13 replies to this topic

[Abid]

We want the regeneration of Molecular Sieve bed through hot air, for the desorption of CO2. The air will be forced through the adsorber by a blower in closed loop, means the same air will be recirculated in the bed untill it reachs the desorption temprature. Size of the bed is 300 kg.Bed Heat requirement is 101800 BTU. Regeneration cycle time is 1 hr. Any one who can help me in calculation of heat transfer rate for this particular problem,by which we can actually know the time it take to reach at desorption temprature. I have gone through these calcualtion but still unable to find out the exact time for bed regeration, feeling problems particularly in heat transfer area of the Molecular sieve beads and heat transfer coefficieint for in packed beds. any help in this regards will be very appreciable and valuable.

Abid

[rosa]

Hi abid,

regarding to your statement,your molsieve column used to adsorption of CO2.
and your heat requierment=101800 BTU that is equal to m*cp*(T2-T1) that
m=mass of molsieve
cp=heat capacity of molsieve
T2=Final temp that you need to reach at end of desorption
T1=START temp of desorption
the time of adsorption is depended of the final CO2 concentration in your process gas.I mean when gas passed through column,what is your process requierment to be reach at the end of adsoption?
In the gas processing book of camble you can find the time of adsoption that is depend on your column number,initial CO2 concentration and final CO2 concentration.hope it helps!!!!

Kind regards
rosa

[Art Montemayor]

Abid:

One big reason why it is difficult to calculate the heating requirement for your application is that you are not accurate nor specific in your query.

I believe we both know that the term "regeneration" involves not only the heating of the adsorbent bed, but also the cooling of the same and possibly the venting and purging of the regen gas (this depends on the type of regen cycle). So, what are you designing as a typical NEMA cycle? Tell us all of your details such as the heating, cooling, etc. times. Otherwise, we don't know your heating (or cooling) rates.

You also fail to tell us your terminal bed temperatures as well as your initial temperatures. It is well understood that Molecular Sieves require the highest of all regeneration temperatures among the various adsorbents. Regen terminal temperatures for Mol Sieve beds are usually 500-600 ^oF as a minimum. I always design my regeneration heaters for an exit temperature of 700 ^oF as a minimum in order to secure the required bed temperature. This means that you must consider a special alloy metal for your heater vessel - especially if you are using an electric heater. I usually specify Stainless Steel for my heaters because of these higher heater requirements.

Give us all of your basic data for the regeneration of your 300 kg of mol sieves and then we can discuss more specifically and accurately instead of just guessing what it is that you are doing or designing. For example, I calculate 71,000 Btu required to heat your bed from 70 to 550 ^oF. This heat plus the amount to heat the vessel, the internal steel, piping, heater, and insulation should be the duty of your heater. With this data you then calculate the heater rate, knowing the regeneration fluid flow rate.

Await your reply.

[Abid]

Dear 'Art Montemayor'
Thank you very much for taking the time out and replying this particular problem. First of all i would like to pay my excuses for not giving detailed information for your kind consideration.
Yes, you are absolutely right "Regeneration" involves both heating, cooling alongwith purging of desorbed gas. Please let's have a look on the detailed data regarding Mol. Sieve bed.

Adsorbent: 4A Mol. Sieve
Material required:300 kg
Bed initial temprature: 25 0C (77 0F)
Bed desorption temprature: 250 -270 0C (482 -518 0F)
Heating mode : forced convection of air through blower
Heating source: Electric heater
Heating time: 40 minutes
Cooling time/ purging: 20 minutes
Heat requirement of bed: 1.123 x 105 Btu ( It includes all the heat loads, as bed heating, vessel heating loss, heat of water evaporation loss, piping heating loss etc)
Mol Sieve specific heat: 0.23 Btu/lb. 0F
Thermal conductivity of Mol Sieve: 0.0183 Btu/hr ft2, 0F (Approx.) as i could not get the exact value for 4A mol sieve.this value is of 13-X mol. sieve which might be close to that of 4A mol.sieve.
Bed Dia.: 2.07 ft ( according to my calculation, so you can chech it for yourself)
Bed length: 4.5 ft
I hope this data will help you for sake of your calculation. However if you find anything missing, do ask me.
Now question arises what would be the air flow rate to regenerate the bed in 1.0 hr and how can one calculate it for this particular bed.

