Scrubbing Systems
- 1. 1 Scrubbing Systems Dr. Akepati S. Reddy School of Energy and Environment Thapar University, Patiala INDIA
- 2. 2 Categories of Scrubbing Systems Depending on whether liquid is used for scrubbing or not categorized as • Wet scrubbers – liquid is used for scrubbing • Dry scrubbers – either solid sorbent or slurry is used Based on the manner of bringing the gas phase in contact with the liquid phase categorized as • Gas phase contacting scrubbers – use gas stream energy • Liquid phase contacting scrubbers – use liquid stream energy • Wet film scrubbers – use energy of both liquid and gas streams • Combination liquid phase and gas phase scrubbers – use energy of both liquid and gas streams • Mechanically aided scrubbers – mechanically driven rotors are used for the contact
- 3. 3 Categories of Scrubbing Systems Based on gas side pressure drop categorized as • Low energy scrubbers – pressure drop <5” water column – spray towers – particles of >5µ size are removed • Medium energy scrubbers – pressure drop 5-15” • High energy scrubbers – pressure drop >15” – venturi and condensation scrubbers But many scrubbers operate over a wider range pressure drop Depending on the use categorized as • Scrubbers primarily used for collecting particles • Scrubbers primarily used for gaseous pollutants removal Dry scrubbers are categorized as • Dry sorbent injectors (DSI) • Spray dryer absorbers (SDA) – also known as semi-dry scrubbers or spray dryers
- 4. 4 Venturi Scrubbers A gas phase contact scrubber used to collect both particles and gaseous pollutants Has 3 sections: converging, throat and diverging section Exhaust stream is forced to move through throat at extremely high velocity (30-120 m/sec.) • Liquid is introduced into the throat either at the throat or at the entrance of the converging section • At the throat the liquid is sheared from walls and atomized into very small droplets • Gas exiting the diverging section is forced to slow down for removing the liquid droplets • Entrainment separator (cyclonic or mesh pad or blade separator) is used for the removal of entrained liquid droplets – Cyclonic separator when used is connected to the venturi usually by a flooded elbow
- 6. 6 Venturi Scrubbers • Non-wetted (dry throat) approach venturi – liquid is introduced at the throat – in case of high temperature exhaust caking of throat can occur – appropriate for cool moist exhaust • Venturi with wetted approach – liquid is introduced at the entrance of the converging section • Venturi with round throat can not handle larger flows (> 88000 m3/hr) - Venturi with long, narrow, rectangular throat can be used
- 7. 7 Venturi Scrubbers • For handling variable gas flows adjustable throat venturis (with varying throat areas) are used – Venturi with a plunger or adjustable disc moving up and down the throat (decreasing and increasing annular opening of the throat) – Rectangular throat venturis with movable plates that decrease or increase throat area • Venturi rod or rod deck scrubber – a number of pipes parallel to each other in the exhaust flow path create a series of rectangular throat openings
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- 9. 9 Venturi Scrubbers Highest particle collection efficiency (90-99%) among scrubbers • Operated at pressure drops of 5 to 100 inches (gas velocity in the throat is 30 to 120 m/S) – Increasing pressure drop across increases efficiency • Liquid to gas ratios employed are 0.4 to 1.3 L/m3 – Higher ratios do not significantly increase efficiency and lower ratios may not sufficiently wet the throat Removal efficiency for gases is 30-60% • Very short gas-liquid contact time limit gas absorption • Efficient removal occurs if the gas is chemically reactive with or it is highly soluble in the liquid • Lower exhaust velocities and higher liquid to gas ratios can enhance the removal efficiencies – Liquid to gas ratios of 2.7 to 5.3 L/m3
- 10. 10 Venturi Scrubbers Wear or abrasion of the scrubber shell is the primary maintenance problem with venturis – Lining the throat with silicon carbide or using replaceable liner can minimize the problem – A pre-cleaner through removing larger particles can also reduce abrasion problem • Flooded elbow at the bottom of the scrubber can reduce abrasion downstream to the throat Method of liquid injection (spray nozzles/weirs) can also cause problems • Spray nozzles can clog specially with recirculated liquid – Nozzles holes twice the size of openings of the strainers used in the liquid recycle line can minimize clogging
