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EARVIN L. JINANG

BSECE-2





o Material and energy balances are fundamental to many engineering disciplinesand have a major role in decisions related to sustainable development. This text,which covers the substance of corresponding undergraduate courses, presents thebalance concepts and calculations in a format accessible to students, engineeringprofessionals and others who are concerned with the material and energy future ofour society.

o Following a review of the basic science and economics, the text focuses on materialand energy accounting in batch and continuous operations, with emphasis ongeneric process units, flow sheets, stream tables and spreadsheet calculations.There is a unified approach to reactive and non-reactive energy balancecalculations, plus chapters dedicated to the general balance equation andsimultaneous material and energy balances. Seventy worked examples show the

elements of process balances and connect them with the material and energyconcerns of the 21st century.



Material quantities, as they pass through processing operations, can be described by material

balances. Such balances are statements on the conservation of mass. Similarly, energy quantities can be described byenergy 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 truefor continuous operation over any chosen time interval.

Material and energy balances are very important in an industry. Material balances are fundamental to the control of

processing, particularly in the control of yields of the products. Thefirst material balances are determined in the exploratory stages of a new process, improved during pilot plantexperiments 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 the process, the material balances need to be determined

again.

The increasing cost of energy has caused the industries to examine means of reducing energy consumption inprocessing. 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 rawmaterial to the finished product.

Material and energy balances 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 and manipulated quite readily and therefore used in

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Material balances can be based on total mass, mass of dry solids, or mass of particularcomponents, for example protein.

Material balances are one of the most basic and useful tools in the air pollution engineeringfield. Stated simply, a material balance means "what goes in, must come out." Matter isneither created nor destroyed in industrial processes (nonradioactive only).

Material balances are used in a wide variety of air pollution control calculations. For example,they are used to evaluate the following:

Formation of combustion products in boilers

Rates of air infiltration into air pollution control systems

Material requirements for process operations

Rate of ash collection in air pollution control systems

Humidities of exhaust gas streams

Exhaust gas flow rates from multiple sources controlled by a single air pollution controlsystem

Gas flow rates from combustion processes



Having decided which constituents need consideration, the basis for the calculations has

to be decided. This might be some mass of raw material entering the process in a batch

system, or some mass per hour in a continuous process. It could be: some mass of a

particular predominant constituent, for example mass balances in a bakery might be all

related to 100 kg of flour entering; or some unchanging constituent, such as in

combustion calculations with air where it is helpful to relate everything to the inert

nitrogen component; or carbon added in the nutrients in a fermentation system becausethe essential energy relationships of the growing micro-organisms are related to the

combined carbon in the feed; or the essentially inert non-oil constituents of the oilseeds

in an oil-extraction process. Sometimes it is unimportant what basis is chosen and in

such cases a convenient quantity such as the total raw materials into one batch or passed

in per hour to a continuous process are often selected. Having selected the basis, then the

units may be chosen such as mass, or concentrations which can be by weight or can be

molar if reactions are important.



Concentrations :Concentrations can be expressed in many ways: weight/ weight (w/w),weight/volume

(w/v), molar concentration (M), mole fraction. The weight/weight concentration is the

weight of the solute divided by the total weight of the solution and this is the fractional

form of the percentage composition by weight. The weight volume concentration is the

weight of solute in the total volume of the solution. The molar concentration is the

number of molecular weights of the solute expressed in kg in 1 m 3 of the solution.

The mole fraction is the ratio of the number of moles of the solute to the total number of

moles of all species present in the solution. Notice that in process engineering, it is usual

to consider kg moles and in this chapter the term mole means a mass of the material

equal to its molecular weight in kilograms. In this chapter percentage signifiespercentage by weight (w/w) unless otherwise specified.



A solution of common salt in water is prepared by adding 20 kg of salt to 100 kg of

water, to make a liquid of density 1323 kg/m³

Calculate the concentration of salt in this .

solution as a (a) weight fraction, (b) weight/volume fraction, (c) mole fraction, (d) molal

concentration.

(a) Weight fraction:

20 / (100 + 20) = 0.167: % weight / weight = 16.7%Bureau of Energy Efficiency

864. Material and Energy Balance

(b) Weight/volume:

A density of 1323kg/m³

means that lm³

of solution weighs 1323kg, but 1323kg of salt

solution contains(20 x 1323 kg of salt) / (100 + 20) = 220.5 kg salt / m³

1 m³ solution contains 220.5 kg salt.

