Flammability & Toxicity in ChlorAlkali Industry

The chlor-alkali industry is a cornerstone of chemical manufacturing, producing chlorine (\(\text{Cl}_2)\), sodium hydroxide (\(\text{NaOH})\), and hydrogen (\(\text{H}_2)\) through the electrolysis of saltwater (brine).

The core chemical reaction is:

$$ 2 \text{ NaCl} + 2 \text{ H}_2\text{O} \xrightarrow{\text{electrolysis}} 2 \text{ NaOH} + \text{Cl}_2 \uparrow + \text{H}_2 \uparrow $$

The Hydrogen Hazard: Flammability 🔥 #

The main danger from hydrogen is its extreme flammability. It is a colorless, odorless gas that is much lighter than air, causing it to rise and collect in ceilings, roof spaces, and the upper enclosures of process equipment.

Lower Explosive Limit (LEL) #

The Lower Explosive Limit (LEL) is the lowest concentration of a combustible gas or vapor in air that can propagate a flame. For hydrogen, the LEL is approximately 4% by volume in air. This means that if the concentration of hydrogen in air exceeds 4%, it can form an explosive mixture.

The LEL for chlorine is not typically a concern in terms of flammability, as chlorine is not flammable. However, it is highly toxic and can form explosive mixtures with other substances.

Mathematical Representation of LEL #

The LEL can be mathematically represented as follows:

$$ \text{LEL} = \frac{\text{Volume of combustible gas}}{\text{Total volume of gas mixture}} \times 100% $$

For hydrogen:

$$ \text{LEL}_{\text{H}_2} = 4% $$

For chlorine, while it does not have an LEL in terms of flammability, its toxic properties must be considered. The permissible exposure limit (PEL) for chlorine is 1 ppm (parts per million) over an 8-hour workday.

Converting LEL to PPM #

To appreciate the scale, we can convert the LEL from a percentage to parts per million (ppm).

Since \(1\% = 10,000 \text{ ppm}\) the LEL of \(\text{H}_2\) is:

$$ \text{LEL}_{\text{H}_2} = 4.0\% \times \frac{10,000 \text{ ppm}}{1\%} = 40,000 \text{ ppm} $$

Alarm Setpoints and %LEL #

In industrial safety, gas detectors are set to alarm at a fraction of the LEL. This provides an early warning before a dangerous concentration is reached.

A typical two-stage alarm system might be:

• Low Alarm: 10% of LEL
• High Alarm: 25% of LEL

Let’s calculate the actual hydrogen concentration for these alarms:

Low Alarm (10% LEL): $$ 0.10 \times \text{LEL} = 0.10 \times 4.0% = 0.4% \text{ H}_2 \text{ (or 4,000 ppm)} $$

High Alarm (25% LEL): $$ 0.25 \times \text{LEL} = 0.25 \times 4.0% = 1.0% \text{ H}_2 \text{ (or 10,000 ppm)} $$

A detector reading “100% LEL” means the atmosphere has reached \(4.0%\text{ H}_2\) and is at the minimum concentration to explode.

Example Calculation #

Suppose a room has a volume of 1000 m³ and contains 50 m³ of hydrogen. The concentration of hydrogen in the room can be calculated as:

$$ \text{Concentration of H}_2 = \frac{50 \ \text{m}^3}{1000 \ \text{m}^3} \times 100% = 5% $$

Since 5% is above the LEL of 4% for hydrogen, the mixture is explosive and immediate action must be taken to dilute the hydrogen concentration.

The Chlorine (\(\text{Cl}_2\)) Hazard: Toxicity ☠️ #

Chlorine presents a completely different challenge. It is not a flammable gas; it is a strong oxidizer. Its primary hazard is extreme toxicity.

\(\text{Cl}_2\) is a greenish-yellow gas with a distinct bleach-like odor. It is approximately 2.5 times heavier than air, meaning it will sink and collect in low-lying areas like trenches, pits, and basements.

Understanding Toxicity Limits (PPM) #

Because \(\text{Cl}_2\) is a toxic gas, we don’t measure it in \(%\text{LEL}\). We measure it in \(\text{ppm}\) and compare it to occupational exposure limits.

• TLV-TWA (Threshold Limit Value - 8-hr average): \(0.5 \text{ ppm}\)
• STEL (Short-Term Exposure Limit - 15 min): \(1.0 \text{ ppm}\)
• IDLH (Immediately Dangerous to Life or Health): \(10 \text{ ppm}\)

The IDLH is the concentration that can cause death or irreversible health effects from a short exposure.

Comparing the Hazards: \(\text{H}_2\) vs. \(\text{Cl}_2\) #

This is the most critical safety concept in the chlor-alkali industry. The two primary hazards exist on completely different scales.

• \(\text{H}_2\) Flammable Alarm (Low): \(4,000 \text{ ppm}\)
• \(\text{Cl}_2\) Lethal Level (IDLH): \(10 \text{ ppm}\)

Let’s find the ratio of these two hazard levels:

$$ \text{Ratio} = \frac{\text{H}_2 \text{ Low Alarm}}{\text{Cl}_2 \text{ IDLH}} = \frac{4,000 \text{ ppm}}{10 \text{ ppm}} = 400 $$

This calculation shows that an atmosphere becomes lethal from chlorine 400 times over before hydrogen even reaches its first flammable alarm level.

Safety Implication: A plant cannot rely on a single type of gas detector. Dedicated systems for both flammable (\(\text{H}_2\)) and toxic (\(\text{Cl}_2\)) gases are mandatory, and their placement must account for their different physical properties (light vs. heavy).

The Combined Hazard: \(\text{H}_2 + \text{Cl}_2\) Mixture #

A unique and severe hazard in this industry is the potential for the \(\text{H}_2\) and \(\text{Cl}_2\) gas streams to mix without air. This mixture is explosively reactive and can be initiated by static electricity, a spark, or even strong sunlight (UV light).

$$ \text{H}_2 + \text{Cl}_2 \rightarrow 2 \text{ HCl} + \text{Energy (Explosion)} $$

This reaction does not require air (oxygen). The explosive limits for hydrogen in pure chlorine are approximately \(4\%\) to \(90\%\) \(\text{H}_2\).

Summary of Hazards #

Understanding the mathematics behind these LEL and PPM values is not just academic; it is the basis for designing the engineering controls, gas detection systems, and emergency procedures that keep a chlor-alkali plant safe.