With best regards

Abid

[Art Montemayor]

Abid:

You haven’t said if this is a new design or an existing designed and operating adsorption system. If it is a new design, then you have to state your design sorptivity and your design superficial velocity through the bed for both the adsorption portion as well as for the heat up and cooling portion.

I use 0.25 Btu/lb-^oF as the Mol Sieve specific heat, but that’s a personal preference based on prior experiences.

As I stated in my prior posting, once you get the total heat load requirement and having established your maximum superficial velocity for heat up, your regeneration gas and its heat rate, you calculate the size of heater required. You are going to have to stay below your maximum superficial heat up velocity, so you may find that what you have previously picked as your time duration for the heat up portion may not be enough time. I suspect that you will not be able to heat up the bed in 40 minutes – in fact, I’m willing to bet you that you can’t do it. I haven’t done the calculations for your application, but I’m citing my personal experiences in this field of design and operation. Of course, you haven’t stated what you are designing as your regeneration gas temperature exiting the heater. I’m betting that you would have to heat beyond 700 ^oF in order to stay below the maximum superficial velocity during the heat up. I’m also willing to bet that you will never cool the bed from 550 ^oF down to 77 ^oF in 20 minutes. Again, this is just from experience.

If this is a new design, what have you calculated as the mass of vessel, piping, valves, insulation, and internals? What have you allowed for the construction of the heater?

You state that the heat requirement of the bed = 1.123 x 105 Btu. This results in a ridiculously small heat up load (118 Btu !). My calculation for the adsorbent heat load is in the attached Excel file. Note that I only calculate for the adsorbent heat up – I do not have any water or other adsorbed substance included. You mention water, but you never mentioned any drying (water adsorbed) in your original post. Please give us ALL of the basic data. You haven’t stated the fluids – other than the regen air. You also have to calculate the pressure drop across the adsorbent bed for the adsorption period as well as the regeneration period.

[Abid]

Dear Art Montemayor
Thank you very much for your reply. yes this is a new design, i am in preliminary phace of process design of Adsorber. I have not selected or calculated the size of heater, so you can take it sort of unfinished item, which can be modified according to the guidance and instruction given by the experienced and kind people like you.For adsorption the Volumetric flow rate of air is 300 CMH (162.15 SCFM), and superficial face velocity is 06. ft/sec. adsorption capacity of the media is 23 lit. of CO2/1 kg of resin. but for the heat up and cooling rate i have not done the calculation. this what i want to consult with you that at which air flow rate and exit temp. of heater, we can meet our requirement of 1.0 hr heating and cooling time.
Heat requirement of the bed is 112300 Btu (not 118 Btu, i did not pick it how did you see it).
Mass of vessel i have calculated is 450 kg. for piping i have selected the 2.0 in. pipe size with max. 95 ft/see velocity in pipe in regeneration mode, is it correct or not, if no, we will change it.. and the piping and insulation mass is still to be calculated. Do suggest me if you can. The inlet air mixed with CO2, which have to be removed through this adsorber is the other fluid in this process. for regeneration existing water in the mol sieve would evporated during heating, that heat load i have added in final heat load.I have calculated the pressure drop in the adsorption phase, don't worry it would not be too high to bother (5.0 -6.0 In. H2O).
In the end i want to mention that i have not fixed anything in my design, because these are the preliminary calculations i have done. It can be changed or modified in the light of your guidance, if you think it as. I could not open your attached file of exel so can not comment on it right at this time.

With best regards,
Abid

[Art Montemayor]

Abid:

Please refer to the attached revised workbook. My comments are as follows:

