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06-‐321 ChemE Thermodynamics
Chemical Reac9on Equilibrium in Aspen v8
Bruno A. Calfa
Last Update: October 27th, 2014
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Basics of Chemical Reac9on Equilibria (I/II) • Given general equilibrium reac9on
its equilibrium is determined by the equilibrium constant K. • Rela9onship between K and standard Gibbs free energy of the reac9on:
• The equilibrium is defined in terms of the fugacity of the components:
• Using fugacity coefficients (typically in gas-‐phase reac9ons):
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Basics of Chemical Reac9on Equilibria (II/II) • Extent of reac9on (coordinate of reac9on): ε
– Extent to which the reac9on has taken place.
where ν is the stoichiometric coefficient of species i (posi9ve if product, nega9ve if reactant). • See deriva9ons in textbook (sec9on 13.1).
See also extension to mul9ple reac9ons. • Procedure: subs9tute yi into expression for K and solve for ε. • Minimiza9on of Gibbs Free Energy (see sec9on 13.9 for deriva9on): results
in solving the following algebraic equa9ons for ni.
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Numerical Example • Water-‐Gas shi_ reac9on
• Given: stoichiometric feed, P = 8,300 kPa, T = 500 K, so that K = 148.4.
• Calculate equilibrium composi9on. • A solu9on by-‐hand is easily achievable
– Introduce the extent of reac9on ε. – Express number of moles of each component as func9ons of ε. – Use the expression of K to solve for ε. – Calculate the equilibrium composi9ons based on the value for ε.
• Aspen Plus modeling – Use REquil or RGibbs models/blocks.
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Solu9on with Aspen • Add components.
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Solu9on with Aspen • Select thermodynamic model. Use Method Assistant. Let us use SRK. Go to
the subfolder Parameters under Methods to confirm that you will be using Aspen’s built-‐in temperature-‐dependent binary parameters.
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Solu9on with Aspen • Switch to Simula9on environment. Add REquil under Reactors. • You may rename the block (right click -‐> Rename Block).
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Solu9on with Aspen • Click on Material. Red arrows are mandatory. Blue arrows are op9onal.
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Solu9on with Aspen • Draw streams. You can rename streams (right click -‐> Rename Stream).
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Solu9on with Aspen • Configure feed stream. Go to folder Streams, then 1 (feed stream).
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Solu9on with Aspen • Configure reactor block. Go to folder Blocks, then B1. • Specifica9ons (use same as feed for this example) + Vapor-‐Only.
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Solu9on with Aspen • Configure reactor block. Go to folder Blocks, then B1. • Reac9ons. Provide guess for extent of reac9on and ask Aspen to evaluate
equilibrium constant exactly at the given T (i.e., no ΔT).
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Solu9on with Aspen • Run simula9on. Click on blue play bujon. It converged! We are not
performing economic evalua9ons at this point, so click Close.
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Solu9on with Aspen • Check results (Stream Summary).
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Solu9on with Aspen • To see molar frac9ons, go to folder Setup -‐> Report Op9ons -‐> Streams.
Check Mole for Frac9on basis.
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Solu9on with Aspen • Rerun simula9on. View Streams (Custom).
• See file ChemReacEqExampleREquil.apwz. 16
Solu9on with Aspen • Now try with RGibbs block. Add and configure streams and block.
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Solu9on with Aspen • Rerun simula9on. View Streams (Custom). Same solu9on as with REquil.
• See file ChemReacEqExampleRGibbs.apwz. 18
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