Effect of Oxygen on Cyclohexane Oxidation - American Chemical

Apr 30, 2009 - In this study, we examined the effect of oxygen availability on the volumetric productivity, yield, and selectivity of this reaction by...
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Effect of Oxygen on Cyclohexane Oxidation: A Stirred Tank Study Radmila Jevtic, P. A. Ramachandran, and Milorad P. Dudukovic* Chemical Reaction Engineering Laboratory (CREL), Department of Energy, EnVironmental and Chemical Engineering (EEC), Washington UniVersity in St. Louis (WUSTL), St. Louis, Missouri 63130

Cyclohexane oxidation is the first step in the currently used technology for production of Nylon-6 and Nylon6,6 which employs a two-stage process. In the first stage, cyclohexane is oxidized with air to 4-8% conversion at about 80% selectivity to cyclohexanol and cyclohexanone as desired products. In this study, we examined the effect of oxygen availability on the volumetric productivity, yield, and selectivity of this reaction by using a stirred autoclave operated free from mass transfer effects in a batch and “dead-end” semibatch mode. Both uncatalyzed and catalyzed systems were used. The experimental and the modeling results lead to the conclusion that increased oxygen availability improves the productivity and selectivity at the fixed cyclohexane conversion (4%) as the residence time required declines with the increase in oxygen concentration. The positive effect on the reaction rate of increased oxygen concentration is the same when such an increase is achieved at constant pressure by raising the mole fraction of oxygen in the feed or by raising the total pressure in the system at fixed oxygen mole fraction. 1. Introduction Cyclohexane oxidation is practiced on a large scale globally. Approximately 106 tons/y of cyclohexanone and cyclohexanol, also known as KA oil,1 are made worldwide by this process as chemical intermediates in the production of adipic acid and caprolactam which are ultimately used in the manufacture of Nylon-6 and Nylon-6,6.2 Yet, this process is among the least efficient major industrial chemical processes. In stage one of this process, the desired products (i.e., cyclohexanol and cyclohexanone) are intermediates generated by a sequence of complicated, multiple, free-radical chain reactions.3-5 Both of these intermediate products, cyclohexanol and cyclohexanone, are more readily oxidized than cyclohexane which is the original reactant. Overoxidation of the intermediates is highly nonselective and results in a number of byproducts, such as succinic, oxalic, caproic, glutaric, and adipic acids.6 This can be minimized only by keeping cyclohexane conversions low, a serious disadvantage for large-scale processing due to the large recycle it demands. Typically, in industrial processes, cyclohexane is oxidized in a series of stirred tank reactors, or staged bubble reactors, in the liquid phase with air or a mixture of oxygen and nitrogen where the oxygen concentration is less than 21%. After about 40 min of reaction time, in the presence of cobalt or manganese salts as catalyst, at a temperature of 125-165 °C and at a pressure of about 8-15 bar, 4% conversion of cyclohexane and 80% selectivity to KA oil (cyclohexanone and cyclohexanol) are typically achieved.1 Reviews1,3,7-10 of the commercial practice of cyclohexane oxidation reveal that various improvements could be made within this process. The best would be to selectively oxidize cyclohexane directly to adipic acid at good yield. This so far has not been accomplished. Another more modest improvement is to increase 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). In order to do that one has to examine and understand the role and the effect of oxygen on the rates and selectivity. This is the objective of our study. * To whom correspondence should be addressed. E-mail: dudu@ wustl.edu. Website: http://crelonweb.eec.wustl.edu.

In general, it is expected that the use of oxygen-enriched air or pure oxygen should increase the rates of a hydrocarbon oxidation process. For example, it is reported11 in the oxidation of p-xylene to terephthalic acid that increasing the oxygen concentration by 2% in the feed air results in a 10% increase in the production capacity of terephthalic acid. Although the use of pure oxygen or oxygen-enriched air could benefit the overall productivity of cyclohexane oxidation, the possibility of deflagration has been the major obstacle in assessing the effect of oxygen not only on the pilot-scale but in laboratory reactors as well. Greene et al.12 reported the first cyclohexane oxidation with pure oxygen. The reaction was performed in the Liquid Oxidation Reactor (LOR) designed and patented by Kingsley et al.13 The LOR uses high efficiency stirring, which ensures high conversion of oxygen in the liquid and limits the oxygen escape from the liquid phase into the gas phase. The LOR uses a nitrogen purge in the vapor space above the liquid thereby keeping the oxygen concentration in the gas phase at a safe level. The findings of Greene et al.12 were compared with the data obtained from the conventional process, and the improvement is clear. The required mean reaction residence time is reduced from 36 to 8 min, the temperature of operation is slightly reduced, and the selectivity and productivity are increased while keeping the same cyclohexane conversion of 4%. One should note that neither was LOR used for conventional oxidation with air nor has the conventional equipment been used for oxidation with oxygen or oxygen-enriched air. Thus, the observed improvement might not be the direct result of the increased oxygen partial pressure in the gas phase but also due to the different transport effects in the two reactors. Therefore, whether pure oxygen or increased oxygen content should yield benefits that would overcome the safety concerns remains to be established. Moreover, Suresh et al.14 claim that cyclohexane oxidation is zero-order with respect to oxygen at lower conversion of cyclohexane. Hence, in the range of conversion considered, the concentration of oxygen would not have an impact on the rate of cyclohexane oxidation. To assess the effect of increased oxygen availability on productivity and selectivity to KA oil is the objective of this study.

