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
  • $\text{Hydrocarbon} + O<em>{2} \xrightarrow{\ \Delta\ } CO</em>{2} + H_{2}O + \text{heat}$
  • 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
  • $CO<em>{2},\ CH</em>{4},\ N<em>{2}O,\ \text{HFCs},\ \text{PFCs},\ SF</em>{6}$
• Philippine Clean Air Act Criteria (parallels EPA list)
  • Total Suspended Particulates (TSP), PM₁₀
  • Photochemical Oxidants (as $O_{3}$)
  • $CO,\ Pb,\ NO<em>{2},\ SO</em>{2}$
• Ozone-Depleting Substances (Montreal Protocol)
  • Class I:
  • Chlorofluorocarbons (CFCs), Halons, Carbon Tetrachloride, Methyl Chloroform, Methyl Bromide.
  • Class II: Hydrochlorofluorocarbons (HCFCs).
• Precursors
  • Photochemical smog: $NO<em>{x} + VOC$ in sunlight → $O</em>{3}$ & PANs.
  • Acid deposition: $SO<em>{2},\ NO</em>{x}$ → $H<em>{2}SO</em>{4},\ HNO_{3}$ 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:
  $a<em>{c} = \frac{v</em>{t}^{2}}{r}$
• Residence time for combustion:
  $t = \frac{V<em>{\text{chamber}}}{Q</em>{\text{gas}}}$
• Condenser heat duty (simplified):
  $Q = m<em>{v} (h</em>{v} - h_{l})$

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.