Layer-by-Layer Crystallization Phenomena in Urea Solution: An Experimental Study.

In the SCR systems of heavy-duty diesel engines, urea deposits formed under real operating conditions differ significantly from laboratory-prepared samples in morphology, composition, and structure. Under real operating conditions, the urea solution undergoes multiple dynamic deposition cycles, forming dense and compositionally complex stubborn deposits. Conventional laboratory methodssuch as single evaporation or single-layer liquid film modelsfail to replicate this process. To address this limitation, this study proposes a multilayer liquid-film heating crystallization method. The method is used to investigate the fundamental differences between crystallization on pre-existing crystal layers and static single-layer crystallization. Crystals derived from single-layer and multilayer liquid films of equivalent total thickness were analyzed using thermogravimetric analysis (TGA), Fourier-transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). The results indicate that (1) the first urea crystal layer in multilayer crystallization impedes heat transfer to the second liquid film, promoting the formation of lower-temperature crystal components; (2) multilayer urea crystals require higher temperatures for thermal decomposition compared to single-layer crystals; (3) when the temperature decreases (with the first layer's crystallization temperature being not lower than the heating temperature of the second layer), the crystal mass from multilayer crystallization exceeds that from single-layer crystallization of equivalent liquid film thickness; (4) multilayer crystallization, involving secondary deposition and a superimposed thermal history, facilitates the development of ordered columnar or layered microstructures with relatively clean surfaces.