Here is a compilation of essays on ‘Material and Energy Balance in Industries’ for class 9, 10, 11 and 12. Find paragraphs, long and short essays on ‘Material and Energy Balance in Industries’ especially written for school and college students.
Essay on Material and Energy Balance in Industries
Essay Contents:
- Essay on the Introduction to Material Balance and Energy Balance
- Essay on Material Balances
- Essay on Energy Balance
- Essay on the Process Flow Diagram/Chart for Energy and Material Balance
- Essay on the Energy System of Utilities in Industry
- Essay on Material & Energy Balance Procedure
Essay # 1. Introduction to Material Balance and Energy Balance:
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In a process operation; material quantities as they pass through processing operations can be described by material balances. Such balances are statements on the conservation of mass. On the similar line, energy quantities can also 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 and also equally true for the continuous operation over any chosen time interval.
Material and energy balances are very important in an industrial process. 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. If there is any change 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.
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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.
Basic Concepts of Mass & Energy Balance:
The mass and energy going into any operation unit must balance with the mass and energy coming out as shown diagrammatically in the fig. 4.1 below:
The law of conservation of mass leads to what is called a mass or a material balance.
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Mass In = Mass Out + Mass Stored
Raw Materials = Products + Wastes + Stored Materials.
∑MR = ∑MP + ∑MW + ∑Mg
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where ∑ (sigma denotes the sum of all terms) and:
∑MR = ∑MR1 + ∑MR2 + ∑MR3 + ∑MR4 = Total raw material
∑MP = ∑MP1 + ∑MP2 + ∑MP3 + ∑MP4 = Total product
∑Mw = ∑MW1 + ∑MW2 + ∑MW3 + ∑MW4 = Total waste products
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∑MS = ∑MS1 + ∑MS2 + ∑MS3 + ∑MS4 = 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 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
∑ER + ∑EP + ∑EW y + ∑EL + ∑ES
where
∑ER = ER1 + ER2 +ER3 + ER4 … = Total Energy Entering
∑EP = EP 1 + EP2 + EP3 … = Total Energy Leaving with Products
∑EW = EW1 + EW2 + EW3 + EW4 … = Total Energy Leaving with Waste Materials
∑EL + EL1 + EL2 EL3 + EL4 … = Total Energy Lost to Surroundings
∑ES = ES1 + ES2 + ES3 + ES4 … = Total Energy Stored
Energy balances are often complicated because forms of energy can be inter converted, for example mechanical energy to heat energy, but overall the quantities must balance.
Sankey Diagram for Energy Flow in a System:
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.
From the Sanky diagram for reheating furnace as shown above, it is clear that exhaust flue gas losses are of 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 pre-heater has to be necessarily part of the system. The lower the exhaust temperature, higher is the furnace efficiency.
Essay # 2. Material Balances:
In case of the material balance analysis, the first step is to consider 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 for Material Balance:
After 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.
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 is 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. Material balances can be based on total mass, mass of dry solids, or mass of particular components, for example protein.
Example 1:
Skimmed milk is prepared by the removal of some of the fat from whole milk. This skimmed milk is found to contain 90.5% water, 3.5% protein, 5.1% carbohydrate, 0.1% fat and 0.8% ash. If the original milk contained 4.5% fat, calculate its composition assuming that fat only was removed to make the skim milk and that there are no losses in processing.
Solution:
Basis: 100 kg of skim milk.
This contains, therefore, 0.1 kg of fat. Let the fat which was removed from it to make skim milk be × kg.
Total original fat = (x + 0.1 )kg
Total original mass = (100 + x) kg and as it is known that the original fat content was 4.5 % so
(x + 0.1) / (100 + x) = 0.045
where = x + 0.1 = 0.045(100 + x)
x = 4.6 kg
So the composition of the whole milk is then fat = 4.5%, water = 90.5/104.6 = 86.5 %, protein = 3.5/104.6 = 3.3 %, carbohydrate= 5.1/104.6 = 4.9% and ash = 0.8%.
Essay # 3. Energy Balance:
There are many forms of energy such as heat, kinetic energy, chemical energy, potential energy. But because of inter-conversions, 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 the heating need not be considered. Therefore practical applications of energy balances tend to focus on particular dominant aspects and so a heat balance.
