November 2021

Sustainability

H2O2 and its hydrocarbon nitridation/oxidation to produce caprolactam and propene oxide

Traditional hydrocarbon nitridation and oxidation technologies, such as caprolactam (CPL) and propene oxide production technologies, present low atom utilization and serious environmental pollution problems.

Traditional hydrocarbon nitridation and oxidation technologies, such as caprolactam (CPL) and propene oxide production technologies, present low atom utilization and serious environmental pollution problems. An urgent need exists for green hydrocarbon oxidation and nitridation technologies. As a well-known green oxidant, hydrogen peroxide (H2O2) is widely used in green hydrocarbon oxidation and nitridation, with water as the only byproduct.

However, the low capacity of one single fixed-bed unit of anthraquinone hydrogenation for H2O2 production—which is widely adopted in China—has severely restricted the supply of H2O2, further hampering the development of the green chemical industry in China. The authors’ company endeavors to develop slurry bed technology of H2O2 production to promote production capacity, reduce production costs and environmental pollution. Furthermore, green production technologies of CPL and propene oxide with H2O2 have also been developed. The nitrogen atom utilization was enhanced from 60% to nearly 100%, and the carbon atom utilization was also promoted from 80% to nearly 100%.

Basic organic chemicals, organic intermediates and fine chemicals usually contain nitrogen or oxygen atoms, and their production involves hydrocarbon nitridation or oxidation reactions. Traditional nitridation or oxidation reactions have poor atom utilization, and cause serious pollution due to the unsatisfactory oxidants that are used, such as dichromate, permanganate, hypochlorite and nitric acid. Ammonia (NH3) is the fundamental source of nitrogen atom in hydrocarbon nitridation reactions, but it must experience the oxidation process to be converted into nitric acid, hydroxylamine, azide or highly toxic cyanide to participant in traditional hydrocarbon nitridation reactions. These complicated processes bring huge energy consumption and pollutants emissions. For example, more than 300 kt of nitrogen oxides (NOx) are emitted in the NH3 oxidation process used in nitric acid production each year.1 Therefore, the selection of oxidant or active N-containing agent is central to promoting nitrogen or carbon atom utilization, and eliminate the generation of pollutants.

As an environment-friendly oxidant, H2O2 is widely used in the chemical industry, bleaching processes, wastewater treatment, exhaust air treatment and for various disinfection applications. Considering the transportation risks and costs of H2O2, the factory must build an H2O2 production unit to support the operation of the green chemical production unit. At least two H2O2 production units adopting fixed-bed technology should be constructed to support the normal operation of a 300 kt·a–1 propene oxide green production unit, significantly increasing construction and operating costs. Conversely, green hydrocarbon nitridation or oxidation technologies with H2O2 as the oxidant are developing rapidly around the world. China should also develop green chemical technologies with independent intellectual property rights to solve environmental pollution problems in its self-development process.

The authors’ company has been working for more than 20 yr to develop the slurry bed technology for H2O2 production with completely independent intellectual property rights, supporting China’s green chemical industry development.

Slurry bed technology of H2O2 production

The industrial production of H2O2 widely adopts the anthraquinone hydrogenation method due to its advantages in industrial efficiency, environmental protection and economic benefits. Its production process includes anthraquinone hydrogenation, hydrogenated anthraquinone oxidation, H2O2 extraction and anthraquinone working liquid purification. Two anthraquinone hydrogenation technologies are now in use: fixed-bed technology and slurry bed technology. The industrial production of H2O2 is also done with these two technologies.

Compared with the slurry bed technology for H2O2 production, the fixed-bed technology is easy to implement and the catalyst does not need to be separated. It also has the following drawbacks:

  1. Heat transfer performance is poor. As the anthraquinone hydrogenation reaction is an exothermic reaction, local hot spots or flying temperatures occur in the fixed bed. The working liquid degrades easily in the high-temperature area, resulting in poor selection performance and poor product quality of the reaction, which causes subsequent processing problems and limits the capacity of the production unit.
  2. Catalysts with a fine particle size cannot be used, so the inner surface with active sites is not fully utilized, resulting in low catalyst utilization efficiency. More importantly, the hydrogenation reaction is limited by heat and mass transfer.

