Potable sea water from discontinuous reverse osmosis4 min read
Through an article taken from Perdue University, iraic.info tells us about a breakthrough in reverse osmosis that could lead to more energy efficient desalination of seawater.
Obtaining fresh water from seawater often requires enormous amounts of energy. The most widespread process for desalination is called reverse osmosis, which works by flowing seawater over a membrane at high pressure to remove minerals.
Now, Purdue University engineers have developed a variant of the process called “batch reverse osmosis,” which promises greater energy efficiency, longer equipment life, and the ability to process water of much higher salinity.
Reverse osmosis is used in many countries; In places like the Middle East, more than half of the drinking water supply comes from desalination facilities. But to maintain the high level of pressure needed for the process – up to 70 times atmospheric pressure – a desalination plant consumes a lot of energy.
About a third of the lifetime cost of a desalination plant is energy. Even small process improvements – a few percentage points difference – can save hundreds of millions of dollars and help keep CO2 out of the atmosphere.
David Warsinger, an associate professor of mechanical engineering at Purdue.
During his doctoral work at MIT, Warsinger first developed the idea of ”batch reverse osmosis.” Instead of maintaining a constant flow of seawater at such high pressure levels, a batch process takes in a set amount of water at a time; processes it; the discharge; and then repeat the process with the next batch.
Each batch lasts between one and two minutes. We increase pressure over time, reduce volume over time, and end up using much less energy to produce the same amount of fresh water.
Although some desalination plants have attempted to use semi-batch techniques, none have ever implemented a full-batch system, in part due to pauses between batches.
It takes time and energy to pump each batch of water, and then pump the next batch of water for processing. The expense of that time and energy often offsets the efficiency gains that would be gained by using the batch process. That is why we have developed a solution called ‘double action reverse osmosis’.
This new process uses a piston tank, a high-pressure vessel with a piston in the center. While one side of the piston sends seawater into the processing circuit, the other side of the piston simultaneously fills with the next batch of seawater in the queue. When a batch process is complete, the piston seamlessly injects the next batch of seawater into the system while simultaneously filling its other side with the next batch of seawater in the tail, and the process repeats continuously.
Instead of completely emptying the piston each time or using some other liquid or gas to pressurize it, we fill it with the next batch of seawater. So instead of one side of the piston being essentially dead space, we’re using the seawater itself to get double duty out of this piston, so there’s almost no downtime.
According to our models, this proposed system offers the lowest energy consumption ever seen for seawater desalination. It is a milestone in its class.
Downtime is something you really want to avoid. If the system has to be repaired after each cycle, all energy efficiency is lost. Reducing or eliminating that downtime is the key to making batch reverse osmosis viable.
Sandra Córdoba, Purdue mechanical engineering master’s student and first author of the paper.
Córdoba also developed the theoretical hydraulic models used in the work.
Reverse osmosis is a complex process. To measure its success, many variables must be tracked: water pressure, volume, salinity, recovery rate, time, and energy. With these models, we were able to determine the correct amount of pressure over time to achieve the best results using the least amount of energy.
What size is the piston tank? It depends on the size of the system.
Reverse osmosis works on a wide range of scales. Indian households often have a micro reverse osmosis system for their own house. For our experiments, we have built a model system in which the piston tank is the size of a fire extinguisher. In a large-scale plant, it could be 30 meters long. But the good thing is that it is not a complex team; it is essentially a pipe, with an airtight piston in the center. But that piston tank changes everything.”
Warsinger’s lab has used this development of double action batches to drive several new advances in desalination. Abhimanyu Das, a mechanical engineering doctoral student at Purdue, has published research describing a variant of the process called “batch counterflow reverse osmosis.” By recirculating certain concentrations of water on both sides of the membrane, the Das process is shown to be the most energy-efficient desalination process for high-salinity water, while requiring fewer components. And Purdue master’s student Michael Roggenburg has published research showing that a combination of batch reverse osmosis and renewable energy could supply fresh water to the entire 1,954-mile border between the United States and Mexico.
Water safety is a huge issue around the world, one to which I have dedicated my entire career. These results with batch reverse osmosis are really exciting. If we lower the cost a bit, desalination will become a viable option for more places. It could be transformative.
Taken from Perdue University