In this post, David Warsinger, an assistant professor in the College of Engineering and a member of the Purdue Institute for a Sustainable Future, discusses his research “Temporally multi-staged batch counterflow reverse osmosis” which was recently published in Desalination with the support of the U.S. Department of the Interior Bureau of Reclamation.
What did you want to know?
Desalination of high-salinity waters and high-recovery desalination are energy intensive processes that usually employ multiple stages to dewater the saline stream. Existing “spatially multi-staged” processes require separate membranes, pumps and energy recovery devices for each stage that increase capital and energy costs per unit volume of water desalinated.
Previous research on Batch Reverse Osmosis (RO) has shown that if the brine exiting the membrane module is recirculated back to the inlet via a mixing tank, the unsteady, gradually increasing tank salinity and hence pressure required for permeation can reduce the specific energy consumption of the process compared to steady processes with a constant feed pressure requirement.
With our research, we wanted to extend the range of salinities and water recovery ratios that Batch RO could achieve. This would be an extension of our previous work on Batch Counterflow Reverse Osmosis (BCRO) that was already proven to have high second law efficiency. With this work, we wanted to introduce the first “temporally multi-staged” desalination process that achieved low energy consumption while using the same apparatus for subsequent stage.
What did you achieve?
Operating a transient pressure-driven separation process that can be scaled up, is difficult without a good model since the operating conditions change continuously during the cycle. If the controllable parameters are not set correctly, it can lead to downtime and damage to equipment.
A temporally multi-staged or cascaded process, is an interesting problem since the terminal conditions for one stage become the initial conditions for the next stage. Also, some of the terminal conditions for a stage are not free but need to match the terminal conditions of previous temporal stage. And to add to the complexity, the process dynamics for each stage is non-linear for a Batch CFRO process.
While we started by posing the problem as an optimization problem and tried iteratively solving for all temporal stages at once, the final solution was rather elegant and came from a simple mass balance. With a few simplifying assumptions, we were able to solve for the operating conditions directly that would lead to a cyclically steady process even though the stages operated under unsteady conditions. We also innovated on the process design and replaced the expensive double acting piston tank with inexpensive tanks, while using just one set of pumps, membrane and energy recovery device, and smart valving.
What is the impact of this research?
Batch CFRO system can reduce the environmental impact of desalination and can achieve high water recovery with low operating and capital expenditure. Replacing the existing inefficient brine concentrators, such as mechanical vapor compression systems, with batch CFRO reduces the energy consumption. Also, the temporal multistaging design reduces the number of components and hence the initial cost. Thanks to the ability of the system to reach high water recovery, the process can be used to eliminate the brine effluent of RO plants and other industrial facilities.