Chapter_8_ppt - CHAPTER 8 BALANCES ON NONREACTIVE PROCESSES...

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CHAPTER 8 BALANCES ON NONREACTIVE PROCESSES
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We have seen that for an open system in which shaft work and kinetic and potential energy changes can be neglected , the energy balance reduces to - = = in i i out i i H n H n H Q ˆ ˆ where the ’s are the specific enthalpies of the inlet and outlet stream components at their respective process conditions (temperatures, pressures, and states of aggregation) relative to those components at some reference conditions. i H ˆ
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In Chapter 7, all enthalpies and internal energies could be found in tables. In this chapter, we present methods for evaluating or when tables of these proper- ties are not available . H U 8.1 ELEMENTS OF ENERGY BALANCE CALCULATIONS In this section, we outline a procedure for solving energy balance problem that will be applied to both nonreactive processes (this chapter) and reactive processes (Chapter 9).
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8.1a Reference States – A review We can never know the absolute values of and for a species at a given state. Fortunately, we never need to know the absolute values of and at specified states; we only need to know and for specified changes of state , and we can determine these quantities experimentally. We may therefore arbitrarily choose a reference state for a species and determine for the tran- sition from the reference state to a series of other states. U ˆ U ˆ H ˆ U ˆ H ˆ H ˆ ref U U U ˆ ˆ ˆ - =
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If we set equal to zero, then for a specified state is the specific internal energy at that state relative to the reference state . The specific en- thalpies at each state can then be calculated from the definition, , provided that the specific volume ( ) of the species at the given temperature and pressure is known. The values of and in the steam tables were generated using this procedure. The reference state was chosen to be liquid water at the triple point [H 2 O ( l , 0.01 , 0.00611 bar)] , at which point was defined to be zero. ref U ˆ ) ˆ ( ˆ U U = V P U H ˆ ˆ ˆ + = V ˆ U ˆ H ˆ
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According to Table B.7, for water vapor at 400 and 10.0 bar, . This means kg kJ 2958 U / ˆ = kg kJ U bar C v O H bar C l O H o o / 2958 ˆ ) 01 . 10 , 400 , ( ) 00611 . 0 , 01 . 0 , ( 2 2 = ref U U U ˆ ˆ ˆ - = 0 ) 00611 . 0 , 01 . 0 , ( ˆ = bar C l U o ref kg kJ bar C v U o / 2958 ) 101 , 400 , ( ˆ = kg kJ U / 2958 0 2958 ˆ = - =
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400 450 500 550 600 650 700 750 50 75 100 150 200 250 300 350 Table B.7 Properties of Superheated Steam
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8.1b Hypothetical Process Paths and are state properties of a species; that is, their values depend only on the state of the species – temperature , state of aggregation ( solid, liquid or gas ), and pressure . A state property does not depend on how the species reached its state. Consequently, when a species passes from one state to another, both and for the process are independent of the path taken from the first state to the second one.
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Chapter_8_ppt - CHAPTER 8 BALANCES ON NONREACTIVE PROCESSES...

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