depressurization of a gas dryer 2

[giruffo]

Hallo to the forum.

I (shortly) introduce myself.
I am a chemical engineer.
I have been working about 10 years in the field oil&gas as process engineer and commissioning engineer.

I would like to know if anybody in the forum has already treated the problem of depressurizing a a gas drier containing a bed of silica gel.

A supplier informed me that short time of depressurization could damage the particles of silica gel that could "explode" for the internal pressure.

The recommendation of the supplier was not to exeed 3 bar per minute for the depressurization.

This, of course give me some problem for what concerns the respect of API indication. (to reach half pressure or 100 psi in 15 minutes)


Can anybody confirm that silica gel get damaged for too fast depressurization? Does anybody know any literature reference about it?

Thanks in advance for your help

Giruffo

 

[Montemayor]

Giruffo:

I started out using Silica Gel in 1960. I’ve used adsorbents in drying Natural Gas, CO₂, air, nitrogen, nitrous oxide, oxygen, Hydrogen, ethane, and many other gases. I stopped using Silica Gel when Alcoa developed their H-151 Activated Alumina adsorbent and since that time – around 1962-63 – I’ve only used the Activated Aluminas and Molecular Sieves. With the advent of higher pressures and lower dew point demands, the need for more sophisticated and higher temperature regeneration has put too much pressure on the silica gels in my opinion. I don’t recommend silica gel to any one in the industrial applications – except in some rare cases for -40 ^oF dew point instrumentation air. The remaining industrial drying applications are too demanding to suit silica gel, in my opinion.

Like many before you, you’ve stumbled onto silica gel’s “Achilles’ Heel” (actually, bed movement). Any bed movement or agitation greatly increases the “dusting” of the silica gel. The adsorbent has an inherent propensity for “dusting” – breaking up into minute particles – and causing a lot of grief and pressure drop. Plus a lot of bed degeneration.

When you match the regeneration temperature limitations, the dusting characteristic, and the lack of producing a really dry gas (-90 ^oF dew point or better) you suddenly realize that the lower price of silica gel is the only thing going in its favor. If you are in the industrial gas business or in liquid hydrocarbon processing plants, then you need reliability and consistent performance. With their ability to withstand 500 to 650 ^oF regeneration temperatures, the Activated Aluminas can produce -100 ^oF dew points. And they do this by withstanding very high pressures and minimal dusting. For better quality product than this I go to the Mol Sieves – which starts to get expensive as adsorbents go.

Anyone besides me can confirm that Silica Gel has a propensity for attrition and dusting – which is why you are hearing stories of depressurization problems. It is not a rugged adsorbent. Some of these problems may also be caused or compounded because of bad design: the adsorbent beds should never be depressurized in an upward direction; this causes a “lift” of the bed and really causes a lot of problems.

The literature going back to the 1960’s is full of stories and studies on the attrition and dusting properties of adsorbents. Silica Gel heads the list.

I hope this experience is of some help.

 

[dcasto]

Yes, the silca gel will break apart and creat a dust that will enter the downstream process. The breaking up will also mean shorter bed life.

As for depressureing the beds to conform with some recommended practice, this would be for an emergency only, not a normal way to operate. If you elect to follow the API practice, the frequency should be very small and the damage to the silica gel minimal over its typical useful life. If plan on depressureing more than once per month or so, I'd rethink following the recommended API practice.