Mixed Refrigerant Precool Loop for Lower LNG Compressor Power
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Solution Overview
Problem
Current gas cooling and liquefaction systems face inefficiencies due to complex equipment and high operating costs, particularly in achieving the necessary temperature range for natural gas liquefaction, with cascaded multilevel processes being the most efficient but still complex, and single mixed refrigerant processes consuming more power due to thermodynamic inefficiencies.
Innovation Solution
A mixed refrigerant system employing simple equilibrium separation of a heavy fraction, which is not entirely vaporized at the primary heat exchanger, reducing compressor power and mechanical complexity, and using the heavy fraction in a pre-cool refrigeration loop to achieve near closure of heating/cooling curves, thereby improving efficiency and reducing energy consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If cascaded multilevel pure component cycles are used to approximate the cooling curves, then the refrigeration efficiency is improved, but the mechanical complexity becomes overwhelming due to additional compressor trains
Solution Approach 1:
The patent extracts the heavy fraction from the mixed refrigerant stream at the interstage drum, separating it from the lighter components. This extracted heavy fraction is then routed to a precool loop where it provides refrigeration in a separate cycle, eliminating the need for additional compressor trains in the main cascaded system while maintaining the efficiency benefits of multilevel refrigeration.
Solution Approach 2:
The refrigeration system is segmented into two distinct loops: the main mixed refrigerant cycle and a separate precool loop using the heavy fraction. This segmentation allows each loop to be optimized independently, with the precool loop handling the warm-end refrigeration requirements and the main cycle focusing on the cold-end liquefaction, thereby reducing overall mechanical complexity.
2Ease of operation
If a single mixed refrigerant process is used to reduce mechanical complexity, then the ease of operation is improved, but the power consumption increases due to thermodynamic inefficiencies
Solution Approach 1:
The single mixed refrigerant process is segmented by extracting the heavy fraction and routing it through a separate precool loop. This creates a two-loop system that maintains operational simplicity while improving thermodynamic efficiency, as the heavy fraction operates in a temperature range where it provides more effective refrigeration.
Solution Approach 2:
The patent changes the operating parameters of the heavy fraction by routing it through a precool loop where it undergoes a different pressure-temperature trajectory. This parameter change allows the heavy fraction to vaporize at temperatures that better match the natural gas cooling curve in the warm-end region, reducing the temperature differences and improving thermodynamic efficiency.
3Use of energy by moving object
If the heavy fraction is not entirely vaporized at the primary heat exchanger to reduce compressor power, then the energy consumption is reduced, but the refrigeration efficiency decreases due to thermodynamic irreversibility
Solution Approach 1:
The heavy fraction is extracted from the mixed refrigerant stream at the interstage drum before it would normally be fully vaporized in the primary heat exchanger. This extraction allows the heavy fraction to be processed separately in a precool loop, where its partial vaporization does not create the same thermodynamic irreversibilities in the main refrigeration cycle.
Solution Approach 2:
The precool loop acts as an intermediary system that handles the heavy fraction separately. This intermediary loop mediates between the need to reduce compressor power (by not fully vaporizing the heavy fraction) and the need to maintain refrigeration efficiency (by providing a dedicated path for the heavy fraction to contribute to cooling in the warm-end region).
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 system reduces compressor power by about 5%, lowers capital costs, and minimizes environmental emissions, resulting in significant savings for natural gas plants by optimizing refrigerant composition and process flow.
Implementation Method 1
simple equilibrium separation of a heavy fraction, which is not entirely vaporized at the primary heat exchanger
Implementation Method 2
Liquefaction is typically accomplished by chilling the gas through indirect heat exchange by one or more refrigeration cycles
Implementation Method 3
the refrigerants vaporize at rising temperatures following the gas cooling curve
Data Source
AI summary
A system and method for cooling and liquefying a gas in a heat exchanger that includes compressing and cooling a mixed refrigerant using first and last compression and cooling cycles so that high pressure liquid and vapor streams are formed. The high pressure liquid and vapor streams are cooled in the heat exchanger and then expanded so that a primary refrigeration stream is provided in the heat exchanger. The mixed refrigerant is cooled and equilibrated between the first and last compression and cooling cycles so that a pre-cool liquid stream is formed and subcooled in the heat exchanger. The stream is then expanded and passed through the heat exchanger as a pre-cool refrigeration stream. A stream of gas is passed through the heat exchanger in countercurrent heat exchange with the primary refrigeration stream and the pre-cool refrigeration stream so that the gas is cooled. A resulting vapor stream from the primary refrigeration stream passage and a two-phase stream from the pre-cool refrigeration stream passage exit the warm end of the exchanger and are combined and undergo a simultaneous heat and mass transfer operation prior to the first compression and cooling cycle so that a reduced temperature vapor stream is provided to the first stage compressor so as to lower power consumption by the system. Additionally, the warm end of the cooling curve is nearly closed further reducing power consumption. Heavy components of the refrigerant are also kept out of the cold end of the process, reducing the possibility of refrigerant freezing, as well as facilitating a refrigerant management scheme.