10.1021/ie900093q CCC: $40.75  2009 American Chemical Society Published on Web 04/30/2009

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Figure 1. Experimental setup for the oxidation of cyclohexane in the 25 mL Parr autoclave reactor and with Agilent 6890 gas chromatograph for analysis.

2. Experimental Details 2.1. Materials. Cyclohexane, together with cyclohexanone and cyclohexanol that are needed for the gas chromatograph calibration, were supplied from Sigma-Aldrich. The oxygen and nitrogen gas with purity >99.9% were procured from Air Gas and were used directly from the cylinders. In the experimental runs performed with the catalyst, cobalt naphthanate, obtained from Sigma-Aldrich, was used. 2.2. Apparatus. All experiments were performed in a 25 mL Parr hastelloy autoclave equipped with Parr 4843 controller (Figure 1). The reactor consists of an agitated vessel, equipped with the gas inducing impeller that consists of a hollow shaft with holes in the impeller blades. The stirred reactor also has a gas inlet and outlet, automatic temperature control, variable agitation speed, high pressure cut off, and pressure read-out by a transducer. Experiments were performed, with and without the catalyst, following the procedure described below. 2.3. Procedure. The reactor was operated either in the batch mode (batch liquid and batch gas) or “dead-end” mode (batch liquid and continuous make up of gas) as indicated below. In the first case, the reactor was charged with 7.5 mL of cyclohexane then pressurized with nitrogen, heated to the reaction temperature (130-160 °C depending on the run), and finally the oxygen was added to raise the initial total pressure to 15 bar. The gas phase oxygen is then being depleted continuously through the batch run. When the reactor was operated in the dead-end mode, the initial procedure was the same. But in this case, the reactor was equipped with the solenoid valve, an additional pressure transducer, and the process control unit. When the total pressure in the reactor drops below the set point, due to oxygen consumption, the valve opens and additional oxygen is supplied to the reactor. In this manner, the partial pressure of oxygen in the reactor is kept constant. The experimental runs in both modes of operation were performed without the use of catalyst first and then with the use of catalyst. In the latter case, cobalt naphthanate at a 5 ppm level was used. This catalyst is fully dissolved in cyclohexane at the employed operating conditions. 2.4. Analytical. Quantitative analysis of the liquid reaction mixture is performed after various reaction times using the Agilent 6890 gas chromatograph (GC) with a flame ionization detector (FID). Helium was used as a carrier gas. A 30 m Alltech AT-5 ms column (0.25 mm × 0.25 mm) was used for separation of oxidation products. The following measurement conditions

