MOLECULAR SIEVE HEAT REGENERATION 3

[mmo]

I am designing a furnace for MS regeneration, what is intended is to eliminate sorbed water by means of a hot air or nitrogen stream. MS are used in oxygen enriching devices (PSA systems).Would like to know about tips and facts about of someone who walked this way before me. Gas temperature, and humidity measure in the hot gas are worrying me.Thanks

 

[TD2K]

We regenerated our mole sieve for gas dehydration with a counter current stream of bone dry gas stream off the back end of the process (if you are using wet gas to regenerate, which you can, you have to regenerate co-currently). Regeneration procedure was to take a bed off adsorption, depressure it and then start regen gas flow through it. The regeneration gas was heated using steam to about 450F I believe (it's been a long time since I worked in that plant). Once the outlet bed temperature reached the target temperature (I think around 400F), we would hold it there for a few hours with hot regen gas flow continuing to strip off the final traces of water. Then the bed would be cooled down, repressurized and ready to be put back on line.

We did not actually attempt to measure the water content of the regen gas leaving the bed (the moisture probe was blocked in as hot gas destroyed it). During the heat up cycle, the outlet temperature will show a small plateau (in our case) as adsorbed hydrocarbons came off the MS. There was a much longer plateau later as adsorbed water came off. Once the bulk of the adsorbed water came off, the outlet temperature would continue to rise and approach the inlet temperature.

 

[Samiran]

What I understand you're trying to design a furnace which will provide the regeneration duty. This duty will represent the primary operating cost of the dehydration system unless waste heat is available.

The total duty should equal the heat for heating the steel of the vessel + losses + sensible heat to bring the water to about boiling temp + heat of desorption. Heat Of desorption in MS could be ~4200KJ/kg. Typical outlet temp should be about 270- 290 degC for 4A/5A.

The overall equation is mCpdelT, where m is regen gas flow, Cp is sp heat of regen gas, deT is th temp diff of regen gas between initial and final temp.

The furnace would typically have some efficiency, so use this to estimate the final furnace heat duty.

 

[saidsaz]

why u need a furnace to supply regeneration duty ..most mol sieves are regenerated using steam heaters or electric heaters ...unless u have some waste heat recovery potential ..

what type of mol seives u have ...do u need a preload step after regeneration ( safety related step ) ,,,what is maximum temperature ur regen gas can have ,,,what is normal direction for adsorption versus ur plan for regen operation ..what is allowed back pressure for regeneration operation ,,,is your control system adequate ?

i think u should go carefully through this and other considerations before trying to just provide a hot enough regen gas ...

the vendor should be of great help to u ...

regards ..

 

[mmo]

Thank you everybody for your useful information.I am building a furnace working prototype with a 2Kw heating element.
The regenerated MS needs to be repoured on small cilinders.
I am working now on how to obtain a "humidity zero room" to perform this operation into.
RGDS

mmo

 

I also have a question about the regeneration temperature profile of the molecular sieve. It first rises, then at about 130C a plateau (when the water desorps), and after that it rises again. How can you explain that properly in terms of equations (Samiran gave an explanation which was useful) The molecular sieve as well as the percentage of water it contains has to be heated. How can you calculate this? How do you know the percentage of water the sieve contains exactly? Do Langmuir isoterms apply? Does coupled mass and heat transport apply? I would very much appreciate it, if someone helped me out and gave me some tips and equations...