Article pubs.acs.org/IECR
Hydrogen Peroxide Generation in Low Power Pulsed Water Spray Plasma Reactors Robert J. Wandell and Bruce R. Locke* Department of Chemical and Biomedical Engineering, FAMU-FSU College of Engineering, Florida State University, Tallahassee, Florida 32310-6046, United States ABSTRACT: The energy yield for hydrogen peroxide formation from pure water sprayed into a low power, pulsed, gliding arc plasma reactor under various experimental conditions was determined. The water flow rate, carrier gas flow rate, electrode shape, and pulse frequency were varied, all of which affect the overall power of the discharge. Results show that the energy yield for hydrogen peroxide formation increased as the mean discharge power decreased, and the trend was independent of the variables that affected the power when a pulsed power supply was utilized to generate the discharge.
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applied electric field, and also imposing large cooling gradients in the system. The addition of a liquid phase in which the desired product is highly soluble was a significant advancement in chemical synthesis with plasma and well-studied in the synthesis of hydrazine from ammonia where it was shown that the introduction of a liquid ethylene glycol solution can significantly increase the energy yield for hydrazine.9 In that work, the liquid phase acts only as an absorbent that sequesters and protects the hydrazine molecule after it has been formed in the gas phase. Similarly, it has been shown that formaldehyde, methanol, and hydrogen peroxide can be synthesized from methane−water vapor mixtures where the liquid water acts as the absorbent.33 This work implies that the production of hydrogen peroxide alone could be achieved without the use of methane, where the liquid water serves as both a source of reactive chemical species and a sink to collect the desired products; an elegant and simple situation. Since the publication of these classic papers, hydrogen peroxide production by subjecting only water and an inert gas to a plasma discharge has proven successful, but improvements in energy yields are still required for consideration for industrial applications. Nonetheless, a unique and vital benefit to this synthesis route compared to others is that the limited chemistry involved in subjecting only liquid water and inert gas to plasma discharge restricts the liquid chemical species produced to only hydrogen peroxide, making purification of the liquid effluent unnecessary. Additionally, the reactant, water carried by an inert gas, is abundant, easily transported, and can be stored for long periods of time. Furthermore, because of the inert nature of the carrier gas, this component could be recycled. A significant amount of research has been conducted to measure hydrogen peroxide production rates in a variety of plasma reactor configurations due to its easy quantification and use as an indicator for the presence of hydroxyl radicals. Review
INTRODUCTION Electrical discharge plasma reactors have been developed and studied for the synthesis of a wide range of compounds and materials.1−3 Some examples of relatively low molecular weight products that have been synthesized by plasma include ozone from oxygen,4−6 hydrazine from ammonia,7−11 ammonia from hydrogen and nitrogen,12−14 acetylene from methane,15−19 formaldehyde from methane,20 and hydrogen peroxide from water vapor or hydrogen and oxygen.21 In spite of the large amount of research conducted on chemical synthesis with plasma, pilot and commercial scale plasma synthesis applications are mainly limited to ozone generation and plasma polymerization.22 It can be noted, however, that pilot and commercial scale plasma processes have been successful in air pollution control23−29 and some pilot scale systems for water pollution control have also been developed.30 Many factors, from the power supply to the reactor design, are important in developing full scale applications; of particular concern in chemical and environmental processes is energy cost. A major hurdle in the development of viable chemical synthesis with plasma discharge, which complicates commercialization, is that overall production of the product of interest is limited by degradation of that product by electron or radical attack before it can be collected. This issue is a fundamental problem in that many synthetic processes are dominated by endothermic reactions where electron impact dissociation of the parent compound is rapidly followed by a series of radical chain reactions leading to the product, which itself is susceptible to additional attack by radical species.31 Strategies to reduce product losses that occur from further degradation of the desired species generally include reducing residence time of the product in the plasma and imposing methods to rapidly quench undesired radical species.31,32 The latter can be achieved by the addition of gas and/or liquid phase radical quenchers, which prevent undesired reactions and potentially enhance desired ones. The former can be achieved by decreasing residence time of all species by adjusting reactor geometry and flow rates, imposing large spatial or temporal gradients on the plasma itself, addition of a liquid absorbent to collect the desired product, adjusting characteristics of the © 2014 American Chemical Society
Received: Revised: Accepted: Published: 609
August 22, 2013 December 23, 2013 December 26, 2013 January 6, 2014 dx.doi.org/10.1021/ie402766t | Ind. Eng. Chem. Res. 2014, 53, 609−618
Industrial & Engineering Chemistry Research
Article
Figure 1. Schematic of reactor design.
the impact of these variables on the energy yield of the process. Although the experimental data in the present paper follows similar trends reported in previous studies conducted in our laboratory,35−39 a wider range of conditions, most notably the electrode geometry, were varied in a single specific reactor system, and thus the interaction of the variables and their effect on the mean discharge power could be analyzed.
of these studies indicates there may be a trend relating the energy yield for hydrogen peroxide to the mean power of the discharge, where energy yields increase as the discharge power decreases.21 However, this trend is clouded by the large variability in reactor design and operating conditions upon which the data was collected. Additionally, there exists a lack of data in the low power regime (
