LNG Subcooling Using Liquid Nitrogen to Prevent Flash Evaporation
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Solution Overview
Problem
The evaporation of liquefied natural gas during transfer to storage facilities is a challenge due to temperature and nitrogen content variations, leading to energy loss and operational inefficiencies in existing methods, especially with the decline of steam propulsion and the complexity of reliquefaction units.
Innovation Solution
A process involving a refrigeration cycle that includes liquefying a natural gas stream and a nitrogen stream in a main heat exchanger, followed by subcooling the liquefied natural gas using a countercurrent liquid nitrogen flow, with the nitrogen stream being reintroduced and vaporized to adjust subcooling temperatures based on nitrogen content, utilizing a Turbo-Brayton nitrogen cycle.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Volume of moving object
If the liquefied natural gas is stored and transported in tanks, then it occupies smaller volume and is easier to transport, but heat losses through insulation cause evaporation and excess pressure
Solution Approach 1:
The patent applies preliminary action by subcooling the liquefied natural gas below its equilibrium temperature before storage and transport. This pre-cooling ensures that even with heat losses from insulation, the gas remains below the evaporation point. The system includes a subcooling unit that cools the LNG to a temperature lower than the equilibrium temperature corresponding to the storage pressure, preventing evaporation during transit.
Solution Approach 2:
The patent changes the temperature parameter of the liquefied natural gas by implementing dynamic subcooling. The system adjusts the subcooling temperature based on the nitrogen content and pressure conditions. By lowering the temperature parameter below the equilibrium point, the system compensates for heat losses and prevents the gas from reaching evaporation conditions during storage and transport.
2Use of energy by moving object
If steam propulsion is used to evaporate and burn boil-off gas, then evaporation is utilized for energy production, but steam propulsion is disappearing and being replaced by more energy efficient methods
Solution Approach 1:
The patent converts the harmful evaporation (boil-off gas) into a beneficial cooling resource. Instead of burning or venting the evaporated gas, the system captures the cold nitrogen-rich vapor and uses it in a heat exchanger to subcool incoming liquefied natural gas. This transforms waste heat loss into a useful cooling effect, reducing the need for additional refrigeration and improving overall energy efficiency.
Solution Approach 2:
The system implements self-service by using its own boil-off gas to provide cooling for the liquefied natural gas. The nitrogen-rich vapor that would normally be wasted is recirculated through the heat exchanger system, where it automatically cools the incoming LNG without requiring external energy input. This self-sustaining cooling mechanism adapts to various propulsion methods without requiring steam engines.
3Object-generated harmful factors
If reliquefaction units are used to treat boil-off gases, then evaporation is eliminated, but the units become more complex and expensive with additional safety and environmental concerns
Solution Approach 1:
The patent merges the subcooling function with the existing refrigeration cycle by integrating a heat exchanger that utilizes nitrogen-rich vapor from the system. Instead of adding a separate reliquefaction unit, the system combines the cooling of incoming LNG with the utilization of boil-off gas in a single integrated heat exchange process. This eliminates the need for complex additional equipment while achieving the same effect.
Solution Approach 2:
The patent introduces a heat exchanger as an intermediary device that facilitates the transfer of cold from nitrogen-rich vapor to incoming liquefied natural gas. This intermediary component enables the system to utilize boil-off gas for subcooling without requiring direct contact or complex mixing, simplifying the overall system architecture compared to traditional reliquefaction units while effectively preventing evaporation.
4Object-generated harmful factors
If subcooling temperature is adjusted based on nitrogen content, then evaporation is prevented, but this requires modifying liquefaction unit parameters which reduces efficiency
Solution Approach 1:
The patent segments the temperature control function into two independent parts: the liquefaction unit maintains its fixed, optimized parameters for efficient gas liquefaction, while a separate subcooling unit adjusts the final temperature based on nitrogen content and pressure conditions. This segmentation allows each unit to operate independently at its optimal parameters, preventing evaporation without compromising liquefaction efficiency.
Solution Approach 2:
The system performs preliminary subcooling after the main liquefaction process. The liquefaction unit produces LNG at its standard operating temperature, and then a separate subcooling stage further reduces the temperature to the appropriate level for the specific nitrogen content and pressure conditions. This preliminary action in a separate stage avoids the need to modify liquefaction unit parameters, maintaining its efficiency while achieving the required temperature adjustment.
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 effectively prevents evaporation of liquefied natural gas during transfer by adjusting subcooling temperatures according to nitrogen content, optimizing energy expenditure and maintaining process efficiency without altering liquefaction unit parameters.
Implementation Method 1
cooling of the liquefied natural gas stream from step a) in a second heat exchanger by circulation of said liquefied natural gas stream countercurrent to a liquid nitrogen flow that is vaporized while cooling said liquefied natural gas stream
Implementation Method 2
liquid nitrogen flow that is vaporized while cooling said liquefied natural gas stream
Implementation Method 3
liquefaction, by means of a refrigeration cycle, of a natural gas stream and of a nitrogen stream in a main heat exchanger
Data Source
AI summary
Process for eliminating the evaporation of a liquefied natural gas stream during the transfer thereof into a storage facility, comprising the following steps:Step a): liquefaction, by means of a refrigeration cycle, of a natural gas stream and of a nitrogen stream in a main heat exchanger;Step b): cooling of the liquefied natural gas stream from step a) in a second heat exchanger by circulation of said liquefied natural gas stream countercurrent to a liquid nitrogen flow that is vaporized while cooling said liquefied natural gas stream;wherein the liquid nitrogen flow used in step b) is from step a).