Detoriation of silicagel/glass fibre rotor 1

[MaxPlanck]

Dear all,

We are using a desiccant rotor for drying our compressed air. This rotor is build up of glass fibre paper, impregnated silica gel to adsorb moisture. The silica gel itself also acts as a binder to hold the glass fibres together.
After some time, we see that this rotor becomes soft and brittle. The rotor operates in conditions with temperatures up to 150°C during the regeneration phase and around 30°C during the adsorption phase.
Has anyone experienced similar reactions?
Has anyone an explanation for this phenomenon? Some already mentioned that a hydrothermal proces occured, but does this proces affect the silicagel or the glass fibre?

Many thanks in advance!

Best regards,
Max

 

[IRstuff]

And what did the manufacturer tell you?

TTFN

FAQ731-376

 

[MaxPlanck]

Actually our manufacturer does not have a solid fysico-chemical explanation.
His opinion is that our regeneration phase is not sufficient enough to dry the desiccant rotor completely (our calculations, simulations and measurement tell us the other thing).
Also according to our manufacturer, it is possible to wash the rotor with water.

I find this strange: if water is detrimental for the rotor (e.g. insufficient regeneration), why is it allowed to wash the rotor (dipping) in water?

 

[btrueblood]

Is the glass fiber breaking/cracking, or is just the silica gel deteriorating?

If the former, is there any chance that chlorides have contaminated/are contaminating your air system? Oil mist from compressor?

 

[MaxPlanck]

We did some SEM analysis and we mainly saw that the silica gel was deteroriating. Because this silica gel also acts as a binder, it seems that this deteroriating of the silica gel is the reason for the weakening/softening of the rotor.
The same SEM analysis didn't show any signs of chlorides, oils are impossible because of the oil-free compressor and filters (and also no oil was found in the SEM).
Is it possible that a combination of high temperatures (in the regeneration phase) and high humidities cause the deteroriation of the silica gel (or glass fibres), and in what way? What could be the physico-chemical reaction behind this phenomenon?

 

[btrueblood]

If silicate becomes over-hydrated (wet), it can become a gel again, which obviously is not as strong as the dry material.

 

[unclesyd]

Analyze for Sodium in your gel.

Less than 1% Na will cause the deterioration of glass fibre when heated above 140F.

 

[btrueblood]

Star from me, Syd. I'd said chloride attack, but I believe you are right that the sodium does more damage. In any case, it is a surprising fact that glass fiber can "corrode".

 

[quark]

We generally use recuperative type dryers for HVAC applications and PSA dryers for compressed air application. It is interesting to know that you are using rotary dryers for compressed air application. Silicagel is quite susceptible to degradation. You have three kinds of cycles that are repetitive in nature viz., adsorption-desorption, heating-cooling and pressurisation-depressurisation which can speed up the degradation.

Can I know what is the reason for going with a rotary dryer? How are you sealing the process and reactivation side? The cup seals used for sealing these two sides may not be very effective at higher pressures. PSA systems could have been economic.

 

[MaxPlanck]

Hi quark,

Thank you for your reply.

Regarding the operation of this type of rotary dryer, I can not go into detail.

You mentioned three types of desiccant detoriation, do you have an idea which one is the most agressive towards silicagel? Personally I think the combination of heat (150°C) and high humidity is the worst combination.
Are you familiar with the science behind this degradation proces?

Best regards,
Max

 

[quark]

If you observe softening then the problem is predominantly with adsorption and desorption. I am new to rotary compressed air dryers. Just check whether the quantity of silica gel is ok or not from adsorption isotherms. Check with manufacturer whether the system suits for compressed air application.