4 replies to this topic

[abacaba]

I have been tasked with designing a flash evaporation vessel  (Knock Out vessel) with the purpose of cooling and decompressing pressurised Ammonia from inlet conditions: 8.3kPa, 19^oC to AtmP, -33.3^oC (Ammonia boiling point) so it can be easily transported and stored. All flashed Ammonia is sent to a compressor & heat exchanger where it is re-compressed  heated & recycled to inlet. The vessel has to process a mass flow-rate of roughly 3500 kg/hr.

1. From Perry's, CRC handbook and Felder & Rousseau I found, plotted and interpolated vapour pressures, density and enthalpy data for liquid and vapour ammonia at the different temperatures.
2. After this I assumed the inlet and liquid outlet flow-rate was 100kg/hr and found the vapour flash fraction using enthalpy data.
3. The vapour and liquid flow rates where then multiplied by the inverse of their respective densities at the said pressures and temperatures to give the  total volumetric flow rates.

My problem now is that I don’t know how to determine the required height and volume of the column. I assumed that I could just set a diameter and determine the height by dividing the volumetric flow-rate by it. That way I get a velocity but this does not shed any light on scale because of the time component for example if I were to do the calculations in seconds I would get a much smaller height than if I did it in using hours which has left me thinking I am missing something, probably obvious. I think I need to incorporate acceleration due to gravity in my calculations. I would appreciate if someone could confirm whether I am on the right tracks or if I am missing something. If anyone could shed light on this problem it will be greatly appreciated and thanks taking the time to read!!

Attached Files

•  6.Ammonia Vapour Pressure data.xlsx   147.58KB   56 downloads

Edited by abacaba, 07 April 2013 - 02:21 PM.

[Art Montemayor]

You have prepared a good-looking workbook, but I can't make out what you are trying to do.  You write you are basically trying to design a liquid-vapor separator, but you show 5 vessels that appear to be separators with saturated liquid ammonia being exapanded into them (although no expansion valve is shown).  All this makes for a very complicated explanation - expecially with all the streams you show on what appears to be a mass balance of all the streams.

 

Why do you show a centrifugal compressor receiving all the flash streams at the same suction port (or stage).  In other words, you show a single stage centrifugal compressor.  Is this correct?  If so, how do you obtain a pressure driving force between the subsequent flash separators?  What is the meaning of the vertical columns with what appears as trays and different streams?  That is very confusing without a detailed explanation.

 

You don't need Perry's Handbook or the CRC or any other book to obtain thermodynamic data on Ammonia all you have to do is go to the NIST website and you have everything you need - in tabulated form.  Search our Forum threads on this subject and you will find out how to obtain all this free thermo information.

 

Which of the five separators do you want to design?   Just show a simple sketch of the separator and we can refer you to existing information on how a flash separator is calculated in detail..

[abacaba]

Thank you for the reply Art and apologies, my workbook is messy and I didn't explain what I have done. I am not sure whether my method is correct as so far I have just been following my intuition and the units so some criticism would be appreciated.

 

Yes I am trying to design 5 flash vessels for the reasons explained below however information on calculating the volume and dimensions of a single vessel would be more than enough. I am feeling like I might need to re-evaluate my calculations so far or use a correlation or some rule of thumb method to continue. I have since used the Souders-Brown Equation with a K value of 0.107m/s and a length to diameter ratio of 4 however this gave me a diameter of 0.32m and a height of 1.61m which seems rather small for a flow-rate of 3.5 tonnes/hour. I can’t help but feel like I am over complicating the process and missing something key.

 

The unit that I am designing is an existing refrigeration tower which flashes ammonia over 5 stages. For the purpose of the design I have treated each stage as an individual flash vessel for simplicity. These 5 stages are each incrementally lower in pressure than the previous until atmospheric pressure is reached which provides the driving force. I have also done calculations for 1,3,and 7 stage system to compare my design with.

 

The main reason for the stages is to reduce the duty on the compressor(s), so that it is compressing smaller vapour streams which are already at higher pressures and temperatures as opposed compressing the whole vapour stream. To answer your question the flow diagram is just a basic diagram expressing the system whether or not I mix the vapour streams before feeding the compressor or use multiple compressors is something I will consider later in my design.

 

I have calculated the Inlet, Outlet & recycle Liquid and Vapour mass and volumetric flow rates and performed and energy balance It’s the scaling of the vessel i am now stuck on.

 

I have searched with little result for information of designing a flash vessel the information I have been able to find is on determining the vapour flow rate which I already have.

 

Again any help will be greatly appreciated & thank you for taking the time to read all of this!!

Edited by abacaba, 09 April 2013 - 10:05 AM.

[abacaba]

I have since assumed a hold-up time of 1/2 an hour and divided by volumetric flow (7291m^3/hr) by it to get the volume of the vessel then by varying the diameter to height ratio making sure its between 3-5 (this is the range most sources I see have it at being) I came up with a height of 38.4m and a diameter of 11m. This seems like a huge vessel so I dropped the hold-up time to 12mins which gives me a volume of height of 28.3m and diameter of 8.1m do these values seem reasonable?

[Bobby Strain]

If you google my name you can find my website which has software to calculate such things online.

 

Bobby Strain