When unfamiliar with the relative magnitudes of the various forms of energy entering into a particular processing situation, it is wise to put them all down. Then after some preliminary calculations, the important ones emerge and other minor ones can be lumped together or even ignored without introducing substantial errors. With experience, the obviously minor ones can perhaps be left out completely though this always raises the possibility of error.
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. In the SI system there is only one energy unit, the joule. However, kilocalories are still used by some nutritionists and British thermal units (Btu) in some heat-balance work.
Heat Balances:
The most common important energy form is heat energy and the conservation of this can be 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 be written round the various items of equipment or process stages, or round the whole plant, and it is assumed that no appreciable heat is converted to other forms of energy such as work.
Enthalpy (H) is always referred to some reference level or datum, so that the quantities are relative to this datum. Working out energy balances is then just a matter of considering the various quantities of materials involved, their specific heats, and their changes in temperature or state (as quite frequently latent heats arising from phase changes are encountered). Figure 4.3 below illustrates the heat balance.
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 processing such as sensible heat and latent heat.
Latent heat is the heat required to change, at constant temperature, the physical state of materials from solid to liquid, liquid to gas, or solid to gas. Sensible heat is that heat which when added or subtracted from materials changes their temperature and thus can be sensed.
The units of specific heat are J/kg K and sensible heat change is calculated by multiplying the mass by the specific heat by the change in temperature, (m × c × AT). The units of latent heat are j/kg and total latent heat change is calculated by multiplying the mass of the material, which changes its phase by the latent heat.
Having determined those factors that are significant in the overall energy balance, the simplified heat balance can then be used with confidence in industrial energy studies. Such calculations can be quite simple and straightforward but they give a quantitative feeling for the situation and can be of great use in design of equipment and process.
Other forms of energy:
Motor power is usually derived, in factories, from electrical energy but it can also be produced from steam engines or from waterpower. The electrical energy input can be measured by a suitable wattmeter, and the power used in the drive estimated.
There are always losses from the motors due to heating, friction and windage; the motor efficiency, which can normally be obtained from the motor manufacturer, expresses the proportion (usually as a percentage) of the electrical input energy, which emerges usefully at the motor shaft and so is available.
When considering movement, whether of fluids in pumping, of solids in solids handling, or of foodstuffs in mixers, the energy input is largely mechanical. The flow situations can be analysed by recognising the conservation of total energy whether as energy of motion, or potential energy such as pressure energy, or energy lost in friction.
Similarly, chemical energy released in combustion can be calculated from the heats of combustion of the fuels and their rates of consumption. Eventually energy emerges in the form of heat and its quantity can be estimated by summing the various sources.
Example 2:
It is desired to freeze 10,000 loaves of bread each weighing 0.75 kg from an initial room temperature of 18°C to a final temperature of -18°C. The bread-freezing operation is to be carried out in an air-blast freezing tunnel. It is found that the fan motors are rated at a total of 80 horsepower and measurements suggest that they are operating at around 90% of their rating, under which conditions their manufacturer’s data claims a motor efficiency of 86%.
If 1 ton of refrigeration is 3.52 kW, estimate the maximum refrigeration load imposed by this freezing installation assuming (a) that fans and motors are all within the freezing tunnel insulation and (b) the fans but not their motors are in the tunnel. The heat-loss rate from the tunnel to the ambient air has been found to be 6.3 kW.
Extraction rate from freezing bread (maximum) = 104 kW
Solution:
Fan rated horsepower = 80
Now 0.746 kW = 1 horsepower and the motor is operating at 90% of rating,
And so (fan + motor) power = (80 × 0.9) × 0.746 = 53.7 kW
(a) With motors + fans in tunnel
Heat load from fans + motors = 53.7 kW
Heat load from ambient = 6.3 kW
Total heat load = (104 + 53.7 + 6.3) kW = 164 kW
= 46 tons of refrigeration
(b) With motors outside, the motor inefficiency = (1- 0.86) does not impose a load on the refrigeration
Total heat load = (104 + [0.86 x 53.7] + 6.3)
= 156 kW
= 44.5 tons of refrigeration
Essay # 4. Process Flow Diagram/Chart for Energy and Material Balance:
The identification and drawing up a unit operation/process is prerequisite for energy and material balance. Flow charts are schematic representation of the production process, involving various input resources, conversion steps and output and recycle streams. The process flow may be constructed stepwise i.e., by identifying the inputs / output / wastes at each stage of the process, as shown in the Figure 4.4 below.
i. Inputs of the process could include raw materials, water, steam, energy (electricity, etc.);
ii. Process Steps should be sequentially drawn from raw material to finished product. Intermediates and any other byproduct should also be represented. The operating process parameters such as temperature, pressure, % concentration, etc. should be represented. The flow rate of various streams should also be represented in appropriate units like m3 /h or kg/h. In case of batch process the total cycle time should be included.
iii. Wastes / by products could include solids, water, chemicals, energy etc. For each process steps (unit operation) as well as for an entire plant, energy and mass balance diagram should be drawn.
iv. Output of the process is the final product produced in the plant.
