Hazards of process materials: flammability, reactivity and toxicity

Process safety starts with understanding the materials. A plant that handles a flammable solvent needs different safeguards from one that handles a toxic gas or a chemical that can react violently when it gets too hot. Most process hazards fall into three groups: materials that burn, materials that react, and materials that poison. This lesson explains the properties that matter in each group, how they are measured, and how to use them in simple calculations. HSE-102 covered labels and safety data sheets; here we look at the numbers behind them and what they mean for a process.

Where the information comes from

The first source is the safety data sheet (SDS), especially sections 2 (hazard identification), 9 (physical and chemical properties), 10 (stability and reactivity) and 11 (toxicological information). For process work, the SDS is a starting point rather than the whole story. Process conditions are often hotter, more concentrated or under higher pressure than the conditions the SDS describes, and mixtures can behave differently from pure substances. Process safety information also includes reaction data from calorimetry testing, the maximum intended inventory, and the safe upper and lower limits for temperature, pressure, flow and composition. Lesson 5 explains how process safety management rules require this information to be compiled and kept up to date.

Flammability

A flammable liquid does not burn as a liquid. It evaporates, and the vapor mixes with air. If the mixture is within the flammable range and meets an ignition source, it burns. Several properties describe how easily this happens.

Two features make a material especially hazardous. A low lower flammable limit means that a small leak is enough to create a burnable mixture: toluene can burn at just over 1 percent in air. A wide flammable range, as hydrogen has, means that a release can burn over a wide range of concentrations as it disperses. Gas detectors in process areas are usually set to alarm at 10 to 25 percent of the LFL, so that people have time to act before the mixture can burn.

In the United States, OSHA's flammable liquids standard (29 CFR 1910.106) uses the GHS categories adopted in the Hazard Communication Standard: Category 1 for flash points below 23 °C (73.4 °F) with an initial boiling point at or below 35 °C (95 °F), Category 2 for flash points below 23 °C with a higher boiling point, Category 3 for 23 to 60 °C (140 °F) and Category 4 for 60 to 93 °C (199.4 °F). Fire codes based on NFPA 30 still use the older Class I, II and III system for storage quantities and building design.

Mixtures: Le Chatelier's rule

Process streams are often mixtures of flammable gases or vapors. The lower flammable limit of a mixture can be estimated with Le Chatelier's rule, which combines the limits of the components weighted by their share of the flammable part of the mixture.

Here each y is the percentage of a component in the flammable part of the mixture, so the y values add up to 100. The same form gives an estimate of the upper limit using the UFL values. The rule works well for similar hydrocarbons and less well for mixtures containing hydrogen or for very different compounds, so it is an estimate rather than a design value.

# Lower and upper flammable limits of a fuel gas mixture by Le Chatelier's rule.
mix = {            # percent of flammable gas, LFL %, UFL %
    "methane": (50, 5.0, 15.0),
    "ethane":  (30, 3.0, 12.4),
    "propane": (20, 2.1, 9.5),
}
assert sum(v[0] for v in mix.values()) == 100
lfl = 100 / sum(y / L for y, L, U in mix.values())
ufl = 100 / sum(y / U for y, L, U in mix.values())
print(f"Estimated LFL of mixture: {lfl:.2f} % by volume")
print(f"Estimated UFL of mixture: {ufl:.2f} % by volume")
print(f"25 % of LFL detector alarm point: {0.25 * lfl:.2f} % by volume ({0.25 * lfl * 10_000:.0f} ppm)")

Estimated LFL of mixture: 3.39 % by volume
Estimated UFL of mixture: 12.73 % by volume
25 % of LFL detector alarm point: 0.85 % by volume (8468 ppm)

Oxygen: the other side of the fire triangle

A fire needs fuel, oxygen and an ignition source. Process plants often control the oxygen side by blanketing tanks and reactors with nitrogen. For each fuel there is a minimum oxygen concentration (MOC), sometimes called the limiting oxygen concentration, below which a flame cannot spread whatever the fuel concentration. A rough estimate is the LFL multiplied by the number of moles of oxygen needed to burn one mole of fuel. For propane, which needs 5 moles of oxygen, the estimate is about 2.1 × 5 ≈ 10.5 percent oxygen; measured values are a little higher, around 11 to 12 percent. Inerting systems usually keep oxygen several percentage points below the MOC to leave a safety margin. Inerting has its own hazard: nitrogen is an asphyxiant, and entering an inerted vessel without breathing air has killed many workers (HSE-211 covers confined space entry).

Reactivity

Reactive hazards are chemical reactions that release energy or gas faster than the process can handle. They include runaway reactions in reactors, decomposition of unstable chemicals, polymerization of monomers, reactions with water or air, and reactions between incompatible materials mixed by mistake. Reactive chemical incidents are a large share of serious process accidents. The CSB studied 167 reactive incidents in the United States over two decades and found that many involved materials that were not covered by process safety management rules at the time.

