Making fresh water out of seawater usually requires huge 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 the minerals.
Now, Purdue University engineers have developed a variant of the process called “batch reverse osmosis,” which promises better energy efficiency, longer-lasting equipment and the ability to process water of much higher salinity. It could end up a difference-maker in water security around the world.
Reverse osmosis is used in many countries; in arid places like the Middle East, more than half of the fresh drinking water supplies come from desalination facilities. But to maintain the high level of pressure required for the process – up to 70 times atmospheric pressure – a desalination plant must employ large numbers of pumps and other equipment. And that uses a lot of energy.
“About a third of the lifetime cost of a desalination plant is energy,” said David Warsinger, a Purdue assistant professor of mechanical engineering. “Even small improvements to the process – a few percentage points of difference – can save hundreds of millions of dollars and help to keep CO2 out of the atmosphere.”
During his doctoral work at MIT, Warsinger first developed the idea of “batch reverse osmosis.” Rather than keeping a constant flow of seawater at those high pressure levels, a batch process takes in a set quantity of water at one time; processes it; discharges it; and then repeats the process with the next batch.
“Each batch runs for about one to two minutes,” Warsinger said. “We ramp up the pressure over time, reduce the volume over time, and we end up using much less energy to produce the same amount of fresh water.”
Though some desalination plants have attempted to use semibatch techniques, none has ever implemented a full batch system—partly because of the time breaks between batches.
“It takes time and energy to pump each batch of water out, and then pump the next batch of water in for processing,” Warsinger said. “Expending that time and energy generally cancels out the efficiency gains you would get from using the batch process. That’s why we developed a solution called ‘double-acting batch reverse osmosis.'”
Double-duty piston
This new process uses a piston tank – a high-pressure vessel with a piston in the middle. While one side of the piston sends seawater forward into the processing loop, the other side of the piston simultaneously fills up with the next batch of seawater in the queue. When one batch process ends, 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 queue, and the process repeats continuously.
“Instead of fully emptying the piston each time or using some other liquid or gas to pressurize the piston, we’re filling it with the next batch of seawater,” Warsinger said. “So rather than one side of the piston being essentially dead space, we are 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 for seawater desalination. It’s a best-in-class milestone.”
Their research has been published in Desalination.
“Downtime is really something you want to avoid,” said Sandra Cordoba, a Purdue master’s student in mechanical engineering and first author of the paper. “If you have to service the system after every cycle, you lose all your energy efficiency. Reducing or eliminating that downtime is the key thing that makes batch reverse osmosis feasible.”
Cordoba also developed the theoretical hydraulic models used in the paper.
“Reverse osmosis is a complex process,” Cordoba said. “To gauge its success, you have to track many variables: water pressure, volume, salinity, recovery ratio, time and energy. With these models, we were able to determine the right amount of pressure over time to achieve the best results using the minimum amount of energy.”
How big is the piston tank? It depends on the size of the system.
“Reverse osmosis operates on a wide range of scales,” Warsinger said. “Households in India often have a micro reverse osmosis system for their own home, where you could hold it in your hands. For our experiments, we’ve built a model system where the piston tank is about the size of a fire extinguisher.
In a full-scale plant, it could be a hundred feet long. But the beauty of it is that it’s not a complex piece of equipment; it’s essentially a pipe, with a water-tight piston in the middle. But that piston tank changes everything.”
Warsinger’s lab has used this double-acting batch development to fuel several new advances in desalination. Abhimanyu Das, a Purdue Ph.D. student in mechanical engineering, 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, 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 conceivably deliver fresh water to the entire 1,954-mile border between the U.S. and Mexico.
“Water security is a huge issue around the world, which I’ve spent my whole career working on,” Warsinger said. “These results with batch reverse osmosis are really exciting. If we bring the cost down just a little bit, then desalination becomes a viable option for more places. It could be transformative.”
