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Integrated Process of H2O2 Generation through Anthraquinone Hydrogenation-Oxidation Cycles and the Ammoximation of Cyclohexanone Tengfei Liu,† Xiangkun Meng,‡ Yaquan Wang,*,† Xinhua Liang,† Zhentao Mi,† Xiangjuan Qi,† Shiyu Li,† Wei Wu,‡ Enze Min,‡ and Songbao Fu§ State Key Laboratory of C1 Chemical Technology, School of Chemical Engineering, Tianjin University, Tianjin 300072, People’s Republic of China, Research Institute of Petroleum Processing, SINOPEC, Beijing 100083, People’s Republic of China, and Research Institute of Yingshan Petrochemical Plant, Yueyang 414003, People’s Republic of China
The liquid-liquid-phase equilibrium of a five-component system of methanol-water-H2O2trimethylbenzene-trioctyl phosphate was studied, and the data were correlated with UNIQUAC and NRTL models, respectively, and used for the simulation of the extraction of H2O2 with an aqueous methanol solution in the H2O2 production process based on anthraquinone hydrogenation-oxidation cycles. The results showed that, with an aqueous methanol solution for the extraction of H2O2, after a few cycles the concentration of methanol in an anthraquinone working solution becomes constant and would not influence the H2O2 production process. All of the impurities in the H2O2 solution obtained with an aqueous methanol solution as the extractant from the H2O2 generation process as well as the stabilizers and corrosion inhibitors that should be added in the H2O2 solution did not appreciably affect the direct ammoximation of cyclohexanone with NH3 and H2O2 catalyzed by TS-1. The results indicated that the processes of cyclohexanone oxime production through the direct ammoximation of cyclohexanone and H2O2 generation through the anthraquinone hydrogenation-oxidation cycles could be integrated. 1. Introduction Cyclohexanone oxime is an important intermediate of -caprolactam for nylon-6 production. Currently, the most widely used technologies for the production of cyclohexanone oxime have some disadvantages. Especially large amounts of ammonium sulfate, which is a low-value byproduct and causes many environmental problems, are produced.1 Enichem Co. developed a new route based on the direct ammoximation of cyclohexanone with NH3 and H2O2 using TS-1 as the catalyst.2 This is an environmentally friendly process in which nearly complete conversion of cyclohexanone and above 99% selectivity to cyclohexanone oxime can be obtained.3-14 However, the profitability of this process depends on the cost of H2O2. Currently, the widely used method for the production of hydrogen peroxide is based on the anthraquinone (2-ethylanthraquinone is often used) hydrogenation and oxidation cycles. In the process, the H2O2 formed is extracted with water and purified by several steps. To store it for longer time before use, several kinds of stabilizers and corrosion inhibitors must be added. Here we studied an integrated process of the ammoximation of cyclohexanone and H2O2 generation through anthraquinone hydrogenation-oxidation cycles as described in Figure 1. In this process, anthraquinone in the mixture of 1,3,5-trimethylbenzene (TMB) and trioctyl phosphate (TOP) (together called an anthraqui* To whom correspondence should be addressed. Tel.: +8622-27406335. Fax: +86-22-27406335. E-mail: yqwang@ tju.edu.cn. † Tianjin University. ‡ SINOPEC. § Research Institute of Yingshan Petrochemical Plant.
Figure 1. Integrated process of ammoximation of cyclohexanone and hydrogen peroxide generation through anthraquinone hydrogenation-oxidation cycles.
none working solution) is hydrogenated into anthrahydroquinone, and then the anthrahydroquinone formed is oxidized with oxygen back to anthraquinone with the generation of H2O2. The H2O2 formed is extracted with an aqueous methanol solution and used directly in the ammoximation of cyclohexanone. After the ammoximation reaction, the excess NH3 can be separated and fed back into the ammoximation reactor, and methanol and partial water are separated and reused for the extraction of H2O2. The oxime can be purified, and the water formed in the ammoximation reaction is separated in the purification step and discharged, as practiced in the conventional processes. The advantage of this integrated process is that the H2O2 formed in the oxidation step can be extracted and used directly in the cyclohexanone ammoximation reaction without the usual several steps of purification, as practiced in the conventional process. To examine the possibility of this integrated process, the liquid-liquid-phase equilibrium of the five-component system of methanol-water-H2O2-TMB-TOP was
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measured and the results were used in the simulation to determine the levels of impurity compounds in the H2O2 solution. Then, the influence of these impurity compounds on the ammoximation of cyclohexanone was investigated.