- 11. 11 Ejector Venturi Actually a spray tower capable of moving exhaust without the aid of any blower or fan – Preferred in explosive or extremely corrosive environment High pressure (15-120 psig) and high rate (7-13 L/m3) liquid spray (with a lone nozzle) into throat creates vacuum, sucks exhaust from the side duct and moves through the venturi Entrainment separator is usually used Effective in removing particles of >1µ size but gas absorption efficiencies are very low For extremely high collection efficiencies multiple stages of ejector venturis are used Pressure drop across is 1.3 to 13 cm water Abrasion can be problem in nozzles and throat
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- 13. 13 Orifice Scrubbers Medium energy (5 to 25 cm water pressure drop across) devices used for particals removal (and also gases) Exhaust stream is forced through a pool of liquid at high velocity (15.2 m/S) creating liquid droplets • Larger particles are removed by impingement on scrubber liquid surface - smaller particles by impacting liquid droplets – Baffles/air foils are provided for the turbulent mixing of liquid droplets with the exhaust – Baffles placed in the path of cleaned exhaust stream serve as impingement surfaces for entrainment removal • Sludge accumulated in the scrubber needs liquid circulation through a sludge removal facility
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- 15. 15 Orifice Scrubbers Designed to operate with a specific liquid level for a given gas velocity – Gas velocities should not fluctuate beyond 10-15% of design values – Low gas flows or reduced liquid levels reduce atomization and particle collection efficiencies – High gas flows can blow the liquid chamber dry Designed for maximum exhaust flow rate and makeup air is introduced when flows become less
- 16. 16 Orifice Scrubbers Maintaining water level at desired level (in the face of sludge separation and removal) is an important and difficult maintenance problem Plugging and scale buildup are not problems – Reactive scrubbing (gaseous pollutants chemically react with liquid) can still produce scales and plug the internals Collection efficiencies are moderate and used for the removal of >1 µ particles Liquid gas ratio is – 1.3 to 5.3 L/m3 for particles removal – 0.07 to 0.7 L/m3 for gases
- 17. 17 Plate Towers Effective in removing gaseous pollutants – Particulates can also be simultaneously removed – Can prove good for flue gas desulphurization One or more plates are mounted horizontally inside a vertical column Exhaust enters at the bottom and flows upwards passing through the plates Liquid enters at top and travels across each plate to a down comer and from their to next plate below and ultimately to the tower bottom – Liquid to gas ratio followed is 0.7 to 2.0L/m3 These are medium energy scrubbers (2.5 to 20 cm pressure drop per plate)
- 18. 18 Plate Towers Plates are sieve, impingement, bubble cap and valve types • Sieve plates: Have 1800-9000/m2 openings of 0.32-2.5 cm size and form 10-100µ size droplets • Impingement plates: plate openings have impaction targets above to increase the gas-liquid contact • Bubble cap plates: around each hole a riser is provided – over the riser a cap with several slots is provided • Valve plates: have liftable caps above the openings and plates attached to legs limit the caps’ vertical movement Smaller openings can increase the particle collection efficiency but can get plugged Cost wise sieve plates are cheapest, impingement plates come next, and bubble cap plates are the costliest
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- 20. 20 Plate Towers Gas removal efficiencies of > 98% can be easily achieved and for increasing efficiencies following can be done – Increasing number of plates – Using higher liquid to gas ratios – Increasing pressure drop across the plates Particle collection efficiencies are moderate – Decreasing hole size and increasing number of holes increase particle collection efficiencies – – Increasing number of plates beyond 2 or 3 do not increase efficiency significantly
- 21. 21 Plate Towers Susceptible to plugging and/or scale build-up problems – high solid content in scrubber liquor can be a cause – can not be used for exhaust with dust or sticky materials – access to plates for cleaning is important – water sprays underside the lower most plate can avoid wet-dry interface and scaling – low pH wash can dissolve and remove scales Higher liquid injection rates and higher exhaust gas velocities can cause flooding of plates – this in turn increase pressure drop but decreases gas-liquid mixing Too low gas velocities cause weeping (liquid dripping through holes) and decrease the gas-liquid contact Poor scrubbing liquor distribution can be a problem
- 22. 22 Spray Towers Low energy (pressure drop is 1.3 to 7.6 cm water) inexpensive devices Used as pre-scrubbers for removing – larger particles (of size >10-25µ) – highly soluble gases (with 50-80% efficiency - plate towers and packed towers are superior for gas absorption removal) chemical reagent addition to the liquid (KMnO4 for odor removal) can also increase the efficiency Used to handle flows upto 50 m3/sec