To avoid excessive hydrogenation of the working liquid, the hydrogenation degree and hydrogen efficiency of anthraquinone are generally controlled within a reasonable range in the actual H2O2 production with fixed-bed technology. Taking the current typical fixed-bed process as an example, if 120 g·L–1 of effective anthraquinone is completely hydrogenated, the theoretical hydrogen efficiency can reach 17 g·L–1. However, due to the above drawbacks, it is difficult to increase the actual hydrogen efficiency to more than 12 g·L–1 in a factory.

To minimize the degradation of working liquid, the hydrogen efficiency is often controlled at 5 g·L–1–8 g·L–1, and the hydrogenation degree of anthraquinone is controlled at 40%–50%. Thereby, energy consumption and production costs increase. Limited by technical factors, China’s H2O2 production has been using fixed-bed technology for many years, and the production capacity of a single unit with fixed-bed technology has never exceeded 50 kt·a–1.

The slurry bed technology for H2O2 production shows good performance on heat and mass transfer, and the production capacity of a single unit with slurry bed technology always exceeds 100 kt·a–1. Compared with the fixed-bed technology of H2O2 production, the industrial implementation of slurry bed technology is relatively difficult. Its technical difficulty lies in the slurry bed reactor, high-strength microsphere catalyst and solid-liquid separation system. Several chemical companies (e.g., DuPont, Solvay, Degussa, BASF) have developed slurry bed technologies of H2O2 production, each with their own characteristics.

The schematic of the authors’ company’s slurry bed technology for H2O2 production is shown in FIG. 1. The technology highlights are reflected in the following aspects:

FIG. 1. Slurry bed technology for H2O2 production.
  1. It is a reaction-filtration system with a simple structure and outstanding performance in heat and mass transfers. The slurry bed reactor is equipped with a separator on the upper section that can avoid the air-resistor in the conveying pipeline and prevent the gas from entering the filter to ensure the efficient and stable operation of the filter. The slurry-containing solid catalyst particles at the bottom of the separator flow into the filter, and then solid catalyst circulates back to the bottom of the slurry bed reactor.2,3 The slurry bed reactor lowers circulating energy consumption, and the industrial amplification is easily achieved with the advantages of high integration, simple structure and small footprint.
  2. As the core of slurry bed technology for H2O2 production, a hydrogenation catalyst with high strength and selectivity is required. The microsphere catalyst of Pd/Al2O3, developed independently by the authors’ company, exhibits high wear resistance, high selectivity and high activity with the synergistic effect of non-noble metals.4 Industrial tests show the hydrogenation efficiency of the microsphere catalyst reaches 12 g·L–1–13 g·L–1, and no obvious change of the catalyst has been observed after the industrial test.
  3. The proprietary oxygen-enriched cyclic oxidation technology improves the environmental protection, economic benefits and safety of the H2O2 production unit.5 Through a compressor-assisted cycle of the oxidation tail gas, the tail gas emissions in the oxidation process are eliminated without the need for a solvent recovery device. To ensure constant oxygen supply, an oxygen-rich gas is continuously added into the recycle gas according to the consumption of oxygen. At the same time, the water brought into the oxidation tower is reduced, and the amount of residual liquid at the bottom of the oxidation tower is significantly reduced. This technology makes the H2O2 production greener.
  4. The catalytic regeneration technology for the working liquid effectively converts anthrone into anthraquinone.6,7 Compared with traditional regeneration technologies, the conversion rate of the catalytic regeneration technology is increased by 10 times. Moreover, the fully acidic environment not only improves the intrinsic safety of the H2O2 production unit, but also avoids the generation of basic alumina solid waste.

A comparison between the authors’ company’s slurry bed technology for H2O2 production and a fixed-bed technology for H2O2 production is shown in TABLE 1. Compared with the fixed-bed technology, the single-unit capacity of the slurry bed technology for H2O2 production is increased by 140%, hydrogen (H2) consumption per ton of product decreased by 5%, the energy consumption and material consumption can be reduced by ~20%, wastewater and solid waste are reduced by more than 70%, and no tail gas emissions are produced. The slurry bed technology has obvious advantages in economy, greenness, intrinsic safety and productivity.

