Abstract
Thermal crackers are mostly modeled as plug flow systems, disregarding the lateral gradients present. In this paper, a 2-dimensional model has been established for ethane cracking in a thermal cracker in laminar flow, using a molecular mechanistic model for ethane cracking. The model, consisting of 9-coupled partial differential equations, is solved using the backward implicit numerical scheme. The resulting product distribution and temperature profiles are predicted throughout the reactor. The concentrations of acetylene and propylene show a maximum within the reactor. The effect of certain operational parameters — tube radius, wall temperature and mass flow rate — is also studied on these profiles. The parameters are varied in the range of 0.005–0.0125 m for tube radius, 1.25 kg/hr-2.5 kg/hr for mass flow rate and 850–1,050 ‡C for tube wall temperature. It is observed that an increase in wall temperature and an increase in tube radius or decrease in flow rate favours the conversion of ethane.
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Abbreviations
- A:
-
ethane
- B:
-
ethylene
- C:
-
propane
- CI,J :
-
conc of species i (discretized form) [gmol/m3]
- Ci :
-
concof species i [gmol/m3]
- Ci0 :
-
conc of species i at entrance of reactor [gmol/m3]
- CAVG :
-
average concentration [gmol/m3]
- Cp :
-
specific heat [kJ/kg‡C]
- D:
-
propylene
- Dm, i :
-
diffusivity of species i [m2/s]
- E:
-
acetylene
- Ei :
-
activation energy [kcal/mol]
- F:
-
butadiene
- G:
-
methane
- H:
-
hydrogen
- h:
-
step length in axial direction
- δHr,i :
-
heat of reaction i [kJ/gmol]
- k:
-
step length in radial direction
- kth :
-
thermal conductivity [W/m ‡C]
- ki :
-
reaction rate constant of reaction i [conc1-n/s]
- ki0 :
-
frequency factor [conc1-n/s]
- L:
-
length of reactor [m]
- r:
-
radius (local), radial coordinate [m]
- R:
-
pipe radius [m]
- q:
-
heat flux [J/m2]
- T:
-
process temperature [‡C]
- < T>:
-
average process temperature [‡C]
- T0 :
-
inlet temperature [‡C]
- Tw :
-
wall temperature [‡C]
- um :
-
max. velocity [m/s]
- vm :
-
stoichiometric coefficient of species ‘m’ in reaction ‘i’
- vz :
-
local velocity [m/s]
- va :
-
average velocity [m/s]
- V:
-
mass flow rate [kg/hr]
- X:
-
conversion
- z:
-
reactor axial dimension [m]
- ρ :
-
density [kg/m3]
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Garg, R.K., Krishnan, V.V. & Srivastava, V.K. Prediction of concentration and temperature profiles for non-isothermal ethane cracking in a pipe reactor. Korean J. Chem. Eng. 23, 531–539 (2006). https://doi.org/10.1007/BF02706790
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DOI: https://doi.org/10.1007/BF02706790