were used: The injection port temperature was held at 220 °C. The initial temperature of the column was 70 °C with an immediate rise in temperature of 10 °C/min for 10 min. After that, when temperature reached 170 °C, the final temperature ramp was 50 °C/min up to 230 °C at which point the temperature was held constant for 2 min. The MSD Productivity ChemStation software was used to record and integrate the output from the FID. It should be noted that for each run two levels of internal standard were used. One was used in as a sample for determination of oxidation products. The other much more diluted sample was used to determine the concentration of remaining cyclohexane. In this manner, the concentration and conversion of cyclohexane could be determined accurately. Other details are available elsewhere.15 3. Results and Discussion 3.1. Absence of Mass Transfer Limitations. The first step in the experimental study was to check the operation for possible mass transfer limitations. Cyclohexane oxidation was performed in the batch reactor at temperature of 130 °C, pressure of 15 bar, and with 20% oxygen in the gas phase initially. No catalyst was used. Experiments were carried out at different agitation speeds and the reaction was stopped after 15 min in each case. The reaction mixture was then analyzed by GC for the two main desired products, cyclohexanol (ROH) and cyclohexanone (RO). The results are presented in Figure 2. Yields, the amount of products formed relative to the amount of cyclohexane at the beginning of the reaction, of the two main products were found to be independent of agitation speed beyond 300 RPM, indicating the absence of gas-liquid mass transfer. As mentioned, these experimental runs were carried out at a lower concentration of oxygen and without catalyst. One can expect that the rate of reaction will increase with the use of catalyst or by increasing the initial concentration of oxygen in the gas phase. Using the correlations available in the literature for the mass transfer coefficient,16,17 it was confirmed that the value of the mass transfer coefficient is increased by an order of magnitude by increasing the agitation speed from 300 to 900 rpm. Thus, even with the potential increase in the rates of reactions, the highest agitation speed possible in this stirred tank should ensure the absence of gas-liquid mass transfer limitations. All subsequent experiments have been performed at 900 rpm to ensure the kinetic regime. After the completion of the experiments, we have confirmed that all the experimentally

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Figure 2. Yield (moles of product formed per mole of cyclohexane fed) of cyclohexanol (ROH) and cyclohexanone (RO) as a function of repetitions per minute in the batch operation of cyclohexane oxidation at 130 °C and 15 bar using 50% oxygen in the initial gas phase as an oxidant.

Figure 4. Normalized oxygen pressure in the gas phase during the batch oxidation of cyclohexane with 20% and 50% oxygen in the initial gas phase (T ) 130 °C, P ) 15 atm).

obtained rates were well below the mass transfer rates expected at 900 rpm. Moreover, estimates of activation energies were in agreement with the range of values reported in the literature thus indirectly confirming the absence of transport effects. 3.2. Effect of Oxygen Concentration: Experiments without the Catalyst. 3.2.1. Batch Mode of Operation. Cyclohexane oxidation was first monitored in the batch system with 20% and 50% oxygen in the initial gas phase. The results in terms of yields of the two main products, cyclohexanol and cyclohexanone, as a function of time are presented in Figure 3. It can be noted that higher yields of the products are formed when 50% oxygen is used in the initial gas mixture. The fact that, at each instant of time, cyclohexane conversion is higher at 50% initial oxygen gas content than at 20% indicates that the reaction proceeds faster when there is more oxygen. The rate of oxidation seems to be positively influenced by increased oxygen concentration. The plateau in conversion and yields that can be observed in both figures about after 60 min is due to the oxygen consumption and its ultimate depletion in the reactor. This was confirmed by calculating the partial pressure of oxygen

left in the system at each experimental point. Since the drop in the total pressure in the system is due solely to oxygen consumption, the partial pressure of oxygen available in the reactor is the difference between partial pressure of oxygen available at the beginning of the reaction and the recorded pressure drop. The calculated partial pressure of oxygen in the system, normalized with the initial partial pressure of oxygen, is presented in Figure 4 as a function of time. The line clearly decays to essentially zero in about 60 min. The fact that the data from both runs overlap and fall on the same line indicates conclusively that the rate of the oxygen consumption in oxidation of cyclohexane is first order with respect to oxygen since the evolution of partial pressure in time is independent of initial oxygen concentration. The summary of the experimental results on cyclohexane oxidation in the batch system without the catalyst with different initial concentration of oxygen in the gas phase is provided in Table 1. The maximum conversion of cyclohexane and the maximum yield of the desired products (KA oil) are higher when more oxygen in the gas phase is used. However, the final

Figure 3. Yield of cyclohexanol (ROH), cyclohexanone (RO), and conversion of cyclohexane obtained experimentally in batch cyclohexane oxidation at 130 °C and 15 bar using 20% oxygen (left) and 50% oxygen (right) initially as an oxidant with no catalyst. (Note: the scale for cyclohexane conversion is the same as that for the yields.)

