Kinetic modeling and simulation of high-temperature by-product formation from urea decomposition

A Study on Byproducts in the High-Pressure Melamine Production Process

The industrial production of melamine is carried out by the thermal decomposition of urea in two technological processes, using high or low pressure. The reaction may be accompanied by the formation of undesirable byproducts, oxoaminotriazines, and so-called polycondensates, mainly melam, melem, and melon, as well as their hydrates and adducts. Their presence leads to the deterioration of the quality of the final product and may lead to the release of troublesome deposits inside the apparatus of the product's separation node. With the limited possibility of controlling the crystallization of the byproducts of the process, improving the technological process requires the precise determination of the composition of the separated insoluble reaction byproducts, which is the main objective of this work. This work presents the results of qualitative and quantitative analyses of the composition of deposits sampled in the technological process of melamine production. The full characterization of the deposits was performed using inductively coupled plasma optical emission spectroscopy (ICP-OES) and inductively coupled plasma mass spectrometry (ICP-MS) techniques. The elemental analysis (EA) of carbon, hydrogen, and nitrogen allowed us to obtain characteristic C/H, C/N, and H/N ratios. X-ray diffraction (XRD) and attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy were also performed to confirm the obtained data. In addition, the morphology of the solid byproducts of the reaction was investigated, and the characteristics of the structures were determined using a scanning electron microscope. The elemental composition was investigated using scanning electron microscopy and the energy-dispersive X-ray spectroscopy (SEM-EDS) technique. The key finding of this research is that about 95% of the deposits are a mixture of melem and melem hydrate. The soluble part of the deposits contains melamine, urea, and oxyaminotriazines, as well as trace inorganic impurities.

Study on decomposition and by-products formation mechanism of urea solution impinging on heated plate in SCR system

Selective catalytic reduction technology is one of the most effective methods to control NOx emissions from diesel engines. However, the generation of solid urea and by-products, which will lead to the incomplete decomposition of urea and reduce NOx reduction efficiency. The accumulation of deposits will also affect the back pressure and even block the catalyst, causing a vicious cycle. In this paper, a numerical model coupled with a detailed decomposition mechanism of urea was established. Based on this model, four heat transfer methods and chemical reactions in the film were calculated. The results show that the main heat transfer methods between the film and the plate are conduction and evaporation, both of which have maximum values higher than 10⁷ W/m². The mass of the film varies with heat flux can be divided into four stages. In the first stage, the film forms rapidly, and intense heat exchange occurs. In the second stage, the film increases slowly and the decomposition mass of urea increases. The main component of the film is solid urea. In the third stage, the solid urea mass begins to decrease. In the fourth stage, film mass decreases, and a large amount of urea is decomposed. The main byproduct of urea decomposition is biuret in the first three stages and cyanuric acid in the fourth stage. The solid urea and byproducts are mainly distributed in regions with low heat flux. With an increase in the initial plate temperature, the heat flux increases, the film and solid urea masses decrease, the byproducts appear earlier and their mass increases faster. When the temperature is 523 K, 528 K, and 533 K, the maximums of biuret mass occur at about 3.1s, 1.4s and 0.5s, respectively. The present work provides guidance understanding the relationship between heat transfer characteristics and byproducts formation of urea decomposition.

Theoretical Study of the Thermal Decomposition of Urea Derivatives

An extensive theoretical study of the thermal decomposition of alkyl- and phenylureas, which are widely used in the pesticides, pharmaceuticals, and materials industries, has been carried out using electronic structure calculations and reaction rate theories. Enthalpies of formation and bond dissociation energies (BDE) of 11 urea derivatives have been calculated using different levels of theory (CBS-QB3, CCSD(T)/CBS//M06-2X/6-311++G(3df,2pd), and CBS-QM062X) according to the size of the system. Potential energy surfaces for the unimolecular decomposition pathways of these urea derivatives were also systematically computed for the first time. Several pericyclic reactions can be envisaged, as a function of the size and the nature of the N substituents, and all of these pathways were explored. Our calculations show that these compounds are solely decomposed by four-center pericyclic reactions, yielding substituted isocyanates and amines, and that initial bond fissions are not competitive. Based on the set of urea derivatives studied, a new reaction rate rule for their thermal decomposition was defined and involves the nature of the transferred H atom (primary or secondary/alkyl or benzyl) and the nature of the N-atom acceptor (primary, secondary, or tertiary). This new reaction rate rule allows us to determine the product branching ratios in the thermal decomposition of a given urea derivative and its total rate of decomposition. Applications on urea derivatives used in the chemical industry are presented and illustrate the usefulness of this new rate rule that allows to predict the previously unknown thermal decomposition kinetics of a large number of these compounds.