7 replies to this topic

[autumnleave]

Hi All,
We need the molecular sieve beds to dry the associated gas before sending it to the pipeline. The inlet (to the beds) water content is around 8000 ppmv and requirement of outlet (from the beds) water content is in the range of 100 to less than 1 ppmv (depends on the options).

I am trying to find out how the outlet gas spec impact on the bed design, as well as operation cost later on.

Your advices are appreciated,

Thanks.

[Zauberberg]

Mol Sieves are used when deep dehydration is required (0.1 ppmv moisture in the outlet stream), usually when the gas undergoes cryogenic processing downstream of dehydration. In your case, TEG unit could be much more appropriate but we don't have enough process data in order to make more thorough evaluation.

Also, it is worth mentioning that cooling/chilling the inlet gas reduces water content substantially, but again we don't know at what conditions the inlet gas is available and is it feasible to consider chilling, or not.

If you want to receive more detailed comments from the Forum members, please upload a sketch of the system listing all the key inlet/outlet parameters.

[autumnleave]

Dear Zauberberg,
Many thanks for your advice.
We are aware of availability of alternatives options. However, due to some reasons, the mol sieve has been selected and the only question is how the outlet gas spec (for case of 50-100 ppm and case of less than 1ppm water content) impact on the bed design.
Some input data: Inlet gas flow: 50 mmscfd @ P=62 bar, T max 80 oC, water content 8000ppmv
Thanks again!

Edited by autumnleave, 20 March 2011 - 07:44 AM.

[Zauberberg]

It is not practical nor required to feed the Mol Sieve adsorbers with 0.8% water in the feed gas. I don't know who has done the conceptual work for the project but this is wrong.

For example, if you cool the feed gas stream (@ 62 bar abs) to 30 degC, almost 90% of water present in the feed gas can be knocked out as liquid water and this is a standard arrangement for all Mol Sieve dehydration units: cooling/chilling of inlet gas, followed by removal of liquid water, prior to processing the gas in the adsorber(s). Removing such large percent of water in the feed gas results in approximately 9 times smaller Mol Sieve beds which is definitely a way to go.

[autumnleave]

Dear Zauberberg,
I understand your concern. However, our gas is associated and hence very heavy (M=32) with large portion of C2, C3, C4. We have to maintain the temp of 80oc so that the hydrocarbon liquid would not dropout within the beds.

I am still looking for the response to my query.
Rgds,

[Art Montemayor]

You have to regenerate any adsorbent, so you have to heat and then cool the bed. The end result is that you are going to condense any hotter, heavy gas within the bed.

You also have to transport the product gas (if you ever succeed in drying it) through a pipeline; the pipeline is not adiabatic - therefore, what is even worse is that any condensables will drop out in the pipeline with resultant 2-phase flow.

Molecular Sieves are definitely not called for here. There are other adsorbents who could do the same - or better job - IF THE IDEA WERE PRACTICAL. I agree with Zauberberg: Whoever thought of this one either isn't telling all of the story or doesn't know enough of the adsorption Unit Operation.

[Zauberberg]

Dear Zauberberg,
I understand your concern. However, our gas is associated and hence very heavy (M=32) with large portion of C2, C3, C4. We have to maintain the temp of 80oc so that the hydrocarbon liquid would not dropout within the beds.

I am still looking for the response to my query.
Rgds,

Listen to Art's advice. Something is basically wrong in your approach, or some of the key considerations are missing.

You can definitely cool and condense a portion of the gas (including water as well) and then reheat it, if it is required for some reason. But feeding the Mol Sieve bed with 0.8% water in the feed gas is conceptually wrong.

[autumnleave]

Dear Friends,
Very appreciate your comments. Further to the discussion, could you please advice:
1) For 01 ppm specification, is there other preferred technology, other than mol sieve?
2) I am agreed that reducing inlet water inlet will proportionally reduce the size of mol sieve bed. Since there is no option for NGL export other than sending to gas export pipeline, is it economical to cool the gas and dehydrate the gas and NGL separately before sending them to the pipeline.

I am eager for learning from you.
Thanks.