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Table 1. Summary of the Experimental Results on Cyclohexane Oxidation in the Batch System with Different Initial Concentrations of Oxygen in the Gas Phase initial oxygen in the gas phase (%)

maximum cyclohexane conversion (%)

maximum KA yield (%)

selectivity of KA (%)

20 50

2.0 4.5

1.8 3.4

90.2 76.7

selectivity is lower with an increased initial oxygen concentration. This was expected since it is known that selectivity decreases in cyclohexane oxidation with an increase in cyclohexane conversion. Cyclohexanol and cyclohexanone oxidize further into the mixture of higher order acids, like adipic, glutaric, and succinic acid. This was confirmed in this study by NMR analysis. It remains to be examined whether selectivity and productivity are affected with the increased availability of oxygen at the same level of cyclohexane conversion. The best way to assess this is to keep the partial pressure of oxygen constant during reaction in a dead-end (semibatch) mode of reactor operation which is described later. We proceeded to establish experimentally in the batch mode of operation that the effect of increased oxygen availability is the same irrespective of how it was achieved (e.g., by raising the mole fraction of oxygen at constant pressure or by raising the pressure at constant mole fraction of oxygen). Theortically, we know that this should be the case, but in view of the discrepancies in the literature regarding the role of oxygen, this seemed worthy of confirming via a couple of experiments. To examine this, cyclohexane oxidation is performed at the same temperature in the batch system, but at higher pressure. The total pressure is chosen in such a way so that oxygen concentration in the liquid phase, when 20% of oxygen in the gas phase is initially present, is the same as the oxygen concentration in the liquid when 50% of oxygen in the gas phase is used at 15 atm. There was no discernible difference in the results, which means that there is no effect of the manner in which the increased oxygen concentration is achieved. This is important since, if it is proven that increased oxygen availability can lead to higher productivity, the improvement in the process can be achieved by increasing the total pressure of the system without increasing the oxygen content in the reactor. 3.2.2. Semibatch (Dead-End) Mode of Operation. The experimental results for cyclohexane oxidation with different oxygen contents in the gas phase (20% and 50%) in the semibatch mode are presented in Figures 5 and 6. The same reaction conditions were employed as in the batch study: 130 °C, 15 atm, no catalyst. Again, higher yields and higher cyclohexane conversion are achieved with more oxygen in the gas phase, but also, selectivity is lower. Overall, it is evident that reaction proceeds faster with 50% oxygen. After 30 min, cyclohexane conversion is around 7% if 50% oxygen is used as opposed to around 3% when 20% oxygen is in the gas phase. This confirms the conclusion that an increased oxygen concentration has a positive effect on the rate of reaction. Still, it remains unclear by how much actual productivity is increased with increased oxygen availability since it is very difficult to stop the experiments at the same level of cyclohexane conversion. That can be accomplished more readily with a model provided that the model is successful in capturing the reaction trends and outcomes in the experimental reactor. 3.3. Model for Batch and Dead-End Operation: Model Development. To extract the maximum value from the experimental studies performed in our setup, we modeled both the batch and semibatch operation. It is known that models that

Figure 5. Yield of cyclohexanol (ROH), cyclohexanone (RO), and conversion of cyclohexane obtained experimentally in the semibatch operation of cyclohexane oxidation at 130 °C and 15 bar using 20% oxygen as an oxidant with no catalyst.

Figure 6. Yield of cyclohexanol (ROH), cyclohexanone (RO), and conversion of cyclohexane obtained experimentally in the semibatch operation of cyclohexane oxidation at 130 °C and 15 bar using 50% oxygen as an oxidant with no catalyst.

capture the reaction well in a given experimental setup can be then used to predict the outcome at any other condition of interest. To set up an appropriate reactor model for the experiments described so far, we started with the simplest one. We assumed constant temperature and fully backmixed gas and liquid phase in our small bench-scale autoclave. We relied on the validity of the ideal gas law for the gas phase at conditions studied and assumed that all the reactions occur in the liquid phase only. The species mass balances in the gas phase for the batch operation are then given as

(

)

pi VG dpi ) -kLia - ci VL RT dt Hi

(1)

The species mass balances in the liquid phase are the following:

(

)

pi dci ) kLia - ci + dt Hi

NR

∑ν r

ki k

(2)

k)1

In the equations above, VG is the volume of the gas phase in the reactor; pi is partial pressure of the component i; R is the

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Table 2. Values of the Parameters Used in the Models VG (m3) VL (m3) T (°C) P (atm) kLa (1/s) HO2 (Pa m3/mol) HN2 (Pa m3/mol) HRH (Pa m3/mol) HROH (Pa m3/mol) HRO (Pa m3/mol)

