9 replies to this topic

[dreambigger777]

Hello everyone,

 

I was studying about Flash Drums and trying to understand the process. I understand that there is a separation of two phases taking place and for which it is required that the flash  drum pressure at a constant temperature is between the bubble point and dew point pressure of the feed or the flash drum temperature at a constant pressure is between the bubble point and dew point temperature of the feed. But while going through some posts on this site about designing flash drums, I found that one could set the operating conditions of the flash drum same as the feed stream conditions. I am really confused as to how this could lead to phase separation. 

 

Thanks

 

 

[breizh]

http://www.nt.ntnu.n...sh_edition_2009

 

Consider this paper , it should help you.

Good luck.

 

Breizh

[dreambigger777]

Thank you very much  Breizh for the paper. What i still dont understand is if the flash drum is kept at the same temperature and pressure as the feed, how can the feed separate into a 2 phase mixture without any reduction in pressure or increase in temperature? I dont know if I am right or not,but could it be because the feed is passing through the throttle valve where the pressure reduction is taking place and then entering the flash drum and therefore the operating conditions of the flash could be kept same as the entering feed stream/

Edited by dreambigger777, 26 May 2014 - 02:06 AM.

[fallah]

Hi,

 

Appears you are discussing about a multicomponent mixture of liquid-vapor rather than a single-component one. Then, because the goal is liquid and vapor separation why one cannot set the operating conditions of the flash drum same as those of the feed?. Of course, pressure reduction on the feed could normally be done across a valve before entering to the drum allows the pressurized liquid to be partially vaporized.

In fact, the flash drum with a confined space well larger than a line makes the moving turbulent fluid nearly stagnant and will provide a room for such separation and allows the liquid-vapor mixture to reach an equilibrium . Obviously for multicomponent liquid-vapor mixture for which it's supposed to be in equilibrium, the flash temperature at a constant pressure is between the bubble point and the dew point temperatures of the feed.

Edited by fallah, 26 May 2014 - 03:22 AM.

[dreambigger777]

Thank you Fallah for the reply. But what if there is no a valve kept before the flash drum for vaporization. The vaporization has then to be initiated within the flash drum and for that we do need the operating pressure of the flash drum to be below the incoming feed stream pressure at a constant temperature. So how can we set the operating conditions for the drum to be same as the feed stream then?

[fallah]

Hi,

 

Let's separate two cases from each other:

 

1-If a valve or RO kept before a low pressure flash drum against a high pressure liquid, then main flashing is already happened across the valve and the drum is mostly applied for separation. But if the valve is close enough to the drum, the pipe segment between the valve and drum and the drum itself can be considered as a whole in which partial vaporization to be happened causes the pressure and temperature of the fluid to be reduced.

 

2-If no valve kept before flash drum, then the feed has already been a two phase flow and will be separated inside the downstream drum with almost the same operating conditions as the feed.

[dreambigger777]

Thank you so much Fallah. This means that a flash drum alone without a valve can never initiate the vaporization.

[fallah]

Hi,

 

If there would be no pressure reduction on the feed flow via a valve, then even though the pressure a little bit reduces inside the drum due to expansion and might followed by a little vaporization, the operating conditions of the drum would be considered same as those of the feed and the downstream drum operates as a separator rather than a flash drum.

[dreambigger777]

Thank you so much Fallah for explaining it so well. 

[Dacs]

"Flash" drums does little "flashing" and it only acts as a separator to separate the vapor from the liquid in your feed.

 

The "flashing" part comes from somewhere (as fallah has already pointed out) upstream either through a throttling valve or an orifice.

 

Looking at the whole picture, you need to have a driving force to push your fluid going to the drum. And with that, you need to have a higher pressure upstream of your drum. Having a feed with the same OP with the drum will lead to nowhere since it won't go inside the drum by itself

 

All I want to point out is that you need to have a mechanism for pressure letdown for flashing to occur. It's almost always the case that your driving force (drum OP  - Feed P) is higher than the frictional losses in your feed line (plus expansion inside the drum), so you need to either have a valve or an orifice in your feed line. This will also serve to control the flowrate going into your flash drum.

 

[EDIT]: I should have been more concise. There are indeed flash drums (which have throttling valves at vessel entry). The reply was in the context of having the letdown outside the drum.

Edited by Dacs, 03 June 2014 - 11:12 PM.