4. Fundamentals of Material Balance Ch-4 Part 1 2014 E.pdf

Chemical engineering

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Bahir Dar Institute of Technology
Bahir Dar University
Faculty of Chemical and Food Engineering
Introduction to Chemical Engineering
Prepared By:-Mequanint D.
Year 2014 E.C.
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Fundamentals of Material Balance
Process Classifications
•Three Types of Chemical Processes:- Concept of Boundary of the Process
1. Batch process
✓Feed is charge to the process unit at once
✓Product is removed when the process is completed
✓No mass is fed or removed from the process during the operation
✓Used for small scale production
✓Operate in unsteady state

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Process Classifications
•2. Continuous process
➢Input and output is continuously fed and remove from the process
➢Operate in steady state
➢Used for large scale production
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3. Semibatch Process
✓Neither batch nor continuous
✓During the process a part of reactant can be
✓fed input side or a part of product can be removed in output side
Process Classifications

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Two Type of Process Operations
•
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Process Classifications
Class Activity
A. Say True or False for Definition type and operation of processes given:
1.A balloon is being filled with air at steady rate of 2 g/min. It is
Semibatch and unsteady state
2.A bottle of milk is taken from the refrigerator and left in the kitchen.
It is Batch and unsteady state
3.Water is boiled in an open flask. It is Semibatch and unsteady state
B. Give examples for the Batch, Semibatch and Continuous Processes
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BALANCES
•The General Balance Equation
A balance on a conserved quantity(total mass, mass of a particular species,
energy, momentum) in a system (a single process unit, a collection of units, or
an entire process) may be written in the following general way:
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Example
Each year 50,000 people move into a city, 75,000 people move out, 22,000 are
born, and 19,000 die. Write a balance on the population of the city.

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Example
Suppose methane is a component of both the input and output streams of a
continuous process unit, and that in an effort to determine whether the unit is
performing as designed, the mass flow rates of methane in both streams are
measured and found to be different (Inlet Flowrate not equal Outlet Flowrate).Why
Inlet Flowrate not equal Outlet Flowrate?
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Possible Explanations for Observed Difference between Measured Flow Rates
➢Methane is being consumed as a reactant or generated as a product within unit.
➢Methane is accumulating in unit—possibly adsorbing on walls/other surfaces in
vessel.
➢Methane is leaking from the unit.
➢The measurements are wrong.

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Two types of balances
1. Differential balances indicate what is happening with a system at an instant in
time. Each term of the balance equation is a rate (rate of input, rate of generation,
etc.) and has units of the balanced quantity unit divided by a time unit
This is the type of balance usually applied to a continuous process
2. Integral balances describe what happens between two instants of time. Each
term of the equation is an amount of the balanced quantity and has the
corresponding unit
This type of balance is usually applied to a batch process, with the two instants of
time being the moment after the input takes place and the moment before the
product is withdrawn
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Main Points while Balancing
✓ If the balanced quantity is total mass, set generation=0 and consumption =0.
Except in nuclear reactions, mass can neither be created nor destroyed.
✓ If the balanced substance is a nonreactive species (neither a reactant nor a
product), set generation =0 and consumption=0
✓ If a system is at steady state, set accumulation =0, regardless of what is being
balanced. By definition, in a steady-state system nothing can change with time,
including the amount of the balanced quantity.
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Balances on Continuous Steady-State Process
•Steady state; Accumulation = 0
Input + Generation = Output + Consumption
N.B If balance is on steady-state & non-reactive processes;
•Generation = Consumption = 0; Input = Output
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Example
One thousands kilogram per hour of a mixture of Benzene (B) &To
luene (T) containing 50%Benzenebymassis separatedby a distillation
into two fractions. The mass flow rate of Benzene in the top stream
is 450 kg/h and that of Toluene in the bottom stream is 475 kg/h.
The operation is a steady state. Calculate the unknown components f
low rates in the output streams.

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Integral Balances on Batch Processes
•Ammonia is produced from nitrogen and hydrogen in a batch reactor.
•At time t0 there are n0 mol of NH3 in the reactor, and at a later time tf the
reaction terminates and the contents of the reactor, which include nf mol
of ammonia, are withdrawn.
•Between t0 and tf ammonia does not enter or leave through the reactor
boundaries, and ammonia is not consumed in a reaction.
•The general balance equation is simply generation = accumulation
•Moreover, the quantity of ammonia that builds up (accumulates) in the
reactor between t0 and tf is simply nf - n0 (the final amount minus the
initial amount).
•The same reasoning may be applied to any substance participating in a
batch process to obtain
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Integral Balances on Batch Processes
•.
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The input and output terms denote the initial and final amounts of the balanced
substance

