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Isothermal Water Ga Sshift-Simulation And Design


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#1 daraj

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Posted 16 November 2021 - 05:26 AM

Hi, need help with isothemral water gas shift which is different from high temo or low temp shift.

 

I wanted to knowwhether it can be modeled as RGIBBS (or RSTOIC?) for the latter you need to know conversion beforrehand.

 

does anybody have any links/info on how typical iso shift reactors work(Linde, Lurgi etcZ) and what is preferred steam/CO ratio at inlet and typical per passconversion of CO? how close is it to equilibrium?

 

Just to add, these are used in hydrogen productrion plants for methanol and ammonia.

 

I am designing a reactor starting from simulation for one such hydrogen production plant of worldscale capacity( 70 KTPY).

 

The isothermal temperature I have seen in literature is around 250-260C for these reactors.

 

Please let me know the simulation approach fpor design. the inlet feed is coming from a primary reformer and has steam in excess, almost 4:1, steam : CO ratio on molar basis.

 

Pressure is around 22 bar



#2 PingPong

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Posted 19 November 2021 - 08:38 AM

An isothermal WGS reactor is merely a reactor with cooling tubes in it to remove the exothermal heat of reaction by generating steam (or whatever).

 

You can use them for high or medium or low temperature shift, depending on the type of catalyst you put into it.

In that respect it is not different from a conventional adiabatic reactor.

The molecules don't care about what kind of reactor it is, all they notice is temperature, pressure and catalyst.



#3 Pilesar

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Posted 20 November 2021 - 04:48 AM

The shift and methanation reactions have well-defined stoichiometry and the conversion is essentially at equilibrium for the reaction temperature. Because just a few of the components take part in the reaction, i would choose REquil instead of RGibbs. It is possible that the results would be the same, but RGibbs may include reactions that you would prefer to be excluded. With REquil, you have better control over the resulting composition.



#4 daraj

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Posted 20 November 2021 - 05:30 AM

Thanks Ping Pong/Piledsar.

 

Pilesar, industrially, do reactors approach equilibrium conditions? or should i use aproach to tempersature of say +10C to be closer to reality?

 

also in water gasshift reactors, do methanol formation happen? these reactors use copper based catalyst



#5 daraj

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Posted 20 November 2021 - 05:33 AM

reactors are generally kinetically controlled in industrial operations, thats why i was asking, almost close to equilibrium but not exactlky at equilibrium, right?



#6 Pilesar

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Posted 20 November 2021 - 09:33 AM

It's been three decades since I designed ammonia plants and details in my mind have gotten overwritten by later data input. For a student project, I doubt you need to put much approach, if any, to the shift reactions. If you want to put an approach in the model to let the professor know you understand the concept, I would suggest making it a very small one (maybe -5 C?). Note that the approach would be negative. Your literature search should find some documentation of how others model this. I am not sure what you mean by 'kinetically controlled.' Industrial operations want these conversions to equilibrium as much as possible. I tracked these reactors when working in an ammonia plant to monitor catalyst activity and I calculated the approach to equilibrium for all the reactions as part of my responsibilities.  I remember my primary and secondary reformers had approaches in the general vicinity of -40 F. The shift and methanation reactor approaches were much smaller than that. The calcs never showed exactly zero approach and I remember on occasion I would even see a slight positive approach which just indicated to me the uncertainty of relying on field data.


Edited by Pilesar, 22 November 2021 - 12:55 AM.


#7 PingPong

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Posted 20 November 2021 - 02:40 PM

If you want to put an approach in the model to let the professor know you understand the concept, I would suggest making it a very small one (maybe -5 C?). Note that the approach would be negative.

That is not correct.

 

In any case for WGS the ATE (approach to equilibrium) has to be added as a positive number to the actual reactor outlet temperature,

but moreover the ATE is always to be specified as a positive number because it is applied by the process simulator as follows:

 

For exothermic reactions (like WGS) the equilibrium constant Keq is calculated at T = Treaction + ATE


For endothermic reactions the equilibrium constant Keq is calculated at T = Treaction - ATE

 

For WGS an ATE of 5 to 10 oC is customary for design calculations.
 



#8 daraj

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Posted 21 November 2021 - 05:25 PM

Pingpong, if i model SMR as RGIBBS say, reformer has both endothermic(reforming) and exothermic(shift) reactions happening at the same time. so how do you decide approach to temperature here? since the overall reformer is endothermic, do you just say mention it as -10C? as ATE for SMR?



#9 PingPong

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Posted 22 November 2021 - 08:50 AM

I don't know what simulator you are using, but in any case I advice you to study your reference manual and keyword manual.

Could be that there is special option for methanation (reverse of reforming) and shift reactors in it.

Verify also how your specific simulator applies the user entered ATE.

 

In PRO/II (which I use) there is option to select an equilibrium reactor that simulates a methanator. That covers both the shift and methanation (reverse of reforming) equilibria. Because shift and methanation are both exothermic a user specified ATE is applied to both in the same way. ATE to be a positive number.

 

How reforming/methanation can be modeled in your simulator I can't tell, check your manuals.


Edited by PingPong, 22 November 2021 - 09:12 AM.


#10 daraj

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Posted 22 November 2021 - 09:52 AM

Pingpong, I use aspen plus which uses RGIBBS model block that uses Gibbs minimization of free energy. you dont need to mention reactions explicitly. This is generally good in situations where yopu dont know all the reactions that are going on inside the reactor, but know that the reactor is operating close to equilibrium.



#11 PingPong

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Posted 23 November 2021 - 08:18 AM

I know what a Gibbs reactor model is as PRO/II also has it, but I never felt the need to use it. I always use an equilibrium reactor and apply suitable ATE's.

 

What I wrote earlier about how a simulator like PRO/II applies user specified ATE in an equilibrium reactor will however not apply to a Gibbs reactor.

 

Not sure how it works in AspenPlus but I suspect that you can only specify one approach (ATE) which is then added to the temperature that is used for the Gibbs free energy minimization calculation. That's not ideal for simulating a steam reformer.

 

It should however be possible to specify the relevant reactions in RGibbs and apply an ATE to those, but you would need to study the AspenPlus reference guide how to apply that.

 

An old AspenPlus 10 reference guide that I found on the internet states:

"For temperature approach specifications, RGibbs evaluates the chemical
equilibrium constant at T + ΔT, where T is the actual reactor temperature
(specified or calculated) and ΔT is the desired temperature approach.


You can have RGibbs consider only a specific set of reactions. You can restrict the
chemical equilibrium by specifying temperature approach or molar extent for the
reactions. You must specify the stoichiometric coefficients for a complete set of
linearly independent chemical reactions, even if only one reaction is restricted."

 

The latest AspenPlus reference guide will be more accurate and useful.


Edited by PingPong, 23 November 2021 - 08:26 AM.





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