Ro Rh

This propagation of the hydroperoxide is of great importance because it produces both the ketone and alcohol from the start of the reaction. As soon as Q=O (cyclohexanone) is formed, an autoacceleration effect is observed, due to the reaction: ROOH + Q = O ! RO' + Q_aH- = O + H2O

in which an 'OH radical abstracts a weakly bonded aH atom from the ketone, which also is an efficient bimolecular initiation reaction.

It was also proposed that starting from RO', ring opening may proceed efficiently:

Figure 7.1 shows a simplified flow sheet of the process ofcyclohexane oxidation with air. The reaction is carried out in three in-series reactors, with cyclohexane being fed in the first reactor and air being distributed to the three reactors. This operation allows a better control of the reaction and improves safety.

A variation of this step currently adopted in several plants, is characterized by the addition of substantial quantities of anhydrous meta-boric acid as a slurry in

Figure 7.1 Simplified flow-sheet of the process for the oxidation of cyclohexane with air.

cyclohexane to the first of a staged series of oxidation reactors (Bashkirov Oxidation). No other catalyst is necessary. Boric acid reacts with cyclohexanol to give a borate ester that stabilizes the product and reduces its tendency to be oxidized further to form either cyclohexanone or degradation products. This results in a higher once-through conversion (up to 10 or even 15%), and selectivity (90%, with a molar ratio Ol/One of around 10). The borate ester formed is easily hydrolyzed by hot water to boric acid and cyclohexanol. After distillation, a 99.5% Ol/One mixture is obtained.

The cyclohexane volatilizes immediately when depressurized from reaction conditions (e.g., 155 °C, 8atm) to atmospheric conditions. This leads to potentially dangerous operations; for instance, in June 1974 an accident occurred at Flix-borough, England, with several casualties and injuries. Because of the inadequate support of a flexible bellow-type piping between two reactors, installed to replace a reactor that had been removed for maintenance, a rupture occurred, and an estimated 30 tons of cyclohexane volatilized and formed a large vapor cloud. The cloud was ignited by an unknown source approximately 45 seconds after release.

The use of oxygen instead of air might lead to improvements, such as a higher rate of oxidation and an increased productivity, the use of milder reaction conditions, a lower capital and operating energy costs, lower purge streams and a reduced cost for the treatment of vent gas. In fact, several companies already use O2 instead of air. Safety concerns are greater than with air operation, but the safety issues have been solved.

A special reactor design, called the liquid oxidation reactor (LOR) allows the use of oxygen or oxygen-enriched air [3e-g]. The LOR has a special enclosure around the stirrer where most ofthe oxygen is fed and consumed. The remaining oxygen passes into the vapor space and is diluted with nitrogen before being vented. Although the formation of flammable compositions in the overhead space is unlikely, because of the high oxygen conversion, there are still concerns regarding the potential explosions that may occur inside the vapor bubbles in the reactor.

A paper by the China Petrochemical Dev Co [3h] reported that the use of pure oxygen for cyclohexane oxidation leads to an increased yield and selectivity to Ol/One with respect to the traditional air-based technology, under inherently safe conditions. The latter are achieved by the addition of water, which avoids the formation of flammable mixtures in the overhead vapor space and in the vapor bubbles. In fact, cyclohexane and water form a minimum-boiling azeotrope, the vapor pressure ofwhich is higher than that of cyclohexane. The increased vapor pressure acts as an inert component.

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