Aqueous urea solution is a chemical compound used across various industries. While often unseen by the general public, it plays a significant role in numerous industrial and environmental applications. Its relevance comes from its unique chemical properties. Understanding this solution provides insight into processes that contribute to cleaner air and more efficient industrial operations.
Understanding Aqueous Urea Solution
Aqueous urea solution consists of urea dissolved in deionized water. Urea is a synthetic organic compound with the chemical formula CO(NH₂)₂. This compound is also known as carbamide and is a colorless, odorless solid in its pure form. When dissolved in water, the resulting solution appears colorless and has a mild, almost undetectable odor.
The most common concentration for applications such as in diesel engines is a 32.5% by weight solution of urea in water. This concentration optimizes its performance and freezing point. The solution is considered non-toxic, though direct contact with skin or eyes should be avoided.
Its Role in Diesel Engines
Aqueous urea solution, known as Diesel Exhaust Fluid (DEF), is used in modern diesel engines. Its primary purpose is to reduce harmful nitrogen oxide (NOx) emissions from vehicle exhaust. Environmental regulations worldwide have mandated a substantial reduction in these pollutants.
To meet these emission targets, many diesel vehicles use Selective Catalytic Reduction (SCR) systems. DEF is injected into the hot exhaust stream before it enters a catalyst. This allows the solution to interact with exhaust gases, preparing them for chemical transformation over the catalyst. SCR technology has made DEF a necessary component for compliant diesel engine operation.
How It Transforms Pollutants
The transformation of pollutants by aqueous urea solution occurs through Selective Catalytic Reduction (SCR). After DEF is injected into the hot exhaust gas, heat causes the urea to decompose into ammonia (NH₃) and carbon dioxide (CO₂). This ammonia is then adsorbed onto the surface of the SCR catalyst.
As nitrogen oxides (NOx) in the exhaust gas pass over the catalyst, they react with the adsorbed ammonia. This reaction converts the toxic nitrogen oxides into harmless nitrogen gas (N₂) and water vapor (H₂O). The catalyst facilitates this reaction without being consumed, ensuring continuous reduction of harmful emissions. This chemical conversion lowers the environmental impact of diesel engines.
Safe Handling and Storage
Proper handling and storage of aqueous urea solution, or DEF, are important to maintain its effectiveness and ensure safety. While considered to have low toxicity, it is important to avoid prolonged skin contact, eye contact, and ingestion. If contact occurs, affected areas should be washed with soap and water.
Storing DEF correctly prevents degradation and freezing. The recommended temperature range for storage is between -5°C and 25°C (23°F to 77°F) to prevent crystallization or decomposition. Contamination with foreign substances, such as fuel, should be avoided as it can impair the solution’s performance. DEF has a shelf life of 12 months when stored under proper conditions. It is also important to use containers made from compatible materials like stainless steel, certain plastics, or fiberglass-reinforced plastic, as DEF can corrode some metals such as mild steel, iron, and nonferrous metals.