Electrochemical generation of Fenton´s reagent to treat spent caustic wastewater in oil production
Advancing the chemical engineering fundamentals
Electrochemical Engineering (T2-14P)
Keywords: Oxidation, phenols, mercaptans, electric current, Fenton´s Reagent
An important wastewater stream from oil refineries is the spent caustic. Caustic solutions are used as scrubbing agent during the desulfurization process to eliminate sulfur and mercaptans from oil and gasses. Spent caustic is classified as D003 (reactive sulfide) hazardous waste under the US Resource Conservation and Recovery Act (RCRA). Spent caustic is a highly specific efluent. Generally, there is exists two types of spent caustic waste streams depending on the origin – sulfidic spent caustic from scrubbing operations and phenolic spent caustic from heavy gasoline sweetening. Typically, relative small volumes (0.1 to 8 m3/h) are discharged, and these vary depending on the refinery´s size and layout. The harmful effects are considerable due to the concentration in S2- and phenols. Around 550 t of of spent caustic is produced when processing 1 Mt of crude oil.
An efficient treatment is the oxidation with Fenton´s reagent. Fenton's reagent is a solution of hydrogen peroxide and an iron catalyst that is used to oxidize contaminants or waste waters. Mixing iron and hydrogen peroxide in the right manner, it results in the generation of highly reactive hydroxyl radicals (.OH) or peroxide radicals (.OOH). Ferrous iron(II) is oxidized to ferric iron(III) by hydrogen peroxide to a hydroxyl radical and a hydroxyl anion. In order to have an efficient process, the procedure requires: a) adjusting the wastewater to pH 3-5, b) adding the iron catalyst (for example as a solution of FeSO4), and c) adding slowly the H2O2. If the pH is too high, the iron precipitates as Fe(OH)3 and catalytically decomposes the H2O2 to oxygen.
Fenton´s reagent can be produced by electrochemical oxidation of iron(0) and adding H2O2 simultanously.If iron plates are used in an electrolytic cell, continuos anodic oxidation of iron will occur at the plate surface and liberate the catalyst for Fenton´s reagent. The advantages with the electrochemical in-situ production of Fe2+ are:
• No generation of residual products.
• The process do not need to be heated.
• Better dosage of Fe2+ by controlling the electric current.
• Less operational costs (no chemicals).
In an experiemtnal cell, the electrochemical production of Fenton´s reagent and the use in the spent caustic treatment was tested. Both batch and continuous operation was investigated. During the process different parameters were analysed such as pH, temperture, electric current density and H2O2 addition rate.
From the results a kinetic model was suggested and validated by the experiments. The results showed that the electro assisted process to treat spent caustic was a promissing alternative to wet air oxidation that operates at 25-90 bar and 200-300 C.
Presented Wednesday 19, 13:30 to 15:00, in session Electrochemical Engineering (T2-14P).
