Mixed refrigerant system and method
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Solution Overview
Problem
Current gas cooling and liquefaction systems for natural gas are complex, costly, and inefficient due to the need for multiple refrigerant cycles and thermodynamic irreversibilities, particularly in achieving a net heating curve that approximates the natural gas cooling curve, leading to high power consumption and mechanical complexity.
Innovation Solution
A mixed refrigerant system with a multi-stream heat exchanger and compressor system that separates and recombines refrigerant fractions at different pressures to optimize temperature ranges and reduce thermodynamic losses, using a single compressor and minimizing separations to enhance efficiency and reduce complexity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If cascaded multilevel pure component refrigeration cycles are used to approximate the natural gas cooling curve, then the heating curve can closely match the cooling curve, but the mechanical complexity increases due to additional compressor trains
Solution Approach 1:
The refrigeration cycle is divided into multiple temperature levels (first, second, and third temperature levels) with separate heat exchangers for each level. Each level handles a specific temperature range, allowing the system to approximate the natural gas cooling curve without requiring cascaded compressor trains. The segmentation of the refrigeration process into distinct temperature zones reduces mechanical complexity while maintaining effective cooling.
2Ease of manufacture
If pure component refrigerants are used in cascaded cycles, then the refrigeration process can be simplified, but thermodynamic efficiency decreases due to constant temperature vaporization instead of following the natural gas cooling curve
Solution Approach 1:
The patent uses mixed refrigerants comprising multiple components (C1-C6 hydrocarbons and nitrogen) instead of pure component refrigerants. This composite approach allows the refrigerant mixture to exhibit a vaporization curve that follows the natural gas cooling curve more closely, improving thermodynamic efficiency. The mixed refrigerant system maintains the simplicity of the refrigeration process while eliminating the energy losses associated with constant temperature vaporization of pure components.
3Temperature
If the number of refrigeration levels is increased to improve cooling efficiency, then the heating curve can better approximate the cooling curve, but power consumption increases
Solution Approach 1:
The patent merges multiple refrigeration functions into a single integrated system with three temperature levels operating simultaneously. Instead of using cascaded cycles that would require separate compressor trains for each level, the system combines the refrigeration levels into one unified process. This merging approach maintains effective cooling across the required temperature range while reducing power consumption by eliminating the energy losses associated with multiple independent compression systems.
4Ease of operation
If refrigeration valves are used to flash liquid into vapor in cascaded cycles, then the phase change can be controlled, but thermodynamic irreversibility increases
Solution Approach 1:
The patent replaces the mechanical refrigeration valve flashing system with a thermodynamically more efficient approach using mixed refrigerants and controlled heat exchange. Instead of using valves to irreversibly flash liquid into vapor, the system uses the natural phase behavior of mixed refrigerants across different temperature levels, allowing for more reversible phase changes and reduced thermodynamic irreversibility while maintaining operational control.
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 achieves significant reductions in power consumption and mechanical complexity by optimizing refrigerant fractionation and separation, allowing for more efficient cooling and liquefaction of natural gas with improved thermodynamic performance.
Implementation Method 1
a main heat exchanger including a warm end and a cold end with a feed stream cooling passage extending therebetween
Implementation Method 2
indirect heat exchange in one or more refrigeration cycles
Implementation Method 3
counter-current heat exchange with the primary refrigeration stream
Implementation Method 4
a first expansion device and a second expansion device
Implementation Method 5
cooling and expanding the cold separator vapor stream
Implementation Method 6
a mixed refrigerant compressor system including a compressor first section and a compressor second section
Implementation Method 7
compressing and cooling a mixed refrigerant
Implementation Method 8
a cold vapor separator having a liquid outlet and a vapor outlet
Implementation Method 9
separating the mixed refrigerant after the first and last compression and cooling cycles so that a high pressure liquid stream and a high pressure vapor stream are formed
Data Source
AI summary
A system and method for cooling a gas using a mixed refrigerant includes a compressor system and a heat exchange system, where the compressor system may include an interstage separation device or drum with no liquid outlet, a liquid outlet in fluid communication with a pump that pumps liquid forward to a high pressure separation device or a liquid outlet through which liquid flows to the heat exchanger to be subcooled. In the last situation, the subcooled liquid is expanded and combined with an expanded cold temperature stream, which is a cooled and expanded stream from the vapor side of a cold vapor separation device, and subcooled and expanded streams from liquid sides of the high pressure separation device and the cold vapor separation device, or combined with a stream formed from the subcooled streams from the liquid sides of the high pressure separation device and the cold vapor separation device after mixing and expansion, to form a primary refrigeration stream.