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Chapter 3 Mass Balance Balance on Reactive Processes System.

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Presentation on theme: "Chapter 3 Mass Balance Balance on Reactive Processes System."— Presentation transcript:

1 Chapter 3 Mass Balance Balance on Reactive Processes System

2 Content  Stoichiometry  Limiting and Excess Reactant, Fractional Conversion and Extent of Reaction  Chemical Equilibrium  Multiple Reaction, Yield and Selectivity  Balance on Reactive System

3 Stoichiometry  Stoichiometry – theory of proportions in which chemical species combine with one another.  Stoichiometric equation of chemical reaction – statement of the relative number of molecules or moles of reactants and products that participate in the reaction. 2 SO 2 + O 2 ---> 2 SO 3  Stoichiometric ratio  ratio of species stoichiometry coefficients in the balanced reaction equation  can be used as a conversion factor to calculate the amount of particular reactant (or product) that was consumed (produced). 2 mol SO 3 generated2 mol SO 2 consumed 2 mol SO 2 consumed1 mol O 2 consumed

4 Test Yourself C 4 H O > 4 CO H 2 O 1.Is the stochiometric equation balance?  Yes 2.What is stochiometric coefficient for CO 2 44 3.What is stochiometric ratio of H 2 O to O 2 including it unit  4 mol H 2 O generated/ 6 mol O 2 consumed 4.How many lb-moles of O 2 reacted to form 400lb-moles CO 2  600 lb-moles O 2 reacted mol/min C 4 H 8 fed into reactor and 50% is reacted. At what rate water is formed?  200 mol/min water generated

5 Limiting Reactant & Excess Reactant  The reactant that would run out if a reaction proceeded to completion is called the limiting reactant, and the other reactants are termed excess reactants.  A reactant is limiting if it is present in less than its stoichiometric proportion relative to every other reactant.  If all reactants are present in stoichiometric proportion, then no reactant is limiting.

6 Example C 2 H 2 + 2H > C 2 H 6 Inlet condition: 20 kmol/h C 2 H 2 and 50 kmol/h H 2 What is limiting reactant and fractional excess? (H 2 :C 2 H 2 ) o = 2.5 : 1 (H 2 :C 2 H 2 ) stoich = 2 : 1 H 2 is excess reactant and C 2 H 2 is limiting reactant Fractional excess of H 2 = (50-40)/40 = 0.25

7 Fractional Conversion  Fractional Conversion (f)

8 Extent of Reaction  Extent of Reaction, ξ ξ = extent of reaction n i = moles of species i present in the system after the reaction occurred n io = moles of species i in the system when the reaction starts v i = stoichiometry coefficient for species i in the particular chemical reaction equation

9 Example N 2 + 3H > 2NH 3 Reactor inlet: 100 mol N 2 /s; 300 mol H 2 /s; 1 mol Ar/s If fractional conversion of H 2 0.6, calculate extent of reaction and the outlet composition. Unreacted H 2 or H 2 outlet= (1-0.6) 300 = 120 mol H 2 /s Solve for extent of reaction : 60 mol/s

10 Test Yourself Page C 2 H 4 + O >2 C 2 H 4 O The feed to a reactors contains 100kmol C 2 H 4 and 100kmol O 2. a) which is limiting reactant? C2H4C2H4 b) Percentage of excess? {(100-50)/50 }x100%=100% c) O 2 out? C 2 H 4 formed? Extent of reaction? 50kmol100kmol C 2 H 4 50kmol d) if fractional conversion for limiting reactant is 50%, what is outlet composition and extent of reaction? 50kmol C 2 H 4 ; extent of reaction = 25 kmol;75 kmol O 2 50 kmol C 2 H 4 O e) if reaction proceed to a point where 60kmol O 2 left, what is fractional conversion for C 2 H 4 ? Fractional conversion of O 2 and extent of reaction? f C2H4 =0.8f O2 =0.4extent of rxn=40 kmol

11 Great work is done by people who are not AFRAID to be great

12 Class Discussion Example 4.6-1

13 Chemical Equilibrium  For a given set reactive species and reaction condition, two fundamental question might be ask: 1.What will be the final (equilibrium) composition of the reaction mixture? – chemical engineering thermodynamics 2.How long will the system take to reach a specified state short of equilibrium? – chemical kinetics  Irreversible reaction  reaction proceeds only in a single direction (from reactants to products)  the concentration of the limiting reactant eventually approaches zero.  Reversible reaction  reactants form products for forward reaction and products undergo the reverse reactions to reform the reactants.  Equilibrium point is a rate of forward reaction and reverse reaction are equal  However the discussion to get the chemical equilibrium point is not covered in this text- learn in chemical engineering thermodynamic

14 Class Discussion Example CO + H2O CO2 + H2 nco= 1-ξ nH2O=2- ξ n CO2= ξ nH2= ξ ntotal= 3 K=yCO2 yH2 / y CO y H2O=1 yY CO2= ξ/3 yH2= ξ/3 y CO= (1- ξ)/3 y H2O= (2- ξ)/3

15 Multiples Reaction, Yield & Selectivity  Some of the chemical reaction has a side reaction which is formed undesired product- multiple reaction occurred.  Effects of this side reaction might be: 1.Economic loss 2.Less of desired product is obtained for a given quantity of raw materials 3.Greater quantity of raw materials must be fed to the reactor to obtain a specified product yield. selectivity= moles of desired product moles of undesired product

16 Yield  3 definition of yield with different working definition Yield= Moles of desired product formed Moles that would have been formed if there were no side reaction and the limiting reactant had reacted completely Yield= Moles of desired product formed Moles of reactant fed Yield= Moles of desired product formed Moles of reactant consumed

17 Extent of Reaction for Multiple Reaction  Concept of extent of reaction can also be applied for multiple reaction  only now each independent reaction has its own extent.

18 Class Discussion Example 4.6-3

19 Balance of Reactive Processes  Balance on reactive process can be solved based on three method: 1.Atomic Species Balance 2.Extent of Reaction 3.Molecular Species Balance


21 Atomic Species Balance No. of unknowns variables -No. of independent atomic species balance -No. of molecular balance on indep. nonreactive species -No. of other equation relating the variable ============================= No. of degree of freedom =============================

22 Extent of Reaction No. of unknowns variables +No. of independent chemical reaction -No. of independent reactive species -No. of independent nonreactive species -No. of other equation relating the variable ============================= No. of degree of freedom =============================

23 Molecular Species Balance No. of unknowns variables +No. of independent chemical reaction -No. of independent molecular species balance -No. of other equation relating the variable ============================= No. of degree of freedom =============================


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