Ψi )
II γIi xIi ) γII i xi
zi )
zIi
+
(1)
zII i
(2)
where z, zIi , and zII i are the moles of component i in the system and phases I and II and γIi and γII i are the corresponding activity coefficients of component i in phases I and II, respectively. γIi and γII i can be calculated from the UNIQUAC or NRTL models. The UNIQUAC equation of the multicomponent system is given by16
ln γi ) ln γCi + ln γRi
()
γCi ) ln
Ψi xi
[
()
(3)
Ψi C θi Z h + qi ln + li xjlj 2 Ψi xi j)1 C
∑ j)1
ln γRi ) qi 1 - ln(
∑
C
θjTji) -
∑ j)1
( )] θjTij
C
∑ θkTkj k)1
(4)
(6)
C
xiri ∑ i)1
2. Experimental Section 2.1. Materials. Cyclohexanone, H2O2 (28 wt %), an ammonia solution (25 wt %), methanol, TMB, TOP, phosphoric acid, sodium nitrate, and tert-butyl alcohol were all analytical grade. TS-1 was supplied by Research Institute of Petroleum Processing, SINOPEC, Beijing, People’s Republic of China, and was prepared according to ref 15, and the atomic ratio of Si/Ti is 39. Dongfang Chemical Factory, Tianjin, People’s Republic of China, provided the industrial anthraquinone working solution. 2.2. Phase Equilibrium and Extraction Simulation. Liquid-liquid-phase equilibriums of the fivecomponent system of methanol-water-H2O2-TMBTOP were measured. The phase equilibrium experiments were performed in a 100-mL glass flask with a septum cap and equipped with a mechanical stirrer. When the mixture was prepared, the flask was placed in a water bath at 40 ( 0.1 °C. After agitation for 1 h, the mixture was left to settle for 4 h without stirring. Then syringes were used to withdraw samples from the organic phase layer and the water phase layer, respectively. The concentration of H2O2 was determined by iodimetry. The concentrations of methanol, TMB, and TOP were measured by gas chromatography (GC) with a packed column and a flame ionization detector (FID). The column was coated with 5% E-30 on Chromosorb W. The concentration of H2O was deduced from concentrations of other components and affirmed by GC analysis of the organic phase with a thermal conductivity detector on the assumption that hydrogen peroxide was all decomposed. According to the principles of thermodynamics, when a liquid mixture is separated into two phases at equilibrium, the compositions of the two phases can be calculated from eqs 1 and 2
xiri
θi )
xiqi
(7)
C
xiqi ∑ i)1 Tij ) exp[-(uij - ujj)/RT]
(8)
(Z2h )(r - q ) - (r - 1)
(9)
li )
i
i
i
Z h ) 10
(10)
(uij - ujj)/R ) bij
(11)
[ ( )]
The NRTL equation of the multicomponent system is given by16 C
ln γi )
(τjiGjixj) ∑ j)1
C
C
+
C
∑
Gkixk
k)1
∑ j)1
xjGij
C
∑
τij -
xkGkj
k)1
∑ xkτkjGkj k)1 C
∑ xkGkj
k)1
(12)
Gij ) exp(-Rijτij) τij )
gij - gjj ) bij/T RT
(13) (14)
All of the liquid-liquid-phase equilibrium data of the five-component system were correlated with both UNIQUAC and NRTL models using the software Aspen Plus. 2.3. Ammoximation of Cyclohexanone. The ammoximation reaction was performed in a 250-mL autoclave with a Teflon vessel and equipped with a mechanical stirrer. In a typical run, 1.5 g of the TS-1 catalyst, 15.2 g of cyclohexanone, 26.5 mL of methanol, 26.5 mL of water, and 22.9 g of ammonia solution were added to the reactor. After the desired temperature was reached, 24 g of H2O2 was continuously added using a feed pump over 4 h. The reaction mixture was stirred for an additional 1 h and finally cooled to room temperature. Then the catalyst was separated from the mixture using a centrifuge. Qualitative and quantitative analyses of the products were performed by GC (HP-5890) with an OV-17 column and a FID detector. Conversion of cyclohexanone and the selectivity to cyclohexanone oxime were calculated as follows: conversion ) 100 - (moles of unreacted cyclohexanone/moles fed) × 100; oxime selectivity ) (moles of oxime/moles of cyclohexanone reacted) × 100. 3. Results and Discussion
