Transcript Slide 1

The Oxidation of Cyclohexane in a Capillary
R. Jevtic, P.A. Ramachandran, M. P. Dudukovic
Motivation
Results
Mass transfer correlation for Taylor flow:
Bercic and Pintar, 1997
OH
O
O
2
+
+
HNO
3
Caprolactam
kGL aGL
& Adipic acid
>120 C
>120 C
~15 bars
~8-15 bars
1.19
0.111uTP

.57
L0SLUG
Van Baten and Krishna, 2004
kGL a  (k L a) cap  k L a film
KA - mixture
KA oil
adipic acid
VR=50 ml
D =2.1 mm
kGL a  0.001s-1 to 0.08 s-1
If smaller (than predicted by correlations from the literature)
mass transfer coefficient is used, agreement between model
and experimental results gets better.
Nylon -6,6
Source: http://www.uni-regensburg.de
Mass transfer might be the reason for the discrepancies
Traditional cyclohexane oxidation (“1st step”) process
operates at:

3-8% cyclohexane conversion

85% selectivity to cyclohexanol and cyclohexanone

adiabatic condition
between the model and the experimental results
Experimental set up: capillary reactor (D=2.1 mm), T= 1600C, P=15 atm, QL=0.1-1.0 ml/min
The interest in cyclohexane oxidation has not diminished
in years:
Concentrations of the products obtained
experimentally are an order of magnitude lower that
those obtained by PFR model (conversion at 20 min
(model)=36% conversion (exp) = 4%).
2003-2006: 61 papers;
32 in Chinese and 21 in English
Mass transfer correlation for Taylor flow in a capillary used
VG dpi
pi
uG
 k L a (
 ci )VL
RT dz
Hi
Kinetics from Kharakova et al, 1989
* Source: SciFinder
NR
dci
pi
uL
 kLa (
 ci )   ki rk
dz
Hi
k 1
Two possible modifications to improve the current process:
1. Selectively oxidize cyclohexane directly to adipic acid in
one step, or
2. Increase volumetric productivity in the first step without
sacrificing selectivity toward cyclohexanol and
cyclohexanone
A small improvement in the product yield can lead to
significant impact on the process economics.
Figure 1. Comparison of experimental and modeling results for cyclohexanol (ROH) and cyclohexanone
(RO) concentrations in cyclohexane oxidation in the capillary at 160ºC and 15 atm without the use of a
catalyst.
Taylor flow in a capillary:
• 3 different mixers used.
• Similar results observed-Taylor flow erratic and almost independent of the gas and the liquid flow rates
used
1
3
Summary
 Design, set up and the experimental study in the capillary
reactor is completed.
 There is discrepancy between model and experimental
results, which is, most likely, due to poor mass transfer in the
capillary
 Better mixing of gas and liquid is needed.
References
Goals
1. Schaefer, R.; Merten, C.; Eigenberger, G., Autocatalytic Cyclohexane
Oxidation in a Bubble Column. The Canadian Journal of Chemical
Engineering 2003, 81, (741-748).
Improved understanding and quantification of
the effect of
oxygen
availability
2
Figure 2. Experimental and modeling results for cyclohexanol
(ROH) and cyclohexanone (RO) concentrations obtained in the
capillary reactor at 160ºC and 15 atm. Mass transfer coefficient
used in the model was an order magnitude lower then the one
predicted from the correlations available in the literature.
the effect of
the reactor
type
on rates and selectivity in cyclohexane oxidation
Gas flow rate: 1.2 ml/min
Liquid flow rate: 3.6 ml/min
(nylon tubing, 1/8’’ OD)
2. Bercic, G.; Pintar, A., The role of gas bubbles and liquid slug lengths on
mass transport in the Taylor flow through capillaries. Che. Eng. Sci. 1997,
52, (21/22), 3709-3719.
3. Kreutzer, M. T.; Du, P.; Heiszwolf, J. J.; Kapteijn, F.; Moulijn, J. A., Mass
transfer characteristics of three-phase monolith reactors. Chem. Eng. Sci.
2001, 56, (21-22), 6015-6023.
4. van Baten, J. M.; Krishna, R., CFD simulations of mass transfer from
Taylor bubbles rising in circular capillaries. Chemical Engineering Science
2004, 59, (12), 2535-2545.
Chemical Reaction Engineering Laboratory