Very small particles
Download 69.34 Kb.
|
- Navigate this page:
- Small particles
- Upper part of tower
- Water drops large relative to particle size
- Water drops a little larger or smaller than the particles
| Week 13 - Control of Air Pollution
Natural Cleansing of the Atmosphere Physical characteristics - influence natural removal mechanisms Very small particles - bounce around randomly, collide, coagulate, and fall out as large particles Particles with an electrical charge - grow by attracting particles of opposite charge Small particles - act as nuclei within a raindrop Raindrops - collide with particles which adhere to the surface of the drop and fall out Air Quality Control Objective: Maintain an atmosphere in which pollutants have no negative impact on human activities Best way: Don't produce the pollutants to begin with Example: Eliminate lead emissions by burning unleaded fuels Alternative: Add-on devices Example: Automobiles use carbon canisters to absorb hydrocarbons emitted from the carburetor and gas tank Catalytic converters chemically reduce hydrocarbon emissions Other add-ons: Scrubbers and bag houses used in industrial applications Dispersion: Used to control local air quality Shift emissions to tall stacks which allow more time to disperse before reaching the ground Less desirable than pollution elimination or add-on alternatives Example: Electric heating Shifts the emissions from short residential stacks to tall utility stacks in remote areas Particle Emission Control Equipment must handle solid and liquid particles Size: 1 µm to larger than 100 µm Smaller particles - much harder to collect Gravitational Settling Chambers Simple, inexpensive collectors Use the laws of gravity to settle the particles Simple duct that expands, forcing the horizontal velocity to slow to allow time for the particles to settle Forces acting on the particle Horizontal viscous force is negligible Particle moves at the horizontal velocity of the gas stream Simple gravitational settling chamber Efficiency of the settling chamber
SUB g=1-exp[-{u SUB t L} OVER {uH}] Where:
L = Length of the collector, m H = Depth of the collector, m u = horizontal velocity of the gas and particles through the collector, m/sec ut = terminal settling velocity of the particles, m/sec dp = diameter of the particle, m d = density of the particle, kg/m3 g = gravitational constant, m2/sec µ = dynamic viscosity, kg/m-s Example: Estimate the 50 percent cutoff diameter for particles of CaO suspended in an airstream at 100oC and at atmospheric pressure for a gravitational settling chamber 3 m long and 1 m high when the gas velocity in the collector is 1 m/sec. p for CaO = 3310 kg/m3 µ for air = 2.17x10-5kg/m-s (@100oC) Note: The 50% cutoff diameter is defined as the particle diameter at which the efficiency is 50%.
0.5=exp[-{9.81x3310x3xd SUB p SUP 2} OVER {18x2.17x10 SUP {-5}}] d SUB p SUP 2=ln(0.5)x[-{18x2.17x10 SUP{-5}} OVER {9.81x3310x3}] d SUB p = 53µm d SUB p SUP 2=28x10 SUP {-10}m SUP 2 Note: The cutoff diameter is not sharp In practice, particles larger and smaller will be collected due to turbulence and flow variations Inertial Collectors Rely on centrifugal forces to separate the heavier particles from the lighter gas molecules Two types - Skimmers and Cyclones Skimmers - increase the particle concentration in a separated gas stream - remove particles by cyclones or settling chambers Cyclones - Particle laden gases enter tangentially - Forced in a downward spiral motion - Particles migrate outside - Gases forced back up through center
Where: mp = mass of the particle uT = tangential velocity r = radius of curvature p = density of the particle dp = diameter of the particle Removal efficiency varies: directly with diameter cubed directly with density directly with the square of the velocity inversely with the radius Cyclones with large r: traditional Cyclones with r < 7.5 cm: high eff. Uses for inertial and gravitational settlers May be fabricated using metals that can withstand high temperatures and resist corrosion Effective for solid or liquid particles Wet collectors Also called scrubbers Designed to increase particle sizes using water or slurry droplets, because larger particles are easier to collect Many different types (look at 2, conventional and venturi scrubbers) Sketch of an absorber or scrubber Absorber or scrubber Upper part of tower - water droplets collide with and collect particles from the upward-flowing gases Packed section of tower - special shapes are added to increase the area of contact between the liquid and aerosol (gas + particles) Below the packed section - flooded perforated disc which supports several cm of water - allows contact between bubbles carrying particles and water Below flooded disc - water drains through perforations to develop another falling-drop collector section Absorber or scrubber (Continued) Demister collects droplets that are left in the stream Not every collision between water and particles results in collection - due to surface tension of droplet and particle wettability characteristics Chemicals may be added to the water to reduce the surface tension OR to improve the ability of the particles to absorb gases in addition to particles Liquid containing particles is collected at the bottom of the tower and pumped to a settling basin or filter Liquid is recirculated to reduce makeup water and disposal water Difference between a scrubber and an absorber Absorber - designed primarily for gas removal Scrubber - designed primarily for particle removal Performance Size of water drop is critical to performance of the scrubber Water drops large relative to particle size - aerodynamic forces (drag) will displace the particles out of the path of the falling drops - number of collisions will decrease significantly Performance (Continued) Water drops same size as particles - collisions also decrease because drag causes the collecting liquid droplets to move with the gas stream and particles Water drops a little larger or smaller than the particles - optimal Scrubber must be well maintained to ensure that the correct droplet size is achieved Performance also depends on the physical and chemical characteristics of the particles, the collecting liquid, and the final droplet collector (demister) Venturi scrubbers * Gases and particles accelerate in the throat followed by rapid deceleration as expansion occurs * Liquid is injected into the throat - solid stream perpendicular to the gas flow * Relative difference in velocity between gas and liquid tears apart the liquid and forms drops * Particles collide and become entrained * Some additional particle collection in the expansion section * Droplets are removed as large particles as velocity decreases
15 to 30 cm in diameter Up to 10 m in length Woven (most common) or made of felt Fabric - cotton, synthetics, and fiberglass Material selection - depends on gas and particle properties Summary of data on the common filter media used in industrial baghouses Fabric and fibrous mat collectors (Continued) May be designed with multiple cells to allow for maintenance during operation Cloth may have holes exceeding 100 µm across - collector will perform with 99%+ efficiency for particle diameters > 1µm Small particles are trapped in the filter cake As thickness of cake increases, power costs increase If cake becomes too thick, pressure may collapse the cake and make it nonporous (moisture also) Cleaning Remove filter cake by shaking the bag in small collectors Large collectors - reverse the flow of air To avoid frequent shaking and to achieve an effective cake thickness - operate at a volume flowrate of 0.5 to 2 m3/sec per m2 of cloth Pressure drops - 5 to 40 cm of water Shaking periods - 4 to 5 per hour up to once in several hours Bag life - 2 to 3 years Electrostatic precipitators Voltage difference (field strength) between the electrode and the collector plates is maintianed at as high a level as possible (but below spark-over) Electrons are released and attach themselves to particles Particles then migrate to grounded surface due to electrostatic forces Electrostatic precipitators (Continued) Efficiencies as high as 99% for particles > 2 µm at pressure losses of 5 cm of water or less Units built of metal Used extensively on processes that discharge: corrosive gases elevated temperatures very large volumes high percentage of particles >1 µm
Silica gel: 175 m2 per g Regenerated using hot steam Adsorptive capacity of activated carbon
Comparison of Air Pollution Control Equipment Source: Handbook of Environmental Management and Technology, Chapter 8, G. Holmes, B. Singh, and L. Theodore, John Wiley and Sons, Inc., 1993. USE 455/555 - Class 12 Download 69.34 Kb. Share with your friends:
|