2-Propanol ISO Tank Loading Parameters: Net Weight, Filling Ratio and the 18.5 t Benchmark

2-Propanol (isopropyl alcohol, IPA; CAS 67-63-0) is transported as UN 1219, Class 3, Packing Group II, with a closed-cup flash point of approximately 11.9 °C and an initial boiling point of 82.5 °C. The ISO tank loading parameters for this product are not simply a matter of available shell capacity; they couple net weight, filling ratio and the 18.5 t benchmark to the liquid’s thermal expansion behaviour. At 20 °C, industrial-grade isopropyl alcohol has a density of 0.785–0.786 kg/L, a vapour pressure of approximately 4.4 kPa, a dynamic viscosity of approximately 2.4 mPa·s, and a cubic expansion coefficient of approximately 0.0011 K⁻¹. These values are referenced to the temperature at the loading arm, not to the tank certificate temperature, because both net weight and filling ratio shift when the liquid temperature changes.

The 18.5 t benchmark is an operational planning figure frequently used for a 20 ft ISO tank container carrying isopropyl alcohol. It is not a shell limit under ISO 1496-3:2019, nor is it a universal allowable mass for every tank. A standard 20 ft ISO tank shell can have a water capacity from 24,000 L to 26,000 L, with tare masses commonly between 3,700 kg and 4,100 kg. If the same 18.5 t mass is applied to a 24,000 L shell, the volume at 20 °C is 23,567 L, which is 98.2 % of water capacity and leaves almost no practical outage. Therefore, the benchmark must always be checked against the individual tank shell and the product temperature.

Why Is Net Weight Not Simply Shell Volume Multiplied by 0.785 kg/L?

Net weight on a bill of lading for isopropyl alcohol is defined by weighbridge gross mass minus tare mass, or by a Coriolis mass-flow meter reading if the terminal has verified the meter against a calibrated weighbridge. The net weight cannot be derived from tank capacity alone because the tank cannot be filled to its full water capacity: a liquid outage must remain for expansion from the filling temperature to the reference temperature of 50 °C. A volumetric reading taken at 20 °C must also be density-compensated. If a fixed density input of 0.785 kg/L is used while the actual product is at 25 °C, the density is approximately 0.781 kg/L, and the resulting mass error can exceed 0.5 %, or approximately 90–100 kg on an 18.5 t batch.

A full 24,000 L shell filled to the top at 20 °C would contain 18,840 kg of isopropyl alcohol, and a 26,000 L shell would contain 20,410 kg. Those values illustrate why the 18.5 t figure is not a fixed volume: it is equivalent to 23,567 L at 0.785 kg/L. The benchmark is closer to the safe loaded volume of a 25,500–26,000 L shell than to a 24,000 L shell when the filling ratio is applied. Density should be confirmed by ASTM D4052-22, while flash point is corroborated by ASTM D56-22.

Thermal Outage, Filling Ratio, and the 25,626 L Shell Threshold

The maximum safe charged volume at 20 °C is calculated from the maximum permitted filling degree at 50 °C, which is 95 % for this Class 3, Packing Group II liquid in the absence of toxic or corrosive subsidiary hazards. The working filling ratio \( F_t \) at a lower fill temperature is \( F_t = 0.95 / [1 + 0.0011\,K^{-1} \times (T_{50} - T_f)] \). For \( T_f = 20\,^\circ C \), \( F_t = 0.95 / 1.033 = 0.920 \). The allowed charged volume at 20 °C is therefore 92.0 % of the tank water capacity, not 95 % of the water capacity.

At a loading-arm temperature of 25 °C, the factor becomes \( 0.95 / 1.0275 = 0.925 \); at 15 °C, it falls to approximately 0.915. The terminal must compute the filling ratio at the actual product temperature and convert it to net weight using the density at that same temperature. Failure to do so creates a systematic underfill or overfill that cannot be detected by a weighbridge alone unless the expected net mass is compared with the calculated volume limit.

The table demonstrates the narrow capacity margin around the 18.5 t benchmark. A 25,000 L shell carries only 18,055 kg at the 20 °C filling ratio of 0.920. The 18.5 t benchmark enters only when the shell water capacity reaches approximately 25,626 L, which corresponds to a loaded volume of 23,566 L at 0.920. Terminals that use the 18.5 t benchmark without confirming the shell capacity are implicitly assuming a 25,626 L or larger vessel.

On a nitrogen-padded 25,000 L loading station equipped with a Coriolis mass-flow meter and a calibrated weighbridge, the mass reading is cross-checked against the thermal outage formula. With the product at 25 °C, the allowed charged volume is 23,125 L at the 0.925 filling ratio, and the net mass is approximately 18,050 kg at a density of 0.781 kg/L. A terminal that instead loads the commercial 18.5 t benchmark into this 25,000 L shell creates an overfill risk relative to the 95 % reference maximum at 50 °C. The back-pressure valve downstream of the meter is held at approximately 1.5 bar g to prevent dissolved nitrogen breakout and two-phase flow, and the loading arm is bonded to earth before connection. Such mass-flow systems provide direct net-weight output, but they cannot replace the shell capacity check.

If the 18.5 t Setpoint Is Applied Without a 20 ft Shell Capacity Check

The arithmetic creates a hard process limit. At 20 °C, an 18.5 t net mass of isopropyl alcohol occupies 23,567 L at 0.785 kg/L. In a 24,000 L shell, that is 98.2 % of water capacity before any temperature rise; after heating to 50 °C, the volume becomes 24,345 L, exceeding the shell by 345 L. In a 25,000 L shell, the same product at 50 °C occupies 97.4 % of water capacity, which exceeds the 95 % reference filling degree by 595 L. The 18.5 t benchmark is therefore a valid mass target only when the shell water capacity is at least 25,626 L, assuming a density of 0.785 kg/L and an expansion coefficient of 0.0011 K⁻¹.

If the density is higher because the isopropyl alcohol has absorbed water, the volume needed for 18.5 t falls, but the expansion risk remains significant because the thermal coefficient does not decline proportionally for dilute water mixtures. Published data for this specific water-contaminated configuration is limited, so the loading calculation should use measured density rather than an assumed 0.785 kg/L value. The maximum filling degree of 95 % at 50 °C is applied under the tank filling-degree provisions of ADR 4.3.2.2 and IMDG 4.2.1.9 for this Class 3 liquid, and the tank shell itself is designed and tested to ISO 1496-3:2019.

Internal cleanliness also interacts with the 18.5 t benchmark. A residual heel of methanol, methyl ethyl ketone or ethyl acetate at 0.5 % by volume in a 25,000 L shell may alter the density and flash point enough to shift the mass and hazard classification if not managed. Before loading, the shell is drained, dried, and checked for odour and liquid pools behind the surge plate; the wash procedure uses warm air or nitrogen and cannot use water alone because residual water increases the density of the next isopropyl alcohol load. If the tank is not completely dried, the payload density rises and the same volumetric filling ratio produces a higher net weight on the weighbridge, which can push a nominally 18.0 t filling into the 18.5 t documentation range without any volume gain. The loading supervisor therefore records product temperature, density, shell water capacity, calculated filling ratio, meter batch total, and weighbridge gross–tare–net for each movement.