A3 Axen Mol Sieve

[10815L]

Hi,
In our new facility for LPG/LNG production, we have selected A3 Axen mol sieve for gas deep dehydration, during startup some upsets were faced due to activation of FDIs and plant is getting shut down.after normalising every thing and now moisture is getting break through from mol sieve beds before completion cycle timer. our gas is heavier and consist on following main components.
C1 to C6 plus Benzene, toluene, methylcyclopentane in trace quantities (0.003), some of wells are waxy nature also.
as per design these heavy hydrocarbon should not deposit on mol sieve but now looks that some thing went wrong,can any one have such type of experience.
Thanks
10815l

 

[EmmanuelTop]

1) Have you checked the dew point instrument analyzers if they are working OK? Check with portable meter if the dew point readings are correct. I don't think they are the problem, but checking will do no harm.

2) Are you exceeding the design gas flow rate or moisture content of the inlet gas to Mol Sieve? That can definitely cause moisture breakthrough.

3) If the plant operates within the design envelope and the analyzer instrument readings are correct, this means the bed is likely damaged. It could be due to numerous reasons: upset of the adsorbent layers causing channeling of gas, carryover of (Amine?) from upstream rendering the bed unusable, etc.

If there are several adsorbers in parallel, I believe it is highly unlikely to see exactly the same phenomenon and parameters in all vessels. This could point in some direction.
You said the plant is new - check with Axens representative and don't try to save money by not calling him. This guy can possibly save you millions.


Dejan IVANOVIC
Process Engineer, MSChE

 

[georgeverghese]

Check upstream preconditioning units for some kind of failure

a) Loss of precooling and / or liquids separation in precooling unit

b) Inadequate liquids filtration downstream of precooling unit

c) What about CO2 removal and feed concentration compared to design ?

 

[10815L]

Hi,
Analyzer checked and validated many time but no change observed, moisture checked with dragger tube found 0.3 ppm while analyzer was reading 6 ppm. Gas is sweet (no h2s or CO2)and there is no treatment unit involved in this plant. We are operating this plant with 50% load as our turbo not yet commissioned due to technical reasons, we operating on JT mode.
10815l

 

[georgeverghese]

Okay.

Next,

a) what about regen gas temp - is it in accordance with Axen requirement ? Should be about 250-300degC.

b) Is the regen gas exit temp from the mol sieves unit (after hot regen is completed) also as required in Axens operating instructions?

Poor design of the pretreating unit in the online moisture analyser can also result in premature condensation of HCs', leading to wrong analyser reading - this is a common fault in online moisture analysers. Analyser vendors are notorious for cutting corners on sample stream pretreating provisions.

 

[10815L]

Axen requirement is 240 to 260 deg C and we are maintaining 255 deg C at inlet and out let just getting 210 deg C. And regarding the preheating for moisture analyzer no preheating unit employed yet. its a amteck 3050-DO and 3050-OW analyzers.
regards
10815l

 

[georgeverghese]

That sounds similar to the unit in the plant I was operating - 270degC regen temp and 210-220degC on completion of regen at mol sieve bed exit for Union Carbide mole sieves 5A type ( for water , CO2 and trace heavy HC removal).

As ET suggests also, the sample stream preconditioning unit for the analyser may not be dropping out the required HC condensate before moisture dewpoint check - if this is the case, several weeks of work and supervision of the corrections seems to be on the cards.

 

[10815L]

George,
what is your suggestion regarding the preconditioning, should i apply heating or cooling system. our gas contain around 15 % C3 to C5 and trace quantities of earlier said components (mainly C3 is 10 % and rest of other gases)
regards
10815l

 

[georgeverghese]

Precooling will drop out the pesky hydrocarbons that interfere with the moisture analyser sensor operation. At the precooling temps applied and given that most of the moisture in the gas is already removed from the process gas, no water would drop out in the precooling unit.

If you pre heat, it may work for some types of sensors, and not for others. Also the tubing arrangement after the preheating should not present any low point where liquids can condense and pool upstream of the analyser sensor during shutdown or on cool tubing surfaces.

Many process engineers pay little attention to these preconditioning units during the bid and design phases.