課程名稱︰質能平衡甲
課程性質︰系定必修
課程教師︰諶玉真
開課學院：工學院
開課系所︰化學工程學系
考試日期（年月日）︰97.06.10
考試時限（分鐘）：1:30 pm ~ 3:30 pm 120 分鐘
是否需發放獎勵金：是
（如未明確表示，則不予發放）
試題 :
Mass and Energy Balance
Final Exam
1. Superheated steam at 40 bar absolute and 500 ℃ flows at a rate of
250 kg/min to an adiabatic turbine, where it expands to 5 bar. The turbine
develops 1500kW. From the turbine the steam flows to a heater, where it is
reheated isobarically to its initial temperature. Neglect kinetic and potential
energy changes.
a) Write an energy balance on the turbine and use it to determine the outlet
steam temperature.
b) Write an energy balance on the heater and use it to determine the required
input (kW) to the steam.
c) Verify that an overall energy balance on the two-unit process is satisfied.
2. Wet solids pass through a continuous dryer. Hot dry air at rate of 400 kg/min
and picks up the water that evaporates from the solids. Humid air leaves the
dryer at 50℃ containing 2.44 wt% water vapor and passes through a condenser
in which it is cooled to 10℃. The pressure is constant at 1 atm throughout
the system.
a) At what rate (kg/min) is water evaporating in the dryer?
b) Use the psychrometric chart to estimate the wet-bulb temperature, relative
humidity, dew point, and specific enthalpy of the air leaving the dryer.
c) Use the psychrometric chart to estimate the absolute humidity and specific
enthalpy of the air leaving the condenser.
d) Use the results of part (b) and (c) to calculate the rate of condensation
of water (kg/min) and the rate at which heat must be transferred from the
condenser (kW).
3. A liquid mixture containing 35.0 mole% n-pentane and the balance n-hexane
at 10℃ enters a heat exchanger at a rate of 25 kmol/h. The exchanger has been
designed to heat and vaporize the liquid and heat the vapor mixture to 180℃.
The heating fluid is a high molecular weight liquid with a constant heat
capacity Cp - 2.62 kJ/(kg‧℃). It enters the exchanger at 215℃ and flows
countercurrently to the hydrocarbon mixture.
a) Calculate the enthalpy change (kJ/h) undergone by the hydrocarbon mixture
in the heat exchanger.
b) According to the heat exchanger design, the heating fluid exit temperature
should be 45℃. Assuming that all the heat lost by the heating fluid is
transferred to the hydrocarbon mixture, what is the required mass flow rate
of the heating fluid?
4. A 10.0 wt% H2SO4 solution at 60℉ is to be concentrated to 40.0 wt% by
evaporation of water. The concentrated solution and water vapor emerge from
the evaporator at 160℉ and 1 atm. Calculate the rate at which heat must be
transferred to the evaporator to process 1000 ibm/h of the feed solution.
5. An aqueous solution containing 85 wt% H2SO4 at 60℉ (specific gravity =
1.78) is diluted with pure liquid water at the same temperature to form 30
wt% H2SO4 solution. The feed solution volume is 350mL. The mixing may be
considered adiabatic, and the pressure is constant at 1 atm.
a) Use the enthalpy-concentration chart to estimate the specific enthalpies
(Btu/ibm) of the feed solution and the water. Then write an energy balance
on this closed system constant-pressure process and solve it for the
specific enthalpy of the product solution. Finally, use the enthalpy-
concentration chart to verify the calculated value and to estimate the
product solution temperature.
b) Use the enthalpy-concentration chart to estimate the maximum temperature
that could be attained by mixing the feed solution with pure water and the
concentration (wt% H2SO4) of the product solution.
c) Good laboratory practice calls for adding acid to water when carrying out
dilutions rather than vice versa. Use the enthalpy-concentration chart to
justify this rule for the dilution of the feed solution in this problem.
P.S.
有附上質能平衡課本上面後面的表
1. Table B.7
2. Figure 8.4-1 Psychrometric Chart / Enthalpy-concentration chart
3. Table B.2 (compounds from Cumene to Water)
4. Table B.1 (compounds from methyl ethyl ketone to methyl chloride)
5. Table F.3 Saturated Steam, English units. -->不是課本的
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