Methyl tert-butyl ether (MTBE; CAS 1634-04-4) is assigned UN 2398, Class 3, Packing Group II and is loaded into 20 ft ISO tank containers certified to ISO 1496-3:2019. The custody-transfer density basis is 0.740 kg/L at 20°C, equivalent to 740 kg/m³, and is normally measured by ASTM D4052-18a. MTBE is a volume-limited ISO tank cargo because its density is only 0.740 times the density of water. A nominal 24,000 L tank barrel filled to a fixed 0.90 ratio at 20°C contains 21,600 L; the calculated net weight is 15,984 kg. On a 25,000 L barrel, the same ratio yields 22,500 L and 16,650 kg. These weights remain below the road gross mass limit of 30,480 kg after tare, leaving thermal expansion control and density correction as the governing loading parameters.

Why Does the 0.740 kg/L Density Basis Control Net Weight in ISO Tank Loading?

The 0.740 kg/L value is a temperature-specific basis tied to 20°C. If the liquid is loaded at 25°C and the volume is multiplied by the 20°C density, the true density is approximately 0.7349 kg/L, so the use of 0.740 kg/L overstates the net weight by approximately 0.69%. For a 21,600 L loaded volume, this error is approximately 110 kg of artificial mass. For a 25,000 L barrel filled to 90%, the same error is approximately 127 kg. The correction formula used in tank loading calculations is ρ_T = 0.740 / [1 + 0.0014 (T − 20)], where T is the bulk liquid temperature in degrees Celsius. The coefficient of volumetric expansion for MTBE is typically 0.0014 K⁻¹, but the supplier certificate must be consulted because the value may shift between 0.0013 K⁻¹ and 0.0015 K⁻¹ depending on C4 raffinate impurities, methanol content, and water content.

Net weight is calculated as m = V_loaded × ρ_T, where V_loaded is the volume measured at the loading temperature. The loading volume itself is governed by the regulatory fill ratio. Under ADR 4.3.2.2.4, the maximum degree of filling is derived from the relationship V_max = V_fill [1 + α (T_max − T_fill)]. Setting V_max at 0.95 of the nominal tank capacity gives F = 0.95 / [1 + α (T_max − T_fill)]. For MTBE with α = 0.0014 K⁻¹, T_max = 50°C, and T_fill = 15°C, the fill ratio is 0.9056. The resulting loaded volume in a 24,000 L barrel is 21,735 L. The density at 15°C is 0.7452 kg/L, producing a net weight of approximately 16,200 kg. At 25°C, the fill ratio increases to 0.9180, the loaded volume becomes 22,029 L, and the density falls to 0.7349 kg/L, again yielding a net weight of approximately 16,200 kg. This mass ceiling remains nearly constant because the fill-ratio formula restricts the expanded volume to 95% of capacity at the 50°C service temperature. For a 24,000 L barrel, the expanded volume at 50°C is 22,800 L and the density is approximately 0.710 kg/L, giving a mass of 16,200 kg.

If the Loading Temperature Is Not 20°C, Net Weight Correction Must Be Applied

A loading operation that records gross volume at ambient temperature but declares weight using the 20°C density basis introduces a systematic error. At 15°C, the MTBE density increases to 0.7452 kg/L; at 25°C, it decreases to 0.7349 kg/L. For a fixed fill ratio of 0.90 in a 24,000 L barrel, the loaded volume is 21,600 L. The net weight at 15°C is therefore 16,096 kg, at 20°C it is 15,984 kg, and at 25°C it is 15,874 kg. A terminal that fails to correct density for a 10°C difference can create a declared weight discrepancy of approximately 220 kg on the same physical volume. This discrepancy is large enough to trigger customs or dangerous goods document checks. The correction must be applied using the measured bulk temperature, not the ambient air temperature, because the tank wall and liquid thermal mass can lag the surrounding temperature during early morning or late evening loading.