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Integral Balances on Batch Processes
•Two methanol–water mixtures are contained in separate flasks. The first
mixture contains 40.0 wt% methanol, and the second contains 70.0 wt%
methanol. If 200 g of the first mixture is combined with 150 g of the
second, what are the mass and composition of the product?
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Integral Balances on Semibatch Processes
➢The procedure is to write a differential balance on the system and then to
integrate it between two instants of time
➢Required calculations are more complex; however, some problems are
relatively straightforward.
Example
Air is bubbled through a tank of liquid hexane at a rate of 0.100 kmol/min. The gas
stream leaving the tank contains 10.0 mole% hexane vapor. Air may be considered
insoluble in liquid hexane. Use an integral balance to estimate the time required to
vaporize 10.0 m3 of the liquid.
Specific gravity of liquid hexane is 0.659
Molecular weight of hexane is 86.2kg/kmol
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Flowcharts
Example
The catalytic dehydrogenation of propane is carried out in a continuous packed-bed
reactor. One thousand pounds per hour of pure propane is preheated to a
temperature of 670°C before it passes into the reactor. The reactor effluent gas,
which includes propane, propylene, methane, and hydrogen, is cooled from 800°C
to 110°C and fed to an absorption tower, where the propane and propylene are
dissolved in oil. The oil then goes to a stripping tower in which it is heated,
releasing the dissolved gases; these gases are recompressed and sent to a distillation
column in which the propane and propylene are separated. The propane stream is
recycled to join the feed to the reactor preheater. The product stream from the
distillation column contains 98% propylene, and the recycle stream is 97% propane.
The stripped oil is recycled to the absorption tower.
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Flowcharts
➢1. Learn how to organize information about process variables:- Flow
Chart drawing and Labeling (in a way convenient for calculations)
➢2. Choose a Basis
➢3. Set up material balance equations
➢4. solve the equations for unknown variables.
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Steps for Material Balance Calculations
The best way to do this is to draw a flowchart
➢Using boxes to represent process units
➢Using lines with arrows to represent inputs and outputs

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Flowcharts
•The flowchart of a process can help get material balance calculations started
and keep them moving.
•Flowchart must be fully labeled when it is first drawn, with values of known
process variables and symbols for unknown variables being written for each
input and output stream.
• Flowchart will functions as a scoreboard for the problem solution: as each
unknown variable is determined its value is filled in, so that the flowchart
provides a continuous record of where the solution stands and what must still
be done.
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Flowcharts
Labeling a Flowchart:- Two Suggestions for Labeling Flowchart
1. Write the values and units of all known stream variables at the
locations of the streams on the flowchart.
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Flowcharts
Labeling a Flowchart
Process stream can be given in two ways:
a) As the total amount or flow rate of the stream and the
fractions of each component or
b) directly as the amount or flow rate of each component.
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Flowcharts
2. Assign algebraic symbols to unknown stream Variables [such as
m (kg solution/min), x (lbm N2/lbm), and n (kmol C3H8)] and write
these variable names and their units on the flowchart.
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Flowcharts
•If that the mass of stream 1 is half that of stream 2, label the
masses of these streams as m and 2m rather than m1 and m2.
•If you know that mass fraction of nitrogen is 3 times oxygen, label
mass fractions as y g O2/g and 3y g N2/g rather than y1 and y2.
•When labeling component mass fraction or mole fraction, the last
one must be 1 minus the sum of the others.
•If volumetric flow rate of a stream is given, it is generally useful to
label the mass or molar flow rate of this stream or to calculate it
directly, since balance are not written on volumetric qualities.
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Problem
Two methanol water mixtures are contained in separate
flasks. The first mixture contains 40wt% methanol, and
the second contains 70% methanol. If 200 g of the fir
st mixture is combined 150 g of the second, what are t
he mass and composition of the product?
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Flowchart Scaling
A procedure of changing the values of all stream amounts or flow
rates by a proportional amount while leaving the stream
compositions unchanged. The process would still be balance
1.Scaling-up – if final stream quantities are larger than the
original quantities
2.Scaling down – if final stream quantities are smaller than the
original quantities.
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Flowchart Scaling & Basis of Calculation
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Flowchart Scaling & Basis of Calculation
A.Suppose you have balanced a process and the amount or flow
rate of one of the process streams is n1.You can scale the flow
chart to make the amount or flow rate of this stream n2 by
multiplying all stream amounts or flow rate by the ratio n2/n1.
B.You cannot, however, scale masses or mass flow rates to molar
quantities or vice versa by simple multiplication; conversions of
this type must be carried out using the methods as discussed in
mass fraction and mol fraction section.
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Basis of Calculation
• A basis of calculation is an amount (mass or moles) of flow rate (mass or
molar) of one stream or stream component in a process. All unknown
variables are determined to be consistent with the basis.
• If a stream amount or flow rate is given in problem, choose this quantity
as a basis
• If no stream amount or flow rate are known, assume one stream with
known composition. If mass fraction is known, choose total mass or mass
flow rate as basis. If mole fraction is known, choose a total moles or molar
flow rate as basis.
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Self-Quiz
The catalytic dehydrogenation of propane is carried out in a continuous
packed-bed reactor. One thousand pounds per hour of pure propane is
preheated to a temperature of 670°C before it passes into the reactor. The
reactor effluent gas, which includes propane, propylene, methane, and
hydrogen, is cooled from 800°C to 110°C and fed to an absorption tower,
where the propane and propylene are dissolved in oil. The oil then goes to a
stripping tower in which it is heated, releasing the dissolved gases; these
gases are recompressed and sent to a distillation column in which the
propane and propylene are separated. The propane stream is recycled to
join the feed to the reactor preheater. The product stream from the
distillation column contains 98% propylene, and the recycle stream is 97%
propane. The stripped oil is recycled to the absorption tower.
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Self-Quiz
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