Most reaction rates increase rapidly with temperature. A familiar rule of thumb is that the rate roughly doubles for each 10 °C rise. The exact factor depends on the activation energy and can be estimated with the Arrhenius equation. Because heat release grows exponentially with temperature while cooling grows only in proportion to the temperature difference, there is a point beyond which the cooling system cannot keep up. Above that point the temperature climbs faster and faster: a thermal runaway.

Another key number is the adiabatic temperature rise: how much the reaction mass would heat up if all the reactant reacted with no cooling at all. If that rise would take the mixture above its boiling point or above the temperature where a dangerous secondary decomposition begins, the reactor depends entirely on its cooling and protection systems.

import math
# Rate increase with temperature (Arrhenius) and adiabatic temperature rise.
R = 8.314                     # J/(mol K)
Ea = 80_000                   # activation energy, J/mol (illustrative)
T1 = 60 + 273.15
for dT in (10, 20, 30):
    T2 = T1 + dT
    ratio = math.exp(Ea / R * (1 / T1 - 1 / T2))
    print(f"{dT:>2} C hotter -> reaction about {ratio:4.1f} times faster")

dH = -120_000                 # heat of reaction, J/mol (exothermic)
C = 2_000                     # reactant concentration, mol/m3
rho, cp = 900, 2_000          # kg/m3, J/(kg K)
dT_ad = (-dH) * C / (rho * cp)
print(f"Adiabatic temperature rise: {dT_ad:.0f} K")
T_proc, T_boil = 60, 111      # process temperature and solvent boiling point, C
print(f"Maximum temperature of the synthesis reaction: {T_proc + dT_ad:.0f} C "
      f"(solvent boils at {T_boil} C)")

10 C hotter -> reaction about  2.3 times faster
20 C hotter -> reaction about  5.1 times faster
30 C hotter -> reaction about 10.9 times faster
Adiabatic temperature rise: 133 K
Maximum temperature of the synthesis reaction: 193 C (solvent boils at 111 C)

In this illustrative case, losing cooling would let the batch heat up by more than 130 °C, well past the solvent's boiling point. The pressure would rise, and if a secondary decomposition starts at those temperatures, the situation could escalate. A reactor like this needs reliable cooling, a high-temperature trip that stops the feed, a way to stop the reaction (for example, a quench or inhibitor), and an emergency relief system sized for the runaway case. Reaction calorimetry testing gives the real numbers.

Simple tools help identify incompatible combinations before they meet. The free Chemical Reactivity Worksheet, part of the CAMEO software developed by the US Environmental Protection Agency and the National Oceanic and Atmospheric Administration, predicts hazards when pairs of chemicals are mixed. Many plants build a compatibility matrix for every material on site and use it when assigning tanks, hoses and waste drums. HSE-312 covers chemical storage and segregation in more detail.

Toxicity

Toxic process materials can harm people at concentrations far below their flammable limits. For process safety, the concern is usually short, high exposures during a release rather than the long, low exposures covered by workplace exposure limits. Several sets of guideline values describe short-term toxic effects.

Concentrations in air are given either in parts per million by volume or in milligrams per cubic meter. At 25 °C and normal atmospheric pressure, one mole of gas occupies about 24.45 liters, which gives a simple conversion.

# Converting ppm to mg/m3 at 25 C and 1 atm, and comparing with guideline values.
gases = {   # molar mass g/mol, ERPG-2 ppm, IDLH ppm (approximate values)
    "ammonia":           (17.03, 150, 300),
    "chlorine":          (70.90, 3, 10),
    "hydrogen sulfide":  (34.08, 30, 100),
}
for name, (M, erpg2, idlh) in gases.items():
    f = M / 24.45
    print(f"{name:<17} ERPG-2 {erpg2:>4} ppm = {erpg2 * f:7.1f} mg/m3 | "
          f"IDLH {idlh:>4} ppm = {idlh * f:7.1f} mg/m3")

ammonia           ERPG-2  150 ppm =   104.5 mg/m3 | IDLH  300 ppm =   209.0 mg/m3
chlorine          ERPG-2    3 ppm =     8.7 mg/m3 | IDLH   10 ppm =    29.0 mg/m3
hydrogen sulfide  ERPG-2   30 ppm =    41.8 mg/m3 | IDLH  100 ppm =   139.4 mg/m3

Chlorine is about 50 times more toxic than ammonia by these measures, which is why a small chlorine leak at a water treatment plant can be as serious as a much larger ammonia leak. Toxic materials also differ in warning properties: ammonia is irritating at low concentrations and people move away, while hydrogen sulfide deadens the sense of smell at dangerous levels (HSE-244 covers hydrogen sulfide). Lesson 4 shows how these guideline values are used to estimate how far a toxic cloud could travel.

Inventory: how much is too much?

The amount of hazardous material on site matters as much as its properties. A 1-liter bottle of chlorine water is a laboratory chemical; a 1-metric ton chlorine container is a major hazard. Process safety laws use threshold quantities to decide which sites must have a formal program (Lesson 5), and inherently safer design starts by asking whether the inventory can be reduced. Bhopal stored tens of metric tons of methyl isocyanate that could have been made and used continuously in much smaller amounts.