CO2 Separation Heat Exchanger Segmentation to Reduce Exergy Loss
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Solution Overview
Problem
Current methods for separating carbon dioxide-rich gases at subambient temperatures suffer from significant exergy losses due to inefficient heat exchange, leading to energy inefficiencies and mechanical stresses in cryogenic applications, particularly in the cold box of CO2 capture units.
Innovation Solution
The integration of a sub-cooler in the cold box to optimize heat exchange by transferring cold from low-temperature fluids to CO2 vaporized at low pressure, allowing for indirect heat exchange between two fluids in a tube-shell exchanger, and utilizing this sub-cooled CO2 to liquefy CO2 efficiently, reducing gas production and energy consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If a multi-pass exchanger is used to condense CO2 using hot fluids, then CO2 condensation is achieved, but exergy losses increase and mechanical stresses occur due to significant temperature differences
Solution Approach 1:
The heat exchange process is divided into two separate exchangers: a first exchanger for initial cooling and a second exchanger for final condensation. This segmentation allows each exchanger to operate with optimized temperature differences, reducing exergy losses while achieving CO2 condensation near the triple point
Solution Approach 2:
The first exchanger acts as an intermediary device that pre-cools the CO2 before it enters the second exchanger. This intermediate cooling step enables the second exchanger to operate with smaller temperature differences, thereby reducing mechanical stresses and exergy losses during the critical condensation phase
2Temperature
If liquid CO2 is expanded to reach low temperatures, then cold is generated, but gas production increases which reduces cooling efficiency
Solution Approach 1:
The liquid CO2 is subcooled in the first exchanger before expansion. This preliminary action of subcooling reduces the temperature of the liquid CO2 prior to expansion, which minimizes gas production during expansion and maximizes the cooling efficiency of the expanded fluid
3Temperature
If CO2 is condensed against hot fluids with large temperature differences, then CO2 condensation is achieved, but mechanical stresses increase on heat exchanger components
Solution Approach 1:
The condensation process is segmented into two exchangers, allowing the second exchanger to operate with optimized temperature differences close to the CO2 triple point. This segmentation reduces the thermal stress on brazing and heat exchanger components while achieving effective condensation
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 approach minimizes exergy losses, optimizes energy consumption, and reduces mechanical stresses by effectively utilizing low-temperature cold in the cold box, enabling operation closer to the triple point of CO2, thereby enhancing the efficiency of the CO2 separation process.
Implementation Method 1
indirect heat exchange between two fluids in a tube-shell exchanger
Implementation Method 2
condense the CO2 efficiently
Implementation Method 3
the production of the cold of the unit is done by the vaporization of a liquid at sufficiently low pressure
Implementation Method 4
subcooling the liquid before expansion
Data Source
Figure 1
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Figure 3
AI summary
In a purification method, a carbon dioxide-rich gas is cooled in a first brazed aluminium plate-fin heat exchanger (9), the cooled gas or at least one fluid derived from the cooled gas is sent to a purification step comprising a distillation step (23), the purification step produces a carbon dioxide-rich liquid (33) which is cooled, then expanded, then sent to a second heat exchanger (35) where it is heated by means of a fluid of the method (13), the exchanger carrying out an indirect heat exchange only between the carbon dioxide-rich liquid and the fluid of the method, the carbon dioxide-rich liquid at least partially vaporises in the second exchanger and the vaporised gas (37) formed heats up again in the first exchanger to form a carbon dioxide-rich gas.