(5)
3.1. Phase Equilibrium and Extraction Simulation. The liquid-liquid-phase equilibrium of the fivecomponent system of methanol-water-H2O2-TMBTOP was measured at 40 °C, and the results were correlated with both UNIQUAC and NRTL models. The
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Table 1. UNIQUAC and NRTL Parameters for the Quinary System composition i
composition j
UNIQUAC model: bij
H2O2 TMB H2O2 TOP H2O2 H2O TMB TOP TMB H2O TOP H2O methanol H2O2 methanol H2O methanol TMB methanol TOP
TMB H 2 O2 TOP H 2 O2 H2O H 2 O2 TOP TMB H 2O TMB H 2O TOP H 2 O2 methanol H 2O methanol TMB methanol TOP methanol
-845.1780 -6491.765 -326.9868 -614.2074 -4922.030 -88.9049 245.6158 -5323.895 -832.7327 -134.4000 -648.8970 -62.0726 -86.5270 -49.6676 76.2575 2.1180 -235.9765 -340.4896 -430.0344 -66.6478
NRTL model bij Rij 2231.488 1533.354 3924.416 0.1212 1059.427 236.6289 515.6625 2692.777 864.5823 2147.579 -363.3545 4123.733 322.8638 89.3591 -286.5039 482.7731 1261.454 187.0753 4343.399 -787.2419
0.30 0.30 0.25 0.20 0.20 0.20 0.30 0.30
Figure 2. Influence of the mass feed ratio of anthraquinone working solution to extractant on the concentration of H2O2 in raffinate.
0.30 0.25
Table 2. Deviation between the Calculated and Experimental Values for the Quinary System organic phase
aqueous phase
composition
UNIQUAC (%)
NRTL (%)
UNIQUAC (%)
NRTL (%)
H2O2 methanol TMB TOP H2O
3.3287 3.5671 0.9213 2.4570 3.3150
3.0852 3.1345 0.8525 2.2543 3.2169
2.3963 2.2564 3.1378 0.7139 1.4327
2.3185 1.7726 3.2898 2.0670 1.5342
UNIQUAC and NRTL models’ parameters and the deviation between the calculated and experimental values are reported in Tables 1 and 2, respectively. It was found that the deviation of the calculation was less than 4% by both models. Therefore, the reliability of both models is confirmed. The NRTL equation and the obtained parameters in correlation were used to simulate the extraction of hydrogen peroxide with the software ChemCAD 5.2.1. Because 2-ethylanthraquinone has very low solubility in water, it was assumed that 2-ethylanthraquinone only stays in the anthraquinone working solution. Figure 2 shows the influence of the mass feed ratio of anthraquinone working solution to extractant on the concentration of H2O2 in raffinate. In the simulation, a 60 wt % methanol aqueous solution was chosen as the extractant. It is seen in Figure 2 that the feed ratio greatly influences the concentration of H2O2 in raffinate, but when the feed ratio is less than 30:1 and the theoretical plate number is more than 10, the concentration of H2O2 in raffinate is less than 200 ppm. This corresponds to 2% loss of H2O2, which is permitted in the industrial production of H2O2. Also, in the production of H2O2, before the hydrogenation step, the working solution should be treated with a saturated K2CO3 solution. In this process, all of the H2O2 left in the working solution is decomposed. So, it will not influence the hydrogenation step. A total of 10 theoretical plates may equal to about 40 real plates, which can meet the requirement of H2O2 extraction in the industrial process. With a 30:1 feed ratio, 20.99 wt % H2O2 solutions can be obtained and can meet the requirement of the ammoximation reaction of cyclohexanone, which requires a 9.92 wt % H2O2 solution (see the second paragraph of section 3.2.1).
Figure 3. Influence of the methanol concentration in extractant on the concentration of H2O2 in raffinate.
