Melamine Offgas Quenching Process Using Liquid Ammonia
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Solution Overview
Problem
Existing melamine synthesis processes generate offgas containing ammonia and carbon dioxide, which must be recycled to a urea plant, but also include melamine that needs to be removed to avoid losses, with current washing methods either introducing water into the urea synthesis or requiring additional stripping stages.
Innovation Solution
A two-zone quenching process using liquid ammonia to precipitate melamine from offgas in an upper zone and a solvent to remove the solidified melamine in a lower zone, producing an anhydrous washed offgas and a melamine solution free of ammonia and CO2, suitable for recycling and further processing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of substance
If aqueous washing is used to remove melamine from offgas, then melamine removal is achieved, but water is introduced into the urea synthesis and the melamine solution contains significant CO2 requiring additional stripping stages
Solution Approach 1:
The washing process is segmented into two distinct zones: an upper zone using liquid ammonia for anhydrous washing, and a lower zone using aqueous solvent for melamine removal. This segmentation allows each zone to perform its specific function optimally without the drawbacks of using a single washing medium throughout the entire process.
Solution Approach 2:
Liquid ammonia serves as an intermediary substance in the upper zone, providing a medium that enables melamine removal without introducing water into the offgas. The ammonia washes the offgas and can be subsequently removed, leaving anhydrous washed offgas that does not require additional drying or stripping stages.
2Ease of manufacture
If water-based washing is used, then equipment cost is reduced, but the washed offgas becomes saturated with water which is undesirable for recycling to urea plant
Solution Approach 1:
The washing process is segmented into two distinct zones: an upper zone using liquid ammonia for anhydrous washing, and a lower zone using aqueous solvent for melamine removal. This segmentation allows each zone to perform its specific function optimally without the drawbacks of using a single washing medium throughout the entire process.
3Loss of substance
If urea melt washing is used, then offgas washing is effective, but the process requires corrosion-resistant equipment and is less suitable for non-integrated plants
Solution Approach 1:
The invention changes the parameter of washing medium from urea melt to liquid ammonia in the upper zone. This parameter change maintains effective melamine removal while avoiding the corrosion issues associated with urea melt, as liquid ammonia is less corrosive to standard equipment materials.
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 process achieves an anhydrous washed offgas reducing water introduction into the urea synthesis and eliminates the need for a dedicated stripping stage, allowing for efficient recycling and processing of the melamine solution in both ammonia-based and metal-alkali processes.
Implementation Method 1
The liquid ammonia introduced in the vessel is colder than the melamine-containing input gas... the melamine contained in the offgas cools down and precipitates in a solid form
Implementation Method 2
The liquid ammonia introduced in the vessel is colder than the melamine-containing input gas... the offgas are quenched and scrubbed with the liquid ammonia
Implementation Method 3
Ammonia vaporizes in contact with the offgas stream and is removed together with washed offgas
Implementation Method 4
the precipitated solid melamine is contacted with a liquid solvent forming a solution of melamine or a melamine slurry
Data Source
Figure 1
AI summary
A process for the synthesis of melamine from urea, preferably of the high- pressure type, wherein offgas quenching is performed in a quenching vessel wherein in the upper part of the vessel, a rising flow of offgas is contacted in counter-current with liquid ammonia to obtain precipitation of melamine contained in the offgas and melamine-free anhydrous washed offgas; in the lower part of the vessel, the solid melamine is contacted with a liquid solvent to form a solution of melamine or a melamine slurry.