Method for treating a liquefied natural gas stream obtained by cooling using a first refrigerating cycle and related installation
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Solution Overview
Problem
Existing LNG treatment processes using semi-open reverse Brayton cycles suffer from limited efficiency, typically around 40%, and are difficult to implement effectively.
Innovation Solution
A method involving a first refrigeration cycle for liquefying natural gas, followed by subcooling in a first heat exchanger, and a second semi-open refrigeration cycle with staged compression and expansion, where the refrigerant fluid is separated into main and sub-cooling streams, expanded, and reused to enhance efficiency and ease of implementation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If a semi-open reverse Brayton cycle is used for LNG subcooling, then the process can be implemented, but the efficiency is limited to approximately 40%
Solution Approach 1:
The refrigeration cycle is divided into two independent cycles: a first cycle for LNG liquefaction and a second cycle for LNG subcooling. The second cycle is further segmented into a compression section and an expansion section with separate refrigerant streams, allowing independent optimization of each function and improving overall efficiency beyond the limitations of a single integrated reverse Brayton cycle.
Solution Approach 2:
The invention changes the operating parameters of the refrigeration system by using two different refrigerant compositions in the two cycles, optimizing the temperature-pressure profiles for each specific function (liquefaction vs. subcooling). This parameter optimization enables efficiency greater than 44% by matching refrigerant properties to specific process requirements rather than using a single fixed-parameter cycle.
2Ease of operation
If a semi-open reverse Brayton cycle is used for LNG subcooling, then the process can operate, but the operation is difficult to implement
Solution Approach 1:
By separating the refrigeration system into two independent cycles with distinct functions, the invention makes operation easier to control and optimize. Each cycle can be independently adjusted and maintained, reducing the operational complexity compared to a single integrated system while achieving superior efficiency.
Solution Approach 2:
The invention introduces an intermediary heat exchanger that couples the two refrigeration cycles, allowing heat transfer between them without direct fluid mixing. This intermediary approach simplifies operational control by providing thermal coupling while maintaining system independence, making the overall process easier to implement and operate.
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
The process achieves efficiency gains greater than 44%, allowing for increased production of nitrogen-free LNG and improved operational simplicity, with the ability to be implemented in various unit structures and existing LNG production units.
Implementation Method 1
subcooling the LNG stream in the first heat exchanger by heat exchange with a refrigerant fluid
Implementation Method 2
the stream of compressed refrigerant fluid from the second heat exchanger is expanded substantially to the low pressure in a main turbine
Implementation Method 3
a staged compressor and an expansion turbine
Data Source
Figure 1
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AI summary
The invention concerns a method which consists in cooling the LNG stream (11) with a coolant (83) in a first heat exchanger (19). The coolant (83) is subjected to a second semi-open refrigerating cycle (21), independent of the first cycle (15). The method includes a step of introducing the under-cooled LNG stream (59) in a distillation column (49) and a step of recovering a gas stream (69) at the head of the column (49). The second refrigerating cycle (21) includes a step of forming a coolant stream (73) from part of the head gas stream (69), a step of compressing the coolant stream (73) up to a high pressure, then a step of expanding part (81) of the compressed coolant stream (75) to form an essentially liquid under-cooling stream (83). The essentially liquid stream (83) is evaporated in the first heat exchanger (19).