The selective catalytic reduction technique for automotive applications depends on ammonia production from a urea-water solution by thermolysis and hydrolysis. In this process, undesired liquid and solid by-products are formed in the exhaust pipe. The formation and decomposition of these by-products have been studied by thermogravimetric analysis and differential scanning calorimetry. A new reaction scheme is proposed that emphasizes the role of thermodynamic equilibrium of the reactants in liquid and solid phases. Thermodynamic data for triuret have been refined. The observed phenomenon of liquefaction and re-solidification of biuret in the temperature range 193–230 °C is explained by formation of a eutectic mixture with urea

Amsler, Jonas

Börnhorst, Marion

Deutschmann, Olaf

Schoch, Günter

Tischer, Steffen

English

Selective catalytic reduction (SCR) for automotive applications depends on ammonia production from a urea-water solution by thermolysis and hydrolysis. In this process, undesired liquid and solid by-products are formed in the exhaust pipe. The formation and decomposition of these by-products have been studied by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC).
						Based on a previously published reaction mechanism by Brack et al. [1], a new reaction scheme is proposed that emphasizes the role of thermodynamic equilibrium of the reactants in liquid and solid phases [2].  The observed phenomenon of liquefaction and re-solidification of biuret in the temperature range 193–230 °C can be explained by formation of a eutectic mixture with urea. According to DSC data, the direct decomposition of urea to ammonia and isocyanic acid can be ruled out. The dominant route is a self-polymerisation of urea to biuret and triuret. Biuret and triuret decomposition are dominated by thermodynamic equilibria with gaseous isocyanic acid. For this, thermodynamic data of triuret have been refined.  The apparent melting point of biuret at 193 °C is explained by the formation of a eutectic mixture within the urea-biuret-triuret-cyanuric acid ensemble. Furthermore, DSC data shows that cyanuric acid sublimates without decomposition at temperatures above 300 °C. 
						Numerical simulations of the TGA and DSC experiments are performed by a multi-phase tank reactor model (DETCHEMMPTR [3]). The new reaction mechanism describes well the main features (decomposition steps and calorimetry) and dependencies (on heating rate and surface area) of the decompositions of urea, biuret, triuret and cyanuric acid.
						[1] W. Brack, B. Heine, F. Birkhold, M. Kruse, G. Schoch, S. Tischer and O. Deutschmann, “Kinetic modeling of urea decomposition based on systematic thermogravimetric analyses of urea and its most important by-products”, CES 106, 1–8 (2014).
						[2] S. Tischer, M. Börnhorst, J. Amsler, G. Schoch and O. Deutschmann, “Thermodynamics and reaction mechanism of urea decomposition”, PCCP, in press, DOI: 10.1039/C9CP01529A (2019).
					[3] www.detchem.co

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KIT – The Research University in the Helmholtz Associationsolid depositliquid filmspray impingementfilm formationexhaustgasurea-water solutionSCRcatalysttall crucible, 2 K/minflat crucible, 2 K/mintall crucible, 10 K/minInstitute of Catalysis Research and TechnologyInstitute for Chemical Technology and Polymer ChemistryThermodynamics and reaction mechanism of urea decompositionSteffen Tischer, Marion Börnhorst, Jonas Amsler, Günter Schoch, Olaf DeutschmannDr. Steffen TischerInstitute of Catalysis Research and Technology (IKFT)Karlsruhe Institute of Technology (KIT)76128 Karlsruhe, Germany+49 721 608-42114steffen.tischer@kit.edudownload full article[1] W. Brack, B. Heine, F. Birkhold, M. Kruse, G. Schoch, S. Tischer and O. Deutschmann: Kinetic modeling of urea decomposition based on systematic thermogravimetric analyses of urea and its most important by-products. Chemical Engineering Science, 2014, 106, 1–8.[2] S. Tischer, M.Börnhorst, J. Amsler, G- Schoch, O.Deutschmann: Thermodynamics and reaction mechanism of urea decomposition. Physical Chemistry Chemical Physics, 2019, 21, 16785 – 16797.[3] www.detchem.com+ HNCObiuret triureturea+ HNCOcyanuricacidammelideammelinemelamineSample ReferenceThermocouplesHeat Resistors TemperatureControlVheatflux TemperatureureaMotivation Experiments Numerical modelThermogravimetricAnalysis (TGA)Differential ScanningCalorimetry (DSC)tall crucible(6 mm)flat crucible(15 mm)tall crucible, 2 K/minbiu triu cyaurea biu triu cyaurea biu cyaurea biu triu cyauwsMulti-phase tank reactor model (0-dim.) DETCHEMMPTR [3]GasphasePhase j=1(e.g. liquid)Phases j=2..n(e.g. solids)Gasphase ReactionsHomogeneousReactionsInterphase ReactionsInterphase Reactionsk-th reaction:Homogeneous reactions:Interphase reactions:Species conservation (index i):Energy conservation:Total enthalpy:Eutectic mixtureurea biuMelting of biuretApparent melting temperature: 193 °CbiuEquilibrium driven reactionsbiu triuequilibrium reactions kinetically controlledInitial step of biuret decompositionis controlled by equilibriumAnalogy for triuret: New thermodynamic data for triuretDecomposition stepsurea biu1 urea melting, 2 formation of urea-biuret eutectic, 3 urea gone4 biuret melting & decomposition, 5 sublimation of cyanuric acidProposed reaction mechanismeutectic mixtureNumerical simulation HomogeneousReactionsureaTGA, 6.2 mg urea, 2 K/min, tall crucible TGA, 6.2 mg urea, 2 K/min, flat crucible TGA, 10.8 mg urea, 10 K/min, tall crucible DSC, 14.2 mg urea, 2 K/min, tall crucible

Thermodynamics and reaction mechanism of urea decomposition

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Thermodynamics and reaction mechanism of urea decomposition

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