Humanity is facing the challenge of clean water resource depletion as predictions show that half of humanity may live in regions with water stress problem by 2030 [1]. This situation appears worse if other factors are included such as population growth, the evolving economy, water resource pollution and climate change. Consequently, it is critical to find solutions to increase fresh water production and to provide safe drinking water for the world’s growing population while limiting energy requirements.
Seawater desalination has attracted growing attention in the last few decades as an alternative technology for fresh water augmentation. However, seawater desalination inevitably costs significantly more than treatment of any other surface water resource.
Indeed, with consideration of the difference in salinity between raw water (seawater) and fresh water (under World Health Organization tap water regulations), desalination induces such a great difference in chemical potential that it inevitably consumes, from a thermodynamics point of view, a high amount of energy to remove dissolved salt.
Nowadays, the most energy-efficient seawater desalination technology is reverse osmosis (RO). This technology has improved considerably in the last five decades and
is at present the most developed seawater desalination technology at industrial scale [2]. These improvements are mainly due to the enhancement of membrane performance (with a quite good compromise between permeability and selectivity), pump efficiency and the implementation of energy recovery devices (ERDs), which result in considerable decreases in energy consumption (from approximatively 15 kWh/m3 in the early 1970s to less than 2 kWh/m3 today).
Nevertheless, this consumption can be further reduced by optimizing the pilot design and its associated operating mode. A new trend is to work with the batch system where the recirculation of the rejected brine goes back into the feed tank. This process is named batch and semi-batch RO configurations. Additionally, there is room for improvement from an energetic standpoint considering the size of the feed tank, the profile pressure applied to the ERD and pump efficiencies.
In continuous mode, the feed pressure depends on (1) the desired conversion yield and (2) the salt concentration to guarantee a minimum permeate flow at the end of the spiral wound. Whereas batch RO is, in theory, the only configuration where the required minimum energy is equal to the thermodynamic theoretical minimal specific energy con- sumption (SEC), by matching/adapting the applied pressure to the increasing osmotic pressure [3].
Thus, by having the possibility to reduce the difference between pump pressure and osmotic pressure, batch RO makes it possible to control and minimize the polarization layer. It has to be noted that, in reality, it is impossible to reach such a limit due to many potential energy losses such as electrical energy conversion into mechanical energy efficiency (pump efficiency) as well as the concentration of polarization (selective mass transfer), pressure loss (friction) and ERD energy loss. However, the minimum practical energy consumption is reduced by nearly 30% when one passes from a continuous RO configuration to a batch configuration (from 1.54 kWh/m3 to 1.1 kWh/m3) [3].
Indeed, in a conventional continuous configuration (Figure 1, Type A), the pressure is fixed according to the osmotic pressure of the outlet of the last pressure vessel module; this is to satisfy the objective of treatment in terms of water recovery. In the batch configuration, feed water is pumped and contained in a feed tank, which can be pressurized or not de- pending on the configuration. The feed is, then pumped through a pressurized membrane vessel where the RO filtration occurs.
The permeate is recovered, while the retentate is recirculated to the feed tank resulting in an increase of its concentration. This operation, named a pass, is reconducted several times until reaching the desired water recovery. Then the feed tank is emptied (corresponding to the final concentrate) and refilled to start a new cycle.
Two configurations can be adapted in batch mode. The first one requires an ERD to recover the pressure and transfer it to the feed stream (Figure 1, Type B), and the second one requires a pressurized feed tank (Figure 1, Type C). This last configuration, with the pressurized tank, seems, at first sight, easier, but it might be very constraining and difficult to set up at a larger scale.
The feed pump in Type A delivers constant pressure, while it delivers time variable pressure in the rest of the processes, to keep producing permeate flux as feed osmotic pressure increases with time. ERDs are used to recover energy from the brine in processes A and B. The pressurized tank is schemed as a tank with a piston that retains the brine’s energy, acting as an ERD.
