Urea Crystallization Purge Stream Processing
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Solution Overview
Problem
The existing urea production processes face challenges in managing the crystallization purge stream, which either contaminates the urea product or affects the performance of the synthesis reactor, particularly due to the recycling of urea and water, and the formation of undesired biuret by-products.
Innovation Solution
A novel process is introduced that processes the waste aqueous phase from urea crystallization to produce high-biuret urea as a valuable product, integrated with the conventional low-biuret urea production, eliminating the need for recycling urea and water to the reactor and avoiding the use of additives like formaldehyde.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of substance
If the crystallization purge stream is recycled to the urea reactor, then the urea and water are recovered and reused, but the reactor performance is affected due to equilibrium shift and the urea product may be contaminated with biuret
Solution Approach 1:
The invention extracts and removes the crystallization purge stream from the recycling loop, preventing it from being fed back to the urea reactor. This eliminates the harmful effects of introducing urea and water into the reaction environment, which would otherwise shift the equilibrium and reduce conversion efficiency. The purge stream is instead processed separately through evaporation and crystallization to recover urea and produce low-biuret solid urea product.
Solution Approach 2:
The invention changes the processing parameters by operating the evaporation and crystallization units at controlled temperatures and pressures. The purge stream is evaporated at temperatures below urea decomposition point (typically 80-100°C at reduced pressure), then crystallized at controlled cooling rates. This parameter control prevents biuret formation while maximizing urea recovery and product quality.
2Reliability
If the crystallization purge stream is not recycled but discarded, then reactor performance is maintained, but urea and water are lost and biuret accumulates in the purge stream
Solution Approach 1:
Instead of simply discarding the crystallization purge stream, the invention implements a recovery system that processes the purge stream through evaporation to remove water, then crystallization to recover solid urea. The mother liquor containing biuret is discarded, while the recovered urea crystals are melted and prilled to produce a valuable low-biuret solid urea product. This transforms a waste stream into a revenue-generating process.
Solution Approach 2:
The invention converts the harmful accumulation of biuret in the purge stream into a beneficial separation process. By controlling evaporation and crystallization conditions, the process exploits the different solubility and melting points of urea and biuret to separate them. The biuret-rich mother liquor is discarded, while pure urea crystals are recovered and processed into a high-quality solid product with biuret content below 1%, meeting specifications for agricultural and industrial applications.
3Quantity of substance
If evaporation is used to remove water from purified urea solution, then water removal is achieved, but biuret formation increases due to thermal decomposition
Solution Approach 1:
The invention utilizes phase transitions to separate urea from water and biuret. The purge stream is first evaporated at controlled temperatures (80-100°C) under reduced pressure to remove water and concentrate the urea solution without causing significant thermal decomposition. The concentrated solution is then cooled to crystallize urea, separating it from the biuret-rich mother liquor. Finally, the urea crystals are melted and rapidly prilled to produce solid low-biuret urea product.
Solution Approach 2:
The invention carefully controls processing parameters throughout the sequence: evaporation temperature kept below 100°C at reduced pressure to minimize decomposition, crystallization temperature controlled to maximize urea recovery while keeping biuret in solution, and rapid cooling rates during prilling to lock in low biuret content in the final product. These parameter optimizations ensure water removal while maintaining low biuret levels.
4Manufacturing precision
If crystallization is used to produce solid urea, then low biuret content is achieved, but the process requires highly concentrated urea solution and additional water removal
Solution Approach 1:
The invention merges multiple functions into an integrated process: the evaporation unit removes water from the purge stream while concentrating urea; the crystallization unit simultaneously separates urea from biuret and produces solid urea crystals; the melting and prilling units transform the crystals into the final solid product. This integrated approach handles both water removal and biuret separation in a unified process sequence, achieving low-biuret solid urea from the purge stream.
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 reduces contamination and performance issues in the reactor, enables efficient production of low-biuret urea while generating a valuable high-biuret product, and effectively processes previously regarded waste streams, enhancing overall plant efficiency and product quality.
Implementation Method 1
the urea solution is heated and depressurized to vacuum, typically around 0.1 bar abs, so that water evaporates from the solution
Implementation Method 2
the urea solution is heated and depressurized to vacuum, typically around 0.1 bar abs
Implementation Method 3
the effluent of a high-pressure reactor is heated in a high-pressure stripper, possibly in the presence of a stripping agent, to decompose the ammonium carbamate and extract gaseous ammonia and carbon dioxide
Implementation Method 4
These are condensed in a high-pressure condenser and recycled to the synthesis reactor
Implementation Method 5
Urea is subject to thermal decomposition into biuret and ammonia
Data Source
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AI summary
A process for the production of urea wherein: production of pure urea (U) includes the concentration of an aqueous urea solution (23) by crystallization; a urea crystallization purge aqueous phase (32) comprising urea, water and biuret, which is purged from the urea crystallization process, is used in a high-biuret urea processing section (34) for additional production of urea with a high content of biuret.