Membrane Gas Separation with Dynamic Bypass Control
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Solution Overview
Problem
Existing membrane processes for gas separation struggle to maintain constant composition of the product gas when dealing with gas streams having varying composition or flow rate, particularly in applications like biomethane production from biogas.
Innovation Solution
A single-stage three-step membrane separation process with recycle of the third-step permeate to the feed stream, and varying the fraction of the second-step permeate recycled to maintain target composition and compensate for changes in membrane efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the membrane separation device is dimensioned to the maximum possible gas flow rate, then the product gas specification is maintained at maximum load, but investment costs increase and recovery of the slower permeating gas component decreases at lower flow rates
Solution Approach 1:
The patent applies dynamics by making the membrane separation system adaptable to varying operating conditions. Instead of a fixed membrane area sized for maximum capacity, the system dynamically adjusts its effective membrane area through a bypass line that can redirect feed gas around the membrane modules. This allows the system to optimize membrane utilization at different flow rates, maintaining specification compliance while reducing the required total membrane area compared to a permanently oversized design.
Solution Approach 2:
The system changes the operating parameter of membrane area utilization by incorporating a bypass line controlled by a control unit. The control unit adjusts the bypass ratio based on actual flow rate and composition, effectively changing the active membrane area parameter dynamically. This resolves the contradiction by allowing the system to use less membrane area at lower flows while maintaining the capability to handle maximum flows, thereby reducing investment costs without compromising reliability.
2Reliability
If the membrane separation device is dimensioned to the maximum possible gas flow rate, then the product gas specification is maintained at maximum load, but recovery of the slower permeating gas component decreases when operated at lower flow rates
Solution Approach 1:
The dynamic bypass control enables the system to optimize recovery at different operating points. At lower flow rates, the control unit increases the bypass ratio, which maintains the partial pressure gradient across the membrane and prevents dilution of the retentate stream. This dynamic adjustment ensures that recovery of the slower permeating gas component (e.g., methane in biogas) remains high even at reduced flows, resolving the contradiction between maintaining specification and maximizing recovery across varying loads.
Solution Approach 2:
The control unit implements feedback control by continuously monitoring flow rate and composition parameters and adjusting the bypass ratio accordingly. This feedback mechanism ensures that recovery of the slower permeating gas component is optimized at each operating point, preventing the recovery degradation that would occur with a fixed membrane area design operated at varying loads.
3Stability of the object's composition
If permeate pressure is regulated to compensate for increases in gas flow or content of faster permeating gas components, then product gas composition remains constant, but the device must operate at higher than necessary permeate pressure during most of the time
Solution Approach 1:
The patent segments the feed gas flow into two paths: one through the membrane modules and one through the bypass line. This segmentation allows independent control of the membrane processing portion, enabling the system to maintain stable product composition by adjusting only the membrane-active portion of the flow rather than increasing pressure across the entire system. The control unit modulates the bypass ratio to compensate for composition variations, reducing the energy penalty associated with maintaining high permeate pressure.
4Device complexity
If a single membrane separation unit is used, then the device is simpler, but it cannot compensate for changes in feed gas flow or composition to maintain constant product purity
Solution Approach 1:
The patent introduces dynamics into a single membrane unit system through the bypass line and control unit. This dynamic configuration allows the single membrane unit to effectively compensate for feed variations by adjusting the bypass ratio, achieving composition stability without requiring multiple membrane units in series or parallel. The control unit calculates the appropriate bypass ratio based on feed conditions and adjusts accordingly, enabling a single membrane unit to perform the function that would otherwise require a more complex multi-unit configuration.
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 approach ensures a product gas enriched in the slower permeating gas component with constant purity of the faster permeating gas component, even under varying conditions, while minimizing membrane area and energy consumption.
Implementation Method 1
a gas separation membrane which has a higher permeance for the first gas component than for the second gas component
Data Source
AI summary
A device for separating a gas stream which has a compressor and three membrane separation units in series, connected to pass the retentate stream of each of the first two units to the subsequent membrane separation unit, comprises conduits for recycling the permeate streams of the second and the third membrane separation unit to upstream of the compressor and a control device controlling the fraction of the second permeate stream which is recycled. Adjusting which fraction of the second permeate is recycled can be used to maintain a target composition of the retentate obtained in the third membrane separation unit when the flow rate or the composition of the gas stream changes.