The Oxidation of Cyclohexane in a Stirred TankR. Jevtic, P.A. Ramachandran, M. P. Dudukovic
Chemical Reaction Engineering Laboratory
Motivation
Experimental Setup
Results
Summary
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
•Green et al. (US 5,780,683 patent 1998) performed the oxidation of cyclohexane with pure oxygen in a Liquid-phase Oxidation Reactor (LOR).
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
Experimental setup:
Parr Mini autoclave (operated in a batch and a semibatch mode), T=1300C, P=15 bar and 30 bar, mol. fraction of O2 in gas phase=0.2 and 0.5, VG/VL=2.33,
total liquid volume=7.5ml, 900 RPM
References1. Greene, M. I.; Sumner, C.; Gartside, R. J. Cyclohexane oxidation. 5,780,683, 1998.2. Jevtic et al., AIChE Annual Meeting, Cincinnati, OH, November, 20053. Jevtic et al., AIChE Annual Meeting, San Francisco, CA, November, 20064. Jevtic et al. NASCRE-2, Houston, TX, February, 2007
Yield (Y): Product formed relative to the amount of cyclohexane at the beginning of the reactionSelectivity (S): Amount of desired products formed relative to the reacted cyclohexane
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.
Assumptions:constant temperature; both gas and liquid phase fully back-mixed; ideal gas law valid
VG
VL
Lii
iL
iG VcH
pak
dt
dp
RT
V)(
NR
kkkii
i
iL
i rcH
pak
dt
dc
1
)(
G:
L:
Volumetric mass transfer coefficient
Henry’s constants for component i
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
RHk0
+O2ROOH
k1
-0.5O2
k2
ROH
RO
k3 +0.5O2
+(n-1)O2
k4
Byproducts
Kinetic models
Spielman(1964), Alagy et al. (1964)
Kharkova et al. (1989) Figure 1. Concentration of products and reactant of cyclohexane oxidation using kinetic model from Alagy et al(1964)
Figure 2. Concentration of products and reactant of cyclohexane oxidation using kinetic model from Kharakova et al (1989)
21% O2, 160ºC, P=15 atm
Reaction condition: 50% O2, T=160ºC, P=15 atm
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
Obtain kinetic constants:
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
130ºC160ºC
% 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
ODRPACK, a collection of Fortran subroutines for fitting a model to data, is used to determine kinetic constants
Reaction condition: 50% O2, T=130ºC, P=15 atm
For fixed conversion of cyclohexane, higher oxygen content in gas phase yields lower residence time and higher selectivity.
