The Oxidation of Cyclohexane in a Stirred Tank

1
The Oxidation of Cyclohexane in a Stirred Tank

• R. Jevtic, P.A. Ramachandran, M. P. Dudukovic

Results
ODRPACK, a collection of Fortran subroutines for
fitting a model to data, is used to determine
kinetic constants
Motivation
Reaction condition 50 O2, T130ºC, P15 atm
Yield (Y) Product formed relative to the amount
of cyclohexane at the beginning of the
reaction Selectivity (S) Amount of desired
products formed relative to the reacted
cyclohexane

• Green et al. (US 5,780,683 patent 1998) performed
  the oxidation of cyclohexane with pure oxygen in
  a Liquid-phase Oxidation Reactor (LOR).

Assumptions constant temperature both gas and
liquid phase fully back-mixed ideal gas law valid
Obtain kinetic constants
Conventional technology Invention
Oxygen 21 100
Temperature, 0C 160 149
Residence time, min 36 8
Conversion, cyclohexane 4 4
Selectivity
Cyclohexanol 42 44
Cyclohexanone, 20 34
Total 78 84
Productivity, gmol/hrL 0.45 1.85
G
Henrys constants for component i
VG
k0 (m3mol-1s-1) k1 (s-1) k2 (s-1) k3 (m3mol-1s-1) k4 (m3mol-1s-1)
6.19e-6 1.01e-3 5.58e-4 8.60e-5 3.15e-4
2.66e-5 4.8e-3 1.3e-3 5.0e-4 3.7e-4
L
130ºC
VL
160ºC
Volumetric mass transfer coefficient
oxygen in the gas phase Time (min) to achieve 4 cyclohexane conversion Selectivity () for cyclohexanol, cyclohexanone, and cyclohexyl-hydroperoxide
20 34.4 89.4
50 14.2 92.6
75 9.8 93.9
100 7.4 94.9

• The model equations for the gas and the liquid
  phase solved simultaneously with all the
  parameters
• A stiff ODE solver from Netlib library (LSODE)
  used

• Conclusion from Green et al Reaction
  temperature and residence time are reduced while
  selectivity and productivity are increased.
• LOR results using pure oxygen were compared to
  those of the existing process, which uses air.
• No conclusive evidence for the cause of
  improvement might be due to higher mass transfer
  rate in the LOR reactor.
• Complete set of temporal data on cyclohexane
  conversion and selectivity is necessary to
  systematically investigate the effect of
  increased oxygen availability

21 O2, 160ºC, P15 atm
Kinetic models
Spielman(1964), Alagy et al. (1964)
For fixed conversion of cyclohexane, higher
oxygen content in gas phase yields lower
residence time and higher selectivity.
Summary
Kharkova et al. (1989)
Figure 1. Concentration of products and reactant
of cyclohexane oxidation using kinetic model from
Alagy et al(1964)

• Design, set up and experimental study in stirred
  autoclave operated in the batch mode is
  completed.
• Increased oxygen content in the gas phase yielded
  higher concentration of desired products but also
  lower selectivity.
• To draw any definite conclusion on the effect of
  oxygen availability, oxygen partial pressure
  should be kept constant.
• Design, set up and preliminary experimental
  study in stirred autoclave operated in the
  semibatch mode is completed.
• Again, increased oxygen percentage in the gas
  phase will result in higher yields of
  cyclohexanol and cyclohexanone but lower
  selectivity.
• However, for fixed conversion of cyclohexane,
  higher oxygen content in gas phase requires lower
  residence time and leads to higher productivity.

Experimental Setup
Figure 2. Concentration of products and reactant
of cyclohexane oxidation using kinetic model from
Kharakova et al (1989)
Reaction condition 50 O2, T160ºC, P15 atm
Experimental setup Parr Mini autoclave
(operated in a batch and a semibatch mode),
T1300C, P15 bar and 30 bar, mol. fraction of O2
in gas phase0.2 and 0.5, VG/VL2.33, total
liquid volume7.5ml, 900 RPM

• References
• Greene, M. I. Sumner, C. Gartside, R. J.
  Cyclohexane oxidation. 5,780,683, 1998.
• Jevtic et al., AIChE Annual Meeting, Cincinnati,
  OH, November, 2005
• Jevtic et al., AIChE Annual Meeting, San
  Francisco, CA, November, 2006
• Jevtic et al. NASCRE-2, Houston, TX, February,
  2007

Figure 3. Experimental and modeling results for
yields of cyclohexanol (ROH) and cyclohexanone
(RO) in time for catalytic reaction
Figure 4. Experimental results for yields of
cyclohexanol (ROH) and cyclohexanone (RO) in time
for non-catalytic reaction
Chemical Reaction Engineering Laboratory