ENVI ENG Particulate Emissions

Wastewater-to-Air Interface

  • Typical Stages of Wastewater Treatment
    • Preliminary → Primary → Secondary → (Tertiary/Polishing)
    • Purpose: progressively reduce solids & organic loads before discharge or reuse.
    • Relevance to air quality: aeration, stripping and sludge handling can release odorous/VOC emissions; therefore air-pollution devices are often co-designed with WWTP units.
  • Shared Control Philosophy
    • Whether treating water or air, engineers try to:
    • Reduce pollutant load as early as possible (pretreatment).
    • Combine simple, low-cost removal first (screens, cyclones).
    • Follow with higher-efficiency or high-cost devices (baghouse, ESP, scrubbers, etc.).

Particulate Control Devices (Dry Phase)

  • Cyclone or Centrifugal Collector
    • Swirling gas imparts centrifugal force; particles migrate to wall, fall into hopper.

    • \text{Typical cut size } d_{50} \approx 10\,\mu\text{m}
    • Advantages: no moving parts, high-temperature tolerance, low pressure drop.
    • Limitation: mainly removes coarse particles; fine PM usually needs a polishing step.
  • Multicyclone (Cluster of Small Cyclones)
    • Many parallel small-diameter tubes ⇒ higher centrifugal acceleration.
    • Widely used as pre-cleaner in cement & boiler industries.
    • Still considered easier & cheaper than equivalent wet scrubber.
  • Fabric Filter (Baghouse)
    • Dirty gas passes through cloth; dust cake forms and becomes the primary filter.
    • Cleaning: shaking, reverse air, or pulse-jet; transcript mentions “rapping-vibration of collection plates” (analogy with ESP).
    • Designers often specify “cyclone + baghouse” tandem: cyclone protects bags from large/abrasive dust ⇒ longer bag life, smaller baghouse.
  • Hopper
    • Under every dry collector; serves as dust receiver.
    • Must be air-tight to avoid re-entrainment.

Gas/Vapor → Liquid Phase Controls

  • Wet Scrubbing Basics
    • Pollutant gas contacts liquid (usually water) and is absorbed, condensed or chemically reacted.
    • Example stated: VOC stream → scrubber (water or chemical) produces CO₂ + H_{2}O downstream (if followed by incineration).
    • For NOₓ:
    • NO/NO₂ sparingly soluble, but can be oxidised/reduced → more soluble forms.
    • Transcript note: “Nitrogen gas → water → Ammonia” hints at selective catalytic/non-catalytic reduction (SCR/SNCR) where ammonia reacts with NOₓ.
  • Disorption / Desorption
    • Opposite of absorption: liberating a species from liquid to gas, often by heating or stripping with air/steam.
    • Used when solvent is regenerated chemically or thermally.

Condensation (Pretreatment Device)

  • Process Definition
    • Convert gas/vapor → liquid by:
    • Lowering temperature, and/or
    • Raising pressure.
  • Types
    • Contact condenser: gas mixes directly with cold liquid.
    • Surface condenser: gas is separated from coolant by a heat-transfer surface (e.g., shell-and-tube).
  • Role in Air Pollution Control
    • Reduces flowrate & organic load before expensive devices (absorber, adsorber, incinerator).
    • Removal efficiency: 50–95% depending on design & vapor pressure of contaminant.
    • Energy recovered as warm condensate can be reused.

Incineration / Combustion Controls

  • Fundamental Reaction
    • \text{Hydrocarbon} + O{2} \xrightarrow{\ \Delta\ } CO{2} + H_{2}O + \text{heat}
    • Converts VOC, odorous sulfides, etc., to relatively innocuous products.
  • Direct Combustion (Flares)
    • Waste gas + air burn at nozzle; no residence chamber.
    • EPA-measured destruction efficiency ≈ 98 %.
    • Common for emergency releases or continuous vent gases with high heating value.
  • Thermal Incinerator
    • Burner flame heats a combustion chamber; residence time t \ge 0.5\,\text{s} at T \ge 760\,^{\circ}C typical.
    • Achievable > 99 % destruction if time, temperature, turbulence, and oxygen (the “3 T + O” rule) are adequate.
  • Catalytic Incinerator
    • After the flame, gases pass through catalyst (noble metal or metal oxide).
    • Lowers required temperature (≈ 250\text{–}400\,^{\circ}C) ⇒ fuel savings.
    • Typical destruction efficiency > 95 %.
    • Sensitive to poisons (Pb, S, halogens) ⇒ requires pretreatment.

