1. Terry A. Ring Chemical Engineering University of Utah
AIChE Project Week 3
Terry A. Ring
Chemical Engineering
University of Utah

2. Questions for the Week
What is MT as in “An assumption around the cost of Methanol ($420/MT) can be used for this analysis.”
Ask Google
MT=Metric Ton =1,000 kg
You’re a getting a deal!
Long term contract price?

3. Do I need Purges on All Recycle Loops?
Generally yes, but
No, If Reaction is equilibrium reactions
A  B+C , could be forward only or equilibrium
A  D (impurity)
Then back reactions can take place and an impurity will end up as B and C after multiple passes.
No Purge is required in this case
If AD (impurity) forward only
Then you need a Purge because any D in recycle loop will build up.

4. How do I optimize the reactor? (wrt Cost)
Your reactor needs to be “optimized” to get the highest single pass conversion.
This usually sets the length for a given number of tubes, Nt(F)
F is Feed flow when there is no recycle – just reactant flow – single pass
You are controlling the total residence time
This can be done by
Longer Tubes (typical not more than 20 ft)
But you can have reactors in Series to get longer tubes
Larger Tubes (typically not more than 1.5 in due to internal heat transfer with exothermic Rxn)
More Tubes in the Bundle (typically less than 2,000)
But you can have reactors in Parallel to get more tubes
As you add Recycle you increase the number of tubes
Based upon a ratio of new flow (R+F) to reactor to feed flow (F)
Nt(R+F)=Nt(F)*(R+F)/F

5. What to do about 50% turn down.
This is operation at ½ production rate.
Run simulation at ½ production rate.
What Breaks?
Trays Drying up?
Heat Transfer is too slow for reaction?
Heat Transfer Area needs to be larger for the lower flow rates
Heat transfer coefficient decreases with flow rate or Reynolds number
Fix what Breaks!
Do you need two smaller reactors that are in parallel?
One operates at ½ production
2 operate at full production
Bigger Heat Exchangers
Run Distillation with Larger Reflux Ratio

6. What interest rate should I use for borrowed capital?
Problem states
“Minimum Acceptable Rate of Return (discount rate) of 8%“
This = im or imin
In this case you are borrowing from your own company so they set the interest rate which is acceptable to them and the stock market
Federal Tax = 24%
State Tax (Louisiana)

7. Questions How To Deal With Impurities
1) How much impurity will Reactor Generate
Reactors
RGibbs to see if a Species will be formed under reaction Conditions
2) How will impurity effect Separation
Separation Train
Add a small amount of impurity
Determine where the impurity ends up in the separation train

8. Does impurity forces a different separation sequence?
Non Condensable
Purge it out
Flash it out before the Separation Train
“Light Flash”
Flash To Remove Just the Impurity
Can you sell the Secondary Product in an impure state?
Impurity
Condensable
See How Separation Train Handles the Condensable Impurity
Contamination of Desired Product?
Improve Separation
Higher Reflux Ratio
More Trays
Add Addition Separation Units To Specifically Remove Impurity
Contamination of Recycle?
Recycle it and let the back reaction return this impurity to feed for subsequent reaction to products
Separate it out before recycle
Contamination of Waste of Water?
Alternative waste treatment plan
Contamination of Secondary Product?
Separate out Impurity before sale
Can you sell the impurity on its own?

9. Other Questions?

10. Other Slides

11. Questions How do we enter the rate data? Go to Aspen Help!
Type Kinetics
LHHW Kinetics
Kinetic Factor is forward Rate Constant
Backward Rate constant is
Kinetic Factor*K2
K2 is the Reaction Equilibrium Constant

12. Concentration Various Options Equilibrium Constants
Keq=Aeq exp(ΔGeq/RT)
To convert
Ln(Keq) = ln(Aeq) + ΔGeq/RT
In general
ΔGeq= ΔH – TΔS
ΔH = ΔHo + Δ𝐶𝑝 𝑑𝑇
ΔCp=Rg*(Δa+ ΔbT+ ΔcT2+ Δd/T2)

13. Questions How To Deal With Impurities
1) How much impurity will Reactor Generate
Reactors
RGibbs to see if a Species will be formed under reaction Conditions
2) How will impurity effect Separation
Separation Train
Add a small amount of impurity
Determine where the impurity ends up in the separation train

14. Various Reactors, Various Reactions

15. Optimum Inlet Temperature Exothermic Rxn
CSTR
PFR