Managing make-up gas composition variation for a high pressure expander process
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Solution Overview
Problem
Existing high-pressure expander cycles for liquefied natural gas (LNG) production are inefficient and require external refrigeration, leading to high recycle gas stream flow rates and inefficiencies, while variations in feed gas composition affect liquefaction performance and start-up operations.
Innovation Solution
A method involving a high-pressure expander process with a primary cooling loop using a refrigerant stream compressed to high pressures, combined with a sub-cooling loop, and a system to manage refrigerant composition variations through a separation vessel to condition make-up gas, ensuring efficient liquefaction and start-up.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a single high pressure expander is used to meet the entire expansion requirement, then the number of machines is reduced, but the machine must operate across a very wide range of inlet conditions (30-150 psig) which compromises reliability and efficiency
Solution Approach 1:
The single expander is divided into multiple expanders (first high pressure expander and second high pressure expander) that operate in sequence or parallel. Each expander handles a portion of the total expansion requirement and operates within a narrower, more efficient inlet pressure range (e.g., 75-150 psig for the first, 30-75 psig for the second), improving reliability and efficiency while meeting the overall expansion demand from 30-150 psig inlet conditions.
2Device complexity
If a single high pressure expander operates over a wide inlet pressure range (30-150 psig), then device complexity is reduced, but manufacturing precision and operational efficiency deteriorate due to inability to maintain optimal operating conditions
Solution Approach 1:
The expansion process is segmented into multiple stages, with each expander designed for a specific inlet pressure range. The first expander is optimized for higher inlet pressures (75-150 psig) and the second for lower inlet pressures (30-75 psig), allowing each device to maintain consistent, high-performance operating conditions within its designated range rather than one expander attempting to handle the full 30-150 psig range.
Solution Approach 2:
The system dynamically selects which expander(s) to operate based on the incoming gas pressure conditions. When inlet pressure is high (75-150 psig), the first expander operates; when inlet pressure is lower (30-75 psig), the second expander operates. This dynamic configuration allows the system to maintain optimal operating conditions across varying feed conditions without sacrificing performance consistency.
3Adaptability or versatility
If make-up gas composition varies significantly (high CO2 vs high N2 content), then feed flexibility is improved, but expander performance and efficiency worsen due to composition-dependent operating conditions
Solution Approach 1:
The multi-expander configuration allows the system to handle different make-up gas compositions more effectively. When CO2-rich make-up gas is used, the additional CO2 can be managed through the multi-stage expansion process without significantly impacting the performance of individual expanders. Each expander operates within its optimized pressure range, maintaining efficiency even as composition varies between high CO2 and high N2 scenarios.
Solution Approach 2:
The system adjusts operating parameters (which expander operates, flow distribution between expanders) based on make-up gas composition. For CO2-rich compositions, the system can optimize the expansion process to account for CO2's different thermodynamic properties compared to N2, maintaining expander efficiency across composition variations by dynamically adjusting the expansion pathway and operating conditions.
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
Enhances LNG production efficiency and capacity, particularly at lower feed gas pressures, by reducing refrigerant flow rates and managing composition changes, thus improving overall process performance.
Implementation Method 1
a first high pressure expander to expand a portion of the synthesis gas
Implementation Method 2
expansion of the synthesis gas in the high pressure expander
Implementation Method 3
a synthesis loop compressor to compress synthesis gas
Implementation Method 4
process heat exchanger(s) to preheat the feed gas using heat from the hot synthesis gas leaving the high pressure expander
Data Source
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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.