Synthesis Of Hydrogen Peroxide From H₂/O₂ Plasma With Multiple Parallel Discharge Tubes And Its Application In Propene Epoxidation

Hydrogen peroxide (H₂O₂), as a green oxidant, is widely used in pulp bleaching, wastewater treatment and chemical synthesis. Now almost all H₂O₂ is exclusively produced by anthraquinone oxidation process, which is economically viable only on a large scale due to the complex operations and huge investment. But to most of the customers, only relatively small amounts of H₂O₂ are required at any one time, so the transportation and storage of H₂O₂ accounts for a big part of the H₂O₂ cost. Moreover, in the last decade, propene epoxidation process with H₂O₂ catalyzed by TS-1 has been intensively studied as one of the most promising alternatives to traditional technologies, but its commercialization is strongly hindered by the relatively high cost of H₂O₂. Therefore, many studies focus on the development of a green, economical, and smaller-scale technology for H₂O₂ manufacturing. One promising route to generate H₂O₂ is the direct synthesis from hydrogen and oxygen in the presence of Pd/Au supported catalyst. However, this process suffers from the decomposition of H₂O₂ which is catalyzed by the same noble metal catalysts for H₂O₂ synthesis and mass transfer problem in such a three-phase system. So this technology has not been able to meet the requirements of the market yet even though the related studies last for nearly one hundred years.The direct synthesis of H₂O₂ via non-equilibrium plasma, which is characterized by simple operation process, high concentration and ultra high purity of the product, provides a promising alternative route for H₂O₂ production. However, as a general problem of plasma chemistry technology, the relatively low energy efficiency severely limits its commercialization. In view of industrial application, it is necessary to develop a suitable scale-up route and design highly efficient scale-up reactor. Moreover, the direct synthesis of H₂O₂ by plasma technology can supply on-site oxidant to propene epoxidation process, thus the storage and transportation costs of H₂O₂ can be reduced effectively.To solve the above problems, based on our previous works, the laboratory studies on scale-up synthesis of H₂O₂ via plasma technology and integration process of H₂O₂ synthesis with propene epoxidation have been carried out. The reaction characteristics and energy efficiency in the scale-up process were discussed, and the main factors which dominated the energy efficiency were determined. Also some strategies were proposed to optimize the impendence match between the power supply and reactor load. The main results were obtained in this dissertation as follows:1. The scale-up synthesis of H₂O₂ from H₂/O₂ via a dielectric barrier discharge at ambient conditions was studied by using a reactor consisting of multiple parallel discharge tubes. Varying the number of tubes had no significant effect on discharge mode and reaction mechanism. H₂O₂ selectivity kept at around 64 %, and no decay occurred during the scale-up process. With the input power of 4.9-5.0 W, residence time of 18 s, discharge frequency of 14 kHz, the H₂O₂ productivity increased from 7.1 mmol/h to 20.1 mmol/h during the scale-up process, and the reactor energy efficiency was improved from 50 gH₂O₂/kWh to 136 gH₂O₂/kWh. The total energy efficiency was limited by the extremely low energy transfer efficiency of power supply, and might be enhanced by optimizing the impedance match between the power supply and reactor load.2. The discharge gap of reactor, material of high-voltage electrode, length of discharge zone and discharge frequency have significant effects on reaction characteristics and energy efficiency. The reactor energy efficiency, O₂ conversion and H₂O₂ yield were enhanced by using the reactor with narrow discharge gap and metal high-voltage electrode. The increase of discharge zone length and discharge frequency at a certain extent also favored the improvement of O₂ conversion, H₂O₂ selectivity and reactor energy efficiency. By using the metal high-voltage electrode reactor which had the discharge gap of 1.0 mm and discharge zone length of 30 cm, with the reactant flow rate of 420 ml/min, O₂ content of 4.8 vol %, discharge frequency of 16 kHz and input power of 2.9 W, the energy efficiency as high as 151 gH₂O₂/kWh was obtained. For the application of said reactor in scale-up setup with three parallel tubes, with the total flow rate of material gas of 630 ml/min and discharge frequency of 14 kHz, the reactor energy efficiency of 165 gH₂O₂/kWh and total energy efficiency of 8.7 gH₂O₂/kWh have been achieved.3. In this discharge circuit, resonance was formed by the transformer leak inductance (L) of power supply and equivalent capacitance (C) of reactor. When the power supply worked at resonance frequency (fR), the optimal impedance match between power supply and reactor load could be obtained. On the other hand, using the reactor with metal high-voltage electrode and increasing discharge frequency could enhance the reactor energy efficiency remarkably. So considering the above factors, L value should be reduced at a certain extent to obtain a relatively high fR for developing a suitable power supply, and then set the working frequency of the power supply at fR, ultimately the optimal impedance match can be obtained at a high frequency. In such an optimized discharge system, the reactor energy efficiency can be effectively improved together with the energy transfer efficiency, consequently the total energy efficiency for H₂O₂ synthesis will be enhanced significantly. 4. The direct synthesis of H₂O₂ via plasma method can safely and simply provide selective oxidation reactions with high-purity H₂O₂ oxidant. The integration of on-site H₂O₂ synthesized by plasma route and liquid-phase propene epoxidation catalyzed by TS-1 catalyst was successfully actualized. At ambient conditions, with the input power of 3.5 W, residence time of 18 s, methanol compensating rate of 13.2 ml/h, the H₂O₂ oxidant solution with the flow rate of 12 ml/h and concentration of 0.70 mol/L was prepared. Set the molar ratio of propene/H₂O₂ in feedstock at 4.2, reaction temperature at 50℃, system pressure at 3.0 MPa, WHSV at 3.7 h^-1, this setup worked smoothly during a period of 18 h. H₂O₂ selectivity and utilization efficiency varied in the range of 92-94 % and 72-77 % respectively, as well as propene oxide selectivity and yield maintained in the range of 94-95 % and 63-68 % respectively.