Green production technology for CPL

As the monomer for nylon-6 fiber and engineering plastics, CPL is an important basic organic chemical that is widely used in textile, automobile, electronics and other industries. More than 90% of CPL worldwide is manufactured by the rearrangement of cyclohexanone oxime. This route mainly includes hydrogenation of benzene to cyclohexane, oxidation of cyclohexane to cyclohexanone, ammoximation of cyclohexanone to cyclohexanone oxime, rearrangement of cyclohexanone oxime to CPL, and the subsequent multi-step refining processes.

Traditional CPL production technology drawbacks include:

  1. Cyclohexanone oxime is synthesized by cyclohexanone hydroxylamine oxidation, which involves the oxidation of ammonia to NOx, the absorption and reduction of NOx to hydroxylamine, and the reaction of hydroxylamine and cyclohexanone to produce cyclohexanone oxime. This process is complex with only 60% utilization of ammonia. During the whole process, noble metal catalyst is consumed, and highly toxic NOx is produced.
  2. Cyclohexanone oxime to CPL adopts the liquid-phase Beckmann rearrangement technology. Fuming sulfuric acid is used as the catalyst, resulting in serious equipment corrosion and a large production of low-valued ammonium sulfate.
  3. Unstable Raney nickel is used in the CPL purification process. This hydrogenation process is complex with a high catalyst consumption and low hydrogenation efficiency.

In view of the above deficiencies of traditional CPL production technology, the green CPL production technology developed by the authors’ company provides solutions:

  1. A direct cyclohexanone ammoximation technology with cyclohexanone, H2O2 and NH3, in which hollow TS-1 zeolite is used as the catalyst and H2O is the only byproduct. This technology integrates micro-sized hollow TS-1 zeolite with a slurry bed reactor fitted with a membrane separation component. A good mass transfer performance of the reaction system has been developed. The micro-sized hollow TS-1 zeolite is prepared by hydrothermal synthesis with secondary structural modification technology, and its stability is improved by adding silicon-containing additives.8,9,10 In this direct cyclohexanone ammoximation technology, the cyclohexanone conversion is more than 99.9%, and the cyclohexanone oxime selectivity is more than 99.5%. The new technology markedly simplifies the cyclohexanone ammoximation process, improves the utilization of NH3 from 60% to > 90%, decreases the plant investment by > 70%, reduces 99.5% of exhaust emissions, and eliminates the production or use of corrosive NOx. As a result, the production cost of cyclohexanone oxime is reduced by 800 CNY·t−1.
  2. A gas-phase Beckmann rearrangement of cyclohexanone oxime to CPL by integrating silicalite-1 zeolite with a moving-bed reactor.11,12 The use of fuming sulfuric acid is avoided, and no ammonium sulfate is produced. As a result, there is no equipment corrosion nor pollutant emissions. The cyclohexanone oxime conversion is higher than 99.9%, and the CPL selectivity is around 96.5%. The nitrogen atom utilization is increased from 36% to near 100%. The production cost can potentially be reduced by 1,000 CNY·t−1.
  3. Integration of amorphous nickel with a magnetically stabilized bed. The large-radius, rare-earth atoms are introduced into amorphous nickel to improve the catalyst thermal stability, and the surface area is improved by the method of “adding-leaching aluminum.”13 Magnetic retainer internals are developed to ensure the uniformity of the magnetic field in the magnetically stabilized bed.14 As a result, unstable Raney nickel catalyst is not used, and the operation cost of the CPL production unit is reduced significantly.

The first industrial plant based on the authors’ company’s CPL green production technology with a capacity of 70 kt·a–1 was built in 2003. FIG. 2 shows a 400-kt·a–1 CPL production plant with green production technology. Compared with traditional CPL production technology, the green CPL production technology reduces the exhaust gas by 95%, no low-valued ammonium sulfate is produced, and the overall investment can drop by 70%. A CPL production unit with a capacity of 50 kt·a–1 can reduce 2.4 × 108 m3 of waste gas emissions and 80 kt of low-valued ammonium sulfate production every year.

FIG. 2. A 400-kt·a–1 CPL industrial production plant. Source: Sinopec.

The proprietary green CPL production technology has strongly supported the technological upgrading of CPL production in China. In 2020, China’s CPL capacity based on this green production technology reached 4,000 kt·a–1, giving China a global market share that exceeds 50% and making it the world’s largest CPL producer—a huge leap from China’s original position of relying almost totally on CPL importation. The green CPL production technology also sets a successful example of green hydrocarbon nitridation.

