Material balances on processes involving chemical reactions may be solved by
applying: 1. ... balance equation is usually obtained from chemical stoichiometry.
CHE 31. INTRODUCTION TO CHEMICAL ENGINEERING CALCULATIONS
Lecture 10
Solving Material Balances Problems Involving Reactive Processes
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños
LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Material Balances on Reactive Processes
Material balances on processes involving chemical reactions may be solved by applying: 1. Molecular Species Balance – a material balance equation is applied to each chemical compound appearing in the process. 2. Atomic Species Balance – the balance is applied to each element appearing in the process. 3. Extent of Reaction – expressions for each reactive species is written involving the extent of reaction.
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños
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LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Molecular and Elemental Balances
For steady-state reactive processes,
Input + Generation = Output + Consumption The generation and consumption terms in the molecular balance equation is usually obtained from chemical stoichiometry.
But for an atomic balance, for all cases
Input = Output
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños
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LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Dehydrogenation of Ethane
Consider the dehydrogenation of ethane in a steady-state continuous reactor,
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños
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LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Dehydrogenation of Ethane
Total Balance:
Input = Output
Molecular Species Balance: C2H6:
Input – Consumed = Output
C2H4:
Generated = Output
H2:
Generated = Output
Atomic (Elemental) Species Balance: C-Balance:
Input = Output
H-Balance:
Input = Output
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños
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LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Degrees of Freedom of Analysis for Reactive Processes Molecular Species Balance
+ No. identified/labeled unknowns + No. independent chemical reactions – No. of independent molecular species – No. other equations relating unknown variables ------------------------------------------------------------------------= No. degrees of freedom
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños
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LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Degrees of Freedom of Analysis for Reactive Processes Atomic Species Balance
+ No. identified/labeled unknowns – No. independent atomic species – No. of independent nonreactive molecular species – No. other equations relating unknown variables ----------------------------------------------------------------------------= No. degrees of freedom
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños
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LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Degrees of Freedom of Analysis for Reactive Processes Extent of Reaction
+ No. identified/labeled unknowns + No. independent chemical reactions – No. of independent reactive molecular species – No. of independent nonreactive molecular species – No. other equations relating unknown variables ----------------------------------------------------------------------------= No. degrees of freedom
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños
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LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Independent Chemical Reactions, Molecular and Atomic Species
Chemical reaction: A chemical reaction is independent if it cannot be obtained algebraically from other chemical reactions involved in the same process.
Molecular Species: If two molecular species are in the same ratio to each other wherever they appear in a process, then these molecular species are not independent.
Atomic Species: If two atomic species occur in the same ration wherever they appear in a process, balances on those species will not be independent equations.
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños
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LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Independent Chemical Reactions, Molecular and Atomic Species
Consider the following reactions:
A =======> 2B B =======> C A =======> 2C Are these chemical reactions independent?
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños
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LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Independent Chemical Reactions, Molecular and Atomic Species
Consider a continuous process in which a stream of liquid carbon tetrachloride (CCl4) is vaporized into a stream of air.
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños
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LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Independent Chemical Reactions, Molecular and Atomic Species
Molecular Species Analysis Total:
3 (O2, N2, CCl4)
Independent:
2 (O2 or N2, CCl4)
Atomic Species Analysis Total:
4 (O, N, C, Cl)
Independent
2 (O or N, Cl or C)
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños
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LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Example 10-1. Production of Chlorine (Deacon Process)
In the Deacon process for the manufacture of chlorine, HCl and O2 react to form Cl2 and H2O. Sufficient air (21 mole% O2, 79% N2) is fed to provide 35% excess oxygen and the fractional conversion of HCl is 85%. Determine the amount of air required per mole of HCl fed into the process.Calculate the mole fractions of the product stream components using: a. molecular species balances b. atomic species balances c. extent of reaction
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños
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LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Example 10-1. Production of Chlorine (Deacon Process)
Identify the components of the product stream: HCl
since not all will be converted (based on fractional conversion)
O2
since it is supplied in excess
N2
it goes with the O2 in air but not consumed during the reaction
Cl2
produced during the process
H2O
produced during the process
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños
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LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Example 10-1. Production of Chlorine (Deacon Process)
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños
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LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Example 10-1. Production of Chlorine (Deacon Process)
To get mole fractions of components in the product stream:
yi = ni/nt For the identified components:
yHCl = n2/nt yO2 = n3/nt yN2 = n4/nt yCl2 = n5/nt yH2O = n6/nt where nt = n2 + n3 + n4 + n5 + n6
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños
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LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Example 10-1. Production of Chlorine (Deacon Process)
DEGREES OF FREEDOM ANALYSIS: Molecular Balance
Unit: Reactor unknowns (n1,n2,n3,n4,n5,n6)
