Staged CO2 Cooling With Brazed Aluminum Exchangers and Mercury Control
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
The use of brazed aluminum heat exchangers in cooling flows containing carbon dioxide and mercury is problematic due to mercury solidification, leading to plugging and embrittlement, and the use of sulfur-doped activated carbon for mercury removal is expensive and ineffective in the presence of NOx, which can cause combustion.
Innovation Solution
Cooling the flow to a temperature above −38.6° C. in a brazed aluminum exchanger to prevent mercury accumulation, using sloped passages and specific solders to prevent corrosion, and switching to stainless steel exchangers for lower temperatures to avoid mercury deposition, along with optional adsorption steps to remove residual mercury.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If a brazed aluminum heat exchanger is used to cool the flow, then heat exchange efficiency is improved, but mercury accumulates and solidifies causing plugging and embrittlement
Solution Approach 1:
The cooling process is divided into multiple stages with different temperature ranges. The first heat exchanger operates at temperatures above -38.6°C where mercury remains liquid and can be drained, while subsequent exchangers handle lower temperatures. This segmentation allows each stage to be optimized for its specific temperature range and mercury handling requirements.
Solution Approach 2:
The first heat exchanger performs preliminary cooling and mercury condensation at temperatures above -38.6°C. By removing the majority of mercury in this preliminary stage, the subsequent exchangers are protected from mercury accumulation and solidification issues, enabling them to operate reliably at lower temperatures.
2Reliability
If sulfur-doped activated carbon is used to remove mercury, then mercury removal effectiveness is improved, but cost increases and combustion risk arises in the presence of NOx
Solution Approach 1:
The heat exchangers perform preliminary mercury removal through condensation and drainage before the gas reaches the adsorbent bed. By removing the bulk of mercury in the liquid phase at higher temperatures, the amount of mercury reaching the activated carbon is significantly reduced, improving safety and reducing adsorbent consumption.
Solution Approach 2:
The invention extracts mercury from the gas stream in the liquid phase through condensation and gravitational drainage in the heat exchangers. This physical separation method removes mercury before it can interact with the adsorbent, eliminating the need for sulfur-doped activated carbon and its associated risks.
3Productivity
If cooling temperature is lowered to increase mercury removal, then mercury condensation efficiency is improved, but mercury solidifies and plugs the exchanger
Solution Approach 1:
The cooling process is segmented into stages: first cooling to above -38.6°C for liquid mercury drainage, then further cooling at lower temperatures. Each stage has appropriate drainage and purification measures matched to the mercury phase and concentration at that temperature level.
Solution Approach 2:
The invention changes the operating temperature parameter strategically. By maintaining the first heat exchanger at temperatures above -38.6°C (the melting point of mercury), mercury remains liquid and drainable. This parameter control prevents solidification while maximizing mercury removal efficiency.
4Ease of manufacture
If aluminum heat exchanger is used, then cost is reduced, but mercury causes embrittlement during shutdown phases
Solution Approach 1:
The first heat exchanger performs preliminary mercury removal through condensation and drainage before the gas enters subsequent exchangers. By removing mercury in advance, the aluminum exchanger is protected from mercury embrittlement during shutdown phases when liquid mercury could accumulate and cause damage.
Solution Approach 2:
The first heat exchanger acts as an intermediary that removes mercury from the gas stream before it reaches the aluminum exchanger. This protective stage prevents direct contact between mercury and the aluminum, eliminating the embrittlement risk while allowing cost-effective aluminum construction.
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
Enables the use of brazed aluminum exchangers in the presence of mercury without corrosion, reduces the need for expensive adsorbents, and improves mercury removal efficiency by minimizing accumulation and using temperature-dependent adsorption processes.
Implementation Method 1
Cooling the flow to a temperature above −38.6° C. in a brazed aluminum exchanger to prevent mercury accumulation
Implementation Method 2
using sloped passages and specific solders to prevent corrosion
Implementation Method 3
switching to stainless steel exchangers for lower temperatures to avoid mercury deposition
Implementation Method 4
along with optional adsorption steps to remove residual mercury
Data Source
AI summary
In a method for cooling a flow containing at least 35% carbon dioxide and at least 0.2 μg/Nm3 of mercury, the mercury being in liquid and/or gas form, the flow is cooled in a first brazed aluminum plate-fin heat exchanger from a first temperature to a second temperature higher than −38.6° C. to form a cold flow at the second temperature, and the flow cooled to the second temperature is cooled in a second heat exchanger, which is a tube and shell heat exchanger, to a third temperature lower than −38.6° C.