CO2 Liquefaction with Recycle Compression and Liquid Subcooling
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Solution Overview
Problem
Existing methods for liquefying CO2 streams with high CO2 content and impurities, such as H2 and N2, fail to achieve subcooling of the liquid CO2, which is necessary for use at pressures lower than the liquefaction pressure, and are inefficient in terms of capital and operational expenses.
Innovation Solution
The process involves cooling the CO2 stream through indirect heat exchange, followed by successive expansions in valves to self-cool and recycle gases, allowing subcooling in a heat exchanger, and recycling vaporized CO2 to earlier stages of the compressor to optimize liquefaction and reduce costs.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If the CO2 stream is cooled to partially liquefy it and sent to phase separators, then liquefaction is achieved, but the liquid cannot be subcooled and must be used at liquefaction pressure
Solution Approach 1:
The cooling and liquefaction process is divided into multiple stages with intermediate phase separation. The CO2 stream undergoes sequential cooling in multiple heat exchangers with phase separators between stages, allowing progressive liquefaction and subcooling while managing pressure changes through controlled expansion steps.
Solution Approach 2:
The process utilizes controlled changes in pressure and temperature parameters through expansion valves and heat exchangers. By expanding the CO2 stream to lower pressures and then cooling it in subsequent stages, the liquid CO2 achieves subcooling below its saturation temperature, enabling versatile use at different pressures.
2Reliability
If conventional liquefaction methods are used, then CO2 liquefaction is achieved, but capital and operational expenses are high
Solution Approach 1:
The process incorporates feedback loops where vaporized CO2 from phase separators is recycled back to earlier compression stages. This feedback mechanism improves overall liquefaction efficiency by utilizing the thermal energy and mass flow from later stages to assist in earlier cooling requirements, reducing external energy input needs.
Solution Approach 2:
The system uses its own output streams to serve its input requirements. The vaporized CO2 from phase separators and the cold CO2 from later cooling stages are reused to pre-cool and assist compression in earlier stages, making the system self-sufficient and reducing external utility requirements.
3Productivity
If gas is recycled upstream of compression stages, then liquefaction yield is improved, but device complexity increases
Solution Approach 1:
The compression system and heat exchangers serve multiple functions simultaneously. The compressors not only pressurize the feed stream but also receive recycled vapor streams. The heat exchangers perform both cooling of the feed stream and heating of recycled streams, reducing the need for separate dedicated equipment for each function.
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 method enables the production of subcooled CO2 at the required pressure while minimizing capital and operational expenses by recycling gases and optimizing the use of heat exchangers, achieving 100% liquefaction yield.
Implementation Method 1
The process for liquefying a flow 1 containing at least 95% mol, or even at least 99% mol of carbon dioxide is carried out by cooling by indirect heat exchange with a cold source
Implementation Method 2
The CO2 thus condensed at high pressure will undergo a succession of expansions in valves V1, V2
Implementation Method 3
The CO2 thus condensed at high pressure will undergo a succession of expansions in valves V1, V2, in order to self-cool by generation of a gas
Implementation Method 4
A part 13 vaporizes against the liquid 18 in the heat exchanger 9, after expansion in a valve V4. Expansion in valve V4 brings the liquid to a temperature as close as possible to that of the triple point (-56.5°C). The vaporized low-pressure CO 2 15 is then recycled to the first stages 3A, 3B of the cycle compressor 3
Data Source
Figure 1
AI summary
The invention relates to a device for the liquefaction of a gas flow containing at least 95 mol-% carbon dioxide, comprising at least a first compression stage (C3, C4) in which the feed gas flow (1) is compressed; means for condensing the compressed flow, formed by two in-series phase separators, in order to partially condense said flow and produce a liquid flow; a first heat exchanger (7); a second heat exchanger (9) which is a shell and tube heat exchanger; means for conveying at least part of the liquid flow into the tubes of the first heat exchanger; means for discharging, as a liquid product (11), a first part of the liquid cooled in the second exchanger, said part being subsequently expanded; a valve (V4); means for conveying a second part of the liquid cooled in the second exchanger, said second part being expanded in the valve and vaporised in the shell of the first exchanger in order to form a vaporised flow (15); and means (3A, 3B) for compressing at least part of the vaporised flow and mixing same with the feed gas flow.