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• υi,j stoichiometric coefficient

for ith species in jth reaction• Aj chemical formula of jth

species• R chemical reactions

C

jjji RiA

1, ,...,2,1,0

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– ξi= (ni,j-in – ni,j-out)/ νi,j

)(1

1

11

1

12

)(

2

2

2

,2

3

2

,1

22

32

sC

OH

COH

v

OHCH

COH

v

sCOHHCO

OHCHHCO

j

j

Cjij ,...,2,1, n nR

1iiin-jout-j

CjXk

jk ,...,2,1,n- n n in-kin-jout-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

– ai activity of component I

• Gas Phase, ai = φiyiP, – φi== fugacity coefficient of i

• Liquid Phase, ai= γi xi exp[Vi (P-Pis) /RT]

– γi = activity coefficient of i – Vi =Partial Molar Volume of i

2

ln,exp

RT

H

dT

Kd

RT

G

aa

aaK

orxneq

orxn

aB

aA

sS

rR

eq

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

i

C

iiTot GnG

1

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 O2 CO

– C + O2 CO2

• 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.