The search for suitable methods for treating wastewater generated in gas and oil production, also termed as produced water, is on the rise. Generally, flocculation magnetic separation systems are applied to eliminate oil as well as solid mater from produced water in off-shore oil plants. Unfortunately, the treated water still contains soluble organic pollutants, among them formic, acetic and propionic acids with acetic acid making up the bulk of the acid volume.
Acetic acid is a persistent organic compound which cannot be decomposed by ozone. However, advanced oxidation process is an efficient approach for decomposing such organic compounds using hydroxide radicals. There are several advanced oxidation processes including hydrogen peroxide-ozone method, in which total organic carbon reduction rate tends to increase with increasing amount of oxidants. Plasma generation in contact with water is another approach although it has the limitation that hydrogen peroxide produced by hydroxide self-quenching is found to scavenge the hydroxide. Therefore, the present paper proposes a combination of ozone-plasma systems that reacts with hydrogen peroxide and ozone to replenish the hydroxide.
Professors Nozomi Takeuchi, Hideaki Mizoguchi from Tokyo Institute of Technology in Japan investigated the appropriate hydrogen peroxide and ozone ratio through numerical simulations as well as experiments for a reaction model for treating water containing acetic acid at a concentration of 100mgTOC/L. Their work is published in Chemical Engineering Journal.
The authors used 1L of acetic acid solution which was treated as model water. They bubbled ozone and injected hydrogen peroxide solution to the model water. In the course of the experiment, the authors analyzed both liquid and gas phase by-products in a bid to track the decomposition path of the acetic acid. They varied the hydrogen peroxide and ozone supply rates in order to get the optimum condition for total organic carbon reduction.
Ozone was varied between 50 and 100g/m3 and hydrogen peroxide between 19.7 to 197.2g/L making hydrogen peroxide supply rate almost similar to that of ozone. The treatment time was 90min and the obtained results were helpful in building a model for the decomposition path of the model water.
Formic and oxalic acids were formed as by-products after model water decomposition. As decomposition took place, oxalic and formic acid concentrations increased then decreased slowly. The calculated concentration of total organic carbon was almost similar to the measured value, therefore, no other by-products were considered. Approximately 90% total organic carbon reduction was realized after 90 minutes.
The researchers measured carbon dioxide concentration in the exhaust gas using FR-IR. They observed that the time delay in the measurement was based on gas cell purge. The concertation of the carbon-dioxide corresponded to the total organic carbon reduction. In addition, the total carbon-dioxide emitted increased as the concentration of carbon in the solution decreased. Therefore, acetic acid was converted to carbon-dioxide through the formation of oxalic and formic acids.
Liquid-phase reactions in the reaction model were simulated to analyze the optimum ratio between ozone and hydrogen peroxide absorption rates. Analyzing the reaction rates, the authors realized that two hydroxide radicals were generated from one hydrogen peroxide and two ozone molecules. Thus, the optimum ratio was about 0.5. About 94% total organic carbon reduction was realized in 1L model water treatment after 90 minutes. The reduction rate wasn’t sufficient, however this is expected to improve when supply rate for both hydrogen peroxide and ozone is increased at the optimum ratio.
Nozomi Takeuchi and Hideaki Mizoguchi. Study of optimal parameters of the H2O2/O3 method for the decomposition of acetic acid. Chemical Engineering Journal, volume 313 (2017), pages 309–316.
Department of Electrical and Electronic Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8552, Japan.Go To Chemical Engineering Journal