Dessicants for drying air 1

[minty]

Hi.

I am looking for a suitable dessicant/drying method to reduce the amount of water in air to as little as possible. Can anyone please suggest cost effective methods to achieve this?

With regards to the final water content of the air, I'm basically looking to reduce it to as low as possible: the air will be used to cool a recently dehydrated substance and I don't want any water getting back in there!!!

Thanks a lot,
Minty

 

[quark]

Minty!

In other words "As dry as possible" is a vague expression and could cost you a fortune. (you may think spoiling the product is better at a later date)

Basically dehumidification by desiccant is constant enthalpy process. So sensible heat of air increases, adding heat load of air, which requires to be cooled. Further to this some portion of regeneration heat of desiccant bed gets added to the air.

You have to clarify the following points.
1. What is the moisture level in your product?
2. What is the ambient condition (/conditions)?
3. Do you recirculate the air or not?
4. What temperatures you require for cooled air?
5. What is your product?

I hope I may be of some help with these details.

 

[kenvlach]

It sounds like you are only dealing with small amounts of air, maybe on a vent line to a desiccator, so refrigerated air driers and self-regenerating dessicating equipment are probably not under consideration.

Desiccants and their efficiencies are listed in the CRC Handbook of Chemistry and Physics. The ones commonly used are calcium sulfate ‘Drierite,’ CaSO₄, which is available in indicator form (doped with cobalt to change color when saturated) and magnesium perchlorate Mg(ClO₄)₂. The magnesium perchlorate is more efficient at removing moisture; however, when preparing a drying tube, normally both are used, since the magnesium perchlorate turns into a mess when wet. On the inlet (the ‘raw’ air side), put some glass wool, then Drierite (with blue, indicating Drierite mixed in), then some more glass wool, then the magnesium perchlorate, then some final glass wool to keep things in place. This set-up should reduce the moisture to less than 0.002 mg per liter of air at ambient temperature.
Include a stopcock valve on the inlet to protect the desiccating material when not in use. You should be able to find further instructions if necessary in quantitative chemical analysis books.

You can get materials and equipment from the lab suppliers, like

https://www1.fishersci.com/index.jsp

http://www.spectrumchemical.com/retail/

 

[TD2K]

As said, a dew point or water content you need would be helpful.

Silica gel and activated alumina are commonly used in air driers, pressure dewpoints of -40F with either material are possible.

The lowest dewpoints are typically reached with molecular sieves. Activated alumina has a similar potential dewpoint but the water capacity of activated dewpoint falls off sharply at low water vapor concentrations while molecular sieve stays much higher. Pressure dewpoints much lower than -100F with molecular sieves are quite common for cyrogenic air separation plants or ethylene plants chilldown trains. Molecular sieves however require higher temperatures to regenerator compared to activated alumina.

 

[Montemayor]

Minty:

Both Quark and Kenvlach are right. You need to be more specific to obtain some sort of usable recommendation or response. TD2K is also on the right track - if you are using compressed air (>100 psig).

You need to tell us the air flow rate and its conditions. For example, you may only need atmospheric or slightly pressurized air. In that case, forget about fixed bed adsorption with silica gel or activated alumina. Also, do you intend to recirculate the dried air? You may simply have a situation where the indicated economic process is a refrigeration system that removes the water moisture by simply refrigerating the air and subsequently passing it through a heat exchanger to precool the entering air while warming up to near ambient temperature. Under this process, your dehydration level is not very low (the water content is the vapor presssure at the refrigeration temperature and pressure) -- but it may be enough for your purposes. We simply don't know. We need more specific informtion as to what you want to do and what is your scope of work. Like TD2K says, you can adsorb with Activated Alumina (I've done this down to a -75 oF dew point) or use Molecular Sieves and produce air with less than 1 ppm of water. It's all up to you. Let us know and a recommendation should be very easy and straight forward.

 

[Chuck56]

The best guide is this: determine if the product must just avoid condensation; if something akin to Instrument Quality Air (18 deg F dewpoint below the minimum recorded ambient in the area), or if something as low as cryogenic (<0.1 ppmv, or -100 deg F dewpoint ) is required. Cost will increase with increasing purity.

A number of dryer companies can provide packaged air drying systems. Depending on your requirements, you may just need an off-the-shelf unit or something that is custome-designed. Let me know about what you are trying to do and I will try to steer you in the right direction. I used to design and market dehydration systems and still work in the adsorbents area.

 

[minty]

Dear Everyone,

Thanks very much for your help and advice. It was very greatly appreciated.

However ...!!!

The problem has been solved on-site.

As I said, thanks a lot for your help.
Yours,
Minty