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Degrees Of Freedom Of A Process

process flowsheet dof degree of freedom chemical engineering process engineering process design

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#1 chemical_teo


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Posted 10 October 2018 - 01:10 PM

Hi all, 


I am currently doing an exercise of a process to make styrene from ethylbenzene and benzene. First task is to analyze the degrees of freedom to see if they are saturated. And then saturate the degrees of freedom for both distillate columns. The flowsheet is the following:



Attached File  flowsheet_process.png   108.01KB   0 downloads


The reactions taking place in the gas-phase reactor at a pressure Ptot of 1.3 bar are the following: 

- C8H10 -> C8H8 + H2

- C6H6 + 3 H2 -> C6H12 


We know both reaction conversion (XA and XB). The feed is only ethylbenzene and benzene (molar ration Xf known). 


We remove H2 as soon as possible as it is dangerous. Both distillations columns contain a partial reboiler and total condenser. Styrene is removed by the first column at low pressure and low temperature. The second distillation column  is working at atmospheric pressure. 



What I've done for the degrees of freedom (in order of the flowsheet): 


- Mixer : d = 4 . Saturate with both inlet streams (uand u5.2) as well as PMix and TMix

- RXN : d = 3 . Saturate with inlet stream (u) as well as PRxn and TRxn

- Flash : d = 3 . Saturate with inlet stream (u as well as TFlash and the recovery of one key component (eg: we suppose that this is Hydrogen and recovery of 1% (to be realistic) since we want to take it off).

 - Dist 1 : d = (P+Q) +5 (from my notes...). Saturate with inlet stream (u3.2 ) as well as Pdist1 , receovery of light key component and also the one of the heavy key component. The two remaining we can fix reflux ratio to infinite at the top and the bottom if we assume Fenske conditions. (Q denotes thermal behaviour that we fix at 0 except from reboiler and condenser because of adiabatic conditions). 

 - Dist 2 : Same as Dist 1 except inlet stream and pressure are different. 



I was first wondering if all of this was correct. Moreover, I will need to guess some parameters before doing my shortcut model I suppose. How could I guess the temperature of the Flash if I don't know the temperature of the reaction? I suppose it will be around the cooling water temperature (so maybe 37°C ??). I guess the choice needs to be done according to hydrogen to avoid any explosion. 


Thanks in advance for your help. 


Edited by chemical_teo, 10 October 2018 - 01:11 PM.

#2 chemical_teo


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Posted 17 October 2018 - 03:57 PM



It's me again. No answer, I guess the question was too vague ? I actually managed to do some work but I actually have some troubles when solving the (linear) system of equations in an iterative way... I've as a condition abs(T(old, 5.1) - T(new, 5.1)) < 0.1 K but I don't really know how to use this information. Should I create another post for this? (the flowsheet is the same). I get no corrections by the instructor for these exercises, that's why I try to do these by myself but I never know if it's correct.

#3 Nikolai T

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Posted 21 October 2018 - 09:15 PM



Please, try to explain what is a goal your work (what are you trying to do?). I don't understand what you mean when you say: "First task is to analyze the degrees of freedom to see if they are saturated".  By now I observed that you showed reaction hydrogenation of benzene. Styrene is produced by dehydrogenation of ethylbenzene as you showed. Is hydrogen for hydrogenation of benzene trace in the feed (ethylbenzene)? Also your scheme doesn't have a column for distillation styrene from tar.




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