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080103__CHE_101_Full_Class_Notes_1

080103__CHE_101_Full_Class_Notes_1 - CHE 101 Chemical...

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CHE 101: Chemical Engineering Concepts 2 Processes Involving Phase Change + Energy Balances CLASS NOTES 1. Introduction Question 1: What are chemical engineers? What do they do? How are they different from chemists? Answers: Question 2: What are some of the unit operations that chemical engineers use? Answers: Question 3: Where does energy come in for the above operations? Why is it important? Answers: 1
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In this course we will understand how to perform both material and energy balances around these unit operations (in particular operations involving phase change). 2. Multiphase Systems It is common for chemical engineers to separate individual components from mixtures of those components. This is accomplished by exploiting differences in the chemical properties of the components. Often these separations are carried out in a multiphase unit operation which involve changing the phase (solid, liquid, gas) of one of the components. CONCEPTS AND DEFINITIONS : Phase : Region of uniform properties. Properties can refer to anything: chemical composition, density, colour, viscosity, heat capacity. Phase does not only refer to solid liquid and gas!!!! Homogeneous mixtures of substances can be considered one phase. EXAMPLE 1: Separation of water from a frozen wet towel. Previously (CHE 100), in order to complete materials balances around unit operations you would have to be given detailed information, such as mole fractions, about the different process streams. However it is not always feasible to acquire this information in real life. As such, we need to use process variables that can be measured and knowledge of the chemical properties of the components to calculate these values that can then be used in material balances. For operations involving phase change the most common variables that can be measured are temperature and relative humidity. Temperature of a liquid will tell us information about the vapour pressure which can be used to determine the vapour phase mole fraction. 2
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Consider the following unit operation (humidifier): 2.1. Gibbs Phase Rule Question: W hen do you have enough information to describe a system? Example: Your friend tells you that the best type of water for cleaning cars is water pressurized to 3mmHg. You try it on your car and find that it does not clean very well. Was your friend wrong? Or did you make a mistake? CONCEPTS AND DEFINITIONS The Gibbs phase rule tells us how many intensive variables we need to specify for a system in order to describe it. Extensive Variables : A variable which depends on the size of the system. Eg. Total mass, total volume. Intensive Variable : A variable which is independent of the size of the system. Extensive Variable Intensive Variables Total Volume Pressure Total Mass Temperature 3
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Figure 1: Volume has increased. What variables will change?
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