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, 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. ______________________________________________________________________________________Bureau of Energy Efficiency 824. 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: MA = (mAP + mAW + mAU) where mAU is the unknown loss and needs to be identified. So the material balance is ______________________________________________________________________________________Bureau of Energy Efficiency 834. 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. Figure 4.2: Energy Balance for a Reheating Furnace ______________________________________________________________________________________Bureau of Energy Efficiency 844. 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