BOG Reliquefaction with Nitrogen Extraction to Reduce Compression Load
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Solution Overview
Problem
Conventional BOG reliquefaction systems suffer from nitrogen accumulation, leading to decreased natural gas quality and inefficient energy consumption due to the increased nitrogen content, which affects the system's efficiency over time.
Innovation Solution
A method involving multi-stage compression and cooling, followed by isenthalpic expansion and phase separation, effectively separates nitrogen-enriched gas for use as a fuel, while reducing the load on compression stages by extracting flash gas, utilizing existing cooling capacity, and optimizing energy use.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional BOG reliquefaction systems are used, then BOG can be reliquefied and returned to the LNG tank, but nitrogen accumulates in the gas mixture over time, decreasing natural gas quality and system efficiency
Solution Approach 1:
The patent extracts nitrogen from the BOG mixture by utilizing the temperature difference during compression and cooling processes. The nitrogen, having a lower boiling point than methane, remains in the gaseous phase while methane condenses into liquid, allowing nitrogen to be separated and removed from the system through the compression and phase separation process.
Solution Approach 2:
The patent changes the temperature and pressure parameters during the compression and cooling process to exploit the different boiling points of nitrogen and methane. By controlling the cooling temperature to be above nitrogen's boiling point but below methane's condensation point, the system selectively condenses methane while leaving nitrogen in the gas phase for removal.
2Quantity of substance
If nitrogen content in the gas mixture increases, then compression unit capacity is consumed by nitrogen, but this leads to decreased system efficiency and increased energy consumption
Solution Approach 1:
The patent removes nitrogen from the compression cycle by separating it during the cooling and phase separation process. This extracted nitrogen does not re-enter the compression unit, thereby preserving compression capacity for methane and reducing the energy required to compress nitrogen, which has different compression characteristics than methane.
3Loss of substance
If multi-stage compression and cooling is implemented, then nitrogen can be separated from the gas mixture, but the system complexity increases
Solution Approach 1:
The patent merges the nitrogen separation function with the existing BOG compression and cooling system. By integrating phase separation vessels and utilizing the temperature differences already present in the compression-cooling process, the system achieves nitrogen removal without adding completely separate and complex separation equipment.
Solution Approach 2:
The system utilizes its own compression and cooling processes to create the temperature conditions necessary for nitrogen separation. The compressed gas naturally cools during the compression process, and this temperature drop is leveraged to induce selective condensation of methane while nitrogen remains gaseous, requiring minimal additional active cooling infrastructure.
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
Efficiently reduces nitrogen enrichment, enhances system efficiency by minimizing compression load, and optimizes energy consumption, allowing for flexible fuel adaptation to engines and improved reliquefaction rates.
Implementation Method 1
Compressing the BOG in a first compression stage to a first pressure p1 between 8 and 18 bara and diverting a first portion of this gas; further compressing a second portion of the gas from step b) in a final compression stage to a second pressure p2 ≥ 120 bara
Implementation Method 2
Cooling at least a portion of the further compressed gas from step c) to a first temperature T1 between -20°C and -100°C
Implementation Method 3
Expanding the gas from step d) to a third pressure p3 between 8 and 20 bara; separating the gas from step e) into a liquid phase and a gaseous phase
Implementation Method 4
Separating the gas from step e) into a liquid phase and a gaseous phase in order to combine the gaseous phase with the diverted first part of the gas from step c) and return the liquid phase to the LNG tank
Data Source
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AI summary
A process and apparatus for reliquefaction and recycling of boil-off gas (BOG) into a liquefied natural gas (LNG) tank, wherein the process comprises: removing BOG (F2) from the headspace of an LNG tank (3); compressing the BOG in a first compression stage (70a) to a first pressure p1 between 8 and 18 bara and branching off a first portion of said gas; further compressing a second portion of the gas from step in a last compression stage (70b) to a second pressure p2 ≥ 120 bara; cooling at least a portion of the further-compressed gas to a first temperature T1 between -20°C and -100°C; expanding the gas from step to a third pressure p3 between 8 and 20 bara; and separating the gas from step into a liquid phase and a gaseous phase in order to combine the gaseous phase with the first portion of the gas branched off from the first compression stage and in order to recycle the liquid phase into the LNG tank (3).