6 replies to this topic

[niazali]

Hello there;

                 I have been asked to design a flash tank separator for Ammonia...

i used the k from Watkin's curve and used it to find the maximum allowed vapor velocity,followed by the Diameter of the drum. Calculated the length of the drum for the typical ratio used ... L/D =3/1

now my Question is: are there parameters enough when it comes to design of a flash separator..?

 

I have noticed in design of throttle, that its unit is in velocity ...So i have to use that maximum throttle velocity to find the diameter of the opening or valve ,as the volumeteric flow rate of feed is known to me...or the maximum throttle velocity itself is a design parameter that i need to show.?

 

last question.:

i was told by an engineer from industry that the temperature is considered to be same thought the separator ?

why is that ? ist the temperature gonna drop due to joule-Thomsom effect ?

and is the pressure drop is only due to the fact that the components are being separated and the overall pressure drops due to the decrease in the partial contribution of the separated component ...?

if this is the case then where is the pressure drop that occurred at the throttle ,regained ?

 

[Bobby Strain]

The separator you must size based on the flow of vapor and liquid at the separator operating temperature and pressure. So, starting with the stream conditions you must first determine these values. For pure ammonia, use a Mollier chart. Then you can size the separator. You will find lots of information on the subject if you use this sites search engine.

 

Bobby

Edited by Bobby Strain, 21 May 2016 - 09:47 PM.

[niazali]

Thanks bobby for your reply;

                                               Yes, i have the feed temperature and pressure based on which i have the vapour and liquid density, and i have used these densities to find THE SEPARATION FACTOR  and then k. I am using aspen HYSIS to get these required values.

can you please inform me about the throttle design and why is its unit ft/s

Also the temperature and pressure change through the flash separator 

[Art Montemayor]

Please be specific.  Are you "flashing" (adiabatically expanding) a stream of liquid ammonia?  If so, specifically state all the process conditions - inlet and outlet.   Additionally, you are merely throwing terms around without understanding wht they represent.  The Joule-Thomson effect is not applicable when you adiabatically expand a liquid stream.  The Joule-Thomson effect applies to a gas stream adiabatic expansion.  The term "flashing" applies to expansion of liquids, not gases.  So there is some confusion here - either on your part or mine.

 

If you, indeed, are expanding a compressed ammonia liquid stream (as probably for refrigeration purposes), then you must calculate the amount of ammonia vaporized as a result of the adiabatic expansion.   This is the usual process done in ammonia mechanical refrigeration systems.  Our Search Engine has a lot of threads dealing with this subject in the past and I have posted the derivation of the necessary equation for calculating the amount of the gas stream resulting when you expand ("flash") a liquid.

[niazali]

THANKS ART;

                       i will try to be more specific and sorry for not being able to clarify myself enough.

i am trying to design a separator where i can separate ammonia from the outlet steam from ammonia synthesis reactor. The attached pic shows the process variables:

(and i mentioned the Joule thomson effect because i have gases in the feed)

can you please explain to me why are we considering no pressure drop and no temperature change across the separator.

 

Attached Files

•  pic.png   24.06KB   5 downloads

[Art Montemayor]

niazali:

 

Even if you are a student, you still have to be specific.  Otherwise, you can't communicate what you mean to say.

 

All you furnish is a Hysys simplified separator print out (simulators being the worst type of flow diagram generators) that shows a gas mixture stream entering what appears to be a mechanical separator.  But your simulator print out fails to identify the PHASE of each stream.   This is why I HATE simulator sketches.  Why don't students just take the time and make the effort to draw their own, accurate sketch???

 

You (or someone you know) entered the basic data into the simulator, so how is it that you have a gas stream entering a separator at the same pressure exiting at the same pressure?????   If the stream entering the separator is a pure gas mixture, WHAT are you separating????  You can't possibly expect to separate one gas component from another in a mechanical separator!

 

How can you establish ANY flow whatsoever if you don't have a pressure drop????   Any fluid flow is impossible without a driving force (a pressure drop).  I don't believe your simulator sketch and I don't understand your query.  If you don't have any liquids or solids in your feed gas stream, there is absolutely nothing to separate ........

[niazali]

Sir ;

Ammonia is in liquid form ( there is a pressure difference between the feed stream  and the tank itself)

but still  i want to know is that what would happen to the pressure across the separator ? will it change if yes then why..? same question for the temp. across the separator 

Attached Files

•  pic 2.png   7.99KB   6 downloads