(744f) A Dual-Membrane Airlift Reactor for Cyclohexanone Ammoximation Reaction Over TS-1

Cyclohexanone oxime
is one of the most important intermediate products of caprolactam which was
used to produce Nylon-6^1,2. The coupling technology of reaction and ceramic membrane
separation had been employed to realize in situ product removal³.
Previous researches adopted
cyclohexanone, ammonia gas and hydrogen peroxide to be interactants,
tert-butanol as a solvent, to run one-step synthesis of oxime along with TS-1⁴ in the split-membrane reactor. A large amount of tert-butanol
was needed to recycle in the current process, which resulted in an increase of
subsequent separation energy consumption. Without any solvents, it upsets the mass transfer of
reactants to the pore system leading to low activity. In addition, concentration polarization and membrane
pollution could affect stability of membrane separation process in long-term simultaneous
reaction and separation processes.

In order to cope with difficulties
mentioned above, a noval dual-membrane airlift reactor with
a ceramic outer-membrane as a reactant ammonia distributor and another ceramic
inner-membrane as a TS-1 separator was developed for the cyclohexanone
ammoximation reaction over TS-1 without any solvents, as shown in Figure 1. Ammonia gas was dispersed into the reaction
system through a ceramic outer-membrane. Another tubular porous ceramic
membrane was employed as a membrane separator for in situ separation of
catalysts from the products. Aeration was introduced to form two-phase flow on membrane surface, overcoming concentration polarization, increasing
the shear force and local mixing velocity and controlling of membrane fouling caused by long-term consecutive reaction. To investigate the feasibility of the consecutive
reaction in this new proposed dual-membrane reactor, the effect of membrane
distributor, influence of aeration on membrane fouling, and optimal operation
conditions were further studied.

It was interested to
find that the introduction of membrane distribution could increase the selectivity
by 34.6%, because many tiny ammonia bubbles could be produced with a membrane
distributor to strengthen the mix of gas phase and organic phase. Membrane
for separation could effectively prevent TS-1 passing micropores to realize the
coupling reaction and separation. Aeration had little effect on conversion and
selectivity, but had positive on controlling resistant. The conversion,
selectivity, and membrane resistance are all closely related to the operation
conditions. The optimum operating conditions were that residence time, pressure,
temperature and catalyst concentration were 100 min, 0.3 MPa, 343 K and 30-35 g/L
respectively, the molar ratio of cyclohexanone, ammonia and hydrogen peroxide
was 1:1.59:1.2. On these conditions, the convertion of 87% and selectivity of 76%
were obtained in continuous reaction process of 25h. Due to serious membrane
fouling,the
catalyst concentration was reduced to 10 g/L to inspect continuous cultivation stability
of 100h. The reaction stability decreased with time while the membrane resistance
increased, as shown in Figure 2. The experiments gave the further direction of
investigation on the separation process stability.

References

1. Thomas JM, Raja R. Design of a "green" one-step
catalytic production of epsilon-caprolactam (precursor of nylon-6). Proceedings of the National Academy of
Sciences of the United States of America. Sep 2005;102(39):13732-13736.

2. Mokaya R, Poliakoff M. Chemistry - A cleaner way to nylon? Nature. 2005;437(7063):1243-1244.

3. Lu C, Chen R, Xing W, Jin W, Xu N. A submerged membrane
reactor for continuous phenol hydroxylation over TS-1. Aiche J. 2008;54(7):1842-1849.

4. Yip ACK, Hu X. Formulation of Reaction Kinetics for Cyclohexanone
Ammoximation Catalyzed by a Clay-Based Titanium Silicalite-1 Composite in a
Semibatch Process. Ind. Eng. Chem. Res. 2011;50(24):13703-13710.

Acknowledgments

Financial
supports from the National Natural Science Foundation of China (No. 21125629), the
National Basic Research Program (No.2009CB623406), the National High-Tech Research
and Development Program of China (2012AA03A606). ADDIN EN.REFLIST