Balance Masa

4. Material and Energy Balance

4. MATERIAL AND ENERGY BALANCE
Syllabus
Material and Energy balance: Facility as an energy system, Methods for preparing
process flow, Material and energy balance diagrams.

Material quantities, as they pass through processing operations, can be described by
material balances. Such balances are statements on the conservation of mass. Similarly,
energyquantities can be described by energy balances, which are statements on the
conservation of energy. If there is no accumulation, what goes into a process must come
out. This is true for batch operation. It is equally true for continuous operation over any
chosen time interval.
Material and energy balances are very important in an industry. Material balances are
fundamental to the control ofprocessing, particularly in the control of yields of the
products. The first material balances are determined in the exploratory stages of a new
process, improved during pilot plant experiments when the process is being planned and
tested, checked out when the plant is commissioned and then refined and maintained as a
control instrument as production continues. When any changes occur in theprocess, the
material balances need to be determined again.
The increasing cost of energy has caused the industries to examine means of reducing
energy consumption in processing. Energy balances are used in the examination of the
various stages of a process, over the whole process and even extending over the total
production system from the raw material to the finished product.
Material and energybalances can be simple, at times they can be very complicated, but
the basic approach is general. Experience in working with the simpler systems such as
individual unit operations will develop the facility to extend the methods to the more
complicated situations, which do arise. The increasing availability of computers has
meant that very complex mass and energy balances can be set up andmanipulated quite
readily and therefore used in everyday process management to maximise product yields
and minimise costs.

4.1 Basic Principles
If the unit operation, whatever its nature is seen as a whole it may be represented
diagrammatically as a box, as shown in Figure. 4. 1. The mass and energy going into the
box must balance with the mass and energy coming out.______________________________________________________________________________________
Bureau of Energy Efficiency
82

4. Material and Energy Balance

Products out
mP1mP2mP3

Raw
Materials in
mR1mR2mR3

Waste products

Unit
Operation

mW1mW2mW3

Stored Materials
mS1mS2mS3

Energy in
products

EP1EP2EP3

Stored Energy
ES1ES2ES3
Energy in
Heat, Work,
Chemical, Electrical
ER1ER2ER3Energy in
Waste
EW1EW2EW3
Energy losses
To surroundings
EL1EL2EL3

Figure 4.1: Mass and Energy Balance

The law of conservation of mass leads to what is called a mass or a material balance.
Mass In = Mass Out + Mass Stored
Raw Materials = Products + Wastes + Stored Materials.
ΣmR = ΣmP + Σ mW + ΣmS
(where Σ (sigma) denotes the sum of all terms).
ΣmR = ΣmR1 + Σ mR2 + ΣmR3 = Total RawMaterials
ΣmP = ΣmP1 + Σ mP2 + ΣmP3

= Total Products.

ΣmW= ΣmW1 + Σ mW2 + ΣmW3 = Total Waste Products
ΣmS = ΣmS1 + Σ mS2 + ΣmS3

= Total Stored Products.

If there are no chemical changes occurring in the plant, the law of conservation of mass
will apply also to each component, so that for component A:
mA in entering materials = mA in the exit materials + mA stored in plant.
Forexample, in a plant that is producing sugar, if the total quantity of sugar going into
the plant is not equalled by the total of the purified sugar and the sugar in the waste
liquors, then there is something wrong. Sugar is either being burned (chemically
changed) or accumulating in the plant or else it is going unnoticed down the drain
somewhere. In this case:
MA = (mAP + mAW + mAU)
where mAU is...