Nitric Acid Startup NOx Reduction via Gas Recirculation
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Solution Overview
Problem
Current methods for reducing NOx nitrogen oxides during the shutdown and start-up of nitric acid production plants are inadequate, leading to excessive emissions that exceed regulatory limits, as the residual gas cleaning system is often unable to operate effectively due to temperature limitations and instability in the absorption column.
Innovation Solution
A method involving temperature control of residual gas using a heat exchanger or heat accumulator to maintain optimal conditions for the residual gas purification system, allowing the residual gas cleaning system to operate longer and reduce NOx nitrogen oxide concentrations through controlled heating and recirculation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-generated harmful factors
If the residual gas cleaning system operates during shutdown/start-up, then NOx reduction is achieved, but the system fails due to temperature dropping below the minimum operating limit
Solution Approach 1:
The patent applies preliminary action by pre-heating the residual gas using a heat exchanger before it enters the SCR catalyst. This ensures the gas temperature remains above the minimum operating temperature (170-200°C) required for the residual gas cleaning system to function properly during shutdown and start-up phases, preventing system failure due to temperature drop.
Solution Approach 2:
The patent changes the temperature parameter of the residual gas by introducing a heat exchanger that heats the gas to and maintains it above the minimum operating temperature threshold. This parameter change enables the SCR system to operate reliably during transient phases when temperature would naturally drop below operational limits.
2Object-generated harmful factors
If the absorption column operates during depressurization, then NOx absorption is achieved, but the column becomes unstable causing sharp drop in absorption efficiency
Solution Approach 1:
The patent introduces a heat exchanger as an intermediary device between the residual gas flow and the SCR catalyst. This intermediary maintains optimal temperature conditions for the SCR reaction, stabilizing the NOx reduction process during depressurization when the absorption column becomes unstable and absorption efficiency drops sharply.
3Loss of time
If the plant depressurizes quickly, then shutdown/start-up time is reduced, but NOx emission concentration increases significantly
Solution Approach 1:
The patent ensures continuous useful action by maintaining the residual gas cleaning system operation throughout the entire depressurization process through pre-heating. The heat exchanger continuously heats the residual gas to keep SCR catalyst temperature above the minimum operating limit, allowing uninterrupted NOx reduction even during rapid plant depressurization and start-up.
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 effectively reduces NOx nitrogen oxide concentrations in the residual gas to below visible limits, achieving emissions compliance by maintaining the residual gas cleaning system's efficiency and stability during plant shutdown and start-up.
Implementation Method 1
Tempering the residual gas during the shutdown and/or start-up of the nitric acid production plant with a temperature control device; and treating the residual gas tempered in step (a) in a residual gas purification plant, wherein the temperature control device is a heat exchanger or a heat storage device
Data Source
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AI summary
The invention relates to a method and to a plant for reducing the concentration of NOx nitrogen oxides in the residual gas during the production of nitric acid, wherein, as the plant is started up and/or shut down, the temperature of the residual gas is controlled by using a temperature control device (15, 18), wherein the residual gas is circulated and, in the process, flows through the temperature control device (15, 18) and the residual gas purification plant (9), such that colorless starting up and/or shutting down of the plant is made possible.