LNG Sub-Cooling Cycle With Distillation for Higher Denitrogenation Yield
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Solution Overview
Problem
Existing methods for processing LNG using semi-open inverted Brayton cycles have limited yield and are difficult to implement, with maximum yield limited to approximately 40% and operation complexities.
Innovation Solution
The method involves sub-cooled LNG expansion in an intermediate turbine, followed by cooling and expansion, introduction into a distillation column to separate denitrogenated LNG and gas, with a second refrigeration cycle that includes compressing a portion of the gas to high pressure, separating streams for cooling and sub-cooling, and reintroducing them into the system for reheating and compression, optimizing the refrigeration cycle for improved efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If a semi-open inverted Brayton cycle is used for LNG sub-cooling, then the process can be implemented, but the yield is limited to approximately 40% and the operation is difficult to implement
Solution Approach 1:
The refrigeration system is divided into multiple independent circuits: a first circuit using natural gas condensation/evaporation for initial cooling, and a second circuit using nitrogen cycle for sub-cooling. This segmentation allows each circuit to operate independently with optimized parameters, achieving over 40% yield while simplifying operational control through modular architecture
Solution Approach 2:
Nitrogen is introduced as an intermediary refrigerating fluid in the second circuit. The nitrogen cycle acts as a mediator between the compression system and the LNG, enabling efficient heat transfer and expansion processes that improve yield while maintaining operational simplicity through well-understood nitrogen properties
2Ease of operation
If the refrigeration cycle is simplified for easier implementation, then ease of operation improves, but yield may be reduced
Solution Approach 1:
The system segments refrigeration functions into two independent circuits with distinct purposes: the first circuit handles bulk cooling through hydrocarbon phase changes, while the second circuit handles precision sub-cooling through nitrogen expansion. This segmentation achieves high yield (>40%) while maintaining operational simplicity through clear functional separation
Solution Approach 2:
The system utilizes parameter changes in the nitrogen cycle, particularly the phase transition and expansion of nitrogen from high-pressure gaseous state to low-pressure state in the turbine. This parameter change enables efficient energy conversion and high yield while keeping the operational protocol straightforward through predictable thermodynamic behavior
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 increases the yield beyond 44% and simplifies implementation by continuously supplying refrigerating fluid, allowing for higher denitrogenated LNG production and improved efficiency in LNG production units with equal power consumption.
Implementation Method 1
the stream of sub-cooled LNG is expanded in a dynamic manner in an intermediate turbine, maintaining this stream substantially in the liquid state
Implementation Method 2
the stream from the intermediate turbine is cooled and expanded and then introduced into a distillation column; a stream of denitrogenated LNG at the bottom of the column and a stream of gas at the top of the column are recovered
Implementation Method 3
the stream of LNG is sub-cooled in the first heat-exchanger by means of heat-exchange with a refrigerating fluid
Implementation Method 4
the top stream of gas is compressed in a stage compressor, and, at an intermediate pressure stage of the compressor, a first portion of the top stream of gas which is compressed at an intermediate pressure PI is extracted
Implementation Method 5
the main cooling stream is expanded substantially to the low pressure PB in a main turbine
Implementation Method 6
the compressed stream of refrigerating fluid is cooled in a second heat-exchanger
Implementation Method 7
the substantially liquid sub-cooling stream is evaporated in the first heat-exchanger in order to form a reheated sub-cooling stream
Data Source
AI summary
In this method, the LNG stream is cooled using a refrigerating fluid in a first heat-exchanger. The refrigerating fluid is subjected to a second semi-open refrigeration cycle which is independent of the first cycle. The method comprises a step for introducing the stream of sub-cooled LNG into a distillation column and a step for recovering a stream of gas at the top of the column.The second refrigeration cycle comprises a step for forming a stream of refrigerating fluid from a portion of the top stream of gas, a step for compressing the stream of refrigerating fluid to a high pressure, then a step for expanding a portion of the stream of compressed refrigerating fluid in order to form a substantially liquid sub-cooling stream. The substantially liquid stream is evaporated in the first heat-exchanger.