What desiccant should I use to remove siloxanes from landfill gas (LFG)?
Most of our customers use white, granular silica gel with a 1-3mm grain size. Beaded silica gel is also available, although it is not as widely used as the granular form for this application. White (non-indicating) silica gel is normally specified because the indicating chemical in blue (indicating) silica gel slightly reduces its adsorption capacity.
What effect does particle size (for either beaded or granular silica gel) have on the system?
Particle size affects pressure drop through a bed of desiccant. Typically, the larger the particles, the lower the pressure drop at a given flow rate through the bed. The beaded form of silica gel will generally have a smaller pressure drop across the bed for a given particle size than granular silica gel.
How can I maximize the adsorption of siloxane by the desiccant?
The lower the moisture in the gas stream, the higher the adsorption of siloxane by the silica gel. This is because the desiccant prefers the H2O molecule due to its polar nature.
What is the effect of pressure on siloxane adsorption by the desiccant?
We do not know of a correlation between pressure and siloxane adsorption, and our experience has shown that pressure has minimal effect on H2O absorption. However, the LFG combustion system that we supply silica gel for in Southern California is run at 80 psig. This system pressure is believed to not be related to siloxane adsorption, but a result of other system requirements. Another user has had good results using a system pressure of 5-10 psig.
How do I know when the desiccant is saturated with siloxanes?
Downstream gas monitoring is required to watch for an increase in siloxane levels. When the silioxane levels begin to trend upward, the bed needs to be replaced. There is no change in the physical appearance of the desiccant when it is saturated.
Can I regenerate the desiccant when it is saturated to drive off the siloxanes?
Regeneration is possible. However, due to the large amount of desiccant used in a normal LFG system, it is usually not economically feasible.