Flash Drum VLE

[GasProcessingEngineer]

Hello Everyone,

I had a practical question related to Flash Drum design and VLE. If I have a two phase mixture entering a flash drum, and I drop the pressure of the entering mixture using a throttle, will the new V-L mixture equilibrate at the same pressure that I drop it to, or will it equilibrate at some other slightly higher pressure. The reason I am asking this is because once we drop the pressure, more of the volatile component in the liquid phase will vaporize, thereby increasing the moles of vapor and causing the overhead pressure to rise again. Thanks in advance for any clarification that you can provide.

 

[Mohammad Ka]

What are the inlet composition,P, T and outlet P of the valve?
Can you explain more about the overhead pressure?

 

[Latexman]

GPE,

In the ideal world (classroom), yes, equilibrium is achieved at the flash drum pressure. In the real world, it may not reach equilibrium at the speed in the process. That's why we have separation efficiencies. If you have an existing or similar operation, one can use that data to get realistic separation efficiencies.

Good luck,
Latexman

Engineers helping Engineers

 

[GasProcessingEngineer]

Thank you for the responses. Unfortunately, I don't have data to support the question as this questions was something I was randomly pondering.

I am just confused because once we drop the pressure of the entering mixture and flashing occurs, wouldn't the vapor overhead pressure begin to increase as more of the liquid is being flashed? At equilibrium, the pressures of the two phases should be equal, so that should mean that the equilibrium pressure will NOT be the initial throttling pressure, even in an infinitely large drum?

 

[Latexman]

The greatest determinate of the pressure in a flash drum is the pressure transmitter (PT) on the flash drum, a controller, and the control valve (CV) in the vapor line. If larger quantities of vapor are generated, the PT detects this, sends a signal to a controller, which requests the CV to open further. It strives to maintain the pressure set point. The PT, controller and CV will make one to several corrections per second. The residence time of the drum depends on size and phase, but it will probably be >> 1 second. So, while the process inside the drum is dynamic, so is the standard control loop.

Good luck,
Latexman

Engineers helping Engineers

 

[GasProcessingEngineer]

Thanks Latexman. Just to clarify, so by opening the PCV on the overhead line further, we are increasing the pressure inside the drum to reduce the amount of flashing?

 

[Latexman]

By opening closing the PCV on the overhead line further, we are increasing the pressure inside the drum to reduce the amount of flashing.


Good luck,
Latexman

Engineers helping Engineers

 

[MortenA]

One thing is the composition another thing is the pressure. Your system pressure is a function of flow and vice versa. So when you drop your pressure it wont increase downstream - but decrease as flow creates further pressure drop in piping. The gasses that flashes off will result in a large volume flow and thus increase pressure drop - but it cant make the pressure go up - the gas will always move towards the lowest pressure. But of course your valve may have to open further to maintain your requested pressure and at the end it may be fully open and your pressure not low enough.