1. Main Points of the Presentation

■The treatment of produced water generated in conjunction with oil and natural gas extraction is an issue of major global interest from the perspective of environmental conservation. In this research, we selectively absorbed oil content from model produced water (model water created from engine oil and distilled water) using exfoliated graphite (EG)<1> obtained from natural graphite, and discovered that 100 ppm of oil can be reduced to a concentration of 0.1 ppm.

■It is hoped that the results of this research will be put into practical use as a primary treatment process of produced water, and that the reduced oil-containing water will then enable advanced treatment using various kinds of membrane, such as RO, thus facilitating more effective environmental conservation measures to be taken when exploiting natural resources.

■The results of this research are due to be published in the international science journal, "Journal of Water Process Engineering (Elsevier)."

<1>Graphite possesses a layered structure of sheets of carbon atoms bonded to form hexagons. While these hexagonal carbon bonds are as strong as diamonds, the bonds between the sheets consist of weak Van der Waals bonds (a type of chemical bond that mainly works between molecules), meaning that these sheets have a tendency to peel off easily. By inserting sulfuric acid between the sheets and then rapidly heating them, the sheets expand, creating exfoliated graphite (EG) that includes many apertures.

2. Summary of the Presentation

Shinshu University's COI Research Team is engaged in the research and development of cutting edge membrane technologies that allow a broad range of purification operations for contaminated water, including the desalination of sea water. While we are also developing robust nanocarbon membranes for use in the treatment of produced water, a byproduct of oil, natural gas and shale oil/gas exploitation, primary treatment methods of removing the oil content from produced water are essential to perform advanced water treatment using such membranes. As such, our group set out to utilize the high potential of carbon science to develop a primary treatment method for produced water.

The exfoliated graphite (EG) used in this research was obtained by subjecting graphite derived from natural sources to sulfate treatment and turning it into a foam. We added EG to the model produced water (distilled water containing engine oil), stirred it and then evaluated how well it absorbed the oil content. As a result, we showed that 100 mg/l (ppm) of oil in the model produced water can be reduced to a concentration of a 0.1-few mg/l (ppm). This is a low concentration, several tenths of the oil content concentration obtained through conventional primary treatments for produced water; at this level, it is possible to perform advanced treatments with nanofilter (NF) membrane or reverse osmosis (RO) membrane separation as post-processes. Furthermore, the fact that EG is an extremely cheap (1,000 Japanese yen or below per kg) carbon body of natural origin makes it a highly viable material for putting into practical use as a primary treatment process of produced water.

Analysis of the amount of absorbed oil content in EG showed how EG absorbs oil content preferably, and that even in cases when the oil content is low, it is absorbed by the EG in a concentration over 250 times higher than that of the model produced water. In addition, we confirmed that EG's this function also worked in another kinds of model produced sea water (salt concentration: 3.5wt%), showing how its absorption performance exceeds conventional activated carbon and others.

Furthermore, through molecular dynamics simulation using our super computer systems, we elucidated on the mechanisms through which the oil content is preferably absorbed from model produced water in the gaps of EG.

We also examined methods of recovering oil sorbed EG, and found that we can effectively recover oil sorbed EG by using magnetic field when EG was incorporated with micro-size iron particles.

Note that we have applied for patents relating to the content of this paper.


SEM image of the original EG

There is also the potential for methods of recovering oil content from oil sorbed EG, and to use this as fuel for thermal power generation, which will be the subject of a future investigation. Natural graphite, the raw material used in this experiment, is widely produced, particularly in Asia and South America; at present, there are no issues in terms of the acquisition of this resource and its cost, and we firmly believe in its potential for development as an effective method for these.

The results of this research have been heralded as a new achievement linking low cost natural graphite with the treatment of produced water, and are due to be published in the international science journal, "Journal of Water Process Engineering (Elsevier)."


Change of oil concentration as a function of time when EG (T-1) was added in simulated engine oil (oil; synthetic oil, 100 ppm).

3. Background behind the Presentation

The results of this research form part of the research initiatives of Shinshu University and other institution that seek to establish innovative desalination and water reclamation systems in line with the vision "establish a sustainable society with vitality". Shinshu University is the core hub for the project "Global Aqua Innovation Center for Improving Living Standard and Water-sustainability," funded by Japan Science and Technology Agency (JST)'s Center of Innovation (COI) Program.

As part of efforts to resolve global water shortages, the project team has been focusing on three key water resources, all of which contain salt: seawater, produced water and blackish water.

We are involved in the research and development of reverse osmosis (RO) membranes that use nanocarbons instead of conventional polyamide, which is our key technology for desalination. However, including oil and natural gas components, produced water contains such things as salt content, a diverse range of organic and inorganic substances, precious metals and radioactive materials of natural origin, meaning that it can easily damage membranes. This is why we have also investigated prior treatment for produced water.

In previous surveys, the discharge volume of produced water has been as much as 3 ? 6 times that of oil ? or, several tens of thousands of tons per day. The method of treatment is decided on a case by case basis, and is then either reinjected in an oil well for oil production, or disposed of in the ocean or underground. While current treatment methods can reduce the concentration of oil content to 5 ppm or less through three stages, such as specific gravity difference separation and filter medium filtration, when digging submarine oil fields from the ocean surface, there is no space to install processing machinery, meaning that there is a limit of around 15 ppm.

Production volumes of oil and natural gas will increase in the future; given this, there is an increasing demand for the development of effective treatment methods for produced water, and we believe our achievements will contribute to environmentally friendly resource extraction. If the oil content concentration is lowered to 1 ppm or under and desalination using newly developed robust RO membranes is carried out, it will lead to dramatic improvements in the rate of reuse, such as through reinjection in oil wells.

* Journal of Water Process Engineering
Kenji Takeuchi, Masatsugu Fujishige, Hidenori Kitazawa, Noboru Akuzawa, Josue Ortiz Medina, Aaron Morelos-Gomez, Rodolfo Cruz-Silva, Takuya Hayashi, Mauricio Terrones, Morinobu Endo*, Oil sorption by exfoliated graphite from dilute oil-water emulsion for practical applications in produced water treatments, Journal of Water Process Engineering 8, 91-98(2015). doi:10.1016/j.jwpe.2015.09.002
*Corresponding author

** Center of Innovation (COI) Program
This is an open recruitment type R&D program operated by the Japan Science and Technology Agency (JST). Here, the premise is based on the vision to anticipate the structure and lifestyle of society that should be aimed for considering the needs of future society that are dormant at present --- however, as this requires groundbreaking innovation that companies alone cannot hope to achieve, the initiative has been taken to tackle R&D via an industry-academia collaboration.
Shinshu University is the core institution of the "Aqua Innovation Center for Improving Living Standards and Water-sustainability" within Vision 3 Establish a sustainable society with vitality (visionaly leader: Masaharu Sumikawa, Hitachi, Ltd., Executive Adviser).

- Project leader (PL): Shinjiro Ueda (Hitachi Infrastructure Systems Company, Executive Technology Adviser)
- Research leader (RL): Morinobu Endo (Shinshu University, Distinguished Professor)

■ Core institutions
Shinshu University, National Institute for Materials Science and Nagano prefecture
■ Core companies
Hitachi Infrastructure Systems Company, Toray Industries, Inc. and Showa Denko K.K.
■ Paticipants
RIKEN, Research Organization for Information Science and Technology, Kitagawa Industries Co., Ltd. and Toclas Corporation
■ COI-S satellite
Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Sony Computer Science Laboratories, University of Tokyo, Chuo University and Japan Aerospace Exploration Agency (JAXA)

- R&D period: FY2013 to FY2021 (maximum duration)