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Tsa Bed Regeneration Heat Energy Calculation

heat of desorption

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#1 Bitan729

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Posted 17 March 2022 - 01:58 PM

Respected Members,

 

We are operating a cryogenic nitrogen generation unit with product specification as 2600 Nm3/hr gaseous nitrogen and 400 nm3/hr liquid nitrogen. For this the design air flowrate is 9750 Nm3/Hr at 7.6 Kg/Cm2(g) pressure at coldbox.

 

In front end , we are operating a TSA unit contains two adsorption bed (one online + one under regeneration).

The cycle time for adsorption/desorption is 4 hour ( 10 minute for depressurization from 7.75 kg/cm2(g) to 0.2 kg/cm2(g) pressure, 85 minute heating with 1350 nm3/hr regeneration gas flow at 190 Deg.C temperature, 130 minute cooling to bring back bed temperature to 35 Deg.C  and lastly re-pressurization (15 min)).

 

For that, each bed contains 985 kg alumina+ 1350 kg 13X zeolite molecular sieve.  Air inlet temperature is 20 Deg.C  and pressure 7.8 kg/cm2G pressure with 100 % RH. Regeneration electric heater rating 90KW.

 

Although bed is operating at design condition (9700 nm3/hr air ,400 ppm CO2, 20 Deg air at 7.8kg/cm2G and 100% RH) but CO2 seepage is happening up to 10 PPM near the end of bed run time. We are thinking about increasing regeneration flow and temperature.

 

Sir, my requirement is I would like to know heat of desorption of CO2, Methane etc ,(which are commonly present in air) for 13X molecular sieves at 0.05 kg/cm2G . Also heat of desorption calculation for Water from alumina bed.

 

Also dynamic adsorption capacity of H20,CO2 in Alumina bed and molecular sieve bed respectively data would be highly helpful.

 

The regeneration gas is bone dry waste air which contains approx 67% nitrogen, 32.8% oxygen and rest is argon (in mole %). In datasheet of adsorbent materials not much data are given.

 

 

Thanks in advance



#2 Art Montemayor

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Posted 17 March 2022 - 05:03 PM

Bitan729:

 

I presume you are dealing with an adsorption unit operation that is dependent on temperature bed regeneration – commonly called a “TSA”.  I’ve designed and built this type of adsorption unit and have dealt with the subject at length.  I don’t understand why you’ve posted this thread in the Process Simulation section.  What you ask has little or nothing to do with simulating adsorption.

 

First and foremost, your needs to define the sorption and desorption abilities of your adsorbents should be addressed to your adsorbent supplier.  Today, there are many – in different countries, and with different degrees of performance and credibility.

 

I don’t know whose process and cold box you are using to produce both gaseous and liquid Nitrogen, but I note that your feed air supply is at a rather low 110 psig.  What this indicates is that your water load is much larger than what I’m used to in the past: 1,000 to 2,000 psig.  Your adsorbers are going to be larger beds.

 

I’ve designed and operated my adsorbers for superficial velocities of 20-50 ft/min and I’ve heated my regenerated beds to 400 – 500 ºF, far in excess of your 375 ºF gas flow.  I started with 350 ºF gas flow temperatures as regen gas and as I got more experienced and obtained more professional advice and knowledge, I wound up with getting the beds to reach the higher 400-500 ºF.  This not only produced better operating results, it also increased the adsorbent operating life.  At the end of my career in compressed gases, I fully expected and got 3-5 years of operating life from my adsorbents which included ALCOA’s Activated Alumina and Linde’s Molecular Sieves.

 

Not knowing what you are operating – or how – I can’t offer any more comments.  I offer you the attached interesting article on the adsorption operation in compressed gases.

 

Some recommended adsorbent bed regeneration temperatures for common adsorbents are as follows:

 

Adsorbent

Regeneration Temperature

Molecular Sieve

450 °F to 600 °F

Activated Alumina

400 °F to 500 °F

Silica Gel

400 °F to 500 °F

 

 

The required regeneration gas temperature exiting the heater is a function of the regeneration gas water content, the regeneration pressure and the desired level of dehydration or final product gas water content.  An adsorbent supplier should be consulted for specific cases.  Note: the regeneration bed temperature is different from the regeneration gas temperature.

 

Attached File  Mol Sieve Dehydration Design and Operation.docx   329.61KB   64 downloads



#3 breizh

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Posted 17 March 2022 - 09:21 PM

Hi,

You may find pointers in the document attached 

Breizh 



#4 Bitan729

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Posted 17 March 2022 - 10:49 PM

First of all I would like to apologize for posting in wrong place. 

 

Art Montemayor sir in reading: Sir it is design and constructed by BOC. One thing I would like to mentioned that the bed life is already 6 year and running continuously. This might be one reason for such problem apart from operational problem (if any).

 

Breizh sir in reading: Sir, I would like to thank you for sharing this reference document and It would help me a lot.

 

In page 11-12 it is written that " For both process concepts, sufficient hot gas must be admitted to heat the bed to TR , and that can be approximated by an energy balance. A rule-of-thumb is to add the enthalpies required to heat  the adsorbent plus the column wall (if not internally insulated) from TF to TR , plus the enthalpy of desorption, then multiply the sum by 2.5. Certain desiccants have a minimum TR that is required to be effective, e.g., 200°C (400°F) for activated alumina. Another rule-of-thumb is to use 30 to 50°C (50 to 100°F) above the highest boiling point present." ...... Sir I do not have this enthalpy of desorption (activation energy) data for CO2 in 13X molecular sieve bed. If you can kindly indicate from where this can be obtained, that would be highly helpful.

Anyway sir, you always help us by providing relevant information.



#5 breizh

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Posted 17 March 2022 - 11:38 PM

Hi,

take a look , It might help you :

https://journals.sag...263617418772665

Good luck

Breizh 






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