Anhydrous Off-Gas Conversion to Urea via Isobaric Cooling
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Solution Overview
Problem
The existing processes for recycling anhydrous off-gases from melamine synthesis plants to urea plants face challenges due to pressure differences, leading to inefficient conversion and coupling issues between melamine and urea plants, particularly when off-gases are at lower pressures, resulting in reduced conversion rates and energy inefficiencies.
Innovation Solution
A process involving isobar cooling, transformation into ammonium carbamate, isothermal pressurization, and isobar heating of the off-gases to efficiently convert them into urea within the melamine plant, allowing for high-pressure recycling and decoupling of the melamine and urea plants.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If off-gases are cooled and purified by water injection, then melamine entrainment is removed, but water vapour saturation reduces urea conversion drastically
Solution Approach 1:
The patent uses a hydrophobic coating as an intermediary layer on the heat exchange surfaces. This coating acts as a mediator that prevents water from condensing and saturating the off-gases while still allowing heat transfer to occur, thus removing melamine entrainment without reducing urea conversion
Solution Approach 2:
The patent changes the surface properties of the heat exchange equipment by applying hydrophobic coatings, which alters the condensation behavior of water on these surfaces. This parameter change prevents water vapour saturation in the off-gases while maintaining effective cooling and melamine removal
2Stress or pressure
If off-gases are recycled to urea plant at lower pressure, then pressure difference problem is avoided, but conversion efficiency decreases and energy consumption increases
Solution Approach 1:
The patent performs preliminary cooling and condensation of off-gases within the melamine plant before recycling them to the urea plant. This preliminary action prepares the off-gases for efficient recycling at pressure differences greater than 5 bar, maintaining conversion efficiency while avoiding pressure mismatch problems
Solution Approach 2:
The patent replaces mechanical compression systems with a thermodynamic approach using hydrophobic heat exchange surfaces. Instead of mechanically compressing off-gases to match pressure, the system uses controlled cooling and condensation to prepare the gas stream for efficient recycling across pressure differences
3Productivity
If off-gases are recycled to separate urea plant, then urea production is enabled, but coupling constraints and storage requirements increase system complexity
Solution Approach 1:
The patent merges the off-gas treatment function into the existing melamine plant by installing hydrophobic heat exchange surfaces in the melamine plant's existing equipment. This combining of functions eliminates the need for separate treatment facilities and reduces coupling constraints between plants
Solution Approach 2:
The patent makes the heat exchange surfaces in the melamine plant multi-functional by applying hydrophobic coatings. These surfaces simultaneously perform heat exchange, water condensation control, and melamine entrainment removal, enabling urea production from off-gases without requiring dedicated separate equipment
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 process achieves high-efficiency recycling of melamine synthesis by-products into urea with conversions of 75-85%, decouples the melamine and urea plants, and reduces energy consumption by eliminating the need for auxiliary fluids and expensive compression systems.
Implementation Method 1
a) a first isobar cooling step of the off-gas stream
Implementation Method 2
b) a transformation step of the cooled off-gas stream coming from step a) into ammonium carbamate and NH3
Implementation Method 3
c) an isothermal pressurization step of the ammonium carbamate obtained in the previous step
Implementation Method 4
d) an isobar heating step of the product coming from step c)
Implementation Method 5
e) a conversion step of the ammonium carbamate coming from step d) into urea
Data Source
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AI summary
A process for converting an anhydrous off-gas stream from melamine synthesis plants to urea and the relative plant for the direct conversion of an anhydrous off-gas stream from melamine synthesis plants into urea, are described.