1.75 × 10-5 7.5 × 10-6 130-160 15 0.1 5965.9 8748.5 49.2 15.2 7.7

ideal gas constant; T is reaction temperature; t is time; kLi is the mass transfer coefficient from the liquid side for component of interest (obtained directly from the literature16 for oxygen and calculated for other components by multiplying it with the square root of the ratio of the diffusivities for the component of interest and for oxygen); a is the ratio of surface area to volume of the liquid; ci is the concentration of the component i in the liquid phase; Hi is Henry’s constant in (Hi ) pin i /c i ) for component i determined using Aspen Plus; VL is the volume of the liquid phase in the reactor; νki is the stoichiometric coefficient of the component i in the reaction k; and rk is the reaction rate of the reaction k. All the values used in the model are shown in Table 2. For a dead-end semibatch mode of operation, oxygen is being supplied as it gets consumed in the reactor. Thus, the partial pressure of oxygen is assumed constant or close to constant for the lower cyclohexane conversion achieved in this study. It is also assumed that cyclohexane is equilibrated at all times between the two phases. The other species balances in the liquid phase are described without the use of the mass transfer term as the evaporation rate of the products can be neglected. Lastly, it remains to be determined what kinetic model is the most appropriate to use. It is widely accepted that the mechanism of the cyclohexane oxidation consists of complicated, multiple, free-radical chain reactions that include initiation, chain propagation, and chain termination steps3-5 with total number of reactions exceeding 150.8 There are a number of different kinetic schemes and models proposed in the literature ranging from the ones that use free radical mechanism itself18,19 to the lumped models.7,8,20-23 On the other hand, the kinetic constants necessary for the use of these models are not widely available. Upon consideration of all the pertinent factors, we selected the lumped kinetics proposed by Spielman23 and Alagy et al.21 schematically shown below for our reactor model. The main reasons for this choice are as follows: (1) Our experiments are not detailed enough and do not contain direct measurements of active intermediates to warrant the use of a more sophisticated and detailed model. (2) We are only interested in determining the initial stage of reaction and the maximum in ROH and RO concentrations and not in the detailed description of higher oxidation products. (3) The kinetic constants for this scheme have been reported by Pohorecki et al.7

Thus, for each of the steps shown in the mechanism above the law of mass action is assumed and the kinetic constants are obtained from the literature7 (Table 3). These constants have been estimated from the operation of the industrial reactor at 160 °C for the catalytic cyclohexane oxidation and allow,

Table 3. Kinetic Constants for the Lumped Kinetic Model Proposed for Catalytic Cyclohexane Oxidation7 k0 (m3/mol · s) -5

2.66 × 10

k1 (1/s) -3

4.8 × 10

k2 (1/s) -3

1.3 × 10

k3 (m3/mol · s) -4

5.0 × 10

k4 (m3/mol · s) 3.7 × 10-4

combined with the above mechanistic scheme, for reactor simulation at this temperature. 3.4. Model Use for Catalyzed Cyclohexane Oxidation. 3.4.1. Strategy for Model Use. Since we have found quantitative information on the simplified mechanism only for the catalyzed oxidation of cyclohexane, we have demonstrated that the catalyzed reaction also is first-order in oxygen.15 We have then adopted the following strategy for the use of the above model for interpretation of our experimental results performed with the soluble catalyst and for prediction of the effect of oxygen availability on productivity and selectivity of KA oil. We first had to determine whether the reported model is capable of matching the results of our experiment at 160 °C in the presence of the catalyst in a batch system. This may require adjustments as the catalyst concentration for Pohorecki’s constants was not clearly reported. We used 5 ppm of the catalyst. Second, we needed to establish the values of the constants at different temperatures for experiments with the same amount of catalyst in the batch system in order to extract the values of apparent activation energies, which were not reported by Pohorecki et al.7 Then with so determined constants by fitting our batch data, we compared the ability of the model to predict the performance of our reactor operated in the semibatch (deadend) mode. Finally, upon accomplishing the above, we used the model to assess the effect of increased oxygen availability on productivity and selectivity. This has been accomplished as described below. 3.4.2. Evaluation of Model Parameters from Catalyzed Cyclohexane Oxidation in a Batch System. The model equations for the gas and liquid phases were solved simultaneously. It was assumed that at the beginning of the reaction the concentrations of all products are equal to zero and only pure cyclohexane is present in the liquid. The stiff ordinary differential equation (ODE) solver from Netlib library (LSODE) was used. To determine the kinetic constants for catalytic cyclohexane oxidation at temperatures other than 160 °C, ODRPack, a collection of Fortran subroutines for fitting a model to data, also available from Netlib, was used. Orthogonal distance regression (ODR) is a method for linear regression that determines the values of unknown quantities in a statistical model by minimizing the sum of the squared residualssthe orthogonal difference between the predicted and observed values. So, for the temperatures other then 160 °C, the experimental results at that temperature are given as input variables. Initial guesses for the kinetic constants are needed and available values for 160 °C are used. The ODE solver is used to solve the model equations until the model solution matches the experimental results. Finally, as the results, the set of kinetic constants at the new desired temperatures are given as the solution. First, the model predictions were compared with the appropriate experimental results obtained at 160 °C using cobalt naphthanate as the catalyst. The concentration of the catalyst is not mentioned in the published papers, but since the constants were obtained from the operation of the industrial reactor, the commonly used catalyst concentration in the commercial process is assumed. The typical concentration of catalyst in cyclohexane oxidation practiced on the large-scale can be found in the patent24,25 and open literature26 and is equal to 0.1-5 ppm. In our experimental study, the concentration of cobalt naphthanate

