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Module 1: Basic Concepts - Material Balance
Lesson MaterialPractice Problems
Objectives
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Give some examples of the types of air pollution control problems that material balance can help solve.
- Apply the concept of material balance in solving air pollution control problems.
Material balances are one of the most basic and useful tools in the air pollution engineering field. Stated simply, a material balance means "what goes in, must come out." Matter is neither created nor destroyed in industrial processes (nonradioactive only).
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Material balances are used in a wide variety of air pollution control calculations. For example, they are used to evaluate the following:
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Formation of combustion products in boilers
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Rates of air infiltration into air pollution control systems
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Material requirements for process operations
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Rate of ash collection in air pollution control systems
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Humidities of exhaust gas streams
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Exhaust gas flow rates from multiple sources controlled by a single air pollution control system
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Gas flow rates from combustion processes
This principle, called the conservation of matter, can be applied in solving problems involving the quantities of matter moving in various parts of a process, and is illustrated in Example Problem 1.
Example Problem 1.
Conservation of Matter
This problem illustrates how a mass balance calculation can be used to check the results of an air emission test.
During an air emission test, the inlet gas stream to a fabric filter is 100,000 actual ft3/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 particulate loading 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?
Solution:
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Calculate the inlet and outlet particulate quantities in pounds mass per hour.
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Calculate the quantity of ash that will have to be removed from the hopper per hour.
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The use of material balances is illustrated in Example Problem 2.
Example Problem 2.
Material Requirements for Process Operations
How much water must be continually added to the wet scrubber shown in Figure 2 in order to keep the unit running? Each of the streams is identified by a number located in a diamond symbol. Stream 1 is the recirculation liquid flow stream back to the scrubber and it is 20 gallons per minute (gpm). The liquid being withdrawn for treatment and disposal (stream 4) is 2 gpm. Assume that inlet gas stream (number 2) is completely dry and that the outlet stream (number 6) has 10 lbm/min of moisture evaporated in the scrubber. The water being added to the scrubber is stream number 5.
Solution:
Step 1. Conduct a material balance around the scrubber.
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For Stream 6, convert from pounds per minute to gallons per minute (gpm) to keep units consistent. The conversion factor below applies only to pure water.
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Set up the material balance equation and solve for Stream 3.
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Step 2. Conduct a material balance around the recirculation tank. Solve for Stream 5.
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One of the key steps in solving Example Problem 2 was drawing a simple sketch of the system. This is absolutely necessary so that it is possible to conduct the material balances.
TIP: Drawings are a valuable first step when solving a wide variety of problems, even ones that appear simple.
The drawing is a very useful way to summarize what we know and what we need to know. It helps visualize the solution. If the problem involves dimensional quantities (such as stream flow quantities), the dimensions should be included on the sketch. They serve as reminders of the need to convert the data into consistent units.
Practice Problems
Material Balance
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Instructions:
- Complete the Practice Problems before proceeding to the next lesson. Click on the button below.