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

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?

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.

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

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

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)

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

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?

Other Questions?

Other Slides

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

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)

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

Various Reactors, Various Reactions

Optimum Inlet Temperature Exothermic Rxn CSTR PFR