Toshiba Develops a Concentration Technology that Reduces Energy Consumption to One-fourth and Improves the Concentration Rate 2.4-fold and under Normal Temperature and Pressure Conditions

TOKYO─Toshiba Corporation (TOKYO: 6502) has successfully developed draw solutes (*1), an osmotic pressure material for the forward osmosis membrane method (*2) to concentrate and extract components or substances (valuable materials) from aqueous solutions. This material both reduces energy consumption to one-fourth (*3) and increases the concentration rate by 2.4-fold (*4) compared with conventional concentration methods. Conventional concentration technologies rely on evaporation or application of high pressure, and thus require a huge amount of energy. On the other hand, this newly developed technology uses the principle of osmotic pressure causing spontaneous removal of water from aqueous solutions, thereby achieving energy-saving concentration. After being used for concentration treatment, the newly developed osmotic material can be separated from water and then reused.
This technology can be used in the manufacturing of chemicals and pharmaceuticals, effluent treatment, and recovery of rare metals. It will also enable the reuse of water and the sustainable use of natural resources and reduce the burden on the environment, thus contributing to a recycling society.
Toshiba will present details of this technology online at the 52nd Autumn Meeting of the Society of Chemical Engineers, Japan (September 22-24, 2021, Tsushima Campus, Okayama University). 

Concentration is performed in various settings such as the manufacturing of chemicals and pharmaceuticals, effluent treatment, and recovery of rare metals. Conventional concentration processes include the evaporation method, that extracts valuable materials by heating the aqueous solution to evaporate water, or the reverse osmosis membrane method, which separates valuable materials from water by applying high pressure to the aqueous solution. However, a large amount of thermal energy is required for heating in the evaporation method and a large amount of electric energy is required for pressurization in the reverse osmosis membrane method. Another problem is that the valuable materials can be degraded by heating and pressurization. To address these problems, an advanced concentration technology is needed that uses less energy and causes less deterioration of valuable materials. Further, given that the SDGs set forth by the United Nations in 2015 aim for the sustainable use of natural resources and minimization of the waste burden on the environment, the reuse of limited resources is an urgent issue.

To overcome the problems described above, Toshiba focused on the forward osmosis membrane method enabling concentration treatment at low temperature without application of pressure (Figure 1). The forward osmosis membrane method utilizes the phenomenon that water spontaneously diffuses across a forward osmosis membrane that separates aqueous solutions with different solute concentrations (diffusing from a low solute concentration solution to a high solute concentration solution) to keep the concentrations of both solutions equal. The energy consumption of the conventional reverse osmosis membrane method is approximately 4 kWh/m³, and this can be reduced to one-fourth by using the forward osmosis membrane method (1 kWh/m³). Toshiba independently developed draw solutes for the forward osmosis membrane method: dissolving of this material into a solution with a high solute concentration enhances spontaneous water transport, thereby achieving highly effective concentration and separation of water in an energy-saving manner. The draw solutes can be separated from water and used repeatedly by desorption of pre-absorbed carbon dioxide in the aqueous solution containing the draw solutes.

Toshiba tested this draw solutes using an aqueous solution containing salt (sodium chloride) as a model, and found that water moved to the aqueous solution containing the draw solutes, resulting in an increase in the salt concentration to 19% (*5), which was 2.4 times higher than the salt concentration achievable by the conventional reverse osmosis membrane method (8%: Figure 2 (a)).

When the aqueous solution containing the draw solutes, which had been diluted after movement of water molecules, was heated at 70 °C, desorption of carbon dioxide occurred and this led to the separation into an draw solutes layer and a water layer (Figure 2 (b)), demonstrating that this material can be regenerated and well separated from water at less than 100 °C. Exhaust heat can be used to separate the draw solutes from water. After desorption, the carbon dioxide generated can also be recycled to use in the preparation of the aqueous solution containing the draw solutes.

The forward osmosis membrane method developed achieves a higher concentration rate than the conventional reverse osmosis membrane method, as well as good separation from water. It can be used for a system that concentrates valuable materials to high concentration under normal temperature and pressure conditions and for a system that highly concentrates effluents for reuse of water.

Toshiba will conduct a pilot study to evaluate continuous operation of this technology and to validate it in individual applications such as manufacturing of chemicals and pharmaceuticals, effluent treatment, and recovery of rare metals, with the aim of rapid commercialization.

*1: A material to be dissolved into the solution of a higher solute concentration. The material developed by Toshiba has well balanced characteristics (water-attracting character, separability from water at low temperature, and low diffusion into the solution to be concentrated).

*2: The membrane method utilizing the phenomenon of spontaneous movement of water molecules across the osmosis membrane from a solution with a higher solute concentration to a solution with a lower solute concentration.

*3: Non-membrane solvent extraction desalination (SED) technology using solubility-switchable amine, J. Hazard. Mat., 403, 123636, 2021

*4: RO systems make their case for brine concentration applications, Global Water Intelligence, Vol. 20, 6, 2019 

*5: Weight/volume percentage. The maximum membrane salt concentration is approximately 25%.