Method and apparatus for pre-heating LNG boil-off gas to ambient temperature prior to compression in a reliquefaction system
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Current LNG reliquefaction systems face inefficiencies due to low-temperature BOG compression, which results in high power demands and unpredictable cooling duties, leading to reduced performance and increased energy consumption.
Innovation Solution
A method and apparatus that pre-heat the BOG to ambient temperatures before compression, using a closed-loop coolant circuit for heat exchange, and control the pressure and composition of vent gas, allowing for efficient heat rejection and optimized operation by separating the BOG compressor's discharge pressure from the reservoir pressure.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Volume of moving object
If low-temperature BOG compression is used, then compressor size is reduced, but power demand increases
Solution Approach 1:
The patent applies preliminary action by pre-cooling the BOG stream before compression using a dedicated pre-cooler heat exchanger. This pre-cooling step reduces the suction temperature to the compressor, increasing the gas density and allowing for a more compact compressor design while maintaining acceptable power consumption levels. The pre-cooler is integrated into the overall heat exchange network, utilizing cold refrigerant from the reliquefaction process.
Solution Approach 2:
The patent employs parameter changes by optimizing the suction temperature and pressure parameters of the BOG compressor. By controlling the pre-cooling degree and adjusting operating parameters, the system achieves an optimal balance between compressor size and power demand. The compressor operates at elevated pressures (e.g., 20-50 bar) with controlled suction temperatures to maximize volumetric efficiency while managing power consumption.
2Productivity
If low-temperature BOG compression is used, then compressor efficiency is improved, but cooling duty becomes unpredictable
Solution Approach 1:
The patent implements feedback control through temperature and pressure sensors that continuously monitor the BOG stream conditions before compression. This feedback mechanism allows the control system to adjust the pre-cooling duty and compressor operation in real-time, maintaining stable and predictable cooling requirements. The feedback loop ensures that the system adapts to varying BOG generation rates and environmental conditions.
Solution Approach 2:
The system applies self-service by integrating the BOG compression cooling duty into the overall reliquefaction heat exchange network. The pre-cooler and compressor intercooler are thermally coupled with the refrigeration system, allowing the compression heat to be automatically utilized for the reliquefaction process. This self-service approach stabilizes cooling duties by making the compression heat rejection an integral part of the heat balance.
3Loss of energy
If BOG is compressed at low temperature, then heat of compression is retained in the system, but system efficiency decreases
Solution Approach 1:
The patent converts the potentially harmful heat of compression into a beneficial resource by integrating it into the reliquefaction heat exchange network. The intercooler and aftercooler of the BOG compressor are thermally coupled with the refrigerant system, allowing the compression heat to be recovered and utilized for cooling the BOG stream during reliquefaction. This heat recovery approach eliminates energy losses and improves overall system efficiency.
Solution Approach 2:
The patent merges the BOG compression process with the reliquefaction heat exchange network by integrating the compressor intercoolers and aftercoolers into the refrigeration circuit. This merging allows simultaneous heat rejection from compression and heat absorption for reliquefaction, creating a thermally integrated system that improves efficiency. The compressed BOG stream and refrigerant streams are thermally coupled in heat exchangers, combining two functions into a unified thermal management system.
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 reduces energy consumption, stabilizes compression conditions, and enhances the overall efficiency of the reliquefaction process by controlling vent gas generation and composition, thereby optimizing the operation of LNG reliquefaction systems.
Implementation Method 1
pre-heating the BOG to substantially ambient temperatures, by heat exchanging the BOG with said coolant in a first heat exchanger
Implementation Method 2
the necessary duty to heat the BOG prior to compression is transferred from the coolant stream
Implementation Method 3
using a closed-loop coolant circuit for heat exchange
Implementation Method 4
compressing the BOG
Implementation Method 5
heat exchanging the compressed BOG against a coolant in a cold box
Implementation Method 6
cooling an LNG boil-off gas (BOG) stream
Implementation Method 7
then expanded to further cool and at least partially liquefy the boil-off stream
Implementation Method 8
control the pressure and composition of vent gas
Data Source
Figure 1
AI summary
A method and an apparatus of pre-heating LNG boil-off gas (BOG) stream (1) flowing from a reservoir (74) in a reliquefaction system, prior to compression (C11, C12, C13). The method comprises heat exchanging the BOG stream in a first heat exchanger (H10), against a second coolant stream (59) having a higher temperature than the BOG stream (1), where the second coolant stream (59) is obtained by selectively splitting a first coolant stream (56) into said second coolant stream (59) and a third coolant stream (57), said third coolant stream being flowed into a first coolant passage in a reliquefaction system cold box (H20), whereby the BOG has reached near-ambient temperatures prior to compression and the low temperature duty from the BOG is substantially preserved within the reliquefaction system, and thermal stresses in the cold box (H20) are reduced. Prior to the compression step, the BOG is pre-heated to substantially ambient temperatures, by heat exchanging (H10) the BOG with said coolant, said coolant prior to the heat exchange having a higher temperature than the BOG.