LECTURE #7b Particulate Scrubbers March 6, 2017 ATMO.5230

Introduction Collect: fumes, mists, suspended dusts Classified as: f(liquid contacting mechanism) Spray-chamber scrubber (use spray nozzles / preformed spray scrubbers) Cyclone spray chambers (influent enters tangentially) Orifice & wet-impingement scrubbers (self-induced or gas atomized spray) Venturi & Venturi jet scrubbers (maximizes relative velocity between liquid droplet and suspended particles) Classified by energy: Low-, moderate- or high-energy units Energy requirements expressed: Pressure drop (inches / centimeters of water) Level of contacting power (hp/1000 cfm or kWh/1000 m3 treated) Liquid circulation rates: gallons per 1000 cf3 (1 gal/1000 ft3 = 0.134 L/m3)

Advantages/disadvantages Can handle flammable/explosive dusts Provide gas absorption and dust collection in single unit Can handle mists Provide cooling of hot gases CE can be varied/optimized Corrosive gases/dusts can be neutralized Disadvantages High potential for corrosion Effluent liquid can create water disposal problems May need protection against freezing; need for exhaust reheat Contaminated collected particulate Disposal of waste sludge may be expensive

Spray-chamber scrubbers Like a packed tower only with spray nozzles Optimized water droplet size separates particles from gas stream Maintenance: nozzle clogging CE of 90% on particles > 8um Applications: paper dust control, latex aerosol control in fiberglass production Power consumption: low  1 – 4 inches water (0.5-2 hp/1000acfm) at Vg of 3-6 fps

Cyclone spray chamber Influent enters tangentially allowing higher Vsuperficial Centrifugal force promotes droplet separation  smaller size w/ greater CE (95% at 5um) Vg(in) =100-200 fps; Vsuperficial =10 fps Liquid recirculation: 2-10 gal/1000 ft3 Pressure drops: 4-8 inches water Power inputs: 1-3.5 hp/1000 cfm

Orifice and wet-impingement scrubbers Self induced or gas atomized spray where combined air-droplet mixture impinges on solid surface Orifice: air impinges on liquid surface then on baffles Low water recirc., 0.5 gal/1000ft3 P drop: 8-12 inches water Power inputs: 2-4 hp/1000 cfm Impingement: air-droplets flow through perforated tray with liquid/froth then impinges on plate Same P drop Liquid requirements: 2-3 gal/1000ft3 Power input: 2-4 hp/1000 acfm

Typical Venturi Scrubber CE = f(relative velocity between liquid-particle and particle size) Water injected at low P at throat with Vg of 150-500 fps  CE of 98% at 0.5 um) Power: 3-12 hp/1000 cfm Liquid recirculation rate of 3-15 gal/1000ft3 Venturi jet scrubbers: Water injected in throat at high velocities P drop: 2-4 inches water Water rates: 30-80 gal/1000 ft3 Nozzle pressures: 50-150 psig CE of 92% for 1 um particles

Contacting power approach to design

Energy dissipated / unit volume of gas

Effective friction loss

CE as f(particle size & pressure) in Venturi scrubber

Typical scrubber applications

Other considerations Elimination of liquid entrainment Humidification of scrubbed gases Cost High-energy Venturi . Low-energy Venturi

Power plants: coal to gas &end-of-pipe controls

China halts coal power plants

China offers bonus to curb emissions