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Hi,I have some problems in setting up a dryer in ChemCAD...I want to simulate drying of gasous ethylene in a molecular sieve column,how do I do it ?
I tried choosing SCDS column but got folowing message :
Error: SCDS 1 No of outputs from topo != No of outputs from spec.
I only have one input and one output-so what unit do I choose ?
Thx in advance !
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Dryer In Chemcad
Started by popay73, Jun 23 2005 05:42 AM
4 replies to this topic
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#2
Guest_Kiran Parihar_*
Guest_Kiran Parihar_*
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Posted 24 June 2005 - 01:50 AM
Can you please ellaborate your problem. Maybe I can help.
#3
Guest_Guest_*
Guest_Guest_*
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Posted 24 June 2005 - 05:32 AM
Ethylene production : after steam cracker,quenching and gas compresson and before cold section and separation columns,there is drying with molecular sieves,becaouse there should be no water when gas enters cold section ( -100°C,30bars )..this conditions are needed in order to get ethylene separated from other gases
My question is how to simulate this dryer in ChemCAD,what unit do I choose ?
I tried using SCDS column (packed tower ) because it can serve as absorber (drying with molecular sieves )...but the problem is that column has feed,top and bottom products,so 1 input,2 outputs...and I have only one input (wet gas) and one output (dry gas)
Thx !
My question is how to simulate this dryer in ChemCAD,what unit do I choose ?
I tried using SCDS column (packed tower ) because it can serve as absorber (drying with molecular sieves )...but the problem is that column has feed,top and bottom products,so 1 input,2 outputs...and I have only one input (wet gas) and one output (dry gas)
Thx !
#4
Guest_Guest_*
Guest_Guest_*
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Posted 24 June 2005 - 10:11 AM
One problem that may arise while using SCDS is that it is modeled for an absorber, not an adsorber. Mol sieves are adsorption columns and are different from an absorbtion column. I'm also trying to model an adsorption column within CHEMCAD and have been hitting some roadblocks. I am trying to model the dehydration of ethanol (used because of the azeotrope formed from common distillation). I would also appreciate any help that anybody can give me.
Thanks
Thanks
#5
Guest_Guest_*
Guest_Guest_*
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Posted 24 June 2005 - 11:51 AM
popay73:
What is your purpose in simulating an adsorption of water vapor from an ethylene stream? What is it that you are looking for from a simulation program?
If your intentions are to "size" an adsorption fixed bed dryer, I believe you're applying a totally inaccurate overkill to the problem. Adsorption dryers are designed manually and the process has many issues and characteristics that make simulators spin off in different, inaccurate and sometimes illogical directions. Simulators must try to consider the following routine adsorber characteristics:
1) Selective, dynamic sorptivity; this keeps changing in accordance with bed aging, contaminants, dusting, regeneration effectiveness, adsorbent quality, channeling of both the process gas and the regen gas, etc.
2) type of process; is it PSA or TSA?
3) Number of beds or vessels;
4) type of regen cycle;
5) the accuracy of the dynamic sorptivity.
Besides all the above, you have the factor that the process is inherently a BATCH process - and not a steady-state, continuous one. The above are all bad news for anyone trying to simulate the adsorbers. What is normally done is that the beds are designed by manual calculations to yield the highest purity required (& with a contingency factor) and meet the pressure drop constraints imposed as well as comply with the regeneration cycle applied. This all results in a unit that will comply with the process requirements of flow, purity of product, and consistancy within the limits given.
If you believe you can create and operate an accurate simulation program that will sense and detect any variances and flaws in the operation of an adsorption dryer, I believe you are going in the wrong direction. Simulators are good for other, well-established and dominated processes like distillation and overall process heat and material balances. But when it comes to detailed design of units such as adsorption and heat transfer, simulators start to fall apart in my opinion. For example, I would never depend on Aspen to produce a practical and competitive heat exchanger design.
Art Montemayor
What is your purpose in simulating an adsorption of water vapor from an ethylene stream? What is it that you are looking for from a simulation program?
If your intentions are to "size" an adsorption fixed bed dryer, I believe you're applying a totally inaccurate overkill to the problem. Adsorption dryers are designed manually and the process has many issues and characteristics that make simulators spin off in different, inaccurate and sometimes illogical directions. Simulators must try to consider the following routine adsorber characteristics:
1) Selective, dynamic sorptivity; this keeps changing in accordance with bed aging, contaminants, dusting, regeneration effectiveness, adsorbent quality, channeling of both the process gas and the regen gas, etc.
2) type of process; is it PSA or TSA?
3) Number of beds or vessels;
4) type of regen cycle;
5) the accuracy of the dynamic sorptivity.
Besides all the above, you have the factor that the process is inherently a BATCH process - and not a steady-state, continuous one. The above are all bad news for anyone trying to simulate the adsorbers. What is normally done is that the beds are designed by manual calculations to yield the highest purity required (& with a contingency factor) and meet the pressure drop constraints imposed as well as comply with the regeneration cycle applied. This all results in a unit that will comply with the process requirements of flow, purity of product, and consistancy within the limits given.
If you believe you can create and operate an accurate simulation program that will sense and detect any variances and flaws in the operation of an adsorption dryer, I believe you are going in the wrong direction. Simulators are good for other, well-established and dominated processes like distillation and overall process heat and material balances. But when it comes to detailed design of units such as adsorption and heat transfer, simulators start to fall apart in my opinion. For example, I would never depend on Aspen to produce a practical and competitive heat exchanger design.
Art Montemayor
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