8 replies to this topic

[lokesh_09]

Dear all,

In our project we are using molecular sieve adsorber for gas dehydration. Since product gas is routed to cryogenic unit. It requires very low water content gas of around 1ppmv. Molecular sieve adsorbers are configured has 3 x 50 % (i.e 2 x 50 % adsorbers for adsorption mode and one for regeneration mode).Inlet to each adsorber has water content of around 91 kg/h. what will be the amount of water has to considered for regeneration and how operation will takes place, since during regeneration hot gas will push the liquid on bed slowly from the bottom of the bed to top?

I have considered this by taking 91 kg/h x 12 hrs for adsorption, which will be regenerated in 4 hrs of heating. Amount of water considered for regeneration is 273 kg/h.

Adsorption Mode : 12 hrs
Regeneration Mode : 6 hrs (4 hrs heating and 2 hr cooling)

please correct me,if my consideration is wrong.

Thanks in Advance.

Regards,
Lokesh.

[Erwin APRIANDI]

Dear Lokesh,

for the amount of water which is adsorp by the sieve, I think it is ok
but for regeneration time???

for me it is better for you to consult with the molecular seive manufacturer
as the time required for regeneration will be affected by the rate of hot air,
quality of hot air and the characteristic of the molecullar sieve.

I'm sure your molecullar sieve manufacture can give the correct estimated time if you can give them
the condition of regeneration, volume of molecular sieve (vessel dimension), and hot air flowrate
and they may have to help you, since you are buying the mol sieve from them cheers

[lokesh_09]

Dear Erwin,

Thanks for your prompt response on this.

Actually we have fixed the adsorption (12 hrs) and regeneration time (6 hrs) as per the GPSA standard and this has been agreed by the molecular sieve vendor. Adsorbers are sized based on this.

We need to design the regeneration system (Regen gas cooler and Regen Gas separator) based on the regen water load.

My intention is to consider that the water load of 273 kg/h( based on 4 hrs regen cycle) or we need to design the regen system for 1092 kg/h (91 kg/h x 12 hrs), since the amount of water removed from the bed in short time.

Regards,
Lokesh.

[ankur2061]

Lokesh,

You may refer to the general methodology for regeneration gas calculations from the following link:

http://www.cheresour...h__1#entry46282

Also specific guidelines are given for calculating regeneration gas load for natural gas dehydration based on various heat loads in Section 20, "Dehydration" of GPSA Engineering Databook,.

Regards,
Ankur.

[Erwin APRIANDI]

Thank you Ankur,

I gained even more, as previously I give it all up to the vendor to calculate

[lokesh_09]

Dear ankur,

Thank you very much for your link and section reference on GPSA Engineering Databook.

I have gone through the link, they have specified how to calculate the regeneration gas flow rate to regenerate the bed.

we normally used to follow the the GPSA Engineering Databook for sizing the adsorbers and gas dehydration system but GPSA does not have information about the regen water load which removed from the adsorber bed.

My intention is to know the regen water load from the adsorbers not gas load( we already calculate the regen gas load.

Regards,
Lokesh.

Edited by lokesh_09, 18 May 2011 - 08:35 AM.

[Art Montemayor]

Lokesh:

If I understand your query, you want to know how much water will be required to be stripped off from the spent adsorbers during their regeneration. If you say that each adsorber has a water content of around 91 kg/h and you are using two (2) adsorbers during adsorption drying, then each adsorber will require sufficient heat to strip out the corresponding 91 kg/h of water – plus all the other heat for other purposes. I have to assume this is an existing and operating adsorption system. Is that correct? If not, kindly tell us the scope of work that you have at present. It is difficult to answer a query with accuracy when we are not told ALL the essential basic data.

If you are using a 3-tower configuration for your adsorption unit, then you are probably using 2 of the towers in adsorption (in series) while you regenerate the third, spent tower. In order to continue to discuss this topic, please furnish us with:

• A detailed PFD drawing showing the 3 towers and the interconnecting piping as well as the block valves used to isolate each tower.
• A detailed listing of the sequence of operation during the adsorption drying cycle, the regeneration heating period, the regeneration cool down period, and the switching of towers.
• Go through a complete NEMA cycle for the process. A NEMA cycle is a detailed explanation of what each tower goes through in a sequential manner, in order to complete the cycle from start up with drying to start up with drying. This is important to know exactly what you are doing in your operation.
• A full description of your regeneration system – regen fluid, type of heater, controls, temperature settings, and flow rates as well as time for all the cycle phases.

[ankur2061]

Lokesh,

I think the answer is quite obvious for the regeneration water load. You have an inlet stream of gas to the adsorber at its saturation pressure and temperature. You can calculate the water content of the gas at the saturation conditions. Again you have an outlet stream based on the desired dew point of the gas at the corresponding saturation pressure for which you can calculate the water content. The difference in the water content of the inlet and outlet streams is the regeneration water load that you need to desorb during the regeneration cycle of the adsorber.

This to me is the fundamental basis of design for any molecular sieve adsorption system.

Am I still missing something?

Regards,
Ankur.

[lokesh_09]

Dear Art sir,

Thank you for your response.

What you have said is correct. But we are doing design for a new plant not existing. Adsorbers are configured as 3 x 50 %. 2 Adsorbers are designed for adsorption mode and one is for regeneration mode.

Feed gas flow rate is around 185 MMSCFD (92.5 MMSCFD for each adsorber (2 x 50 %))
Adsorber operating pressure and temperature 52.5 bar and 35 deg c (adsorption) and 50.5 bar and 230 deg c(regeneration)

Cycle time, i have already mentioned on this thread above (i.e Adsorption = 12 hrs and Regeneration = 6 hrs (4 hrs for heating and 2 hrs for cooling)

we are utilizing the part of dry gas from the adsorber(10 MMSCFD) for regeneration, which is heated by using electric heater.
On the regen system, we have regen gas cooler, regen gas separator and regen filters. Regen gas cooler is used to cool the gas from 210 deg C to 40 deg C.
my intention is know how much water will be stripped from the adsorber. Each Adsorber inlet has water content of 91 kg/h, since adsorption cycle is 12 hrs, amount of water will be adsorbed is 1092 kg(91 kg/h x 12). we are regenerating (removing water in the bed) the adsorbers in 4 hrs.
Could please tell me the water load to be considered for designing regen equipments such as regen gas cooler and regen gas separator.
We have to consider the 1092 kg/h water load for deisgn the regen equipment or 273 kg/h water load(1092 kg for 4 hrs (1092/4).
Also please explain the regenerator operation, how it will remove the liquid from the bed.
For my assumption the gas will act as piston which will slowly push the liquid from bottom zone of the bed to Mass transfer zone and then to top zone. please correct me if i am wrong.


Regards,
Lokesh.

Edited by lokesh_09, 22 May 2011 - 09:33 AM.