The advantage of working with the batch RO configuration is that the pressure can be modulated and adapted precisely according to the osmotic pressure evolution (Figure 2). Batch RO can operate similar to an N-stage configuration by increasing pressure like a staircase function to overcome increasing osmotic pressure between stages. An alternative pressure profile is to place an osmotic pressure sensor (conductometer) to apply enough pressure that would keep the net driving pressure (NDP; NPD = ∆P ∆π) constant to maintain a constant flux.
Any random pressure profile that is greater than the osmotic pressure would be suitable for the batch RO. Figure 2 was drawn to compare the two different operating modes. The mean permeate flux was fixed at 12 L m−2 h−1 for both configurations. The osmotic pressure stays parallel to the pump pressure for batch RO, whereas the osmotic pressure tends to reach the pump pressure for continuous RO.
In continuous RO, the permeate flux is also a function of the module position in the pressure vessel (from 26 to 3 LMH), while in continuous RO, it remains constant as the batch RO pressure was set to deliver constant flux. Thus, the recovery ratio depends on the module’s place in the pressure vessel (PV) for continuous configuration, whereas in batch RO it is a function of the process time. What is also important to note is that the salt convective flux (JS = JW·CS) is different, showing that the scaling risk is not the same.
Another alternative to continuous RO desalination is the semi-batch configuration (Figure 1, Type D). The main difference with the batch process is that the recirculation stream is mixed instantly with the feed stream, instead of being stored in a feed tank. Feed salinity increases with time; thus, the pump pressure also increases to keep a positive permeate flux. While the main focus of our study is the modeling of batch RO desalination, it is worth mentioning that the semi-batch process, also known as closed circuit reverse osmosis (CCRO), is patented and commercialized by Desalitech Company under the name of Reflex CCRO [4]. The company claims a high recovery ratio of up to 98%, energy savings as well as less fouling and scaling. The main findings regarding the performance of CCRO were published in a series of papers exploring all aspects of this technology [5–7]. CCRO is now incorporated in different RO software such as ROSA [8], LewaPlus [9] and PROTON [10].
Figure 2. Batch RO and continuous RO (a) feed pump pressure and osmotic pressure and (b) permeate flux and convective salt flux. (Initial conditions: Salinity = 35 g/L; mean permeate flux = 12 LMH.).
Research on batch configuration is still limited, and large-scale use remains under in- vestigation. Some patents were introduced by Szucz et al. [11], Oklejas [12] and Warsinger et al. [13]. Warsinger et al. [14] modeled the batch configuration and semi-batch config- urations and found that they can save up to 64% and 37% of energy, respectively, for brackish water at high water recovery.
They explained that the batch configuration exhibits higher energy efficiency than CCRO does because of the high entropy generated in CCRO caused by mixing brine with feed water, which is lessened in the batch process where the concentration difference between the brine and the feed is much lower (both stream con- centrations increase).
Another advantage of the batch mode is the less fouling propensity due to better control of the effective driving force, which allows to control the polarization concentration phenomena and thus reduces fouling. Warsinger et al. [15] explored the effect of batch configuration on scaling.
They concluded that due to the shorter residence time of scalants and the cyclic concentration of the seawater feed, batch RO is more likely to resist inorganic fouling of Gypsum CaSO4 and could reach high recoveries greater than 75% while continuous RO is limited to 60% in order to avoid scaling under the same conditions.
Our paper proposes an approach to modeling the batch RO process that is based on the works of Slater et al. [16] and is different than recent models. We opted for this model because it allows the use of time-dependent pressure profiles and detailed description of process variable dynamics, using a forward and direct analytical approach delivering differential equation describing the whole batch RO system concentrations.
A Python algorithm was developed to that end. Validation is conducted by comparison to existing models and experimental data. An energetic comparison between batch RO and continuous RO configurations is discussed to highlight energetic performances. The batch RO process is also simulated under a wide set of parameter variations and under different pressure profiles to explore its energetic response.
reference link : https:// doi.org/10.3390/membranes11030173
More information: Sandra Cordoba et al. Double-acting batch reverse osmosis configuration for best-in-class efficiency and low downtime, Desalination (2021). DOI: 10.1016/j.desal.2021.114959
