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Cross Temperature In Heater Exchanger (Lng)

lng heater exchanger temperature cross minimum approach warming

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#1 Yogan

Yogan

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Posted 21 February 2024 - 09:41 AM

Dear all,

My name is Yogan Felipe Sganzerla, and I am a Mechanical Engineer, writing from Brazil.

I am simulating a Linde-Hampson Cycle to liquefy hydrogen. The cycle is as shown in the attached photo. Additionally, I am adding a photo of my simulation.

You can find the .hsc file at this link (my Google Drive): https://drive.google...ew?usp=sharing.

Let me talk a little bit more about our trouble...

I need to define the maximum effectiveness of the HEX2 - bath so that the minimum approach is higher than zero (as close as possible to zero). For example, when I have in the stream LN2 10% of N2 and 90% of methane (set in spreadsheet composition), the temperature cross appears, and it is because in my ADJ-2, the Specific Target Value is 0.98 (effectiveness), meaning that it is too high for this mixture, so I should decrease it. I would like to do it automatically because I will run many scenarios for compositions.

How can I implement this?

Thank you for your attention, and if you need additional information, just let me know.

Kind regards,

Attached Thumbnails

  • Photo2.png
  • photo1.png


#2 Pilesar

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Posted 21 February 2024 - 10:06 AM

The simulator should be able to use minimum internal temperature approach (MITA) as a parameter. It is useful to start the simulation at much more cold fluid than you need so that the MITA is large. Then sneak up on a close MITA by using a controller with small steps to adjust cold fluid flowrate. That way you never get a temperature cross which is difficult for the simulator model to recover from automatically. If the simulator has a 'case study' feature, then this would be an ideal candidate.


Edited by Pilesar, 21 February 2024 - 10:08 AM.


#3 Yogan

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Posted 22 February 2024 - 09:07 AM

Hello Pilaser, 

 

First of all, thank you for your reply. 

 

Yes, the approach that you wrote is the same as we pretend to use. However we don't know how to implement it on HYSYS. Were you able to acess the file stored here:https://drive.google...ew?usp=sharing.?

 

Let me talk a little bit more about the variables that influence this trouble...

 

The LN2 stream and GN2 stream are mixed fluid refrigerant (10% Nitrogogen and 90% CO2). It is defined that this blend will enter as liquid (title =0) and will leave as gás (title =1) and the pressure of the stream is defined as 1 bar and to simplify, we are considering he pressure drop in the Heater exchanger iqual to Zero. 

In the stream 3 and 4 the compostion is Hydrogen and the pressure and mass flor id defined. 

 

So the unkown variables in the HEX2 are:

  1. Mass flow of the stream LN2
  2. Exist themeprature of Hydrogen (4)

The effectiveness will be used to difine the Temperature 4. 

The Energy balance is going to be use to define the mass flow in the stream LN2. 
 

However, when I do it, i have themperature cross! To solve it, I realized that I need to decrease the effectiveness until a point where the Min Approach becomes positive.






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