Nitrogen Stripping Column Design for Low-Energy LNG Purification
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Solution Overview
Problem
Existing methods for liquefying natural gas face challenges in maintaining a nitrogen content of ≤1 mol % in LNG while managing high nitrogen levels in the raw gas, often requiring the release of a highly concentrated nitrogen stream, which is energy-intensive and costly.
Innovation Solution
A method involving a nitrogen stripping column and a high-pressure nitrogen column, thermally coupled via a heat exchanger, where the liquefied hydrocarbon-rich fraction is expanded and processed to achieve a nitrogen content of ≤1 mol % without releasing nitrogen/hydrocarbon mixtures, using a refrigeration circuit for cooling and expansion, and a reflux collector for high-purity nitrogen stream extraction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If a highly concentrated nitrogen stream is drawn off and released to the atmosphere, then the nitrogen content in LNG can be maintained at ≤1 mol %, but the process becomes energy-intensive and costly
Solution Approach 1:
The patent applies parameter changes by operating the nitrogen stripping column at elevated pressures (typically 20-50 bar) rather than atmospheric pressure. This pressure parameter change improves the gas-liquid equilibrium, enhancing nitrogen separation efficiency and reducing the energy required for compression and refrigeration, while still achieving the required nitrogen content specification of ≤1 mol % in LNG
Solution Approach 2:
The patent utilizes phase transitions in the nitrogen stripping column where nitrogen is selectively stripped from the liquid hydrocarbon phase into the vapor phase. By controlling temperature and pressure parameters during this phase transition process, the system achieves efficient nitrogen removal with reduced energy consumption compared to conventional atmospheric pressure operations
2Quantity of substance
If nitrogen is separated from the overall raw gas stream in a column, then a highly concentrated nitrogen fraction is obtained, but the separation intensity and energy intensity become comparatively high
Solution Approach 1:
The patent applies preliminary action by performing initial cooling and partial liquefaction of the raw natural gas before feeding it to the nitrogen stripping column. This preliminary preparation at elevated pressure creates favorable conditions for subsequent nitrogen separation, reducing the energy intensity required during the actual separation process while achieving high nitrogen concentration in the stripped stream
3Manufacturing precision
If the nitrogen content in raw gas is high, then it is difficult to simultaneously comply with maximum permissible nitrogen content in both LNG and fuel gas
Solution Approach 1:
The patent applies the extraction principle by removing nitrogen from the raw gas stream in a dedicated nitrogen stripping column operated at elevated pressure. This extraction process produces a highly concentrated nitrogen stream that can be separately managed, while the depleted gas stream meets both LNG and fuel gas nitrogen specifications, simplifying the overall process compared to multiple separation units
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 method reduces nitrogen content in the product fraction to ≤1 mol % with minimal apparatus complexity and energy intensity, achieving on-spec LNG and technical grade purity nitrogen without additional product streams, thus lowering capital and operating costs.
Implementation Method 1
the hydrocarbon-rich fraction is liquefied and subcooled with a refrigeration circuit
Implementation Method 2
the two columns are thermally coupled via a heat exchanger which serves as reboiler for the nitrogen stripping column and as tops condenser for the high-pressure nitrogen column
Implementation Method 3
a) the liquefied and subcooled hydrocarbon-rich fraction is expanded and fed to a nitrogen stripping column
Data Source
AI summary
A method of obtaining a liquefied hydrocarbon-rich fraction (product fraction) having a nitrogen content of ≤1 mol %, wherein the hydrocarbon-rich fraction is liquefied and subcooled with a refrigeration circuit and then subjected to a rectificative removal of nitrogen is disclosed.