In the extraction, the concentration of methanol in extractant can also influence the concentration of H2O2 in raffinate. As shown in Figure 3, with an increase of methanol in extractant of up to 50 wt %, the concentration of H2O2 in raffinate increases slowly, but at above 50 wt %, with an increase of methanol in extractant, the concentration of H2O2 in raffinate increases sharply. With 12 theoretical plates and a 30:1 feed ratio, when the concentration of methanol in extractant is less than 60%, the concentration of H2O2 in raffinate can be kept below 200 ppm. Methanol and water can be taken into an anthraquinone working solution in extraction and may influence the hydrogenation of anthraquinone. As shown in Figure 4, with a 60 wt % methanol solution extracting a typical anthraquinone working solution, which after the anthrahydroquinone oxidation step has the compositions 0.93 wt % H2O2, 67.63 wt % TMB, 22.68 wt % TOP, and 8.76 wt % 2-ethylanthraquinone, as the anthraquinone working solution recycles in the process, the concentrations of methanol and water first increase and then become constant. After five cycles, the concentrations of methanol and water in an anthraquinone working solution are 3.81 and 0.48 wt %, respectively. In the industrial H2O2 production process, after extraction there is a drying step, using a near-saturated K2CO3 solution to remove the water in the anthraquinone working solution. The 0.48 wt % water is below that permitted in the industrial H2O2 production process, so it would not influence the H2O2 production process. Our previous results showed that the concentration of
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Figure 4. Change of methanol and water concentrations in the anthraquinone working solution with recycle numbers.
Figure 6. Influence of the reaction temperatures on the ammoximation: 1.5 g of TS-1, 15.2 g of cyclohexanone, 24 g of 28% H2O2, 22.9 g of a 25% ammonia solution, 26.5 cm3 of CH3OH, and 26.5 cm3 of H2O. Cyclohexanone:H2O2:NH3 ) 1:1.27:2.17 (mole ratio). Reaction temperature ) 80 °C.
Figure 5. Influence of the methanol concentration in extractant on the concentrations of TMB and TOP in the extract.
methanol below 10 wt % did not affect the hydrogenation of anthraquinone in an anthraquinone working solution.17,18 Therefore, an aqueous methanol solution can be used as the extractant in the H2O2 production process based on anthraquinone hydrogenation-oxidation cycles. Next to be determined are the amounts of TMB and TOP turned up in the H2O2 solution obtained using an aqueous methanol solution as the extractant and whether these compounds would influence the ammoximation of cyclohexanone. Figure 5 shows the changes of the concentrations of TMB and TOP in the H2O2 solution with the concentration of methanol in extractant. With a 30:1 mass feed ratio of anthraquinone working solution to extractant, 12 theoretical plates, and a 60 wt % methanol aqueous solution as the extractant, the concentrations of TMB and TOP are 0.63 and 0.04 wt % in the H2O2 solution, respectively. At these conditions, the concentration of H2O2 in the extract is 20.99 wt %. The effects of these compounds on the ammoximation of cyclohexanone are discussed in the next section. 3.2. Ammoximation of Cyclohexanone. 3.2.1. Ammoximation of Cyclohexanone with Aqueous Methanol as the Solvent. Figure 6 shows the cyclohexanone ammoximation results at different temperatures. It is seen that, with an increase of reaction temperatures, the conversions of cyclohexanone increase rapidly; at 80 °C it increases up to 99.6%, while the selectivity to cyclohexanone oxime is above 99.9% at reaction temperatures from 70 to 90 °C. These results agree with those of Petrini et al.2 So, in the following studies, all of the reactions were conducted at 80 °C.
Figure 7. Influence of the methanol contents after the feeding of H2O2 in the reaction solution on the ammoximation reaction: 1.5 g of TS-1, 15.2 g of cyclohexanone, 24 g of 28% H2O2, and 22.9 g of a 25% ammonia solution. Cyclohexanone:H2O2:NH3 ) 1:1.27: 2.17 (mole ratio). Reaction temperature ) 80 °C.
