Multistage Membrane Cascade for Reverse Osmosis Energy Reduction
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Solution Overview
Problem
Current reverse osmosis technologies require high pressure to produce potable water from saline solutions, resulting in high specific energy consumption and limited water recovery, due to the large osmotic pressure differential across membranes.
Innovation Solution
A novel multistage membrane configuration combining single-stage reverse osmosis with a counter-current membrane cascade and recycle (CMCR) system, where the retentate from the high-pressure side is introduced between stages, and permeate is recycled counter-currently, reducing osmotic pressure differential and increasing water recovery.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional single-stage reverse osmosis is used, then salt rejection is achieved, but high pressure (40 bars or more) is required resulting in high specific energy consumption
Solution Approach 1:
The patent divides the single-stage reverse osmosis process into multiple stages (typically three stages). The first stage operates at high pressure to achieve initial salt rejection, while subsequent stages operate at progressively lower pressures. This segmentation allows the system to maintain effective salt rejection across all stages while reducing the overall energy consumption compared to single-stage operation at continuously high pressure.
Solution Approach 2:
The patent introduces a temporal dimension to the pressure application by varying pressure across different stages of the process. Instead of maintaining constant high pressure throughout, the system applies high pressure only in the first stage and progressively reduces pressure in subsequent stages, effectively using time-based pressure modulation to reduce energy consumption while maintaining separation effectiveness.
2Quantity of substance
If conventional single-stage reverse osmosis is used, then potable water is produced, but water recovery is limited to typically 50%
Solution Approach 1:
The patent recovers water that would otherwise be discarded in the brine stream from the first stage by feeding it to subsequent stages. The multi-stage configuration allows the system to extract additional potable water from the concentrate stream of the first stage, thereby increasing overall water recovery beyond the 50% limitation of single-stage systems while still producing potable quality water.
Solution Approach 2:
The patent maintains continuous useful action by feeding the concentrate stream from one stage to the next stage, ensuring that water extraction continues throughout the entire process rather than stopping after a single stage. This continuous extraction process maximizes water recovery by utilizing the full concentration gradient available across all stages.
3Productivity
If high pressure is applied to overcome osmotic pressure differential, then water permeation is achieved, but energy consumption increases significantly
Solution Approach 1:
The patent segments the pressure application across multiple stages, with the first stage handling the bulk of the osmotic pressure differential at high pressure to achieve initial water permeation. Subsequent stages operate at lower pressures to extract additional water with minimal energy input, thereby maintaining high overall productivity while significantly reducing total energy consumption compared to single-stage high-pressure operation.
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
This configuration significantly reduces specific energy consumption and increases potable water recovery by minimizing the osmotic pressure difference across membranes, allowing for efficient desalination with reduced energy costs and improved water recovery rates.
Implementation Method 1
Reverse osmosis (RO) has emerged as a major technology for producing potable water from seawater as well as inland brackish water whose salt content ranges from 500 ppm to 30000 ppm. RO uses a salt-rejecting membrane under high pressure in order to force water to permeate through the membrane while rejecting the salt and other solutes.
Implementation Method 2
the current art in RO has a limited potable water recovery, typically 50%. As a result, the specific energy consumption (SEC) for producing potable water from saline water is quite high. The theoretical minimum SEC for conventional single-stage RO (SSRO) is 3.084 kwh/m3 (kilowatt hours of energy per cubic meter of product water) to produce a potable water product containing 350 ppm from seawater containing 35000 ppm of salt at a pressure of 55.5 bar with a water recovery of 50% using a membrane with a salt rejection of 0.993. The SEC for the current RO process technology is high because of the high pressure required and the relatively low recovery of potable water product. The required pressure is high because of the large difference between the salt concentration on the retentate or high pressure side of the membrane and the permeate or low pressure side of the membrane.
Data Source
AI summary
An apparatus for reverse osmosis, the apparatus comprising: a single-stage reverse osmosis (SSRO) unit; and a counter-current membrane cascade with recycle (CMCR) unit comprising a plurality of stages of reverse osmosis including at least a first stage and a second stage wherein permeate from the first stage is configured to be introduced as feed to the second stage; wherein retenate from the SSRO unit is configured to be introduced as feed to the first stage, and wherein product obtained using the apparatus comprises permeate from the SSRO unit and permeate from a last stage of the CMCR unit.