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Determining Chemistry In Reflux Vessel


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#1 HailToPitt

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Posted 05 October 2014 - 11:58 AM

Admittedly, this question is probably a bit elementary, but I've got to get back in practice with this sort of process.

 

I recently inherited a sour water stripper in a water treatment plant.  Feed rate and temperature is controlled, and feed stream has known quantities of NH3, NH4, SCN, HCN, H2S. The feed also has dissolved CO2 (and all constituents) but is not tested for normally. Feed is near the top (stripping only), and overhead vapors are cooled with condensate falling into a reflux drum.  All condensate is refluxed back into the column and excess vapor is taken to a different process.

 

It is crucial to control the temperature in the reflux drum. Pressure is typically controlled outside the process, so the temperature must be high enough that excess gas does not dissolve in the water, and rather goes off with the vapor stream. We have some experimental data for benchmarks, but I wanted to be able to simulate it for more exact answers and see if it fit the existing data.

 

1) Given just this data (known feed stream, pressure and temperature in reflux drum) is it possible to determine the vapor and water chemistry inside the reflux drum. My thoughts are that I'd be able to take and initial guess, solve for the drum chemistry, then add the reflux back into the column, and repeat until I reached convergence.

 

2) If not, the equilibrium condition of the reflux stream is relatively well known and tested, so would that be required data to fully solve.

 

3) The number of equations describing the drum is large, so I wanted to detail my process so far.  I'm not able to back myself into an answer so I'll list them in the next post, and hopefully someone can steer me in the right direction. 

 

Thanks



#2 PingPong

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Posted 05 October 2014 - 12:12 PM

My thoughts are that I'd be able to take and initial guess, solve for the drum chemistry, then add the reflux back into the column, and repeat until I reached convergence.
It is madness to try to calculate the reflux composition by hand, as well as to consider it as a feed in your column calculation.

 

Use a proper process simulator and model the reflux drum as part of the column, not separate.



#3 HailToPitt

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Posted 05 October 2014 - 12:14 PM

Unknowns:
PH2O, PHCN, PH2S, PNH3, PCO2
[H+] [OH-]
[H2S] [HS-] [S--]
[HCN] [CN-]
[CO2] [H2CO3] [HCO-] [CO--]
[NH3] [NH4+]

Count water and that's 19

--------------------------------------

Here's my list of equations

---------------------------------------

Solution Equilibrium equations:

H2O <---> H+ + OH- [1]
NH3 + H2O <---> NH4+ + OH- [2]
HCN <---> H+ + CN- [3]
H2S <---> H+ + HS- <---> 2H+ + S-- [4-5]
CO2 + H2O <---> H2CO3 <---> H+ + HCO3- <---> 2H+ + CO3-- [6-8]

(I'm working under the assumption that no SCN is stripped out)

-----------------------------------------------

Henry's law equations
H2O (not counted since it is set with steam tables *phew*)
HCN [9]
H2S [10]
CO2 [11]
NH3 [12]

------------------------------------------------

Charge balance [13-14]
Mass balance for each around the drum[15-19]


------------------------------------------------

So am I completely overthinking this or is there something I'm missing? The unknowns and equations line out perfect but I'm having trouble finding convergence. Is it supposed to be as simple as hunting for the right answer until you get a convergence or is there a way to get an exact solution for any set of conditions?

Edited by HailToPitt, 05 October 2014 - 12:31 PM.


#4 HailToPitt

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Posted 05 October 2014 - 12:16 PM

All joking aside, finishing this portion of the model is supposed to be my excuse to get a good simulator. Need to be able to do the math first though, else I won't understand and everything will be just numbers.



#5 Pilesar

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Posted 07 October 2014 - 12:49 PM

   What answer could a simulation give that would affect the way the column operates? If you have an operating stripper, you probably already know the highest reflux temperature you can consistently run. The most efficient stripper operation will be when the reflux drum temperature is just at or below the condensing temperature of the water. You should have a temperature safety margin large enough to account for partial pressure effects as well as process upsets. A process model is not necessary as you already know the process optimum. If your pressure changes significantly, you can use a simple equation to calculate how the steam dewpoint temperature changes with operating pressure. If your feed composition changes affect the steam dewpoint significantly, then I think you will just have to include that in your safety margin and live with it. Before I spend my company's time to build a simulation, i try to remember to ask myself "How can these simulation results add value to the company? What will I do with the answer?" Simulation can sometimes be a real benefit, but it can often be an unnecessary drain on company resources.

   You write that you have some experimental data for benchmarks, but want to simulate the stream for more exact answers. If the simulation does not match your sample, which will you believe? If you already know you would believe the lab analysis over the simulation model, then why simulate?






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