Boil-Off Gas Nitrogen Separation for LNG Production Efficiency
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Solution Overview
Problem
Current methods for treating boil-off gas from cryogenic liquid storage tanks are inefficient in recovering liquefied methane and optimizing hydrocarbon composition for use as fuel or regeneration gas in liquefaction plants, leading to suboptimal LNG production and plant efficiency.
Innovation Solution
A process involving the compression and counter-current heat exchange of boil-off gas with a mixed refrigerant to produce a liquid fraction and cooled vapour fraction, where the vapour fraction is depleted of nitrogen, allowing its use as a fuel gas to drive compressors and the liquid fraction is returned to the storage tank, while the vapour fraction, enriched in nitrogen, is used to enhance refrigeration plant efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If boil-off gas is compressed and cooled in a cold box to produce LNG, then LNG production increases, but nitrogen contamination in the LNG occurs
Solution Approach 1:
The patent extracts nitrogen from the boil-off gas before the liquefaction process. A nitrogen separator removes nitrogen-rich gas from the compressed BOG, producing nitrogen-depleted gas that is then cooled in the cold box to produce LNG. This extraction principle resolves the contradiction by removing the harmful nitrogen component before it can contaminate the LNG product.
Solution Approach 2:
The patent segments the boil-off gas treatment process into distinct stages: nitrogen separation, liquefaction, and product separation. By dividing the process, nitrogen removal occurs in a dedicated separation unit, allowing the LNG production system to focus on efficient liquefaction without nitrogen contamination issues.
2Manufacturing precision
If nitrogen separator and control unit are added to remove nitrogen from LNG, then LNG composition purity improves, but system complexity increases
Solution Approach 1:
The nitrogen separator is positioned to perform nitrogen removal before the gas enters the cold box for LNG production. This preliminary action prevents nitrogen from entering the LNG production stream in the first place, eliminating the need for complex post-processing nitrogen removal systems and simplifying the overall configuration.
3Productivity
If boil-off gas is fully liquefied, then LNG production increases, but energy consumption increases
Solution Approach 1:
The patent changes the composition parameter of the gas being liquefied by removing nitrogen before the cold box. Since nitrogen has a lower condensation temperature than methane, removing it allows the liquefaction process to operate more efficiently at the target temperature, reducing the energy required for full liquefaction while maintaining high LNG production.
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 increases LNG production by redirecting boil-off gases for liquefaction, improving the hydrocarbon composition for fuel use and enhancing overall plant efficiency by utilizing the nitrogen-enriched vapour fraction as a fuel and regeneration gas, thereby reducing energy consumption and increasing output.
Implementation Method 1
compressing the boil-off gas
Implementation Method 2
cooling the compressed gas in a refrigeration zone
Implementation Method 3
counter-current heat exchange of boil-off gas with a mixed refrigerant
Implementation Method 4
separating the cooled gas into a liquid fraction and a vapour fraction
Implementation Method 5
compressing the vapour fraction in a second compressor
Data Source
Figure 1~2
AI summary
A flowline system for transferring cryogenic liquids between a cryogenic liquid storage tank and a cryogenic liquid receiving/loading facility, and a method of maintaining the system at or marginally above cryogenic temperature during periods between transfer of cryogenic liquids between the cryogenic liquid storage tank and the cryogenic liquid receiving/loading facility are provided. The flowline system has a main transfer conduit and a vapour return line in fluid communication with the cryogenic liquid storage tank and the cryogenic liquid receiving/loading facility. A cooling medium line is provided that is in fluid communication with the main transfer conduit, the vapour return line, and a source of cooled boil-off gas, wherein the cooled boil-off gas is at or marginally above cryogenic temperature. The cooled boil-off gas is circulated between said tank and said facility through the main transfer conduit and the vapour return line during periods between transfer of cryogenic liquids to maintain the main transfer conduit and the vapour return line at or marginally above cryogenic temperature.