Electrochemical sulfur production from treating petroleum produced water
Graphical abstract
Introduction
Souring of petroleum crude oil and corrosion of pipelines are two major issues for the crude oil industry, both of which are caused due to the sulfides present in petroleum produced water (PPW) or oilfield produced water. This also prevents the oil companies from emplacing the generated PPW back into the reservoir for maintaining reservoir pressure. The oil companies use fresh water for pressure maintenance and discharge the PPW into the environment, thus, incurring a huge cost to the industry [1]. Moreover, the volume of water-to-oil ratio, more commonly known as the “water cut”, increases over the life of a conventional oil or gas well and can go up to 98% of the extracted fluids during the later stages of crude oil production [2]. Conserving water becomes even more important in cases where oil companies are located in regions of water scarcity. Another important concern for treatment of PPW is its high TDS and conductivity, especially if PPW has to be discharged into the environment. Traditionally, for PPW treatment, various physical, biological, chemical and/or membrane-based technologies have been used. The physical technologies usually have high retention time and space requirements. For chemical treatments, secondary pollution due to chemicals is a major concern. Furthermore, PPW has low organic content which coupled with high conductivity creates an impediment for the conventional biological treatment processes. Thus, to tackle the complex nature and large volume of PPW, newer technologies need to be developed and explored [3].
Electrochemical treatment of wastewaters has been explored and used for many years. These systems can also be used remove/reduce sulfide content from wastewaters by subjecting them to a specific applied voltage. Pikaar et al. [4] electrochemically treated sulfide in domestic wastewater using different mixed metal electrodes. They achieved more than 75% sulfide reduction. Pikaar et al. [5], also used electrochemical system for long term treatment of sewage for sulfate reduction. Recently, Selvaraj et al. [6], treated sulfides in pond water using electrochemical system at 20 mA/cm2. An electrochemical pilot scale plant was also reported by Ma and Wang, [7] to treat petroleum produced water. However, they used a very high voltage (15 V) and focused on COD, suspended solids, corrosion rate and bacterial count reduction. Petroleum based brine was also treated by another group using Ti/IrO2–Ta2O5 and BDD electrodes under galvantostatic conditions [8]. Hastie et al. [9], also reported treatment of geothermal brines in packed bed reactors to treat sulfides. However, they converted the sulfides to organosulfates rather than sulfur.
Through this communication, we propose to use electrochemical systems for sulfide and TDS reduction. Furthermore, the high conductivity of PPW is also due to high NaCl concentration, which can be converted to caustic soda (NaOH). Caustic soda is a versatile chemical and has many uses in various industries. In the crude oil industry itself, it is used along with drilling fluids to maintain their pH and viscosity, petroleum refining, etc. Thus, through the electrochemical approach, a chemical of in-house use is being generated along with reduced TDS and sulfide content. Moreover, to the authors’ best knowledge, this is the first attempt to treat PPW in a long term, carbon electrode based electrochemical system.
Section snippets
Reactor set-up and operation
A dual chambered electrochemical cell (EC) with a total/ working volume 350/300 ml was used for experiments. Cation exchange membrane (CMI-7000S; Membranes International Inc., USA), pre-treated with 5% NaCl overnight, was used for partitioning the chambers. The anolyte was purged with oxygen free N2 gas (Sigma gases, India) before sealing the anode chamber with butyl rubbers to maintain anaerobic microenvironment. Open air conditions were maintained in the cathode chamber. Activated carbon
Results and discussion
In this study, syn PPW was electrochemically treated at 0.5 V (vs. Ag/AgCl) in a continuous mode of operation. The treatment efficacy was evaluated in terms of sulfide and TDS removal from this waste stream. Also, concomitant polysulfide and NaOH production were estimated, which would add value to the treatment process.
Conclusion and future remarks
In this study, we investigated the electrochemical treatment of chloride laden brine i.e. PPW by directly oxidising the sulfides to sulfur/polysulfide. The electrochemical cell was run continuously for over 2 months using cheap carbon-based electrodes. PPW is sulfide rich brine and through this study; the authors have tried to treat a feed containing very high sulfide content (0.649 g/L), which hasn’t been reported previously. Even with a high sulfide value, CE of approximately 0.65 was
Acknowledgement
P. Jain is supported by DST-INSPIRE fellowship (DST/INSPIRE/2013/499) from Department of Science and Technology, Government of India.
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