+6
independent chemical reaction
+1
independent molecular species
–5
other equations: 35% excess O2 & fractional HCl conversion
–2
Degrees of freedom
0
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños
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LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Example 10-1. Production of Chlorine (Deacon Process)
Method I: Molecular Species Balance
35% excess O2: 0.5 molO 2 (O 2 )T 100mol HCl 25molO 2 2 mol HCl (O 2 ) A 25mol O 2 1.35 33.75molO 2 1molair n1 33.75mol O 2 160.7 molair 0.21mol O 2 160.7 mol air molair Required air 1.607 100 mol HCl mol HCl Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños
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LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Example 10-1. Production of Chlorine (Deacon Process) HCl Balance: Input – Consumed – Output = 0 (100 mol) – 0.85(100 mol) – n2 = 0 n2 = 15 mol HCl O2 Balance:
Input – Consumed – Output = 0 (33.75 mol) – 85 mol HCl react (0.5/2) – n3 = 0 n3 = 12.5 mol O2
N2 Balance:
Output = Input n4 = 160.7 mol air (0.79 mol N2/1 mol air) n4 = 127 mol N2
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños
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LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Example 10-1. Production of Chlorine (Deacon Process) Cl2 Balance:
Generated – Output = 0 85 mol HCl react (1/2) – n5 = 0 n5 = 42.5 mol Cl2
H2O Balance:
Generated – Output = 0 85 mol HCl react (1/2) – n6 = 0 n6 = 42.5 mol H2O
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños
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LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Example 10-1. Production of Chlorine (Deacon Process)
Calculation for mole fractions: Component i
ni (moles)
y
HCl
15.0
(15.0/239.5) = 0.063
O2
12.5
(12.5/239.5) = 0.052
N2
127.0
(127.0/239.5) = 0.530
Cl2
42.5
(42.5/239.5) = 0.177
H2O
42.5
(42.5/239.5) = 0.177
Total
239.5
1.000
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños
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LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Example 10-1. Production of Chlorine (Deacon Process)
DEGREES OF FREEDOM ANALYSIS: Atomic Balance
Unit: Reactor unknowns (n1,n2,n3,n4,n5,n6)
+6
independent atomic specie(s)
–3
independent nonreactive molecular specie(s)
–1
other equations: 35% excess O2 & fractional HCl conversion
–2
Degrees of freedom
0
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños
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LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Example 10-1. Production of Chlorine (Deacon Process)
From % excess O2
======> n1
From fractional conversion ======> n2 Atomic Species Balance: H-Balance:
100(1) = n2 + 2n6
O-Balance:
n1(0.21)(2) = 2n3 + n6
Cl-Balance:
100(1) = n2 + 2n5
N-Balance:
n1(0.79)(2) = 2n4
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños
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LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Example 10-1. Production of Chlorine (Deacon Process)
DEGREES OF FREEDOM ANALYSIS: Extent of Reaction
Unit: Reactor unknowns (n1,n2,n3,n4,n5,n6)
+6
independent chemical reaction(s)
+1
independent reactive molecular species
–4
independent nonreactive molecular species
–1
other equations: 35% excess O2 & fractional HCl conversion
–2
Degrees of freedom
0
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños
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LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Example 10-1. Production of Chlorine (Deacon Process)
From % excess O2
======> n1
From fractional conversion ======> n2 Extent of Reaction: HCl:
n2 = 100 – (2)
Cl2:
n5 = 0 + (1)
H2O:
n6 = 0 + (1)
N2:
n4 = 0.79n1 ± (0)
O2:
n3 = 0.21n1 – (0.5)
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños
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LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Example 10-2. Production of Ethyl Bromide
The reaction between ethylene and hydrogen bromide to form ethyl bromide is carried out in a continuous reactor.
C2H4 + HBr =====> C2H5Br The product stream is analyzed and found to contain 51.7 mole% C2H5Br and 17.3% HBr. The feed to the reactor contains only ethylene and hydrogen bromide. Calculate the fractional conversion of the limiting reactant and the percentage by which the other reactant is in excess. If the molar flow rate of the feed stream is 165 mol/s, what is the extent of reaction?
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños
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LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Example 10-2. Production of Ethyl Bromide
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños
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LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Example 10-2. Production of Ethyl Bromide
DEGREES OF FREEDOM OF ANALYSIS: Atomic Species
Unit: Reactor unknowns (x and n2)
+2
independent atomic specie(s)
–2
independent nonreactive molecular specie(s)
0
other equations
0
Degrees of freedom
0
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños
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LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Example 10-2. Production of Ethyl Bromide
Determine the limiting reactant:
Stoichiometric Ratio :
C2 H 4 1.0 HBr S
Actual Ratio :
x 165mol / s x C2 H 4 HBr A (1 x)(165mol / s) 1 x
Solve x and n2 using any 2 of the 3 atomic species balances: C-Balance H-Balance Br-Balance
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños
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LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Example 10-2. Production of Ethyl Bromide
C-Balance:
165
mol x molC2 H 4 2mol C n 2 0.310 2 n 2 0.517 2 s mol 1mol C2 H 4
330x 1.654n 2 Br-Balance:
mol 1 x mol HBr 1mol Br 165 n 2 0.1731 n 2 0.517 1 s mol 1mol HBr 165(1 x) 0.69n 2
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños
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LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Example 10-2. Production of Ethyl Bromide
Solving simulateneously, x = 0.545 mol C2H4/mol ; n2 = 108.77 mol/s Solving for the actual ratio of C2H4 and HBr in the feed:
0.545 C2 H 4 1.0 HBr A 1 0.545 Therefore, HBr is limiting.
% excess C 2 H 4
actual stoichiometric 100 actual
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños
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LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Example 10-2. Production of Ethyl Bromide
Actual feed for C2H4:
(165 mol/s)(0.545) = 89.93 mol/s Theoretical requirement for C2H4 based on stoichiometry:
mol 1 0.545 mol HBr 1mol C 2 H 4 mol 165 75.08 1mol HBr s mol s % excess C2 H 4
89.93 75.08 100 19.8% 75.08
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños
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LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Example 10-2. Production of Ethyl Bromide
Fractional conversion of HBr:
X HBr
amount reacted input output amount fed input
165 1 0.545 108.77 0.173 X HBr 0.749 1651 0.545 The can be determined based on C2H4, HBr, C2H5Br: C2H4: HBr: C2H5Br: Solving for :
0.310(108.77) = (165)(0.545) – 0.173(108.77) = (165)(1-0.545) – 0.517(108.77) = 0 –
= 56.2 mol/s
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños
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