Aspen for Process Creation Chapter 4&5 Terry A. Ring

Onion Model of Process Design

Process Synthesis-1 Reaction 3 –Direct Chlorination –Pyrolysis

Reactor Types Ideal –PFR –CSTR Real –Unique design geometries and therefore RTD –Multiphase –Various regimes of momentum, mass and heat transfer

Reactors in Process Simulators Stoichiometric Model –Specify reactant conversion and extents of reaction for one or more reactions Two Models for multiple phases in chemical equilibrium Kinetic model for a CSTR Kinetic model for a PFR Custom-made models (UDF) Used in early stages of design

Stoichiometric Reactor C chemical Species υ i,j stoichiometric coefficient for i th species in j th reaction A j chemical formula of j th species R chemical reactions

Stoichiometric Reactor Example Reactions –1 Methane Synthesis –2 Coking Conversion, X k, of key component, k –X k =(n k-in – n k-out )/ n k-in Extent of Reaction –ξ i = (n i,j-in – n i,j-out )/ ν i,j

Reactions with low conversions? Due to slow kinetics Due to non-favorable Equilibrium –Solution Set up reactor Followed by Separator Recycle reactant to extinction

Equilibrium Reactor-1 Single Equilibrium aA +bB  rR + sS –a i activity of component I Gas Phase, a i = φ i y i P, –φ i= = fugacity coefficient of i Liquid Phase, a i = γ i x i exp[V i (P-P i s ) /RT] –γ i = activity coefficient of i –V i =Partial Molar Volume of i Van’t Hoff eq.

Equilibrium Reactor-2 Total Gibbs Free Energy is minimized at T&P –Specify components that are entering system and T&P of System –Specify possible reaction products Note, no reactions are defined! Gives outlet composition at equilibrium

Equilibrium Reactors To determine the potential products given some reactants –Use Gibbs Reactor To determine equilibrium for a given reaction(s) –Use Equilibrium Reactor To determine what operating conditions for a given reactor –Use Equilibrium Reactor Vary T and P to get highest conversion To determine heat of reaction –Use Stoichiometric Reactor Conversion at 100%

Examples Which reaction takes place? –C + ½ O 2  CO –C + O 2  CO 2 What temperature (or pressure) is required for a reaction? –Run equilibrium reactor at different T and P.

Separations What conditions are to be used for a flash after reaction? Run flash with different T and P.