9 replies to this topic

[Luna D]

 Hello everyone,

I am having a Temperature Cross problem in the simulation attached. The stream 26 has pure nitrogen and the 28 has hydrogen. My objectif is to find the good flow for the nitrogen stream if I put 1 tonne/day for the hydrogen one. I tried to change all parameters possible and I can't find the problem. 

Thank you for the help.

[breizh]

Hi,

No file attached!

Breizh

[latexman]

Luna D., 

 

To put your attachment in the original post (OP), click Edit at bottom of OP, click Use Full Editor, and the Attach File functions are under the Full Editor text window.

[Pilesar]

LNG exchangers usually have a tight temperature approach so that the hot stream temperature stays very close to the cold stream temperature for most of the length. The simplifying assumption that 'heat capacity is constant unless there is a phase change' is not good enough! The heating curves for your streams are not exactly linear. Since the two heat curves are close together, there can be an internal pinch that is not obvious from looking at the inlet and outlet temperatures. I do not know the details of your problem, but you should calculate the heat exchanger using many internal regions. Some modeling software allows the user to specify a minimum internal temperature approach (MITA) which is extremely handy. View the plots of your heat curves on the same graph to get a better feel for what is happening with your calculations.

[Luna D]

Hi everyone, thank you for the replies. Actually I was not able to attach the file and in the end I found the problem : it's happening a change of state inside the LNG (from gas to liquid), that's why the Temperature Cross problem. I would like to know if some of you know some way to solve this problem or should I model a new process using a heater and a cooler to replace de LNG. Thank you !

[Luna D]

Pilesar, I just read read your answer and that's exactly my problem : the phase change. Is this approach you talked about disponible in HYSYS?

[breizh]

Hi Luna ,

Je crois bien que Pilesar t'a donne la reponse , segmente ton echangeur .

Disponible en francais se traduit par available en anglais .

Breizh 

[Pilesar]

(I learned a new word 'disponible' meaning 'available' in English. Thank you!) For heat exchange to take place, there has to be a difference in temperature between the streams (Q=UAdT) and if dT becomes zero, then Q becomes zero. Hysys should have tools to show the heat curves of your streams. It may not be physically possible to transfer the heat you desire with the streams supplied.

[breizh]

Hi,

Google "LNG heat exchanger design with Hysis" , you will find tutorials on Youtube

Breizh

[Luna D]

Thank you very much for the tips and answers, Breizh and Pilesar! Have a good day!