molecular sieve regeneration with "warm nitrogen"

[mrtangent]

Dear All,

I'm looking for some advice on molesieve usage. We use molecular sieves to dry out liquid product down to about -80 dew point / or ppm water. To re-generate the beds we use a cold nitrogen purge at about 50 kg/hr for a 15 m3 bed at 1 barg.

We dont really know how much N2 we should use and /or if moderate heating would work. The outlet of the n2 used for drying needs to remain below 50deg C due the vent handling system.

The nitrogen we use for drying is from a BOC nitrogen plant the measured dew point for the n2 on the system is -25/-30deg C.

The current re-generate does seem to work if we re-generate over two weeks.

We use 4A molesieves.

So,

> Would heating the N2 to say 37 deg C be effective ?
> would N2 pressure swing be more effective , say swinging the bed from 0 to 5 barg as it could help release H2O from the pores of the sieves.
> I've not hand lots of information from the manufacturers of molesieves.
> What would be a good ref/article to read?

Thanks in advance James

 

[ris2]

You can get some information on mol sieves from the John M Campbell Company. The book is Gas conditioning and Processing.

 

[toothless]

That is exactly how we regenerates our mole sieve. We had a 40 F waste stream that was about 85% dry nitrogen. We used a steam heat exchanger to heat it to about 300 F and passed it through the mole sieve bed. When the regen outlet temp reached 250 F, we knew it was dry. then we cut the heater off and cooled the sieve back down to process temp.

To regenerate, you want a warm, dry, low pressure gas.

 

[Montemayor]

James:

You don't state what make/composition of Mol Sieve you're using, so it is folly and not smart to recommend any specific method, temperature or pressure to regenerate the beds at.

However, generic Mol Sieves were originally made by Linde (then Div of Union Carbide) to dry any gas down to -100 oF and lower! I can produce Nitrogen gas with -100 oF dew point merely using Activated Alumina instead of Mol Sieves. I can do better than that with Mol Sieves. Mol Sieves generically require higher regen temperatures than other adsorbents to strip out any H2O moisture and other adsorbed impurities; I traditionally have used 600 - 700 oF where my materials of construction were up to it. It is common sense that an adsorbent that dries down to the fractions of a PPM will require a higher energy level to regenerate itself. This is to be expected in accordance with the Universal Engineering Law: there are no free rides or lunches.

You also don't state your manufacturer of the Mol Sieves; I wouldn't proceed with any regen cycle design without their written approval; dusting or attrition will kill you if the adsorbent can't take certain levels of regen temperature. The designer of your adsorption beds is also unknown; I would consult with them before going to the Mol Sieve manufacturer.

Additionally, you should know (although you haven't mentioned it) that every regen cycle is composed of:

1) a heat up step;
2) a cool-down step; amd.
3) a purge step (only if the regen gas used is different from the process gas).

So, common sense tells you that if you use hotter gas to regen, you will achieve ultra-low dew points in the product; but, you will also have to cool down the regenerated bed before putting it back into service. Again, you haven't said if you have a dual-bed system or a multiple (more that 2) bed system.

I don't care who made your Mol Sieve; Pressure Swing regeneration will never produce a better regenerated bed than Temperature Swing regeneration. I've used Linde, Davidson, & other makes and I feel very confident in making that statement.

If your dew point is of importance, I would insist on regenerating at as high a temperature as possible within mechanical design limits, at as low a regen pressure as possible, and with as dry a regen gas as possible. If your regen gas is not drier than your expected design product gas, then you will have to employ a co-current flow with the regen gas. This is very, very important adsorption process design criteria. If you regenerate a bed with counter-current flow using a relatively moist or wet gas, you will produce an off-spec product due to the equilibrium effect produced by cooling the bed with a wet(ter) gas and leaving the exit portion of the adssorption bed saturated with moisture at the moment of switching the beds. This is an effect that was wrongly and badly misunderstood in the early stages of applying adsorption drying about 40 years ago. It is still misunderstood by a lot of young and inexperienced process engineers who don't have much experience with adsorption beds and their regeneration and cool-down cycles.

I'm not at my home library, otherwise I'd furnish you with a list of recommended books and reading sources on the subject.

Art Montemayor
Spring, TX