Regulatory Lists & Key Pollutants

  • US EPA Criteria Pollutants
    • Particulate Matter (PM₁₀, PM₂.₅)
    • Ground-Level Ozone (O_{3})
    • Carbon Monoxide (CO)
    • Lead (Pb)
    • Nitrogen Dioxide (NO_{2})
    • Sulfur Dioxide (SO_{2})
  • Kyoto Protocol Greenhouse Gases
    • CO{2},\ CH{4},\ N{2}O,\ \text{HFCs},\ \text{PFCs},\ SF{6}
  • Philippine Clean Air Act Criteria (parallels EPA list)
    • Total Suspended Particulates (TSP), PM₁₀
    • Photochemical Oxidants (as O_{3})
    • CO,\ Pb,\ NO{2},\ SO{2}
  • Ozone-Depleting Substances (Montreal Protocol)
    • Class I:
    • Chlorofluorocarbons (CFCs), Halons, Carbon Tetrachloride, Methyl Chloroform, Methyl Bromide.
    • Class II: Hydrochlorofluorocarbons (HCFCs).
  • Precursors
    • Photochemical smog: NO{x} + VOC in sunlight → O{3} & PANs.
    • Acid deposition: SO{2},\ NO{x} → H{2}SO{4},\ HNO_{3} aerosol.

Sampling & Measurement of Particulates

  • Sedimentation / Settling Devices
    • Fallout jars, Petri dishes, coated trays capture large settling dust.
    • Directional samplers: vertical adhesive papers or cylinders with petroleum jelly.
  • Inertial & Centrifugal Samplers
    • Miniature cyclones separate particles > 10\,\mu\text{m}.
  • Electrostatic Precipitator (ESP) Samplers
    • Platinum wire electrode ionizes air; particles migrate to collection surface.
  • Automatic Tape Smoke Sampler
    • Deposits soot on moving filter tape; gives time-resolved “dirtiness” index (not mass-accurate for TSP standard).
  • High-Volume (Hi-Vol) Sampler
    • Flow: 40–60 cfm through quartz/fiber filter (EPA reference).
    • Gravimetric equation:

      PM\,\text{concentration} = \frac{W{\text{after}} - W{\text{before}}}{Q_{\text{avg}} \times t}

      where W in grams, Q in m^{3}\,\text{min}^{-1}, t in minutes.
  • Impingers
    • Force gas to change direction sharply.
    • Wet impinger: high-speed jet into liquid ⇒ captures fine particles.
    • Dry impinger (e.g., Andersen impactor stages): collects coarse particles on plates.
  • Cascade Impactor
    • Sequential nozzles of decreasing diameter; deposits size-segregated fractions on slides; useful for lung deposition studies.
  • Nuclei Counter
    • Air saturated & adiabatically expanded ⇒ supersaturation ⇒ droplets form on nuclei; optical count gives number of condensation nuclei (useful for fog & cloud studies).
  • Pollen Sampler
    • Petroleum-jelly-coated slide exposed 24 h; grains counted microscopically; assists allergen forecasting.

Additional Concepts & Connections

  • Velocity Considerations
    • Low gas velocity in cyclone ⇒ larger cut size (loses fine PM).
    • High velocity ⇒ higher pressure drop & erosion; balance required.
  • Load Reduction Strategy
    • Condenser or cyclone frequently installed upstream of absorber/ESP/Baghouse to "protect" expensive units and reduce OPEX.
  • Environmental & Ethical Context
    • Regulatory compliance built on health-based standards (EPA, Philippine CAA).
    • Engineers must weigh operating cost vs. public health benefit; e.g., catalytic incineration saves fuel but may require precious metals.
    • Proper hopper sealing prevents secondary dust emissions, reflecting the hierarchy: prevent → control → dispose.

Key Equations & Quantities (Compilation)

  • Centrifugal acceleration in cyclone:
    a{c} = \frac{v{t}^{2}}{r}
  • Residence time for combustion:
    t = \frac{V{\text{chamber}}}{Q{\text{gas}}}
  • Condenser heat duty (simplified):
    Q = m{v} (h{v} - h_{l})

Quick Reference: Device Selection Guide

  • Coarse PM (> 10\,\mu m): settling chamber, cyclone.
  • Fine PM (1–10 µm): baghouse, wet scrubber, multicyclone, ESP.
  • Sub-micron & fumes: ESP, fabric filter with membrane, HEPA.
  • Gaseous organics: condenser (pre), adsorber, absorber, incinerator.
  • NOₓ: SCR/SNCR (ammonia), wet scrubber with oxidant.
  • Acid gases (SO₂, HCl): limestone spray tower, packed scrubber.