Hazardous Materials

Handling toxic, corrosive and reactive chemicals safely. Learn GHS classification and SDS, exposure limits (TLV, IDLH, AEGL), toxicology basics, dilution ventilation, Gaussian plume dispersion of toxic releases, reactive chemical hazards, storage and transport.

Bhopal (1984) — a methyl isocyanate release killing thousands — remains the worst industrial disaster. Chemical engineers must know how toxic a substance is, how people may be exposed, and how releases disperse, so that inventories, containment and emergency plans are adequate.

An analogy

A toxic release is like a drop of ink in a flowing river: it spreads and dilutes as it travels, but close to the source it can still be lethal.

GHS and SDS

The Globally Harmonised System classifies chemicals (flammable, toxic, corrosive, oxidiser, health hazard, environmental) with pictograms and hazard statements. The 16-section Safety Data Sheet gives properties, hazards, PPE, first aid, storage and disposal.

Toxicology and limits

Routes: inhalation (most important in industry), skin, ingestion. Dose–response: LD50/LC50. Occupational limits (TLV, PEL, WEL) protect workers for 8-h days; emergency limits (IDLH, ERPG, AEGL) guide response planning. Chlorine: TLV 0.5 ppm, IDLH 10 ppm.

Dispersion

Neutral gases follow Gaussian plume models; σ_y, σ_z grow with distance and depend on atmospheric stability (Pasquill A–F). Stable night-time conditions (F) with low wind give the worst concentrations. Dense gases (chlorine, LPG) slump and hug the ground — use heavy-gas models.

Reactive hazards and inherent safety

Incompatible chemicals, runaway reactions, peroxides and self-heating need screening (DSC, ARC calorimetry). Inherently safer design: minimise inventory, substitute less hazardous materials, moderate conditions, simplify (Kletz: 'what you don't have can't leak').

Formulas

• ppm ↔ mg/m³ (25 °C): ppm = mg/m³ × 24.45 / M
• Ground-level centreline (ground release): C = Q / (π σ_y σ_z u)
• Dilution ventilation: Q_v = G K / TLV (G = vapour generation, K = mixing factor 1–10)
• Mixture exposure: Σ C_i / TLV_i ≤ 1

Worked example: Chlorine release

Given: Q = 0.5 kg/s ground release, u = 3 m/s, stability D; At x = 500 m: σ_y = 39.0 m, σ_z = 22.7 m

Find: Concentration.

• C = 0.5/(π × 39.0 × 22.7 × 3) = 6.0 × 10⁻⁵ kg/m³ = 60 mg/m³
• ppm = 60 × 24.45/70.9 = 21 ppm

Answer: ≈ 21 ppm — above IDLH (10 ppm)

Worked example: Ventilation for toluene

Given: Evaporation 0.5 kg/h toluene (M 92.1); TLV 20 ppm, K = 3

Find: Air flow.

• G = 0.5/92.1 × 24.45 = 0.133 m³/h vapour
• Q_v = 0.133 × 3/20 × 10⁻⁶ = 19 900 m³/h = 5.5 m³/s

Answer: ≈ 5.5 m³/s (local exhaust is often more effective)

Common mistakes

• Wrong: Using neutral-gas models for dense gases like chlorine. — Right: Use heavy-gas dispersion models (DEGADIS, SLAB).
• Wrong: Relying on PPE first. — Right: Hierarchy of controls: eliminate, substitute, engineer, administrate, then PPE.

Applications

• Emergency planning: Toxic zones define shelter-in-place and evacuation areas.
• Workplace hygiene: Ventilation, monitoring and PPE for exposure control.
• Storage & transport: Segregation of incompatibles, ADR/DOT rules, bunding.

Key takeaways

• GHS + SDS communicate hazards.
• TLV for workers, IDLH/AEGL for emergencies.
• C = Q/(πσ_yσ_zu) for ground releases.
• Inherent safety beats add-on protection.

Going further

• Probit models: Convert dose (C^n·t) into probability of injury or death.
• Heavy-gas dispersion: Gravity slumping and entrainment models for dense clouds.
• Reactive chemistry screening: ARC and DSC data feed thermal-runaway assessments.