While solid bed natural gas dehydrators using calcium chloride desiccants are simple to operate and provide gas outlet dew points suitable for many gas drying applications, there will be performance limits based on inlet conditions, and the outlet humidity will vary with time.
This is because the calcium chloride desiccant will be CaCl2 (anhydrous) or CaCl2•2H2O (di-hydrate) when it is fresh, which provides approximately 50% better dew point suppression than after the desiccant reaches an equilibrium steady state CaCl2•4H2O (tetrahydrate)over time. The time can be several hours to several weeks depending on the size of the bed and the inlet conditions of the gas.
The bed will continue to decrease and settle downward until it is several feet in depth and the hydrate is progressing to CaCl2•6H2O (hexahydrate). This will be evident when outlet water content approaches feed conditions (breakthrough) and the bed must be recharged. When possible it is desirable to size the bed to permit convenient recharging times.
A typical calcium chloride solid bed dehydrator will operate with saturated inlet gas temperatures ranging from 45 °F to 70 °F and provide an equilibrium dew point suppression between 20 °F and 27 °F respectively at a fixed inlet pressure. Higher inlet pressures will result in lower water content measured in lbs. H20/MMSCF gas.
Low temperature gas < 45 °F tend to smear the bed with H2O forming a tetrahydrate or hexahydrate brine that cools forming a shell on the desiccants surface speeding the process.
Tall beds with high contact times and 100% RH at higher temperatures will form large amounts of deliquescing brine in the lower portion of the bed and slow the process.
It is desirable to avoid free water and high inlet temperatures at low pressures to avoid heat effects, such as bridging.
Bridging is when adjacent desiccant tablets fuse or join together, which prevents the bed from settling. It is caused by a variety of reasons but mainly it is due to cyclic operating conditions or flooding. By itself, bridging is not a problem but it leads to channeling which will affect performance.
Channeling is when the gas tries to find a path of least resistance through the bridged bed and ultimately is not in contact with the unused desiccant bed causing premature breakthrough of moisture occurs.
Freezing can occur as calcium chloride hydrates form a dilute solution at low temperature on the surface of the desiccant. As the solution super cools it forms a solution and ice.
The problems associated with bridging, channeling, heat effects and freezing are negligible when operating at steady state flow conditions within recommended temperatures and pressures.
In applications where the feed temperature and pressure do not permit an outlet dew point low enough to utilize a calcium chloride solid bed dehydrator on its own; it may be desirable to use the calcium chloride unit as a low cost guard bed in series with a more costly downstream Lithium chloride desiccant gas dehydrator.
The guard bed of calcium chloride will remove some water helping to optimize the use of lithium chloride while protecting the high cost lithium chloride from slugs.