16 replies to this topic

[masino]

Dear members

 

I have a question on flash drum temperature.

 

I attached process diagram, which is showing reactor and flash drum.

 

I have understand that temperature in the usual flash drum goes down when the separation happens.

.which means that flash drum feed temperature is higher than flashed vapor temperature.

 

However, in my simulation, 27K/G of two phase flow at 37C enter into the flash drum just right after pressure drop to 13K/G  and then flashed.

 

Strangely, flash temperature goes up to 40C.

 

I guess this is caused by some amount of HYDROGEN.

 

I cannot draw a clear picture of it.

 

Could you let me know why the temperature inside the flash drum increases???

 

I appreciate for your time and effort in advance

 

Yours

Attached Files

•  hydrogen flash.pdf   19.13KB   1142 downloads

Edited by masino, 12 August 2013 - 10:06 PM.

[breizh]

This resource may help you out :

 

http://www2.questcon...rmo/thermo.html

 

Breizh

[masino]

Yes

I appreciate for sharing webpages.

I will try to find method to relate the diagram to the hydrogen specific trend

 

Yours

[PingPong]

Could you let me know why the temperature inside the flash drum increases???

Probably because there is something not quite right in your simulation input, with respect to the specification of the control valve, the downstream pipe (if included) or the flash drum.

[thorium90]

Might I ask which simulation software you are using? You could post a zip file and we can take a look at it.

[masino]

I am using aspen plus

 

Yours

Edited by masino, 13 August 2013 - 06:53 PM.

[masino]

I am curious that small amounts of hydrogen (0.004mol%) will dramatically drop bubble points.

This will  affect the situation that I want to make it clear.

I cannot connect this bubble point dropping into the temperature increase inside the flash drum.

[thorium90]

I will be able to open aspen plus files. Do zip them up and post for further commenting.

[Art Montemayor]

Has everybody forgotten the notoriety of Hydrogen for having a NEGATIVE Joule-Thomson coefficient?   A negative J-T coefficient indicates that the gas or vapor will HEAT UP instead of cooling down when adiabatically expanded - such as the OP indicates he is doing.

 

To check out the J-T coefficient value, go to the NIST thermodynamics database website and check it out.  I did, and I got a negative value.   I remember Dr. Holland saying something about this one day in Thermo class while I was awake.

http://webbook.nist....Pa*s&STUnit=N/m

 

I insert the attached document as a refresher in free, adiabatic expansion of a compressed gas.

 

 Explaining and Analyzing Joule-Thomson Expansion.pdf   89.6KB   7846 downloads

[curious_cat]

Shouldn't the OP be telling us compositions? How much H2 is in there?

[PingPong]

Amount of hydrogen is very small. See message #7.

Joule-Thomson effect is here not relevant. Moreover vapor will not be pure hydrogen but will also contain vaporized liquid, so J-T coefficient of total vapor will not be negative.

 

However this is a mixture of gas and liquid, that flashes over the control valve and then flows into the drum.

Due to pressure drop over the control valve part of liquid vaporizes and causes therefor temperature drop of mixed phase stream.

 

If the drum has the same pressure as the control valve outlet there will be no further temperature drop.

If the drum has a lower pressure than the control valve outlet there will be some further temperature drop due to further vaporization.

If the drum has a higher pressure than the control valve outlet there will be a temperature increase due to some condensation.

 

Therefor I suspect there is an input error in the simulation.

[thorium90]

Thank you Art for pointing out the negative JT coefficient. OP has also pointed out the amount of H2. I agree with PingPong that there could be something wrong in the simulation. However the OP has elected not to continue the discussion by not posting any replies or his part of the simulation.

[curious_cat]

Amount of hydrogen is very small. See message #7.

Joule-Thomson effect is here not relevant. Moreover vapor will not be pure hydrogen but will also contain vaporized liquid, so J-T coefficient of total vapor will not be negative.

 

 

 

With that low a H2 percent I doubt Joule Thomson is relevant at all. 

[Art Montemayor]

I think you guys are right.  I failed to read his last input of 0.004% H2 and only read his highlight of HYDROGEN and the name of his uploaded file.

 

Without a composition, no one can seriously deduce anything.

[masino]

I appreciate for your participation and valuable knowledge so that I will take a closer look into Joule-Thomson, let alone thermodynamics

 

I will relate the phenomenon with negative value of coefficient

 

P.S

I am not finding any error in simulation input even though I am not sure everything is free of errors in it at the same time.

Afterwards, I will take care of data input and make the intent and purpose of question much clearer

 

Yours

Edited by masino, 04 September 2013 - 07:18 PM.

[PingPong]

masimo,

 

In your drawing in message #1 you show a temperature of 37 ^oC after the control valve, correct?

And then, only a slight distance further, in the drum at 13 kg/cm²(g) the temperature increased to 40 ^oC.

 

1) What is the pressure at the control valve outlet? Is that also 13 kg/cm²(g) ? Or higher? Or lower?

 

2) Did you specify the same thermoset for both the control valve and the flash drum?

[masino]

file that relates to this topic is demaged while formatting.

I will check it out and then try to provide information

 

thank you so much for letting me know what is going on

 

Yours