Gaseous Methane Expander Cycle for Floating LNG Platform Liquefaction
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Solution Overview
Problem
Conventional liquefaction processes for natural gas face challenges in floating platforms due to two-phase flow issues and the need for minimal flammable refrigerant inventory, with existing methods having lower efficiency and larger equipment requirements.
Innovation Solution
A method and system utilizing a gaseous expander cycle with methane or natural gas as refrigerant, employing multiple stages of expansion and end-flash refrigeration to liquefy and sub-cool natural gas, eliminating external refrigerants and minimizing flammable refrigerant usage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional mixed refrigerant cycles are used, then cooling duty for liquefying natural gas is provided, but two-phase flow issues and liquid-vapor maldistribution occur on floating platforms
Solution Approach 1:
The invention changes the refrigerant from a mixed refrigerant to pure methane, fundamentally altering the thermodynamic parameters and eliminating two-phase flow issues in the refrigeration cycle. This parameter change allows the system to operate reliably on floating platforms without liquid-vapor maldistribution problems.
Solution Approach 2:
The invention uses methane as a single-component refrigerant that circulates in a controlled gaseous state, creating a stable and predictable refrigeration environment. This eliminates the complexity of mixed refrigerant composition and phase behavior, providing operational simplicity and reliability on floating platforms.
2Reliability
If liquefied refrigerant is used, then cooling duty is provided, but liquid sloshing causes additional mechanical stresses
Solution Approach 1:
The invention changes the refrigerant state parameter from liquid to gas throughout the entire refrigeration cycle. By maintaining methane in a gaseous state and avoiding liquidification, the system eliminates liquid sloshing and associated mechanical stresses on floating platforms while still providing effective cooling duty.
3Object-affected harmful factors
If nitrogen recycle expander process is used, then flammable refrigerant inventory is minimized, but process efficiency is lower and equipment size is larger
Solution Approach 1:
The invention uses methane, which is inherently present in the natural gas being processed, as the refrigerant. This self-service approach eliminates the need for separate nitrogen inventory while maintaining high process efficiency. The methane refrigerant is obtained directly from the feed gas, turning a potential byproduct into a useful resource.
Solution Approach 2:
The invention makes methane serve dual functions: as the primary component of the natural gas being processed and as the refrigerant for liquefaction. This multi-functionality eliminates the need for separate refrigerant inventory (unlike nitrogen-based systems) while achieving high efficiency through the integrated nature of the process.
4Reliability
If mixed refrigerant cycles are used, then cooling duty is provided, but large quantities of refrigerant inventory are required
Solution Approach 1:
The invention uses methane that is already present in the natural gas feed stream as the refrigerant. This eliminates the need for maintaining large inventories of separate refrigerant components, as the refrigerant is obtained directly from the process feed gas itself.
Solution Approach 2:
The invention changes from using mixed refrigerant compositions to pure methane, fundamentally altering the refrigerant inventory requirements. This parameter change simplifies the system by eliminating the need to store and manage multiple refrigerant components in specific proportions.
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 enhances efficiency, reduces equipment size, and mitigates two-phase flow issues, making it suitable for floating LNG platforms and other applications with space and safety constraints.
Implementation Method 1
the gaseous warm refrigerant is compressed and cooled to form a compressed refrigerant
Implementation Method 2
The compressed refrigerant is then expanded to further cool the refrigerant, resulting in an expanded cold refrigerant
Implementation Method 3
used to liquefy the natural gas via indirect heat exchange between the refrigerant and natural gas
Implementation Method 4
expanding the first LNG stream to further cool and partially vaporize said stream
Implementation Method 5
recovering refrigeration from the second flash gas stream by using said stream to sub-cool, by indirect heat exchange
Data Source
AI summary
Described herein is a method and system for liquefying a natural gas feed stream to produce an LNG product. The natural gas feed stream is liquefied, by indirect heat exchange with a gaseous methane or natural gas refrigerant circulating in a gaseous expander cycle, to produce a first LNG stream. The first LNG stream is expanded, and the resulting vapor and liquid phases are separated to produce a first flash gas stream and a second LNG stream. The second LNG stream is then expanded, with the resulting vapor and liquid phases being separated to produce the second flash gas stream and a third LNG stream, all or a portion of which forms the LNG product. Refrigeration is recovered from the second flash gas by using said stream to sub-cool the second LNG stream or a supplementary LNG stream.