter medium & are retained in a layer which becomes continuously thicker
- The filtration becomes increasingly fine but the volume flowing through decreases continuously
- Cross – flow filtration belong to this type of filtration

Depth filtration
- Increasingly, separation media are used which consist of very porous materials & which as a result of their very large surface & labyrinthine structure compel the liquid to take a very circuitous part
- The particle are thereby held back by a mechanical sieving effect because of their size
- The gradually block the pores & thereby decrease the flow rate through the filter or
- The particles are fixed by adsorption
- This adsorption occurs because of difference in electrical charges between the filter & the material retained
- Sieving & adsorption effects usually occur together

Alternative filtration methods

- Filtration is a process in which a turbid liquid (unfiltered liquid) is separated by a filter into a clear filtrate & a filtered residue or filter cake is left behind
- The driving force for this is always a pressure difference between the filter inlet & filter outlet
- The pressure at the inlet side is always greater than the pressure at the outlet side
- The greater the pressure difference the greater the resistance with which the filter opposes filtration
- It rises greatly towards the end of filtration

The purpose of filtration

Filtration :

The purpose of filtration is to make product so stable that no visible changes occur for a long time & so the product looks the same as when it was made.