1. You still haven’t told us what fluid you are purifying through adsorption. Are you removing CO₂ from air and are you also removing water simultaneously?
2. You cannot recycle your regeneration fluid (in this case, atmospheric air) without removing the desorbed contaminants. When regenerating with atmospheric air, the regen air is sent back to atmosphere with all the desorbed contaminants. You should not recirculate the impurities through the bed during the heat up and subsequent cooling. This is a big mistake because all this does is concentrate your impurities in the bed during cool-down.
3. You yourself wrote: “the heat requirement of the bed = 1.123 x 105 Btu”. That is a well understood mathematical expression that means what it states. If you forgot to indicate that that you were using scientific notation with the last 5 being a power of 10, then that is a mistake on your part. I can only interpret correctly what is written correctly.
4. You don’t state how you arrive at your total heat up quantity. I have told & showed you how I arrive at mine.
5. Look carefully at the schematic in the workbook. In order to operate the given setup of two beds, you must completely regenerate one bed in less time than it takes for the other bed to complete its adsorption step. That time that it takes to do the actual adsorption minus a contingency amount of time to purge, check, and change valves is all you have to do your complete regeneration of a bed --- assuming both beds are identical in size and everything else. Therefore, how do you come up with your heat up time of 40 minutes and your cool down time of 20 minutes?
6. The time used for effectively adsorbing all your impurities is determined by what you decide is a convenient and manageable NEMA cycle. Industrially, the most popular cycle I’ve seen is one that has the adsorption taking place during one operator’s shift – 8 hrs. I’ve also used a 16-hour adsorption cycle as well. It all depends on what you need or want in your operations. Please explain how you are arriving at your NEMA cycle and how it is designed to operate.
7. How have you calculated your pressure drop? Are you using Ergun’s equation?
8. You should describe your NEMA cycle, step-by-step, giving the time required for each.

Await your reply.
 Mol_Sieve_Regeneration_Rev1.xls   222.5KB   1169 downloads

[Abid]

Dear Sir,
Thank you ver much for your reply. I have gonethrough your attached Excel file.
1. As i have mentioned in my previous reply that CO2 mixed air is the inlet stream at the Adsorber inlet, and CO2 free air is the outlet stream at the adsorber outlet. So We want to remove only the CO2 gas from the inlet dry air not water.
2. you are absolutely right desorbed CO2 gas can't be recycled with regeneration air, for that purpose we will pass some inert gas as soon as the desorption temprature (250 oC) of CO2 is achieved during heating cycle that will vent the desorbed CO2 through another line.

3. I have calculated the total energy requirement of the Mol. Sieve bed on the following basis.
a) Heat requirement of the Mol. Sieve bed for CO2 desorption by applying the same formula which you have applied in attached Exel sheet.
heat taken up by the Adsorber vessel of mild steel (Qs = m x Cp x LMTDheating)
c) Moisture load (for that moisture which is already stored in the bed before shipping approx. 5%).
d) Additional energy requirement, Radiation (4-8% of the sum of above three loads).
Then i got the Qgrand heat requirement load.

4. Energy requirement for cooling have been calculated with the same way excluiding the moisture load, radiation load and with LMTDof cooling.

5. You are right that adsorption time should be greater than regeneration time.
I say that adsorption time for this adsorber is 1.5-2.0 hr depending upon the inlet flow rate of CO2 mixed air which is 300 CMH (162.25 SCFM). So the regeneration time should be less than this, this is what i want to calculate or know that the same bed would be regenerated in 1.5 hr or not. If your answer is no, then how much time it will take for regeneration of the adsorber and how can i calculate it. This is the main query i have,??
6. I have not fixed the heating time of 40 minutes and cooling time 20 minutes. This was the only constraint from some one who want it to be in one hour. Please let me know if this not possible then what would be the most minimum regeneration time in which we can attain our objective, considering all the options of higher possible regn. air flow rate.
7. Yes, I have calculated the pressure drop in bed through Ergun equation.
8.Step by step NEMA cycle is follows.
Adsorption time: 1.5- 2.0 hr
Desorption time: 1.5 hr (if possible, otherwise we will adjust it according to regn. cycle time).

Waiting for your reply.

With best regards
Abid

[Abid]

Dear Art Montemayor,
This is my 2nd reminder for you. I am waiting very anxiously for your reply. Pleasse do reply me if you have any solution regarding this problem.

Dear all Memebers,
This is an open forum discussion platform, so anyone who can help me in solving this problem. Any suggestion, idea would be highly appreciated.

Abid

[vicini]

Attached is a mole sieve calculation sheet for CO2 Water and HC. Take alook and the ask questions.

If you try to use y=the same gas in recycle, you will not unload all the CO2 and you will start with more and more and the bed will stop working. You must vent the regengas to stop the recylce of the contaminat. NOW, you can can put in a exchanger that recooperates the heat from the regan gas being used. If you put one of those in, you can recover about 80% of the energy at best.

