Moist Cargoes

The problem is quite different when the container holds a cargo that contains tons of water, such as a cargo of wood, agricultural products or paper. If any significant proportion of this moisture were released into the air, any desiccants would be overwhelmed. The trick is of course to try to make sure that the moisture stays in the cargo.
It is not possible to really control the moisture within a container loaded with a moist cargo, but from a good understanding of the interaction between the cargos, the air and the Container desiccants it is possible to manage conditions in a way to prevent damage.

If any moisture containing cargo is put into an enclosed space and left at a constant temperature for a length of time, water will evaporate or be absorbed by the cargo until an equilibrium is established at a characteristic RH level. This level depends on the particular cargo, its moisture content and the temperature. So long as there is any amount of liquid water in the container, the RH will essentially stay at 100%. For many agricultural products at normal moisture content such as peanuts, the equilibrium RH is typically 70-80%.

If the air should be somehow circulated and conditioned to a level of RH that is below the equilibrium level, the cargo will dry out. If the air one the other hand is made more humid than the equilibrium RH, the cargo will absorb moisture and its moisture content will increase.

Warm relatively dry air from a warmer part of the cargo, cools down and become more moist (RH increases) as it flows into a cooler section of the cargo. This results in a section of the cargo having an elevated RH, often high enough that mould will grow (>RH 80%). As the cargo absorbs moisture at high RH, the net effect is to create a migration of moisture from one part of the cargo to another part.

The moisture processes in the air happen very quickly. The interaction between the air and the cargo happen more slowly, over a period of time. How quickly a cargo evaporates or absorbs moisture depends strongly on how far the air RH is from the equilibrium RH and on the temperature. In fact this dependence has an exponential character.