The influence of the concentration of methanol in the reaction solution (including methanol, H2O, and H2O2), which is calculated after the feeding of H2O2 on the ammoximation reaction, is presented in Figure 7. The concentration of methanol in the reaction solution did not have an appreciable effect on the selectivity, but it greatly affected the conversion. As the methanol concentration in the reaction solution increased above 27.07 wt %, the conversion decreased rapidly. Therefore, it is believed that 27.07 wt % is the upper limit of the methanol content in the reaction solution, and at this time, the content of H2O2 in the reaction solution is 9.92 wt %. When 60 wt % methanol was used for the extraction of H2O2, the H2O2 solution obtained contains 20.99 wt % H2O2, 47.05 wt % methanol, 31.29 wt % H2O, 0.63 wt % TMB, and 0.04 wt % TOP. If the content of H2O2 in the solution is diluted to 9.92 wt %, the concentration of methanol is 22.24 wt %, which can meet the requirement of the ammoximation reaction. On the one hand, H2O2 can be extracted without influencing the H2O2 production process. On the other hand, the methanol concentration is enough to keep the ammoximation reaction homogeneous, making it easy for the products to be further processed. According to our experiments,
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Figure 8. Influence of the TMB contents on the ammoximation reaction: 1.5 g of TS-1, 15.2 g of cyclohexanone, 24 g of 28% H2O2, 22.9 g of a 25% ammonia solution, 26.5 cm3 of CH3OH, and 26.5 cm3 of H2O. Cyclohexanone:H2O2:NH3 ) 1:1.27:2.17 (mole ratio). Reaction temperature ) 80 °C.
Figure 9. Influence of the TOP contents on the ammoximation reaction. Conditions are as shown in Figure 8.
the minimum concentration of methanol required to keep the reaction solution homogeneous is 15.0 wt %. 3.2.2. Influence of Impurities in a H2O2 Solution on the Ammoximation Reaction. If an aqueous methanol solution were used to extract H2O2 and the obtained H2O2 solution were directly used in the ammoximation of cyclohexanone, some compounds such as TMB and TOP would be brought from the anthraquinone working solution into the ammoximation reaction. Therefore, the influences of TMB and TOP on the ammoximation of cyclohexanone were studied. The results are presented in Figures 8 and 9, respectively. Figure 8 shows that TMB does not appreciably influence either the conversion of cyclohexanone or the selectivity to cyclohexanone oxime. At 1 wt % TMB, the conversion and the selectivity are both 99.6%. Figure 9 shows that TOP, up to 1.7 wt %, does not influence the ammoximation of cyclohexanone either. In the simulation of extraction, it was found that, with a 30:1 mass feed ratio of anthraquinone working solution to extractant and a methanol content in the methanol solution of 60%, the levels of TMB and TOP in the H2O2 solution are only 0.63 and 0.04 wt %, respectively (see section 3.2.1). At this condition, the concentration of H2O2 in the extract is 20.99 wt %, which is higher than that used in the ammoximation, 9.92 wt %. The extract may need be diluted by water before it comes into the reaction system. So, in fact the concentrations of TMB and TOP
Figure 10. Influence of the anthraquinone working solution contents on the ammoximation reaction. Conditions are as shown in Figure 8.
