Chlorine gas (Cl₂) is a greenish-yellow substance with a pungent, irritating odor that plays a foundational role in modern industry. The elemental form is highly toxic, classified as a pulmonary irritant that can cause acute damage to the respiratory tract upon inhalation. Despite this high hazard, chlorine is widely used in essential applications, including municipal water purification, the manufacture of plastics like PVC, and the production of pharmaceuticals and agricultural chemicals. Moving this volatile material safely from production facilities to end-users across vast distances necessitates stringent and specialized transportation standards.
Preparing Chlorine for Transit
The physical state of chlorine must be altered to make bulk transportation economically viable. In its natural gaseous form, chlorine occupies a large volume, making the transport of commercially useful quantities inefficient. Producers therefore employ compression and cooling to convert the gas into liquid chlorine.
Liquefaction is a staged process where the gas is first dried, compressed, and then cooled using refrigeration units. The process converts the chlorine gas into a clear, amber-colored liquid that is approximately one and a half times as dense as water. To achieve this liquid state, the chlorine is cooled to temperatures as low as -20°F to -40°F (approximately -28°C to -40°C) while under pressure.
Maintaining this liquid state during transit allows for the transport of a much greater mass of the substance within a fixed container volume. The liquid is then loaded into containers under its own vapor pressure, ensuring a closed-loop system. This transformation enables massive quantities of chlorine to be shipped across the country.
Standard Transportation Vessels
The movement of liquid chlorine relies on specialized, heavy-duty steel containers designed to maintain high pressures and prevent leakage. The smallest common containers are seamless carbon steel cylinders, typically holding 100 or 150 pounds of product. These cylinders are constructed to strict specifications, such as DOT3A480 or DOT3AA480, and are intended for localized delivery or smaller-scale operations.
Moving up in size, the one-ton container, a horizontal vessel, is frequently used in water treatment plants and smaller chemical operations. This container holds 2,000 pounds of liquid chlorine and is built to the DOT 106A specification, such as the 106A500X. These containers feature concave heads designed to reverse outward during extreme over-pressurization, providing a visual safety indicator and momentary relief. They are also equipped with multiple fusible plugs designed to melt and release pressure at high temperatures, typically between 158°F and 165°F.
For the largest industrial transfers, specialized rail tank cars are the primary vessel, typically carrying 55, 85, or 90 tons of product in capacities up to 23,000 gallons. These are purpose-built pressure vessels meeting the DOT 105 class specification, such as the DOT105H600W. These cars are thermally insulated and jacketed, built with a heavy-duty steel shell to withstand a high internal test pressure, often up to 600 PSI. They include safety features like internal excess flow valves and pressure relief devices set to activate around 375 psig to manage the risks of transporting a liquefied poison inhalation hazard.
Modes of Delivery and Logistics
The vessels are moved across the country using road, rail, and marine transport, each governed by extensive hazardous materials regulations. Road transport handles smaller volumes, with cylinders and ton containers moved by specialized motor vehicles. Larger cargo tank motor vehicles, such as those built to DOT MC-330 and MC-331 standards, can transport between 15 and 20 tons of liquid chlorine.
These motor vehicles are required to use specific routes that avoid densely populated areas and must be equipped with excess flow check valves to mitigate the risk of a catastrophic release during an accident. Drivers transporting these materials must hold specialized endorsements and undergo rigorous training specific to chlorine hazards.
Rail transport is the dominant method for moving the largest bulk quantities in the 90-ton tank cars. The Department of Transportation Hazardous Materials Regulations govern this movement, requiring analysis of the safety and security risks of the routes used. To enhance safety, the industry utilizes dedicated hazmat routes and adheres to best practices regarding train composition and speed restrictions for cars carrying poison inhalation hazard materials.
For extremely large shipments, marine transport is utilized, with specialized barges moving liquid chlorine along inland waterways. Regardless of the mode of transport, a comprehensive logistical framework is mandated, including the use of highly visible placards, detailed emergency response documentation, and the requirement for shippers to provide a staffed 24-hour emergency telephone number. This regulatory oversight ensures the material remains safely contained throughout its journey.