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Table 4. Kinetic Constants for Cyclohexane Oxidation at 130°C Obtained by Fitting the Batch Isothermal Model to the Experimental Results k0 (m3/mol · s) 6.19 × 10-6

k1 (1/s)

k2 (1/s)

1.01 × 10-3 5.58 × 10-4

k3 (m3/mol · s) k4 (m3/mol · s) 8.60 × 10-5

3.15 × 10-4

Table 5. Pre-exponential Factors (k) and Activation Energies (E) for Kinetic Constants in Proposed Cyclohexane Oxidation Kinetic Scheme

k0 k1 k2 k3 k4 a

k (m3/mol · s)

E (kJ/mol)

× × × × ×

70.59 75.38 40.90 85.12 7.70

8.75 5.95 1.12 9.30 3.00

3

10 106 a 102 a 106 10-3

k (1/s).

Figure 7. Cyclohexane oxidation experimental results from the batch stirred tank operation at 160 °C and 15 atm with 50% oxygen in the gas phase and with cobalt naphthanate as catalyst compared with the model predictions at the same conditions using kinetic scheme of Alagy et al. and kinetic constants from the work of Pohorecki et al.7

Figure 9. Experimental results for the yields of the two main products in cyclohexane oxidation performed in the dead-end batch reactor at 140 °C and 15 atm with 50% oxygen in the gas phase, and with cobalt naphthanate as catalyst compared to the model predictions at the same conditions (kinetic constants previously obtained).

Figure 8. Cyclohexane oxidation model fitted to data from the batch operation at 130 °C and 15 atm with 20% oxygen in the gas phase and with cobalt naphthanate as catalyst using ODRPack.

was 5 ppm, which is equal to 10-5 mol/L. The role of the catalyst is twofold: it expedites the conversion of absorbed oxygen into hydroperoxide, in other words increases the rate of the initial step, and it intensifies the decomposition of the hydroperoxide into cyclohexanol and cyclohexanone.26 The comparison of the model generated and experimental results can be seen in Figure 7. The model follows the trend obtained in the batch operation of the reactor at 160 °C and 15 atm with cobalt naphthanate and captures reasonably the initial part of the curve. Clearly, more data in the initial time period would be needed for rigorous evaluation of the model at this temperature. However, the level of the agreement achieved is sufficient for our studies at lower temperatures with slower rates. As already mentioned, the kinetic constants for the lumped model are given at only one temperature (160 °C). With the use of ODRPack, the kinetic constants for the system in question at different temperatures have been estimated and further used for assessment of system behavior. Figure 8 shows the fit of the model to the experimental results obtained in the batch mode of the stirred tank operation at 130 °C. ODRPack uses the initial

guesses of the kinetic constants that are provided as well as the concentrations of the two desired products as a function of time to fit the model predictions to the observed data. The outcome is the set of kinetic constants at the stated temperature (see Table 4). From the results at two different temperatures, we obtain the estimates of activation energies and preexponential factors for the constants (Table 5). 3.4.3. Comparison of Model Predictions and Data for Catalyzed Cyclohexane Oxidation in a Semibatch (DeadEnd) System. To check the validity and the utility of the newly obtained constants in the Arrhenius form, the experimental results obtained for the catalyzed oxidation of cyclohexane in the dead-end operated stirred tank are compared with the model predictions at 140 °C. The comparison is shown in Figure 9. It can be observed from this figure that cyclohexanol and cyclohexanone concentrations after reaching a peak decrease in time. This is due to the previously mentioned fact that these two desired intermediate products get readily oxidized further (more susceptible to oxygen attack than cyclohexane itself) as the reaction progresses. Thus, as mentioned previously, the selectivity in cyclohexane oxidation decreases when cyclohexane conversion increases past a certain point. For that reason, cyclohexane oxidation is usually stopped at 4-8% of cyclohexane conversion. When the model for dead-end operation of cyclohexane oxidation with air in a stirred tank is used for

