Water Recovery with Silicone Hollow Fiber Membranes

Water vapor permeation – membrane method


1. the problem of water scarcity

In the 21st century, water scarcity has become a worldwide problem due to factors such as global population growth, increased water use, and climate change. 2.1 billion people worldwide lacked access to safe water in 2017, and 844 million of them are reported to lack access to even drinking water.

In response to the serious water shortage problem, the water shortage problem can be solved if seawater, which accounts for about 97% of the water present on the earth, can be converted into fresh water necessary for human activities. Desalination of seawater is one of the most promising solutions to the water shortage problem, especially for drinking water.

2. Seawater Desalination

Desalination methods can be broadly classified into evaporation, which utilizes the phase change from liquid to vapor, and reverse osmosis (RO), which does not utilize the phase change while the water remains in the liquid phase. The evaporation method is further classified into the multi-stage flash method (MSF), multiple-effect method (MED), and vapor compression method (MVC). The electrodialysis (ED) method, which does not use phase change, also exists as a desalination technology, although it is less proven.

The number of desalination plants under planning and construction around the world has been particularly high in recent years, with desalination plants currently in operation in more than 120 countries. Reverse osmosis accounts for 65% of the world’s desalination plant capacity, a significant share.

3. water vapor permeation membrane method

Apart from the methods mentioned above, there is the water vapor permeable membrane method, which uses a gas separation membrane to separate water vapor.

Polydimethylsiloxane has high gas permeability among polymer membrane materials due to its high molecular mobility and extremely large molecular chain spacing. Among these materials, polydimethylsiloxane has extremely high water vapor permeability, making it possible to selectively permeate water vapor from seawater to obtain fresh water.

By supplying seawater to the housing side of the silicone hollow fiber membrane module NAGASEP and passing dry air through the hollow fiber membrane, water vapor is permeated from the liquid phase to the vapor phase. Since water vapor moves several hundred times faster than oxygen, the gas phase is quickly saturated with vapor and water can be recovered. By reducing the air flow rate as much as possible, this desalination method consumes less energy.

  At present, adsorption with activated carbon is the most mainstream separation method for recovering organic vapors from exhaust gas. However, adsorption has some problems, such as the need for regeneration system and the large size of the equipment. As the development of simpler and more energy-efficient organic vapor recovery and recycling technology is expected, the recovery method using membrane separation operation has become an attracting choice.

  By supplying ambient air to the silicone hollow fiber membrane module NAGASEP and depressurizing the permeate side, VOCs in the environment can be concentrated and recovered. As an actual application, NAGASEP has been put to practical use in a metering device that collects gasoline vapor leaked at gas stations when refueling. Gasoline vapor leaked into the atmosphere is suctioned through a double-tube refueling nozzle, then concentrated by a hollow fiber membrane, and finally collected as liquid gasoline. The system is also being considered for practical use in the recovery of organic vapors such as hexane discharged into the environment from factories and as a monitor of odors (VOC) in the ambient air. If you wonder whether NAGASEP could be a solution for your problem, feel free to contact us and have a discussion.


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