7. Storage

Storage and Handling Considerations

A thorough knowledge of the chemical and physical properties, federal and local regulations, and building codes, is
necessary for the safe handling and storage of TDI.

TDI is not considered a corrosive chemical; however, the selection of materials for TDI handling systems plays a crucial role in maintaining TDI product quality. Trace amounts of metals, including iron (rust), copper, brass or aluminum, may affect the reactivity of TDI in sensitive applications. In general, mild steel, epoxy-phenolic lined steel, or stainless steels are the recommended materials of construction for TDI handling and storage systems. If unlined mild steel is selected, the surface must be maintained clean and rust-free to maintain product quality. Presently, plastic materials are not recommended for handling TDI. TDI may migrate into some plastics causing them to become brittle with age.

When designing storage systems for TDI, extreme care must be exercised to avoid exposure of TDI to water, strong bases, or other active hydrogen-containing compounds. Acids, bases, and other polyurethane catalysts should not be stored in the same area as TDI.

The reaction of TDI with moisture, even from ambient air, will produce polyurea solids and CO2 gas. These insoluble polyureas will deposit on surfaces of pipes and tanks causing line restrictions and filtration problems. The generated CO2 could present a pressure hazard, including the potential of a violent rupture of an under-vented tank or vessel.

Although TDI is relatively non-flammable (flash point 135°C / 275°F), TDI should not be stored adjacent to highly flammable materials. Water, dry chemical, protein foam, or CO2 fire extinguishers should be available in all storage and processing areas. Automatic fire or smoke detection equipment, as well as automatic sprinklers should be installed in all TDI processing and storage areas.

Storage Tank Design

All TDI storage tanks must be blanketed with nitrogen or dry air. Dry nitrogen or dry air must have a new dew point below –40°C (-40°F). Storage tanks should be maintained under a slight pressure (1mbar / 1-2 psi). Storage tank vents must be sized to adequately protect the tank against pressure build positive up during unloading operations or the generation of pressure from moisture contamination. As a minimum, TDI vents must be directed outside, away from ventilation systems, or into plant exhaust vent systems. Activated carbon filters have been successfully used to reduce TDI emissions from storage tank vents. In all instances, TDI
venting procedures must comply with applicable codes, regulations and permits.

To maintain the desired product temperature, TDI storage tanks should be equipped with a temperature indicator, heat tracing, and insulation. The preferred method of temperature control is external heat exchangers using an inert heating medium.

External tempered water or electric tracing has been successfully used. Steam should not be used due to the possibility of overheating. Any moisture contamination with TDI must be prevented. To eliminate any potential of a coil leak, internal coils are not recommended. Heating coils and exchangers should be checked for corrosion regularly.

The TDI storage tank temperature should be maintained between 18ºC (65ºF) and 41ºC (105ºF) to ensure consistent product quality. At extended periods above 41ºC (105ºF), TDI may begin to discolor and dimerize. At extended periods above 100ºC (212ºF), TDI may begin to trimerize; the trimerization reaction is exothermic and will generate CO2 gas, which may create a pressure hazard in a sealed or under-vented vessel.

If TDI is shipped or stored below the recommended storage temperature, freezing or partial freezing may occur. TDI can be easily reliquified without any degradation by heating 23 – 35ºC (73 – 95ºF). Always be certain that all of the TDI has reliquified and that the product is thoroughly mixed. The 2,4-isomer has a higher melting point than the 2,6-isomer. Unless the TDI is thoroughly melted and mixed, the liquid isomer ratio may vary. Off ratio TDI may produce processing problems because the reactivities of the isomers vary.

TDI tanks should be equipped with level indicators and separate high-level alarms as well as cutoffs to prevent accidental overflow. Tank areas must be diked to prevent runoff in the event of a TDI release. Diking must be sufficient to contain potential spills and leaks, and prevent accidental release of TDI into sewers, waterways, or public thoroughfares. Dikes must be designed for 1½ times the tank capacity, or as directed by codes and regulations for handling hazardous chemicals. Storage tanks should be designed to hold an entire TDI shipment (i.e., if the shipment is typically a 20,000-gallon rail car, the tank capacity should be greater than the capacity of the rail car).

Transfer pumps should be of compatible materials and of a seamless design. Canned pumps and magnetic drive pumps have been successfully used. Appropriate hazard labels may be required on storage and transfer systems containing TDI. In the United States, OSHA requires hazard communication labels for all containers containing TDI. Please note that facilities that store, or otherwise have onsite, TDI in quantities of 10,000 lbs. or more are required to register a Risk Management Plan with the EPA. Information on EPA’s RMP may be obtained at the following website: https://www.epa.gov/rmp.

Drum Storage

Drum storage areas should be covered and well ventilated. Ideally, TDI drum storage areas should be diked and separated from materials reactive with TDI. All storage areas should be arranged in an orderly manner, leaving doorways and exit routes clear. Local codes may have specific requirements for the storage of hazardous chemicals. Many local authorities reference the International Fire Code when making decisions on TDI drum storage. Information on the International Fire Code may be obtained at the following website: https://codes.iccsafe.org/content/IFC2015/toc