Chapter 3 Mass Balance Balance on Reactive Processes System

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

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 SO2 + O2 ---> 2 SO3 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 SO3 generated 2 mol SO2 consumed 2 mol SO2 consumed 1 mol O2 consumed

C4H8 + 6 O2 --------> 4 CO2 + 4 H2O Test Yourself C4H8 + 6 O2 --------> 4 CO2 + 4 H2O Is the stochiometric equation balance? Yes What is stochiometric coefficient for CO2 4 What is stochiometric ratio of H2O to O2 including it unit 4 mol H2O generated/ 6 mol O2 consumed How many lb-moles of O2 reacted to form 400lb-moles CO2 600 lb-moles O2 reacted 100 mol/min C4H8 fed into reactor and 50% is reacted. At what rate water is formed? 200 mol/min water generated

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.

Example C2H2 + 2H2 ------> C2H6 Inlet condition: 20 kmol/h C2H2 and 50 kmol/h H2 What is limiting reactant and fractional excess? (H2:C2H2) o = 2.5 : 1 (H2:C2H2) stoich = 2 : 1 H2 is excess reactant and C2H2 is limiting reactant Fractional excess of H2 = (50-40)/40 = 0.25

Fractional Conversion Fractional Conversion (f)

Extent of Reaction Extent of Reaction, ξ ξ = extent of reaction ni = moles of species i present in the system after the reaction occurred nio = moles of species i in the system when the reaction starts vi = stoichiometry coefficient for species i in the particular chemical reaction equation

N2 + 3H2 ------------> 2NH3 Example N2 + 3H2 ------------> 2NH3 Reactor inlet: 100 mol N2/s; 300 mol H2/s; 1 mol Ar/s If fractional conversion of H2 0.6, calculate extent of reaction and the outlet composition. Unreacted H2 or H2 outlet= (1-0.6) 300 = 120 mol H2/s Solve for extent of reaction : 60 mol/s

Test Yourself Page 119 2 C2H4 + O2 ------->2 C2H4O The feed to a reactors contains 100kmol C2H4 and 100kmol O2. a) which is limiting reactant? C2H4 b) Percentage of excess? {(100-50)/50 }x100%=100% c) O2 out? C2H4 formed? Extent of reaction? 50kmol 100kmol C2H4 50kmol d) if fractional conversion for limiting reactant is 50%, what is outlet composition and extent of reaction? 50kmol C2H4; extent of reaction = 25 kmol; 75 kmol O2 50 kmol C2H4O e) if reaction proceed to a point where 60kmol O2 left, what is fractional conversion for C2H4? Fractional conversion of O2 and extent of reaction? fC2H4=0.8 fO2=0.4 extent of rxn=40 kmol

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Class Discussion Example 4.6-1

Chemical Equilibrium For a given set reactive species and reaction condition, two fundamental question might be ask: What will be the final (equilibrium) composition of the reaction mixture? – chemical engineering thermodynamics 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

CO + H2O <----> CO2 + H2 Class Discussion Example 4.6-2 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

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: Economic loss Less of desired product is obtained for a given quantity of raw materials 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

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

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.

Class Discussion Example 4.6-3

Balance of Reactive Processes Balance on reactive process can be solved based on three method: Atomic Species Balance Extent of Reaction Molecular Species Balance

MUFLIS DALAM HARTA HANYA KEMISKINAN YANG SEMENTARA DI DUNIA, MUFLIS DALAM WATAK ADALAH KEMISKINAN TERBURUK DI DUNIA. IA AKAN DI BAWA BERSAMA KE AKHIRAT

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

============================= 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

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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