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Modeling and optimization of thermally coupled reactors of naphtha reforming and propane ammoxidation with different feed distributions

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Abstract

In this paper, the thermal coupling of naphtha reforming with propane ammoxidation was simulated using a one-dimensional homogenous model for two processes. By this technique, the required heat for the endothermic naphtha reforming process is provided by the exothermic propane ammoxidation which caused the related furnaces to be removed. The propane ammoxidation takes place in the tube side while the naphtha reforming occurs in the shell side of thermally coupled reactors. Depending on the feed distribution, four configurations including (1) series naphtha-series ammoxidation, (2) series naphtha-parallel ammoxidation, (3) parallel naphtha-parallel ammoxidation, and (4) parallel naphtha-series ammoxidation were investigated to select the best configuration which yields the highest efficiency. The modeling results showed that the first configuration is the best configuration in which produces the aromatics 4 kmol h−1 greater than the conventional naphtha reforming (CNR). Hence the optimization of the first configuration with the genetic algorithm method was done to obtain the optimal value of its key variables. The molar flow rate of aromatics was achieved to 141.9 kmol h−1 at optimized input temperature of naphtha reforming (776.94 K), input temperature of propane ammoxidation (779.9 K) and the number of tubes (395).

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Abbreviations

a :

Catalyst activity

Ac :

Cross-section area (m2)

AP :

Heat transfer area (m2)

B :

3 in r13, 1 in other rates of propane ammoxidation

b i :

Stoichiometric coefficient

C :

Concentration (mol m−3)

C j0 :

Inlet concentration of component J (mol m−3)

C p :

Specific heat capacity at constant pressure (kJ kmol−1 K−1)

CT :

Total concentration (mol m−3)

C v :

Specific heat capacity at constant volume (kJ kmol−1 K−1)

d p :

Particle diameter (m)

D:

Diameter (m)

Dis:

Distribution of mass catalyst (wt %)

E i :

Activation energy for ith reaction (kJ kmol−1)

Fi :

Molar flow rate of component (kmol h−1) for naphtha reforming and mol h−1 for propane ammoxidation

k :

Thermal conductivity (W m−1 K−1)

k w :

Thermal conductivity of wall (W m−1 K−1)

kc :

Kinetic constant in propane ammoxidation (mol g−1 h−1)

Kp :

Adsorption constant for propylene in propane ammoxidation (m3 mol−1)

Ko :

Adsorption constant for oxygen in propane ammoxidation (m3 mol−1)

KN :

Adsorption constant for ammonia in propane ammoxidation (m3 mol−1)

k eff :

Effective thermal conductivity (W m−1 K−1)

\(k_{{c_{i} }}\) :

Mass transfer coefficient for component i, m h−1

\(k_{{f_{1} }}\) :

Rate constant for reaction (1) (kmol h−1 kgcat−1 MPa−1)

\(k_{{f_{2} }}\) :

Rate constant for reaction (2) (kmol h−1 kgcat−1 MPa−2)

\(k_{{f_{3} }}\) :

Rate constant for reactions (3) (kmol h−1 kgcat−1)

\(k_{{f_{4} }}\) :

Rate constant for reactions (4) (kmol h−1 kgcat−1)

\(K_{{e_{1} }}\) :

Equilibrium constant (MPa3)

L :

Reactor length (m)

M :

3 for propylene, 2 for ACN, 10 for AcCN, 5 for HCN, 15 for C2 in rates of propane ammoxidation

MWi :

Molecular weight of component i (kg kmol−1)

n:

Number of tubes

nf:

Normalization factor

p i :

Partial pressure of component i (kPa)

p t :

Total pressure (kPa)

R :

Gas constant (kJ kmol−1 K−1)

r i :

Rate of reaction for ith reaction (kmol kg−1 h−1)

T:

Temperature (K)

u 2 :

Feed velocity (ms−1)

U:

Overall heat transfer coefficient between two sides of the reactor (W m−2 K−1)

z :

Axial coordinate (m)

ε :

Void fraction of catalyst bed

μ :

Viscosity of gas (kg m−1 s−1)

ρ :

Density of gas (kg m−3)

H :

Heat of reaction (kJ mol−1)

Ar:

Aromatic

endo:

Endothermic side

exo:

Exothermic side

Hy:

Hydrogen

I:

Propylene in r7–r10, ACN in r11, AcCN in r12, HCN in r13, C2 in r14

i:

Numerator for reaction

j:

Numerator for component

Li:

Light ends

N:

Ammonia

Na:

Naphthene

O:

Oxygen

opt:

Optimized

P:

Propane

Pa:

Paraffin

ss:

Steady state

t:

Total

ACN:

Acrylonitrile

AcCN:

Acetonitrile

CNR:

Conventional naphtha reforming

Config. 1:

Co-current flow, both naphtha reforming and propane ammoxidation in series

Config. 2:

Co-current flow, naphtha reforming in series and propane ammoxidation in parallel

Config. 3:

Co-current flow, both naphtha reforming and propane ammoxidation in parallel

Config. 4:

Co-current flow, naphtha reforming in parallel and propane ammoxidation in series

HCN:

Hydrogen cyanide

OF:

Objective function

Pt:

Platinum

TCR:

Thermally coupled reactor

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Acknowledgements

The authors would like to thank Iran National Science Foundation (INSF) for supporting the research (Grant number: 98012467).

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Correspondence to Davood Iranshahi.

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Ebrahimian, S., Iranshahi, D. Modeling and optimization of thermally coupled reactors of naphtha reforming and propane ammoxidation with different feed distributions. Reac Kinet Mech Cat 129, 315–335 (2020). https://doi.org/10.1007/s11144-019-01682-x

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Keywords

  • Naphtha reforming
  • Hydrogen and aromatic boosting
  • Mathematical modeling
  • Propane ammoxidation
  • Co-current flow
  • Thermally coupled reactors