- 23. 23 Spray Towers Counter-current, co-current and cross-current configurations are used • Cross-current configurations (horizontal spray scrubbers) can have more than one spray sections – For better results the liquid can flow counter-current and cleanest liquid can be sprayed in the last section • Counter-current configurations can have multilevel liquid spray nozzles • For similar collection efficiencies co-current configuration is smaller than a counter-current configuration – Exhaust relative velocity is higher in the counter-current configuration
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- 25. 25 Spray Towers Exhaust enters the bottom and moves upwards in counter- current flow configuration • Exhaust gas velocity is within 0.3 to 1.2m/sec. to prevent liquid carry over Liquid at pressure is 10-400 psig is sprayed through nozzles downwards into the spray tower • Produces droplets of size 500 to 1000µ (venturis produce 10 to 50µ size droplets) – injection of liquid at 300 to 450 psig pressure creates fog like droplets • Liquid to gas ratios employed are 0.7 to 2.7 L/m3
- 26. 26 Spray Towers • Decreasing liquid droplet size and increasing liquid to gas ratio can increase the absorption removal • Liquid droplets after falling short distance tend to agglomerate into bigger drops or hit walls and comedown affecting efficiency • Completely open design – have least scale buildup and plugging problems • Nozzle plugging or eroding can occur specially with recycled liquid
- 27. 27 Baffled Spray Scrubbers Similar to spray towers both in design and operation – In addition to spray nozzles, baffles (that allow further atomizing of liquid) are added to boost treatment Used mostly as pre-cleaners for the removal of >10µ size particles – Not specifically used for the removal of gases Pressure drop is 2.5 to 7.5 cm water Liquid to gas ratio is 0.13 L/m3 Liquid inlet pressure is <15 psig Maintenance problems are the least – solids buildup on baffles can occur
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- 29. 29 Cyclonic Spray Scrubbers Low to medium energy devices (pressure drop is 4 to 25 cm of water) – Uses the features of both a dry cyclone and spray tower for collecting particulate matter – More efficient than spray towers but less efficient than venturi scrubbers – Particles >5 µ are removed with >90% efficiency – Particle velocity is 60-180 m/sec. comparable to venturis (in spray towers it is 0.6 to 1.5 m/sec. – Turbulence level is much lesser than that generated in a venturi Less efficient in removing gaseous pollutants and not chosen for gaseous pollutant removal
- 30. 30 Cyclonic Spray Scrubbers Include Irrigated cyclones and Cyclonic spray scrubbers Irrigated cyclone – Exhaust enter tangentially near the top into water sprays and swirls downward, then change direction return upward in a tighter spiral Cyclonic spray scrubber – Exhaust enters the chamber tangentially near the bottom, swirl through the chamber moves upwards and exit from top – Liquid is sprayed from nozzles on a central post and directed toward the chamber walls – Straightening vanes are provided at the top of the chamber – Can also act as entrainment separators
- 31. 31 Cyclonic Spray Scrubbers Nozzle plugging and corrosion or erosion side walls are the maintenance problems – Better install the nozzles in a way for ease of access for maintenance – Abrasion resistant materials may be used to protect the cyclone body Liquid to gas ratio employed is 0.3 to 1.3 L/m3 Liquid inlet pressure is 280 to 2800 kPa
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- 33. 33 Packed Tower Scrubber Medium energy scrubbers used for gas absorption – Particles are also removed, but submicron particles not efficiently – Both liquid phase and gas phase provide the energy needed for the scrubbing Used for handling smaller flows and easy to handle gases – For larger flows and difficult gases plate towers are preferred Cheaper but weight is more than that of plate tower Packing material supported over trays is sprayed with liquid to form liquid film coat Exhaust is passed through packing for contact with liquid and absorption removal of gaseous pollutants
- 34. 34 Packed Tower Scrubber Absorption removal is enhanced by – Liquid film surface – Turbulent contact between the liquid and gas – Contact time – Solubility of the gaseous pollutants Higher gas velocity through packing ensures good mixing but can cause flooding of packing Increasing liquid injection increases absorption efficiency but can cause flooding
