Three-Stage Membrane Nitrogen Rejection Process
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Solution Overview
Problem
Existing methods for separating methane (CH4) and nitrogen (N2) in natural gas or biogas are costly and equipment-intensive, particularly for small volumes, and often require cooling the feed stream, which is inefficient and uneconomical.
Innovation Solution
A three-stage membrane separation process that does not require cooling the feed stream, using membranes selective for methane over nitrogen, where the first permeate is collected as a methane-rich product, the first residue is processed to recover additional methane, and the resulting permeate is recycled back to the feed, with optional compression and temperature adjustment of the residue.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If cryogenic separation is used to remove nitrogen, then nitrogen rejection efficiency is improved, but equipment size and capital cost increase significantly
Solution Approach 1:
The separation process is divided into multiple membrane stages (first membrane stage, second membrane stage, third membrane stage) that work in sequence. Each stage performs a portion of the nitrogen rejection task, with permeates and residues being redirected to subsequent stages. This segmentation allows achieving high nitrogen rejection efficiency without requiring a single large-scale cryogenic unit, thereby reducing overall equipment size and capital cost.
2Measurement precision
If low temperature operation is used to increase CH4/N2 selectivity, then separation performance is improved, but cooling requirements and energy consumption increase
Solution Approach 1:
The membrane system self-regulates temperature through the Joule-Thomson effect. As gas passes through the membranes from high to low pressure, the expansion cooling effect naturally lowers the temperature of the gas stream, increasing CH4/N2 selectivity without requiring external cooling systems. The process uses its own pressure differential to generate the necessary cooling, eliminating the need for separate refrigeration equipment and reducing energy consumption.
3Productivity
If multi-stage membrane process is used to achieve high methane recovery, then methane recovery is improved, but process complexity and number of heat exchangers increase
Solution Approach 1:
Multiple membrane stages are combined into an integrated system where permeates and residues from earlier stages are fed into subsequent stages. The first membrane stage produces permeate and residue; the residue goes to the second membrane stage, and its permeate goes to the third membrane stage. This merging of stages allows high methane recovery by capturing methane that passes through one stage in subsequent stages, while sharing common infrastructure and reducing overall process complexity compared to separate treatment systems.
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 effectively reduces nitrogen levels to pipeline quality, achieving methane purity of less than 4 mol% without the need for extensive cooling or heating schemes, making it more economical for handling modest nitrogen levels in natural gas or biogas processing.
Implementation Method 1
a first membrane stage for processing a nitrogen and methane-containing feed to produce a first permeate and a first residue, the first membrane stage comprising membranes selective for methane over nitrogen
Implementation Method 2
The incoming feed gas is cooled to a sub-ambient temperature by a combination of residue and permeate streams; the cooling is generated by the Joule-Thomson effect of the membranes.
Data Source
AI summary
A feed containing methane and nitrogen gas is processed in a three-stage membrane system, each stage of which is selective for methane over nitrogen. The methane enriched permeate from the first stage is removed as product gas. The methane-depleted residue from the second stage is purified in second and third cascaded stages to provide second and third permeates and second and third residues. The third stage permeate is recycled to the feed.