4 replies to this topic

[mykid]

Hi, i am currently lookign for information on the design of molecular sieve adsorber.
Upstream of the molecular sieve bed, a TEG contactor and Sulfatreat has been installed to remove water and H2S to 3lb/MMSCFD and 5ppmv respectively. The molecular seive has an outlet specification of 1ppm for max gas flow of 5MMSm3/day.

Can anyone advise on the design of the molecular sieve bed? Can we do a preliminary sizing based on loading criteria or superficial velocity or we have to get feedback from vendor?

For regeneration flowrate, it can be estimated by the total heating requirement to heat the bed from normal operating temp to the required regen temp of 200-300C. Similarly for cooling duty. However how do we determine the amount of molecular sieve required?

Any input will be very much appreciated.

[HMAlmosa]

The diameter of the bed is determined by the superficial velocity (Vs) at actual conditions:
Vs = 55/sqrt(gas density) ..... for 1/8" pellets. Use factor 39 for 1/16" pellets
where Vs is in ft/minute
density in lb/cf
Round up the diameter to the next 0.5'. If diameter exceeds 12', then use more beds in parallel.

Amount of mole sieve required is a function of how long you want your cycle to be. Considering that you are dehydrating upstream to 3 lb/mmscf, then I suggest a long cycle (24 hours). In this case the amount per bed would be:

mole sieve amount (in lbs) = (total water to be removed per day in lb)/0.07

From the total mole sieve amount and the bed diameter, you can calculate the bed height (use actual mole sieve density or 45 lb/cf). Height should be at least 1.5 times the diameter.

I suggest you size the regeneration cycle to complete in 8 hours just in case you need to reduce the adsorption cycle in the future.

If your inlet conditions are as good as you describe, then your mole sieve should last for a very long time (many years).

[Andrei]

You can find a detailed and very effective sizing procedure in GPSA Engineering Databook, Section 20 - Dehydration.

Regards

[mykid]

Thanks for the quick reply.
I have one more question, if the molecular sieve is to remove only from inlet of 15ppm to 1ppm, do we normally install a regenerative type molecular sieve or non regenerative? any idea of the life cycle?

[Art Montemayor]

Mykid:

I must have addressed this same adsorption thread over a hundred times on this forum in the last eight years. If you use the "SEARCH" function on these forums you will find all the relevent threads - complete with my sketches and calculations on different adsorption tower configurations and different regen methods.

I don't believe anyone in his right mind would use Mol Sieves without a regen cycle (a TSA - Temperature Swing Adsorption). There is no such thing as "a regenerative type molecular sieve or non regenerative type". At the price you have to pay for Mol Sieve, you can't afford NOT to regenerate it. Look at the flow diagrams I've drawn in the past and you will find out how this is done.

What do you mean by "life cycle"? Do you mean how long a charge of Mol Sieve will continue to be effectively regenerated? If so, the effective life of the Mol Sieve depends on the design of the beds (agitation and movement), the purity and cleanliness of the feed gas, the cleanliness of the regen gas, the quality of the regen cycle, and the quality of the operation. I've operated some beds for as long as 3.5 years.