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論文題目「Decomposition of Insoluble Organics by DC Water Plasma at Atmospheric Pressure」
Sun Qiran
1. Introduction
Thermal plasma technology for hazardous waste treatment has recently attracted
increased attention, due to its high energy density, high temperature,
and rapid quenching rate (105–10E6 K/s). The oxidation and reduction atmosphere
can be selected depending on the requirements of chemical reactions.
Direct current (DC) water plasma is a lightweight plasma generation system without a requirement for inert gas supply, pressure-controlled and cooling-controlled units. The water plasma can be generated using only water as plasma gas. The water plasma is highly destructive and energy efficient in a short period. Therefore, this technique provides a new incentive for waste treatment using thermal plasmas.
Insoluble oily wastewater is discharged from many industrial activities
such as machining, cosmetic production, food processing, plating, etc.
The oily wastewater is usually in the form of an o/w emulsion. The complex
physicochemical characteristics of o/w emulsions render oily wastewater
particularly challenging to treat compared to other types of wastewater.
The low biodegradability of insoluble oily wastewater results in poor performance
in traditional biological treatment processes .
Thermal plasma treatment technology shows great potential for treating emulsion wastewater. The treatment of typical oily emulsion wastewater was demonstrated using water plasma in this study. The simulated emulsion was prepared using 1-decanol with different surfactants. The effect of surfactant propreties, concentrations and currents were discussed in detial.
2. Experiment
2.1. Emulsion preparation
The o/w emulsion can be described as a collection of tiny droplets of oil dispersed in water. Surfactant prevents suspended droplets from coalescing and breaking the emulsion. The hydrophilic–lipophilic balance (HLB) of surfactant is a measure of the degree to which it is hydrophilic or lipophilic. In the case of nonionic surfactants, o/w emulsions are formed using nonionic surfactants with HLB values between 8 and 18. Nonionic surfactant Tween 20, Tween 60, and Tween 65 were chosen to maintain o/w emulsion in this study due to their appropriate HLB values of 16.7, 14.9, and 10.5, respectively. The utilization of nonionic surfactants can avoid the contaminant from elements like Na, S, and P that are commonly present in ionic surfactants. The total concentration of the emulsion was fixed at 16 g/L to avoid the effect of concentration. The mass ratio of nonionic surfactant to 1-decanol is controlled to be approximately surfactant:1-decabol = 1:15, 2:14, and 3:13.
The effect of total concentration on the emulsion was also investigated
under the condition of Tween 20:1-decanol = 1:15. The concentrations of
1-decanol used for the formation of emulsion to be decomposed were 5–20
g/L, which were 0.06–0.23mol%. The molar ratio of Tween 20 to 1-decanol
is 1:116, by which a stable emulsion can be easily obtained.
The emulsions were obtained by mixing 1-decanol and surfactant dispersed separately in the aqueous phase, stirring at 40°C for 1 hour, followed by ultrasonic treatment for 10 minutes.
2.2. Plasma treatment
The main part of the process consisted of a non-transferred nozzle-type
plasma torch, equipped with a reaction tube. A DC power supply was used
to generate plasma. The torch consists of a hafnium cathode and a copper
anode, the cathode is embedded into a copper rod.
Emulsion was fed into the system by a pump with a stable feed rate, and then be introduced into the electrode region as mist generated by ultrasonic atomizing. The mist feed rate can be controlled by changing the power supply of a mist generator. Disintegration of emulsions with a water plasma device with a current of 6.0 A. The exhaust gas was analyzed by a gas chromatograph (GC-14B, Shimadzu) equipped with a thermal conductivity detector. The flow rate of gas effluent was analyzed by a soap flowmeter. The effluent liquid was analyzed by HPLC (V-550, Jasco, Japan). The total organic carbon (TOC) was analyzed by a TOC analyzer (TOC-L, Shimadzu). The decomposition rate of 1-decanol was calculated from the difference of concentration measured by HPLC.
3. Results and discussion
The feed rate during the decomposition is in the range of 46–56 mg/s. The
feed rate decreased with increasing surfactant concentration. The decrease
in feed rate is mainly due to the increase in viscosity, which lead to
a lower mist generation rate. Gas generation rate also decreased as less
carbon is fed into the plasma, resulting in reduced generation of CO and
CO2.
High decomposition rate of more than 99.99% is achieved in all conditions,
as 1-decanol is not detected in the liquid effluent by HPLC with a detection
limit of 1 mg/L. TOC removal rate is in the range of 98.7%–99.7%, increased
with increasing Tween 65 concentration. The difference between decomposition
rate and TOC removal rate is the percentage of carbon remained in liquid
effluent as by-product. The by-product conversion rate is less than 2%,
suggesting water plasma has good performance on the mineralization of organic
compounds.
The gas product from the water plasma treatment mainly consists of H2,
CO, and CO2, with concentrations around 70%, 20% and 10% respectively.
The concentration of H2 increased slightly with increasing Tween 65 concentration,
while concentrations of both CO and CO2 decreased.
4. Conclusion
The decomposition of 1-decanol emulsion was successfully demonstrated using
mist-type water plasma. High decomposition rate of higher than 99.99% and
high TOC removal rate of higher than 98.7% are achieved. The effects of
different surfactants, concentrations and input currents on the decomposition
are discussed. The presence of large amounts of hydrogen and carbon monoxide
in the generated gas has the potential to be converted into clean energy
through further treatment. This study provides an idea for solving the
problem of oily wastewater containing large amounts of insoluble organics.
国際学会
- Qiran Sun, Manabu Tanaka, and Takayuki Watanabe: Decomposition Mechanism of Insoluble Organics by DC Water Plasma at Atmospheric Pressure, Proceedings of 25th International Symposium on Plasma Chemistry, POS-12-108 (2023.5.22 Miyako Messe, Kyoto).