- 35. 35 Packed Tower Scrubber Common used configurations include counter-current, co- current and cross-flow Counter-current flow configuration – Liquid is introduced at the top of packing by sprays/weirs – Exhaust is entered at bottom and flown through upward Co-current flow configuration – Both exhaust and liquid enter at top and move downward – Can operate at higher liquid & gas flow rates without flooding – Removal efficiencies are limited – For similar flows and absorption efficiencies, diameter of the tower is relatively smaller and pressure drop is lower
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- 37. 37 Packed Tower Scrubber Cross flow configuration – Exhaust moves horizontally while liquid moves vertically – liquid sprays at the face of packing on the inlet side may also be used – packing can be slanted in the direction of incoming exhaust for ensuring complete wetting of packing – can include more than one sections – liquid flow rate can be higher in the front section for particulate washout – last section can be left dry to act as entrainment separator – considered as better for handling exhausts with high particulate concentration – can be designed to have lower pressure drop Fiber bed scrubber with cross flow scheme
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- 39. 39 Packed Tower Scrubber Material used as packing include stoneware, porcelain, metal, high density thermoplastics (polyethylene and polypropylene), etc. – Dimensions can be of 0.6 to 10 cm in size – Smaller size offers larger specific surface area but increases pressure drop Shapes of packing material used – Raschig rings (5 cm) – Tellerette packing (2.5 cm) – Tripacks – Pall rings – Intelox metal – Bari saddle, etc.
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- 41. 41 Packed Tower Scrubber Factors influencing selection of packing material include nature of contaminants, geometric mode of contact, size of the absorber, and scrubbing objectives Cost, pressure drop, corrosion resistance, specific area, structural strength, weight, design flexibility, etc., also influence Packing can be either random or stacking systematically – Random packing provides higher specific surface area but cause higher pressure drop and poor liquid distribution – Stacked packing provides better liquid distribution but installation costs are higher
- 42. 42 Packed Tower Scrubber Proper design of support trays can ensure uniform distribution of exhaust through the packed bed Metals plates or grids used to support the packing also act as distribution baffles Liquid should be distributed over the entire the top surface of the packed bed Channeling of the liquid should be avoided Liquid tend to flow towards tower walls and result in short circuiting – Require redirecting from the tower walls back to the center of the packed column by liquid redistributors placed at intervals of <3 m or at 5 tower diameters from the top
- 43. 43 Packed Tower Scrubber Liquid distribution is by weirs, tubes or spray nozzles Distribution Weirs: – Do not plug and can handle dirty liquids but require leveling – Can be easily inspected and maintained Perforated tubes: – Can be buried within the packing – allows the packing above the perforated tube to act as entrainment separator – Avoids liquid blowing against the side walls of the tower – Tubes (and perforations) can clog and knowing it is difficult Spray nozzles: – Nozzle clogging and erosion can be a problem – Use of fewer relatively large nozzles can avoid the clogging – Can be easily inspected – Pressure drop is the highest among the three
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- 45. 45 Packed Tower Scrubber Buildup of solids in the packing is a serious problem • Particulate matter tend to get removed and accumulate in the packing - pre-cleaning spray can reduce the solids buildup • Buildup could also result from chemical reactions between the liquid and absorbed gases • Liquid draining through the packing carry the collected particles and plug the void spaces • Tower internals are not easily accessible and cleaning requires shutting the system and removing, cleaning and reinstalling the packing
- 46. 46 Packed Tower Scrubber Maintaining proper liquid and exhaust flows is needed - increasing any of the two can result in flooding – Flooded packing result in higher pressure drop, pulsating air flow and dramatically reduced efficiencies – Optimal operating flow rates should be 60-75% of the flooding flow Lower gas flow rates can result in gas channeling Liquid to gas ratio employed is 0.13 to 2.0 L/m3 and liquid inlet pressure is 5-15 psig Pressure drop is 1.7 to 5 cm water per foot or 2 to 8.5 cm of water per meter of installed packing and 5 to 22 cm of water across the packed tower
