5 replies to this topic

[anj3]

Hi,

I know this question has been asked several times but I really don't know where to begin. I am given several scenerios where the flow rate of the incoming stream is constant. I know the incoming stream conditions (P and T) but the composition is not given. The incoming stream has heavy boiler material, HTF and low boiler material and the flash drum is used to separate the HTF and low boiler material from the heavy boilers. How do I chose the operating temperature and pressure for the flash vessel? after proceeding, how do i find the specifications without knowledge of streams' compositions?

Thank you for help in my first engineering design job.

[S.AHMAD]

Two important properties for sizing flash drum namely
1. density of liquid
2. density of vapor.
From these densities, we then can calculate the critical velocity from which we can calculate the drum diameter.
Liquid holdup of 5 mins would be sufficient.

Edited by S.AHMAD, 07 October 2011 - 02:41 AM.

[anj3]

How do I find the operating conditions first? Should I keep the temperature or pressure of the incoming stream constant? Do i need to know the mass fraction composition of the stream ?

Thank you so much for your help!

[S.AHMAD]

You can use the upstream conditions (P&T), compositions and flowrate. You need all these data.
1. Once you have the critical velocity, use the vapor volumetric flowrate to determine the diameter of the vessel.
2. Once diameter available, use the liquid flowrate to determine the liquid level so that the liquid residence time is about 5 minutes.

[anj3]

using the information above, my calculations show a result of : 1.916 in. for the radius.

The data I used is:

vapor stream: .03 kg/s
density of liquid: 52.8 lb/ft3
vapor density: .325 lb/ft3
K value= .2 for a system without a wire mesh

my radius is too small and inconsistent with known vessel sizes. Are there other methods to size a flash vessel? Are there models that might apply specifically for certain heat transfer fluids(HTF), such as therminol in this case?

[S.AHMAD]

1. The vapor flowrate is too small
2. In this case, use liquid hold up time (e,g 2 minutes) to determine the liquid hold up volume.
3. Size the vessel using L/D = 4
4. Search internet for more info. One of them as below

http://www.chemicalf...00ab&topic=8913