Liquefaction process for producing subcooled LNG
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Solution Overview
Problem
The depletion of natural gas wells is challenging to predict, and existing methods for extending their lifetime are costly and inefficient, particularly due to high liquefaction energy consumption and contamination of natural gas with nitrogen during extraction.
Innovation Solution
A method involving a variable speed liquid LNG expander in series with a variable speed two-phase LNG expander, optimizing rotational speeds to maximize liquid LNG production and minimize vapor and boil-off, thereby reducing energy consumption and extending well lifetime.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If a two-phase LNG expander is used to subcool LNG, then the temperature of liquefied gas is reduced below condensation temperature, but the pressure reduction is relatively small and vaporization heat is consumed
Solution Approach 1:
The system divides the LNG expansion and subcooling process into two separate stages: a first two-phase LNG expander performs initial expansion and partial subcooling, while a second two-phase LNG expander performs additional expansion and subcooling. This segmentation allows each expander to operate optimally and reduces the total vaporization heat consumption compared to a single expander handling the entire pressure reduction.
Solution Approach 2:
The patent employs variable speed drives on both expanders to dynamically adjust their rotational speeds based on process conditions. This dynamic control optimizes the expansion ratio and refrigeration effect of each expander, maximizing subcooling while minimizing unnecessary vaporization and energy consumption.
2Stress or pressure
If nitrogen gas is injected into the gas well to maintain pressure, then the well pressure is sustained, but the natural gas is contaminated with nitrogen and overall liquefaction costs increase
Solution Approach 1:
The system utilizes the phase transition properties of nitrogen and natural gas components during expansion. The two-phase expanders are designed to selectively condense natural gas components while keeping nitrogen in the vapor phase, which is then separated and removed. This phase transition-based separation eliminates nitrogen contamination without requiring nitrogen injection.
Solution Approach 2:
The patent extracts and removes nitrogen from the gas stream using the two-phase expansion process. By operating the expanders at specific conditions, nitrogen is preferentially vaporized and separated from the liquid LNG phase, effectively taking out the harmful nitrogen component and delivering nitrogen-free LNG.
3Device complexity
If a single two-phase LNG expander is used, then the equipment complexity is reduced, but the amount of liquid LNG produced is limited and boil-off downstream increases
Solution Approach 1:
The system segments the LNG production process into two expansion stages, with each expander contributing to liquid LNG production. The first expander produces an intermediate amount of liquid LNG, which is then further processed by the second expander to maximize total liquid production. This segmentation increases overall productivity while keeping each individual expander relatively simple.
Solution Approach 2:
The patent combines the output streams from both expanders and integrates them into a unified LNG production system. The liquid LNG from both expanders is merged and directed to storage or transport, while the vapor streams are combined and reused for refrigeration or fuel, maximizing the utility of both expanders and reducing downstream boil-off.
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 the required feed gas supply, decreases boil-off losses, and extends the lifetime of gas wells while maintaining production levels, with a payback time of less than six months for investment in LNG expanders.
Implementation Method 1
Evaporation cooling occurs at the liquid-vapor interface. A liquid-to-vapor phase change process requires vaporization heat, which is extracted from the remaining liquid part. Consequently any partial vaporization of a liquid cools the remaining part of the liquid.
Implementation Method 2
A liquid-to-vapor phase change process requires vaporization heat, which is extracted from the remaining liquid part. Consequently any partial vaporization of a liquid cools the remaining part of the liquid.
Implementation Method 3
The reduction of pressure in two-phase expanders is relatively small compared to the pressure difference across a single phase LNG expander
Data Source
AI summary
A variable speed liquid LNG expander (X1) and a variable speed two-phase LNG expander (X2) in line, downstream from X1. The rotational speed of both expanders can be controlled and changed independent from each other. The speed of expander X1 and expander X2 is determined in such way that the amount of liquid LNG downstream from the PHS compared to the feed gas supply is maximized and the amount of vapor and boil-off downstream of X2 is minimized.