16.1 Flammability, Toxicity, and Industrial Hygiene

In chemical manufacturing, safety, health, and environmental considerations are foundational to engineering practice. On the NCEES FE Chemical exam, safety questions test quantitative and qualitative aspects of flammability, toxicity, and exposure control. The primary source for hazard communication is the Safety Data Sheet (SDS), structured into 16 standardized sections under the Globally Harmonized System (GHS). The GHS utilizes standard pictograms, hazard statements, and signal words (either 'Danger' or 'Warning'). Chemical engineers extract physical properties, exposure limits, and flammability parameters from the SDS to design processes, size venting equipment, and specify personal protective equipment (PPE).

Flammability Hazards: Key Definitions and Principles

Flammability is characterized by parameters describing a material's capability to vaporize and burn in the presence of air and an ignition source.

• Flash Point: The lowest temperature at which a liquid generates sufficient vapor to form an ignitable mixture with air near its surface. A spark will cause a temporary flash, but combustion is not self-sustained.
• Fire Point: The temperature at which the vapor concentration is high enough to sustain continuous combustion for at least 5 seconds after ignition (typically slightly higher than the flash point).
• Autoignition Temperature: The minimum temperature at which a substance will spontaneously ignite in air without an external ignition source (spark or flame), driven by thermal energy alone.
• Lower Flammable Limit (LFL) & Upper Flammable Limit (UFL): The concentration range (in volume percent of fuel in air) within which a mixture is flammable. Below the LFL, the mixture is 'too lean' to propagate a flame. Above the UFL, the mixture is 'too rich' (insufficient oxygen).

Flammability of Gas Mixtures

On the FE Chemical exam, you are often required to calculate the flammability limits of a multi-component gas mixture using Le Chatelier's Law:

$LFL_m = \frac{1}{\sum_{i=1}^n \frac{y_i}{LFL_i}}$

where $y_i$ is the volume (or mole) fraction of the combustible component $i$ on a combustible-only basis (excluding air and inert gases), and $LFL_i$ is the lower flammability limit of component $i$ in air (in volume percent).

Worked Example 1:

A process stream contains 65 vol% methane ($LFL = 5.0%$), 25 vol% ethane ($LFL = 3.0%$), and 10 vol% nitrogen. Calculate the Lower Flammable Limit of the mixture.

1. Determine the combustible-only fractions ($y_i$), excluding inert nitrogen: Total combustible volume = $65% + 25% = 90%$. $y_{\text{methane}} = 65 / 90 = 0.7222$ $y_{\text{ethane}} = 25 / 90 = 0.2778$

2. Apply Le Chatelier's Law: $LFL_m = \frac{1}{\frac{0.7222}{5.0} + \frac{0.2778}{3.0}} = 4.22 \text{ vol}\%$

The mixture is flammable when its concentration in air is $4.22%$ or greater.

Toxicity and Exposure Limits

Toxicity represents a chemical's capacity to cause injury to a living organism. Exposure routes include inhalation, dermal absorption, ingestion, and injection. Toxicological metrics include:

• $LD_{50}$ (Lethal Dose 50%): The dose administered (typically orally or dermally) that kills 50% of a test population, in mg chemical per kg body weight.
• $LC_{50}$ (Lethal Concentration 50%): The concentration in air (inhalation) that kills 50% of a test population over a specified time, in ppm or mg/m³.

In industrial hygiene, exposure limits are defined as:

• Threshold Limit Value - Time-Weighted Average (TLV-TWA): The average concentration for a normal 8-hour workday and 40-hour workweek to which workers can be repeatedly exposed without adverse effects.
• Short-Term Exposure Limit (TLV-STEL): A 15-minute average exposure that must not be exceeded during a workday.
• Ceiling Limit (TLV-C): The concentration that must not be exceeded at any instant.

The TWA concentration is calculated as:

$TWA = \frac{\sum_{i=1}^n C_i t_i}{8}$

where $C_i$ is the concentration during time interval $t_i$ (in hours).

For exposures to mixtures of toxic chemicals acting on the same organ, the mixture exposure index ($E_m$) is used:

$E_m = \sum_{i=1}^n \frac{C_i}{TLV_i}$

If $E_m > 1$, the mixture threshold limit is exceeded.

Worked Example 2:

An operator is exposed to hexane ($TLV = 50 \text{ ppm}$) and heptane ($TLV = 400 \text{ ppm}$) over an 8-hour shift:

• 3 hours: Hexane = 30 ppm, Heptane = 150 ppm
• 5 hours: Hexane = 40 ppm, Heptane = 200 ppm Calculate individual TWAs and check the mixture index.

1. Hexane TWA: $TWA_{\text{hexane}} = \frac{(30 \times 3) + (40 \times 5)}{8} = 36.25 \text{ ppm}$

2. Heptane TWA: $TWA_{\text{heptane}} = \frac{(150 \times 3) + (200 \times 5)}{8} = 181.25 \text{ ppm}$

3. Mixture Index: $E_m = \frac{36.25}{50} + \frac{181.25}{400} = 1.178 > 1$

The mixture limit is exceeded, requiring corrective action.

Industrial Hygiene Controls and Ergonomics

To protect workers, chemical engineers apply the Hierarchy of Controls:

1. Elimination: Physical removal of the hazard.
2. Substitution: Replacing the hazard with a safer alternative.
3. Engineering Controls: Isolating people from the hazard (e.g., local exhaust ventilation, secondary containment).
4. Administrative Controls: Changing the way people work (e.g., worker rotation, training, audits).
5. Personal Protective Equipment (PPE): Protecting workers with physical barriers (e.g., respirators, goggles). PPE is the least effective control because it relies on human compliance and can fail without warning.

Ergonomics focuses on designing the workplace, equipment, and tasks to match the physical capabilities of workers, minimizing musculoskeletal disorders (MSDs) and repetitive strain injuries. This includes optimizing valve heights, control panel displays, lifting techniques, and work-rest schedules to prevent physical fatigue and injury in industrial operations.