Make-Up Refrigerant Conditioning for Stable High-Pressure LNG Expansion
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Solution Overview
Problem
The high-pressure expander process for natural gas liquefaction faces inefficiencies and logistical challenges due to the need for external refrigerants and sensitivity to feed gas composition variations, particularly at pressures below 1,200 psia, which affects the primary cooling loop's performance and requires managing variations in nitrogen and heavy hydrocarbon contents.
Innovation Solution
A method involving a high-pressure expander process that uses a compressed refrigerant stream expanded in a work-producing expander, mixed with a make-up refrigerant stream to condition and condense heavy hydrocarbon components, forming a cold primary refrigerant mixture for indirect heat exchange with the feed gas stream, optimizing the liquefaction process by allowing the use of methane-rich feed gas and boil-off gas as refrigerants, and adjusting the composition dynamically to match incoming feed gas compositions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If external refrigerants are used in the high-pressure expander process, then cooling performance is improved, but logistical complexity and handling requirements increase
Solution Approach 1:
The system uses the feed gas itself as the refrigerant medium. The feed gas is compressed, cooled, expanded through a turboexpander to produce cold temperatures, and then this cold expanded gas is used to cool the incoming feed gas in the heat exchanger. This eliminates the need for separate external refrigerants and their associated logistics.
Solution Approach 2:
The feed gas serves multiple functions: it is the material to be liquefied, the working fluid for compression, and the refrigerant for cooling. By making the feed gas multi-functional, the system eliminates the need for separate refrigerant systems and reduces logistical complexity.
2Use of energy by moving object
If the primary cooling loop is sensitive to feed gas composition variations, then cooling efficiency is improved under optimal conditions, but process stability deteriorates with composition changes
Solution Approach 1:
The system dynamically adjusts operating parameters including compression pressure, expansion ratio, and heat exchanger conditions to maintain optimal performance despite variations in feed gas composition. This dynamic adaptation allows the system to preserve cooling efficiency while accommodating composition changes.
Solution Approach 2:
The system changes key parameters such as compression pressure (operating at high pressures above 6895 kPa), expansion temperature, and heat exchange conditions to optimize performance for different feed gas compositions. These parameter adjustments maintain both efficiency and stability.
3Volume of stationary object
If high pressure expansion is used for natural gas liquefaction, then equipment size is reduced, but sensitivity to composition variations increases
Solution Approach 1:
The system incorporates feedback mechanisms where the composition and conditions of the feed gas are monitored, and this information is used to adjust compression and expansion parameters. This feedback loop reduces sensitivity to composition variations while maintaining the compact high-pressure design.
Solution Approach 2:
The feed gas undergoes preliminary compression and cooling before expansion. This preliminary preparation stabilizes the gas conditions and reduces the impact of composition variations during the subsequent expansion and liquefaction processes, allowing the compact high-pressure design to function reliably.
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 the thermal efficiency and reduces the size of the main cryogenic heat exchanger, offering flexibility in energy use and production rate optimization, while minimizing the impact of feed gas composition variations, thus improving the overall efficiency and cost-effectiveness of the liquefaction process.
Implementation Method 1
The compressed refrigerant stream is expanded in at least one work producing expander, thereby producing an expanded, cooled refrigerant stream
Implementation Method 2
mixing the expanded, cooled refrigerant stream with a make-up refrigerant stream in a separator, thereby condensing heavy hydrocarbon components from the make-up refrigerant stream
Implementation Method 3
passing the cold primary refrigerant mixture through a heat exchanger zone to cool the feed gas stream by indirect heat exchange with the expanded, cooled refrigerant stream
Data Source
AI summary
A method for liquefying a feed gas stream. A refrigerant stream is cooled and expanded to produce an expanded, cooled refrigerant stream. Part or all of the expanded, cooled refrigerant stream is mixed with a make-up refrigerant stream in a separator, thereby condensing heavy hydrocarbon components from the make-up refrigerant stream and forming a gaseous expanded, cooled refrigerant stream. The gaseous expanded, cooled refrigerant stream passes through a heat exchanger zone to form a warm refrigerant stream. The feed gas stream is passed through the heat exchanger zone to cool at least part of the feed gas stream by indirect heat exchange with the expanded, cooled refrigerant stream, thereby forming a liquefied gas stream. The warm refrigerant stream is compressed to produce the compressed refrigerant stream.