- 47. 47 Mobile Bed Scrubbers Similar to packed towers but packing is in constant motion – Gas stream powers motion of the packing – Liquid is sprayed over the packing – Bottom sprays are used for saturating the gas (also removes large particles) – Entrainment separators are required Preferred when high collection efficiency of particulates and gaseous pollutants is required Provides effective absorption like packed and plate towers but without plugging problems Because of high gas velocities these are much more compact than packer towers and plate towers but these are not as energy efficient as the latter
- 48. 48 Mobile Bed Scrubbers Based on the degree of movement of the packing mobile bed scrubbers are classified as – Flooded bed scrubbers – Fluidized bed scrubbers Packing used in the scrubbers – Spheres of plastics, glass or marble in flooded beds – Hollow polypropylene/polyethylene balls in fluidized beds Flooded bed scrubbers: – Packing gently moves and rotates – Packing of 10 to 20 depth is used – Bubbles formed in the bed create a layer of froth over the bed twice the depth of packing
- 49. 49 Mobile Bed Scrubbers Fluidized bed scrubbers (also known as turbulent contact absorbers): • Packing is suspended or fluidized within the bed • For this exhaust gas velocity of 1.8 to 4.8 m/sec. is used • packing of 0.3 to 0.6 m depth is used and froth zone is about 0.6 m thick over the packing • More than one fluidized packing sections (upto 6) may be used for efficient gas absorption – If gas absorption is not needed then a single packed section is sufficient
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- 51. 51 Mobile Bed Scrubbers Particulates can be collected – At inlet below the bed by bottom sprays – At the packing by impingement on the wetted surface – In the froth layer over the packing Maintenance problems – Plugging and scale buildup can occur specially at the scrubber inlet and on the packing support grid – Nozzle maintenance is of concern in lime or limestone scrubbing systems – Deterioration of spheres of packing can occur from high temperature and constant rubbing against each other
- 52. 52 Mobile Bed Scrubbers • Pressure drop in mobile bed may range from 5 to 15 cm water per section • Liquid to gas ratio is 2.7 to 8 L/m3 for gases and 0.4 to 0.7 L/m3 for particulates removal • Increasing liquid-to-gas ratio, increasing the depth of packing and increasing the number of packing stages enhance adsorption – Removal efficiencies are 99% for gases – For SO2 removal from flue gases 8L/m3 ratio is used • Particles >2 to 3µ are removed
- 53. 53 Mechanically Aided Scrubbers Motors are used to supply the energy needed for scrubbing More compact but have higher overall power requirement Capable of high particle collection efficiencies and can remove particles of >1µ size – Generally not used for gas absorption Operate at low liquid to gas ratio – 0.07 to 0.2 L/m3 for centrifugal fan scrubbers – 0.5 to 0.7 L/m3 for spray rotor scrubbers Because of moving parts maintenance problems are high – Moving parts are susceptible to corrosion and fouling – Rotating parts are subject to vibrations – induced fatigue or wear cause them to become unbalanced Not used with corrosive or sticky materials
- 54. 54 Mechanically Aided Scrubbers Centrifugal fan scrubbers • Serve as both air movers and particle collection devices • Water is sprayed (at 20-60 psig pressure) on blades to form droplets and to impart centrifugal motion to droplets • Rotating blades force both liquid and particles off the blades towards the walls of the casing • Extra power consumption equivalent to 10.2 to 15.2 cm water occurs from use as particulate collection devices Induced spray scrubbers • Vertical whirling rotors submerged in a liquid pool are used to produce fine droplet sprays • Exhaust is forced to move through the spray • Pressure drop across the device is 10-20 cm water
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- 56. 56 Dry Sorbent Injection Scrubber Used to control acid gas (SO2, HCl, HF, etc.) emission Have advanteges over the wet scrubbers which can be alternative – Release of steam plume is avoided – Requirement of a system of piping and pumps for the collection, treatment and recirculation of scrubbing liquid associated with wet scrubbers is avoided Powdered sorbent is injected into flue gas at – Furnace of the boiler – Duct work or reaction chamber prior to the air pollution control device Reaction chamber increases residence time of acid gases to react with the sorbent
- 57. 57 Dry Sorbent Injection Scrubber Includes two main sections: • Device to introduce the sorbent – consists of – a dry sorbent storage tank – a weight feeder to meter the sorbent – a blower and transfer line – an injection device (venturi) Sorbent is blown through pneumatic line to the injection area Injection is done countercurrent to the flue gas to create turbulence and promote mixing • Particulate matter control device to remove the reaction products, excess sorbent and other SPM of the exhaust – Fabric filters or Electrostatic precipitators are used
