Aspen for Process Creation

1
Aspen for Process Creation

• Chapter 56
• Terry A. Ring

2
Onion Model of Process Design
3
Reactor Types

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

4
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
5
Stoichiometric Reactor

• C chemical Species
• ?i,j stoichiometric coefficient for ith species
  in jth reaction
• Aj chemical formula of jth species
• R chemical reactions

6
Stoichiometric Reactor Example

• Reactions
• 1 Methane Synthesis
• 2 Coking
• Conversion, Xk, of key component, k
• Xk(nk-in nk-out)/ nk-in
• Extent of Reaction
• ?i (ni,j-in ni,j-out)/ ?i,j

1 Rxn
7
Reactions with low conversions?

• Due to slow kinetics
• Due to non-favorable Equilibrium
• Solution
• Set up reactor
• Followed by Separator
• Recycle reactant to extinction

8
Equilibrium Reactor-1

• Single Equilibrium
• aA bB ? rR sS
• ai activity of component I
• Gas Phase, ai fiyiP,
• fi fugacity coefficient of i
• Liquid Phase, ai ?i xi expVi (P-Pis) /RT
• ?i activity coefficient of i
• Vi Partial Molar Volume of i

Vant Hoff eq.
9
Equilibrium Reactor-2

• Total Gibbs Free Energy is minimized at TP
• Specify components that are entering system and
  TP of System
• Specify possible reaction products
• Note, no reactions are defined!
• Gives outlet composition at equilibrium

10
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

11
Example
12
Separations

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