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Filtration

Filtration. By Sidra Jabeen Department of Chemical Engineering, University of Engineering & Technology Lahore. Lecture includes:. What is filtration? Understanding of the basic terms How filtration is done? Types of filtration Rate of filtration Filter Media Filter Aid. Filtration.

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Filtration

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  1. Filtration By Sidra Jabeen Department of Chemical Engineering, University of Engineering & Technology Lahore

  2. Lecture includes: What is filtration? Understanding of the basic terms How filtration is done? Types of filtration Rate of filtration Filter Media Filter Aid

  3. Filtration Removal of solidparticlesfrom a fluidby passing it through a septumon which the solids are deposited

  4. 1. Solid Particles Concentration Valuability Solubility Size Traces to High % Dissolved Desired Waste Undissolved Ultra filtration Micro filtration Nano Filtration

  5. 2. Fluids A continuous substance which can Flow Liquid Gas

  6. 3. Septum or Filtering Media Membrane Filter Paper

  7. 4. Solid Deposition In the form of cake on the septum Or Trapped inside the pores of the septum

  8. Example • The solid remaining in the filter paper is called the residue. • The residue can be dried by spreading it out on the filter paper and allowing the liquid to evaporate. • The liquid which has passed through the filter paper is called the filtrate.

  9. How filtration is done? Through Feed Side • Pump • Blower • Liquid height of column etc Through Product Side • By generating Vacuum Fluid flows through the filter medium by virtue of pressure difference across the medium.

  10. Types of filtration Two basic types of filtration are Cake filtration  Solid particles form a layer of solids on filtering media (called as cake). Deep or Depth bed filtration  Solid particles are caught inside the pores of filtering media.

  11. 1. Cake Filtration Solid particles are retained on septum as a layer of solids called as “cake”

  12. 2. Deep Bed Filtration Further of two types Clarifying filtration Cross flow filtration

  13. a. Clarifying Filtration Solid particles are trapped inside the septum

  14. b. Cross flow Filtration Some of the liquid passes through the septum leaving behind conc. solution

  15. Factors affecting Rate of Filtration Drop in pressure from the feed to the far side of septum Area of the filtering surface Viscosity of the solution Resistance of the filter cake

  16. Filter Media Common names • Filter Paper • Membrane • Screen • Porous Solids Desired Properties • Good Filtration • Should not Plug • Chemically Resistant • Physically Strong • Cheap

  17. Material of construction of filter media • Woven Materials (Cotton, Wool, Silk etc) • Perforated Metal Sheeting • Granular Materials (Sand, Asbestos etc) • Porous Solid • Felted-Fiber materials (Porous Paper)

  18. Filter Media Structure • Cloth • Metal

  19. Filter Aids • Filter aids are granular solids which are used in combination with filtration to enhance rate of filtration. For example, Diatomaceous silica, perlite, purified wood cellulose etc. • Filter aids are used in two different ways: • Added before filtration • As a pre-coat

  20. Filter Aids • Added before filtration This increases the porosity of the cake and reduces resistance of the cake during filtration. • As a pre-coat They can be used as a precoat before the slurry is filtered. This will prevent solids from plugging the filter medium and also give a clearer filtrate. Use of filter aids is usually limited to cases where the cake is discarded.

  21. Types of Filters

  22. Classification of Filters Filters are classified on following three different basis: • Mechanism • The driving force • Operation

  23. On the basis of mechanism Cake filters Clarifying filters Cross flow filters On the basis of driving force Pressure Filters Vacuum filters Centrifugal filters On the basis of operation Continuous filters Batch or discontinuous filters

  24. Classification of filters Pressure filters  Pressure applied at feed side Vacuum filters  Vacuum applied at product side Centrifugal separators  Spinning the slurry to force separation Continuous filters  Flow of fluid and discharge of solids (cake) is continuous or uninterrupted. Batch filters Flow of fluid is continuous but it has to be interrupted to permit discharging of the accumulated solids.

  25. Cake filters They can be pressure filters (filter press, shell & leaf filter) Or they can operate under vacuum (rotary drum filter) Or cake filtration can be carried in centrifugal filters (suspended batch centrifuges) Either type can be batch or continuous.

  26. 1.Filter Press • Pressure filter operate in batch mode • Sets of plates (circular or square) • Plate faces covered by septum • Slurry enters at one end • Solids (cake) collect in chamber and liquid pass out • ‘Jammed’ filter • Wash liquid, air or steam used to remove residual liquid • Press opened and cake removed • Washing is time consuming and labor intensive

  27. Filter Press Pink: Plates Yellow : filter Cloth Blue : Filtrate

  28. Filter Press

  29. 2. Shell and leaf filter • Pressure filter operating in batch mode • Stack of leaves inside the closed tank • Feed enters in the side of the tank • Pass through leaves covered with filter • Liquid leaves into discharge manifold • Cake is removed by gravity force and vibration • Cake is effectively removed and economize labor

  30. 3. Shell and leaf filter

  31. Shell and leaf filter cake disposal

  32. 3. Rotary Drum Filter • Vacuum filter with continuous operation • Drum turns at 0.1 – 2 rpm in slurry trough • Filter covers the face of the drum • Rotary drum filter has four zones: • Filtering zone • Washing and drying zone • Cake removal zone • Dead zone • Vacuum is applied in first two zones • Permeate and washing liquid discharge into separate compartments • Doctor blade is used in third zone

  33. Rotary Drum Filter

  34. Rotary Drum Filter

  35. Rotary Drum Filter

  36. Rotary Drum Filter

  37. 4. Suspended Batch Centrifuge • Centrifugal filter operating in batch mode • Perforated baskets 750 – 1200 mm in dia. and 18 – 30 inch deep • Basket rotates at 750 – 1200 rpm • Feed taken from inlet is centrifuged • Clear liquid pass out leaving behind solid cake in basket • Wash liquid sprayed to remove soluble solids and cake dried • For cake removal, basket is turned at 30 – 50 rpm and cake is cut off with unloader knife

  38. Suspended Batch Centrifuge

  39. Clarifying Filtration

  40. Clarifying Filtration Principle of Clarification • Particles are trapped inside the filter media or on its surfaces • Caught by the surface forces • Reduce the active dia. of the channel Removal of the Small amounts of solids or liquids droplets from the fluids.

  41. Liquid Clarification: Cartridge Filter Series of thin disks with close clearances in a vertical stack Disks carried on a vertical hollow shaft Liquid admitted under pressure Trapping of the solids between disks Liquid flows out through the casing Comb cleaner pass between the disks and drop solids to the bottom of casing

  42. Cartridge Filter

  43. Gas Clarification For atmospheric dust – Pad filters For process dusts – Bag Filters

  44. Pad Filter • Air passed through pads of filter media • Pad may be dry or coated with viscous oil • For light duty – pads are disposable • For heavy duty – reused by rinsing and recoating After Before

  45. Bag Filter or Bag House • Contains one or more large bags of fabric material inside metal casing • Dust laden gas enters the filter at the bottom • Solid particles trap inside the bag and clean gas leave at the top • Solids are discharged by shaking mechanism

  46. Cross flow Filtration

  47. Definition Filtration in which flow of the suspension is parallel to the filtering media used to get concentrated slurry

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