Membrane Permeate Recycle for Pressurized Anaerobic Digesters
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Solution Overview
Problem
Existing methods for upgrading methane content in raw biogas from pressurized anaerobic digesters do not utilize membrane separation technologies to increase methane content and recirculate the membrane permeate gas back to the anaerobic digester, leading to inefficiencies in biogas purification and pH management.
Innovation Solution
A membrane separation process involving multiple stages of gas separation membranes made of fluoropolymers and polyimides, where the permeate stream enriched in CO2 is recycled back to the anaerobic digester, enhancing methane enrichment and preventing pH drops by releasing dissolved CO2, thereby improving biogas quality and efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If membrane separation process is used to upgrade biogas, then methane content is enriched to over 98%, but CO2 removal creates pH management issues in the anaerobic digester
Solution Approach 1:
The patent recycles the CO2-enriched permeate stream back to the anaerobic digester instead of discarding it. This recovery approach prevents pH drops by replenishing CO2 that would otherwise be removed, while still achieving high methane enrichment in the residue stream.
Solution Approach 2:
The system implements a feedback loop where the permeate stream enriched in CO2 is returned to the anaerobic digester. This feedback mechanism maintains pH stability by compensating for CO2 removal in the membrane separation process, allowing continuous operation at high methane purity.
2Manufacturing precision
If CO2 is removed from biogas through membrane separation, then methane purity increases, but pH drops occur in the anaerobic digester
Solution Approach 1:
Rather than discarding the CO2-containing permeate stream, the system recycles it back to the anaerobic digester. This recovery strategy maintains pH stability by replenishing CO2 in the digester, enabling sustained high methane purity in the upgraded biogas product.
Solution Approach 2:
The patent converts the potentially harmful effect of CO2 removal (pH drops) into a benefit by recycling the CO2-enriched permeate back to the digester. The CO2 that would otherwise be waste becomes a valuable pH buffer, maintaining stable operating conditions.
3Productivity
If conventional biogas upgrading methods are used, then CO2 is removed, but the process lacks efficiency in purification and pH management
Solution Approach 1:
The patent merges the biogas purification function with pH management by integrating the permeate recycle loop. The membrane separation unit and anaerobic digester operate as a coupled system where CO2 removal for purification simultaneously triggers a feedback mechanism that maintains pH, improving both efficiency and ease of operation.
Solution Approach 2:
The system employs feedback control where the permeate stream is monitored and recycled back to the digester to maintain pH stability. This automated feedback mechanism eliminates the need for separate pH management operations, simplifying process control while maintaining high purification efficiency.
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 process effectively enriches the methane content in the biogas residue stream to over 98% and recycles CO2-enriched permeate to the anaerobic digester, maintaining pH stability and increasing methane yield, while reducing CO2 emissions and operational costs.
Implementation Method 1
a first membrane stage, configured to separate the biogas into a first residue stream, enriched in CH4... and a first permeate stream, enriched in CO2
Implementation Method 2
membrane separation process comprising... gas separation membranes
Implementation Method 3
a compressor, configured to compress the second permeate stream from the second membrane stage to a pressure slightly greater than the operation pressure
Implementation Method 4
an anaerobic digester, operated under an operation pressure higher than atmosphere pressure, configured to generate the biogas from a feedstock containing an organic content
Implementation Method 5
maintaining pH stability and increasing methane yield, while reducing CO2 emissions
Data Source
AI summary
A system for upgrading biogas through a membrane separation process comprises an anaerobic digester, operated under an operation pressure higher than atmosphere pressure, preferably greater than 4 baba, generating the biogas, a first membrane stage and a second membrane stage, separating the biogas into a residue stream, enriched in CH4, and a permeate stream, enriched in CO2, a compressor compressing the permeate stream to a pressure slightly greater than the operation pressure, and recycling the permeate stream back to the bottom volume of the anaerobic digester.