Multistage Reverse Osmosis System for High Salinity Concentration
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Solution Overview
Problem
Existing reverse osmosis systems for desalination are limited in concentrating saltwater due to maximum operating pressure constraints, leading to salinity levels below 80,000 mg/L, and are energy-intensive when thermal evaporation is used for further concentration.
Innovation Solution
A multistage reverse osmosis system comprising a high salt-rejection upstream unit followed by a low salt-rejection downstream unit, with heat exchangers to recycle and cool permeate, allowing for increased salinity concentration beyond 200,000 mg/L and reducing energy consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If a single reverse osmosis unit with high salt rejection membranes is used, then salt rejection is improved, but the maximum salinity concentration is limited to approximately 80,000 mg/L due to operating pressure constraints
Solution Approach 1:
The system divides the concentration process into two separate stages: a first RO unit with high salt rejection membranes (95%+) for initial desalination, and a second RO unit with low salt rejection membranes (30-75%) for concentrating the first RO concentrate to salinities above 200,000 mg/L. This segmentation allows each unit to operate within its optimal pressure range while achieving overall concentration levels that would be impossible with a single unit.
2Quantity of substance
If thermal evaporation is used to concentrate RO concentrate beyond RO limits, then salinity concentration is improved, but energy consumption increases significantly
Solution Approach 1:
The invention replaces the thermal evaporation process (thermal energy-intensive) with a mechanical pressure-driven process. By using a second RO unit operated at high pressure (1,000-2,000 psi) with low salt rejection membranes, the system achieves salinity concentrations above 200,000 mg/L through mechanical filtration rather than thermal evaporation, significantly reducing energy consumption.
3Productivity
If high pressure is applied to increase water recovery percentage, then productivity is improved, but membrane durability and system reliability deteriorate
Solution Approach 1:
The system segments the pressure application into two stages: the first RO unit operates at moderate pressure (1,000-2,000 psi) with high salt rejection membranes to produce permeate, while the second RO unit operates at higher pressure (1,000-2,000 psi) with low salt rejection membranes to concentrate the first RO concentrate. This segmentation allows high pressure to be applied selectively in the second stage where it enhances concentration without compromising membrane durability in the first stage.
4Productivity
If the second RO permeate is recycled to the first unit inlet, then water recovery is improved, but temperature management becomes more complex
Solution Approach 1:
A heat exchanger is introduced as an intermediary component between the second RO unit outlet and the first RO unit inlet. The heat exchanger cools the second RO permeate before it is recycled and mixed with the saltwater feed, preventing temperature-related issues in the first RO unit while enabling efficient water recovery through permeate recycling.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The system effectively concentrates saltwater to salinity levels above 200,000 mg/L, more than twice the typical limit, while minimizing energy usage by leveraging heat exchangers for thermal management and recycling permeate, enhancing water recovery and membrane durability.
Implementation Method 1
Reverse osmosis (RO), which utilizes hydraulic pressure to drive a feed saltwater through an RO unit
Implementation Method 2
The difference in salt concentration between the RO permeate and RO concentrate creates an osmotic pressure difference across the membrane, which the applied hydraulic pressure overcomes
Implementation Method 3
Water in the saltwater permeates through a semipermeable RO membrane to generate RO permeate, while salts are rejected by the membrane
Implementation Method 4
heat exchangers within the system for transferring heat between various fluids in the system
Data Source
AI summary
A reverse osmosis-based method and system for concentrating a saltwater. The system includes at least first and second reverse osmosis units fluidly connected in series. The membrane of the first reverse osmosis unit has at least a 95% rejection rate for sodium chloride, and the membrane of the second reverse osmosis unit has a 30% to 75% rejection rate for sodium chloride. Permeate from the second reverse osmosis unit is recycled to the first reverse osmosis unit for increased water recovery. Temperatures for the saltwater and/or the recycled permeate are controlled using heat exchangers to help ensure the reverse osmosis units' nominal performance.