8 replies to this topic

[ShaneL]

hi, 

 

I have been assigned to design a water gas shift reactor. I first have to do mass balances but am unsure where to start. I know that I have to find the equilibrium constant and extent of reaction, but there are 2 ways to calculate the equilibrium constants, thermodynamically and kinetically from literature. I also know the composition at the inlet.

 

Can you please advise which method to use for mass balances as I am stuck and have not made any progress.

 

 

Shane

 

 

[PingPong]

James,

 

I don't think anybody understands what you have posted.

 

but there are 2 ways to calculate the equilibrium constants, thermodynamically and kinetically from literature.

No, there is only one way to calculate the equilibrium constant: thermodynamically. Kp is a function of temperature.

 

In reality however the equilibrium is never reached as that would require an infinitely large reactor. So the reactor is designed for an approach to equilibrium of, say, 10 ^oC, at the reactor outlet. On that basis  the required catalyst volume can then be calculated using kinetics.

[ShaneL]

ping pong

 

thanks for your reply mate. I've done mass balances over the high temperature shift reactor thermodynamically now. But my equilibrium constant is really small(almost no products formed), can you please advise how I can increase the equilibrium constant?

 

Shane

[PingPong]

I have no idea what you have done, or how.

 

Are you doing the mass balance by hand or using a simulator?

 

Note that, as the name says, the equilibrium constant K_p is a constant, dependent on temperature. You have to get values for K_p at different temperatures from literature, if you are doing the calculations by hand.

[EdoRm]

Remember that the WGS reaction is exothermic. If you're working at high temperatures your conversion will be low, no matter how big the reactor is.

 

At this link you can find some rate expressions for the WGS reaction, which consider the thermodynamics into their expressions.

 

http://www.sciencedi...378775303006141

Edited by EdoRm, 07 March 2014 - 02:39 PM.

[ShaneL]

I did my Mass balances working forward since I knew the inlet compositions and temps, I found Keq(by using thermodynamic eqns from literature) first and then extent of reactions. By using K=[ξ^2/(1-ξ)^2]. 

 

However, I am unsure cause I read from literature that it has to be 2:1 for H20:CO. So I would like to ask is the 2:1 ratio for H2:CO or H20:C0, also is that for feed? Cause if its 2:1 does the equation change to K=[ξ^2/(1-ξ)(2-ξ)], and does pressure affect it now? Since initially pressure had no effect as it is equimolar feed and volume didnt change.

 

Thanks a lot 

 

Shane

[PingPong]

The reaction will occur in any ratio of H₂O/CO however the catalyst will rapidly deactivate if there is the H₂O/CO ratio is too low at any point in the catalyst.

 

Therefor I suspect that what you read was about the H₂O/CO ratio recommended by the catalyst vendor.

In my experience with shift catalyst vendors however a much higher H₂O/CO ratio than 2 is required.

[ShaneL]

hi guys

 

I have found Keq at 350C to be 20 and have selected a empirical rate expression to model the kinetics of the high temp shift, I then equated the rate value I found with the Plug flow reactor design equation to find the weight of catalyst.

 

I also found the space velocity, by dividing the weight of catalyst over the molar flow rate of CO.

 

Can you please advise what I should do next for the reactor design, like how do I find the volume of reactor and height etc using the space velocity?

 

Many thanks

 

Shane

[asedhai1]

Hey Shane any progress on designing the WGSR? I am working on same stuff. I was able to get the amount of catalyst but the volume i got is small. 

Thanks