Green production technology of propene oxide

As the third-largest propene derivative, propene oxide is widely used in unsaturated resins, surfactants and polyurethanes. In 2020, the actual output of propene oxide in China was 2.9 × 106 t, compared to a global output of approximately 1.05 × 107 t.

Traditional industrial production processes of propene oxide include the chlorohydrin process and co-oxidation process. Hypochlorous acid is used as oxidant in the chlorohydrin process, resulting in serious equipment corrosion and environmental pollution. When producing 1 t of propene oxide, the chlorohydrin process consumes 1.35 t–1.85 t of chlorine gas, and produces 40 t–80 t of chlorine-containing wastewater and more than 2 t of calcium chloride (CaCl2). Though the construction of new plants for producing propene oxide with the chlorohydrin process has been strictly controlled since 2011, the chlorohydrin process still accounts for more than 50% in the production of propene oxide in China.

To overcome the disadvantages of the chlorohydrin process, the co-oxidation process was developed. Though the co-oxidation process solves the equipment corrosion and environmental pollution problems, the co-oxidation process with ethylbenzene or isobutane as a co-reductant is complicated, and requires harsh reaction conditions that necessitate high-quality equipment material. This results in high equipment costs. Additionally, the quality of propene must be high, and the economic benefit is restricted by the co-products, as 2.2 t–2.5 t of styrene or 2.3 t of tert-butanol is produced when producing 1 t of propene oxide. The co-oxidation route with cumene as co-reductant does not produce co-product, but it does consume significant energy due to the separation and conversion of intermediate products. Additionally, it requires the construction of a large-scale oxidation unit of cumene.

Technologies for propene oxide production by direct epoxidation of propene have been developed in recent years. Among them, the direct epoxidation technology with H2O2 as the oxidant and TS-1 zeolite as the catalyst (HPPO process) is the most mature, and it has been industrialized. Compared with the chlorohydrin method, the C atom utilization of the HPPO process is close to 100%, with reductions in equipment investment (25%), wastewater discharge (70%–80%) and energy consumption (> 35%). The authors’ company began research on its HPPO process in 2003, and put it into industrial demonstration on a 100-kt·a–1 unit, which is shown in FIG. 3. Process highlights include:

FIG. 3. A 100-kt·a–1 propene oxide industrial production unit. Source: Sinopec.
  1. A hollow TS-1 zeolite with Si enriched on the surface.15,16 The company found that the acidity of catalyst is the main factor that accelerates the solvolysis of propene oxide, and the acidity comes from Ti active centers, trace Al, defects of surface and lattice in TS-1 zeolite. Therefore, the synthesis process of the hollow TS-1 zeolite was modified to enrich Si on the surface of hollow TS-1 zeolite—which is different from the hollow TS-1 zeolite used in the cyclohexanone ammoximation reaction—increasing the selectivity of propene oxide to higher than 95%.
  2. Amorphous SiO2 and other additives are added into the hollow TS-1 catalyst to increase the crushing strength of the catalyst to higher than 120 N·cm–1 without decreasing the catalyst activity and fixed-bed reactor utilization.17,18,19 The conversion of H2O2 could reach higher than 96% with 95% selectivity of propene oxide.
  3. The in-situ catalyst regeneration technology of deactivated catalyst with solvent extraction was developed by the authors’ company.17 The life span of regenerated catalyst is equivalent to fresh catalyst, and the catalyst activity is well restored.
  4. A two-stage, fixed-bed reactor in series was adopted.17 This technology not only solves problems caused by the strong exothermic effect of the HPPO process, but also realizes the continuous production of propene oxide combined with the in-situ catalyst regeneration technology.

The company has successively developed the synthesis technology of hollow TS-1 zeolite with Si enriched on the surface, the preparation technology of high-strength hollow TS-1 catalyst with high-selectivity, the in-situ regeneration technology of deactivated catalyst, the design and manufacturing technology of large-scale tubular reactor, and the safety control technology of the whole process. The 1 kt·a–1 pilot-scale test shows that the HPPO process can convert 96%–99% H2O2 with a 96%–98% selectivity of propene oxide. After the catalyst runs for 6,000 hr, no significant change in activity was detected.17 In 2020, the technology package of the HPPO process with a capacity of 300 kt·a–1 passed the technical appraisal.