The table indicates that the regulatory maximum net weight is substantially independent of loading temperature when the fill ratio is set by ADR 4.3.2.2.4. This does not mean that density correction is unnecessary. It means that the two corrections offset each other for the regulatory fill limit: a lower loading temperature reduces the permitted volume but increases the liquid density, while a higher loading temperature increases the permitted volume but reduces the density. If a terminal instead uses a fixed fill ratio of 0.85 or 0.90, the offset is no longer automatic and the net weight must be corrected explicitly. At a fixed 0.85 ratio and 20°C, a 24,000 L barrel contains 20,400 L and has a net weight of 15,096 kg. Under the same fixed ratio, a 25,000 L barrel contains 21,250 L and has a net weight of 15,725 kg. These fixed-ratio values are used by operators that prefer additional vapour space beyond the regulatory minimum.

Vapour Space, Outage Volume, and Fill-Ratio Limit Under ADR Thermal Expansion

At 15°C and a fill ratio of 0.9056, the initial outage is 9.44% of nominal capacity, or 2,266 L in a 24,000 L barrel. As the liquid warms to 50°C, MTBE expands into this outage and leaves only 5% vapour space. MTBE vapour pressure at 20°C is commonly reported as approximately 26.7 kPa and rises as the liquid approaches its atmospheric boiling point of 55.2°C. The vapour space must accommodate thermal expansion, but the pressure relief valve is sized separately for fire exposure and tank design pressure. The stamped design pressure on the ISO tank frame plate and the pressure relief valve set pressure must be verified against the maximum vapour pressure of the loaded MTBE at the highest transport temperature. The fill-ratio calculation alone does not substitute for pressure relief sizing under ISO 1496-3:2019 and the applicable ADR tank code.

MTBE is carried under ADR tank code LGBF as a flammable liquid with a boiling point above 35°C. The tank container must carry the correct approval number, tank code, and test date on the vessel plate. A 24,000 L barrel loaded to the regulatory maximum contains approximately 16,200 kg of MTBE; a 25,000 L barrel contains approximately 16,900 kg. The lower fill ratios of 0.90 and 0.85 reduce the declared net weight to 15,984 kg and 15,096 kg respectively for a 24,000 L barrel at 20°C. These reductions increase outage volume but also increase free-liquid surface motion because ISO tank containers do not have internal baffles. Fill ratios below 0.80 can amplify surge forces on road and rail handling, while fill ratios near the regulatory limit restrict surge but reduce thermal buffer volume.

Pressure Relief Settings, Loading Line Velocity, and Bottom Valve Compatibility

ISO tank equipment for MTBE should use a stainless steel barrel or an internal coating qualified for ether service. Bottom valve seals and top gasket elastomers must be specified for MTBE because ether exposure can swell or extract certain natural rubber and nitrile compounds. During bottom loading, the initial liquid velocity should be limited to 1 m/s until the fill pipe outlet is submerged. After the outlet is covered, the velocity may be increased to a maximum of 3 m/s in accordance with API RP 2003. MTBE is a low-conductivity flammable liquid, and loading at higher velocities increases the risk of static charge accumulation in the bulk liquid and vapour space. The bulk liquid temperature should be measured near the tank sump and at the middle liquid level. If the two readings differ by more than 3°C, the lower temperature should be used for conservative density correction. A fixed use of 0.740 kg/L without temperature correction is valid only when the bulk liquid is at 20°C.

For a 24,000 L tank barrel loaded to the ADR-derived fill ratio, the regulatory maximum net weight remains approximately 16,200 kg across the normal ambient loading range. The corresponding value for a 25,000 L barrel is approximately 16,900 kg. Loading at a fixed 0.85 ratio at 20°C gives a conservative net weight of 15,096 kg for a 24,000 L barrel and 15,725 kg for a 25,000 L barrel. The declared net weight on the dangerous goods transport document must match the corrected weight after density and temperature adjustment. A mismatch exceeding 0.5% of the declared mass can trigger ADR or customs verification, particulary when the quantity is derived from a volume meter rather than a weighbridge.