Absorption of moisture and heavier hydrocarbons in a flowing Nitrogen stream is the most effective at low pressures - it's that simple. The results of deriming are not as nearly as effective at pressures close to the unit operating conditions.

There may be other reasons why the unit pressure should be maintained low - there could be differential pressure limits on heat exchangers or other equipment, or it could be the case that pressurizing the unit above 1 barg could result in a violent moving-breaking of the ice/condensate plug inside piping or equipment, that could behave like a projectile and damage the equipment.

-70 degC dew point is probably the safe threshold value below which water content does not represent any problem in the process, based on the unit licensor experience. For various cryogenic processes and technologies, these threshold values for dew point vary between -40 and -80 degC.

Dejan IVANOVIC
Process Engineer, MSChE

The vapor pressure of ice is constant at any given temperature. To remove this water vapor from your equipment a nitrogen purge is used. If you reduce your system pressure by a factor of 10, you will use 10 times less nitrogen to accomplish the same result.

1.0kg/cm2 may be the upper limit operating constraint related to the design pressure of the upper LP column.