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[Not Really a ChE]

To Anyone Who Can Help,

First, About Me:
I'm posting this here because I'm just barely not a student and I don't have a ChE background. I graduated in May with a BS in Metallurgical Engineering. I now work mostly with extractive processing, but find myself needing to do a little bit of everything (of course.) The only ChE related class I have taken is Intro to ChE Thermo, geared towards non-ChE's, and that was almost 4yrs ago now.


Now to the Questions...
I am trying to design a flash drum for with the following conditions:
1. Input Stream: 2.5 wt% (2.64 mol%) soln of NH3 in H2O at 110ºC. (I do not know the pressure, I also don't know what a reasonable real world assumption might be.)
2. Liq. Output Flow rate: 50 gpm liquid required
3. Vapor Output Flow rate: Whatever is necessary, this will be a recycle stream.


I have gone through a few other posts on flash drums, and attached my attempt in excel.
-Questions About the Souders-Brown Eqn.-
(I believe that most of my problem stems from my total lack of ever having done this before...)
1. I am not sure of the conditions to use for the liq and vapor densities. For the spreadsheet I looked up the density of saturated ammonia in "The Thermodynamic Properties of Ammonia" by F.G. Keyes and R.B. Brownlee. I chose the value for NH3 at 200ºF as it was the closest to the temperature of the input stream, from this table I found dV and a corresponding pressure, ~800psi. Then I looked up dL for water under the same conditions. I am a little uneasy with this due to the high pressure.
--What conditions do I use to determine dL and dV?

2. It doesn't seem like the Souders-Brown eqn. could give you the internal pressure of the vessel, how do you determine this? (Need to know in order to figure out the pressure vessel part of the design)


-Other Things I don't know, but would like to.-
1. Ammonia is pretty soluble in water, do you think the output liquid will have a significant difference in composition from the input stream?
2. Output temperatures, esp. the liquid phase (known: NH3 absorbs 327 cal/g upon evaporation)
3. Output composition of the liquid and vapor would probably be useful too. (Not sure if you have this particular book but...I have borrowed the Intro to ChE Thermo text by Smith and Van Ness and pg 368, ex 10.5, seems like the way to go. But then I have questions about that too.)


I know there is a lot in here, but if anyone can set me on the right path that would be greatly appreciated.

Thanks,
Brea

Attached Files

•  Flash_Drum_Sizing.xls   17.5KB   84 downloads

[siretb]

Hi,
Your problem looks to me much underspecifiedf, but we can try to make a few assumptions
1) inlet pressure; If it comes from a pump you can use 2-3 bars. at 110°C you xill be 100% liquid (no vapor)

the second assumption is that your flash drum will operate under atmospheric pressure. This is by no way obvious. And be be adiabatic (no heating, no cooling)

Under these assumptions, you get the vapor composition by looking at the equilibrium tables;
Ammonia is very soluble in water, but if NH3/H2O are the only two components (no air), then the liquid outlet stream will have a weaker ammonia concentration. A full flash calculation can be made, and under the above assumptions I find that the outlet concentration is slightly less than 2%wt
The temperature is about 93°C
The vapor stream will have a very significant amount of ammonia (because the initial temp is high).
Specific gravity will be much under 1. Since the molar weight of ammonia and of water are about equal, you can easily calculate that from T and P.

3) vapor flow rate. the Vfraction will be small(just because NH3 is so soluble isn water) You should no flash more than 3-5%.

I hope this helps, but again your problem is not well defined. It is important that you understand that you will have to take decisions, make choices. The most important one will be the operating pressure of your flash drum. Why do you need to flash this 110°C stream?