4 replies to this topic

[Enginerdo]

Hi all,

I'd like some validation to some basic concepts behind how flash drums operate.

From my understanding:

The idea is to bring a fluid to saturation conditions so that the fluid can separate into the 2 phases.

The size of the drum is based on residence time required.

Are there any other important aspects that I am missing?

How are they typically controlled (pressure/temp/level)?

Thanks.

Edited by Enginerdo, 27 January 2011 - 01:16 PM.

[Art Montemayor]

Enginerdo:

Your assumptions are not correct. I don’t know where you obtained them – and I would like to know how you came about that understanding.

The best way I know of to explain a flash unit operation is to start with basics. A flash drum is used as a vessel to enable a change of phase as well as a mechanical separation of the two resulting phases. This is the general and most prevalent description of what a “Flash Drum” does – or is meant to do.

The basic underlying requirement for a fluid to undergo a flashing operation is that it must be in a condition that allows it to comply with a driving force (a pressure drop) in order to allow a change of phase. The best way that I know how to illustrate what I just described is to present a Mollier Diagram – or better still, a Temperature-Entropy diagram.

Please refer to the uploaded Mollier diagram for Methane. In this diagram you will see the plot of Pressure versus Enthalpy for Methane. Let us suppose that you have methane compressed to a rather high pressure and a relatively ambient or cool temperature and you want to flash this stream in order to obtain cooling and a separation of the resultant phases in order to recompress the vapor formed and continue to create a cold sink in that manner. This is actually what we do in real life refrigeration cycles. Here, I am assuming you are knowledgeable in thermodynamics and fully understand refrigeration cycles.

Look at the Methane Mollier Diagram and identify the “dome” curve that represents the saturated conditions of Methane. The left-hand side of the dome curve (the 0% line) represents the saturated liquid methane line while the right-hand side of the dome (the 100% line) is the saturated vapor line. Where these two lines meet (at the very top of the dome) is where we identify the Critical Point. For Methane, the Critical Pressure is 4,599.2 kPa; the Critical Temperature is 190.56 K.

If you look carefully at the “dome”, you will see that it leans slightly to the right-hand side.

A fluid expansion process is basically an adiabatic, irreversible process and it is described as an ISENTHALPIC process (the enthalpy is constant). Therefore, a “flashing” (actually, an adiabatic expansion of the fluid) is represented by a straight, vertical line on the chart. This is an important point because it means that you can take any fluid condition that lies on top of the “dome” and draw a straight vertical line that ends inside the dome at the terminal pressure condition and it will represent the process of flashing. You will note that you can flash from the supercooled liquid region at the top-left, the supercritical region at the very top of the dome, and even some of the superheated gas that is slightly to the top-right part of the chart and still wind up inside the dome. You do not have to start with a saturated fluid in order to “flash”. Also note that the moment you reduce the pressure and get inside the dome, you are winding up with a colder fluid mixture. This is the refrigeration effect of flashing. THE PROCESS IS DEFINITELY NOT ISOTHERMAL. IT IS ISENTHALPIC.

If I knew your background I could go into further details regarding this process. I don’t want to make assumptions because I could be wasting both yours and my time and not accomplish anything of value to you.

I hope I have succeeded in explaining the process.

Attached Files

•  Methane Mollier Diagram.pdf   905.57KB   100 downloads

[Enginerdo]

Art,

It has been quite some time since I took this in school and most of my work experience has been post EDS phase, so I needed a refresher. Thank you for clearing any uncertainties up.

My assumptions were based on a PFD and material balance that I was reviewing this morning. The numbers didn't seem to make sense and that is why I had started the topic (I was trying to reason through the process based on what I assuming the data was correct, after a closer look I realized there was a mistake on the material balance).

I had then edited the post, however, based on your response it doesn't seem that it went through.

[Art Montemayor]

Enginerdo:

Yes, we probably crossed each other with your editing and my posting. That’s OK. We seem to be on the same page. Allow me to further answer your edited queries:

After the high-pressure fluid (usually a liquid) is expanded, the resulting product falls inside the T-S dome and, as such is a mixture of saturated liquid and vapor. The position it occupies within the dome sets the “quality” of the mixture (% saturated vapor);

The flash drum size is NOT BASED ON RESIDENCE TIME. Rather, it is based on the superficial velocity allowed to the vapor inside the flash drum as it is separating and ascending up and out at the top of the drum. The Brown-Souders equation is used to size the diameter of the vertical flash drum and the amount of residence time is what determines the volume in the sump of the drum. Usually, we design this part depending on the operator’s reaction time required to mitigate a drain valve failure or similar incident.

This subject has been discussed many times within various threads in our Forums in the past. If you use the SEARCH feature, you will be able to pick up specific details and variations on the sizing and calculations involved.

I hope this helps.

[joesteam]

Art,

Another well informed post, keep it up.