Lecture 7 Material and Energy Balance

CHENG5163 :PLANT DESIGN AND ECONOMICS
CHAPTER
2 PROCESS DESIGN DEVELOPMENT
Lecture 7 :Mass and Energy Balance

Tesfa Nega
Faculty of Chemical and Food Engineering, Bahir Dar
Institute of Technology, Bahir Dar, Ethiopia
Email: tesfanega510@gmail.com
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Faculty of Chemical and Food Engineering
Outline
Mass and Energy Balance
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CHENG5163– PLANT DESIGN AND ECOMONICS
Mass Balance
Material balances are the basis of process design.
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Mass Balance
Material balance is also useful tool for the following:

 The study of the plant operation & troubleshooting.
 Check performance against design.
 Check the instrument calibration.
Chemical processes may be classified into as batch, continuous, or semi
batch, or either steady state or transient.
 Batch process: the feed is charged (fed) into a vessel at the beginning of
the process & the vessel content are removed sometimes later.
 Continuous process: the inputs and outputs flow continuously throughout
the duration of the process.
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Mass Balance
 Semi-batch process: any process that is neither batch nor continuous.

 If the values of all variables in the process do not change with time,
Steady state process.
 If any of the process variables change with time, transient or unsteady
state.
 A balance on a certain conserved quantity can be written as:
Material out = Material in + Generation - Consumption – Accumulation
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Mass Balance
 For steady-state non reactive system: Input = Output

[Number of equations = Number of components]
 For steady-state reactive system: Input – output + generation –
consumption = 0.0
A balance equation can be written for each separately identifiable
species present,
elements, compounds and for the total material.
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Mass Balance
Important Parameters for Reactive System
Limiting Reactant:
Stoichiometry:
Fractional Conversion:
a-Single pass conversion =
b-Overall conversion =
Selectivity: Measure of the efficiency of the reactor in converting reagent

to the desired product
= =
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Mass Balance
Yield: A measure of the performance of a reactor or a plant.
a- Reaction yield (Chemical yield)
Reaction yield = Conversion*Selectivity =
b- Plant Yield: A measure of the overall performance of the plant and
includes all chemical & physical losses (during separation process).
Plant yield=
Stoichiometric factor = theoretical moles of reagent required per moles of

product produced in the reaction balanced equation.
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Mass Balance
Excess: A reagent may be supplied in excess to promote the desired reaction
to:
 1- Maximize the use of an expensive reagent.
2- Ensure complete reaction of a reagent, as in combustion.
% Excess = = =
Note: Excess component actual feed = Theoretical feed* (1+ fraction excess)
Tie Component
Bypass: A flow stream may be divided and some part diverted (bypassed)
around some units. This procedure is often used to control stream
composition or temperature.
Recycle:
Purge:
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Mass Balance
Choice of the system boundary
System boundary: The part of the process being considered.
The system chosen must have a degree of freedom = 0.0 in order to be able to
be solved (i.e. No. of equations = No. of unknowns).
Note: For any reactive system, the following parameters
should be defined:
1- Yield or conversion.
2- % Excess if present.
3- Concentrations (strength) of reactants & products streams.
4- Recovery.
Recovery =
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Mass Balance
The standard procedures for material balances are:
1. Make any necessary assumptions (E.g. steady state, no rxn, etc.)
2. Draw a flowchart for the process & fill in all known variables and
values. Label unknown stream variables on the chart. Include either:
Total mass & stream composition
Total mole & stream composition
The mass or molar flow rate of each stream components.
3. Choose as a basis of calculation an amount or flow rate of one of the
process stream.
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Mass Balance
4. Express what the problem statement asks you to determine in terms of
the labeled variables.
5. If you are given mixed mass and mole units for a stream, convert all
quantities to one basis.
6. Do the degree of freedom analysis. Count the unknowns and identify
equations
 No of df = No of unknown – No of indp. Equ
✓ If No of df = 0, the problem can in principle be solved
✓ If No of df > 0, the problem is underspecified
✓ If No of df < 0, the problem is over specified
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Mass Balance
 Sources of equations relating unknown process stream variables include
the following:
Material balances
Energy balances
Process specifications
Physical properties and laws
Physical constraints
Stoichiometric relations
7. If the number of unknowns equal the number of equations then start
solving the equation.
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Mass Balance: Process with recycles

Recycling is used to:
Improve yield,
purity
Recover catalyst
 Without recycle stream: compute mass balance Dilute feed to
sequentially. control its
With recycle stream: Follow any one approach concentration
 Estimate the recycle stream flows and proceed with Conserve heat
calculation. Compare estimated flows with the Control of a process
calculated and a better estimate is then made. variable
 The formal algebraic method. The equations are set up
with the recycle flows as unknown and solved
simultaneously
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using standard methods. 14
Mass Balance: Process with recycles: Examples
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The HCl from the pyrolysis step is
recycled. The conversion in the
pyrolysis reactor is limited to 55%.
The unreacted dichloroethane is
separated and recycled.
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Mass Balance: Purge

Purge stream used to:
 Maintain the steady state
conditions in the system
 Prevent the accumulation of inert
or undesired materials [To rid the
process of the undesired material]
It is necessary to bleed off a portion of a recycle stream to prevent
the accumulation of inert impurities or unwanted material in the
recycle stock.
 Suppose, the reactor feed contains inert components. If these
inerts are not separated from the recycle stream in the separation
units, these inerts will accumulate in the recycle stream and the
recycle stream will finally consist of inerts only.
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Mass Balance: Purge
Concentration in the purge

stream = concentration in
the recycle stream
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Mass Balance: Purge
Consider production of ammonia from

nitrogen and hydrogen. Feed contains
0.2% argon which is inert. What should
be the purge rate to hold the argon in the
recycle stream below 5%?
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Energy Balance
Process industries have always recognized that wasting energy leads to

reduced profits.
 If a plant uses more energy than its competitors, its product could be
priced out of the marketplace.
 A chemical process may consist of reactors, pumps, compressors,
distillation columns, mixing tanks, evaporators, filter presses, particle
size reduction and transportation apparatuses. Each of these units either
requires or releases energy.
 So, the total amount of energy required in the plant should be estimated
and always need to design and operate each unit operations in such a
way that the energy requirement is minimum
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Energy Balance
 Energy can exist in several forms: heat, mechanical energy, electrical

energy, and it is the total energy that is conserved.
 The final stage of all designs is writing a design report which will
present the results of the design works.
 This report shows both engineering and economics calculations and
designs.
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Energy Balance
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THANK YOU..
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CHENG5163 :PLANT DESIGN AND ECONOMICS

2 PROCESS DESIGN DEVELOPMENT

Lecture 7 :Mass and Energy Balance

Mass and Energy Balance

Material balance is also useful tool for the following:

 Semi-batch process: any process that is neither batch nor continuous.

 For steady-state non reactive system: Input = Output

Selectivity: Measure of the efficiency of the reactor in converting reagent

Stoichiometric factor = theoretical moles of reagent required per moles of

Mass Balance: Process with recycles

Mass Balance: Process with recycles: Examples

Mass Balance: Process with recycles

Mass Balance: Process with recycles

Mass Balance: Process with recycles

Mass Balance: Process with recycles

Mass Balance: Process with recycles

Mass Balance: Process with recycles

Mass Balance: Purge

Mass Balance: Purge

Concentration in the purge

Mass Balance: Purge

Consider production of ammonia from

Process industries have always recognized that wasting energy leads to

 Energy can exist in several forms: heat, mechanical energy, electrical

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