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Dew Point Calculation

ammonia hydrogen dewpoint

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#1 Student234


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Posted 01 March 2022 - 10:16 AM


Edited by Student234, 01 March 2022 - 06:59 PM.

#2 MrShorty


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Posted 01 March 2022 - 01:02 PM

I don't usually think of "dew point" relating to a single component -- unless that component is the only "condensible" in the gas mix. In that case, perhaps a simple look at your favorite vapor pressure equation for ammonia will be accurate enough.


The other scenario when I see us talk about "dew point of a single component" is when we don't really want to calculate dew point, but want an alternative method of expressing concentration. From what I see, this is usually an "idealized" calculation like above where you assume that the component is the only condensible component and use its vapor pressure equation to convert partial pressure to temperature.


Assuming that you really want to know the actual dew point of this gas mixture, I note that you have only told us what is in 1 mol% of your gas mix. What is in the other 99 mol%? I know ammonia can be among the tricky compounds to model, especially when present with water and other acid gases. You will need to know what is in the rest of your gas mix so you can choose an appropriate thermodynamic VLE model for calculating the dew point of the gas mix. Once you have chosen your thermo VLE model, then you can start working on how to solve that model for dew point.


As for the question of "most accurate", obviously that's the model that provides the most reliable solution. In the case of ammonia, I know that there has been quite a bit of research and discussion over the years trying to decide what the "best" model for ammonia that can handle its interactions with water and acids and other bases correctly. So much of choosing "the most accurate model" (assuming there is a single model that is most accurate for ammonia in all scenarios) will depend on what is in the rest of your gas mix.


I'm not sure how else to help. It seems that the information in the OP is insufficient to really talk about dew point calculations. Help us understand in more detail what you are trying to do, and we should be able to help.

Edited by MrShorty, 01 March 2022 - 01:03 PM.

#3 Art Montemayor

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Posted 01 March 2022 - 01:25 PM


You have - as usual - hit the nail on the head.


I received a message from Harry (our OP) detailing more Basic Data than that posted here.  I have replied requesting him to be more specific and detailed in describing his need for assistance in what is basically a phase separation problem.


Your reply and appropriate reaction is exactly what I predicted to Harry regarding replies to such a weak and non-descriptive kind of request for help or assistance.  I hope that Harry replies to your sincere and professional attempt to help him out because of the the knowledge and assistance he can reap and gather from our experienced members such as you.  Basically, I believe the problem is how to manually calculate the cricondentherm temperature involved in condensing ammonia from a mixture with H2 and N2 in the Haber Process.



#4 Bobby Strain

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Posted 01 March 2022 - 03:22 PM

It's obvious that those are fractions; 0.4, 0.4, 0.2. Total is 1. It's simply a language translation error. And it's also obvious that the first condensate is nearly pure ammonia.



#5 MrShorty


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Posted 01 March 2022 - 04:00 PM

@Bobby: Now that you mention it, it should have been obvious that he/she meant 40%:40%:20% H2:N2:NH3. And, as you say, it should also be obvious that any condensate is going to be almost pure ammonia.


Seeing that now, it still boils down (ha ha) to choosing a VLE model. If I didn't want to settle for the simplistic "ideal" model (where I get the partial pressure of ammonia then assume that the dew point is along ammonia's vapor pressure curve), I might be inclined towards a simple cubic equation of state. I'd want to make sure my EOS would accurately predict the vapor pressure curve (since the calculation, in many ways, simplifies to "find the dew point of pure NH3 followed by small corrections for the total pressure and for the small amount of dissolved, inert gas." Interaction parameters for the mixing rule would be important for the most accurate calculation, and I would wonder if I had access to good interaction parameters for my chosen EOS/mixing rule.

#6 MrShorty


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Posted 02 March 2022 - 12:55 PM

After reviewing your PM, I would recommend that you share that information here in the public part of the forum. In part because much of your question is beyond my expertise, so putting the details out publicly will allow the other engineers here who are more familiar with different aspects of your query can comment on those. My expertise is phase equilibrium, so I felt I could comment on the dew point part of the question, but your PM included problems related to other engineering aspects of the problem in addition to the phase equilibrium part.


I would note that your PM included composition information very different (mostly in that it included a significant amount of water) from your now deleted OP.


We are happy to help, but I find that the best help comes in the public areas of the forum where all of our experts can bring their expertise to a question. It also helps significantly to make sure you share enough detail about your question to allow us to understand the problem you are having.

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