23-07-2012, 01:35 PM
MATERIAL AND ENERGY BALANCE
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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 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 of processing, 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 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 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 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 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.
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4. Material and Energy Balance
Raw Materials inmR1mR2mR3Energy inHeat, Work,Chemical, ElectricalER1ER2ER3UnitOperationStored MaterialsmS1mS2mS3Stored EnergyES1ES2ES3Products outmP1mP2mP3Waste productsmW1mW2mW3Energy in productsEP1EP2EP3Energy inWasteEW1EW2EW3Energy lossesTo surroundingsEL1EL2EL3
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 Raw Materials
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.
For example, 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:
M
A = (mAP + mAW + mAU)
where mAU is the unknown loss and needs to be identified. So the material balance is
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4. Material and Energy Balance
now:
Raw Materials = Products + Waste Products + Stored Products + Losses
where Losses are the unidentified materials.
Just as mass is conserved, so is energy conserved in food-processing operations. The energy coming into a unit operation can be balanced with the energy coming out and the energy stored.
Energy In = Energy Out + Energy Stored
ΣER = ΣEP + ΣEW + ΣEL + ΣES
where
ΣER = ER1 + ER2 + ER3 + ……. = Total Energy Entering
ΣEp = EP1 + EP2 + EP3 + ……. = Total Energy Leaving with Products
ΣEW = EW1 + EW2 + EW3 + … = Total Energy Leaving with Waste Materials
ΣEL = EL1 + EL2 + EL3 + ……. = Total Energy Lost to Surroundings
ΣES = ES1 + ES2 + ES3 + ……. = Total Energy Stored
Energy balances are often complicated because forms of energy can be interconverted, for example mechanical energy to heat energy, but overall the quantities must balance.
4.2 The Sankey Diagram and its Use
The Sankey diagram is very useful tool to represent an entire input and output energy flow in any energy equipment or system such as boiler generation, fired heaters, furnaces after carrying out energy balance calculation. This diagram represents visually various outputs and losses so that energy managers can focus on finding improvements in a prioritized manner.
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4. Material and Energy Balance
Example: The Figure 4.2 shows a Sankey diagram for a reheating furnace. From the Figure 4.2, it is clear that exhaust flue gas losses are a key area for priority attention.
Since the furnaces operate at high temperatures, the exhaust gases leave at high temperatures resulting in poor efficiency. Hence a heat recovery device such as air preheater has to be necessarily part of the system. The lower the exhaust temperature, higher is the furnace efficiency.
4.3 Material Balances
The first step is to look at the three basic categories: materials in, materials out and materials stored. Then the materials in each category have to be considered whether they are to be treated as a whole, a gross mass balance, or whether various constituents should be treated separately and if so what constituents. To take a simple example, it might be to take dry solids as opposed to total material; this really means separating the two groups of constituents, non-water and water. More complete dissection can separate out chemical types such as minerals, or chemical elements such as carbon. The choice and the detail depend on the reasons for making the balance and on the information that is required. A major factor in industry is, of course, the value of the materials and so expensive raw materials are more likely to be considered than cheaper ones, and products than waste materials.
Basis and Units
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 because the 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.