Molecular Sieve Considerations
April 1, 2010
- Ambient temperatures are recommended for moisture removal. At temperatures above ambient, the absorbent effectiveness decreases. Temperatures below ambient can be used until the viscosity reaches 5 cP, but lower temperatures require special equipment considerations. The sieves can lose their effectiveness if the viscosity increases above 2 cP.
- Optimum flow rates can vary depending on system size and water concentration. In general, higher flow rates require larger molecular sieve beds due to reduced contact time. Slow flows (less than 10 gal/min) usually are acceptable in typical beds provided they do not channel. The molecular sieve manufacturers should be consulted on the operating conditions for the specific heat transfer system using the organic heat transfer fluid.
- The molecular sieves can be reactivated with heated nitrogen flow through the molecular sieve bed followed by cooling with ambient temperature nitrogen. Reactivation with heated nitrogen can be accomplished with an electric heater or through a combination of heating with steam and electric heat for larger molecular sieve towers.
- Generally, a small pressure drop will occur through a molecular sieve bed. For example, a bed with a diameter of 1.5′ and a height of 4.5′ could have a pressure drop of 2 psi for a flow of 50 gal/min and 0.13 psi for a flow of 5 gal/min.
- The removal of moisture from organic heat transfer fluids that have high moisture solubility generally will require longer time periods for the moisture-removal processes. Larger quantities of molecular sieves will be needed to remove moisture effectively in these cases. With some fluids that have a strong odor, nitrogen purging of the system expansion tank vapor space to remove moisture is not recommended.
- Molecular sieves are not efficient at removing free water. Free water will saturate the molecular sieves, which then will require frequent molecular sieve replacement or regeneration. To bring the water level down to the saturation level where molecular sieves are effective, a coalescer or another free-water removal technique needs to be employed first.