Bead versus Pellet form of molecular sieve

[gks]

Hey all,

Can any one suggest which form of molecular sieve is superior - the bead form or the pellet form?

Thank you.

 

[TD2K]

There's a lot of arguement over that, pellets have more surface area than a bead, beads are stronger.

I'm not sure I've seen much difference in service, I've had good service with both types and had both types fail because of problems with the sieve batch itself.

 

[Samiran]

I would say TD2K is right. I've seen each vendor claiming tall in each type of sieve.

However, I can say pellets are more prone to breaking and also induce greater pressure drop. But well designed and manufactured pellets offer very good performance.

 

[gks]

Dear Mr. TD2K and Mr.Samiran,
Thank you for your valuable comments.

 

[TD2K]

We had pretty oversized driers so when we would dump the beds because of dP, we'd screen the old sieve to remove fines, and just reload the sieve and top up the driers with fresh sieve. The broken chunks of pellets, other than if they got really small, worked just fine. DP would be a little higher but that wasn't a concern to us versus the cost savings.

 

[TomOC]

For a given size pellets tend to have a higher pressure drop. When sizing vessels beads allow smaller diameters because they are harder to fluidize.

Tom

 

[Aldream]

TD2K:

Pretty interesting with re-using the seive! Our dryers are activated alumina desiccant type. This desiccant is expensive and we've been replacing it every 3 to 4 years. Do you think we can screen the old desiccan and re-use it? If so, how do you screen it?

Thanks
KAD

 

[TD2K]

It depends (don't you just love that phrase?).

The plants I was involved in had large oversized dessicant beds so we could really run out into the plateau area of adsorption capacity versus time and typically, we had to shut down due to pressure drops across the bed rather than cycle times. Thus, the material in the beds could be reused after the fines causing the high dP were removed.

The molecular sieve was simply screened first to remove the large hold-down and support balls and then screened again to remove the fines. We simply contracted with a local company who came in with the necessary equipment and we unloaded the beds into their equipment. After screening, we made up the loss of dessicant fines with new material (I can't remember if we loaded the new dessicant on top to take advantage of its higher water capacity or on the bottom to take advantage of a shorter mass transfer zone, I think it went on the top).

What material are you drying with your alumina? You want to make sure there isn't any hazardous material on the dessicant, pyrophoric material, when the contractor is handling it. I'll see if I can track down some specifics on the procedure for you on the area of safety. I was more involved with this at arm's length than the actual planning and execution.

 

[Chuck56]

Screening alumina is less practical than sieve, because sieve tends to break up an cause the increased pressure drop mentioned. 3-5 years is a typical bed life for alumina (as siad earlier, it depends), so you are getting reasonable life.

As a rule, if one is 'topping off' a bed with fresh adsorbent, put the freshest material on the outlet portion of the bed.

Chuck