Two-Stage LNG Nitrogen Rejection With Flash Drum Reflux
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Solution Overview
Problem
Conventional LNG processes struggle to efficiently remove nitrogen from liquefied natural gas streams, leading to excessive nitrogen rejection in fuel gas, which is not compatible with low nitrogen oxide burners, and require additional equipment like heat pumps and compressors, impacting plant performance.
Innovation Solution
A two-stage nitrogen removal process using a distillation column and a flash drum, where the liquefied natural gas is first fractionated to produce a high-purity nitrogen-enriched overhead vapour, and then further processed in a flash drum to achieve a nitrogen-freed LNG product with minimal additional equipment and heat transfer modifications.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If conventional fractionation with refrigeration or heat pump streams is used to remove nitrogen from LNG, then nitrogen removal efficiency is improved, but device complexity and energy consumption increase due to additional heat pumps and compressors
Solution Approach 1:
The invention extracts the nitrogen removal function from the complex refrigeration/heat pump system and implements it through a simple flash drum separator. The flash drum utilizes pressure reduction and phase separation to remove nitrogen without requiring additional compressors or heat pumps, thereby maintaining nitrogen removal efficiency while significantly reducing equipment complexity
Solution Approach 2:
The flash drum system uses the natural phase behavior of nitrogen in LNG under pressure reduction to achieve separation. The system serves itself by utilizing the inherent properties of the LNG stream rather than requiring external refrigeration or heat pump assistance, eliminating the need for additional complex equipment
2Object-affected harmful factors
If conventional fractionation with refrigeration streams is used to remove nitrogen from LNG, then nitrogen content in fuel gas is reduced, but energy consumption increases due to additional refrigeration requirements
Solution Approach 1:
The invention replaces the mechanical refrigeration system with a thermodynamic pressure reduction process. The flash drum uses pressure reduction to induce phase separation, substituting the need for mechanical refrigeration compressors and heat pumps with a simpler pressure control mechanism, thereby reducing energy consumption while achieving the same nitrogen removal objective
Solution Approach 2:
The invention changes the operating parameters from constant temperature refrigeration to variable pressure flash separation. By controlling pressure reduction in the flash drum, the system achieves nitrogen separation through phase equilibrium changes rather than requiring continuous refrigeration, significantly reducing energy consumption
3Loss of substance
If high concentration nitrogen removal is achieved through fractionation, then nitrogen rejection to atmosphere is reduced, but plant performance is impacted due to additional equipment requirements
Solution Approach 1:
The invention segments the nitrogen removal process into two stages: a distillation column for high-concentration nitrogen removal and a flash drum for additional nitrogen rejection control. This segmentation allows the system to achieve both high nitrogen removal efficiency and flexible control over atmospheric nitrogen rejection without compromising overall plant performance, as each stage operates independently with optimized function
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 effectively reduces nitrogen content in LNG to environmentally friendly levels for gas turbines, allowing for efficient nitrogen rejection without significant equipment changes, optimizing plant performance and utilizing the LNG product for refrigeration in the nitrogen separation process.
Implementation Method 1
subjecting the liquefied natural gas stream to a first fractionation in a distillation column to provide a first nitrogen-enriched overhead vapour stream and a nitrogen-containing bottoms liquid stream
Implementation Method 2
refluxing said distillation column with at least a portion of the condensed portion of the first nitrogen-enriched overhead vapour
Implementation Method 3
subjecting said sub-cooled, pressure reduced portion to a second fractionation in a flash drum to provide a second nitrogen-enriched overhead vapour stream that is of lower purity than said first overhead vapour stream and a purified liquefied natural gas stream
Data Source
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AI summary
Nitrogen is removed from a liquefied natural gas feed (41 ) by a two stage separation in which the liquefied natural gas (41 ) is first fractionated (23) to provide a first nitrogen-enriched overhead vapour stream (46) and a nitrogen- containing bottoms liquid stream (19) and then at least a portion of said bottoms liquid stream (19) is fractionated (25) to provide a second nitrogen-enriched overhead vapour stream (36) that is of lower purity than said first overhead vapour stream (46) and a purified liquefied natural gas stream (50). The first fractionation is conducted in a distillation column (23) refluxed (45) with nitrogen overhead (43) condensed in a condenser (24) located in a flash drum (25) in which the second fractionation is conducted. The provision of two nitrogen-containing streams (26, 36) of different concentration permits control of the nitrogen content of fuel gas for use in the natural gas liquefaction plant.