4 replies to this topic

[Victor_process_Engineer]

Dear colleagues,

 

I'm designing the molecular sieve adsorption bed for drying of gas containing H2O at 56 barg @ 40degC. Water content of dry gas at outlet should be below 77 ppmv.

 

The regeneration gas is around 10% of outlet dry gas. The regeneration conditions is 53 barg @ 240degC.

The molecular sieve type is 4A.

 

My concern is about regeneration conditions. Is it possible to regenerate molecular sieve at such high pressure 53 barg with regeneration gas temperature 240 degree C? 

 

I have checked the water against regeneration conditions and found out that it is total liquid phase due to high pressure. Will the water evaporate under these conditions or is it necessary to increase the temperature or reduce the pressure in the regeneration step?

 

I kindly ask you not to criticize the choice of drying technology, since the decision was made at the FEED stage and it could not be changed. Although it is clear the best option for this case is TEG dehydration.

I will appreciate any support and suggestions

Thanks everyone in advance.

[Pilesar]

My guess is the FEED regen pressure was a mistake which no one caught. Regeneration is usually done at lower pressure. Efficiency is much improved since the required velocity through the bed can be achieved with reduced mass of regen gas. While lower pressure regen is better, your 'water is a liquid at such pressure' conclusion is not relevant. When the regen gas is in the vessel, the water will have a much lower partial pressure than the total pressure. I suggest getting design advice from an expert molecular sieve supplier such as UOP. 

[breizh]

Hi,

To add to the discussion:
https://www.chemenvi...o-regeneration/

Should be low pressure!

Breizh

[Pilesar]

If the plant has uses for the high pressure regen gas then the FEED may not be mistake. You can use high pressure regen gas effectively, but low pressure regen is much more common. The flow of regen gas will depend on the pressure. The GPSA Tech Data book has a graph showing how regen flow should vary with regen pressure and lists equations to use in calculation. Getting outside guidance from the mole sieve experts is advised. Once they teach you, then you will be the expert!

[Victor_process_Engineer]

Dear Pilesar,

 

My guess is the FEED regen pressure was a mistake which no one caught. Regeneration is usually done at lower pressure. Efficiency is much improved since the required velocity through the bed can be achieved with reduced mass of regen gas. While lower pressure regen is better, your 'water is a liquid at such pressure' conclusion is not relevant. When the regen gas is in the vessel, the water will have a much lower partial pressure than the total pressure. I suggest getting design advice from an expert molecular sieve supplier such as UOP. 

 

 

You are absolutely right. I have missed the point about partial pressure for the mixture. Indeed, the water will be in the vapor phase at 53 barg @ 240degC as per composition of regeneration gas and actual water vapor pressure.

 

The regeneration gas pressure is defined in our case by pressure of dry gas and pipe losses in the regeneration loop. Regeneration gas is sent back to the inlet of the unit by means of Compressor.

 

Thanks for your reply