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.