are below 0.63 and 0.04 wt %, respectively. Of course, the extra methanol and water used in the dilution can be recycled back in the ammoximation reaction after separation. Therefore, the above results demonstrate that TMB and TOP would not have any harmful effects on the ammoximation reaction. In the industrial H2O2 production process, anthraquinone partially degrades into its derivatives. Anthraquinone and its derivatives may also be brought into the ammoximation reaction. However, these derivatives are very complex. It is difficult to find one that has the representative properties of all. To study the effects of these compounds, the industrial anthraquinone working solutions were added in the ammoximation reaction. The results in Figure 10 show that, with an increase of the industrial anthraquinone working solution, the conversion of cyclohexanone is not influenced but the selectivity to cyclohexanone oxime decreases gradually. However, at 1 wt % industrial anthraquinone working solution, the selectivity to oxime is still above 99.2%. The industrial anthraquinone working solution consists of anthraquinone, its derivatives, TMB, and TOP, and the concentrations of anthraquinone and its derivatives are maintained at about 120 and 30 g/L, respectively. If it is assumed that the mass of a 1-L working solution is equal to 1000 g, the mass ratio of anthraquinone and its derivatives to TMB plus TOP is 15:85. According to the simulation results (see section 3.1), the total concentrations of TMB and TOP in the extract are 0.63 and 0.04 wt %, respectively. In the extraction process, small amounts of anthraquinone and its derivatives may also be extracted into the H2O2 aqueous solution along with TMB and TOP. On the assumption that the mass ratio of anthraquinone and its derivatives to TMB plus TOP in an aqueous solution is also equal to the one in the working solution, the total concentration of anthraquinone and derivatives is (0.63% + 0.04%) × 15/85 ) 0.12 wt %. So, the total concentration of the anthraquinone working solution in the H2O2 solution would be about 0.63% + 0.04% + 0.12%, i.e., 0.79 wt %. Therefore, it can be concluded that anthraquinone and its derivatives have no appreciable negative effects on the ammoximation of cyclohexanone. 3.2.3. Influence of H2O2 Additives on the Ammoximation Reaction. H2O2 itself is not stable. It tends to decompose. So, in the industrial H2O2 production process, a stabilizer must be added in order to store it for longer time before use. On the other hand, H2O2
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of cyclohexanone and the selectivity to cyclohexanone oxime. Therefore, the processes of the ammoximation of cyclohexanone and hydrogen peroxide generation through anthraquinone hydrogenation-oxidation cycles can be integrated. This integration process can save the several steps of hydrogen peroxide purification as used in the conventional process. The production cost of cyclohexanone oxime, and further that of nylon-6, can be greatly reduced. Acknowledgment The present work has been supported by a State Key Fundamental Research Project of China (Grant G2000048005). Figure 11. Influence of the phosphoric acid contents on the ammoximation reaction. Conditions are as shown in Figure 8.
Nomenclature zi ) number of moles of component i C ) number of components xi ) equilibrium mole fraction of component i T ) absolute temperature, K qi ) relative surface area per molecule in the UNIQUAC model ri ) number of segments per molecule in the UNIQUAC model Tij ) binary interaction parameter in the UNIQUAC model u ) interaction energy in the UNIQUAC model Z h ) lattice coordination number in the UNIQUAC model, set equal to 10 g ) interaction energy within a pair of molecules in the NRTL model G ) binary interaction parameter in the NRTL model bij ) adjustable parameter
Figure 12. Influence of the sodium nitrate contents on the ammoximation reaction. Conditions are as shown in Figure 8.
itself is capable of causing corrosion of the metal equipment. Moreover, the stabilizers, such as the common one, phosphoric acid, are strong complexants of transition-metal cations including Fe(III) and also cause corrosion. So, in the industrial process, certain amounts of corrosion inhibitors, such as sodium nitrate, should also be added to the reaction system. In the present work, the effects of a typical stabilizer, phosphoric acid, and a typical corrosion inhibitor, sodium nitrate, on the ammoximation reaction were studied. The results are presented in Figures 11 and 12, respectively. In hydrogen peroxide production, the stabilizers and corrosion inhibitors are both added in less than 0.005 wt %. Figure 11 shows that, with an increase of the concentrations of phosphoric acid, the conversion of cyclohexanone does not change but the selectivity to cyclohexanone oxime decreases slightly. However, at 0.005 wt % phosphoric acid, the selectivity is almost not changed. Figure 12 shows that sodium nitrate does not have any influence on the ammoximation reaction at all. So, both phosphoric acid and sodium nitrate would not affect the ammoximation of cyclohexanone. 4. Conclusions The liquid-liquid-phase equilibrium measurements and H2O2 extraction simulation results indicate that an aqueous methanol solution can be used for the extraction of H2O2 without influencing the H2O2 production process. The H2O2 solution obtained can be used in the ammoximation of cyclohexanone with NH3 and H2O2 catalyzed by TS-1 without influencing the conversion
Greek Letters γ ) activity coefficient Ψ ) segment fraction in the UNIQUAC model θ ) area fraction in the UNIQUAC model τ ) adjustable parameter in the NRTL model R ) adjustable parameter in the NRTL model Superscripts C ) combinatorial part of the activity coefficient R ) residual part of the activity coefficient Subscripts i ) counter for compounds j ) counter for compounds
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Received for review July 10, 2003 Revised manuscript received October 30, 2003 Accepted November 6, 2003 IE030578S
