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 closely 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 includes interstage and high-pressure separation devices, allowing for efficient separation and recombination of refrigerant fractions to follow the natural gas cooling curve, reducing mechanical complexity and power consumption by using a single compressor and minimizing thermodynamic losses.
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 process is divided into multiple temperature levels with separate refrigerant loops, each handling a specific temperature range. This segmentation allows each loop to operate independently with its own heat exchangers, achieving a composite heating curve that approximates the natural gas cooling curve without requiring cascaded compressor trains across all levels.
Solution Approach 2:
A single mixed refrigerant composition is used across all temperature levels, performing multiple functions simultaneously. The same refrigerant mixture provides cooling in the de-superheating region, condensation in the two-phase region, and subcooling in the liquid region, eliminating the need for separate pure component systems at each temperature level.
2Device complexity
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 cooling curve
Solution Approach 1:
The refrigerant composition is changed from pure components to a mixed composition with specific ratios of different hydrocarbons. This parameter change allows the vaporization temperature to vary continuously throughout the heat exchanger, following the natural gas cooling curve and maximizing heat transfer efficiency while maintaining a relatively simple process configuration.
3Quantity of substance
If refrigeration valves are used to flash liquid into vapor, then the phase change can be achieved, but thermodynamic irreversibility increases
Solution Approach 1:
The refrigeration valve is extracted and replaced with an expansion device that operates isentropically, such as an expansion turbine or throttling valve with minimal pressure drop. This removal of the irreversible flashing process reduces thermodynamic losses while still achieving the necessary phase change from liquid to vapor.
4Device complexity
If a single mixed refrigerant process is used with one compressor, then mechanical complexity is reduced, but power consumption increases due to difficulty in matching the heating curve to the cooling curve
Solution Approach 1:
The refrigerant composition parameters are optimized to achieve a heating curve that closely matches the natural gas cooling curve. By adjusting the proportions of different hydrocarbons in the mixed refrigerant, the system achieves efficient heat transfer across all temperature levels with a single compressor, minimizing power consumption while maintaining mechanical simplicity.
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 efficiently separating and recombining refrigerant fractions, improving the process's ability to follow the natural gas cooling curve and enhance thermodynamic efficiency, resulting in a more cost-effective and efficient gas cooling and liquefaction process.
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
countercurrent heat exchange with the refrigeration streams
Implementation Method 4
one or more expansion devices, each having an inlet and an outlet
Implementation Method 5
interstage separation device with a liquid outlet and a vapor outlet; high pressure separation device with a liquid outlet and a vapor outlet
Implementation Method 6
separating the mixed refrigerant after compressing and cooling the mixed refrigerant so that a high pressure liquid stream and a high pressure vapor stream are formed
Implementation Method 7
compressing and cooling a mixed refrigerant using first and last compression and cooling cycles
Implementation Method 8
first section cooler with an inlet and an outlet; second section cooler with an inlet and an outlet
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.