Attached Files

•  regen_calcs.xls   34.5KB   686 downloads

[wai]

Dear 'Art Montemayor'

I dealing with mol sieve 4A that absorb moisture to below 1ppm in crude argon stream (96%Ar,3.5%H2,balance O2,N2 & H2O), currently facing moisture breakthrough of >20ppm. My case is as below

The mol sieve is 150kg/ bed. I have 2 beds.

Regen heating source temperature: 400F
Regen terminal temperature: 350F
Regen flow: 45sm3/hr pure N2
Heating time: 3 hours
Cooling time: 4.5 hours
Pressure buildup and letdown time: 0.5 hour

This moisture breakthrough problem happens within 15days after changing of new mol sieve. Troubleshooting carried out and the possibility of inaccurate analyzer reading, valve/piping leaking, upstream water separator malfunction and cooler malfunction has been eliminated. Could it be the problem of regen flow terminal temperature as you mention that the 500F-600F is required for regen terminal temperature? Or due to other reasons as well?

Please kindly advise any solution to solve the moisture breakthrough problem.

[Art Montemayor]

Wai:

This thread is a year and a half old. It was started by, and belongs to Abid. I believe I know what is the answer to your problem, but I don’t respond to queries that have “hijacked” another thread. I would prefer that you start your own, original thread with this query and furnish us ALL of the basic data. You haven’t said if this is an existing adsorption unit that has worked successfully in the past to the specifications you require. And if the unit operated to satisfaction, what are the regeneration condtions that have been applied as far as regen bed and exit temperature? What has been the regen gas flow rate? How long has the existing Mol Sieve been in service? When was the last time it was inspected and checked? Has the pressure drop increased across the bed during regen?

Await your reply in a new thread in this Forum.

[wai]

Dear Art Montemayor,

Both mol sieve beds have serviced since Jan 1997. Moisture breakthrough problem started to occur 15days after the mol sieve was commisioned, ~1.2ppm H2O. Until now, the problem become worse. Both beds can't be completely regenerated. After regeneration, moisture breakthrough at ~12ppm. End of service, ~25ppm. The mol sieve never achieve the specification below 1ppm.

For details sequence of events and mol sieve specifications, please kindly find the attached files. Simplified drawing illustrate the design case.

There is no pressure drop transmitter available on the bed. Is the pressure drop across bed a critical issue in affecting the moisture breakthrough?

Please kindly advise. Thanks.

Attached Files

•  Design_specification.pdf   12.04KB   452 downloads
•  Event_Sequence.pdf   92.98KB   373 downloads
•  Simplified_Drawings.xls   36.5KB   367 downloads

[Art Montemayor]

Wai:

Thank you for responding quickly. It demonstrates your concern and the importance you give to the problem. However, you still have not followed my request to start your own, original thread so that we can dedicate all responses only to you and your problem. Please follow this request and don't hijack this thread. It creates a lot of confusion in archiving the threads and in sorting out responses.

You also fail to answer all my questions. What are the operating temperatures of the regen bed and the regen outlet gas? Who designed the adsorption unit and why was the the adsorbent changed? Are there any existing adsorber calculations on the application? Have you or anyone else made any calculations on the existing operation?

Why is the feed moist argon being introduced at the bottom of the bed? This is very unorthodox in adsorber bed applications. That is why I am asking WHO designed and built the adsorption unit. The last thing you want to do is to move or agitate an adsorption bed; and when you feed the moist gas at the bottom, you have a tendency to "lift" the bed and cause movement and attrition - which lead to dust. That is why I asked about when you have last inspected the beds and how long have they been in operation. Additionally, I note that there are FOUR additional valves on the unit - two on the inlet and two on the outlet. I have to assume that these are for "bleeding" the beds or re-pressurizing them. Is that correct? You don't give us any data on the regeneration gas stream: its condition, pressure, temperature, composition, and source. It is difficult to make sense of how you are carrying out the simple practice of adsorption and regeneration. Please give us ALL of the Basic Data.

Without the original or recent calculations, we can't know what the basis for selecting the installed adsorbent was and we also don't know the adsorption loading that was employed in arriving at the installed amount of adsorbent in each bed. Therefore, we can't check out the NEMA cycle for adsorption and regeneration. There are many additional questions and comments that I have, but I will offer them up when we have our own, specific thread to concentrate all this information on.

Await your reply.