(95g) The Effect of Oxygen Availability on the Oxidation of Cyclohexane

Cyclohexane oxidation process is commercially practiced on a large scale and approximately 106 tons per year of cyclohexanone and cyclohexanol, also known as KA oil1, are made worldwide by this process as chemical intermediates in production of adipic acid and capolactum, which are ultimately used in the manufacture of Nylon-6 and Nylon-6,62. Yet, this process seems to be quite inefficient with only 4-8% cyclohexane conversion per pass1, which requires separation and recycle of most of the liquid reactant. Low conversion is practiced in order to prevent overoxidation of two main products to various byproducts; thus, prevent low selectivity. Significant improvements in current operation of the process could be made by increasing volumetric productivity in the first step in the current two-step oxidation of cyclohexane to adipic acid without sacrificing selectivity toward cyclohexanol and cyclohexanone (KA oil).This requires an improved understanding of the reaction scheme involved, as well as understanding of the effect of transport, reactor design, and most importantly the role of oxygen on the rates and selectivity.

It is expected that the use of oxygen enriched air or pure oxygen could increase the rates of the hydrocarbon oxidation process3. It remains to be determined what effect would that have on reactions. Yet, due to the concern regarding potential explosion or deflagration either in the vapor space or the vapor bubbles, the oxidation of cyclohexane with oxygen enriched air or pure oxygen has not been performed even in small lab-scale reactors. Greene et al (1998)4 reported first cyclohexane oxidation with pure oxygen. The reaction was performed in the Liquid Oxidation Reactor (LOR) designed and patented by Kingsley et al 5 (1994). The improvement was clear. The residence time is reduced from 36 to 8 min, temperature of operation is slightly reduced, and selectivity and productivity are increased while keeping the same cyclohexane conversion of 4%. LOR was not used for conventional oxidation with air and data for comparison are not obtained in the same reactor system. Thus, the improvement might not be the result of the increased oxygen content but due to the different transport effects in the reactor itself.

Therefore, it is still unclear weather pure oxygen or increased oxygen content should yield to benefits that would overcome the safety concerns. To assess this one needs a complete set of data on conversion and selectivity as a function of time in order to conclude what the effect of the increased content of oxygen in cyclohexane oxidation process. This is the scope of our research study.

References

1) Suresh, A. K.; Sharma, M. M.; Sridhar, T., Engineering aspects of industrial liquid-phase air oxidation of hydrocarbons. Ind. Eng. Chem. Res 2000, 39, 3958-3997.

2) Schuchardt, U.; Calvarlho, W. A.; Spinace, E. V., Why is Interesting to Study Cyclohexane Oxidation? Synlett 1993, 10, 713-718.

3) Chen, J.-R., An inherently safer process of cyclohexane oxidation using pure oxygen - An example of how better process safety leads to better productivity. Process Safety Progress 2004, 23, (1), 72-81.

4) Greene, M. I.; Sumner, C.; Gartside, R. J. Cyclohexane oxidation. 5,780,683, 1998.

5) Kingsley, J. P.; Roby, A. K.; Litz, L. M. Terephthalic acid production. 5,371,283, 1994.