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- 59. 59 Dry Sorbent Injection Scrubber Injection at elevated temperature may be needed for the decomposition reaction of the sorbent – CaCO3 decomposition into CaO Efficiency is 50% for SO2 and 90% for HCl in simple systems – can be increased by – Cooling and/or humidifying the flue gas – Recycling portion of the collected particulate matter (has unreacted sorbent) – Operating at higher stoichiometric ratios of sorbent – Introducing an expansion/reaction chamber can increase residence time and efficiency
- 60. 60 Spray dryer systems Used to control acid gas (SO2, HCl, HF, etc.) emission Have advantages over the wet scrubbers which can be alternative – Use lesser quantities of liquid – hot flue gases evaporate the moisture and avoid release of steam plume – Do not require the system of piping and pumps for the collection, treatment and recirculation of scrubbing liquid Mixing of acid gases with alkaline sorbent is more effective than in dry sorbent injectors and can achieve higher removal efficiencies
- 61. 61 Spray dryer systems Typical spray drying includes – Lime storage and slacking system – Lime making and feeding tanks – Atomizer – Spray dryer chamber (absorption tower/reaction chamber) – Particulate control device – Particulate recycle system (optional) Exhaust is introduced into absorbing tower (reaction or drying chamber) for contacting with finely atomized alkaline slurry – Then passed through a fabric filter or an electrostatic precipitator for the particulates removal – Collected solids can be recycled and used in slurry making
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- 63. 63 Spray dryer systems Performance can be affected by – Flue gas flow rate and temperature – Stoichiometric ratio of the alkaline sorbent – Properties of the alkali used For better performance – Gas residence time in the spray dryer should be 10-15 sec. – Spray dryer outlet temperature should be above and close to dew point Amount of water to be used depends on incoming flue gas temperature and moisture content Alkaline slurry loading rate depends on acid gas level in the exhaust and removal efficiency desired Use of 1.5 to 2.5 times stoichiometric ratio can achieve 75- 80% SO2 removal or 95% HCl removal
- 64. 64 Spray dryer systems Rotary atomizers (atomizing wheels) and duel fluid nozzles, are used to inject the alkaline sorbent slurry Rotary atomizers (atomizer wheel) – 8 to 16 inches size wheels of corrosion and abrasion resistant materials – Have higher capacity and simpler piping system and single atomizer is used – Atomation energy is supplied via a motor coupled to the atomizing wheel – Rotational speed is 7000 to 20000 rpm and produce 25-150µ size droplets Duel fluid pneumatic nozzles – Multiple nozzles are used – High velocity and high pressure air is used – Easier to maintain – each nozzle can be isolated – Energy is supplied primarily as high pressure air
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- 66. 66 Spray dryer chamber • Gas and liquid flow scheme may be cocurrent or counter current or mixed – Cocuurent is most commonly used – Counter current dryers have higher drying capacity – Precise control of exit gas temperature is possible in cocurrent dryers • Typical dryer diameter is 25-30 ft. – Design of the dryer chamber is affected by the atomizing method used • Length to diameter ratio for chambers with rotary atomizers is 0.8:1 – droplets should not deposit on walls Spray dryer systems
- 67. 67 Particulate matter collection • Either fabric filters or ESP are used – Fabric filters give better acid gas removal than ESP • Potential heat loss in the particle collection system should minimum – The particulate matter being hygroscopic can cause corrosion problems or plugging – Insulation, hopper heaters and reducing air in-leaks can prevent operational problems Spray dryer systems
- 68. 68 Dry scrubbing systems Except for atomizers, these systems are relatively simple devices and have fewer moving parts Plugging in solid or slurry transport systems is the primary maintenance problem with these systems – Dead areas of solid/slurry piping, valves and atomizers are more prone to plugging – Flexible piping is less susceptible to plugging – T joints & quick connectors allow quick flushing & deplugging – Use of screens ahead of atomizers & valves minimize plugging - screens require frequent cleaning (by acid!) – Atomizers may need frequent flushing with water Draining and flushing of lines and storage tanks during extended downtimes is also helpful Low quality lime and/or water can result in scaling, plugging and reduce acid gas removal efficiencies