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[rsk]

We use silica gel as dessicant in dual cycle dryers for compressed air drying. When one of the bed in adsorption other is being regenerated.Regeneration is done by passing ambient air over heater and then throgh bed till temp of bed reaches 180 d egC. Then it is cooled by continued flow of ambient air but this time putting off the heater. When temp comes down to 65 d egC blower is stopped and dry instrument air from outlet of bed in service is passed for 20minutes.this we call as purging mode.Please why this purging require. Here we are losing lot of instrument air.

[Art Montemayor]

RSK:

 

You don't tell us what your product air moisture content requirement is nor do you furnish a schematic sketch of your adsorber piping and the direction of the different air flows you identify.  Refer to the attached sketch that shows how I would pipe up and operate an air adsorption dryer.

 

Depending on how your regeneration piping is installed and in what direction you are flowing the regen gas, my guess is that the original designer and builder (whoever that is or was) intended to used moist ambient air to start the required cool-down period and arbitrarily use product dried air to finish up the cool-down and achieve a re-generated adsorption bed at the desired approximate ambient temperature.  You should start your adsorption cycles at the same temperature as the incoming, humid atmospheric air.

 

What you are calling a "purge" cycle is in reality a portion of the regenerated bed's cool-down cycle.  You are not purging anything.  You are merely continuing the final cool-down period - but with a drier air flow.  Study the sketch I have furnished very carefully and note that you are regenerating with atmospheric humid air.  This is no problem for the bed in question at the very outset of the heating portion; however, when you have to cool down the bed, you would normally prefer to do it with DRY air - at least air with the same moisture content (dewpoint) as the product air you want to produce.  Think it out logically: when you finish your regeneration you want the bed to be in as dry a condition as you possibly can get it in order to have it adsorb as much moisture as possible in the next, subsequent cycle.

 

I consider this type of air adsorption cycle as a very undesirable and overly complicated one that has little or no advantage over much simpler cycles.  I have designed, built and operated many adsorbers in the past drying a lot of compressed gases (air, CO₂, N₂, O₂, CO, CH₄, N₂O, etc., etc.) producing product gas with an atmospheric water dewpoint of -90 ^oF and lower using process humid gas as the regeneration gas flow with no purging or venting required.  I have done this by designing a portion of the adsorption beds to have sufficient capacity to withhold the quantity of moisture introduced in the regeneration gas stream.  I was able to carry this operation out by selectively routing the humid regeneration gas flow direction in the same direction (top to bottom) as the process drying stream.  The regeneration was done under actual process pressure conditions for both the adsorption and the regeneration portions of the total drying cycle.  Since I don't require any blower or compressor to furnish regeneration atmospheric air to regenerate and I don't require any change-over from atmospheric air to product dry air to cool down the beds, my process is much simpler and easier to control than what you describe.  That's why I consider your system as very awkward and complicated.

 

An additional experienced comment: how do you identify the temperature of the regenerating bed as reaching 180 ^oC?  Do you measure the actual temperature of the adsorbent beds within the bed?  Or do you simply measure the temperature of the regen gas exiting the bed - as most of us would do?  If it is the latter, bear in mind that common sense tells us that the exit gas temperature is not necessarily the temperature of the bed.  It normally is much higher than the bed's temperature - by approximately 20 ^oF - or more.  That is one reason I always designed my adsorption vessels to withstand at least 500 ^oF.  This allowed me to take the beds to approximately +400 ^oF - ensuring an excellent regeneration for the beds and a subsequent superior dewpoint of the product gas.

 

Common recommended regenerating temperatures for various adsorbents - by major operating companies - are as follows:

 

• Molecular Sieve:  204 ^oC to 277 ^oC (400 ^oF to 530 ^oF)
• Silica Gel:  177 ^oC to 316 ^oC (350 ^oF to 600 ^oF)

• Activated Alumina:  177 ^oC to 316 ^oC (350 ^oF to 600 ^oF)

• Sorbead:  177 ^oC to 277 ^oC (350 ^oF to 530 ^oF)

 

 Adsorption System Configuration.xlsx   19.93KB   25 downloads