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Effect Of Recycle On Reaching Equilibrium

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#1 ChemJack2


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Posted 09 November 2022 - 05:02 AM



I have a theoretical question regarding the effect of recycling in a reactive system.
Consider that we have a series of reactions, which are very fast and therefore are considered to be at thermodynamic equilibrium in the typical residence times of the system.
Then consider that among these reactions, there is a much slower one, which is therefore kinetically controlled. Such a reaction is undesirable.
My question is, can reducing the residence time of the fluid at the higher temperature point of the system help reduce the formation of the undesirable species?
Or, since the system includes acomplete recycling of the fluid, without purging, will I after a certain time always bring the system to a thermodynamic equilibrium condition for all reactions, including the slow one, regardless of the reduction in residence time that I impose per every pass in the plant?
Thank you,

#2 Pilesar


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Posted 09 November 2022 - 06:21 AM

Reducing residence time in reactor will affect product composition where there are multiple reactions. This is commonly seen in steam cracking processes where ethylene yield depends partly on reactor residence time. The 'full recycle, no purge, reaction to completion' premise of your question is confusing. Complete recycling of all fluid would be equivalent to a batch process with infinite residence time. I suggest you develop a material balance using real numbers to get your answer. You should be able to calculate this using a spreadsheet tool if you do not have access to a process simulator.

#3 SilverShaded


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Posted 14 November 2022 - 07:31 AM

It would depend on the rates of reaction of all the reactions.  Increasing temperature could easily speed up the rate of reaction of the unwatned product faster than the wanted products (quite likely if activation energy is higher), also changing temeperature will likely affect the equilibrium position of the other reactions.  In some cases larger reactor volumes can be used and run at lower temepartures as this is favourable to the equilibrium position. 

The only way to know is consider the rates of reaction of all the reactions and model it. (in excel if neccesary).

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