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MASS BALANCING

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WHAT IS MASS BALANCE?

MASS BALANCE (Material balance) is An accounting of all mass in a process, generally a

chemical process.

An application of the law of conservation of mass whichstates that,

Mass cannot be created nor destroyed

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CLASSIFICATION OFPROCESSES

Based on how the process varies with time.

1. STEADY-STATE PROCESS There is no change in the value of all process variables

(temperature, pressure, flow rates, heat-transfer rates)except for minor fluctuations about the mean value. Continuous processes may be steady-state.

2. TRANSIENT(UNSTEADY-STATE) PROCESS

The values of process variables change with time. Batch and semibatch process are transient by nature. Continuous processes may be transient.

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CLASSIFICATION OFPROCESSES (cont.)

Based on how the process was built to operate.1. BATCH PROCESS

No mass crosses system boundaries between the timefeed is charged and the time product is removed.

Typically used for making small quantities, particularlythose products of sporadic manufacture.

2. CONTINUOUS PROCESS Feeds and effluents continuously flow across the

system boundary through the duration of the process. Suited for the production of large quantities.

3. SEMI-BATCH PROCESS Any process that is neither batch nor continuous.

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TYPES OF BALANCES DIFFERENTIAL BALANCES

Indicate state of various rates occurring in a system atan instant in time. (Mass flow rate)

Typically applied to a continuous process.

INTEGRAL BALANCES indicate total amounts of a balanced quantity between

two instants of time. (Mass) Typical applied to a batch process.

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GENERAL MASS BALANCEEQUATION

Consider the following continuous process unit for whichmethane is a component of both the input and output,but the measured methane inlet and outlet mass flowrates are not the same.

Maybe methane is consumed as a reactant, or generated as a product

within the process unit; or accumulating within or leaking from the unit; or the measurements are wrong (though we will assume

they are correct).

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GENERAL MASS BALANCEEQUATION

INPUT + GENERATION OUTPUT CONCUMPTION =ACCUMULATION

WHERE:INPUT (enters through the system boundaries)

GENERATION (produced within system boundaries)OUTPUT (leaves through the system boundaries)CONSUMPTION (consumed within system boundaries)ACCUMULATION (build up within system boundaries)

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GENERAL MASS BALANCEEQUATIONEach year, 50,000 people move into a city; 75,000 moveout; 22,000 are born; 19,000 die. Perform a balance onthe population of the city (system).

input + generation

output

consumption =accumulation

input: 50,000 people/year generation: 22,000 people/year ouput: 75,000 people/year consumption: 19,000 people/year accumulation: unknown

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GENERAL MASS BALANCEEQUATIONEach year, 50,000 people move into a city; 75,000 moveout; 22,000 are born; 19,000 die. Perform a balance onthe population of the city (system).

input + generation

output

consumption =accumulation 50,000 P/yr + 22,000 P/yr - 75,000 P/yr - 19,000 P/yr = A

A = -22,000 P/yr

the citys (system) population is decreasing by22,000 people each year.

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RULES OF MBSIMPLIFICATION

If the balanced quantity is total mass, set generation =0 and consumption = 0

If the balance substance is a nonreactive species, set generation =0 and consumption = 0

If a system is at steady state, set accumulation = 0

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PROCEDURE FOR MBCALCULATION1. Draw and label the process flow chart (block diagram).

When labeling, write the values of known streams andassign symbols to unknown stream variables. Use theminimum number possible of symbols.

2. Select a basis of calculation. This is usually the given streamamounts or flow rates, if no given then assume an amountof a stream with known composition.

3. Write material balance equations. Note in here themaximum number of independent equations you can writefor each system is equal the number of species in the inputand output streams of this system. Also note to first writebalances that involve the fewest unknown variables.

4. Solve the equations derived in step 3 for the unknownquantities to be determined.