Essay # 5. Energy System of Utilities in Industry:
There are various energy systems/utility services provides the required type of secondary energy such as steam, compressed air, chilled water etc., to the production facility in the manufacturing plant. A typical plant energy system is shown in Figure 4.5. Although various forms of energy such as coal, oil, electricity etc., enters the facility and does its work or heating, the outgoing energy is usually in the form of low temperature heat.
The energy usage in the overall plant can be split up into various forms such as:
i. Electrical energy, which is usually purchased as HT and converted into LT supply for end use.
ii. Some plants generate their own electricity using DG sets or captive power plants.
iii. Fuels such as furnace oil, coal are purchased and then converted into steam or electricity.
iv. Boiler generates steam for heating and drying demand.
v. Cooling tower and cooling water supply system for cooling demand.
vi. Air compressors and compressed air supply system for compressed air needs.
All energy/utility system can be classified into three areas like generation, distribution and utilisation for the system approach and energy analysis.
Essay # 6. Material & Energy Balance Procedure:
Material and Energy balances are important, since they make it possible to identify and quantify previously unknown losses and emissions. These balances are also useful for monitoring the improvements made in an ongoing project, while evaluating cost benefits.
Raw materials and energy in any manufacturing activity are not only major cost components but also major sources of environmental pollution. Inefficient use of raw materials and energy in production processes are reflected as wastes.
Guidelines for M&E balance:
(i) For a complex production stream, it is better to first draft the overall material and energy balance.
(ii) While splitting up the total system, choose, simple discrete sub-systems. The process flow diagram could be useful here.
(iii) Choose the material and energy balance envelope such that, the number of streams entering and leaving, is the smallest possible.
(iv) Always choose recycle streams (material and energy) within the envelope.
(v) The measurement units may include, time factor or production linkages.
(vi) Consider a full batch as the reference in case of batch operations.
(vii) It is important to include start-up and cleaning operation consumptions (of material and energy resources (M&E).
(viii) Calculate the gas volumes at standard conditions.
(ix) In case of shutdown losses, averaging over long periods may be necessary.
(x) Highlight losses and emissions (M&E) at part load operations if prevalent.
(xi) For each stream, where applicable, indicate energy quality (pressure, temperature, enthalpy, kCal/hr, KW, Amps, and Volts etc.).
(xii) While preparing M&E balances, precision of analytical data, flow and energy measurements have to be accurate especially in case of short time span references.
The material and energy (M&E) balances along the above guidelines, are required to be developed at the various levels.
1. Overall M&E balance:
This involves the input and output streams for complete plant.
2. Section wise M&E balances:
In the sequence of process flow, material and energy balances are required to be made for each section/ department/cost centre. This would help to prioritize focus areas for efficiency improvement.
3. Equipment-wise M&E balances:
M&E balances, for key equipment would help assess performance of equipment, which would in turn help identify and quantify energy and material avoidable losses.
Energy and mass balance calculation procedure:
The Energy and Mass balance is a calculation procedure that basically checks if directly or indirectly measured energy and mass flows are in agreement with the energy and mass conservation principles.
This balance is of the utmost importance and is an indispensable tool for a clear understanding of the energy and mass situation achieved in practice.
In order to use it correctly, the following procedure should be used:
i. Clearly identify the problem to be studied.
ii. 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.
iii. The boundary must be chosen in such a way that:
(a) All relevant flows must cross it, all non-relevant flows being within the boundary.
(b) Measurements at the boundary must be possible in an easy and accurate manner.
iv. Select an appropriate test period depending on the type of process and product.
v. Carry out the measurements.
vi. Calculate the energy and mass flow.
vii. Verify an energy and mass balance. If the balances are outside acceptable limits, then repeat the measurements.
viii. The energy release or use in endothermic and exothermic processes should be taken into consideration in the energy balance.