Weight/volume fraction = 220.5 / 1000 = 0.2205

And so weight / volume = 22.1%



c) Moles of water = 100 / 18 = 5.56Moles of salt = 20 / 58.5 = 0.34Mole fraction of salt = 0.34 / (5.56 + 0.34) = 0.058d) The molar concentration (M) is 220.5/58.5 = 3.77 moles in m

Note that the mole fraction can be approximated by the (moles of salt/moles of water) asthe number of moles of water are dominant, that is the mole fraction is close to 0.34 /5.56 = 0.061. As the solution becomes more dilute, this approximation improves andgenerally for dilute solutions the mole fraction of solute is a close approximation to themoles of solute / moles of solvent.In solid / liquid mixtures of all these methods can be used but in solid mixtures theconcentrations are normally expressed as simple weight fractions.

With gases, concentrations are primarily measured in weight concentrations per unitvolume, or as partial pressures. These can be related through the gas laws. Using the gaslaw in the form:pV = nRTwhere p is the pressure, V the volume, n the number of moles, T the absolute temperature,and R the gas constant which is equal to 0.08206 m³atm / mole K, the molarconcentration of a gas is then

n / V = p/RTand the weight concentration is then nM/V where M is the molecular weight of the gas.The SI unit of pressure is the N/m³called the Pascal (Pa). As this is of inconvenient sizefor many purposes, standard atmospheres (atm) are often used as pressure units, theconversion being 1 atm = 1.013 x 10Pa, or very nearly 1 atm = 100 kPa.



o This problem illustrates how a mass balance calculation can be used to check theresults of an air emission test.

o During an air emission test, the inlet gas stream to a fabric filter is 100,000 actualft3/min (ACFM) and the particulate loading is 2 grains/actual cubic feet (ACF).The outlet gas stream from the fabric filter is 109,000 ACFM and the particulateloading is 0.025 grains/ACF. What is the maximum quantity of ash that will have

to be removed per hour from the fabric filter hopper based on these test results?

Figure 1. Conservation of Matter





Energy takes many form, such as heat, kinetic ,chemical, potential energies

but because of interconversions it is not always easy to isolate separate constituents of

energy balances. However, under some circumstances certain aspects predominate. In

many heat balances in which other forms of energy are insignificant; in some chemical

situations mechanical energy is insignificant and in some mechanical energy situations,

as in the flow of fluids in pipes, the frictional losses appear as heat but the details of theheating need not be considered. We are seldom concerned with internal energies.

Energy balances can be calculated on the basis of external energy used per kilogram

of product, or raw material processed, or on dry solids or some key component. The

energy consumed in food production includes direct energy which is fuel and electricity

used on the farm, and in transport and in factories, and in storage, selling, etc.; and

indirect energy which is used to actually build the machines, to make the packaging, to

produce the electricity and the oil and so on. Food itself is a major energy source, and

energy balances can be determined for animal or human feeding; food energy input can

be balanced against outputs in heat and mechanical energy and chemical synthesis.



o The most common important energy form is heat energy and the conservation of this canbe illustrated by considering operations such as heating and drying. In these, enthalpy(total heat) is conserved and as with the mass balances so enthalpy balances can bewritten round the various items of equipment. or process stages, or round the wholeplant, and it is assumed that no appreciable heat is converted to other forms of energysuch as work.o Heat is absorbed or evolved by some reactions in processing but usually the quantities

are small when compared with the other forms of energy entering into food processingsuch as sensible heat and latent heat. Latent heat is the heat required to change, atconstant temperature, the physical state of materials from solid to liquid, liquid to gas, orsolid to gas. Sensible heat is that heat which when added or subtracted from materialschanges their temperature and thus can be sensed. The units of specific heat are J/kg Kand sensible heat change is calculated by multiplying the mass by the specific heat bythe change in temperature, (m x c x ∆T). The units of latent heat are J/kg and total latentheat change is calculated by multiplying the mass of the material, which changes itsphase by the latent heat. Having determined those factors that are significant in theoverall energy balance, the simplified heat balance can then be used with confidence inindustrial energy studies. Such calculations can be quite simple and straightforward butthey give a quantitative feeling for the situation and can be of great use in design ofequipment and process.



Energy and Mass Balance Calculation Procedure:The Energy and Mass balance is a calculation procedure that basically checks if directlyor indirectly measured energy and mass flows are in agreement with the energy and massconservation principles.This balance is of the utmost importance and is an indispensable tool for a clearunderstanding of the energy and mass situation achieved in practice.In order to use it correctly, the following procedure should be used:• Clearly identify the problem to be studied.• Define a boundary that encloses the entire system or sub-system to be analysed.Entering and leaving mass and energy flows must be measured at the boundary.• The boundary must be chosen in such a way that:a) All relevant flows must cross it, all non-relevant flows being within theboundary.b) Measurements at the boundary must be possible in an easy and accuratemanner.• Select an appropriate test period depending on the type of process and product.

• Carry out the measurements.• Calculate the energy and mass flow.• Verify an energy and mass balance. If the balances are outside acceptable limits,then repeat the measurements.• The energy release or use in endothermic and exothermic processes should betaken into consideration in the energy balance.



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