Urea decomposition

Evaluated by comparision of the results of numerical simulation with experimental measurements by thermo gravimetric analysis (TGA) and differential scanning calorimetry (DSC). Samples substances were urea, biuret, triuret, cyanuric acid, ammelide, and urea-water solution. The temperature range from room temperature to 600°C is covered.
Reaction type
multi-phase
Reactants
urea
Version
1.0 (July 2019)
Reference

S. Tischer, M. Börnhorst, J. Amsler, G. Schoch, O. Deutschmann: Thermodynamics and reaction mechanism of urea decomposition. PCCP, accepted (2019)

DOI:10.1039/C9CP01529A

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<SPECIES>
  <GASPHASE> 
     H2O NH3 CO2 HNCO urea(g) cya(g) HCN(g) N2
  </GASPHASE>

  <PHASE liquid> 
     DCSLMAX=20
     urea(l)  1320  # species name and density in kg/m3
     HNCO(l)  1140
	 NH3(l)    700
	 biu(l)   1470
	 triu(l)  1547
  </PHASE>
  
  <PHASE aq>
     H2O(l)    998
	 urea(aq) 1100
	 NH3(aq)   700
	 HNCO(aq) 1140
  </PHASE>
  
  <PHASE solid> 
     ammn(s)  2220
     ammd(s)  1573
	 melem(s) 1717
	 H2NCN(s) 1140
	 NH(x)    1140
  </PHASE>

  <PHASE urea_s>
     urea(s)  1320
  </PHASE>
  
  <PHASE biu_s>
	 biu(s)   1470
  </PHASE>  
  
  <PHASE triu_s>
	 triu(s)  1547
  </PHASE>

  <PHASE cya_s>
     cya(s)   2500
  </PHASE>
	 
</SPECIES>

<MECHANISM>
  <HOMOGENEOUS> 
     <GLOBAL>  
         3 ammd(s) > 2 ammn(s) + H2NCN(s) + 2 CO2
         <ARRHENIUS>
		     ammd(s)  1
             A/cm_units = 1e10
             Ea/kJ_mol  = 165.67
	     </ARRHENIUS>
     </GLOBAL>

     <GLOBAL>  
         ammn(s) > HNCO + 2 H2NCN(s)
         <ARRHENIUS>
             A/cm_units = 5.0e9
             Ea/kJ_mol  = 165.67
	     </ARRHENIUS>
     </GLOBAL>

     <GLOBAL>  
         ammn(s) + 3 H2NCN(s) > melem(s) + H2O(l) + NH3
         <ARRHENIUS>
             A/cm_units = 8.0e13
             Ea/kJ_mol  = 140.67
	     </ARRHENIUS>
     </GLOBAL>

     <GLOBAL>  
         melem(s)  > 6 HCN(g) + 2 N2
         <ARRHENIUS>
             A/cm_units = 6.0e6
             Ea/kJ_mol  = 165.67
	     </ARRHENIUS>
     </GLOBAL>

     <GLOBAL>  
         H2NCN(s) > NH(x) + HCN(g)
         <ARRHENIUS>
             A/cm_units = 2.0e5
             Ea/kJ_mol  = 105.67
	     </ARRHENIUS>
     </GLOBAL>

     <GLOBAL>  
         3 NH(x) > NH3 + N2
         <ARRHENIUS>
             A/cm_units = 1.0e5
             Ea/kJ_mol  = 50.67
	     </ARRHENIUS>
     </GLOBAL>

	 <REACTION> 
       urea(l) + HNCO(l) = biu(s)
       A/SIunits = 1e-4 
       Ea/kJ_mol = 0     
     </REACTION>

     <REACTION> 
       urea(l) + HNCO(l) = biu(l)
       A/SIunits = 1e-4  
       Ea/kJ_mol = 0     
     </REACTION>

     <REACTION>     
        biu(l) + HNCO(l) = triu(s)
        A/SIunits = 1e-4  
        Ea/kJ_mol = 0    
     </REACTION>

     <REACTION> 
         triu(s) > cya(s) + NH3
         A/SIunits  = 1.2e2 
         Ea/kJ_mol  = 45    
     </REACTION>

     <REACTION>  
         triu(s) > ammd(s) + H2O
         A/SIunits  = 0.3e2 
         Ea/kJ_mol  = 45    
     </REACTION>
	
  </HOMOGENEOUS>

  <INTERPHASE>
 
     <REACTION> 
        cya(s) > cya(g) 
	    A/SIunits =  3e4
        Ea/kJ_mol  = 141.3
     </REACTION>
 
     <REACTION>
        urea(l) + urea(l) > biu(l) + NH3
	    A/SIunits =  3.5e0    
        Ea/kJ_mol  = 99     
     </REACTION>
	
	 <REACTION> 
	    biu(l) + urea(l) > triu(s) + NH3
		A/SIunits =  2e2       
		Ea/kJ_mol  = 116.5          
	 </REACTION>
	
# Phase transitions 

	 <REACTION>
       H2O = H2O(l)
       A/SIunits = 0.086
       beta=0.5
     </REACTION>
	
	 <REACTION>
	   NH3 = NH3(l)
	   A/SIunits = 0.088
	   beta=0.5
	 </REACTION>
	
	 <REACTION>
       NH3 = NH3(aq)
       A/SIunits = 0.088
       beta=0.5
     </REACTION>
	
	 <REACTION>
	   NH3(aq) = NH3(l)
	   A/SIunits = 0.088
	   beta=0.5
	 </REACTION>
	 
	 <REACTION>
       urea(g) = urea(l)
       A/SIunits = 0.047
       beta=0.5
     </REACTION>
	
	 <REACTION>
       urea(aq) = urea(s)
       A/SIunits = 0.047
	   beta=0.5
     </REACTION>

	 <REACTION>
       urea(aq) = urea(l)
       A/SIunits = 0.047
	   beta=0.5
     </REACTION>
	
	 <REACTION>
	   HNCO = HNCO(l)
	   A/SIunits = 0.055
	   beta=0.5
	 </REACTION>
	
	 <REACTION>
	   HNCO = HNCO(aq)
	   A/SIunits = 0.055
	   beta=0.5
	 </REACTION>
	
	 <REACTION>
	   HNCO(aq) = HNCO(l)
	   A/SIunits = 0.055
	   beta=0.5
	 </REACTION>

	 <REACTION>
       urea(l) = urea(s)
       A/SIunits = 1e-6
     </REACTION>
	
	 <REACTION>
       biu(l) = biu(s)
       A/SIunits = 1e-6
     </REACTION>

  </INTERPHASE>
	
</MECHANISM>
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