Molecular Sieve Adsorption Dryer Trouble Shooting

Hi All,

Our DHU is designed on TSA based and we are facing problem in moisture break through. here is some information for your consideration and feedback on this topic.

MOL Sieve type: 4 A (UOP)

Bed: 2+1 (one on regeneration +one adsorption+ one standby)

Feed gas flow: 98 MMSCFD

Regeneration gas flow: 10 MMSCFD (design)

Mole Sieve height: 5.79m (4A mol sieves loaded in beds).

Liquid recovery in scrubber: very low

Gas inlet temperature: 68 0F

Feed gas is cooled with propane chiller before entering in inlet scrubber.

When regenerating on low pressure it work fine for three days and then start moisture breakthrough on normal (high pressure regeneration mode). I attached a DCS screen shot that is indicating bed inlet and outlet temp difference is almost 40 0F but outlet temperature start decreasing after 40-45 minutes. What i understand water is not fully removed because outlet temperature remain high for longer time.

Forum experts i need your advice how to trouble shoot this problem.

Regards

Root 

Edited by Root, 01 July 2018 - 10:14 PM.

You probably have liquid carryover that has fouled the sieves. You need a proper filter-separator upstream of the dryers. The only cure is to install the filter-separator and replace the sieves.

Bobby

Hi Bobby,

Mol Sieve dryer inlet we have coalescer filter 10micron size. hence no liquid carryover. feed composition also does not have olefins , no sulphur as well.

Toor

Root:

What is a "DHU"?  I need this identity to edit the title of this thread.  Otherwise, it won't contribute anything to our Search Engine.

Give us the history of the sieves. And what is the gas being processed along with conditions. Unless the sieves are old, it still can be due to liquid carryover if the filter separator is damaged, or not draining.

Bobby

Sir,

Dehydration unit (DHU).

LPG recovery plant started this year march. Mol sieve dryers are filled with UOP 4A type moles. inlet of Mol sieve dryers are cool down to 68F in chiller, then gas flow to inlet scrubber and then to coalescer filter and then to mol sieve dryers. 

Dryer fill up drawing is attached for consideration.

Root

I assume that all three dryers show the same behavior. Is that true? If so, it's likely due to some contaminant in the gas. You will eventually have to replace the beds sooner than you planned.

Bobby

Yes all three bed behave similarly, Inlet gas C6+ is .009 and dry gas also have same as inlet. once again i repeat there is no pressure drop during regeneration time. the change is only in temperature. additionally no valve passing. 

when we regenerate at low pressure then bed is working very well and dew point goto below -98 F.

Root 

What do you mean by "the change is only in temperature"? What is the normal operating pressure? What is the regeneration gas? What is the low pressure regeneration pressure? We need lots more information. Including gas composition and source. It still looks to be some contaminant. What are the cycle times? When did this begin to happen? Have you consulted with UOP?

Bobby

Here is more information,

Gas is coming from separator, you can read my 1st post for more information.

Adsorption time: 8 hrs

Heating time : 5 hrs

Cooling Time: 3 hrs

Hold time: 5 minutes

Normal operating pressure: 1050 psig

Regeneration normal operating pressure: 1050 psig

In case of low pressure regeneration then pressure: 510 psig.

Feed composition: (mole%)

C1:84.398

C2:7.695

C3:3.283

iC4:0.298

nC4:0.444

iC5:0.061

nC5:0.039

C6+:0.014

N2;1.002

CO2;1.428

Inquiry has sent to uop but waiting for their reply.

Root

Look likes you have channeling problem due water carry over from upstream separator

No water carryover, keep focus on my input information.

Do you recycle cooled regeneration gas to dryer feed chiller? If there are other similar operations within the field, you should talk with them.

Bobby

Regeneration gas after cooler and regeneration scrubber going back to feed inlet line, the aim is max hydrocarbon recovery.

Root

So, if you have some strange contaminant then it will build concentration via the regenation gas recycle. Must be something unusual, because I haven't heard of problems that you are experiencing. UOP should have some suggestions for you. And, UOP should check the sizing of your dryers. But at this early operation the sieves should be quite active.

Bobby