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Table 6. Selectivity at 4% Cyclohexane Conversion and Time Needed to Achieve That Conversion at Different Oxygen Percentages in the Gas Phase Predicted by the Model for the Fed-Batch Catalyzed Cyclohexane Oxidation percent oxygen time (min) to achieve selectivity (%) for in the gas 4% cyclohexane cyclohexanol, cyclohexanone, phase conversion and cyclohexyl-hydroperoxide 21 50 75 100

34.4 14.2 9.8 7.4

89.4 92.6 93.9 94.9

reaction conditions of 160 °C and 15 atm, maximum selectivity is obtained at 6.6% cyclohexane conversion. The developed model is then used to predict the outcome of catalyzed cyclohexane oxidation performed at various oxygen concentrations in the gas phase in a dead-end operated stirred reactor. The selectivity to cyclohexanol and cyclohexanone and the mean reaction residence time needed to achieve 4% cyclohexane conversion, as predicted by the model, are shown in Table 6. The temperature in the reactor was chosen to match the reaction temperature stated in the patent by Green et al.12 (149 °C). From the results in Table 6, one can conclude that by increasing oxygen availability in cyclohexane oxidation, the reaction time to achieve 4% cyclohexane conversion is 4-5 times shorter when oxygen is increased from 21% (air) to 100% (pure oxygen) in the gas phase. Thus, productivity is increased as well. This agrees with the findings by Greene et al.12 Reaction rates are higher at increased oxygen concentration, and we did not find any evidence that at conditions used in this study cyclohexane oxidation is zero-order with respect to oxygen as suggested by some authors.14,27 5. Concluding Remarks Our experimental study of cyclohexane oxidation in a stirred mini-autoclave was conducted in absence of oxygen masstransfer limitations to determine the effect of oxygen concentration on the rate of reaction, yields, and selectivity. In the batch mode of operation for the uncatlyzed reaction system, the finite initial amount of oxygen provided to the system is eventually consumed and the oxygen partial pressure and concentration in the liquid phase decay in time to essentially zero. Our experimental results established that an increase in initial oxygen concentration in the vapor phase from 20% to 50% has a positive effect on the rate and yields. The effect on the final selectivity was not clear due to the different conversions of cyclohexane reached. The results in the batch system confirmed that the rate of oxygen consumption is first-order in oxygen as the oxygen partial pressure decay in time was independent of the initial partial pressure of oxygen. Our experiments in the semibatch dead-end mode of operation kept oxygen partial pressure constant during the runs. From these runs, it was also evident that reaction proceeds faster with 50% oxygen than with 20% oxygen. After 30 min, cyclohexane conversion is around 7% if 50% oxygen is used as opposed to around 3% when 20% oxygen is used in the gas phase. This confirms that increased oxygen concentration has a positive effect on the rates of reaction. Moreover, higher yields are achieved at higher reaction rates, i.e. when higher concentrations of oxygen in the gas phase is used. The positive effect of increased oxygen concentration on the rate, productivity, and selectivity is observed for both the uncatalyzed and catalyzed cyclohexane oxidation To determine the optimal selectivity and yield conditions for the catalyzed cyclohexane oxidation, we fit the lumped kinetics

