2 replies to this topic

[chemengineer111]

Hello!

 

I'm Chemical Engineer and I would like to improve my knowledge regarding the design of Heatless Air Dryer for instrument air supply at power plants.

 

At my engineering company, I had experience in engineering Under Pressure Vessels, interconnecting piping, instruments and valves, providing clients with all sort of engineering documents, such as datasheets, sketches, isometrics, layouts, support drawings, wiring diagrams, specific control software, etc.

Besides, I already had a huge previous experience on heatless air dryers, but always sized by others. Now the goal is to design them completely by myself.

What I would like to verify and improve is the know-how for sizing adsorption columns, pre and after filters, choosing all needed dimensions for the good operation and maintenance of the dryer. Moreover, is it more suitable to use activated alumina, silica gel, or (somehow) a mix of both?

Regarding the operative conditions, let’s assume I have compressed air coming from one (or more) air compressors, with following characteristics:

• Operating pressures 6,5-10 barg
• Inlet temperatures 50-65°C
• Inlet flowrates 400-3000 Nm3/h
• HR inlet air up to 100%
• Discharge dew point up to -40°C at operating pressure

Usually, for heatless regeneration we used the purge (15% of outlet dried air) with a 5+5 minutes.

 

Do you have any suggestion, excel calculation sheet, or specific literature sources in order to do the complete design of this kind of dryer?

 

Thank you in advance,

Bye

[Art Montemayor]

 

Andrea:

 

I have done this before, so I can pass on my experience and some documents on the subject.  If you are dealing with adsorption-based air dryers, then I believe that expecting a consumption of 15% of the feed air as purge loss is optimistic.  I believe that 20-25% of the feed air as purge loss is more realistic in practice when dealing with heatless dryers.

 

The design of pre and after filters has already been done by a variety of companies in the business and I always simply just order these items from a catalog.  I have never spent time in designing these.

 

The appropriate adsorbent for drying air depends on what dew point you require for the product air.  For -40 ^oC atmospheric dew point, activated alumina would be my choice.  Silica gel is out of the question because it has dusting tendencies and the constant pressuring and de-pressuring of the beds generates a lot of adsorbent attrition and dust.

 

An inlet air temperature of 50-65 °C is totally impractical.  I don’t think it is possible to dry the air to your specifications using air as hot as that.  If you are going to design for clients, then you should force the client to cool down his air to a normal level of 20-30 ^oC.

 

Drying 3,000 Nm³/h with a heatless dryer will require some relatively large piping and valves.  This requires special attention to the type of valves, seals, instrumentation, etc.

 

If you are applying an instrument air dryer at a power plant, then I challenge the use of a heatless (and relatively inefficient) dryer rather than a HSA (heat swing adsorber).  You would find a plentiful heat source in the normal operation of a power plant – either in the flue gas or in the steam + condensate – enough to heat regen gas up to 300 ^oF.  I would spend some time investigating that form of adsorbent regeneration.

 

I hope this experience helps.

 

 Adsorption Design Guide - US Army Corp of Engrs.pdf   754.64KB   111 downloads

 Heuristic Design of PSA.pdf   565.49KB   86 downloads

 How-To-Guide for Adsorber Design.pdf   510.39KB   91 downloads

 

[chemengineer111]

Dear Mr. Art Montemayor,

Thank you for the careful and prompt response! 

 

I will look deeper into your documentation and I will write here again hoping to have some conclusions, in order to have one further confirmation by you, since you seem to be used dealing with such kind of topic.

Until soon!