Takeaway

The authors’ company has developed full-process green chemical technologies of CPL and propene oxide production, including an important supporting technology: the slurry bed technology for H2O2 production, which provides a stable and reliable source of H2O2 for the development and industrial implementation of green chemical technologies. A 120-kt·a–1 slurry bed production unit of H2O2 is sufficient to support the effective operation of a 300-kt·a–1 production unit of propene oxide.

The slurry bed technology for H2O2 production and green hydrocarbon nitridation/oxidation technologies with H2O2 to produce CPL and propene oxide developed by the authors’ company have provided the whole-process technical support for two chemical production bases: Gulei and Baling. The Baling offsite construction project was regarded as a benchmark by the Chinese government for the relocation of hazardous chemical production plants along the Yangtze River. The production technologies for H2O2, CPL and propene oxide have greatly promoted the development of China’s green chemical industry. HP

ACKNOWLEDGMENTS

The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (U19B6002) and National Key R&D Program of China (2016YFB0301600).

LITERATURE CITED

  1. Isupova, L. A. and Y. A. Ivanova, “Removal of nitrous oxide in nitric acid production,” Kinetics and Catalysis, February 2019.
  2. Li, H., K. Yang, G. Gao, et al., “Method for producing hydrogen peroxide,” China patent: CN104418309B, September 2013.
  3. Li, H., K. Yang, G. Gao, et al., “Slurry bed reactor and its application method,” China patent: CN104415716B, September 2013.
  4. Zheng, S., Z. Pan, X. Meng, et al., “Palladium-based hydrogenation catalyst and application in anthraquinone hydrogenation,” China patent: CN104549246B, October 2013.
  5. Gao, G., H. Li, K. Yang, et al., “Anthraquinone oxidation method for producing hydrogen peroxide,” China patent: CN105271131B, July 2014.
  6. Wang, W., Z. Pan, B. Zheng, et al., “Regeneration method of circulating operating fluid in production process of hydrogen peroxide by anthraquinone method and method for producing hydrogen peroxide,” China patent: CN106542502B, September 2015.
  7. Wang, W., Z. Pan, B. Zheng, et al., “Catalyst preparation method and its application, regeneration method of working solution in hydrogen peroxide production by anthraquinone process and production method of hydrogen peroxide,” China patent: CN106540685B, September 2015.
  8. Lin, M., X. Shu, X. Wang, et al., “Titanium-silicalite molecular sieve and the method for its preparation,” U.S. patent: US6475465B2, December 2000.
  9. Wu, W., B. Sun, Y. Li, et al., “Process for ammoximation of carbonyl compounds,” U.S. patent: US7408080B2, May 2003.
  10. Sun, B., W. Wu, E. Wang, et al., “Process for regenerating titanium-containing catalysts,” U.S. patent: US7384882B2, May 2003.
  11. Cheng, S., S. Zhang, X. Mu, et al., “Method for preparing catalyst containing molecular sieve of MFI structure,” China patent: CN101468319B, December 2007.
  12. Cheng, S., S. Zhang, L. Xie, et al., “A gas-phase Beckmann rearrangement process of cyclohexanone oxime for preparing caprolactam,” China patent: CN103896839B, December 2012.
  13. Pan, Z., M. Dong, X. Zhang, et al., “Process for preparing modified amorphous nickel alloy catalyst,” China patent: CN101199934B, December 2006.
  14. Meng, X., X. Mu, B. Zong, et al., “Process for refining aqueous caprolactam solution by hydrogenation,” China patent: CN1249031C, May 2003.
  15. Zhu, B., C. Xia, M. Lin, et al., “A micro-meso porous TS-1 zeolite and its synthesis method,” China patent: CN104556112B, October 2013.
  16. Lin, M., C. Shi, J. Long, et al., “Noble metal-containing titanosilicate material and its preparation method,” U.S. patent: US8349756B2, March 2008.
  17. Lin, M., H. Li, W. Wang, et al., “The preparation of propylene oxide by propylene epoxidation with hydrogen peroxide in 1.0 kt/a pilot plant,” Petroleum Processing and Petrochemicals, 2013.

The Authors

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