of Alagy et al.21 and Spileman et al.,23 with the kinetic constants provided by Pohorecki et al.,7 to our data at the temperature at which the needed kinetic constants were given and at an additional temperature. This allowed us to estimate the activation energies and pre-exponential factors. The model captures the experimentally determined evolution in the peaks of the yield curves in time in our dead-end (semibatch) experiments satisfactorily at different temperatures. From the experimental and modeling results, it can be concluded that increased oxygen availability improves the productivity of cyclohexane oxidation at the fixed cyclohexane conversion (4%). Thus, the reaction (residence) time needed to achieve such conversion declines considerably with the increase in oxygen concentration. This agrees with the findings of Greene and his collaborators.12 One should note that the way in which the increased oxygen availability is achieved does not have an effect. In other words, instead of increasing oxygen concentration in the gas phase, the same benefits can be accomplished by increasing the total pressure in the system. Acknowledgment The authors are grateful for the support of the National Science Foundation through the Center for Environmentally Beneficial Catalysis (CEBC) at Kansas University (KU) via Grant EEC-0310689. Fruitful discussions with professors B. Subramaniam and R. V. Chaudhari at the University of Kansas were particularly appreciated. Industrial support of CREL, where this work was executed, also contributed to its successful completion. Our special gratitude to professor P. L. Mills of TAMU at Kingsville for many useful suggestions regarding experimental design and techniques and for the analytical equipment provided on loan. Literature Cited (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) Berezin, I. V.; Denisov, E. T.; Emanuel, N. M. The oxidation of cyclohexane: Pergamon Press: Elmsford, NY, 1966. (4) Hendry, D. G.; Gould, W. C.; Schueltze, D.; Syz, M. G.; Mayo, F. R. Autoxidation of Cyclohexane and Its Autoxidation Products. J. Org. Chem. 1976, 41 (1), 1–10. (5) Walling, C. Limiting Rates of Hydrocarbon Autoxidations. J. Am. Chem. Soc. 1969, 91 (27), 7590–7594. (6) Wen, Y.; Potter, O. E.; Sridhar, T. Uncatalysed oxidation of cyclohexane in a continuous reactor. Chem. Eng. Sci. 1997, 52 (24), 4593– 4605. (7) Pohorecki, R.; Baldyga, J.; Moniuk, W.; Krzysztoforski, A.; Wojcik, Z. Liquid-phase oxidation of cyclohexane - modeling and industrial scale process simulation. Chem. Eng. Sci. 1992, 47 (9-11), 2559–2564. (8) Pohorecki, R.; Baldyga, J.; Moniuk, W.; Podgorska, W.; Zdrojkowski, A.; Wierzchowski, P. T. Kinetic model of cyclohexane oxidation. Chem. Eng. Sci. 2001, 56 (4), 1285–1291. (9) Kroschwitz, J. I.; Howe-Grant, M. Kirk-Othmer Encyclopedia of Chemical Technology; John Wiley & Sons, Inc: New York, 1993. (10) Sato, K.; Aoki, M.; Noyori, R. A “Green” Route to Adipic Acid: Direct Oxidation of Cyclohexenes with 30% Hydrogen Peroxide. Science 1998, 281 (5282), 1646–1647. (11) 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 Progr. 2004, 23 (1), 72–81. (12) Greene, M. I.; Sumner, C.; Gartside, R. J. Cyclohexane oxidation. U.S. Patent 5,780,683, 1998. (13) Kingsley, J. P.; Roby, A. K.; Litz, L. M. Terephthalic acid production. U.S. Patent 5,371,283, 1994. (14) Suresh, A. K.; Sridhar, T.; Potter, O. E. Autocatalytic Oxidation of Cyclohexane-Modeling Reaction Kinetics. AIChE J. 1988, 34 (1), 69– 80.

Ind. Eng. Chem. Res., Vol. 48, No. 17, 2009 (15) Jevtic, R. The Effect of Oxygen on Cyclohexane Oxidation. PhD Thesis, Washington University in St. Louis, St. Louis, MO, 2008. (16) Tekie, Z.; Li, J.; Morsi, B. I. Mass Transfer Parameters of O2 and N2 in Cyclohexane under Elevated Pressures and Temperatures: A Statistical Approach. Ind. Eng. Chem. Res. 1997, 36 (9), 3879–3888. (17) Tekie, Z.; Li, J.; Morsi, B. I.; Chang, M. Gas-liquid mass transfer in cyclohexane oxidation process using gas-inducing and surface-aeration agitated reactors. Chem. Eng. Sci. 1997, 52 (9), 1541–1551. (18) Khar’kova, T. V.; Arest-Yakubovich, I. L.; Lipes, V. V. Kinetic model of the liquid-phase oxidation of cyclohexane. I. Homogeneous course of the process. Kinetika i Kataliz 1989, 30 (4), 954–8. (19) Schaefer, R.; Merten, C.; Eigenberger, G. Autocatalytic Cyclohexane Oxidation in a Bubble Column. Can. J. Chem. Eng. 2003, 81, 741– 748. (20) Krzysztoforski, A.; Wojcik, Z.; Pohorecki, R.; Baldyga, J. Industrial contribution to the reaction engineering of cyclohexane oxidation. Ind. Eng. Chem. Process Des. DeV. 1986, 25 (4), 894–8. (21) Alagy, J.; Defoor, F.; Franckowiak, S. Oxidation of cyclohexane to cyclohexanone. ReV. Inst. Franc. Pet. Ann. Combust 1964, 19 (12), 1380– 1390.

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ReceiVed for reView January 19, 2009 ReVised manuscript receiVed March 31, 2009 Accepted April 9, 2009 IE900093Q