GAV’s obtained by regression to a vast

set of calculated data (400+ compounds)

and validated by comparison with

experimental and calculated data.

*Laboratory for Chemical Technology

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OUT

M2dcR2 Advisory board meeting, Gent, June 19th 2012.

Modelling complex free radical processes involving hydrocarbons and

organosulfur compounds Aäron G. Vandeputte*

http://www.lct.UGent.be E-mail: aaron.vandeputte@UGent.be

Krijgslaan 281 (S5), 9000 Ghent, Belgium

European Research Institute of Catalysis

Free radicals

RMG

tolerance ε

initial conditions

thermo/rate rules reaction mechanism

structures

IN

A

B

C

D

E

F

A

B

C

D

E

F

Rate based algorithm: products are added to the core mechanism

if their flux is sufficiently high, i.e. Rprod > ε Rchar

Partition functions bridge the gap between the microscopic

world and macroscopic, measurable parameters.

microscopic world macroscopic world

thermodynamic properties:

enthalpy, entropy, heat capacity…

Benson group additivity

Group additive modelling of Arrhenius parameters

Reaction mechanism generation

Computational chemistry

Properties of compounds can be obtained by summing the contributions of the

constituting groups: f = Σi GAVf(groupi) with f = H , Sint and cp

= S–(C)(H) + C–(S)(C)(H)2 + 2 C–(C)2(H)2

+ C–(C)(H)3

Activation energies and pre-exponential factors can be obtained by adding

contributions of the transition state group to the reference reaction:

f = f ref +Σi ∆GAV f (TS groupi) with f = Ea, logÃ

Models have been constructed for additions/β-scissions, hydrogen abstractions,

homolytic substitutions and scission/recombination reactions.

Applications

Steam cracking of ethane and butane Pyrolysis of organosulfur compounds

Conclusions

Free radicals are species that contain one or more unpaired electrons and

because of this tend to be very reactive.

Radical intermediates are involved in many chemical processes (e.g. steam

cracking, radical polymerization, combustion…)

a

e

( )

a( ) ( , ( ))

E T

RTÃk T T n eTE

depth of network

nre

acti

on

s The high reactivity of radicals translates to

a wide variety of reactions that can occur.

As a result, reactions networks involving

radicals easily contain thousands of

reactions involving more hundreds of

components.

Simulating these processes requires a vast set of thermochemical and

kinetic data. Most of these data are hard and expensive to obtain by

means of experiment. Recent developed computational methods allow to

accurately reproduce the required data at a fraction of the cost and time

required for experimental work.

Experimental and simulated DES (exp:▲; sim: full

line) and ethylene (exp:■; sim:•) concentration during

the pyrolysis of DES (T=1013K, F0= 0.25 mol s-1,

mol% DES = 150 ppm)

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Experimental and simulated CS2 (exp:▲; sim: full line),

ethane (exp:■; sim:•), methane (exp:●; sim: dotted

line) and ethyne (exp:; sim: ••) mole fractions during

the thermal decomposition of DES (T = 1013 K, F0 =

0.25 mol s-1, mol% DES = 150 ppm)

Group additivity methods allow to obtain accurate estimates for

missing thermodynamic or kinetic data.

Using automated reaction network generation packages and the

constructed group additivity models, detailed reaction networks are

obtained that succeed to reproduce the experimental data well.

R2R1

X1

X2

X3

Y1

Y2

Y3

aa 1 1 2 2 1 2a ref 3a 3, GAV ((R )-(X )(X GAV ((R )-(Y )(Y )(Y ))( )(X )( ))) EEET TE

MAD (∆fH )

kJ mol-1

MAD (∆S and cp )

J mol-1 K-1

CxHy < 2.0 5.0

CxHySz < 2.0 3.0

Agreement between GA modelled and experimental/calculated rate coefficients

amounts to a factor 3.

ref 1 1 2 1 22 3lo 3loggGAV ((R )-(X )( Glog AV ((R )-(Y )(Y )(l Y ))( og ( ) X )) (X ))

à Ãà T à T

Mean absolute deviation (MAD) between GA modelled

and experimental/calculated thermochemical data.

0

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Experimental yield [-]

Ethene

Ethane

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Dihydrogen

Methane

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Experimental yield [-]

Propene

n-Butane

Ethene

Methane

1,3-Butadiene

Accurate prediction of

experimental yields

+ +

R

RH RH

R

2CH3

+ H

R

RH

1.1%

29.3%

1.1%

CH3 +

0.7% 0.1%

27.3%

28.0%

67.5%

65.6%

1.1%1.2%

1.8%

0.7%RH

RR

RH

Reaction path analysis during steam cracking of n-butane

(COT = 1073 K, COP = 1.9 105 Pa, δ = 0.55, Fn-hexane =

0.58 g s-1, τ = 0.8 s-1, axial position = 15 m)

SEMK models allow an enhanced

understanding of the chemistry involved

during the process.

Rv,n-butane = 100%