Question 16.CSGP.94: Redo the previous Problem but include the dissociation of ox......

Redo the previous Problem but include the dissociation of oxygen and nitrogen.

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1kmol air (0.78N2,0.21O2,0.01Ar) at 2600K,1MPa1 \,kmol \text { air }\left(0.78 N _2, 0.21 O _2, 0.01 \,Ar \right) \text { at } 2600 \,K , 1 \,MPa \text {. }

 (1) O22OlnK=7.52,K1=0.000542=(yO2/yO2)P/Po\text { (1) } O _2⇔2 O \quad \ln K =-7.52, \quad K _1=0.000542=\left( y _{ O }^2 / y _{ O 2}\right) P / P _{ o }

 (2) N2>2NlnK=28.313,K2=5.056×1013=(yN2/yN2)P/Po\text { (2) } N _2 \Leftrightarrow>2 N \quad \operatorname{lnK}=-28.313, \quad K _2=5.056 \times 10^{-13}=\left( y _{ N }^2 / y _{ N 2}\right) P / P _{ o }

 (3) N2+O22NOlnK=5.316,K3=0.00491=(yNO2/yN2yO2)\text { (3) } N _2+ O _2 \Leftrightarrow 2 NO \quad \operatorname{lnK}=-5.316, \quad K _3=0.00491=\left( y _{ NO }^2 / y _{ N 2} y _{ O 2}\right)

Call the shifts a,b,c respectively so we get

nO2=0.21ac,nO=2a,nN2=0.78bc,nN=2b,nNO=2c,nAr=0.01,ntot =1+a+b\begin{aligned}& n _{ O 2}=0.21- a – c , \quad n _{ O }=2 a , \quad n _{ N 2}=0.78- b – c , \quad n _{ N }=2 b , \\& n _{ NO }=2 c , \quad n _{ Ar }=0.01, \quad n _{\text {tot }}=1+ a + b\end{aligned}

From which the mole fractions are formed and substituted into the three equilibrium equations. The result is

0.000542×0.1=yO2/yO2=4a2/[(1+a+b)(0.21ac)]5.056×1014=yN2/yN2=4b2/[(1+a+b)(0.79bc)]0.00491=yNO2/yN2yO2=4c2/[(0.79bc)(0.21ac)]\begin{aligned}& 0.000542 \times 0.1= y _{ O }^2 / y _{ O 2}=4 a ^2 /[(1+ a + b )(0.21- a – c )] \\& 5.056 \times 10^{-14}= y _{ N }^2 / y _{ N 2}=4 b ^2 /[(1+ a + b )(0.79- b – c )] \\& 0.00491= y _{ NO }^2 / y _{ N 2} y _{ O 2}=4 c ^2 /[(0.79- b – c )(0.21- a – c )]\end{aligned}

which give 3 eqs. for the unknowns (a,b,c). Trial and error assume b = a = 0 solve for c from the last eq. then for a from the first and finally given the (a,c) solve for b from the second equation. The order chosen according to expected magnitude K3>K1>K2K _3> K _1> K _2

a=0.001626,b=0.99×107,c=0.01355a =0.001626, b =0.99 \times 10^{-7}, c =0.01355 \Rightarrow

nO2=0.1948,nO=0.00325,nN2=0.7665,nN=1.98×107,nNO=0.0271,nAr=0.01yNO=nNO/ntot =0.0271/1.00165=0.02706\begin{aligned}& n _{ O 2}=0.1948, \quad n _{ O }=0.00325, \quad n _{ N 2}=0.7665, \quad n _{ N }=1.98 \times 10^{-7}, \\& n _{ NO }=0.0271, \quad n _{ Ar }=0.01 \\& y _{ NO }= n _{ NO } / n _{\text {tot }}=0.0271 / 1.00165= 0 . 0 2 7 0 6\end{aligned}

Indeed it is a very small effect to include the additional dissociations.

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