Method for purifying biogas through membranes at negative temperatures
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Solution Overview
Problem
Current biogas purification methods using polymer membranes require high electricity consumption and a large number of membranes due to limited intrinsic performances and selectivity, leading to increased operating pressure and multiple stages, which affects productivity and efficiency.
Innovation Solution
A process involving compressing biogas to 5-20 bars, cooling to 0-15°C, drying, further cooling to -60°C, and using three membrane stages for separation, with thermal integration and recycling of permeate and retentate to reduce energy consumption and membrane count, achieving high methane yield and purity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If polymer membranes are used for biogas purification, then separation between CO2 and CH4 is achieved, but electricity consumption is high (≥ 0.25kWh/Nm3 raw biogas)
Solution Approach 1:
The patent applies parameter changes by cooling the biogas feed stream to sub-ambient temperatures (e.g., 5°C or lower) before membrane separation. This temperature reduction modifies the physical properties of the gas components, enhancing the selectivity and permeance of the polymer membrane for CO2 over CH4. As a result, high separation efficiency is achieved with reduced compression requirements and lower electricity consumption, resolving the contradiction between reliable separation and energy use.
2Reliability
If high selectivity is used to limit methane losses, then methane recovery is improved, but productivity is limited and number of membranes increases
Solution Approach 1:
The patent changes the temperature parameter to sub-ambient conditions, which fundamentally alters the permeation characteristics of the membrane. This enables high CO2 selectivity without requiring excessive membrane area, thus maintaining both high methane recovery and acceptable productivity. The temperature modification allows fewer membrane modules to achieve the same separation performance, resolving the contradiction between methane recovery and productivity.
Solution Approach 2:
The patent employs a multi-stage membrane configuration where the permeate from one stage becomes the feed for the next stage. This segmentation approach allows progressive enrichment of biomethane while recycling CO2-rich streams. The staged configuration optimizes the balance between methane recovery and productivity by distributing the separation burden across multiple units operating at optimized conditions.
3Reliability
If operating pressure is increased to improve separation, then membrane performance is enhanced, but electricity consumption increases
Solution Approach 1:
The patent changes the temperature parameter instead of increasing pressure to improve separation performance. By cooling the feed gas to sub-ambient temperatures, the membrane achieves higher CO2 selectivity and permeance at moderate pressures. This approach enhances separation performance while avoiding the exponential increase in electricity consumption that would result from higher compression ratios, thus resolving the contradiction between separation performance and energy use.
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 process reduces electrical consumption and the number of membranes needed, achieving methane yields of 90-99.99% and biomethane purity greater than 97% while maintaining thermal autonomy and reducing operational costs.
Implementation Method 1
Method for purifying biogas through membranes... permeation by membrane(s) of a gas stream containing at least methane and carbon dioxide... the permeate is enriched in CO2 and the retentate is depleted of CO2
Implementation Method 2
cooling the gas stream to a temperature between 0 and -60°C... drying the cooled and compressed gas stream making it possible to essentially separate all the water vapor
Data Source
Figure 1
AI summary
The invention relates to a method for membrane permeation of a gas flow including methane and carbon dioxide, wherein said gas flow is cooled to a temperature of 0°C to -60°C before being fed into a membrane separation unit.