課程名稱︰質能平衡甲
課程性質︰系定必修
課程教師︰蔡偉博
開課系所︰化學工程學系
考試時間︰2006/6/22 13:00~15:20
是否需發放獎勵金：是的 謝謝^^
（如未明確表示，則不予發放）
試題 :
Note: When you solve the following questions, please specify the sources of
data that you use in solving the questions. (For example, from the classmate
next to me!)
1.(20 points) A stream of air at 37°C and 50% relative humidity flowing at a
rate of 1250 kg/h is to be cooled to 15°C and dehumidified in a spray tower.
The air is saturated as it emerges from the tower. Liquid water leaves the
tower at 12°C; some is withdrawn, and the rest is cooled and recirculated. No
heat is transferred between the tower and its surroundings. Calculate the rate
(kg/h) at which water must be withdrawn from the recirculation loop and the
heat duty on the cooler (kW).
2.(25 points) Molten sodium chloride is to be used as a constant-temperature
bath for a high-temperature chemical reactor. Two hundred kilograms of solid
NaCl at 300 K is charged into an insulated vessel, and a 3000 kW electrical
heater is turned on, raising the salt to its melting point of 1073 K and
melting it at a constant pressure of 1 atm.
(a) The heat capacity (Cp) of solid NaCl is 50.41J/(mol*K) at T=300K,and 53.94
J/(mol*K) at T=500K, and the heat of fusion fo NaCl at 1073 K is 30.21 kJ/mol.
Use these data to determine a linear expression for Cp (T) and to calculate
^
ΔH (kJ/mol) for the transition of NaCl from a solid at 300 K to a liquid at
1073 K.
(b) Write and solve the energy balance equation for this closed system isobaric
process to determine the required heat input in kilojoules.
(c) If 85% of the full power of 3000 kW goes into heating the salt, how long
does the process take?
3.(15 points) An aqueous ammonia solution is in equilibrium with a vapor phase
in a closed system at 160°F and 1 atm. The liquid phase accounts for 95% of
the total mass of the system contents. Determine the weight percent of ammonia
in each phase and the enthalpy of the system per unit mass of the system
contents.
4.(20 points) Ethylene oxide is produced by the catalytic oxidation of
ethylene:
C2H4 + 0.5 O2 => C2H4O
(g) (g) (g)
An undesired competing reaction is the combustion of ethylene to CO2. The feed
to a reactor contains 2 mol C2H4/mol O2. The conversion and yield in the
reactor are respectively 25% and 0.70 mol C2H4O produced/mol C2H4 consumed. A
multiple-unit process separates the reactor outlet stream components: C2H4 and
O2 are recycled to the reactor, C2H4O is sold, and CO2 and H2O are discarded.
The reactor inlet and outlet streams are each at 450°C, and the fresh feed and
all species leaving the separation process are at 25°C. The combined fresh-
recycle stream is preheated to 450°C.
(a) Taking a basis of 2 mol of ethylene entering the reactor, draw and label a
flowchart of the complete process (show the separation process as a single
unit) and calculate the molar amounts and compositions of all process streams.
(b) Calculate the heat requirement (kJ) for the entire process and that for the
reactor alone.
Data for C2H4O (g):
^。
ΔHf = -51.00 kJ/mol
Cp[J/(mol*K)] = -4.69+0.2061T-9.995*10^-5*T^2
Where T is in Kelvin.
5.(30 points) Ammonia scrubbing is one of many process for removing sulfur
dioxide from flue gases. The gases are bubbled through an aqueous solution of
ammonium sulfite, and the SO2 reacts to form ammonium bisulfite:
(NH4)2SO3 + SO2 + H2O => 2 NH4HSO3
(aq) (g) (l) (aq)
Subsequent process steps yield concentrated SO2 and regenerate ammonium
sulfite, which is recycled to the scrubber. The sulfur dioxide is either
oxidized and absorbed in water to form sulfuric acid or reduced to elemental
sulfur. Flue gas from a power plant boiler containing 0.30% SO2 by volume
enters a scrubber at a rate of 50000 mol/h at 50°C. The gas is bubbled through
an aqueous solution containing 10.0 mole% ammonium sulfite that enters the
scrubber at 25°C. The gas and liquid effluents from the scrubber both emerge
at 35°C. The scrubber removes 90% of the SO2 entering with the flue gas. The
effluent liquid is analyzed and is found to contain 1.5 moles (NH4)2SO3 per
mole of NH4HSO3. The heat of formation of (NH4)2SO3 (aq) at 25°C is -890.0
kJ/mol, and that of NH4HSO3 (aq) is -760 kJ/mol. The heat capacities of all
liquid solutions may be taken to be 4.0 J/(g*°C) and that of the flue gas may
be that of nitrogen. Evaporation of water may be neglected. Calculate the
required rate of heat transfer to from the scrubber (kW).
Bonus question (15 points): (This question is only for those students who may
fail in this course. It is not applicable to those who definitely pass this
course.)
Pure water at 60°F is mixed with 100g of an aqueous 80 wt% H2SO4 solution,also
at 60°F. The mixing vessel is insulated well enough to be considered
adiabatic.
1. If 250 g H2O is mixed with the acid, what will the final solution
temperature be?
2. What is the maximum attainable solution temperature and how much water must
be added to achieve it.
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