Method and unit for processing a gas mixture containing nitrogen and methane
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Solution Overview
Problem
Existing methods for liquefying gas mixtures containing nitrogen and methane, such as natural gas, face inefficiencies in achieving high nitrogen purity and methane recovery, particularly when nitrogen content is high, leading to excessive methane loss and inefficient use of vapor phases.
Innovation Solution
The method involves separate heat exchangers for liquefying the gas mixture and partially liquefying the vapor phase, with a mixed refrigerant circuit that allows for flexible refrigerant composition and independent temperature control, enabling the production of a nitrogen-rich top fraction with at least 99 mol% nitrogen without additional compressors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a single mixed refrigerant circuit is used for both liquefaction of gas mixture and partial liquefaction of vapor phase, then device complexity is reduced, but temperature control precision deteriorates
Solution Approach 1:
The single mixed refrigerant circuit is segmented into two separate heat exchangers with independent temperature control systems. The first heat exchanger is dedicated to liquefying the gas mixture, while the second heat exchanger handles partial liquefaction of the vapor phase. This segmentation allows each heat exchanger to be optimized for its specific function, maintaining temperature control precision while using a unified refrigerant circuit architecture.
2Manufacturing precision
If separate heat exchangers are used for liquefaction and partial liquefaction, then temperature control precision is improved, but device complexity increases
Solution Approach 1:
While two separate heat exchangers are employed for precise temperature control, they are integrated into a single mixed refrigerant circuit system. The refrigerant flows sequentially through both heat exchangers, sharing common refrigerant supply and return lines. This merging approach allows independent temperature control in each heat exchanger while avoiding the full complexity of two completely separate refrigerant circuits.
3Manufacturing precision
If high nitrogen purity is achieved through conventional methods, then nitrogen purity is improved, but methane loss increases
Solution Approach 1:
The method utilizes controlled phase transitions in two stages: first, the gas mixture is liquefied in the first heat exchanger, then the liquid is partially vaporized and partially re-liquefied in the second heat exchanger. This two-stage phase transition process exploits the different volatility characteristics of nitrogen and methane, allowing nitrogen to be preferentially separated in the vapor phase while methane remains in the liquid phase, achieving high nitrogen purity with minimal methane loss.
4Productivity
If vapor phase is fully utilized in conventional processes, then productivity is improved, but methane recovery efficiency deteriorates
Solution Approach 1:
The method extracts and separates the vapor phase from the liquid phase in the second heat exchanger, taking out the nitrogen-enriched vapor for purification while retaining the methane-enriched liquid for further processing or storage. This extraction approach allows the vapor phase to be fully utilized for nitrogen production without sacrificing methane recovery, as the methane remains in the liquid phase and can be recovered separately.
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 enhances the efficiency of gas processing by achieving high nitrogen purity and optimizing temperature control, allowing for the economical recovery of methane and reducing nitrogen content in the liquefied gas, thereby improving the overall process efficiency.
Implementation Method 1
the gas mixture is at least partly liquefied using a mixed refrigerant circuit
Implementation Method 2
liquefying the gas mixture and partially liquefying the vapor phase in separate heat exchangers
Implementation Method 3
a liquid phase, which is depleted in nitrogen and enriched with methane relative to the gas mixture, and a vapor phase, which is enriched with nitrogen and depleted in methane relative to the gas mixture, are formed in the storage tank
Implementation Method 4
subjected to low-temperature rectification, wherein a top fraction rich in nitrogen and lean in methane and a bottom liquid lean in nitrogen and rich in methane are formed
Implementation Method 5
at least some of the vapor phase is compressed, at least partly liquefied
Data Source
AI summary
A method for processing a gas mixture containing nitrogen and methane, the gas mixture being at least partly liquefied using a mixed refrigerant circuit and is expanded in a storage tank, wherein: formed in the storage tank are a liquid phase, which is depleted in nitrogen and enriched with methane relative to the gas mixture, and a vapour phase, which is enriched with nitrogen and depleted in methane relative to the gas mixture; at least some of the vapour phase is compressed, at least partly liquefied, and subjected to low-temperature rectification; and formed in the low-temperature rectification are a top gas rich in nitrogen and lean in methane, and a bottom liquid lean in nitrogen and rich in methane. The invention provides that the partial liquefaction of the vapour phase is caused by cooling by means of heat exchange using the mixed refrigerant circuit.